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# Water Film-Mediated Hydrolysis of Pyrophosphate on a Nanosized Mn Oxide

Tao Chen,* Tao Luo, Tra My Bui Thi, Khalil Hanna, and Jean-François Boily

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Cite This: Langmuir 2025, 41, 24336–24343

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## ACCESS

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Supporting Information

**ABSTRACT:** The geochemical behaviors of phosphate-containing species at mineral surfaces are of fundamental importance for controlling phosphorus (P) mobility, fate, and bioavailability. Understanding these interfacial behaviors in water-unsaturated environments, where minerals are covered by thin water films, is of special importance in the context of soil vadose zone geochemistry. This study resolved the transformation of pyrophosphate to orthophosphate within the confines of nanometer-thick water films condensed on nanosized birnessite (MnO₂). Time-resolved vibrational spectroscopy indicated that PP was effectively degraded into monodentate mononuclear complexes on MnO₂ and at rates that followed zero-order kinetics. We found a direct link between PP degradation kinetics and water film thickness, which revealed the role of sorbed water as the key nucleophilic agent needed for P–O–P bond rupture. By highlighting the essential roles of surface complexation and water loadings in controlling pyrophosphate transformation, this work adds new insight needed to advance knowledge on the global cycling of P in nature. It can also have broader applications to catalysis and energy storage solutions involving MnO₂ nanoparticles.

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## INTRODUCTION

Phosphorus is an essential nutrient for all living organisms, and is present in various forms in nature.¹,² This includes polyphosphates, which is a group of phosphate-bearing compounds joined by phosphoanhydride (P–O–P) bonds. Polyphosphates represent important fractions of the total P in planktonic organisms (1–13%),³,⁴ dissolved and particulate pools of seawater (0–8%),⁵,⁶ marine sediments (0–8%),⁷ lake sediments (1.5–11.4%),⁸ and soils (0.4–7%).⁹ They can also be produced biogenically¹⁰ (e.g., extracellular release, lysis, and death) as well as by industrial discharge (e.g., water treatment processes, medicines, fertilizers, flame retardants, food additives).¹¹ The biogenic and anthropogenic release of dissolved and particulate polyphosphates in natural watersheds (e.g., lakes, seawater, and sediments)¹² calls for a new understanding of their fate and stability in nature. This knowledge has important implications in the study of geological and anthropogenic P cycles.¹³–¹⁵

Hydrolysis reactions of polyphosphates to orthophosphate (orthoP) are of great interest in this context. Other than the well-recognized enzymatic dephosphorylation by phosphatases,⁷,¹⁶,¹⁷ this process can be abiotically facilitated by extremely acidic¹⁸,¹⁹ or basic²⁰ media. The nucleophilic attack of H₂O or OH⁻ on the activated P–O–P bonds is one of the key steps toward bond cleavage,²¹,²² and therefore the production of shorter-chained polyphosphates:

$$H_{n+2}P_nO_{3n+1} + H_2O \rightleftharpoons H_{n+1}P_{n-1}O_{3n-2} + H_3PO_4 \quad (1)$$

However, they can also be catalyzed under circumneutral conditions through Lewis acid/leaving group activation by complexation with divalent metal cations²³ or with metal ion centers at mineral surfaces (e.g., Mn,²⁴ Fe,²⁵,²⁶ and Al oxides.¹³,²⁷ By relaxing local P–O–P bonds, complexation facilitates attacking by weak nucleophiles, water¹⁸,²⁴ or hydroxyl groups,²²,²⁸ also giving way to bond cleavage.²¹,²⁸ It is consequently possible that polyphosphate hydrolysis could be sensitive to water availability and therefore to air moisture, such as in water-unsaturated environments, including vadose zones of soils.²⁶ This becomes especially important considering that minerals capture air moisture in the form of thin water films, whose thickness is directly affected by natural variations in humidity.²⁹,³⁰

