Enhanced Production of in Situ Keggin Al₁₃⁷⁺ Polymer by a Combined Fe-Al Coagulation Process for the Treatment of High Alkalinity Water

Treatment of high alkalinity water by coagulation is one of the challenges for drinking water treatment. The solution of acidifying the raw water or adding more coagulants would inevitably increase the corrosive tendency and treatment costs. In this study, a combined Fe-Al coagulation, during which a small amount of Fe was introduced prior to Al, promoted the coagulation efficiency of Al salts significantly. In contrast to the individual Al process, the combined Fe-Al coagulation accelerated the floc growth rate with the turbidity removal increased from 46% to 73% at the Fe:Al mole ratio of 1:10 and time interval of 10 s, and better tolerated the high basicity. The Keggin Al137+ polymer of the combined process was confirmed by the surface-enhanced Raman spectroscopy, X-ray photoelectron spectra, and 27Al nuclear magnetic resonance analyses, and its enhanced generation induced by the Fe addition was investigated by the Ferron assay. The favored local pH at the colloid-water interface governed by the hydrolysis of Fe was proposed to be responsible for the enhanced coagulation of Al salts. The potential applicability of the combined Fe-Al coagulation was further demonstrated by its desirable impurity removal efficiency for the treatment of a high alkaline/pH algae containing source water