The characteristics of flocs and zeta potential in nano-TiO2 system under different coagulation conditions

Three coagulants were used to investigate the characteristics of flocs formed in TiO2 system, and the effects of humic acid (HA) on the coagulation process were also investigated. The results indicated that flocs could not regrow to former size in all coagulation conditions. The floc sizes after re-growth decreased with increasing the breakage period. Polyacrylamide (PAM) could significantly increase the floc size, and the fractal dimension decreased with increasing the concentration of N-PAM. Zeta potentials of flocs formed without N-PAM as coagulant decreased when the flocs were broken at 400 rpm. Zeta potential using N-PAM as flocculant did not significantly change during breakage period, and this maybe caused by the adsorption of N-PAM molecules on new exposed TiO2 surfaces. The results of coagulation process using nano-Al2O3 + AS as coagulant showed that nano-Al2O3 particles could increase the floc size and fractal dimension in TiO2 system, and the recovery factor was larger than only using AS. The further addition of Ca2+ and Al2O3 particles could enhance the floc size after regrowth. In the enhanced coagulation process (Al 1.0 mmol/L), PACl could produce larger flocs due to electrostatic patch effects.