## 3 Results and Discussion
### Transport and Retention Behavior of ZnO-NPs with SRHA
To remove the SRHA remaining in the column pores, NaCl with the same ionic strength was supplied at 2 PV. All the tests were conducted at pH 9 and two levels of ionic strength (0.1 and 10 mM NaCl). The transport and retention behaviors of both type of _n_ZnO with the presence/absence of suspended SRHA (5 mg L\({}^{-1}\)) are shown in Figure 6. In the absence of suspended SRHA, _n_ZnO-1 showed breakthrough with SRHA-coated quartz sand, and the breakthrough amount decreased with the increase in ionic strength (0.1 and 10 mM). The reason was that with the increase in ionic strength, the electrostatic repulsive force between NPs and SHRA-coated sand decreased, and more and more NPs were deposited, resulting in a low level of breakthrough. This corresponded to the results of previous studies that confirmed an increase in the deposition of various nanoparticles (C\({}_{60}\), TiO\({}_{2}\), and Fe\({}^{0}\)) with decreasing ionic strength [25, 61, 62]. The _n_ZnO-1 breakthrough curves showed a ripening pattern due to the aggregation among NPs in the porous media. However, _n_ZnO-2 showed no breakthrough at any ionic strength regardless of the SRHA coating. The reason for the difference in the transport behavior between the two NPs was due mainly to the difference in the zeta potential between _n_ZnO-1 and _n_ZnO-2. In the presence of suspended SRHA, no difference in the breakthrough amount of _n_ZnO-1 was observed regardless of sand coating or ionic strength. In _n_ZnO-2, no difference in the breakthrough amount was observed regardless of sand coating or change in the ionic strength either. The reason for the lack of difference regardless of SRHA coating was that the 5 mg L\({}^{-1}\) suspended SRHA could be sufficiently adsorbed into the heterogeneous sites that were present in sand in a patch form. Based on these results, deposition site competition between _n_ZnO and suspended SRHA was confirmed as the main retention mechanism of _n_ZnO. In addition, because the rate of SRHA depositing to sand was higher than that of _n_ZnO, the _n_ZnO transport improved in the presence of suspended SRHA. Figure 5: DLVO interaction forces of two types of _n_ZnO for quartz sand surfaces–nanoparticle (**a**) and nanoparticle–nanoparticle (**b**) and in NaCl solution (0.1 mM). In the absence of SRHA, the \(n\)ZnO was largely retained in the column inlet. To investigate the reason, the NPs-NPs DLVO interaction energy profiles were calculated, and the results are shown in Figure 5b. As a result of confirming the NPs-NPs interaction energy profiles, an active NP aggregation developed because the significantly low energy barrier in the absence of SHRA was overcome by \(n\)ZnO Brownian motion or collision force [63]. The massive \(n\)ZnO deposition in the column inlet area could be explained by NP aggregation. In the presence of SRHA, a high level of energy barrier was confirmed due to the NPs-NPs interaction energy (unfavorable conditions). The significantly strong repulsive force reduced NPs aggregation, resulting in no mass deposition in the column inlet area. While no difference in breakthrough curves and retention profiles was observed between the two types of \(n\)ZnO according to the change in ionic strength in the absence of SRHA, differences in the transport behavior were observed between \(n\)ZnO-1 and \(n\)ZnO-2 in the presence of 1 mg L\({}^{-1}\) SRHA (Figure 7a,b). More specifically, the \(n\)ZnO-1 breakthrough amount was confirmed at 47-52%, and \(n\)ZnO-2, 34-40%. In the presence of 5 mg L\({}^{-1}\) SRHA, the breakthrough amount of \(n\)ZnO-1 was 48-53%, and that of \(n\)ZnO-2, 41-48%. The transportability of \(n\)ZnO-1 was confirmed to be higher than that of \(n\)ZnO-2. To explain the difference in the transport of the two types of \(n\)ZnO, the zeta potential of \(n\)ZnO was investigated.