Bioprospecting of coffee (Coffea arabica L.) husk as an eco-friendly biosorbent for the removal of Congo red dye

With increasing contamination of water bodies by synthetic dyes, conventional treatments are often costly and harmful. This study investigates the biosorption of Congo Red dye using raw and acid-treated coffee husk (RCH and ATCH). Characterization using Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Brunauer–Emmett–Teller (BET) surface area analysis confirmed that acid treatment enhanced the functional groups and surface area of the husk, improving its dye adsorption capacity. The maximum adsorption occurred at pH 2, with ATCH achieving a higher dye removal capacity (94%) compared to RCH (82%). Kinetic studies revealed that the adsorption followed both pseudo-first-order and pseudo-second-order models. The pseudo-first-order rate constants were 0.024 min⁻1 for RCH and 0.030 min⁻1 for ATCH, while the pseudo-second-order rate constants were 0.064 g/mg·min for RCH and 0.076 g/mg·min for ATCH. Isotherm models, including Langmuir, Freundlich, and Dubinin-Radushkevich (D-R), were applied to analyze the adsorption data. The Langmuir isotherm provided the best fit for both adsorbents, with ATCH showing a higher maximum adsorption capacity (25.30 mg/g) than RCH (20.5 mg/g). The Freundlich isotherm indicated a favorable adsorption process for both RCH and ATCH, with 1/n values suggesting good adsorption. The D-R model confirmed that the adsorption was predominantly physical in nature. Thermodynamic analysis showed that the adsorption processes were spontaneous and endothermic, with negative standard Gibbs free energy (ΔG°) values of -8.3 kJ/mol for RCH and -9.1 kJ/mol for ATCH, indicating the favorable nature of the adsorption. It can be concluded that, ATCH demonstrated superior biosorption efficiency for Congo Red dye removal, with enhanced surface properties and adsorption kinetics, positioning it as an effective and eco-friendly material for wastewater treatment.