Continuous fixation of dissolved arsenite from flooded soil by cooperating ferrihydrite with Geobacter sulfurreducens.

The fixed arsenic in soil is easy to be released into the aquatic environment in the form of arsenite (As(III)) with high toxicity and mobility due to the eutrophication of environment under anaerobic conditions. However, As(III) is difficult to be fixed in situ continuously by traditional methods, especially for the most efficient fixation method by iron ores. Based on that Fe(II) could promote the fixation of As(III), this study investigated the possibility that Geobacter sulfurreducens (G. sulfurreducens) cooperates with ferrihydrite to fix released As(III) from flooded soil in a glass column continuously under anaerobic conditions. During 42 days of operation of reactors that simulated the actual flooded soil environment, the concentration of released As(III) in the reactor with adding G. sulfurreducens and ferrihydrite is always lower than that in reactors with adding ferrihydrite or no treatment. Compared with reactors without treatment, the accumulated content of released As(III) (2455.0 ± 313.1 µg) decreased by 39.4% in the reactor with adding G. sulfurreducens and ferrihydrite on the last day, while that in reactors with adding ferrihydrite only decreased by 11.6%, respectively. These were caused by the cooperation of G. sulfurreducens and ferrihydrite, which increased the relative abundance of iron-reducing microorganisms to inhibit metabolisms of As-reducing microorganisms, inhibited the quick release of As(III) from solid soil, and promoted the release of iron to accelerate the formation of stable secondary ores with As. This study could provide an environmentally friendly method to fix dissolved As(III) pollutants from soil continuously.

Immobilization and redistribution process of As(V) during As(V)-bearing ferrihydrite reduction by Geobacter sulfurreducens under the influence of TiO2 nanoparticles.

The redistribution process of arsenate (As(V)) and the variation in As(V) content in different locations must be clarified to ensure low mobility of As(V) during microbial ferrihydrite reduction. In this study, we investigated As(V) immobilization and redistribution processes when ferrihydrite was incubated with Geobacter sulfurreducens in the presence of titanium dioxide (TiO2) nanoparticles. Our study results showed that, As(V) in the aqueous phase and ferrihydrite were redistributed on light minerals (goethite), heavy minerals (ferrihydrite and magnetite), and extracellular polymeric substances (EPS) induced by G. sulfurreducens during ferrihydrite reduction. Interestingly, we found that As(V) in the form of arsenate ion (AsO43-) was adsorbed by the functional groups of the EPS, while the formed FeII3(AsVO4)2 was wrapped in the network structure of EPS. Moreover, the addition of TiO2 nanoparticles did not promote but delayed the entire ferrihydrite reduction, As(V) immobilization and redistribution processes. Furthermore, changes in the aqueous arsenic and iron concentrations are closely related to the formation time of secondary minerals. Our study findings provide new insights into the As(V) immobilization process mediated by G. sulfurreducens under anaerobic conditions.

Effects of anode/cathode electroactive microorganisms on arsenic removal with organic/inorganic carbon supplied.

This study reports the effects of an external voltage (0 V, 0.4 V and 0.9 V) on soil arsenic (As) release and sequestration when amended with organic carbon (NaAc) and inorganic carbon (NaHCO3), respectively, in a soil bioelectrochemistry system (BES). The results demonstrated that although an external voltage had no effect on the As removal capacity in an oligotrophic environment fueled with NaHCO3, 93.6% of As(III) in the supernatant was removed at 0.9 V with an NaAc amendment. Interestingly, the content of As detected on the electrodes was higher than that removed from the supernatant, implying a continuous release of soil As under external voltages and rapid adsorption onto the electrodes, especially the cathode. In addition, the species of As on the cathode were similar to those in the supernatant (the As(III)/As(V) ratio was approximately 3:1), indicating that the removal capacity was independent of preoxidation. From the viewpoint of electroactive microorganisms (EABs), the relative abundances of the arrA gene and Geobacter genus were specifically enriched at the anode, thus signifying stimulation of the reduction and release of soil As in the anode region. By comparison, Bacillus was particularly abundant at the cathode, which could contribute to the oxidation and sequestration of As in the cathode region. Additionally, specific extracellular polymeric substances (EPSs) secreted by EABs could combine with As, which was followed by electrostatic attraction to the cathode under the effect of an electric field. Furthermore, the formation of secondary minerals and coprecipitation in the presence of iron (Fe) may have also contributed to As removal from solution. The insights from this study will enable us to further understand the biogeochemical cycle of soil As and to explore the feasibility of in situ As bioremediation techniques, combining the aspects of microbial and physicochemical processes in soil bioelectrochemical systems.