Combined process of visible light irradiation photocatalysis-coagulation enhances natural organic matter removal: Optimization of influencing factors and mechanism

Photocatalysis or coagulation process has been separately studied for the removal of natural organic matter (NOM) from surface water, and the reaction mechanism of organic fractionations of treated effluent has been clarified. Although coagulation-based process has been extensively studied for NOM removal, few attempts have been made based on a combined process that can not only efficiently degrade NOM, but also enhance the coagulation performance. The hypothesis is that a combination of photocatalysis and coagulation process in a single unit might potentially enhance NOM removal efficiency. This is because the nano-photocatalyst can serve as both pre-oxidant and flocculation core. Herein, a combined process of visible light photocatalysis prior to coagulation is conducted, with the transformation efficiency of the organics fractionations revealed. The as-prepared nano-sized photocatalyst consisting of Bi2O3-TiO2 (molar Bi/Ti ratio 0.04:1) had a stronger visible light absorption ability. Compared to coagulation alone, the combined photocatalysis-coagulation process greatly improved NOM removal, with synergistic effect, while the pre-photocatalysis reduced the required dosage of polyaluminium chloride (PACl) coagulant and increased the optimal pH of solution. A mathematical model optimized by response surface methodology of the combined process was developed that produced a good correlation between predicted and experimental values. The optimal conditions were calculated as follow: 2.0 g/L photocatalyst, a photocatalytic reaction time of 10 min 18 sec, 0.04 mM PACl coagulant, and a pH of 4.2. These conditions particularly enhanced removal of acidic hydrophobic substance, humic-/protein-like substances and fractions with molecular weight between 3 and 44 kDa. The XRD spectra and SEM images of the produced flocs identified that these flocs consisted of a core of photocatalyst, which could contribute to an effective flocculation and enhanced NOM removal.