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 isrecovered and reused. Around 18-20% of the world's silver needs are supplied by recyclingphotographic waste. Since silver is linked to gelatin in the emulsion layer, it is possible to break thesame and release the silver using proteolytic enzymes. Alkaline protease from Conidioboluscoronatus was investigated for enzymatic hydrolysis of gelatin from waste X-ray films. At the endof the treatment, gelatin layer was completely removed leaving the polyester film clean and silverwas 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. Gelatinhydrolysis was monitored by measuring increase in turbidity in the hydrolysate, which wasaccompanied by release of protein and hydroxyproline. Gelatin layer was stripped completelywithin 6 min with 1.35 U ml-1 of protease at 40°C, pH 10. Rate of gelatin hydrolysis increased withincreased in protease concentration. The enzyme could be effectively reused for four cycles ofgelatin hydrolysis. Silver in hydolysate was around 3.87% (w/w) based on total weight of sludge.Key words: Silver recovery; X-ray films; gelatin hydrolysis; alkaline protease; Conidiobolus coronatusDOI: 10.3126/kuset.v6i1.3311 Kathmandu University Journal of Science, Engineering and Technology Vol.6(1) 2010, pp60-69