KINETICS AND MECHANISM OF ENZYMATIC-HYDROLYSIS OF GELATIN LAYERS OF X-RAY-FILM AND RELEASE OF SILVER PARTICLES

Used X-ray film contains a large number of silver particles in its gel atin layers. With the aim of developing an efficient enzymatic process for the recovery of silver and polyester film from used X-ray film, h ydrolysis experiments were performed using a stirred-tank reactor in b atch operation. On the bottom of the tank a sheet of X-ray film was fi xed so that only one of the film surfaces would be exposed to a soluti on containing the alkaline protease B21-2 from alkalophilic Bacillus s p. The time courses of gelatin hydrolysis, measured by using unexposed film which was prepared by developing X-ray film without exposure to light and which did not contain any silver, showed that at any stirrin g speed there existed induction periods in which hydrolysis was very s low. When the stirring speed was higher, the induction period and the time required to complete the hydrolysis were shortened due to an incr ease in the transfer rate of the enzyme through the liquid film. The r ate of gelatin hydrolysis of unexposed film increased with the enzyme concentration. The time courses measured using exposed film, which was prepared by developing X-ray film after it had been exposed homogeneo usly to light and which contained fine silver particles, showed that t he induction periods for the release of silver particles were much lon ger than those for gelatin hydrolysis. The presence of silver particle s did not significantly affect the gelatin hydrolysis. The above exper imental results on gelatin hydrolysis were well explained by a propose d model which took into consideration a number of physical processes, such as diffusion of the enzyme and hydrolysis products through the li quid film on the surface of the gelatin layer, in addition to the chem ical processes.