An overview of Chromium history, discovery, properties, uses, sources, toxicology, methods of analysis and its removal techniqueAn overview of Chromium history, discovery, properties, uses, sources, toxicology, methods of analysis and its removal technique

An overview of Chromium history, discovery, properties, uses, sources, toxicology, methods of analysis and its removal technique

The intricate chemistry of chromium, its prevalence in various compounds, and its contrasting roles as both a vital nutrient and a hazardous pollutant make it a subject of significant scientific interest and concern. In this exploration, we delve into the multifaceted world of chromium, delving into its properties, applications, and the challenges it poses in both natural and industrial contexts. Here is an An overview of Chromium history, discovery, properties, uses, sources, toxicology, methods of analysis and its removal technique.

Table of Contents

Authors:

Izaz Ul Islam

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1.1. Heavy Metals

Various industrial discharge waste is laden with a considerable amount of inorganic and organic pollutants. Among the inorganic pollutants, the concentration of heavy metals is extremely high. Heavy metals events at low concentrations are highly toxic due to their accumulation in the human body and therefore extremely harmful to living organisms and human beings. Once the heavy metals are introduced into the food chain they are biomagnified regularly and their concentration is increased to a harmful level. Arsenic, Lead, Chromium, Copper, Cadmium, Zinc, Mercury and Nickel are the heavy metals that are frequently released by various industrial operation. 0.050, 0.006, 0.05, 0.25, 0.01, 0.08, 0.00003 and 0.2 are the maximum contaminant limit (MCL) for Arsenic, Lead, Chromium, Copper, Cadmium, Zinc, Mercury and Nickel. For heavy metals removal, various techniques have been introduced that include chemical reduction, oxidation, ion exchange, chemical precipitation, reverse osmosis, ultrafiltration, and electrodialysis. Amongst the heavy metals, chromium is one of the extremely noxious, carcinogenic, and mutagenic metals that adversely affect all the biotic components of the ecosystem (Huang et al 2016; Renu et al 2017). (An overview of Chromium history)

1.2. History and discovery of chromium

The Chinese Terracotta Army in the Qin Dynasty (259-210 B.C.E) used chromium (III) oxide for coating their weapons such as steel swords and bronze crossbow bolts.

The German geologist and mineralogist Johann Gottlob Lehmann (1719-1767) in the Ural Mountain in the Beryozovskoye mines on July 26th, 1971, found an orange-red mineral which he called the Siberian red lead. This orange-red compound was misjudged as a compound of Lead with iron and selenium component which we called now crocoite PbCrO. Later on, the French chemist and pharmacist Louis-Nicolas Vauqelin pointed out that Siberian red lead was composed of a new element. In 1971 he become success full in the extraction of chromium (III) oxide Cr2O4 from chrocite. In 1798, the first contaminated elemental chromium was obtained by the reduction of Cr2O3 with charcoal. The word chromium was descended from the Greek word “chroma” implying color.

Figure. 1. Represents Teracotta Army (A), Johan Gottlob Lehmann (B), Louis-Nicolas Vauqelin (C) and Crocite (D). (An overview of Chromium history)

1.3. Properties of Chromium

Elemental chromium having the symbol Cr and atomic number 24 is a steel-grey, shiny, rigid, and brittle transition metal. Chromium is exceedingly resistant to oxidation even at elevated temperature. Due to their anti-corrosive properties, it is the basic component of stainless steel. Chromium possesses the ability to be well polished while opposing corroding. Round about 90 % of infrared light and 70% of the visible spectrum has been reflected by the polished chromium. Electroplating with chromium conjointly comprehends 85% of the commercial consumption. (An overview of Chromium history)

1.4. Uses of chromium         

Chromium is mostly used in Ferrochrome (about 95 %), besides this chromium is also used in refractory, chemical, and foundry with a percentage of 0.8, 1.6, and 2.4 &, respectively (Lunk et al., 2015).

1.4.1. Use of chromium as a catalyst

The compounds of chromium can be used as a catalyst for hydrocarbon processing. Phillips catalyst that is formed by the combination of silica, SiO2 with chromium (VI) oxides, CrO3 commonly uses for polyethylene production. Similarly, chromium oxide in combination with iron oxides can be used as a catalyst in water gas shift reactions.

For hydrogenation copper, Chromate Cu2Cr2O5 is used as a catalyst (Lunk et al., 2015).

1.4.2. Chromium use in tanning

For leather tanning chromium sulfate dodecahydrate, Cr2 (SO4)3. 12H2O and chromium Alum KCr(SO4)2. 12 H2O are used. Approximately 4 – 5 % of chromium is present in chromium–tanned leather. The collagen fibers present in skin protein are cross-linked by the chromium and hence make it stable. Recently in the tanning process, the recycling of tanning floats has been introduced. This recycling intensity depends upon the manufacturing quality of leather. As a result of this recycling process per Kg of tanned leather, 18 L of water can be saved (Morera et al., 2011; Lunk et al., 2015). (An overview of Chromium history)

1.4.3. Chromium use in Metallurgy

Due to its hard nature and elevated corrosion resistance, chromium is the basic component of metallurgy. Recent studies indicated that steel corrosion resistance properties can be enhanced by chromium addition that leads to the formation of stainless steel. The combination of chromium with steel and electroplating of chromium constitute the greatest use of metal by volume. The chromite ores act as a source for ferrochromium and chromium. The carbide of chromium is of wide importance. There are 3 carbides of chromium that are Cr3C2, Cr7C3, and Cr23C6. Among these carbides, Cr3C2 is of great importance. Cr3C2 is used in the manufacturing of hard metals and thermally sprayed coating preparation (Lunk et al., 2015).

