Evaluating adsorption and biodegradation mechanisms during the removal of microcystin-RR by periphyton

Microcystin-RR (MCRR) is among the cyanobacterial toxins of significant concern due to their negative effects on water quality and human health. In this study, periphyton dominated by bacteria and diatoms was applied to remove MCRR from water. The maximum removal rate of MCRR by periphyton was observed in the first day (the latent adaptation period). Within this period, 85.2%, 73.3%, 83.5%, and 86.5% of the total MCRR removed (through adsorption and biodegradation) was by the adsorption of periphyton when the periphyton biomasses were 1.32 g, 3.96 g, 6.60 g, and 9.24 g, respectively. The amount of MCRR adsorbed increased with the increasing ratio of periphyton biomass to MCRR in solution. The adsorption process fitted well to the Freundlich, Langmuir, and Dubinin-Radushkevich (D-R) models, implying that the bioadsorption process has mechanistic relevance. The MCRR adsorption by periphyton is physical in nature and thermodynamically spontaneous. This study provided strong evidence that adsorption was the main mechanism for the removal of MCRR and other microcystins by periphyton and similar microbial aggregates in the latent adaptation period. Thereafter, biodegradation of periphyton dominated the toxin removal process. These results show that periphyton can be employed for an environmentally benign and effective solution for MCRR removal.