Bio-remediation as a solution for common textile dyes in soil

Textile dyes are artificial or natural substances used to dye fabric. Artificial dyes are one of the worst contributors to soil pollution as they contain mutagenic, cytotoxic, cancer and allergy causing properties (Khandare & Govindwar 2015). This is aggravated by the fact that the textile dye industry release by-products in the form of effluent, causing extensive pollution. However, existing physio-chemical technologies to clean up polluted water or soil are expensive and cause secondary problems in terms of disposal. Nevertheless, studies have shown that certain group of microorganisms produce enzymes to degrade dyes, decolorize and metabolize them (Singh et al., 2015). Therefore, bioremediation is a technique to address this. More specifically, it is a process by which pollutants and waste are removed using microbes and plants or their by-products. Subsequently, such microbial properties can be used to help in large scale cleaning up of textile dye effluent.

Prevalence of dye pollution on global and national scale

Common textile dyes fall into cationic, anionic or non-ionic class and include more than 100,000 types of compounds (Husain 2006). They are made up of different chromophores, such as azo, anthraquinone and triarylmethane of which the azo dyes are the biggest (Husain 2006). Their structural properties ensure that they are resistant to fading when exposed to light, water or chemicals. A study has shown that during the dyeing process, 10%-25% of the dyes are wasted. However, approximately 2%-20% of dye is directly released as aqueous effluent during the process (Carmen & Daniela 2010). The amount of textile dye effluent varies across countries, as shown in the table below.

Country Type of dye waste Percentage of dye waste
USA Synthetic dye 28% (Robinson & Nigam 2008)
USA Indigo dye 30% (Wambuguh & Chianelli 2008)
Egypt Synthetic dye, Acid dyes, Indigo dyes 26% (El-Kassas & Mohamed 2014)
China Indigo dyes, Synthetic dyes 16% (Chen et al. 2016)
China Supranol Red 3BW 22% (Lim et al., 2010)
Turkey Reactive Red 198 (RR198) 12% (Akar, et al., 2009)
Slovenia Anthraquinone dye Reactive Blue 19, Diazo dye Reactive Black 5 and Acid Orange 7 dye 18% (Mohorčič, et al., 2006)
India Tamil Nadu Orange 3R 19% (Ponraj et al., 2011)
Mumbai Azo dye and Reactive dye 22% (Verma et al. 2010)
Jaipur Synthetic dyes, Reactive dyes, Direct dyes etc 38% (Mathur et al., 2005)
Gujarat Remazol Black B 15% (P Shah 2012)
Bangalore Reactive blue MR 12% (Thangamani et al. 2007)
Reactive red 198 13% (Mona et al., 2011)
Solapur Reactive textile dye Red BLI 10% (Kalyani et al., 2008)

Table: Reported levels of dye waste globally and in India

Microbial diversity in bioremediation of textile dyes

Bioremediation of dyes can be achieved either through degradation, biosoprtion, or enzymatic reduction (Kandelbauer & Guebitz 2005). During degradation, aerobic and anaerobic microbes degrade the recalcitrant dyes, either oxidising or reducing them to decolorized forms. Degradation microbes include bacteria, yeasts and fungi, even though fungi and agriculture residues have shown biosorption properties (Kandelbauer & Guebitz 2005). The table below lists microbial diversity capable of dye degradation.

Microbe Example Function
Aerobic Bacteria Pseudomonas sp.,

Proteus sp.,

Enterococcus sp.

Alcaligenes sp., Sphingomonas sp., Rhodococcus sp., Mycobacterium,

Aeromonas sp.,

Achromobacter,

Proteus sp. 

Degrade hydrocarbons, pesticides, dyes (azo dyes) and PCBs
Anaerobic Bacteria Dechloromonas sp. Degrade PCBs, dechlorination of TCE and Chloroform.
Endophytic and Rhizospheric Bacterias Rhodococcus sp.,

Azospirillum sp.,

Lipoferum,

Lysini bacillus,

Pseudomonas sp., etc.

Heavy metals degradation, industrial wastes, dyes and other toxic components.
Fungi, Ligninolytic bacteria and Mycorriza Cladophialophora sp., Exophiala sp.,

Leptodontium sp., Pseudeurotium zonatum; Yeasts like S. cerevisiae,

Rhodotorula pilimanae, Candida lipolytica,

Rhodotorula aurantiaca and C. Ernobii and

Aspergillus sp., etc.

