New insight into wastewater treatment by activation of sulfite with humic acid under visible light irradiation.

Sulfite (S(IV)), as an alternative to persulfate, has demonstrated its cost-effectiveness and environmentally friendly nature, garnering increasing attention in Advanced Oxidation Processes (AOPs). Dissolved organic matter (DOM) commonly occurred in diverse environments and was often regarded as an interfering factor in sulfite-based AOPs. However, less attention has been paid to the promotion of the activation of sulfite by excited DOM, which could produce various reactive intermediates. The study focused on the activation of sulfite using visible light (VL) - excited humic acid (HA) to efficiently degrade many common organic pollutants, which was better than peroxydisulfate (PDS) and peroxymonosulfate (PMS) systems. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that the triplet states of HA (3HA*) activated sulfite through energy transfer, resulting in the production of SO4·-, O2·-, and 1O2. The most significant active species found in the degradation of roxarsone (ROX) was 1O2, which was a non-radical pathway and exhibits high selectivity for pollutant degradation. This non-radical pathway was not commonly observed in traditional sulfite-based AOPs. Additionally, the coexistence of various inorganic anions, such as NO3-, Cl-, SO42-, CO32-, and PO43-, had little effect on the degradation of ROX. Furthermore, DOM from different natural water demonstrated efficient activation of S(IV) under light conditions, opening up new possibilities for applying sulfite-based advanced oxidation to the remediation of organic pollution in diverse sites and water bodies. In summary, this research offered promising insights into the potential application of sulfite-based AOPs, facilitated by photo-excited HA, as a new strategy for efficiently degrading organic pollutants in various environmental settings.

Zeolite facilitates sequestration of heavy metals via lagged Fe(II) oxidation during sediment aeration.

Aeration of sediments could induce the release of endogenous heavy metals (HMs) into overlying water. In this study, experiments involving FeS oxygenation and contaminated sediment aeration were conducted to explore the sequestering role of zeolite in the released HMs during sediment aeration. The results reveal that the dynamic processes of Fe(II) oxidation play a crucial role in regulating HMs migration during both FeS oxygenation and sediment aeration in the absence of zeolite. Based on the release of HMs, Fe(II) oxidation can be delineated into two stages: stage I, where HMs (Mn2+, Zn2+, Cd2+, Ni2+, Cu2+) are released from minerals or sediments into suspension, and stage II, released HMs are partially re-sequestered back to mineral phases or sediments due to the generation of Fe-(oxyhydr) oxide. In contrast, the addition of zeolite inhibits the increase of HMs concentration in suspension during stage I. Subsequently, the redistribution of HMs between zeolite and the newly formed Fe-(oxyhydr) oxide occurs during stage II. This redistribution of HMs generates new sorption sites in zeolite, making them available for resorbing a new load of HMs. The outcomes of this study provide potential solutions for sequestering HMs during the sediment aeration.

Photocatalytic activation of sulfite by maghemite (γ-Fe2O3) for iohexol degradation and alleviation effect of HCO3- on water acidification.

Photo-catalyzing sulfite (S(IV)) for the generation of sulfate radical (SO4•-) has emerged as a novel advanced oxidation process (AOP) recently. However, both the potential of soil minerals as effective photocatalysts and the process of water acidification due to S(IV) oxidation have been overlooked. Herein, maghemite (γ-Fe2O3), a typical soil iron oxide with excellent photocatalytic reactivity like hematite and magnetic-collectible property like magnetite, was successfully used to activate S(IV) for iohexol degradation under visible light irradiation. As a result, 91.3% of iohexol was eliminated within 15 min at 0.1 g/L maghemite and 0.5 mM S(IV) under neutral conditions. The influencing factors, including initial pH, catalyst dosage, S(IV) amount, co-existing substances and water matrix, were systematically investigated. The maghemite/S(IV)/vis system exhibited superior performance in iohexol degradation at a wide pH range (3-10). It was found that the released proton via S(IV) oxidation led to severe water acidification. Interestingly, a low dose of HCO3- could evidently resist water acidification with little influence on iohexol elimination. Radical quenching experiments and electron spin resonance (ESR) analysis confirmed that SO4•-, •OH and •O2- were involved in iohexol abatement with SO4•- being the dominant reactive species. Compared with hydrogen peroxide, persulfate and peroxymonosulfate, the established maghemite/S(IV)/vis system achieved much more remarkable degradation performance. Furthermore, the reactivity of the catalyst was not obviously reduced even after 10 runs of reaction. This study expands the application of soil iron oxide mineral in S(IV) activation in water treatment and proposes an approach to regulate water acidification in S(IV)-based AOP.

