Production of high-performance biochar using a simple and low-cost method: Optimization of pyrolysis parameters and evaluation for water treatment

Abstract

Elephant grass is a plant that grows easily and rapidly, with low nutrient requirements. One of the possible applications of this raw biomass is as a renewable source for simple and low-cost production of carbon materials. The objective of this work was to use elephant grass biomass to produce biochar intended for the treatment of contaminated water. Biochars were produced with variation of the pyrolysis parameters, using three different temperatures (350, 600, and 900 °C) and two different residence times (30 and 120 min). The products were characterized by DRIFT spectroscopy, XRD, Raman imaging, EPR, TGA, SEM, TEM, nitrogen adsorption at 77 K, potentiometric titration to obtain the pKa values, and elemental CHN analysis. Continuous flow adsorption tests were used to evaluate the potential of the material for practical applications. The infrared spectra and pKa values revealed structural changes of the carboxylic groups (pKa 3–6), forming lactones (pKa 6–9) or phenols (pKa >9). The diffractograms, low H/C and O/C ratios, and Raman spectra indicated the formation of aromatic and graphitic structures in the materials. Environmentally persistent free radicals, mainly formed by constituents of lignin, were centered near to carbon atoms in samples produced at higher temperatures. Continuous flow adsorption tests showed that the sample with the highest surface area (∼550 m² g−1) and the largest pore volume (∼0.34 cm³ g−1) achieved a removal efficiency of 96.5 % and presented good reusability, indicating its suitability as a promising material for the treatment of water contaminated with aromatic pollutants.