Effect of OH −/Al 3+ ratio on the coagulation behavior and residual aluminum speciation of polyaluminum chloride (PAC) in surface water treatment

Abstract

The coagulation behavior, residual aluminum ratio of three PAC coagulants as well as content and proportion of different residual Al speciation during the treatment of a specific surface water were investigated in this paper. Experimental results suggested that surface adsorption and entrapment effect worked effectively in turbidity removal regarding this specific water. PAC 20 was appropriate to remove hydrophilic natural organic matter (NOM), while PAC 15 and PAC 23 were appropriate to remove hydrophobic fraction NOM. PAC, especially PAC 20, could reduce the content of total dissolved Al and dissolved monomeric Al with comparatively higher toxicity. PAC 20 showed lower concentration for every residual Al species. The majority of total residual Al existed in the form of unsoluble particulate Al (about 62–93% for PAC 15, 65–92% for PAC 20 and 54–78% for PAC 23). Among total dissolved Al in the effluent treated with PAC 15 and PAC 23, dissolved organically-bound Al was almost the major speciation (about 40–92% for PAC 15 and 37–86% for PAC 23). There existed no dissolved organically-bound monomeric Al in purified water treated with three PAC samples. Residual Al ratio of PAC varied in the following order: PAC 15 > PAC 23 > PAC 20. Besides, Al speciation in coagulants likely remained in treated water at lower dosage.