Fe-modified hydrochar from orange peel as adsorbent of food colorant Brilliant Black: process optimization and kinetic studies

The main aims of this work were to produce and characterize Fe-modified hydrochar from orange peel waste, optimize the adsorption through response surface methodology, investigate the role of treatment time, dye concentration, adsorbent dose and temperature, and determine the dominant mechanisms through kinetics analysis. Orange peel waste was hydrothermally carbonized at 200 °C for 8 h, and the hydrochar was embedded with magnetite nanoparticles. The composite adsorbent was characterized through spectrometric and surface analytical methods. Subsequently, analysis of variance was used to design the experimental runs, propose a polynomial equation describing the adsorption process and finally optimize the adsorption conditions. The results indicated that 99% removal can be theoretically achieved at the following conditions: dye initial concentration of 6.08 mg/L, treatment time of 26.30 min, temperature of 44.79 °C and adsorbent concentration of 2.27 g/L. The dominant factors were the dye and adsorbent concentration, whereas time and temperature variations had a much lesser impact. Among examined models, the Langmuir model showed a better match to the experimental data. The maximum monolayer adsorption capacity was determined as 10.49 mg/g. The mechanism of interaction was largely based on surface chemisorption between the dye and adsorbent. Fe-modified hydrochar exhibited a positive adsorption behavior, and it was shown that a new valorization option for orange peel waste is available. This option may follow other valorization pathways, such as isolation of biologically active molecules, therefore offering a complete solution to this type of waste.