## 3 Results and Discussion
### Ni\({}^{2+}\) Adsorption on Sand and Soil Colloids  
#### 3.2.1 pH Effect  
Figure 3a shows that there were no significant changes in the adsorption behaviors of Ni\({}^{2+}\) on quartz sand under different pH values (because the adsorption of Ni\({}^{2+}\) on quartz sand was weak due to the small surface area (only 10 m\({}^{2}\cdot\)g\({}^{-1}\)) of sand). Although the high pH values of the solution may decrease the zeta potential of sand, the weak sorption capacity of sand leads to no significant effect of pH on Ni\({}^{2+}\) adsorption on a small scale.  
Noticeably, the Ni\({}^{2+}\) adsorption by soil colloids had no significant changes with different pHs when the IS was 0 or 3 mmol\(\cdot\)L\({}^{-1}\). However, the Ni\({}^{2+}\) adsorption by soil colloids slightly increased with the increase of pH from 5.5 to 7.5 when the IS was 30 mmol\(\cdot\)L\({}^{-1}\) since the final concentration of Ni\({}^{2+}\) in the solution slightly decreased from 0.55 to 0.50 mg\(\cdot\)L\({}^{-1}\) with an increase of the pH from 5.5 to 7.5. Some studies reported that the increase in pH led to more negative charges on the surfaces of soil particles, indicating the high pH was  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Materials**} & \multirow{2}{*}{**pH**} & \multirow{2}{*}{**CEC a**} & \multirow{2}{*}{**Mechanical Composition**} & **Average** & **Zeta** & **Mass Concentration** & \multirow{2}{*}{**TOC b**} \\  & & & & **(mm\%)** & & **Particle Size (nm)** & **Potential** & **of Soil Colloid** \\  & & & & & **Size (nm)** & **(mV)** & **(g\(\cdot\)L\({}^{-1}\))** & \\ \hline Soil colloids & 5.81 & — & — & — & 433.3 & \(-\)26.23 & 0.22 & 5.72 mg/L \\ \hline \multirow{2}{*}{Soil samples} & \multirow{2}{*}{7.32} & \multirow{2}{*}{34} & 2–0.02 & 32.94 & \multirow{2}{*}{1270} & \multirow{2}{*}{\(-\)25.7} & \multirow{2}{*}{—} & \multirow{2}{*}{27.17\%} \\  & & & & \(<\)0.002 & 11.34 & & & \\ \hline \hline \end{tabular}  
* \({}^{\text{a}}\) CEC: cation exchange capacity (meq/100 g); b TOC: total organic carbon.  
\end{table}
Table 1: The basic physicochemical characteristics of soil samples and soil colloids.  
Figure 2: (a) SEM images of soil colloids; (b) EDX of soil colloids and the embedded tables are the main element compositions and the ion content in the soil samples.  
favorable for heavy metal adsorption by soil colloids in porous media [34; 35]. Generally, the surfaces of soil colloids were predominantly negatively charged, implying that most Ni\({}^{2+}\) adsorption occurred through electrostatic attraction. The increasing pH in the amount of H\({}^{+}\) became low, which further reduced the H\({}^{+}\) competitive adsorption between H\({}^{+}\) and Ni\({}^{2+}\) in the presence of soil colloids [36]. In addition, the amount of cation at the bonding sites of the soil colloids was also reduced with high pHs; therefore, the number of negative sites increased to contribute to Ni\({}^{2+}\) adsorption.  
High H\({}^{+}\) concentration can produce compression of the electric double layer and repulsive surface interaction energy, which is why the corresponding zeta potential of the soil colloids and sand decreased with the increase of pH [37]. The hydroxides of most heavy metals are insoluble; therefore, the OH\({}^{+}\) in the solution could precipitate with heavy metal ions, which may further adsorb heavy metal ions in the precipitation process when the pH increases [36]. Moreover, the adsorption affinity of soil to heavy metal hydroxyl ions is greater than that of free ions and the proportion of hydrated ions will increase with pH increasing, indicating more heavy metals will be adsorbed by soil colloids.