Chromium: in Soil, Plant, Animal, and Human

Introduction

Chromium element was first discovered in the Siberian red lead ore (crocoite) in 1798 by the French chemist Louis – Nicholas Vauquelin. Chromium is a Greek word (Chroma = color), which means colored compounds. Chromium (Cr) is the 17^th most abundant element in the Earth’s mantle. Chromium exists in several oxidation states, but the most stable and common forms are Cr (0), Cr (III), and Cr (VI) species. Cr (0) is the metallic form, produced in industry and is solid with a high fusion point usually used for the manufacturing of steel and other alloys. Cr (VI) in the forms of chromate (CrO₄²⁻), dichromate (Cr₂O₇²⁻), and CrO₃ is considered the most toxic forms of chromium, as it presents high oxidizing potential, high solubility and mobility across the membranes in living organisms and in the environment. Cr (III) in the forms of oxides, hydroxides, and sulfates is less toxic as it is relatively insoluble in water, presents lower mobility, and is mainly bound to the organic matter in soil and aquatic environments. Moreover, Cr (III) tends to form hydroxide precipitates with Fe at typical groundwater pH values. At high concentrations of oxygen or Mn oxides, Cr(III) can be oxidized to Cr(VI) [1, 2].

As Cr (III) and Cr (VI) are present in different chemical, toxicological, and epidemiological characteristics, they are differently regulated by EPA, which constitutes a unique characteristic of Cr among the toxic metals [3]. Cr (VI) is a powerful epithelial irritant and is considered a human carcinogen [4]. Cr (VI) is also toxic to many plants [5] aquatic animals [6], and microorganisms [7]. Contrarily to Cr (VI), Cr (III) is considered a micronutrient in humans, being necessary for sugar and lipid metabolism [8], and is generally not harmful. In crops, Cr at low concentrations (0.05– 1 mg L⁻¹) was found to promote growth and increase yield, but it is not considered essential to plants [2, 9].

Chromium also affects several physiological processes such as seed germination, growth, photosynthesis, the status of mineral elements, water balance, and nitrogen metabolism. The toxic property of Cr (VI) originates from the action of this form itself as an oxidizing agent as well as from the formation of Reactive Oxygen Species (ROS) such as superoxide radical (O^–), hydrogen peroxide (H₂O₂) and hydroxyl radical (OH) during the reduction of Cr (VI) to Cr (III) occurring inside the cell [10]. ROS at higher concentrations leads to oxidative damage in cells due to the oxidation of lipids and proteins [11]. Thus, plants exposed to Cr suffer from oxidative stress and this is an important factor leading to altered metabolic processes and cell death. Plants as other organisms can use antioxidants to counteract Cr ^– induced oxidative stress [10].

Soil and aquatic ecosystems near Cr-releasing sources are adversely affected by it, making arable land unproductive and unfertile. Unlike other heavy metals such as Cd, Al, Pb, Cu, Zn, etc Cr. detoxification via mycorrhiza fungi or other mechanisms as well as its phytol-accumulation and phytoremediation has been studied very little. Mycorrhizas can alleviate Cr toxicity and supports greater plant growth in Cr-rich soils [12, 13]. It has been demonstrated in sorghum that transcription rates of MTs are specifically high under Cr stress and the H₂O₂ generated as a result can act as a signal, inducing MT-mRNA transcription [14].

Due to wide industrial use, chromium is considered a serious environmental pollutant. Contamination of soil and water by chromium is of recent concern; chromium occurs naturally in bound forms that constitute 0.1 – 0.3 mg kg^-1 of the Earth’s Crust. A maximum acceptable concentration of Cr 0.05 mg L^-1 (50 g L^-1) in drinking water has been established based on health considerations. Chromium is a major source of aquatic pollution in India, especially in Tamil Nadu, Uttar Pradesh, and West Bengal states [15]. The toxicity of chromium to plants depends on its valence state, Cr (VI) is highly toxic and mobile [(which usually occurs associated with oxygen as chromate (CrO₄^2-) or dichromate (Cr₂O₇^2-)]. Whereas, Cr (III) is less mobile, less toxic, and is mainly found in bound to the organic matter in soil and aquatic environments [1].

In India, Cr (VI) contamination is a big problem in various industries using Cr compounds, which causes a considerable negative impact on crop production [16]. Farmers in irrigation exacerbate this problem further due to the use of Cr-contaminated water. Thus, the cleanliness of the environment for safer food production is a major concern. Therefore, methods are needed to alleviate Cr toxicity and to decrease the Cr content in crops, which may be helpful to minimize health risks. In this context, the accumulation of chromium in edible plants may represent a potential hazard to animals and humans [17].

Position of Chromium in the periodic table

Chromium (atomic number 24, atomic mass 51.99) has an outer electronic configuration of 3d⁵ 4S¹ and belongs to VI B group or chromium group on the periodic table. In soil, the ionic form of chromium that is absorbed by plants is Cr⁺³ and Cr⁺⁶. Cr (III) was absorbed more rapidly than Cr (VI) [15]. The different oxidation states of chromium are given in below (Table 1).

