Torrefaction of Cassia fistula seeds for sequestration of aqueous and gaseous pollutants: Experimental and computational approach

Abstract

Contamination of water bodies due to pharmaceutical pollutants including antibiotics is growing day by day due to enhanced consumption of these molecules. Biochar is a competent material for wastewater treatment due to ease of preparation as well as high adsorption capability towards desired pollutants. Cassia fistula biochar (CFB) was prepared by torrefaction process in an inert atmosphere. Owing to a large surface area of 672.3 m²/g, the purpose of this work is to carry out adsorption studies of three antibiotics, namely, ciprofloxacin (CFX), levofloxacin (LVX) and diclofenac sodium (DFC) in aqueous phase as well as for the sequestration of carbon dioxide in gaseous phase. The batch adsorption studies were carried and effects of various operational conditions were studied. The maximum adsorption capacities for CFX, LVX, and DFC were found to be 607.86 mg/g, 68.27 mg/g and 160.51 mg/g respectively under the optimum pH 6.0 for all the three adsorbates, contact time of 60 minutes for CFX, LVX and 20 minutes for DFC at room temperature condition of 298 K. Various operational parameters were optimized using Response Surface Methodology (RSM). Isotherm and kinetics studies for the adsorption of all three drugs followed Langmuir model (R² >0.99) and pseudo-second order kinetics (R² >0.95). Thermodynamic studies show the adsorption of all three drugs were enthalpy driven spontaneous processes. Fixed bed studies were performed showing the applicability of CFB for larger sample volumes. DFT calculations showed strong attractive interaction of CFB with all the three drug molecules. The same material has been applied for capture of carbon dioxide at different temperatures. The CO₂ capture studies showed maximum adsorption capacity of 64.78 cc/g at 273 K owing to activation of CFB with high CO₂ selectivity of 14.29 with respect to nitrogen. Hence, a multipurpose adsorbent has been thoroughly studied with environmental sustainability factor of 0.03.