Hydrated metal charge density as a universal descriptor explaining mechanistic variations in periodate activation toward pollutant degradation

Domain: neuroscience
Published: 2026-02-12
Editorial outcome: minor_revision

Abstract
Although activation mechanisms of oxidants diverge significantly across different transition metals, whether the universal descriptor exists that can unify these pathways remains unknown. Herein, taking periodate (PI) as an example, we systematically explored the connection among Group VIII (8) metals (Fe(II), Ru(III), Os(III)). While all three metals activate PI effectively, Os(III) exhibits fundamentally different activation behavior compared to Fe(II) and Ru(III). PI activation by Os(III) primarily generated ·OH, promoting hydroxylated products formation; whereas high-valent metal-oxo species dominate in Fe(II) and Ru(III) systems, yielding epoxy products. Theoretical calculations indicate that Os(III) possesses a significantly higher positive charge density than that of Fe(II) and Ru(III). The high charge density of Os(III) enhances oxygen affinity, and reduces the energy barrier for single-electron transfer to water ligands, thereby promoting radical generation. Conversely, the lower charge density of Fe(II) and Ru(III) indicates a stronger affinity for electrons, favoring the oxygen atom transfer process. This work identifies metal charge density as a key descriptor modulating reaction mechanisms of oxidants and informs the design of tunable oxidation processes.

Introduction
Advanced oxidation processes (AOPs) have been widely used for the degradation of organic pollutants due to their advantages in terms of ease of storage and transport1,2. Various methods have been proposed for the activation process of oxidants, including photoactivation, ultrasonic activation, carbon-material-mediated activation, and transition metal activation3–6. Among them, transition metal activation is a sustainable strategy without requiring consumable chemicals or energy input7. The most common activation mechanisms involving transition metals includes single electron transfer (SET) and oxygen atom transfer (OAT)2,8,9. In the SET process, the transition metal bound to the oxidant can provide electrons to the antibonding orbitals of the oxidant10,11, thus weakening the I-O or O-O bond, and ultimately generating reactive species12,13. Conversely, in the OAT process, transition metals have the capacity to accept an oxygen atom from the oxidant, thereby generating high-valent metal-oxo species (HVMO)9,14. At present, there is no theoretical framework capable of unifying the apparent mechanistic differences and explaining the connections between the disparate activation pathways exhibited by various transition metals. This deficiency severely limits our in-depth understanding of the activation mechanism and hinders the rational design of oxidant activators.

In contrast, specific descriptors (such as d-band centers) have been successfully applied to characterize catalytic differences and guide catalyst design in the field of nitrogen reduction reactions15. However, descriptors that clarify the differences in the mechanisms of metal-activated oxidants are still lacking. It is noteworthy that even adjacent fourth-period metals (e.g., Mn, Fe, and Co) exhibit distinctly different dominant mechanisms (radical pathway vs. nonradical pathway) for persulfates, which is primarily attributed to differences in their electronic structures, particularly differences in spin states16–18. Given that the pronounced mechanistic differences observed among elements within the same period stem predominantly from their intrinsic elemental properties (e.g., electron configuration, spin state), a fundamental question pertinent to the periodic law arises: Do elements within the same group, despite possessing analogous valence electron configurations, exhibit different mechanistic patterns when activating oxidants? Since Fe(II) and Ru(III) were found to exhibit considerable efficiency in periodate (PI) activation2,9, and Os(III) in the sixth period has a similar electronic structure to Fe(II) and Ru(III)19, it is crucial to investigate whether metal ions in the same group activate PI via similar or fundamentally different mechanisms.

In this study, PI is used as the model oxidant to systematically investigate PI activation by metal ions in Group VIII (8) (i.e., Fe, Ru, and Os), aiming to reveal the commonalities and differences in their activation processes. The periodic law states that elements within the same group, yet situated in different periods, exhibit analogous chemical properties20,21. However, when investigating the effect of Os(III) on PI activation for different pollutant degradation, experimental phenomena are encountered that cannot be explained by the reaction mechanism proposed in the Fe(II)/PI and Ru(III)/PI systems. For instance, the degradation of pollutants shows no significant selectivity in the Os(III)/PI system, quite different from the high selectivity in the Fe(II)/PI and Ru(III)/PI systems. This prompts us to conduct an in-depth comparative study of the PI activation with transition metals in the same group. We hypothesize that a fundamental property of transition metals governs the bifurcation of activation pathways, and the hydrated metal center’s positive charge density is identified as this key universal descriptor, establishing the “Charge Density Control Mechanism”.

Results
Degradation of CBZ by PI activated with different transition metals
To systematically evaluate the performance of transition metals in PI-based AOPs, we conducted a comparative analysis of 12 metal ions in PI activation for carbamazepine (CBZ) degradation. As illustrated in Fig. 1A, Fe(II), Ru(III) and Os(III) (abbreviated as Group VIII (8)) demonstrated exceptional PI activation efficiency, achieving significant CBZ degradation within 40 min. In contrast, activation of PI with other transition metals, such as Co(II), Ni(II), and Cu(II), showed negligible removal rates towards CBZ (<10%) (Fig. S1, see supporting material for details). This limited reactivity may be attributed to their relatively weak oxidative potentials or poor oxophilicity, which hinder both electron transfer and I–O bond cleavage during PI activation9,22. Remarkably, significant reactivity divergences were observed within Group VIII (8) congeners. Ru(III) demonstrated ultrafast activation efficiency towards PI, achieving complete CBZ degradation within 1 min, consistent with previous reports2,9. In contrast, Fe(II) exhibited limited activation capacity with only 20% CBZ removal after 40 min. Of particular significance is the intermediate activation performance of Os(III), which attained 50% pollutant elimination within 40 min. This study is among the first to identify Os(III) as a competent PI activator.

