Abstract: Not available
Properties
Chromium, one of the toxic metals is detectable in earth's crust in small
quantities associated with other metals, particularly iron. The atomic weight
of chromium is 51.996. It melts at 1,875 °C and boils at 2,680 °C. The
specific gravity of chromium is 7.19 g cm-3. It forms a number of
salts, which are characterized by a variety of colors, solubilities and other
properties. The name “chromium”
is from the Greek word for color. Most important chromium salts are sodium and
potassium chromates and dichromates and the potassium and ammonium chrome alums.
Oxidation States
Chromium can occur under several oxidation states from Cr (0) to Cr (VI).
Trivalent chromium Cr (III) occurs naturally in rocks, soil, plants, animals
and volcanic emissions (Duffs, 2005). This form is believed by many to play
a nutritional or pharmaceutical role in the body, but its exact mechanism of
action is unknown. Cr (III) is used industrially as a brick lining for high-temperature
industrial furnaces and to make metals, metal alloys and chemical compounds
(Barantsevs et al., 2004). Hexavalent chromium, Cr(VI), is produced industrially
when Cr(III) is heated in the presence of mineral bases and atmospheric oxygen
(for instance, during metal finishing processes). It is the form of chromium
that has proven to be of the greatest occupational and environmental health
concern. Cr(VI) compounds are emitted into the air, water and soil by a number
of different industries. In the air, chromium compounds are present mainly as
fine dust particles that eventually settle over the land and water.
Entrance into the Environment
Chromium enters environmental waters from anthropogenic sources such as
electroplating factories, leather tanneries, wood preserving, metal processing,
chromium plating, alloy preparation, leather tanning, petroleum refining and
manufacturing of automobile parts and textile manufacturing facilities (Panov
et al., 2003; Huang et al., 2004). Chromium also enters groundwater
by leaching from soil. In aqueous environments, however, Cr mainly exists in
two oxidation states: Cr(VI) and Cr(III). Aqueous Cr(VI) is present in anionic
forms whose compounds are generally soluble over a wide pH range. Hydrolysis
of Cr(VI) yields a number of pH-dependent species, that is, H2CrO4,
HCrO-4, CrO4-2 and Cr2 O4-2.
On the other hand, the main aqueous Cr(III) species are Cr+3, Cr
(OH)+2 Cr(OH)+2, Cr(OH)30 and Cr(OH)4¯,
while polymeric species such as Cr2 (OH)2+4,
Cr3 (OH)4+5 and Cr4 (OH)6+6
are insignificant in natural systems. In the pH range encountered in natural
waters, most Cr(III) exists in the least insoluble form of Cr (OH)3 (Smith
andGhiassi, 2006).
Industrial Application
It has been estimated that workers in some 80 different professional categories
may be exposed to Cr(VI). Various Cr(VI) compounds are used in leather tanning,
the production of textiles, dyes, pigments and chrome plating (Cristian et
al., 2005). Other sources of chromium emissions include oil and coal combustion,
stainless steel welding, steel production, cement plants, industrial paint and
coating manufacture and cooling towers, which use Cr(VI) as a rust inhibitor
for their submerged moving parts (Potgieter et al., 2003). Occupational
exposures to Cr(VI) compounds can be quite acute (Proctor et al., 2004).
Manufacturing of Chromium Salts
Chromium (II) and (III) chemicals are also manufactured but in small amounts
as compared to chromium (VI). Chromium dichloride (CrCl2) and chromium
sulfate (CrSO4) are examples of the former and chromium trifluoride
(CrF3), chromium trichloride (CrCl3) and chromium nitrate
(Cr (NO3)3 are examples of the latter. Chromium platers
use heated baths of H2Cr2O7 to plate chromium
onto pieces of other metals. Under an electric current, the hydrated dichromate
ions move toward the positive electrode, which is the metal to be plated. The
hydrogen and oxygen gases formed escape the bath forming aqueous acidic chromate
laden mists.
