## 3 Results and Discussion
### Ni\({}^{2+}\) Transport in the Saturated Porous Media  
#### 3.3.1 Effect of Soil Colloids  
The column breakthrough curves (BTCs), i.e., the normalized Ni\({}^{2+}\) concentrations in the column effluent (C/C0) plotted as a function of PV, are presented in Figure 5. The concentration of Ni\({}^{2+}\) increased quickly at the initial phase of transport and reached the maximum value at about 2 PV, which was then maintained at a steady concentration. The normalized Ni\({}^{2+}\) concentrations are less than 1, indicating that Ni\({}^{2+}\) was retained in the sand column by adsorption on the surface of the sand or adsorption on the soil colloids being caught in the sand column together during transport.  
Compared to the breakthrough curve of the Ni\({}^{2+}\) transport without soil colloids, the retention of Ni\({}^{2+}\) in the sand column increased in the presence of soil colloids. Moreover, the migration rate of Ni\({}^{2+}\) with soil colloids in the initial phase was slightly faster than that of Ni\({}^{2+}\) transport without soil colloids; the IS was 0 mmol\(\cdot\)L\({}^{-1}\) (Figure 5a). This phenomenon can be explained by the following mechanisms [13]: The motion rates of large molecules may be greater than the average velocity of the average groundwater flow line in fine granular soil or aquifers, caused by the volume exclusion effect. When the particle sizes of molecules or ions are too large to pass through smaller pores, they are limited to a larger velocity than the average velocity of the groundwater flow line. The size of the colloid is larger than that of the soluble Ni\({}^{2+}\) and the colloid-associated Ni\({}^{2+}\) may be trapped in the sand column due to the blocking effect in the late transport [45], causing the transport efficiency of Ni\({}^{2+}\) to decrease.  
Moreover, Ni\({}^{2+}\) could act as a bridge to reduce the repulsive forces between the quartz sand and soil colloids and, therefore, have an inhibiting effect on Ni\({}^{2+}\) as well as soil colloid transport in the sand column [46; 47]. It can be seen from Figure 5 that, despite the presence of soil colloids resulting in a faster penetration rate of Ni\({}^{2+}\) at the initial phase of transportation, the final transport efficiency of Ni\({}^{2+}\) was suppressed. It can be speculated that a part of Ni\({}^{2+}\) was retained in the sand column due to the direct adsorption by quartz sand, and the other part of Ni\({}^{2+}\) was retained in the sand column just because the colloid-associated Ni\({}^{2+}\) was captured by the sand column. The effect of colloid on the transport of pollutants in porous media is two-sided [48]. On the one hand, colloids can move faster than that of solute in porous media due to size exclusion, and promote the transport of some contaminants as a carrier; on the other hand, colloid-associated contaminants could be \"immobilization\" in the porous media if the porosity is small or the affinity between porous media and the colloids is strong.  
Figure 5: The effect of soil colloids on Ni\({}^{2+}\) transport in the sand column. (**a**) IS = 0; (**b**) IS = 3 mmol\(\cdot\)L\({}^{-1}\). The initial concentration of Ni\({}^{2+}\) was 0.6628 mg\(\cdot\)L\({}^{-1}\), the different ISs were adjusted with NaNO\({}_{3}\), the concentration of soil colloids was 220 mg\(\cdot\)L\({}^{-1}\).