Simulation of vertical migration behaviors of heavy metals in polluted soils from arid regions in northern China under extreme weather.

Heavy metal migration behaviors and mechanisms in soils are important for pollution control and remediation. However, there are few related studies in arid areas under extreme weather patterns. In this study, we developed a one-dimensional continuous point source unsaturated solute transport model, and utilized Hydrus-1D to simulate the transport of Cu, As and Zn, in the pack gas zones of soils within the impact areas of two typical mining areas in Inner Mongolia. The results show that the soil has a significant interception capacity, with a short heavy metal vertical migration distance of ≤100 cm. Soil texture and heavy metal sorption affinity are two key factors that influence heavy metal transport. In soils with high contents of sands but low contents of clays, heavy metals have large mobility and thus migrate deeper and are more evenly distributed in the soil profile. The migration of different heavy metals in the same soil also varies considerably, with large migration depth for metals having low binding affinities onto soils. Scenario analysis for extreme drought and rainfall shows that, rainfall amount and intensity are positively correlated with heavy metal transport depth and negatively correlated with the peak concentration. Increasing rainfall/intensity results in a more uniform distribution of heavy metals, and lower profile concentrations owing to enhanced horizontal dispersion of surface runoff. When the total amount and intensity of rainfall remain constant, continuous or intermittent rainfall only affects the transport process but has almost no effect on the final pollutant concentration redistribution in the soil. These results provide theoretical data for estimating the degree of heavy metal pollution, and help design control and remediation strategies for polluted soils.

Controls of Mineral Solubility on Adsorption-Induced Molecular Fractionation of Dissolved Organic Matter Revealed by 21 T FT-ICR MS.

Mineral adsorption-induced molecular fractionation of dissolved organic matter (DOM) affects the composition of both DOM and OM adsorbed and thus stabilized by minerals. However, it remains unclear what mineral properties control the magnitude of DOM fractionation. Using a combined technique approach that leverages the molecular composition identified by ultrahigh resolution 21 T Fourier transform ion cyclotron resonance mass spectrometry and adsorption isotherms, we catalogue the compositional differences that occur at the molecular level that results in fractionation due to adsorption of Suwannee River fulvic acid on aluminum (Al) and iron (Fe) oxides and a phyllosilicate (allophane) species of contrasting properties. The minerals of high solubility (i.e., amorphous Al oxide, boehmite, and allophane) exhibited much stronger DOM fractionation capabilities than the minerals of low solubility (i.e., gibbsite and Fe oxides). Specifically, the former released Al3+ to solution (0.05-0.35 mM) that formed complexes with OM and likely reduced the surface hydrophobicity of the mineral-OM assemblage, thus increasing the preference for adsorbing polar DOM molecules. The impacts of mineral solubility are exacerbated by the fact that interactions with DOM also enhance metal release from minerals. For sparsely soluble minerals, the mineral surface hydrophobicity, instead of solubility, appeared to be the primary control of their DOM fractionation power. Other chemical properties seemed less directly relevant than surface hydrophobicity and solubility in fractionating DOM.

Characteristics of iron (hydr)oxides and Cr(VI) retention mechanisms in soils from tropical and subtropical areas of China.

