Introduction

All metals are toxic and our bodies require special transport and handling mechanisms to keep them from harming us. The toxicity occurs in humans due to environmental pollution via soil or water contamination or to occupational exposure. Some of these metals are useful to us in low concentrations but are highly toxic in higher concentrations, causing serious morbidity and mortality. Among these heavy metals, pollution by chromium is of considerable concern. Chromium, a beautiful element, was first isolated in 1797–98 by L.N. Vauquelin (France). Chromium is a gray-colored lustrous, hard metal and is the seventh most abundant element in the earth. It has atomic number 24 and mass number 52. It is situated in group 6 and period 4. It can exist in different oxidation states from 0 to +6. But the stable states are 0, +3, and +6. The major source of chromium is chromite ore. In this ore, chromium exists in +3 state, but industrial processes produce hexavalent chromium and elemental metal. Aerial oxidation of chromite gives sodium chromate which upon treatment with conc. H2SO4 gives sodium dichromate.

$$ 4 {\text{FeCr}}_{ 2} {\text{O}}_{ 4} + {\text{ 8Na}}_{ 2} {\text{CO}}_{ 3} + {\text{ 7O}}_{ 2} = 8 {\text{Na}}_{ 2} {\text{CrO}}_{ 4} + {\text{ 2Fe}}_{ 2} {\text{O}}_{ 3} + {\text{ 8CO}}_{ 2} $$
$$ 2 {\text{Na}}_{ 2} {\text{CrO}}_{ 4} + {\text{ H}}_{ 2} {\text{SO}}_{ 4} = {\text{ Na}}_{ 2} {\text{SO}}_{ 4} + {\text{ Na}}_{ 2} {\text{Cr}}_{ 2} {\text{O}}_{ 7} + {\text{ H}}_{ 2} {\text{O}} $$

The sodium dichromate is reduced to Cr2O3 by heating with carbon (wood charcoal).

$$ {\text{Na}}_{ 2} {\text{Cr}}_{ 2} {\text{O}}_{ 7} {\text{ + 2C}} = {\text{Cr}}_{ 2} {\text{O}}_{ 3} + {\text{ Na}}_{ 2 } {\text{CO}}_{ 3} + {\text{CO}} $$

Cr2O3 is reduced to metallic chromium by aluminium powder (thermite process) (Fig. 1)

$$ {\text{Cr}}_{ 2} {\text{O}}_{ 3} + 2 {\text{Al }} = {\text{Al}}_{ 2} {\text{O}}_{ 3} {\text{ + 2Cr}} $$

Cr(III) is an essential dietary element. It is required to potentiate insulin and for glucose metabolism. The biologically active form of a Cr(III) organic complex, often referred to as a glucose tolerance factor, is believed to function by facilitating the interaction of insulin with its cellular receptor sites. Studies have shown that Cr(III) supplementation in deficient subjects can result in the rapid reversal of many of the symptoms of chromium deficiency [1, 2]. Cr(III) deficiency causes cardiovascular disease, decreased lean body mass, elevated percent body fat, glucosuria-impaired fertility, etc. [3]. Cr(VI) is used in stainless steel and non-iron alloy production for plating metals, development of pigments, leather processing, wood preservatives, and in refractories [3]. It is also used in cooling tower water to inhibit corrosion. Hexavalent chromium is widely used in the laboratory as an oxidant, as the reduction potential value for the couple Cr2O7 2−/Cr3+ is 1.33 V, a quite high value. In dichromometry, K2Cr2O7 is used as a primary standard.

Fig. 1
figure 1

Chromite ore

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But today, chromium is a primary contaminant at over half of all Superfund hazardous waste sites. The +6 oxidation state of chromium is highly toxic and carcinogenic [4, 5]. After lead, cadmium and mercury, chromium is the next entry in major toxic metal series. The discharge limit of chromium from industries is less than 1 mg/L. Chromium is hazardous to health when its limit in portable water exceeds 0.5 mg/L. The imposed presence of chromium(VI) in natural bodies of water is harmful, as the metal ions are extremely toxic to fauna and flora, and the ecology of streams may be irreversibly damaged [4]. Ingestion of a lethal dose of chromate can result in cardiovascular collapse. Oral exposure to Cr(VI) compounds may result in hematological toxicity. Potential reproductive effects of chromium in humans have not been adequately investigated. Data indicate that Cr(VI) compounds are teratogenic in animals. Cr(VI) compounds induced DNA damage, gene mutation, sister chromatid exchange, and chromosomal aberrations in a number of targets, including animal cells in vivo and human cells in vitro. Lung cancer is a potential long-term effect of chronic Cr(VI) exposure [5]. Besides the lungs and intestinal tract, the liver and kidney are often target organs for chromate toxicity from chronic exposure.

