Microbial Transformation of Arsenic and Organoarsenic Compounds in Anaerobic Environments

Abstract

Arsenic (As) is a common occurring environmental pollutant. The USEPA has a stricter regulation for arsenic in drinking water (10 ug/L). Small drinking water suppliers from regions with high arsenic backgrounds levels, will need to remove arsenic from drinking water in order to meet the new standard. The proposed treatment by the USEPA is the oxidation of arsenite (AsIII) to arsenate (AsV) followed by the adsorption onto metal oxides (e.g. granular ferrihydrite (GFH)). Large amounts of arsenic-bearing solid waste will be generated and disposed in municipal landfills based on EPA's recommendation. The alkaline and anaerobic conditions prevailing in mature landfills combined with the high content of organic matter and the microbial activity prevailing in landfill could play an important role in the biotransformation and mobilization of sorbed arsenic in landfills. The extensive use of organo arsenical compounds such as pesticides in agriculture has become an emerging source of arsenic contamination in the environment. Roxarsone (4-hydroxy-3-nitrophenylarsonic acid) is a compound supplemented in the poultry feed to enhance growth by controlling coccidian parasites. Chickens excrete the roxarsone without changes in its chemical structure. Land application of chicken manure in agricultural fields is a common practice. Approximately 900 metric tons of roxarsone is estimated to be released into environment in the U.S. annually. The environmental impact is significant when considering that these quantities of arsenic are spread onto relative small land areas in the direct vicinity of poultry houses. The aim of this study is to evaluate the role of anaerobic bioconversion on the fate and mobility of inorganic and organic arsenic compounds. The results taken as a whole suggest that arsenic in drinking water residuals is susceptible to enhanced mobilization due to anaerobic microbial activity, and therefore As mobilization should be expected in mature landfills. Roxarsone was readily biotransformed by a methanogenic consortium to its corresponding amino compound, 4-hydroxy-3-aminophenylarsonic acid (HAPA). A fraction of the HAPA went on to be converted to AsIII after long term incubations. Therefore, land application of roxarsone-containing wastes could potentially lead to the formation of more toxic and mobile forms of arsenic which will have negative impact on the environment.