Remediation of Soils Polluted by Heavy Metals using Salts of Organic Acids and Chelating Agents

Abstract

Various weak organic acids and/or their salts were tested for the remediation of a loam and a sandy clay loam naturally polluted by heavy metals for over three years against two strong synthetic chelating agents (EDTA and DTPA). Among 7 weak organic acids and/or their salts, citrate, tartarate and oxalate were found to effectively remove Cd, Cu, Pb and Zn from the two soils in double extractions, at a wide range of pH. Citrate removed 80 to 99.9% of all four metals within 24 h at pH from 2.3 to 7.5. Tartarate removed 84 to 99.9% of all metals from both soils within 24 h at pH from 2.1 to 6.7. Oxalate alone is not effective in removing metals particularly Pb but with ammonium citrate (1:1 ratio) removed effectively 82, 70, 99.9, and 99.9% for Cd, Pb, Cu and Zn, respectively at pH from 2.6 to 5.8. Zinc met the Quebec A level soil clean-up criteria after two extractions whereas Cu and Pb reached the B level. Probably, two more extractions with these weak organic acids and/or their salts are required to meet the A level for the soil clean-up criteria for Cd, Cu and Pb. EDTA and DTPA removed 75 to 99.9% of the heavy metals within 24 h at pH from 3.5 to 9.0 for both soils. Citrate and tartarate were found to remove heavy metals from two contaminated soils, as effectively as EDTA and DTPA. But compared to EDTA and DTPA they removed 80% less macronutrients from the soil and improve its structure. Weak organic acids and/or their salts therefore can be used as an environmentally friendly remediation technique.