Eco Friendly Recovery of Silver from Used X-ray Films by Alkaline Protease of Bacillus Cereus Strain S8

Abstract

Silver is a precious metal used in photographic/X-ray film industry. The waste X-ray films contain 1.5-2%(w/w) black metallic silver which is recovered and reused. Around 18-20%of the world's silver needs are supplied by recycling photographic waste. Since silver is linked to gelatin in the emulsion layer, it is possible to break the same and release the silver using proteolytic enzymes. In the present work an environmentally friendly recycling system to peel away the base coat from the X-ray film by using an proteolytic enzyme was developed. Alkaline protease from Bacillus cereus strain S8(MTCC NO: 11901) was studied for silver recovery from used X-ray films. Enzyme extract was obtained by using the medium supplemented with Molasses,1%(w/v); Potassium nitrate, 0.75%(w/v); salt solution- 5%(v/v) {MgSO4.7H2O, 0.5%(w/v); KH2PO4,0.5%(w/v)}; FeSO4.7H2O, 0.01%(w/v) and CaCO3, 0.5% respectively which has optimum activity at pH 10.0 and 75°C. The silver having purity of 98.6% was recovered by smelting the obtained slurry in the presence of borax. The metal impurities (Al, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb and Sn) in the recovered silver were determined using the ICP-MS method.