Though both iron (hydr)oxides and soil organic matter (SOM) significantly influence heavy metal behaviors in soils, studies on the characteristics of natural minerals and the synergic effects of the two on Cr(VI) transformation are limited. This study investigated Cr(VI) retention mechanisms in four soils from tropical and subtropical regions of China based on a comprehensive characterization of Fe (hydr)oxides. These soils exhibited varying quantities of hematite, ferrihydrite and goethite, with distinct Al substitution levels and varied exposed crystallographic facets. Adsorption experiments revealed a positive correlation between Fe (hydr)oxide content and Cr(VI) fixation amount on colloid, which was influenced by the mineral types, Al substitution levels and facet exposures. Further, Cr(VI) was sequestered on soil by adsorption and reduction. In soils enriched with crystalline Fe (hydr)oxides, Cr(VI) reduction was primarily governed by SOM, while in soils enriched with poorly crystalline Fe (hydr)oxides, mineral-associated Fe(II) also contributed to Cr(VI) reduction. Aging experiments demonstrated that SOM and mineral-associated Fe(II) expedited Cr (VI) passivation and diminished the Cr leaching. These results improve our understanding of natural Fe (hydr)oxide structures and their impact on Cr(VI) behavior in soils, and shed light on complex soil-contaminant interactions and remediation of Cr(VI) polluted soils.

Adsorption Mechanisms of Glyphosate on Ferrihydrite: Effects of Al Substitution and Aggregation State.

Ferrihydrite is one of the most reactive iron (Fe) (oxyhydr)oxides in soils, but the adsorption mechanisms of glyphosate, the most widely used herbicide, on ferrihydrite remain unknown. Here, we determined the adsorption mechanisms of glyphosate on pristine and Al-substituted ferrihydrites with aggregated and dispersed states using macroscopic adsorption experiments, zeta potential, phosphorus K-edge X-ray absorption near-edge structure spectroscopy, in situ attenuated total reflectance Fourier transform infrared spectroscopy coupled with two-dimensional correlation spectroscopy, and multivariate curve resolution analyses. Aggregation of ferrihydrite decreases the glyphosate adsorption capacity. The partial substitution of Al in ferrihydrite inhibits glyphosate adsorption on aggregated ferrihydrite due to the decrease of external specific surface area, while it promotes glyphosate adsorption on dispersed ferrihydrite, which is ascribed to the increase of surface positive charge. Glyphosate predominately forms protonated and deprotonated, depending on the sorption pH, monodentate-mononuclear complexes (MMH1/MMH0, 77-90%) on ferrihydrites, besides minor deprotonated bidentate-binuclear complexes (BBH0, 23-10%). Both Al incorporation and a low pH favor the formation of the BB complex. The adsorbed glyphosate preferentially forms the MM complex on ferrihydrite and preferentially bonds with the Al-OH sites on Al-substituted ferrihydrite. These new insights are expected to be useful in predicting the environmental fate of glyphosate in ferrihydrite-rich environments.

Solutions for an efficient arsenite oxidation and removal from groundwater containing ferrous iron.

Manganese (Mn) oxides are extensively used to oxidize As(III) present in ground, drinking, and waste waters to the less toxic and more easily removable As(V). The common presence of multiple other cations in natural waters, and more especially of redox-sensitive ones such as Fe2+, may however significantly hamper As(III) oxidation and its subsequent removal. The present work investigates experimentally the influence of Mn(III) chelating agents on As(III) oxidation process in such environmentally relevant complex systems. Specifically, the influence of sodium pyrophosphate (PP), an efficient Mn(III) chelating agent, on As(III) oxidation by birnessite in the presence of Fe(II) was investigated using batch experiments at circum-neutral pH. In the absence of PP, competitive oxidation of Fe(II) and As(III) leads to Mn oxide surface passivation by Fe(III) and Mn(II/III) (oxyhydr)oxides, thus inhibiting As(III) oxidation. Addition of PP to the system highly enhances As(III) oxidation by birnessite even in the presence of Fe(II). PP presence prevents passivation of Mn oxide surfaces keeping As and Fe species in solution while lower valence Mn species are released to solution. In addition, reactive oxygen species (ROS), tentatively identified as hydroxyl radicals (•OH), are generated under aerobic conditions through oxygen activation by Fe(II)-PP complexes, enhancing As(III) oxidation further. The positive influence of Mn(III) chelating agents on As(III) oxidation most likely not only depend on their affinity for Mn(III) but also on their ability to promote formation of these active radical species. Finally, removal of As(V) through sorption to Fe (oxyhydr)oxides is efficient even in the presence of significant concentrations of PP, and addition of such Mn(III) chelating agents thus appears as an efficient way to enhance the oxidizing activity of birnessite in large-scale treatment for arsenic detoxification of groundwaters.