The activation capacity of Fe(II), Ru(III), and Os(III) in Group VIII (8) towards PI exhibits no consistent increase or decrease across periods. The observed kinetic measurements reveal significant reactivity disparities that Ru(III) demonstrates 10-fold higher activity than Os(III), and Os(III) shows 2.5-fold greater activity than Fe(II). According to Pauling’s valence bond theory, these congeners should possess analogous valence electron configurations and consequently similar activation capabilities23,24. The non-periodic variation implies the absence of a discernible pattern governing their activation efficiency and reveals a lack of a suitable theoretical framework to explain the underlying mechanism. These results suggest that factors beyond valence configuration (such as ionic radius and charge) may critically influence PI activation, necessitating deeper investigation into periodic trends for the activation process.

Comparison of group VIII (8) in PI activation
Degradation of different micropollutants. The reactivity patterns of Group VIII (8)-activated PI systems were systematically investigated using different micropollutants with contrasting electronic properties. As depicted in Fig. 1B, rapid and non-selective degradation of all selected micropollutants was observed in the Os(III)/PI system, irrespective of their electron-donating or electron-withdrawing functional groups. In contrast, strong substrate selectivity was observed in Fe(II)/PI and Ru(III)/PI systems, with >95% removal of tetracycline achieved within 5 min and low efficacy against carboxyl-substituted benzoic acid and nitro-substituted nitrobenzene (Fig. S2). Such behavior implies fundamentally distinct activation mechanisms between Os(III)-based systems and Fe(II)/Ru(III)-based systems.

Impact of metal ion and PI concentrations. The concentration-dependent behaviors of Group VIII (8)/PI systems were systematically evaluated using CBZ as the model pollutant. Notably, a distinct volcano-shaped curve for CBZ degradation was observed in the Os(III)/PI system in Fig. 1C, D, where CBZ degradation was accelerated with increasing Os(III) (0–50 μM) and PI (0–100 μM) concentrations, yet declined sharply at higher concentrations (>50 μM Os(III) or >100 μM PI). At higher concentrations of Os(III), accelerated PI activation leads to a rapid initial decay of CBZ within the first 0–5 min, followed by a pronounced decrease in the reaction rate as the oxidizing equivalents become depleted. In contrast, when concentrations of PI are further increased to 250 µM, the overall degradation rate decreases from the beginning because excess PI scavenges reactive species. This scavenging represents a classical self-quenching effect, which continuously reduces the availability of ROS throughout the reaction.

The reverse concentration dependence at high concentration suggests a self-quenching mechanism analogous to those reported in H2O2-based AOPs25,26, where oxidants scavenge excess reactive species, thereby reducing the availability of reactive species towards contaminant oxidation. In the Fe(II)/PI and Ru(III)/PI systems, however, the contaminant degradation was reported to increase monotonically with both metal ion concentration and oxidant dosage2,9. The self-quenching behavior observed in the Os(III)/PI system stands in sharp contrast to the monotonic activation patterns of Fe(II)/PI and Ru(III)/PI systems, emphasizing the unique activation mechanism of Os(III).

Impact of pHs. The degradation of CBZ in the Os(III)/PI system exhibited a distinct pH-dependent activation pattern. As shown in Fig. 1E, the degradation efficiency of CBZ increased remarkably from 45% to 95% within 40 min as the solution pH elevated from 3.0 to 9.0. This pH-dependent activation mechanism maybe attributed to two synergistic effects: (1) Progressive ionization of PI (H4IO6− to H3IO62−) enhances its redox potential from 1.7 V to 2.3 V27, thereby strengthening its electron abstraction capacity. (2) Increased hydroxide coordination forms Os(III)(OH−)n complexes that elevate the electron density of the metal center. In contrast, the inverse pH dependency was observed in the Fe(II)/PI and Ru(III)/PI systems, characterized by accelerated CBZ degradation under acidic conditions but significantly suppressed reactivity under alkaline environments (Fig. S3). Previous studies have established that HVMO (Fe(IV)=O and Ru(IV)=O) are primarily generated in these systems, with their formation decreasing at higher pH due to hydroxide precipitation and oxidant decomposition2,9. These combined effects suggest that the Os(III)/PI system was likely to proceed through a fundamentally different pathway compared to the Fe(II)/Ru(III)-based systems.

Impact of water matrices. The impact of common water matrices (Cl−, NO3−, HCO3− and humic acid (HA)) on CBZ degradation was compared in the Group VIII (8)/PI systems. As illustrated in Fig. S4, the presence of Cl− (1–100 mM) and NO3− (1–100 mM) showed no significant inhibitory effect on CBZ degradation in the Os(III)/PI system, consistent with previous reports on Fe(II)/PI and Ru(III)/PI systems2,9. The addition of HCO3− significantly inhibited the degradation of CBZ, suggesting that HCO3− may have reacted with the reactive species present in the system. Notably, the Os(III)/PI system exhibited superior resistance to HA interference compared to the Fe(II)/PI and Ru(III)/PI systems. As depicted in Fig. 1F, >80% degradation efficiency of CBZ was maintained across HA concentrations of 5–50 mg/L in the Os(III)/PI system, contrasting sharply with the significant inhibitory impact of HA observed in Fe(II)/PI and Ru(III)/PI systems2,9. This difference in experimental phenomena suggests that the activation mechanism of Os(III) likely differs from that of Fe(II) and Ru(III). In the Fe(II)/PI and Ru(III)/PI systems, electron-rich phenolic moieties in HA competitively interact with HVMO, thereby quenching these primary oxidants and suppressing contaminant degradation. In contrast, the anomalous behavior observed in the Os(III)/PI system suggests two possible mechanistic explanations: (1) the primary reactive species may not be HVMO, but radical-type intermediates generated through the SET process; and (2) the interaction between Os(III) and organic ligands may modulate the electronic structure of the metal center, thereby facilitating electron transfer28,29 and, to some extent, compensating for the quenching effect of HA on reactive species.

