Optimized removal process and tailored adsorption mechanism of crystal violet and methylene blue dyes by activated carbon derived from mixed orange peel and watermelon rind using microwave-induced ZnCl2 activation

Releasing wastewater containing organic dyes into water bodies generates a variety of hazards for humans and marine life. Thus, developing effective adsorbents to remove organic dyes from wastewater is critical. Herein, a mixture of the orange peel (OP) and watermelon rind (WR) wastes was converted into a mesoporous activated carbon (OPWRAC) via microwave-induced ZnCl2. Several analytical techniques such as XRD, N-2 adsorption-desorption isotherms, FTIR, pH(pzc), and SEM-EDX were applied to characterize the physicochemical properties of OPWRAC. Subsequently, the adsorptive efficiency of OPWRAC was comprehensively explored towards the removal of two structurally different organic dyes, namely, crystal violet (CV) and methylene blue (MB). The operational adsorption conditions such as OPWRAC dose (0.04-0.1 g) coded as (A), solution pH (4-10) coded as (B), and contact time (10-60 min) coded as (C) were statistically optimized using the response surface methodology-Box-Behnken design (RSM-BBD). The adsorption isotherm data for CV and MB dyes agree with the Freundlich model, and the kinetic data can be explained by the pseudo-second-order model. Thus, OPWRAC displays remarkable adsorption capacity for capturing CV (137.8 mg/g) and MB (200.7 mg/g). The tailored adsorption mechanism of CV and MB by the OPWRAC indicates the involvement of several types of electrostatic forces, pi-pi stacking, pore filling, and H-bonding. The output of this research shows the feasibility of converting the mixture of OP and WR into promising activated carbon with potential application for capturing two structurally cationic dyes from an aqueous environment.