## 4 Conclusions  
The effects of soil colloids on the adsorption and transport of Ni\({}^{2+}\) in saturated porous media were explored in this study. The main conclusions are as follows:  
1. Soil colloids have greater potential for Ni\({}^{2+}\) adsorption than quartz sand, suggesting the important role of soil colloids in the subsurface environment. From this, we infer that most contaminants, such as heavy metals, could be immobilized by colloids in soil instead of transported from surface soil to groundwater.
2. Relatively higher pHs and smaller ISs were favorable for Ni\({}^{2+}\) adsorption by soil colloids, indicating the hydrochemical conditions of groundwater greatly affected the fate of heavy metals in the subsurface environment. Human activities, such as artificial pumping, water injection experiment, or rapid infiltration of rainfall, may lead to the release of colloids and change the environmental conditions, which  
Figure 7: The effect of HA on Ni\({}^{2+}\) transport in the sand column. (**a**) In the presence of soil colloids; (**b**) in the absence of soil colloids. The initial concentration of Ni\({}^{2+}\) was 0.6628 mg\(\cdot\)L\({}^{-1}\), the concentrations of soil colloids and HA were 220 and 20 mg\(\cdot\)L\({}^{-1}\), respectively; IS = 0 mmol\(\cdot\)L\({}^{-1}\).  
should be considered to protect the groundwater safety and to take steps, carrying out contaminant remediation strategies.
3. A higher IS could slightly improve the initial transport rate of Ni\({}^{2+}\), but could not improve the final transport efficiency of Ni\({}^{2+}\) in the absence (or presence) of soil colloids. HA has a negative effect on Ni\({}^{2+}\) transport in the presence of soil colloids, while there is no obvious difference when Ni\({}^{2+}\) transports with and without HA in the absence of soil colloids. This result indicates that HA is negligible when heavy metals transport without soil colloids.
4. All final transport efficiencies of Ni\({}^{2+}\) were less than 100% when Ni\({}^{2+}\) was transported in the sand column, which means that Ni\({}^{2+}\) was retained in the sand column more or less. This result reveals that the soil or aquifer could act as a filter to remove heavy metals in the subsurface environment.  
Therefore, soil colloids are important compositions in the subsurface environment, which affect the adsorption, transport, and fate of many contaminants. A future investigation should focus on the interaction mechanisms of different kinds of contaminants with natural colloids as well as solid media. At the same time, factors, such as colloids, types, hydrogeochemical conditions, and organic matter, should be considered when natural colloids are applied to study the heavy metal contamination of groundwater.  
Conceptualization, Z.W. and Y.W. ([PERSON]); methodology, Y.Z. and Y.W. ([PERSON]); validation, Z.S. and Y.W. ([PERSON]); formal analysis, W.M. and Y.H.; investigation, Z.W.; data curation, W.Z.; writing--original draft preparation, Z.W.; writing for review and editing, Y.Z. and Y.Y. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Science and Technology Planning Project of Qinghai Province (grant no. 2019-ZJ-7063); Open-end Fund of Fujian Provincial Key Laboratory of Ecology-toxicological Effects & Control for Emerging Contaminants (PY19004).  
Not applicable.  
The authors declare no conflict of interest.