## 3 Results and Discussion
### Heavy Metals
#### 3.2.2 Equilibrium Tests  
Equilibrium tests demonstrated that for both metals temperature influences contaminant removal Figures 4 and 5. However, the shape of curves showing the dependence for Zn and limestone sand at 22 \({}^{\circ}\)C and 40 \({}^{\circ}\)C (Figure 5), potentially pointing to a precipitation process, did not allow for the determination of the maximum adsorption capacity of this material with regard to Zn ions. The concentration of heavy metals may be regulated by processes depending both on the metal type, but also geochemical conditions influencing metal speciation. Dissolution of Ca\({}^{2+}\) and release of HCO\({}_{3}\)' linked with temperature increase may lead to precipitation of Zn in form of ZnCO\({}_{3}\). As shown by previous studies, the formation of smithsonite is the main mechanism of Zn\({}^{2+}\) ion removal by limestone materials [41]. Additionally, the intensity of heavy metal removal on limestone materials may be linked with chemisorption of metal ions resulting from the rough surfaces of the material, precipitation of metals in the form of metal hydroxides and carbonate, and ion exchange with calcium [59]. In the studies performed herein, precipitation of heavy metals could take place at low initial concentrations, because the pH of equilibrium solutions was below 8.74 for Zn and 8.04 for Cu, whereas in a higher range of initial concentrations, the pH of equilibrium solutions was below 6.88 for Zn and 5.91 for Cu. In the case of the remaining materials, precipitation of Zn and Cu was less significant, because pH was within 5.26 to 7.55 for activated carbon, 5.07 to 7.52 for halloysite, 5.08 to 6.56 for zeolite, and 5.05 to 6.95 for diatomic. Removal of heavy metals on zeolite is generally via ion exchange [36], but also takes place through electrostatic adsorption to negatively charged sites on the zeolite surface [31]. Moreover, heavy metals are removed on halloysite and diatomic via ion exchange and surface complexation respectively on permanently charged sites and variably charged sites [40; 60]. An analysis of heavy metal (Cu and Zn) removal equilibrium studies showed a greater influence of temperature on the removal of copper ions compared to zinc ions. Comparison of maximum adsorption capacities of the analyzed reactive materials \(Q_{max}\) for Cu and Zn and the constants of adsorption equilibrium \(K_{L}\) determined by describing the obtained results with the Langmuir isotherm equation (Equation (1)) is presented in Figure 6 and Table 4, respectively. Removal of heavy metals (Cu and Zn) on all reactive materials was well described by the Langmuir isotherm with the determination coefficient of \(R^{2}\)\(\geq\) 0.932 (Table 4). The only exception was removal of Cu on activated carbon and on zeolite at 40 \({}^{\circ}\)C, described by the Langmuir isotherm with determination coefficient at 0.415 and 0.799, respectively. The material with the best ability to remove Cu ions was activated carbon, followed by limestone sand, halloysite and diatomic, whereas Zn ions were best removed by limestone sand, zeolite, halloysite, diatomic and activated carbon. It cannot be stated unequivocally that the copper ions were removed with larger intensity than zinc ions for all materials. Such a relationship was observed only for activated carbon. Halloysite and limestone sand were more useful for zinc removal. However, it should be emphasized that the maximum adsorption capacity of limestone sand with regard to zinc at 22 \({}^{\circ}\)C and 40 \({}^{\circ}\)C was not determined in the range of the analyzed concentrations A decrease of temperature from 22 \({}^{\circ}\)C (standard temperature for batch analyses) to 3 \({}^{\circ}\)C, reduced the maximum adsorption capacity of Cu by approximately 41% for halloysite (by 2.585 mg/g). The next reactive material in the series was zeolite (by 2.407 mg/g), next--limestone sand (by 2.062 mg/g), activated carbon (by 1.462 mg/g) and diatomite (by 1.431 mg/g).