Contrasting effects of dissimilatory Fe(III)/As(V) reduction on arsenic mobilization of Al coprecipitated ferrihydrite in simulated groundwater

Arsenic pollution caused by dissimilatory As(V)/Fe(III)-reducing microorganisms in oxygen-starved sediments has aroused widespread concern. During the alternate redox cycles, precipitation and dissolution of iron (oxy-hydr)-oxides often introduce various impurities, among which aluminum (Al) is the most prevalent one. Therefore, it is of great significance to devote some of our attention to the arsenic contamination induced by the bio-reduction of aluminum-iron (oxyhydr)-oxides. In this study, we synthesized aluminum-ferrihydrite (AlFh) with Al/(Al + Fe) mol% of 0, 10, 20 and 30 successfully. After discussing the adsorption mechanism of arsenate by these materials, the biotic experiment with the presence of Shewanella oneidensis MR-1 (MR-1) or As(V) reducing bacteria Citrobacter sp. JH012-1 (JH012-1) were conducted. Our results suggested that the copreci-pitated of Al inhibited the bio-reduction of Fe(III). In MR-1 incubation systems, due to the impaired Fe(III) reduction, As(V) release in the initial stage of Al coprecipitate Fh was inhibited. The released As(V) was then immobilized into the secondary mineral phases. In JH012-1 incubation systems the incorporated aluminum result in a higher degree of As dissolution and As(III) accounts for a relatively high proportion in the liquid phase. Spectroscopic analysis demonstrated that with the increase of Al content in AlFh, the relative proportions of surface As(III) were 63.53%, 58.60%, 54.91% and 50.35% respectively. The enhanced release of As in JH012-1 experiment was mainly ascribed to the poor adsorption capacity of As(III) on AlFh. These findings will help to complement the bio-migration behavior of As in natural aluminum-iron (oxyhydr)-oxides.