Table 1: Different oxidation state of chromium and its compounds

Note:   Cr⁺³ are maximum stable, because it is not both oxidizing and reducing agent

Table 2: Physicochemical properties

Chromium is a polyvalent element, found naturally in the air, soil, water, and lithosphere [18]. It can exist in several chemical forms displaying different oxidation states from zero to six, but in the natural environment, only trivalent and hexavalent chromium are stable [19, 20]. Cr (III) is the most stable state in water with a standard potential of -1.74V. Therefore, a considerable amount of energy would be needed to convert it to lower or higher oxidation states. The chromium oxidation states of I – III have negative potentials thus, oxidation is favored, while, oxidation states of IV-VI have positive potentials thus reduction is favored [21].

In acidic solutions, Cr (VI) has a very high positive redox potential (1.38 V) denoting that it is strongly oxidizing and unstable in the presence of electron donors. Under common environmental conditions of pH and Eh, Cr (III) compounds are sparingly soluble in water but Cr (VI) compounds are quite soluble [22]. The resulting Cr (VI) solutions are powerful oxidizing agents under acidic conditions, but less so under basic conditions. For example, H₂CrO₄ is used for cleaning glassware in chemical laboratories by oxidizing organic residues. Thus, Cr (VI) is much more toxic and mobile in groundwater than the relatively immobile Cr (III). Depending on the concentration and acidity, Cr (VI) can exist either as a chromate ion (CrO₄^2-) or as a dichromate ion (Cr₂O₇^2-). The common dissolved Cr entities of Cr (VI) are the hydrogen chromate ion (HCrO₄^–), CrO₄^2-, and Cr₂O₇^2- [23]. The entity that will dominate in a particular environment depends upon specific conditions like pH, EC, the total concentration of Cr, and the aqueous chemistry [24].

At pH >6.5, CrO₄^2- species dominate while at pH <6.5, HCrO₄^– counterparts dominate at low concentrations (<0.03 mol L⁻¹), but at concentrations greater than 0.001 mol L⁻¹, HCrO₄^– ions begin to change to Cr₂O₇^2-  which becomes the dominant entity at concentrations greater than 0.03 mol L⁻¹ [25]. HCrO₄^– imparts a yellow color to the water while Cr₂O₇^2- imparts an orange color [23]. In an aqueous solution, Cr (III) dominates as soluble Cr³⁺ at pH<3.0. As pH increases, Cr (III) hydrolyzes to Cr (OH) ²⁺, Cr (OH)_3, and Cr (OH)₄ ^– species. In slightly acidic to alkaline conditions, Cr (III) precipitates as amorphous Cr (OH)_3, which subsequently crystallizes to Cr (OH)₃⋅H₂O [24].

Chromium (VI) is strongly oxidizing only under high redox potentials, and it reacts rapidly with numerous reducing agents found commonly in the environment. Eary and Rai [26, 27] reported that the Cr (VI) was reduced in seconds by ferrous ions and in a matter of hours to days by ferrous iron-containing oxide and silicate minerals. Similar reactions were reported to occur in low-pH soils that contain small amounts of ferrous iron in clay minerals [28]. The reduction might occur rapidly even in the presence of dissolved oxygen. Chromium (VI) is also reduced by organic matter [29, 30] and by H₂S(g) [31]. Ferrous iron and organic matter are ubiquitous in soils and ground waters. Consequently, Cr (VI) is reduced to Cr (III) in many natural environments. The oxidation potential for transforming the trivalent chromium into the hexavalent one is high, and the probability of transformation into a higher oxidation form in environmental conditions is reduced. Due to the high redox potential of the Cr (VI)/Cr (III), there are few oxidants present in natural systems that are capable of oxidizing Cr (III) to Cr (VI). Common oxidants include dissolved oxygen and manganese oxides. However, the oxidation of Cr (III) by dissolved oxygen is reported to be very slow [26, 29], while the oxidation of Cr (III) by manganese oxides are reported to be more rapid. Therefore, manganese oxides are more important oxidants for Cr (III) in groundwater systems [32]. Despite the wide range of chromium in the soil and plants, hexavalent chromium is rarely found in natural water above the concentration of the natural background (1 µg L⁻¹).

