Upcycling  <i>Citrus limetta</i>  peel into a high‑efficiency adsorbent for methylene blue removal: a waste-to-worth approach

In this study, NaOH-modified <i>Citrus limetta</i> peel (NCLP) was prepared and utilized as an effective adsorbent to remove methylene blue (MB) dye from synthetic wastewater. The prepared adsorbents were characterized using a range of techniques, including SEM/EDS, BET, TGA, XRD, proximate and component analyses, and Point of Zero Charge. Based on compositional and proximate analyses, NCLP was found to have high cellulose and fixed carbon contents, along with lower concentrations of volatile matter, moisture, hemicellulose, lignin, ash, and extractives. The studies were carried out by varying the adsorbent dose, contact time, pH, initial adsorbate concentration, and temperature. Different isotherm and kinetic models were applied; among them, the Langmuir isotherm and pseudo-second-order kinetics most accurately described the adsorption behavior of MB by NCLP. The adsorption of MB onto the NCLP adsorbent was found to be spontaneous and endothermic, accompanied by an increase in entropy. Using HCl as an eluent, five adsorption-desorption cycles were successfully carried out on MB-loaded NCLP. The present study confirms that NCLP is a suitable adsorbent for removing MB from wastewater.