Spatially decoupled electrochemical strategy for lime passivation prevention and sustainable phosphate recovery

Domain: chemistry
Published: 2026-01-06
Editorial outcome: minor_revision

Abstract
Lime-based precipitation, though widely adopted for wastewater phosphorus (P) removal, suffers from surface passivation. The passivation layer inhibits Ca2+ release, forcing excessive dosing while yielding low-quality sludge and effluent with elevated hardness and pH. Limestone, despite its economic and environmental benefits, exhibits limited efficacy under fluctuating alkalinity. Here, we develop a limestone-integrated electrochemical system that spatially decouples the dissolution and precipitation reactions. By strategically positioning limestone at the acidic anode, we ensure sustained Ca2+ release without passivation, while cathodicalkalinity enables efficient P recovery (85.7%). The system produces high-purity products (15.2 wt% P) at low energy consumption (14.8 kWh kg P–1) and delivers superior effluent quality. Beyond its robustness and capacity flexibility over long-term operation, the electrochemical strategy reduces overall costs by 73.2% and carbon emissions by 29.1%, positioning it as a cost-effective and sustainable alternative to traditional lime-based wastewater treatment with remarkable passivation resistance and resource recovery efficiency.

Introduction
The over-exploitation of phosphate rock and inefficient phosphorus (P) use have exacerbated P scarcity and recurrent eutrophication1,2. To promote a more sustainable P cycle, P removal and recovery from industrial, agricultural, and municipal wastewater have been widely advocated3,4. Currently, lime-based chemical precipitation remains the dominant treatment for wastewater containing P, heavy metals, and fluoride5–7. Owing to its operational simplicity and cost-effectiveness, this method is considered a gold standard for neutralizing acidic phosphate-rich wastewater and removing phosphate (PO 3–)7–9. The process typically involves dosing quicklime (CaO), hydrated lime (Ca(OH)2), or limestone (CaCO3). On dissolution, CaO generates Ca(OH)2 (Eq.1), releasing Ca2+ and OH– into wastewater (Eq. 2). Under optimal conditions, this induces supersaturation of calcium phosphate (Ca-phosphate) species, leading to their precipitation (Eq.3)10.

CaO+H2O!Ca(OH)2 (1)

Ca(OH)2!Ca2++2OH– (2)

5Ca2++3PO43–+OH–!Ca5(PO4)3(OH)# (3)

However, effective PO43– removal via lime-based precipitation typically requires either excessive CaO dosing or pre-treatment of CaO with large volumes of water to generate sufficient Ca(OH)2 and ensure adequate Ca2+ availability. For example, in treating highly acidic PO43–-laden wastewater, removing just 1.3 kg of P necessitates 14 kg CaO and 120 kg of water—representing a CaO dosage roughly four times the stoichiometric requirement7. This inefficiency likely stems from surface passivation of lime particles, whether CaO, Ca(OH)2, or CaCO3. In soil systems, PO43– is known to nucleate with Ca2+ and precipitate on calcite mineral surfaces11. Similarly, when CaO-based adsorbents (e.g., fly ash or CaO-biochar) are used for aqueous P removal, Ca-phosphate forms on the adsorbent surface12,13, impairing reactivity. A parallel mechanism occurs in ammonium recovery via struvite precipitation, where brucite14,15 or MgO16 dissolution and struvite precipitation are coupled at the solid-liquid interface. This process creates a thin fluid boundary layer that inhibits further Mg2+ release. Likewise, during lime-based precipitation, in-situ Ca-phosphate likely coats the reagent surface, inducing passivation and reducing reagent utilization efficiency for PO43– removal17.

Unfortunately, the passivation process and its impacts on wastewater P removal efficiency remain poorly understood. Surface passivation is not unique to lime-based P removal—it also critically impairs other environmental processes, including struvite recovery14–16, lime-based heavy metal removal5,18, zerovalent iron oxidative corrosion19,20, and mineral carbonation21–23, where practical solutions are urgently needed. Beyond reducing P removal efficiency, surface passivation compromises the quality and recovery of the valuable products, as unreacted lime contaminates the recovered Ca-phosphate. For instance, in treating acidic PO43–-laden wastewater, removing 1.3 kg of P requires excessive lime dosing and water pre-treatment, ultimately producing 129 kg of poorly setting, Ca-rich sludge7. A fundamental understanding of passivation mechanisms and practical mitigation strategies is essential to optimize reagent utilization and enable high-purity fertilizer production.

Electrochemical methods have garnered significant attention as next-generation wastewater treatment technologies24,25, with electrolysis emerging as an up-and-coming platform26. This process splits water molecules via direct current, generating H+ at the anode and OH– at the cathode. These electrochemically produced ions have been successfully employed in diverse applications ranging from acid and base production27, water softening28, sludge leaching29, and electrochemically mediated precipitation30–33.

Building on this foundation, we present an innovative limestone-integrated electrochemical system designed to overcome the persistent challenges of surface passivation in conventional lime-based precipitation. Our system creates two functionally distinct zones: (1) an anodic region where dissolution is enhanced by locally generated H+ (Eqs.4–5), ensuring continuous Ca2+ release while preventing particle passivation, and (2) a cathodic region where HER (hydrogen evolution reaction, Eq. 6) creates alkaline conditions for Ca-phosphate precipitation and collection (Eq.3)34.

Anode: 2H2O!4H++4e–+O2 (4)

2H++CaCO3!Ca2++H2O+CO2 (5)

Cathode: 4H2O+4e–!4OH–+2H2 (6)

This system achieves three key advantages by overcoming surface passivation. First, Ca release is enhanced, and overdosing is avoided. The acidic anode region prevents surface passivation, maximizing reagent Ca release while eliminating the need for lime overdosing. This breakthrough makes low-solubility but cost-effective CaCO3 (Ksp=10–9)35 (previously unsuitable for conventional precipitation) now fully applicable. Second, high-quality phosphate products are recovered: the cathodically formed Ca-phosphate precipitates as dense solids rather than sludge through homogeneous precipitation. The absence of lime overdosing further ensures product purity. Lastly, the system induces Ca-phosphate precipitation by creating a localized supersaturation at the cathode without elevating the bulk solution pH. This avoids the adjustment of the whole bulk pH and discharge of strongly alkaline effluent, as found in conventional lime-based precipitation.

This study investigates wood-based activated carbon (WAC) wastewater as a representative PO43–-laden stream. In the activated carbon industry, phosphoric acid activation generates acidic, PO43–-rich effluents7. We first evaluate three conventional lime-based precipitation methods (CaO, Ca(OH)2, and CaCO3), elucidating their surface passivation mechanisms and limitations. Subsequently, we demonstrate the effectiveness of our limestone-integrated electrochemical system in overcoming these challenges through targeted experiments and mechanistic analysis. A comprehensive comparison between electrochemical and conventional methods is presented, assessing PO43– removal efficiency, reagent utilization, product quality, and effluent characteristics. The system’s additional benefits, particularly in terms of chemical oxygen demand (COD) reduction, are also quantified. Techno-economic analysis (TEA) and Life Cycle Assessment (LCA) highlight the operational advantages of the electrochemical approach, supporting its practical implementation. This work provides an innovative solution to surface passivation in phosphate recovery and offers insights applicable to other processes (i.e., mineral carbonation) plagued by similar interfacial limitations.

Results
Thermodynamic insights
The WAC wastewater exhibits strong acidity (pH 1.9) due to residual phosphoric acid from the activation process, with phosphate (1244.4 mg L–1) as the dominant anion (Supplementary Table 1). Minor metal cations (Ca2+, K+, Na+, Mg2+, and Al3+) are present, alongside high water turbidity (255.6 NTU) from activated carbon fines and elevated COD (438.0 mg L–1) from tail gas treatment7. The existing lime-based precipitation systems struggle with three key inefficiencies: (1) excessive lime demand due to surface passivation, (2) voluminous, poor-settling sludge, and (3) highly alkaline effluents, undermining economic and ecological sustainability.

To address these limitations, Ye et al. proposed fluidized-bed struvite crystallization for WAC wastewater treatment7. The authors highlighted that the wastewater’s firm acidity enhances MgO dissolution, lowering reagent demand while producing struvite as a slow-release fertilizer7. However, both CaO and MgO dosing face inherent particle passivation issues. Given CaO’s cost advantages over MgO, our study explicitly targets surface passivation in lime precipitation. Before conducting lime dosing experiments, we modeled the effects of Ca2+ supply and pH elevation using Hydra-Medusa software. Phosphate speciation in WAC wastewater, initially dominated by H3PO4 and H2PO4–, progressively shifted to HPO42– and PO43– with increasing pH (Supplementary Fig. 1a). Notably, precipitation only occurred upon Ca2+ introduction, as pH adjustment alone proved insufficient. At a Ca/P molar ratio of 1:1, brushite (CaHPO4·2H2O) began precipitating at pH ~3.5 (Supplementary Fig. 1b). The system favored the formation of thermodynamically stable hydroxyapatite (Ca5(PO4)3OH, HAP) under neutral to basic conditions10. Accordingly, complete P removal was realized at an elevated Ca/P ratio of 1.67:1 to account for HAP formation (Supplementary Fig. 1c). Interestingly, excess Ca2+ (Ca/P ratio of 3:1 or 4:1) lowered the pH threshold due to enhanced thermodynamic driving forces (Supplementary Fig. 1d, e), demonstrating the critical role of Ca2+ concentration in precipitation kinetics.

Mechanistic insights on lime surface passivation
While thermodynamic modeling suggested that lime dosing at a Ca/P of 1:1 should achieve almost complete P removal (based on brushite’s stoichiometry, Supplementary Fig. 1b), experimental results revealed only 2.3% of P removal at this ratio. Even at the theoretically optimal Ca/P ratio for HAP formation (1.67:1), merely 60.1% P removal was achieved (Fig. 1a). Significant P removal efficiencies (91.9–100.0%) required substantial lime overdosing at Ca/P ratios of 3:1 and 4:1, respectively.

Notably, despite using fine lime powder (<0.15 mm) with vigorous mixing, granules with varying sizes were generated after dosing (Fig. 1b), exhibiting a broad particle size distribution up to 6.0 mm (Fig. 1c). At Ca/P 1:1, only 28.4 wt% of particles were below 0.15 mm, with the remainder distributed across multiple size fractions: 19.9 wt% (0.15–0.5 mm), 16.8 wt% (0.5–1.0 mm), 18.8 wt% (1.0–2.0 mm), 4.1 wt% (2.0–3.0 mm), 9.6 wt% (3.0–4.0 mm), and 2.4 wt% (4.0–5.0 mm) (Fig. 1c).

The observed reduction in P removal and the formation of granular products stem from surface passivation during lime precipitation. When lime is introduced to the aqueous system, it rapidly hydrates to Ca(OH)2, releasing substantial heat (ΔH =–104 kJ mol–1)36 and consuming water. This exothermic reaction dramatically decreases Ca(OH)2 solubility with increasing temperature, promoting rapid nucleation and growth of Ca(OH)2 crystals that readily agglomerate (Fig. 1d)—a phenomenon well-documented in lime-based thermochemical heat storage materials37,38.