Redox reactions of Chromium

The oxidation-reduction or redox reactions occur in both soil and aquatic environments. The Fenton-type reactions involve transition metals and hydrogen peroxide or hydroperoxide leading to the formation of oxidizing species. The redox reactions that occur in the soil are important in altering mobility and phytotoxicity. The involvement of chromium in the Fenton reaction is not very well understood or studied [33]. Besides the well-known redox metals like Cu and Fe, Cr also appears to participate in Redox or Fenton reactions [34]. The oxidation state of Cr is important because the common triplet oxidation state (Cr III) is not toxic as compared to the hexavalent form [35]. Mn in both the trivalent and tetravalent state oxidizes Cr (III) to Cr (VI), while FeS and organic matter in the soil can reduce Cr (VI) to the more stable and less phytotoxic Cr (II). Cr (III) and H₂O₂ can cause breakage of the DNA strand at pH 6 to 8, but not at pH 4 and it is believed that Cr (III) and Cr (II) enter the Fenton reaction. During radical oxidation of Cr (III), Cr (IV) and Cr (V) intermediates are thought to be involved [33]. Besides Cr (IV) and Cr (V), other species like they (GS^–) and hydroxyl radicals (OH^–) are considered to be toxic and carcinogenic [36]. The ESR studies at pH 5.8 and 7.1 for Cr (III)/H₂O₂ showed that the lower oxidation states of Cr could effectively participate in generating oxidizing species without generating Cr (IV) [33]. At higher pH (8.9), the oxidation of Cr (III) is rapid. Although the concentration of Cr (VI) reaches a steady state, the production of oxidizing species like Cr (IV) and Cr (V) proceeds at a slow measurable rate. This result indicates that both Cr (IV) and Cr (V) are catalytically active and possess the capability to generate ROS such as the hydroxyl radical (OH^–). The catalytic activity of Cr (III) is much higher in a Fenton reaction system compared to other metals like Co (II), Cd (II), Zn (II), and Fe (III) but lower than Cu (II) [33].

Uses of chromium

Chromium and its compounds are very useful in everyday life, as presented in Table 3. It is used on a large scale in many different industries, including metallurgical, electroplating, production of paints and pigments, tanning, wood preservation, Cr chemicals production, and pulp and paper production [20]. Chromium is resistant to ordinary corrosive agents at room temperature, which explains its uses as an electroplated, protective coating. It is also used in ferrous and non-ferrous alloys, in refractories, and in chemicals. Ferrous alloys, mainly stainless steel, account for most of the consumption. These steels have a wide range of mechanical properties as well as being corrosion and oxidation-resistant [37].

Table 3: Uses of Chromium

Chromium in the Environment

Chromium exists in the environment in several diverse forms such as trivalent [Cr (III)] and hexavalent, of which hexavalent chromium [Cr (VI)] is a carcinogen and a potential soil, surface water, and groundwater contaminant, while its reduced trivalent form (Cr³⁺) is much less toxic, insoluble and a vital nutrient for humans. Cr (III) occurs naturally in the environment and is an essential nutrient required by the human body [38. 39].

The sources of trivalent chromium include many fresh vegetables and fruits, meat, grains, and yeast [40]. Relatively insoluble, it is the most prevalent form in surface soils where reduction processes (which convert chromium from the hexavalent to trivalent form) are most common. Hexavalent chromium also occurs naturally and most notably in water-saturated (reducing) conditions and is an indicator of human pollution. Inside cells, Cr (III) can complex with organic compounds and this interferes with metallo-enzyme systems at high concentrations [41]. Chromium is found in all phases of the environment including air, water, and soil (Table 4).

Table 4: Chromium concentrations in the environment

The chromium concentration in soil ranges from 10 to 50 mg kg^-1 [42]. Cr concentration varies widely in the atmosphere from background concentration of 5.0 x 10^-6 to 1.2 x 10^-3g m^-3 in air samples from remote areas such as Antarctica and Greenland to 0.015 to 0.03 g m^-3 in air samples collected over urban areas [43].

Chromium in Soil

The concentration of Cr in the soils may vary considerably according to the natural composition of rocks and sediments that compose them [3]. The levels of chromium in the soil may increase mainly through anthropogenic deposition, for example, atmospheric deposition [44], also dumping of Cr-bearing liquids and solid wastes as chromium byproducts, ferrochromium slag, or chromium plating baths [3]. Generally, Cr in soil represents a combination of both Cr (III) and (VI). As in an aquatic environment, once in the soil or sediment, Cr undergoes a variety of transformations, such as oxidation, reduction, sorption, precipitation, and dissolution [3]. The oxidants present in the soil (e.g., dissolved oxygen and MnO₂) can oxidize Cr (III) to Cr (VI) [45]; however, it seems that oxidation of Cr (III) by dissolved O₂ is residual when compared with MnO₂. On the other hand, Cr (VI) is reduced by iron, vanadium, sulfides, and organic materials [46]. However, the reducing capacity of the soil is overcome, Cr (VI) may persist in the soil or sediment for years, especially if the soils are sandy and/or low in organic matter [47].

Lopez-Luna [48] compared the toxicity of Cr (VI), Cr (III), and Cr tannery sludge respecting Cr mobility in the soil and toxicity in wheat, oat, and sorghum plants and found that Cr (VI) was more mobile in soil and caused higher toxicity on those plant seedlings, while tannery sludge was the least toxic.

Chromium in Water

Chromium may enter the natural waters by weathering of Cr-containing rocks, direct discharge from industrial operations, leaching, etc. In the aquatic environment, Cr may suffer reduction, oxidation, sorption, desorption, dissolution, and precipitation [3]. The aqueous solubility of Cr (III) is a function of the water pH. Under neutral to basic pH, Cr (III) will precipitate, and conversely while in acidic pH it will tend to solubilize. The forms of Cr (VI) as chromate and dichromate are extremely soluble under all pH conditions, but they can precipitate with divalent cations [3]. The recommended limits for Cr concentration in water are 8.0μg L⁻¹ for Cr (III) and 1.0 μg L⁻¹ for Cr (VI).

