Relative importance of hydrolyzed Al species (Ala, Alb, Alc) on residual Al and effects of nano-particles (Fe-surface modified TiO2 and Al2O3) on coagulation process

Al-based coagulants are widely used in water treatment plants, and residual Al has detrimental effects on human health. Response surface method (RSM) was used to investigate the residual Al produced in the coagulation process. Aluminum sulfide (Al2(SO4)3·18H2O) (AS) or polyaluminum chloride (PACl) and high performance polyaluminum chloride (HPAC) were used as coagulants. Fe-surface modified TiO2 (100 mg/L) and Al2O3 (100 mg/L), after mixing with AS, were used as another two new coagulants. The results indicated that PACl and HPAC produced lower dissolved residual Al than AS in most experimental conditions, but AS produced the lowest dissolved residual Al (0.07 mg/L) under the optimum pH value (6.83). Ala may have significant effects on dissolved residual Al. Alb formed in situ performed more efficiently than pre-hydrolyzed Alb. The results of HPSEC and 3D-EEM indicated that the addition of nanoparticles could increase the removal efficiency of organic matter in the solution, and the concentration of organic matter with molecular weight smaller than 1500 Da was decreased. The residual Al with a MW range of 0–1.0 kDa was of a larger portion in all water samples. The element mapping proved that the nanoparticles connected the particles together as a bridge, resulting that the floc size after re-growth with nanoparticles remains larger than when using AS.