In wastewater treatment, these agglomerates release Ca2+ and OH–, creating localized conditions favorable for Ca-phosphate precipitation. Due to its low solubility product (Ksp=10–6.59 for brushite)39, Ca-phosphate readily forms a passivation layer on particle surfaces, inhibiting further Ca2+ release and limiting P removal (Fig. 1d). Solid-phase characterization confirmed this mechanism: XRD analysis identified that brushite and Ca(OH)2 coexist as primary phases (Fig. 1e). Cross-sectional SEM revealed a distinct core-shell structure (Fig. 1f), with a shell thickness of around 20 μm (Supplementary Fig. 2). The associated elemental mapping showed that Ca and O are present evenly throughout the particles, while P is exclusively concentrated in the shell layer (Fig. 1g–i). The flake-like morphology of the shell layer of the generated granule (Supplementary Fig. 3), as shown in Supplementary Fig. 4a, corroborated its brushite composition40,41. Moreover, the even distribution of Ca, P, and O on the shell layer (Supplementary Fig. 4b–d), with nearly identical atomic percentages of Ca (10.7 atom%) and P (10.1 atom%), further confirm that the shell layer is composed of brushite (Supplementary Fig. 4e). In contrast, the core of the granules only contains Ca and O, with a ratio (1:2) consistent with that of Ca(OH)2 (Supplementary Fig. 5a, b).

Surface passivation significantly reduces lime utilization efficiencies in P removal. Consequently, at a Ca/P of 1:1, only 2.0% of the dosed lime is effectively utilized (Fig. 1j). The passivation process renders a substantial portion of Ca(OH)2 inactive, and even the dissolved fraction fails to elevate pH (4.0) sufficiently for adequate Ca-phosphate precipitation despite available Ca2+ (Fig. 2a, b). While increasing the Ca/P of 1.67:1 improves P removal and raises lime use efficiency to 32.1%, further overdosing to Ca/P=4:1 (required for complete PO43– removal) reduces efficiency to 20.9% (Fig. 1j). The fundamental mechanism of surface saturation-induced passivation appears widely applicable across chemical systems. For example, in the realm of struvite precipitation, MgO/Mg(OH)2 surfaces experience passivation through struvite overgrowth, limiting dissolution and reagent efficiency14,16,42,43. Likewise, silicate minerals or alkaline solid waste show comparable carbonate-induced passivation during CO2 capture21–23.

Notably, our study reveals a distinct phenomenon not reported in the literature: the formation of mm-scale granules during lime treatment. This unique observation suggests that lime’s characteristic exothermic conversion to Ca(OH)2, combined with the subsequent agglomeration caused by Ca(OH)2’s retrograde solubility behavior, may constitute the critical mechanistic step leading to passivation. To verify this hypothesis, we performed comparative experiments using preformed Ca(OH)2 (Supplementary Fig. 6). Strikingly, at a Ca/P molar ratio of 1:1, Ca(OH)2 dosing achieved 98.2% PO43– removal efficiency (Supplementary Fig. 6a), representing a dramatic improvement over the mere 2.3% efficiency observed with lime under identical conditions (Fig. 1a). Simultaneously, the reagent usage efficiencies of hydrated lime (37.9–106.1%) were notably higher than that of lime (Supplementary Fig. 6c). This improved performance originates from the fundamental difference in dissolution behavior: Ca(OH)2 directly provides Ca2+ and OH– ions without undergoing the agglomeration associated with lime’s exothermic hydration process. Consequently, Ca(OH)2 dosing facilitates phase-pure brushite production when dosed appropriately (Supplementary Fig. 6d). Still, owing to the intrinsic nature of homogeneous crystallization in chemical precipitation, marked by instantaneous high supersaturation and the generation of dispersed small nuclei, even hydrated lime—despite avoiding passivation—inevitably forms poorly settling sludge (Supplementary Fig. 6e) with high water content (Supplementary Fig. 6f). This ultimately results in a low P content in the collected sludge (3.7–6.1 wt% P, Supplementary Fig. 6g).

Overall, these findings provide conclusive evidence that lime passivation originates from three sequential phenomena: (1) the exothermic hydration of lime to form Ca(OH)2, followed by (2) particle agglomeration induced by the retrograde solubility characteristics of Ca(OH)2, and (3) the complete coverage of the agglomerates by the Ca-phosphate layer. This mechanistic understanding represents a significant advancement in our knowledge of phosphate precipitation processes, explaining the previously observed limitations of lime-based treatment approaches.

Lime passivation compromises effluent and product quality
Lime passivation not only reduces P removal efficiency but also compromises effluent and product quality. Achieving adequate P removal (e.g., at Ca/P=4:1) requires excessive lime dosing, yielding an alkaline effluent (pH 12.7) with elevated Ca2+ (936.7 mg L–1, Fig. 2a, b), increasing scaling risks as well as post-treatment efforts in the downstream process44. Moreover, lime precipitation consistently produces low-quality sludge across all Ca/P dosing ratios (Fig. 2c, d), thereby dictating their current destination as hazardous alkaline waste in practical applications7. The passivation was a key driver in reducing P content, leading to unreacted Ca(OH)2 contamination in the generated sludge. At Ca/P=1:1, the P content is only 0.6 wt% (Fig. 2d), which is far below the theoretical value of brushite (18.0 wt% P). At Ca/P=1.67:1, purity improves to 6.9 wt% P with higher P removal. Further overdosing (Ca/P=4:1) ensures P removal but reduces purity to 2.9 wt% P due to excess Ca(OH)2.

Another factor contributing to the inferior quality of the sludge turns out to be the high moisture content. The collected product exhibited elevated moisture contents (52.0–74.2%), showing a distinct trend with varying Ca/P ratios (Fig. 2e). Moisture levels remained high at Ca/P=1:1 (70.0%), decreased to 52.0% at 1.67:1, and peaked at 74.2% under overdosed conditions (Ca/P=4:1). This non-monotonic trend inversely correlated with both CaO utilization efficiency (Fig. 1j) and product purity (Fig. 2d), suggesting a mechanistic link to the Ca(OH)2 content in the solids.

We attributed this behavior to two key factors: (1) the hygroscopic nature of Ca(OH)2, whose hydroxyl groups promote stronger water retention than crystalline Ca-phosphate; (2) the amorphous structure of Ca(OH)2-rich solids, which enhances moisture adsorption capacity. At a low Ca/P ratio (1:1), passivation leaves substantial unreacted Ca(OH)2 in the product, elevating moisture. Improved CaO efficiency at an intermediate ratio (1.67:1) increases crystalline brushite content in the obtained product, reducing hygroscopicity. However, excessing dosing (4:1) reintroduces Ca(OH)2 dominance, maximizing moisture retention. These findings align with known material properties—Ca-phosphates like brushite exhibit lower water affinity than Ca(OH)2, consistent with our observed moisture trends.

Limited efficacy of limestone dosing
Lime and hydrated lime production are highly carbon- and energy-intensive processes. Direct use of their precursor material, limestone, could substantially reduce carbon emissions, manufacturing costs, and transportation hazards associated with corrosive chemicals (Fig. 3a). However, our experiments revealed severe limitations in limestone’s P removal capability: at Ca/P ratios of 1:1, 1.67:1, and 3:1, only 0.3%, 0.8%, and 0.4% P removal were achieved, respectively, after settling (Fig. 3b). Even at a 4:1 ratio, P removal efficiency plateaued at merely 11.0%, with CaCO3 remaining the dominant solid phase (Fig. 3d).

This poor performance stems from fundamental chemical constraints. While limestone can dissolve to release Ca2+ and create localized high-pH microenvironments at its surface—enabling transient Ca-phosphate precipitation—it cannot directly supply sufficient OH– to maintain bulk wastewater alkalinity (Fig. 3e). Consequently, precipitates re-dissolve as they encounter the low-pH bulk solution (Fig. 3e and Supplementary Fig. 7). This mechanism is supported by two key observations: initial P removal (5.6–94.4%, after 1-h mixing) was significantly higher than final removal (0.3–11.0%, after proper settling) (Fig. 3b); corresponding increases in aqueous Ca2+ concentrations (e.g., from 203.2 to 961.8 mg L–1 at Ca/P=4:1, Fig. 3c) confirmed precipitate instability.

These findings highlight a fundamental limitation of chemical precipitation approaches: the inseparable coupling of Ca2+ release and phosphate precipitation. In lime systems, this coupling causes surface passivation on Ca(OH)2 agglomerates. For limestone, it leads to precipitate redissolution unless spatial separation can be achieved, where limestone dissolves in acidic zones while phosphate products remain stabilized in alkaline regions.

Spatially decoupled electrochemical strategy for enhanced calcium release and phosphate recovery
Water electrolysis offers a viable solution by establishing a sustained and distinctly differentiated acidic condition at the anode and an alkaline condition at the cathode. Water oxidation delivers an acidic region at the anode, in which limestone undergoes continuous dissolution, steadily supplying Ca2+. Furthermore, the acidity suppresses Ca-phosphate formation on the limestone surface, effectively preventing surface passivation. Meanwhile, the HER creates an alkaline condition at the cathode for Ca-phosphate precipitation and collection (Eq.3)34.

Anodic acidity for passivation elimination
To implement this configuration, limestone was positioned adjacent to the anode, with a Ru-Ir mesh basket functioning as both the anode and a porous container (Fig. 4a). The Ca2+ release kinetics were initially evaluated in a 10 mM Na2SO4 electrolyte without PO43–, with the pH pre-adjusted to 1.9. At a current density of 18 Am–2, the Ca2+ concentration steadily increased, reaching 470.6 mg L–1 in 2 h (Supplementary Fig. 8a), while the solution pH rose to 6.1 (Supplementary Fig. 8a)—consistent with limestone dissolution driven by electrochemically generated H+. Notably, Ca2+ release correlated with current density: higher currents produced more H+, intensifying the H+-limestone reaction and enhancing Ca2+ release (Supplementary Fig. 8b). These results underscore the critical role of anodic acidity in sustaining Ca2+ supply.

