# Immobilised Humic Substances as Low-Cost Sorbents for Emerging Contaminants
## 3 Results and Discussion
### Sorption Efficiency
The sorbed amount of As(V) was more than 15 mg/g (see Figure 7). At the pH range of 5-7, the sorbed amount of As(V) decreased from 12 mg/g to 2.4 mg/g. The arsenic species were negatively charged at the studied pH range. According to the determined pHpzc values, the sorbent surface was positively charged at a pH range of 3-4.5, the interaction between negatively charged species and the positively charged surface area were favorable, and the sorbed amount was relatively high, while at a pH? 4.5, the negatively charged species did not interact, and the sorption capacity of the sorbent decreased. The Cr(III) ion sorption was negligible in acidic media (pH 3) but increased at a pH \(>\) 3.5 (see Figure 8). At this pH level, the predominant chromium species in the solution were Cr\({}_{2}\)(OH)\({}_{2}\)\({}^{4+}\) and Cr(OH)\({}^{2+}\) (see Figure 9), while with a pH value increased to 5.5, the predominant species were Cr\({}_{3}\)(OH)\({}_{4}\)\({}^{5+}\) and Cr(OH)\({}_{2}\)\({}^{+}\). All of the positively charged chromium species may react with the negatively charged sorbent surface (the HSph surface was negatively charged at a pH \(>\) 3.5). The obtained results indicated that a pH of 5.5 was more favorable for sorption, with Cr\({}_{3}\)(OH)\({}_{4}\)\({}^{5+}\) as the predominant form. Experiments were not performed at higher pH values due to the formation of insoluble Cr(OH)\({}_{3}\) at a pH \(>\) 6. Figure 8: The effect of pH on the sorption of Cr(III) onto the humic substance sorbent immobilized with iron compounds (HSFe), phenol (HSph) and resorcinol (HSrz), with a sorption time of 24 h at room temperature with an initial Cr(III) concentration of 45 mg/L. Figure 7: The effect of the pH on the sorption of As(V) onto the humic substance sorbent immobilized with iron compounds (HSFe), with a sorption time of 24 h at room temperature and an initial As(V) concentration of 100 mg/L. Theorbents used for sorbing chlorpromazine in this study showed variability with respect to the sorption capacity, depending on the pH. At the pH range of 5-9, chlorpromazine mainly occurred in the cationic form [36]. The obtained results (Figure 10) indicate that the amount of chlorpromazine sorbed onto HSph slightly increased in the pH range of 3-5.5 and did not significantly increase in a more basic environment. Similarly, the HSFe and HSrz sorbents appeared to be more suitable for the removal of chlorpromazine from aqueous solutions under neutral or basic conditions. A relatively rapid increase in the amount of sorbed chlorpromazine (14.6-31 mg/g) was observed with the use of the HSFe sorbent in the pH range of 3-6, while HSrz had a constant sorbed amount at a pH of 3-5.5, and the sorbed amount increased at pH values above 5.5. Concerning the effect of the contact time, according to the sorption kinetics study results (Figure 11), the rate of chromium sorption was very high at the beginning of the contact time. Most of the Cr(III) was sorbed within the first 2 h. The sorbed amount reached 1.9, 2.7 and 3.9 mg/g with the use of HSrz, HSFe and H  
Figure 10: The effect of the pH level on the sorption of chlorpromazine HCl onto immobilized humic substance sorbents (HSrz, HSph and HSFe), with a sorption time of 24 h at room temperature with an initial chlorpromazine HCl concentration of 80 mg/L. Figure 9: Cr(III) speciation forms, prepared using Visual Minteq 3.1 [35]. at the initial concentration of 8 mg/L with 0.1 g of immobilized HSs. After that, the Cr(III) sorption rate gradually reduced, and the maximum sorption capacity was reached. During the first 2 h, the sorption capacity increased rapidly, which could have been related to the availability of sorption sites on the sorbent surface. However, when the sorbent surface became saturated with sorbate molecules, the sorption rate tended to decrease. The sorption of As(V) occurred slower on the HSs immobilized with Fe compounds (HSFe), in contrast to Cr(III).