Activity of Cr(VI) within the cell

Cr(VI) exists in solution as CrO4 2− and, due to the structural similarity between CrO4 2− and SO4 2−, the chromate anion can overcome the cellular permeability barrier, entering via transport pathway(s) for SO4 2− [6]. Unless rapidly reduced, CrO4 2− can oxidatively damage DNA via the production of more reactive and oxidizing transient Cr(V) and Cr(IV) species that are formed during one electron reduction reactions by hydrogen peroxide (H2O2), glutathione (GSH) reductase, ascorbic acid, and GSH. Simultaneously, free radicals such as RS− and OH are also formed. These radicals attack DNA, producing bond cleavage and, ultimately, faulty gene expression. The more inert Cr(III) is produced via reduction of the transient species and can also result from the reduction and consequent irreversible binding of Cr(IV) to phosphate groups on the DNA or free nucleotides, and this also affects gene function [7]. However Cr(III) cannot readily enter the cell, and this reduced form, applied exocellularly, is not known to be carcinogenic [8]. Chromate is only metastable in the presence of reducing organic compounds under physiological conditions. p53, a tumor suppressor protein, plays an important role in protecting cells from tumorgenic alternation. It has been reported that more than 50 % of human cancers contain mutations in the p53 gene. It has been found that hexavalent chromium is able to activate p53 in human lung epithelion cells (A549) by increasing the protein level and enhancing both DNA binding activity and trans-activation ability of the protein [9, 10]. Hexavalent chromium is able to cause apoptosis [11]. Apoptosis is a process in which cell death is initiated and completed in an orderly manner through activation and/or synthesis of gene products necessary for cell destruction. In cancer, there is an imbalance between the rate of cell division and death, influencing the anomalous accumulation of neoplastic cells (Fig. 2).

Fig. 2
figure 2

Possible structures of the Cr(III)–guanine and Cr(III)–His–guanine adducts formed at the –NGG– sequence

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Exposure pathway

Cr(VI) can enter the body when people breathe air, eat food, or drink water containing it. Cr(VI) is also found in house dust and soil, which can be ingested or inhaled. Chromium-bearing wastes are discharged from rinsing of metals, chrome plating, anodizing, electroplating, dip solutions and bright drips. Occupational exposures to Cr(VI) compounds can be quite acute. Breathing in Cr(VI) at concentrations as low as 2 μg/m3 can cause sneezing and irritation of the nasal mucosa, and air concentrations of Cr(VI) compounds can get much higher than that in certain workplace settings. In chrome plating workshops with local exhaust, for example, concentrations generally range from 10 to 30 μg/m3; in shops without local exhaust, concentrations can climb to 120 μg/m3. Arc, stainless steel, and alloy steel welding can produce even higher concentrations; according to IARC, depending on the process, welding fumes have been found to contain concentrations as high as 1,500 μg/m3. Home-based toxic exposures can happen when people who work in certain industries go home at night, in what are known as worker-to-family exposures. Family members may be exposed to Cr(VI) and other hazardous materials through contact with contaminated clothes, shoes, and other items. Home exposures can also come from living near hazardous waste sites. Acute poisoning is likely to occur through the oral route, whereas chronic poisoning is mainly from inhalation or skin contact [12]. Severe exposures to Cr(VI) compounds are usually accidental or intentional (suicide), and are rarely occupational or environmental.

Aqueous chemistry of chromium

Chromium remains mainly as +III and +VI oxidation states in aqueous solution. Chromium(III) remains as hexa aqua ion [Cr(H2O)6]3+ in aqueous solution [13]. The aqua ion is acidic (pK a = 4) and the hydroxo ion condenses to give a dimeric hydroxo bridged species. The process of condensation through the formation hydroxo bridged species is known as “olation”. In aqueous solution above pH 6, Cr(VI) forms tetrahedral yellow chromate ion CrO4 2− ion; between pH 2 and 6, HCrO4 − ion and the orange red dichromate ion Cr2O7 2− are in equilibrium [13]. At pH <1 the main species is H2CrO4 [13]. Acidic solutions of dichromate are strong oxidants [13]. During oxidation Cr(VI) is reduced to Cr(III) through the formation of intermediates Cr(V), Cr(IV), and Cr(II); however, basic solutions of chromate are less oxidizing [13], as evident from the E 0 values. Cr(VI) does not give rise to an extensive complex series of poly acids and polyanions [13, 14], characteristics of somewhat less acidic oxides, such as those of V(V), Mo(VI), or W(VI). The reason for this is perhaps the greater extent of multiple bonding (CrO) for the smaller chromium. Polymerization beyond dichromate is apparently limited to formation of tri- (Cr3O10 2−) and tetra- (Cr4O13 2−) chromate [14].

Removal technologies of chromium

Existing chemical treatment processes for the lowering of Cr(VI) concentrations generally involve the aqueous reduction of Cr(VI)–Cr(III) using various chemical reagents, with the subsequent adjustment of the solution pH to near-neutral conditions, for the precipitation of the Cr(III) ions produced. The forming precipitate can be separated from water by sedimentation or filtration. Treated water is than decanted and appropriately discharged or reused. Reduction followed by precipitation is an effective and by far the most widely used process in industry because it is relatively simple and inexpensive to operate. The conventional chemical processes include chromium hydroxide precipitation. Other methods are ion exchange [15], reduction [16], electrochemical precipitation [17], solvent extraction [18], membrane separation [19], cementation [20], evaporation [21], foam separation [22], ultrafiltration [23], electrodialysis [24], and biosorption [25].

