Metal leachability from sewage sludge-amended Thai soils

Determining mobility and availability of metals in sewage sludge amended soil is an environmental concern. Potential leachability and bioavailability of metals following sludge applied to agricultural soil was evaluated. Geochemical forms of metal occurring in sludge-amended soil were subjected to fractionation for understanding heavy metal transformation and remobilization in sludge-amended soil. Metal leachability was determined using reconstructed soil profiles where dewatered sewage sludge was incorporated into the surface 0-10cm of soil. Two-sludge application rates; 150 and 300 kgN/ha, equivalent to sludge at 4 and 8 ton/ha, were applied to soil columns representing typical agricultural clay soils of Thailand (Rangsit acid sulfate soil). The soil columns were leached with 321 equivalent to 600 mm of surface water using different leachants (distilled water pH 6, distilled water adjusted to pH 3 and rainwater pH 5). Among metals measurement, results showed Mn leachability from sludge-amended Rangsit soil were high at both sludge application rates (18-29% of total Mn applied). The leachability of other metals was less than 2.5 and 7.2% following application of 150 and 300 kgN/ha of sludge, respectively. Results from the experiments indicated that the leachant at pH 3 had the most effect on potential leachability of Cu, Zn, Cd, and Ni, except Fe and Mn, at low sludge application rate. Whereas, only the leachability of two metals was influenced by the lowest pH (pH 3) when sludge applied was increased. Besides pH of leachant, it appeared that leachability of elements from the soil column depended on rate of sludge applied, the particular metal, and metal form or fraction. The soil studied had organic matter, CEC, pH, clay content, etc., that resulted in high buffering capacity, which favors metal retention. Less than 0.5 and 1.8% of the added Cu and Zn applied at the 150 and 300 kgN/ha application rates, respectively, were detected in the leachate. Approximately 100% of the added Cu and Zn in the sludge remained in the surface 10 cm at each sludge application rate. Sequential extraction of sludge-amended soil following leaching (at the high sludge application rate) revealed that most of the Cu remained in the surface sludge-amended soil layer (0.2 cm depth) in the form of organic and residual Cu fractions that are not easily mobilized. An exchangeable fraction of Zn increased, approximately representing 60% total Zn applied in sludge-soil layer as compared with its native soil Zn fractions. These results demonstrate that Zn availability in the soil would increase as a result of sludge application. However, the total Zn in the leachate is safe for agricultural use, because it represents an amount of less than 2% of total Zn applied.