## 3 Results and Discussion
### Influence of pH on the Dye Sorption Efficiency on MES and MMS  
RB5 and RY84 sorption efficiency on MES and MMS peaks at pH 2 and deteriorates as the pH increases, with the worst performance noted at pH 11. The highest drop in dye adsorption is observed between pH 2 and 4. In the RB5 groups, a slight increase in sorption intensity is noted at pH 9. Dye sorption of RB5 and RY84 to MES and MMS is the lowest at pH 11 (Figure 3a,b). Figure 2: FTIR spectra of MES and MMS. The high efficiency of anionic dye sorption at low pH resulted from a strong positive charge acquired by the sorbent. The hydronium ions of the acidic solution caused some functional groups (e.g., amine acetamide, hydroxyl) to protonate. -NHCOCH3+H3O+--NH2 COCH3++H2O  
Figure 3: The effect of pH on the sorption effectiveness of (**a**) RB5 and (**b**) RY84 onto MES and MMS (mean + range). The effect of MES and MMS on solution pH during dye sorption: (**c**) RB5 and (**d**) RY84. (**e**,**f**) Determination of pHFZC for MES and MMS with the drift method. \[\text{-NH}_{2}+\text{H}_{3}\text{O}^{+}\rightarrow\text{-NH}_{3}{}^{+}+\text{H}_{2} \text{O}\]  
\[\text{-OH}+\text{H}_{3}\text{O}^{+}\rightarrow\text{-OH}_{2}{}^{+}+\text{H}_{2} \text{O}\]  
The positively charged functional groups of the sorbents (-NH\({}_{3}\)+, -OH\({}_{2}\)+) electrostatically interact with the negatively charged groups of the dyes (-SO\({}_{3}\)-), greatly promoting sorption. As the pH increases, there are fewer and fewer hydronium ions in the system, limiting the protonation of functional groups. The lower number of positively charged functional groups translates to lower sorption efficiency of the anionic dyes. At pH > 4, the dye binding rate became relatively low, as only the primary amine groups (fairly sparse on the MES and MMS surface) are still being protonated. This explains the significant decrease in RB5 and RY84 sorption on the tested sorbents at pH 2-4. At high pH (pH > 9), dye sorption may have been further impeded by the negative charge attained by the sorbent, potentially caused by the deprotonation of certain (e.g., hydroxyl or carboxylic) functional groups upon interaction with the dissolved hydroxyl ions. \[\text{-COOH}+\text{OH}^{-}\rightarrow\text{-COO}^{-}+\text{H}_{2} \text{O}\]  
\[\text{-OH}+\text{OH}^{-}\rightarrow\text{-O}^{-}+\text{H}_{2} \text{O}\]  
The negatively charged functional groups electrostatically repulse the anionic dyes, suppressing sorption. RB5 and RY84 removal at high pH may have also been inhibited by competition with OH- ions for sorption centers. The positive effect of low pH on anionic dye sorption efficiency has been corroborated by studies on RB5 sorption onto purified chitin [27], feathers [28], and activated carbon [29], as well as RY84 sorption onto cotton fiber [30] and sunflower hulls [31]. The slight increase in RB5 sorption efficiency observed at pH 9 (Figure 3a) is caused by the primary amine groups in the dye structure. At pH 9, MES and MMS already possessed a total negative charge, while a significant amount of the amine groups of RB5 have been ionized (-NH\({}_{3}\)+). Despite the generally anionic nature of the RB5, its protonated amine functional group generates a local positive electric charge, which may have electrostatically interacted with the negatively charged sorbent's surface, driving RB5 sorption. The positive effect of pH 9 on RB5 sorption has also been reported in studies investigating its sorption on eggshell membranes [32] and cotton fiber [30]. However, the same positive effect is not noted for the RY84 group due to the lack of primary amine groups in the dye. Sorbents based on mealworm exoskeletons and molts affect solution pH during sorption (Figure 3c,d). With starting pH of 4-10, the solution pH after sorption is 6.88-7.49 for MES and 6.77-7.40 for MMS. The type of dye has little effect on pH trends in the system. The pH-modifying capacity of the sorbents results from their high content of readily-ionized functional groups, such as amine or carboxyl groups.