Spectroscopic study of aluminium speciation in removing humic substances by Al coagulation

Removal of humic substances (HS) isolated from swamp water, surface soil, peat and brown coal by aluminium (Al) coagulation under different conditions was studied using the jar test method and spectroscopic techniques. At lower pHs<4.5, humic substances formed soluble complexes with Al3+ and the amounts of these HS–Al complexes increased with increasing concentration of the humic substances. Maximum removal of the humic substances was found to occur in the pH range from 5.0 to 7.0, due to the adsorption of humic substances on the surface of Al(OH)3(s) crystal. Sequential extraction of the humic substances adsorbed on Al(OH)3(s) using the Soxhlet technique with different solvents showed that the adsorption forces involved were great. The effects of humic substances on the Al speciation are discussed and a model is proposed to explain the mechanism for the removal of humic substances during the Al coagulation process.