Photocatalysis-enhanced coagulation for removal of intracellular organic matter from Microcystis aeruginosa: Efficiency and mechanism

Photocatalysis is an efficient advanced oxidation process that provides a promising alternative for pollution control during algal blooms in lakes or water reservoirs. Previous research has mainly focused on the destruction of algal cells by photocatalysis, whereas the release and control of concomitant secretions specially intracellular organic matter (IOM) is undervalued. Herein, we studied the removal efficiency and mechanism of algal IOM extracted from Microcystis aeruginosa during stationary phase by the combined process of pre-photocatalysis with a novel composite of Bi2O3-TiO2/PAC (Bi-doped TiO2 nano-composites supported by powdered activated carbon) followed by enhanced coagulation with aluminum sulfate (AS) coagulant. Optimized photocatalysis and coagulation combined treatment (PC) was indicated to enhance IOM removal with 64%, 92% and 100% for DOC, absorbance at 254 nm (UV254) and the optical density at 680 nm (OD680) respectively. Pre-photocatalysis reduced the required AS coagulant dose and weakened the effect of solution pH. Photocatalysis was more inclined to remove humic substances and building blocks than high-molecular weight components, while coagulation alone displayed preference for high-molecular weight biopolymers and protein-like substance. Pre-photocatalysis was complementary with subsequent coagulation for efficient removal of protein-like sub-stances and fractionation with medium molecular weight of 0.5-1 kDa. The produced flocs revealed that the Bi2O3-TiO2/PAC adsorbing IOM acted as a flocculation core to improve coagulation performance. Besides, the combined PC process was proved to perform well to co-existing algal pollution without damaging cells integrity and enhanced algal cells removal from 26% by coagulation alone to 61%. Therefore, this combined treatment was an effective method for the removal of IOM of Microcystis aeruginosa and control cyanobacteria bloom.

Automated summary

This study investigated the efficiency and mechanism of Bi2O3-TiO2/PAC composite combined with photocatalysis and coagulation for the removal of intracellular organic matter (IOM) from Microcystis aeruginosa. It was found that the optimized combined treatment effectively enhanced the removal of algal IOM, reducing the required coagulant dose and pH dependence. Pre-photocatalysis was effective in removing humic substances and building blocks, while coagulation preferred high-molecular weight biopolymers. The combined treatment resulted in improved coagulation performance, and was able to remove co-existing algal cells without damaging their integrity.