Abstract: This paper studied the effects of applying citric acid, oxalic acid and acetic acid on the removal of heavy metals Cd, Pb, Cu and Zn from sewage sludge, and the form transformation and bioavailability of these heavy metals before and after the application of tested organic acids. The results showed that the removal rates of tested heavy metals except Cu increased with increasing acid concentration and action time. Among the tested organic acids, citric acid was more effective in removing heavy metals from sewage sludge, with the removal rate of 52.0% for Pb and 74.2% for Zn after a 7-hour action, and 76.0% for Pb and 92.5% for Zn after 24-hour action. The removal rates of the heavy metals by oxalic acid and acetic acid were relatively low. The Pb and Zn removed by citric acid were mainly in stable form, resulting in a relative increase of their unstable and exchangeable forms in the sludge. Though the removal rate of Cu and Cd was lower, their exchangeable forms still had somewhat increase after the action of tested organic acids.

Key words: landscape ecological pattern, optimization strategy, urban forest, gradient analysis