The mechanism involved is based on the formation of covalent bonds between the inorganic support (silica) and cells in the presence of crosslinking agents. A said joint is needed for the modification of the support surface. The reaction requires the introduction of reactive organic groups on the silica surface for the attachment of cells to the support. As coupling agent generally used aminopropyl triethoxy silane; this organic functional group condenses with hydroxyl groups of the silica and the group as a result becomes available for covalent bond formation on the surface. Covalent bonds can also be established by treating the silica surface with glutaraldehyde or isocyanate.

The advantage of this method is that the support can be generated without the limitations on physical and chemical conditions imposed by the biocatalyst, which can be optimized by the characteristics of mechanical stability, porosity, strength of the support, etc.

When you want to form covalent bonds between the substrate and cells, the problem is how to promote adhesion of cells to relatively large surface without damaging its stability and resistance to washing. The support may have pores of greater diameter than the cell to allow the latter to penetrate the internal surfaces. Porous supports are used which are embedded by immersion in cell suspensions [60].

Another important matrix being used for immobilization for metal removal is silica. Silica- immobilized preparations offer advantage in terms of reusability and stability. The silica immobilized product is mechanically strong and exhibits excellent flow characteristics [68]. A silica immobilized algal preparation AlgaSORBR (Bio-Recovery Systems, Inc., Las Cruces, NM 80003, USA) which is being used commercially retains approximately 90% of the original metal uptake efficiency even after prolonged use (> 18 months) [57].