乙二胺四乙酸在重金属污染土壤修复过程的降解及残留

Degradation and residue of EDTA used for soil repair in heavy metal-contaminated soil

为分析乙二胺四乙酸(ethylenediaminetetraacetic acid,EDTA)在修复重金属污染土壤中的环境风险,通过田间调查和培养试验研究EDTA在不同重金属污染土壤中的降解及其残留。田间调查结果表明,乐昌试验田EDTA施用6 a后,表层土壤及深层土壤中均没有检出EDTA残留。佛冈试验田在施用EDTA4个月后,表层土壤EDTA残留量为0.039~0.056 mmol/kg,仅为施入量的2%~5%,施用1a后土壤中未检测到EDTA。翁源试验田在EDTA施用45 d后,表层土壤中EDTA残留量约为施用量的一半,1 a后残留量为施入量的2,6%,深层土壤监测到EDTA残留,但地下水中并没有检测到EDTA,另外地下水中重金属含量并没有升高。因此,深层土壤对离子态和螯合态重金属具有较强的固定能力,可保护地下水免遭重金属的污染。培养试验结果表明,EDTA在土壤中降解遵循一级动力学方程,EDTA在赤红壤、褐土和重金属污染土壤中的降解速率常数分别为4.6×10^(-3)、1.4×10^(-2)和5.8×10^(-3),其降解的半衰期分别为71、25和53 d。EDTA在土壤中降解半衰期与土壤有机质含量和土壤阳离子交换量(cation exchange capacity)之间表现较好的相关性。微生物对EDTA在土壤中的降解具有显著的影响。总之,EDTA可在土壤中降解,建议在中国重金属污染土壤修复过程中可采用EDTA强化修复技术,EDTA的环境风险是可控的。

According to the bulletin of the national soil pollution reported by Ministry of Environmental Protection and Ministry of Land and Resources of the Peoples Republic of China, the total above standard rate is 16.1% in national soil, and above standard soils of Cd, Zn, Pb and Cu are 7.0%, 0.9%, 1.5% and 2.1%, respectively. Soil washing with chelating agents and phytoextraction by chelator-enhanced is potentially useful technique for remediating the heavy metal-contaminated soils. EDTA is the most frequently cited chelating agent in these techniques because of its strong chelating ability for different heavy metals. However, the slow degradation rate and persistence of residual EDTA in soil potentially increases the metal leaching risk which may cause groundwater contamination. But the environmental risk of EDTA reported in literature is from pot and column leaching experiments in laboratory scale. In order to understand the environmental risk of residual EDTA in the remediation of metal-contaminated soil, the field investigation and the incubation experiments were conducted to investigate the residue and degradation of EDTA in soil. The results of Lechang field investigation revealed that EDTA residue was not detected in the topsoil and deep soil after EDTA applied for 6 years. In Fogang field, the concentration of EDTA in soil was 0.039 - 0.056 mmol/kg soil, which was 2% to5% of the applied amount in the 4th month after application. However, the EDTA was not detected in soil after 1 year. In Wengyuan field, the concentration of EDTA in topsoil was approximately 50% of added amount （3.3 mmol/kg soil） after 45 d of EDTA application, while it was only 2.6% of added amount after application in one year. EDTA residue was detected in the deep soil. However, the EDTA was not detected in groundwater. In addition, the concentration of heavy metals in groundwater was not increased after EDTA application. The deep soils have considerable fixation capacity for the heavy metal-chelator complexes, which help preventing the metal-chelator complexes from leaching down to groundwater. Incubation experiments were carried out to evaluate the degradation of EDTA in different soils. The air-dried soil （2 kg with 〈 5 mm particle size） was placed in plastic pots with the rate of 10 mmol/kg soil EDTA addition. Soils were incubated at room temperature and about 60%-70% soil water-holding capacity. Then soil samples were taken in 0, 3, 10, 20, 30, 50 and 72 d after incubation. The results indicated that degradation of EDTA in soils followed the first-order kinetic equation. Degradation rate constant of EDTA in latosolic red soil, cinnamon soil and metal-contaminated soil was 4.6x10-3, 1.4x10-2 and 5.8~10-3, and the half-life was 71, 25 and 53 d for each soil, respectively. The half-life of EDTA had a good correlation with the organic matter content and CEC in soil. Microorganisms had a marked influence on the degradation of EDTA in soils. The finding suggested that EDTA-enhanced remediation technology can be used on remediating heavy metals-contaminated soil, but the added amount should be controlled. As such, the environmental risk of EDTA is minimum.