Enhanced adsorption of anionic dyes onto NaOH modified green coffee waste biochar: characteristics and performance evaluation

Synthetic dyes are resistant to self-degradation in aqueous systems that pose severe health hazards to marine life. This work focuses on the comparative adsorption evaluation of synthesized biochar derived from green coffee wastes (GCB) and NaOH-modified GCB (NaOH-GCB) for the removal of anionic dyes (Eriochrome Black T and Congo Red) in aqueous solution. The characterization results of NaOH-GCB indicate higher thermal stability, improved surface functionality with -OH groups, smaller particle size (from 2–15 µm decreased to 0.5–1 µm), higher pore volume (increase from 0.0027 to 0.003 cm³/g), and higher surface area (increase from 2.91 to 3.22 m²/g) which enhanced dye adsorption. The maximum adsorption capacity was attained using NaOH-GCB with values of 119.37 mg/g and 91.54 mg/g recorded for EBT and CR, respectively. For both adsorbents, the optimal removal for EBT and CR dyes was obtained at solution pH 2 and 7 due to electrostatic and hydrogen bonding interactions. CR had slightly lower removal than EBT due to its hydrophobic nature. Pseudo-second order and Redlich-Peterson were the most effective models in providing insights about the adsorption kinetics and isotherm, with the system being heterogeneous and involving multi-layer behavior. Additionally, thermodynamics elucidated the adsorption to be spontaneous and exothermic. After five reusability cycles, the NaOH-GCB indicated excellent performance of over 90% removal for both dyes. The results show that NaOH-GCB is potentially an economical adsorbent for efficient and effective remediation of dye-contaminated wastewater streams.