Application of sludge dewatered products to soil and its effects on the leaching behaviour of heavy metals

The leaching behaviour of eight heavy metals from the dewatered products of sludge and lignite mixtures before and after application to the soil was investigated using batch tests. It was found that for most of the heavy metals the content in the mixtures fell within relevant EPA guidelines for biosolids application. Leaching tests showed that the leached fraction of most of the metals was low. The addition of the sludge-lignite mixtures to soils to increase the soil carbon content by 0.5%, 1% and 2% affected the leaching behaviour of some heavy metals possibly due to the changes in the conditions, such as pH and redox, of the mixtures. For arsenic and chromium, which are present in the environment as various species with different mobilities, the effect of the mixing on their leaching behaviours was the most significant. Cadmium, mercury and lead generally had low mobility. The lignite may have acted as an adsorbent which bonded the heavy metals, e.g. zinc and lead, and hence reduced their leachability. Together with the availability of nutrients in the sludge-lignite mixtures, the addition of sludge and lignite mixtures as a soil amendment is beneficial in improving soil characteristics and presents the potential to be applied to agricultural soils.