In PO43–-rich WAC wastewater, Ca2+ concentrations displayed a characteristic rise-and-fall trajectory across all tested current densities (Fig. 4b). Under conditions of stable H+-limestone reactivity, this trend reflected progressively intensifying Ca-phosphate precipitation throughout electrolysis. During the initial phase (<8 h), the system exhibited net Ca2+ accumulation, as precipitation kinetics were constrained by the low baseline Ca2+ concentration (79.3 mg L–1 Ca compared to 1244.4 mg L–1 P, Fig. 4b and Supplementary Table 1). This accumulating Ca2+ elevated the saturation index (SI) of Ca-phosphate species (Eq.8), thereby increasing the thermodynamic driving force for precipitation. As the reaction progressed, Ca2+ consumption eventually outpaced supply, leading to a net concentration decline. Critically, higher current densities amplified this effect: by enhancing the thermodynamic driving force at the cathode, they accelerated Ca-phosphate precipitation34, ultimately yielding lower residual Ca2+ levels. Notably, one may argue that the inherent acidity of WAC wastewater may also play a role in Ca supply. Indeed, while Ca2+ release correlated with current density in the absence of PO43-, the trend observed is relatively modest. For instance, increasing the current fourfold from 4.5 to 18 Am–2 only increased Ca2+ release from 455.2 to 479.2 mg L–1 in 2 h (Supplementary Fig. 8b). As demonstrated in the limestone chemical dosing, the low pH of the WAC wastewater itself could indeed contribute to limestone dissolution and Ca2+ supply in the first place. However, the subsequent precipitation and redissolution of Ca-phosphate remained an inevitable limit (Fig. 3e). Thus, while the acidity of WAC wastewater could contribute to limestone dissolution, the key to stable and efficient Ca2+ supply lies in sustained anodic acidity, specifically by preventing surface passivation.

We noticed that continuous operation for 36 h under 18 Am–2 preserved the original limestone morphology with no observable alterations (Fig. 4c, e). Post-reaction characterization confirmed the maintained bulk composition (Ca, C, and O) without P enrichment (Fig. 4d, f), as evidenced by both surface analysis and acid-digestion elemental comparisons (Supplementary Table 2). The passivation resistance stemmed from the persistently acidic anodic environment (pH<3.5 at 18 Am–2, Supplementary Fig. 9), which remained below the threshold required for Ca-phosphate precipitation—consistent with the thermodynamic predictions (Supplementary Fig. 1)—while still being sufficiently acidic, ensuring Ca2+ supply via limestone dissolution.

The absence of passivation was further verified by spectroscopic analyses. Raman spectroscopy revealed preserved calcite signatures, with the characteristic ν1 symmetric CO32– stretch at 1087 cm–145, along with associated peaks at 155, 282 (ν2), and 714 cm–1 (ν4) (Fig. 4g). Similarly, XRD patterns matched reference calcite profiles (Fig. 4h). Notably, unlike lime, which readily forms Ca-phosphate passivation layers, the electrolysis-integrated limestone showed no evidence of secondary mineral phases, confirming the effectiveness of anodic acidification in maintaining surface reactivity.

Cathodic alkalinity for enhanced phosphate recovery
The limestone-integrated electrochemical system maintains stable anodic acidity while generating cathodic alkalinity via HER, creating a localized high-pH environment at the cathode without significantly increasing the bulk pH34. Combined with Ca2+ supplied from the anode, this configuration establishes ideal supersaturation conditions for Ca-phosphate crystallization at the cathode surface, enabling efficient P precipitation and recovery (Fig. 4a). As primary ions in this framework, the migration of Ca²⁺ and H3-xPO4(x-3) toward the cathode is primarily driven by mass diffusion, driven by their consumption due to precipitation (Fig. 4a). For cations such as Ca²⁺, electromigration also plays a role in their movement toward the cathode. Importantly, in addition to mass diffusion, H3-xPO4(x-3), as a buffer, tends to react with OH– generated, further promoting its migration toward the cathode (Fig. 4a). While the electric field opposes H3-xPO4(x-3) migration, among the three current-related driving forces, the combined influence of mass diffusion and buffering predominates, as evidenced by the positive correlation between current and P removal (Fig. 5a), rather than a decline, which would be expected if electromigration prevailed.

Specifically, P removal efficiency demonstrated strong current-dependence: 36.0% P at 4.5 Am–2 (36 h), increasing to 47.3% at 9 Am–2 and reaching 85.7% at 18 Am–2 (Fig. 5a). This enhancement arises from intensified electrode reactions at higher currents: accelerated H+ generation at the anode improves limestone dissolution kinetics and Ca2+ release; enhanced HER at the cathode creates more pronounced local alkalinity. The synergistic effect of increased Ca2+ availability and elevated local pH provides a greater thermodynamic driving force for Ca-phosphate crystallization, ultimately boosting P recovery efficiency.

XRD and SEM-EDS analyses confirmed Ca-phosphate crystallization in the recovered products. As shown in Fig. 5b, the product obtained at 4.5 Am–2 displayed distinct brushite patterns. SEM reveals the characteristic stacked plate-like morphology of this crystalline phase with even distribution of Ca, P, and O (Supplementary Fig. 10a–d). EDS analysis of selected plate-like products further verified brushite formation, showing consistent Ca/P ratios (~1.0) across sampled regions (Supplementary Fig. 10e, f). However, at higher current densities (18 Am–2), the appearance of a broadened peak around 2θ=30°—accompanied by sphere particles in SEM images with uniform distribution of Ca, P, and O (Fig. 5b and Supplementary Fig. 11a–d)—indicated the formation of amorphous calcium phosphate (ACP). This current-dependent crystallinity transition aligns with established literature demonstrating that rapid pH elevation in complex wastewater matrices promotes ACP formation10,32,46,47. Specifically, the combination of high local alkalinity at elevated current densities and the presence of interfering species (e.g., Mg2+ and NOMs) can inhibit the transformation of ACP into crystalline phases, explaining our observed predominance of amorphous products under more intensive operating conditions.

Notably, the electrochemical approach achieves superior Ca-phosphate recovery (13.9–15.2 wt% P) compared to conventional lime precipitation (0.6–6.9 wt% P) by fundamentally addressing its key limitations (Fig. 5c). Unlike lime systems, where surface passivation necessitates reagent overdosing and compromises product purity, the electrochemical method prevents passivation through spatially separated reaction zones. The physical segregation enables Ca-phosphate crystallization to proceed without limestone interference, yielding high-purity products. More importantly, electrochemically induced heterogeneous precipitation provides sustained supersaturation conditions that preferentially promote post-nucleation growth, forming larger Ca-phosphate particles that aggregate into dense, low-moisture precipitates (Fig. 5d). In contrast, conventional chemical precipitation creates instantaneous high supersaturation, generating numerous small nuclei dispersed as high-water-content sludge with poor settling properties—a direct consequence of rapid, uncontrolled homogeneous nucleation32,48.

The electrolysis process induced gradual pH elevation in WAC wastewater, reaching 4.4, 4.2, and 4.0 after 36 h at current densities of 4.5, 9, and 18 Am–2, respectively (Fig. 5e). This pH increase results from two coupled processes: H+ consumption during limestone dissolution (Eq.5) and OH– accumulation through cathodic HER (Eq.6). All current conditions displayed similar temporal patterns—rapid pH increase within the initial 8 h followed by stabilization and eventual slight decline at 4.5 and 9 Am–2. This behavior confirms our previous observation that OH– consumption by Ca-phosphate crystallization begins to dominate over H+-consumption (from Ca2+ release) after approximately 8 h of operation. The inverse current density-pH relationship reflects enhanced OH– consumption at higher current densities, where accelerated Ca-phosphate precipitation occurs.

More importantly, the electrochemical approach demonstrates significant advantages over conventional lime precipitation in effluent quality (Figs. 4b and 5e). At optimal P removal conditions (18 Am–2), the system produced effluent with markedly low residual Ca2+ (88.6 mg L–1) and pH (4.0), compared to lime treatment (936.7 mg L–1 Ca2+, pH 12.7 at Ca/P=4:1), not to mention its low specific energy consumption (14.8-19.5 kWh kg P–1, Fig. 5f) and competitive cost efficiency ($1.4-1.9 kg P–1 removed) within a wider range of processes targeting P removal/recovery ($1.4-422.3 kg P–1 removed at 5.6-1300.0 mg L–1 P) (Supplementary Fig. 12 and Supplementary Table 3). This breakthrough eliminates two major drawbacks of lime-based systems: scaling risks from high Ca2+ concentrations44 and two-step pH swing (alkalinization and neutralization). On top of passivation elimination and enhanced P recovery, the electrochemical system demonstrated a notable added capability for COD removal. In chemical precipitation, lime, hydrated lime, and limestone dosing exhibited only modest COD removal of 6.7–13.0%, 8.4–11.0%, and 7.4–15.9% (Supplementary Fig. 13), respectively, likely due to co-precipitation by the generated sludge47,49. In contrast, even at a low current density of 4.5 Am–2, the electrochemical system achieved a COD removal efficiency of 79.3%, which increased to a maximum of 87.0%, as the current density was elevated to 18 Am–2 (Supplementary Fig. 13). This enhanced COD removal may be attributed to anodic direct oxidation and active chlorine species generation at the inert anode50, with chloride ions (Cl–) present in the WAC wastewater at 127.5 mg L–1 (Supplementary Table 1).

Long-term robustness and implementation outlook
To evaluate the long-term performance and stability of the spatially decoupled electrochemical strategy, we conducted a 864-h (36-d) continuous operation with the same setup. At an initial setting (9 Am–2; HRT 36 h), the system attained 60.0% P recovery within the first few days (Supplementary Fig. 14). Upon increasing the current to 18 Am–2, the P recovery remained stable at 86.2–96.5% for over a week (Supplementary Fig. 14). Anticipating scenarios that require higher treatment capacities, the HRT was shortened to 6 h, and the current density was further elevated to 72 Am–2 following a decline in recovery efficiency. Subsequently, the system shows promising P recovery stabilizing between 72.9–93.3% under 72 Am–2 and HRT 6 h for a further 20 days (Supplementary Fig. 14). Throughout the operation, limestone was replenished on days 21 and 35 upon noticing a decline in P recovery, highlighting the importance of adequate limestone supply in the system’s practical implementation. Overall, the system displayed good stability and flexible recovery capacity via the conditions tuning.

The continuous anti-passivation and utilization of limestone were further validated under prolonged operation. Even excluding the limestone that was fully consumed, the remaining limestone exhibited a notable decrease in particle size, shifting from an initial dominant range of 2.0–3.0 mm to 1.0–2.0 mm, with a minor fraction falling below 1.0 mm (Supplementary Fig. 15). Throughout the dissolution, the surface of the limestone remained consistent (Supplementary Fig. 16a). The limestone before and after long-term operation both exhibited strong Ca 2p signals, indicative of CaCO3, with no P detected (Supplementary Fig. 16b, c). These findings collectively illustrate that limestone underwent continuous dissolution from the outside in and maintained its resistance to passivation through utilization.

The electrochemical cell’s durability is pivotal for practical applications. Inspiringly, the Ru-Ir coated Ti anode, renowned for its electrochemical stability, exhibited exceptional performance with no detectable Ru or Ir leaching over the extended operation (Supplementary Table 4). Importantly, the precipitates/products remained on the cathode over extended use. These precipitates/products can be effectively harvested via physical scraping without compromising the integrity of the cathode, allowing its prompt cleaning and reuse for the next cycles (Supplementary Fig. 17). Foreseeing more automated implementations, scraper integration51 and polarity reversal52 are promising options for product collection and electrode refreshing.

