柠檬酸对重金属复合污染土壤的浸提效果研究

以浙江富阳某炼铜厂附近受Cd、Cu、Pb和Zn复合污染的土壤为研究对象,采用振荡、静置和超声3种不同浸提方式,通过研究柠檬酸在不同浸提方式下对重金属的去除效果,筛选出最佳柠檬酸浓度、处理时间及功率,并采用Tessier连续提取法分析浸提前后4种重金属形态变化.结果表明,柠檬酸对4种重金属均具有较好的去除效果,其中对Zn的去除效果最好,Cd的去除效果最差,而Cu和Pb的去除效果相当.Pb和Zn的去除率均随着柠檬酸浓度的增加而增加,但增幅随之减小,Cu的去除率则先增加后基本保持不变,而Cd的去除率变化不明显.综合考虑成本和浸提效果后,选用0.2 mol·L-1柠檬酸浸提2次为最优条件,最佳振荡时间和静置时间分别为4 h和24 h,最佳超声时间和功率分别为30 min和500 W.柠檬酸能够在很大程度上改变土壤中重金属的形态分布,主要去除以可交换态和碳酸盐结合态形式存在的Cd及以碳酸盐结合态和铁锰氧化物结合态形式存在的Cu、Pb和Zn.通过对比3种浸提方式,在同等去除效果的前提下,静置浸提相比振荡浸提需要更长的时间,而将超声波应用于重金属污染土壤淋洗,所需时间较少,因此,超声波辅助浸提土壤中重金属是可行的,具有良好的应用前景.

Removal of heavy metals from co-contaminated soil by washing with citric acid

Soil contaminated with heavy metals Cd, Cu, Pb and Zn was collected from the surroundings of a copper smelting plant in Fuyang, Zhejiang Province, China. Batch extractions, including oscillation extraction, static equilibrium extraction and ultrasonic vibration extraction, were conducted to evaluate the effectiveness of citric acid in 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. Washing conditions, including citric acid concentration, contact time and ultrasonic power, were tested and optimized. The results showed that 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. It was also noteworthy that an increasing concentration of citric acid resulted in a corresponding increase in the quantity of Pb and Zn extracted, while 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. Based on the cost and benefit analysis, two separate extractions with using 0.2 mol · L^-1 of citric acid were found to be the optimal procedure. The optimal contact time was 4 hours for the oscillation extraction, and 24 hours for the static equilibrium extraction. A combination of a contact time of 30 min and an ultrasonic power of 500 W achieved the best result in the ultrasonic vibration extraction. Citric acid treatment can 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. The static equilibrium extraction requires more contact time than the oscillation extraction to achieve the same removal rate for heavy metals. Ultrasonic vibration extraction requires less contact time than oscillation and static balance extractions in washing the soil contaminated with heavy metals. Therefore, ultrasonic assisted soil washing has potential for a practical application.