Effects of pH and cyclodextrins on pentachlorophenol degradation (mineralization) by white-rot fungi

Abstract

White-rot fungi (WRF) such as Trametes hirsuta completely degrade (mineralize) pentachlorophenol (PCP) and many other organopollutants. This has led to them being used to decontaminate various substrates (e.g. soil) through biorememediation. However, because PCP is a biocide, it can inhibit fungal growth and thereby its own degradation. It was hypothesized that substrate pH might affect PCP degradation, because when the pH is lower than 4.7 (the pKa for PCP) the phenol predominates, while at higher pH the phenate does. These two PCP species differ markedly in physical and biological properties. The effect of cyclodextrins was also investigated since it is known that the inclusion complexes these form with PCP differ in bioavailability and toxicity from non-complexed PCP. Tests were first made in liquid and agar media (where conditions are relatively easy to control), and then in a sawdust, because it is a common target matrix for WRF bioremediation. Results with the liquid and agar media showed that growth in the presence of inhibitory PCP concentrations decreased as the pH decreased, consistent with the phenol being more toxic. Growth in sawdust was less affected by PCP regardless of the pH, presumably because the PCP sorbed to the wood which decreased its bioavailability. Some cyclodextrins markedly decreased the PCP's toxicity in liquid, agar and sawdust media. Rates of PCP mineralization (measured from production of 14CO 2 from 14C PCP) in liquid cultures containing 0.5 mg kg −1 PCP (a sub-inhibitory concentration) were similar from pH 2.5–7.5, indicating that the phenol and the phenate were equally degradable. Degradation of a growth inhibiting concentration on sawdust (1000 mg kg −1) could be increased slightly by lowering the pH below the pKa, this increasing sorption to the wood. Degradation increased more when the pH was raised well above the pKa, presumably due to the phenate being less toxic and more soluble, making it more available to the degradative system. Although some cyclodextrins decreased growth inhibition, they also interfered with degradation. If this interference could be overcome, cyclodextrins could be used to increase the maximal PCP concentration that could be treated by WRF bioremediation.