Humic Acid Removal from Water with PAC-Al30: Effect of Calcium and Kaolin and the Action Mechanisms

Polyaluminum chloride with a dominant species of Al-30 (PAC-Al-30 ) was prepared in laboratory and used for humic acid (HA) removal from water. The action properties and mechanisms of PAC-Al-30 HA, calcium, and kaolin were tested and discussed. The results showed that the existence of calcium or kaolin contributed to the HA removal when the PAC-Al-30 dosage was deficient and had no obvious effect when the amount of PAC-Al-30 was sufficient. When the PAC-Al-30 dosage was 0.01 and 0.02 mmol/L, the HA removal rate was increased by 66.59 and 42.20%, respectively, with a calcium concentration of 2.0 mmol/L, or increased by 53.31 and 40.92%, respectively, with the kaolin particle concentration of 150 mg/L. Calcium could compress the double electrical layers or complex with HA to neutralize a part of the surface negative charge of HA, but could not make the water system reach its isoelectric point. The mechanisms of calcium and kaolin's promoting coagulation effect were adsorption neutralization and collision aggregation respectively, but these actions were much weaker than that of PAC-Al-30 with HA. The adsorption neutralization capacity of PAC-Al-30 was calculated to be nearly 60 times than that of calcium, and the higher gamma value of calcium modified by the Sips equation may indicate that the adsorption or neutralization sites of calcium on HA were pickier than PAC-Al-30.