Study On The Efficiency And Mechanism Of Lead And Atrazine Contaminated Soil Remediation By Dual Membrane Electrokinetics Remediation System

The prevention and remediation of soil pollution is the focus of environmental remediation research all over the world.Improper discharge of industrial waste water and relocation of old pesticide factories have led to serious soil heavy metal pollution and pesticide pollution,or even their combined pollution.Lead(Pb)is a common heavy metal pollutant in soil environment.Human activities such as mining and battery development often lead to serious soil lead pollution.Atrazine is a kind of pesticide widely used in agricultural production process,its large migration and long residue in soil environment will seriously harm human health.Electrokinetic remediation technology has been widely concerned for its good adaptability in soil pollution remediation.However,the traditional electrokinetic remediation technology will form acidified and alkalized areas in the soil,which is not conducive to the removal of heavy metals in the soil and its combination with other technologies.In this study,a dual-membrane electrokinetic remediation system(DMEK)was designed combining anion/cation exchange membrane and coupled with atrazine degradation bacteria DNS32,which was applied to remediation of Pb contaminated soil and atrazine contaminated soil.Finally,Pb-Atrazine mixed contaminated soil was repaired by gradient remediation.The main findings are summarized as follows:(1)DMEK can efficiently remove heavy metal Pb in contaminated soil.Under the voltage gradient of 3 V/cm,Pb content in most areas of soil decreased from 7066.64 mg/kg to less than1000 mg/kg within 4 days.Under these conditions,the removal rate of Pb in soil was faster,but the change of soil p H was obvious.Under the voltage gradient of 2 V/cm,the removal efficiency of Pb in soil and the stability of soil p H can be balanced.The anion/cation exchange membrane can effectively block the hydrogen ions and hydroxide ions generated by electrolysis at the electrode,ensuring the stability of soil p H in the process of soil remediation.(2)In DMEK,the removal of Pb in soil mainly depends on electroosmotic flow caused by pore water movement driven by electric field.Pb in soil can chelate with EDTA-2Na and be recovered in the cathode with electroosmotic flow.Free Pb ions can also be dissolved in electroosmotic flow and recovered.In this process,the electroosmotic flow is positively correlated with the voltage gradient,and the corresponding electroosmotic flow can be obtained by regulating the voltage gradient.(3)DMEK coupled with DNS32 degradation bacteria can effectively repair atrazine contaminated soil.Between voltage gradients of 1-2 V/cm,the anion/cation exchange membrane in the system effectively balances soil p H,creating a favorable environment for microbial growth and reproduction.Compared with single microbial remediation,DMEK/DNS32 can remove atrazine from soil more efficiently.Under the action of electric field,the activity of DNS32 degradation bacteria was promoted and the growth rate was accelerated.At the same time,the driving effect of electric field on pore water and the electrophoretic effect of bacteria in electric field make it easier to obtain nutrients in soil.(4)Under the voltage gradient of 1 V/cm and 1.5 V/cm,DNS32 degradation bacteria could completely degrade atrazine in 28 and 20 days(initial concentration was 216.32 mg/kg),respectively.However,at the voltage gradient of 2 V/cm,the degradation efficiency of DNS32-degrading bacteria was lower than that of 1.5 V/cm,because the high voltage gradient inhibited the activity of DNS32-degrading bacteria.(5)The soil contaminated by Pb and atrazine could be repaired by gradient remediation method.DMEK reduced Pb concentration in soil from 5216.62 mg/kg to 281.16 mg/kg within 16 days.Subsequently,after 18 days of remediation,DMEK/DNS32 reduced soil atrazine concentration from 232.75 mg/kg to below the detection limit.The gradient remediation method could completely remove Pb and atrazine from the composite contaminated soil within 34 days.(6)During the gradient remediation process,the activities of urease,sucrase and neutral phosphatase in soil increased first and then decreased due to the removal of heavy metals and the addition of exogenous bacteria DNS32.However,catalase activity continued to decrease due to the decrease of microbial stress response.After remediation,the microbial diversity in the soil recovered and the relative abundance of some bacteria increased.At the phylum level,the relative abundance of Firmicutes and Proteobacteria increased significantly.At class level,the relative abundance of Gammaproteobacteria and Clostridia was enhanced.In conclusion,with the decrease of the two pollutants in soil,the toxic effect decreased gradually,the relative abundance of some organisms in soil increased gradually,and the community diversity also improved.