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

Reduction of Fe(III) from iron-rich sludge by Geobacter to reconstruct MIL-100(Fe)@Fe3O4 to accelerate electron transfer and organic pollutants mineralization in Fenton-like system

Iron-rich sludge contains abundant iron resources, yet lacks high-value processing methods. Iron-based metal-organic frameworks (Fe-MOFs), such as MIL-100(Fe), are widely used but suffer from considerable attrition and hardly recycle. Integrating iron extraction from sludge with Fe-MOF modification is promising for solid waste management. The redox capabilities of Geobacter, along with its production of Extracellular polymeric substance (EPS), hold potential for enabling this process. Nevertheless, the underlying mechanisms remain unclear, and the performance of the resulting product needs evaluation. In this study, Geobacter successfully loaded reduced Fe3O4 from iron-rich sludge onto MIL-100(Fe), creating MIL-100(Fe)@Fe3O4 (MF), a mesoporous nanomaterial with magnetic recovery. MF demonstrated excellent catalytic performance and repeatability, with removal efficiencies 1.4-2.5 times than those of MIL-100(Fe) in terms of pollutants. After seven cycles, the catalytic performance of MF remained stable because of the exist of EPS produced by Geobacter. Density Functional Theory (DFT) calculations confirmed that Fe3O4 loading enhanced charge transfer, improving catalytic efficiency. This study offers important insights on Geobacter for the pivotal role between iron-rich sludge and Fe-MOFs, which achieve the sustainable recovery and efficient utilization of solid waste.

Properties and Reactivity of Iron-Organic Matter-Arsenic Composites and their Influence on Arsenic Behavior in Microbial Reduction and Oxidation Processes

The biogeochemistry of arsenic in soils is strongly controlled by iron oxides and soil organic matter (SOM). The present study intends to elucidate the behavior of arsenic in Fe-SOM-As composites formed through adsorption or coprecipitation under redox conditions. The X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) showed that crystalline minerals were generated during Fe-HA-As coprecipitation, while other composites exhibited an amorphous structure. In an anoxic environment, iron-reducing bacteria reduced Fe(III) and As(V) to Fe(II) and As(III), respectively, enhancing the mobility of arsenic. The presence of SOM increased the concentrations of dissolved Fe(II) and As(III) through complexation. Notably, elevated As(III) and reduced Fe(II) were observed in the HA-containing coprecipitation group due to the weak adsorption capacity of crystalline minerals, which released As(V) into solution and competed with Fe(III) for electrons. Under oxic conditions, superoxide, hydrogen peroxide, and hydroxyl radical (•OH) were formed through the oxidation of Fe(II) and reduced SOM. As(III) was subsequently oxidized by superoxide and •OH, and the process was dominated by •OH. Substantial •OH in the HA-containing coprecipitation group mainly oxidized dissolved As(III), while limited •OH in other groups contributed greater to adsorbed As(III). These findings contribute substantially to understanding the mechanisms of the coupled transformation of iron and arsenic in soil under fluctuating redox conditions.

Impact of iron and sulfur cycling on the bioavailability of cadmium and arsenic in co-contaminated paddy soil

The biogeochemical cycling of iron (Fe) or sulfur (S) in paddy soil influences the cadmium (Cd) and arsenic (As) migration. However, the influence of coupled reduction effects and reaction precedence of Fe and S on the bioavailability of Cd and As is still not fully understood. This study aimed to reveal the influence of Fe and S reduction on soil Cd and As mobility under various pe + pH conditions and to elucidate the related mechanism in subtropical China. According to the findings, higher adsorption from Fe reduction caused high-crystalline goethite (pe + pH > 2.80) to become amorphous ferrihydrite, which in turn caused water-soluble Cd (62.0%) to first decrease. Cd was further decreased by 72.7% as a result of the transformation of SO42- to HS-/S2- via sulfate reduction and the formation of CdS and FeS. As release (an increase of 8.1 times) was consequently caused by the initial reduction and dissolution of iron oxide (pe + pH > 2.80). FeS had a lesser impact on the immobilization of As than sulfate-mediated As (V) reduction in the latter stages of the reduction process (pe + pH < 2.80). pe + pH values between 3 and 3.5 should be maintained to minimize the bioavailability of As and Cd in moderate to mildly polluted soil without adding iron oxides and sulfate amendments. The practical remediation of severely co-contaminated paddy soil can be effectively achieved by using Fe and S additions at different pe + pH conditions. This technique shows promise in reducing the bioavailability of Cd and As.

Role of microbial iron reduction in arsenic metabolism from soil particle size fractions in simulated human gastrointestinal tract

Gut microbiota provides protection against arsenic (As) induced toxicity, and As metabolism is considered an important part of risk assessment associated with soil As exposures. However, little is known about microbial iron(III) reduction and its role in metabolism of soil-bound As in the human gut. Here, we determined the dissolution and transformation of As and Fe from incidental ingestion of contaminated soils as a function of particle size (<250 μm, 100-250 μm, 50-100 μm and < 50 μm). Colon incubation with human gut microbiota yielded a high degree of As reduction and methylation of up to 53.4 and 0.074 μg/(log CFU/mL)/hr, respectively; methylation percentage increased with increasing soil organic matter and decreasing soil pore size. We also found significant microbial Fe(III) reduction and high levels of Fe(II) (48 %-100 % of total soluble Fe) may promote the capacity of As methylation. Although no statistical change in Fe phases was observed with low Fe dissolution and high molar Fe/As ratios, higher As bioaccessibility of colon phase (avg. 29.4 %) was mainly contributed from reductive dissolution of As(V)-bearing Fe(III) (oxy)hydroxides. Our results suggest that As mobility and biotransformation by human gut microbiota (carrying arrA and arsC genes) are strongly controlled by microbial Fe(III) reduction coupled with soil particle size. This will expand our knowledge on oral bioavailability of soil As and health risks from exposure to contaminated soils.

Effect of goethite-loaded montmorillonite on immobilization of metal(loid)s and the micro-ecological soil response in non-ferrous metal smelting areas

In this work, the immobilization stabilization and mechanism of heavy metal(loid)s by goethite loaded montmorillonite (GMt) were investigated, and the soil microbial response was explored. The simulated acid rain leaching experiment showed that GMt had a higher acid tolerance and the more stable heavy metal(loid)s fixation ability. The soil incubation demonstrated that GMt significantly decreased the available Cd, Zn, Pb and As concentration. Interestingly, higher immobilization of heavy metals was observed by GMt in highly acid leached and acidic soils. The richness and diversity of bacterial communities improved after the addition of GMt. GMt induced the enrichment of the excellent functional bacteria of the phylum Proteobacteria as well as the genus Massilia and Sphingomonas. The main immobilization mechanisms of heavy metal(loid)s by GMt include electrostatic interaction, complexation, precipitation and oxidation. The addition of the GMt also optimizes the soil bacterial community structure, which further facilitates the immobilization of heavy metal(loid)s. Our results confirm that the novel GMt has a promising application in the immobilization and stabilization of heavy metal(loid)s contaminated soils in non-ferrous metal smelting areas.

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.