Biosorption : An eco-friendly alternative for heavy metal removal

Heavy metals occur in immobilized form in sediments and as ores in nature. However due to various human activities like ore mining and industrial processes the natural biogeochemical cycles are disrupted causing increased deposition of heavy metals in terrestrial and aquatic environment. Release of these pollutants without proper treatment poses a significant threat to both environment and public health, as they are non biodegradable and persistent. Through a process of biomagnification, they further accumulate in food chains. Thus their treatment becomes inevitable and in this endeavor, biosorption seems to be a promising alternative for treating metal contaminated waters. This technology employs various types of biomass as source to trap heavy metals in contaminated waters. The biosorbent is prepared by subjecting biomass to various processes like pretreatment, granulation and immobilization, finally resulting in metal entrapped in bead like structures. These beads are stripped of metal ions by desorption which can be recycled and reused for subsequent cycles. This technology outperforms its predecessors not only due to its cost effectiveness but also in being ecofriendly i.e., where other alternatives fail.


INTRODUCTION
Water bodies are being overwhelmed with bacteria and waste matter.Among toxic substances reaching hazardous levels are heavy metals (Regine and Volesky, 2000).Heavy metals of concern include lead, chromium, mercury, uranium, selenium, zinc, arsenic, cadmium, silver, gold, and nickel (Ahalya et al., 2003).Heavy metal pollution in the aquatic system has become a serious threat today and of great environmental concern as they are non-biodegradable and thus persistent.Metals are mobilized and carried into food web as a result of leaching from waste dumps, polluted soils and water.The metals increase in concentration at every level of food chain and are passed onto the next higher level-a phenomenon called bio-magnification (Paknikar et al., 2003).Heavy metals even at low concentrations can cause toxi-  (Ahalya et al., 2003).With increasing environmental awareness and legal constraints being imposed on discharge of effluents, a need for cost-effective alternative technologies are essential.In this endeavor, microbial biomass has emerged as an option for developing economic and eco-friendly waste water treatment process.Biosorption can be defined as "a non-directed physicochemical interaction that may occur between metal /radionuclide species and microbial cells" (Shumate and Stranberg, 1985).It is a biological method of environmental control and can be an alternative to conventional contaminated water treatment facilities.It also offers several advantages over conventional treatment methods including cost effectiveness, efficiency, minimization of chemical/biological sludge, requirement of additional nutrients, and regeneration of biosorbent with possibility of metal recovery.
The biosorption process involves a solid phase (sorbent or biosorbent; usually a biological material) and a liquid phase (solvent, normally water) containing a dissolved species to be sorbed (sorbate, a metal ion).Due to higher affinity of the sorbent for the sorbate species the latter is attracted and bound with different mechanisms.The process continues till equilibrium is established between the amount of solid-bound sorbate species and its portion remaining in the solution.While there is a preponderance of solute (sorbate) molecules in the solution, there are none in the sorbent particle to start with.This imbalance between the two environments creates a driving force for the solute species.The heavy metals adsorb on the surface of biomass thus, the biosorbent becomes enriched with metal ions in the sorbate.
Mechanisms involved in biosorption can be classified taking into account various criteria that are, based on cell metabolism, they are classified as metabolism dependent and non-metabolism dependent while based on location of the sorbate species it is classified as extra cellular accumulation/precipitation, cell surface sorption /precipitation and intra cellular accumulation.The adsorbed ions are transported across the membrane in the same mechanism by which metabolically important ions such as potassium, magnesium, and sodium are conveyed.These mechanisms comprise (i) physical adsorption e.g., electrostatic interaction has been demonstrated to be responsible for copper biosorption by bacterium Zooglea ramigera and alga Chorella vulgaris (Aksu et al.,1992), (ii) ion exchange e.g., biosorption of copper by fungi Ganoderma lucidium and Asperigillus niger (Muraleedharan and Venkobachr, 1990), (iii) complexation e.g., biosorption of copper by C. vulgaris and Z. ramigera takes place through both adsorption and formation of co-ordinate bonds between metals and amino or carboxyl groups of cell walls (Aksu et al., 1992).Various biosorption mechanisms mentioned above can take place simultaneously.Figure 1 shows a gene-ralized schematic process of biosorption for heavy metal removal.
A successful biosorption process requires preparation of good biosorbent.The process starts with selecting various types of biomass.Pretreatment and immobilization are done to increase the efficiency of the metal uptake.The adsorbed metal is removed by desorption process and the biosorbent can be reused for further treatments.

