Influence of methyl groups in triphenylmethane dyes on their adsorption on biochars from coffee husks

Abstract

Biochars (BC), whose properties are highly dependent on the pyrolysis temperature used, have been proposed for the efficient removal of a variety of contaminants from wastewater. In this work, pristine biochars were produced by the pyrolysis of coffee husks at temperatures of 400, 500, 750, and 900 °C, for use in the adsorption of pararosaniline (PRA) and methyl violet 10B (MV10B), which are triphenylmethane dyes with similar structures, but different numbers of methyl groups. The biochars were characterized and the dye adsorption kinetics and equilibria were investigated. FTIR and Raman spectroscopy analyses indicated that a higher pyrolysis temperature increased the aromaticity of the biochar surface structure, while decreasing the number of oxygenate functional groups. Higher adsorption capacities were generally observed at pH 7.5, with the maximum adsorption amounts increasing in the order BC900 ≈ BC750 < BC500 < BC400 for both dyes, being 1.3 times higher for PRA on BC400 (97.22 mmol kg−1), but 1.9 times higher for MV10B on BC900 (5.49 mmol kg−1). The Langmuir model provided the best fit to the adsorption isotherms for BC400 and BC900, while the Dubinin–Radushkevich model satisfactorily fitted the isotherms for the other biochars. These results showed that increase of the pyrolysis temperature resulted in a decrease in the number of adsorption sites with which the dyes interacted more favorably by means of hydrogen bonds. Although hydrophobic interactions were not important driving forces for adsorption of the dyes, the higher number of methyl groups in MV10B favored its adsorption on the more hydrophobic biochar.