Natural Attenuation of Arsenic by Sediment Sorption and Oxidation

Abstract

Arsenic sorption onto aquifer sediments was investigated in anaerobic laboratory batch and column uptake experiments and characterized by As, Fe, and Mn X-ray absorption spectroscopy (XAS) to estimate the extent and mechanism of abiotic sorption and oxidation of As(III). Batch experiments at pH 6 showed that the amount of As(III) or As(V) sorption from synthetic background porewater to sediments was similar as a function of total As concentration, but slightly more As(V) was sorbed than As(III) with increasing As concentrations. Column experiments with As(III) solutions in the absence and presence of dissolved Fe2+ showed more As uptake in the presence of Fe but also more Fe desorption during flushout with As-free solutions such that net As uptake was similar to, or less than that of, the Fe-free experiment. Fits to bulk Fe X-ray absorption near-edge spectroscopy (XANES) spectra showed no change between unreacted and reacted sediments. Manganese XANES revealed small increases in absorption in the spectral region associated with Mn(II) after reaction, indicating sediment Mn reduction. However, XANES spectra showed that Mn is not present as MnIVO2(s) but is probably substituted into other sediment minerals as a mixture of Mn(II,III). Quantitative analyses of As XANES spectra, which indicated mixtures of As(III) and As(V) after reaction with As(III) solutions, were used to estimate a fraction of As(V) in excess of native As(V) in the sediment (0.2 mmol kg−1) that corresponds to sorbed As(III) oxidized to As(V). The spectroscopic and solution data indicate that the aquifer sediments have a limited abiotic capacity to oxidize As(III), which did not exceed 30% of the total amount of As sorbed and was estimated in the range of 0.025−0.4 mmol kg−1 sediment. In the presence of dissolved Fe2+, the precipitation of Fe(III) hydrous oxide phases will be an effective mechanism for As scavenging only if there exists sufficient dissolved oxygen in groundwater to oxidize Fe. Once the aqueous oxidative capacity is exhausted, dissolved Fe2+ may compete with As(III) for the limited abiotic oxidation supplied by sediment Mn-bearing phases.

Oxidation of As(III) by MnO2 in the absence and presence of Fe(II) under acidic conditions

“…This increased the exposure of the activated sites on MnO 2 for As(III), and thus increasing its oxidation rate. Power et al (2005) confirmed competitive sorption of Zn 2+ on birnessite surfaces decreased the oxidation rate of As(III), and Choi et al (2009) also assumed competition between Fe(II) and As(III) for sorption sites on manganese oxides further blocked oxidation rate of As(III).…”

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

“…Geochemical simulation using the reaction progress approach to provide thermodynamic constraints (solubility and sorption) during changes from oxidized to reduced conditions, with model parameters generalized from field observations, lend insight into dynamic processes and factors that may control dissolved concentrations. Arsenic mobilization may occur in the zone affected by seasonal fluctuations of groundwater level potentially during two conditions: when ground water level rises, flooding reduced sediments and promoting oxidative dissolution of sulfide; and when water level falls and oxidized sediments become progressively reduced as microbial action consumes O 2 and other electron acceptors, reducing arsenic, iron, and manganese (the latter being particularly important in lowFe systems, e.g., Amirbahman et al, 2006;Choi et al, 2009). During an oxidative cycle, oxidation of iron and arsenic, leading to (co-)precipitation of Fe(III)-(hydr)oxides and arsenate and/or pH-controlled arsenate sorption, will be an effective attenuation mechanism as long as oxidative capacity is maintained and the ratio of arsenic to labile iron (and potentially manganese) is low.…”

The effects of iron(II) on the kinetics of arsenic oxidation and sorption on manganese oxides

“…Power et al [14] confirmed competitive sorption of Zn(II) on birnessite decreased the oxidation rate of As(III). Choi et al [36] assumed competition between Fe(II) and As(III) for sorption sites on manganese oxides further blocked the oxidation rate of As(III). Parikh et al [15] showed a reduced As(III) oxidation in the presence of phosphate (PO 4 3À ) caused by competitive adsorption between As(III) and PO 4 3À .…”