Mobility and distribution of cadmium, nickel and zinc in contaminated soil profiles from Bangladesh

We investigated the mobility and distribution of cadmium (Cd), nickel (Ni) and zinc (Zn) in four contaminated soil profiles from Bangladesh. The sources of contamination of these profiles were tannery wastes, city sewage and the wastes of pharmaceutical and paper mill factories at different locations in Bangladesh. The samples were collected from the A-, B- and C-horizons of each profile: two sub-samples from the A-horizon at depths of 0-5 cm (A1) and >5 cm (A2), and one sample each from the B- and C-horizons. Soil samples were analysed for total metal content by dissolution in aqua regia followed by sequential extraction of the fractions based on their varying solubility. There were six operationally defined groups of extraction sequences: water soluble (F1), exchangeable (F2), carbonate (F3), oxide (F4), organic (F5) and residual (F6). The total concentration of Cd, Ni and Zn in the A-horizon (0-5 cm) ranged from 0.10 to 0.62, from 31 to 54 and from 85 to 838 mg kg-¹, respectively. In the B- and C-horizons, the concentrations of these metals decreased many fold, particularly in the city sewage profile where the decrease for Cd and Zn was approximately fourfold. The distribution of metals among the chemical fractions depended on their total concentrations. In the A1-horizon, Cd had the highest mobility factor [MF = (F1+F2+F3)/(F1+F2+F3+F4+F5+F6) x 100] at 41-43% and Ni had the lowest at 3-13%, while Zn, showed intermediate values at 8-25%. The MF decreased with depth in all soil profiles. Among the fractions, the residual fraction contained the lowest levels of Cd but the highest levels of Ni and Zn. This state affected their relative mobility and distribution in soil profiles. Of the soil profiles, the mobility of all metals was higher in the city sewage soil than in any of the other three soils investigated, suggesting that the former may create a health risk by contaminating agricultural products and ground water as it also contained higher amounts of Cd and Zn.