Nostoc calcicola Immobilized in Silica-coated Calcium Alginate and Silica Gel for Applications in Heavy Metal Biosorption

The present study reports the preparation and characterization of silica-based immobilization matrices for the purpose of metal accumulation using immobilized cyanobacterium Nostoc calcicola. Silica gel was prepared using aqueous sodium silicate and colloidal silica. Calcium alginate (CAG) beads were coated with silica using sodium silicate solutions. Microscopy observations and TTC tests confirmed that the immobilized cells were intact and viable. Ultrastructural studies with electron microscopy revealed a membrane thickness of approximately 10 mu m around the CAG and the silica gel to be of mesoporous nature. BET surface area of silica gel-immobilized N. calcicola was 160 m(2) g(-1). The porous volume and average pore diameter were 0.40 cm(3) g(-1) and ca. 100 angstrom, respectively, as calculated using the BJH model. Studies on silica-coated calcium alginate immobilized cells showed that these were superior to the uncoated CAG beads in terms of mechanical strength and metal accumulation. The silica matrices were found to be stable for repeated cycles of metal removal and with commonly used eluants for desorption processes. These matrices have potential applications in immobilization of industrially important biocatalysts.