The toxicity of Cr (VI) gained notoriety in the book and subsequent movie Erin Brockovich, released in 2000, that it was a major contaminant in the drinking water of the town of Hinckley in California responsible for a cluster of illnesses and cancers. A later study by Kirpnick-Sohol [49] reported that both the contaminant Cr (VI) and nutritional supplement Cr III caused large-scale and irreversible genome damage in yeast and mice when ingested in drinking water.

Chromium in Air

Some studies exposed to airborne chromium (VI) have found to be increased levels of low- molecular-weight urinary proteins, such as retinol-binding protein, β2-microglobulin, and tubular antigens, indicative of early kidney changes, for example, one study identified the Lowest – Observed – Adverse – Effect-Level (LOAEL) of 4 µg m-3 Cr (VI). Work-related cough or dyspnea, production of phlegm, and shortness of breath were also noted in workers exposed to dust containing chromium oxide at an approximate concentration of 240 to 480 µg m^-3 Cr (III) [50].

Anthropogenic source of chromium

Cr and its compounds have multifarious industrial uses. They are extensively employed in leather processing and finishing [43] in the production of refractory steel, drilling muds, electro-planting cleaning agents, catalytic manufacture, and the production of chromic acid. Cr (VI) compounds are used in industry for metal planting, cooling tower water treatment, hide tanning, and until recently, wood preservation. These anthropogenic activities have led to the widespread contamination that Cr shows in the environment and have increased its bioavailability and bio mobility [41].

Further, the leather industry is also the major cause of high influx of Cr to the biosphere, accounting for 40% of the total industrial use [51]. In India, about 2000 to 32000 tons of elemental Cr annually escape into the environment from tanning industries. Even if the recommended limit for Cr concentration in water is set differently for Cr (III) (8 g L^-1) and Cr (VI) (1 g L^-1), it ranges from 2 to 5 g L^-1 in the effluents of these industries [52].

The chromium content as well as other heavy metals in any soil depends initially on the nature of parent materials. Flanagan [53] reported that the concentration of heavy metals was higher in basaltic rocks and comparatively low in granite rocks and Vine and Tourlets [54] reported that the heavy metal composition in coal ash (Table 5). Some fertilizers and soil amendments also contain chromium in the soil (Table 6).

The anthropogenic sources of Cr in the environment stem from the use of Cr in the metallurgy, refractory, and chemical industries [20]. Chromium from anthropogenic sources can be released to soils and sediments indirectly by atmospheric deposition, but releases are more commonly from dumping of Cr-bearing liquid or solid wastes such as chromate by-products (“muds”), ferrochromium slag, or chromium plating wastes. Such wastes can contain any combination of Cr (III) or Cr (VI) with various solubilities [55].

Table 5: Concentration of chromium and other heavy metals in some types of igneous rocks and coal ash

Table 6: Chromium concentration in selected fertilizers and soil amendments

Chromium in Plants

The pathway of Cr uptake in plants is not yet clearly elucidated. However, being a nonessential element, Cr does not have any specific mechanism for its uptake and is also dependent on Cr speciation. Plant uptake of Cr (III) is a passive process, that is, no energy expenditure is required by the plant [20, 60]. The uptake of Cr (VI) is thought to be an active mechanism performed by carriers for the uptake of essential elements such as sulfate [61]. Cr also competes with Fe, S, and P for carrier binding [5].

Cr (VI) has higher solubility and thus bioavailability is more toxic at lower concentrations than Cr (III), which tends to form stable complexes in the soil [48]. There are conflicting results concerning the uptake and translocation. Thus, Cr toxicity is dependent on metal speciation, which is a determinant for its uptake, translocation, and accumulation. Cr is toxic for agronomic plants at about 0.5 to 5.0 mg L⁻¹ in nutrient solution and 5 to 100 mg g⁻¹ in soil [62]. Under normal conditions, the concentration of Cr in plants is < 1.0 μg g⁻¹ [63].

Independent uptake mechanisms for Cr (VI) and Cr (III) have been reported in barley. The use of metabolic inhibitors diminished Cr (VI) uptake whereas it did not affect Cr (III) uptake, indicating that Cr (VI) uptake depends on metabolic energy [60]. Active uptake of both Cr species, slightly higher for Cr (III) than Cr (VI), was found in the same crop [64].

Chromium toxicity in plants

In plants, high levels of Cr supply can inhibit seed germination and subsequent seedling growth. Peralta et al., [65] found that 40 ppm of Cr (VI) reduced by 23% the ability of seeds of Lucerne (Medicago sativa) to germinate and grow in the contaminated medium. Reductions of 32-57% in Sugarcane bud germination were observed with 20 and 80 ppm Cr, respectively [66].

The toxicity of Cr to plants depends on its valence state Cr (VI) is highly toxic and mobile whereas Cr (III) is less toxic. Toxic effects of Cr on plant growth and development include alterations in the germination process as well as in the growth of roots, stems, and leaves, which may affect total dry matter production and yield. Cr also causes deleterious effects on plant physiological processes such as photosynthesis, water relation, and minerals nutrition [15].

