Cocoa Pod Husk–Derived Biochar for High-Efficiency Removal of Congo Red and Methylene Blue from aqueous solutions

Abstract

The persistent discharge of dye-contaminated effluents from textile activities necessitates the development of low-cost and sustainable adsorbents for wastewater treatment. In this study, cocoa pod husk, an abundant agricultural waste, was valorized into biochar via thermal pyrolysis and applied for the removal of Congo Red (CR) and Methylene Blue (MB) from aqueous solutions. The prepared biochar was characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), revealing a porous surface enriched with oxygen-containing functional groups. Adsorption performance was strongly pH-dependent, with optimum CR removal at pH 2 and MB at pH 8. Equilibrium was achieved within 120–150 min, yielding removal efficiencies exceeding 90% at an initial concentration of 100 mg L⁻¹ of CR and MB. Kinetic data followed the pseudo-second-order model (R² > 0.99), indicating chemisorption-controlled adsorption. Equilibrium data were better described by the Freundlich isotherm, with Langmuir monolayer capacities of 75.99 mg g⁻¹ for CR and 67.43 mg g⁻¹ for MB. These results highlight cocoa pod husk–derived biochar as an effective material for textile wastewater remediation.