Optimization of coffee grounds-based activated carbon catalyst for ozone water treatment: A Box-Behnken design approach

Abstract

This study addresses two pressing environmental issues—resource conservation and waste valorization—while advancing water cleaning solutions. Activated carbon derived from coffee grounds (ACCG) was synthesized to optimize oxalic acid (OXL) removal via catalytic ozonation, as OXL is not effectively degraded by ozone alone, which can lead to its persistence in the environment. A Box-Behnken design approach was used to optimize synthesis conditions, namely dwell temperature (°C), dwell time (h) and %CO₂ in gas flow rate, through response surface methodology (RSM). The resulting materials were characterized by thermogravimetric analysis (TGA), specific surface area measurement by nitrogen adsorption-dessorption isotherms at −196 °C, Fourier transform infrared (FTIR) and Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). The statistical model that describes the response adjusts perfectly to the experimental data with R² = 0.994 and the analysis of variance (ANOVA) confirms that it is statistically predictive and significant. Optimal synthesis conditions for the highest OXL degradation rate constant (k', min⁻¹) were identified as: 400 °C of dwell temperature, 2 h of dwell time and 70 % of CO₂ (v/v). The ACCG prepared under these conditions enabled a k’ 14-fold higher than that achieved with single ozonation. Selective quenching experiments suggest that singlet oxygen (¹O₂) is the main reactive oxygen species formed during catalytic ozonation. The best performing ACCG was submitted to three reutilization cycles, and although a more prominent activity loss was observed after the 1st cycle, the catalyst maintained good catalytic activity across all cycles, consistently achieving higher OXL removal than ozonation alone. Overall, this study provided a sustainable approach to managing waste by valorizing coffee grounds into effective catalysts while enhancing water treatment efficiency.