Cutting costs and alleviating emissions
To assess the potential of the electrochemical process for the iteration of the typical industrial lime-based precipitation process, the ongoing WAC wastewater treatment (20.8 m3 h–1) was selected as a scenario, where a TEA was performed for both the existing processes and the electrochemical process. The detailed calculations of capital expenditure (CAPEX) and operating expenditure (OPEX) were outlined in Supplementary Note 1 and Supplementary Tables 5, 6. The electrochemical process inevitably entails a higher CAPEX ($398,300) than the existing lime/hydrated lime precipitation process ($312,000), with the anode representing the major share (72.0%) (Supplementary Table 5). Allocating the CAPEX as depreciation over the system’s lifespan yields a cost of $0.109 m–3 at a 5% depreciation rate (Supplementary Table 6).

Inspiringly, by overcoming the key limitations of lime-based precipitation, the limestone-integrated electrochemical technique notably reduces the OPEX towards WAC wastewater, far offsetting the CAPEX (Supplementary Table 6 and Supplementary Note 1). Specifically, the lime precipitation currently represents the highest total cost ($3.503 m–3), with substantial portions allocated to limestone expenditure (23.0%) and calcination (25.2%), both of which can be attributed to the substantial increase in limestone consumption due to reagent surface passivation. More importantly, lime overdosing derives a considerable amount of hazardous alkaline sludge, leading to inevitably high sludge dewatering and disposal costs, which are the dominant cost factors (36.7%) (Supplementary Table 6). While reagent passivation would not occur in hydrated lime precipitation and the total cost is reduced to $1.699 m–3, it still cannot circumvent the alkaline sludge, there covered dense, high-purity Ca-phosphate by the electrochemical method serves as P fertilizer, obtaining a revenue of $1.802 m–3. Thankstotheseimprovements,thelimestone-integratedelectrochemicalsystemnotablyreducestheoverallcostto$0.939m–3 (SupplementaryTable6).Itisworthnotingthatthemaincostdriverfortheelectrochemicalmethodremainselectricalenergyconsumption (72.8%), which holds great potential to be optimized in future studies by enhancing the interaction between H⁺ and limestone (e.g., via limestone grinding)53. Collectively, the process iteration is expected to bring 73.2% overall cost savings in its lifespan.

Furthermore, environmental benefits are anticipated by implementing a limestone-integrated electrochemical system to replace the existing lime precipitation. The two LCA scenarios are depicted in Fig. 6b, with calculation details documented in Supplementary Note 2. As illustrated in Fig. 6c, the electrochemical strategy (12.60 kg CO2 eq) results in lower CO2 emissions than the routine lime treatments (17.78 kg CO2 eq), for each m3 of wastewater processed. The CO2 emissions in the current situation primarily originate from lime manufacture (11.24 kg CO2 eq), which is carbon-intensive and further exacerbated by passivation-induced excessive lime demand. Simultaneously, the disposal of hazardous alkaline sludge resulting from passivation contributes 1.63 kg CO2 eq, alongside the manufacture of Ca-phosphate fertilizer (4.93 kg CO2 eq). By circumventing lime passivation and its associated detriments, the electrochemical method has demonstrated notably reduced CO2 emissions (12.60 kg CO2 eq) (Fig. 6c). While the construction of new electrochemical plants does incur certain carbon emissions (14258.17 kg CO2 eq), this impact becomes minimal (0.07 kg CO2 eq) when normalized per m3 of wastewater, given the system’s high processing capacity. Notably, since the predominant emissions from the electrochemical method stem from electricity consumption (12.51 kg CO2 eq), integrating renewable energy could further reduce carbon emissions to 7.6% of those associated with the routine lime precipitation process (Fig. 6c). Despite the fact that electrode manufacturing contributes significantly to mineral resource scarcity (54.3%), the electrochemical strategy (0.02 kg Cu eq per m3 wastewater processed) is characterized by substantially less mineral resource scarcity than the routine method (0.19 kg Cu eq per m3 wastewater processed), as it reduces P fertilizer production and the corresponding ore extraction via efficient wastewater P recovery (Fig. 6d).

Methods
Chemicals
Analytical-grade and powdered CaO, Ca(OH)2, or CaCO3 were purchased from Macklin Biochemical Co., Ltd (Shanghai, China). The limestone (1–2 mm) was received from Jinxin Mineral (Xuancheng, China), and its elemental composition is depicted in Supplementary Table 2. The WAC wastewater investigated in this research was collected from the regulating tank in a typical WAC factory in Nanping (Fujian, China), with physicochemical properties displayed in Supplementary Table 1.

Experimental procedure
Conventional lime-based precipitation. The performance of three typical lime (CaO, Ca(OH)2, and CaCO3) in WAC wastewater was systematically investigated. Four dosing ratios were implemented based on Ca/P molar ratios of 1:1, 1.67:1, 3:1, and 4:1, corresponding to CaHPO4·2H2O formation, Ca5(PO4)3OH precipitation, and calcium-excess conditions, respectively. For each lime-based precipitation experiment, reagents were added to 500 mL of WAC wastewater at designated dosing ratios under continuous agitation at 500 rpm. All precipitation experiments were processed with 1-h mixing to ensure a complete reaction, followed by 24-h settling.

Water samples were collected at three stages: before reaction, after mixing, and post-settling, for comprehensive analysis of critical parameters such as pH, element (e.g., Ca and P), and COD. The generated sludge was collected via vacuum filtration, air-dried, and subsequently subjected to characterization, including moisture content and P content quantification and solid-phase analyses (SEM, XRD, and Raman spectroscopy).

Limestone-integrated electrochemical method. The electrochemical system consisted of a 1.0 L reactor equipped with a Ru-Ir coated titanium mesh basket anode (∅=29 mm, h=118 mm; coated density of 8 g m–2 Ru and 2 g m–2 Ir) and a tubular stainless steel 304 cathode (∅=90 mm, h=120 mm; effective surface area of 111.1 cm2). Each experiment was conducted with 25 g of limestone loaded in the basket anode. We systematically investigated the effects of current density (4.5, 9, and 18 Am–2, corresponding to applied currents of 50, 100, and 200 mA, respectively) on system performance and reaction mechanisms. All electrochemical experiments were conducted under constant current conditions using a DC power supply MN-155D from Zhaoxin (Shenzhen, China) at room temperature (20±0.5 °C).

All experiments were performed using 0.5 L of WAC wastewater under continuous mixing (500 rpm) for 36 h. We conducted periodic sampling to monitor the temporal variations in critical parameters, including pH, element concentrations, and COD. Following reaction completion, the generated solids were air-dried for 24 h and then collected by gentle scraping. Subsequently, the recovered solids were subject to subsequent characterization of moisture content and P content quantification and solid-phase analyses. Unless stated otherwise, all experiments were performed in triplicate, and data are presented as mean with standard deviation (s.d.).

Sample analysis
Liquid analysis. Solution pH was measured immediately after sampling using a calibrated pH meter (SevenExcellence, METTLER TOLEDO). For elemental analysis, samples were first filtered through 0.45 μm membranes and appropriately diluted before measurement by ICP-OES (iCAP 7400 Duo MFC, Thermo Scientific) to determine Ca, P, Mg, and other relevant element concentrations. For trace metals (e.g., Ru and Ir), ICP-Mass (iCAP RQ, Thermo Scientific) was employed. Turbidity was quantified at 860 nm using a Formazin calibration standard (Supelco Analytical, Germany) under ISO 7027 standards. COD was determined using HACH cuvette tests (2038215-CN) following the manufacturer’s protocols.

Solid analysis
The moisture content of the collected sludge was determined via 105 °C drying. The composition, including the P content of all products, was quantified by ICP-OES after nitric acid digestion. Scanning electron microscopy (SEM, SU3800 Hitachi, Japan) coupled with energy-dispersive X-ray spectroscopy (EDS, XFlash 6130 Bruker, USA) and X-ray diffraction (XRD, Rigaku SmartLab, Japan) was used to examine the morphology, elemental distribution, and phase composition of the collected products. Raman spectra were recorded using a Thermo Fisher Scientific DXR3 Laser Microscopic Raman Spectrometer, with spectra collected in the 300-1200 cm–1 range at a resolution of 2 cm–1 using a 532 nm excitation laser. For passivated granules generated in lime precipitation, particle size distribution was determined by sieve analysis. The limestone’s chemical composition was determined by an X-ray photoelectron spectrometer (XPS, PHI 5000 Versaprobe III, ULVAC-PHI) with the monochromatic Al Kα radiation at a voltage of 15 kV and current of 5 mA.

Software simulation and calculational methods
The software package of Hydra and Medusa (version: Jan., 2016)54 was applied to simulate phosphate speciation in WAC wastewater in response to Ca2+ release and pH increase, enabling comprehensive prediction of phosphate species distribution across the experimental conditions examined.

The P removal efficiency in % was calculated with Eq. 7:

P removal efficiency (%) = 100 * (C0 - Ct) / C0 (7)

C0 and Ct mean the P concentrations before and after treatment (mg L–1).

The saturation index (SI) was calculated by Eq. 8:

Saturation Index (SI) = log IAP - log Ksp (8)

IAP refers to the ion activity of the associated lattice ions, and Ksp refers to the solubility product.

As P in the sludge from lime-based precipitation exists exclusively in CaHPO4·2H2O, the reagent usage efficiency in lime-based precipitation was determined as follows (Eq. 9):

Reagent usage efficiency (%) = (Psludge * MWcalcium) / (Careagent * MWphosphorus) (9)

Careagent refers to the Ca content in the dosed reagent, and Psludge is the P content in the sludge collected from lime-based precipitation.

The moisture content of the sludge collected was calculated with Eq. 10:

Moisture content (%) = 100 * (m0 - mt) / m0 (10)

where m0 and mt mean the weight of sludge before and after 105 °C drying (g).

The energy consumption (kWh) of the electrochemical system was calculated by Eq. 11:

W (kWh) = U * I * t (11)

where U and I are the recorded voltage (V) and current (I) on average over the electrolysis of a specific time (h).

Lifecycle assessment and economic aspect
TEA was conducted to assess and compare the economic feasibility of the limestone-integrated electrochemical method to existing lime-based precipitation processes, based on lab-scale results and sourced inventory data (Supplementary Table 5 and 6). Calculation details can be found in Supplementary Note 1. The LCA was performed to evaluate the environmental impacts of both the limestone-integrated electrochemical method (proposed scenario) and conventional lime-based precipitation processes (baseline scenario). The assessment followed the framework and standards outlined in ISO 14040/44 with background information from both lab-scale results and the Ecoinvent version 3 database. SimaPro software was employed to establish scenarios and assess environmental impacts using the ReCiPe Midpoint (H) v1.05 methodology. The system boundary and calculation details are presented in Fig. 6b and Supplementary Note 2, respectively.

