PHENOL CONVERSION AND DIMERIC INTERMEDIATES IN HORSERADISH PEROXIDASE-CATALYZED PHENOL REMOVAL FROM WATER

Phenol was removed from water by horseradish peroxidase-catalyzed polymerization. Five dimeric and one trimeric products from the reaction were identified in the aqueous solution. The trimer had a structure of 4-(4-phenoxyphenoxy)phenol (VI) determined from its NMR spectrum. The dimers such as p,p'-biphenol (I), o,o'-biphenol (II), and p-phenoxyphenol (III) were the reaction intermediates. With more than 95% phenol removal from an initial phenol concentration of 188 mg/L, the final concentrations of the three dimers were each below 1 mg/L. About 7% of the precipitate mass was attributed to the three dimers (I, II, III), and the rest consisted mainly of the compounds of higher hydrophobicity and molecular mass. With an equimolar ratio of phenol to hydrogen peroxide, the phenol conversion behaved as a first-order reaction with respect to phenol concentration. A peroxidase inactivation model for the reaction in the presence of poly(ethylene glycol) (PEG) was proposed, and the inactivation rate constant was found to have a logarithmic relationship with the ratio of PEG to enzyme doses. The three dimers were the substrates of peroxidase, and their conversion could also be depicted with a first-order model with respect to the dimer concentrations. A comparison of the specific reaction rates indicated that p-phenoxyphenol was the best substrate of peroxidase (2.172 nM(-1) min(-1)) followed by p,p'-biphenol (0.671 nM(-1) min(-1)), phenol (0.0105 nM(-1) min(-1)), and o,o'-biphenol (0.00453 nM(-1) min(-1)). Therefore, the predominant polymerization bonds in the products may be the oxygen-para connection whereas the ortho-ortho connection would hardly be found in the higher oligomers.