Role of aluminum speciation in the removal of disinfection byproduct precursors by a coagulation process

Humic acid (HA) was extracted and separated into different molecular weight (MW) fractions, then coagulated by aluminum chloride and polyaluminum chloride (PACI). The removal of disinfection byproduct (DBP) precursors and the aluminum speciation variation of the coagulants were investigated in detail. In particular, the role of aluminum speciation in the removal of DBP precursors was discussed. During the coagulation process, AlCl3 hydrolyzed into dominating in situ Al-13 species at pH 5.5. The in situ Al-13 species exhibited better removal ability for haloacetic acid (HAA) precursors than PACI. At pH 7.5, in situ hydrolyzed Al-13 species of AlCl3 decomposed into dimeric Al species. In this case, preformed Al-13 of PACI had a high removal ability of HAA precursors. Specially,the greatest reduction of HAA precursors with a low MW (<30 kDa) was through charge neutralization at pH 5.5, and that of HAA precursors in high MW (>30 kDa) fractions was through adsorption at pH 7.5. Different from HAA precursors, the in situ Al-13 species did not have a high removal ability of trihalomethane (THM) precursors. Therefore, PACI exhibited a better removal ability of THM precursors than AlCl3 at different pH values. In the different MW fractions, the greatest reduction of THM precursors was through charge neutralization at pH 5.5.