Differences in degradation products. The degradation products of CBZ were further explored with LC/MS/MS (Table S1, Fig. S5 and Text S1, see supporting material for details). In the Os(III)/PI system, the hydroxylated CBZ derivatives such as dihydroxy-CBZ (TP 270a, TP 270b) and ring-opening small-molecule products including benzoic acid (TP 122) were detected (Fig. S5). These degradation products exhibited significantly reduced ecotoxicity compared to the parent compound (Fig. S6). In contrast, epoxy-CBZ and less hydroxylated intermediates were produced in both Fe(II)/PI and Ru(III)/PI systems, and the formation of epoxy-CBZ originates from the OAT reactions of HVMO2,9. The complete absence of epoxy-CBZ in the Os(III)/PI system strongly excludes the participation of high-valent Os-oxospecies. This could be attributed to either the lack of detectable Os(V)=O intermediates under the studied conditions or the lower reactivity of Os(V)=O toward OAT compared to Fe and Ru. The formation of hydroxylated products and small-molecule fragments instead implies a distinct reaction pathway that favors sequential hydroxylation and mineralization processes.

Overall, the Os(III)/PI process exhibits fundamentally different activation behavior compared to the Fe(II)/PI and Ru(III)/PI systems. Specifically, contaminants are oxidized non-selectively in the Os(III) process, in contrast to the selectivity toward select electron-rich pollutants observed in the Fe(II)/PI and Ru(III)/PI systems. Furthermore, HA showed a negligible impact on contaminant degradation in the Os(III)/PI system but exerted significant quenching effects on that in the Fe(II)/PI and Ru(III)/PI systems. Notably, self-quenching behavior occurs at elevated metal concentrations in the Os(III)/PI system, diverging from the linear concentration dependence observed with Fe(II) and Ru(III). Product analysis further highlights this mechanistic divergence, that oxygen-transfer adducts dominate in Fe(II)/PI and Ru(III)/PI systems, whereas hydroxylation products prevail in the Os(III)/PI process. Collectively, these distinct behaviors across pollutant selectivity, impact of pH and HA, concentration dependence, and product profiles strongly suggest a fundamentally different activation mechanism in the Os(III)/PI system, challenging the assumption of similar chemistry among these group elements and prompting further investigation into the reactive species involved in Os(III)-mediated PI activation.

Group VIII (8)/PI: nonradical vs. radicals
Previous studies9,30,31 have shown that reactive species such as HVMO, reactive oxygen species (ROS), and reactive iodine species (RIS) are generated in the PI activation system. The electron spin resonance spectroscopy (ESR) analysis, molecular probe experiments and quenching experiments were carried out to investigate the reactive species responsible for contaminant degradation in the Group VIII (8)/PI system.

HVMO. Benzoic acid (BA) was used to determine the potential generation of ROS in the Os(III)/PI system due to its inertness to HVMO32,33. The results indicated rapid degradation of BA (>80% within 40 min, Fig. 1B) in the Os(III)/PI system, implying that ROS other than HVMO are likely operative in this system. In contrast, only marginal BA degradation (<20% within 40 min) was observed in the Fe(II)/PI and Ru(III)/PI systems (Fig. S2). This minimal degradation provides strong evidence that HVMO dominates contaminant degradation in the Fe(II)/PI and Ru(III)/PI systems.

Phenyl methyl sulfoxide (PMSO) is a specific probe for HVMO, generating the product of PMSO2 via OAT2,9. In the Os(III)/PI system at neutral pH (both buffered and unbuffered), no significant PMSO2 formation was detected (Fig. 2A, S7). This absence of PMSO oxidation was further supported by the lack of epoxy-CBZ product formation (Fig. S5), indicating no involvement of Os(V)=O species in substrate oxidation. To further investigate whether HVMO were generated in the Os/PI system, NaOsCl6 was used as a potential source of HVMO. It is noteworthy that in the Os(IV)/PI system, the characteristic signal of HVMO (PMSO2) remains undetectable (Fig. 2B). However, the observed degradation of PMSO indicates that Os(IV) can still activate PI to generate reactive species, yet these species are not HVMO. Overall, in the presence of either Os(III) or Os(IV), the Os/PI activation system mediates PMSO degradation without producing any detectable PMSO2 intermediates. This necessitates two plausible explanations: (1) Os(V)=O species do not form during activation, or (2) they exhibit intrinsically low OAT reactivity toward PMSO. An in-depth investigation at the mechanistic level was required through theoretical calculations in the following sections.

In sharp contrast, significant PMSO2 accumulation was observed in both Fe(II)/PI and Ru(III)/PI systems (Fig. 2C, D), confirming HVMO generation along PI activation2,9. To validate their dominance in contaminant degradation, dimethyl sulfoxide (DMSO), a selective quencher for HVMO34, was introduced in the Fe(II)/PI and Ru(III)/PI systems. The addition of DMSO could nearly completely inhibit CBZ degradation (Fig. 3D, E), confirming HVMO as the primary reactive species, which is consistent with the previous studies2,9.

