Bioclogging Study Of Porous Media During The Groundwater Nitrate Contamination Bioremediation By Nano-emulsified Vegetable Oil

During the in-situ bioremediation process of contaminated aquifers,biogenic gas production,microbial cells proliferation and extracellular polymeric substances?EPS?accumulation are often the main causes of pore media clogging.These three functions are usually closely linked,resulting in reduction or even failure of repair efficiency.In order to reveal the synergistic effect of the three and explore feasible mitigation measures,the reaction system was inoculated by commercial denitrifying bacteria and nano emulsified vegetable oil?Nano-EVO?was used as carbon source in simulated batch experiments on the removal of nitrate nitrogen?NO₃-N?pollution from groundwater.The main conclusions are as follows:?1?In 81 days'?I^V cycles?operation of simulated experiments,average unit volume of Nano-EVO?mL?could remove 64.69 mg NO₃-N and the total removal efficiency was 83.13%.CO₂ and N₂ were the main gases,with 0.16 mL and 0.51 mL for 1 mg of nitrate,respectively.The growth curve of denitrifying bacteria showed a downward trend as a whole,and could be devided into three stages:adaptation period,stability period and extinction period.All the dominant bacteria have characteristics of producing viscous metabolites.Microbial species richness and diversity were lower in the early stage,but showed obvious superiority in the later stage.The EPS of microbial metabolites were mainly composed of polysaccharides?PS?and proteins?PN?,and the average content of PS and PN produced 0.16 mg and 0.80 mg per g of nitrate,respectively.?2?In the process of in-situ bioremediation of nitrate nitrogen pollution by Nano-EVO,staggered peaks of the three bioclogging factors?gas production,microbial cells and EPS?appeared.The peak of gas production is in the front?I cycle?,while the peak of EPS is in the back?IV cycle?,and the microbial cell decreases from the initial level.The synergistic effect was obvious.The whole clogging process could be divided into four stages:0¹5d,15³0d,30⁵8d and 58⁸1d,the effective porosity(n_Mobile)of porous media system went through a rapid decline,a biggish recovery,a small decline and a large recovery,respectively.Assuming that the closed system exhausts only during the lag period when the nitrate nitrogen concentration no longer decreases,the model predicted that the maximum permeability loss caused by synergistic effect of the three could reach 72.12%,and the contribution of gas production to pore bioclogging was the largest,with n_Gasas was above 75%most of the time.?3?Different opening degrees of groundwater aquifer system were simulated and various exhaust scenarios were designed.Assuming that the system can effectively exhaust gases at a constant rate(7.5 mL·d^-1),n_Gasas would be 1² orders of magnitude lower and the permeability loss caused by the three could be reduced by26%at most.Assuming that gas can be discharged with production,the final predicted permeability loss of pore medium caused by microbial biomass?cells and EPS?was 24.43%.The contribution rate of gas production in permeability loss gradually decreased with the increase of aquifer opening.Hence,for a relatively closed aquifer system,exploring the ways to enhance its opening condition to promote effective exhaust is an effective means of bioclogging alleviation measure.