## 4 Micromechanics
### Effect of Zeolite Doping on Metal Adsorption Properties of Mixed Soils  
(1) Zeta potential  
Zeta potential value is an essential indicator of the stability of a constant colloidal system, which is judged by measuring the strength of mutual repulsion or attraction between particles. The more stable the particle dispersion system is, the higher the absolute value of zeta potential, while the particle condensation or coalescence is an unstable state. The results of the potential variation of the stone chip-bentonite-zeolite mixture under different zeolite doping and ion concentration conditions are shown in Figure 12a. The potential of bentonite in pure water was also measured to be \(-39\) mV, and the bentonite particles indicated a negative potential. As can be seen from the Figure, without bentonite doping, in the Zn\({}^{2+}\) solution, the absolute value of the bentonite potential decreases with increasing concentration, such as when the Zn\({}^{2+}\) concentration increases from \(10\) mmol/L to \(50\) mmol/L, the potential is \(-11.80\) and \(-7.60\) mV. This is because the exchange of Zn\({}^{2+}\) in solution with the exchangeable cation Na\({}^{+}\) is indicated by the bentonite particles, making the bentonite double electric layer thickness decrease, resulting in a decrease in the negative charge on the surface of the particles. Moreover, as the ionic solution concentration increases, the number of cations near the bentonite particles indicates an increase, and the thickness of the double electric layer is compressed, which shows that the absolute value of zeta potential decreases; the weakening of the negative potential on the surface of the soil particles indicates that the adsorption of Zn\({}^{2+}\) by the mixed soil increases. When the solution ion concentration is specific, the absolute value of the potential of mixed soil decreases with the increase of zeolite admixture. For example, when the initial concentration of Zn\({}^{2+}\) was \(3\) mmol/L, the surface potential of bentonite particles was negative, with a potential value of \(-15.20\) mV. After the incorporation of zeolite, the potential of mixed soil samples continued to rise with the increase of zeolite incorporation and was \(-13\), \(-11.50\), and \(-11.10\) mV at \(12.50\), \(25\), and \(50\%\) incorporation, respectively. With solid adsorption capacity, zeolite added to bentonite will share part of the negative effect of pollutants on bentonite and weaken the compression effect of metal ions on the double bentonite layer. Therefore, under the same concentration conditions, the increase of zeolite doping increases the stone chip-bentonite-zeolite mixture's overall adsorption capacity, as shown by the absolute value of the potential on the surface of the stone chip-bentonite-zeolite mixture decreases with the increase of zeolite doping. (2) FTIR spectra  
The infrared spectrum analysis of adsorbed different concentrations of Zn\({}^{2+}\) bentonite is shown in Figure 12b. The bentonite layers between the scanning range of \(4000\) cm\({}^{-1}\) and \(400\) cm\({}^{-1}\) and the elastic vibration of the water molecule H-O-H corresponds to a peak at \(3450\) cm\({}^{-1}\). The peak near \(1425\) cm\({}^{-1}\) corresponds to the unique vibration of the  
Figure 12: Microscopic changes in adsorption tests under different conditions. (**a**) change of zeta potential; (**b**) FTIR spectra. Si-O-Si bond. As a whole, the basic frame of bentonite remains unchanged before and after adsorption, and the absorption peak changes the same, indicating that heavy metal ions enter the interlayer after adsorption. When the concentration of Zn\({}^{2+}\) increases, the peak intensity of the M Al-OH (M metal cation) shock peak at 877 cm\({}^{-1}\) decreases, which means that ion exchange has occurred between bentonite and Zn\({}^{2+}\). The aforementioned modifications become more pronounced with the addition of zeolite, indicating that zeolite increases interlayer adsorption and ion exchange to a greater extent.