Degradation of nylon finshing wastewater using UV/H2O2 and photo-Fenton treatment

Abstract

In nylon finishing a final step is a thermal treatment to fix auxiliary chemicals to fibers, to remove volatile compounds and monomers and to relax the textile structure. For this purpose stenters with natural gas burners are applied. The hot exhaust air contains monomers, solvents, lubricants and specific textile auxiliary chemicals that have to be removed with large amounts of fresh or process water by exhaust air washers. To reduce water costs, process-integrated water treatment and recycling processes are demanded. Annular photoreactors with low and middle-pressure mercury arc lamps and a solar non concentrating tubular receiver reactor have been applied to examine the treatment of exhaust air washer liquor and its components in the laboratory and up to semi-technical scale. The application of sole UV radiation, the combination of UV radiation with oxidants, semiconductor photocatalysts and the light enhanced Fenton reaction were compared. The photo Fenton conditions were optimized in respect of throughput and hydrogen peroxide consumption. The degradation pathway of epsilon-caprolactam (the main constituent of the air washer liquor) was analyzed by ion chromatography and HPLC-MS. The overall degradation was controlled by total organic carbon analysis. Intermediates like epsilon-amino capronic acid, N-hydroxy-epsilon-caprolactam, epsilon-nitroso capronic acid have been confirmed as initial intermediates also like acetate, oxalate and formiate as intermediates before complete mineralization. In comparison to photolytic hydrogen peroxide oxidation, light enhanced Fenton turned out to be much faster and to consume lower amounts of hydrogen peroxide. It could be demonstrated that the iron catalyst can be recycled with minor activity losses with middle-pressure mercury arc lamps. Photochemically enhanced Fenton reaction has proved to be most effective with respect to the consumption of time, energy and chemicals for the application in exhaust air treatment of nylon finishing.