Study On Migration And Transformation Of Roxarsone In Vadose Zone

Roxarsone(3-nitro-4-hydroxyphenylarsone)is a kind of organic arsone compound with low toxicity and high water solubility.It is widely used as an additive in livestock feed because of it can promote pigmentation,livestock growth and egg production.However,roxarsone is less absorbed in livestock and most of it is excreted through livestock faece.Roxarsone in the environment will rapidly degrad into a variety of arsenic compounds,including toxic As(III)and As(V),under a series of biological and abiotic actions such as microorganisms and light.However,when inorganic arsenic enters the soil environment,it will migrate and accumulate vertically under the action of leaching,etc..When it enters the water environment,it will spread.It is likely that the use of roxarsone will lead to the abnormality of arsenic in the key zone of the whole shallow surface of the earth and cause corresponding environmental geological problems.Vadose zone is the "transfer station" of contaminants moving from surface to groundwater,thus this thesis focuses on the transition and transformation of roxarsone in the vadose zone.By selecting a representative chicken farm in the Lubei Plain as study area,and combining the field survey in the study area,indoor unsaturated soil column leaching experiment and metal oxide indoor batch adsorption experiments of sand rock arsine.Through the detection of arsenic morphology and content in the gas-encased zone section of chicken manure accumulation in the study area and the surrounding groundwater,it was found that the Open-air stacking of livestock feces will cause the increase of arsenic content in the vadose zone over time.and As(V)is more likely to be accumulated in the vadose zone of the study area.The accumulation position of different forms of arsenic in vadose zone is related to soil lithology and the transformation of arsenic form.Through 112 days of unsaturated soil column simulation experiment,it was observed that roxarsone in the soil column was rapidly degraded to As(III),and finally accumulated in the vadose zone in the form of As(V).By observing the change of microbial community structure,it was found that there was no g__norank_f__XVIII bacteria in the beginning,and then its abundance increased first and then decreased with the extension of time,and the abundance of g__norank_f__XVIII bacteria had a strong positive correlation with the content of As(III),indicating that there was a strong correlation between the bacteria andarsenic compounds.According to the indoor adsorption experiment on roxarsone,the thesis finds that the adsorption effect of three kinds of metal oxides on roxarsone is better,which is mainly physical adsorption.The equilibrium adsorption amount of aluminum oxide > manganese dioxide > iron oxide,and the adsorption process is in line with Freundlich model.Metal oxides in the gas zone affect the migration of roxarsone and arsenic compounds,which are affected by temperature and pH.