Investigation of Use of Hydrochars Obtained From Legume Wastes as Fuel and Their Conversion into Activated Carbon for Amoxicillin Removal

Legume wastes, pinto bean peel (PBP) and pea shell (PS), were hydrothermally carbonized in subcritical water at various temperatures (200-240 °C) with the aim of obtaining a solid fuel, hydrochar. Fuel characteristics and chemical properties of hydrochars were determined by standard fuel analysis methods. Hydrochar yield decreased sharply with the increase of temperature due to the enhanced degradation of legume wastes. The weight percent of initial carbon in the legume wastes retained in the obtained hydrochars was lower than those in the literature due to the low hydrochar yields. The effect of temperature on carbon content and hence higher heating value (HHV) of hydrochar became noticable at 240°C. As a result of this effect, bituminous coal-like and lignite-like hydrochars with HHV of 31.2 and 28.1 MJ.kg-1were obtained from PBP and PS, respectively. Hydrochars obtained at 220 °C were chemically activated with ZnCl2 to produce activated carbons (PBP-AHC and PS-AHC). The activated carbons were characterized by elemental analysis, FTIR spectroscopy, BET surface area analysis and Scanning Electron Microscopy (SEM). BET surface area, total pore volume, and mesopore volume of PS-HC were determined as 1205 m2. g-1, 0.686 m3. g-1 and 0.144 m3. g-1, respectively. PBP-AHC was found to have higher BET surface area (1350 m2. g-1), total pore volume (0.723 m3. g-1), and mesopore volume (0.249 m3. g-1) than PS-AHC. Activated carbons were tested as adsorbent for removal of amoxicillin (AMX) from aqueous solutions with the batch adsorption studies carried out at different initial concentrations, adsorbent dosage, and contact time. The compatibility of the adsorption data with the Langmuir and Freundlich isotherm models was checked to determine the adsorption capacity of activated carbons. The maximum Langmuir adsorption capacity (Qmax) was calculated as 188.7 and 70.9 mg. g-1 for PBP-AHC and PS-AHC, respectively. Adsorption kinetic analysis revealed that AMX adsorption on PBP-AHC and PS-AHC best fits with the pseudo-second order kinetic model. AMX adsorption was found to be faster on PBP-AHC than PS-AHC due to its higher surface area and more mesoporous character. ZnCl2 activation of PBP-derived hydrochar produced a potential adsorbent for amoxicillin removal.