Iranian Journal of Environmental Health, Science and Engineering Iranian Association of Environmental Health (IAEH) ISSN: 1735-1979 Vol. 7, Num. 3, 2010, pp. 182-190

Iranian Journal of Environmental Health, Science and Engineering, Vol. 7, No. 3, 2010, pp. 182-190

Review

Chromium Removal Using Various Biosorbents

* ¹ M. Sen , ² M. Ghosh Dastidar

¹Department of Applied Chemistry, Amity School of Engineering & Technology (ASET), Bijwasan, New Delhi, India
²Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi, India

*Corresponding author: E-mail: mousumi1976@gmail.com, Tel: +91 11 28062106/ Fax: + 91 11 28062105

Received 3 November 2009; revised 20 February 2010; accepted 25 June 2010

Code Number: se10022

ABSTRACT

Pollution of water due to presence of certain heavy metal ions is a severe socio-environmental problem caused by the discharge of industrial wastewater. In view of their toxicity, non-biodegradebility and persistent nature, their removal becomes an absolute necessary. Chromium is one of the major pollutants in the environment and is frequently present in wastewaters from various industrial units. Several conventional physical and chemical treatment techniques may be used for the removal of chromium. However, such processes are not only expensive and highly energy intensive, but also lead to production of harmful by-products and end-products, the ultimate disposal of which again causes secondary pollution. Hence, the potential application of microorganisms as biosorbent for the removal of chromium has been recognized as an alternative to the existing conventional physico-chemical methods. The aim of the present study is to review the removal of chromium from aqueous solution using various materials of agricultural and biological origin , which have been studied as potential chromium biosorbent (plant leaves, saw dust, sugar cane bagassa, sugar beet pulp, maize cob and rice hulls). Also reported cases on chromium removal from aqueous solution by using fungal, algal and bacterial biomass under the growing, resting and dead conditions in batch as well as in continuous bioreactors are reviewed.

Key words : Chromium; Biosorbents, Microorganisms, Metal removal

INTRODUCTION

Chromium as one of the major pollutants of the environment is available in nature as an odourless, steel grey hard metallic element. It is the seventh most abundant element on the earth and twenty first most abundant element in the rocks (McGrath and Smith, 1990). Elemental chromium is not usually found pure in nature and principally occurs as the mineral chromite FeOCr₂O₃ or chrome iron stone in which form it is extremely stable.

Chromium exists in nature as stable hexavalent and trivalent forms. The hexavalent form of chromium is more toxic than trivalent chromium and is often present in wastewater as chromate (CrO₄^2-) and dichromate (Cr₂O₇^2-). This is of serious environmental concern as Cr(VI) persists indefinitely in the environment complicating its removal. The persistant nature makes it accumulate in the food chain which with time reach harmful levels in living beings resulting in serious health hazards such as irritation in lungs and stomach, cancer in digestive tract, low growth rates in plants and death of animals. Therefore, removal of Cr(VI) from wastewater prior to its discharge into natural water systems, adjoining landmasses and sewer systems. requires serious and immediate attention.

The conventional physico-chemical techniques used for the removal of Cr(VI) include chemical reduction followed by precipitation with caustic soda. This process requires a large excess of chemicals and produces voluminous sludges, disposal of which again create secondary pollution. Other available treatments include ion-exchange, electrolysis and reverse osmosis. which are not only expensive and high energy processes, but are also ineffective in removal of metal ions present at lower concentration in large volume of wastewaters (Bai et al., 2000). Environmentally friendly processes, therefore, need to be developed to clean-up the environment without creating harmful waste by-products. Biosorption involves application of microorganisms in removal of heavy metals and has been recognized as a potential alternative to the conventional methods for treatment of contaminated wastewaters (Stratten,1987; Volesky, 1990).

