Influence of natural organic matter on the transport and deposition of zinc oxide nanoparticles in saturated porous media

The significance of natural organic matter (NOM, both humic acid and alginate) on the transport and deposition kinetics of ZnO nanoparticles (NPs) in irregular quartz sand was examined by direct comparison of both breakthrough curves and retained profiles with NOM present in NPs suspension versus those obtained without NOM. Packed column experiments were conducted in both NaCl and CaCl2 solutions under a series of environmentally relevant ionic strengths. Under all examined conditions, breakthrough plateaus with NOM even at concentration as low as 1 mg L−1 of total organic carbon (TOC) were higher than those without NOM, indicating that presence of NOM in NPs suspensions enhanced ZnO NPs transport. Although hyper-exponential retained profiles were observed both in the presence and absence of NOM, the amount of retained ZnO NPs acquired in the presence of NOM decreased slowly as the transport distance increased. Straining induced by concurrent aggregation is found to cause the hyper-exponential decrease. In the presence of NOM, electrosteric interaction effectively reduced the ZnO NPs deposition on collector surfaces and NPs–NPs aggregation. Subsequently, the amount of NPs that jammed in the column inlet in the absence of NOM were markedly decreased, which therefore exhibited as flatter retained profiles.

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► Distinct effects of NOM on ZnO NPs transport and spacial distribution was observed.
► Both SRHA and Alg markedly increased the mobility of ZnO NPs in porous media.
► The retained profiles with NOM exhibited more flat than those without NOM.
► Electrostatic force and steric repulsion contributed to less retention with NOM.
► Decreased concurrent aggregation induced more flat retained profiles with NOM.