Effect of pH on metal coagulation with alum and the ability of the metals to leach out from synthetic and natural alum sludge was evaluated. The potential of pH on coagulation was studied using cadmium, chromium, copper, iron, manganese, nickel, lead and zinc with alum solution at pH 2, 4, 6, 7, 8, 10 and 12. Leachability of metals from synthetic alum sludge (SAS) and natural alum sludge (NAS) were investigated using the extraction test. The NAS was obtained from a water treatment plant in Malaysia. Leaching eluent consisted of distilled water, 0.1 M solution of hydrochloric acid, acetic acid, ammonium hydroxide and sodium hydroxide. The results showed that more than 90% of the metals were effectively removed using alum in the pH range of 8-12. All the metals were found to leached out easily from the SAS in all eluents used except for Mn and Ni which leached out in base eluents suggesting that the binding within in the prepared SAS is weak and easily dissolved in acidic medium. For NAS, the level of leached metal was found to be below 5 ppm, except for Fe due to their presence in alum and soil colloids during the coagulation process. The experimental results showed that metals found in NAS did not leach out easily, thus it can be considered safe for reusing purposes
