Investigation On The Degradation And Biotransformation Of Roxarsone In The Anaerobic Sludge System

Roxarsone (3-nitro-4-hydroxy benzene arsenic acid) is an organoarsenic feed additive and has been widely used in the poultry industry to prevent coccidiosis and improve feed efficiency. Large amount of roxarsone entered into the environment through manure, which was further degraded by microorganisms in the environment. The degradation of roxarsone in anaerobic granular sludge (AGS) system, degradation mechanism of roxarsone with different carbon sources,and the effects of zerovalent iron addition on the degradation were investigated in this paper. The following results were obtained:(1) The degradation and speciation of roxarsone in an anaerobic granular sludge (AGS) system and the impacts of roxarsone were studied. The results showed that when sucrose was added in AGS system, roxarsone was rapidly degraded into 4-hydroxyl-3-amino phenyarsonic acid (HAPA) and HAPA was further degraded.20 mg/L and 350 mg/L of roxarsone caused 74.2% and 96.6% of inhibition of methanogens, respectively. Speciation and distribution of inorganic arsenic varied in the solution and AGS in the assays of 20 mg/L and 350 mg/L roxarsone addition with short-term and long-term incubation.(2) The degradation of roxarsone with different carbon sources was investigated. The results showed that the addition of carbon source accelerated the degradation of roxarsone. When no carbon source or sodium acetate was added, roxarsone was only degraded to HAPA. While sucrose was added, roxarsone could be biotransformed to HAPA and HAPA was further degraded to o-aminophenol and unknown organoarsenic. No inhibition effects were found with roxarsone addition in the assays with sodium acetate as carbon source. But in the assays with sucrose as carbon source, the addition of roxarsone inhibited the consumption of VFAs and caused 90% of inhibition to methangens, resulting in the structural damage, enlarged cavities and change of microorganisms’ forms in the AGS.(3) The effects of zerovalent iron addition on the degradation of roxarsone in was investigated. The results showed that the addition of zerovalent iron slowly improved the degradation of roxarsone through reducing nitro to amino. When dissolve organic carbon existed in the aqueous solution, zerovalent iron decreased the concentration of inorganic arsenic from roxarsone degradation, but could not relieve the methanogenic inhibition by roxarsone.