1.4.4. Chromium use in electroplating

Electroplating is a process in which one metal is deposited over another metal. Similarly, Chromizing is a process in which chromium is coated on the surface of various metals for decoration and resistance to corrosion. Electroplating for decorative purposes involves the coating of chromium usually 13 – 1.3 µm thickness over nickel. Electroplating having 5 – 250 µm thickness is referred to as hard plating and uses chromic acid solution (Lunk et al., 2015). (An overview of Chromium history)

1.4.5. Chromium use in the preservation of wood

The salts of Cr(VI) are highly toxic for this purpose it is used for wood preservation. To prevent the wood from decomposition by various fungi, termites, insects attacking wood and marine borers Chromated Copper Arsenate (CCA) is used. SupaTimber®, Celcure®, and Tanalith® are the brands in which CCA are available. For the wood lignin and cellulose potassium dichromate (K2Cr2O7) and sodium, dichromate dehydrates Na2Cr2O7. 2H2O is used as a chemical fixing agent (Lunk et al., 2015).

1.4.6. Chromium uses in dyes and pigments

Before 1939 the color of school was pure yellow similar to lemon color. According to the scientist, as compared to red, the visibility of yellow color is 1.24 times more. In 1939 in North America, a special color was introduced for the school buses that were called School bus yellow. In Canada and the US officially this color was known as National School Bus Glossy Yellow. PbCrO4, Lead monochromate is the pigment for this color. Besides their use for buses, it is also used on the roadside for safety and flashlights, etc. The advantage of chrome yellow is that it cannot vanishes over time. However, it becomes darken because of Cr2O3 formation (Lunk et al., 2015).

1.4.7. Chromium use in refractory materials

The chromium oxide (Cr2O3) and chromite (FeCr2O4) due to high melting point and high resistance to heat can be used as refractory materials. The refractory application of chromium is in, cement kiln, blast furnace, bricks firing molds, etc., where the temperature is extremely high. A mixture of magnesite, MgCO3, and chromite are employed for the preparation of refractory materials (Lunk et al., 2015). (An overview of Chromium history)

1.5. Sources of chromium

1.5.1. Natural sources
1.5.1.1. In Atmosphere

Volcanic activities, meteoric dust, forest fire, salt spray from the sea, and windblown sands are the agent responsible for the exemption of chromium into the atmosphere. Among these agents volcanic eruption and wind-blown dust are of prime importance. It has been forecast that above 4.6 ×104 metric ton Y-1 of chromium is a drop-off to soil globally. Deposition of chromium by mass in urban, rural, and remote areas are 10Kg-2KmY-1, 5KgY, and 0.5KgY, respectively (Saha et al., 2011).

1.5.1.2. Ground and surface water

Weathering of rocks containing chromium and leaching of soil are the natural sources for the introduction of chromium in water. Chromium in the aquatic environment may undergo oxidation, reduction, precipitation, dissolution, sorption and desorption (Kimbrough et al., 1999). 8µ/L and 1 µ/L are the acceptable amounts of Cr (III) and (VI) concentration in water (Chandra et al., 1997). (An overview of Chromium history)

In water the naturally occurring Chromium exists in (III) and (VI) oxidation states. However, Chromium (III) is the most frequent form in natural water. Under normal conditions this form of naturally occurring chromium is extremely firm unless the pH is extremely low. In strong oxidizing conditions, Chromium appears as hexavalent chromium and exist in poly-atomic form I.e. CrO4-2 (Saha et al., 2011)

Figure. 2. Illustration of the natural sources of Cr in surface and groundwater. (Re-printed fromTumolo et al., 2020). (An overview of Chromium history)

1.5.1.3. In Rock and soil

Anorthosite and ultrabasic rocks are the natural deposits of chromium, a considerable amount of chromite is found in ultramafic rocks. Chromium in these rocks exists in ally with Nickel (Ni) and magnesium (Mg). In ultrabasic rocks, these chromium deposits come into existence as a result of magmatic sequestration takes place as masses and lenses. The formation of chromite by magma cooling occurs via gravitational liquid aggregation or through the matutinal setting of chromium. Deposits of chromium appear in two conical forms: pod-shaped and stratiform. However stratiform constitute 98% of natural deposits. There are large deposits of stratiform in Pakistan, India, Cuba, Greece, and Brazil. The concentration of chromium in chromium rocks is 1000-3000ppm, and the concentration of chromium in granite and gabbros are 5ppm and 200ppm, respectively. Chromium secondary deposits come into existence as a result of heaping of minerals. During suitable Subtropical and tropical conditions, a soil called lateritic is form enclosing 2-4 % of chromium and 50 % of Fe. Lateritic soil is reddish and formed from ultramafic rocks that contain FeCr2O4 from which magnesium silicates leaches out. Chromite and Chrocites also called lead chromate are basic sources of elemental Chromium.

