Electrokinetic Remediation Of Cadmium Contaminated Soil Using Low Voltage Gradients Coupled With Adsorption

There is an urgent need for an efficient and low-cost technology to control soil cadmium(Cd)contamination as a threat to the ecological environment safety and human health.Electrokinetic remediation is one of the most potential technologies to remediate contaminated soil.In conventional electrokinetic remediation,the migration path of Cd(Ⅱ)is long,and the removal efficiency is usually higher when a high voltage gradient(≥1V/cm)is used,but the high voltage gradient will cause a series of side effects and high energy consumption.Besides,when a low voltage gradient is used,the migration of Cd(Ⅱ)is weak and the long path migration is difficult,so the remediation efficiency is low.Herein,the aim of this study is to develop a combined electro-adsorption remediation technique for effective removal of Cd(Ⅱ)from soil at low voltage gradients,and to achieve adsorption and capture of Cd(Ⅱ)at low voltage gradients and short migration distances by shortening the trapping distance of Cd(Ⅱ)in the electric field.The effects of voltage gradient(0.2 V/cm,0.6 V/cm and 1.0 V/cm)and polarity exchange on soil water content,pH,conductivity,temperature,and Cd(Ⅱ)removal during the electrokinetic remediation coupled with columnar activated carbon were investigated.The feasibility of remediation of Cd(Ⅱ)contaminated soil with the combination of electrokinetic-adsorption(columnar activated carbon)remediation technology,electrokinetic-array adsorption zone remediation technology and the combination of main-auxiliary electrode system electrokinetic-array adsorption zone remediation technology under the condition of 0.2 V/cm voltage gradient and polarity exchange was determined.The main conclusions are as follows:(1)Compared with the higher voltage gradients(0.6 V/cm and 1.0 V/cm),when the voltage gradient of 0.2 V/cm was used,the side effects such as dehydration effect and thermal effect in the conventional electrokinetic remediation process were weakened,and the energy loss caused by Ohmic heat was reduced.It is difficult to activate inactive Cd(Ⅱ)when the applied voltage gradient is 0.2 V/cm.Therefore,the removal efficiency of total Cd(Ⅱ)depends on the bioavailable Cd(Ⅱ)content in the soil.(2)Polarity exchange kept the distribution of soil moisture and temperature more uniform and avoided the extreme change of soil pH.Polarity exchange weakened the possibility of inducing and enhancing asymmetric distribution of soil electrical conductivity.However,in conventional electrokinetic remediation coupled with polarity exchange,the reciprocation of Cd(Ⅱ)due to polarity exchange without electrolyte chamber resulted in lower Cd(Ⅱ)removal efficiency than remediation without polarity exchange.When the electrolyte chamber was installed,the reciprocation of Cd(Ⅱ)resulted in more Cd(Ⅱ)remaining in the middle part of the soil than in other parts.The amount of reciprocation of Cd(Ⅱ)was reduced when electrokinetic remediation coupled with polarity exchange and array adsorption zone.In addition,the reciprocation of Cd(Ⅱ)in the vicinity of the adsorption columns may increase the chance of being captured by the adsorption columns.(3)In a 28-day electrokinetic-adsorption(columnar activated carbon)combined remediation,the average removal efficiency of total Cd(Ⅱ)was 61%,and the energy consumption was 5.65 kWh/m~3,the energy utilization efficiencyβwas 43.26.In a 30-day electrokinetic-adsorption combined remediation process,an average removal efficiency of83%of total Cd(Ⅱ)and an energy consumption of 7.72 kWh/m~3were achieved and the energy utilization efficiencyβwas 10.75 by spraying the enhanced reagent citric acid.In the 20-day combined remediation of main-auxiliary electrode system electrokinetic-array adsorption zone remediation process,electrolyte chambers were set with 0.1 mol/L citric acid as the electrolyte,the average removal efficiency of total Cd(Ⅱ)was 83%,and the energy consumption was 0.83 kWh/m~3and the energy utilization efficiencyβwas 101.19.The main-auxiliary electrode system made the removal efficiency of Cd(Ⅱ)in each part of the soil more uniform.In previous studies,the energy utilization efficiencyβof enhanced electrokinetic remediation of Cd(Ⅱ)-contaminated soil by other enhanced methods ranged from 0.13 to 7.07.The three combined electrokinetic-adsorption remediation methods proposed in this study could effectively remove Cd(Ⅱ)from soil under a low voltage gradient and improve energy utilization efficiency.