Behavior and Fate of Chromium and Carbon during Fe(II)-Induced Transformation of Ferrihydrite Organominerals.

The mobility of chromium (Cr) is controlled by minerals, especially iron (oxyhydr)oxides. The influence of organic carbon (OC) on the mobility and fate of Cr(VI) during Fe(II)-induced transformation of iron (oxyhydr)oxide, however, is still unclear. We investigate how low-weight carboxyl-rich OC influences the transformation of ferrihydrite (Fh) and controls the mobility of Cr(VI/III) in reducing environments and how Cr influences the formation of secondary Fe minerals and the stabilization of OC. With respect to the transformation of Fe minerals, the presence of low-weight carboxyl-rich OC retards the growth of goethite crystals and stabilizes lepidocrocite for a longer time. With respect to the mobility of Cr, low-weight carboxyl-rich OC suppresses the Cr(III)non-extractable associated with Fe minerals, and this suppression is enhanced with increasing carboxyl-richness of OC and decreasing pH. The presence of Cr(III) mitigates the decrease in total C associated with Fe minerals and increases the Cnon-extractable especially for Fh organominerals made with carboxyl-rich OC. Our study sheds new light on the mobility and fate of Cr in reducing environments and suggests that there is a potential synergy between Cr(VI) remediation and OC stabilization.

Long-term organic carbon preservation enhanced by iron and manganese.

The balance between degradation and preservation of sedimentary organic carbon (OC) is important for global carbon and oxygen cycles1. The relative importance of different mechanisms and environmental conditions contributing to marine sedimentary OC preservation, however, remains unclear2-8. Simple organic molecules can be geopolymerized into recalcitrant forms by means of the Maillard reaction5, although reaction kinetics at marine sedimentary temperatures are thought to be slow9,10. More recent work in terrestrial systems suggests that the reaction can be catalysed by manganese minerals11-13, but the potential for the promotion of geopolymerized OC formation at marine sedimentary temperatures is uncertain. Here we present incubation experiments and find that iron and manganese ions and minerals abiotically catalyse the Maillard reaction by up to two orders of magnitude at temperatures relevant to continental margins where most preservation occurs4. Furthermore, the chemical signature of the reaction products closely resembles dissolved and total OC found in continental margin sediments globally. With the aid of a pore-water model14, we estimate that iron- and manganese-catalysed transformation of simple organic molecules into complex macromolecules might generate on the order of approximately 4.1 Tg C yr-1 for preservation in marine sediments. In the context of perhaps only about 63 Tg C yr-1 variation in sedimentary organic preservation over the past 300 million years6, we propose that variable iron and manganese inputs to the ocean could exert a substantial but hitherto unexplored impact on global OC preservation over geological time.

Cooperative microbial interactions drive spatial segregation in porous environments.

The role of microbial interactions and the underlying mechanisms that shape complex biofilm communities are poorly understood. Here we employ a microfluidic chip to represent porous subsurface environments and show that cooperative microbial interactions between free-living and biofilm-forming bacteria trigger active spatial segregation to promote their respective dominance in segregated microhabitats. During initial colonization, free-living and biofilm-forming microbes are segregated from the mixed planktonic inoculum to occupy the ambient fluid and grain surface. Contrary to spatial exclusion through competition, the active spatial segregation is induced by cooperative interactions which improves the fitness of both biofilm and planktonic populations. We further show that free-living Arthrobacter induces the surface colonization by scavenging the biofilm inhibitor, D-amino acids and receives benefits from the public goods secreted by the biofilm-forming strains. Collectively, our results reveal how cooperative microbial interactions may contribute to microbial coexistence in segregated microhabitats and drive subsurface biofilm community succession.

Zinc Stable Isotope Fractionation Mechanisms during Adsorption on and Substitution in Iron (Hydr)oxides.

The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (Δ66/64Znsolid-solution = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with Δ66/64Znsubstituted-stock solution of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.

Potential use of engineered nanoparticles in ocean fertilization for large-scale atmospheric carbon dioxide removal.

