ABSTRACT
Twelve samples collected from 0-15 cm depths at different locations in Bangladesh were designated as contaminated, noncontaminated, and background soils to investigate the solid-phase speciation of Cd, Ni, and Zn with widely differing soil properties. Total Cd, Ni, and Zn in the soils ranged from 0.01-0.69, 21-52, and 32-939 mg kg1, respectively. Total contents of Ni except in two hilly soils and Zn in the city sewage (939 mg kg1) and pharmaceutical soils (162 mg kg1) exceeded the Dutch limit for clean soil. Sequential extraction was used to speciate Cd, Ni, and Zn from 12 Bangladeshi surface soils to assess metal mobility. The sequences of extractions were six operationally defined groups: water soluble (F1), exchangeable (F2), carbonate (F3), oxide (F4), organic (F5), and residual (F6). The contaminated, noncontaminated (control), and other background soils contained 42, 32, and 26% of Cd; 4, 1, and 1% of Ni; and 11, 3, and 2% of Zn in mobile (F1-F3) fractions, respectively. Higher proportions of metals in mobile fractions in the contaminated soils indicate higher mobility of anthropogenically added metals. Cadmium was found predominately associated with the oxide (44%) fraction irrespective of soil types. In contrast, higher percentages of Ni (83%) and Zn (72%) were found in the residual fraction. The relative extractability, expressed as the ratio of DTPA to aqua regia-extractable contents, was 29% for Cd, 2% for Ni, and 5% for Zn in an average of 12 soils. Significant correlations between mobile and immobile fractions of metals (p 0.01 and 0.05, respectively) with soil properties were found only with organic carbon. Total contents and mobile fractions of metals were much higher in the city sewage soil than in other soils, so this soil should be of greater concern for the contamination of agricultural products and groundwater.
