Valorization of Pomelo Peel into a Carboxyl-Rich Adsorbent for Charge-Selective Removal of Cationic Dyes

<title>Abstract</title> <p> Efficient treatment of multicomponent dye effluents requires low-cost adsorbents that combine high adsorption capacity, selectivity, and reusability. In this study, a carboxyl-functionalized pomelo peel biosorbent (CPP) was prepared through alkali treatment, oxidative bleaching and esterification grafting. The modification introduced abundant carboxylate groups, converted the compact biomass matrix into a porous nanoparticulate architecture, increased the specific surface area from 8.19 to 17.41 m ² ·g ^-1 and produced a superhydrophilic, negatively charged surface. CPP preferentially adsorbed cationic dyes, achieving removal efficiencies exceeding 90% for methylene blue (MB) and methyl violet (MV), while exhibiting limited affinity for anionic dyes. The maximum adsorption capacities estimated using the Langmuir model were 954.20 mg·g ^-1 for MB and 1170.96 mg·g ^-1 for MV. The adsorption process followed the pseudo-second-order kinetic model and was thermodynamically spontaneous and endothermic. Competitive adsorption and column separation experiments further demonstrated the effective separation of cationic dyes from mixed-dye systems. After six adsorption-desorption cycles, CPP retained adsorption capacities of 566.37 mg·g ^-1 for MB and 809.10 mg·g ^-1 for MV. Mechanistic analyses indicated that electrostatic attraction was the predominant driving force, with hydrogen bonding providing an additional contribution. These findings demonstrate the potential of CPP as a reusable biosorbent for the selective treatment of dye-contaminated water. </p>