Data availability
The data supporting the findings of the study are included in the main text, Supplementary information and Source Data files. Raw data can be obtained from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript resents a novel limestone-integrated electrochemical system that spatially decouples Ca²⁺ supply (via anodic dissolution) from phosphate precipitation (via cathodic alkalinity). By strategically positioning limestone in the acidic anode chamber, the authors achieve sustained Ca²⁺ release without surface passivation, while cathodic alkalinity enables efficient phosphorus recovery (85.7% in 36 h). The system produces high-purity products (15.2 wt% P) with low energy consumption (14.8 kWh/kg P) and delivers superior effluent quality (88.6 mg/L Ca²⁺, pH 4.0), thereby eliminating scaling risks in downstream treatments. The integrated economic analysis and LCA further reveal that this electrochemical strategy can reduce costs by 87.0% and carbon emissions by 29.5%, positioning it as a sustainable and cost-effective alternative to current lime-based processes. Overall, the manuscript is well written, and the concept is both innovative and impactful. My specific comments are as follows:\n1. The Results section dedicates substantial space to elucidating the mechanisms of surface passivation in conventional lime-based precipitation. While this context is valuable, it may overshadow the novel aspects of the proposed system. I suggest providing a more detailed discussion for Section 3.5 and revising the manuscript title to better reflect the innovation and core findings.\n2. A Ru–Ir mesh basket was used as the anode and porous container. Could the authors clarify whether Ru or Ir leaching into the solution was monitored during the experiments?\n3. Figure 4a: The illustrated ion migration pathways appear counterintuitive. In the current diagram, Ca²⁺ moves toward the cathode, while PO₄³⁻ also moves toward the cathode. Could the authors clarify the driving forces for these movements in the figure and whether they are influenced primarily by electron transfer or other factors?\n4. Section 3.5: The reported system performance is based on 2-h experiments. It remains unclear how long this performance can be sustained under continuous or repeated operation. Have the electrodes been tested for stability over multiple operational cycles? Additional data on the long-term performance and durability of the electrochemical cell would strengthen the manuscript.", "reviewer_2": "The manuscript titled “Electrochemical Prevention of Lime Passivation: Mechanism and Application for Sustainable Phosphate Recovery from Wastewater” proposes a novel but simple electrochemical approach to improving phosphorus removal and recovery from wastewaters with lime dosing. The authors have developed a membrane-less electrochemical system where calcium release is spatially decoupled from the Ca-phosphate precipitation step that follows. Using the anode of the electrochemical setup for improving Ca release facilitates the use of limestone, which requires much less pre-processing, as the Ca source over quicklime and hydrated lime. By decoupling the Ca release step from Ca-phosphate precipitation, lime passivation and need for overdosing is avoided, which results in more efficient use of the limestone for phosphate precipitation. This leads to the formation a high-quality, reusable Ca-phosphate precipitation product and an effluent water stream with low pH and residual Ca, which reduces needs for further processing.\nI find the experimental part of the study to be rigorously planned and executed and the manuscript to be well written. I think the findings of the study are exciting and highly relevant to the field of wastewater treatment and nutrient recovery. The advantages compared to the traditional lime/hydrated lime dosing are apparent and well argued. As a small comment, it would be good to compare the “remarkably low energy consumption” (lines 556-557) values of the proposed electrochemical system to other phosphate production/recovery alternatives to increase the credibility of the claim. All in all, I believe the novelty and relevance of the findings make this manuscript highly suitable for Nature Communications.\nI do, however, have greater concerns regarding the economic and life cycle analyses that need to be addressed before the manuscript can be accepted for publication. Both analyses seem relatively simple and, frankly, skewed. The authors argue they have only considered the operational costs/aspects of the different scenarios in their analyses because the capital costs would not be significant considering the “long lifetime” of the facilities. No more details are given on what the expected lifetimes of the equipment/plants in the different scenarios would be. How long is a long lifetime? I would argue the capital costs of the different scenarios – lime/hydrated lime precipitation and limestone-integrated electrochemical system – would be very different and thereby affect the economic analysis in particular. The electrochemical scenario can be expected to require much higher capital investment, as it involves the use of much more advanced materials, namely the electrodes. Leaving the capital costs out unfairly favours the electrochemical scenario proposed by the authors. I strongly encourage the authors to redo the analysis with capital costs included, with appropriate lifetime, depreciation rate etc. considerations.\nFurthermore, I can’t currently find details on how the Ca-phosphate precipitate is recovered from the cathode surface and how this might affect the operational costs of the electrochemical system. I encourage the authors to add details and discussion on this as well.\nSimilarly to the economic analysis, I strongly believe the life cycle assessment should also include the construction of an electrochemical plant. The baseline scenario requires no new infrastructure, whereas building a completely new electrochemical plant is a major contributor to the impact of the limestone-integrated electrochemical scenario.\nAdditionally, I have a couple of minor comments that I think would improve the readability of the manuscript:\n1. Line 242, Figure 1(c): I find the figures throughout the manuscript to be clear and easy to follow, except for Figure 1(c). The colours and overlapping shading make the figure hard to read. It is particularly difficult to distinguish between graphs Ca:P=1.67:1, Ca:P=3:1 and Ca:P=4:1. I recommend the authors re-design this figure.\n2. Line 387: I believe the authors meant to say “displayed” instead of “displaced”."}, "author_response": "Response to Reviewers' Comments\nDear Editors and Reviewers,\nWe sincerely appreciate your time and effort in evaluating our manuscript. Your valuable comments and suggestions have contributed significantly to improving the quality of our work.\nIn this revised version, we have carefully addressed your comments, particularly regarding the long-term performance, life cycle, and economic analysis.\nFor clarity, your comments are presented in regular dark font, and our response is shown in blue font. Changes made to the revised manuscript are highlighted in yellow for easy identification.\nWe hope that our revisions meet your expectations and adequately address your concerns.\nWe would be delighted to provide further details if you have any further questions or require additional clarification.\nThank you!\nBest regards,\nDr. Yang Lei, also on behalf of all co-authors,\nResponse to Reviewer #1,\nComments:\nThis manuscript resents a novel limestone-integrated electrochemical system that spatially decouples Ca²⁺ supply (via anodic dissolution) from phosphate precipitation (via cathodic alkalinity). By strategically positioning limestone in the acidic anode chamber, the authors achieve sustained Ca²⁺ release without surface passivation, while cathodic alkalinity enables efficient phosphorus recovery (85.7% in 36 h). The system produces high-purity products (15.2 wt% P) with low energy consumption (14.8 kWh/kg P) and delivers superior effluent quality (88.6mg/L Ca²⁺, pH 4.0), thereby eliminating scaling risks in downstream treatments. The integrated economic analysis and LCA further reveal that this electrochemical strategy can reduce costs by 87.0% and carbon emissions by 29.5%, positioning it as a sustainable and cost-effective alternative to current lime-based processes. Overall, the manuscript is well written, and the concept is both innovative and impactful. My specific comments are as follows:\nReply:\nWe are grateful for your acknowledgment and have carefully revised the manuscript based on your insightful comments.\n1. The Results section dedicates substantial space to elucidating the mechanisms of surface passivation in conventional lime-based precipitation. While this context is valuable, it may overshadow the novel aspects of the proposed system. I suggest providing a more detailed discussion for Section 3.5 and revising the manuscript title to better reflect the innovation and core findings.\nReply:\nWe appreciate your valuable suggestion. In light of your comments and those of Reviewer #2, we have supplemented additional data and elaborated on the discussion beyond lime surface passivation, particularly in Section 3.5 (newly updated as Section 2.5). Please refer to the highlighted contents throughout the revised manuscript.\nFurthermore, we have updated the title to Spatially Decoupled Electrochemical Strategy for Lime Passivation Prevention and Sustainable Phosphate Recovery to emphasize the novel aspects and key findings.\n2. A Ru–Ir mesh basket was used as the anode and porous container. Could the authors clarify whether Ru or Ir leaching into the solution was monitored during the experiments?\nReply:\nThank you. The Ru-Ir coated Ti material, a typical Dimensionally Stable Anode (DSA), was selected for its superior electrochemical stability and high efficiency in water oxidation, balancing cost-effectiveness and stability compared to materials like Pt, BDD, and graphite. To address your concern, we monitored the Ru and Ir content in the WAC wastewater over long-term operation (Table R1) via ICP-MS.\nTable R1 (newly updated as Table S4 in the supporting information). Monitoring of Ru and Ir in the influent and effluent over long-term operation.\nInfluent Effluent\nDay Ru Ir Ru Ir Unit\n1 <0.3a <0.3a <0.3a <0.3a\n7 <0.3a <0.3a <0.3a <0.3a\n13 <0.3a <0.3a <0.3a <0.3a\n18 <0.3a <0.3a <0.3a <0.3a ng/L\n24 <0.3a <0.3a <0.3a <0.3a\n30 <0.3a <0.3a <0.3a <0.3a\n36 <0.3a <0.3a <0.3a <0.3a\na Below detection limit.\nAs shown in Table R1, in the 36-day experimental duration, the Ru and Ir concentrations in the influent and effluent consistently remained below the detection limit (0.3 ng/L), thus eliminating concerns related to potential coating degradation and water contamination.\nFollowingly, we have included the data and discussion on anode stability in the revised manuscript (Please see Table S4 and Lines 434-436 and 576-577).\nRevised content:\nLines 434-436:\n“The electrochemical cell's durability is pivotal for practical applications. Inspiringly, the Ru-Ir coated Ti anode, renowned for its electrochemical stability, exhibited exceptional performance with no detectable Ru or Ir leaching over the extended operation (Table S4).”\nLines 576-577:\n“For trace metals (e.g., Ru and Ir), ICP-Mass (iCAP RQ, Thermo Scientific) was employed.”\n3. Figure 4a: The illustrated ion migration pathways appear counterintuitive. In the current diagram, Ca²⁺ moves toward the cathode, while PO₄³⁻ also moves toward the cathode. Could the authors clarify the driving forces for these movements in the figure and whether they are influenced primarily by electron transfer or other factors?\nReply:\nThank you for raising this important point.\nFigure R1 (newly updated as Figure 4c in the main text). Schematic diagram of the passivation mitigation by anodic acidity and stable precipitation by cathodic alkalinity, with driving forces of Ca²⁺ and H₃₋ₓPO₄⁽ˣ⁻³⁾ ions labeled.