Artificial ocean fertilization (AOF) aims to safely stimulate phytoplankton growth in the ocean and enhance carbon sequestration. AOF carbon sequestration efficiency appears lower than natural ocean fertilization processes due mainly to the low bioavailability of added nutrients, along with low export rates of AOF-produced biomass to the deep ocean. Here we explore the potential application of engineered nanoparticles (ENPs) to overcome these issues. Data from 123 studies show that some ENPs may enhance phytoplankton growth at concentrations below those likely to be toxic in marine ecosystems. ENPs may also increase bloom lifetime, boost phytoplankton aggregation and carbon export, and address secondary limiting factors in AOF. Life-cycle assessment and cost analyses suggest that net CO2 capture is possible for iron, SiO2 and Al2O3 ENPs with costs of 2-5 times that of conventional AOF, whereas boosting AOF efficiency by ENPs should substantially enhance net CO2 capture and reduce these costs. Therefore, ENP-based AOF can be an important component of the mitigation strategy to limit global warming.

Mineralogical control on methylotrophic methanogenesis and implications for cryptic methane cycling in marine surface sediment.

Minerals are widely proposed to protect organic carbon from degradation and thus promote the persistence of organic carbon in soils and sediments, yet a direct link between mineral adsorption and retardation of microbial remineralisation is often presumed and a mechanistic understanding of the protective preservation hypothesis is lacking. We find that methylamines, the major substrates for cryptic methane production in marine surface sediment, are strongly adsorbed by marine sediment clays, and that this adsorption significantly reduces their concentrations in the dissolved pool (up to 40.2 ± 0.2%). Moreover, the presence of clay minerals slows methane production and reduces final methane produced (up to 24.9 ± 0.3%) by a typical methylotrophic methanogen-Methanococcoides methylutens TMA-10. Near edge X-ray absorption fine structure spectroscopy shows that reversible adsorption and occlusive protection of methylamines in clay interlayers are responsible for the slow-down and reduction in methane production. Here we show that mineral-OC interactions strongly control methylotrophic methanogenesis and potentially cryptic methane cycling in marine surface sediments.

Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides.

Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ114/110Cdsolid-solution) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd2+ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO6] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ114/110Cdsolid-solution of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth's critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applying Cd isotopes as environmental tracers.

Combining local knowledge and soil science for integrated soil health assessments in conservation agriculture systems.

The challenges of soil degradation and climate change have led to the emergence of Conservation Agriculture (CA) as a sustainable alternative to tillage-based agriculture systems. Despite the recognition of positive impacts on soil health, CA adoption in Africa has remained low. Previous soil health studies have mainly focused on 'scientific' measurements, without consideration of local knowledge, which influences how farmers interpret CA impacts and future land management decisions. This study, based in Malawi, aims to 1) combine local knowledge and conventional soil science approaches to develop a contextualised understanding of the impact of CA on soil health; and 2) understand how an integrated approach can contribute to explaining farmer decision-making on land management. Key farmers' indicators of soil health were crop performance, soil consistence, moisture content, erosion, colour, and structure. These local indicators were consistent with conventional soil health indicators. By combining farmers' observations with soil measurements, we observed that CA improved soil structure, moisture (Mwansambo 7.54%-38.15% lower for CP; Lemu 1.57%-47.39% lower for CP) and infiltration (Lemu CAM/CAML 0.15 cms-1, CP 0.09 cms-1; Mwansambo CP/CAM 0.14 cms-1, CAML 0.18 cms-1). In the conventional practice, farmers perceived ridges to redistribute nutrients, which corresponded with recorded higher exchangeable ammonium (Lemu CP 76.0 mgkg -1, CAM 49.4 mgkg -1, CAML 51.7 mgkg -1), nitrate/nitrite values (Mwansambo CP 200.7 mgkg -1, CAM 171.9 mgkg -1, CAML 103.3 mgkg -1). This perception contributes to the popularity of ridges, despite the higher yield measurements under CA (Mwansambo CP 3225 kgha-1, CAML 5067 kgha-1, CAM 5160 kgha-1; Lemu CP 2886 kgha-1, CAM 2872 kgha-1, CAML 3454 kgha-1 ). The perceived carbon benefits of residues and ridge preference has promoted burying residues in ridges. Integrated approaches contribute to more nuanced and localized perceptions about land management. We propose that the stepwise integrated soil assessment framework developed in this study can be applied more widely in understanding the role of soil health in farmer-decision making, providing a learning process for downscaling technologies and widening the evidence base on sustainable land management practices.

