Abiotic association of soil-borne monomeric phenols with humic acids

Monomeric phenols including phenolic acids occur ubiquitously in soil and can be stabilized by soil matrices; however, the stabilization mechanisms are still unclear. Using a dialysis equilibrium system, we quantitatively studied the abiotic association of uniformly ring-14C-labelled catechol, p-hydroxybenzaldehyde, protocatechualdehyde, vanillin, p-coumaric acid, ferulic acid, and caffeic acid with soil humic acid fractions (> 1000 Da) at pH values 1.5, 3.0, 5.0, 7.0 and 10.0. The association of the diphenols was strong at all pH values tested, with a maximum at neutral or basic pH, and was mainly irreversible, whereas the association of the monophenols was weak, being present only at pH values 1.5 and 3.0 and completely reversible. The association was independent of the ionic strength, the Ca2+ concentration of the medium, and the existing state of the humic acids (dissolved or solid). The present study indicates that humic substances may play an important role in the abiotic immobilization of soil-borne monomeric phenols, especially diphenols, in soil or in the gut of soil-feeding animals, such as earthworms and termites.