## 3 Results and Discussion
### Biogenic Components
#### 3.3.2 Equilibrium Tests  
The dependence of the ammonium ion concentration in the solution and the sorption capacity in equilibrium conditions is presented in Figure 8. The assumed initial concentration range was too small to exhaust the maximum sorption capacity of the zeolite. Therefore, the parameters of the Langmuir equation were determined at \(R^{2}\) < 0.833 and fitted Langmuir isotherms where not presented. Ammonium ions represent only part of the nitrogen compounds present in stormwater and their content increases in agricultural areas [8; 9; 12]. According to [PERSON] et al. [65], the contribution of nitrates in total nitrogen in stormwater is about 58%, whereas all other forms, including ammonium ions, contribute to 42%. The maximum permissible concentration of ammonium ions according to the World Health Organization (WHO, Geneva, Switzerland) is 10 mg/L. Emission control of this compound is particularly crucial with regard to the oxidation of ammonium ions, resulting in the production of nitrites and nitrates harmful for human health and at the same time extremely difficult to remove in in-situ conditions [7; 20]. The usefulness of zeolites in the removal of ammonium ions during ion exchange processes has been widely documented [65; 66]. An important property influencing the intensity of these processes is the cation exchangeable capacity (CEC) [67; 68; 69; 66], which was at 118.02 meq/100 g for the zeolite studied. CEC of the remaining material was in the range from 1.12 meq/100 g for limestone sand to 13.88 meq/100 g for diatomic (Table 2), and the intensity of NH\({}_{4}\)-N removal on these materials was lower than for zeolite. Statistical analysis showed a positive correlation of CEC both with the maximum adsorption capacity \(Q_{max}\) (\(R\) = 0.996), as well as the difference of the \(Q_{max}\) values calculated for extreme temperatures \(\Delta Q_{max}\) (\(R\) = 0.971). A similar degree of reduction of the ammonium ion concentration due to contact with halloysite, limestone sand, activated carbon and diatomic was not the result of gaseous ammonia release caused by solution pH increase. The average value of the pH of equilibrium solutions was from 6.8 for diatomic to 7.9 for activated carbon, and in such conditions the percentage content of ammonium was below 1.5%. There was a positive relationship between K\({}_{2}\)O and MnO, and the maximum adsorption capacity and \(\Delta Q_{max}\) (\(R\) respectively larger than 0.938 and 0.984), and the CEC value was positively correlated with K\({}_{2}\)O (\(R\) = 0.938) and MnO (\(R\) = 0.984) contents. The curves showing the relationship of equilibrium concentration and sorption capacity for diatomic attained a specific shape (Figure 8). A significant influence of temperature on the equilibrium  
Figure 8: Adsorption isotherm of ammonium ions at 3 \({}^{\circ}\)C, 10 \({}^{\circ}\)C and 22 \({}^{\circ}\)C (error bars mark the standard deviation for \(n\) = 3; line—fitting of Langmuir isotherm). concentration of ammonium ions was observed for this material at low initial concentrations of this contaminant, whereas at initial concentrations exceeding 20 mg NH\({}_{4}\)-N/L the influence of temperature was insignificant. [PERSON] and [PERSON] [70] indicated difficulties in the interpretation of the results resulting from the influence of temperature and pH on the shapes of equilibrium isotherms. As shown by [PERSON] et al. [71], ammonium ions are removed on diatomic due to physical adsorption. They also showed that diatomic is negatively charged in an environment with pH exceeding 3.0, whereas according to [PERSON] et al. [72] this takes place at pH above 6. It was observed herein that the pH of equilibrium solutions increased with increase of contamination load, attaining a maximum of 7.53.