*3.2. 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 Pb2+ 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 Pb2+ in the

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. A comparison of the average pH values for the three typical media revealed that pH at different grain diameters generally followed the order of: coarse sand > medium sand > fine sand.

**Figure 4.** Effects of pH on adsorption (**a**) and desorption (**b**) on different media, including coarse, medium, and fine sand.

## *3.3. Migration Law of Pb Due to Water Table Fluctuations*

Due to the strong adsorption capacity of fine sand, the migration ability of Pb in fine sand is weak. The Pb of each sample solution in the columns did not reach the instrument's detection limit (the instrument's detection limit (TAS-990A) is 0.01 mg/L). Therefore, we only analyzed the migration law of Pb in coarse and medium sand in this article. During groundwater table fluctuations, the variation in Pb concentration at different sample solutions is shown in Figure 7a (coarse sand) and Figure 7b (medium sand). The results of how Pb concentration is altered with water table fluctuations are shown in Table 3 and can be used to study the migration of Pb when the water table rises and falls in the experiment. Our analysis is as follows. (1) In the sample solution, when the height was 30 cm high in coarse sand, the water table height increased from 20 to 40 cm, and Pb2+ concentration increased in the range of 8.29–42.42% in the early-stage relative to the initial concentration on the first day. Then, with the water table fluctuations, Pb2+ concentration decreased in the range of 37.12–94.34%. This concentration declined by 9.54% on average within 8 days. (2) At the height of 35 cm for sample solutions in coarse sand, Pb2+ concentration decreased in the range of 4.01–97.47% with water table fluctuations. This concentration decreased by 6.68% on average within 14 days. (3) At the height of 40 cm for the sample solution in coarse sand, the concentration declined by 5.54% on average within 12 days. (4) At the height of 45 cm for the sample solution, Pb2+ was absorbed by coarse sand after 4 days. (5) At the heights of 30 and 40 cm for the sample solution in medium sand, Pb2+ was only detected on the first day. Afterward, Pb2+ was completely absorbed by the medium. (6) At the 30 and 40 cm heights for the sample solution in medium sand, Pb2+ concentration decreased in the range of 24.26–100%. This concentration decreased by 7.57% on average until the water table fluctuated at between 50 and 60 cm.

**Figure 5.** pH variation at different heights of sampling locations in the first and second cycles: (**a**) pH variation of coarse sand at sampling positions with heights of 20, 30, and 35 cm; (**b**) pH variation of coarse sand at sampling positions with heights of 35, 40, 45, 50, 55, and 60 cm; (**c**) pH variation of medium sand at sampling positions with heights of 20, 30, and 35 cm; (**d**) pH variation of medium sand at sampling positions with heights of 35, 40, 45, 50, 55, and 60 cm; (**e**) pH variation of fine sand at sampling positions with heights of 20, 30, and 35 cm; (**f**) pH variation of fine sand at sampling positions with heights of 35, 40, 45, 50, 55, and 60 cm.

**Figure 6.** Average pH at different heights of sampling locations.

**Figure 7.** The Pb2+ concentrations vary in water table fluctuations. (**a**) The Pb2+ concentrations vary in columns filled with coarse sand; (**b**) the Pb2+ concentrations vary in columns filled with medium sand.

As seen in the analysis in Figure 5 of the pH and the water table, the closer the sample solution was to the pollutant in the rising water table, the lower the pH became, the greater the activity of Pb became, and the more desorption quantities resulted from polluted sand. Therefore, at the height of 35 cm for the sample solution, Pb2+ concentration reached its maximum compared with the others. When the sample solutions were further away, the maximum concentration of Pb2+ was smaller. In a column of coarse sand, when the water table rose to 30 cm, the desorption quantity of Pb2+ reached its maximum, and the Pb2+ concentration in the solution reached its maximum. Throughout the whole experiment, the maximum concentration of Pb2+ at the 20, 30, 35, 40, and 45 cm sample solutions were 0.072, 36.061, 38.973, 16.941, and 0.042 mg·L<sup>−</sup>1, respectively. Figure 7b shows that the adsorption capacity of medium sand is greater than that of coarse sand; therefore, the migration capacity of Pb2+ in medium sand becomes weak compared with that in coarse sand. Based on Figure 7b, Pb2+ is only detected at the heights of 30, 35, and 40 cm. In these sample solutions, the highest concentrations are 8.619, 18.862, and 0.164 mg·L<sup>−</sup>1, respectively.

**Table 3.** Variation range of concentration of Pb in coarse and medium sand sampling ports with water table fluctuations.


Note: During the whole experiment, the concentrations in the solution sample at each sampling port did not reach the detection limit for columns filled with fine sand because of the strong adsorption capacities of fine and medium sand; therefore, they are not listed in this table. The "-" in the table indicates that the water sample was not obtained due to water table fluctuation limitations.

The experimental results of groundwater fluctuation showed that the underground water table rose to a height of 30 cm in coarse and medium sand after coming into contact with pollutants. The capillary band rose when the moisture content of the medium increased at the height of 30 cm. Then, Pb2+ in the medium dissolved into water under the effect of desorption. Pb2+ in the solution was still detectable since the water table fluctuated rapidly and Pb2+ in the solution had not been fully adsorbed by the medium. Reasons for why the concentration of Pb2+ decreased as the water table rose are as follows. (1) Due to the alkaline soil, the solution's pH increased with the rising water table, resulting in OH<sup>−</sup> and Pb2+ combining in the water to form precipitation. (2) The adsorption quantity of the unpolluted medium was stronger than that of the pollution medium. The rising of the water table brought about Pb2+ adsorption one more time. (3) The adsorption and desorption of Pb is affected by hydrodynamic conditions. The change in hydrodynamic conditions caused Pb2+ to be desorbed and Pb2+ adsorbed by the medium. These two reactions were mutual until the adsorption equilibrium was reached.