The growing, resting and non-living cells of microorganisms are reported to remove Cr(VI) from aqueous solutions (Llovera et al., 1993), (Muter et al., 2001), (Sen et al, 2002), (Srinath et al., 2002), (Sen et al., 2005), (Sen et al., 2007), (Sen & Dastidar, 2007), (Sen et al., 2010). However, most of the works to remove Cr(VI) have been carried out using non-living fungal cells and a very little information is available on use of growing and resting cells. The application of non-living cells has advantages over growing and resting cells due to the absence of both toxicity limitations and requirements of growth media and nutrients. Both growing and resting cells can be maintained biochemically active. However, growing systems have the advantage over the non-living and resting cells that the simultaneous removal of metal is obtained during growth of the organism. On the other hand, the major limitation of using growing systems for biosorption of metals is that cell growth is inhibited when the metal concentration is high. This problem can be overcome by the use of metal tolerant organism. The tolerance and removal capacities are the essential characteristics of growing biomass used in a metal ion removal process .

The relation between the hexavalent and the trivalent states of chromium can be described by the following equation:

Cr₂O₇^2- +14 H⁺+6e →2 Cr(III)+7 H₂O+1.33 V (1)

The reduction–oxidation potential (1.33 V) of the two states indicates the strong oxidizing properties of hexavalent chromium (Cotton and Wilkinson, 1980). The oxidation state has important consequence on toxicity and bioavailability by the microbial biomass.

Biological treatment techniques

Biosorption of Cr(VI) using fungal, algal or bacterial biomass (growing, resting and dead cells) and biological and agricultural waste materials has been recognized as a potential alternative to the existing conventional methods for detoxification of industrial wastewaters. The major advantages of the biosorption process over the conventional treatment methods include:

• Low cost
• Increased metal removal
• Regeneration of biosorbent
• Possibility of metal recovery

The removal of Cr(VI) from the wastewaters using biological materials have long been used. Several authors have studied the adsorption of Cr(VI) from aqueous solution using cellulosic, lignocellulosic and agricultural waste materials, briefly reported in Table 1.

Higher plants have been used for many years as bioindicator of metal due to their bioaccumulation properties. Living plants have been used for the decontamination of both water and soil. Water hyacinth (Eichhornia Crassipes) has been shown to remove Cr(VI) from water (Lytle et al.,1998), while terrestrial plants such as maize have been presented to adsorb Cr(VI).

Many higher plants biomass materials such as plant, parts of the plants or derivatives have been shown to have good biosorbent properties. These include maize cob (Sharma & Forster, 1994), leaves (Mahvi et al., 2007; Gholami et al., 2006), saw dust (Sharma and Forster,1994), sugar cane baggase (Sharma and Forster, 1994) and rice hulls (Cici and Keles, 1990). The cellulosic materials of Chitin and Chitosan have considerable potential for Cr(VI) binding.

The relation between the amount of metal adsorbed by the adsorbent and unadsorbed component in solution at a constant temperature can be represented by both Langmuir and Freundlich adsorption isotherms, which provide the equilibrium data required for the designing of the adsorption system. The Langmuir adsorption isotherm, which is applicable to monolayer sorption onto a surface having homogeneously distributed identical binding sites over the surface sorbent , is given by

or

or:

where : q_e is the amount of metal adsorbed per gram of dried biomass at equilibrium (mg Cr(VI)/g of dried biomass) and C_e is the residual (equilibrium) metal concentration in the solution after sorption [mg Cr(VI)/L]. The Langmuir constants, Q^o and b, indicate the maximum amount of metal ion bound per g of adsorbent to form a monolayer (mg/g) and the adsorption affinity (l/mg) for binding of Cr(VI) on the adsorbent sites, respectively. The values of Q^o and b can be calculated from the slope and intercept of the plot C_e /q_e against the residual concentration , C_e (Aksu et al., 2001; Gupta et al., 2001).