Metagenomic and 14 C tracing evidence for autotrophic microbial CO2 fixation in paddy soils.

Autotrophic carbon dioxide (CO2 ) fixation by microbes is ubiquitous in the environment and potentially contributes to the soil organic carbon (SOC) pool. However, the multiple autotrophic pathways of microbial carbon assimilation and fixation in paddy soils remain poorly characterized. In this study, we combine metagenomic analysis with 14 C-labelling to investigate all known autotrophic pathways and CO2 assimilation mechanisms in five typical paddy soils from southern China. Marker genes of six autotrophic pathways are detected in all soil samples, which are dominated by the cbbL genes (67%-82%) coding the ribulose-bisphosphate carboxylase large chain in the Calvin cycle. These marker genes are associated with a broad range of phototrophic and chemotrophic genera. Significant amounts of 14 C-CO2 are assimilated into SOC (74.3-175.8 mg 14 C kg-1 ) and microbial biomass (5.2-24.1 mg 14 C kg-1 ) after 45 days incubation, where more than 70% of 14 C-SOC was concentrated in the relatively stable humin fractions. These results show that paddy soil microbes contain the genetic potential for autotrophic carbon fixation spreading over broad taxonomic ranges, and can incorporate atmospheric carbon into organic components, which ultimately contribute to the stable SOC pool.

Characteristics and mechanisms of Pb(II) sorption onto Fe-rich waste water treatment residue (WTR): A potential sustainable Pb immobilisation technology for soils.

Pb contamination of soils is a global problem. This paper discusses the ability of an Fe-rich waste, water treatment residual (WTR), to adsorb Pb(II). This was investigated using batch sorption experiments, X-ray diffraction, electron microprobe microanalysis, PHREEQC modeling and Extended X-ray Absorption Fine Structure (EXAFS) analysis. The WTR is composed of approximately 23 wt. % natural organic matter (NOM), 70 wt. % ferrihydrite and <10 wt. % silicate material. Pb(II) sorption to WTR was dependent on initial Pb(II) load, particle size, time and pH, but not on ionic strength. EXAFS analysis at the Pb LIII-edge confirmed that Pb(II) sorbed to WTR by co-existing bidentate edge-sharing and monodentate or corner-sharing complexes, with 2 O at ∼2.31-2.34 Å, 1 Fe at ∼3.32-3.34 Å, 2 Fe at ∼3.97-3.99 Å and 1 Pb at ∼3.82-3.85 Å. Linear combination showed that the Pb(II)-sorbed spectra were best fit with a ∼0.9 ± 0.1 and 0.1 ± 0.1 contribution from Pb(II)-sorbed ferrihydrite and Pb(II)-sorbed humic acid end members, respectively. Overall, we show that Pb(II) sorbs via strong inner-sphere complexation of Pb(II) to the ferrihydrite component of the WTR, which itself is stable over a wide pH range. Therefore, we suggest that Fe-rich WTR wastes could be used as effective adsorbents in Pb(II)-contaminated soils to help ensure sustainable terrestrial ecosystems.

Outer Membrane c-Type Cytochromes OmcA and MtrC Play Distinct Roles in Enhancing the Attachment of Shewanella oneidensis MR-1 Cells to Goethite.

The outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC in Shewanella are key terminal reductases that bind and transfer electrons directly to iron (hydr)oxides. Although the amounts of OmcA and MtrC at the cell surface and their molecular structures are largely comparable, MtrC is known to play a more important role in dissimilatory iron reduction. To explore the roles of these outer membrane c-Cyts in the interaction of Shewanella oneidensis MR-1 with iron oxides, the processes of attachment of S. oneidensis MR-1 wild type and c-type cytochrome-deficient mutants (the ΔomcA, ΔmtrC, and ΔomcA ΔmtrC mutants) to goethite are compared via quartz crystal microbalance with dissipation monitoring (QCM-D). Strains with OmcA exhibit a rapid initial attachment. The quantitative model for QCM-D responses reveals that MtrC enhances the contact area and contact elasticity of cells with goethite by more than one and two times, respectively. In situ attenuated total reflectance Fourier transform infrared two-dimensional correlation spectroscopic (ATR-FTIR 2D-CoS) analysis shows that MtrC promotes the initial interfacial reaction via an inner-sphere coordination. Atomic force microscopy (AFM) analysis demonstrates that OmcA enhances the attractive force between cells and goethite by about 60%. As a result, OmcA contributes to a higher attractive force with goethite and induces a rapid short-term attachment, while MtrC is more important in the longer-term interaction through an enhanced contact area, which promotes interfacial reactions. These results reveal that c-Cyts OmcA and MtrC adopt different mechanisms for enhancing the attachment of S. oneidensis MR-1 cells to goethite. It improves our understanding of the function of outer membrane c-Cyts and the influence of cell surface macromolecules in cell-mineral interactions.IMPORTANCEShewanella species are one group of versatile and widespread dissimilatory iron-reducing bacteria, which are capable of respiring insoluble iron minerals via six multiheme c-type cytochromes. Outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC are the terminal reductases in this pathway and have comparable protein structures. In this study, we elucidate the different roles of OmcA and MtrC in the interaction of S. oneidensis MR-1 with goethite at the whole-cell level. OmcA confers enhanced affinity toward goethite and results in rapid attachment. Meanwhile, MtrC significantly increases the contact area of bacterial cells with goethite and promotes the interfacial reaction, which may explain its central role in extracellular electron transfer. This study provides novel insights into the role of bacterial surface macromolecules in the interfacial interaction of bacteria with minerals, which is critical to the development of a comprehensive understanding of cell-mineral interactions.

Adsorption of Cr(VI) on Al-substituted hematites and its reduction and retention in the presence of Fe2+ under conditions similar to subsurface soil environments.

Aluminum substitution is common in iron (hydr)oxides in subsurface environments, and can significantly modify mineral interactions with contaminants. However, few studies investigate Cr(VI) adsorption and its subsequent mobility on Al-substituted iron (hydr)oxide surfaces. Here shows that Al substitution gradually modifies hematite crystals from {101}, {112}, {110} and {104} faceted rhombohedra to {001} faceted plates, resulting in a general decrease in Cr(VI) adsorption density and favoring of monodentate mononuclear over bidentate binuclear Cr(VI) adsorption complexes. Consequently, the mobility of Cr(VI) might be increased in environments with an abundance of Al-containing iron (hydr)oxides. However, pre-adsorption of Fe2+ on hematite promotes Cr(VI) adsorption, reduction and fixation, and Al-substituted hematite removes more Cr(VI) than pure hematite. Similarly, although addition of Fe2+ to Cr(VI)-adsorbed hematite remobilizes a small proportion of Cr, it greatly increases the proportion of Cr fixed. As the coexistence of Fe2+ and iron (hydr)oxides is common in subsurface environments, Al-containing iron (hydr)oxides will promote Cr(VI) uptake and retention, with a significant proportion fixed as Cr(III), limiting Cr mobility and toxicity. These results offer new insights into how iron (hydr)oxides might control the behaviors of other high-valence redox-sensitive contaminants, and provide a platform for modeling such processes in complex soil and sediment systems.

Arsenite and arsenate binding to ferrihydrite organo-mineral coprecipitate: Implications for arsenic mobility and fate in natural environments.