Essentiality and Toxicity
Chromium is present in food and feed plants, but the form is not well characterized
(Cary, 1982; Das et al., 2005). The likely form is soluble chromium (III)
organic compounds such as chromium (III) oxalate in plants (Smith et al.,
1989). Chromium is an important micronutrient for animals and humans (Bahijri
and Mufti, 2002). Humans must consume organically bound or chelated chromium
as part of the proper metabolism of Glucose Tolerance Factor (GTF). Although
chromium (VI) can be rapidly absorbed through the intestinal wall, any ingested
chromium (VI) is believed to be quickly reduced in the stomach where the pH
is around 1 and numerous organic reducing agents can be found.
Chromium plays a key role in the biological life but above critical level it is toxic (Balamurugan et al., 2004; Han et al., 2004;) mutagenic (Gili et al., 2002; Puzon et al., 2002; Wise et al., 2005), carcinogenic (Codd et al., 2003; Reddy et al., 2003; Sato et al., 2003) and teratogenic (Asmatullah et al., 1998). Trivalent form of chromium is more common and its compounds are less soluble and less toxic than hexavalent chromium (Smith andGhiassi, 2006). Trivalent chromium forms stable complex with legends on DNA, proteins and small molecules such as glutathionein (Adach and Cielak-Golonka, 2005). Trivalent chromium bounds to the DNA template cause increased DNA polymerase processivity and decreased DNA replication fidelity. These alterations in DNA function can result in greatly increased bypass of oxidative DNA lesions, which are promutagenic (Adach and Cielak-Golonka, 2005).
Toxicity Towards Plants
In chromium contaminated sites not only human, animal and microbial life
damage but the effects on plants are also equally pronounced. Due to chromium
accumulation reduction in plant production along with toxicity in the nutritional
contents are also observed (Pandey and Sharma, 2003; Klumpp et al., 2002).
Some plants show tolerance against chromium, but some have acquired the ability
to accumulate chromium (Tripathi and Chandra, 1991). The root and shoot growth
rate and leaf chlorosis could be elicited in hyacinth (Eichornia crassipes)
by exposure to chromium and copper for several weeks (Hafeez and Ramzan, 2002).
In Triticum aestivum, hexavalent chromium showed adverse effect on the
growth parameters and also caused accumulation of chromium in the plants (Faisal
et al., 2005). The hexavalent chromium salt has more adverse effect on
germination and growth of Helianthus annuus (Faisal and Hasnain, 2005)
and Vigna radiata (Hsu and Chou, 1992). The effect of chromium on the
cortical cells of meristematic zone was observed in Pisum sativum L.
(Gabara et al., 1992). Chromium toxicity on the seed germination and
growth of Phaseolus vulgaris was markedly increased with an increase
in its concentration (Zeid, 2001). Plants growing in higher concentration of
chromium take up more chromium, which accumulate in the plant tissues and via
plants enter in the food chain. At a concentration of >0.2 mM Cr(VI), Citrullus
plant showed growth depression with chlorosis and loss of turgor of middle leaves
(Dube et al., 2003). Growth of mustard (Sorghum bicolor L.) plants
decreased by increasing the concentration of Cr(VI) from 50 to 100 μM (Shanker
andPathmanabhan, 2004). Lamina minor accumulated Cr(III) in smaller quantities
as compared to Cr(VI) which posses more toxic effects on growth parameters as
compared to Cr(III). In Brassica juncea Cr(VI) application caused growth
retardation, reduces the number of palisade and spongy parenchyma cells in leaves,
results in clotted depositions in the vascular bundles of stems and roots and
increases the number of vacuoles and electron dense materials along the walls
of xylem and phloem vessels (Han et al., 2004). When Vigna radiata
plants were treated with Cr(VI), chlorophyll and protein content decreased while
activity of catalase, peroxidase, glucose-6-phosphate dehydrogenase and superoxide
dimutase increased. Concentration of chlorophyll a and b, soluble protein and
grain production decreased significantly in maize plants exposed to Cr(VI) (Sharma
et al., 2003).
