Arsenic Transformation And Its Microbial Mechanism During The Sludge Anaerobic Digestion

As a highly toxic matalloid,arsenic(As)contamination in environment has been a research hotspot in recent years.The toxicity and bioavailability of arsenic depends on the total content in environment as well as its species and transformation.Arsenic in sludge has a potential threat to environment for its wide distribution.Anaerobic digestion is a stable and effective biological chemical conversion process.In addition,microbes can affect the migration and transformation of arsenic by regulating its speciation,showing an important role in the cycle of arsenic.Current studies mainly focused on the species and bioavailability of arsenic,thus,it is necessary to study the process and mechanism of arsenic oxidation,reduction and methylation during the anaerobic digestion of sludge.The change of arsenic species and microbial community structure after the field anaerobic digestion of sludge was firstly investigated.Further,a laborsatory study of anaerobic digestion of sludge to certify the arsenic transformation was carried in this study.The potential functional microorganisms of arsenic were also studied according to the high throughput sequencing and Real-time PCR results.The results could provide a theoretical basis for the biological removal of arsenic in sewage sludge.Firstly,the transformation of arsenic during field anaerobic digestion was investigated.The results showed that As(V)was the dominant specie accounting for 41.73-74.27%in sludge while As(III)decreased for about 8.92-19.57%after anaerobic digestion.MMA increased dramatically reaching 21.35-30.61%of the four arsenic content.Secondly,in the laboratory study,As(?)decreased about 26.42%while As(V)increased slightly about 16.85%.MMA was detected at the third day.The content and form of arsenic were different obviously while the variation tendency was similar in two anaerobic digestion due to the different sources of sludge.Besides,arsM gene was found in the digested sludge and the copies of arsM genes were more than triple after the process of field anaerobic digestion,indicating it might be a dominant metabolism gene in the arsenic functional genes.Change of the microbial community structure was observed by 16S rRNA gene sequencing after the digestion.The decrease of Chaol,ACE and Shannon index indicated lower species abundance of microbial diversity after anaerobic digestion.Forty seven OTUs chose from sludge matched with arsM genes(similarity ? 96%)reported previously and the relative abundance were shown.Euryarchaeota represented primarily by Methanosarcinales increased to 8.06-10.90‰ after anaerobic digestion.Chloroflexi represented primarily by Anaerolineales also increased by 3.50-3.59‰,suggesting that these phyla might be the functional microbes for the methylation of arsenic during the anaerobic digestion,however,further study is still needed for validation.Finally,in the experiment in which a inhibitor(bromoethanesulfonate)was added to inhibit the digestion process,no methylated arsenic could be found during the digestion.The results confirmed that the methylation process of arsenic occurred during the anaerobic digestion of sludge.Overall,the change of arsenic transformation,microbial communities and the abundance of metabolism genes of arsenic were preliminary discussed during the anaerobic digestion of sludge in this paper.The results could provide more knowledges about the behavior of arsenic in sludge and help to develop technologies to remove the arsenic from sludge.