Arsenic (As) and antimony (Sb) are in the same group, and both are listed as priority pollutant. Microorganisms play an important role in the biogeochemical cycle of As/Sb, by regulating As/Sb species or the transformation of adsorbent. Sulfate-reduing bacteria (SRB) is usually assumed to facilitate Sb immobilization via Sb2S3 precipitation. On the contrary, our recent study suggested that SRB mobilize Sb by the formation of thioantimony. To study the bio-transformation of As/Sb mediated by bacteria, the aims of the present work are: 1) to explore the effect of Pantoea sp. IMH with ars gene on As mobilization in spent nanoscale zero-valent iron (nZVI) waste residue; 2) to study the fate of adsorbed As mediated by coexisting arsenate-reducing and arsenite-oxidizing bacteria; 3) to investigate Sb intake of residents near an active mining area and Sb speciation in human biomarkers; 4) to study the effect of SRB on adsorbed Sb; 5) to compare the priority in the formation of thioarsenic and thioantimony, and the effect of microbial sulfate reduction on the mobility of As and Sb.
Firstly, our incubation results showed that IMH preferentially reduces soluble As(V), not solid-bound As(V), and was innocent in elevating total dissolved As concentrations. μ-XRF and As μ-XANES spectra clearly revealed the heterogeneity and complexity of the inoculated and control samples. Nevertheless, the surface As local coordination was not affected by the presence of IMH as evidenced by similar As-Fe atomic distance (3.32-3.36 Å) and coordination number (1.9) in control and inoculated samples. The Fe XANES results suggested that magnetite in nZVI residue was partly transformed to ferrihydrite, and the IMH activity slowed down the nZVI aging process. IMH distorted Fe local coordination without changing its As adsorption capacity as suggested by Mössbauer spectroscopy. Arsenic retention is not inevitably enhanced by in situ formed secondary Fe minerals, but depends on the relative As affinity between the primary and secondary iron minerals.
Secondly, Bacteria with arsenate-reducing (ars) and arsenite-oxidizing (aio) genes usually co-exist in aerobic environments, but their contrast impacts on arsenic (As)speciation and mobility remain unclear. To identify which bacteria, dominate As speciation under oxic conditions, we studied the biotransformation of adsorbed As on goethite in the co-existence of Pantoea sp. IMH with ars gene and Achromobacter sp. SY8 with aio gene. The incubation results show that SY8 dominated the dissolved As speciation as As(V), even though aio exhibited nearly 5 folds lower transcription levels than ars in IMH. Nevertheless, our XANES results suggest that SY8 showed a negligible effect on solid-bound As speciation whereas IMH reduced adsorbed As(V) to As(III). Our Mössbauer spectroscopic results suggest that the incubation with SY8 reduced the degree of crystallinity of goethite, and the reduced crystallinity can be partly compensated by IMH.
Thirdly, to identify human biomarkers for Sb exposure, we analyzed 480 environmental samples from an active Sb mining area in Hunan, China. Elevated Sb concentrations exceeding the acceptable level were detected in drinking water (70% of n = 83 total samples), foods (80%, n = 188), urine (95%, n = 63), saliva (44%, n = 48), hair (80%, n = 51) and nails (83%, n = 47). Drinking water contributed 85%-100% of the average daily dose (ADD) of Sb, and the total ADD (11.7 μg/kg bodyweight/day) was up to thirty times higher than the oral reference dose (0.4 μg/kg bodyweight/day) as recommended by USEPA. A distinct positive correlation was found between ADD and Sb content in hair (p = 0.02), but not in urine (p = 0.051), saliva (p = 0.52) or nails (p = 0.85), suggesting that hair is the best non-invasive biomarker. μ-XRF analysis indicated that Sb is distributed in discrete spots in hair and nails, and Sb distribution is correlated with other metals. Methylated Sb species were predominant in urine (46%-100%) and saliva (74%-100%) in collected samples, implying that the human metabolic system adopts methylation as an effective pathway to detoxify and excrete Sb.
Fourthly, the biogenic sulfide is usually assumed to facilitate Sb immobilization via Sb2S3 precipitation. Here, to the contrary, we discovered that SRB mobilize adsorbed Sb(V). When SbV(OH)6--bearing goethite was incubated anaerobically with Desulfovibrio vulgaris DP4, elevated antimony was released due to the formation of thioantimonate which is the dominant Sb species in solution. Our fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis observed multiple six- or five-coordinate thioantimonate intermediates, suggesting stepwise ligand exchange of hydroxyl groups on SbV(OH)6- by biogenic sulfide. Direct H2S elimination reactions resulted in four-coordinate thioantimonate species as the stable end product, which was confirmed by our density functional theory (DFT) calculations. The thiolation of antimonate is pH-dependent and occurs in neutral environments. The thiolation changed Sb(V) from six-coordinate octahedral to four-coordinate tetrahedral coordination, weakening its affinity for iron oxides and thus facilitating its release into the aquatic environment.
Finally, biogenic sulfide from microbial sulfate reduction under anaerobic conditions may enhance the release of coexisting As and Sb through the formation of labile thioarsenate (AsV-S) and thioantimonate (SbV-S). To understand the competition between As and Sb for thiolation and its effect on their mobility, we investigated the speciation and fate of As and Sb in the presence of indigenous microbiota in sediments during multiple oxic-anoxic cycles. Our incubation results indicated that As was released under anoxic conditions whereas Sb was mobilized during oxic cycles. The As release was not driven by the Fe(III) reductive dissolution, but by biogenic sulfide through the formation of soluble AsV-S. AsV-S formation outcompeted SbV-S because the limited available sulfide preferentially reacted with As rather than Sb. The Sb mobility was regulated by its redox transformation between antimonate (SbV-O) and antimonite (SbIII-O) where microbial SbV-O reduction resulted in Sb immobilization.