RIS. As illustrated in Fig. S8, the addition of excess phenol (a scavenger of RIS (IO4·, IO3·, IO2·, and IO·) and ·OH35,36) strongly inhibited CBZ degradation in the Os(III)/PI system, indicating the potential involvement of RIS or ·OH in the contaminant degradation. In contrast to the Os(III)/PI system, studies have proven that in both Fe(II)/PI and Ru(III)/PI systems, RIS either fails to form or requires prohibitively high activation energy for generation2,9. To further detect the generation of RIS in the Os(III)/PI system, ESR analysis with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping agent was conducted. As shown in Fig. S9, the six peaks of equal intensity representing the signals of DMPO-IO3·37, were observed in the ESR spectra, indicating the presence of IO3· in the Os(III)/PI system. However, multiple studies have reported the low reactivity of IO3· toward CBZ2, implying that RIS likely plays a minor role in this system. Given that TBA does not react with IO3·38, the almost complete inhibition of CBZ degradation after the addition of TBA suggests that other reactive species (e.g., ·OH) can promote the degradation of CBZ in the Os(III)/PI system (Fig. S10A), which will be discussed later.

ROS. The ESR analysis was also employed to probe the generation of ROS in the Os(III)/PI system (Figs. S9, S11). The DMPO-·OH adduct with the characteristic 1:2:2:1 quartet signal was observed in the ESR spectra of the Os(III)/PI system, confirming the generation of ·OH. Degradation of terephthalic acid (TA) was further conducted to corroborate ·OH production, because TA selectively reacts with ·OH to form highly fluorescent 2-hydroxyterephthalic acid (HTA)1,38. Intense HTA fluorescence signals were clearly detected in the Os(III)/PI/TA system (Fig. 3A), providing strong complementary evidence consistent with the ESR results for ·OH generation. Separately, the ESR spectrum displayed the characteristic three-line signal (1:1:1 intensity ratio) of the TEMP-1O2 adduct (2,2,6,6-tetramethyl-4-piperidone-N-oxyl, TEMPO), which confirms the generation of 1O2 (Fig. S9).

Furthermore, the quenching experiments demonstrated that both tert-butanol (TBA, a ·OH scavenger39,40) and methanol (MeOH, an effective scavenger of both HVMO and ·OH) significantly suppressed CBZ degradation (>80%, Fig. S10A), which illustrates that ·OH serves as the predominant reactive species responsible for CBZ degradation in the Os(III)/PI system. Crucially, subsequent theoretical calculations (presented below) confirmed the low reactivity of Os(V)=O species, effectively eliminating the possibility of Os(V)=O quenching by these scavengers. Consequently, the observed inhibition of CBZ degradation (>80%, Fig. S10A) upon DMSO addition (k·OH+DMSO = 7.0 × 109 M−1 s−1) should therefore be attributed specifically to the scavenging of ·OH. Besides, the addition of furfuryl alcohol (FFA), a scavenger of ·OH and 1O238, also reduced the degradation of CBZ (>80%, Fig. S10B). Given the established dominance of ·OH as the primary oxidant, the significant degradation inhibition observed upon FFA addition was primarily attributed to ·OH scavenging. In general, these observations collectively confirm that ·OH serves as the predominant reactive species involved in CBZ degradation, with RIS (IO4·, IO3·, IO2·, and IO·) and 1O2 playing a negligible role in the Os(III)/PI system.

In contrast, the markedly different active species were observed in the Fe(II)/PI and Ru(III)/PI systems compared to the Os(III)/PI system. Specifically, no HTA fluorescence signal was observed in these systems (Fig. 3A), indicating the absence of ·OH generation. This observation was further supported by quenching experiments that the addition of TBA caused negligible inhibition of CBZ degradation (Fig. S10C, D). Significantly, although the addition of MeOH substantially suppressed CBZ degradation efficiency (>80%), given that MeOH can react with Fe(IV) species (kFe(IV)+MeOH = 2.5 × 103 M−1 s−1)2, the observed inhibition is therefore attributable specifically to MeOH’s reaction with HVMO, rather than ·OH. Therefore, these findings collectively demonstrate that ROS play insignificant roles in the Fe(II)/PI and Ru(III)/PI systems, where HVMO dominates the reaction pathways.

The combined evidences demonstrate that while HVMO dominates in Fe(II)/PI and Ru(III)/PI systems, ·OH is the primary reactive species in the Os(III)/PI system. Considering that the suitable descriptor to explain this mechanistic divergence remains elusive, further investigation into the molecular reaction mechanisms underlying the differential activation of PI by Group VIII (8) elements is motivated.

Activation mechanism of PI with Os(III)
Origin of ·OH in the Os/PI system. Among Group VIII (8) investigated in this study, prominent ·OH production was observed only in the Os(III)/PI system (Fig. 3A). This unique behavior raises a critical question regarding why Os(III), despite sharing a similar electron configuration with Ru(III) in the same group, demonstrates unique ·OH production activity. We first sought to identify the oxygen source for ·OH formation, as it holds the key to understanding Os’s exceptional activation behavior. Within the Os(III)/PI system, both water molecules (H2O potentially oxidized to ·OH) and PI (IO4− potentially cleaved to ·OH) could serve as possible ·OH precursors. An 18O isotopic tracing experiment was performed by replacing H2O with H218O while maintaining natural abundance oxygen in PI and tracking the potential incorporation of 18O in the hydroxyl oxygen in the PMSO-OH. Strikingly, LC-MS analysis revealed a definitive absence of 18O enrichment in PMSO-OH (Figs. 3B, C, S12), thereby excluding water as the oxygen source. This provides conclusive evidence that PI serves as the exclusive oxygen donor for ·OH generation in this system, likely through cleavage of the I–O bond.

