Kinetics and mechanism of enzymatic hydrolysis of gelatin layers of X-ray film and release of silver particles

Abstract

Abstract Used X-ray film contains a large number of silver particles in its gelatin layers. With the aim of developing an efficient enzymatic process for the recovery of silver and polyester film from used X-ray film, hydrolysis experiments were performed using a stirred-tank reactor in batch operation. On the bottom of the tank a sheet of X-ray film was fixed so that only one of the film surfaces would be exposed to a solution containing the alkaline protease B21-2 from alkalophilic Bacillus sp. 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 stirring speed there existed induction periods in which hydrolysis was very slow. When the stirring speed was higher, the induction period and the time required to complete the hydrolysis were shortened due to an increase in the transfer rate of the enzyme through the liquid film. The rate 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 homogeneously to light and which contained fine silver particles, showed that the induction periods for the release of silver particles were much longer than those for gelatin hydrolysis. The presence of silver particles did not significantly affect the gelatin hydrolysis. The above experimental results on gelatin hydrolysis were well explained by a proposed model which took into consideration a number of physical processes, such as diffusion of the enzyme and hydrolysis products through the liquid film on the surface of the gelatin layer, in addition to the chemical processes.