Comparison of the coagulation performance of tetravalent titanium and zirconium salts with alum

• ZrCl4 achieved better DOC removal near to its isoelectric point than alum and TiCl4.
• For TiCl4, DOC removal at pH 4.5 was dominantly due to adsorption-enmeshment mechanisms.
• ZrCl4 is more efficient for the removal of low molecular weight organics than alum.
• TiCl4 produced relatively larger, faster initial growth and heterogeneous size flocs.
• The composite of Ti and Zr salts with alum could be a good option for improved removal.

With occurrences of high concentrations of dissolved organic carbon (DOC) in drinking water supplies and consequential increased health risks from halogenated disinfection by-products following chlorine disinfection, there is need to develop improved cost effective methods that lead to low residuals of organics in drinking waters. In conventional treatment of drinking waters, alum and ferric based salts are generally used removing low to moderate percentages of the organics present. Where raw waters are high in DOC, the residuals may also then be at comparatively high levels. Performances of titanium tetrachloride (TiCl4) and zirconium tetrachloride (ZrCl4) for drinking water treatment were studied and compared with alum. Jar test experiments were performed at various coagulant doses and pH levels to determine the optimum conditions based on removal efficiencies of organic content (DOC), zeta potential for assessing the destabilization mechanism and the kinetics of coagulation. The Ti(IV) and Zr(IV)-based coagulants showed greater capacity for DOC removal at near their isoelectric points, with ZrCl4 providing the highest DOC removal. Negative zeta potentials of Ti flocs at pH 4.5, indicated that dominant destabilization of organics occurred by adsorption-enmeshment mechanisms, whereas, charge neutralization was the dominant mechanism at pH between 3 and 3.5. Fluorescence spectroscopy and HPSEC showed that ZrCl4 was more efficient for the removal of low to medium MW range organic compounds (<2000 Da) than TiCl4 or alum. TiCl4 produced relatively larger flocs with faster initial growth and with more heterogeneous sizes, than alum and ZrCl4. The results indicate that these Ti and Zr salts could be used as alternative coagulants in water treatment processes with some capacities exceeding that of alum, providing there are no health risks from their application.