This study uncovered the roles that air moisture plays in the hydrolysis of pyrophosphate (P₂O₇⁴⁻, PP) associated with birnessite (MnO₂), a highly reactive and abundant Mn(IV)-oxide nanomineral in nature.³¹ Focus on this simpler polyphosphate, which is present in nature³²,³³ at up to hundreds of µmol/L,³³,³⁴ provides an opportunity to follow

Received: May 16, 2025

Revised: August 26, 2025

Accepted: August 26, 2025

Published: August 29, 2025

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the very last step in polyphosphate degradation into two orthoP species,³⁵ a process that has received less attention in the scientific literature. Monitoring these reactions was made possible by *in situ* vibrational spectroscopy, which uncovered a direct link between PP degradation kinetics and water loadings. This study deepens our understanding of polyphosphate degradation by exploring the water film-driven decomposition of PP in terms of surface complexation and activation reactions. This knowledge can be directly tied to processes in vadose zones of soils,³⁶ and contributes to the understanding of P cycling¹ and primary production in ecosystems.³⁷ It can also have implications for catalysis and energy storage solutions involving MnO₂.

## EXPERIMENTAL SECTION

**Materials.** Stock solutions of sodium orthophosphate (Na₃PO₄) and sodium pyrophosphate decahydrate (Na₄P₂O₇·10H₂O) (Sigma-Aldrich) were prepared in ultrapure water. Standard solutions of premade 1 M NaOH or 1 M HCl (Sigma-Aldrich) were used to adjust the pH of the solutions. Acid birnessite was synthesized by reacting KMnO₄ and HCl in boiling water.³¹,³⁸ The resulting suspension (26 g/L) was purged from dissolved atmospheric CO₂(g) using a stream of N₂(g) for 2 h, then stored in the dark at 4 °C. The study of Li et al.³¹ contains a detailed report on the synthesis and purification procedures, as well as salient physicochemical properties of the high specific surface area (85 m²/g) MnO₂ nanoparticles measured by Brunauer–Emmett–Teller (BET) N₂ adsorption.

**PP Degradation in Water Films on Nanosized MnO₂.** Vibrational spectroscopy was used for *in situ* tracking of water film populations and surface species transformation on dry MnO₂ coatings exposed to humid air at 25 °C. MnO₂ coatings were obtained by pipetting a 5 mL droplet of 2.6 g/L MnO₂ suspension on a thermostatic optical diamond window of an Attenuated Total Reflection (ATR) accessory (Golden Gate, diamond single bounce N29328, Specac) in a closed chamber. The wet MnO₂ coatings were dried under a flow of 500 mL/min dry N₂(g) (~ 0% RH, 0.0095 kPa H₂O). Continuous monitoring of water O–H stretching and H–O–H bending bands indicated that the sample lost all free water within 20 min.

A 5 mL aliquot of a 2 mM PP solution adjusted to pH 7 was thereafter cast onto the dried MnO₂ coating. The resulting sample was then dried under a stream of dry N₂(g) for no more than 20 min. Note that because PP hydrolysis is a slow process operating over the course of hours, it is unlikely that this initial sample preparation step generated any orthoP. PP hydrolysis reactions were thereafter monitored by continuously acquiring spectra as the samples were exposed to a stream of moist N₂(g) at 10, 30, 60, 80, and 95% RH. Gas composition was generated and controlled using a humidity generator module (proUmid MHG32).

All vibrational spectra were collected on samples prepared on the ATR cell using a Fourier Transform Infrared (FTIR) spectrometer (Bruker, Vertex 70/v). Measurements were acquired in the 600–4500 cm⁻¹ range at a resolution of 4 cm⁻¹, and at a forward/reverse scanning rate of 10 kHz. Spectra were generated with a Blackman-Harris 3-term apodization function with 16 cm⁻¹ phase resolution and the Mertz phase correction algorithm. Each spectrum was obtained by coadding 200 spectra, which were collected over a 178 s period.

