## 4 Conclusions  
The effectiveness of sorption of dyes on the bark of beech and birch sorbent depends on the type of cationic dye. The tested sorbents showed a higher sorption efficiency of Basic  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Dye**} & \multirow{2}{*}{**Sorbent**} & \multirow{2}{*}{**Sorption Capacity** **[mg/g DM]**} & \multicolumn{2}{c}{**Time**} & \multirow{2}{*}{**pH**} & \multirow{2}{*}{**Source**} \\  & & & & & \\  & & & & **[min]** & & \\ \hline \multirow{8}{*}{BR46} & beech tree bark & 131.5 & 300 & 5 & own research \\  & birch tree bark & 111.7 & 300 & 5 & own research \\  & ceramic clay & 28.05 & 120 & 6 & [57] \\  & rapeseed husks & 49.00 & 60 & 8 & [58] \\  & bone meal & 24.56 & 90 & 6 & [59] \\  & biochar prepared from Chrysanthemum & 32.26 (180 \({}^{\circ}\)C) & & & \\  & morifolium Ramat straw & 49.5 (200 \({}^{\circ}\)C) & 60 & 10 & [60] \\  & clay–wood sawdust mixture & 53.19 (220 \({}^{\circ}\)C) & & & \\  & pyrolyzed by-product biomass & 30.12 & 30 & 7 & [61] \\  & carbon and silica-based composite & 135.0 & 5 & 7.5 & [62] \\  & & 41.3 (293 K) & & & \\  & carbon and silica-based composite & 87.31 (313 K) & 120 & 4.7 & [1] \\  & & 176.1 (333 K) & & & \\  & raw and purified clay & 54.0 (raw) & 10 (raw) & 6 & [63] \\  & & 72.0 (purified) & 20 (purified) & & \\ \hline \multirow{8}{*}{BV10} & beech tree bark & 18.0 & 240 & 3 & own research \\  & birch tree bark & 17.3 & 240 & 3 & own research \\  & cedar cone & 17.2 & 480 & 5 & [64] \\  & exhausted coffee ground & 2.5 & 180 & 2 & [65] \\  & fly ash & 1.9 & 4320 & & [66] \\  & sugarcane bagasse & 88.7 & - & 4 & [67] \\  & pine bark–compost & 126 & - & & [68] \\  & compost & 41.7 & 1440 & 5.3 & [69] \\  & banana peels & 9.5 & 60 & 2 & [70] \\ \hline \hline \end{tabular}
\end{table}
Table 9: Summary of sorption properties of various unconventional biosorbents.  
Red 46 than Basic Violet 10. The bark of beech and birch trees has proven to be an effective sorbent for cationic dyes.  
The pH correction is a prerequisite for sorption. The pH value of sorption for the cationic dyes Basic Red 46 and Basic Violet 10 should be determined individually.  
The time of equilibrium for sorption of dyes on the beech and birch bark sorbent depends on the type of dye and must be redetermined each time. This is due to differences in the type of dye and its molar mass.  
The adsorption of cationic pigments on beech and birch barks occurs in a single layer, as evidenced by a better fit of the experimental data to the Langmuir isotherm.  
Conceptualisation, U.F.; methodology, U.F., T.J. and M.D.; software, U.F. and T.J.; formal analysis, U.F. and M.F.; investigation, U.F., T.J., M.F. and M.D.; resources, U.F.; data curation, U.F. and T.J.; writing--original draft preparation, U.F.; writing--review and editing, T.J. and M.F.; visualisation, U.F. and T.J.; supervision, U.F.; project administration, U.F.; funding acquisition, 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 authors.  
The authors declare no conflicts of interest.