Figure. 3. Illustrate Ultabasic rock (A), Granite (B), Gabbro (C), Chromite (D) and Lateritic solil (E). (An overview of Chromium history)

1.5.2. Anthropogenic Sources
1.5.2.1. In Atmosphere

Coal, oil and natural gas combustion, industries manufacturing chemicals, and metallurgical operations are the anthropogenic sources for the introduction of chromium into the atmosphere in the form of suspended particles. About 471.7, 1418.8, and 1478.7 metric tons.Y-1 of chromium are released into the environment by steel production, combustion of coal, and production of refractory bricks. In a decade the recent studies indicate that steel and iron industries are the massive man-made sources of chromium emissions globally (McGrath et al., 1990). (An overview of Chromium history)

1.5.2.2. Chromium in water

The untreated wastewater from several industries such as leather, paints, tannery, dyeing, and paper pigmentation are the principal sources for chromium discharge in water bodies. The effluents located nearby these industries contain 2 to 5gL-1 of Cr (Oliveira et al., 2012). The concentration of chromium in surface water and groundwater are estimated as 1ppb and 0.6ppb, respectively. When pH of water changes or chromium oxidation state changes as a result of some factors chromium convert into immobilized form by changing into solid phase through precipitation. In a reducing environment chromium (III) is the predominant form of chromium that is highly immobilized. However, Chromium (VI) is the prominent form under oxidizing conditions that is highly toxic and mobile. There are several processes for the formation of immobile chromium (III) from highly mobile and toxic Chromium (VI) through reduction. Once chromium (III) are formed they are locked in a solid phase which cannot be re-oxidized to chromium(VI) easily (Saha et al.,2011).

Figure. 4. Representation of Cr emission in percentage by various industrial processes (Re-printed from Tumolo et al., 2020). (An overview of Chromium history)

1.5.2.3. In agricultural materials

Approximately 4.35105 to 1.18106 metric tons of chromium are introduce into the environment by different agricultural sources per year (McGrath et al., 1990). Among different agricultural sources concentration of chromium in the manure of animals, fertilizers and limestone are of prime importance. However, phosphate fertilizers contain a considerable amount of chromium (Steritt et al.,1981).In 1976 the National research council of Canada conducted research that findings indicated that around 30-3000 mg kg-1 of chromium is present in phosphate fertilizers.

1.5.2.4. In Soil;

Chromium contamination of soil as a result of anthropogenic activities is a global problem (Shahid et al., 2017). The prominent agents that are responsible for the befouling of soil by chromium are units manufacturing leather, metal fishing, and wood preservation (Xu et al., 2020). The principal chemical use in the leather industry is chromium sulfate as a result the sludge and effluents coming out may contain a considerable amount of chromium (Zhang et al., 2017). Tannery sites also hoped up a significant amount of chromium in soil. In Sialkot, Pakistan soil close to the discharge site of tannery wastewater contains 21-675 mg Kg-1of Cr at a pH of 7.1-10.6 (Ali et al., 2015).

The figure below show the chromium deposits in different form globally.

Figure. 5. Reprinted from Chromium- A national mineral commodity perspective by Papp and John F 2007. (An overview of Chromium history)

The below figure show different ores of chromium and the uses of chromium in various industries.

Figure. 6. Material flow of chromium and there end uses (Reproduced from Chromium- A national mineral commodity perspective by Papp and John F 2007). (An overview of Chromium history)

1.6. Chromium standards

Chromium is extremely toxic and carcinogenic therefore its surplus amount in water, air or food may cause severe damage to biota. For this purpose various environmental protection agencies set an acceptable limit of chromium at that no acute or chronic effect of chromium on human health has been reported.

1.6.1. Chromium standards for water

According to various agencies the acceptable limit of chromium in water is;

1.6.1.1. WHO standards for Chromium in water

The permissible amount of Chromium in water according to the World health organization (WHO 1958) is 0.05mg/L.

1.6.1.2. US EPA standards for chromium in water

The United States Environmental and Protection Agency (US EPA 2012) set the Maximum Contaminant Limit (MCL) and Maximum Contaminant Limit Goal (MCLG) of Chromium in water at 0.1ppm (100ppb).

1.6.1.3. California Standards for Chromium in water

According to the report of the California department of public health (2013), 0.05 mg/L is considered as a Maximum Contaminant Limit (MCL) for chromium in drinking water. However, for Cr (VI) the maximum contaminant limit is set at 10ppb and the public health goal at 0.02ppb.

1.7. Analytical methods for chromium analysis

The analysis of chromium can be carried out if the chromium in the solution is in soluble form. We can find out the chromium total concentration by using acids (Hot acids) that vanish the physical and chemical interactions edged in chromium and sample matrix and transformed into soluble form in water. Once chromium is converted into a soluble form in water its analysis is carried out by several instruments. Most frequently used are Atomic Absorbance Spectrometry (Graphite Furnace Atomic Absorbance Spectrometry GFAAS, Electrical Atomic Absorbance Spectrometry), Electrothermal or Graphite Furnace, Inductive Coupled Plasma – Optical (or atomic) Emission spectroscopy (ICP – OES or ICP- AES), Inductively Coupled Mass Spectrometry (ICP – MS), X-ray fluorescence and Flame Atomic Absorbance Spectrometry. The most reliable among the above instruments for chromium analysis are inductively Coupled Plasma – Mass Spectrometry and Electrical Atomic Absorbance Spectrometry. Currently, a new instrumental technique X-ray fluorescence (XRF) is prevailing for the analysis of chromium that is not destructive (Kimbrough et al., 1999).