**PP and orthoP Sorption Experiments.** Effects of surface loadings on the vibrational spectra of PP and orthoP on MnO₂ surfaces were measured to assist in the interpretation of the hydrolysis experiments. Those for PP were conducted in the same manner as in the degradation experiments, but only the starting spectra were collected. Dry MnO₂ coatings previously prepared on the ATR cell were treated with 5 mL droplets of 1, 2, 5, and 10 mM PP solutions at pH 7. The samples were dried under N₂(g), and ATR-FTIR spectra were then collected in 20 min as the samples were exposed to a stream of 80% RH. PP loadings were obtained by batch adsorption.

Suspensions of 2.6 g/L MnO₂ were mixed with 1, 2, 5, 10 mM PP solutions, and the mixtures were adjusted to pH 7 using 1 M NaOH or HCl. The resulting suspensions were equilibrated at pH 7 for 24 h at 25 °C on an end-to-end rotator in the dark.

Surface loadings of orthoP were studied by batch adsorption in the same fashion. Here orthoP solutions at concentrations of 0.5, 1, 5, 10, and 20 mM were each mixed with a 1 g/L suspension of MnO₂. The resulting mixtures were then adjusted and equilibrated at pH 7 for 24 h. The effects of pH on orthoP adsorption mechanisms were also investigated by reacting 1 mM orthoP solutions with 1 g/L MnO₂ at pH 4, 7, and 9 for 24 h. The centrifuged wet pastes of the resulting suspensions were applied on the ATR cell using a spatula, and dried under N₂(g). ATR-FTIR spectra were thereafter collected as the dried pastes were exposed to 80% RH.

The resulting supernatants above were filtered (0.22 µm hydrophilic polytetrafluoroethylene, H-PTFE-20/13, Duren-Germany), and concentrations of PP and orthoP in the filtrate were measured by Ion Chromatography (IC). The IC instrument (DX-120) was operated with an anion separation column (Dionex IonPac AS19 – 4 × 250 mm) coupled with a conductivity detector. Elution was performed at a flow rate of 1 mL/min of deionized water. Surface loadings of PP (Γ<sub>PP</sub>) and orthoP (Γ<sub>orthoP</sub>) on MnO₂ were then calculated from the difference between the initial and final concentrations.

**Water Loadings by Microgravimetry and Vibrational Spectroscopy.** Water loadings on the synthetic MnO₂ powder were determined by dynamic vapor sorption (DVS), using a DVS Advantage ET 2 instrument (Surface Measurement systems). These were obtained through an 11-point adsorption isotherm between 0 and 95% RH (0.0095 – 3.011 kPa H₂O) at 25 °C. Measurements were made using ~ 30 mg samples initially dried at ~ 0% RH (25 °C) for 10 h to remove adsorbed water. The samples were thereafter exposed to a flow of moist N₂(g) at preselected levels of RH. Equilibration times of 60 min for samples exposed up to 60% RH were extended to 180 min at higher RH to allow masses to reach time-independent values before each adsorption step. Gravimetric measurements were continuously taken every 1 min, and a complete adsorption isotherm took up to 28 h. The gravimetrically determined water loadings were expressed in terms of H₂O/nm², using 85 m²/g as the specific surface area of MnO₂. The number of equivalent water monolayer (ML) was estimated assuming that 1 ML contains 12 H₂O/nm², which is a typical value of water films on (hydr)oxo-terminated metal oxide surfaces.³⁹,⁴⁰

ATR-FTIR spectra of water films formed on PP-free and PP-bearing MnO₂ were also collected at 0 – 95% RH, using the same experimental setup as in the PP hydrolysis experiments. Equilibration times were the same as in the microgravimetric measurements.

**Vibrational Spectral Analysis.** The time-resolved PP degradation spectra were analyzed by (i) Multivariate Curve Resolution (MCR)⁴¹ and (ii) Gaussian deconvolution. All procedures were performed on spectra with absorbances offset to zero at (i) 1250 cm⁻¹ for the P–O(H) stretching region (850–1250 cm⁻¹),¹⁵ (ii) 1800 cm⁻¹ for the water bending region (1300–1800 cm⁻¹), and (iii) 4000 cm⁻¹ for the O–H stretching region (2500–4000 cm⁻¹). All vibrational spectra were normalized to the Mn–O stretching band intensity at 600 cm⁻¹ to account for variations in sample quantity prior to the analysis.

