Kinetics and characteristics of humic acids produced from simple phenols

Abstract

In the presence of H 2O 2 as donor, horseradish peroxidase was used to catalyze the polymerization of seven monomeric phenols. Yields of humic acid (HA) polymers from meta phenols—resorcinol and phloroglucinol—were insignificant. Of the five ortho and para phenols—phenol, catechol, hydroquinone, pyrogallol and 1,2,4-trihydroxybenzene—all except hydroquinone inhibited the enzyme at high concentration. The kinetics of polymerization of the ortho and para compounds were complex and dependent on the concentration of both electron acceptor and donor. The percentage yield of HA before dialysis was far greater from pyrogallol than from catechol or hydroquinone. After dialysis, the yield of the catechol HA was higher than those of the hydroquinone and pyrogallol HAs. A higher molecular weight for the catechol HA over those of the hydroquinone and pyrogallol HAs was also indicated by the lowest E 4/E 6 ratio and highest free radical content. All of the synthetic HAs were relatively rich in free radicals, suggesting that their synthesis occurred via free radicals, i.r. and 13C NMR spectra showed that the HAs were molecularly complex polymers or mixtures of complex aromatic structures rich in phenolic OH groups and to a lesser extent in CO 2H groups. The only HA which showed fine structure in the i.r. spectrum was the pyrogallol HA; the presence of aryl ethers was indicated. 13C NMR spectra showed that all synthetic HAs were highly aromatic, that aromatic rings of the initial phenols had been built into the HAs, but that molecular environments around phenolic OH groups had changed during the formation of the HAs.