Arsenic mobility in soils, sediments and groundwater systems is strongly controlled by adsorption occurring at iron oxide/water interfaces, and the extent of this adsorption may be influenced by the presence of natural organic matter (NOM). This study aims to investigate the adsorption of As(III) and As(V) onto coprecipitates made with ferrihydrite (Fh) and humic acid (HA) with two organic carbon (OC) loadings of 5 and 15 wt% OC. We show that the coprecipitation of HA with Fh can significantly reduce the retention of both As(III) and As(V) over a wide pH range (4-11), and with increased OC loading, there is reduced arsenic adsorption. On pure Fh, As(III) is adsorbed to a greater extent than As(V) at pH > 6.5 (the crossover pH), whereas the crossover pH shifts to more acidic pH in the presence of HA, implying that the binding of As(III) is more favorable than As(V) in the presence of NOM. Both As(III) and As(V) are complexed with the ferric hydroxyl functional groups, and no ternary Fh-HA-As complexes are detected. We observe that ∼40% of the adsorbed As(III) is oxidized to As(V) on pure Fh, compared to only ∼29% of As(III) oxidation on the Fh-HA coprecipitate, indicating that NOM hinders As(III) oxidation on iron (hydr)oxide. The results of this study suggest that NOM interacts with arsenic in ways that promote arsenic mobility and especially promote the mobility of arsenate relative to arsenite, which is of great significance for evaluating the migration and bioavailability of arsenic in both natural and contaminated environments.

Competitive binding of Cd, Ni and Cu on goethite organo-mineral composites made with soil bacteria.

Soil is a heterogeneous porous media that is comprised of a variety of organo-mineral aggregates. Sorption of heavy metals onto these composite solids is a key process that controls heavy metal mobility and fate in the natural environment. Pollution from a combination of heavy metals is common in soil, therefore, understanding the competitive binding behavior of metal ions to organo-mineral composites is important in order to predict metal mobility and fate. In this study, batch experiments were paired with spectroscopic studies to probe the sorption characteristics of ternary CdNiCu sorbates to a binary organo-goethite composite made with Bacillus cereus cells. Scanning electron microscopy shows that goethite nano-sized crystals are closely associated with the bacterial surfaces. Sorption experiments show a larger adsorptivity and affinity for Cu than Cd/Ni on goethite and B. cereus, and the goethite-B. cereus composite. X-ray photoelectron spectroscopy reveals that carboxylate and phosphate functional moieties present on the bacterial cell walls are primarily responsible for metal sorption to the goethite-B. cereus composite. Synchrotron-based X-ray fluorescence shows that Cu and Ni are predominately associated with the bacterial fraction of the goethite-B. cereus composite, whereas Cd is mainly associated with the goethite fraction. The findings of this research have important implications for predicting the mobility and fate of heavy metals in soil multi-component systems.

Towards a better understanding of the aggregation mechanisms of iron (hydr)oxide nanoparticles interacting with extracellular polymeric substances: Role of pH and electrolyte solution.

Extracellular polymeric substances (EPS) are ubiquitous in the soil and water environment and interact strongly with mineral surfaces. However, these interactions and their impacts on the behavior and fate of minerals remain poorly understood. Here, for a better understanding of the colloidal stability of minerals in the environment, we investigated the aggregation of goethite (α-FeOOH) nanoparticles (NPs) in the presence of EPS from Bacillus subtilis under different environmental conditions (pH, ionic strength and ionic valence). Results showed that the aggregation processes of goethite NPs are determined by the solution chemistry, and the colloidal stability of goethite NPs is strongly influenced by the addition of EPS. In the absence of ionic strength, the addition of EPS promotes the aggregation of goethite NPs only when the pH (pH = 6) is less than the point of zero charge for the goethite nanoparticles (pHpzc ≈ 8). In the presence of ionic strength, the aggregation rate of goethite NPs increases with increasing concentration of NaCl, NaNO3 and Na2SO4 solutions, and after the addition of EPS solution, the critical coagulation concentrations (CCC) of goethite NPs are increased from 43.0, 56.7 and 0.39 mM to 168.0, 304.9 and 126.2 mM in the three electrolyte solutions, indicating that the addition of EPS inhibits the aggregation of goethite NPs. While in Na3PO4 solution, when the concentration of Na3PO4 solution ranged from 0 to 1 mM, the aggregation rate of goethite NPs increases first, followed by a decrease, and with the concentration of Na3PO4 solution exceeding 1 mM, the aggregation rate of goethite NPs increases again, due to the charge screening by sodium counter ions. This study provides a fundamental understanding of the behavior of goethite NPs in natural soil and water environments.

