Valorization of lemon peel waste into a polyacrylamide hydrogel composite for efficient adsorptive removal of Sunset Yellow FCF from aqueous media

Abstract

Abstract The uncontrolled discharge of synthetic azo dyes into aquatic environments poses a serious threat to ecosystem integrity and public health, necessitating the development of sustainable and efficient remediation strategies. In this study, a novel bioderived hydrogel composite (PAA@LP) was synthesized by integrating waste lemon peel (LP) biomass into a polyacrylamide network using free‐radical polymerization and evaluated for the adsorption of the anionic dye Sunset Yellow FCF (SY) from aqueous solutions. Structural and surface characterizations using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy–energy dispersive X‐ray (SEM–EDX) spectroscopy confirmed the successful incorporation of LP into the polymer matrix and revealed a porous, functionalized architecture rich in hydroxyl, carboxyl, and amide groups. Batch adsorption experiments demonstrated that SY removal was highly pH dependent, with maximum adsorption occurring under acidic conditions (pH 2–3), consistent with the composite's point of zero charge (pH p zc = 3.87). Equilibrium data were best described by the Langmuir isotherm model ( R 2 = 0.983), indicating monolayer adsorption with a maximum adsorption capacity of 89.5 mg g −1 . Kinetic analysis showed that the adsorption process followed a pseudo‐first‐order model, suggesting surface‐controlled chemisorption, and intraparticle diffusion contributed as a secondary rate‐limiting step. Thermodynamic parameters revealed that the adsorption process was spontaneous Δ( G ° = −4.58 to −11.8 kJ mol −1 ) and endothermic Δ( H ° = +53 kJ mol −1 ), with increased randomness at the solid–liquid interface Δ( S ° = +207 J mol −1 K −1 ). Regeneration studies indicated that the PAA@LP composite retained substantial adsorption performance over three adsorption–desorption cycles, achieving up to 62% desorption efficiency using 0.1 mmol dm −3 HCl. The adsorption mechanism was governed by a combination of electrostatic attraction, hydrogen bonding, π – π interactions, and partial chemisorption between the dye molecules and functional groups of the composite. Overall, this work demonstrates an effective waste‐to‐resource strategy by valorizing LP biomass into a low‐cost, reusable, and environmentally benign hydrogel adsorbent. The developed PAA@LP composite shows strong potential for sustainable treatment of dye‐contaminated wastewater and contributes to the advancement of circular bioeconomy‐oriented water remediation technologies.