# Immobilised Humic Substances as Low-Cost Sorbents for Emerging Contaminants
## 3 Results and Discussion
### Sorption Efficiency  
The impact of the sorbent dosage was investigated using different doses of the immobilized HSs (0.05-1.2 g) while keeping the sorbate concentration and volume constant (100 mg/L, 50 mL). The obtained results indicated that the most appropriate sorbent dosage for As(V) removal was 0.05 g in the case of using the HSs immobilized with phenol (HSph) or resorcinol (HSrz), while the trend differed when using the HS immobilized with Fe  
Figure 3: Thermogravimetric (TG) and DTG curves of immobilized humic substance samples (humic substances immobilized by polycondensation with phenol (HSph), resorcinol (HSrz) and iron compounds (HSFe)). compounds (HSFe). The most appropriate dose of the HSFe sorbent for As(V) removal was 0.1 g, because at this sorbent concentration, it was possible to reach the highest sorbed amount of arsenic. Chromium sorption had a similar trend to the arsenic sorption, as all three immobilized HSs showed the highest amount of sorbed chromium at a dose of 0.05 g. The obtained results with the dosage that allowed reaching the highest sorbed amount of the elements in this step were used in further experiments. For the sorption isotherms, the obtained results of arsenate (As(V)) sorption for three kinds of immobilized HSs indicated that only the HS that was immobilized with iron compounds could be a prospective sorbent material for the removal of arsenic compounds from polluted waters. The As(V) sorption isotherm using HSFe as a sorbent is shown in Figure 4, where the sorbed amount of arsenic increased and exceeded 16 mg/g at the initial As(V) concentration of 750 mg/L. Compared with the results of As(V), the sorbed amount of Cr(III) onto the HSFe, HSph and HSrz sorbents was considerably lower. Figure 5 shows the sorption isotherms of Cr(III) when using the HS that was immobilized with iron compounds, resorcinol or phenol as a sorbent. The maximum sorbed amount reached 6 mg/g at the initial chromium concentration of 60 mg/L when using the HSs immobilized with Fe compounds and phenol, while the HSs immobilized with resorcinol showed a considerably lower sorption capacity, reaching the highest sorbed amount of 2.45 mg/g at an initial Cr(III) concentration of 100 mg/L. The possible mechanism of binding may include the formation of bonds between the metal ions and the hydroxyl groups of the sorbent. Compared with the sorption capacities of the inorganic substances investigated in this study, the pharmaceutical exemplar yielded even more promising results. Chlorpromazine HCl sorption was investigated on three kinds of immobilized HSs, and the results indicated that all the studied materials had similar sorption capacities. The highest sorbed chlorpromazine HCl amount was 55.2 mg/g for the HSFe sorbent, followed by 46.8 mg/g for HSrz and a maximum of at least 45.7 mg/g for HSph (Figure 6). Figure 4: As(V) sorption onto the humic substance sorbent immobilized with iron compounds (HSFe) at a sorption time of 24 h at room temperature and a pH of 6. In summation, the immobilized HSs synthesized in this study can be used for removal of metals and metalloids from water. Up to now, immobilized HSs have not been widely studied as sorbents. However, other natural materials modified with iron compounds have been widely studied, and the amounts sorbed by these materials are comparable to those obtained in this study. The maximum sorption capacity of As(V) was 70.42 mg/g when using modified (iron (III)-loaded) orange juice industrial residue [28]. In the study by [PERSON] et al. [29], the sorption capacity was 0.0977 mg/g for siderite-coated sand and 0.0519 mg/g for hematite-coated sand. Furthermore, the study by [PERSON] et al. [30] found that the sorption capacities of iron oxide-coated sand and ferrihydrite used for arsenic removal from natural water were 0.0183 and 0.285 mg/g, respectively.