# An Experimental Study on the Migration of Pb in the Groundwater Table Fluctuation Zone
## 3 Results and Discussion
### pH Variation of Sample Solution on Water Table Fluctuation Zone  
In Section 3.1, pH is one of the major factors affecting the adsorption and desorption capacities of Pb. Figure 5 illustrates pH variation at different sampling port heights in the water table fluctuation experiment on three typical media. The average pH variation at different sampling port heights is shown in Figure 6. The vertical direction of the column shows the distribution properties of pH in the sample solution. Firstly, pH decreased and then increased from the top to the bottom. The lowest pH often occurred at the height of 35 cm (pH = 4.7), where the location of original pollutants was consistent. The average pH of the three typical media followed the sequence of fine sand > medium sand > coarse sand. The aforementioned changes may be attributed to the increasing groundwater table since Pb\({}^{2+}\) was not only adsorbed by the adsorption sites on the surface of soil particles, but also possibly combined with OH\({}^{-}\) ions to obtain Pb(OH)\({}_{2}\) precipitation. Consequently, the concentration of OH\({}^{-}\) ions decreased, so that the pH decreased in the solution, which resulted in a higher pH the further the distance. As the water table rose, the Pb\({}^{2+}\) in the \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multirow{2}{*}{**Media**} & \multicolumn{2}{c}{**Elovich Equation**} & \multicolumn{2}{c}{**Hyperbolic Diffusion**} & \multicolumn{2}{c}{**Pseudo-Second-**} & \multicolumn{2}{c}{**Weber-Morris**} \\  & & & & \multicolumn{2}{c}{**Equation**} & & \multicolumn{2}{c}{**Order Equation**} & \multicolumn{2}{c}{**Equation**} & \\ \cline{3-13}  & & \(a_{1}\) & \(b_{1}\) & \(R^{2}\) & \(a_{2}\) & \(b_{2}\) & \(R^{2}\) & \(k_{1}\) & \(R^{2}\) & \(k_{2}\) & \(c\) & \(R^{2}\) \\ \hline \multirow{4}{*}{adsorption} & Coarse sand & 437.20 & 163.15 & 0.96 & 0.5513 & 0.0159 & 0.81 & 0.00005 & 0.72 & 24.52 & 851.0 & 0.81 \\  & Medium sand & 667.04 & 179.76 & 0.97 & 0.6171 & 0.0140 & 0.74 & 0.00006 & 0.83 & 25.81 & 1139.9 & 0.74 \\  & Fine sand & 799.14 & 252.62 & 0.90 & 0.6326 & 0.0143 & 0.60 & 0.00005 & 0.95 & 33.89 & 1497.1 & 0.60 \\ \hline \multirow{4}{*}{desorption} & Coarse sand & 15.18 & 10.21 & 0.94 & 0.5208 & 0.0176 & 0.69 & 0.0008 & 0.96 & 1.43 & 42.50 & 0.69 \\  & Medium sand & 26.14 & 5.39 & 0.94 & 0.6465 & 0.0124 & 0.72 & 0.0022 & 0.81 & 0.77 & 40.33 & 0.72 \\ \cline{1-1}  & Fine sand & 21.00 & 1.24 & 0.97 & 0.8298 & 0.0062 & 0.76 & 0.0165 & 0.55 & 0.18 & 24.21 & 0.76 \\ \hline \hline \end{tabular} Note: The data were fitted to the Elovich equation: \(Q=a_{1}+b_{1}\)Inf, the hyperbolic diffusion equation: \(Q/Q_{\text{max}}=a_{2}+b_{2}\)\(t^{1/2}\), the pseudo-second-order equation: \(Q=k_{1}\)Qmax\({}^{2}\)\(t/(1+k_{1}\)Qmax\(t)\) and the Weber-Morris equation: \(Q=k_{2}\)\(t^{1/2}+c\), respectively, where \(Q\) is the adsorption/desorption capacity; \(t\) is time; \(a_{1}\) and \(a_{2}\) refer to constant associated with maximum adsorption/desorption amount for the Elovich equation and the pseudo-second-order equation, respectively, \(b_{1}\) and \(b_{2}\) refer to adsorption/desorption rate coefficient, \(k_{1}\) and \(k_{2}\) refer to adsorption/desorption rate coefficient for pseudo-second-order model and Weber–Morris model, respectively, \(c\) is constants related to the medium for Weber–Morris model, \(Q_{\text{max}}\) refers the equilibrium adsorption/desorption capacity for coarse sand, medium sand and fine medium.  
\end{table} 
Table 2: Kinetic parameters for adsorption and desorption of Pb in different media. Figure 3: Curves of adsorption and desorption kinetics of Pb in different media: (**a**) the adsorption quantity variation of Pb based on adsorption kinetics; (**b**) the desorption quantity variation of Pb based on desorption kinetics. solution was absorbed to saturation by the medium. A pH range from 4.5 to 7.5 was detected in the sample solution by coarse sand, at pH range from 5.1 to 7.8 by medium sand, and a pH range from 7.1 to 7.7 by fine sand.