Generation mechanism of ·OH in the Os(III)/PI system. Two potential mechanisms have been proposed to explain the enhanced degradation observed in the Os(III)/PI system under alkaline conditions, including the influence of increased OH− coordination number on the oxidation state of Os(III) and the enhanced oxidizing capability of PI itself. To elucidate this phenomenon, we first investigated how the OH− coordination number affects the electronic properties of the Os(III) center. Theoretical calculation was conducted to investigate the mechanistic link between the Os(III) coordination structure, specifically the increase in OH− coordination number, and its SET capacity. Computational results show that the addition of OH− to Os(III) progressively elevates the HOMO energy level (Table S2). According to Frontier Orbital Theory, an elevated HOMO energy level weakens the binding capacity of the active center for electrons41, rendering the Os(III) center more likely to lose electrons. Considering that the number of OH− coordinating with Os(III) increases with the increase of pH value, Marcus theory calculations were performed to analyze the SET energy barrier between Os(III) and IO4− (Fig. 3D). As shown by the blue curve in Fig. 4A, B, the reaction energy barrier decreases with increasing OH− coordination (or alkalinity). However, since the product energies exceed the reactant energies, and the energy barriers exceed 200 kJ/mol during this reaction (considering the effects of H4IO6− and H3IO62−), the reaction is unlikely to occur at room temperature. Nevertheless, the calculation results indicate that the increase in the number of OH− coordinating with Os(III) facilitates SET from Os(III).

Molecular-level analysis of the reaction system showed that the Os(III)/PI system was mainly composed of Os(III), IO4−, OH−, and H2O, which might be involved in the ·OH generation process. The calculations have shown that SET processes between Os(III) and IO4− are thermodynamically unfavorable (Fig. 4A, B), and SET from OH− to Os(III) (reducing Os(III) to unstable Os(II)) obviously will not occur either. Given that the aforementioned pathways are unlikely to occur, the water molecule-mediated SET process involving Os(III)-H2O-PI was identified as a candidate pathway warranting further investigation (Fig. 3E). In aqueous solution, Os(III) predominantly exists as hydrated complexes such as [Os(H2O)6]3+ or hydroxide-coordinated species like [Os(OH)3(H2O)3]. The Os-bound water molecule (Os(H2O)) can be conceptualized as Os(OH−)(H+). Guided by these observations, we conducted calculations of the energy barrier for SET between Os(III) and H+ under different OH− coordination numbers. As depicted in the red curve in Fig. 4A, B, the reaction energy barrier decreases markedly with increasing OH− coordination (119.1 kJ/mol), rendering the reaction thermodynamically feasible. The observed decrease in the energy barrier with increasing basicity (increase in the number of OH− coordinations) suggests that alkalinity facilitates PI activation and subsequent pollutant degradation, consistent with the experimental observation in Fig. 1E that CBZ degradation accelerates at higher pH. The SET from [Os(OH)3(H2O)3] to H+ leads to the simultaneous oxidation of Os(III) to Os(IV) and the formation of the unstable intermediate H·-IO4− (i.e., Os(III)-H2O-IO4− → Os(IV)-OH− + H·-IO4−). The ΔG of this reaction step becomes significantly less than 0, rendering it thermodynamically favorable. Subsequently, H·-IO4− dissociates into ·OH and IO3−, whereby the O in ·OH is derived from IO4−, which is consistent with the results of the isotope labeling experiment (Fig. 3B, C). After Os(III) undergoes electron transfer to form Os(IV), the resulting Os(IV) species can further participate in electron transfer reactions with PI to promote pollutant degradation. This OH−-enhanced mechanism contrasts with Fe(II)/PI and Ru(III)/PI systems, whose higher energy barriers hinder analogous pathways, as discussed below.

Os(III)/PI system versus Fe(II)/PI and Ru(III)/PI systems
Electron transfer between Group VIII (8) and coordinated water. As ·OH is dominant in the Os(III)/PI system but is absent in both the Fe(II)/PI and Ru(III)/PI systems, we calculated the energy barriers for SET from Fe(II) and Ru(III) species to H+ in coordinating water molecules to elucidate the reason for the lack of ·OH formation. The results (Table S3) revealed that the energy barriers for SET in both Fe(II)/PI and Ru(III)/PI systems gradually decreased with increasing pH. Under alkaline conditions, the energy barriers for Fe(OH)2(H2O)4 and Ru(OH)2(H2O)4+ were calculated to be 208.1 and 194.9 kJ/mol, which are still higher than the 119.1 kJ/mol observed in the Os(III)/PI system. These high barriers, generally considered insurmountable for reaction initiation under common experimental conditions, provide a compelling mechanistic explanation for the absence of ·OH in the Fe(II) and Ru(III) systems. While this resolves the divergence in ·OH generation pathways, the observed variations in HVMO formation remain an open question, necessitating further comparative analysis of these species to fully illuminate the distinct activation mechanisms across the Group VIII (8).

Reactions of HVMO with PMSO. Given the limited existing studies on Os(V)=O reactivity with PMSO, we conducted transition state calculations for interactions between Os(V)=O and PMSO alongside comparative analyses of Fe(IV)=O and Ru(V)=O systems. Under acidic conditions, theoretical results show the energy barrier for PMSO2 formation from PMSO oxidation by Fe=O(H2O)5 is 14.1 kJ/mol, compared to 26.3 kJ/mol for Ru=O(H2O)5 and 90.5 kJ/mol for Os=O(H2O)5 (Fig. 4C). Hence, the Os=O(H2O)5 exhibits a substantially higher reaction barrier with PMSO, indicating greater difficulty in the generation of PMSO2. Crucially, the 90.5 kJ/mol barrier remains thermally accessible under acidic conditions, allowing slow PMSO2 formation consistent with experimental detection of trace signals of PMSO2 at pH 3 (Fig. 2A and S5). This observation further supports the presence of the Os(IV)=O species in the reaction system, as this high-valent intermediate enables limited oxygen atom transfer to PMSO under acidic conditions. In contrast, under neutral or alkaline conditions, the dominant Os=O(OH)3(H2O)2 species exhibits an energetically prohibitive barrier (>164.8 kJ/mol) for PMSO oxidation to PMSO2 (Fig. 4D). This high activation barrier explains the absence of PMSO2 formation above pH 3. Although these energy barriers explain the observed differences in reactivity between elements in Group VIII (8), no suitable descriptors have yet been found to explain these differences.