\nIn the limestone-integrated electrochemical system, calcium (Ca2+) and phosphate ions (H 3- PO (x-3)) are the primary species involved (Figure R1). For H PO (x-3), a key driving force toward the cathode is mass diffusion, driven by its consumption due to precipitation. Moreover, H 3- PO (x-3), as a buffer, tends to react with OH- produced, further promoting its migration toward the cathode. While the electric field indeed opposes the migration of H PO (x-3) to the cathode, 3-x 4 among the three current-related driving forces, the combined influence of mass diffusion and buffering predominates. This dominance is supported by the observed enhanced P removal with increased current density, rather than a decline, which would be expected if electromigration were the primary factor. For cations such as Ca2+, in addition to mass diffusion, electromigration also contributes to their movements toward the cathode. In response, we have revised Figure 4a with driving forces labelled and expanded the discussion in the revised manuscript (Please see the revised Figure 4c and Lines 340-349).\nRevised content:\nLines 340-349:\n“As primary ions in this framework, the migration of Ca²⁺ and H₃₋ₓPO₄⁽ˣ⁻³⁾ toward the cathode is primarily driven by mass diffusion, driven by their consumption due to precipitation (Figure 4a). For cations such as Ca²⁺, electromigration also plays a role in their movement toward the cathode. Importantly, in addition to mass diffusion, H₃₋ₓPO₄⁽ˣ⁻³⁾, as a buffer, tends to react with OH⁻ generated, further promoting its migration toward the cathode (Figure 4a). While the electric field opposes H₃₋ₓPO₄⁽ˣ⁻³⁾ migration, among the three current-related driving forces, the combined influence of mass diffusion and buffering predominates, as evidenced by the positive correlation between current and P removal (Figure 5a), rather than a decline, which would be expected if electromigration prevailed.\nSpecifically…”\n4. Section 3.5: The reported system performance is based on 2-h experiments. It remains unclear how long this performance can be sustained under continuous or repeated operation. Have the electrodes been tested for stability over multiple operational cycles? Additional data on the long-term performance and durability of the electrochemical cell would strengthen the manuscript.\nReply:\nThank you. To further explore the long-term performance and stability of this strategy, we performed an 864-h (36-d) continuous operation with the same setup. Within this framework, the continuous anti-passivation and utilization of limestone, as well as the electrochemical cell's durability, were also further highlighted.\nFigure R2 (newly updated as Figure S14 in the supporting information). Long-term performance of an 864-h (36-d) operation, where the P concentrations (recovery efficiency), pH, and Ca concentration at the inlet and outlet, as well as the voltage, were monitored.\nAt an initial setting (9 A/m²; HRT 36h), the system attained 60.0% phosphorus removal within the first few days (Figure R2). Upon increasing the current to 18 A/m², the P removal efficiency remained stable at 86.2-96.5% for over a week (Figure R2). Foreseeing scenarios that require higher removal capacities, the HRT was shortened to 6 h, and the current density was elevated to 72 A/m² following a drop in recovery efficiency. The system was subsequently run under 72 A/m2 and HRT 6 h for a further 20 days, with P recovery stabilizing between 72.9-93.3% (Figure R2). Throughout the operation, limestone was replenished on days 21 and 35 upon noticing a decline in P recovery, highlighting the importance of adequate limestone supply in the system's practical implementation. Overall, the system displayed good stability and flexible recovery capacity via the conditions tuning.\nFigure R3 (newly updated as Figure S15 in the supporting information). The particle size distribution (in wt%) of the limestone before and after long-term operation.\nFigure R4 (newly updated as Figure S16 in the supporting information). (a) XPS survey spectrum of the limestone before and after long-term operation. The Ca 2p (b) and P 2p (c) high-resolution spectra of limestone before and after long-term operation.\nThe continuous anti-passivation and utilization of limestone were also confirmed. Over the long-term operation, apart from the limestone that was completely consumed, the remaining limestone exhibited a notable decrease in particle size, shifting from a dominant range of 2.0-3.0 mm to 1.0-2.0 mm, coupled with a small fraction falling below 1.0 mm (Figure R3). Throughout the dissolution, the surface of the limestone remained consistent (Figure R4a). The limestone before and after long-term operation both exhibited strong Ca 2p signals, indicative of CaCO , with no P detected (Figure 3 R4b-c). These results collectively illustrate that limestone was continuously dissolved from the outside in and remained resistant to passivation throughout its utilization.\nRegarding the electrodes' stability, Ru-Ir was selected as the anode coating due to its well-known electrochemical stability in the first place. The anode showed no signs of scaling after long-term operation, thanks to the very low pH on its surface. Along with the integrity of the Ru-Ir coating (no leaching recorded, Table R1), the long-term durability of the anode should not be a concern.\nFigure R5 (newly updated as Figure S17 in the supporting information). The image of the cathode after long-term operation, along with an illustration of product collection and cathode refreshing/reuse anticipated in practical applications.\nAs for the cathode, we acknowledge that a layer of precipitates/products remained on the cathode after long-term operation, which might impact the electrochemical reactions (Figure R5). To investigate, we also monitored the cell voltage over long-term operation (Figure R2). Based on the stable voltages and solution pH under respective condition (Figure R2), the impact of precipitates on the system is considered negligible. This is likely due to the large cathode surface area (for precipitation) provided by the tubular stainless-steel cathode. Furthermore, in our previous study, we examined the long-term stability of conventional electrochemically induced precipitation from dairy wastewater over 173 days of operation. Surprisingly, the system maintained stable performance even though the precipitates filled the reactor. This is likely because the initial precipitates served as seed material for subsequent precipitation1. In addition, the H₂ gas generated by the cathode creates channels/holes in the precipitation layer (Figure R5), enabling the diffusion of OH⁻ from the cathode surface to the outer layer of precipitates, which allows for continuous and stable precipitation during long-term operation. Certainly, if product collection or electrode refreshing is necessary, physical scraping serves as a fundamental method without compromising the cathode's integrity, and the cathode can then be readily cleaned and reused for the next cycle (Figure R5). To facilitate easier physical scraping, a scraper (i.e., a ring) can be integrated. The deposited solids can be removed from the cathode by automatically pushing the scraper down along the inner side of the tubular cathode. To our knowledge, this technique has been applied in the cooling water treatment industry2. As a more automated option, periodic polarity reversal has also shown great promise in product collection or electrode refreshing, as demonstrated by our group3 and other research groups4.\nTo address your concern, we have provided a new section 2.5.3 Long-term Robustness and Implementation Outlook integrating the above results and discussions (Please see Figure S14-17 and Lines 414-441, 576-577, and 589-591).\nRevised content:\nLines 414-441:\n“2.5.3 Long-term Robustness and Implementation Outlook\nTo evaluate the long-term performance and stability of the spatially decoupled electrochemical strategy, we conducted an 864-h (36-d) continuous operation with the same setup. At an initial setting (9 A/m²; HRT 36h), the system attained 60.0% P recovery within the first few days (Figure S14). Upon increasing the current to 18 A/m², the P recovery remained stable at 86.2-96.5% for over a week (Figure S14). Anticipating scenarios that require higher treatment capacities, the HRT was shortened to 6h, and the current density was further elevated to 72 A/m² following a decline in recovery efficiency. Subsequently, the system shows promising P recovery stabilizing between 72.9-93.3% under 72 A/m2 and HRT 6h for a further 20 days (Figure S14). Throughout the operation, limestone was replenished on days 21 and 35 upon noticing a decline in P recovery, highlighting the importance of adequate limestone supply in the system's practical implementation. Overall, the system displayed good stability and flexible recovery capacity via the conditions tuning. The continuous anti-passivation and utilization of limestone were further validated under prolonged operation. Even excluding the limestone that was fully consumed, the remaining limestone exhibited a notable decrease in particle size, shifting from an initial dominant range of 2.0-3.0 mm to 1.0-2.0 mm, with a minor fraction falling below 1.0 mm (Figure S15). Throughout the dissolution, the surface of the limestone remained consistent (Figure S16a). The limestone before and after long-term operation both exhibited strong Ca 2p signals, indicative of CaCO , with no P detected (Figure 3 S16b-c). These findings collectively illustrate that limestone underwent continuous dissolution from the outside in and maintained its resistance to passivation throughout utilization.\nThe electrochemical cell's durability is pivotal for practical applications. Inspiringly, the Ru-Ir coated Ti anode, renowned for its electrochemical stability, exhibited exceptional performance with no detectable Ru or Ir leaching over the extended operation (Table S4). Importantly, the precipitates/products remained on the cathode over extended use. These precipitates/products can be effectively harvested via physical scraping without compromising the integrity of the cathode, allowing its prompt cleaning and reuse for the next cycles (Figure S17). Foreseeing more automated implementations, scraper integration51 and polarity reversal52 are promising options for product collection and electrode refreshing.”\nLines 576-577:\n“For trace metals (e.g., Ru and Ir), ICP-Mass (iCAP RQ, Thermo Scientific) was employed.”\nLines 589-591:\n“The limestone's chemical composition was determined by an X-ray photoelectron spectrometer (XPS, PHI 5000 Versaprobe III, ULVAC-PHI) with themonochromatic Al Kα radiation at a voltage of 15 kV and current of 5 mA.”\nWe are deeply grateful for your thorough review and constructive feedback throughout the revision. Your insightful comments and suggestions have significantly improved the quality of our manuscript.\nResponse to Reviewer #2,\nComments:\nThe manuscript titled “Electrochemical Prevention of Lime Passivation: Mechanism and Application for Sustainable Phosphate Recovery from Wastewater” proposes a novel but simple electrochemical approach to improving phosphorus removal and recovery from wastewaters with lime dosing. The authors have developed a membrane-less electrochemical system where calcium release is spatially decoupled from the Ca-phosphate precipitation step that follows. Using the anode of the electrochemical setup for improving Ca release facilitates the use of limestone, which requires much less pre-processing, as the Ca source over quicklime and hydrated lime. By decoupling the Ca release step from Ca-phosphate precipitation, lime passivation and need for overdosing is avoided, which results in more efficient use of the limestone for phosphate precipitation. This leads to the formation a high-quality, reusable Ca-phosphate precipitation product and an effluent water stream with low pH and residual Ca, which reduces needs for further processing.\nI find the experimental part of the study to be rigorously planned and executed and the manuscript to be well written. I think the findings of the study are exciting and highly relevant to the field of wastewater treatment and nutrient recovery. The advantages compared to the traditional lime/hydrated lime dosing are apparent and well argued. As a small comment, it would be good to compare the “remarkably low energy consumption” (lines 556-557) values of the proposed electrochemical system to other phosphate production/recovery alternatives to increase the credibility of the claim. All in all, I believe the novelty and relevance of the findings make this manuscript highly suitable for Nature Communications.