SELECTION AND TYPES OF BIOMASS
While choosing the biomass for metal biosorption, its origin is a major factor to be taken into account.Biomass can come from, activated sludge or fermentation waste from industries like those of food, diary and starch.Also, organisms (e.g., bacteria, yeast, fungi and algae) coming from their natural habitats are good sources of biomass.Fast growing organisms that are specifically cultivated for biosorption purposes (e.g., crab shells, seaweeds) (Regine and Volesky, 2000) can be used as biosorbents.Apart from the microbial sources even agricultural products such as wool, rice, straw, coconut husks, peat moss, exhausted coffee (Dakiky et al., 2002), waste tea (Ahluwalia and Goyal, 2005), walnut skin, coconut fibre, cork biomass (Chubar et al., 2003), seeds of Ocimum basilicum (Melo and D'Souza, 2004), defatted rice bran, rice hulls, soybean hulls and cotton seed hulls (Teixeria et al., 2004), wheat bran, hardwood (Dalbergia sissoo) sawdust, pea pod, cotton and mustard seed cakes, (Saeed et al., 2002) are also proven as good biomass sources.However, sea weeds, molds, yeasts, bacteria have been tested for metal biosorption with encouraging results (Regine and Volesky, 2000).

Seaweeds
Seaweeds are large group of marine benthic algae.They offer several advantages for biosorption because of their larger surface area.This feature offers a convenient basis for the production of biosorbent particles suitable for sorption process.They contain many polyfunctional metal-binding sites for both cationic and anionic metal complexes.Potential metal cation-binding sites of algal cell components include carboxyl, amine, imidazole, phosphate, sulphate, sulfhydryl, hydroxyl and chemical functional groups contained in cell proteins and sugars (Crist et al., 1981).Brown algae stand out as very good biosorbent of heavy metals (Romera et al., 2006).Their cell walls contain fucoidin and alginic acid.The alginic acid offers anionic carboxylate and sulfate ions at neutral pH.Table 2a shows examples of various heavy metals adsorbed by seaweeds.

Fungi and yeasts
The majority of fungi show filamentous or hyphal growth.Cell walls of fungi present a multi-laminate architecture where up to 90% of their dry mass consists of amino or non-amino polysaccharides.The fungal cell walls can be considered as a two phase system consisting of chitin framework embedded on an amorphous polysaccharide
Saccharomyces cerevisiae can remove toxic metals, recover precious metals and clean radio-nuclides from aqueous solutions to various extents.S. cerevisiae is a product of many single cell and alcohol fermentations, it can be procured in large quantity at low cost.Saccharomyces has the ability to differentiate between different metals such as selenium, antimony and mercury based on their toxicity.This property makes S. cerevisiae useful in analytical measurements (Wang and Chen, 2006).Tables 2b, 2c show examples of heavy metals adsorbed by various fungi and yeast respectively.

Bacteria
A great deal of heterogenecity exists among different bacterial species in relation to their number of surface binding sites, binding strength for different ions and the binding mechanisms (Paknikar et al., 2003).Cell walls of bacteria and cyanobacteria are principally composed of peptidoglycans which consist of linear chains of the disaccharide N-acetylglucosamine, -1,4-Nacetylmuramic acid with peptide chains.Gram positive cell walls and surfaces have a negative charge density owing to the peptidoglycan network, a macromolecule consisting of strands of alternating gluosamine and muramic acid residues, which are often N-acetylated.Carboxylate groups at the carboxyl terminus of individual strands provide bulk of anionic character to the cell wall.The phosphodiesters of teichoic acid and the carboxyl groups of teichuronic acid contribute to the ion exchange capacity of cell walls (Paknikar et al., 2003).Table 2d shows examples of various heavy metals adsorbed by bacteria.

