Mobility and bioavailability of selected heavy metals in coal ash- and sewage sludge-amended acid soil

A sequential extraction procedure has been used to study the changes in the distribution and mobility of Cd, Cr, Cu, Ni, Pb and Zn in an acid lateritic soil amended with alkaline coal ash and neutral sludge individually and with their mixture of equal proportions at 25, 50 and 75 Mg/ha application rates and grown in a crop with peanuts. A separate set of experiments consisting of the same treatments was repeated with the addition of lime at 2 Mg/ha. Increases in total heavy-metal levels with application of various amendments were mainly associated with increases in the insoluble and less mobile forms of metals except for Cd, which showed an increase in its exchangeable form. An increase in pH of the amended soil restricted the metal mobility in their labile forms and was more pronounced after liming the treatments. Positive yield responses were observed in the amended soil, the magnitude being higher in only sludge-applied treatments. The vegetative plant parts showed maximum accumulation of metals indicating a physiological barrier in the transfer of metals from the root to the kernel. Linear relationships of total concentrations of heavy metals in soil with that in the crop were observed. Lime addition to the treatments further reduced the transfer and accumulation of metals from the soil to the plant, even though the relationship remained linear.