Bioprocessing of Used X-ray Films for Silver Recovery by Thermoalkaliphilic Protease from Thermus aquaticus YT-1

Abstract

An environmentally benign enzymatic method for recovery of silver and polyethylene from used X-ray films was developed. This industrially scalable enzymatic process for the recovery of silver was investigated using the thermophilic alkaline protease from Thermus aquaticus YT-1. The rate of gelatin hydrolysis of X-ray film increased with the temperature till 70 ºC, suggesting enzyme stability till 70 ºC. Further, pH 10 was optimum for this process. Transparent polyethylene film and black colloidal silver was obtained after complete enzymatic hydrolysis. Estimated inactivation rate constant of 0.006 min^-1 and half-life of 115 min, of enzyme during process indicate good stability under extreme conditions. Enzyme re-usability studies showed that the enzyme (83.3U ml^-1) could be used for more than 5 cycles. Atomic absorption spectroscopy results confirmed recovery of 1.3% (w/w) of 1.5% (w/w) silver in total weight of the X-ray film used (i.e., 86.67% recovery). The method is quick, easy and economical for further bioprocessing and industrial applications.