Heavy metals like Pb, Cd, As, Se, Cr, and Al are biologically non-essential and toxic above certain threshold levels. Cr is toxic to plants and does not play any role in plant metabolism [67]. Accumulation of Cr by plants can reduce growth, induce chlorosis in young leaves, reduce pigment content, alter enzymatic function damage root cells that cause ultra-structural modifications of the chloroplast and cell membrane [68, 69, 70]. Cr toxicity can reduce seed germination and radical growth in plants [71, 72]. Growth inhibition in plants can be due to the inhibition of cell division by inducing chromosomal aberrations. However, in many plants, an increase in DNA content has been observed under Cr and the amount of DNA increased with increasing Cr concentration [73]. During seed germination, hydrolysis of proteins and starch takes place, providing amino acids and sugars. Under Cr treatment, a decrease in both α and β- amylase has been reported, which is one of the important factors for germination inhibition in many plants.

Phytotoxic effects of Cr on plant growth have been thoroughly studied in many plant species like mosses, rice, pea, wheat, etc. in relation to oxidative stress. Cr exposure at the micro molar range can lead to severe phytotoxic symptoms in a plant cell. Both Cr (III) and Cr (VI)can reduce chlorophyll content and thereby inhibit growth [74, 75]. It can cause ultra-structural changes in the chloroplast leading to the inhibition of photosynthesis. Such alterations in the chloroplast have been observed in the case of plants like Lemna minor, Pistia sp., Taxithelium Nepalese [76]. Cr can affect the roots of plants causing wilting and plasmolysis in root cells [76, 77]. Cr can also inhibit the Hill reaction, affecting both the dark and light reactions [73, 78]. Aquatic plants like Vallisneria spiralis can accumulate significant amounts of Cr in their tissues, which decreases the biomass of the plant [79, 80]. In aquatic plants uptake and bioaccumulation of Cr can influence many physiological and biochemical processes and in many plant species, photosynthetic pigments are affected by Cr [80]. Decreases in total chlorophyll, Chlorophyll a, and b, and carotenoids have been well documented under Cr stress in plants [68, 70, 81]. Cr mostly in its hexavalent form can replace Mg ions from the active sites of many enzymes and deplete chlorophyll content [82]. Like other heavy metals Cr can induce the degradation of carotenoids in plants [83].

Lipid peroxidation, which is considered an indication of oxidative stress in plants, can be induced via free radicals or Reactive Oxygen Species (ROS) that are generated because of heavy metal toxicity in plants. Lipid peroxidation can degrade biological membranes making them susceptible to oxidative damage [84]. Under Cr, lipid peroxidation can be initiated resulting in oxidative stress. ROS which is a common consequence of most biotic and abiotic stress also formed as a result Cr toxicity [67, 75]. In many crop plants like rice, wheat, and pea and also in lower plants significant increases in ROS production can be observed with concomitant increases in lipid peroxidation [69].

Cr can degrade proteins, which results in the inhibition of nitrate reductase (NR) activity [82]. The correlation between NR activity and proteins has been well documented in plants by [83]. The amino acid cysteine is an important component of phytochelatin [80].

The Antioxidant response in plants

The Antioxidant enzymes catalase (CAT), guaiacol peroxidase (GPx), glutathione reeducate (GR), ascorbate peroxidase (APx), and superoxide dismutase (SOD) have been thoroughly studied for plant-like rice, wheat, pea and even in lower plants like mosses [69, 72, 74, 75]. CAT is an important heme-containing enzyme that catalyzes the dismutation of H₂O₂ to H₂O and oxygen and it is localized in the peroxisomes. CAT is also an indispensable enzyme required for ROS detoxification in plants. In rice, Cr can either induce CAT activity or suppress it. Treatment of developing wheat seedlings with different concentrations of Cr showed varied responses. In most of the studies conducted, a gradual decrease in CAT activity was observed in plants while in mosses both alleviation and decline were observed. Exposure of Cr to developing wheat seedlings decreased the CAT activity after 7d and 9d of Cr treatment at 0.01, 0.1, and 1mM [74]. Moreover, at much higher concentrations (10 and 100mM) severe inhibition in CAT activity was observed after 2d and 4d of Cr treatment [75]. A similar inhibition of CAT was reported when Cr was supplied in addition to nitrogen nutrition in wheat [72].

