# The Effect of Cu (II) on Swelling and Shrinkage Characteristics of Sodium Bentonite in Landfills
## 1 Introduction  
Landfilling is the most common way to eliminate solid municipal wastes. Consequently, the issue of the production of leachates was accompanying [1]. Leachate, referred to as a kind of liquid with complex composition and a great threat to the ecological environment, was formed by the infiltration of groundwater and surface water due to garbage centralized in landfills and that had been buried. Furthermore, the liquid in waste by anaerobic decomposition were also produced [2; 3; 4; 5; 6]. In this process, heavy metals and various organic compounds were the main pollutants in leachate, in which, many heavy metals such as cadmium (Cd), lead (Pb), arsenic (As), and copper (Cu) can be found [7]. Once those copper intruded into the soil, they may be absorbed by plants and then enriched, which can cause great harm to human health [8; 9; 10]. Therefore, it was important to control the seepage of leachate and avoid the pollutants from erosion. As clays, such as bentonite, show many advantages in cost-effectivity, a wide range of sources, and practicality, it was considered as one of the commonly used raw materials to make impervious buffer liner [11; 12; 13]. Herein, bentonite was a layered silicate clay with a large specific surface area, and its main component was montmorillonite. Bentonite shows extremely high swellingability due to its microstructure: when it reacts with water, leading to the squeeze of the volume of micro-pore in the clay, and caused a great decrease in permeability [14; 15; 16; 17; 18]. There were many exchangeable cations between layers of montmorillonite [19], which had strong cation exchange capacity and were prone to unequal cation substitution. It also can effectively adsorb heavy metal ions in water [8]. Therefore, the discharge and leakage of pollutants were effectively controlled. The performance of bentonite liner was highly affected by the swell-shrinkage characteristic of bentonite. When the swelling property of bentonite decreased, the compaction response of the micro-pore in the liner becomes weaker and the permeability becomes higher, which weakens its buffer effect [20; 21]. At the same time, the insufficient swelling property of bentonite proved that the interlayer space of montmorillonite in bentonite was relatively narrow, and the cation exchange capacity was impaired as well. The adsorptive capacity of bentonite can be influenced by many factors [22]. The swelling and shrink properties of bentonite are related to the adsorption capacity. Usually, the leachate contained many different heavy metal ions, the competition among them would take place at the same adsorption sites in the mineral structure. Consequently, some certain heavy metal ions were adsorbed preferentially, which influences the treatment effect of different heavy metal ions [8; 22; 23; 24]. Previous studies had shown that the action of competitive adsorption can be changed due to the presence of H+ under different pH conditions. When the pH ranged from 2 to 9, bentonite would preferentially adsorb copper ions over lead and cadmium ions. Once pH was less than 4, heavy metal ions were easily desorbed by clay due to the protonation of Si-O- and Al-O- on the clay surface [25; 26]. When the pH is higher than 9, the Si and Al in the bentonite are dissolved, which can reduce the expansibility and increase the permeability [27]. Aiming at the mechanical behavior of the bentonite, many studies can be found in literature including compactibility, expansibility, permeability, and so on [28; 29]. However, the influence of heavy metal pollution on the swelling and shrinkage behavior of the clay is seldomly reported. Therefore, our work in this paper focuses on understanding the effect of Cu (II) on the swelling and shrinkage characteristics of sodium bentonite. The Dose Response model was used to predict the swelling and shrinkage behavior. The results of XRD and XRF further revealed the influence of Cu on the expansibility, which can provide the guideline of implementation of liner in landfills.