Cotransport and deposition of biochar with different sized-plastic particles in saturated porous media

Plastic particles recently are reported to be ubiquitous in natural environments, while biochar is widely employed as an amendment into natural environments e.g. soil for pollution remediation. It is highly likely their transport behaviors in porous media would be altered due to interaction with each other. In this study, the cotransport and deposition behaviors of biochar and plastic particles (0.02 μm nanoplastics (NPs), 0.2 μm and 2 μm micrometer-plastics (MPs)) in quartz sand were investigated at both low (5 mM) and high ionic strength (25 mM) in NaCl solutions at pH 6. The results demonstrated that smaller sized plastic particles (0.02 μm NPs and 0.2 μm MPs) increased biochar transport under both 5 and 25 mM NaCl solutions, while large plastic particles (2 μm MPs) increased biochar transport in 5 mM NaCl solutions yet decreased biochar transport in 25 mM NaCl solutions. For all sized plastic particles, biochar significantly decreased their transport at both low (5 mM) and high (25 mM) ionic strengths in NaCl solutions. The change of zeta potentials of biochar due to heteroaggregation with plastic particles, and the steric repulsion induced by the suspended plastic particles contributed to the enhanced biochar transport with the copresence of smaller sized plastic particles (0.02 μm NPs and 0.2 μm MPs). While, the cotransport of biochar with 2 μm MPs due to their heteroaggregation contributed to the varied transport behaviors of biochar observed at low and high ionic strength with the copresence of 2 μm MPs. The formation of plastic particles-biochar heteroaggregates drove to the decreased transport of all three different sized plastic particles with the copresence of biochar. Considering the coexistence of biochar and plastic particles in natural environment, they would interact with each other (form heteroaggregates) and thus lead to the change of transport behaviors in porous media.