## 3 Results and Discussion
### Transport and Retention Behavior of ZnO-NPs with SRHA  
To compare and analyze the transport behaviors of both types of the _n_ZnO in the packed column in the presence of SRHA (1 and 5 mg L\({}^{-1}\)), experiments were conducted using NaCl (0.1-10 mM) and at pH 9. The breakthrough curves and retention profiles of both type of _n_ZnO affected by SRHA (1 and 5 mg L\({}^{-1}\)) are shown in Figure 3a-f as well as Figure 4c-f, respectively. In the presence of SRHA (1 and 5 mg L\({}^{-1}\)), the _n_ZnO-1 breakthrough curve showed a breakthrough over the whole ionic strength range (0.1-10 mM) unlike the case in the absence of SRHA, but no difference in the breakthrough amount was observed as a function of ionic strength or SRHA concentration (1 and 5 mg L\({}^{-1}\)). Similar to _n_ZnO-1, _n_ZnO-2 showed a breakthrough with the presence of SRHA (1 and 5 mg L\({}^{-1}\)), but there was no difference in the breakthrough amount as a function of change in ionic strength (0.1-10 mM) or SRHA concentration (1 and 5 mg L\({}^{-1}\)). The patterns of both types of _n_ZnO breakthrough curves were flat breakthrough plateaus or non-ripening blocks. To further investigate the transport behavior of both types of _n_ZnO, the profiles of _n_ZnO retained in the column were analyzed. Unlike the case in the absence of SRHA, no large amount of deposition at the column inlet was observed in the presence of SRHA (1 and 5 mg L\({}^{-1}\)) as a function of the change in the _n_ZnO-1 ionic strength (0.1-10 mM). No difference in retained _n_ZnO was observed as a function of change in ionic strength (0.1-10 mM), and no difference as a function of the change in SRHA (1 and 5 mg L\({}^{-1}\)) was seen either. In the presence of SRHA (1 and 5 mg L\({}^{-1}\)), the retention profiles of _n_ZnO-2 and _n_ZnO-1 showed no difference in retained _n_ZnO regardless of the change in ionic strength (0.1-10 mM), and no difference as a function of the change in SRHA (1 and 5 mg L\({}^{-1}\)) was seen either. The reasons for the increase in the transport of both types of _n_ZnO in the presence of SRHA (1 and 5 mg L\({}^{-1}\)) include first that the suspended SRHA is adsorbed into the surface of _n_ZnO, resulting in the _n_ZnO zeta potential being more negative, and eventually, the repulsive electrostatic force between _n_ZnO and the sand surface increases. Second, there is deposition site competition between NPs and suspended SRHA. These two reasons were verified as follows. To verify the first point, DLVO interaction energy profiles were calculated; the results are shown in Figure 5a. Because the difference in transport as a function of the change in the ionic strength and SRHA content was not significant between the two types of _n_ZnO, the results of representative conditions, such as 0.1 mM ionic strength and 5 mg L\({}^{-1}\) SRHA, are shown. The interaction energy profiles for other conditions (1 mg L\({}^{-1}\) SRHA in 1 and 10 mM NaCl) can also be found in the Supplementary Information (Figures S4 and S5). In Figure 5a, the energy barrier was observed in all conditions; the level was significantly low in the absence of SRHA, whereas the level was higher in the presence of 5 mg L\({}^{-1}\) SRHA. In the absence of SRHA, the NPs-sand energy barrier was low, NPs could probably overcome the energy barrier, all the _n_ZnO were deposited on the column, and consequently, no breakthrough occurred. In the presence of 5 mg L\({}^{-1}\) SRHA, the NPs-sand energy barrier level was significantly higher, so _n_ZnO could not deposit on the sand, due to the stronger repulsive force, and consequently, breakthrough occurred [19, 21]. To verify the second point, SRHA was coated on the sand for the column test. If deposition site competition was the main mechanism, the column packed with SRHA-coated sand might show improved _n_ZnO transport versus the column packed with bare sand. NaCl with a desired ionic strength was supplied to the column packed with sand to reach equilibrium; then, 5 mg L\({}^{-1}\) SRHA was supplied in as much as 5 PV to coat the quartz sand.