Chromium removal by ion exchange

Sorption operations, including adsorption and ion exchange, are potential alternatives for wastewater treatment. In an adsorption process, atoms or ions (adsorbate) in a fluid phase diffuse to the surface of a solid (adsorbent), where they bond with the solid surface or are held there by weak intermolecular forces [26]. Ion exchange resins hold great potential for the removal of heavy metals from water and industrial wastewater. Thus, 1200H, 1500H and IRN97H cation exchange resins have been used for the removal of chromium from aqueous solution. The concentration of metal on a medium is calculated as the difference between the original concentration in the solution and the concentration in solution, after contact. The mass balance may be expressed as

$$ m\left( {N_{\text{e}} - N_{0} } \right) = V\left( {C_{0} - C_{\text{e}} } \right) $$
(1)

and N 0 = 0; therefore,

$$ N_{\text{e}} = V\left( {C_{0} - \, C_{\text{e}} } \right)/m $$

where N e and C e, are, respectively, the adsorbent phase metal concentration (mg/L) and solution phase concentration (mg/L), C 0 is the initial metal concentration (mg/L), V is the solution volume (l), and m is the mass of the adsorbent (g).

The adsorption process, which is pH-dependent, shows maximum removal of chromium in the pH range 2–6 for an initial chromium concentration of 10 mg/l. The metal ion adsorption obeyed linear, Langmuir and Freundlich isotherms. The adsorption of chromium on these cation exchange resins follows first-order reversible kinetics and pseudo-first-order kinetics. The intraparticle diffusion of chromium on ion exchange resins represents the rate-limiting step. The uptake of chromium by the ion exchange resins was reversible and thus has good potential for the removal/recovery of chromium from aqueous solutions. By increasing the resin, the removal efficiency increases but adsorption density decreases. The decrease in adsorption density can be attributed to the fact that some of the adsorption sites remain unsaturated during the adsorption process, whereas, the number of available adsorption sites increases by an increase in adsorbent and this results in an increase in removal efficiency. As expected, the equilibrium concentration decreases with increasing adsorbent doses for a given initial chromium concentration, because, for a fixed initial solute concentration, increasing the adsorbent doses provides a greater surface area or adsorption sites [27].

Commercially available chelating resin, namely Amberlite IRA 743 (AMB), is a macroporous polystyrene N-methylglucamine with free base form which bis also used for removal of chromium from aqueous solution.

Removal by solvent extraction

The solvents used for the extraction of Cr(VI) are diethyl ether, isobutyl ketone, ethyl acetate, hexane, tri-n-butyl phosphate, and chloroform [28]. Nowadays, for extracting metal ions from waste water, new extractants, based on macrocyclic ligands having high selectivity for metal ions, are used. Calixarenes and similar macrocyclic ligands, e.g., cyclophanes, etc., having a hydrophobic platform are used for solvent extraction as the use of template effects are protective groups can effectively control the synthesis [29–31].

Previously, the synthesis of Calix[n]arenes (n = 4–6) having mixed functionalities including carboxylic acid groups, as well as their selective extraction behavior towards noble metal ions [32] and f block element ions [33–35], were known. It was noted that the cavity size, the position and kind of donor groups, and the hydrophobicity of the ligands have a pronounced impact on the extraction power and selectivity. Calixarenes having a variety of possible structures are used in solvent extraction [36].

The substitution of the methylene bridges in a calix[4]arene by sulfide linkage results in a high extractability towards divalent transition metal ions [37]. Again, the introduction of diphenyl phosphino groups into calix[4]arenas promotes the extraction of transition metals as well as Cd(II) and Hg(II) [38].

Here, the extractions of divalent heavy metals and transition metals from weakly acidic media into toluene by using the extractants (Scheme 1) are shown. Ligands (1, 2, 3) are characterized by a crown linkage at the lower rim of the molecule [39]. The linkages work in two ways: (1) it restricts the conformational freedom of the calixarene thus defining the cavity size, and (2) it shields the encapsulated cation from the solvent molecules.

Scheme 1
scheme 1

Structures of the extractants (ligands)

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Removal by membrane-based hybrid process

Membrane-based hybrid processes have been developed to maximize the efficiency of Cr(VI) removal from aqueous wastes. One such process comprises emulsion liquid membrane (ELM) extraction reduction–precipitation. ELM is a one-step process involving extraction and stripping, simultaneously. The liquid membrane constitutes of an extractant and surfactant dissolved in organic diluents. The addition of strip phase into the organic phase results in the formation of an emulsion. The selection of surfactant is decisive in the ELM extraction process, as the concentration of the target metal inside the strip phase heavily relies on it. Aqueous waste containing Cr(VI) (300 mg/L) was treated with the emulsion to gain higher concentrations of Cr(VI) inside the strip solution in one step. The ELM extraction of Cr(VI) is governed by several parameters. The major parameters include the concentration of surfactant and extractant, speed of agitation, treatment ratio of the aqueous to emulsion phase volumes, strip phase base concentration and pH of the feed phase. Among all the parameters, the contact time of the emulsion and the feed phase is very important in achieving the highest concentration of Cr(VI) inside the strip phase. A prolonged contact time for extraction results in more transfer of water inside the internal phase, which causes the membrane to swell and initiates the breakage of the emulsion phase [40].