The hydrated metal charge density: a universal descriptor
Variations in oxygen-binding affinity at HVMO metal centers likely underlie the differential OAT energy barriers observed during PMSO2 generation. Theoretical calculations demonstrate that the Os(V) center spontaneously abstracts an oxygen atom from PI to form a stable Os(V)=O species (Fig. S13), indicating significantly stronger Os(V)=O bonding compared to its Fe(IV)=O and Ru(V)=O analogs. Mulliken charge analysis of the optimized HVMO structures (Fig. 4E) reveals progressively higher positive charge densities on the metal centers, with values of +1.11 for Fe, +1.28 for Ru, and +1.39 for Os. This trend correlates with enhanced electrophilicity at the Os(V) center, which intensifies stabilization of the Os(V)=O bond. Consequently, the Os(V)=O species exhibits diminished chemical reactivity toward CBZ, as evidenced by undetectable levels of epoxy-CBZ products (Fig. S5). Also, despite the structural homology of the Os(V)=O species with its reactive Fe(IV)=O and Ru(V)=O counterparts, the charge effect of the Os(V) center severely attenuates its OAT reactivity toward PMSO. Consequently, these probes fail to generate the PMSO2 signal for Os(V)=O, highlighting the limitation in detecting HVMO of transition metals using OAT-based methodologies.

The divergent reactivity patterns among Group VIII (8) arise from periodic variations in atomic electronic structure. Descending the periodic table to the sixth period, Os exhibits an increased atomic radius compared to its Fe and Ru congeners. This expanded electron cloud reduces the effective nuclear charge attraction toward valence electrons at the Os center. Consequently, Os develops substantially higher positive charge density (+1.39 vs. +1.28 for Ru and +1.11 for Fe; Fig. 4E), thereby intensifying its oxophilicity through reinforced electrostatic stabilization. This higher positive charge center suppresses conventional OAT pathways, directly accounting for the absence of both PMSO2 generation and epoxy-CBZ formation observed experimentally. Simultaneously, the weakening of electron retention promotes the SET process of metal ions, causing the dominant reaction mechanism to shift from OAT to free radical reactions triggered by SET. These contrasting behaviors, namely the suppression of OAT by high charge density coupled with the promotion of SET due to weakened electron retention, are unified under the Charge Density Control Mechanism. This intrinsic property of the hydrated metal center acts as the primary switch, directing the activation flux either towards radical-mediated oxidation (high charge density, e.g., Os) or HVMO (lower charge density, e.g., Fe, Ru).

Preliminary computational and literature evidence further suggest that this charge-density-based descriptor may possess broader generality beyond Group VIII (8) metals. For instance, in the case of Mn(II)-mediated PI activation, the Mn(II)-EDTA complex exhibits an OAT pathway42 with a calculated metal center charge of +1.020, whereas the Mn(II)-picolinic acid complex follows a SET mechanism43, with a corresponding charge density of +1.305. These results are fully consistent with the mechanistic boundary (around 1.3) predicted by the Charge Density Control Mechanism, demonstrating that the same descriptor can effectively distinguish between OAT and SET-dominated pathways in other transition-metal systems as well. This cross-system consistency underscores the theoretical generality and predictive utility of the metal charge density descriptor as a unifying electronic parameter governing PI activation. It not only bridges the mechanistic understanding across different metal centers but also provides a rational basis for designing coordination environments that emulate precious-metal-like reactivity using earth-abundant metals, thereby enhancing the translational relevance of this work to practical catalytic systems.

As summarized in Fig. 5, the fundamental divergence in reactivity between the Os(III)/PI system and the Fe(II)/PI or Ru(III)/PI systems stems from the higher charge density of the Os(III) center. The high-valent Os-oxospecies formed by this high charge density is kinetically inert due to the strong bonding of the Os=O bond. The weaker electron retention ability of the Os(III) center causes the system to tend toward the SET mechanism, which produces non-selective ·OH. Consequently, alkaline pH enhances reactivity by weakening the Os center’s electron retention capability, facilitating SET and ·OH production. However, this radical pathway also leads to self-quenching at high oxidant or catalyst concentrations. While HA can quench ROS, it simultaneously promotes the SET process in the Os system via coordination, resulting in retained efficiency and hydroxylated products as the dominant outcome. In stark contrast, the lower charge density of Fe(II) and Ru(III) centers enables the facile generation of reactive HVMO via the OAT mechanism. This nonradical pathway exhibits high substrate selectivity, yielding OAT products. The OAT pathway is favored under acidic conditions but is significantly inhibited by HA, which quenches the key HVMO. Thus, the intrinsic metal charge density dictates the operative mechanism (SET vs. OAT), explaining the distinct reactivity patterns, selectivity, pH dependence, HA effects, and product distributions.

Methods
Materials
Sodium periodate (NaIO4, 99%) and Osmium(III) chloride hydrate (OsCl3·xH2O, 99.9%) were purchased from Aladdin Biochemical Technology Co., LTD. (Shanghai, China). The ultrapure water was generated from the Milli-Q Water Purification System (18.25 MΩ·cm−1, Millipore, Bedford, USA). Other reagents used were summarized in Text S2.

