Characterization and quantification of reversible redox sites in humic substances

Cyclic oxidation and reduction reactions using oxygen and palladium with H-2, respectively, of dissolved humic and fulvic acids (HA and FA) and model quinone compounds were used to structurally characterize and quantify the electron-carrying capacity (ECC) of reversible redox sites present in humic substances. This technique was used to examine 8 quinone compounds and 14 HA and FA samples and identified 3 redox sites as a function of their stability against the Pd-catalyzed hydrogenolysis process. Six highly aliphatic HA and FA isolated from landfill leachate did not contain redox sites under any conditions; however, the other HA and FA demonstrated reversible redox properties characterized by a combination of three redox sites. On the basis of the model compound results, it is proposed that one site consists of a non-quinone structure (NO.) and the other two sites have quinone structures. The two quinone sites differ in that one group (M) has electron-withdrawing groups adjacent to the quinone functional group while the second group (Q2) contains either no substituents near the quinone or has nearby electron-donating groups with additional substitutents hindering hydrogenolysis through steric interactions. The reversible ECC of NO. sites ranged from 25 to 265 mu equiv e(-) transferred/g HA or FA, representing 21-56% of the total ECC of the HA and FA when measured with the mildest reducing method (pH 8.0, pure Pd). Q1 redox sites resistant to hydrogenolysis at pH 8.0 using Pd/Al2O3 accounted for 13-58% of the total ECC and ranged from 40 to 120 mu equiv e(-)/ g HA or FA. The most sensitive Q2 reversible redox sites accounted for 8-50% of the total ECC (20-220 mu equiv e(-)/ g HA or FA). These results directly demonstrate that HA and FA are capable of acting as reversible electron-transfer agents using different functional groups, some of which may not be quinones.