Effect Of Bi-directional Pulsed Electric Field On Lead Species In Soil And Enhanced Soil Washing

Soil washing remediation for heavy metal contaminated soils is an effective technique for removing heavy metals from the soil,The efficiency of soil washing is not only related to the type of chemical agents,but also directly related to the chemical speciation of heavy metals in the soil,Citric acid is a promising and environmentally friendly eluent,however,due to its weak ability of activation,the repair effect is not ideal,especially for the actual contaminated soil.It has been reported enhance soil washing with DC electric field,However,it practical applications is limitated because that the DC electric field is prone to electrode polarization.Compared with DC electric field,bi-directional pulsed electric field can change the positive and negative poles periodically,the current has a strong electric shock,consumes polarized material of electrode,eliminates the phenomenon of electrode passivation,and improves the transmission efficiency of the material and energy of the system.Therefore,by studying the transformation of Pb on different electric field parameters in soil,including power-on time,pulse voltage,pulse frequency and duty cycle,on the basis,the pulsed electric field was applied to soil washing and explore the removal efficiency of lead by the combined technology,so as to provide a theoretical basis for enhanced soil washing remediation by electric field.The main conclusions of this study are as follows:(1)The bi-directional pulsed electric field can cause the change of soil pH,showing an acid zone moving from anode to cathode and an alkali zone moving from cathode to anode as a whole,and the acid zone is much larger than the alkali zone,the range of soil pH varies from 4.4 to 8.12;Different electric field conditions have different effects on soil pH,among which the effect of electrification time and pulse voltage on soil pH is more significant.(2)Bi-directional pulsed electric field can cause the transformation of lead speciation.pulse time,pulse voltage,pulse frequency and duty cycle are the main electric field factors to promote the transformation of lead speciation.The longer of pulse time,the larger the pulse voltage,the more obvious the transformation of lead speciation.Under the pulsed electric field,the transformation of lead speciation is mainly from the more difficult to speciation extract to the easier to extract morphology,which promotes the transformation of lead speciation as a whole.The activity and migration of lead were promoted.(3)When the chemical leaching by citric acid enhanced with pulsed electric field,when the concentration of citric acid was 0.2 mol/L,the removal efficiency of Pb reached 59.4%;Applying the bi-directional pulsed electric field,under the condition of duty ratio 75%,0.04 Hz and voltage of 16 V,the maximum removal rate of lead could reach 77.8% after 6 hours of electrification.Compared with the single citric acid leaching,the removal rate of lead could reach 77.8%,increased by16.4%.(4)BCR speciation analysis of heavy metals in soils before and after leaching showed that citric acid leaching mainly took two forms: acid extractable form and Fe-Mn bound form,in which acid extractable form had the highest leaching removal rate and reaching 93.5%;After bi-directional pulsed electric field strengthening,the removal rates of various forms of lead were enhanced in different degree,which the removal efficiency of Fe-Mn bounded Pb and residual Pb was enhanced obviously,the removal efficiency increased from 61% to 88% and from 4.5% to 17%,respectively.The removal efficiency of organic bounded Pb increased slightly,and the removal rate increased from 30% to 33%.