The ATR-FTIR spectra collected in water sorption experiments on the PP-free and PP-bearing MnO₂ were also analyzed by MCR to relate MCR concentration profiles to water loadings. More details on this procedure are described in the Supporting Information.

MCR extracted spectral profiles and their relative concentrations of end-member components. This was achieved by expressing the matrix (A) of time-resolved spectra in terms of a linear combination of the product of the spectral (e) and concentration (C) profiles, which is related through the Beer–Lambert law (A = eC). This was achieved using the MCR-ALS program⁴¹ in the computational environment of MATLAB (The Mathworks, Inc.). Gaussian deconvolution was performed on the same dataset, however corrected by a linear background. The resulting datasets (A') were deconvoluted with
<figure><img src="image_10.png" /><figcaption>**Figure 1.** (a) ATR-FTIR spectra of PP adsorbed on MnO₂ exposed to a flow of 80% RH N₂(g) for 53 h. The sample was initially prepared by reacting 2 mM PP with 2.6 g/L MnO₂ at pH 7, leading to a surface loading of Γ = 0.34 PP/nm². MCR (b) spectral components and (c) concentration profiles from the data in (a). Gaussian deconvolution on the initial and final spectra (d) in (a), and (e) time-resolved concentration profiles. Note that the asymmetric stretching bands (900–920 cm⁻¹) of the bridging P–O–P bond (νₚ–O–P,as) of aqueous PP partly overlap with the δₘₐₗOH band (Figure S4 and Table S4).³⁵ (f) Proposed surface configurations of adsorbed PP (MCRₐ I) and orthoP (MCRₐ II).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 2.** XPS spectra and fitting results of the O 1s region of (a) pure sodium pyrophosphate (Na₄P₂O₇·10H₂O), (b) pure MnO₂, and (c-f) PP-bearing MnO₂ (2 mM PP, pH 7) reacted for 0–7 days at 80% RH.</figcaption></figure>

$$A(n)' = \sum_{1}^{n} A(n)'_{n,max} e^{-(v-v_{n})/\sigma_{n}} \quad (2)$$

where A'_{n,max} is the maximal absorbance of the nth component centered at wavenumber v_n with the width distribution σ_n.

**Surface and Solid Characterizations.** To probe the surface state of the mixtures in the PP degradation experiments, MnO₂ layers were cast and dried on glass slides by pipetting 200 mL of a suspension of 2.6 g/L MnO₂. Another 200 mL of a 2 mM PP solution at pH 7 was then cast over the dried MnO₂ layers. The mixtures were dried and exposed to a stream of 80% RH in N₂(g), and the solids were collected after reaction of 0, 1, 2, and 7 days. The surface composition of the samples was analyzed using X-ray photoelectron spectroscopy (XPS). The instrument was a Kratos Axis Ultra electron spectrometer, equipped with a monochromatic Al K_α X-ray (1486.7 eV)

source operating at 10 mA and 15 kV. Our analysis was focused on the O 1s region to resolve the bonding environment of P-bound O. Reference spectra of sodium pyrophosphate decahydrate salt powder and dried pristine MnO₂ were also acquired for comparison.

To exclude possible surface PP/orthoP precipitation or intercalation into MnO₂ interlayers, the reacted solids were also investigated using X-ray diffraction (XRD). XRD patterns of pristine and reacted PP-loaded MnO₂ were acquired on a Bruker D8 Advance diffractometer equipped with a copper anode tube (Cu K_α radiation). All XRD data were recorded in the 2θ = 5–80° range at an angular step size of 0.006°. Slightly different from the XPS measurements, pH 7 solutions of 1, 2, 5, and 10 mM PP were mixed with MnO₂ coated on a low-background silica (Si) sample holder. The resulting mixtures were then measured immediately after a 20 min period of drying.
<figure><img src="image_12.png" /><figcaption>**Figure 3.** (a) ATR-FTIR spectra of adsorbed PP (Γ<sub>PP</sub> in PP/nm<sup>2</sup>) on MnO<sub>2</sub>. The samples were initially prepared by reacting 1–10 mM PP with 2.6 g/L MnO<sub>2</sub> at pH 7, and the spectra were obtained at 80% RH after 20 min of drying. Note that this drying time is too short for any significant PP hydrolysis (Figure 1). MCR (b) spectral components and (c) concentration profiles from the data in (a). Gaussian deconvolution of the spectra of samples reacted with (d) 10 and (e) 2 mM PP. (f) Concentration profiles of Gaussian components as a function of PP concentrations. Dashed lines are linear regressions on the PP bands.</figcaption></figure>

