Hexavalent Chromium: Toxic and Genotoxic Effects and its Bioremediation Strategies

A silver-colored heavy metal Chromium (Cr) which exhibits different oxidation states (-2 to +6) has physical properties like being lustrous, brittle, odourless and, tasteless. It has many physicochemical and biochemical properties due to its trivalent chromium (Cr(III)) and chromium (Cr(VI)) electronic structure. Being an extensive metal it has many industrial uses such as in electroplating, leather tanning, pigments, petroleum refining etc. It is a major concern to us as it is non-biodegradable in nature, its chemical treatment results in production of more secondary pollutants. Consequently, when the heavy metals are emitted out of the industries into the environment their concentration should be brought down to an admissible level. The chemical remediation of the Cr(VI) residues is expensive. Hence, bioremediation of this toxic Cr(VI) metal is the focus of this article. Microorganisms or microcosms that can remediate Cr(VI) exhibit distinctive strategies (depends upon the quality, bio-sorbent and further estimating the presence of reductants) such as biosorption, biotransformation and bioreduction. It is very beneficial for Cr(VI) removal in this way, overcoming all the difficulties related between research facilities and modern applications for chromium remediation. Genotoxic


Introduction
Chromium was first discovered by Louis Nicholas Vauquelin as an element (PbCrO 4 ) along with molybdenum and tungsten that belong to the transition group (VI-B) in the modern periodic table [1][2][3][4][5]. This heavy metal is toxic to environment as it contaminates the soils and sediments along with surface and groundwater due to man-made activities. ATSDR (Agency for Toxic Substances and Diseases Registry) states that Cr(VI) is listed under the 17 most toxic chemicals. Chromium occur in several states but the two most stable states are: one being soluble and highly toxic Cr(VI) and the other being comparatively less soluble and toxic Cr(III), which is 10-100 times less bio-available and more stable. Cr(VI), most toxic form of chromium as it is a strong clastogen because it has high oxidation potential, greater solubility in water and further can easily penetrate through biological surfaces. Cr(VI) before being ejected out from the tanneries into the external environment needs to be treated properly before outflow in the environment. Cr (VI) also has an adverse impact on plants such as like alteration in seed germination, reduced growth of roots, stems and leaves. Waste like phenol, chloride, sulphide, tannins and formaldehyde are generated by tanning of Chromium. Even, less toxic Cr(III) gets converted into highly toxic Cr(VI) upon oxidation during this process. Chromium sulphate [Cr 2 (SO 4 ) 3 ] is used in treating leather. As Cr(VI) is a very stable compound it can sometimes get deposited in the food chain which leads to diseases like stomach irritation, cancer in digestive tract, brain damage, diarrhea, premature death in mammals etc. that can even be even fatal to higher organisms. According to USEPA (United States Environmental Protection Agency) hexavalent chromium is regarded as priority pollutant and listed as class ' A' human carcinogen [6][7][8][9][10].
Some of the methods like reverse osmosis, precipitation , chemical reduction followed by ion exchange and absorption (coal, activated carbon and alum) are generally used to purify the Cr(VI) residues released from the industrial effluents. Besides, being economically expensive the major drawback of these methods is partial removal of metal, high reagent and requires too much energy which leads to the production of toxic material which again need disposal. Most practical methods for reducing Cr(VI) which is toxic in nature to less toxic form Cr(III) is by biological

Essentiality of Trivalent Chromium
Cr can occur in several oxidation states, depending upon the human exposure. Cr(III) is a part of GTF(Glucose Tolerance Factor), which has an important dietary component that increases the action of insulin and hence regulate the carbohydrate metabolism.
Insulin activity is also enhanced by Cr(III), when it attaches to insulin, further boosts its impact by triple times. Thus, Cr deficiency may cause diseases related with Carbohydrates and weight loss [16]. Even nutritional supplements, foods and multivitamins consist of Cr(III) carrying compounds. The supplementation products are as follows: CrPic (Chromium Picolinate), CrHis (Chromium histadinate), CRDC (Chromium dinicocysteinate), and NBC (Niacin-bound Chromium). As Cr(III) compounds cannot cross the membranes, so they get collected around the cells which leads to the variations in functioning of cells and damage of membrane in cell. It was reported that Cr(III) containing healthy addition for example CrPic can encourage DNA damage outcomes. Within biological media, series of chemical changes can also undergo in Cr (III). In 2016, Cr(III) was the fourth most selling supplement in USA.
Behind calcium, magnesium, and iron one can take Cr(III) in their diet within the range of 23-29 µg/day .The dietary intake of Cr(III) has been beneficial for diabetes and in the process of anabolism of muscle and loss of weight according to some studies. But the evidence of it is not very strong and pretty controversial, moreover some studies even revealed that too much intake of chromium is carcinogenic [17][18][19][20][21][22].

Physical and Chemical Methods for Remediation of Cr(VI)
Physio-chemical properties of substances for remediation is explained through physical and chemical methods. Physical Hence, bioremediation is seen to be an effective methodology for eliminating Cr from polluted environment in an eco-friendly and economical manner (Table 1).    Corynebacterium paurometabolum which has properties of biosorption, just as biotransformation, also contributes to limit the poisonousness of chromium mixes can also be utilized (Table 3).

Fungi Being used in Bioremediation
Fungi has also been reported and introduced for remediating the sites which are contaminated by Chromium. In case of fungi biosorption has been found to be productive than bioreduction.
In Aspergillus oryzae, Aspergillus niger, Trichoderma species flavus and Aspergillus niger were tested for its capability of its capacity and accumulation of Cr(VI). The results were found to be more than 25% of the supplied Cr(VI). Some fungi like Hypocrea tawa and Trichoderma inhamatum can reduce Cr(VI) to Cr(III) [32].

Conclusion
Uncontrollable and uneven excretion of heavy metals contaminated industrial effluent into the environment has led to a major concern for the environment of 21st century. Some techniques like biosorption and bioreduction has become effective for the remediation of highly toxic and carcinogenic Cr(VI). Biomass of microorganisms, fungi, plants, algae has been reported effective for the compelling of highly toxic hexavalent chromium reduction. The molecular remediation system also gave a better vision about Cr(VI) removal. These all helped in building up the current advances of chromium remediation to be progressively proficient.
There is a wide gap between lab discoveries and efficient usage Till now we all are working on the culture dependent techniques which involves the culturing of organisms in the artificial laboratory conditions but this technique is not totally reliable for the remediation of toxic heavy metals as we know that very low number of microbes can be grown or cultured in artificial media in laboratories so we need to switch our work towards the culture independent techniques which involves metagenomics etc so that we can work in a broader way and on more microorganisms. By using culture independent technique basically we can explore more on an environmental community of microorganisms and not only on a single organism. This technique may reduce the gap between the laboratory work and its application on the remediation of Cr(VI) on the actual site in environment.