Bacillus sp. are prominent sources of extracellular enzymes, especially proteases, which have been proposed for the eco-friendly recovery of silver from used X-ray films, as the conventional methods of incineration and chemical leaching are related to high costs, as well as environmental and health hazards.
Scientists explored low-cost biomasses as substrates for protease production by Bacillus sp. CL14. Among 18 low-cost substrates, canola meal yielded the highest protease production, which was 2.32 times that of common peptone. Canola meal is a very low-cost substrate, with a global production reaching 39 million tons.
The silver content in X-ray films is approximately 1.5%–2%, and the silver recovered from such films accounts for about 10% of global silver recycling, which brings considerable economic benefits. Researchers found that the crude enzyme produced by Bacillus and its dilutions can effectively remove the gelatin–silver emulsion from X-ray films. Specifically, the undiluted crude protease completely removed the gelatin–silver emulsion within 5 minutes and retained strong enzymatic activity even after 5 cycles. The results suggest the crude protease is an effective biocatalyst to recover silver and the PET base from waste X-ray films, representing a safer, greener and cost-effective approach when compared to conventional methods.
The work entitled “Production and applications of Bacillus sp. CL14 crude protease: approaches for biomass valorization and silver recovery” was published on Systems Microbiology and Biomanufacturing (published on January 02, 2026).
DOI:10.1007/s43393-025-00401-z