# 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  
The sorption effectiveness of acidic dyes (AR18, AY23) onto RH was the highest at pH = 2 and decreased along with pH increasing up to pH = 11. The greatest decrease in the binding effectiveness of these dyes onto the tested sorbent was recorded in the pH range of 2-4. In the pH range, of pH 5-10, the AR18 and AY23 sorption effectiveness onto RH was similar and low (Figure 2a). Figure 1: FTIR spectra of rapeseed husks (RH). Figure 2: Effect of pH on the effectiveness of sorption of (**a**) acidic dyes (AR18/AY23) and (**b**) basic dyes (BR46/BV10) onto RH (average + range). Effect of RH on changes in solution pH after sorption of (**c**) AR18, (**d**) AY23, (**e**) BR46, and (**f**) BV10. (**g.h**) Determination of pHpzc of the RH using the “drift” method. The high sorption effectiveness of anionic dyes on RH in the acidic environment might have been due to the positive charge gained by sorbent's surface. At pH < 6, most of the amine groups [36] underwent protonation, whereas the hydroxyl groups started to undergo protonation presumably at pH < 3. -OH + H3O+ -OH2+ + H2O  
-NH2 + H3O+ -NH3+ + H2O  
Positively charged RH functional groups attracted electrostatically the ionized sulfone groups of AR18 and AY23, which significantly intensified their sorption. Since the number of protonated functional groups increased with pH decrease, the highest binding effectiveness was obtained at the lowest pH tested (pH = 2). Rapeseed husks have a high content of polysaccharides (cellulose, hemicellulose) and lignin (rich in hydroxyl groups) and a fairly low content of proteins (having amine groups). Due to the low content of the amine groups (despite their high ionization susceptibility at pH < 6), the total number of functional groups protonated on the RH surface in the pH range of 4-6 was relatively low, which explains the low sorption effectiveness of AR18 and AY23 in this pH range. In an alkaline environment, the sorbent's surface might have gained a negative charge, presumably due to deprotonation of certain functional groups (e.g., hydroxyl ones). - + H2O  
A negatively charged RH surface could repel the dyes electrostatically. With the increase in pH, the number of deprotonated functional groups of RH increased, which translated into an increasing total negative charge of the sorbent and, thus, a stronger inhibition of dye sorption. This explains the lowest sorption effectiveness of both AR18 and AY23 determined at pH = 11. The high sorption effectiveness of AR18 at low pH was also noted in the studies addressing the treatment of colored aquatic solutions on sunflower husks [37], chitosan flakes [38], and also activated carbons [39]. In the case of AY23, the positive effect of low pH was also observed during decolorization of solutions on sawdust [40] and chitin [41]. The sorption effectiveness of the cationic dye BR46 onto RH increased with pH increasing up to pH = 6 (Figure 2b). The greatest leap in the binding effectiveness of this dye on the tested sorbent was noted in the pH range of 2-4. At pH > 7, the sorption effectiveness of BR46 on rapeseed husks was observed to decrease (Figure 2b). Due to the spontaneous decolorization of BR46 solutions at pH > 8, the results related to its sorption in the pH range of 9-11 are not presented in Figure 2b. The relatively high effectiveness of basic BR46 sorption onto rapeseed husks over a broad pH range (pH 4-8) may be due to the generally acidic nature of the sorbent. Ionized carboxyl groups located on RH surface can serve as sorption centers for BR46. The binding of BR46 on the analyzed sorbent can also proceed through hydrogen bonds (between hydrogen and nitrogen atoms, and between hydrogen and oxygen) and Van der Waals forces. The low sorption effectiveness of BR46 at low pH (pH = 2) was due to a strong positive charge on the surface of the sorbent, which electrostatically repelled BR46 cations. With the increase in pH, the number of protonated functional groups of RH decreased.