## 3 Results and Discussion
### Ni\({}^{2+}\) Transport in the Saturated Porous Media
#### 3.3.3 Effect of Humic Acid  
It was found that complexing with dissolved organic matter (DOM) has more significant effects on Cu and Pb than Cd, Zn, and Ni [52]. Figure 7 shows that there is little  
Figure 6: The effect of ion strength 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\({}^{-1}\), different ISs were adjusted with NaNO\({}_{3}\), the concentration of soil colloids was 220 mg\(\cdot\)L\({}^{-1}\)).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \
\begin{tabular}{c} **Column** \\ **Experiment** \\ \end{tabular} & **Colloids** &
\begin{tabular}{c} **IS** \\ **(mmol\(\cdot\)L\({}^{-1}\))** \\ \end{tabular} & **HA (mg\(\cdot\)L\({}^{-1}\))** & **AnSi\({}^{2+}\) Injection** & **Concentration of Ni\({}^{2+}\)** & **Rate of Ni\({}^{2+}\)** \\  & & & **(mg\(\cdot\)L\({}^{-1}\))** & **at Plateau (mg\(\cdot\)L\({}^{-1}\))** & **(\%)** \\ \hline
1 & \(\times\) & 0 & 0 & 0.6628 & 0.54 & 81.39 \\
2 & \(\times\) & 3 & 0 & 0.6628 & 0.49 & 73.96 \\
3 & \(\surd\) & 0 & 0 & 0.6628 & 0.47 & 71.30 \\
4 & \(\surd\) & 3 & 0 & 0.6628 & 0.48 & 72.92 \\
5 & \(\times\) & 0 & 20 & 0.6628 & 0.54 & 80.88 \\
6 & \(\surd\) & 0 & 20 & 0.6628 & 0.38 & 58.01 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Results of the Ni\({}^{2+}\) transport in the sand column with different conditions.  
difference in Ni\({}^{2+}\) transport with or without HA in the absence of soil colloids (Figure 7b). In the presence of soil colloids (Figure 7a), although there is no significant difference in the migration rate of Ni\({}^{2+}\) with or without HA at the initial phase, the final transport efficiency of Ni\({}^{2+}\) without HA in the sand column was 71.30%, which is greater than the 58.01% of Ni\({}^{2+}\) with HA (Table 3). This result is probably due to the co-effect of HA and soil colloids in the porous media [53]. HA can enhance the desorption of Ni\({}^{2+}\) in the sand column in the presence of soil colloids. In fact, the decrease of metal ions in the solution is due to the adsorption of free metal ions by colloids, the chelation of metals with colloid-associated HA, and the chelation of metals with free HA [53]. The formation of insoluble metal complexes promotes the adsorption of more heavy metal ions. It was reported in the previous study that HA is easy to dissociate and it complexes with heavy metals, which has a certain binding ability with colloid [54]. In addition, HA can be adsorbed on the surface of colloids, forming stronger ion-exchange centers for metals. Thus, insoluble complexes are formed between heavy metal ions, HA, and colloids to further generate larger particles, resulting in the deposition of heavy metal ions in porous media.  
In the presence of soil colloids, HA plays an important role in the fate and transport of heavy metals in a subsurface environment. It was reported that organic matter usually affects the chemical properties of the colloidal surface, and even changes the surface characteristics of porous media [55; 56]. Similarly, HA can increase the repulsive force between illite colloids and solid media and, thus, promote the transport of illite colloids [57]. However, in this study, HA inhibited the overall transport efficiency of Ni\({}^{2+}\) with soil colloids, which may be related to the properties of soil colloids and the complex interaction between heavy metals, colloids, and sand, as discussed above.