Immobilization of cells as biocatalysts is almost as common as enzyme immobilization. Immobilization is the restrict ion of cell mobility within a defined space. Immobilized cell cultures have the following potential advantages over suspension cultures.

• Immobilization provides high cell concentrations.

• Immobilization provides cell reuse and eliminates the costly processes of cell recovery and cell recycle.

• Immobilization eliminates cell washout problems at high dilution rates.

• The combination of high cell concentrations and high flow rates (no washout restrictions) allows high volumetric productivities.

• Immobilization may also provide favorable microenvironmental conditions (i. e., cell­cell contact, nutrient-product gradients, pH gradients) for cells, resulting in better performance of the biocatalyst» (e. g., higher product yields and rates).

• In some cases, immobilization improves genetic stability.

• For some cells, protection against shear damage is important.

The major limitation on immobilization is that the product of interest should be excreted by the cells. A further complication is that immobilization often leads to systems for which diffusional limitations are important. In such cases the control of microenvironmental conditions is difficult, owing to the resulting heterogeneity in the system. With living cells, growth and gas evolution present significant problems in some systems and can lead to significant mechanical disruption of the immobilizing matrix.

The primary advantage of immobilized cells over immobilized enzymes is that immobilized cells can perform multistep, cofactor-requiring, biosynthetic reactions that are not practical using purified enzyme preparations.

Adsorption of cells on inert support surfaces has been widely used for cell immobilization. The major advantage of immobilization by adsorption is direct contact between nutrient and support materials. High cell loadings can be obtained using microporous support materials. However, porous support materials may cause intraparticle pore diffusion limitations at high cell densities, as is also the case with polymer-entrapped cell systems. Also, the control of microenvironmental conditions is a problem with porous support materials. A ratio of pore to cell diameter of 4 to 5 is recommended for the immobilization of cells onto the inner surface of porous support particles. At small pore sizes. Accessibility of the nutrient into inner surfaces of pores may be the limiting factor, whereas at large pore sizes the specific surface area may be the limiting factor. Therefore, there may be an optimal pore size, resulting in the maximum rate of bioconversion.

Adsorption capacity and strength of binding are the two major factors that affect the selection of a suitable support material. Adsorption capacity varies between 2 mg/g (porous silica) and 250 mg/g (wood chips). Porous glass carriers provide adsorption capacities (l08 to 109 cells/g) that are less than or comparable to those of gel-entrapped cell concentrations (109 to 1011 cells/mL). The binding forces between the cell and support surfaces may vary, depending on the surface properties of the support material and the type of cells. Electrostatic forces are dominant when positively charged support surfaces (ion exchange resins, gelatin) are used. Cells also adhere on negatively charged surfaces by covalent binding or H bonding. The adsorption of cells on neutral polymer support surfaces may be mediated by chemical bonding, such as covalent bonding, H bonds, or van der Waals forces. Some specific chelating agents may be used to develop stronger cell-surface interact ions. Among the support materials used for cell adsorption are porous glass, porous silica, alumina, ceramics, gelatin, chitosan, activated carbon, wood chips, polypropylene ion — exchange resins (DEAE-Sephadex, CMC-), and Sepharose [54].

Various reactor configurations can be used for immobilized cell systems. Since the support matrices used for cell immobilization are often mechanically fragile, bioreactors with low hydrodynamic shear, such as packed-column, fluidized-bed, or airlift reactors, are preferred. Mechanically agitated fermenters can be used for some immobilized-cell systems if the support matrix is strong and durable. Any of these reactors can usually be operated in a perfusion mode by passing nutrient solution through a column of immobilized cells [54].

Since the design of reactors for the removal of heavy metals from liquid effluent must consider optimum contact between these and the biomass, it has been considered the use of different types of support for the immobilization of the biomass with the aim of achieving greater efficiency in removing heavy metals. This achieves prevent biosorbent is removed from the reactor in the output current and at the same time is obtained a greater mechanical stability thereby reducing the shear stresses that could damage the structure of the microorganism which affects removal efficiency heavy metals [53].

Living biomass immobilized, must first take the form of biofilm on supports prepared from a variety of inert materials. One of the materials that have been studied as biomass support is activated carbon by porosity and high surface area, besides being an abundant product is obtained as a byproduct of the production of oil from coconut, olive and processing sugarcane [53]. Other materials have been used as biomass support such as silica, polyacrylamide gel and polyurethane include agar, cellulose, alginates, polyacrylamides, the silica gel, sand, textile fibers, calcium alginate, polysulfone, glutaraldehyde and other organic compounds, and have been used for removing heavy metals [55,56].

There are other materials that could be used for biomass carriers; such as the natural zeolites are known important industrial applications due to its high affinity for water and that the cavities only allow passage of molecules of a certain size. Have been used as additives in animal feed, such as soil improvers in agriculture due to increased nitrogen retention and soil moisture, and as catalysts in industrial processes of refining, petrochemicals and fine chemicals [57].