## 2 Materials and Methods
### Suwannee River Humic Acid (SHRA) Solution Preparation  
In this study, humic substances, which are abundant in the freshwater environment, were used as an NOM. SRHA (Cat. #2S101H, International Humic Substances Society), which is used frequently in colloid transport studies, was used as a humic substance. The SRHA stock solution was prepared according to a method reported previously [15; 19]. First, 25 mg of dried SRHA was placed into 50 mL Milli-Q water (Q-Gard) and was stirred for 24 h. The solution was filtered using a 0.22 \(\upmu\)m filter, and its pH was adjusted to 8.0 using 0.1 M NaOH. This filtered and pH-adjusted solution was kept in a darkroom at 4 \({}^{\circ}\)C to be used as the SRHA stock solution. The concentration of the SRHA stock solution was analyzed using a TOC meter (TOC-VCPH, Shimadzu Co., Kyoto, Japan), the median diameter of which was \(\sim\)475 \(\upmu\)m, and the uniformity factor 1.5. The porous media were cleansed to remove chemical heterogeneity using the method suggested in previous studies [38; 39]; the soil media were placed into Milli-Q water and shaken to remove impurities, then soaked in 37% HCl at 1:1 ratio for 24 h. The media were cleaned again with Milli-Q water, and the above process was repeated one more time before baking them at 800 \({}^{\circ}\)C for 8 h. To rehydrate the baked soil media prior to the column test, they were placed into Milli-Q water and boiled for at least 1 h. The zeta potential was measured at the desired NaCl solution ionic strength after the soil media were ground. Column experiments were conducted with NaCl as a salt at various ionic strengths, ranging from 0.1 mM to 10 mM. The transport and retention behaviors of the two types of \(n\)ZnO were compared and analyzed. Cleansed sand was wet-packed in the cylindrical glass chromatography column (10 cm in length \(\times\) 2.5 cm in inner diameter (Omnififit, UK)). The wet sand was packed little by little (at an interval of \(\sim\)1 cm) with a slight wobble and layering or minimal air inflow. The porosity of the packed column was measured at 0.36. After packing, deionized water (DI) was supplied for -10 pore volumes (PV, defined as t/VQ; here, t, V, and Q represent time, column void volume, and volumetric flow rate)  
Figure 1: (**a**) XRD patterns of the synthesized \(n\)ZnO obtained from synthesis 1 of \(n\)ZnO-1 and synthesis 2 of \(n\)ZnO-2. TEM images of the synthesized \(n\)ZnO obtained from synthesis 1 (**b**) \(n\)ZnO-1 and synthesis 2 (**c**) \(n\)ZnO-2. to remove impurities in the column, and the NaCl set at the desired ionic strength and pH was injected for at least 10 PV to reach an equilibrium prior to the experiment. Upon completion of equilibration, the _n_ZnO suspension with or without SRHA was injected for 5 PV. Then, particle-free NaCl with the same ionic strength and pH was injected for 5 PV. The _n_ZnO and salt solution were injected in up-flow mode using a peristaltic pump (Cole-Parmer, Thermo Fisher Scientific, Waltham, MA, USA). The concentration of the injected _n_ZnO suspension was 20 mg L\({}^{-1}\). All the flow rates in the experiments were set at 1.144 mL min\({}^{-1}\). The _n_ZnO released from the column were continuously (-8 mL) sampled in a 10 mL test tube. To investigate the distribution of the retained NPs in the column, the column was dissected at approximately every 1 cm after the transport experiment, and soil media were placed into 0.5 M HNO\({}_{3}\) to dissolve _n_ZnO. The concentrations of the _n_ZnO released from the column and the _n_ZnO retained in the column were analyzed using ICP. The areas below the breakthrough curves represent the relative concentration of the _n_ZnO released from the column against the concentration of the _n_ZnO that was injected initially. The obtained retention profiles represent the fraction (i.e., retained _n_ZnO amount/total injected _n_ZnO amount) of the _n_ZnO retained in the unit mass of the soil media.