Electron paramagnetic resonance (EPR) spectroscopy as a tool for the characterization of biochar from guava waste
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The ability of biochars prepared from pyrolysis of guava pulp and seed industrial wastes to form complexes with the ions vanadyl (VO2+), a Pearson hard acid, and copper divalent (Cu2+), an intermediate acid, was used to characterize the ligand groups of biochars prepared in a laboratory oven and in a pilot plant. These biochars are highly suitable for application as soil conditioners, due to their porous structures and the presence of functional groups on their surfaces.
Materials and methods
The biochars from guava pulp and seeds industrial wastes were obtained in two ways, using a batch process in a laboratory oven (sample BG1) and a continuous flow process in a pilot plant (sample BG2), in both cases employing a final temperature of 350 °C and off. The characterization techniques used were elemental analysis, Fourier transform infrared (FTIR) spectroscopy,, and electron paramagnetic resonance (EPR) spectroscopy.
Results and discussion
The FTIR spectroscopy results showed that carboxylic acid groups acted as ligands in the complexes with Cu2+ ions (samples BG1Cu and BG2Cu), with decreases in the intensities of these bands and increased intensities of carboxylate bands after complex formation. The EPR parameters indicated that the complexes with VO2+ ions (samples BG1VO and BG2VO) presented C4V symmetry, with coordination to oxygen ligands in the equatorial plane. The BG1Cu and BG2Cu complexes showed slightly distorted octahedral geometry, with stretching of the bonds on the z-axis, due to the Jahn-Teller effect.
The combination of spectroscopic analyses showed that the BG2 biochar was more effective in complexing with ions, compared to the BG1 biochar, because it underwent a milder pyrolysis that preserved greater amounts of oxygenated functional groups, as observed in the elemental analysis.
KeywordsBiomass waste Complexation Paramagnetic probes Pyrolysis Responsible editor: Yong Sik Ok
The authors are grateful to CNPq and CAPES for research fellowships.
This study was supported by INCT E&A.
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