Sudipta Chakraborty

U(VI) sorption and reduction by Fe(II) sorbed on montmorillonite.

The influence of surface-bound Fe(II) on uranium oxidation state and speciation was studied as a function of time (6 min-72 h) and pH (6.1-8.5) in a U(VI)-Fe(II)-montmorillonite (Ca-montmorillonite, MONT) system under CO(2)-free, anoxic (O(2) <1 ppmv) conditions. The results show a rapid removal of U(VI) from the aqueous solution within 1 h under all pH conditions. U L(III)-edge X-ray absorption near-edge structure spectroscopy shows that 96% of the total sorbed U(VI) is reduced at pH 8.5. However, the extent of reduction significantly decreases at lower pH values as specifically sorbed Fe(II) concentration decreases. The reduction kinetics followed by X-ray photoelectron spectroscopy during 24 h at pH 7.5 demonstrates the presence of partially reduced surface species containing U(VI) and U(IV). Thermodynamically predicted mixed valence solids like U(3)O(8)/beta-U(3)O(7)/U(4)O(9) do not precipitate as verified by transmission electron microscopy and extended X-ray absorption fine-structure spectroscopy. This is also supported by the bicarbonate extraction results. The measured redox potentials of Fe(II)/Fe(III)-MONT suspensions are controlled by the Fe(II)/hydrous ferric oxide [HFO(s)] couple at pH 6.2 and by the Fe(II)/lepidocrocite [gamma-FeOOH(s)] couple at pH 7.5. The key finding of our study is the formation of a sorbed molecular form of U(IV) in abiotic reduction of U(VI) by sorbed Fe(II) at the surface of montmorillonite.

Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh.

BackgroundThe origin of the spatial variability of dissolved As concentrations in shallow aquifers of the Bengal Basin remains poorly understood. To address this, we compare here transects of simultaneously-collected groundwater and aquifer solids perpendicular to the banks of the Hooghly River in Chakdaha, India, and the Old Brahmaputra River in Araihazar, Bangladesh.ResultsVariations in surface geomorphology mapped by electromagnetic conductivity indicate that permeable sandy soils are associated with underlying aquifers that are moderately reducing to a depth of 10–30 m, as indicated by acid-leachable Fe(II)/Fe ratios <0.6 in the solid phase and concentrations of dissolved sulfate >5 mg L-1. More reducing aquifers are typically capped with finer-grained soils. The patterns suggest that vertical recharge through permeable soils is associated with a flux of oxidants on the banks of the Hooghly River and, further inland, in both Chakdaha and Araihazar. Moderately reducing conditions maintained by local recharge are generally associated with low As concentrations in Araihazar, but not systematically so in Chakdaha. Unlike Araihazar, there is also little correspondence in Chakdaha between dissolved As concentrations in groundwater and the P-extractable As content of aquifer particles, averaging 191 ± 122 ug As/L, 1.1 ± 1.5 mg As kg-1 (n = 43) and 108 ± 31 ug As/L, 3.1 ± 6.5 mg As kg-1 (n = 60), respectively. We tentatively attribute these differences to a combination of younger floodplain sediments, and therefore possibly more than one mechanism of As release, as well as less reducing conditions in Chakdaha compared to Araihazar.ConclusionSystematic dating of groundwater and sediment, combined with detailed mapping of the composition of aquifer solids and groundwater, will be needed to identify the various mechanisms underlying the complex distribution of As in aquifers of the Bengal Basin.

Reactivities of Fe(II) on calcite: selenium reduction.