Effects and mechanisms of Al substitution on the catalytic ability of ferrihydrite for Mn(II) oxidation and the subsequent oxidation and immobilization of coexisting Cr(III).

Al(III)-substituted ferrihydrite existing in natural soils is more common than pure ferrihydrite; however, the effects of Al(III) incorporation on the interaction between ferrihydrite, Mn(II) catalytic oxidation, and coexisting transition metal (e.g., Cr(III)) oxidation remain elusive. To address this knowledge gap, Mn(II) oxidation on synthetic Al(III)-incorporated ferrihydrite and Cr(III) oxidation on the previously formed Fe-Mn binaries were investigated in this study via batch kinetic studies combined with various spectroscopic analyses. The results indicate that Al substitution in ferrihydrite barely changes its morphology, specific surface area, or the types of surface functional groups, but increases the total amount of hydroxyl on the ferrihydrite surface and enhances its adsorption capacity toward Mn(II). Conversely, Al substitution inhibits electron transfer in ferrihydrite, thereby weakening its electrochemical catalysis on Mn(II) oxidation. Thus, the contents of Mn(III/IV) oxides with higher Mn valence states decrease, whereas those of lower Mn valence states increase. Furthermore, the number of hydroxyl radicals formed during Mn(II) oxidation on ferrihydrite decreases. These inhibitions of Al substitution on Mn(II) catalytic oxidation subsequently cause decreased Cr(III) oxidation and poor Cr(VI) immobilization. Additionally, Mn(III) in Fe-Mn binaries is confirmed to play a dominant role in Cr(III) oxidation. This research facilitates sound decision-making regarding the management of Cr-contaminated soil environments enriched with Fe and Mn.

Transformation of zinc oxide nanoparticles in the presence of aluminum oxide with pre-sorbed phosphorus ligands.

Naturally occurring oxides could react with zinc oxide (ZnO) nanoparticles (NPs) and then change its transformation and toxicity to ecological receptors. The reaction may be affected by a variety of environmental factors, yet the relevant processes and mechanisms are limitedly investigated. Natural prevalent ligands, as an important factor, can sorb on natural oxide minerals and change its surface property, finally affecting ZnO NP transformation. This study investigated the interactions of ZnO NPs with phosphorus ligands (i.e., phytate and orthophosphate) pre-sorbed γ-alumina (γ-Al2O3) via batch experiments and multi-technique analyses. A limited amount of aqueous Zn2+ is observed when the concentration of ZnO NPs is relatively low (<64.8 mg L-1) in the presence of phytate pre-sorbed γ-Al2O3. Solid Zn(II) species includes binary/ternary surface Zn(II) complexes on γ-Al2O3 with minor amounts of zinc phytate precipitates. As the concentration of ZnO NPs increases, surface Zn(II) complexes gradually transform into zinc phytate and Zn-Al layered double hydroxide (Zn-Al LDH) precipitates. The quantitative analysis indicates that, as the concentration of ZnO NPs increases from 32.4 to 388.8 mg L-1, the proportion of Zn(II) species as binary/ternary surface complexes decreases from 81.9 to 30.2%; and the proportion as zinc phytate and Zn-Al LDH increases from 17.9 to 27.6% and 0 to 43.8%, respectively. The pre-sorption of orthophosphate can also inhibit ZnO NP transformation into Zn-Al LDH precipitates on γ-Al2O3. This study suggests that natural ligands pre-existed on natural oxide minerals could greatly influence the solubility, stability, transformation, and fate of easily dissoluble metal oxides (e.g., ZnO) in the environments.

