Cadmium biosorption by cells of Spirulina platensis TISTR 8217 immobilized in alginate and silica gel

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Abstract

The biosorption of cadmium by immobilized Spirulina platensis on alginate gel and silica gel was studied. The maximum biosorption capacities for alginate immobilized cells and silica immobilized cells were 70.92 and 36.63 mg Cd/g biomass, respectively. Temperature did not have an influence on metal sorption, whereas an initial pH solution did. Sorption occurred in a wide pH range (pH 3–8). The highest adsorption of alginate immobilized cells was at pH 6, while silica immobilized cell adsorption was not affected at pH between 4 and 7. The immobilized cells were reused in consecutive adsorption–desorption. The results showed that immobilized cells could be repeatedly used in the sorption process up to five times.

Introduction

The presence of heavy metals in the aquatic environment is a source of great environmental concern. Conventional techniques, such as chemical precipitation, ion exchange, activated carbon adsorption and membrane separation processes have limitations for the removal of heavy metals from wastewater. They become inefficient and expensive especially when the heavy metal concentration is less than 100 ppm Leusch et al., 1995, Yan and Viraraghavan, 2001. The ability of microorganisms to accumulate metal ions from aqueous solutions has been widely reported. Ting and Sun, 2000, Rangsayatorn et al., 2002, Tangaromsuk et al., 2002. It is a potential alternative to conventional processes for the removal of metals. Some researchers have found that stripping metals out of the biosorbents was difficult (Macaskie et al., 1997). However, Gaedea-Torresday et al. (2002) and Manju et al. (2002) have shown that desorption and regeneration of biosorbent can be easily done. The low strength and small particle size make it difficult to use in column application as well (Yan and Viraraghavan, 2001). Cell immobilization, which is applied to various biotechnological processes, is one approach that can avoid the biomass–liquid separation requirement. It can prevent the independent movement of cells during the aqueous phase of the system. Cell immobilization is a general term that describes many different forms of cell attachment or entrapment (Lopez et al., 1997). Various techniques are used for cell or biomass immobilization, such as flocculation, adsorption on surfaces, covalent bonding to carriers, cross-linking of cells, encapsulation in a polymer gel, and entrapment in a polymeric matrix. The physical entrapment of organisms inside a polymeric matrix, generally described as a gel, is one of the most widely used techniques for immobilization because a polymeric matrices can be made as beads with optimize mechanical strength, rigidity, and porosity characteristics Lu and Wilkins, 1995, Annadurai et al., 2000. Many diverse gel matrices have been proposed as possible carriers. Either natural biopolymers (polysaccharides, alginate, carrageenan, agar) or synthetic polymer (polyacrylates, polyurethanes, polyethers) can be used as gel-forming agents (Lozinsky and Plieva, 1998).

Alginate gel and silica gel are two commonly used entrapment matrices. Alginate is a heteropolymer of l-guluronic acid and d-monouronic acid. It is extracted from several species of marine algae and, after processing, is available as water-soluble sodium salts (Tampion, 1987). When the monovalent counter ion of sodium is replaced by divalent calcium, ionic cross-linking among carboxylic acid groups occurs and gives a gelatinous substance, calcium alginate. As a result of cross-linking, a polymeric network of polysaccharide molecules is formed in which about 60% of water is entrapped in gel (Phillips and Poon, 1998). Another inorganic synthetic polymeric matrix often used to entrap cells is porous silica gel. The use of silica gel cell for entrapment is called the sol–gel technique (Weller, 2000). The silica gel is generated by decreasing the pH of alkali silicate solution to less than 10. The solubility of silica is then reduced to form the gel. As the silica begins to gel, cells in silica are trapped in a porous gel, which is a three-dimensional SiO2 network (Chaiko et al., 1998).

The kinetics of metal uptake, assumed to be a physical adsorption to the cell surface, is very rapid and occurs shortly after the biosorbents contact the metal ions in solution. Kinetic studies of sorption are significant since the data can be used: (1) for determining the time required to reach equilibrium, and (2) to evaluate the maximum adsorption capacity (Singh et al., 2001). The metal uptake (q), for the construction of sorption isotherms, is determined as follows:q=(Ci−Cf)V/Mwhere Ci and Cf are the initial and final metal concentrations (mg/l), respectively; V is the volume of sample solution (l); and M is the dry weight of added biomass (g). Traditionally, the Langmuir model has been used to evaluate maximum metal uptake. It is based on the basic assumptions that: (1) metal ions are chemically adsorbed at a fixed number of sites, (2) each site can hold one sorbed ion, (3) all sites are energetically equivalent, and (4) there is no interaction between ions adsorbed on neighboring sites (Inthorn et al., 1996). The Langmuir equation has the following formq=qmaxbCf/(1+bCf)where b is a constant related to the energy of adsorption/desorption and qmax is the maximum uptake.

In this study, immobilized cyanobacterial cells, Spirulina platensis, were entrapped in two kinds of gel, alginate gel and silica gel. The cadmium adsorption capacity and reusability of the biosorbents were investigated.

Section snippets

Culture of cynobacteria

The cyanobacteria, S. platensis TISTR 8217, were obtained from the Thailand Institute of Scientific and Technology Research (TISTR), Bangkok, Thailand. S. platensis was inoculated onto Zarrouk medium (Becker and Venkataraman, 1984) and incubated under the continuous illumination of a cool white fluorescent lamp. Cell growth was determined by measuring the optical density of S. platensis at 560 nm. After 12 days, the culture was in the linear growth phase. The cells were collected and washed

Biosorption rates

Fig. 1 shows the changes in cadmium adsorbed onto beads with time. Rapid biosorption rates were observed at the beginning (first 5 min), which then reached the equilibrium stage at about 45 min for both alginate and silica gels. At the equilibrium stage, the cadmium adsorption was higher than 95% for both immobilized cells on alginate and on silica gels. For the negative control, which was the gel bead alone with no biomass entrapped in it, cadmium was adsorbed by 5.75% and 7.88% for the silica

Conclusion

The removal of heavy metal ions from aquatic systems is carried out using classical adsorption techniques. S. platensis immobilized on alginate and silica gels were applied to remove cadmium from the solution. The results of this study showed a high cadmium sorption capacity of immobilized cyanobacterial cells. They could be repeatedly used in multiple adsorption–desorption cycles. Metal sorption decreased after the first desorption, but sorption capacity was still high. Temperature did not

Acknowledgements

This work was supported by the Royal Golden Jubilee PhD program of the Thailand Research Fund (grant no. 00093/2541) and the Development and Promotion of Science and Technology Talents Project (DPST).

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