# The Use of Rapeseed Husks to Remove Acidic and Basic Dyes from Aquatic Solutions
## 3 Results and Discussion
### Maximum Sorption Capacity of RH
In turn, the maximum sorption capacities of the alkaline dyes were determined at 59.07 mg/g and 20.93 mg/g for BR46 and BV10, respectively. As mentioned in Section 3.3, the lower sorption capacity of the tested sorbent towards BV10 compared to BR46 was probably due to the acidic functional group (-COOH) being untypical for the basic dye, which generated the local negative charge. Two opposing charges in the BV10 structure significantly impaired the possibility of its sorption through electrostatic interactions with the sorbent's surface. The higher sorption effectiveness of the basic dye BR46 onto RH compared to the acidic dyes AR18 and AY23 may be due to the generally acidic nature of the sorbent (pH\({}_{\text{PZC}}\) = 6.62), which promotes the binding of cationic sorbates. The smaller size of the BR46 molecules also contributed to its better sorption effectiveness compared to AR18 and AY23, which allowed them to occupy active sites inaccessible to the acidic dyes. The relatively small values of \(K_{1}\) and \(K_{2}\) constants (<0.5), determined from the Langmuir 2 model, confirm earlier findings suggesting the low affinity of the dyes to the active sites of RH. This finding indicates that the sorbent will be able to bind a large amount of dye only at high concentrations of the sorbate in the solution. In turn, the percentage removal effectiveness of dyes at their low concentrations will be relatively small in this case. Table 5 lists the sorption capacities of various biosorbents and activated carbons towards the dyes analyzed in the present study. \begin{table}
\begin{tabular}{l l c c c} \hline \hline \multirow{2}{*}{**Dye**} & \multirow{2}{*}{**Sorbent**} & \multirow{2}{*}{**Sorption**} & \multirow{2}{*}{**pH of Sorption**} & \multirow{2}{*}{**Duration of**} \\  & & **Capacity [mg/g]** & & **Sorption [min]** \\ \hline \hline  & Activated carbon WG-12 & 100 & – & – & [55] \\  & Activated carbon from almond shell & 51.6 & 2 & 60 & [39] \\  & Rapeseed husks & 49.4 & 2 & 150 & This work \\  & Chitosan flakes & 39.9 & 4 & 180 & [38] \\  & Cellulose & 29.7 & 6 & 120 & [38] \\ AR18 & Activated Carbon from Poplar Wood & 29.41 & 5 & 120 & [48] \\  & _Sargassum glaucescens_ biomass & 15.0 & 6 & 60 & [56] \\  & Agar & 13.6 & 6 & 120 & [38] \\  & Activated carbon from wood & 3.9 & 7 & 120 & [48] \\  & (polar tree) & 1.8 & 3 & 90 & [37] \\  & Cosonut shells & 0.66 & 2 & 45 & [51] \\ \hline  & Activated carbon from coconut courí & 132.07 & 6 & 120 & [49] \\  & Rapeseed husks & 41.52 & 2 & 150 & This work \\  & Chitin flakes & 24.2 & 2 & 120 & [41] \\  & Sawdust & 4.7 & 3 & 70 & [40] \\  & Sunflower seed hulls & 2.3 & 3 & 90 & [37] \\  & Cosonut shell activated carbon & 2.3 & 1.7 & 60 & [57] \\  & Cosonut shells & 0.53 & 2 & 45 & [51] \\ \hline  & Granular activated carbon & 333.3 & 8 & \textless{60} & [44] \\  & Activated carbon from biomass & 65.7 & 7 & 90 & [58] \\  & Rapeseed husks & 59.1 & 6 & 180 & This work \\  & Spent green tea leaves & 58 & 6 & 240 & [45] \\  & Lemon peels & 54 & 6 & 240 & [42] \\  & Molts of mealworm & 50.9 & 6 & 150 & [46] \\ BR46 & Cosonut shells & 49.4 & 6 & 120 & [51] \\  & Active carbon ROW 08 & 45 & 8 & 60 & [59] \\  & _Palourain tomentosa_ tree leaves & 43.1 & 8 & 72 & [43] \\  & Exoskeletons of mealworm & 31.5 & 6 & 180 & [46] \\  & Nut sawdust & 30.1 & 7 & - & [60] \\  & Bone meal & 24.6 & 6 & 90 & [61] \\  & Wood sawdust & 19.2 & - & 120 & [50] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Sorption capacity of various biosorbents and activated carbons towards AR18, AY23, BR46, and BV10 dyes. The sorption effectiveness of AR18 and AY23 dyes on the tested rapeseed husks is higher than on other investigated sorbents based on plant biomass, such as sawdust, seed husks or coconut shells (Table 5). Interestingly, the sorption capacity of RH towards the acidic dyes is higher than the sorption capacities of chitin sorbents and also certain activated carbons.