Simultaneous oxidation of Mn(II) and As(III) on cupric oxide (CuO) promotes As(III) removal at circumneutral pH.

Oxidation of Mn(II) or As(III) by molecular oxygen is slow at pH < 9, while they can be catalytically oxidized in the presence of oxide minerals and then removed from contaminated water. However, the reaction mechanisms on simultaneous oxidation of Mn(II) and As(III) on oxide mineral surface and their accompanied removal efficiency remain unclear. This study compared Mn(II) oxidation on four common metal oxides (γ-Al2O3, CuO, α-Fe2O3 and ZnO) and investigated the simultaneous oxidation and removal of Mn(II) and As(III) through batch experiments and spectroscopic analyses. Among the tested oxides, CuO and α-Fe2O3 possess greater catalytic activity toward Mn(II) oxidation. Oxidation and removal kinetics of Mn(II) and As(III) on CuO indicate that O2 is the terminal electron acceptor for Mn(II) and As(III) oxidation on CuO, and Mn(II) acts as an electron shuttle to promote As(III) oxidation and removal. The main oxidized product of Mn(II) on CuO is high-valent MnOx species. This newly formed Mn(III) or Mn(IV) phases promote As(III) oxidation on CuO at circumneutral pH 8 and is reduced to Mn(II), which may be then released into solution. This study provides new insights into metal oxide-catalyzed oxidation of pollutants Mn(II) and As(III) and suggests that CuO should be considered as an efficient material to remediate Mn(II) and As(III) contamination.

The impacts of aging pH and time of acid mine drainage solutions on Fe mineralogy and chemical fractions of heavy metals in the sediments.

Fe (oxyhydr)oxides are the main components that accumulate heavy metals (HMs) in the acid mine drainage (AMD) sediments, but how the aging pH and time of AMD solution affects the Fe mineralogy and HMs speciation remains ambiguous. Herein, we determined the impacts of aging pH and time on the Fe mineralogy and chemical fractions of HMs in the sediments from Dabaoshan mining area using mineral characterizations, chemical extraction, and AMD solution incubation. For the natural AMD sediments, jarosite and goethite are the major Fe (oxyhydr)oxides in sample S1 with solution pH 2.68, while schwertmannite is dominant in sample S2 with solution pH 6.78, co-existing minor ferrihydrite. With increasing the AMD solution pH, the total contents of HMs (expect for As) and the reducible fraction of HMs (expect for Pb) in the sediments both increase. The HMs of Mn, Zn, Ni, and Cd are mainly associated with Fe (oxyhydr)oxides, while Pb possibly exists as Pb-bearing minerals (e.g., PbSO4) in the sediments. The oxidizable fraction of all HMs is negligible in both sediments. When the AMD solution of S1 was aged at different pHs, schwertmannite is dominant initially at all pHs, with a higher crystallinity being at a lower pH. With increasing aging time, the pre-formed schwertmannite transforms to goethite and jarosite at pH ≤ 3, while it keeps stable at pH 5 and 7 due to the accumulation of more HMs. These new insights are essential to assess the mobility and availability of HMs in the AMD-affected areas.

Co-sorption of metal ions and inorganic anions/organic ligands on environmental minerals: A review.

