Interactions of triazine herbicides with biochar: Steric and electronic effects

Water Research 80 (1),179–188, 2015

Link: http://www.sciencedirect.com/science/article/pii/S0043135415002778

We studied the adsorption of triazine herbicides and several reference heteroaromatic amines from water onto a temperature series of hardwood biochars (300–700 °C, labeled B300–B700). Adsorption on biochars correlated poorly with pyrolysis temperature, H/C, O/C,mean minimum fused ring size, surface area (N₂ orCO₂), microporosity, and mesoporosity, but correlated well with a weighted sum of microporosity and mesoporosity. Steric effects were evidentby the negative influence of solute molecular volume on adsorption rate. For agiven compound, adsorption rate maximized for the biochar with the greatest mesoporosity-to-total-porosity ratio, suggesting that mesopores are importantfor facilitating diffusion into pore networks. The cationic forms of aminesadsorb more slowly than the neutral forms. To further probe steric andelectronic effects, adsorption on a biochar (B400) was compared to adsorption on graphite—a nonporous reference material with an unhindered, unfunctionalized graphene surface—and in comparison with reference compounds (benzene,naphthalene, pyridine, quinoline and 1,3-triazine). Relative to benzene, the surface area-normalized adsorption of the triazine herbicides was disfavored on B400 (favored on graphite) by 11–19 kJ/mol, depending on concentration. Itis estimated that steric suppression of B400 adsorption comprises 6.2 kJ/mol of this difference, the remainder being the difference in polar electronic effects. Based on the behavior of the reference amines, the difference in polar effects is dominated by π−π electron donor–acceptor (EDA) interactions with sites on polyaromatic surfaces, which are more electropositive and/or more abundant on graphite. Overall, our results showthat mesoporosity is critical, that adsorption rate is a function of solute molecular size and charge, that steric bulk in the solute suppresses equilibrium adsorption, and that π−π EDA forces play a role in triazine polar interactions with biochar.