Chromium detoxification

Heavy metals like Cr, Cd, Zn, Fe, Pb, and As are highly reactive and toxic to living cells. Some heavy metals particularly Cu, Zn, and Fe are essential micronutrients involved in various physiological processes but become toxic above certain threshold concentrations. Plants have developed complex mechanisms by which they control the uptake and accumulation of heavy metals [85, 86]. These mechanisms involve the chelation and sequestering of metals ion by a particular class of metal-binding ligands denominated phyto-chelatins (PCs) and metallothioneins (MTs) [86, 87] MTs have a possible role in Cr detoxification in plants and it has been reported for sorghum that MT-like proteins are expressed under Cr stress [10]. MTs are the product of mRNA translation and are characterized as low molecular weight cysteine-rich metal-binding proteins [88]. The role of MTs or even PCs in Cr detoxification in plants has not been thoroughly studied compared to that of other heavy metals like Cd, Hg, Cu, etc. and consequently, there is very little information about the involvement of these metal-binding ligands in Cr detoxification in plants. A study of Cr (IV) effects on the MT3 gene expression using Cr tolerant and susceptible varieties revealed a high-intensity band matching the gene of interest in the tolerant variety compared to the susceptible one [10]. This suggests that under Cr stress there could be high transcription rates of MTs, particularly in the tolerant variety [10]. The role of PCs in regulating metal toxicity has been reported in plants. It was suggested that the production of ROS and H₂O₂, as a result of Cr exposure might have triggered signals to induce MT mRNA transcription [10]. Thus, MTs may have a very important role in Cr detoxification in plants, possibly by binding Cr ions and making them non-toxic. However, the role of MTs in Cr detoxification in plants is not well understood nor thoroughly studied, so their role in this respect still remains a challenge for the future.

The root contains organic acids that bind to metals from highly insoluble forms in the soil and acids like citric acid and malic acids can act as essential ligands for metals. The role of citric acid in regulating A1 (III) and Ni (II) detoxification in plants has been clearly demonstrated by [89]. The root exudates are very important agents that form complexes with trace metals and affect their redox behavior [90]. Root exudates containing organic acids can form complexes with Cr compounds, making them available for plant uptake [91]. Studies on the role of organic acids in Cr toxicity in Lycopersicon esculentum showed that in the presence of organic acids like carboxylic acid and amino acids, Cr uptake in roots is enhanced [92]. However, these types of organic acids and amino acids have been found to be less effective in mobilizing chromium [92]. Organic acids like citric acid, aspartic acid, and oxalic acid can convert inorganic Cr to organically bound Cr, making it soluble for a longer period of time and thereby available to plants [93]. Whether organic acids can play a significant role in Cr detoxification is still not completely understood.

Chromium in Human and Animal

A recent study in Australia by Wu et al., [94] raised concerns regarding the safety of nutritional supplements containing Cr (III). Such supplements are widely consumed for treating such metabolic disorders as insulin resistance, and type 2 diabetes, and as muscle development agents. Using a combination of X-ray fluorescence microscopy (XFM) and X-ray absorption near edge structure (XANES) studies, Wu et al., [94] found that Cr (III) injected into mice fat cells (adipocytes) was oxidized into the carcinogenic forms of chromium, Cr (VI) and Cr (V). The long -latency time of Cr-induced cancers in humans makes it difficult to extrapolate from animal studies to humans [95].

Table 7. Symptoms of Cr deficiency in humans and animals [96]

Papers dealing with the experimental study of Cr deficiency are relatively scarce and most of the existing ones quote results of experiments on laboratory animals. Anderson [96] has summed up the results of a number of trials on humans, rats, mice, and other animal species in a review of physiological and biochemical symptoms of Cr deficiency that we present in Table 7. Frank [97] have studied experimentally induced Cr deficiency in goats. The population with a Cr deficiency showed higher weight gains (31.1±11.7vs.20.0±7.3kg) for the period of monitoring (84 weeks) compared with the control group. The authors explain this unexpected defect by the possibility that Cr deficiency has impaired glucose tolerance and increased insulin release subsequently leading to hyperinsulinemia. Cr deficiency has also led to an increase in hematological parameters (hemoglobin, hematocrit, erythrocytes, leucocytes, and mean erythrocyte volume); increased total protein concentrations and hyperinsulinemia were observed compared with the group of controls as well [98].

Exposure to Cr (VI) exerts toxic effects on biological systems [40]. Inhalation of Cr (VI) has been shown to cause perforation of the nasal septum, asthma, bronchitis, pneumonitis, inflammation of the larynx and liver, and increased incidence of bronchogenic carcinoma while, exposure due to dermal contact of Cr (VI) compounds can induce skin allergies, dermatitis, dermal necrosis, and dermal corrosion [37]. Cr (VI) is a powerful epithelial irritant and is also considered a human carcinogen [4]. Studies of workers in various industries with exposure to chromium compounds (including production of chromate and chromate pigments and chromium plating) showed that they are at risk of developing various cancers such as of the nasal or sinonasal cavity, the lung and the stomach [99]. Several chromium (VI) compounds like calcium chromate, lead chromate (and its derived pigments), chromium trioxide, and sodium dichromate have been tested for carcinogenicity by several routes in several animal species and strains and were shown to cause various cancerous tumors [4]. Other Cr (VI) containing compounds such as sodium dichromate dehydrate [100], and potassium chromate [101] have also been shown to be carcinogenic.

