Anaerobic Biotransformation Of Roxarsone And In Situ Immobilization Of Its Degradation Products

Roxarsone(ROX,4-hydroxy-3-nitro-phenylarsonic acid)has been widely used as a typical organoarsenic feed additive because of growth promotion,feed utilization enhancement and coccidiosis prevention.Its widespread use in livestock and poultry farms causes an increasing production of arsenic-bearing wastewater.Anaerobic digestion is a conventional technique for livestock wastewater treatment.However,anaerobic biotransformation of ROX and the associated arsenic contamination control remain poorly understood.Therefore,this dissertation aims to explore biodegradation of ROX and its impacts on anaerobic granular sludge(AGS),to clarify arsenic accumulation in AGS and the immobilized arsenic forms,to evaluate the factors affecting arsenic volatilization in ROX-loaded digestors,and to investigate the feasibility and mechanism of enhancing ROX degradation and in situ arsenic immobilization using a bio-electrochemical system(BES)coupled with sulfate reduction.The main findings were outlined as follows:Firstly,the anaerobic degradation of ROX and its impact on methanogenic activity,AGS structure and microbial community in an uplow anaerobic sludge blanket(UASB)reactor were investigated.During the 420-day operation,methane production decreased by?40%on day 260 and gradually restored,which was mainly caused by the influence of ROX and its metabolites on the acetotrophic methanogenic activity.According to the analysis of ROX degradation metabolites,the nitro and arsenate groups of ROX were successively reduced to an amino group and arsenite group,followed by the deamination,dehydroxylation and the cleavage of C-As bond.With the feeding of 19.0?M ROX,AGS size was gradually reduced and stabilized at 1.20-1.26 mm,equivalent to 69.4-72.8%of the initial size.A coating of proteins and carbohydrates was formed on the surface of AGS,and the spalling of surface layer was observed.ROX loading exerted little impact on bacterial community,but a significant change in the abundance and composition of archaea.Methanothrix,an acetotrophic methanogen,was inhibited first and became the domin methanogen eventually.These findings provide useful information to treat livestock wastewater contaminated with organoarsenicals.Secondly,arsenic accumulation in AGS treating ROX-contaminated wastewater and the accumulated arsenic forms were investigated in a 260-day cultured UASB reactor.The total arsenic concentrations in the influent and effluent were 3.76 mg·L-1 and 2.88 mg·L-1.Volatile arsenic was released at a rate of 134.93 ng·d-1.The result of arsenic balance analysis indicated that 23.40%of influent arsenic accumulated within the AGS.The total arsenic concentrations in the AGS at the top and bottom of the anaerobic sludge bed were 5,276.90 mg·kg-1 and 10,661.92 mg·kg-1(based on dry mass),respectively.Of which,half of arsenic was insoluble under acidic conditions.Over 65%of soluble arsenic in the AGS was extracellular and the dominant species was As(?)and As(V).Based on the analysis of field emission scanning electron microcopy-energy disperse spectroscopy(FESEM-EDS)and X-ray diffraction(XRD),the precipitated arsenic in the AGS mainly existed in the form of crystalline As4S4.The results provide important information on evaluating the biotoxicity of arsenic accumulated in AGS and the risk of arsenic leaching during the disposal of anaerobic sludge.Thirdly,arsenic volatilization from ROX-loaded anaerobic digestor was investigated in terms of abiotic factors,arsMgenes and microbial community.ROX loading,exposure time of AGS to ROX,and volatile fatty acids(VFAs)were selected as the primary factors affecting arsenic volatilization.Addition of 20 mg·L-1 ROX triggered the maximum volatile arsenic yield of 6.78 ng·g-1-VSS·d-1,which was nearly 5 times compared to the ROX-free assay.Arsenic volatilization was significantly facilitated after the degradation of arylarsenicals to inorganic arsenic.The 160-day and 270-day exposure of AGS to ROX caused 6-fold and 8-fold increase in volatile arsenic yield,respectively,compared to the 0-day exposure.After the degradation of ROX into inorganic arsenic,VFA level was positively associated with arsenic volatilization(r=0.832-0.950;p<0.05).The abundance of arsM genes in AGS increased by 34.62-129.05%after the 100-day incubation,and presented strong correlation to arsenic volatilization.The microorganisms related to arsenic transformation were enriched with ROX loading and long-time exposure of AGS to ROX.The results provide valuable information for evaluating the risk of arsenic volatilization from organoarsenic-contaminated environments.Finally,the sulfate reduction was combined with BES for enhancing the degradation of ROX and in situ immobilization of the generated inorganic arsenic.The feasibility of this process and its mechanism were investigated.ROX and its metabolite,4-hydroxy-3-amino-phenylarsonic acid(HAPA),were completely degraded within 13-22 days in the BES coupled with sulfate reduction,while the degradation efficiency of arganoarsenicals was less than 85%within 32 days in the anaerobic digestion.Approximately 75.0-83.2%of arsenic was immobilized in the sludge in the BES coupled with sulfate reduction,significantly higher than that in the anaerobic digestion(46.7-57.3%).The combination of sulfate reduction and electrochemical stimulation exerted a synergetic effect on enhancing HAPA degradation and sulfide-driven arsenic precipitation.The analysis of an additional double-cell BES experiment indicated that As(V)and sulfate were transported from the anode to cathode chamber,and co-precipitated in the form of crystalline alacranite in the cathode chamber.These findings suggest that the BES coupled with sulfate reduction is a promising technique for organoarsenic-contaminated manure/wastewater treatment.