Synthetic Dyes
Synthetic dyes, pervasive in various industrial effluents, pose significant ecological risks to aquatic ecosystems. These anthropogenic compounds, characterized by their complex aromatic structures, exhibit high stability and low biodegradability, leading to persistent environmental contamination. Upon entry into aquatic environments, synthetic dyes can significantly impair the quality of the water bodies through several mechanisms.
Firstly, the introduction of synthetic dyes into waterways can lead to a marked reduction in sunlight penetration. This diminution in light availability is critical, as it directly affects photosynthetic activities of aquatic flora, including phytoplankton and macrophytes. The inhibition of photosynthesis results in decreased oxygen production, thereby exacerbating hypoxic conditions. Such conditions are detrimental to aerobic aquatic organisms, leading to shifts in species composition and reductions in biodiversity.
Secondly, many synthetic dyes and their degradation products are known to bioaccumulate in the tissues of, and exhibit toxic effects on, aquatic biota. These substances can interfere with various physiological and biochemical processes in organisms. For example, exposure to certain dyes has been associated with oxidative stress, resulting in the generation of reactive oxygen species (ROS) and subsequent cellular damage. Additionally, some dyes possess endocrine-disrupting capabilities, altering hormonal regulation and affecting reproductive functions in wildlife.
Moreover, the degradation of synthetic dyes can lead to the formation of aromatic amines, compounds known for their mutagenic and carcinogenic properties. These metabolites can induce genetic mutations in aquatic organisms, potentially leading to population-level effects over prolonged periods.
The ecological impact of synthetic dyes is further compounded by the challenges associated with their removal from wastewater. Conventional wastewater treatment methods often prove inadequate in completely eliminating these compounds, necessitating the development and implementation of advanced treatment technologies, such as photocatalytic degradation, ozonation, and advanced oxidation processes, to mitigate their environmental footprint.
In conclusion, synthetic dyes represent a substantial threat to aquatic ecosystems, with their persistence, bioaccumulation potential, and toxicity posing severe risks to aquatic life and biodiversity. The environmental impact of these substances underscores the urgent need for stringent regulatory frameworks, improved industrial practices, and the development of efficient wastewater treatment technologies to safeguard aquatic ecosystems from anthropogenic contamination.
| Fungi | Dye | Decolorisation [%] | References |
|---|---|---|---|
| Aspergillus bombycis | Reactive Red 31 | 99.02 | Khan and Fulekar (2017) |
| Aspergillus flavus SA2 | Acid Red 151 and Orange II | 67 | Ali et al. (2018) |
| Aspergillus niger | Malachite Green | 82.6 | Alam et al.(2018) |
| Aspergillus sp. | Methyl Violet | 95 | Kumar et al. (2012) |
| Aspergillus sp. | Remazol Brilliant Blue R | 90 | Soares et al. (2002) |
| Aspergillus sp. | Reactive black 5 and reactive blue 114 | 90 | Cristovao et al. (2011) |
| Aspergillus sp. CB-TKL-1 13 | Brilliant Green | 100 | Kumar et al. (2012) |
| Ceriporiala cerata | Congo Red | 90 | Wang et al. (2017b) |
| Cerrena sp. | Malachite Green | 91.6 | Yang et al. (2015) |
| Coriolopsis sp. | Crystal VioletCotton Blue | 85.179.6 | Munck et al. (2018) |
| Curvularia clavata NZ2 | Congo red | 80 | Neoh et al. (2015) |
| Ganoderma lucidum IBL-05 | Sandal-fix black CKF | 95.7 | Bilal and Asgher (2015) |
| Irpex lacteus F17 | Malachite green | 96 | Yang et al. (2016) |
| Mixed fungal cultures ofPleurotus ostreatus (BWPH), Gloeophyllum odoratum (DCa), and Fusarium oxysporum (G1) | Brilliant Green and Evans Blue | 74.3 | Przystaś et al. (2013) |
| Myceliophthora vellerea | Reactive Blue 220 | 80 | Patel et al. (2013) |
| Paraconiothyrium variabile | Acid red 18 | 97 | Ashrafi et al. (2013) |
| Peroneutypa scoparia | Acid red 97 | 75 | Pandi et al. (2019) |
| Peyronellaea prosopidis | Scarlet RR | 90 | Bankole et al. (2018) |
| Phanerochaete chrysosporium | Mixture of Malachite Green, Nigrosin and Basic Fuchsin | 78.4 | Rani et al. (2014) |
| Pichia sp. | Acid Red B | 95 | Qu et al. (2012) |
| Pleurotus ostreatus | Acid Red 27 | 85 | Ali and El-Mohamedy (2012) |
| Pleurotus sajor-caju | Reactive black-5 | 84.4 | Murugesan et al. (2007) |
| Scheffersomyces spartinae | Acid Scarlet 3R | > 90 dec | Tan et al. (2016) |
| Trametes trogii SYBC-LZ | Remazol Brilliant Blue R | 85.2 | Zeng et al. (2011) |
| Trametes versicolor | Sulphur blue 15 | 81.6 | Nguyen et al. (2016) |
| Trametes versicolor | Reactive blue 19 | 97.5 | Champagne and Ramsay (2007) |
The diagram opposite outlines the biochemical degradation pathway of Remazol Brilliant Blue R (RBBR), an anthraquinone dye, by fungal enzymes. Here's a step-by-step explanation of the process:
Initial Reduction: The dye undergoes an initial reduction step where the molecule accepts electrons (e⁻) and protons (H⁺), leading to the cleavage of the azo bond (-N=N-) and resulting in the formation of aromatic amines.
Formation of Amines: The reduction step produces sodium 2-((3-aminophenyl)sulfonyl)ethyl sulfate and 1-amino-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate, which are intermediate amines.
Deamination: The intermediate 1-amino-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate undergoes deamination, losing an NH₄⁺ ion, which further alters the structure.
Desulfonation and Ring-Opening: The molecule then experiences desulfonation (loss of -SO₃Na group) and ring-opening reactions that lead to the breakdown of the aromatic rings, a crucial step for mineralization.
Further Breakdown: The smaller fragments formed through these steps continue to undergo various reactions, including further deamination, ring-opening, and oxidation, leading to smaller and less complex structures.
Complete Degradation: Finally, these products go through successive steps that lead to complete mineralization into CO₂, H₂O, and inorganic salts.
Each step is associated with different enzymes and occurs under specific conditions facilitated by the fungal metabolic machinery. The m/z values indicate the mass-to-charge ratios of the molecular ions detected at various stages, which are characteristic of the intermediates formed during the degradation process.
Fungi produce a variety of enzymes, including laccases, peroxidases, and ligninolytic enzymes, which can degrade or modify the chemical structure of dyes and other pollutants. Laccases, in particular, are copper-containing enzymes that are widely used in decolourisation processes due to their ability to oxidize a wide range of substrates, including phenols and aromatic compounds.
The decolourisation process by fungal enzymes involves the enzymatic oxidation of the dye molecule, which breaks down the chemical bonds responsible for its colour. The breakdown products are then further degraded or transformed into less harmful substances by other enzymes or microorganisms present in the environment.
