Extraction of Heavy Metals from Water-Stable Soil Aggregates Using EDTA

Abstract

Surface soil samples (0-15cm) were collected from a lead/zinc tailing site in Nanjing, China, and separated into four size fractions of water-stable aggregates. Metal contents in the bulk soil were 7479±370 mg·kg-1 for Zn, 4189±82 mg·kg-1 for Pb, 58.0±0.4 mg·kg-1 for Cd and 544±22mg·kg-1 for Cu, respectively. Contents of Zn, Pb, Cd and Cu in 0.02-0.002mm fractions were lower than them in other size fractions. Extraction of heavy metals from each size fractions of aggregates as function of extraction time and pH value of EDTA was performed. Also, the effects of the extraction process on metal fractions in the samples were investigated. The molar ratio of EDTA/total heavy metals (Zn+Pb+Cu+Cd) was adopted as 1 for all extraction experiments. The extraction kinetics of the four metals in each fraction could be well fitted with Elovich formula. Pb had the highest extraction efficiency at time 24h, 82.7% (2- 0.2mm), 73.6% (0.2-0.02mm), 75.0% (0.02-0.002), and 72.2% (<0.002mm), among the four tested metals. The highest desorption rate for four metals in different size fractions was Zn in <0.002mm fraction and Pb, Cd, and Cu in 2-0.2mm fraction. Extraction of the tested metals negatively depended on the original pH value of extraction solution and the relationship could be described linearly. Exchangeable/acid soluble Zn, exchangeable/acid soluble and reducible fraction of Pb, Cd and Cu were influenced greatly by the extraction using EDTA.