Toxicity Towards Animals
Chromium also shows toxicity towards different animals (Fatima et al.,
2005; Geetha et al., 2005). Exposure of Rana ridibunda to either
Cr or a mixture of Cr and Cd caused a decrease in liver GST and P450-MO and
renal GST activities (Kostaropoulos et al., 2005). It was observed that
treatment of rats with chromium (0.8 mg/100 g body weight/day) for a period
of 28 days caused significant increase in chromium content while lowering the
body weight along with the organ (kidney, testis, spleen, cerebrum and cerebellum)
weight, (Dey et al., 2003). Beside this there was a significant decrease
in the DNA and RNA content and in liver, cerebrum and cerebellum significant
decreases in total protein content was also observed (Dey et al., 2003).
The application of higher dose of chromium in drinking water during embryonic
development in mice results in reduced fetal weight, retarded fetal development
and higher incidence of dead fetuses (Junaid et al., 1995). Animal studies
on the carcinogenicity of various chromium species have generally suggested
that water-insoluble species, CaCrO4 in particular (Levy et al.,
1986), are the causative agent of respiratory cancers (Gad, 1989). Studies have
showed the toxicity of chromium picolinate in renal impairment, skin blisters
and pustules, anemia, hemolysis, tissue edema, liver dysfunction; neuronal cell
injury, impaired cognitive, perceptual and motor activity; enhanced production
of hydroxyl radicals, chromosomal aberration, depletion of antioxidant enzymes
and DNA damage in mice (Bagchi et al., 2002). In mice, potassium dichromate
treatment (20 mgCr kg-1), as Cr(VI) compound, significantly elevated
the level of lipid peroxidation as compared with the control group. This was
accompanied by significant decreases in nonprotein sulfhydryls (NPSH) level,
superoxide dismutase (SOD) and catalase (CAT) enzyme activities as well as a
significant chromium accumulation (Bosgelmez and Güvendik, 2004).
Toxicity Towards Humans
Certain Cr(VI) compounds have been found to be carcinogenic in humans, but
the evidence to date indicates that the carcinogenicity is site-specific-limited
to the lung and sinonasal cavity and dependent on high exposures, such as might
be encountered in an industrial setting (Feng et al., 2003). It has been
reported that trivalent chromium salts are poorly absorbed through the gastrointestinal
tract. Hexavalent chromium compounds are approximately 1000-fold more cytotoxic
and mutagenic than trivalent chromium compounds in cultured diploid human fibroblasts,
but both hexavalent and trivalent chromium compounds induce dose-dependent anchorage
independence in human diploid fibroblasts (Biedermann and Landolph, 1990). Cr
(VI) can cause a wide range of other health effects. Inhaling relatively high
concentrations of some forms of Cr(VI) can cause a runny nose, sneezing, itching,
nosebleeds, ulcers and holes in the nasal septum (Banerjee et al., 2003).
Short-term high-level inhalational exposure can cause adverse effects at the
contact site, including ulcers, irritation of the nasal mucosa and holes in
the nasal septum (Stohs et al., 2001). Hexavalent chromium compounds
induce dose-dependent cytotoxicity and anchorage independence in cultured human
diploid fibroblasts (Biedermann and Landolph, 1987). Cr(VI) is one of the most
highly allergenic metals, second only to nickel (Bock et al., 2003).
