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Development of bioleaching-based sustainable processes to recover silver from photographic film and film development
Secondary Supervisor(s): Dr Fatemah Pourhossein Alamdari
University of Registration: Coventry University
BBSRC Research Themes: Renewable Resources and Clean Growth (Industrial Biotechnology)
Project Outline
The global demand for metals is growing, while primary resources are being depleted. The need for the development of sustainable approaches to recover metals from waste streams, while protecting the environment, has been recognised by both DEFRA and the UN. Silver is one of these metals, with major economic implication.
Despite its benefits, silver is a regulated element because free silver ions are highly toxic to aquatic organisms. While silver is generally considered non-toxic to humans due to its inertness, high concentrations of silver salts can cause organ damage and skin discoloration. It is also classified as a secondary contaminant in drinking water. As a result, photographic waste containing significant amounts of silver can be hazardous if improperly disposed of (Adie et al., 2022). Silver is designated as a "priority pollutant" under the Clean Water Act due to its potential environmental impact. Additionally, under the Resource Conservation and Recovery Act (RCRA), silver is considered an indicator of hazardous solid waste when leachate contains concentrations exceeding threshold limit values. These limits are set at 0.1 mg/m³ for metallic silver and 0.01 mg/m³ for soluble silver compounds, based on the permissible exposure limit (PEL)(Drake and Hazelwood, 2005).
Despite the rise of digital photography, film waste is still generated from photography, medical imaging, and industrial processes, and remains an environmental concern. However, its sustainable recycling can minimise environmental impact, and would provide an alternative source for silver. Traditional recycling methods are unsustainable, as pyrometallurgy emits hazardous gases and particles, and hydrometallurgy produces heavy metal-contaminated wastewater, often exceeding permissible exposure limits for silver. Contrarily to these methods, Bio-hydrometallurgy, a novel approach in waste treatment, uses microorganisms for metal extraction, with minimal environmental impact and low resource requirements. The aim of this project is to develop sustainable bioleaching-based processes for silver recovery from films and film processing.
This proposal will utilise the ability of microorganisms to solubilise metals into technological solutions to recover silver, a metal of fundamental importance in multiple technologies. As such, the connection between microbiology and bio-engineering will translate fundamental science into a higher technology readiness level. Ultimately, this proposal is targeting a closed-loop system with environmental, economic and social impact, in agreement with the UN Sustainability Development Goals 7 to 13. As only a few studies have investigated bioleaching of silver, this is a novel application of bioleaching to the waste industry and an addition to the Bioleaching Research Group’s successful track record in developing bioleaching processes for metal recovery from waste, alongside their translation into industrial applications. This work introduces an integrated, sustainable solution for silver recovery from solid waste, utilizing a semi-continuous bioreactor part of a closed-loop system without fumes and below the permissible limit exposure. In this process, silver is bio-chemically dissolved and efficiently recovered through biosorption. The development of mathematical modelling for bio-processes represents a significant innovation that provides a deeper understanding of mechanism of microbial-mediated silver extraction, allowing for the efficient optimization and scaling up of the process, while providing closed-loop recycling systems. The approach emphasizes environmental sustainability and resource efficiency, making it a promising method for urban waste management and circular economy practices.
The innovative aspects of this research plan include the use of bioleaching and advanced microbial engineering, customized solutions for diverse waste streams, integration of multi-step recovery processes, mathematical modelling for optimization, the establishment of closed-loop recycling systems, and interdisciplinary collaboration. This project highlights our commitment to innovation and sustainability, and reinforces our leadership in the field of environmental biotechnology. The research excellence of adding mathematical modelling to the emerging field of bioleaching, will have industrial, economic and environmental Impacts. The project will benefit from existing collaboration with the Bioleaching Research Group within CU, including John Graves from CMM for the electrochemistry, Matthew England from CSM for the mathematical modelling and Craig Ashley from FAH for providing photographic material.
Project objectives
1) Development of a two-steps silver bioleaching/biosorption process for the treatment of waste from films.
2) Investing biosorption and electro-chemical processes to recover solubilised silver from bioleaching solutions.
3) Development of mathematical models of the bioleaching/biosorption processes.
4) Combining the biological methods to establish closed-loop recycling systems that promote circular economy principles for sustainable resource management.
References
Adie, G.U., Shoneye, H.O., Iniaghe, P.O., 2022. Optimizing Silver Extraction Potential from Waste X-Ray Films Using Acid and Alkaline Leaching Agents. Annals of Science and Technology 7, 21–28. https://doi.org/10.2478/ast-2022-0006.
Drake, P.L., Hazelwood, K.J., 2005. Exposure-related health effects of silver and silver compounds: A review. Annals of Occupational Hygiene. https://doi.org/10.1093/annhyg/mei019.
