Converting Hylocereus undatus (white dragon fruit) peel waste into a useful potential adsorbent for the removal of toxic Congo red dye

Dragon fruits are considered as exotic fruits and have recently gained popularity due to their health benefits in having high nutritional values and good antioxidant properties. The peels, being inedible, are discarded as wastes. In this study, white dragon fruit peel (WDFP) was utilized as a potential low-cost natural adsorbent for the removal of toxic Congo red (CR) dye. This is the first report on the use of WDFP as an adsorbent. Previous reports on white dragon fruit were based on utilizing its foliage. The WDFP-CR system achieved equilibrium within 3 h. WDFP, whose point of zero charges was determined to be at pH 5.02, exhibits good overall resilience to changes in both pH medium and ionic strength. Adsorption isotherm indicated the Temkin model to be the best fit followed by the Sips model with high maximum adsorption capacity (qmax) of 76.6 mg g-1 as compared to many other reported natural adsorbents. Adsorption process using 100 mg L-1 CR dye confirmed the pseudo- second-order kinetics with rate constant k2 of 1.87 g mmol-1 min-1. The Weber and Morris model indicated that intraparticle diffusion of CR into WDFP could be the rate-determining step. WDFP can be regenerated and was found to be reusable up to 4 consecutive cycles when using acid as the desorbing solvent.