Bio-Mediated Flocculation of Freshwater Microplastics: Effects of Microalgae With Exopolymer Attachments

Transparent exopolymer particles (TEPs) are crucial for enhancing the flocculation of microplastics (MPs). However, quantitatively evaluating the influence of TEP on the flocculation process and addressing these effects in a flocculation model are challenging. In this study, three freshwater microalgae (Scenedesmus sp., Aulacoseira granulata, and Melosira varians) with various levels of TEP production were incubated to investigate the biologically mediated flocculation process with MPs in a mixing chamber. The results revealed that the three microalgal species significantly increased flocculation, with floc size increasing notably (one-way analysis of variance, p value < 0.001) at later incubation periods (12, 16, 20, 24, and 30 days), compared with the early incubation periods (after 6 and 9 days), when TEP production was lower. A critical TEP concentration (0.42 mg/L) was observed, beyond which further increases in TEP production had minimal effects on the flocculation process. Among the selected microalgae, the Scenedesmus sp.-MPs mixture presented a faster floc growth rate than Aulacoseira granulata and Melosira varians. Furthermore, a modified population balance equation model was proposed to incorporate the ratio of the TEP concentration to the microplastic concentration into the aggregation and maximum specific growth rate parameters. The modified model revealed that the floc growth rate and equilibrium mean size are dependent on the TEP concentration when the MP concentration is fixed, which is in good agreement with the experimental data. The modified model illustrates the potential to simulate exopolymer-driven interactions between microalgae and MPs and provides insights into the mechanisms of bio-mediated flocculation.