Effect of temperature and ageing on sewage sludge decomposition, organic acids transformation and cadmium release

Sewage sludge is being heaped in congested urban cities, and the safe disposal of this waste on to agricultural soils is becoming a serious environmental concern around the world. The present incubation study of 135 days duration aims at evaluating the effect of ageing and temperature on the release of cadmium (Cd) from the sewage sludge. The study was carried out at different temperatures (22 and 45°C) to estimate Cd availability and organic matter (OM) degradation in soils receiving sewage sludge at 0, 15 and 30 g kg^-1 soil. Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed that sludge addition and high temperature (45°C) caused obvious changes in fulvic acid spectra mainly at 1050-1250 cm^-1 region. While humic acid spectra showed more humification and aromatic character with ageing, that are chemically and biologically more stable for Cd retention. Sequential extraction of Cd revealed a significant decrease in OM bound and reducible Cd fractions at the higher temperature (45°C). While residual and exchangeable Cd fractions increased with ageing, indicating that high temperature had a major influence on the availability and redistribution of different Cd forms. It is proposed that regular monitoring of sewage sludge amended soils is required for better assessment of Cd availability and OM contents in hot and dry regions.