Adsorption of Microcystins onto Synthetic and Natural Clay Minerals

Abstract

The proliferation of cyanobacterial harmful algal blooms and their toxins, especially microcystins, represents a significant threat to water quality worldwide. Clay-based adsorption offers a promising method to mitigate this threat. This study compares the adsorption of microcystins onto a synthetic clay (laponite) and two natural clays (bentonite and kaolinite) under various chemical conditions. We specifically examined the effects of solution pH, salinity, and cation type on the adsorption process. The difference between the initial and equilibrium concentrations of microcystins, divided by the clay concentration, determines the toxin’s removal efficiency. Our laboratory experiments demonstrate that at neutral pH, laponite exhibits greater toxin-removal efficiency than bentonite, followed by kaolinite. Consequently, laponite is more effective at removing microcystins compared to natural clays. All three clays exhibit reduced adsorption efficiency as pH increases from 7 to 9, with kaolinite showing the most significant decrease. Additionally, higher salinity levels and the presence of divalent cations such as Ca²⁺ boost the adsorption ability of all three clays. Across all tested chemical scenarios, laponite consistently outperforms bentonite and kaolinite in capturing microcystins, demonstrating its greater efficiency across diverse water chemistries. Our results highlight the potential of laponite for removing microcystins and demonstrate that both the clay type and the water’s chemical characteristics play a critical role in toxin removal efficiency. These findings pave the way for future applications of clays to lower microcystin pollution across various aquatic environments, from freshwater sources to brackish estuaries.