Role of Al substitution in the reduction of ferrihydrite by Shewanella oneidensis MR-1.

Substitution of aluminum under natural environmental conditions has been proven to inhibit the transformation of weakly crystalline iron (oxyhydr)-oxides towards well crystalline iron oxides, thereby enhancing their long-term stability. However, exploration on the role of aluminum substitution in bacteria-mediated iron oxides transformation is relatively lacking, especially in the anaerobic underground condition where iron (oxyhydr)-oxides are easy to reduced. In this study, we selected four different levels of substitution aluminum prevalent in iron oxides under natural conditions, which are 0 mol%, 10 mol%, 20 mol%, and 30 mol% (mol Al/mol (Al + Fe)) respectively. With the presence of Shewanella oneidensis MR-1, we conducted a 15-day anaerobic microcosm experiment in simulated groundwater conditions. The experiment data suggested that aluminum substitution result in a decrease in bio-reduction rate constants of ferrihydrite from 0.24 in 0 mol% Al to 0.17 in 30 mol% Al. Besides, when containing substituted aluminum, secondary minerals produced by biological reduction of ferrihydrite changed from magnetite to akaganeite. These results were attributed to the surface coverage of Al during the reduction process, which affects the contact between S. oneidensis MR-1 and the unexposed Fe(III), thus inhibiting the further reduction of ferrihydrite. Since iron (oxyhydr)-oxides exhibit a strong affinity on multiple kinds of pollutants, results in this study may contribute to predicting the migration and preservation of contaminants in groundwater systems.

Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater.

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca2+ on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca2+ were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca2+ mainly exists on the surface through the outer-sphere complex. Adsorbed Ca2+ entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca2+ inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca2+ adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.

Interactions between arsenic adsorption/desorption and indigenous bacterial activity in shallow high arsenic aquifer sediments from the Jianghan Plain, Central China.

Exposure to arsenic in the environment is hazardous to biota. Three types of sediments from a shallow, high arsenic aquifer were collected from Jianghan Plain, Central China to investigate interactions between indigenous bacterial activity and arsenic adsorption/desorption. For the same level of bacterial activity, the As(III) or As(V) adsorption rate in sediments decreased from clay loam to loamy sand to silty sand. However, the arsenic desorption rate from these sediments followed the reverse sequence. For the same arsenic speciation and content, bacterial activity decreased from clay loam to loamy sand to silty sand. Overall, arsenic adsorption/desorption rates were related to both bacterial activity and sediment texture. The bacteria present could reduce As(V) to As(III), thereby favoring its release from sediment into solution and increasing the ratio of As(III) to total arsenic. These results indicate indigenous bacteria strongly affect the adsorption/desorption and oxidation-reduction of arsenic, and are actively involved in the dynamic equilibrium of arsenic between sediment and groundwater in this shallow aquifer.