The Effects Of Environmental Conditions On The Process Of Roxarsone Biotransformation In Porous Media

Roxarsone(3-nitro-4-hydroxyphenylarsonic acid),as one of arsenic feed additives,has been extensively used in animals feeding,which was usually excreted unchanged in manure and eventually entered in soil and water via animal manure and animal wastewater.Roxarsone might be converted to diverse organic arsenic or inorganic arsenic in environment.It brings potential risks to the ecological environment and had aroused widespread concern.In this study,we aimed to discover the process and mechanism of roxarsone biotransformation in porous media.Environmental conditions including substrate level,moisture contents and bacterial strains,were investigated.The main conclusions were obtained as follows:(1)Roxarsone could not be used as the sole carbon source for Shewannella oneidensis MR-1 growth.The presence of exogeneous carbon source promoted bacterial growth and roxarsone degradation,along with HAPA(4-hydroxy-3-amino phenylarsonic acid)as the only reductant product detected.The fluctuations of water content in porous media obviously affected roxarsone transformation.At moisture content lower than 25.00%,roxarsone nearly kept unchanged,while roxarsone transformation could be observed as water content was higher than 28.75%.Roxarsone biotransformation was enhanced by the higher moisture content,for instance,the degradation half-life of roxarsone in the water content of 28.75% and 100% experimental groups were 148.5 and 17.2 hours.(2)Collecting the soil microbes from different depth of soil showed that no roxarsone transformation happened in aerobic environment after 108 hours.In anaerobic environment,the deeper the soil in which the microbes were located,the slower the transformation efficiency of roxarsone was,and the biotransformation efficiency of roxarsone by soil surface microbes was the highest among three groups.Results showed that higher moisture content dramatically promoted roxarsone biotransformation.After 144 hours of cultivation,roxarsone in the experimental group with water content higher than 40.25% was completely transformed,the roxarsone conversion rate in the 30.00% water content group was 30%,and roxarsone in 12.50% water content group hardly ever transformed.In experimental groups with water content of 40.25% and 100%,roxarsone was firstly reduced to HAPA,and then 37% of the HAPA was converted into inorganic arsenic,MMA(monomethylarsine)and DMA(dimethylarsine)in solution,along with 6% of arsenic inside microbial cells.(3)The effect of moisture content on the dynamics of bacterial community diversity was investigated.High-throughput sequencing indicated that higher moisture content caused lower bacterial community diversity.The dominant species in soil environment were Enterobacter(59.15%),Aeromonas(9.25%)and Pseudomonas(6.63%),after cultivation of a period of time,the dominant species in 100% water content group became to Pseudomonas(52.87%),Enterobacter(25.82%)and Stenotrophomonas(7.56%),the dominant species in 12.50% water content group became to Enterobacter(37.19%),Pseudomonas(37.09%)and Stenotrophomonas(11.12%).It indicated that the above three genus of bacteria could be well adapted to roxarsone.High-throughput sequencing also indicated that higher roxarsone concentrations caused lower biodiversity,and Buttiauxella?Lysinibacillus and Pseudomonas were large diversity genus between 2.3 and 0.5 mmol/L roxarsone concentrations experimental groups.