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Vol. 11 No. 1 - January 2005

Chromium Pollution and Bioremediation

By: Sumit Yadav, O. P. Shukla & U. N. Rai

Introduction:

Tannery effluent is a major source of aquatic pollution in India with high chemical oxygen demand (COD), biological oxygen demand (BOD), and hexavalent chromium. There are a large number of tanneries scattered all over the country but the main areas of their concentration are Tamilnadu, Uttar Pradesh and West Bengal.

 Chromium, a steel-grey, lustrous, hard and brittle metal, occurs in nature in bound forms that constitute 0.1–0.3 mg kg-1 of the Earth’s crust. It has several oxidation states ranging from Cr (-II) to Cr (+VI), the trivalent and hexavalent states are the most stable ones. A maximum acceptable concentration of 0.05 mg/L (50 µg / L) for chromium in drinking water has been established on the basis of health considerations.

  Hexavalent chromium [Cr (VI)] compounds are being used in a wide variety of commercial processes and unregulated disposal of the chromium containing effluent has led to the contamination of soil, sediment, surface and ground waters. In trace amounts, chromium is considered an essential nutrient for numerous organisms, but at higher level, it is toxic and mutagenic. Nearly 80% of the tanneries in India are engaged in the chrome tanning processes. Most of them discharge untreated wastewater into the environment. In such aqueous waste, Cr(VI) is present as either dichromate (Cr2O72-) in acidic environments or as chromate (CrO4- ) in alkaline environments. 

Impact On Environment:

 In humans, exposure to hexavalent chromium salts for periods of 2 to 26 years has been implicated as a cause of cancer of the digestive tract. High levels of chromium and zinc in soil have been correlated with regional incidences of stomach cancer. Based on exposure to chromium via inhalation, the International Agency for Research on Cancer has categorized “chromium and certain chromium compounds” in Group 1: sufficient evidence for carcinogenicity in humans and animals.

In plants, high levels of Cr supply can inhibit seed germination and subsequent seedling growth. The deleterious effect of Cr is less pronounced on seed germination than on seedling growth. Barley seeds germinated and grew well at Cr(VI) levels of up to 100 mg kg-1 in soil but were always slower in development due to Cr inhibition of diatase, which is responsible for mobilizing the reserve starch necessary for initial growth. At 500 mg kg-1 Cr no seed germination occurred. In another study, Cr(VI) concentrations up to 2 mM supplied as K2Cr2O7 (588 mg kg-1 Cr) did not affect germination of pea seeds significantly. However, high levels of organic matter (5%) and/or low levels of soil pH significantly reduced Cr toxicity on germination due to the extremely low bio-availability of Cr under these conditions.

One potentially important source of increasing Cr levels in plant shoots is foliar application of Cr. Interestingly, research has shown that when Cr(III) or Cr(VI) is applied through the leaf surface, on lettuce and bean plants, they were not translocated from the leaves to other plant parts.. Surprisingly, Cr(III) was absorbed more rapidly than Cr(VI). The lack of Cr transport in plant tissues might be due to the localization of Cr in leaf cells as well as the tendency of Cr ions to bind or precipitate in an insoluble form.

Bioremediation of chromium:

