2.3.1. Adsorption and Desorption Experimental Methods of Pb

In this work, we completed the adsorption and desorption experiments, including the adsorption kinetics experiment, the desorption kinetics experiment, and the Pb adsorption and desorption quantities due to pH variation, in the laboratory. The experimental principle for adsorption was as follows: For the experiment, 50 mL of a 200 mg·L−<sup>1</sup> Pb(NO3)2 solution was prepared mixing sample sand (the sample sand was divided into coarse, medium, and fine sand). After mingling, the supernatant liquids, which were samples obtained in 5, 10, 15, 20, 30, 60, 90, 120, 180, 240, 360, 480, 720, and 1440 min, passed through the oscillator for centrifugal filtration. The experimental principle for desorption was as follows. The desorption process resembles the first step of the adsorption process, i.e., the pollutant was

mixed with the sand medium. After shaking this mixture for 24 h, the filtered samples were passed through centrifugal filtration. Then, 50 mL of a 0.01 mg·L−<sup>1</sup> NaNO3 solution appended the filtered samples. The desorption time was the same as the adsorption time. The supernatant liquids were left behind by the oscillator via centrifugal filtration. The adsorption experiment at pH range of 4 to 6 and desorption experiment at pH range of 4 to 9 were as follows: Combining 50 mL of 500 mg·L−<sup>1</sup> Pb(NO3)2 solution with 10 g of sample sand, the pH of the background solution was adjusted to target values, such as pH 4, 5, 6, with either HCl or NaOH. When reaching the equilibrium (1440 min), the supernatant liquids and residue for adsorption were obtained through centrifugal filtration. The residue was obtained at the end of the experiment on the adsorption due to pH variation. The solutions at pH 4, 5, 6, 7, 8, 9 were added to the residue. Then the mixture was shaken using oscillator (200 rpm·min−1), the supernatant liquids for desorption were obtained through centrifugal filtration. The following equations reveal the calculation formulae for the adsorption and desorption capacities [32]:

$$Q\_{\rm ads} = \frac{V(\mathbb{C}\_1 - \mathbb{C}\_2)}{M} \tag{1}$$

$$Q\_{\rm des} = \frac{V\mathbb{C}\_3}{M} \tag{2}$$

where *<sup>Q</sup>*ads is the adsorption capacity for the medium (mg·kg−1), *<sup>Q</sup>*des is the desorption capacity for the medium (mg·kg−1), *<sup>V</sup>* is the supernatant volume (mL), *<sup>C</sup>*<sup>1</sup> is the original concentration for adsorption aspect (mg·L−1), *<sup>C</sup>*<sup>2</sup> is the supernatant concentration for adsorption aspect (mg·L−1), *<sup>M</sup>* is the mass of the sand sample (g), *<sup>C</sup>*<sup>3</sup> is the supernatant concentration for desorption aspect (mg·L<sup>−</sup>1).
