pH effects on sorption of nonionic compounds to soils: Possible involvement of π-π charge-transfer interactions for PAHs and other strong electron donor aromatic compounds

Sorption of pi-donors, pentamethylbenzene (PMB), naphthalene (NAPH), and phenanthrene (PHEN), to soils increases with decreasing pH, whereas a reversed trend is observed for 2-nonanol and 2-nonanone, which are non pi-donors but capable of H-bonding. No significant pH effect is observed for non pi-donor, hydrophobic compounds: trans-1,2-dichlorocyclohexane, hexachloro-1,3-butadiene, 1,2,4-trichloro-benzene and 2,2′,5,5′-tetrachlorobiphenyl. These results cannot be explained by pH-dependent H-bonding or by hydrophobicity of soil organic matter (SOM). Instead, pi-pi charge-transfer interactions are proposed between donors and SOM pi-accepting sites, including N-heteroaromatic cations and aromatic rings with multiple carboxyl groups. The pi-pi interaction between donors, PMB, NAPH, and PHEN and model SOM acceptors, 1,3,5-benzenetricarboxylic acid (BTA) and 1,4,5,8-naphthalene-tetracarboxylic acid, was verified by upfield ¹H NMR chemical shifts in methanol and methanol-water. The donor affinity of BTA for PMB was reduced by deprotonation of BTA.