1.7.1. Inductively Coupled Plasma – Mass Spectrometry

For the analysis of an extremely minute amount of toxic elements ICP – MS is a very potent instrument. ICP – MS having high demands due to its accuracy and precision in the quantitative analysis of chromium and other toxic substances. This technique uses plasma produced from Argon gas that is a mixture of neutral particles, electrons, and ions. The advantages of ICP – MS are highly sensitive, short time of analysis, and multi-elemental analysis (Jeong et al., 2005). (An overview of Chromium history)

1.7.2. Atomic Absorption Spectrometry

Quantification of chromium in various samples like water, urine, blood, and even air samples in the range of (10 – 1000 µg/L) can be carried out by using atomic spectrometry (Hayes et al., 1980; Torgrimsen et al., 1982). The atomic absorption spectrometry used in the estimation of chromium may be Electrical Atomic Absorbance Spectrometry or Graphite Furnace Atomic Absorbance Spectrometry (Kimbrough et al., 1999).

1.7.3. Graphite Furnace Atomic Absorbance Spectrometry

The Graphite furnace atomic absorbance spectrometry uses a set of temperature regulating systems that are used for temperature stabilization. It retard the process of atomization unless the steady condition of temperature is attain (Jeong et al., 2005).

1.7.4. Electrical Atomic Absorption Spectrometry

The principle of EAAS is based on direct flame atomization. In EAAS the stand burner head replaces graphite furnace and electrical heated atomizer. The sample volume required for EAAS is very small possessing a high limit of detection and sensitivity (20 – 1000 times) more as compared to the technique involving flame (Jeong et al., 2005). (An overview of Chromium history)

1.8. Various techniques for the removal of Chromium from wastewater

Chromium is considered a contaminant of wastewater. Numerous industrial operations such as dyeing, tannery, mining of chromium, electroplating, leather and metal disposal, etc. are responsible for their introduction into wastewater. As we discuss that Chromium is highly toxic, carcinogenic, teratogenic, and mutagenic so its removal from wastewater is of prime importance. For this purpose, numerous removal techniques have been introduced that are discussed below (Zhang et al., 2018).

1.8.1. Physio-chemical treatment
1.8.1.1. Chemical precipitation

Due to its inexpensive and easier in operation chemical precipitation is considered a good technique for the management of chromium contaminated water. During this process the chromium existing in wastewater is precipitated with the help of chemicals, as a result, the contaminants are settled down which are then removed by various separation techniques such as centrifugation filtration, etc. The chemical precipitation use coagulating agent as a precipitant. This coagulating agent aggregates the smaller particle into a larger particle, as a result, there occurs an increase in the size of suspended particles and finally, they settle down as sludge. Mostly hydroxides are used as a precipitant because hydroxides are simple, cheaper and their pH can easily be controlled. Hydroxides precipitant and sulfide precipitant are the prevalent chemical precipitation technique (Nur-E-Alam et al., 2020).

Adding Alum, organic polymers, and salts of iron to the hydroxide precipitation process as a coagulant can enhance the remediation of chromium from wastewater (Fu et al., 2011).In tanneries sodium hydroxide, Magnesium hydroxide, or calcium hydroxides are widely used for the precipitation of Cr (Hintermeyer et al., 2008 ). It has been found experimentally that calcium hydroxide in combination with sodium hydroxide removed 99.7% of Cr from wastewater at a pH of 7 and adsorbent dosage of 100mg/L (Ramakrishnaiah et al., 20120). The combination of Ca(OH)2 with MgO for the remediation of chromium from wastewater gives a good result with minimum sludge generation (Abbas et al., 2020). The chemical precipitant with their characteristic is described in below table (Nur-E-Alam et al., 2020).

Table. 7. Chemical precipitants and their Characteristics (An overview of Chromium history)

1.8.1.2. Membrane filtration

The membrane is a Latin word that means skin. It can be defined as a phase that possesses selective properties and is placed in among two adjoint phases that allow the transfer of energy, matter and information between the phases (Drioli et al., 2016). The chief membrane separation method for the treatment of wastewater are Ultrafiltration (UF), nanofiltration (NF), Reverse osmosis (RO) Electrodialysis (ED), and micro-filtration (MF). During these separation techniques, the membrane behaves as a filter that grips out the solid suspended materials and allows water molecules to pass through it. The separation achieved through the membrane may depend upon operating pressure or other driving forces (Tansel et al., 2008). An experiment was conducted in wastewater from tanneries was subjected to RO. The results indicated that 98.66% of Cr was recovered at a temperature of 25C, pressure of 150Psi, and 5.6-7 pH (Tripathi et al.,2012). The use of numerous membranes for the Cr(VI) removal is shown below (Nur-E-Alam et al., 2020).