Some research workers suggest that by using Cr (VI) contaminated groundwater to irrigate organic matter rich soil they could remove Cr from water by reduction and precipitation in the soil as Cr (III). Their initial investigations in a soil column study indicated that application of relatively large volumes of water spiked with 1 mg l1 Cr (VI) yielded outflow Cr levels. Chromate adsorption accounted for <1% of the total immobilized Cr and the amount taken up by alfalfa shoots was <0.5% of the total added. In a study Losi et al. (1994) examined the processes responsible for Cr (VI) reduction in soil. They found that organic matter content, bioactivity, and oxygen status were among the important factors. Under aerobic, field-moist conditions, organic matter rich soil reduced 96% of added Cr (VI). Sterile soils receiving similar amendments reduced only 75% of the original Cr (VI), demonstrating the importance of the presence of soil microorganisms in conjunction with a readily available carbon source. These studies emphasize the role of soil organic matter in the reduction of Cr (VI) to Cr (III). Organic matter enhances the reduction of chromate in soil by increasing microbial activities. Realizing the potential importance of soil microorganisms in reducing Cr (VI) in contaminated soils, several groups attempted to identify and isolate microorganisms. Recent researchers suggest that Cr-resistant microorganisms are present in all soils even in those with no history of Cr-contamination. Bacterial populations resistant to as much as 500 mg L1 Cr (VI) and fungal populations resistant to 1000 mg L1 Cr (VI) were directly isolated from 2 uncontaminated soils. In an attempt to evaluate the use of Cr-resistant bacteria for the bioremediation of Cr (VI)-contaminated soils researchers have isolated a population of P. mendocina from a sewage sludge and used it for the reduction of Cr (VI) to Cr (III) in a soil microcosm study. Their results indicate that P. mendocina was able to immobilize 100 mg kg1 Cr (VI) in 8 h by reducing it to Cr (III). The Cr (VI)- contaminated soils, after the microbiological treatment, supported growth of wheat seedlings without exerting any toxic effects, illustrating the usefulness of the microbiological treatment in the bioremediation of chromate-contaminated sites.

Bioaccumulation and bio-sorption:

Many microbes by cellular activities and/or their products significantly contribute in these biogeochemical cycles. Biotechnological approaches to the abatement of toxic metal pollution consist of selectively using and enhancing these natural processes to treat particular wastes. The processes by which the microorganisms interact with the toxic metals enabling their removal/and recovery are bio-sorption, bioaccumulation and enzymatic reduction.

Phytoremediation:

Phytoremediation of Cr pollution can be achieved by extraction of the metal from polluted soils into harvestable plant tissues (phytoextraction), by the accumulation of the element in the root tissues of aquatic plants growing in contaminated water (rhizofiltration), or by the in situ detoxification of the metal through plant-based chelation, reduction, and oxidation mechanisms (phytodetoxification).

Phytoextraction:

Research on the phytoextraction of Cr from contaminated soils and sediments has been scarce. Very few plant species such as Sutera fodina, Dicoma niccolifera and Leptospermum scoparium have been reported to accumulate Cr to high concentrations in their tissues. Attempts are being made  to use promising aquatic plant species for the phytoextraction of Cr from contaminated tannery sludge, the ability of three plant species (Scirpus lacustris, Phragmites karka and Bacopa monnieri) to absorb, translocate and concentrate Cr in their tissues. For the phytoextraction process to be effective substantial amounts of Cr removed from the root medium must be translocated to the harvestable plant parts so that it can be completely removed from the contaminated site. Clearly, more research needs to be done in this area to utilize available Cr hyper-accumulator plant species. 

Molecular approaches in remediation of chromium:

Another approach is to use molecular techniques to genetically engineer plants that can hyperaccumulate Cr and other heavy metals. In a recent study it was found that there is a high correlation in Cr content in shoots of many plant species with the content of other heavy metals such as cadmium, copper, nickel and zinc. Traits impacting the accumulation of these heavy metals in plant shoots were found to be associated. Using recent molecular approaches, it will be possible to evolve plants suitable for phytoremediation of soils and waters contaminated with Cr and/or other heavy metals.

Conclusion:

Thus chromium bioremediation through micro-organism or plants may be the best-suited technology in present context to clean up Cr contaminated sites and these technologies are eco-friendly and cost effective. Cr (VI) is readily immobilized in soils by adsorption, reduction, and precipitation processes, with only a fraction of the total Cr concentrations available for plant uptake. When taken up by plants, >99% of the absorbed Cr is retained in the roots where it is reduced to Cr (III) species in a short time. Phytotoxic levels of Cr in most plants seem to limit its accumulation in the food chain. Because most plants have low Cr concentrations, even when grown on Cr rich soils, the food chain is well protected against Cr toxicity. In regions, where Cr (VI) contamination of the environment represent a major area of concern, the use of Cr-hyper-accumulator plant species or Cr-reducing microorganisms may represent a cost efficient and highly effective technology for the removal and detoxification of the toxic forms of Cr.

The authors  Sumit Yadav, O.P. Shukla and U.N. Rai, are associated with the Ecotoxicology and Bioremediation Group of the National Botanical Research Institute, Lucknow -226 001, India


This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.


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