## 3 Results and Discussion
### Biogenic Components
#### 3.3.2 Equilibrium Tests
Therefore, it may be concluded that at lower pH values (corresponding to lower initial concentrations of NH\({}_{4}\)-N), the surface of diatomic was less negatively charged, and temperature increase could influence the slight increase of the removal processes intensity. Moreover, [PERSON] et al. [64] have observed that removal of NH\({}_{4}\)-N on halloysite is not the result of electrostatic interaction. They have evidenced that halloysite had a higher negatively charged surface, but the maximum sorption capacity was relatively small, similarly as in the analyses presented herein. The intensity of phosphate removal from aqueous solutions and the influence of temperature on these processes were more variable than in the case of ammonium ions (Figures 9 and 10). In samples with a zero value of the phosphate initial concentration, the phosphate concentration in solutions being in contact with zeolite and halloysite attained values above zero after reaching equilibrium. These observations were concordant with the results of studies on the oxide content in selected reactive materials. The presence of P\({}_{2}\)O\({}_{5}\) was observed only in these two materials: 0.39% in zeolite and 0.54% in halloysite. Average removal of phosphates was calculated at 0.14 mg/g of zeolite and 0.19 mg/g for halloysite, respectively. The mechanisms of binding or removal of phosphorus compounds may be very variable and depend on pH, temperature, presence of competing compounds, salinity, occurrence of organic matter or grain-size distribution [73]. For example, high values of environmental pH may stimulate phosphorus adsorption through ligand exchange of HPO\({}_{4}\)\({}^{2-}\) or H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) ions present in the solution with OH\({}^{-}\) ions present on the surfaces of solid particles [74]. [PERSON] and [PERSON] [75] stated that temperature increase causes increase of the energy at which phosphorus sorption is bound in soils and sediments and shortening of the time needed for the formation of crystalline forms. Moreover, as pointed out by [PERSON] and [PERSON] [75], in the presence of limestone rocks, orthophosphates are  
Figure 9: Adsorption isotherm of phosphates 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). adsorbed on the surface of the material forming CaHPO\({}_{4}\) or replacing CO\({}_{3}\)\({}^{2-}\) by HPO\({}_{4}\)\({}^{2-}\). However, taking into account the measured maximum sorption capacities of limestone sand (from 0.205 to 0.300 mg/g), it can be concluded that such processes did not take place. This is most probably related with the pH of equilibrium solutions below 7.5, because most mentioned processes take place in an alkaline environment. Moreover, according to [PERSON] et al. [76], the zeta-potential of halloysite is at pH range of 6 to 7, whereas [PERSON] et al. [77] suggested that the external surface of halloysite exhibits a negative zeta-potential over a pH range of 2.5-8.5. The pH of equilibrium solutions for this material is in the range of 6.07 to 6.93 and for zeolites, i.e., in the range of 6.74 to 7.74. In the case of the second material, phosphorus removal may be caused by electrostatic adsorption, because the point of zero charge of this material can range from 8.5 to 9.0 [30]. Equilibrium tests for ammonium and phosphate ions were performed only in the temperature range from 3 \({}^{\circ}\)C to 22 \({}^{\circ}\)C; therefore, the relationship of the maximum adsorption capacity and temperature has not been described by a polynomial function (Figure 10). Similarly, as in analyses of series I, ammonium ions were removed most intensely by zeolite (\(Q_{max}\) in the range from 8.450 to 9.723 mg/g); the maximum sorption capacities of the remaining materials attained similar values from 0.960 (activated carbon) to 1.233 mg/g (diatomite).