# The Use of Rapeseed Husks to Remove Acidic and Basic Dyes from Aquatic Solutions
## 4 Conclusions  
Rapeseed husks can serve as a cheap sorbent for both anionic and cationic dyes. The maximum sorption capacity of RH towards AR18 and AY23 acidic dyes was 49.37 and 41.52 mg/g, respectively, while towards BR46 and BV10 basic dyes it reached 59.07 mg/g and 20.93 mg/g, respectively.  
A prerequisite for achieving a high effectiveness of dye sorption onto RH is the appropriate pH of the solution during the process. The optimal pH for the sorption of AR18 and AY23 acidic dyes was established at pH = 2, whereas that of BR46 and BV10 basic dyes--at pH = 6 and pH = 3--respectively.  
The pH of the solutions was observed to change during dye sorption onto rapeseed husks. The system always tended to achieve pH close to the pH PZC value of RH (pHPZC = 6.62). The slightly acidic character of rapeseed husks is probably due to the higher ratio of carboxylic groups to amino groups on the sorbent's surface.  
The time needed to reach the sorption equilibrium of dyes onto RH depended on their initial concentration and ranged from 120 to 150 min for the acidic dyes and from 150 to  
\begin{table}
\begin{tabular}{l l c c c c} \hline \hline \multirow{2}{*}{**Dye**} & \multirow{2}{*}{**Sorbent**} & \multirow{2}{*}{**Sorption**} & \multirow{2}{*}{**pH of Sorption**} & \multirow{2}{*}{**Duration of**} & \multirow{2}{*}{**References**} \\  & & **Capacity [mg/g]** & & & **Sorption [min]** & \\ \hline \multirow{8}{*}{BV10} & Palm shell-based activated carbon & 30 & 3 & - & [47] \\  & Activated carbon from jute fiber & 28 & 8 & 220 & [62] \\  & Spent green tea leaves & 26.7 & 3 & 240 & [45] \\  & Rapeseed husks & 20.9 & 3 & 180 & This work \\  & Banana peels & 20.6 & 7 & 1440 & [63] \\  & Cedar cones & 17.2 & 5 & 480 & [64] \\  & Coconut fiber & 14.9 & 9.2 & 90 & [65] \\  & Sugar came fiber & 10.4 & - & - & [66] \\  & Molts of measulworm & 6.44 & 3 & 210 & [46] \\  & Lemon peels & 5.7 & 3 & 240 & [42] \\  & Mango leaves (powder) & 3.3 & - & 50 & [67] \\  & Orange peels & 3.2 & 4 & - & [42] \\  & Coal-fired coconut fiber & 2.6 & 6.5 & 150 & [52] \\  & Powdered coffee & 2.5 & 2 & 180 & [53] \\ \hline \hline \end{tabular}
\end{table}
Table 5: _Cont._180 min for the basic ones. Shorter sorption equilibrium times were achieved at higher concentrations of dyes, which is associated with a higher probability of dyes colliding with sorption sites and their faster saturation.  
Data obtained from the Langmuir 1 and Langmuir 2 experimental models indicate that only one type of the sorption center, which is presumably the protonated amine groups, played the main role in AR18 and AY23 sorption onto RH. In turn, the binding of basic dyes onto RH took place via at least two different mechanisms. In the case of BR46, it presumably proceeded via hydrogen bonds as well as electrostatic interactions with the ionized carboxyl groups on the sorbent's surface. In the case of BV10, in addition to hydrogen bonds, the binding occurred through the ionic bonds between the carboxyl group of the dye and the protonated functional groups of RH.  
Conceptualization, [PERSON]; methodology, [PERSON]; software, [PERSON]; formal analysis, U.F.; investigation, [PERSON]; resources, T.J. and U.F.; data curation, [PERSON]; writing--original draft preparation, [PERSON]; writing--review and editing, U.F.; visualization, [PERSON]; supervision, [PERSON]; project administration, U.F.; funding acquisition, T.J. and U.F. All authors have read and agreed to the published version of the manuscript.  
This study was financed under Project No. 29.610.023-110 of the University of Warmia and Mazury in Olsztyn, Poland.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.