Reference patterns of pure MnO₂ were obtained following the same procedure. The sample prepared with 2 mM PP was, additionally, reacted *in situ* on the Si holder for 0, 1, and 7 days under a stream of 80% RH at 25 °C.

■ RESULTS AND DISCUSSION

The inherently low nucleophilicity of water, combined with a
high activation energy barrier,¹⁸,²¹,⁴² makes the hydrolysis of
PP (H₄P₂O₇ + H₂O ⇌ 2H₃PO₄) kinetically sluggish in
circumneutral aqueous media.⁴³ To assess how PP hydrolyzes
in thin MnO₂ water films under these conditions, we began by
tracking the vibrational P–O stretching and P–O–H bending
of PP as samples were exposed to a stream of 80% RH (Figure
1). The spectra initially revealed characteristic PP bands (~
993, 1070, and 1165 cm⁻¹), which gradually shifted to those of
orthoP over the course of 53 h. This was detected through the
growth of the P–O stretching band of orthoP (1030 cm⁻¹)
and the concurrent loss (993 cm⁻¹) and redshift (1165 cm⁻¹
→ 1100 cm⁻¹) of the other PP bands (Figure 1a). Accordingly,
XPS (Figure 2 and Tables S1–S3) showed that P/Mn ratios of
the samples concomitantly increased from 0.03 to 0.11 (Table
S2). Additionally, P–O–Mn/P–O–P (Table S3) ratios,
which signaled the degradation of P–O–P oxygens to orthoP
oxygens, increased from 1.07 to 1.68.

These transformations were also captured by two MCR<sub>a</sub>
vibrational spectral components, with MCR<sub>a</sub> I representing
PP<sup>44</sup> and MCR<sub>a</sub> II orthoP (Figure 1b). Their corresponding
concentration profiles followed zero-order kinetics (Figure 1c),
with MCR<sub>a</sub> I linearly decreasing and MCR<sub>a</sub> II increasing over
time. A Gaussian deconvolution of MCR<sub>a</sub> I resolved the four
PP bands (993, 1056, 1086, ~ 1165 cm<sup>-1</sup>), as well as two
others (883 and 915 cm<sup>-1</sup>) from the bending modes of surface
and nonstoichiometric Mn-OH groups (δ<sub>MnOH</sub>) of Mn-
(OH)<sub>2</sub>.<sup>45</sup> Spectral component MCR<sub>a</sub> II was, on the other
hand, deconvoluted into only two bands, one sharp (1030
cm<sup>-1</sup>) and the other broad (~ 1100 cm<sup>-1</sup>) (Figures 1d,e and
S5).

To resolve the mechanisms of decomposition, we inves-
tigated a plausible coordination geometry for the incipient

surface PP on MnO₂ in a set of loading-dependent adsorption
experiments (Figure 3). We note that dried PP solutions (0 –
10 mM) showed no signals at 80% RH, suggesting no
contributions from free PP species to the spectra obtained. A
MCR analysis of the resulting spectra (Figure 3a) extracted
only two components: (i) MCR<sub>b</sub> I representing the nonreacted
MnO₂, seen through its Mn–OH bending modes (Figure 3b),
and (ii) MCR<sub>b</sub> II revealing surface PP species (Figure 3b). The
concentrations of these components also directly scaled with
PP concentrations (Figure 3c). A Gaussian deconvolution of
MCR<sub>b</sub> II revealed that the PP complex could be described by
four comparable bands (993, 1056, 1086, ~ 1065 cm<sup>-1</sup>;
Figures 2d,e and Table S4) to MCR<sub>b</sub> II, with their respective
concentrations profiles scaling with PP concentrations (Figures
3f and S6). These results consequently revealed the existence
of only one PP species on MnO₂.

