Electrokinetic Remediation Of Soil Contaminated By Heavy Metals And Its Mechanism

Electrokinetic remediation?EKR?,as a kind of technology that can separate and remove heavy metals from soil,has always been of concern.Laboratory researches on the technology has been widely done in the world,but great difficulties still exist for its engineering application.This paper is based on the problems faced in engineering application,with focus on process and mechanism of electrode corrosion and salt crystallization,thermal effects,energy allocation and electrolyte enhanced remediation characteristics,its advancements and deeper research.The main conclusions are as stated below:?1?Under the action of an electric field,Ca in the test soil would move to the cathode,and crystals would generate in alkaline condition,resulting in the degradation of the electrode conductivity.The adsorption amount on the electrode may be related to current,voltage gradient,soil pH,treatment time and properties of working fluid.The concentrations distribution of Pb/Cu appeared to be related to the distribution of soil pH in the cell,which might be associated with the desorption and hydroxide precipitation of Pb/Cu.Polyaspartic acid?PASP?obviously inhibited the corrosion of stainless steel electrode.Citric acid?CA?and PASP could clearly destroy the formation of CaCO₃ crystal,while barely effectively disrupting the formation of Ca?OH?₂ crystal.Both CA and PASP could promote the removal of Pb[?cA=?32.52±2.57?%;?PASP=?21.91 ±4.58?%],but the influence of PASP on the removal of Cu was not obvious,and the effect of CA was very significant[?=?45.81±1.85?%].?2?There was a positive correlation between the change of temperature in each region of soil cell and the electric current,and exhibited an obvious differential distribution in the process of EKR for heavy metal contaminated soil.The temperature became higher in the soil at/near the cathode as opposed to at/near the anode,which may be related to differential change of soil properties?e.g.pH,MC?and formation of heavy metal hydroxide precipitation.In the EKR,the useless power is mainly caused by the thermal effect,transport of non-target materials and reaction of water electrolysis.At the same time,in order to compare the effective energy and the optimize degree of different electrokinetic remediation processes,this paper designs an optimized parameter??_?to evaluate experimental reault.It was found that proper soil acidification(?_pH=6.02= 33.08±3.92>?_pH=8.24 =3.75±1.33),proper increase of voltage gradient(?_2V·cm^-1=33.08 ± 3.92>?_V.cm^-1 =13.85 ± 1.98)and addition of suitable electrolyte(?_CA=41.56±0.69>?_DW=33.08±3.92)could be beneficial to obtain higher energy consumption ratio and improve energy utilization efficiency for target pollutants?Pb/Cu?.?3?Compared with deionized water?DW?,using CA in the contaminated soil was found to effectively improve the removal efficiencies of both the total Cr[?=?49.54±5.09?%]and Cr⁶⁺[?=?94.27±0.51?%].However,using PASP only improved the Cr⁶⁺[?=?93.26±0.49?%]removal efficiency,which was mainly because the CA and PASP could restore Cr⁶⁺ to Cr3+,and the a-OH and-COOH groups could play a key role in this process.The relatively lower removal efficiency of the total Cr with PASP was partially due to the transformation of the exchangeable portion of Cr into carbon-bounded Cr which has a lower migration rate than that of the exchangeable portion.Additional energy consumption was increased in the EK remediation after adding CA?0.1 M?or PASP?1%?to reach the same removal rates of Cr.Consequently,utilization efficiency of the accumulated electrical energy consumption decreased.