The influence of structural properties on the adsorption capacities of microwave-assisted biochars for metazachlor removal from aqueous solutions

Abstract

Biochars, carbonaceous materials prepared without the usage of chemical agents, are porous materials capable of adsorbing pollutants from ground- and surface waters. In this study, biochars prepared from various types of agricultural biomass were tested for the adsorptive removal of herbicide metazachlor from an aqueous environment. Banana wastes, red mombin seeds, corncob, cocoa pod husk, and coffee husk were used as precursors. Biochars were prepared with the aid of microwave treatment. The effect of precursor type on structure and adsorption was examined. Adsorption was controlled by a multistep mechanism, adsorption kinetics followed predominantly the pseudo-second-order model, adsorption isotherms suited to both Langmuir and Freundlich isotherms, depending on the particular biochar. Significant differences between the structural properties and adsorption capacities of the examined biochars were observed. The best adsorption properties for metazachlor uptake were observed for banana waste-based biochar, which had large, elongated pores, highest volume of micropores and one of the highest contents of polar functional groups. The maximum adsorption capacity, calculated from Langmuir isotherm, was 146.01 mg·g−1, The adsorption capacity at equilibrium, obtained by kinetic measurements, was 27.25 mg·g−1, the kinetic constant was 5.14·10−3 dm3·min−1 (both calculated from pseudo-second order model). Molecular modeling revealed that metazachlor molecules preferably entered two layer wide cavities containing one COOH group with their pyrazole rings.