Co-sorption of metal ions and anions/ligands at the mineral-water interface plays a critical role in regulating the mobility, transport, fate, and bioavailability of these components in natural environments. This review focuses on co-sorption of metal ions and naturally occurring anions/ligands on environmentally relevant minerals. The underlying mechanisms for their interfacial reactions are summarized and the environmental impacts are discussed. Co-sorption mechanisms of these components depend on a variety of factors, such as the identity and properties of minerals, pH, species and concentration of metal ions and anions/ligands, addition sequence of co-sorbed ions, and reaction time. The simultaneous presence of metal ions and anions/ligands alters the initial sorption behaviors with promotive or competitive effects. Promotive effects are mainly attributed to surface electrostatic interactions, ternary surface complexation, and surface precipitation, especially for the co-sorption systems of metal ions and inorganic anions on minerals. Competitive effects involve potential complexation of metal-anions/ligands in solution or their competition for surface adsorption sites. Organic ligands usually increase metal ion sorption on minerals at low pH via forming ternary surface complexes or surface precipitates, but inhibit metal ion sorption via the formation of aqueous complexes at high pH. The different mechanisms may act simultaneously during metal ion and anion/ligand co-sorption on minerals. Finally, the potential application for remediation of metal-contaminated sites is discussed based on the different co-sorption behaviors. Future challenges and topics are raised for metal-anion/ligand co-sorption research.

Facet-Dependent Photoinduced Transformation of Cadmium Sulfide (CdS) Nanoparticles.

Microbial-mediated transformation of anthropogenic Cd2+ controls its distribution, bioavailability, and potential risks. However, the processes readily form CdS nanoparticles (CdS-NPs), which exhibit dissolution behavior different from that of larger sized particles. Here, we investigated the effects of morphologies and facets of CdS-NPs on their photoinduced dissolution. Three CdS-NPs, CdS-sphere, CdS-rod, and CdS-sheet, and one nanosized biogenic CdS (Bio-CdS) were synthesized with different dominant facets of {101}, {100}, {001}, and {111} and thus distinct surface chemistry. As explored by HRTEM, EPR, and DFT calculations, photogenerated e-/h+ pairs were more likely to generate on CdS-sheet surfaces due to higher surface energies and a narrower band gap, facilitating the formation of •OH and thereby faster dissolution (kobs = 6.126-6.261 × 10-2 h-1). The wider band gaps of CdS-sphere and CdS-rod caused less formation of O2•- and •OH, leading to slower oxidative dissolutions (kobs = 0.090-0.123 and 2.174-3.038 × 10-2 h-1, respectively). Given the similar surface energy as that of CdS-sheet, the dissolution rate of Bio-CdS was close to that of CdS-rod and CdS-sheet, which was 1.6-3.5 times faster than that of larger sized CdS, posing higher environmental risks than thought. Altogether, this work revealed the facet effects on the dissolution of CdS-NPs, manifesting a deeper understanding of metal sulfides' environmental behaviors.

Kinetics of Mn(II) adsorption and catalytic oxidation on the surface of ferrihydrite.

Mn(II) adsorption-oxidation on iron (Fe) oxides (e.g., ferrihydrite) occurs in various soils and sediments, significantly affecting the toxicities and bioavailabilities of Mn and other associated elements. However, the detailed processes of Mn(II) adsorption-oxidation on ferrihydrite remain elusive. In this study, the Mn(II) (2 mM) adsorption-oxidation kinetics on different masses of ferrihydrite (0.25, 0.50, 1.00, and 1.25 g) at pH 7 were determined using batch kinetic studies combined with X-ray diffraction, transmission electron microscopy, and wet chemistry analyses. The results indicated that the low-concentration Mn(II) adsorption-oxidation on ferrihydrite occurred in two steps. First, Mn(II) was adsorbed onto ferrihydrite, where it was partially oxidized by the catalytic effect of ferrihydrite, within ~0-60 min; subsequently, the remaining Mn(II) underwent autocatalytic oxidation on the previously generated Mn (oxyhydr)oxides. The initial adsorption-oxidation behaviors of Mn(II) on the ferrihydrite surface determined the kinetics of Mn(II) removal and oxidation, and therefore the amounts and types of Mn (oxyhydr)oxides formed. Furthermore, the specific characteristics of Mn(II) adsorption-oxidation on ferrihydrite showed a strong dependence on the Fe/Mn molar ratio. When this ratio was below 16.35, the initial process was dominated by Mn(II) adsorption onto ferrihydrite, with slight oxidation generating hausmannite (~0-60 min), followed by the catalytic oxidation of Mn(II) on the formed hausmannite, generating manganite or groutite. Conversely, when the Fe/Mn molar ratio was above 32.7, the reactions primarily involved Mn(II) adsorption onto ferrihydrite with minor oxidation to form Mn(III/IV) (oxyhydr)oxides (~0-60 min), followed by the autocatalytic oxidation of Mn(II) on the freshly-generated Mn(III/IV) (oxyhydr)oxides, forming Mn(III) (oxyhydr)oxides, i.e., feitknechtite. These results provide further insight into the interaction between Fe and Mn, Mn(II) removal, and Mn (oxyhydr)oxide formation in the environment.

