Zeta potential distributions of model waters containing silica, humic acid, and differing ionic composition were evaluated. These distributions showed that under high ionic strength and in the presence of divalent cations, positive colloids exist in suspension with neutral and negative colloids. Positive colloids do not form at lower ionic strength or when polyvalent cations are absent. The existence of the positive colloids in suspension with the negative colloids suggests that, while electrodynamically unstable, some stabilizing factor is preventing coagulation. This stabilizing factor is likely adsorbed humic acid causing steric hindrance. Further tests indicated that under the conditions that lead to the existence of positive colloids, removal of humic acid by ferric chloride coagulation is diminished as compared to coagulation in lower ionic strength water or water containing just sodium chloride as the ionic matrix. As positively calcium–humic complexes can increase fouling of desalting membranes through the formation of densely packed fouling layers, understanding the mechanisms of coagulation in high ionic strength waters is crucial to improving performance of desalination operations.

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