The Freundlich adsorption isotherm is applicable to adsorption of Cr(VI) on a heterogeneous surface and is expressed as:

q_e= K_F C_e^1/n (3)

or :

where: K_F and n are the Freundlich constants and are related to the adsorption capacity and adsorption intensity of the adsorbent, respectively. Equation (2) can be linearized in logarithmic form and Freundlich constants n and K_F can be determined from the slope and intercept which are equal to 1/n and K_F , respectively (Aksu et al., 2001 ).

Bacteria, fungi, yeast and algal biomass under growing, resting and dead conditions have been reported to remove Cr(VI) from aqueous solution in substantial quantities (Tzezos, 1996; Sudha Bai and Abraham, 2000; Solisio et al., 2000; Gupta et al., 2001; Muter et al., 2001; Aksu et al., 2001; Muter and Rapport., 2001; Srinath et al., 2002; Sen et al., 2002; Dursan et al., 2003; Sen et al., 2005; Sen et al., 2007; Sen and Dastidar, 2007; Sen et al., 2010).

The use of bacterial species as biosorbents (Table 2) has raised the possibility of using these microorganisms as a biotechnological tool for removal of Cr(VI) from wastewaters because of their small size, their ubiquity and their ability to grow under wide range of environmental situations.

The complexity of microorganisms structure implies that there are many ways for the metal to be captured by the cell. Various mechanisms involved in metal removal by bacteria include cell surface binding, extra cellular precipitation, intra-cellular accumulation, oxidation and reduction and methylation/demethylation. The ability of bacterial cells to bind metal ions is associated with its cell envelop, which is formed by many layers and it separates the cell protoplasm from the surrounding area.

Several types of structures may form the cell envelop. The most important one is the cell walls, on which capsules, S-layers and sheaths are commonly found to be superimposed. The metal binding to the surface occurs through a passive mechanism which involves stoichiometric interaction of the metal with the reactive chemical groups, followed by the intracellular accumulation of metal due to the simultaneous effects of growth and surface biosorption.

Algae

Algae are photosynthetic organisms. Both growing and non-living cells of algae are capable of removing Cr(VI) and shown in Table 3.

The cell surface binding is the first step involved in the binding of Cr(VI) ions to algal species. This is a rapid process and is metabolized independently. This is followed by the second step of intracellular accumulation of metal due to the simultaneous effect of growth and surface biosorption. This step is metabolism dependent and is a much slower process.

Fungi

Fungi and yeasts are the eukaryotic organisms which are used as biosorbents for the removal of heavy metals due to the production of high yields of biomass. They grow easily under wide range of environmental situations and can also be modified genetically to produce enzymes (reductase, DNA polymerase etc.), which are helpful in higher metal removal from the wastewaters. The fungal organisms, in general, are resistant to higher metal ion concentrations. Heavy metals such as zinc, copper, manganese, nickel and cobalt in trace amount serve as micronutrients for the growth of the fungus. The fungi can also accumulate non-nutrient metals such as cadmium, mercury, lead, uranium, silver and gold in substantial amounts.

The fungi and yeasts ( A. niger, T. reessii, R. arrhizus, R. nigricans, S. cerevisiae, etc.) are used in a variety of industrial fermentation processes (Volesky, 1990) and can serve as an economical and constant supply of biomass for biosorption of heavy metals. They can also be grown using inexpensive media and unsophisticated fermentation techniques. Therefore, the cost of a biosorbent (obtained from an industrial fermentation process in which biomass is generated as waste) will be significantly lowered as compared to the cost of the conventional adsorbent.

A wide range of fungal species under non-living condition have been studied by different researchers for the removal of Cr(VI) from the wastewaters. The fungal cells can be killed for biosorption by physical and chemical methods. The physical methods include boiling, autoclaving, vaccum and freeze drying, and mechanical disruption (Galun et al, 1983a ; Townsley et al., 1986). The chemical methods include the treatment of biomass with various organic and inorganic compounds (Kapoor et al., 1999). Table 4 shows the Cr(VI) biosorption potential of various fungal biomass under growing, resting and non-living conditions.