Removal by ultrafiltration

A batch complexation–ultrafiltration process was used to concentrate and recover chromium from sulfate solution. Factors affecting the rejection rate and permeate flux are pH, concentration ligand, chloride and sulfate concentration, membrane pore size, applied pressure. As the chromium ions are too small to be retained by the filter, they are first complexed with a water-soluble macroligand (polyethylene–imine). Then, decomplexation is obtained so that metal can be separated from macroligand by a second ultrafiltration plant to reuse the macroligand. Polymer-enhanced ultrafiltration technique is also used for removal of chromium from aqueous solution. Water soluble polymers, namely chitosan, polyethyleneimine (PEI) and pectin are selected for this study. For Cr(III), high rejections approaching 100 % were obtained at pH higher than 7 for the above three polymers. With PEI, the retention of Cr(VI) approached 100 % at low pH and decreased when the pH was increased. The concentration of polymer was found to have little effect on rejection. The application of micellar-enhanced ultrafiltration (MEUF) for removing heavy metal ions from water is gaining importance [41]. MEUF is done to analyze the adsorption of chromate ions on the surfaces of the surfactant micelles. In the MEUF process, the surfactant, having charge opposite to target ions, is added to the effluent stream containing the metal ions at a concentration greater than the critical micellar concentration, so that they form aggregates of around 50–150 of monomer molecules, called micelles [42]. Therefore, a large fraction of the metal ions get electrostatically attached to the micelle surface. Retention of such metal ions attached to the micelles is possible if the resulting solution is passed through an ultrafilter, having a pore size smaller than the micelle diameter [43]. Precipitation of surfactant using mono or multivalent counter ions has been proposed [44]. The removal of hexavalent chromium by micellar-enhanced ultrafiltration, using cetyl tri-methyl ammonium bromide as well as cetylpyridinium chloride, has been studied [45, 46]. Rejection coefficients, higher than 99 %, were obtained as long as the feed concentration was less than or equal to 200 times the standard. Hexavalent chromium remains in solution as [CrO4]−2 and as [HCrO4]−. Hence, anionic surfactant cannot be used. The increase in surfactant concentration provides increased percent retention of chromate ions. Pressure was found to have no significant effect on the percent retention of chromate. However, the permeate flux was found to increase linearly with increase in pressure, indicating negligible presence of concentration polarization. Cationic starch-enhanced ultrafiltration is also used to remove chromate from an aqueous stream.

Removal by foam separation

Many methods are available for the separation of liquid mixtures and metal ions. Among them, the foam separation technique holds great promise especially when the concentration involved is very low. The basis for the separation is surface absorption phenomena. Foam is made of films and plateau borders, and holdup variations occur because of the liquid drainage involving these two. Foam fractionation is a method of separating the components of a liquid mixture based on the difference in the concentration between the liquid in contact with a gaseous interface and that of the bulk liquid. Thus, foam separation processes are based on the partition of chemical species between an aqueous solution and a foam phase. Chromium ions were separated from water by using the ferrous co-precipitation foam separation method. When chromium ion concentration was 8 mg/L, the coefficient of removal was 97.1 %. The process of co-precipitation foam separation could be regarded as a first-order reaction [47]. Recovery of chromium(VI), copper(II), and zinc(II) ions from electroplating effluent was carried out in a continuous foam separation process using sodium laurylsulfate as surfactant. Sodium laurylsulfate is an anionic surfactant and it can be ionized into C12H25OSO3− and Na+. The three metal ions of Cr6+, Cu2+, and Zn2+ can be exchanged with Na+ and combined with C18H29SO3 to form complexes [48]. These complexes are then adsorbed into the gas–liquid interface and then removed by concentrating the solute. Thus, the three metal ions in the effluent are removed and concentrated. With the increase of surfactant concentration, the removal efficiency of chromium increases. Better removal efficiency can be reached when the feed solution is acidic, since the adsorbability between Cr2O7 2− and the surfactant is enhanced [49].

Electrocoagulation

Effluents issuing from surface finishing and plating industries usually contain hexavalent chromium concentrations much higher than the permissible levels. There are various techniques employed for the treatment of waste waters. Among them, precipitation is most applicable and most economical [50]. It has two steps: first, chemical coagulation by adding lime to raise the pH and aluminium salt to remove colloidal matter as gelatinous hydroxide, and second, this precipitate is adsorbed onto activated carbon to complete the metal removal at the ppm level [51]. Although precipitation is quite effective in treating industrial effluents, the chemical coagulation may induce secondary pollution due to added chemical substances.

