# The Use of Rapeseed Husks to Remove Acidic and Basic Dyes from Aquatic Solutions
## 3 Results and Discussion
### The Kinetics of Dye Sorption onto RH  
The time needed to reach sorption equilibrium for the acidic dyes on RH depended on their initial concentration and ranged from 120 min for their concentration of 250 mg/L to 150 min for their concentrations of 50-100 mg/L (Figure 3a,b, Table 2). The sorption intensity of AR18 and AY23 dyes on the tested material was the highest in the initial stage of the process. After 20 min, the amount of the sorbent-bound AR18 ranged from 59 to 72% of \(q_{e}\) and that of AY23 from 65 to 75% of \(q_{e}\). A similar sorption equilibrium time has also been noted in studies on the sorption of AR18 on cellulose flakes (120 min) [38] and activated carbon from poplar wood (120 min) [48]. In the case of AY23, a similar equilibrium time was obtained in experiments determining its removal on chitin flakes (120 min) [41] as well as activated carbon from coconut co  
Figure 3: Sorption kinetics of (**a**) AR18, (**b**) AY23, (**c**) BR46, and (**d**) BV10 onto RH (average + range). Pseudo-first-order model and pseudo-second-order model. A similar sorption equilibrium time was also observed in the studies analyzing BR46 sorption onto sawdust (120 min) [50], coconut shells (120 min) [51], as well as mealworm molds (150 min) [46] and mealworm exoskeletons (180 min) [46]. In the case of BV10, a comparable sorption equilibrium time was recorded in the experiments on its binding on carbonized coconut fibers (150 min) [52], and spent coffee grains (180 min) [53]. In general, shorter sorption times achieved at the higher initial concentrations of dyes may be due to a higher probability of dye particle collisions with sorbent sorption centers. Faster saturation of active RH sites resulted in faster reaction completion. The longer sorption times of the alkaline dyes onto RH, compared to the acidic dyes, may have been the result of their higher molecular weight (AR18--604.5 g/mol; AY23--534.4 g/mol vs. BR46--321.4 g/mol; BV10--479.0 g/mol). The smaller particle sizes of the BR46 and BV10 dyes could theoretically allow them to reach the sorption centers in deeper sorbent's layers inaccessible to AR18 and AY23. However, the migration of basic dye molecules to the active sites located in hardly accessible sites required a relatively large amount of time. In the case of AR18 and AY23, the dye molecules were likely to quickly clog the entries to RH pores, which blocked the access to functional groups located in deeper sorbent's layers and could speed up sorption completion. Experimental data from studies on the kinetics of dye sorption onto RH were described with the pseudo-first order and pseudo-second order models. In each experimental series, the pseudo-secondary model showed the best fit to experimental data, regardless of the initial dye concentration (Figure 3, Table 2). The values of \(k_{2}\) constant and \(q_{e}\) determined from the model indicate that for each tested dye, the intensity of its sorption onto RH increased along with the increase in its initial concentration. The strong correlation between dye concentrations and sorption rate constants may indicate a rather weak affinity of the functional groups of dyes to the active sites of the sorbent. The sorption kinetics of the analyzed anionic dyes AR18 and AY23 was similar, which could be due to the same nature of the dyes, their similar molar mass and the same number of their acidic functional groups. The lower sorption effectiveness of the basic dye BV10 onto RH compared to the BR46 dye may have been caused by the different pHs applied during sorption as well as by the carboxyl functional group present in dye's structure, which is untypical of the acidic dyes. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**Dye**} & \multirow{2}{*}{**Dye Conc. **} & \multicolumn{2}{c}{**Pseudo-First Order Model**} & \multicolumn{2}{c}{**Pseudo-Second Order Model**} & \multirow{2}{*}{**Exp.