Influence of titanium dioxide nanoparticles on the transport and deposition of microplastics in quartz sand

The influence of titanium dioxide nanoparticles (nTiO) on the transport and deposition of polystyrene microplastics (MPs) in saturated quartz sand was investigated in NaCl solutions with ionic strengths from 0.1 to 10 mM at two pH conditions (pH 5 and 7). Three different-sized polystyrene (PS) MPs (diameter of 0.2, 1, and 2 μm) were concerned in present study. We found that for all three different-sized MPs in NaCl solutions (0.1, 1 and 10 mM) at both pH 5 and 7, lower breakthrough curves and higher retained profiles of MPs with nTiO copresent in suspensions relative to those without nTiO were obtained, demonstrating that the copresence of nTiO in MPs suspensions decreased MPs transport and increased their deposition in quartz sand under all examined conditions. The mechanisms contributing to the increased MPs deposition with nTiO in suspensions at two pH conditions were different. The formation of MPs-nTiO heteroaggregates and additional deposition sites provided by previously deposited nTiO were found to drive to the increased MPs deposition with nTiO in suspensions at pH 5, while the formation of MPs-nTiO aggregates, additional deposition sites and increased surface roughness induced by the pre-deposited nTiO on quartz sand surfaces were responsible for the enhanced MPs deposition at pH 7. The results give insights to predict the fate and transport of different-sized MPs in porous media in the copresence of engineered nanoparticles.