Anaerobic Bioprecipitation Of Roxarsone And Its Enhanced Removal By Electrocoagulation

Roxarsone（4-hydroxy-3-nitrophenylarsonic acid）,an organoarsenic feed additive,has been widely used to prevent coccidial intestinal parasites,promote animal growth and improve feed efficiency.Most of the added roxarsone is excreted via urine and feces without chemical change,which would enter the environment and cause arsenic contamination.To control the contamination of organoarsenicals,in this work,the precipitation of roxarsone and precipited arsenic species in anaerobic sludge,the anaerobic bioprecipitation mechanism of roxarsone,and the enhanced removal of residual arsenic products via biodegradation of roxarsone by electrocoagulation were studied,respectively.The results are listed as follows:（1）The precipitation of roxarsone and precipited arsenic species were investigated.High concentration arsenic was accumulated in the anaerobic granular sludge（AGS）which was collected from a continuous flow anaerobic bioreactor operated with synthetic wastewater containing roxarsone.After 300-day operation,the concentration of total arsenic in the AGS was 8246.69±378.93μg·g^-1（based on arsenic and dry weight）.In the AGS,cellular bonded arsenic had the highest percentage（42.2%）,followed by insoluble arsenic（25.4%）,extracellular soluble arsenic（24.6%）and intracellular soluble arsenic（7.8%）.It was comformed that the insoluble arsenic existed in the AGS in in form of AsS,and arsenite（As（ⅡI））was the predominant specie of other precipited arsenic.Under anaerobic condition,the arsenic precipited in the AGS trended to bond to cellular material or form insoluble arsenic,and the latter could be used for stabilization of organoarsenicals to control the arsenic contamination.（2）The anaerobic bioprecipitation mechanism of roxarsone was explored.Under anaerobic biological condition,roxarsone in the solution was reduced to4-hydroxy-3-aminophenylarsonic acid（HAPA）and then was degradated to inorganic arsenic by electrochemical stimulation,in which arsenate（As（V））was the dominant inorganic specie.In the solid phase（Fe₃O₄）,the anaerobic bioprecipitation of roxarsone was finished successively as follows:adsorption of roxarsone,transformation and biodagradation of roxarsone and formation of insoluble arsenic.Significant As（ⅡI）existed in Fe₃O₄ even under electrochemical stimulation condition,and As₂S₃ was detected when the applied voltage was removed.The enhancement of reducing condition due to the removal of applied voltage led to the formation of AsS.The existence of microorganism and sulfate was the key to form sulfur-bearing insoluble arsenic.（3）The removal of residual arsenic products via biodegradation of roxarsone by electrocoagulation was studied.The highest removal rate of arsenic by chemical coagulation of poly aluminium chloride or polymerization ferric chloride was less than 10%.Electrocoagulation was efficient for removal of arsenic and highest removal rate reached 90%.Enhancement of applied voltage,reduction of electrode distance and application of aeration could increase the removal rate of arsenic,in which application of aeration was the most effective.Two new arsenic compounds were generated during the electrocoagulation but the species were unknown.When the generation of coagulant was limited,the maldistribution of coagulant would cause the different rate in the same electrocoagulation reactor.The precipitation in electrocoagulation reacter with applied aeration was yellow and mainly consisted of FeO（OH）.But the precipitation in electrocoagulation reacter without applied aeration was green and amorphous,which was speculated as Fe（Ⅱ）（oxyhydr）oxides.