To overcome this problem, electrocoagulation is used where the flocculating agent is generated by electro-oxidation of a sacrificial anode generally made of iron or aluminium. In this process, the treatment is done without adding any chemical coagulant, thus reducing the amount of sludge which must be disposed of [52]. It gives removal efficiencies as high as 99 % when treating dye-containing solutions [53–55], potable water [56], and urban and restaurant waste water [57–59].

Electrocoagulation mechanism

Electrocoagulation is based on the in situ formation of the coagulant as the sacrificial anode corrodes due to an applied current with simultaneous evolution of hydrogen at the cathode allows for pollutant removal by floatation. Electrochemistry, coagulation, and hydrodynamics are the three main synergistic steps to remove pollutants. The main reactions occurring at the electrodes are:

$$ {\text{Al}} = {\text{Al}}^{3 + } + \, 3e \, \left( {\text{anode}} \right) $$
(2)
$$ 3{\text{ H}}_{2} {\text{O}} + 3e = 3/2{\text{ H}}_{2} + \, 3{\text{OH}}^{ - } $$
(3)

In addition, Al3+ and OH− ions generated at electrode surfaces react in the bulk waste water to from Al(OH)3

$$ {\text{Al}}^{ 3+ } + {\text{3OH}}^{ - } = {\text{ Al }}\left( {\text{OH}} \right)_{ 3} $$

The aluminium hydroxide flocks act as adsorbents and/or traps for metal ions and so eliminate them from the solution. In addition, a direct electrochemical reduction of hexavalent chromium to trivalent chromium may occur at the cathode surface [52, 60, 61]. Simultaneously, the hydroxyl ions which are produced at the cathode increase the pH in the electrolyte and may induce co-precipitation of Cu(II), Zn(II) and Cr(III) as their hydroxides [52, 56, 59]. This acts synergistically to remove pollutants from water.

Influence of polyaniline for removal and recovery of chromium

Various methods are used for the removal of chromium from aqueous solutions. Among these methods, surface adsorption has been considered more effective than others when low concentrations are present [62]. “Polyaniline” has shown good potential for absorbing heavy metals from effluents [63, 64]. Polyanilines have about three times more removal efficiency than has powder-activated carbon [65]. Nitrogen atoms in amine derivatives make a co-ordinate bond with positive charge of metals due to the presence of an electron in the s 2 p 3 orbit of nitrogen. This co-ordinate bond is the plausible mechanism for adsorption of Cr(VI) and Cr(III) from solution by polyanilines. Under acidic conditions, the surface of polyanilines is highly protonated due to its nature [66]. The protonated form of polyanilines can form bonds with solution anions (chromate and dichromate) by electrostatic attraction. Synthesized polyaniline in water has a steady pore on its surface and has best hexavalent chromium removal efficiency, but synthesized polyanilines in a mixture of water with other solvents have scale-like surfaces [65]. This variation of polyanilines surfaces affects the capacity of prepared polyaniline for chromium removal due to changes of the interface between polyanilines and the chromium solution.

Bioremediation technology

The cited conventional chromium elimination processes are costly or ineffective at small concentrations and may also lead to environmental problems from waste disposal. In recent years, biosorption research has focused on using readily available biomass that can accumulate heavy metals [67]. This approach involves the use of biomaterials that form complexes with metal ions using their ligands or functional groups. It is particularly the cell wall structure of certain algae, fungi, bacteria and plants that is responsible for this phenomena. This process can be applied as a cost-effective way of purifying industrial waste water whereby drinking water quality can be attained. The major advantages of biosorption over conventional methods include: low price, high effectiveness, minimization of chemical and/or biological mud, restoration of biosorbent, and the possibility of metal recovery.

Factors affecting biosorption

Many factors can affect biosorption. The type and nature of the biomass of the derived product is very important, including the nature of its application as, e.g., freely suspended cells or biomass, immobilized preparation, living biofilms, etc. Physical and chemical treatments such as boiling, drying, autoclaving, and mechanical disruption will all affect binding properties, while chemical treatments such as alkali treatment often improve biosorption capacity, especially evident in some fungal systems because of deacetylation of chitin to form chitosan–glucan complexes with higher metal affinities. Growth and nutrition of the biomass and age can also influence biosorption due to changes in cell size, wall composition, extracellular product formation, etc. The surface area to volume ratio may be important for individual cells or particles, as well as available surface area of immobilized biofilms. In addition, the biomass concentration may also affect the biosorption efficiency with a reduction in sorption per unit weight occurring with increasing biomass concentration. Apart from this, physico-chemical factors such as pH, the presence of other anions and cations, metal speciation, pollutant solubility and form, and temperature may also have an influence. With living cell systems, the provision of nutrients and optimal growth conditions is an obvious requirement.