\nReply:\nWe greatly appreciate the time and effort you have dedicated, as well as your positive feedback. Following your comments, we have carefully revised the manuscript as follows.\nWe agree with you that a comprehensive cost comparison would strengthen the credibility. For this reason, we have normalized the energy consumption into operational costs and compared it with ~20 previous studies, including both non-electrochemical and electrochemical methods commonly applied in lab-scale and industrial settings (Figure R6). Detailed information including the types of methods, wastewater characteristics (P level), operational costs, and product quality, is provided in Table S3.\nFigure R6 (newly updated as Figure S12 in the supporting information). Normalized operation cost of the reported studies targeting P removal/recovery in diverse waste streams (●, non-electrochemical methods; ▲, electrochemical methods; ■, conventional electrochemically mediated precipitation methods; Detailed comparison is included in the supporting information (Table S3).\nOur findings indicate that the operational cost (energy consumption) is competitive with existing techniques targeting P removal/recovery in diverse waste streams, and we have included the results and discussion in the manuscript accordingly (Please see Figure S12, Table S3, and Lines 399-402).\nRevised content:\nLines 399-402:\n“…not to mention its low specific energy consumption (14.8-19.5 kWh/kg P, Figure 5f) and competitive cost efficiency ($1.4-1.9/kg P removed) within a wide range of processes targeting P removal/recovery ($1.4-422.3/kg P removed at 5.6-1300.0 mg/L P) (Figure S12 and Table S3).”\nTable S3:\nTable S3. Listing and comparison of reported phosphate removal/recovery technologies in terms of wastewater type, phosphate level, operation cost, and product quality.\nCost per\nType of method Waste stream Product Reference\nkg P removeda\nWAC wastewater\nLime dosing $3.6/kg P Ca-rich sludge Ye et al. 20215\n(1300.0 mg/L P)\nStored urine\nHydrated lime dosing $11.6/kg P 1.5 wt% P containing sludge Pradhan et al. 20176\n(328.0 mg/L P)\nStored urine Antonini et al.\nMgO dosing $13.4/kg P 12.6 wt% P containing precipitates\n(310.0 mg/L P) 20117\nSecondary effluent\nChemical coagulation (FeCl ) $118.3/kg P Ferric sludge Akhtar et al. 20258\n3\n(8.9 mg/L P)\nSecondary effluent\nChemical coagulation (Alum) $119.5/kg P Alum sludge Akhtar et al. 20258\n(8.9 mg/L P)\nSynthetic wastewater\nFluidized bed homogeneous crystallization $4.9/kg P 11.1 wt% P containing K-struvite Le et al. 20209\n(850.0 mg/L P)\nSynthetic wastewater\nElectrodialysis $1.6-8.9/kg P 0.075 mol/L H PO Wang et al. 201310\n3 4\n(100.0 mg/L P)\nDomestic influent 4.2-13.2 wt% P containing\nMicrobial electrolysis cell $2.0-23.7/kg P Lei et al. 201911\n(7.1-23.6 mg/L P) precipitates\nSynthetic wastewater\nFlow-electrode capacitive deionization $2.7/kg P P-rich solution Xu et al. 202012\n(62.0 mg/L P)\nFlow-electrode capacitive deionization Synthetic wastewater\n<$1.4/kg P 73.9% purity phosphoric acid Xu et al. 202113\ncoupled with a liquid membrane chamber (526.6 mg/L P)\nFresh urine\nFe electrocoagulation $4.6/kg P Iron phosphate species Zheng et al. 200914\n(49.0 mg/L P)\nSynthetic urine\nAl electrocoagulation $2.5/kg P Aluminium phosphate species Zheng et al. 201015\n(52.7 mg/L P)\nToilet wastewater\nElectrochemically mediated precipitation $422.3/kg P 9.8 wt% P containing precipitates Cid et al. 201816\n(18.6 mg/L P)\nDomestic wastewater\nElectrochemically mediated precipitation $10.6/kg P 4.0-8.0 wt% P Ca-phosphate Lei et al. 201917\n(7.5 mg/L P)\nSynthetic wastewater\nElectrohydromodulation $18.9/kg P 15.1 wt% P Ca-phosphate Perera et al. 202018\n(5.6 mg/L P)\nCheese wastewater\nElectrochemically mediated precipitation $6.2/kg P 9.4-13.4% purity Ca-phosphate Lei et al. 202119\n(827.0 mg/L P)\nWAC wastewater\nThis work $1.4-1.9/kg P 13.9-15.2 wt% P Ca-phosphate\n(1244.4 mg/L P)\na In non-electrochemical methods, only the cost related to P removal was considered. For electrochemical methods, only electricity consumption ($0.096/kWh) was considered due to their chemical-free nature.\nb The MgO price is normalized at $5.7/kg20.\nI do, however, have greater concerns regarding the economic and life cycle analyses that need to be addressed before the manuscript can be accepted for publication. Both analyses seem relatively simple and, frankly, skewed. The authors argue they have only considered the operational costs/aspects of the different scenarios in their analyses because the capital costs would not be significant considering the “long lifetime” of the facilities. No more details are given on what the expected lifetimes of the equipment/plants in the different scenarios would be. How long is a long lifetime? I would argue the capital costs of the different scenarios – lime/hydrated lime precipitation and limestone-integrated electrochemical system – would be very different and thereby affect the economic analysis in particular. The electrochemical scenario can be expected to require much higher capital investment, as it involves the use of much more advanced materials, namely the electrodes. Leaving the capital costs out unfairly favours the electrochemical scenario proposed by the authors. I strongly encourage the authors to redo the analysis with capital costs included, with appropriate lifetime, depreciation rate etc. considerations.\nReply:\nThank you for your insightful comments. Followingly, we have re-performed the techno-economic analysis with detailed Capital Expenditure (CAPEX) and expanded Operating Expenditure (OPEX). The calculation details of CAPEX were illustrated in Note S1 and Table S5. The overall cost with updated OPEX were highlighted in the updated Note S1 and Table S6.\nAs you highlighted, the total CAPEX of the proposed electrochemical system ($398,300) is indeed higher than the current lime/hydrated lime precipitation system ($312,000), with the Ru-Ir coated Ti anode representing the major share (72.0%) (Table S5). It is worth noting that the total CAPEX of the lime/hydrated lime treatment process is not as low as one might assume, as it involves a series of equipment, including mixing tanks, settling tanks, and sludge processing systems (Table S5)5. The lifespan of the electrochemical system is set as 20 years, corresponding to a depreciation rate of 5%, with maintenance and labor now incorporated into the overall cost for all three methods (Table S6).\nThe common approach involves allocating the CAPEX over its lifespan with its depreciation rate, ensures a rational distribution of the CAPEX across the system's lifespan. Based on this approach, the depreciation attributed to the overall cost is calculated as $0.109/m³ (Table S6). With depreciation (new method only), maintenance, and labor included, the overall costs are now updated to $3.503/m³, $1.699/m³, and $0.939/m³ for lime, hydrated lime, and the limestone-integrated electrochemical system, respectively (Figure R7 and Table S6). That is, while the construction of a new electrochemical plant indeed requires high capital investment, an overall cost saving of 73.2% will be obtained via process updating.\nFigure R7 (newly updated as Figure 6a in the main text). Total cost comparison and analysis of lime, hydrated lime, and the limestone-integrated electrochemical system.\nCertainly, we have recognized the importance of reducing electrode costs, particularly the substantial cost associated with the anode. Here, we initially demonstrated our concept using a basket-shape anode. In our follow-up work, we are developing an upgraded anode design featuring a rod anode surrounded by a plastic basket. In this new design, all the H⁺ produced from the anode were forced to diffuse into the system through the limestone, ensuring maximum limestone dissolution and anti-passivation while reducing the anode surface area-to-effective volume ratio from 9.8 m2/m3 in this initial work to 0.1 m2/m3. Owing to this improvement, the anode cost has been notably reduced by two orders of magnitude.\nWe have followingly revised the section Cutting Cost and Alleviating Emission (Please see revised Figure 6a and Lines 444-454 and 472-473). The calculation details of economic analysis were illustrated in Note S1 and Table S5-S6 (Please refer to the revised supporting information).\nRevised content:\nLines 444-454:\n“To assess the potential of the electrochemical process for the iteration of the typical industrial lime-based precipitation process, the ongoing WAC wastewater treatment (20.8 m³/h) was selected as a scenario, where a techno-economic analysis (TEA) was performed for both the existing process and the electrochemical process. The detailed calculations of capital expenditure (CAPEX) and operating expenditure (OPEX) were outlined in Note S1 and Table S5 and S6. The electrochemical process inevitably entails a higher CAPEX ($398,300) than the existing lime/hydrated lime precipitation process ($312,000), with the anode representing the major share (72.0%) (Table S5). Allocating the CAPEX as depreciation over the system's lifespan yields a cost of $0.109/m3 at a 5% depreciation rate (Table S6).\nInspiringly, by overcoming the key limitations of lime-based precipitation, the limestone-integrated electrochemical technique notably reduces the OPEX towards WAC wastewater, far offsetting the CAPEX (Table S6 and Note S1). Specifically, the…”\nLines 472-473:\n“Collectively, the process iteration is expected to bring 73.2% overall cost savings in its lifespan.”\nNote S1:\n“Capital Expenditure (CAPEX)\nImportant factors of CAPEX acquired from the long-term lab test\nHRT: 36.0 h.\nEfficiency: ≥85% phosphate recovery from WAC wastewater (1.244 kg P/m3);\nOperational current and energy consumption: 18 A/m2; 19.5 kWh/kg P;\nAnode surface area-to-effective volume ratio: 9.8 m2/m3;\nCathode surface area-to-effective volume ratio: 22.2 m2/m3;\nWAC wastewater volume: 20.8 m3/h;\nThe WAC wastewater treatment was selected to represent the current common lime-based precipitation process, which is proposed to be replaced by limestone-integrated electrochemical method. The WAC wastewater treatment plant currently has a treatment capacity of 20.8 m³/h, primarily achieved by three sequential processes: lime precipitation, solid-liquid separation, and coagulating sedimentation, completed within 2 h.\nAccording to lab-scale tests, the limestone-integrated electrochemical system requires an HRT of 36 h. For the comparison with lime-based process, an HRT of 2 h is recommended. In light of this, we need to build 18 reactors with a total volume of 20.8 m³.\nConsequently, the required anode size is about 203.8 m². Given that the price of 1 m² Ru/Ir-Ti electrode is $586.0 (quotation provided by Wenghe Metal Materials, Hefei), the cost of purchasing 203.8 m² Ru/Ir-Ti anode is about $119,450.2. Similarly, the required cathode size is around 461.8 m². The price of 1 m² stainless-steel electrode is $13.8 (quotation provided by Maize Wire Mesh Product, Anping), resulting in a cost of $6,372.3 for purchasing 461.8 m² cathode. Additionally, the purchase of 18 PMMA reactors with a total volume of 20.8 m³, costs approximately $40,000 (quotation provided by Dayou Instrument Equipment, Shanghai).\nCollectively, the total investment in reaction equipment amounts to $165,822.5. Other direct costs (e.g., construction, piping, electrical, and instrumentation) and indirect costs (e.g., engineering and contingency) are estimated based on the reaction equipment investment, as per the USEPA manual2. Detailed CAPEX is illustrated in Table S5.\nOperating Expenditure (OPEX)”\nNote S1:\n“On top of the aforementioned operation costs, labor ($0.29/m³) and maintenance ($0.14/m³) expense were applied to all three processes1. Eventually, with a 20-year lifespan and a 5% depreciation rate, the CAPEX of the electrochemical reactors allocated to the overall cost is calculated as $0.109/m³.”\nTable S5:\nTable S5. Comparison of the estimated capital cost for the current lime-based and proposed electrochemical treatments (US$ in 1000's).