Recovery of silver from used X-ray film using alkaline protease from <i>Bacillus subtilis</i> sub sp. <i>subtilis</i>

Abstract

Silver is an important industrial metal used in several areas such as photographic and x-ray films, jewelries, silver wares and electronic objects. Silver is used for photographic film/x-ray film because of its matchless quality as a light-sensitive material for making a photographic image. Silver is not destroyed in the photographic process and it can be reused and recovered. Results have proven that, bacterial alkaline protease can be used to extract silver in 30 min, but its activity decreases with increasing incubation period. Gelatin hydrolysis was monitored by measuring the increase in turbidity of the hydrolysate, which was accompanied by release of protein and hydroxyproline. The protease of the culture filtrate used was 97 U/ml after 30 min, but it decreased to 86.5U/ml after 60 min. After 90 min, it reached 85 U/ml. A great inactivation was recorded after 120 min; it got to 39.5 and 36.5% (U/ml) after 180 min. Gelatin layer was stripped completely within 30 min with 97 U ml-1 protease at 50°C and pH 8. At the end of the treatment, gelatin layer was completely removed and the polyester film was left clean. In addition, silver was recovered in the hydrolysate, both of which can be reused.

Keywords: Silver recovery, x-ray films, gelatin, alkaline protease, Bacillus subtilis