The reductive immobilization of Se(IV) by micrometer-sized (100-200 microm) calcite containing sorbed or coprecipitated Fe(II) was investigated at pH 7 under anoxic conditions (O(2) < 1 ppmv) using X-ray absorption near-edge structure (XANES) spectroscopy. The Se(IV) sorption on calcite increased in the presence of sorbed Fe(II) compared to that of Fe-free pure calcite. XANES spectra of Se K-edge shows that nearly half of the total sorbed Se(IV) is reduced to Se(0) by Fe(II) sorbed on calcite within 24 h. The extent of reduction decreases with increasing equilibration time of calcite with Fe(II) solution before Se(IV) addition. The combined results of field emission scanning electron microscopy and X-ray diffraction have shown that needle-shaped red monoclinic elemental Se with diameters of 30-50 nm and lengths of up to 100 nm is precipitated on the calcite surface. Fe(II) coprecipitated calcite does not contribute to Se(IV) reduction within 72 h. Therefore, the reduction capacity of Fe(II) linked to calcite critically depends on its location (either on the surface or in the bulk solid), and less extensively on the pre-equilibration time of calcite with Fe(II) solution. Such understanding is important to predict the transport, transformation, and attenuation of Se in subsurface and in nuclear waste repositories.

Arsenic mobilization in the aquifers of three physiographic settings of West Bengal, India: understanding geogenic and anthropogenic influences.

A comparative hydrogeochemical study was carried out in West Bengal, India covering three physiographic regions, Debagram and Chakdaha located in the Bhagirathi-Hooghly alluvial plain and Baruipur in the delta front, to demonstrate the control of geogenic and anthropogenic influences on groundwater arsenic (As) mobilization. Groundwater samples (n = 90) from tube wells were analyzed for different physico-chemical parameters. The low redox potential (Eh = -185 to -86 mV) and dominant As(III) and Fe(II) concentrations are indicative of anoxic nature of the aquifer. The shallow (<100 m) and deeper (>100 m) aquifers of Bhagirathi-Hooghly alluvial plains as well as shallow aquifers of delta front are characterized by Ca(2+)HCO3(-) type water, whereas Na(+) and Cl(-) enrichment is found in the deeper aquifer of delta front. The equilibrium of groundwater with respect to carbonate minerals and their precipitation/dissolution seems to be controlling the overall groundwater chemistry. The low SO4(2-) and high DOC, PO4(3-) and HCO3(-) concentrations in groundwater signify ongoing microbial mediated redox processes favoring As mobilization in the aquifer. The As release is influenced by both geogenic (i.e. geomorphology) and anthropogenic (i.e. unsewered sanitation) processes. Multiple geochemical processes, e.g., Fe-oxyhydroxides reduction and carbonate dissolution, are responsible for high As occurrence in groundwaters.

An Insight into the Spatio-vertical Heterogeneity of Dissolved Arsenic in Part of the Bengal Delta Plain Aquifer in West Bengal (India)

Naturally occurring, carcinogenic, arsenic (As) is omnipresent in hydrological systems, and is considered as the most serious abiotic contaminant of groundwater in several parts of the world (Smedley and Kinniburgh, Appl Geochem 17:517–56, 2002; Chatterjee et al., Environ Geol 49:188–206, 2005; Charlet et al., Appl Geochem 22:1273–1292, 2007; Mukherjee et al., J Contam Hydrol 99:1–7, 2008a; Neumann et al., Nat Geosci 3:46–52, 2010 and references therein). Holocene aquifers of south-east Asia (mostly shallow, <50 m) often contain high As groundwater. The groundwater is predominantly used for irrigation and domestic purposes, e.g., cooking, drinking and bathing (Bhattacharya et al., J Water Resour Dev 13:79–92, 1997; Bhattacharyya et al., Mol Cell Biochem 253:347–355, 2003a; Charlet et al., Appl Geochem 22:1273–1292, 2007). In south-east Asia, As-rich groundwaters are often found in alluvial plains of regional rivers (Fendorf et al., Science 328:1123–1127, 2010). Prolonged consumption of groundwater with elevated levels of As may cause a formidable threat to human health and millions of people are now at risk (Bhattacharyya et al. Mol Cell Biochem 253:347–355, 2003a; Chatterjee et al., Water Res 44:5803–5812, 2010; Nath et al., Water Air Soil Pollut 190:95–113, 2008a). Arsenic contamination in groundwater and related health issues is considered as the greatest mass poisoning in human history (Smith et al., Bull World Health Organ 78:1093–1103, 2000).