Many wastewaters are currently being considered as energy or nutrient sources, and thus the research paradigm has shifted from simply organic and nutrient removal to resource recovery. In contrast, metal recovery from wastewaters has hardly been considered. Wastewaters have not commonly been characterized for their PGM and REE content. This can be attributed to the fact that utilities and industries primarily focus on the metals included in environmental quality standards imposed by environmental legislation when monitoring the quality of their wastewaters. The few full-scale biotechnologies applied to waste streams from mining or metal refining industries today were primarily developed to remove toxic elements in order to comply with regulations.
Preliminary data and the scientific literature indicate that a large number of high volume waste streams can contain sufficient PGMs and REEs to warrant recovery from both an economic and an environmentally beneficial viewpoint. For example, municipal wastewater has long been recognized as a source of PGMs, which comes from road dust present in storm water run-off. Deterioration of catalytic converters due to thermal and mechanical strain and to acid fume components leads to the emission of particles containing Pt and Pd at mg kg?1 concentrations. In addition, wastewaters from hospitals and dental clinics contain significant amounts of PGMs as well. For example, Pt can occur in hospital wastewater from administration of the anticancer drugs cisplatin and carboplatin. Concentrations have been reported to range between 10–100 ng L?1 and 75 ?g L?1 in the effluent of treatment plants receiving wastewater from hospitals and might occur in mg L?1 ranges in the urine of cancer patients. In addition, these effluents contain low concentrations of Gd (up to 100 ?g L?1) that is used in contrast media for magnetic resonance imaging. Based on typical numbers of wastewater produced by hospitals (500 m3 d?1) and using the reported concentrations and the current metal prices (52129 $ kg?1 for Pt and 55 $ kg?1 for Gd), low recovery values of 2 $ d?1 Pt and 0.2 $ d?1 Gd can be calculated. Although this example shows that the recovery would only be beneficial from an environmental point of view, recent studies have shown that metal recovery from wastewater treatment plant sludges could be economically favorable due to the accumulation of multiple metals over time in the sludge.
Water from geothermal resources has also been recognized as a potential source of PGM and REE metals (National Research Council, 2002). During geothermal energy production, large volumes of brine are typically extracted from depth and discharged after cooling. During the extended time the water percolates through and is heated by crustal rocks, significant amounts of metals and minerals dissolve into the geothermal fluids. While the most abundant metals in these fluids are the alkali earth metals (e.g. Na+, Mg2+), geothermal fluids are a potential source of PGM and REE metals, including Pt, Pd, Nd, and Eu. The geothermal fluids of the Salton Sea, one of most metal-rich water-bodies worldwide, contains ~225 mg L?1 Nd and ~300 mg L?1 Eu. Based on the current metal prices, these fluids have a value of 0.2 $ L?1 which is significant given the massive volumes present. Historically, the high metal content of these brines posed a liability of geothermal power plants, as it has led to scaling. Consequently, recovery of PMG and REE metals offers a means of improving the economic viability of geothermal energy. The central challenges posed by geothermal fluids are the need to withstand high temperatures (~70–150°C), the ~100–1000 times greater concentration of low-value metals over desired metals, and the presence of multiple REEs or PGMs.
Metals recovery from many of these fluids is complicated since the concentrations are dilute and a high specificity would be required to extract them from these aqueous streams. Similar to the nutrient recovery practices in wastewater treatment, research needs to focus on source collection of particular streams such as cancer patients’ urine. Examples from the chemical industry include the recovery of Pd used as a homogeneous catalyst in the manufacturing of acetaldehyde via the Wacker process, and the recovery of Pt from effluents containing hexahydroxy platinic acid from the production of auto catalyst wash coats.
Although researchers so far have mainly focused on PGM recovery from synthetic wastewaters using whole cells or products of microorganisms, a few studies have used real wastewater. Two studies reported the Pd recovery from metal refining and catalytic converter production waste streams by reductive precipitation on Desulfovibrio and Cupriavidus strains. Due to the higher concentrations of metals in these waste streams compared to hospital or municipal wastewaters, it becomes economically more interesting to recover these (metal values of 25 $ L?1 and 5 $ L?1 for the refining and converter study, respectively), even though the produced volumes are reasonably low (Janyasuthiwong, 2016). Ngwenya et al. demonstrated the uptake and reductive precipitation of Rh from a wastewater (PGM producer Anglo American Platinum) by a sulfate-reducing consortium and their extracted enzymes. Finally, Ru recovery from a plating industry wastewater was described using selective adsorption on Rhodo-pseudomonas strains. All of these streams contained high salt concentrations (40–300 g L?1) and a range of bacteriostatic metals such as Cu. Moreover, the solutions were highly acidic (pH
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