Electrochemical Stimulation Of Microbial Organoarsenic Degradation And In-situ Recovery Of Inorganic Arsenic In Wastewater

Organoarsenic is widely used as additive in the livestock and poultry industry, owning the good performance in promoting growth and controlling coccidial intestinal parasites. However, most of the added organoarsenic is excreted unchanged in manure, causing arsenic pollution of water environment. In this paper, the electrochemical stimulation of microbial organoarsenic degradation and transformation under anaerobic conditions was investigated. The distribution and speciation of degraded organoarsenic were analyzed. Meanwhile, the cathodic electron donor was combined with the anodic bioelectrochemical system to recover arsenic under anaerobic reductive conditions. The results are listed as follow:(1) Bioelectrochemically stimulated degradation of roxarsone was investigated. Roxarsone was quickly reduced to HAPA under anaerobic conditions. After the carbon sources in the substrate were depleted, the degradation of HAPA was accelerated under 0.5 V voltage. The two-cell membrane reactor assays revealed that the HAPA was degraded in anode chambers, confirming that the anode enhanced the electron transfer as an electron acceptor. The degradation product formed with electrochemical stimulation was arsenate, which facilitates the removal of arsenic from wastewater. Based on the analysis of arsenic species and gas chromatography-mass spectrometry (GC-MS), the pathway and mechanisms for the electrochemically stimulated degradation of roxarsone are proposed.(2) The toxicity of roxarsone for anaerobic sludge and the species analysis of the extracted arsenic were explored. With the addition of 20 mg/L roxarsone, total methane production produced by anaerobic sludge decreased by 17.6%. In the assay without electrochemical stimulation, inorganic arsenic existed as As(III) mainly and combined less with Fe in sludge. However, inorganic arsenic existed as As(V) and mostly combined with Fe in the assay with 0.5 V voltage applied. Moreover, there is large part of arsenic can not be extracted, proving they are combined tightly in the sludge.(3) Combined bioelectrochemical system for stimulating arsanilic acid degradation and in-situ recovery of arsenic under anaerobic conditions was constructed. With 0.5 V voltage applied, graphite anode in stage I was regarded as an electron acceptor for stimulation, and the cathode in stage II performed as an electron donor to make inorganic arsenic fixed on the surface of electrodes. The concentration of toxic As(Ⅲ) and As(V) was hardly detected at the end of the process. Based on the analysis of GC-MS and species analysis instrument, the possible pathway of arsanilic acid degradation was proposed. Field emission scanning electron microscopy (FESEM) analysis confirmed the precipitation of arsenic on electrodes. The characterization of arsenic on electrodes was analyzed using XRD and XPS.