Fabrication and analysis of copper oxide–based hydrogel nanocomposite for the extraction of MB dye from wastewater

This research focuses on the removal of a toxic pollutant viz. methylene blue (MB) from polluted water. Xanthan gum (XGm) was employed as the base polymer, while acrylamide (AAm) and methacrylic acid (MAA) were used as monomers. N-hydroxymethyl acrylamide (NHMA) served as the crosslinking agent in the free-radical polymerization process to synthesize the hydrogel XGmMAN. Three hydrogel variants were developed by varying the concentration of NHMA. To enhance the catalytic performance, copper oxide (CuO) nanoparticles were integrated into the hydrogels via the sol–gel method, forming a nanocomposite (XGmMAN@CuO NCs) that acted as catalysts for the reduction of MB dye. These materials were thoroughly characterized using techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), point of zero charge (ΔpHpzc), and scanning electron microscopy (SEM). The synthesized XGmMAN-3@CuO NCs achieved the highest dye removal efficiency of 98.65% at an optimal pH of 7, with a maximum adsorption capacity of 144.50 mg/g. Additional experiments investigated the effects of adsorbent dosage, pH, contact time, temperature, and dye concentration. The adsorption behavior followed the Langmuir isotherm model, while kinetic studies indicated a pseudo-second-order mechanism. Thermodynamic analysis revealed negative ΔG values, confirming that the adsorption process was spontaneous. The adsorbed MB dye was converted to a less harmful colorless substance leuco-methylene blue using NaBH₄ as a reducing agent. The catalytic activity of the nanocomposite toward MB reduction was assessed through UV–visible spectrophotometry, keeping the amount of NaBH₄ constant while varying the pH levels. The hydrogel nanocomposites showed excellent catalytic efficiency at moderate NaBH₄ concentrations, exhibiting its effectiveness for removal of MB dye from synthetic wastewater. Furthermore, the XGmMAN-3 hydrogel exhibited excellent reusability, achieving a desorption efficiency of 96.62% and maintaining consistent performance across five consecutive adsorption–desorption cycles.

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