Removal of methylene blue by using sodium alginate-based hydrogel; validation of experimental findings via DFT calculations

Removal of commonly used dyes from water bodies has recently gained great interest from the scientific community. Presence of the methylene blue (MB) dye in drinking water poses harmful effects on the human health. The large-scale removal of MB is achievable through highly efficient, inexpensive, renewable, and biodegradable adsorbents. Our research group has recently synthesized a sodium alginate-based hydrogel and explored its application towards the removal of MB. Previous results have shown that the synthesized hydrogel exhibits a high adsorption capacity of 51.34 mg/g under basic conditions. Herein, we employed the density functional theory (DFT) calculations to explore the mechanism of MB removal by using sodium alginate hydrogel at various pH levels. Results of this study have shown that under acidic/neutral conditions the removal of MB is endergonic (ΔGint = 6.10 kcal/mol). Whereas under basic conditions it is highly exergonic (ΔGint = -97.58 kcal/mol). Moreover, the QTAIM and NCI analyses have shown that the MB dye is chemisorbed to the absorbent via strong covalent-like interactions between the polymer's carboxylate groups and the hydrogens in MB. Furthermore, preferability of basic conditions have been confirmed by the large charge transfer (0.104 |e|), as compared to no charge being transferred in the acidic/neutral conditions.