Based on previous work,⁴⁶ PP should adsorb under acidic to
circumneutral conditions through a ligand exchange reaction
with surface Mn–OH sites, forming direct P–O–Mn(IV)
bonds.⁴⁶ We also note that, despite a difference in Mn
oxidation state, the spectra of adsorbed PP were strongly
comparable to the one of the aqueous Mn(III)PP₂³⁻
complex.⁴⁷ This, and other metal-PP complexes (e.g.,
Mg·H₂PP), involves phosphate bidentate coordination with
the metal center. It is also this coordination type that is
believed to facilitate P–O–P bond cleavage²⁴,²⁶,²⁷ by lowering
the activation energies of P–O–P dissociation, producing free
phosphate complexes.⁴⁸

Given this similarity, we propose that PP surface complexes
should also be in bidentate mononuclear or binuclear
coordination with surface Mn sites. Although more direct
information could be provided in a dedicated study (e.g.,
phosphorus and manganic K-edge X-ray absorption spectros-
copy), numerical simulations of mineral surface-bound PP⁴²,⁴⁶
gave support for the binuclear coordination. Here, two
terminal phosphate moieties (Figure S7) are believed to be
complexed to two adjacent Mn(IV) sites, most likely at edges
of MnO₂ nanoparticles,⁴⁹ after ligand exchange:
<figure><img src="image_13.png" /><figcaption>Figure 4. (a-b) ATR-FTIR spectra of adsorbed orthoP (Γ<sub>orthoP</sub> in orthoP/nm<sup>2</sup>) on MnO<sub>2</sub> under a stream of 80% RH N<sub>2</sub>(g). The samples were initially prepared by reacting suspensions of 1 g/L MnO<sub>2</sub> with (a) 1 mM PP at pH 4, 7, and 9 and (b) 0.5–20 mM PP at pH 7 for 24 h in the dark. (c) MCR spectral components and (d-e) concentration profiles from the data in (a-b). (f) Proposed configurations of orthoP surface complexes extracted from MCR<sub>c</sub> II and MCR<sub>c</sub> III.</figcaption></figure>

$$2 \equiv \mathrm{MnOH}^{-1/3} + \mathrm{H}_2\mathrm{P}_2\mathrm{O}_7^{2-} \rightleftharpoons \mathrm{Mn}_2\mathrm{P}_2\mathrm{O}_7^{-8/3} + 2\mathrm{H}_2\mathrm{O} \quad (3)$$

A slight redshift (<4 cm<sup>-1</sup>) in the characteristic P–O–Mn(IV) bond (~1056 cm<sup>-1</sup>) compared with Mn(III)-PP complexes (~1060 cm<sup>-1</sup>) can be explained by differences in the electronic structure of Mn(III) and Mn(IV) (Table S4). In accordance with previous studies,<sup>44</sup> adsorbed PP was not likely to be protonated as deuteration (Figure S8) did not shift the vibrational spectra (Figure 1f, MCR<sub>a</sub> I).

To further investigate the coordination environment of the orthoP species (1030 cm<sup>-1</sup>) formed by PP hydrolysis (Figure 1, MCR<sub>a</sub> II), we acquired FTIR spectra across a range of pH and surface loadings (Figure 4). These measurements revealed the concomitant growth of a pair of bands (~980 and 1150 cm<sup>-1</sup>) both with pH and with surface loadings. These trends were also captured by MCR (Figure 4c) through: (i) MCR<sub>c</sub> I representing MnO<sub>2</sub>, (ii) MCR<sub>c</sub> II corresponding to a low pH and low P loading orthoP species (Figure 4d), and (iii) MCR<sub>c</sub> III corresponding to a high P loading orthoP species (Figure 4e). Most strikingly, MCR<sub>c</sub> III contained the 1030 cm<sup>-1</sup> band manifested during MnO<sub>2</sub>-induced PP hydrolysis (Figure 1e).

