Adsorptive removal of basic cationic dyes from aqueous solution by chemically protonated watermelon (Citrullus lanatus) rind biomass

ABSTRACT

This study presents a study on the potentiality of protonated watermelon rind (PWR), an agro waste, to remove methylene blue (MB), crystal violet (CV), and rhodamine B (RB) dyes from aqueous solution. Protonation of watermelon rind was carried out using hydrochloric acid, and process was evidenced by EDX patterns. The batch experiments were performed systematically by varying contact time, pH, adsorbent dose, initial concentration, and temperature. Effects of salt ionic strength were also studied. Sorption of dyes onto watermelon rind is found to be rapid, and equilibrium has been reached in less than 30 min. The equilibrium data tend to fit better with Langmuir isotherm than Freundlich isotherm. The maximum loading capacity of PWR is found to be 489.8, 104.76, and 86.6 mg g⁻¹, respectively, for MB, CV, and RB. Kinetic studies indicate the adsorption process that follows pseudo-second-order model. Thermodynamic parameters including change in free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) are derived. Desorption of dyes studies was performed with HCl, acetic acid, H₂O, and NaOH. The results suggest that the watermelon rind can be an eco-friendly, low-cost and value-added biosorbent for the removal of MB, CV, and RB from aqueous solution.

Keywords:

• Watermelon rind
• Agro waste
• Protonation
• Biosorption
• Dyes