Biosorption and Charcoal-Based Adsorption of Malachite Green Using Watermelon Rind: A DualApproach Investigation

Abstract

In the present study, two different and complimentary approaches are used for the removal of malachite green (MG) dye from aqueous solutions. These include raw biosorption by unmodified watermelon rind (WMR) powder and adsorption onto charcoal prepared from the same agricultural waste. The watermelon rind, a by-product of the food-processing industry, is mostly made of cellulose, pectin and hemicellulose and has been increasingly considered as a cheap biosorbent due to the presence of a large number of carboxyl and hydroxyl functional groups on its surface. In this study, the raw and carbonised WMR were characterised by FTIR, SEM, BET surface area analysis and pHpzc measurement. The effect of solution pH, contact time, initial dye concentration, adsorbent dosage and temperature on the percentage removal and adsorption capacity for each material were studied by systematic batch experiments. The results show that the charcoal based adsorbent has much greater adsorption capacity than the raw biosorbent, mainly due to its increased surface area and improved pore formation. Equilibrium data fitted best to the Langmuir isotherm model indicating monolayer adsorption. The pseudo second order kinetic model was found to be the best fit of the time dependent uptake curves for both adsorbents. Thermodynamic data corroborated the spontaneity and endothermic character of the adsorption process. These findings indicate that watermelon rind in raw and carbonised states is a potential, inexpensive and environmentally friendly alternative adsorbent for malachite green treatment from industrial effluents.

Highlights
Raw watermelon rind and its derived charcoal were compared for malachite green removal.
Charcoal-based adsorbent showed significantly higher adsorption capacity than raw biosorbent.
Langmuir isotherm and pseudo-second-order kinetics best described both systems.
Thermodynamic analysis confirmed spontaneous and endothermic adsorption behavior.
Watermelon rind is proposed as a sustainable, low-cost alternative for dye remediation.