The appearance of this 1030 cm<sup>-1</sup> band with P loadings align with previous FTIR work on orthoP binding on goethite, which is suggestive of the establishment of a single P–O–Mn(IV) bond in a monodentate mononuclear complex of C<sub>3v</sub> symmetry (≡MnPO<sub>4</sub><sup>-7/3</sup>).<sup>50,51</sup> We consequently propose that the remaining bands of MCR<sub>c</sub> III and all the bands of MCR<sub>c</sub> II, which are highly similar, are from a bidentate binuclear complex of C<sub>2v</sub> symmetry (≡Mn<sub>2</sub>PO<sub>4</sub><sup>-5/3</sup>).<sup>14,52,53</sup> These collective findings therefore align with the previously established concept<sup>46,52,54</sup> that low orthoP loadings favor bidentate binuclear (C<sub>2v</sub>) and high loadings monodentate mononuclear (C<sub>3v</sub>) complexes. MCR<sub>c</sub> III is therefore assigned to a combination of both (Figure 4f). Additionally, as the spectra were unaffected by exposure to D<sub>2</sub>O (Figure S9), these complexes should be completely deprotonated.

Finally, building upon these findings, we resolved the roles that water plays in PP hydrolysis by tracking PP and orthoP

spectral signatures in water films of varied loadings. This was achieved by exposing samples to moist N<sub>2</sub>(g) between 10 to 95% RH (Figures 5 and S10). These gases produced the

<figure><img src="image_14.png" /><figcaption>Figure 5. Dependence of PP hydrolysis kinetics on humidity. (a) Initial and (b) final ATR-FTIR spectra of MnO<sub>2</sub> collected under a stream of 10–95% RH. The samples were initially prepared by reacting coatings of 2.6 g/L MnO<sub>2</sub> with 2 mM PP at pH 7. (c) Color-coded normalized MCR concentration profiles of the spectra in Figure S10. MCR concentration profiles were scaled to unity. Note that the MCR components shared the same characteristics to MCR<sub>a</sub> I and II in Figure 1. (d) Linear correlation between half-lives of MCR<sub>a</sub> I degradation and water loadings (water monolayers).</figcaption></figure>

equivalent of 2.6 to 7.2 monolayers (MLs) of water on PP-bearing MnO<sub>2</sub> (cf. Supporting Information for details). All samples generated highly comparable spectra (Figure 5a,b), suggesting the production of the same orthoP monodentate complex regardless of air moisture.

MCR concentration profiles (Figure 5c) revealed that air moisture controlled (zero-order) PP hydrolysis rates. PP half-
lives taken from these concentration profiles, decreased on the order of 12.2 h per ML, namely from 82.5 h in 2.6 ML at 10% RH down to 25 h in 7.2 ML at 95% RH (Figure 5d). This relationship thus underscored the need of available MnO₂-bound water—including externally adsorbed and condensed water (Text S1)—in catalyzing P–O–P rupture when PP was associated with MnO₂ under circumneutral conditions. This finding is noteworthy as previous experimental<sup>24,26,27</sup> and theoretical<sup>42,46</sup> studies have generally overlooked the contributions from the combined effects of mineral surface complexation and hydration on PP hydrolysis.<sup>55,56</sup> Again, from the vibrational spectra, these reactions are expected to convert bidentate binuclear PP complexes (≡Mn₂P₂O₇⁻⁸/³) into two adjacent orthoP complexes each in monodentate coordination (≡MnPO₄⁻⁷/³).<sup>42</sup> Direct coordination of two PP oxygens with two surface Mn atoms creates a double Lewis acid-base interaction. This activates P–O–P bonds by drawing electron density away from the P atoms and increasing P–O–P electrophilicity.<sup>28</sup> Our results support the idea that, instead of surface hydroxyl groups,<sup>22</sup> surface complexation increases the susceptibility of P–O–P rupture by H₂O nucleophiles supplied by the water films. We thereby conclude that PP hydrolysis kinetics operating through the reaction:

$$≡Mn_2P_2O_7^{-8/3} + H_2O \rightleftharpoons 2 \equiv MnPO_4^{-7/3} + 2H^+ \quad (4)$$

is a zero-order process because the rate-limiting step is controlled by the availability of water molecules in the surface films, not by the concentration of PP.