Degradation of oils, heavy metals, industrial waste such as dyes, toxic elements, plastics, drugs, toxic dyes and human wastes etc.
Algae and Protozoa Chlorella vulgaris, Scenedesmus platydiscus,

S. quadricauda and

S. Capricornutum.

Degrading oils such as diesel, hydrocarbons and few synthetic dyes.

Table: Microbial diversity in bioremediation of textile dyes (Source: Nezha Tahri Joutey et al., 2013)

Microbial degradation of Azo dyes

Microbes possess unique ways of degrading and de-colourising aromatic compounds, especially textile dyes. This degradation takes place under both aerobic and anaerobic conditions in a sequential manner. This is mainly achieved through enzymes that decolourise and degrade dye compounds (Kandelbauer & Guebitz 2005). Since azo dyes are the largest group of textile dyes, extensive work has been carried out on their bio remediation properties. Micro-organisms reduce azo dyes by producing enzymes such as:

  • laccase,
  • azoreductases,
  • peroxidases,
  • monooxygenase,
  • dioxygenase and
  • hydrogenase.

These enzymes catalyses the oxidation or reduction process and further mineralise them into simpler compounds (Kandelbauer & Guebitz 2005).

showing the flowchart of the textile dyes

Figure: Flowchart depicting microbial degradation of textile dyes (Saratale et al. 2011)

Laccases, the most promising enzyme, degrades azo dyes by attacking the phenolic ring, thus, making dyes more susceptible to hydrolysis (Sudha & Saranya 2014). Further, azoreductases catalyses in presence of reducing elements FADH and NADH on the azo cleavages. Another enzyme, peroxidase, catalyses the oxidation of phenolic groups, in the presence of hydrogen peroxide, thereby reducing the compound. Such enzymes make direct contact with dye substrates or redox mediators at the cell surface (Singh et al. 2015).

Bacteria need a combination of both aerobic and anaerobic conditions in order to degrade or decolourise dyes. Furthermore, under anaerobic conditions, azo bond undergoes cleavage to generate aromatic amine, which are mineralised in aerobic conditions through the ring cleavage (Sugumar & Sadanandan, 2010). In case of fungi, lignin peroxidases, laccase and manganese peroxidases are produced. All of which are extracellular enzymes and are unspecific in nature (Husain 2010). As a result of fungi excreting the enzymes, they are highly applicable in bioremediation of textile dye effluents. However, the use of fungi, like the white rot fungi pose several problems, such as their slow growth rate, need for nitrogen limiting conditions, unreliable production of the required enzymes and also the need for a long hydraulic retention time (average amount of time a compound remains in storage unit) for complete decolorisation process (Shah 2014).

Further research required for more efficient microbes

Subsequently, inefficiency of the textile dyeing process has resulted in significant concentration of dyes being released into the environment. This is aggravated by the fact that these dyes are recalcitrant in nature, requiring biological systems that have highly specific enzymes capable of degrading them. Although both bacteria and fungi have proven to be exceptional sources of such enzymes, the process of degradation is complex requiring specific environmental conditions and substrates. In addition, it has to be emphasised that azo dyes are toxic and harmful. Microbes need to produce enzymes degrading them have not been tested extensively in-situ. Subsequently, there is a need to develop consortium capable of completely degrading, with limited environmental maintenance and substrate input. Further research is therefore needed to develop and test the consortium.

In the next article, bioremediation of another industrial effluent, pharmaceutical waste, is studied. Pharmaceutical effluents typically consists of a mixture of raw materials as well as end-products with biological activity, that can pose serious threat to organisms in the environment. Bioremediation of such a mixture of compounds is thus very important.

 

Reference      

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Avishek Majumder

Avishek Majumder

Research Analyst at Project Guru
Avishek is a Master in Biotechnology and has previously worked with Lifecell International Private Limited. Apart from data analysis and biological research, he loves photography and reading. He loves to play football and basketball in his spare time with an avid interest in adventure and nature. He was also a member of the Scouts in his school and has attended Military training.
Avishek Majumder

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