Organic dyes from industrial effluents can cause severe damage to the environment, leading to the destruction of the biodiversity of the aquatic ecosystem (Katheresan et al. 2018). Moreover, they also cause water pollution and desertification and are toxic to aquatic and human lives. Textile industry, dyeing industry, paper and pulp industry, tannery and paint industry are the major contributors of the dye effluents in the environment (Katheresan et al. 2018; Robinson et al. 2001; Forgacs et al. 2004; Ghosh et al. 2019; Pathak et al. 2018; Velusamy et al. 2021). Dyes are chemically stable, inert molecules and are resistant to the action of sunlight, water, soap, bleach, and perspiration (Lellis et al. 2019; Robinson et al. 2001; Langhals et al. 2003)
The colour imparted by the dyes to water causes not only aesthetic damage, but also prevents the penetration of sunlight through water, thereby causing considerable decrease in the rate of photosynthesis and the amount of dissolved oxygen (Lellis et al. 2019; Banat et al. 1996; Hassan et al. 2018; Imran et al. 2015; Roy et al. 2010). In addition to being non-biodegradable, dyes and their degradation products can be carcinogenic, mutagenic and teratogenic and can cause long-lasting health issues (Lellis et al. 2019; Nestmann et al. 1979; Carneiro et al. 2011; Schneider et al. 2004; Christie et al. 2007).
Therefore, it is a necessity that the industrial effluents containing hazardous organic dyes should be treated properly before they are disposed into the water resources. The properties that make the dyes useful in various applications also cause their elimination from the environment a difficult task. A large number of methods have been consequently developed to remove the dye pollutants from the waste water, such as adsorption, chemical precipitation, filtration, coagulation, floatation, oxidation, ozonation, chlorination, bleaching, Fenton oxidation, ion-exchange, reverse osmosis, phytoremediation, aerobic and anaerobic processes etc (Forgacs et al. 2004; Lellis et al. 2019; Robinson et al. 2001; Lin et al. 1994; Tünay et al. 1996; Hao et al. 2000; Ghoreishi et al. 2003; Crini et al. 2006). Most of the techniques above exhibit a low efficiency and require huge costs (Saratale et al. 2011; Abdelrahman et al. 2019). However, degradation of organic dyes present in the effluent water completely using catalysts and visible light or ultraviolet radiation completely into innocuous products has proven to be an efficient method of effluent water treatment. This is due to the ability of this approach to degrade the organic dye molecules in a simple, environmentally benign, sustainable and cost-effective manner (Abdelrahman et al. 2019; Jain et al. 2019). These methods mostly explored the usage of conventional semiconductor nanomaterials and their modifications for the photocatalytic degradation of dyes (Kang et al. 2010; Sun et al. 2020; Zhang et al. 2017; Ding et al. 2017; Dong et al. 2017; Deng et al. 2017).
Recently, metal complexes have shown good potential as photocatalysts in the degradation of toxic organic dyes (Kim et al. 2008; Ardo et al. 2011; Qin et al. 2013; Chang et al. 2017; Chen et al. 2017; Roy et al. 2017; Azam et al. 2018; Wang et al. 2019). There are a few reports of the use of Cu-Schiff base complexes in this regard (Jain et al. 2019; Wu et al. 2013; Wang et al. 2014; Li et al. 2014; Xu et al. 2015; Qiao et al. 2017; Wu et al. 2017; Yang et al. 2018; Ghosh et al. 2019; Wang et al. 2019; Pan et al. 2019; Carvalho et al. 2020; Wang et al. 2020; Li et al. 2020).
These metal complexes are found to be a good alternative to the photocatalysts based on gold, silver, ruthenium and platinum in terms of economy. Therefore, it is the need of the hour to develop copper-based complexes having narrow band gap, that can absorb in the visible and UV region. All the above reasons prompted us to synthesize three new copper (II) complexes (C1, C2 and C3) with the ligand N'-(3-hydroxy-4-methoxybenzylidene)nicotinohydrazide (HL) and explore their use as photocatalysts in the degradation of methylene blue. The degradation kinetics of methylene blue using these complexes is also examined. Furthermore, the antibacterial properties of the compounds were studied against Escherichia coli (gram negative), and Bacillus circulans (gram positive) by disc diffusion method.