Hydrochar derived from Liquorice root pulp utilizing catalytic/non-catalytic hydrothermal carbonization: RSM optimization and cationic dye adsorption assessment

Abstract

The present study targeted producing hydrochar from Liquorice root pulp employing the catalytic/non-catalytic hydrothermal carbonization (HTC) method and evaluating its performance in methylene blue adsorption from an aqueous solution. The response surface methodology (RSM) was utilized to assess the impact of operational variables such as temperature, time, and water/ solid ratio on the yield of hydrochar production and MB adsorption capacity. The maximum yield of hydrochar production and dye adsorption capacity was achieved at 168 °C, 2.016 h, and a water/solid ratio of 9.215. The assessment of the catalytic HTC process at different concentrations of ZnCl2 demonstrated the 0.25 M solution had the most positive effect on MB adsorption capacity. The physicochemical characteristics of hydrochars were evaluated utilizing techniques such as CHN, FTIR, BET, XRD, TGA, and SEM. Also, the peak fitting method presented a semi-quantitative assessment of the impact of the catalytic/ non-catalytic HTC process on the oxygenated functional groups of hydrochars. The methylene blue adsorption process was evaluated by conducting the experiments at different values of effective parameters such as pH, contact time, initial dye concentration, and dose of adsorbent as temperature. Furthermore, the kinetic and isotherm models were studied to describe the dominant mechanisms in the MB adsorption process. The maximum MB adsorption capacity was achieved at 290.65 and 314.62 mg/g for hydrochar and modified hydrochar at pH of 9, initial dye concentration of 400 mg/L, 45 min of contact time, 1 g/L of adsorbent dose, and temperature of 25 °C. Finally, studying the thermodynamic parameters of the adsorption process demonstrated MB adsorption's spontaneity and exothermic nature.