Dye decolorization by yeasts

: insights on pathways towards an innovative solution for textile effluents

Abstract

The textile industry produces high amounts of effluents containing synthetic dyes that are recalcitrant, toxic, potentially hazardous to the environment, and difficult to treat by conventional methods. Biotechnological approaches are generally considered more environmentally friendly and biological methods for dye degradation need to be further investigated. The work described in this PhD thesis aimed to develop a yeast-based solution to decolorize textile dyed effluents that can be safely discharged into the environment. Yeast strains Candida parapsilosis (HOMOGS20B), Yarrowia lipolytica (HOMOGST27AB) and Candida pseudoglaebosa (LIIIS36B), isolated from wastewater treatment plants, were tested for their ability to decolorize textile dyes. A total of 32 commercial textile synthetic dyes and simulated textile effluents were tested with the target yeasts (single strains and consortia) and the decolorization was evaluated spectrophotometrically for 48-72 hours. Yeasts were able to perform decolorization through adsorption and biodegradation for 28 of the dyes and simulated effluents to extents higher than 50%. Y. lipolytica (HOMOGST27AB) and C. pseudoglaebosa (LIIIS36B) presented true decolorization of reactive dyes, above 90% at 100 mg/L, and of simulated effluents at 5 g/L of concentration. Enzyme production was evaluated: oxidoreductase was found in the three yeasts, whereas tyrosinase was only found in Y. lipolytica (HOMOGST27AB) and C. pseudoglaebosa (LIIIS36B). The potential toxicity of the dyed simulated effluents, before and after treatment, was investigated. The toxicity of the yeast-treated effluents was assessed using three different assays to represent different trophic levels. Mutagenicity was also evaluated. C. pseudoglaebosa (LIIIS36B) was the most effective strain, completely decolorizing four of the five simulated effluents. Degradation products, possibly aromatic amines, were detected in the supernatant. Although the toxicity of the original simulated effluents was considered high, treated effluents demonstrated a decrease in toxicity and none exhibited mutagenicity. A yeast-based solution for decolorization of textile industrial wastewater was assessed. The three yeast strains previously selected for their dye decolorization capacity were freeze-dried. Additionally, Y. lipolytica (HOMOGST27AB) was also spray-dried. Skim milk powder and maltodextrin were used as cell protectors, and the freeze-dried products were stored at cold (4 °C) and room temperature for 210 days. The viability of the yeast cells and their decolorization capacity over time were assessed. Dried yeast cells maintained their viability and decolorization capacity for at least 90 days of storage after spray- and freeze-drying with both cell-protecting agents. The dried yeast-based solution for decolorizing textile industrial wastewater combines stability, efficiency, and convenience of production for application in real industrial facilities. A sequencing batch reactor (SBR) inoculated with activated sludge and successfully bioaugmented with the dye-decolorizing yeast strain – Y. lipolytica (HOMOGST27AB) was assembled. The bioaugmented AGS-SBR was operated for the treatment of a synthetic saline wastewater (12 g/L) intermittently fed with the reactive textile dye (Navy Everzol ED) at 25, 15, and 7.5 mg/L. Dye degradation did not occur although some dye adsorbed to the granules. AGS-SBR performance in removing carbon and nitrogen was effective and was not affected by the dye addition. The AGS core microbiome gathered essentially microorganisms from the Proteobacteria and Bacteroidetes phyla. The microbial profile showed a dynamic microbiome established at Phase I of the operation, with a high decrease in abundance of Ignavibacterium from the initial biomass to the granules formed and an increase of Actinobacteria, Cytophagia, Flavobacteria, and Alphaproteobacteria in the remaining phases of the bioreactor operation. The bioaugmented yeast remained present throughout the entire reactor process. Y. lipolytica (HOMOGST27AB) and C. pseudoglaebosa (LIIIS36B) are a potential biotechnological tool for dye degradation in textile wastewaters, especially those containing reactive dyes and a promising tool to integrate in bioremediation solutions, contributing to eco sustainability in the water sector, but further operational scenarios need to be researched.

Date of Award	8 Apr 2024
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Paula M. L. Castro (Supervisor), Maria Manuela Pintado (Co-Supervisor) & Patrícia R. Costa (Co-Supervisor)