Optimized EDTA Washing Procedure to Decontaminate Heavy Metals from Soils in Iron and Steel Works Sites

Abstract

The removal of heavy metal ions from the soil of iron and steel work site with EDTA enhanced washing was investigated through a series of laboratory tests to examine the effect of EDTA concentration, washing time, washing cycles and physical sieving. The heavy metal species in the soil before and after EDTA washing were examined by the sequential extraction procedure . The results are based on comprehensive consideration of associated factors such as high heavy metal removal rates, low loss of soil mineral elements and proper pH value of washing solution, EDTA concentration was controlled around 0.075 mmol g(-1). Considering the heavy metal removal rates and its increasing extent, the washing time and cycles were kept at about 3 h and 3 cycles respectively. Screening out the soil with size of 1-2 mm from the buck soil reduced soil volume by 21.7%. However, the heavy metal removal rates were not significantly reduced. This made EDTA washing economically viable. Under the optimum conditions, a total of 91.7, 35.5 and 21.4 % of Zn, Cd and As, respectively, could be removed from the soil, mainly came from the Fe/Mn fraction (48-82 %). With the lower initial concentration in contaminated soil, Cd and As were more tightly bound to soil particles, which resulted in greatly lower removal rates of Cd and As than Zn. The operationally defined fraction results indicated that heavy metals in the soil after EDTA washing occurred mainly in residue bound fraction (62-68 %) and lowest in exchangable fraction (lower than 0.4 %), to explain that the heavy metal availability in the washed soil did not increase. The results implied that EDTA washing could be used to remediate heavy metal contaminated soil of the iron and steel work site. However, careful control of the process parameters of EDTA washing should be considered to minimize the mobility of the remaining heavy metals.