PRETREATMENT OF BIOMASS
Biosorbents are prepared initially by pretreating the biomass with different methods.The importance of any given group of biosorption of a certain metals by a certain   Muraleedharan et al. (1991) biomass depends on various factors such as the number of sites in the biosorbent material, the accessibility of the sites, the chemical state of the site (i.e., availability) and affinity between site and metal (i.e., binding strength) (Regine and Volesky, 2000).Biomass can be pretreated directly however, if it is larger in size (seaweeds), they are sized into fine particles or granules and they are further treated in several ways.Methods involved in pretreatment include heat treatment, detergent washing, employing acids, alkalies, enzymes, etc.Heat treatment and detergent washing expose additional metal binding groups (Gadd et al., 1988); enzymes destroy unwanted components and increase sorption efficiency (Ting and Teo, 1994).In case of alkali pretreatment, bioadsorption capacity of Mucor rouxii biomass was significantly enhanced in comparison with autoclaving, while pretreatment of biomass with acid resulted in decreased bioadsorption of heavy metals (Kapoor and Viraraghavan, 1998;Yan and Viraraghavan, 2000).This can be attributed to binding of H + ions to biomass after acid treatment resulting in reduced heavy metals adsorption.

IMMOBILIZATION OF BIOMASS
Microbial biomass consists of small particles with low density, poor mechanical strength and little rigidity.However, biosorbents are hard enough to withstand the application pressures, water retention capacity, porous and/or "transparent" to metal ion sorbate species, and have high and fast sorption uptake even after repeated regeneration cycles, also because of immobilization, the biosorbent will have better shelf-life and offers easy and convenient usage compared to free biomass, which is easily biodegradable (Volesky and May-Phillips, 1995).Hence, the biomass is to be immobilized before being subjected to biosorption.The principal techniques available for application of biosorption are based on (i) adsorption on inert supports e.g., activated carbon was used as a support for Enterobacter aerogens biofilm (Scott and Karanjakar, 1992;Wei-Bin et al., 2006); (ii) entrapment in polymeric matrix e.g., polymers used were calcium alginate (Costa and Leite, 1991;Peng and Koon, 1993), polyacrylamide (Macaskie et al., 1987;Michel et al., 1986;Takehiko, 2004;Wong and Kwok, 1992) polysulfone (Sudha and Abraham, 2003;Vijayaraghavan and Yeoung-Sang, 2007) and polyethylenimine (Wilke et al., 2006); (iii) covalent bonds to vector compounds (Holan et al., 1993;Mahan and Holocombe, 1992); (iv) cell cross-linking (Holan et al., 1993).However, the last two techniques are majorly employed for algal immobilization.Table 3 gives examples of various immobilization matrices used for the study of metal adsorption.

DESORPTION AND METAL RECOVERY
The regeneration of the biosorbent may be crucially important for keeping the process cost down and in opening the possibility of recovering the metals extracted from the liquid phase.For this purpose it is desirable to desorbs the sorbed metals and to regenerate the biosorbent material for another cycle of application.The desorption process should yield the metals in a concentrated form, restore the biosorbent close to the original state for effective reuse with undiminished metal uptake and no physical changes or damages to the biomass.Dilute mineral acids (HCl, H 2 SO 4 , HNO 3 ) have been used for the removal of metals from biomass (De Rome and Gadd, 1987;Holan et al., 1993;Puranik and Paknikar, 1997;Zhou and Kiff, 1991) and also organic acids (Citric, acetic, lactic) and complexing agents (EDTA, thiosulphate, etc) can be used for metal elution without affecting the biosorbent (Mattuschka and Straube, 1993).
The technology also has some novel applications like recovering economic heavy metals like silver, tellurium, cadmium, etc, from waste cadmium tellurium photovoltaic cells, which if disposed into landfill sites, may pose severe environmental and health hazards.It can also be used to remove heavy metals like mercury, arsenic, lead, etc sequestered in food and food products caused due to metal accumulation in plants.

CONCLUSION
Despite the fact that the technology also suffers inherent disadvantages like early saturation of biomass, little bio-

Table 1 .
Types of heavy metals and their effect on human health

Table 2a .
Heavy metal adsorbing capability of various sea weeds.

Table 2b .
Some fungal species used in metal biosorption.

Table 2c .
Various yeast species used for metal biosorption.

Table 3 .
Various immobilization matrixes used with biomass for metal adsorption the characteristics of biosorbents.It offers several advantages including cost effectiveness, high efficiency, minimization of chemical/biological sludge, and regeneration of biosorbent with possibility of metal recovery.In countries, with the rush for rapid industrial development coupled with lack of awareness about metal toxicity there is an urgent need for developing an economical and eco-friendly technology which satisfies these demands when other conventional methods fail.