Hexavalent chromium reduction potential of Cellulosimicrobium sp. isolated from common effluent treatment plant of tannery industries
Introduction
The contamination of environments (soil and water) with various toxic metals is a serious threat for ecosystem and human health, and requires the implementation of appropriate remedial measures. Heavy metals, such as chromium, cadmium, mercury, arsenic, lead etc. are considered as major environmental pollutants due to their toxic effects on environment as well as on human health (Ray and Ray, 2009). In developing countries, different types of industrial wastes (solid and liquid) containing a number of toxic metals in high concentration are directly or indirectly discharged into the environment without adequate treatment (Dixit et al., 2015, Chandra et al., 2009). Industries such as metallurgical, chemical, refractory brick, leather, wood preservation, pigments and dyes are the major sources of toxic metals contamination in environment (USEPA, 1998, Ryan et al., 2002).
However, tannery industries are the major source of chromium contamination into the environment. Tannery industries consume a huge volume of water in tanning of hides and skin, as it is wholly a wet process and generate ~ 30–35 L of wastewater per kg skin/hides processed (Nandy et al., 1999). There are ~ 3000 tanneries in India, mainly located in the states of Tamil Nadu, West Bengal, Uttar Pradesh, Andhra Pradesh, Bihar, Gujarat, and Maharashtra, generating total ~ 1,75,000 m3 wastewater per day (Kaul et al., 2005). In Uttar Pradesh, ~ 444 tanneries are in operation mainly in Kanpur and Unnao region generating 22.1 MLD of wastewater per day (CPCB, 2013) and this wastewater is reported to contain 0.01–4.24 mg/L of Cr(VI) (MOWR, 2013). However, most of the tanneries (nearly 80%) are engaged in chrome tanning process that releases ~ 2000–3200 t of Cr into the environment annually (Belay, 2010). The Cr concentration in tannery wastewater ranges between 2000 and 5000 mg/L, which is much higher than the permissible limit of 2 mg/L for wastewater discharge (Belay, 2010).
Like organic pollutants, metals are not degraded and tend to accumulate into the environment, may enter the food chain and cause toxic, genotoxic, mutagenic and carcinogenic effects (Chandra et al., 2011). Chromium compounds are well known to have toxic, genotoxic, mutagenic, and carcinogenic effects on humans, animals, plants, and as well as in microbes (Cheung and Gu, 2007, Mishra and Bharagava, 2016). In nature, chromium exists in several oxidation states ranging from − 2 to + 6, but only trivalent (III) and hexavalent (VI) forms of chromium is most prevalent and stable. Out of these two forms, hexavalent chromium [Cr(VI)] is highly toxic, mutagenic, teratogenic, carcinogenic to human and animals and has been designated as priority pollutant by US Environmental Protection Agency (USEPA) (1998). If Cr(VI) concentration into the environment exceeds > 0.05 mg/L, then it may affect the human physiology and if enter the food chain, it may cause severe health hazards such as skin irritation, nasal irritation, ulceration, eardrum perforation, and lung carcinoma etc. (WHO, 2011, Srinath et al., 2002).
Cr(VI) also acts as a strong oxidizing agent and exists only in oxygenated forms as hydro-chromate (HCrO4-), chromate (CrO4-) and dichromate (Cr2O7−2) ionic species in aqueous systems. Cr(VI) compounds are comparatively more toxic than Cr(III) compounds due to their higher solubility in water, rapid permeability through biological membranes and subsequent interaction with intracellular proteins and nucleic acids (Thacker et al., 2006; Cheung and Gu, 2007). Although a number of conventional/traditional methods are reported either for removal or detoxification of Cr(VI) from industrial wastes such as chemical precipitation, reverse osmosis, ion-exchange, filtration, membrane technologies, evaporation recovery, absorption on coal, activated carbon, alum, kaolinite, and fly ash etc. (Saxena et al., 2016, Ahluwalia and Goyal, 2007). These methods are very costly, less effective and also generate a metal rich sludge as secondary pollutants. Therefore, it becomes very essential to develop an eco-friendly, cost-competitive and effective method for removal/detoxification of Cr(VI) for the safety of environment and human health protection.
However, microbial reduction of toxic Cr(VI) to non-toxic Cr(III) by chromium resistant bacteria (CRB) is the most pragmatic approach that offers an economical as well as eco-friendly option for chromate detoxification and bioremediation. Microbes have diverse resistance mechanisms to cope with chromate toxicity that enable them to survive in such harsh environmental conditions (Cervantes and Campos-Gracia, 2007). These detoxification strategies include biosorption, bioaccumulation and biotransformation by enzymatic reduction, diminished intracellular accumulation through either direct obstruction of ion uptake system or active chromate efflux, precipitation, and reduction of Cr(VI) to less toxic and less mobile Cr(III) (Cheung and Gu, 2003, Ramirez-Diaz et al., 2008). Hence, the objectives of this study were to isolate and characterize chromium resistant bacteria, which should be capable to reduce/detoxify the toxic Cr(VI) into less toxic and less mobile Cr(III) for environmental cleanup and human health safety.
Section snippets
Collection of tannery wastewater
The tannery wastewater was collected from Common Effluent Treatment Plant (CETP) of Jajmau Unit, Kanpur (26°26'59.7228"N and 80°19'54.7335"E), Uttar Pradesh, India in a pre-sterilized conical flask (Cap. 2 L), brought to laboratory, maintained at 4 °C and used in analysis of physico-chemical parameters as well as for the isolation of bacterial strains capable for the reduction of hexavalent chromium.
Physico-chemical analysis of tannery wastewater
The physico-chemical analysis of tannery wastewater was made in triplicate as per the standard
Physico-chemical characteristics of tannery wastewater
Thephysico-chemical analysis of tannery wastewater reveals that it was alkaline in nature (pH 8.49 ± 0.2), light yellowish in color and deficient in dissolved oxygen. In addition, the BOD, COD, total solids, TDS, TSS, phenol, sulphate, and total chromium content was 160 ± 15.8, 322 ± 28.6, 11,028 ± 805.2, 3491.3 ± 239.4, 194 ± 23.5, 12.7 ± 1.2, 1445 ± 67.9 and 5.7 ± 0.2 mg/L, respectively as shown in Table 1. The alkaline pH and high EC of collected tannery wastewater could affect the biological
Conclusion
Metals such as chromium, cadmium, mercury, arsenic, lead etc. are considered as a major environmental pollutant due to their toxic effects on environment as well as on human health. Based on the results of this study, it can be concluded that the isolated bacterium can be a good agent for the reduction/detoxification of hexavalent chromium from contaminated environments. During the chromium reduction experiment, the bacterium was found capable to reduce 99.33% and 96.98% Cr(VI) at 50 and 100
Acknowledgements
Authors are highly grateful to the University Grant Commission (UGC), Government of India (GOI), New Delhi for UGC Fellowship. Authors also acknowledge the support from USIC B. B. Ambedkar University for SEM and FTIR analysis.
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