Of physico-chemical factors, pH is possibly the most important. Metal biosorption has frequently been shown to be strongly pH-dependent in almost all systems examined, including bacteria, cyanobacteria, algae, and fungi. Sorption of Cr(VI) is found to increase with decreasing the pH to acidic values. Low pH values within the 2–3 range have been found to be favorable to Cr(VI) biosorption. Within the 1–4 pH range, HCrO4 − and CrO4 2− are the major species in solution, and their stability depends on the pH and the hexavalent chromium concentration. At low pH, the active sites of the biosorbents are protonated and the anionic species can be bound on the sorbent by electrostatic forces. Maximum uptake values of Cr(VI) were observed at pH 2.0 for tea waste [68], hazelnut shells [69], Agave lechuguilla [25], saltbush [70], yohimbe bark [71], osage orange [72], rice straw [73], and neem bark powder [74]. In other studies, ph 3.0 and 1.5 were found to be optimal for the hexavalent chromium sorption onto cork and grape stalks [71] and Thuja orientalis [75], respectively.

The temperature could be a parameter that affects the sorption of Cr(VI) [76, 77]. Romeo-Gonzalez et al. [78] also found that the sorption capacity of A. lechuguilla leaves, a plant of the Chihuahuan desert, for Cr(VI) increased on increasing the temperature in the 10–40° range. The authors justify the endothermicity of the process with the apparent binding and reduction of Cr(VI) to Cr3+. Malkoc and Nuhoglu [68] also found that the process of Cr(VI) sorption on tea factory waste is endothermic and metal uptake increases on increasing the temperature from 25 to 60 °C. The favorable effect of temperature on sorption may be a result of swelling effect within the internal structure of the sorbent enabling large metal ions like Cr(VI) to penetrate further.

Instruments and techniques used in biosorption studies

Many analytical techniques have been used to study hexavalent chromium binding to biomaterials, including infrared spectroscopy or Fourier transformed infrared spectroscopy (IR, FTIR) [25, 69–71, 77–79], UV–Vis spectroscopy [25, 70, 75, 80–83], atomic adsorption spectroscopy (AAS) [84], scanning electron microscopy (SEM), and transmission electron microscopy (TEM) [71, 85–88], as well as X-ray diffraction (XRD) analysis [89, 90] and X-ray photoelectron spectroscopy (XPS) [91–94]. The functional groups on the sorbent surface that may involve metal ion sorption are usually investigated by FTIR spectroscopy. The energy needed to excite the bonds in a compound, making them vibrate more energetically, occurs in the infrared region of spectrum, rendering IR a most useful technique. IR and FTIR are used to characterize the functional group present in the adsorbent. This sheds very important light for chemical modification of adsorbent for better performance. UV–Vis spectroscopy is used for the detection of hexavalent chromium. The absorbance of pink-colored 1,5-diphenyl carbazide complex of hexavalent chromium is measured at 540 nm. AAS is used for the detection of total chromium (trivalent and hexavalent). Electronic microscope SEM and TEM are used to study the morphology of the adsorbent (before and after adsorption of chromium). Photoelectron spectroscopy is an excellent technique for probing atomic and molecular energy levels, XRD analysis has been studied to understand the nature of adsorbed chromium; XPS verifies the oxidation state of chromium bound to the biomaterials [91]. A colorimetric method has been used to measure the concentration of the different chromium species present in aqueous solution. The pink-colored 1,5-diphenylcarbazide and Cr(VI) in acidic solution has been spectroscopically analyzed at 540 nm [91, 95, 96]. For the determination of Cr(III) (formed due to the reduction of Cr(VI) into Cr(III) during the sorption process), it was again converted to Cr(VI) by the addition of excess potassium permanganate at high temperature (130–140 °C), and thereafter 1,5-diphenylcarbazide was added. The pink-colored complex formed gives the concentration of Cr(VI) and total chromium. The Cr(III) concentration was then calculated by the difference of the total chromium and measured Cr(VI) concentration [91, 95, 96]. Among all the above discussed techniques for the removal of chromium, adsorption is the most promising technique and a feasible alternative [97, 98].

Biosorption mechanism

One of the mechanisms involved in the sorption of positively charged metal species is the ion exchange process between protons and/or alkaline or alkaline earth metals as counterions present in the biomass, and metal ions taken up from water [99]. Vegetal biomaterials (constituted mainly by lignin and cellulose as major constituents and by a non-negligible portion of fatty acid, bearing functional groups such as alcohol, ketone and carboxylic groups that can be involved in complexation reactions with chromium ion) [100] can be viewed as natural ion exchange materials that primarily contain weak acidic and basic groups on the surface. In the 2.5–6.0 pH range, the binding of heavy metal is determined by the degree of dissociation of the acidic groups. This kind of mechanism is normally investigated by analyzing the release of alkaline and alkaline earth metal ions during the sorption process. The release of K+, Mg2+, and Ca2+ has been found to determine the extent of ion exchange mechanism taking place in heavy metal removal by grape stalk waste [101, 102], yohimbe bark [103], and olive stone waste [104]. The functional groups on sorbent surfaces that may involve metal ion sorption are usually investigated by FTIR. In some recent studies, the existence of ion exchange between light metal ions on sorbent surfaces and heavy metals in solution was confirmed by scanning electron microscopy–energy dispersive X-ray (SEM–EDX).

