Significantly Enhanced Adsorption of Herbicides on Biochar after Tailoring By Post-Pyrolysis Air Oxidation.

Biochar is a pyrolysis product of biomass wastes that has attracted interest as a soil amendment for purposes of increasing crop productivity or reducing the mobility of soil contaminants. We present a simple and effective post-pyrolysis method for enhancing adsorption of polar organic compounds by biochar. By reheating the biochar at 400 °C for up to 30 min in air, we observed up to a two-fold increase in both surface area and carboxyl content. The increase in surface area leads to enhanced adsorption of both charged and neutral compounds, including six triazine herbicides, 2,4-dichloro-phenoxyacetic acid (2,4-D) (an anionic herbicide), and several other reference compounds (including the nonpolar naphthalene) that are either neutral or anionic at pH 7. The greatest enhancements were observed for compounds (carboxylic acids and heteroaromatic amines) with pKa of 3-5, close to the intrinsic pKas of surface carboxyl groups. Normalized by surfaced area, post-pyrolysis air oxidation of biochar increased adsorption of this group of compounds by up to an order of magnitude, suggesting that specific interactions with surface carboxyl groups play a role. We postulate that adsorption of this group of compounds is augmented by the formation of exceptionally strong homonuclear (organoanions) or heteronuclear (heterocyclic amines) hydrogen bonds with surface carboxyl groups known as charge assisted hydrogen bonds (CAHB). They can be written (O…H…O)- or (N…H…O)+/-, respectively, and are among the strongest hydrogen bonds known. The CAHB hypothesis is supported by competitive adsorption experiments in which a CAHB-capable target solute is pitted against a CAHB-capable compared to a CAHB-incapable competing solute.