Application of Bacillus sp. and its Keratinolytic Enzyme to Recover Plastic and Silver from Waste X-Ray Films

Abstract

The global adoption of microbial enzymes in various industrial processes has surged, driven by a desire to mitigate the adverse effects of chemicals, with proteases being the most prominent. In the same direction, the present study was dedicated for developing an efficient and eco-friendly way for hydrolyzing this gelatinous layer to leave behind a clean polyester film using isolated bacterial species Bacillus sp. ND6D and by the keratinolytic enzyme produced by it. The isolate tolerated up to 0.07 M silver nitrate and 0.2–0.3% (w/v) pure silver in its growth medium, indicating its potential for direct use in bioleaching. It could hydrolyze the gelatinous layer from X-ray films over a broad temperature range, from 20–70°C, with an optimum temperature of 50°C. The concentration of X-ray films causes variation in the time required, not in the efficiency of the isolate to hydrolyze silver halide embedded-gelatinous layers. Keratinase produced by Bacillus sp. ND6D is more efficient in this process; however, the enzyme is active under broad temperature, pH, and agitation conditions. The optimum conditions were found to be 7.4 pH, 45°C, and 40 rpm, and it is highly stable, allowing for repeated use until exhausted. At optimum conditions, 100% removal of the emulsion layer from the X-ray film was achieved in 40–50 sec with 0.5 U/ml of keratinase. Ultimately, both bioleaching by Bacillus sp. ND6D and enzymatic hydrolysis of silver halides embedded in a gelatinous layer from X-ray films were successful for the recovery of silver and plastic from radiographic films.