\nLime-based Limestone-integrated\nItems Description\ntreatment electrochemical method\nMixing tank-6m3 $20.0\nSettling tank-60m3 $40.0\nSludge processing system $50.0\nAuxiliary system $20.0\nPMMA Reactor $40.0\nDirect\nRu-Ir coated Ti anode $119.5\ncost\nStainless-steel cathode $6.4\nTotal equipment investment $130.0 $165.9\nConstruction & building, 50% $65.0 $83.0\nProcess piping, 30% $39.0 $49.8\nElectrical & instrumentation, 20% $26.0 $33.2\nTotal direct costa $260.0 $331.9\nEngineering cost, 12% $31.2 $39.8\nIndirect\nContingency, 8% $20.8 $26.6\ncostb\nTotal indirect cost $52.0 $66.4\nTotal estimated capital cost $312.0 $398.3\nNote: The equipment costs associated with lime-based precipitation are derived from the existing equipment used at the WAC wastewater treatment plant for lime precipitation, solid-liquid separation, and coagulating sedimentation1.\na: The total direct costs were determined by allocating each cost item proportionally to the total equipment investment2.\nb: The indirect cost was estimated by applying appropriate factors to the total direct cost2.\nFurthermore, I can’t currently find details on how the Ca-phosphate precipitate is recovered from the cathode surface and how this might affect the operational costs of the electrochemical system. I encourage the authors to add details and discussion on this as well.\nReply:\nThank you for highlighting this important point. Inspired by you and Reviewer#1, we have performed an 864-h (36-d) long-term operation of this electrochemical strategy, within which the impact of precipitates was also investigated. Following the continuous operation, we noted that a layer of precipitates/products accumulated on the cathode surface (Figure R5), which might affect the electrochemical reactions.\nFigure R5 (newly updated as Figure S17 in the supporting information). The image of the cathode after long-term operation, along with an illustration of product collection and cathode refreshing/reuse anticipated in practical applications.\nTo explore this further, we also monitored the cell voltage over long-term operation (Figure R2). The stable cell voltages and solution pH under each condition (Figure R2) suggested that the precipitates show negligible impact on the operational costs, which are primarily determined by the power derived from the voltages. This is likely due to the large cathode surface area (for precipitation) provided by the tubular stainless-steel cathode. Furthermore, in our previous study, we examined the long-term stability of conventional electrochemically induced precipitation from dairy wastewater over 173 days of operation. Surprisingly, the system maintained stable performance even though the precipitates filled the reactor. This is likely because the initial precipitates served as seed material for subsequent precipitation1. Moreover, the H₂ gas generated at the cathode created channels/holes within the precipitation layer (Figure R5), enabling the diffusion of OH⁻ from the cathode surface to the outer layer of precipitates, thereby ensuring continuous and stable precipitation during long-term operation. Certainly, if product collection or electrode refreshing is necessary, physical scraping serves as a fundamental method without compromising the cathode's integrity, and the cathode can then be readily cleaned and reused for the next cycle (Figure R5). To facilitate easier physical scraping, a scraper (i.e., a ring) can be integrated. The deposited solids can be removed from the cathode by automatically pushing the scraper down along the inner side of the tubular cathode. To our knowledge, this technique has been applied widely in the cooling water treatment industry2. As a more automated solution, periodic polarity reversal has also shown great promise for product collection or electrode refreshing, as demonstrated by our group3 and other researchers4. Accordingly, maintenance and labor (e.g., for product collection/electrode refreshing) have been incorporated into the updated OPEX (Please see Note S1 and Table S6).\nTo address your concern, we have added details and discussion on product collection/electrode refreshing into the revised manuscript (Please see Figure S17 and Lines 436-441).\nRevised content:\nLines 436-441:\n“Importantly, the precipitates/products remained on the cathode over extended use. These precipitates/products can be effectively harvested via physical scraping without compromising the integrity of the cathode, allowing its prompt cleaning and reuse for the next cycles (Figure S17). Foreseeing more automated implementations, scraper integration51 and polarity reversal52 are promising options for product collection and electrode refreshing.”\nNote S1:\n“On top of the aforementioned operation costs, labor ($0.29/m³) and maintenance ($0.14/m³) expense were applied to all three processes1.”\nSimilarly to the economic analysis, I strongly believe the life cycle assessment should also include the construction of an electrochemical plant. The baseline scenario requires no new infrastructure, whereas building a completely new electrochemical plant is a major contributor to the impact of the limestone-integrated electrochemical scenario.\nReply:\nThank you for your insightful comment. Similar to the economic analysis, we have now incorporated the environmental impacts associated with the electrochemical reactors' construction. The updated data and calculation details are illustrated in the revised Figure 6b-d and Note S2, respectively.\nIndeed, building electrochemical reactors presents a certain global warming potential (GWP) of 14258.17 kg CO₂ eq, with the Ru-Ir anode being a primary contributor (80.3%). Similarly, building electrochemical reactors contributes to a mineral resource scarcity (MRS) of 2704.46 kg Cu eq, due to the extraction and processing of critical metals required for electrode manufacturing. However, given the system's high treatment capacity, building electrochemical reactors account for only 0.5% of the updated GWP (12.60 kg CO₂ eq) when normalized to the functional unit (Figure R8b). While electrode manufacturing contributes significantly to mineral resource scarcity (54.3%), the updated mineral resource scarcity (0.02 kg Cu eq) remains much lower than the routine processes (0.19 kg Cu eq) (Figure R8c), as the new process facilitates efficient wastewater P recovery, minimizing the P fertilizer production and the corresponding ore extraction.\nOverall, while building new electrochemical plants does contribute to certain environmental impacts at the beginning, the process iteration still offers environmental benefits.\nFigure R8 (newly updated as Figure 6b-d in the main text). (a) Schematic representation of processes (bold) and material flows (italicized) in the life cycle assessment of the incorporation of a limestone-integrated electrochemical system (proposed scenario) in comparison to the current lime precipitation (baseline scenario). Evaluation of the environmental impacts of the two scenarios in terms of global warming potential with contribution analyses (b) and mineral resource scarcity (c).\nIn light of your comment, we have revised the discussion and updated the calculation details of the LCA (Please see Lines 492-494 and 497-499 and the revised Note S2).\nRevised content:\nNote S2:\n“The environmental burdens of the electrochemical reactors are allocated to the processed wastewater based on their lifespan and processing capacity with Equation (S1):\n𝑚\n𝑡𝑜𝑡𝑎𝑙\n𝑚 = (𝑆1)\n𝑄∗𝑡\nwhere, m represents material mass allocated per FU (kg/FU) and m refers to total mass of total the material (kg). Q means treatment capacity (20.8 m3/h) and t is reactor's lifespan (set as 20 years).\nOn top of this, the material allocation and modeling assumptions for the components (e.g., titanium substrate, coating load, stainless steel 304, PMMA) of the electrochemical reactor are based on parameters collected from our electrochemical setup:\nTitanium substrate (Anode): 1 mm thickness, density = 4.51 g/cm³; total anode area per FU = 1.18 cm².\nCoating load (Anode): Ru = 0.8 mg/cm², Ir = 0.2 mg/cm²; total anode area per FU = 1.18 cm².\nStainless Steel 304 (Cathode): 1 mm thickness, density = 7.9 g/cm³; total cathode area per FU = 2.53 cm².\nPMMA (Reactor): 100 kg, PMMA sheet, density = 1.18 g/cm².\nTo be noted, to ensure operational reliability and account for electrode degradation, two sets of electrodes (anodes and cathodes) were included in the reactor, assuming a replacement interval of 10 years over the 20-year lifespan.”\nLines 492-494:\n“While the construction of new electrochemical plants does incur certain carbon emissions (14258.17 kg CO₂ eq), this impact becomes minimal (0.07kg CO eq) when normalized per m3 2 of wastewater, given the system's high processing capacity.”\nLines 497-499:\n“Despite the fact that electrode manufacturing contributes significantly to mineral resource scarcity (54.3%)…”\nAdditionally, I have a couple of minor comments that I think would improve the readability of the manuscript:\n1. Line 242, Figure 1(c): I find the figures throughout the manuscript to be clear and easy to follow, except for Figure 1(c). The colours and overlapping shading make the figure hard to read. It is particularly difficult to distinguish between graphs Ca:P=1.67:1, Ca:P=3:1 and Ca:P=4:1. I recommend the authors re-design this figure.\nReply:\nThank you. We have re-designed the Figure 1c (Please refers to Figure R9), and now each condition is displayed separately with clear labels. We hope the revised figure addresses your concern.\nFigure R9 (newly updated as Figure 1c in the main text). The particle size distribution (in wt%) of the granular products obtained from various Ca/P dosing ratios in lime precipitation.\n2. Line 387: I believe the authors meant to say “displayed” instead of “displaced”.\nReply:\nThank you. It has been corrected.\nRevised content:\nLine 301:\n“…Ca2+ concentrations displayed a characteristic rise-and-fall trajectory…”\nWe want to express our sincere thanks for your time and valuable comments throughout the revision. Your insightful comments and suggestions have significantly improved the quality of our manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "All reviewer comments have been well addressed, and I believe the paper is suitable for publication as is.", "reviewer_2": "Reviewer #2 comments to manuscript NCOMMS-25-47301-T:\nI want to thank the authors for the rigorous work they have put into addressing the comments of both reviewers from the previous revision round. The additional longer-term laboratory experiments considerably strengthen the message of the practical applicability of the proposed technology. Similarly, I am glad to see the authors have reworked the techno-economic analysis and life cycle assessment to better capture the impacts of the capital costs/works of the new electrochemical system. In my opinion, these new additions, together with the already well planned and executed initial set of experiments, make the manuscript strong enough to accept for publication in Nature Communications."}, "author_response": "Response to Reviewers' Comments\nResponse to Reviewer #1,\nComments:\nAll reviewer comments have been well addressed, and I believe the paper is suitable for publication as is.\nReply:\nWe sincerely appreciate your time and effort in evaluating our manuscript.\nResponse to Reviewer #2,\nComments:\nI want to thank the authors for the rigorous work they have put into addressing the comments of both reviewers from the previous revision round. The additional longer-term laboratory experiments considerably strengthen the message of the practical applicability of the proposed technology. Similarly, I am glad to see the authors have reworked the techno-economic analysis and life cycle assessment to better capture the impacts of the capital costs/works of the new electrochemical system. In my opinion, these new additions, together with the already well planned and executed initial set of experiments, make the manuscript strong enough to accept for publication in Nature Communications.\nReply:\nWe are deeply grateful for your time and effort, as well as your constructive feedback and encouragement throughout the review process. Your insights have significantly strengthened our manuscript."}]

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