1.8.1.3. Electro-chemical treatment

Electrochemistry is the branch of chemistry that deals with the interdependence of electricity and chemical reaction. While the process in which current is passed via an aqueous solution containing metal ions, cathode, and an anode is called electrochemical treatment. For the treatment of wastewater, it is considered an alternative method. Electro-osmosis, Electro-oxidation, Electro-deposition, Electro-reduction, and Electro-coagulation are various processes involved in electrochemical treatment. Due to being eco-friendly and electron as a cleaner reagent, this process has been the point of interest among researchers (Nur-E-Alam et al., 2020). (An overview of Chromium history)

1.8.1.4. Electro Coagulation

This technique is considered highly effective for the elimination of Cr(VI) from wastewater. It involves the generation of metallic hydroxide flocs through electro dissolution of soluble anodes inside the wastewater. Some experimental work indicates that time, pH and applied current density is the only factor that affects the Cr(VI) removal rate from wastewater through electrocoagulation. However recent studies show that Cr(VI) initial concentration is among the factor that affects the removal rate. As compared to chemical coagulation, electrocoagulation is faster and results in the minimum generation of dissolved salts and lime. Al-Al, Fe-Fe, or Al-Fe are the frequently use pairs of electrodes in electrocoagulation. When we use iron as anode the following reaction will occur;

This reaction indicates that as a result of the combination of OH- ions with Cr(III) and Fe(III) insoluble precipitate is formed that is precipitated out (Tumolo et al., 2020). The treatment of wastewater by the process of chemical coagulation involves the addition of coagulants like iron or aluminum salts that results in the removal of pollutants as gelatinous hydroxide. This means that chemical coagulation is effective but involves the generation of secondary wastes. On the other hand coagulation in the electrocoagulation process does not involve any coagulating agents and hence no generation of secondary wastes. Due to no use of coagulants and no generation of secondary wastes, electrocoagulation is successfully used in numerous industries for the treatment of wastewater (Dermentzis et al., 2018).

1.8.1.5. Electro-flotation

Electro-flotation is a process that involves the generation of bubbles by the passage of electric current through wastewater, the pollutants present in wastewater will be brought up to the surface with the movement of bubbles. In EF due to the passage of electric current electrolysis of water occur that produces the bubbles of oxygen and hydrogen at anode and cathode (Mickova et al., 2015). During the motion of these bubbles, they strike with the suspended particles present in wastewater and bring it up to the surface which then glides off (Pryia et al., 2012). The basic difference between modern and traditional flotation lies in its mechanism. Both involve the generation of bubbles but in one through the electrolysis of water and in the other through the pressure of dissolved air.

1.8.1.6. Electro-oxidation

This process is an advanced form of oxidation that involves the passage of electric current in wastewater through electrodes, thats lead to generation of strong oxidizing species like OH-.. These oxidizing species will interact with the pollutants present in wastewater and will degrade it. EO is carried out in two ways; indirect oxidation and direct anodic oxidation. Indirect anodic oxidation involves the generation of oxidizing species electro-chemically that are engaged in the oxidation of organic pollutants extending in the wastewater. In direct anodic oxidation, the organic pollutants are adsorbed at the surface of the electrode which is then oxidized (Nur-E-Alam et al., 2020).

1.8.1.7. Electrodialysis

Electrodialysis was commercially used for the treatment of Brackishwater in 1950 for the time. ED is an electrochemical process of membrane technology that involves the remediation of pollutants present in ionic form in an aqueous solution. The basic principle of ED is the use of an ion-exchange membrane for the dissociation of ions in the presence of electrical potential between the cathode and anode (Valero et al., 2011, Bernades et al., 2014). With the passage of time advancement occurs in the ED and 1974 highly advance form of ED was introduce called Electrodialysis reversal (EDR). The basic difference and improvement in the EDR is the introduction of Auto-cleaning of membranes by changing the direction of applied constant current (Tanaka et al., 2007). According to the reported study above 90% of Cr(VI) is removed from wastewater by using the ED process (Sadyrbaeva et al., 2016). Recently coagulation is engaged with electrodialysis which is combined called Electrocoagulation-Electrodialysis process (EC-ED). According to the researchers, this hybrid process is highly efficient and possesses the ability to 100 % remove Cr(VI) from tannery wastewater (Deghles et al., 2016).

1.8.1.8. Adsorption

A process in which particles are extracted from the fluid phase and are concentrated at the surface of a solid phase is known as adsorption. The substance which gets adsorbed onto the surface is called adsorbate for example if gas is adsorbed on the surface of a solid, then gas is termed as adsorbate. While the substance on which the process of adsorption takes place is termed an adsorbent that is illustrated in below figure. From the above discussion, we can say that adsorption is a phase transfer process that is commonly used for the elimination of pollutants from wastewater. Adsorption is provident as the best technique that is low in cost, easier in operation, and possesses high removal efficacy for the Cr(VI) elimination from wastewater (Hoang et al., 2019).

In the past decade, activated carbon is the frequently used adsorbent for the remediation of noxious waste from wastewater. However, due to their high cost, it is necessary to introduce such an adsorbent that is cheaper and involve no generation of secondary wastes (Ghouti et al., 2019). Recently the focus of researchers is on the preparation of adsorbents from various agricultural wastes like rice straw, rice husk, corn cob, corn straw, banana peel, date pits, etc. The advantage of such an adsorbent over the other is, it is low in cost, minimum usage of chemicals, sorbents restoration, highly effective (removal efficiency from 90-99 %) and resorption of the metals for the useful purpose (Saha et al., 2010). There is various factors that affects the rate of adsorption that is the preliminary concentration of a metal ions in solution, pH, contact time adsorbent dosage, and interfering substances (Grassi et al., 2012). The studies indicated that rice husk as an adsorbent for the Cr(VI) exclusion from wastewater possess the ability to remove 76.5% of Cr(VI) at a solution pH of 2 from wastewater (Bansal et al., 2009).

