Bio-oxidation of Nylon-6 by Aspergillus niger Isolated from Solid Waste Dumpsites

Abstract

An Aspergillus niger strain (AF3) isolated from dumpsites soil sample of Abule-Egba, Lagos, Nigeria, was studied for its potential to degrade a known weight synthetic aliphatic polyamide-6 (generically known as nylon-6) in a submerged culture. The growth activities of the fungus were monitored for three weeks at the room temperature followed by chemical and structural analysis. The fungus reduced the pH of the medium from 6.4 to 4.8 and oxidized the nylon-6 fibre by turning it to yellowish brown. It caused 23.95% loss in weight and reduced the number-average molecular mass of the nylon-6 fibre by 29.77% relative to the control. Structural changes in the molecular functional group of the nylon-6 fibres were ascertained by Fourier transform infrared (FT-IR) spectroscopy. The decrease in the transmittance intensity coupled with increase in the peak area of characteristics functional groups especially of the peaks at 3,304 cm-1 corresponding to amide A N-H stretch, 1,641 cm-1 and 1,545 cm-1 corresponding to amide I and amide II, respectively, as well as disappearance and formation of new peaks confirmed the degradation of the polymer. The discoloration of polymer fibre and the change of the pH by the fungi to create an acidic condition suggested the hydrolytic-oxidation mechanisms through the production of enzymes that initiated the protonation of the hydroxyl group of an intramolecular hydrogen bond followed with the hydrolysis of the carbonyl to produce molecules of lower molecular weight compounds. This study contributes to the possibility of use of the potential in filamentous fungi for large scale biodegradation of recalcitrant nylon-6.