Microbial transformation of arsenic and organoarsenic compounds in anaerobic environments

Arsenic (As) is a common occurring environmental pollutant. This metalloid is well known for its carcinogenic and teratogenic effects. The U.S. Environmental Protection Agency (USEPA) enacted a new and stricter regulation for arsenic in drinking water. Small drinking water suppliers from regions with high arsenic backgrounds levels, such as the Southwest of the United States 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 since the material is classified as non hazardous according to the toxicity characteristic leaching procedure (TCLP), a test used to determine whether solid wastes are considered hazardous. The acidic and anaerobic conditions in the assay are contradictory with the alkaline and anaerobic conditions prevailing in mature landfills. This combined with the high content of organic matter and the microbial activity typical of mature landfill could play an important role in the biotransformation and mobilization of sorbed arsenic in landfills.; The new standard highlights a national concern on the health effects of arsenic. The extensive use of organoarsenical compounds such as pesticides in agriculture has become an emerging source of arsenic contamination in the environment. Roxarsone (4-hydroxy-3-nitrophenylarsonic acid) is an organic As-containing compound supplemented in the poultry feed to enhance growth by controlling coccidian parasites. Chickens mostly excrete the roxarsone without changes in its chemical structure. Land application of chicken manure in agricultural fields is a common practice. Based on broiler production and roxarsone feed dosage, approximately 900 metric tons of roxarsone is estimated to be released into environment in the U.S. annually, equivalent to 250 metric tons of arsenic. 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. This dissertation work is divided in three sections: (A) The impact of anaerobic microbial activity on the biotransformation and mobilization of sorbed As V from drinking water residuals (Chapters 2 and 3) was evaluated. AsV adsorbed onto GFH was placed in continuous flow through columns and subjected to microbial conditions prevailing in landfills. (B) The biotic and abiotic mechanisms of roxarsone transformation were studied utilizing anaerobic batch experiments (Chapter 4). (C) The methanogenic toxicity (Chapters 5 and 6) of inorganic and organic arsenicals were evaluated utilizing short- and long-term batch bioassays.; The results indicated that arsenic sorbed onto GFH is susceptible to biological mobilization under simulated landfill conditions. Arsenic mobilization was significantly enhanced by microbial activity and by the presence of organic substrates. Inoculated columns supplemented with organic substrates showed higher As mobilization that was many-fold more than non inoculated columns. High rates of arsenic release from a fully packed GFH column was delayed by more than 150 days. Arsenic release from the fully packed column was predominantly in the form of AsV sorbed onto colloids; however 15% of the arsenic was released in the soluble form, mostly made up of AsIII. In a second column study with lower GFH concentrations mixed into a sand bed, the delay time for arsenic release was significantly shorter and fine colloids were filtered by the sand bed. Consequently, the predominant form of arsenic release was soluble AsIII in the samples. The results taken as a w...