Figure. 7. Explains the process of adsorption. (An overview of Chromium history)

1.8.2. Bioremediation          

Bioremediation is the combination of two words. Bios and remediate. Bios refer to life and remediate means to uncover the problem. Thus the bioremediation entangles the use of biological organisms to clean the environment. This process involves the use of microorganisms such as bacteria, fungi, etc. that convert and remove the toxic and harmful pollutants present in soil and water by consuming them as a source of food and energy for their reproduction and growth. For the energy and generation of new cells, these contaminants supply electrons and carbon after their degradation by microorganisms (Nur-E-Alam et al., 2020).

1.8.2.1. Bioreduction

Bioreduction involves the reduction of the contaminant in wastewater via Bacteria. Deinococcus, Anthrobacter, and bacillus are the group of gram-positive bacteria that possess the ability for Cr(VI) reduction. The remediation of Cr(VI) from wastewater through microbial reduction is considered an effective and promising method. The reduction of Cr(VI) in wastewater by bacteria occurs via enzymatic reduction or by chemical reduction. During chemical reduction, the bacteria present in wastewater generate H2S and Fe(II) that are reducing agents and are responsible for the Cr(VI)  reduction in wastewater (Tumolo et al., 2020).

1.8.2.2. Biosorption

Biosorption is a process that involves the sorption of metal ions present in wastewater by the cellular surface of microorganisms. The process of Biosorption is highly affected by various factors that include pH, temperature, presence of other ions, initial concentration of metal ions in solution, biosorbent surface properties, and the quantity of biomass (Shamim et al., 2018).On the cell wall of bacteria phosphate, amine, carboxyl, sulfhydryl, and hydroxyl groups present that act as anionic ligands and are involved in the elimination of metal from wastewater (Asri et al., 2017). The biosorption of metals can occur in both dead and living cells. However, the sorption through the living microorganisms is of prime importance due to their high adsorption rate then sorption by dead cells (Beveridge et al., 1980).

1.8.2.3. Bioaccumulation

The ligands present on the surface of bio sorbents result in the movements of ions from wastewater towards the cells of microorganisms which then enter into the cell by passive and active transport. As a result of this transport the concentration of metal ions inside the microorganism increases (Igiri et al., 2018). The association of passive and active uptake is termed bioaccumulation (Gutiérrez-Corona et al., 2016). The ligands present on the surface of biosorbents are anionic it means that they will show a tendency towards the cat-ionic species. On the contrary, Chromium exists frequently as oxyanion form (CrO4-2) and hence the interaction between chromium oxyanionic form and bacterial anionic ligands is not possible (Volesky et al., 2020). The oxyanion form CrO4-2 is quite similar to the sulphate ion SO4-2. Due to these similarities the sulphate transporters will actively transport the Cr(VI) into the biological membranes. The studies indicated that Bacillus circulans and B.megateriun strains isolated from tannery waste bioaccumulate 32.0mgand 34.5mg Cr g-1 dry weight, respectively.

1.8.2.4. Phytoremediation

Phytoremediation is the combination of two words. Phyto from Greek word which means plant and remedium are from Latin word that means to remove evil. Phytoremediation belongs to bioremediation that involves the usage of plants and their physical characteristic for the removal from wastewater. The mechanisms involved in the process of phytoremediation are phytoextraction, phytostabilization, phytodegradation, phytovolatilization, and photostimulation, etc. This process is low in cost that involves a large variety of plants for the removal, stabilization, and conversion of various contaminants in groundwater and soil. Numerous studies indicated the usage of cost-effective agricultural waste such as rice husk, banana peel, neem leaves, etc. for the remediation of Cr from wastewater (Nur-E-Alam et al., 2020).

1.9. Chromium Exposure Routes

The possible routes through which chromium enter into human bodies are:

1.9.1. Food

Food enriched with chromium content is responsible for its entrance into the human body. According to the report of US EPA, 1984 the intake of chromium in the diet of North Americans was in the range of 60 – 90 µg/day. Similarly in commercial alcoholic beverages, the content of chromium in British spirits, wines, and beer is 0.135 mg/L, 0.45 mg//L, and 0.30 mg/L respectively. That was higher as compared to United States beverages chromium content (EPA, 1984).

1.9.2. Water  

The chromium content of water depends upon the surrounding conditions. The water closed to industrial zones contain a significant amount of chromium. Similarly, the underlying soil nature influences chromium concentration in water. In the United States, 3834 samples were taken from various tap water in selected cities the results show that the concentration of chromium in the range of 0.4 to 8 µg/L (US EPA 1984).

1.9.3. Air

According to the WHO guidelines, 2000 the cardinal point for chromium (VI) carcinogenic effect is bronchi. Like chromium compounds exposure, aerosols containing chromium inhalation is also a serious problem. Air is responsible for the transport of chromium to a wide range among various ecosystems. Chromium exists and is transported in the air in form of droplets and particles. The particle size of chromium is a significant factor because it not only influences the transportation of chromium in the environment but is also strongly correlated with health effects. Only those particles are respirable and carcinogenic that having a diameter of less than 10 µm (Stanin et al., 2004).

