B. Lusk, C. Zhou, B. Rittmann, A. Tomaswick
Keywords: membrane biofilm reactor, MBfR, platinum, precious metal, bioremediation
Summary:Conventional physical and chemical processes for recovery of platinum group metals (PGM) are costly and introduce contamination into the environment. Alternatively, microorganisms can biologically recover soluble PGMs as metallic nanoparticles: e.g., convert Pd(II) to Pd(0) (metallic palladium). Microbial reduction of Pd(II) is advantageous because it yields controllable Pd(0) nanoparticles having large specific surface area and, therefore, high catalytic activity. The Membrane Biofilm Reactor (MBfR) is a technology that enables microbial reduction of PGMs with in situ recovery of Pd(0) nanoparticles. MBfRs are modular, which means that they can be scaled to meet the recovery needs of individual sites. MBfRs also are efficient, customizable, simple to operate, and cost-effective. MBfRs make recovery of PGMs feasible and highly profitable: conservative estimates predict at least a 10-to-1 return in value for PGM nanoparticles recovered from wastewater when compared to the price of operating an MBfR. The MBfR contains a microbial community that is able to reduce and recover specific PGMs at a mining, refining, or manufacturing site. The biofilm employed in the MBfR naturally accumulates PGMs, particularly Pd and platinum (Pt), as nanoparticles that have high economic value due to their high specific surface area and superior catalytic capability. Unlike conventional methods, MBfR technology is relatively low-cost, does not require the addition of hazardous chemicals, and employs a microbial community that specifically enables the recovery of PGMs as high-value nanoparticles. The customer for this innovation is any industry that generates wastewater containing PGM and potentially also gold (Au): primarily miners of precious metals including Pt and Pd and electronic-waste-processing plants. PGMs are widely applied in industry, especially in automotive catalytic converters (∼65% of its consumption). The global demand for Pd has increased and continues to increase, as gasoline-powered vehicles must be manufactured to meet stricter emissions standards. Efficient recovery of PGMs from their major waste streams -- mining, metal refining, waste electrical and electronic equipment (WEEE), and catalytic-converter industries -- is needed for a sustainable means to recycle them in order to meet market demand, maintain an affordable market price, and reduce their environmental impacts. Precient Technologies estimates that PGM miners are losing $2 billion annually in PGMs that are lost to wastewater; almost 10% of the total market value. When recovered as nanoparticles using an MBfR, the PGMs lost in mining waste streams have a potential market value far in excess of $2 billion, a consequence of PGM nanoparticles being worth about six times more than the bulk PGMs due to their unique physical properties and the high costs associated with traditional methods for producing nanoparticles from bulk PGMs. The MBfR is a robust, low-cost, and benign technology for the recovery of PGM. Instead of requiring expensive and hazardous chemicals, the MBfR takes advantage of the ability of microorganisms to reduce the polluting form of the PGMs in waste streams to highly valuable nanoparticles which can be recovered and yield high market value.