Effects of different coagulants in treatment of TiO2–humic acid (HA) water and the aggregate characterization in different coagulation conditions

Effects of coagulant dosage and solution pH on the coagulation behavior of nano-Al13 were studied with respect to the treatment of TiO2–humic acid water. Polyaluminum chloride (PACl) or AS (Al2(SO4)3) and Al2O3 (100 mg/L) + AS were used to compare with nano-Al13. The floc size, strength factor and fractal dimension under different coagulation conditions were investigated. The results indicated that hydrophobic NOM with larger molecular weight could be easily removed by the four coagulants. The strength factor increased with increasing the coagulant dosage for AS under all coagulation pH values. The results using PACl and nano-Al13 as coagulants indicated that the coagulant dosage had no significant effects on the strength factors. The addition of Al2O3 could significantly increase the floc size, and the fractal dimension after breakage was larger than using AS. Nano-Al13 had highest charge neutralization ability, so when the dosage was high, the removal efficiency decreased due to charge reversal. The results of the coagulation performances under three coagulation pH values indicated that nano-Al13 could re-stabilize the particles under pH 6.0 at 0.2 mmol/L calculated as Al. The recovery factors using Al2O3 + AS as coagulant were smaller than using AS, and this may be caused by the adsorption of HA molecules on nano-Al2O3 particles. The HA molecules in the solution could bridge the nano-particles together through bridging effects. The Al2O3 particles had positive charges on the surface under pH 6.0, and it could bridge the particles together.