Optimization of methylene blue adsorption using grapefruit peel-derived active biochar-magnetite/alginate bead composite via box-behnken design

Abstract This study reports the preparation and optimization of a magnetic biochar–alginate bead composite for the removal of methylene blue (MB) from aqueous solutions. Activated biochar (ABC) was produced from grapefruit peel (GP) waste through chemical activation and subsequently modified with Fe 3 O 4 nanoparticles to obtain magnetic biochar (ABC-Mag). The magnetic material was immobilized in sodium alginate to form easily separable hydrogel beads (GP-ABC-Fe@Alg). The composite was comprehensively characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, Barrett–Joyner–Halenda (BJH) pore distribution analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The BET surface area and total pore volume of the composite were determined as 349 m 2 g −1 and 0.251 cm 3 g −1 , respectively. Adsorption performance was optimized using response surface methodology (RSM) based on the Box–Behnken design (BBD), considering solution pH (2–10), initial MB concentration (50–150 mg L −1 ), and adsorbent dosage (0.02–0.10 g) as independent variables. The maximum adsorption capacity was obtained under optimum conditions of pH 6, 150 mg L −1 initial concentration, and 0.10 g adsorbent dosage. Isotherm and kinetic analyses indicated that the adsorption process was best described by the Temkin isotherm and pseudo-first-order kinetic model. Electrostatic interactions, π–π stacking, hydrogen bonding, and pore-filling effects were identified as the primary mechanisms governing MB adsorption. The results demonstrate that GP-ABC-Fe@Alg is a promising, low-cost, and magnetically recoverable adsorbent for dye-contaminated wastewater treatment.