## CONCLUSIONS

Polyphosphates of biogenic and anthropogenic origins tend to be degraded via a terminal-only pathway, contributing to the formation of shorter-chained counterparts, down to orthoP in nature.<sup>57</sup> The interfacial behaviors, including adsorption and hydrolysis, of PP on environmentally abundant Mn oxide are of great relevance to understanding P cycling in both aquatic and humid environments.

Our *in situ* experiments showed that PP hydrolysis on MnO₂ covered with water films obeyed zero-order kinetics with half-lives in the order of hours to days. Decomposition was driven by exposure of water and the binuclear bidentate geometry of PP at MnO₂ surfaces. This coordination is critical for catalyzing hydrolysis reactions which produced two orthoP in monodentate mononuclear coordination. The linear dependence of PP half-lives with water loadings also revealed a significant role of surface water as a key nucleophilic agent for P–O–P bond cleavage.

This work provides molecular-level insight into the adsorption and hydrolysis of polyphosphates at mineral–water interfaces and underscores the importance of interfacial water in mediating phosphorus transformations. Notably, polyphosphate hydrolysis within nanometric water films on minerals could enable phosphorus transformation and mobilization in arid/humid environments through moisture scavenging<sup>58</sup> while supporting biological activity through enhanced nutrient availability. Given that PP represents the final reactive intermediate in polyphosphate degradation, this work also informs broader questions related to phosphorus sequestration and burial<sup>59,60</sup> in water-unsaturated environments. Beyond environmental implications, these findings can benefit redox biogeochemistry,<sup>49</sup> catalysis,<sup>61,62</sup> and energy storage solutions.<sup>63</sup>

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.5c02476.

Water sorption isotherm and changes in water vibrations upon hydration; linear correlations between DVS measurements and water vibrations; XRD patterns of MnO₂; ATR-FTIR spectra and band assignments of aqueous orthoP and PP; band positions in Gaussian deconvolution; calculated aqueous orthoP and PP speciation; spectral response in D₂O experiments; time-resolved ATR-FTIR spectra in humidity experiments; fitting parameters and results from XPS measurements; band assignments for P–O(H) and Mn–O vibrations; adsorption isotherm modeling of DVS data (PDF)

## AUTHOR INFORMATION

### Corresponding Author

Tao Chen – Department of Chemistry, Umeå University, Umeå SE-901 87, Sweden; ● orcid.org/0000-0002-8238-4823; Email: tao.chen@umu.se

### Authors

Tao Luo – Department of Chemistry, Umeå University, Umeå SE-901 87, Sweden; ● orcid.org/0009-0004-8317-8564

Tra My Bui Thi – Univ. Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, ISCR-UMR 6226, Rennes F-35000, France; ● orcid.org/0000-0001-8839-8428

Khalil Hanna – Univ. Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, ISCR-UMR 6226, Rennes F-35000, France; ● orcid.org/0000-0002-6072-1294

Jean-François Boily – Department of Chemistry, Umeå University, Umeå SE-901 87, Sweden; ● orcid.org/0000-0003-4954-6461

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.langmuir.5c02476

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

The authors wish to thank support from the Swedish Research Council (2020-04853, 2024-04694), Formas (2022-01246), the Kempe Foundation (JCSMK23-0172), and the Carl-Tryggers Foundation (CTS 22:2326) to J.-F.B.; the French National Research Agency via the INFLOW project (ANR-21-CE29-0008-01) to K.H.; and the CNRS (IRP CHEMICY 2022-2027) to K.H. and J.-F.B.

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