In the previous article, the hazards and possible solutions for lead and arsenic pollution from the perspective of bioremediation were discussed. This article, on the other hand, explores problems posed by chromium pollution in soil and the possibility of bioremediation as a solution. It is well known that chromium metal exists in various forms in the environment, mainly as trivalent [Cr (III)] and hexavalent [Cr (VI)] cation. However, the water-soluble hexavalent chromium is the most toxic form of the element with strong carcinogenic properties.
Hence, with increasing risks posed by chromium pollution of soil globally, a safe and effective solution eliminating high concentrations of this heavy metal is needed. Bioremediation, a tool by which living organisms assimilate or extract pollutants selectively from the environment, can offer an efficient solution to this widespread problem.
The problem of chromium pollution
Though chromium has wide application in the tanning industries and others, its hazardous byproduct is often released into the environment along with the sludge. Chromium is also used in plating of metals, manufacturing pigments and dyes in rust inhibitors and production of catalysts (1). Chromium pollution has been well documented as a major threat that is responsible for causing damage to the skin, liver, kidney and respiratory tract in human beings (2–4). Typically, trivalent chromium is readily oxidized into the hexavalent form and poses a high risk as soil and groundwater contaminant. More problematic is the fact that hexavalent chromium has high mobility and ability to penetrate cell membranes more readily than the trivalent forms.
According to a 2015 report by Pure Earth and Green Cross Switzerland, chromium toxicity affects 16 million people globally. It is also responsible for 3 million disability-adjusted life years among low and middle-income countries. Thus, it is important to prevent the spread of chromium toxicity more so because of its high-risk nature of chromium poses an immense danger to humans and wildlife alike and requires stringent policies for remediation. Therefore, bioremediation using microbes and plants for extracting and cleaning up of pollutants from the environment offers a solution for large-scale cleaning up of contaminants safely. From the table below one can note the use of different chromium compounds in several industries globally.
Type of Hexavalent Chromium Chemicals
|Pigments in paints, inks, and plastics||Lead chromate (PbCrO4), Zinc Chromate (ZnCrO4), Barium Chromate (BaCrO4), Calcium Chromate, Potassium Dichromate, Sodium Chromate|
|Anti-corrosion coatings (chrome, spray coatings)||Chromic Trioxide (chromic acid), Zinc Chromate (ZnCrO4), Barium Chromate (BaCrO4), Calcium Chromate, Sodium Chromate, Strontium Chromate (SrCrO4)|
|Stainless Steel||Hexavalent Chromium, Ammonium Dichromate ((NH4)2Cr2O7), Potassium Chromate, Potassium Dichromate, Sodium Chromate|
|Wood preservation||Chromium Trioxide|
|Leather tanning||Ammonium Dichromate ((NH4)2 Cr2O7)|
Table: Industries and Type of Hexavalent Chromium Chemicals (Source: Das & Mishra, 2008)
Microbes Used in bio-remediation of Chromium
As previously discussed, microbes are suitable agents for carrying out bioremediation. This is because they can survive and grow in toxic environments, in addition to having an inherent resistance to heavy metals such as arsenic and chromium. By virtue of their resistive property, microbes are able to detoxify the metal contaminants and survive their toxic effects (5). There are some bacterial species which can perform bioremediation of chromium in the soil. P. dechromaticans for instance, was the first microbe discovered by Romanenko and Korenkov in 1977, which was capable of reducing hexavalent chromium. Fungi, yeast and actinobacteria such as Streptomyces griseus are also capable of bioremediation.
For example, Pichia guilliermondi is a yeast that is capable of binding metals (6,7). The table below shows different species of microbes that have shown Chromium bioremediation activity.
|Microbacterium sp.||Mucor hiemalis|
|Enterobacter sp.||Trichoderma viride|
|Exiguobacterium sp.||Candida sp.|
|Bacillus sp.||Lecythophora sp.|
|Thermus scotoductus||Aureobasidium pullulans|
Table: Microbes capable of carrying out Chromium bioremediation (Source: Kanmani et al., 2012)
Mode of action of microbes
It has been documented that microorganisms are able to carry out a biological reduction of the hexavalent chromium under both aerobic and anaerobic conditions. The reduction process can be either direct or indirect depending on the nature of the resistive pathway and it can occur directly through metabolic processes or by the action of metabolites, such as H2S, on chromates. Furthermore, the above figure illustrates various pathways by which microbes bring about the process of reduction using enzymes like reductases for transfer of electrons to convert the toxic hexavalent chromate to non-toxic forms.
The bacteria, Pseudomonas putida, possesses the enzyme ChrR which facilitates one or two electron transfers to convert Cr (VI) to Cr (III), whereas in E. coli the reduction is brought about by four electron transfer facilitated by the enzyme YieF. Besides these, SR (soluble reductase) and MR (membrane-bound reductase) along with cytochromes are associated with electron transfer systems to bring about the reduction reaction (6) in aerobic microbes. Thus, an example is aerobic bacteria, Thermus scotoductus, which carries out the reduction via NAPH dependant enzyme (8). Further, a study by Stewart et al (2010) found bacterial population in groundwater at a site in Northern England and noted that the bacterial population in the sandy clay deposits beneath the waste were capable of carrying out chromate reduction through an abiotic reaction with Fe(II) present in the soil, leading to its replenishment (9).
Bio-remediation an effective solution for chromium pollution
In conclusion, it has been shown that chromium is the most soluble, mobile and notorious heavy metal found in the environment and takes diverse forms such as Cr(0), Cr(III) and Cr(VI) species. Among them, the hexavalent species is the most toxic for living beings. Hence, in-situ bioremediation of chromium pollution is vital and can be done by employing microbes which through their metabolic processes convert the toxic ions into non-toxic Cr(IV). Although microbes possess the capability to detoxify harmful chromates, chromium also has mutagenic effects on the microbial population. Therefore, considering this phenomenon long-term remediation solution is dependent on microbial species with strong reduction properties and that is highly resistant to mutation.
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