Poly (acrylic acid-co-2-hydroxyethyl methacrylate)-grafted gum ghatti hydrogel for capturing heavy metal ions

In this work, a facile route is explored for the synthesis of a novel polymer composite-based hydrogel (PC-hydrogel). The ratio of 2-(Hydroxyethyl) methacrylate (HEMA) and acrylic acid (AA) is optimized first based on Fourier transform infra-red spectroscopy, swelling ratio (SR%) and surface negative charge (PZC). Results indicate that PC-hydrogel composed of copolymer of HEMA: AA in 1:4 ratio is optimized, for grafting on Gum ghatti (Gg) during free-radical graft copolymerization process. Among all other possible combination of HEMA: AA, 1:4 ratio grafted Gg is termed as PC-hydrogel [Poly (AA-co-HEMA)-g-Gg]. PC-hydrogel exhibited negative surface charge over a wide range of pH owing to increase in AA. The swelling (g/g) and water retention ratio (%) of the prepared hydrogel have been found to be 342.6, 385 & 412.6 g/g and 74.83, 65.30 & 57.86 % in grey, tap and distilled water respectively. Furthermore, PC-hydrogel is applied for capturing Cu²⁺ and Co²⁺ ions in aqueous phases. Experimental results showed that adsorption process was pH-dependent, and the maximum capturing of Cu²⁺ and Co²⁺ was observed at neutral pH 7. Among different adsorption isotherms models like Langmuir, Freundlich, and Temkin models, experimental data fitted closely with the Langmuir adsorption model showing a maximum adsorption capacity of 381.67 and 328.94 mg/g for Cu²⁺ and Co²⁺ respectively. The capturing of metal ion followed pseudo-second-order rate model [rate constant k = 1.7 x 10⁻⁴ for Cu²⁺ and 1.5 x 10⁻⁴ for Co²⁺ g/(mg.min)]. The PC-hydrogel property retained its uptake capacity of metal ions up to the three successive adsorption−desorption cycles, and exhibited higher selectivity towards Cu²⁺ and Co²⁺ and other (NaCl, MgCl₂, CaCl₂) coexisting ions.