Preparation of a novel superabsorbent hydrogel based on polyacrylic acid/shellac using gamma irradiation for adsorption removal of malachite green dye

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Abstract
The objective of this study is to prepare a super absorbent hydrogel based on polyacrylic acid and an environmentally friendly material such as shellac to remove malachite green dye from aqueous solution using gamma radiation. The adsorption of malachite green (MG) dyes using polyacrylic acid (PAA)/shellac (SH) hydrogels was studied. These hydrogels were prepared by mixing aqueous solutions of PAA and SH at differing molecular ratios (10%, 20%, and 30% SH content in the final reaction mixture) and varying doses of gamma radiation (10–50 kGy). The water absorption by the PAA/SH hydrogels prepared at a 30-kGy radiation dose increased with an increase in the SH content. The removal of the MG dyes from an aqueous solution using PAA and PAA/SH hydrogels was investigated at different values for selected parameters, such as pH, contact time, SH concentration, and adsorbent dosage. The highest MG adsorption percentage (95.5%) was obtained for the PAA/SH hydrogel with the highest SH content (30%). As the adsorbent dosage increased, the dye adsorption capacity increased, accordingly. The combination of SH and gamma radiation caused the PAA to undergo structural changes verified by X-ray diffraction and Fourier transform infrared spectroscopy. Scanning electron microscopy was employed to investigate the morphology of the PAA/SH hydrogel and revealed that the SH belonging to the PAA matrixes exhibited a homogeneous dispersion. The thermal stability of the hydrogels was investigated by thermogravimetric analysis. The amount of dye adsorbed by the PAA/SH hydrogel was calculated by subtraction based on the ultraviolet–visible (UV–VIS) spectroscopy determination of the concentration of leftover dye in solution using a UV–VIS spectrophotometer.

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