Metal ion uptake by fungal biomass similarly takes place as discussed in algal cells. The first uptake mode involves the surface binding of Cr(VI) ions to the cell wall and extra-cellular material, which is metabolism independent. The second mode of Cr(VI) uptake into the cell across the cell membrane is dependent on the metabolic activity and is referred to as intra cellular accumulation. The first mode of metal uptake is common both in dead and living cells. Cr(VI) uptake by the second process, which is metabolism dependent, occurs only in the living cells.

To date the biosorption mechanism of Cr(VI) ions by fungal biomass has been studied largely in relation to chitin, its deacylated derivatives, chitosan and cellulose. The fungal cell wall also contain glycan, proteins, lipids, polyuronids and melanin. The role played by the cell wall fraction and structural polysaccharides is not fully understood and needs to be studied in greater detail.

The use of non-living biomass in the removal of metal ions has the advantages due to the absences of toxicity limitations and adverse operating conditions (pH and temperature), but the most important limitation is that enzymatic reactions are no longer continued as the cells are dried.

Resting cells can sequester the metal through a combination of surface reactions, intracellular and extra cellular precipitation and extra cellular complexation reactions and can be maintained metabolically active using very low maintenance energy.

The use of growing cells for metal removal has the advantage that the simultaneous removal of metal is obtained during growth of the organism and separate biomass production processes e.g., cultivation, harvesting, drying, processing and storage can be avoided (Dursun et al., 2003). Also, the actively growing systems sequester metal through a combination of surface reactions, intracellular and extracellular precipitation and extracellular complexation reactions. However, major limitation of using growing systems for removal of metals is that cell growth is inhibited when the metal concentration is high, resulting in poor metal removal ( Donmez & Aksu, 1999). This problem can be overcome by the use of metal tolerant organism. The tolerance and removal capacities are the essential characteristics of growing biomass used in a metal ion removal process.

Different researchers have reported different tolerances for Cr(VI) depending on the type of the fungal strain.

SUMMARY

Most of the studies reported in the literature to remove Cr(VI) have been carried out using non-living and resting cells of various microorganisms (algae, bacteria, fungi, etc.) and a very little information is available on the use of growing cells due to the toxicity limitations. However, actively growing system has a number of advantages. The simultaneous removal of metal is obtained during growth of the organism and therefore, separate production processes e.g., cultivation, harvesting, drying, processing and storage can be avoided. Moreover, the growing cells can be maintained enzymatically active to remove the metals from aqueous solution.

Different cell forms have different metal uptake capacities and sensitivities towards the potential toxic metals. Most of the studies in reported literature have been conducted using bacterial cells and scanty information is available on the use of fungal cells. The fungi, in general, have the higher tolerance levels for metals and hence studies are needed on their successful exploitation in biosorption processes.

Batch systems are mostly reported for Cr(VI) removal in the literature. However, the effluents are generated by the industries in very large quantities, treatment of which in the batch systems can no longer be applied. Therefore, a suitable operational strategy needs to be developed for continuous removal of Cr(VI) from the industrial effluents.

Most of the studies in reported literature have been conducted using synthetic Cr(VI) containing solution prepared in the laboratory and scanty information is available on treatment of actual industrial effluents. Keeping in mind large scale applications, studies are required to be carried out on metal removal from actual industrial effluents.

The reported literature information is available on Cr(VI) removal using bacteria, algae and fungus at lower concentrations of Cr(VI). There is a need to examine the tolerance level and the metal removal capacities of the organisms at higher metal ion concentrations.

ACKNOWLEDGEMENTS

The authors are grateful to Centre for Energy Studies (CES) & Department of Biochemical Engineering & Biotechnology (DBEB) , Indian Institute of Technology (IIT), Delhi, for providing the necessary laboratory facilities for the researches which have been used in this paper.

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