Different low cost biomaterial as adsorbent

A variety of materials have been tried as adsorbents for Cr(VI), and a number of studies have been reported using adsorbents like granular activated carbon [105], Soya cake [106], rubber tyres and sawdust [107], activated sludge [108], lignocellular substrate [109], fly ash [110], rice husk-based activated carbon [111], etc.

Removal of chromium(VI) from aqueous solution can be done by using coconut husk fiber and palm pressed fiber [112]. Each of these substrates is made up of 35 % cellulose, 28 % hemicelluloses, and 20–25 % lignin, with the remainder being other constituents [113]. These components are responsible for chromium reduction. Removal of chromium is pH-dependent. Maximum efficiency is obtained at acidic conditions. The uptake mechanism of Cr(VI) by activated carbon, CxO, under strongly acidic conditions [114, 115]:

$$ {\text{HCrO}}_{4}^{ - } + {\text{ C}}_{x} {\text{O }} + {\text{ H}}_{ 2} {\text{O }} \to {\text{ C}}_{x} {\text{OHO}}_{ 3} {\text{Cr}}^{ + } + {\text{ 2OH}}^{ - } $$

Anion exchange under alkaline condition:

$$ {\text{R}}{-}{\text{OH }} + {\text{Cr}}_{ 2} {\text{O}}_{7}^{2 - } \leftrightarrow {\text{ R}}{-}{\text{Cr}}_{ 2} {\text{O}}_{7}^{2 - } + {\text{OH}}^{ - } $$

Regeneration:

$$ {\text{R}}_{ 2}{-}{\text{Cr}}_{ 2} {\text{O}}_{7}^{2 - } + {\text{ NaOH }} \to {\text{ 2R}}{-}{\text{OH }} + {\text{ Na}}_{ 2} {\text{Cr}}_{ 2} {\text{O}}_{ 7} $$

The sorption of Cr(VI) on CHF increases with increasing contact time up to a certain limit when it reaches equilibrium.

Neem leaf powder can also be used as an adsorbent for the removal of Cr(VI) from aqueous solution [116]. Adsorption behavior follows Freundlich and Langmuir isotherms. The adsorption mechanism follows second-order kinetics. The presence of niacin, proline, glutamic acid, aspartic acid, glutamine, tyrosine, and alanine, which contain polar groups like –NH2, –COOH, –OH, etc. in neem powder [117], contribute to the negative surface charge. The ingredients contribute an electronegativity of 35.1 %. Decrease of the size of the adsorbent results in increase of the surface area of it, thereby the number of active sites are better exposed to the adsorbate. With increases in initial concentration of chromium, the percent removal decreases [118, 119]. Evidently, such a behavior can be attributed to the maintenance of a fixed number of binding sites in the dosage while increasing the concentration [120]. Adsorption increases with an increase in pH [121]. At low pH, H+-ions compete with chromium ions for appropriate sites on the adsorbent. But beyond pH 7, % removal of chromium decreases as pH increases. The principal factor for metal ion adsorption is the electrostatic interaction; that is, attraction between the adsorbate and adsorbent. The greater the interaction, the higher is the adsorption of heavy metal. Better removal efficiency is obtained with neem leaf powder as it contains higher electronegative components.

A new mechanism has been proposed for the removal of Cr(VI) by biomaterials [98, 122]. Cr(VI) can be removed from an aqueous solution by biomaterials through both direct and indirect reduction mechanisms [123]. In mechanism I (direct reduction mechanism), Cr(VI) is directly reduced to Cr(III) in the aqueous phase by contact with electron donor groups of the biomaterial, i.e., organic groups having lower reduction potential values than that of Cr(VI), with the reduced Cr(III) remaining in the aqueous phase. Mechanism II (indirect reduction mechanism), however, consists of three steps: (1) the binding of the anionic Cr(VI) to the positively charged groups present on the biomaterials surface, (2) the reduction of Cr(VI)–Cr(III) by adjacent electron donor groups, and (3) the release of the reduced Cr(III) into the aqueous phase due to electronic repulsion between the positively charged groups and the Cr(III), or the complexation of the reduced Cr(III) with adjacent groups. If there are a small number of electron donor groups in the biomaterial or protons in the aqueous phase, the chromium bound onto the biomaterial surface may remain in the hexavalent state. The removal rate of Cr(VI) increases with increasing Cr(VI) and biomaterial concentrations; it is first-order with respect to Cr(VI) concentration and biomaterial concentration, respectively. As solution pH decreases, the removal rate of Cr(VI) increases since protons take part in both anionic Cr(VI) adsorption and Cr(VI) reduction reactions. The increase in temperature increases the removal rate owing to the endothermic nature of the redox reaction. Recently, a new and simple kinetic model, in the form of −d[Cr(VI)]/dt = k[Cr(VI)] (biomaterial), was derived from a basic concept of the redox reaction between Cr(VI) and biomaterial, and successfully described the Cr(VI) removal behavior in aqueous phase under various Cr(VI) and biomaterial concentrations [124, 125]. It was the first model considering the removal mechanism of Cr(VI) by biomaterials. But this model did not consider the effect of pH and temperature. An advanced kinetic model is proposed using pine needles as biomaterial. The basic concept of the redox reaction between them is

$$ {\text{B }} + {\text{Cr}}\left( {\text{VI}} \right) \to {\text{B }}\left( {\text{oxidized}} \right) + {\text{Cr}}\left( {\text{III}} \right) $$

