# Immobilised Humic Substances as Low-Cost Sorbents for Emerging Contaminants
## 3 Results and Discussion
### Sorption Efficiency
Additionally, a comparison can be made with our previous studies, where arsenic sorption on unmodified HSs was low and did not exceed 1 mg/g, while the sorption capacity of the best sorbent (Fe-modified peat) was 15 mg/g [31]. Figure 5: Cr(III) sorption onto the humic substance sorbent immobilized with iron compounds (HSFe) (in green), phenol (HSph) (in blue) and rezorcinol (HSrz) (in orange), with corresponding [PERSON] and Freundlich theoretical sorption models and a sorption time of 24 h at room temperature with a pH of 5.5. Figure 6: Chlorpromazine HCl sorption onto immobilized humic substance sorbents, with theoretical Langmuir and Freundlich sorption models having a sorption time of 24 h at room temperature and a pH of 6. The data of sorption experiments are often compared with theoretical sorption models, such as the Langmuir, Freundlich, Dubinin-Radushkevich and Redlich-Peterson models, among others. In this study, the obtained results were compared with the [PERSON] and [PERSON] theoretical models. To determine the appropriate isotherm, the linear forms of equations are often used, and linearization is possible in many different types. For example, four different types are possible for the Langmuir equation. In any case, the Langmuir equation (Equation (2)) is the most popular and most appropriate for the assessment of experimental data, and it is as follows:  
\[\frac{C_{e}}{q_{e}}=\frac{1}{q_{m}K_{a}}+\frac{C_{e}}{q_{m}} \tag{2}\]  
where \(C_{e}\) is the equilibrium concentration in mg/L; \(q_{e}\) is the amount of substance sorbed onto the solid phase in mg/g; and \(q_{m}\) and \(K_{a}\) are the Langmuir constants related to the sorption capacity and sorption energy, respectively. In addition, \(q_{m}\) is \(q_{e}\) for a complete monolayer (mg/g), and \(K_{a}\) is the sorption equilibrium constant (L/mg) [32; 33]. Another frequently used sorption isotherm model is the Freundlich model. Its linearized form is shown in Equation (3):  
\[logq_{e}=\frac{1}{n}logC_{e}+logK \tag{3}\]  
where \(k\) and \(n\) are the Freundlich constants that correspond to the adsorption capacity and adsorption intensity, respectively. The Freundlich model assumes that the sorbent surface is heterogeneous and the sorption sites have diverse energies. Based on the correlation coefficient, the sorption of arsenic using the HSFe sorbent fit better into the [PERSON] sorption isotherm model (R\({}^{2}\) = 0.91). Nevertheless, the correlation coefficient values were close (R\({}^{2}\) of 0.80 for the Freundlich model), confirming that the experimentally obtained sorption data corresponded to both theoretical isotherm models. It was not even possible to maintain that one of the models was predominant. A similar trend was observed in the sorption results for chromium. The correlation coefficient values were close (R\({}^{2}\) for HSph, HSFe and HSrz were 0.82, 0.95 and 0.82, respectively, for the Freundlich model and 0.998, 0.999 and 0.992 for the Langmuir model), leading to the conclusion that the experimentally obtained data on Cr(III) sorption when using the HSFe sorbent corresponded to both theoretical isotherm models. The effect of the pH was one of the most important factors that influenced the sorption process. It may affect the chemical form of the investigated element or substance in the solution, as well as the surface properties of the sorbents. The pHpzc was 3.50 for the HSph sorbent and 4.50 for the HSFe sorbent. The obtained pHpzc values indicate that the sorbent surface was positively charged at a pH! pHpzc and negatively charged at a pH? pHpzc. The pH range of 3-8 was chosen to represent as similar conditions as possible to those in the environment. Inorganic arsenate species are negatively charged in the studied pH range, and the stable species of As(V) at the proper pH range are H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) (pH 2-7) and HAsO\({}_{4}\)\({}^{2-}\) (pH 7-12) [34]. Acidic media (pH 3-5) are most favourable for the removal of As(V) using HSs immobilized with Fe compounds.