Bacteria have different effects on the transport behaviors of positively and negatively charged microplastics in porous media

Bacteria, biological colloids with wide presence in natural environments, would interact with plastic particles (emerging colloids with great concern recently) and thus would influence the fate and distribution of plastics in environment. In present research, the impacts of bacteria (both Gram (-) E. coli and Gram (+) B. subtilis) on the transport/deposition of model microplastics (MPs) in porous media were examined in NaCl salt solutions (5 and 25 mM, pH = 6). Both negative carboxylate-modified MPs (CMPs) and positive amine-modified MPs (AMPs) were concerned. We found that under both solution conditions, the presence of both types of bacteria decreased CMPs transport and enhanced retention of CMPs in sand columns. In contrast, the presence of bacteria (regardless of cell type) yet increased AMPs transport and decreased their deposition in sand columns under both ionic strength conditions. The mechanisms leading to the altered transport of CMPs and AMPs by bacteria were different. The formation of larger sized CMPs-bacteria clusters and the extra deposition sites resulted from bacteria adsorbed on quartz sand contributed to the decreased CMPs transport and enhanced their deposition in sand columns. Whereas, the formation of AMPs-bacteria clusters with overall negatively surface charge improved AMPs transport in quartz sand.