Transformation of Chlorinated Phenolic Compounds by White Rot Fungi

Abstract

Chlorinated phenolic compounds are among the most abundant recalcitrant wastes produced by the paper and pulp industry, being accumulated in the effluents after secondary treatments. Due to their high toxicity to a wide range of organisms, chlorinated phenolic compounds pose a big concern to human and environmental health. These compounds are produced upon the partial degradation of lignin during bleaching process, and as such they are degradable by white rot fungi, the only organisms capable of degrading lignin to CO2 and H2O. White rot fungi are a group of organisms very suitable for the removal of chlorinated phenolic compounds from the environment. Indeed, they are robust, ubiquitous organisms and may survive also in the presence of high concentrations of various pollutants, even with a low bioavailability. The activity of white rot fungi is mainly due to the action of oxidoreductive enzymes, i.e., laccase, manganese peroxidase and lignin peroxidase, which are deliberately released by fungal cells into their nearby environment. In cases of secondary metabolism and oxidative stress, the fungi produce other intra- and extracellular enzymes to provide substrates for the key enzymes just mentioned. Phanerochaete chrysosporium is one of the most widely studied fungi, for which intermediary products and reactions involved in the degradation of chlorophenols have been identified. Extracellular laccases and peroxidases carry out the first productive step in the oxidation of chlorophenols, forming para-quinones and consequently releasing a chlorine atom. Further degradative steps involving several enzymes and highly reactive, nonspecific redox mediators produced by the fungus render it capable of efficiently degrading several toxic compounds. In soil environment, chlorophenols may be involved in physical–chemical processes such as polymerization and/or adsorption of/on humic substances that may hinder their degradation by microbial cells, even though both processes can result in dehalogenation of the chlorinated compounds. This review analyzes the presence of chlorophenols in the environment, their main chemical and physical properties, and the main processes in their degradation by white rot fungi with particular attention to soil environment. The reactions involved in the process, the intermediary products, the factors that may affect the fungal transformation of chlorophenols, and the possible applications for environmental purposes of both the whole fungal cells and/or their enzymes as isolated catalytic agents are also addressed.