Comment on “the interaction of humic substances with cationic polyelectrolytes”

Abstract

Kam and Gregory (Water Research 35(2001) 3557– 3566) reported on the removal of humic substances (HS) in the coagulation and flocculation process using cationic polyelectrolytes. The investigation was devoted to the dominant mechanism of HS removal in the process. The HS elimination was compared in a Jar test performed with cationic polymers of different charge and molecular weight under constant pH of 7. The experimental results showed that charge neutralization was a significant factor for the removal of aquatic HS in the coagulation/flocculation process, monitored by the colloid titration and the streaming current procedures, and indicated that a bridging mechanism was unlikely to play a major role in this process. The authors concluded that the key mechanism of charge neutrality was applied to the HS removal in the coagulation process of metal coagulants used, as well as cationic polymers. However, the former is questionable. In coagulation using metal coagulants, three kinds of mechanisms were commonly referred for the HS removal: (1) charge neutralization, (2) complexation, and (3) adsorption [1,2]. A dominant mechanism depends on the pH of a solution. Gregor et al. [2] noted that cationic Al ions formed complexes combined with acidic functional groups (carboxylic and phenolic groups) of HS in the pH condition below 5.5, and that the function (defined as complexation) prevalently attributed to the HS reduction under the pH region. The complexation was differentiated from charge neutralization in that the complex would be precipitated due to size increase rather than charge neutrality [2]. On the other hand, Bell-Ajy et al. [3] recently reported that at low pH conditions insoluble complexes could be established through charge neutralization between negative HS and cationic metal hydrolysis products. It is likely that at low pH values the key mechanism of the HS removal is not clarified yet in the coagulation process. It was reported that the adsorption of humic substances to alum flocs played a major role in the HS removal under pH 6–7 favoring Al hydroxide precipitation [4,5]. The adsorption of HS to Al hydroxide occurs through surface complexation or ligand exchange [6]. Here, the surface complexation means interactions between functional groups of HS and surface hydroxyl on Al hydroxide [7,8]. Bose and Reckhow [9] also suggested that the adsorption of HS to Al hydroxide floc was a prevalent process in removing aquatic HS. In this paper, it was concluded that charge neutralization was the key mechanism in the coagulation process using cationic polymers, and the phenomenon extended to the HS removal in the coagulation of metal salts employed without experimental validation. However, it has been demonstrated that the dominant mechanism for the HS removal seriously varies depending on the pH of the solution in the coagulation/flocculation process when metal salts are used as coagulants. It is likely that a supplementary experiment is required to evaluate the HS removal mechanism in the coagulation using metal coagulants.