Study On The Removal Of Heavy Metals From Co-contaminated Soil By Washing With Citric Acid And EDTA

Soil contamination with heavy metals derived from sewage sludge, mining, fertilizer, municipal waste, smelting and atmospheric precipitation has been of great concern. Heavy metals do not undergo microbial degradation, and persist for a long time after their introduction to soils. Heavy metal pollutionis often an irreversible process. It not only degrades the quality of the atmosphere, water and food crops, but also threatens the health of human beings through the food chain. Remediationof the soil contaminated with heavy metals has become an important research topic in the field of environmental protection. As an effective technology for rapid remediation of soils contaminated with heavy metals, soil washing has increasingly received attention in recent years.The current study aims to provide guidance for the research and development of soil washing technology. Soil contaminated with heavy metals of Cd, Cu, Pb and Zn was collected from the surroundings of a copper smelting plant in Fuyang, Zhejiang Province, China. Batch extractions, including conventional mechanical shaking(CMS), static equilibrium extraction(SEE) andultrasound-assisted extraction(UAE), were conducted to evaluate the effectiveness of citric acid and EDTA solution for removing heavy metals from the soil. Changes in fractions of Cd, Cu, Pb and Zn in the soil were analyzed using the Tessier sequential extraction procedures before and after washing by citric acid. Extraction conditions, including the eluant concentrations, contact time and ultrasonic power, were tested and optimized. The main results are as following.1. Citric acid can greatly enhance the removal of Cd, Cu, Pb and Zn from the contaminated soil. The citric acid solution was the most effective in removing Zn, and the least effective in removing Cd. However, Cd removal rate was higher than that of Pb when soil was extracted with low concentrations of citric acid. The removal rate of Pb and Zn increased with the increase of citric acid concentration, regardless of the extraction methods. The removal rate of Cu increased initially and then reached a plateau with the increase of citric acid concentration. The Cd removal seemed to be less dependent on citric acid concentration.2. Citric acid treatment can greatly influence the distribution of heavy metal fractions in the tested soil. The exchangeable and carbonate bound fractions of Cd, and carbonate bound and Fe-Mn oxide bound fractions of Cu, Pb and Zn were the main metal fractions that were washed off from the soil.3. Similar to citric acid, EDTA solution can also enhance the removal of Cd, Cu, Pb and Zn from the contaminated soil. The EDTA solution was more effective in removing Pb and Cu than in removing Cd and Zn. Nearly all of the soil Pb was removed. The higher concentrations of EDTA resulted in a relative lower removal efficiency of Cu, Pb and Zn. Incomplete extractions were observed even using the highest EDTA concentration was tried. However, the Cd removals seem to be less dependent on the concentration of EDTA in CMS.4. When citric acid and EDTA were used as eluants in the extraction experiments, the optimal contact time was 4 hours in the CMS, and 24 hours in the SEE. A combination of a contact time of 30 min and an ultrasonic power of 500 W achieved the best result in the UAE.Based on the cost and benefit analysis, two separate extractions using 0.2 mol·L-1 of citric acid were found to be the optimal procedure for all extraction approaches tested. For EDTA, the highest removal efficiencies of tested heavy metals were obtained with the concentration of 0.03 mol·L-1 in the CMS and UAE, while 0.07 mol·L-1 was needed in the SEE. When the ultrasonic power was applied, UAE requires less contact timethan CMS and SEE in washing the soil. Therefore, ultrasound-assisted extraction provides an effective alternative for extracting heavy metals from soil, and has potential for a practical application.