Sulfate ion in raw water affects performance of high-basicity PACT coagulants produced by Al(OH)3 dissolution and base-titration: Removal of SPAC particles by coagulation-flocculation, sedimentation, and sand filtration

Many PACT (poly-aluminum chloride) coagulants with different characteristics have been trial-produced in laboratories and commercially produced, but the selection of a proper PACT still requires empirical information and field testing. Even PACls with the same property sometimes show different coagulation performances. In this study, we compared PACls produced by AICl(3)-titration and Al(OH)(3)-dissolution on their performance during coagulation-flocculation, sedimentation, and sand filtration (CSF) processes. The removal targets were particles of superfine powdered activated carbon (SPAC), which are used for efficient adsorptive removal of micropollutants, but strict removal of SPAC is required because of the high risk of their leakage after CSF. PACIs of high-basicity produced by AICl(3) -titration and Al(OH)(3)-dissolution were the same in terms of the ferron assay and colloid charge, but their performance in CSF were completely different. High-basicity Al(OH)(3) -dissolution PACls formed large floc particles and yielded very few remaining SPAC particles in the filtrate, whereas high-basicity AICl(3)-titration PACls did not form large floc particles. High-basicity PACls produced by Al(OH)(3)-dissolution were superior to low-basicity PACT in lowering remaining SPAC particles by the same method because of their high charge neutralization capacity, although their floc formation ability was similar or slightly inferior. However, highbasicity Al(OH)(3)-dissolution PACT was inferior when the sulfate ion concentration in the raw water was low. Sulfate ions were required in the raw water for high-basicity PACls to be effective in floc formation. In particular, very high sulfate concentrations were required for high-basicity AICl(3)-titration PACls. The rate of hydrolysis, which is related to the polymerization of aluminum species, is a key property, besides charge neutralization capacity, for proper coagulation, including formation of large floc particles. The aluminum species in the high-basicity PACls, in particular that produced by AICl(3)-titration, was resistant to hydrolysis, but sulfate ions in raw water accelerated the rate of hydrolysis and thereby facilitated floc formation. Normal-basicity Al(OH)(3)-dissolution PACT was hydrolysis-prone, even without sulfate ions. Aluminum species in the high-basicity AICl(3)-titration PACT were mostly those with a molecular weight (MW) of 1-10 kDa, whereas those of high-basicity Al(OH)(3)-dissolution PACls were mostly characterized by a MW > 10 kDa. Normal-basicity Al(OH)(3)-dissolution PACT was the least polymerized and contained monomeric species. (C) 2020 Elsevier Ltd. All rights reserved.