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

Abstract: 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 4.6×10-3, 1.4×10-2 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.