Degradation Of Roxarsone In Anaerobic Biological And Chemical Treatment Processes And Its Mechanisms

Roxarsone?3-nitro-4-hydroxyphenylarsonic acid?has been widely used for decades as an organoarsenic feed additive to prevent parasites,promote growth and improve feed efficiency in animal production.However,barely metabolized in animal bodies,most of the roxarsone ingested is excreted unchanged via urine and feces,which,as a result,would cause arsenic contamination in aqueous environment due to its good mobility and high water solubility.This work focused on the treatment of organoarsenicals.In this thesis,the impact of roxarsone on an anaerobic digestion system with continuous roxarsone addition and its degradation were investigated,the degradation of roxarsone by UV/H₂O₂ and UV/S₂O₈^2-was compared,and the ozonation experiment of roxarsone were conducted.The main contents and results obtained are listed as follows:1.The impact of roxarsone on the performance of the upflow anerobic sludge blanket?UASB?reactor and the degradation and speciation of roxarsone were investigated.The UASB reactor was operated using synthetic wastewater supplemented with roxarsone for a period of 260 days.With continuous roxarsone addition at 25.00 mg·L^-1,severe inhibition to methanogenic activity occurred after 87 days accompanied with an accumulation of volatile fatty acids?VFAs?and a decline in pH.The decrease of added roxarsone concentration to 13.20 mg·L^-1 did not mediate the inhibition.Arsenite?As?III??,arsenate?As?V??,monomethylarsonic acid?MMA?V??,dimethylarsinic acid?DMA?V??,4-hydroxy-3-aminophenylarsonic acid?HAPA?and an unknown arsenic compound were detected in the reactor,and a possible biodegradation pathway of roxarsone was proposed.Mass balance analysis of arsenic indicated that 60%-70% of the arsenic was discharged into the effluent,and 30%-40% was precipitated in the reactor.The results from this study suggest that we should pay attention to the stability in the UASB reactors treating organoarsenic-contaminated wastewater,and the effluent and sludge from the reactor to avoid diffusion of arsenic contamination.2.The degradation of roxarsone by UV/H₂O₂ and UV/S₂O₈^2-was compared.The second order rate constant for roxarsone reacting with hydroxyl radical?OH·?at pH 7.0 was determined to be?2.84 ± 0.02?× 109 L·mol^-1·s^-1 using competition kinetics and it was about ten-fold higher than that with sulfate radical?SO4·-?(?2.63 ± 0.31?× 108 L·mol^-1·s^-1).The effects of environmental conditions on the degradation of roxarsone by UV/H₂O₂ and UV/S₂O₈^2-were investigated.HCO3-/CO32-did not obviously impact roxarsone degradation in the two processes.However,the degradation of roxarsone was inhibited by humic acid in the two processes and stronger inhibition was observed in the UV/S₂O₈^2-process.The presence of Cl-had negligible effect on the degradation of roxarsone in the UV/H₂O₂ process,and remarkably promoted the degradation in the UV/S₂O₈^2-process.The analysis of arsenic species showed that As?V?was the primary inorganic arsenic in the two processes.Moreover,both processes could completely mineralize roxarsone,whereas UV/S₂O₈^2-showed a high efficiency for mineralization.3.The degradation of roxarsone by ozone was also investigated.The second order rate constants for roxarsone reacting with molecular ozone and OH· at pH 7.0 were determined to be?1.70 ± 0.09?× 105 L·mol^-1·s^-1 and?2.96 ± 0.20?×109 L·mol^-1·s^-1,respectively,using competition kinetics.The ozonation of roxarsone was affected by the initial roxarsone concentration,pH,humic acid and Cl-.The increase of solution pH did not always enhance the ozonation of roxarsone.The calculated rate constants were used to model the ozonation of roxarsone in natural waters and the results showed that roxarsone was completely degraded by molecular ozone and OH· had negligible effect on the ozonation of roxarsone in natural waters.The analysis of arsenic species indicated that As?V?was the primary inorganic arsenic.Finally,based on the identified degradation products and the ions released,a possible ozonation pathway of roxarsone was proposed.