Binding of Cd by ferrihydrite organo-mineral composites: Implications for Cd mobility and fate in natural and contaminated environments.

Adsorption and coprecipitation of organic matter with iron (hydr)oxides can alter iron (hydr)oxide surface properties and their reactivity towards nutrient elements and heavy metals. Organo-mineral composites were synthesized using humic acid (HA) and iron oxide, during coprecipitation with ferrihydrite (Fh) and adsorption to pre-formed Fh with two C loadings. The Fh-HA coprecipitated composites have a higher C content and smaller surface area compared to the equivalent adsorbed composites. NanoSIMS shows there is a high degree of spatial correlation between Fe and C for both composites, but C distribution is more uniform in the coprecipitated composites. The C 1s NEXAFS reveals a similar C composition between the Fh-HA coprecipitated and adsorbed composites. However composites at high carbon loading are more enriched in aromatic C, likely due to preferential binding of carboxyl functional groups on aromatic rings in the HA. The amount of Cd sorbed is independent of the composite type, either coprecipitated or adsorbed, but is a function of the C loading. Composites with low C loading show Cd sorption that is almost identical to pure Fh, while composites with high C loading show Cd sorption that is intermediate between pure Fh and pure HA, with sorption significantly enhanced over pure Fh at pH < 6.5. A bidentate edge-sharing binding was identified for Cd on pure Fh and Cd-carboxyl binding on pure HA. These findings have significant implications not only for the sequestration of Cd in contaminated environments but also the coupled biogeochemical cycling of Cd, Fe and C in the critical zone.

In situ arsenic oxidation and sorption by a Fe-Mn binary oxide waste in soil.

The ability of a Fe-Mn binary oxide waste to adsorb arsenic (As) in a historically contaminated soil was investigated. Initial laboratory sorption experiments indicated that arsenite [As(III)] was oxidized to arsenate [As(V)] by the Mn oxide component, with concurrent As(V) sorption to the Fe oxide. The binary oxide waste had As(III) and As(V) adsorption capacities of 70mgg-1 and 32mgg-1 respectively. X-ray Absorption Near-Edge Structure and Extended X-ray Absorption Fine Structure at the As K-edge confirmed that all binary oxide waste surface complexes were As(V) sorbed by mononuclear bidentate corner-sharing, with 2 Fe at ∼3.27Ǻ. The ability of the waste to perform this coupled oxidation-sorption reaction in real soils was investigated with a 10% by weight addition of the waste to an industrially As contaminated soil. Electron probe microanalysis showed As accumulation onto the Fe oxide component of the binary oxide waste, which had no As innately. The bioaccessibility of As was also significantly reduced by 7.80% (p<0.01) with binary oxide waste addition. The results indicate that Fe-Mn binary oxide wastes could provide a potential in situ remediation strategy for As and Pb immobilization in contaminated soils.

EXAFS Study of Sr sorption to Illite, Goethite, Chlorite, and Mixed Sediment under Hyperalkaline Conditions.

Strontium is an important contaminant radionuclide at many former nuclear sites. This paper investigates the effect of changing pH and ionic strength on the sorption of Sr to a range of common soil minerals. Specifically it focuses on the sorption of Sr onto illite, chlorite, goethite, and a mixed sediment. The interplay between ionic strength and pH was determined by varying the background ionic strength of the system using both NaCl (for a constant pH) and NaOH (to also vary pH). Under conditions of moderate pH, Sr sorption decreased with increasing ionic strength, due to competition between the Na and Sr atoms for the outer-sphere complexes. However, where increasing ionic strength was accompanied by increasing pH, Sr sorption remained high. This suggested that Sr was sorbed to the minerals without competition from background Na ions. Extended X-ray absorption fine structure (EXAFS) spectra confirmed that at highly alkaline pH (>12.5) Sr was forming inner-sphere complexes on the surfaces of all minerals. This specific adsorption of the Sr (as SrOH(+)) explains why it was still adsorbed to the minerals under very high ionic strength conditions and was not out-competed by Na.