Enhanced removal of heavy metal ions bound to humic acid by polyelectrolyte flocculation

The enhanced removal of heavy metal ions from solution, such as Pb2+ and Zn2+, was studied by binding the ions to humic acid (HA) and then coagulating–flocculating with the cationic polyelectrolyte polydiallyldimethylammonium chloride (PolyDADMAC). The effect of the dosage of PolyDADMAC, the pH level and the concentrations of HA and metal ions were studied. Ultrafiltration was used to separate bound metal ions from free ions in solution. The removal of bound metal ions was found to increase with the extent of coagulation–flocculation of the HA by PolyDADMAC. The pH affects the removal efficiency of bound (complexed) metal, in so much as it affects the binding strength of metal ions to the HA. The removal efficiency of metal also increases with the initial concentration of HA. The effective coagulation–flocculation region of the HA by PolyDADMAC is affected by the initial concentration of the metal ions; an increase in the concentration leads to a decrease in the amount of PolyDADMAC required. Humic substances have the advantage of being naturally occurring, and the results indicate that such a complexation–flocculation process is of potential interest for the removal of heavy metals during water treatment.