Microorganisms play a significant role in the speciation and mobility of arsenic in the environment. In this study, the oxidation of arsenite [As(III)] to arsenate [As(V)] linked to chlorate (ClO 3 ؊ ) reduction was shown to be catalyzed by sludge samples, enrichment cultures (ECs), and pure cultures incubated under anaerobic conditions. No activity was observed in treatments lacking inoculum or with heat-killed sludge, or in controls lacking ClO ؊ is an alternative electron acceptor to support the microbial oxidation of As(III).The contamination of drinking water with arsenic (As) is a global public health issue. Arsenic is a human carcinogenic compound (2), which poses a risk to millions of people around the world (31). The most common oxidation states of As in aqueous environments are arsenite [As(III), H 3 AsO 3 ] or arsenate [As(V), H 2 AsO 4 Ϫ , and HAsO 4 2Ϫ ]. Microbial processes play critical roles in controlling the fate and transformation of As in subsurface systems (22). As(V) binds to aluminum oxides more extensively than As(III) under circumneutral pH conditions (12, 16). Both As(III) and As(V) are strongly adsorbed on iron oxides (9). However, As(III) is more rapidly desorbed compared to As(V) (35).Aerobic bacteria can oxidize As(III) forming As(V) (14, 28), which potentially is less mobile in the subsurface environment. Also, in environments with dissolved ferrous iron [Fe(II)] the oxidation of Fe(II) (both abiotic and biotic) would result in formation of Fe(III) (hydr)oxides such as ferrihydrite which adsorb As. Oxidation processes, therefore, can decrease the mobilization of As in groundwater. However, oxygen (O 2 ) is poorly soluble in groundwater and may become consumed by microbial activity, creating anaerobic zones. Alternative oxidants aside from O 2 also have the potential to support the microbial oxidation of As(III). Recently, several studies have demonstrated that nitrate-dependent As(III) oxidation is carried out by anaerobic microorganisms to gain energy from As(III) oxidation. As(III)-oxidizing denitrifying bacteria have been isolated from various environments including As-contaminated lakes and soil (21, 25), as well as enrichment cultures (ECs), and isolates from pristine sediments and sludge samples (33, 34). 16S rRNA gene clone library characterization of the ECs indicates that the predominant phylotypes were from the genus Azoarcus and the family Comamonadaceae (34).Beside nitrate, chlorate (ClO 3 Ϫ ) can also be considered as a possible alternative oxidant for microorganisms to promote the bioremediation of contaminated plumes (6, 17). (Per)chlorate is commonly used as a terminal electron acceptor by anaerobic bacteria; as a result, it is completely degraded to the benign end product, chloride (Cl Ϫ ). Microbial reduction of perchlorate proceeds via a three-step process of ClO 4 Ϫ 3 ClO 3 Ϫ 3 ClO 2 Ϫ 3 O 2 ϩ Cl Ϫ (6). Reduction of perchlorate to chlorate, and chlorate to chlorite is catalyzed by respiratory (per)chlorate reductases (3). Subsequent disproportionation of chlorit...
The objective of this study was to explore a bioremediation strategy based on injecting NO3− to support the anoxic oxidation of ferrous iron (Fe(II)) and arsenite (As(III)) in the subsurface as a means to immobilize As in the form of arsenate (As(V)) adsorbed onto biogenic ferric (Fe(III)) (hydr)oxides. Continuous flow sand filled columns were used to simulate a natural anaerobic groundwater and sediment system with co-occurring As(III) and Fe(II) in the presence (SF1) or absence (SF2) of nitrate, respectively. During operation for 250 days, the average influent arsenic concentration of 567 µg l−1 was reduced to 10.6 (±9.6) µg l−1 in the effluent of column SF1. The cumulative removal of Fe(II) and As(III) in SF1 was 6.5–10-fold higher than that in SF2. Extraction and measurement of the mass of iron and arsenic immobilized on the sand packing of the columns was close to the iron and arsenic removed from the aqueous phase during column operation. The dominant speciation of the immobilized iron and arsenic was Fe(III) and As(V) in SF1, compared with Fe(II) and As(III) in SF2. The speciation was confirmed by XRD and XPS. The results indicate that microbial oxidation of As(III) and Fe(II) linked to denitrification resulted in the enhanced immobilization of aqueous arsenic in anaerobic environments by forming Fe(III) (hydr)oxides coated sands with adsorbed As(V).
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