1. Introduction

Water is a one of the basic source for a human to survive on the earth. Among the total availability of water resources, only 1% of water is utilized for human consumption [1]. The adulterants from various industries like dye manufacturing industries, apparel industries, paper pulp mill industries, reed mat industries, tanneries, and printing industries increase a great threat to our natural resources [2]. Textile industry is the one which releases a huge amount of waste i.e. the organic dyes, chemicals, heavy metals, and oil in water bodies create a mass disaster to the environment because of its venomous nature and it acts as an agent for cancer disease [3]. Textile dyes are highly soluble in water, due to this high release of dark dye the entry of sunlight is blocked causing a severe damage to the aquatic organisms and humans in and around the areas of these industries [4]. These dyes are approximately ten thousand in number. On viewing the large scale of usage, the azo dye is the one which comprises of the immense and critical damage [5].

Malachite Green (N-methylated diaminotriphenylmethane) is an organic compound containing a green colour crystal with a metallic lustre used in various dye stuff industries. It is one among the most effective chemicals all over the world [6]. It is highly toxic and should be treated before discharging it into the natural resources. It has a noxious effect on liver, gills, kidney, intestine, and various damages to mammalian cells. It may lead to cancer when inhaled; causes the skin irritation with pain when come into contact [7]. The direct discharge of MG dye in hydrosphere causes an imbalance to environment. This untreated water used for the irrigation purpose might result in reducing the quality of crop production [8]. These dyes are banned in many countries, but are still in use due to its low cost, accessibility, and potency [9].

Treating these dye wastes before discharging into water bodies is the most crucial need to be performed. One in eight people on the earth lack access to clean drinking water. Nearly one billion people have no access to safe drinking water. In developing countries, 3.5 million people die each year due to inadequate sanitation and hygiene. Around 1.5 million children die due to water-borne diseases. By 2025, 1.8 billion people will be living in countries or regions with absolute water scarcity. This problem in water scarcity is going to make a worldwide systematized hazard [10]. To overcome this, research in appropriate material and perfect treatment method/ devices should be implemented in an urgent scale.

This mini review contains the process in literatures from 2018 to 2022 for the degradation of malachite green dye by photocatalytic technique by using various metal oxide nanoparticles.

2. Dye Treatment Methods

For the betterment of human society and to resolve inadequate drinking water availability, both the portable water and waste water should be treated. Various physical, chemical, and biological ways have been employed to remove the contaminants in water (Figure 1). The electrocoagulation process [11], adsorption, enzyme degradation [12], ion exchange, membrane filtration, chemical precipitation, flocculation, chemical oxidation [13], bacterial decomposition, electrochemical decolourisation, ozonisation [14] nano photocatalysis, ceramic nano filtration membrane, biofilms, [15], organic resin, electrolysis, reverse osmosis, hybrid materials, oxidation, and electrolysis, biofoulents [16]. The properties of these toxic dyes make the traditional methods less effective in decolourisation. Yet, the light based adsorbent (photocatalysis) method has come out with an excellent capability of degrading these dyes.

Figure 1. Various methods of treating industrial waste

3. Photocatalytic Treatment of Dyes

The photocatalytic treatment covers a heterogeneous photocatalysis, semiconducting absorbent which degrades numerous impurities upon illumination to the light including the aerosphere organic pollutants. It comes up with several conveniences over the standard methods of treating industrial wastewater. For instance, within a short span of time, the dye material gets completely degraded by photocatalyst even at room temperature. More importantly, there is no formation of toxic residues, it completely breaks down the organic pollutants into unthreatening by-products i.e. water and carbon-dioxide [17]. Some optimal conditions during the photocatalysis reaction also influence the degradation efficiency. They are the light source which is used for irradiation (sunlight is the naturally available light source), the time period which the dye get exposed to the light, pH value of the dye solution, amount of catalyst required for efficient degradation, the distance between light source and dye, the temperature maintained during the reaction, and concentration of the dye solution [18].

In the beginning, the ordinary porous materials and those made under nano size are used as an adsorbent like activated carbon, CNTs, mesoporous silica, and chitosan bead. These materials exposed some constraints such as struggling during operation, less efficient, high cost, need for high amount of energy. An alternate material as photocatalyst is highly recommended which should overcome the previous drawbacks [19].

4. Metal Oxide Nanoparticle as Photocatalyst

Different catalysts have been engineered to enhance the photocatalytic process. Metal oxide nanoparticles are less toxic and can be easily oxidized into hydroxides or oxides. Another main property is its band gap which enables both the oxidation and reduction process by observing energy from the light source. The metal oxides with less band gap make it efficient under the visible light i.e. low of cost and less time consuming because of its absorption shifts to red band [20]. Many metal oxide nanoparticles are employed as an efficient photocatalyst. Some of them are TiO₂, ZnO, Bi₂O₃, Fe₂O₄ [21], WO₃, CuO, Cu₂O [22], SnO₂, CeO₂, BiVO₄, Bi₂WO₆, InTaO₄, Zn_1.7GeN_1.8O, ZnAl₂O₄, and ZnGaNO [23]. These metal oxides have much morphology such as nanoparticle, nano spheres, nanofibers, nanotubes, nano ribbons, nano sheets, etc. These morphologies also affect the efficiency of a photocatalyst [24]. The following table (Table 1) provides the Malachite green dye degradation by various metal oxide nanoparticles, the experimental condition, and its degradation efficiency.

Table 1. Malachite green dye degradation by various metal oxide nanoparticles, experimental condition, and its degradation efficiency

5. Conclusion

Though there are various ways to treat the dye contaminated water, photocatalysis plays a major role in waste water treatment. The industrial dye waste water which makes a great threat to the society can be resolved by the photocatalytic degradation method which has been widely employed. The work presented in this review showed that metal oxide nanoparticles can act as an efficient photocatalyst in treating industrial dye waste. These photocatalytic metal oxides can be reused after the proper cleaning treatment after separation by using centrifuge. By using the abundant naturally available source solar light, these industrial dyes should be treated before discharging into water bodies. The scarcity for the proper and safe drinking water can be resolved by photocatalytic treatment and the society can be benefitted.

Acknowledgment

Author R. Sarathi would like to express her gratitude to Sri Paramakalyani College, Alwarkurichi for providing technical supports.

Orcid:

R. Sarathi

https://orcid.org/0000-0001-6644-506X

Renuga Devi

https://orcid.org/0000-0003-3251-2089

N.L. Sheeba

https://orcid.org/0000-0002-3346-670X

E.Selva Esakki

https://orcid.org/0000-0003-2287-334X

Meenakshi Sundar

https://orcid.org/0000-0001-7164-8689

Citation: R. Sarathi*, L. Renuga Devi, N.L. Sheeba, E. Selva Esakki, S. Meenakshi Sundar, Photocatalytic Degradation of Malachite Green Dye by Metal Oxide Nanoparticles - Mini Review. J. Chem. Rev., 2023, 5(1), 15-30.