Chromium (VI) compounds are known to exert genotoxicity both in vivo and in vitro. Several studies have shown lymphocytes of workers exposed to dust of chromium (VI) compounds to have elevated frequencies of DNA strand breaks [102], sister chromatid exchange [103], and micronuclei [104]. DNA single-strand, as well as double-strand breaks, may arise due to the reaction of chromium (VI) with hydrogen peroxide, forming hydroxyl radicals [105]. This induces mismatches during replication, leading to aberrant mismatch repair. Further, chronic exposure to toxic doses of chromium (VI) provokes the selective outgrowth of mismatch-repair-deficient clones with high rates of spontaneous mutagenesis, and thus, genomic instability [106]. The binding of Cr (VI) to double-stranded deoxyribonucleic acid (DNA), alters gene replication, repair, and duplication, a proposed mechanism for cancer formation [107].

Cr (VI) is capable of penetrating cell membranes and getting reduced to Cr (III), with the released electrons damaging the membrane [108].  This reduction (Cr⁶⁺+ 3e → Cr ³⁺) contributes to mutagenic changes in the organism. It is also one of the ways in which Cr⁶⁺ is removed from organisms. Reduction of Cr (VI) to Cr (III) in the immune system forms reactive intermediates, which, in combination with oxidative stress, tissue damage (the result of oxidation), and a cascade of cell collisions, give rise to cytotoxicity, genotoxicity, and carcinogenicity [109]. It is known that the reduction of Cr (VI) produces the following free radicals: Cr (V), Cr (IV), and Cr (III), which are responsible for the observed toxic and carcinogenic effects [40, 110].

Based on epidemiologic investigations of workers and experimental studies with animals, hexavalent chromium compounds were confirmed to be carcinogenic [40]. Epidemiological studies conducted in past years in the USA indicated a 10 to 30-fold- increased risk of lung cancer among workers in the chromate industry compared to the general population [40]. In most studies, a positive correlation between the duration of exposure and lung cancer death was found. Gastrointestinal bleeding, tuberculosis and asthma infertility, birth defects, skin cancers, skin ulcers, and stillbirths have also been recorded among workers exposed to high levels of chromium (VI), [40]. The International Agency for Research on Cancer [4] concluded that there was sufficient evidence in humans for the carcinogenicity of chromium (VI) compounds as encountered in the chromate production, chromate pigment production, and chromium plating industries for the carcinogenicity of chromium (VI) compounds in humans based on the combined results of epidemiological studies, carcinogenicity studies in experimental animals, and evidence that chromium (VI) ions generated at critical sites in the target cells are responsible for the carcinogenic action observed.

Accidental or intentional ingestion of high doses of chromium (VI) compounds results in acute, potentially fatal, effects in the respiratory, cardiovascular, gastrointestinal, hepatic, renal, and neurological systems [8, 50, 111]. Some of these effects can be attributed to the corrosive nature of the compound [112]. Typically, in one case a 17-year-old male died 14 hours from respiratory distress with severe hemorrhages after ingesting potassium dichromate (29 mg chromium (VI) kg^-1) in an attempted suicide [8]. Caustic burns in the stomach and duodenum and gastrointestinal hemorrhages were noted [8]. Several other cases have also been reported fatalities following ingestion of lower doses of chromium (VI). In one case, a 14-year-old boy suffered gastrointestinal ulceration and severe liver and kidney damage and died 8 days after hospitalization after ingesting potassium dichromate (7.5 mg chromium (VI) kg^-1), while in another case, a 44year- old man died of severe gastrointestinal hemorrhage one month after ingesting chromic acid (4.1 mg chromium (VI) kg^-1) [8].

Developmental toxicity of Cr (VI) has been observed in animals, but there is not enough evidence to determine the potential for developmental effects on humans [113]. Chromium has also been shown to be transferred from the mother through the placenta and mother’s milk which increased birth and developmental defects in children have been informally noted in areas of poorly regulated chromite mining, leather tanning (using Cr), and chrome production [114]. However, no scientific studies investigating the potential relationship between these effects and specific chromium exposures in these locations have been conducted. A study on chrome plating workers occupationally exposed to chromic acid (mean 2 to 200 µg m^-3 Cr (VI) for 8 hours in a day for 0.2 to 23.6 years) found that at low concentrations (mean <2 µg m^-3), Cr (VI) workers developed smeary, crusty and atrophied septum mucosa and at higher concentrations (2 to 200 µg m^-3 Cr (VI)) nasal irritation, mucosa ulceration and atrophy and septum perforation were observed [50].  These effects may not have resulted from exposure levels actually measured, but also from earlier exposures. Another study on electroplating workers exposed to chromic acid (>0.1 mg m^-3 Cr (VI)) reported frequent incidences of coughing, expectoration, nasal irritation, sneezing, rhinorrhea, nose-bleed, nasal septum ulceration and perforation [8]. Further, the evidence also suggests that exposure to Cr (VI) may induce occupational asthma, and chromate-sensitive workers acutely exposed to Cr (VI) compounds may develop asthma and other signs of respiratory distress [8]. For example, a study of 5 individuals with a history of contact dermatitis to Cr, found that exposure via nebulizer to a K₂Cr₂O₇ aerosol containing 0.035 mg ml^-1 Cr (VI) resulted in decreased forced expiratory volume, facial erythema, nasopharyngeal pruritus, blocked nose, coughing and wheezing [8].

