An anionic adsorbent was prepared by grafting citrate onto pomelo peel (PPL) to remove methylene blue (MB) from aqueous solution. The PPL and modified pomelo peel (MPPL) were analyzed by Fourier transform infrared spectroscopy (FTIR) and observed by scanning electron microscopy (SEM). The effects of dye concentration, contact time, and pH on adsorption were studied. The FTIR results confirmed that the carboxyl groups were successfully bound to cellulose molecules in PPL via modification with citrate. SEM indicated that the surface of PPL became clean and the porous structure was well maintained after modification. The adsorption capacities of MB onto PPL and MPPL were 81.7 mg/g and 199.2 mg/g, respectively, thus indicating that the addition of anionic groups significantly improved the adsorption performance. The increase in the initial dye concentration and pH of the dye solution promoted the adsorption process. The adsorption equilibrium on MPPL required approximately 3 h. The adsorption of MB on MPPL was well described by a pseudo-second order kinetic model and Langmuir isotherm model. The thermodynamic parameters indicated spontaneous and exothermic adsorption. This study suggests that PPL modified with citrate can be used as a sustainable adsorbent in wastewater purification.
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