# Immobilised Humic Substances as Low-Cost Sorbents for Emerging Contaminants
## 3 Results and Discussion
### Sorption Efficiency
The amount of sorbed arsenic increased slightly from 2 mg/g after 2 h of sorption and reached 4 mg/g after 24 h at an initial As(V) concentration of 100 mg/L. In this case, the sorption isotherm had a relatively slow growing trend. Thus, a longer time period was required to remove arsenic from polluted areas in comparison with Cr (III). The sorption of chlorpromazine HCl onto HSph and HSFe occurred more rapidly in the first hour (Figure 12). After that, the amount of sorbed chlorpromazine increased over the next 24 h, but at a slower rate. In contrast to this, the HSs immobilized with Fe compounds (HSFe) reached the highest sorbed amount of chlorpromazine HCl over a longer period of time. Figure 4:0 The effect of the contact time on Cr(III) sorption onto immobilized humic substance sorbents (HSph and HSFe), with an initial Cr(III) concentration of 8 mg/L. Kinetic models are often used to characterize the sorption process. In order to explore the mechanisms of sorption and potential rate-controlling steps, we applied chemical reaction processes and the mass transport pseudo first-order and pseudo second-order models to evaluate the experimental data. An appropriate sorption model was determined by comparing the determination coefficients in the pseudo first- and pseudo second-order equations. A pseudo first-order reaction is expressed as follows:  
Figure 4:1  
where \(q_{e}\) is the sorbate sorbed at equilibrium (g/g), \(q_{t}\) is the sorbate sorbed at time \(t\) (g/g), \(t\) is the time (min) and \(k_{1}\) is the reaction rate constant (1/min). A pseudo first-order reaction rate constant can be determined from the slope of the \(ln(q_{e}-q_{t})\) vs. \(t\) graph. A pseudo second-order reaction is expressed as follows:  
Figure 4:2  
where \(k_{2}\) is the reaction rate constant (g/g min). A pseudo second-order reaction rate constant (\(k_{2}\)) can be determined from the slope of the \(t\)/\(q_{t}\) vs. \(t\) graph [37]. The linear regression coefficient of determination (R\({}^{2}\)) and parameters of the pseudo first- and pseudo second-order reactions were calculated. The experimental data showed that the pseudo second-order kinetic model could explain the process of As(V), Cr(III) and chlorpromazine HCl sorption on immobilized HS sorbents better than the pseudo first-order model, since the correlation coefficient R\({}^{2}\) was 0.96 for As(V) on HSFe, 0.9998, 0.9998 and 0.999 for Cr(III) on HsFe, HSrz and HSph, respectively, and 0.996, 0.996 and 0.993 for chlorpromazine HCl when using the HSrz, HSph and HsFe sorbents, respectively. The experimental values for Cr(III) q\({}_{\rm{eq}}\) were consistent with the estimated q\({}_{\rm{eq}}\) values of 4.51 mg/g, 3.0 mg/g and 2.09 mg/g (HSph, HSFe and HSrz, respectively). Moreover, the estimated q\({}_{\rm{eq}}\) values for chlorpromazine HCl were 29.2 mg/g, 23.8 mg/g and 14.6 mg/g (HSFe, HSrz and HSph, respectively). These values were calculated with the pseudo second-order equation. Finally, the estimated reaction rate constant k\({}_{2}\) was 0.0011 mg/g min for HSrz, 0.0016 mg/g min for HSph and 0.0004 mg/g min for HSFe. The Cr(III) sorption results gave the estimated reaction rate constant k\({}_{2}\) of 0.18 mg/g min for HSrz, 0.40 mg/g min for HSph and 0.31 mg/g min for HSFe, while the HSFe estimated reaction rate constant in sorbing As(V) was 0.025 mg/g min.