Transport and deposition of ZnO nanoparticles in saturated porous media

The impact of ionic strength and cation valence on the transport and deposition kinetics of ZnO nanoparticles in saturated porous media was systematically investigated in this research. Packed column experiments were performed over a series of environmentally relevant ionic strength in both NaCl (ranging from 1 to 20 mM) and CaCl2 (ranging from 0.1 to 1 mM) solutions. Solution chemistries (ionic strength and ion types) greatly affected the transport of ZnO nanoparticles in saturated quartz sand. Flat breakthrough plateaus were observed at relatively low ionic strength in both NaCl (1 and 5 mM) and CaCl2 (0.1–0.5 mM) solutions, whereas, ripening was observed at high ionic strength (10 and 20 mM in NaCl, and 1 mM CaCl2) conditions. Deposition of nanoparticle increased with increasing solution ionic strength in both monovalent and divalent salt solutions. The presence of divalent ions in solutions increased nanoparticle deposition in quartz sand. Under all examined conditions, nanoparticles mainly retained at segments near the column inlet. The retained ZnO nanoparticle concentrations versus transport distance decreased faster than the theory prediction of log-linear decrease under all examined conditions. Our study found that concurrent aggregation of ZnO nanoparticles occurred during the transport process, which contributed to the hyper-exponential retained profiles.

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► Solution chemistries greatly affected the transport behavior of ZnO nanoparticles.
► Transport of ZnO nanoparticles in quartz sand is controlled by DLVO theory.
► Retained profiles deviated from theory predicted log-linear decreases.
► Nanoparticle aggregation during travel led to hyper-exponential retained profiles.