Recovery of Silver from Waste X-Ray Film by Alkaline Protease from <i>Conidiobolus coronatus</i>

Abstract

The waste X-ray/ photographic 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. Alkaline protease from Conidiobolus
coronatus was investigated for enzymatic hydrolysis of gelatin from waste X-ray films. At the end
of the treatment, gelatin layer was completely removed leaving the polyester film clean and silver
was recovered in the hydrolysate, both of which can be reused. Various parameters such as pH,
temperature, enzyme concentration, time etc on silver removal from the film were studied. Gelatin
hydrolysis was monitored by measuring increase in turbidity in the hydrolysate, which was
accompanied by release of protein and hydroxyproline. Gelatin layer was stripped completely
within 6 min with 1.35 U ml^-1 of protease at 40°C, pH 10. Rate of gelatin hydrolysis increased with
increased in protease concentration. The enzyme could be effectively reused for four cycles of
gelatin hydrolysis. Silver in hydolysate was around 3.87% (w/w) based on total weight of sludge.