## 1 Introduction  
Metal oxide nanomaterials are widely used in industrial settings [1; 2; 3; 4]. In particular, zinc oxide nanoparticles (ZnO-NPs) are used widely in the fields of cosmetics, optics, and the chemical industry [5; 6] due to their multifunctional properties, and also for nanocomposite development [7]. Adverse effects of ZnO-NPs on living organisms have been reported frequently, e.g., [8; 9; 10; 11; 12; 13; 14; 15]. For example, the toxicity of ZnO-NPs in plants [10; 11], bacteria [8; 16], and rodents [17] has been reported. The environment has been exposed more and more to ZnO-NPs with the increase in the demand for and production of ZnO-NPs. For detailed and accurate analyses on the risk of ZnO-NPs, their exposure probability must be evaluated, and studies on their fate and transport must be conducted [1].  
Studies on the fate and transport of ZnO-NPs have been conducted in various soil environment conditions, such as flow rate, ionic strength, pH, ion valence, and type of porous media, which have been shown to be important factors in nanoparticle transport behavior, e.g., [4; 18; 19; 20; 21; 22; 23; 24]. Humic acid has been reported to improve the ZnO-NPs transport level in the soil environment [14; 16], and one of the main mechanisms causing such trend was confirmed as deposition site competition between natural organic matter (NOM) and nanoparticles [25]. These studies on ZnO-NPs transport have targeted single particles. Although variously synthesized particles were used in previous studies on ZnO-NPs fate and transport, comparisons among their outcomes may not be appropriate due to the differences in the soil environment conditions, such as flow rate, ionic strength, pH, ion valence, and porous media. Although ZnO-NPs are known to be synthesized by various methods [26; 27], no comparative studies on the fate and transport behavior of ZnO-NPs synthesized with diverse methods have been reported so far. In our recent comparative study [28], ZnO-NPs synthesis methods were confirmed to affect the physicochemical properties (particle size, surface functional groups, specific surface area, and particle shape) of ZnO-NPs. Significant differences in their properties, which affected the aggregation and dissolution tendencies of the two types of ZnO-NPs, were also confirmed [28].  
It should be noted that the physicochemical properties of nanoparticles have been reported to be critical factors affecting their fate and transport [4; 29; 30]. Thus, it is reasonable to expect that the nanoparticles synthesized with different approaches could show different transport behavior, but no comparative studies on this issue have been reported; thus, the relevant study is required.  
The first objective of this study was to compare/analyze the transport and retention behaviors of the two types of commercial ZnO-NPs, which were prepared with different methods, in saturated porous media as a function of the presence/absence of the SRHA that is present in the soil media surface and the SRHA that is present in the bulk solution, and to changes in the ionic strength. The second objective of this study was to reverify the deposition site competition that was reported as the mechanism of the coexistence of recently-reported suspended SRHA and NPs in the soil environment using two types of the ZnO-NPs that were synthesized by different methods, and eventually to enhance the reliability of the suggested mechanisms of ZnO-NPs.