In fact, irritation of the skin a most frequently reported human health effect from exposure to Cr(VI), taking the form of skin ulceration (dermatosis) and allergic sensitization (dermatitis) (Hansen et al., 2003). Indeed, dermatosis was the first observed health effect associated with chromium exposure, although improved industrial hygiene practices have greatly reduced its incidence (Refiye et al., 2005). Once absorbed into the blood system there are various antioxidants that act as reducing agents, such as glutathione and ascorbate, which rapidly reduce chromium (VI) to chromium (III). Respiratory cancer is the health effect of most concern and is the basis for the regulation of chromium (VI) (O’Brien et al., 2003). There is also some indication that chromium (VI) may cause cancer of the upper airways and upper gastrointestinal tract, such as the esophagus, larynx, trachea and stomach (Xie et al., 2005). Chromium concentration in the lungs was found to increase with age for both occupationally and environmentally exposed individuals (Xie et al., 2005). The upper lobes tended to have higher concentrations than lower lobes and cancerous portions of lungs showed the highest chromium concentrations. However, large doses of chromium into the blood can result in acute kidney and liver damage (Dey et al., 2003).Research on the carcinogenicity of chromium (VI) has focused on the fact that chromate ions quickly pass through cellular and nuclear membranes, while the trivalent species are many orders of magnitude slower (Pas et al., 2004). Once in the cytoplasm, chromate ions can either pass the nuclear membrane and be reduced to chromium (III) or be reduced in the cytoplasm. Because neither Cr(VI) nor Cr(III) reacts strongly with DNA, it is thought that the reduction of Cr(VI) to Cr(III), either in the cytoplasm, nucleus, or the blood, produces free radicals, which in turn can bind to DNA (Medeiros et al., 2003). Indeed, the DNA-protein crosslinks induced by chromate have been used as a biomarker for chromium exposure (Taioli et al., 1995).
Toxicity Towards Microorganisms
Most microorganisms (protozoa, fungi, algae, bacteria and cyanobacteria)
are able to accumulate chromium (Dursun et al., 2003; Pas et al.,
2004; Faisal and Hasnain, 2005). In general, toxicity for most microorganisms
occurs in the range of 0.05-5 mg Cr L-1 of medium. In Saccharomyces
cerevisiae chromium caused oxidative damage to cellular proteins (Sumner
et al., 2005). Reduced growth of mycelium in many fungi is due to the
toxic effect of hexavalent chromium (Lozovaia et al., 2004). Studies
of Shewanella oneidensis showed that the growth was much inhibited even
at a low concentration (0.015 mM) of Cr(VI) (Viamajala et al., 2004).
Incubation of E. coli cells with 10 mM of hexavalent chromium and 3 mM
hydrogen peroxide caused the degradation of double-strand DNA in vivo,
which was suppressed by the addition of mannitol (Itoh et al., 1994).
Cr(VI) caused decrease in cell size, the structure of cell wall, as well as
the redox reaction caused by the exudate or enzyme when the cultures of Chlorella
pyrenoidosa, Synechococcus, Spirulina maxima, Spirulina
platensis, Selenastrum capriornutum and Scenedesmus quadricauda
were grown in chromium solutions (Chen et al., 2003).
Remediation Strategies
To minimize the environmental contamination, much effort has been made to
treat hexavalent chromium in wastewater. Developing health-based clean up standard
and remediation strategies for chromium-contaminated soils based on the hexavalent
forms of this heavy metal is a complex and controversial task.
Conventional Methods
Conventional methods for treatment of contaminated hexavalent chromium include
chemical reduction followed by precipitation, ion exchange and adsorption on
coal, activated carbon, alum, kaolinite and flyash. However, most of these methods
require either high energy or large quantities of chemicals and therefore more
practical, cost effective methods are being explored (Ohtake and Silver, 1994).
Abiotic Redution
In some natural system conditions exist where oxidation and reduction of
chromium can both occur (Bartlett, 1991). For example, as mentioned above, in
aquatic systems chromate ions may be reduced to Cr3+. Manganese dioxide
can then oxidize Cr+3 back to chromate. If the water is well aerated
and sunlight can penetrate to a sufficient depth, the manganese will be reoxidized,
allowing more reduced chromium to be oxidized. Just as manganese can be reoxidized
by sunlight and oxygen, iron (III) can also be reduced by sunlight to iron (II),
which can then reduce chromium (VI) to (III). Indeed, Kaczynski and Kieber (1993)
reported a diurnal variation in the ratio chromium (III)/(VI) in pH neutral
lakes caused by indirect action of sunlight on chromium via iron and manganese.
