In Situ Allocation of a Monomer in Pectin-<i>g</i>-Terpolymer Hydrogels and Effect of Comonomer Compositions on Superadsorption of Metal Ions/Dyes

Abstract

Pectin-<i>g</i>-(sodium acrylate-<i>co</i>-3-(<i>N</i>-isopropylacrylamido) sodium propanoate-<i>co</i>-<i>N</i>-isopropylacrylamide) interpenetrating polymer networks (PANIPNs) were synthesized through systematic multistage optimization of equilibrium swelling ratio by response surface methodology for individual and/or synergistic removal(s) of cationic safranine (SF), anionic methyl orange, and M(II/III), such as Hg(II), Cd(II), and Cr(III). The relative effects of copolymer compositions on ligand-selective adsorption, strong/weak H-bonds, thermal stabilities, crystallinity, surface properties, swelling abilities, cross-link densities, network parameters, hydrophilic-hydrophobic characteristics, and adsorption capacities (ACs) were measured through extensive microstructural analyses of adsorbed and/or unadsorbed PANIPN41 and PANIPN21 bearing sodium acrylate and <i>N</i>-isopropylacrylamide (SA/NIPAm) in 4:1 and 2:1 ratios, respectively, using Fourier transform infrared, ¹H and ¹³C NMR, X-ray photoelectron spectroscopy, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy, along with measuring lower critical solution temperature, % gel content (% GC), % -COOH, and pH_PZC. Extensive UV-vis measurements were carried out at varying copolymer compositions, initial pH (pH_i), and dyes, interpreted considering monomer-dimer and azonium-ammonium equilibrium of dye, dye-dye complexation, ligand-selective PANIPNs-dye adduct formation, π-π stacking interactions, and orientation effect of dyes. Thermodynamically feasible chemisorption processes showed the maximum ACs of 127.61, 96.78, 103.36, and 99.41 mg g^-1 for SF, Hg(II), Cd(II), and Cr(III), respectively, under optimum conditions.