Soil washing technology for removing heavy metals from a contaminated soil: A case study

Abstract

Although EDTA solution is considered an efficient soil washing liquid for extraction of heavy metals from soils, its low biodegradability may alter soil properties and suppress plant growth. Alternatively, chlorides are safer and cheaper washing liquids than EDTA. To investigate the efficiency of chlorides versus EDTA in extracting heavy metals from contaminated soils, soil samples (pH 6.14±0.11) were collected from a local agriculture soil in Australia, artificially contaminated with either Pb, Cd or Cr at three different levels of 200, 400 and 600 mg kg-1, and then packed in capped plastic flasks. Batch washing techniques were followed with either EDTA or FeCl3 solutions (prepared at 4 different concentrations of 0.05, 0.1, 0.25 and 0.5 M), and soil suspensions were agitated for different time periods (from 5.0 min. to 60.0 min); afterward, the extraction efficiencies of the investigated metals were considered. The removal efficiency of Pb from the contaminated soil (200 mg Pb kg-1) after 5 minutes of EDTA application seemed to be relatively high (≈75%). Afterward this efficiency decreased gradually with time. The efficiencies of the extracted Cd and Cr by soil washing with EDTA increased significantly with increases in the agitating period. Generally, the extraction efficiencies by EDTA decreased noticeably with increasing levels of soil contamination. On the other hand, the efficiencies of Pb, Cd and Cr extractions were high – especially when increasing both the concentrations of applied FeCl3 solution and the time of agitation. The results also highlighted that soil washing with FeCl3 seemed to be more favourable over EDTA for rapid extraction of heavy metals from contaminated soils. The efficiencies of extracting heavy metals by soil washing with 0.5M FeCl3 for only one hour were 93.79±2.35%, 97.4±2.45% and 81.75±7.86% for Pb, Cd and Cr, respectively.