The ratio chromium (III)/(VI) may also be affected by the photo disassociation
of chromium from organic materials (Kaczynski and Kieber, 1993).
To remediate toxic hexavalent chromium from the contaminated environment, it is necessary first to take up this toxic Cr(VI) and then reduced it in to Cr(III).
Cr Phytoremediation
There are conflicting views concerning the uptake and translocation of chromium
(VI) in plants (Citterio et al., 2003; Han et al., 2004). Ramachandran
et al. (1980) suggest that CrO4-2 is reduced to
chromium (III) at the surface of root cells. Other studies found chromium (VI)
in plants, which suggest that dissolved chromium (VI), may be taken up by plants
without immediate reduction. For instance, chromate was found in the xylem sap
of L. scoparium but not in the soluble plant fraction (Cary et al.,
1977). Also, it has been shown that chromium (VI) enters the plant through the
root by active transport in barley (Hordeum vulgare L.) (Cary, 1982).
Uptake of CrO4-2 by intact barley seedlings was stimulated
by Ca+2 but inhibited by SO4-2 and other Group
VI anions. Chromate appears to be reduced during passage from culture solutions
to plant leaves (Cary et al., 1977). Higher concentrations of chromium
have been reported in plants growing in high chromium-containing soils (e.g.,
soil near ore deposits or chromium-emitting industries and soil fertilized by
sewage sludge) compared with plants growing in normal soils. However, most of
the increased uptake of chromium results in accumulation in the roots and only
a small fraction of the total chromium is translocated to the above-ground part
of edible plants (Gardea-Torresdey et al., 2004; Faisal and Hasnain,
2005). Leaves usually contain higher chromium concentrations than grains (Smith
et al., 1989). In an attempt to design crop practices that might increase
the chromium in food and feed crops, Faisal et al. (2005) also found
that plants accumulated chromium from nutrient solutions but retained most of
this chromium in the roots. It was found that plant tissues that tend to accumulate
iron also accumulate chromium (Burd et al., 1998). Chromium uptake was
found to increase with increasing chromate concentration, and most of the chromium
accumulated by the roots was present in a soluble, non-particulate form in the
plant vacuoles. Although chromium is largely retained in the roots of plants,
the oxidation state of chromium, the pH and the presence of humic substances
and plant species affect plant uptake and transport (Smith et al., 1989).
Microbial Reduction
Microorganisms have the potential to accumulate chromium (Ozdemir et
al., 2003) and reduce chromium (VI) to chromium (III) (Desjardin et al.,
2003; Camargo et al., 2004; Ramirez-Ramirez et al., 2004). Although
high levels of chromium (VI) are toxic to microorganisms (Bartlett, 1991). Chromium
is important to yeast metabolism (Coleman, 1988) and sorption of Cr(VI) by several
species of yeast has been reported (Rapoport and Muter, 1995; Babyak et al.,
2005). Chromium accumulation has been shown to occur in bacteria periphytic
to a crab (Helice crassa) carapace and in sewage fungus (Coleman, 1988)
and may contribute to the presence of chromium in the food chain.
Bacterial Reduction
Many types of chromium resistant bacteria, which have developed different
strategies to overcome heavy metal stress in the environment are now known (Faisal
et al., 2005). Bacterial resistance to chromate can be due to chromosomal
borne (Peitzsch et al.,1998; Juhnke et al., 2002) or plasmid mediated
(Peitzsch et al., 1998) or both (Peitzsch et al.,1998; Juhnke
et al., 2002). Some bacterial strains are able to reduce toxic hexavalent
chromium to less toxic trivalent chromium (Konovalova et al., 2003; Faisal
and Hasnain, 2004; Thacker and Madamwar, 2005). Reduction of hexavalent chromium
by microorganisms has been reported under a number of conditions and by a variety
of bacterial species (Faisal and Hasnain, 2004: Faisal et al., 2005;
Thacker and Madamwar, 2005). Although different genera of bacteria are involved
in the reduction of hexavalent chromium to trivalent chromium but E. coli,
Bacillus sp, Pseudomonas fluorescens LB300, Desulfovibrio vulgaris,
Pseudomonas ambigua G-1 and Alcaligenes eutrophus are most pronounced
(Peitzsch et al., 1998).
