Background of the Study
Principally, textile industries discharge to natural water sources colored effluents with pigments and dyes that are toxic and have carcinogenic and mutagenic effects (Demir et al, 2008). Waste water from the textile industries mainly contains moderate concentration (10-200 mg/mL) of dyestuffs which contribute significant contamination of aquatic ecosystem (Neill et al, 1999). They generate a considerable amount of waste water. It is estimated that more than 100,000 commercially available dyes with over 7x105 tonnes of dyestuff produced annually worldwide (Pearce et al, 2003). Color is the first contaminant to be recognized in waste water. The presence of even very small amounts of dyes in water – less than 1 ppm for some dyes – is highly visible and undesirable (Rafatullah et al, 2009).
Malachite green, an N-methylated diaminotriphenylmethane dye is the most widely used dye for coloring purpose among all other dyes of its category (Crini et al, 2007). It is used as a food coloring agent, food additive, a medical disinfectant and antihelminthic as well as dye in silk, wool, jute, leather, cotton, paper and acrylic industries (Saha et al, 2010). However, the consumption of malachite green has many adverse effects due to its carcinogenic, genotoxic, mutagenic and teratogenic properties. It is highly cytotoxic to mammalian cells and acts as a liver tumor promoter. It decreases food intake capacity, growth and fertility rates; causes damage to liver, spleen, kidney and heart; inflicts lesions on skin, eyes, lungs, and bones (Chowdhury & Saha, 2010). Discharge of malachite green into the hydrosphere can cause environmental degradation as it leaves undesirable color to water and reduces sunlight penetration. Malachite green is also environmentally persistent and acutely toxic to a wide range of aquatic and terrestrial animals (Tsai & Chen, 2010). Hence, there is a necessity for treatment of effluent containing such dye due to its harmful impacts on receiving waters. Congo red is a benzidine-based, direct, anionic diazo dye prepared by coupling tetrazotised benzidine with two molecules of napthionic acid. Congo red is the first synthetic azo dye produced that is capable of dying cotton directly. Congo red containing effluents are generated from a number of industrial activities: textiles, printing and dyeing, paper, rubber, plastics industries (Vimonses et al., 2009). Exposure to the dye has been known to cause allergic reactions. The substance is considered as toxic exhibiting acute, algal, bacterial, protozoan, cutaneous, environmental, microbial, yeast toxicity; cytotoxicity; genotoxicity; hematotoxicity; neurotoxicity, as well as carcinogenicity and mutagenicity (Raymundo et al., 2010). The capability of congo red to form carcinogenic amines such as benzidine through cleavage of one or more azo groups is the reason why it falls under the category of banned azodyes (Pielesz, 1999). The recalcitrance of CR has been attributed to the presence of aminobiphenyl group and azo bonds, two features generally considered as xenobiotic (Pinheiro et al., 2009). During the last few decades, a number of physical, chemical and biological methods were studied as coagulation, ultrafiltration, electrochemical adsorption, photo-oxidation and ion exchange method. Among them adsorption technique is generally considered to be an effective method for quickly lowering the concentration of dissolved dyes in waste water (Reife et al, 1996). The adsorption process could be explained via van der Waals forces, electrostatic, H−bonding and hydrophobic-hydrophobic interactions (Ahmad and Kumar, 2010).
Activated carbon is the most widely used adsorbent for dye molecules and dissolved compounds due to its high porosity and good surface area for sorption of organic compounds but high capital problem with handling of spent carbon, limits its widespread application. (Silva et al,...