|Title||Integrated and sequential anaerobic/aerobic biodegradation of azo dyes|
|Source||Wageningen University. Promotor(en): G. Lettinga; J.A. Field. - S.l. : S.n. - ISBN 9789058083746 - 104|
|Publication type||Dissertation, internally prepared|
|Keyword(s)||biodegradatie - bioremediëring - kleurstoffen (dyes) - azoverbindingen - biodegradation - bioremediation - dyes - azo compounds|
|Categories||Bioremediation / Waste treatment / Soil Pollution|
Azo dyes constitute a major class of environmental pollutants accounting for 60 to 70% of all dyes and pigments used. These compounds are characterized by aromatic moieties linked together with azo groups (-N=N-). The release of azo dyes into the environment is a concern due to coloration of natural waters and due to the toxicity, mutagenicity and carcinogenicity of the dyes and their biotransformation products. Therefore, considerable attention has been given to evaluating the fate of azo dyes during wastewater treatment and in the natural environment. Azo dyes require an anaerobic and an aerobic phase for their complete biodegradation. Therefore, the aim of this thesis was to evaluate the mineralization of azo dyes under integrated and sequential anaerobic/aerobic conditions. These conditions were applied on three azo dyes, Mordant Orange 1 (MO1), 4-phenylazophenol (4-PAP) and Mordant Yellow 10 (MY10). Since many azo dyes are substituted with a sulfonic acid group, special attention was paid to the sulfonated azo dyes and their biodegradation products, the sulfonated aromatic amines.
The first step in the biodegradation of azo dyes is the azo dye reduction resulting in the formation of the aromatic amines. The influence of oxygen on the azo dye reduction of MO1 was tested in order to evaluate the integrated anaerobic/aerobic conditions. The co-substrate ethanol was successfully applied as electron donor for azo dye reduction and also created anaerobic microniches to facilitate anaerobic azo dye reduction in the presence of oxygen. Increasing oxygen concentrations showed decreasing azo dye-reduction rates with ethanol as co-substrate. These rates were higher when applying acetate as co-substrate. Moreover, Nedalco granular sludge could withstand better with the applied integrated anaerobic/aerobic conditions than Shell granular sludge.
Since none of the expected aromatic amines from MO1 was degraded, further research was conducted to evaluate the aerobic biodegradation of aromatic amines. MY10 and 4-PAP were tested under integrated anaerobic/aerobic conditions. All aromatic amines were removed if sufficient oxygen was present. Oxygen was primarily used to oxidize co-substrate and if sufficient oxygen was available the formed aromatic amines were further degraded. The removal of 4-aminobenzenesulfonic acid (4-ABS) was only possible after bioaugmentation of a 4-ABS-degrading enrichment culture. The results from the biodegradation study of 4-AP indicated that 4-aminophenol was removed due to autoxidation. Degradation of azo dyes is possible under integrated anaerobic/aerobic conditions if co-substrate and oxygen are in balance.
In the sequential anaerobic/aerobic bioreactor, the azo dye MY10 was successfully mineralized. Its constituent aromatic amines, 5-aminosalicylic acid (5-ASA) and 4-ABS, were both recovered in the anaerobic stage and degraded in the aerobic stage. Biodegradation of the 4-ABS was only possible after bioaugmentation with a 4-ABS-degrading enrichment culture. High recovery of sulfate indicated mineralization of 4-ABS. After long-term exposure to 5-ASA, the anaerobic granular sludge showed the ability to degrade this compound.
It was observed that the degradation of the sulfonated aromatic amine 4-ABS only proceeded after bioaugmentation of a specialized enrichment culture. Therefore, the ultimate biodegradability of the sulfonated aromatic amines is questionable. To investigate this matter, the fate of sulfonated aromatic amines with natural mixed cultures was evaluated in aerobic and anaerobic batch assays and bioreactor experiments. None of the ten tested compounds was degraded under anaerobic conditions and only 2-ABS and 4-ABS were aerobically mineralized. This was observed in batch as well as in bioreactor experiments. Degradation of the 2-ABS and 4-ABS was found with inoculum sources that were historically polluted with sulfonated aromatic compounds. At concentrations up to 1.0 g l -1 , none of the tested sulfonated aromatic amines showed any toxicity towards anaerobic and aerobic biomass.
The results of this research demonstrated that azo dyes are mineralized under integrated and sequential anaerobic/aerobic conditions. Due the difficulties with balancing the supply of co-substrate and oxygen in integrated anaerobic/aerobic systems, the sequential anaerobic/aerobic conditions are recommended for the mineralization of azo dyes. However, during the degradation of sulfonated azo dyes, many different sulfonated aromatic amines will be formed and these compounds are not likely to be degraded aerobically. Therefore, special attention should be paid on the removal of these compounds