Arsenic volatilization in roxarsone-loaded digester: Insight into the main factors and arsM genes

The extensive use of roxarsone (ROX) in livestock and poultry husbandry causes the production of arylarsenic-contaminated manure/wastewater. Anaerobic digestion is a conventional technique for livestock manure/wastewater treatment. However, the factors affecting arsenic volatilization are poorly understood in arylarsenic-loaded anaerobic reactors. The main factors such as ROX loading, exposure time of anaerobic granular sludge (AGS) to ROX, and volatile fatty acid (VFA) levels, affecting arsenic volatilization were investigated in this study. The results indicated that ROX loading of 5.70 mg-As·L^-1 triggered the maximum volatile arsenic yield of 6.78 ng-As·g^-1-VSS·d^-1, which was 4.95 times higher compared to the ROX-free assay. The conversion of ROX into inorganic arsenic was an essential step for arsenic volatilization. The 160-day and 270-day exposure of AGS to ROX caused 6-fold and 8-fold increase in volatile arsenic yield, respectively, compared to the 0-day exposure. With the longer-time exposure to ROX, AGS provided more available arsenic for volatilization and its arsenic-volatilizing capacity was significantly enhanced. VFA level was positively associated with arsenic volatilization (r = 0.832-0.950; p < 0.05). The abundance of arsM genes in AGS increased by 34.62-129.05% after the 100-day incubation, and was strongly correlated to arsenic volatilization. Based on these results, possible pathway of arsenic volatilization in ROX-loaded digesters were proposed. The result from this study improves a better understanding of the potential of arsenic volatilization in arylarsenic-contaminated environments.