Molecular-Scale Understanding of Sulfate Exchange from Schwertmannite by Chromate Versus Arsenate.

Schwertmannite effectively sorbs chromate (Cr(VI)), yet the sorption mechanisms remain elusive. We determined the Cr(VI) sorption mechanisms on schwertmannite at pH 3.2 and 5 using combined macroscopic sorption experiments with molecular-scale characterization and by comparing them to arsenate (As(V)) sorption. Cr(VI) adsorbs as bidentate-binuclear (BB) inner-sphere complexes through exchanging more sulfate and less >Fe-OH/OH2, with 0.59-0.71 sulfate released per Cr(VI) sorbed. While As(V) also forms BB complexes, it exchanges sulfate and >Fe-OH/OH2 equally with 0.49-0.52 sulfate released per As(V) sorbed. At high As(V) loadings, As(V) precipitates as amorphous FeAsO4, particularly at low pH. The abovementioned differences between Cr(VI) and As(V) can be related to their different ionic radii and binding strength. Moreover, Cr(VI) and As(V) preferentially exchange sulfate inner-sphere complexes, increasing the proportion of sulfate outer-sphere complexes in schwertmannite. In turn, the concentration of sulfate outer-sphere complexes increases and then decreases with increasing Cr(VI) loading. Results suggest that an oxyanion, which would form inner-sphere complexes on a mineral surface, preferentially exchanges inner-spherically bound oxyanions than outer-spherically bound ones on the surface, even though both are exchanged. This study improves our understanding of the sorption of oxyanions on schwertmannite and their capabilities to template schwertmannite formation and stabilize its structure.

Oxidation of Mn(III) Species by Pb(IV) Oxide as a Surrogate Oxidant in Aquatic Systems.

Dissolved Mn(III) species have been recognized as a significant form of Mn in redox transition environments, but a holistic understanding of their geochemical properties still lacks the characterization of their reactivity as reductants. Through using PbO2 as a surrogate oxidant and pyrophosphate (PP) as a model ligand, we evaluated the thermodynamic and kinetic constrains of dissolved Mn(III) oxidation under environmentally relevant pH. Without disproportionation, Mn(III) complexes could be directly oxidized by PbO2 to produce Mn oxides. The reaction rates decreased with increasing PP:Mn(III) ratio and became negligible when the ratio was above a threshold value. Particulate manganite could also be oxidized by PbO2 with detectable production of Pb(II). The favorability of Mn(III) oxidation by PbO2 as a function of the PP:Mn ratio could be predicted by the stability constant of the Mn(III)-PP complex. We developed kinetic models that couple multiple pathways of Mn oxidation by PbO2 to simulate the dynamics of Pb release, loss of dissolved Mn, as well as Mn(III) production and consumption. Beyond the context of Mn geochemistry, the interactions between Pb and various Mn species, including its trivalent forms, may also have important implications to the water quality in lead service lines within distribution systems.

Highly enhanced oxidation of arsenite at the surface of birnessite in the presence of pyrophosphate and the underlying reaction mechanisms.

