Effects of earthworms on leachability of lead compounds in soil

Contamination of soils by metals and metalloids is an important environmental problem in many residential and industrial sites around the world. Lead is a common contaminant, which enters the soil through mining, industrial activities and waste disposal. A range of technologies can be used to remediate soil lead, however most remediation technologies adversely affect the environment and particularly soil biota. In this research, I assessed the effects of vermiremediation (the use of earthworms for remediation) as an environmentally friendly alternative for remediation of lead in soil. Earthworms were introduced to a sandy soil spiked with the common lead compounds PbCl2, PbCO3, PbO or PbS at the concentration of 1,000 mg (Pb).kg-1. Concentrations of lead in soil washing from contaminated soil were significantly lower (ANOVA, F=23.56, p<0.05) in soil containing earthworms than in soil without. Earthworms increased slightly the concentration of lead in pore water between soils with and without worms but these differences were not statistically significant (ANOVA, F=0.14, p>0.05), and there were no significant differences (ANOVA, F=0.38, p<0.05) in pore water concentrations between compounds. Earthworms accumulated 241, 276, 235 and 40 mg.kg-1 (dry weight of earthworms) of lead in their bodies, for PbO, PbCl2, PbCO3 and PbS respectively. The concentrations of Pb accumulated by earthworms in PbS soil were significantly (ANOVA, F=8.38, p<0.05) less than those in worms in other soils. Measurement of earthworm weight before and after the experiment showed that earthworms lost weight in all contaminated soils, except for those contaminated with PbS. Reasons for these patterns, and their implications for the use of earthworms for remediation are discussed.