Experimental procedure
All experiments were conducted at a controlled temperature (22 ± 2 °C) in 150 mL quartz beakers containing 100 mL reaction solution, using a magnetic stirrer. The reactions were initiated by adding transition metals (Fe(II), Ru(III) and Os(III)) to solutions containing the target organic contaminant (i.e., carbamazepine (CBZ)) and PI. Considering the observed pH instability in the Os(III)/PI system (pH dropping to 3 within 15 s), pH adjustment was performed using phosphate buffer solution (10 mM PBS, pH 7). Figure S14 illustrates the negligible influence of the PBS on CBZ degradation. The pH was adjusted using 0.1 mM H2SO4 and 0.1 mM NaOH in the Fe(II)/PI and Ru(III)/PI systems. The specific sample processing procedures are described in Text S3. All experiments were repeated at least twice under the same conditions. Data are presented as mean ± standard deviation in figures.

Analytical methods
The concentrations of CBZ and other pollutants were determined using high-performance liquid chromatography (HPLC, Agilent 1260), equipped with a UV diode array detector and a ZORBAX SB C18 column (2.4 mm × 150 mm, 5 μm). The mobile phase and the detection wavelengths are presented in Table S4. The presence of free radicals and 1O2 was confirmed by electron spin resonance spectroscopy (ESR) utilizing 5,5-dimethyl-1-pyrroline-n-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP), as detailed in Text S4. Terephthalic acid (TA) was employed as a probe to capture ·OH, and the presence of ·OH is confirmed by detecting the generated fluorescent compounds. The experimental procedure is outlined in Text S5. Ultra-high-performance liquid chromatography equipped with quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS, Agilent 6545, USA) was employed for the analysis of 16O/18O isotopically labeled methyl phenyl sulfone (PMSO-OH). The detailed methodology is presented in Text S6. The degradation products were identified and characterized using HPLC-triple quadrupole mass spectrometry (TSQ Quantum Ultra EMR, Thermo Fisher Scientific, USA). The specific instrumental parameters and analytical procedures are detailed in Text S7.

Quantum chemical calculations for the activation mechanism of PI
Quantum chemical calculations based on density functional theory were performed at the PBE0/def-2svp level44 using the Gaussian 16 software package45 (Gaussian, Inc., USA), based on density functional theory. The specific details of the reaction mechanism calculations39,46,47 are outlined in Text S8. The key molecular coordinates are provided in the source data file.

