Steric and Electronic Effects in the Interactions of Triazine Herbicides with Biochar.

The roles played by surface/pore dimensions, steric bulk of the solute, and “π−π” interactions in adsorption of organic compounds to carbonaceous materials are poorly substantiated and quantified. We studied adsorption of triazine herbicides and several reference heteroaromatic amines from water onto a temperature series of hardwood biochars (300 – 700 ^oC). Adsorption correlated poorly with surface area (N₂ or CO₂), microporosity, mesoporosity, pyrolysis temperature, H/C, O/C, and mean minimum fused ring size, but correlated well with a weighted sum of microporosity and mesoporosity. Steric effects were evident by the negative effect of solute molecular volume on rate. The cationic forms adsorbed more slowly than the neutral forms. Adsorption rate maximized for the biochar with the greatest mesoporosity-to-total-porosity ratio, indicating that mesopores facilitate intra-particle diffusion. Adsorption of the triazines and the reference compounds on the 400 ^oC biochar (B400) was compared to their adsorption on graphite—a nonporous reference material with an unhindered, unfunctionalized surface. Relative to benzene, the surface area-normalized adsorption of triazines was disfavored on B400 (favored on graphite) by 11−19 kJ/mole, depending on compound and concentration. Steric suppression comprised ~6 kJ/mol of this difference, while the difference in polar electronic effects comprised the remainder. The difference in polar effects is attributed to π−π electron donor–acceptor (π−π EDA) interactions with polyaromatic surface sites, which are more electropositive and/or more abundant on graphite. The role of π−π EDA was supported by the strong correlation between the hydrophobicity-normalized distribution coefficient on graphite and the electrostatic potential at the midpoint of the ring computed by Density Functional Theory. Our results overall show that mesoporosity is critical, adsorption rate is a function of molecular size and charge, steric effects suppress equilibrium adsorption, and π−π EDA forces do play an important role in triazine polar interactions with biochar.