# The Use of Rapeseed Husks to Remove Acidic and Basic Dyes from Aquatic Solutions
## 3 Results and Discussion
### The Effect of pH on the Effectiveness of Dye Sorption onto RH
At pH = 6, their number was already very low, thanks to which BR46 sorption was not impaired by electrostatic interactions with the sorbent's surface. Despite the cationic nature of BR46, the pH increase above pH = 7 had no positive effect on its binding onto RH (Figure 2b), presumably due to the increasing concentration of Na+ cations, which competed with dye cations for free sorption centers. A similar effect of pH on the BR46 sorption effectiveness has also been noted in studies on the decolorization of aqueous solutions using citrus peel [42], crop leaves [43] and granular activated carbons [44]. The effectiveness of BV10 sorption onto the analyzed sorbent was the highest at pH = 3 and decreased along with pH increase. A significant decrease in its binding effectiveness was also noted when the pH dropped from pH = 3 to pH = 2 in the system (Figure 2b). A characteristic feature of the BV10 dye is its carboxyl functional group. Being capable of easy ionization, it is able to generate a local negative charge in the dye molecule. Owing to this property, BV10 can behave like an anionic dye in sorption systems despite its generally cationic nature. This explains the--unusual for the alkaline dye--effect of pH on its sorption effectiveness, i.e., sorption effectiveness increase along with solution pH decrease (Figure 2b). A noticeable decrease in the binding effectiveness of BV10 at pH drop from pH = 3 to pH = 2 may be due to deionization of the carboxyl group of the dye. The loss of the local negative charge in the dye's molecule significantly weakened the electrostatic interactions with the positively charged surface of RH. A very similar effect of pH on the BV10 sorption effectiveness, manifested by the highest intensity noted at pH = 3, has also been demonstrated in studies addressing the sorption properties of spent tea leaves [45], mealworm molds [46], and activated carbon from palm bark [47]. Rapeseed husks had a significant effect on the solution's pH change during sorption (Figure 2c-f). In the case of the solutions with the initial pH of 4-10, their post-sorption pH was set within the range of 6.12-6.90, which was due to the material having easily ionizable alkaline and acidic functional groups. Alkaline groups (e.g., amino groups) started to undergo protonation at pH < 9. In an acidic environment, they took protons from hydronium ions, whereas in a neutral or slightly alkaline environment--from water molecules. In each case, this increased the pH in the system. -NH2 + H3O+ -NH3+ + H2O(pH increase in the acidic environment)  
-NH2 + H2O -NH3+ + OH^-(pH increase in the neutral and slightly alkaline environment). In turn, acidic functional groups, like carboxylic groups, were deprotonated already at pH > 3. The proton released by the functional group attached itself to a hydroxyl ion (in the alkaline environment) or to a water molecule (in the neutral or acidic environment), causing solution's pH to decrease. -COOH2 + OH^- -COO^- + H2O(pH decrease in the acidic environment)  
-COOH + H2O -COO^- + H3O^-(pH reduction in the neutral and slightly acidic environment)  
The nature of pH change in the solution depends mainly on the ratio of alkaline groups to acidic groups on the surface of the sorption material. For this reason, the system always tends to reach the pH value that is close to the pH\({}_{\text{PZC}}\) (the point of zero charge) of the sorbent. The PZC determined for RH with the drift method was pH\({}_{\text{PZC}}\) = 6.62. (Figure 2g,h). This suggests the slightly acidic nature of the sorbent, which may be induced by a higher number of carboxylic groups than amine groups on its surface. Further stages of the study described in Sections 3.3 and 3.4 were accomplished at the optimal sorption pH, which was pH = 2 for the acidic dyes (AR18, AY23) as well as pH = 6 and pH = 3 for the basic dyes (BR46 and BV10, respectively).