Data availability
Source data are provided with this paper. The minimum dataset necessary to interpret, verify and extend the findings of this study is included in the Source Data file or Supplementary Information. All the raw data relevant to the study are available from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Comments of NCOMMS-25-64891\nThis study systematically elucidates the mechanistic divergence of Group VIII (8) metals (Fe, Ru, Os) in periodate (PI)\nactivation. It was discovered that Os(III) primarily generates •OH via a single-electron transfer (SET) pathway. In contrast, the\nFe(II)/Ru(III) systems rely on oxygen atom transfer (OAT) to form high-valent metal-oxo species. Based on these findings, the\nauthors propose the \"hydrated metal positive charge density\" as a unified descriptor, enabling the prediction and control of\nthe mechanism from an electronic structure perspective. This research not only resolves long-standing theoretical\ncontroversies but also provides operable principled guidance for practical applications such as water treatment and\nenvironmental catalysis. By revealing the central role of this unified descriptor, this work offers a valuable reference for the\nrational screening and design of efficient catalysts under complex water quality conditions. Overall, this work possesses\nboth cutting-edge significance and practical utility, and the proposed descriptor and mechanistic framework advance the\nfield of advanced oxidation processes (AOPs) and meet the publication standards of Nature Communications.\nComments for Authors:\n1. The authors' perspective on the \"universal\" of this descriptor is highly novel and forward-looking. It is recommended to\nbriefly elaborate on this in the Discussion section, perhaps by citing relevant literature, to explore whether this descriptor is\nequally applicable for predicting the reaction mechanisms of other metal groups (e.g., Mn, Co) in PI activation. This would\nhelp highlight the descriptor's generalizability and further expand its academic influence within the field.\n2. The authors' theoretical explanation for \"how alkaline conditions promote the shift to the SET mechanism in Os(III)-\nmediated PI activation\" is clear and convincing. To further strengthen the core argument regarding \"charge density control,\"\nproviding a quantitative correlation plot, such as the relationship between OH- coordination number and the SET energy\nbarrier, in the main text would be beneficial. This visualization would make the key information more intuitive, allowing\nreaders to better grasp the trend of mechanism evolution with the changing coordination environment.\n3. The authors' innovative use of Mulliken metal charge density as a unified descriptor successfully correlates the\nmechanistic divergence in PI activation by Group VIII (8) metals, which is a robust finding. However, Mulliken charge\nanalysis is sometimes debated due to its strong basis set dependence. To ensure the general applicability of this key\ndescriptor and make the conclusions more widely acceptable within the computational chemistry community, it is strongly\nrecommended that the authors supplement their analysis with validation using several other, more modern charge analysis\nmethods (e.g., Hirshfeld charges).\n4. The value of truly groundbreaking research lies in its ability to open new directions for a field. This study reveals the\npromotive and regulatory role of OH⁻ coordination number on the reaction mechanism. Could we, through precise\ncoordination engineering of inexpensive metal centers, rationally tune their electronic structures to mimic the catalytic\nfunctions of precious metals, thereby achieving efficient and selective transformations?\n5. This study excellently elucidates the differences in the static electronic structures of various metal ions and their\ncorrelation with reaction mechanisms. However, in actual catalytic processes, the valence state, coordination environment,\nand intermediates of the metal center are constantly evolving dynamically. Does the charge density descriptor play a\ndominant role throughout the entire catalytic cycle in your proposed reaction pathways? For instance, after Os(III) is\nconverted to Os(IV) via the SET process, does the change in its charge density significantly affect the efficiency or pathway\nselectivity of subsequent reactions?\n6. The paper systematically reveals different metal activation pathways and their electronic structure correlations. In real\naqueous environments, organic matter is ubiquitous and may play significant regulatory roles in the reaction mechanism. It is\nrecommended that the authors consider briefly commenting on the potential mechanisms of organic matter influence in the\ndiscussion, such as its effect on electron transfer processes or its ability to alter the charge environment of the metal center\nthrough coordination, thereby affecting the catalytic pathway. Appropriately supplementing the discussion with these\npotential mechanisms would enhance the extrapolation value of the research conclusions to real, complex systems.\n7. The Supporting Information (SI) is an integral part of the paper. It is recommended to add a brief prompt (e.g., \"(see\nSupporting Information for details)\") when Fig. S1, Text S1, etc., are first mentioned in the main text to guide readers.\n8. The term \"high-valent metal-oxo species\" appears multiple times. It is suggested to define its common abbreviation,\nHVMO, at its first occurrence and maintain consistent usage thereafter to enhance conciseness and consistency.\n9. The spellings \"nonradical pathway\" and \"non-radical pathway\" are used interchangeably in the text, and the consistent\nformatting is recommended. Some reference DOIs contain a repeated \"doi.org/https://doi.org\", and the standardization is\nadvised.\n10. The authors might consider supplementing the manuscript with recently published literature on PI activation by other\nmetals in the revised version. Furthermore, contrasting the mechanistic differences in the Discussion section could further\nhighlight the universality and uniqueness of the \"charge density\" descriptor proposed in this study for unifying the\nunderstanding of activation pathways across different metals, thereby more powerfully demonstrating the theoretical value\nand innovation of this work.", "reviewer_2": "It is an interesting but challenge issue. The authors provided quantified evidences to support the divergence of Os(III) from\nFe(II) and Ru(III), and tried to propose an universal descriptor to explain reasonably the divergence. It can be considered for\npublication after careful revision.\n1. Os(III) at high concentration of 100 μM exhibited depressed effect, but it was only observed after 5 min. It is not suitable to\nsimply attribute this to self-quenching mechanism.\n2. Little effect of HA in the Os(III)/PI system is obviously inconsistent with the high inhibition effect observed in hydroxyl\nradicals dominated AOPs. A reasonable explanation should be given.\n3. As a mostly common anion, HCO3- should be investigated in the “Impact of water matrices”.\n4. The Os(III)/PI system is inefficient in oxidation of PMSO to PMSO2 via OAT. Thus, 18O incorporated PMSO2 is lacking as\nspeculated. Instead, the authors should focus the hydroxylated PMSO, in which 18O was not observed. This is not\nconsidered in the mechanism investigations.\n5. The authors provided computational evidences for •OH generation in the Os(III)/PI system, in which hydrogen radical is a\ncritical intermediate for generation of •OH. An experimental evidence is required for hydrogen radical at least. Moreover, fate\nof Os(IV) has been not considered.\n6. From the perspective of chemical equilibrium, generation of more boned hydroxyls as Os(IV)-OH- is inconsistent with the\nfacilitated CBZ degradation at higher pH.\n7. Although hydrated metal charge density as a universal descriptor to explain the divergence of Os(III) from Fe(II) and\nRu(III), it is regrettable that a quantitative relationship was not obtained.\n8. It is unclear whether Os(V)=O is formed in the Os(III)/PI system although the authors tried to reveal its low reactivity with\nPMSO. The oxidation products of Os(III) should be analyzed, providing more convincing evidences to gain the mechanism.\n9. The conclusion of low reactivity of Os(V)=O might be not applicable to other organics.\n10. As the oxidation product of Os(IV), Os(VI) might be produced as a reactive species.\n11. Language editing is required as many errors can be still seen.", "reviewer_3": "In this manuscript, authors have examined the degradation of CBZ by periodate/metals system under different experimental\nconditions. Different metals showed variation in degradation of CBZ and authors have explained using the concept of\nhydrated metal charge density. A number of experiments were carried out and theoretical calculations were performed to\ndemonstrate SET and OAT steps were proposed. Overall, this research has no experimental flaws and this reviewer has\nfollowing comments to enhance the quality of the manuscript.\n(i) Authors have studied only one pollutant and the title should not be generalized. Similarly, authors studied only one\noxidant and the title should have this oxidant.\n(ii) Authors have no consider the speciation of metals with pH. Different hydroxide complexes exist and what type pf metal\nhydroxo species are present in the studied pH range that reacted with PI.\n(iii) It is not clear to this reviewer why Co(II), Ni(II), and Cu(II) did not have any effect.\n(iv) Ruthenium has interesting results and why high-valent ruthenium not considered? This is not clear to this reviewer.\n(v) Does the mechanism change with pH? What pH DFT calculation were performed,"}, "author_response": "We sincerely thank you for the valuable comments, which are very helpful\nfor improving the quality of our manuscript. These suggestions also helped us to clarify\nour arguments and strengthen the presentation of our work. Our detailed responses to\neach point are provided below.\n[... Author responses to all points ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "My comments have been well addressed in the revised version, which can be considered for publication in Nature\nCommunications.", "reviewer_2": "The generalizability of hydrated metal charge density as a descriptor is still problematic. But the authors have carefully\nrevised the manuscript and have adequately addressed most of concerns. So I recommend it for acceptance for publication.", "reviewer_3": "This reviewer is satisfied with the response. These were carefully discussed and responded positively. Authors are\ncomended for their revision."}, "author_response": ""}]

Decision letter