Removal of Malachite Green by Poly(acrylamide-co-acrylic acid) Hydrogels: Analysis of Coulombic and Hydrogen Bond Donor–Acceptor Interactions

Abstract

Water pollution caused by dyes poses a significant threat to life on earth. Poly(acrylamide-co-acrylic acid) hydrogels are widely used to treat wastewater from various pollutants. This study aims to examine the removal of malachite green (MG), a harmful and persistent dye that could cause extensive environmental damage, from an aqueous solution by adjusting the initial concentration of acrylamide (AM) and the degree of copolymer crosslinking. The copolymer hydrogels efficiently eliminate MG in a brief timeframe. The most successful hydrogel accomplished a removal rate exceeding 96%. The copolymer of 4 wt % 1,6-hexanediol diacrylate and a concentration of 100 mg/mL AM was effective. The degree of swelling was affected by crosslinking density as expected, with low crosslinking ratios resulting in significant swelling and high ratios resulting in less swelling. To evaluate the results, a docking approach was used which presented three crosslinked models: low, medium, and high. The copolymer–dye hydrogel system displayed robust hydrogen bonding interactions, as confirmed by the high quantities of both donors and acceptors. It was determined that MG contains six rotatable bonds, enabling it to adapt and interact with the copolymer chains. The dye and copolymer enhance H-bond formation by providing two hydrogen bond donors and 16 hydrogen bond acceptors, respectively. Through capitalizing on cationic and anionic effects, the ionic MG/copolymer hydrogel system improves retention efficiency by enhancing attraction between opposing charges. It is interesting to note that the synthesized copolymer is able to remove 96.4% of MG from aqueous media within one hour of contact time.