Effect of Vermicompost Rates on Mobility of Cd and Pb in Soil Columns

Soil organic matter could be effective on mobility of heavy metals and underground water pollution. This study was conducted in order to evaluate the effect of different levels of vermicompost on the mobility of Pb and Cd in the soil columns. Three levels of vermicompost comprising  0% as the control (V₁), 2% (V₂), and 4 wt% (V₃) were mixed with soil in Polyethylene columns filled with a clay loam soil and irrigated with Cd- and Pb-enriched (20 and 40 mg L^-1, respectively) waste water during 8 periods of 10 days. Drainage waters were collected at the end of each period and characteristics such as pH, electrical conductivity (EC), oxidizable organic carbon, Pb, and Cd concentration were measured. At the end of the 8^th period, soil samples of the depths of 0-20 and 40-60 cm were collected and analyzed. The results showed that the maximum concentration of Pb and Cd, OC, and EC of the drainage water was in the first study period and this trend was reduced in the subsequent periods. The reverse trend was observed for the pH. The highest concentration of Pb and Cd was measured at a depth of 0-20 cm in soil column due to the low mobility of these elements in the soil profile. In both depths (surface and deep), more concentrations of Pb and Cd were adsorbed by soil particles in the control treatment, suggesting the positive role of vermicompost in enhancement of the mobility of these elements and more removal from soil columns. Higher ratio of Pb concentration in surface to deep layers showed lower mobility of this element compared with Cd. Therefore, higher organic matter contents in soils enhance heavy metals mobility, particularly Cd, toward lower soil layers.