Table 8: Effects of Cr (III) and Cr (VI) on humans

Remediation of chromium contamination 

Chromium remediation through microorganisms or plants may be the best technology for Cr-contaminated sites. Yadav [115] give some remediation processes for Cr-contaminated soil and water which are given below:

1.      Bioremediation: The decontamination of polluted or degraded soils by means of enhancing the chemical degradation or other activity of soil organisms. 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). OM enhances the reduction of chromate in soil by increasing microbial activities. Bacterial populations resistant to as much as 500 mg L^-1 Cr (VI) were directly isolated from two uncontaminated soils [15, 116].

2.      Phytoremediation: Phytoremediation of Cr pollution can be achieved by using plants to remediate heavy metal contaminated sites called phytoremediation. Shahandeh and Hossner [117] reported that the Brassica June and Helianthus annus plant species absorbed a maximum Cr concentration of about 1400 mg kg^-1.

3.      Rhizofiltration: Another promising clean-up technology appears to be hemofiltration which involves the use of plant roots to remove contaminants such as heavy metals from contaminated water [118]. Generally, aquatic plants are growing in contaminated water. Examples- are Scirpus lecusteris, Phragmites karka, and Bacopa monnieri.

4.      Phytoextraction: The extraction of metal from polluted soils into harvestable plant tissues (Phytoextraction). Very few plant species Sutera fodina, Dicomanic coliform and Leptospermum scoparium have been reported to accumulate Cr to high concentrations in their tissues [15, 119].

5.      Cheletion: Nutrient culture studies revealed a marked enhancement in uptake and translocation of chelated ⁵¹Cr in P. verlgaris. Cr chelated by DTPA was most effectively translocated followed by ⁵¹Cr-EDTA and ⁵¹Cr-EDDHA [120].

6.      Remediation by reduction of Cr (VI) in soils: Remediation by reduction schemes employing microbiological and chemical processes. Many new techniques and chemical reactions have been developed for the remediation of Cr (VI) – contaminated soils and groundwater including those using carbon-based minerals, zero and divalent Fe, reduced sulfur-containing compounds, and H₂ gas. The rates and extent of reduction of Cr (VI) by each of these are dependent on pH, aeration status, and the concentration and reactivity of the reducing agent.

Manures have been used successfully to reduce Cr (VI) in chromite ore processing, residue–enriched soils [121]. Zero valent Fe has been especially with reactive permeable barrier walls [122] and ferrous ion has been used for the reduction of Cr (VI) in soil and aqueous systems, with Fe (II) in soluble and insoluble forms and with and without light-induced reduction of Fe (III), [123]. Reduced sulfur-containing compounds (eg. Fe (II) sulfides, dithionite etc) have been used to reduce Cr (VI) directly or to create reduced colloids or conditions in soils and groundwater, [124, 125].

Conclusion

Chromium is a metal found in natural deposits. The two largest sources of chromium emission in the atmosphere are chemical manufacturing industries and the combustion of natural gas, oil and coal. The greatest use of Cr is the formation of metal alloys such as stainless steel, protective coatings on metal, magnetic tapes, and pigments for paints, cement, paper, etc. Cr exists mainly in three oxidative states Cr (0), Cr (III), and Cr (VI), which are the most stable forms of Cr. As Cr (0) is the metallic form, the forms of Cr (III) and Cr (VI) are the most preponderant in soils and water. Cr (III) uptake is a passive process, whereas Cr (VI) uptake is performed by carriers of essential elements such as sulfate. The understanding of the basic mechanism involved in chromium uptake, transport, accumulation, and detoxification in plants together with its physiological effects is necessary for the phytoremediation of chromium-polluted environments using molecular and genetic techniques. These approaches may include the identification of hyper-accumulators that can provide efficient phytoremediation of chromium-polluted soils, the study of biochemical and molecular responses of these plants to chromium, and the identification of genes that express PCs or MTs involved in the detoxification of the metal within the plant. These technologies will prove useful in environmental cleanup procedures and subsequent restoration of soil fertility.

According EPA found Cr potentially causes the following health effects when people are exposed to it at levels of skin irritation or ulceration, damage to the liver, kidney, circulatory, and nerve tissues. If the Cr (VI) compounds are present in high concentrations it can increase the risk of lung cancer. Chromium bioremediation through microorganisms or plants may be the best technology at present to clean up Cr-contaminated sites and these technologies are eco-friendly. In human nutrition, chromium is used as a nutritional supplement recommended for impaired carbohydrate metabolism characterized by reduced glucose tolerance and impaired insulin action, for weight reduction, and last but not least as a prevention for the formation of atherosclerotic plaques in blood vessels. In contrast to that, supplementation of livestock animal diets with chromium has currently been banned in EU countries. The positive effects associated with the use of chromium as a nutritional supplement include the reduction of animal sensitivity to negative environmental impacts, enhanced growth, an increased proportion of muscle compared with fat, improved reproduction function, support of the immune function, etc.

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