Arsenite binding to natural organic matter: spectroscopic evidence for ligand exchange and ternary complex formation

Iron and arsenic speciation and distribution in organic flocs from streambeds of an arsenic-enriched peatland

Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. To date, spectroscopic speciation analyses of Fe and trace elements in these mineral-organic matter (OM) associations are missing. In this study, we investigated the speciation and distribution of Fe and As in flocs collected from low-flow streams (pH 5.3-6.3) of the naturally As-enriched peatland Gola di Lago (Switzerland) using (57)Fe Mössbauer spectroscopy and synchrotron X-ray techniques. The flocs were rich in acid carbohydrates and contained up to 22.1 wt % Fe, 34.9 wt % C, and 2620 mg/kg As. Mössbauer analyses revealed small quantities (<5%) of Fe(II) and Fe(III)-OM complexes and the predominance of ferrihydrite (∼ Fe(5)HO(8) · 4H2O, 51-59%) and lepidocrocite (γ-FeOOH, 34-46%). The latter was not observed by synchrotron X-ray diffraction, implying a coherent scattering domain size of <10 nm. Iron X-ray absorption spectroscopy (XAS) confirmed the Mössbauer results, and bulk As XAS indicated the prevalence of arsenate (71-84%) in the flocs. Shell-fit analyses showed that As was entirely sorbed to Fe(III)-(oxyhydr)oxides and that both arsenate and arsenite exclusively formed monodentate-binuclear ("bridging") complexes (R(As-Fe) = 3.31-3.34 Å). Microfocused X-ray fluorescence spectrometry documented a strong correlation between As and Fe in the flocs. These analyses also revealed intense As hotspots coinciding with abundant freshwater green algae (Closterium spp.). Microfocused As X-ray absorption near-edge structure spectra collected at algae-specific points identified up to 29% As(III), which, in combination with ∼ 5% As(III) detected at Fe-rich points, suggests As(V) bioreduction in the algae. Our findings imply that floc (bio)organics serve primarily as nucleation sites for the precipitation of nanocrystalline Fe(III)-(oxyhydr)oxides, rendering flocs effective sorbents for trace metal(loid)s. Thus, Fe-rich freshwater flocs likely play a pivotal role for the speciation and cycling of trace elements in wetlands.

Impact of birnessite on arsenic and iron speciation during microbial reduction of arsenic-bearing ferrihydrite

Elevated solution concentrations of As in anoxic natural systems are usually accompanied by microbially mediated As(V), Mn(III/IV), and Fe(III) reduction. The microbially mediated reductive dissolution of Fe(III)-(oxyhydr)oxides mainly liberates sorbed As(V) which is subsequently reduced to As(III). Manganese oxides have been shown to rapidly oxidize As(III) and Fe(II) under oxic conditions, but their net effect on the microbially mediated reductive release of As and Fe is still poorly understood. Here, we investigated the microbial reduction of As(V)-bearing ferrihydrite (molar As/Fe: 0.05; Fe tot: 32.1 mM) by Shewanella sp. ANA-3 (10(8) cells/mL) in the presence of different concentrations of birnessite (Mn tot: 0, 0.9, 3.1 mM) at circumneutral pH over 397 h using wet-chemical analyses and X-ray absorption spectroscopy. Additional abiotic experiments were performed to explore the reactivity of birnessite toward As(III) and Fe(II) in the presence of Mn(II), Fe(II), ferrihydrite, or deactivated bacterial cells. Compared to the birnessite-free control, the highest birnessite concentration resulted in 78% less Fe and 47% less As reduction at the end of the biotic experiment. The abiotic oxidation of As(III) by birnessite (k initial = 0.68 ± 0.31/h) was inhibited by Mn(II) and ferrihydrite, and lowered by Fe(II) and bacterial cell material. In contrast, the oxidation of Fe(II) by birnessite proceeded equally fast under all conditions (k initial = 493 ± 2/h) and was significantly faster than the oxidation of As(III). We conclude that in the presence of birnessite, microbially produced Fe(II) is rapidly reoxidized and precipitates as As-sequestering ferrihydrite. Our findings imply that the ability of Mn-oxides to oxidize As(III) in water-logged soils and sediments is limited by the formation of ferrihydrite and surface passivation processes.

Fabrication of magnetic porous Fe-Mn binary oxide nanowires with superior capability for removal of As(III) from water

Magnetic porous Fe-Mn binary oxide nanowires were successfully fabricated to efficient removal of As(III) from water. The adsorption capacity of the porous nanowires for As(III) obviously increased with increasing of manganese oxide in the composite, accompanying decrease of the saturation magnetization of the adsorbents. Magnetic porous Fe-Mn binary oxide nanowires with an initial Fe:Mn molar ratio of 1:3 exhibited the highest absorption capacity for As(III) and enable magnetic separation from water. The maximal adsorption capacity value is 171mgg(-1) at pH 7.0. In the initial pH range from 3 to 9, 200μgL(-1) of As(III) could be easily decreased to below 10μgL(-1) by the magnetic porous Fe-Mn binary oxide nanowires (0.05gL(-1)) within 75min, and the corresponding residual As was completely oxidized to less toxic As(V). The coexisting chloride, nitrate and sulfate had no significant effect on arsenic removal, whereas, phosphate and humic acid reduced the removal of As(III) by competing with arsenic species for adsorption sites. The resulting magnetic porous Fe-Mn binary oxide nanowires could be a promising adsorbent for As(III) removal from water.

Arsenite binding to sulfhydryl groups in the absence and presence of ferrihydrite: a model study

Binding of arsenite (As(III)) to sulfhydryl groups (Sorg(-II)) plays a key role in As detoxification mechanisms of plants and microorganisms, As remediation techniques, and reduced environmental systems rich in natural organic matter. Here, we studied the formation of Sorg(-II)-As(III) complexes on a sulfhydryl model adsorbent (Ambersep GT74 resin) in the absence and presence of ferrihydrite as a competing mineral adsorbent under reducing conditions and tested their stability against oxidation in air. Adsorption of As(III) onto the resin was studied in the pH range 4.0-9.0. On the basis of As X-ray absorption spectroscopy (XAS) results, a surface complexation model describing the pH dependence of As(III) binding to the organic adsorbent was developed. Stability constants (log K) determined for dithio ((AmbS)2AsO(-)) and trithio ((AmbS)3As) surface complexes were 8.4 and 7.3, respectively. The ability of sulfhydryl ligands to compete with ferrihydrite for As(III) was tested in various anoxic mixtures of both adsorbents at pH 7.0. At a 1:1 ratio of their reactive binding sites, R-SH and ≡FeOH, both adsorbents possessed nearly identical affinities for As(III). The oxidation of Sorg(-II)-As(III) complexes in water vapor saturated air over 80 days, monitored by As and S XAS, revealed that the complexed As(III) is stabilized against oxidation (t1/2 = 318 days). Our results thus document that sulfhydryl ligands are highly competitive As(III) complexing agents that can stabilize As in its reduced oxidation state even under prolonged oxidizing conditions. These findings are particularly relevant for organic S-rich semiterrestrial environments subject to periodic redox potential changes such as peatlands, marshes, and estuaries.