Preparation and characterisation of biochars from elephant grass and their utilisation for aqueous nitrate removal: Effect of pyrolysis temperature

Abstract

Biochar is a solid material obtainable from biomass pyrolysis and useful for pollution alleviation and soil amendment. In this study, Biochars A and B were produced from elephant grass at pyrolytic temperatures of 400 °C and 600 °C, respectively, for removal of nitrate ion from aqueous solution. The physicochemical characteristics of the biochars were evaluated. The biochars were also characterised using Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), and X-ray Diffraction (XRD). Operational variables such as pH, contact time, and concentration of nitrate ion were varied and optimum variables were obtained. Kinetic and equilibrium data were subjected to kinetic (pseudo-first order, pseudo-second order, Avrami fractional order, Elovich and intraparticle diffusion) and equilibrium (Langmuir, Freundlich, Liu, and Redlich-Peterson) models, respectively, to elucidate the interaction between the nitrate ion and biochars. The yields of Biochars A and B were 41.40 % and 32.25 %, respectively. The two biochars possessed good cation exchange capacity, water-holding capacity, carbon stability, and porosity. Avrami fractional kinetic order was the best model that explained the kinetic data. Maximum adsorption capacities obtained from Liu model (the best equilibrium model) are 140.7 and 237.5 mg g−1 for Biochars A and B, respectively. Adsorption process was spontaneous and exothermic. There was a decrease in the disorderliness in the nitrate-biochar system. Biochar B performed better than Biochar A for removal of nitrate ion from water. In summary, the biochars produced from elephant grass excellently removed nitrate ion from solution and could be utilised for water decontamination.