Water-Soluble Graft Copolymer Synthesized from Collagenous Waste and Polyethylene Glycol (PEG) with Functional Carboxylic Chains: A Highly Efficient Adsorbent for Chromium(III) with Continuous Recycling and Molecular Docking Studies

Abstract

A pragmatic pathway to zero emission is achieved by utilizing solid and liquid wastes completely by recycling and generating value-added auxiliaries from chrome shaving waste through process-intensified unit operations. Graft copolymer was synthesized from chrome shaving waste and polyethylene glycol (PEG), which is used as an adsorbent in the fresh chrome tanning process. The copolymer was characterized via thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy, and particle size analyses. FT-IR results have been characterized, confirming that the presence of free functional carboxylic acid is involved in chromium adsorption. The application of 6% copolymer in chrome tanning showed 95.95% optimal adsorption of chromium in the first cycle and 98.90% optimal adsorption of chromium in the second cycle. 1H NMR investigation revealed a peak at 8.384 ppm (6% copolymer) in the experimental sample, confirming that the participation of free functional carboxylic acid groups plays a primary role in adsorbing chromium. Molecular modeling showed an increased adsorption of chromium and greater participation of ligand–copolymer with collagen, resulting in a docking energy of 3.9 kcal/mol through hydrogen bonding, establishing the spatial arrangement of the active functional groups. A mathematical model for the prediction of concentration of chromium adsorbed by the copolymer has been proposed. Scanning electron microscopy (SEM) and scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDX) studies confirmed an increase in the adsorption of Cr especially: 6.33 wt % of Cr (2.03 at. %) in samples containing 6% copolymer. Atomic force microscopy (AFM) studies from topography and deflection showed clear pictures of increased adsorption of chromium by the copolymer, indicated by the absence of chromium spots on the surface for the experimental samples.