The chemical speciation of arsenic in sediments and porewaters of aquifers is the critical factor that determines whether dissolved arsenic accumulates to potentially toxic levels. Sequestration of arsenic in solid phases, which may occur by adsorption or precipitation processes, controls dissolved concentrations. We present synchrotron x-ray absorption spectra of arsenic in shallow aquifer sediments that indicate the local structure of realgar (AsS) as the primary arsenic-bearing phase in sulfate-reducing conditions at concentrations of 1–3 mmol·kg ^–1 , which has not previously been verified in sediments at low temperature. Spectroscopic evidence shows that arsenic does not substitute for iron or sulfur in iron sulfide minerals at the molecular scale. A general geochemical model derived from our field and spectroscopic observations show that the ratio of reactive iron to sulfur in the system controls the distribution of solid phases capable of removing arsenic from solution when conditions change from oxidized to reduced, the rate of which is influenced by microbial processes. Because of the difference in solubility of iron versus arsenic sulfides, precipitation of iron sulfide may remove sulfide from solution but not arsenic if precipitation rates are fast. The lack of incorporation of arsenic into iron sulfides may result in the accumulation of dissolved As(III) if adsorption is weak or inhibited. Aquifers particularly at risk for such geochemical conditions are those in which oxidized and reduced waters mix, and where the amount of sulfate available for microbial reduction is limited.

Synchrotron X-ray absorption spectroscopy (XAS) is becoming an increasingly popular tool for the analysis of element speciation in complex natural mixtures such as soils and sediments. Identification of a particular mineral or amorphous solid in a heterogeneous mixture by XAS depends on the spectral uniqueness of the element in the bonding environment associated with a component, and on absorption effects from the components and the matrix. A suite of 27 common, Fe-bearing reference compounds, including sulfides, carbonates, phosphates, oxides, oxyhydroxides, and phyllosilicates, was analyzed to empirically assess the utility of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for identifying a particular Fe mineral (or class of minerals) in a soil or sediment mixture. We show that XANES spectral features are useful for distinguishing qualitatively among major mineral classes, but not necessarily for identifying minerals within classes. A practical detection limit (based on empirical mixtures) for most mineral classes is on the order of 5% of the total atomic Fe absorption, but detection limits vary depending on the spectral uniqueness of the components, the number of components, and the matrix. Calibration curves for Fe sulfide and non-sulfide (phyllosilicate ± oxide) component mixtures were made independently from the analyses of XANES and EXAFS fluorescence spectra of binary and ternary mineral mixtures (at 5% total Fe) in a quartz matrix to mimic natural sediments. Because of differences in sample and matrix absorption and fluorescence among sulfide and phyllosilicate minerals, apparent component fractions of pyrite derived from fits deviated significantly from linear binary mixtures. We show that corrections for non-linear fluorescence (as much as 20%) are particularly important for binary mineral mixtures with different densities and absorption characteristics (such as sulfides and phyllosilicates), and for mixtures with similar component abundances (i.e., far from one end-member). An application of the fluorescence calibration for XANES and EXAFS is shown for natural samples. This study points out the importance of a high-quality, experimentally consistent reference compound library, and the need for calibration of fluorescence spectra, in order to quantify accurately the component fractions of complex mixtures using XAS.

Strontium sorption to amorphous silica and goethite was measured as a function of pH and dissolved strontium and carbonate concentrations at 25°C. Strontium sorption gradually increases from 0 to 100% from pH 6 to 10 for both phases and requires multiple outer-sphere surface complexes to fit the data. All data are modeled using the triple layer model and the site-occupancy standard state; unless stated otherwise all strontium complexes are mononuclear. Strontium sorption to amorphous silica in the presence and absence of dissolved carbonate can be fit with tetradentate Sr2+ and SrOH+ complexes on the β-plane and a monodentate Sr2+complex on the diffuse plane to account for strontium sorption at low ionic strength. Strontium sorption to goethite in the absence of dissolved carbonate can be fit with monodentate and tetradentate SrOH+ complexes and a tetradentate binuclear Sr2+ species on the β-plane. The binuclear complex is needed to account for enhanced sorption at hgh strontium surface loadings. In the presence of dissolved carbonate additional monodentate Sr2+ and SrOH+ carbonate surface complexes on the β-plane are needed to fit strontium sorption to goethite. Modeling strontium sorption as outer-sphere complexes is consistent with quantitative analysis of extended X-ray absorption fine structure (EXAFS) on selected sorption samples that show a single first shell of oxygen atoms around strontium indicating hydrated surface complexes at the amorphous silica and goethite surfaces.

Mercury is a toxin that causes neurological impairments in adults, is particularly harmful for fetuses and children, and is deadly in severe cases, making it a worldwide health concern. Methylmercury (MeHg) is the environmentally relevant form of mercury (Hg) because it biomagnifies along the food chain. Methylmercury is mainly produced in aquatic sediments via methylation of inorganic Hg (Hg(II)) and transformed back via demethylation. Because transformation rates determine MeHg concentrations, quantification of methylation and demethylation rates is needed to inform management of MeHg. Published rate constants for Hg(II) methylation ( $k_m$) and MeHg demethylation ( $k_d$) vary greatly, stemming partly from differences in experimental methods. We conducted a comprehensive review of rate laws, evaluated published rate constants, and performed biogeochemical simulations to assess variability in reported $k_m$ and $k_d.$ Based on selected studies employing the same pseudo-first-order rate law and similar experimental methods, we found that $k_m=0.04\pm 0.03\mathrm{ }{\mathrm{d}}^{-1}$ is a reasonable range for wetland sediments. Over a number of environments, maximum $k_d$ was smaller at sites without Hg source ( $k_d=0.5\mathrm{ }{\mathrm{d}}^{-1}$) than at sites with identified Hg source ( $k_d=1.8\mathrm{ }{\mathrm{d}}^{-1}$). Larger variability and higher uncertainty in $k_d$ compared to $k_m$ highlight the need for more research on MeHg demethylation rates. This critical review: (a) aids the design of future experimental studies of $k_m$ and $k_d;$ (b) provides guidance for comparing rate constants from different studies; (c) presents a biogeochemical reaction model to assess rate constants; and (d) informs selection of $k_m$ and $k_d$ values from the literature for use in model simulations. Graphical abstract