Microorganisms Influence Biochemical Transformation Of Roxarsone In Soil

Roxarsone have been extensively used in the feed of animals, which is usually excreted unchanged in the manure and eventually enter in soil and water, resulting in severe arsenic pollution. We investigate the influences of environmental factors on the biotransformation of roxarsone in soil, and the consequences of roxarsone biotransformation on microbial physiology and ecological functioning. The results show that biotic processes dominate roxarsone transformation, taking the widely distributed iron reducing bacteria, Shewanella oneidensis MR-1 as the model microbe, we study the influences of Fe (?) mineral adsorption and phosphate on the transformation of roxarsone by soil microbes. The main conclusions were obtained as follows:(1) Firstly, the biotransformation of roxarsone in a silt loam soil and its toxicity was investigated. Results showed that biotic processes dominate roxarsone degradation, higher soil moisture and temperature promoted roxarsone degradation, associating with emergent pentavalent arsenic. Analysis of fluorescein diacetate (FDA) hydrolysis activity revealed that roxarsone does not exert acute toxic on soil microbes. With the release of inorganic arsenic, FDA hydrolysis activity was inhibited gradually, as evidenced by ecotoxicological assessment.(2) Secondly, Shewanella oneidensis MR-1 facilitates direct and Fe(?)-mediated roxarsone transformation was studied. Results revealed that MR-1 strain was able toefficiently transform roxarsone into 3-amino-4-hydroxyphenylarsonic acid (HAPA) in aqueous systems at the presence of exogenous carbon source, while gain energy (for cell growth) through the transform pathway. Presence of dissolved Fe(?) significantly enhanced roxarsone reduction through Shewanella oneidensis MR-1 induced Fe(?) transformation process, where the formed Fe(?) was able to abiotically transfer electrons to roxarsone and thereby, acted as an efficient chemical reducing reagent for direct roxarsone reduction. Consequently, such biochemical processes yielded violent secondary iron mineralization, creating conditions for enhanced adsorption of organic arsenics to iron minerals and thereby sufficient arsenic immobilization.(3) Lastly, the effects of Fe(?) mineral adsorption and phosphate on microbial reduction of roxarsone by Shewanella oneidensis MR-1 was investigated. Results confirmed that synthesized ferrihydrite adsorbed more roxarsone than synthesized goethite. Release and transformation of roxarsone was observed during the microbial reduction of roxarsone-bearing Fe(?)(oxyhydr)oxides. Both the extent and the rates of Fe (?) reduction were promoted after adsorption of roxarsone. The X-ray diffraction (XRD), scanning electron microscope (SEM) and X-rays photoelectron spectroscopy (XPS) results showed that the presence of phosphate could desorb roxarsone, alter the formation of secondary Fe mineral products and mobility of roxarsone. The XPS results suggested HAPA was immobilized on secondary Fe mineral products. However, more characterizations need to conducted to fully understand the fate of arsenic.