Manganese(IV) oxides, and more especially birnessite, rank among the most efficient metal oxides for As(III) oxidation and subsequent sorption, and thus for arsenic immobilization. Efficiency is limited however by the precipitation of low valence Mn (hydr)oxides at the birnessite surface that leads to its passivation. The present work investigates experimentally the influence of chelating agents on this oxidative process. Specifically, the influence of sodium pyrophosphate (PP), an efficient Mn(III) chelating agent, on As(III) oxidation by birnessite was investigated using batch experiments and different arsenic concentrations at circum-neutral pH. In the absence of PP, Mn(II/III) species are continuously generated during As(III) oxidation and adsorbed to the mineral surface. Field emission-scanning electron microscopy, synchrotron-based X-ray diffraction and Fourier transform infrared spectroscopy indicate that manganite is formed, passivating birnessite surface and thus hampering the oxidative process. In the presence of PP, generated Mn(II/III) species form soluble complexes, thus inhibiting surface passivation and promoting As(III) conversion to As(V) with PP. Enhancement of As(III) oxidation by Mn oxides strongly depends on the affinity of the chelating agent for Mn(III) and from the induced stability of Mn(III) complexes. Compared to PP, the positive influence of oxalate, for example, on the oxidative process is more limited. The present study thus provides new insights into the possible optimization of arsenic removal from water using Mn oxides, and on the possible environmental control of arsenic contamination by these ubiquitous nontoxic mineral species.

Quantitative investigation of ZnO nanoparticle dissolution in the presence of δ-MnO2.

The widespread use of zinc oxide nanoparticles (ZnO NPs), the second most produced nanomaterial, inevitably leads to their release into the environment. In this study, dissolution and transformation of ZnO NPs in the presence of δ-MnO2, an abundant and ubiquitous manganese (Mn) oxide mineral, was investigated via a suite of techniques covering bulk to molecular scales. Dissolution kinetics indicated that the presence of δ-MnO2 significantly affected ZnO NP dissolution rate/trend and equilibrium Zn2+ concentration, which were found to be mainly dependent on the concentration and mass ratio of ZnO NPs and δ-MnO2. Approximately 300 mg ZnO NPs per g δ-MnO2 was expected for ZnO NP uptake at pH 7.0 via ZnO NP dissolution and surface Zn2+ adsorption. X-ray diffraction (XRD), ζ potential, high-resolution transmission electron microscopy (HR-TEM), and Zn K-edge X-ray absorption spectroscopy (XAS) results revealed that when the mole content of ZnO NPs was less than the total adsorption sites of δ-MnO2 surface, ZnO NPs were completely dissolved and adsorbed on δ-MnO2 surface in the form of inner-sphere complexes. A fraction of ZnO NPs persisted when the mole ratio of ZnO to δ-MnO2 further increased. These results suggest that the transformation and fate of ZnO NPs is affected by environment-relevant minerals such as Mn oxides due to their huge capacity of fixing dissolved metal cations at the surface or interlayer structure.

Modeling coupled kinetics of arsenic adsorption/desorption and oxidation in ferrihydrite-Mn(II)/manganese (oxyhydr)oxides systems.

The speciation and mobility of As are controlled by both Fe and Mn (oxyhydr)oxides through a series of surface complexation and redox reactions occurring in the environment, which is also complicated by the solution chemistry conditions. However, there is still a lack of quantitative tools for predicting the coupled kinetic processes of As reactions with Fe and Mn (oxyhydr)oxides. In this study, we developed a quantitative model for the coupled kinetics of As adsorption/desorption and oxidation in ferrihydrite-Mn (oxyhydr)oxides and ferrihydrite-Mn(II)-O2 systems. This model also accounted for the variations in solution chemistry conditions and binding site heterogeneity. Our model suggested that Mn (oxyhydr)oxide and ferrihydrite mainly served as an oxidant and an adsorbent, respectively, when they coexisted. Among the three types of binding sites of ferrihydrite, the adsorbed As(V) was mainly distributed on the nonprotonated bidentate sites. Our model quantitatively showed that the oxidation rates of different reaction systems varied significantly. The rates of As(III) oxidation were enhanced with higher pH values and higher molar ratios of Mn(II)/As(III) in the ferrihydrite-Mn(II)-O2 system. This study provides a modeling framework for predicting the kinetic behavior of As when multiple adsorption/desorption and oxidation reactions are coupled in the environment.