Combined chlorine and natural clay of bentonite for eliminating toxic Microcystis bloom and restoring water quality in an urban shallow pond

The frequent occurrence of cyanobacterial blooms poses challenges to water sources used for drinking water and recreational activities. Modified clays have been applied for bloom control in freshwater but they have well-documented limitations - residual modifiers, resurfaced cyanobacterial flocs, and limited ability to remove cyanotoxins. To address these drawbacks, we developed a combined treatment of bentonite with chlorine to control toxic Microcystis blooms in a shallow urban water, Pond MAXI, through both lab- and field-scale experiments. Lab-scale experiments revealed that calcium hypochlorite as chlorine agent at concentrations of 5-20 mg L⁻¹ enhanced the flocculation process of bentonite, improving the removal efficiency of Microcystis colonies by 62.2-81.5%. The combined treatment achieved a higher photosynthetic inactivation efficiency of Microcystis colonies (57.1-100%) compared to the sole use of calcium hypochlorite (44.7-100%) or bentonite (7.9-8.4%) ascribed to the elevated ROS levels in cells. Additionally, the combined treatment effectively reduced dissolved microcystin to below detectable levels, owing to the synergistic oxidation and adsorption processes. For field application in Pond MAXI, the optimized treatment of calcium hypochlorite (10 mg L⁻¹) and bentonite (200 mg L⁻¹) was employed. This treatment successfully eliminated toxic Microcystis blooms. Furthermore, it promoted the abundance and diversity of eukaryotic algae and water quality, with these benefits persisting for four years. Overall, this study demonstrates that the combined treatment of calcium hypochlorite and bentonite is a promising disposal technology for eliminating toxic cyanobacterial blooms and restoring water quality in situ freshwater.