Figure. 8. Exposure routes, circulation and accumulation of chromium in Human Body. (An overview of Chromium history)

1.10. Health effects of Chromium

1.10.1. Toxicological overview of chromium

The Agency for Toxic Substances and Disease Registry (ATSDR 2003) issues a list of 50 most toxic substances among them chromium is placed in the top 8 most toxic substances. Chromium exists in several oxidation states, However trivalent and hexavalent chromium are the major ones (Zhang et al., 2011). Among them, Chromium(VI) is highly toxic that is responsible for various types of cell injuries such as disorientation of chromosomes, damage to DNA, and instability of microsatellites (Sun et al., 2016). The basic reason for the toxicity of Cr(VI) is its structural similarity to phosphate and sulfate anion, hence the non-specific anion transporters of the cell can easily adsorb it (Zhitkovch et al., 2011).

1.10.1.1. Single (Acute) exposure effects on health           

The effect of chromium single exposure on human health are;

1.10.1.1.1. Inhalation

For the chromium that is inhaled the chief target is the respiratory tract. Whereas the Liver, Kidney, and Gastrointestinal tract were also affected. According to the Toxic Substance and Disease Registry (ATSDR 1993), there is numerous factor that affects the chromium adsorption after inhalation that are oxidation state, solubility and size of chromium particles, alveolar macrophages activity and after deposition in lungs its interactions with biomolecules. According to Health Protection Agency (HPA 2007), the inhalation of chromium compounds in case of single exposure does not involve any fatalities. The chromium that is absorbed as a result of acute inhalation can be detected by the quantification of chromium in urine, hairs, and serum of the victims (Grevatt et al., 1998).

1.10.1.1.2. Ingestion

The compounds of Chromium (VI) single high dose ingestion either intentionally or accidentally may adversely affect the renal, cardiovascular, respiratory, hepatic, neurological, and gastrointestinal system (HPA 2007). The compound’s corrosive nature is responsible for all the above potentially fatal effects (HSE 2005). According to the reported studies of ATSDR 2000, 29mg ingestion of Potassium dichromate by 17 years old boy suffered from a severe respiratory problem and died after 14 hours of chromium (VI) exposure. As a consequence of these ingestion gastrointestinal, stomach, and duodenum hemorrhages were reported. According to another report of ATSDR 2000, ingestion of 7.5mg of potassium dichromate by a 14 years old boy admit severe kidney and liver damage along with ulceration of the gastrointestinal tract and after 8 days of his hospitalization, he died. The European Chemical Bureau (ECB, 2005) reported that 2.5-195 mg of Chromium(VI) Kg-1 ingestion is extremely fatal and lethal to human beings.

1.10.1.1.3. Ocular exposure

According to the reported studies, the Cr(VI) ocular exposure may severely affect the hematological, cardiovascular, and renal system, hyperemia of gastric mucosa, and in some cases death. The chromium (VI) ocular exposure results in high temperature and damage to the skin (ECB, 2005; ATSDR, 2000).

1.10.1.2. Chronic (Repeated) exposure effect on human health
1.10.1.2.1. Inhalation

According to the reported studies inhalation of mists containing Cr(VI)  in aqueous form badly affects the respiratory tract and leads to its inflammation and irritation followed by cyanosis and dyspnea (ECB, 2005).

According to the WHO the exposure time and exposure intensity of Cr(VI) for workers working in chrome plating is 8 h in a day for 0.2-23.6 years and 2-200 µg/m3 of Cr(VI) concentration. Exposure to Cr(VI) at a low concentration that is less than 2µg m-3 leads to crusty, smeary, and atrophied septum mucosa. However, exposure to Cr(VI) in a higher concentration that is 2-200 µg m-3 leads to severe irritation of the nasal cavity, ulcer in mucosa and septum, and atrophy perforation were noticed (IPCS, 2006). Another study reported that workers working in electroplating suffered from severe coughing, bleeding of nose, irritation of nasal cavity, sneezing, perforation, and ulceration of nasal septum and rhinorrhea, when they are exposed for less than one year to chromic acid i.e., Cr(VI) concentration of 0.1 mg m-3. Various research indicated that repeated exposure to Cr(VI) results in Asthma and respiratory distress. The repeated inhalation of Cr(VI) results in the enhancement of low molecular weight urinary proteins that include ß2-microglobulin, retinol-binding protein, and tubular antigen (ATSDR, 2000).

1.10.1.2.2. Ingestion

The repeated ingestion of Cr(VI) is extremely fatal and lethal. According to the report of ATSDR 2000, a village in China consists of 155 persons and is located nearby to a smelting plant of chromium. The well water present in their contains 20 mgL-1 of Cr(VI). Due to the repeated consumption of this chromium contaminated water by the inhabitants of the corresponding village for daily purposes. They suffer from severe problems of the gastrointestinal tract that include vomiting, pain in the abdomen, indigestion, diarrhea, and oral ulcer as well as problems of blood that include immature neutrophils and leukocytosis.

1.10.1.2.3. Dermal exposure

Eczema and dermatitis are the frequent adverse effect of Cr(VI) repeated dermal exposure. Chrome holes also known as dermal ulcers or sores are a consequence of Cr(VI) chronic occupational exposure. These chrome holes may deeply enter into the skin if remain untreated. Moreover, this healing process of this dermal ulcer is very gradual subordinate to continue exposure. Rashes on the skin are another effect of epidermal exposure to chromate salts (IPCS, 2006).

According to the reported studies, Potassium dichromate is an extreme sensitizer as compared to chromium sulphate (IPCS, 2006).

