Heteroaggregation of PS microplastic with ferrihydrite leads to rapid removal of microplastic particles from the water column

Microplastic (MP) particles are ubiquitous in aquatic environments. Therefore, understanding the processes that affect their removal from the water column, such as sedimentation, is critical for evaluating the risk they pose to aquatic ecosystems. We performed sedimentation experiments in which polystyrene (PS) and PS + ferrihydrite, a short-range ordered ferric (oxy)hydroxide, were analyzed in settling columns after 1 day and 1 week of settling time. The presence of ferrihydrite increased sedimentation rates of PS at all pH values studied (pH 3-11). At pH 6 we found that almost all PS particles were removed from the water column after only one day of exposure time. SEM/EDS imaging confirmed heteroaggregation between the PS particles and ferrihydrite. Zeta potential measurements indicated that at acidic pH values the negatively charged PS surface was coated with positively charged ferrihydrite particles leading to charge reversal. Our results demonstrate for the first time that ferric (oxy)hydroxides drive heteroaggregation and subsequent removal of MP from the water column, especially at typical pH values found in natural lake environments. Given their abundance in aquatic systems ferric (oxy)hydroxides need to be regarded as key scavengers of MP.