Hydrodynamics and partitioning of selected heavy metals in surface and subsurface soil

Abstract

The transport of cadmium (Cd), lead (Pb), nickel (Ni) and zinc (Zn) in acid rain leachate, through porous surface soil and subsurface soils were investigated in column experiment. The granullometric and geotechnical properties of the different soil horizons were determined using the American Society for Testing and Materials (ASTM) methods. The physico-chemical properties and heavy metal concentrations were determined using standard methods. Surface and subsurface soil horizons, in replica of the profile pit dimension measured at Ilupeju village study site, were packed in 8 x 120 cm polypropylene plastic material. About 250 ml of contaminant solutions containing 100 mg l-1 each of Cd, Pb, Ni and Zn were leached by saturating the column with simulated acid rain until leachates were collected from each soil horizon outlets. The leachates were digested using standard methods and the levels of Cd, Pb, Ni and Zn were determined using flame atomic absorption spectrophotometer. The concentrations of Cd, Pb, Ni and Zn in the acid rain leachates of soils collected at the end of the top soil column were: 18.40±2.57, 0.5±0.12, 7.43±2.19 and 23.02±4.09 mg l-1 respectively. Significant (p<0.05) levels of Cd, Ni and Zn were attenuated in the fourth soil horizon, with leachate concentration decreasing from 9.60±1.58 to 1.03±0.45; 6.66±2.15 to 0.69±0.37 and 11.80±2.26 to 2.60±0.97 mg l-1 respectively, in leachates collected at the column end of the fifth horizon, while Pb concentrations was less than 0.10 mg l-1. The surface and subsurface soil column down to the fourth horizon demonstrated high metal sequestration capacity, hence there may be significant reduction or cessation in the transport of heavy metals to the deeper subsurface soil.