1.10.1.4. Carcinogenicity

The Cr(VI) to humans has been categorized as a Class A carcinogen by the International Agency for Research on Cancer (IARC, 1997). Epidemiological studies reported that Cr(VI) compounds are strongly correlated to stomach and lungs cancer.  According to the report of IARC, 1997 numerous workers, working in chrome plating and chromate pigments units for at least 10 years suffer from lungs and stomach cancer. A research was conducted in a small village of Liaoning province in China where numerous cases of stomach cancer have been reported and their number increasing day by day. The studies pointed out that water used by the residents for their daily consumption was Cr(VI) contaminated (Cr(VI) concentration 0.5 mg/g) (Zhang et al., 1987). The studies indicated that numerous regions of Greece had been faced the adverse effects of the consumption of Cr(VI) contaminated water. In Greece in the Oinofita region, numerous cases of lung, liver, kidney, and stomach cancer were reported. The researcher relates these adverse effects to the Cr(VI) contamination of water. According to the report, the Cr(VI) concentration in the affected region water was in the range of 41 – 156 µg/L (Linos et al., 2011).

1.10.1.5. Effect on Reproductive system

Exposure to Cr(VI) compounds also having an adverse effect on the male reproductive system. The various studies indicated that 21 workers in chromium electroplating in China were subjected to motility and a significant decrease in the count of sperm. Similarly, Cr(VI) compounds exposure results in the enhancement of follicle-stimulating hormones concentration (IPCS, 2006).  

1.11. Theory of adsorption

To discuss the Cr(VI) adsorption via iron-modified adsorbents from wastewater, it is essential to recapitulate the theory of adsorption concisely. For a long time, the adsorption is used as a treatment process for the remediation of unwanted species from wastewater. It includes the removal of unwanted species from the liquid phase, surface binding of components, and their aggregation at the adsorptive media surface. The main distinction between adsorption and absorption is that one is surface phenomenon and the other is bulk phenomenon that involve the filling of solid pores. Adsorption is classified into chemical adsorption (chemisorption) and physical adsorption (physisorption). Chemical adsorption involves the generation of a strong bond between the active sites of the adsorbent and the molecules of the adsorbate. In chemisorption, we can assess that during chemical reaction how many active sites of the adsorbent are involved. The attachment of the adsorbate molecules on the surface of the adsorbent via electrostatic and Vander Waals forces constitute physical adsorption. The figure illustrates physical and chemical adsorption. Adsorbate concentration and nature, solution pH and temperature, competing solutes presence and adsorbent properties such as the size of pores and the specific surface area are the factors that affect the adsorption. Based on porosity the adsorbents are classified into porous and non-porous adsorbents. In porous adsorbent, the internal surface area for the adsorption is large hence it shows maximum efficiency. On the other hand in non-porous adsorbents, the internal surface area for the process of adsorption is less. Non-porous adsorbents include steel beads, clay, and glass.

Figure. 9. Illustration of physical and chemical adsorption. (An overview of Chromium history)

 Adsorption equilibrium and adsorption rate are the two aspects of the process of adsorption.

1.11.1. Adsorption Equilibrium

For any specific adsorbent-adsorbate equilibrium, adsorption equilibrium is also called adsorption isotherm. At a particular temperature, isotherm is the segregation of solute among the liquid phase and the adsorbed phase. Langmuir, Freundlich, Sips, Brunauer, Emmet, and the teller are the adsorption isotherms that are used to explain the phenomena of adsorption. The fitting of the corresponding model to the equilibrium model system depends on the system characteristics. For a specific adsorbate, the applicability of the corresponding model depends upon the uniformity or the heterogeneity of the surface of the adsorbent. The Freundlich and Langmuir model are the frequently use models for single-solute adsorption.

The basic assumption of the Freundlich isotherm model is that the surface of the adsorbent is heterogeneous that consists of various types of sites for the adsorption. Equation (1) is used to successfully explain the adsorption process via the Freundlich isotherm model.

  Langmuir isotherm model assumes that at a particular homogenous site the solutes adsorption occurs that results in monolayer formation. The equation (2 ) represents the Langmuir eq.

According to the Sips isotherm model, the adsorbent surface is homogenous and the process of adsorption is a combined process of adsorbate-adsorbent interactions. The equation (3) represents the Sips equation.

1.11.2. Kinetic Mechanisms

The adsorption rate is a prominent factor in determining adsorption efficiency. Pseudo-first-order kinetic model and pseudo-second-order kinetic models are frequently employed to evaluate the adsorption rate and to key out the adsorption rate. The equation for the PFO kinetic model and PSO kinetic model are shown in equation 4 and 5 respectively;

1.12. Removal of Chromium through adsorption technique

The metal ions or molecules present in the fluid phase when adhered to the solid phase this process is called adsorption. As compared to other techniques the elimination of Cr from wastewater through adsorption is valuable. Because its design is simple, results in zero sludge production, and expenses on this technique are less as compared to other techniques. For the process of adsorption, numerous adsorbents are used that are low in cost and available in sufficient forms such as Agriculture wastes, clay, coal fly ash, cheap zeolites, biomasses, and sewage sludge. For the exclusion of heavy metals, dyes, eutrophication, etc., agricultural wastes are the main focus for the preparation of adsorbents for the adsorption process by many researchers (Nur-E-Alam et al., 2020).

Also read: A comprehensive overview on Biochar (green adsorbent), its preparation, factors affecting, uses and its modification

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