The Research Of Migration And Transformation Of Chromium In Electrokinetic Remediation Of Contaminated Soil Assisted By CaAl-LDH

Chromium contamination is considered to be the priority heavy metal pollution.Ministry of Environmental Protection and Ministry of Land Resources jointly issued the “National Soil Pollution Survey” in 2014, it showed that chromium contamination exceeded the 1.1% point. Chromium is easier to migrate into the soil environment.Under the force of gravity, chromium will be brought into the groundwater and make a serious threat to the environment. Currently, single soil remediation technology was very difficult to promote in the country. The reasonable combination of various techniques and breakthrough in terms of soil remediation had become one of the hot research topic. Among them, the electrokinetic remediation technique combined with permeable reactive barrier(EK-PRB) showed great prospect.The remediation effect of EK-PRB coupling technique depends on the migration and transformation of chromium during the remediation process. This paper researched the influences of the PRB position, voltage gradient, moisture content and processing time to the migration and transformation of chromium. Through the study of migration and transformation rule of chromium in soil free water, the synergistic effect of EK-PRB coupling technique was explored. Utilized the non-equilibrium solute partition coefficient(Kd*) as reference for the judgment of contaminant migration, the distribution and transformation of Cr(VI) in soil and free water was discussed. Finally, the morphological changes of chromium after EK-PRB remediation were analyzed.The main conclusions are as follows:(1) The best removal efficiencies of Cr(VI) and TCr were 81.47% and 72.26%respectively after 120 h, which were achieved in EK-PRB remediation with the optimal parameters as follows: PRB position close to the anode, moisture content of30%, voltage gradient of 1 V/cm.(2) EK-PRB coupling technique could improve Cr(VI) dissolution from soil particles and efficiently promote the migration and removal of chromium in soil free water. During the remediation process of EK-PRB, Cr(VI) and TCr concentrations in soil free water at each sampling point significantly reduced with the increasing time.After 120 h treatment, both Cr(VI) and TCr concentrations in soil free water were below 5mg/L. The residual Cr(VI) content in soil free water and soil were 126.8 mg and 46.09 mg, respectively.(3) The non-equilibrium solute partition coefficient(Kd*) could effectively reflect the migration performance of Cr(VI). Cr(VI) mobility gradually increased from the cathode to the anode. And in each sampling area, the Cr(VI) mobility decreased with the processing time. However, when the Cr(VI) concentrations in soil and soil free water were too low, the Kd* value should be utilized with residual concentration of Cr(VI) as reference for the judgment of Cr(VI) mobility.(4) In the EK-PRB remediation process, part of the Cr(VI) was reduced to Cr(III). Under the condition of high voltage and high moisture, Cr(VI) were more easily converted into Cr(III). As the processing time increased, Cr(III) was given priority in the soil of EK-PRB device, fixed in the soil particles, and didn’t participate in the migration of chromium in free water. Under the condition of 30% moisture content, 1V/cm voltage gradient and 120 h processing time, Cr(III) concentration in the soil was controlled within 100 mg/kg.(5) Ca Al-Cl-LDH structure gradually disappeared during the remediation process, and was replaced by Ca Al-Cr O4-LDH generated. The formation of Cr-O vibration absorption peaks in FTIR spectra confirmed the immobilization of chromium in the LDH. It was determined by XPS analysis that only hexavalent chromium in the form of the acid radical(M-Cr O4) existed in the Ca Al-LDH after use,Cr(III) compound was not present.(6) After EK-PRB remediation, the residual state became the main speciation of chromium in soil samples near cathode and anode. The residue ratio increased11%~16% than the soil before remediation. Among them, the ratio of the residue state in S6 section was the highest of 46.97%. It could be considered that EK-PRB remediation promoted the stability of Cr(VI) contaminated soil.