The Low Temperature Oxidation of Humic Substances

Abstract

In attempts to uncover a relatively mild method for the oxidative degradation of humic substances, a humic and a fulvic acid were oxidized with peracetic acid at 40 °C for 8 days. The oxidation products were extracted into organic solvents, methylated, separated by column-, thin layer-, and preparative gas-chromatography, and identified by matching their mass and micro-i.r. spectra with those of authentic specimens.The major humic acid oxidation products were phenolic acids (isolated in fully methylated forms, 8.4%), benzenecarboxylic acids (as methyl esters, 16.0%), and aliphatic acids (as methyl esters, 3.4%). The fulvic acid yielded 11.5% phenolic compounds, 8.4% benzenecarboxylic acids (as methyl esters), and 2.6% aliphatic acids (as methyl esters). These data indicated a more phenolic character for the fulvic acid than for the humic acid. In addition, small amounts of 2-methyl-3,4-furandicarboxylic acid were tentatively identified as dimethyl esters among humic and fulvic acid oxidation products. As far as total yields of oxidation products were concerned, peracetic acid oxidation at 40 °C of untreated humic and fulvic acids compared favorably with permanganate oxidation of methylated humic materials at about 100 °C, but was more time consuming.The results of this and earlier oxidative degradation studies show that the oxidation of humic substances produces phenolic and benzenecarboxylic acids as major products in addition to smaller amounts of aliphatic dicarboxylic acids. Whether these products arise from more complex structures of lignin or microbial origin or whether they occur in the forms in which they were isolated but held together by hydrogen-bonding or Van der Waal's forces requires further investigation.