Research On As(Ⅴ) Reduction Behaviour Mediated By Microbes In Landfills

Arsenic,a toxic element,is widely found in nature and has now become a worldwide pollutant,causing both ofecological and human health’s risk.The landfill,as a disposal site for municipal solid waste,is also one of the collection sites for As-containing waste and is considered to be one of the main sources of As pollution.Thus As pollution of landfills cannot be ignored.The risk caused by arsenic contamination in landfills depends not only on the total Ascontent but also on the distribution of As species,since microbial metabolic processes are considered to be the key drivers for arsenic morphological transformation in landfills.The simulated landfill reactors with different As（Ⅴ）contents were constructed to explore the As（Ⅴ）bioreduction behavior and its regulation mechanism in landfills.Based on the above research,the effect of temperature fields and sulphate-reducing bacteria（SRB）were further investigatedon the As（Ⅴ）bioreduction behaviour.The main conclusions are as follows:（1）Waste samples from six typical sanitary landfills in Zhejiang Province were analysed.The results revealedthe distribution of arsenic speciesin landfills in response to the degradation of municipal solid waste.The dominate As species in the landfills was As（Ⅴ）,and the peaking value was appeared in the middle of the landfill process,with an average As（Ⅴ）content of 4.84 mg kg^-1.The environmental factors such as p H,moisture,DOC,NH₄⁺,NO₃^-and SO₄^2-were correlated with As species distribution.The arsenate respiratory reductase（Arr A）and the arsenate reductases（Ars C）were considered as thekey factors to drivethe reduction behavior of As（Ⅴ）.The abundance of arr A genes and ars C genes were also significantly correlated with As（Ⅴ）reduction behaviour.It showed thatthe increase in the abundance of arsenic-reducing genes could result in an increase in high As（Ⅴ）reducing capability.（2）The simulated reactors,adding different contents of As（Ⅴ）,shed light on the bioreduction behavior of As（Ⅴ）in the landfill and its regulation mechanism.Comparison with additional As（Ⅴ）groups（Low and High）,the reduction efficiency of As（Ⅴ）were significantly increased,with As（Ⅴ）reduction efficiency of 32.52%and 33.07%,respectively.Meanwhile,the arr A and ars C genes from the Low and High groups were significantly higher than Initial group,especially the abundance of ars C increased significantly with increasing As（Ⅴ）content in the reactors.Subsequently,identification of putative As（Ⅴ）reduction microorganism（PARM）were identified at different operating stages via ANOVA analysis.Basing on the RMT,co-occurrence network analyzed the ecological functions of PARM,and formed three ecological networks（Initial,Low and High,modularity coefficient>0.4）.The Arsenic-tolerant PARM species such as Moheibacter,Rubellimicrobium,Arenimonas,Muricauda,Sumerlaea,Sporocytophaga,Limnobacte,Phaselicystis,norank＿OTU2511,norank＿OTU309 and the non-arsenic-tolerant PARM species such as Castellaniella and Pseudomonas were peripheral nodes in the ecological network and did not play a critical role in landfill ecosystemical stability.But Sporocytophaga sp.as a core node of the module,was an important’hub’in its own ecological network and playedan important role in the ecosystem.（3）Enrichment culture under different As contamination were used to investigate the effect of temperature fields and SRB on As（Ⅴ）reduction behaviour in landfills.Results showed the As（Ⅴ）reduction rates were significant different under various temperature fields,namely high temperature field（50°C）>medium to high temperature field（35°C）>room temperature field（25°C）>low temperature field（10°C）.Moreover,the addition of Mo was significantly suppressed SRB,which led to decrease As（Ⅴ）reducing behaviour and the As（Ⅴ）reducing rate was suppressed by nearly 70%at 50°C and by approximately 18%at 35°C.（4）Different ARMs were identified underdifferent temperature fields,and Pseudomonas was the dominant genus in the low temperature field（10°C）;Pseudomonas and Aquanacterium at the room temperature field（25°C）;Pseudomonas,Aquanacterium,Bacillus and Clostridium at 35℃;Bacillus and Clostridium were dominant genus at high temperature field（50°C）.（5）The dominant genera of ARM were Pseudomonas,Pseudoxanthomonas,Clostridiumin SRB enrichment cluture at 35°C,and the dominant genera of SRB were Clostridium and Desulfitobacterium.While Bacillus and Clostridium were still the dominant genera of ARM and the dominant genera of SBR were Clostridium and Desulfotomaculum at 50°C.In summary,this study investigated the As（Ⅴ）bioreduction behaviour in landfills and its regulatory mechanism,providing a theoretical basis for controlling As pollution and reducing As toxicity.