Effect of Pyrolysis of Rice Husk–Derived Biochar on the Fuel Characteristics and Adsorption of Fluoride from Aqueous Solution

Biochar obtained from pyrolysis of biomass finds multiple applications in combating environmental problems. However, the quality and quantity of biochar depends closely on the synthesis conditions and nature of the feedstock. The present study investigates the efficacy of employing rice husk–derived biochar for dual application: as a solid fuel and as an adsorbent. This study employs a response surface methodology (RSM) to optimize experimental parameters, temperature, time and heating rate. RSM provides linear and interaction effect amongst variables for selected responses, fuel ratio and percentage of fluoride removal. The optimum conditions for experimental factors (temperature, time and heating rate) were found to be 500 °C, 55 min and 7 °C/min. At the optimum conditions, the fuel ratio and percentage of fluoride removal were found to be 2.44 and 79.2% respectively. Moreover, the percentage of biochar yield at optimum conditions was found to be 40.7%. The Langmuir isotherm model was found to be applicable with a maximum monolayer adsorption capacity (Qm) of fluoride of 1.856 mg/g at 303 K. Thermodynamic studies demonstrated enhanced adsorption at lower temperature, and parameters such as change in free energy (ΔG) − 23.32 kJ mol−1, change in enthalpy (ΔH) 22.82 kJ mol−1 and change in entropy (ΔS) 0.15 kJ mol−1 K−1 indicate spontaneous nature of reaction. This study successfully converted biomass-derived biochar into a value-added product which could be used either as a solid fuel or as a potential adsorbent for effective removal of fluoride.