Influence of Humic Acid on the Transport of Two Types of Synthesized Zinc Oxide Nanoparticles in Quartz Sand

In this study, transport and retention behaviors of the two types of nZnO prepared with separate manufacturing methods were compared/analyzed according to the presence/absence of Suwannee River humic acid (SRHA) adsorbed into the sand surface and the SRHA suspended in bulk solution, and to changes in the solution ionic strength (0.1–10 mM) in sand-repacked water-saturated columns. In the absence of suspended SRHA, nZnO-1 breakthrough was observed only in SRHA-coated soil, and the breakthrough amount decreased with the increase in the ionic strength (23.8% to 17.2% at 0.1 mM to 10 mM, respectively). In contrast, nZnO-2 breakthrough was not observed over the entire ionic strength range, regardless of the SRHA sand coating. With the presence of suspended SRHA, neither nZnO-1 nor nZnO-2 showed a significant difference in the breakthrough amount regardless of sand coating or ionic strength. However, the breakthrough amount of nZnO-1 was higher than that of nZnO-2 (51.5% versus 37.7% at 10 mM with 1 mg/L SRHA). From confirming the difference in transport between the two types of nZnO, the amount of SRHA adsorbed into nZnO-1 was less than the amount adsorbed into nZnO-2 (0.29 mg/g versus 0.64 mg/g at 10 mM with 1 mg/L SRHA). This result was considered to be due to the larger nZnO-1 breakthrough amount than the nZnO-2 breakthrough amount, which was caused by the larger amount of suspended SRHA that could occupy the deposition sites in the nZnO-1 suspension. Because the rate of SRHA deposition on the sand surface was higher than on nZnO, nZnO transport improved in the presence of suspended SRHA, and changed according to the amount of suspended SRHA.