A new biosorbent material, marine micro-algae Isochrysis galbana, has been used as an adsorbent to remove chromium [126]. Increases in aqueous metal concentration increases the metal uptake within a certain range. The presence of acid decreases the metal uptake, probably due to the preferential adsorption of hydrogen ions compared to the chromium ions; the immobilized calcium alginate beads adsorb chromium in the absence of biomass, but in the presence of biomass, the metal uptake increases up to 3- to 4-fold. This type of algae strongly adsorbs chromium due to the presence of polysillicate layers within the cell. Equilibrium distribution data are correlated by Freundlich type equation. The equation is C S = IC 0.5A , where C S = metal concentration in solid phase, I = proportionality constant, and C A = metal concentration in aqueous phase. The proportionality constant (I) varies with pH. The relationship between pH and I is log I = 0.274 pH + 0.3059. So the final equation for the estimation of equilibrium metal concentration and the immobilized algal beads (C S) as a function of pH and aqueous metal concentration is C S = 2.2023·100.274pH C 0.5A [127].

Chromate reductase activity has previously been characterized in aerobic bacteria and facultative anaerobones, such as Escherichia coli, Pseudomonas putida [128–131] Paracoccus denitrificans [132], and Bacilus subtilis [133], as well as anaerobic sulfate-reducing bacteria [134, 135]. Methane oxidizing bacteria are ubiquitous in the environment and they have wide potential for the bioremediation of organic and chlorinated organic pollutants. Methylococcus capsulatus is able to bioremediate chromium(VI) pollution over a wide range of concentrations (1.4–1,000 mg/L of Cr6+) [136]. The chromium(VI) reduction reaction is dependent on the availability of the reducing equivalents from the growth substrate methane and was partially inhibited by the metabolic poison sodium azide. The genome sequence of M. capsulatus suggests at least five candidate genes for the chromium(VI) reductase activity in this organism. The reduction reaction is dependent on the active cellular metabolism rather than merely a reaction between the chromate and cellular constituent [137]. The growth substrate methane is required to supply electrons for reduction of chromate. A bacterial (E. coli) enzyme, using NADH as the reductant, converts Cr(VI) to a soluble NAD+–Cr(III) complex and cytochrome-C-mediated Cr(VI) reduction produces cytochrome–C–Cr(III) adducts [138]. Cr(VI) reduction in the presence of cellular organic metabolites formed both soluble and insoluble organo–Cr(III) end products. Significant amounts of Cr(III) remain in the supernatant of bacterial cultures and do not precipitate after reduction [139–142]. Field studies have identified elevated amounts of soluble Cr(III) in the environment [143, 144], greater than that predicted if Cr(OH)3 and Cr2O3 are the sole species. Enzymatic reduction of chromate have formed organo–Cr(III) complexes, where Cr(III) is bound to the microbial organic reductant, such as cytochrome C7 [145] and NAD+ [131]. Similar results are noted in eucaryotes with reduced Cr(III) bound to organic components such as protein, DNA and certain metabolites [146–150]. Chemical reduction of Cr(VI) by ascorbate also produces an organo–Cr(III) complex, ascorbate–Cr(III) [151]. Several soluble end products are characterized by absorbance spectroscopy and electron paramagnetic resonance spectroscopy. These complexes remained soluble and stable upon dialysis with distilled H2O and over a broad pH range. The ready formation of stable organo–Cr(III) complexes suggests that organo–Cr(III) complexes are rather common, likely representing an integral part of the natural cycling of chromium. Thus organo–Cr(III) complexes may account for the mobile form of Cr(III) detected in the environment.

Conclusion

Hazardous Cr(VI) and other heavy metal pollution of waste water is one of the most important environmental problems throughout the world. In this international year of water, the supply of drinking water is a major challenge to human civilization. A wide range of treatment technologies such as chemical precipitation, coagulation, flocculation, ion exchange, and membrane filtration have been developed for chromium(VI) removal. Biosorption represents an efficient and economic method for low concentration chromium removal. For successful application of biosorption technology, the most effective biomass with biosorption capacity and selectivity should be chosen by using this biomass with real waste water. Again, the water soluble part of the agricultural biomass contains different reducing components that can reduce Cr(VI) to Cr(III). So both the solid biomass and its water soluble part can remove toxic Cr(VI).