Cr(VI) is known to be reduced both aerobically and anaerobically in different bacterial systems (Guha et al., 2001; Megharaj et al., 2003; Viera et al., 2003; Faisal and Hasnain, 2004). In anaerobic systems, membrane preparations reduce Cr(VI) and Cr(VI) has been shown to serve as a terminal electron acceptor (Lovley and Phillips, 1994). Aerobic reduction of Cr(VI) has been found to be associated with soluble proteins (Ishibashi et al., 1990). Lovley and Phillips (1994) reported that the sulfate-reducing bacterium Desulfovibrio vulgaris, which was capable of enzymatically reducing Fe (III) and U (VI), could also act as Cr(VI) reducers in the presence of H2 as the electron donor. Although Fude et al., (1994) also found Cr(VI) reduction by a consortium of sulfate-reducing bacteria in a medium containing reducing agents such as ascorbic acid and thioglycolate, the interpretation of the results must be cautioned because those reducing agents might chemically react with Cr(VI), resulting in conversion of Cr(VI) to Cr(III) (Hamilton and Wetterhahn, 1988). Environmental factors, including pH, temperature and other electron acceptor as well as waste characteristics (metal ions, initial Cr(VI) and biomass concentrations) affecting Cr(VI) removal capability were quantified. With the addition of biological inhibitors such as penicillin, cycloserine, or chloramphenicol, the loss of Cr(VI) reduction activity in microbial cells under reducing conditions clearly indicates that Cr(VI) reduction is an enzymatically catalyzed reaction. Such evidence was found with both growing cultures (Wang et al., 1989) and resting cells (Llovera et al., 1993). Other evidence for enzymatic roles in microbial Cr(VI) reduction includes: Cr(VI) was reduced faster with higher cell densities (Chen and Hao, 1996); Cr(VI) reduction occurred only in the presence of cells or cell extracts and electron donors (Lovley and Phillips, 1994); the Cr(VI) activity was lost when the Cr(VI)-reducing cell extracts were heated at 100°C (Ohtake and Hardoyo, 1992); Cr(VI) reduction was incomplete with inadequate acetate concentration (Chen and Hao, 1996); and the highest rates of Cr(VI) reduction all occurred during the exponential growth phase of three different strain cells (Apel and Turick, 1991). All these observations strongly suggest that Cr (VI) reduction in biological systems is directly related to microbial activities. The reduced Cr existed mainly in the medium, forming insoluble Cr as Cr (OH)3 (Shen and Wang, 1994; Faisal and Hasnain, 2004).
Factors Affecting Chromate Reduction
Factors affecting chromate reduction are temperature, pH, gases and organic
substance and poisons chemicals. Bacteria can reduce hexavalent chromium under
mild conditions. This process may be the best mechanism to assure hexavalent
chromium treatment. Commonly used chemical reduction generally occurs only over
narrow range of pH. These methods are not only costly but the productions of
toxic byproducts are also a problem. Bacterial reduction of hexavalent chromium
as a means of bioremediation has several potential advantages.
| • | Reduction occurs under neutral pH. |
| • | It requires neither chemical additives nor aeration. |
| • | No toxic byproduct is formed. |
| • | Anaerobic reduction minimizes excess sludge production in aqueous systems. |
| • | The activity is reproducible and reusable. |
Conclusions
Reduced chromium forms insoluble chromium hydroxide at a neutral pH. These chromium compounds are very stable and they are unavailable to living organisms. No microbial activity has been found so for that is able to oxidized reduced chromium. The feasibility of the bioremediation process has been examined in the laboratories and is now proceeding to the developmental stage. So it is postulated that the use of microbes creates the right methods for environmental pollution control.