## 3 Results and Discussion
The effect of grain size on sorption efficiency is shown in Figure 4, using two equilibrium concentration values (about 0.1 mg/L and 0.3 mg/L) and two selected process temperatures (10 \({}^{\circ}\)C and 20 \({}^{\circ}\)C) as examples. The magnitudes of phosphate sorption were determined by plotting q = f (Ce) (Figures 1-3). The points are arranged somewhat chaotically, and it is impossible to decide which grain size favors sorption. The scattering of the measurement points may result from variations in the content of active compounds in the individual portions of the sorbent used in this study. It should  
Figure 4: Weight of phosphate (q) retained by Rockfos® material at different grain sizes at temperatures of 10 \({}^{\circ}\)C and 20 \({}^{\circ}\)C.  
be emphasized that natural rocks were used in this study after being burned, crushed, and separated into fractions. The compositions of individual particles could therefore be different depending on the natural crystallization processes occurring over long periods of time, temperature fluctuations during crystallization, and other disturbances influencing the variability of the shares of individual oxides of different metals. Statistical analysis also showed that at the initial concentration of phosphates in the solution, 0.5 mg/L, the tested fractions of the Rockfos(r) material did not differ significantly in terms of phosphate sorption. Statistical analysis also showed that at the initial concentration of phosphates in the solution (0.5 mg/L), the tested fractions of the Rockfos(r) material did not differ significantly in terms of phosphate sorption. This finding was consistent across all the analyzed temperatures. At a concentration of 1.0 mg/L, the sorption capacity for granulations of 1.6-2.5 mm and 2-5 mm across the entire temperature range was significantly higher than that for the 1-1.6 mm granulation. In turn, at a concentration of 2.0 mg/L and low process temperatures (5 \({}^{\circ}\)C, 10 \({}^{\circ}\)C), sorption for the 2-5 mm fraction was significantly lower than that for the 1.6-2.5 mm granulation. At higher process temperatures, no significant differences in phosphate sorption were observed between the granulations. A study carried out by [PERSON] et al. (2019) concluded that grain size affects sorption efficiency, but a closer examination of the text in this publication does not support this conclusion. The smaller the grains, the larger the surface area, revealing a relationship that can increase sorption capacity ([PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2020). In chemisorption processes, however, the chemically active parts of the grains will play a more crucial role (rather than the total surface area). Therefore, it can be stated that in the case of materials with a heterogeneous composition, grain sizes within the ranges applied in this study do not affect the amounts of phosphates retained in the chemisorption process. Temperature is one of the most critical factors affecting the amount of phosphate sorption on Rockfos(r) material. This relationship is illustrated in Figure 5 using the example of a 2.0-5.0 mm granulation with an initial phosphate concentration in solution of 2 mg/L. The results in Figure 5 show an increase in sorption with increasing temperatures. The points corresponding to 5 \({}^{\circ}\)C are the lowest, and those corresponding to 25 \({}^{\circ}\)C are the highest. To statistically substantiate the effect of temperature on the amount of sorption (q) and equilibrium concentration (C\({}_{\rm e}\)), an analysis was performed using the generalized linear model (GLM); sample results are shown in Figure 6.  
Figure 5: Mass of adsorbed phosphate (q) on grains of 2.0–5.0 mm granulation at different process temperatures and phosphate concentration in the starting solution of 2 mg/L. The model showed a statistically significant effect of temperature on sorption, with a probability of less than 0.001, which is less than the assumed significance level of 0.05.