# The Effect of Cu (II) on Swelling and Shrinkage Characteristics of Sodium Bentonite in Landfills
## 4 Mechanism of Bentonite Erosion Caused by Cu (II)  
Montmorillonite was the main clay mineral in bentonite. Al\({}^{3+}\) was replaced by Mg\({}^{2+}\) in the octahedron of this clay mineral. As a result, the balance of internal charge was changed, and the negative charges appeared between layers. The external charges were demanded to balance the negative charges. Generally, the negative charges between layers were mainly balanced by Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), and H\({}^{+}\). For sodium bentonite, the interlayer cation was mainly Na\({}^{+}\), which were easily exchanged by other ions; so, bentonite was widely used in the adsorption treatment of heavy metal. But once the sodium bentonite was corroded by Cu (II), Na\({}^{+}\) of montmorillonite were exchanged by Cu (II), resulted in a decrease of the interlayer space. This process can be illustrated in Figure 9. When the concentration of Cu (II) increased, the ion exchange reaction increased, leading to a great reduction in swelling and shrinkage of bentonite. Furthermore, the performance of the liner has been weakened.  
In general, when clay particles interacted with the surrounding electrolyte, the surface won't only attracted water molecules to form an adsorption layer, but also attracted cations and hydrated cations to form a diffusion double layer. Molecular thermal movement as the driven force attempted to make the ions adsorbed on the surface of soil particles evenly, so these ions tended to diffuse outward. Therefore, the diffusive electric double layer could be divided into the inner layer and outer layer according to the ion diffusion orientation around soil particles. Because of the higher valence of Cu (II) in CuSO\({}_{4}\) solution, the diffusive double layer formed by a water molecule, the copper ion, and other ions that were attracted to the surface of clay particles by the negative charge becomes thinner. The layout of soil particles was therefore more compact. Consequently, the hydrophilicity of bentonite in CuSO\({}_{4}\) solution and the swelling performance decreased. The relation between potential energy and particle spacing can be illustrated in Figure 10.  
As shown in Figure 10, according to the potential energy theory in the soil-water-ion system, when the ion concentration of the environment was high, the potential energy curve was dominated by attracting energy. When the particles were close to each other due to Brownian motion, the particles were condensed rapidly. While the ion concentration of the environment was low, clay particles were difficult to be agglomerated due to the. At this point, the kinetic energy of particles should be large enough to break through the barrier to agglomerate [38, 39]. Therefore, the swell rate of bentonite with a high concentration of copper ions was low. It can be concluded that the landfill liner with bentonite as raw material encountered waste liquid with a high concentration of heavy metal, the swell  
Figure 9: Diagram of interlayer cation exchange.  
performance of soil particles would be significantly reduced, and its ability to prevent pollutants can weaken accordingly.