Angela J. Murray

Biorecycling of Precious Metals and Rare Earth Elements

The six platinum group elements (PGEs), also known as the platinum group metals (PGMs) are platinum, palladium, rhodium, ruthenium, iridium and osmium. These, together with gold and silver, are considered to be “precious” metals due to their high demand coupled with relatively low abundance. Complex processing is required for their extraction and refining from primary ores (Bernardis et al., 2005). Relative to the other precious metals PGMs have high technological importance. Valuable for their resistance to corrosion and oxidation, high melting points, electrical conductivity and catalytic activity, these elements have wide industrial applications (Xiao & Laplante, 2004). The major uses are found in the chemical, electrical, electronic, glass and automotive industries. For example their high catalytic activity for a wide range of substrates has resulted in their use in many industrial synthetic processes, reforming reactions in the petroleum refining industry, hydrogenation and dehydrogenation reactions in the pharmaceutical industry, and both organic and inorganic oxidation reactions (Bernardis et al., 2005) to name but a few. The catalytic properties of PGM are also having a positive impact on the environment through the implementation of automotive emission control catalysts (Whiteley & Murray, 2003, Ek et al., 2004, Zereini & Wiseman, 2009). South Africa is currently the largest producer of PGMs, followed by Russia and North America. In 2008 South Africa supplied 76% of the worlds platinum, 33% of its palladium and 82% of its rhodium. Russia supplied more palladium than South Africa (51%) but its contribution of the other two metals was much smaller. With the exception of 2006 demand for PGMs has constantly exceeded supply, resulting in large price increases within the industry. The disparity in 2006 is thought to have been caused by significantly lower platinum usage in jewellery compared with previous years (in fact the lowest usage since 1993). The last 75 years have seen the overall consumption and uses of platinum expand dramatically. Demand (and hence also its price) and uses are impossible to predict far into the future, but the resources and potential supply of platinum and palladium can be calculated with some degree of confidence. Cawthorn’s calculations (Cawthorn, 1999) indicate about 204 and 116 million ounces of proven and probable reserves of platinum and palladium, respectively, and 939 and 711 million ounces of inferred resources, down to a depth of 2 km. These figures represent about 75 and 50% of the world’s platinum and palladium resources, respectively. These figures for proven and probable reserves in the Bushveld Complex are sufficient for the next 40 years at current rate of production.

Biorecovery of Platinum Group Metals from Secondary Sources

Since 1998 demand for the platinum group metals (PGM) has exceeded supply resulting in large price increases. Undersupply, combined with rising costs prompts environmentally friendly recycling technologies. Leachates containing PGM were produced from secondary waste sources using microwave leaching technology with the aim of recovering precious metals using bacterial biomass. Previous studies showed that metallised biomass exhibits catalytic activity; hence metal is not only recovered but can be converted into a valuable product. Cells of Escherichia coli MC4100 that had been pre-metallised with Pt were more effective at reducing PGM from the leachates. The solid recovered from the leachate onto the bacteria was characterised using X-ray Powder Diffraction (XRD) and Energy Dispersive X-ray Microanalysis (EDX). Metallised biomass was tested for catalytic activity (reduction of Cr(VI) to Cr(III)) to compare the ‘quality’ of polymetallic bacterial-based catalysts versus counterparts made from single and mixed metal model solutions.

Biosynthesis of zinc sulfide quantum dots using waste off-gas from a metal bioremediation process

Dissimilatory reduction of sulfate, mediated by various species of sulfate-reducing bacteria (SRB) and a few characterized species of archaea, can be used to remediate acid mine drainage (AMD). Hydrogen sulfide (H2S/HS−) generated by SRB removes toxic metals from AMD as sulfide biominerals. For this, SRB are usually housed in separate reactor vessels to those where metal sulfides are generated; H2S is delivered to AMD-containing vessels in solution or as a gas, allowing controlled separation of metal precipitation and facilitating enhanced process control. Industries such as optoelectronics use quantum dots (QDs) in various applications, e.g. as light emitting diodes and in solar photovoltaics. QDs are nanocrystals with semiconductor bands that allow them to absorb light and re-emit it at specific wavelength couples, shifting electrons to a higher energy and then emitting light during the relaxation phase. Traditional QD production is costly and/or complex. We report the use of waste H2S gas from an AMD remediation process to synthesize zinc sulfide QDs which are indistinguishable from chemically prepared counterparts with respect to their physical and optical properties, and highlight the potential for a empirical process to convert a gaseous “waste” into a high value product.

6.55 – Today’s Wastes, Tomorrow’s Materials for Environmental Protection

Over the past 30 years, the literature has burgeoned with bioremediation approaches to heavy metal removal from wastes. The price of base and precious metals has also increased. With the resurgence of nuclear energy, uranium has become a strategic resource. Other noncarbon energy technologies are driven by the need to reduce CO2 emissions. The ‘new biohydrometallurgy’ we describe unites these drivers by the concept of conversion of wastes into new materials for environmental applications. The new materials, fashioned, bottom-up, into nanomaterials under biocontrol, can be termed ‘functional bionanomaterials’. This new discipline, encompassing waste treatment along with nanocatalysis or other applications, can be summarized as ‘environmental bionanotechnology’. Several case histories illustrate the scope and potential of this concept. The research highlights biogenic nuclear waste remediation, Pd and Pt bionanocatalysts for environment and energy, Au oxidation bionanocatalysts from jewelery waste, optically active bioproducts from Se oxyanions, and nanoscale magnets biofabricated from Fe (III) wastes.

Hydroxyapatite Biosynthesis by a Serratia sp. and Application of Nanoscale Bio-HA in the Recovery of Strontium and Europium

ABSTRACT A Serratia sp. expresses a high level of acid phosphatase when grown continuously under carbon limitation. In the presence of CaCl2, biosynthesis of nanocrystalline hydroxyapatite (bio-HA) was achieved by utilizing phosphate released via enzymatic cleavage of an applied substrate (glycerol 2-phosphate: G2P). Hydroxyapatite crystals were identified by energy dispersive X-ray emission (EDX) and selected area diffraction (SAD). X-ray powder diffraction (XRD) analysis gave a mean crystallite size of ∼21–32 nm, with the smallest crystals (21–24 nm) obtained using 1 mM Ca2+ and 1 mM G2P. The uptake of Eu3+ and Sr2+ by bio-HA made by continuously pregrown cells (0.42 mg/mg and 0.043 mg/mg respectively) was ∼20% greater for Sr2+ than was previously reported for bio-HA material of size ∼40 nm made by batch-pregrown cells, while the corresponding uptake of Eu3+ was increased by > 1.8-fold. This was attributed to the localization of Eu (III) at grain boundaries by reference to previous work and highlights the potential of bio-HA as a sequestration agent for recovery of rare earth elements and trivalent actinides.

Continuous biocatalytic recovery of neodymium and europium

Batch-grown cells and a continuously-grown biofilm of a Serratia sp. were utilized to recover the rare earth elements (REEs) lanthanum and neodymium from solution. Selectivity was obtained for La(III) over Th(IV) using columns of polyacrylamide gel-immobilized cells challenged at a rapid flow rate, exploiting the different solution chemistries and behaviors of REEs(III) and Th(IV). Biofilm-grown cells had a ten-fold higher activity of mediating phosphatase, which promotes metal deposition as the corresponding metal phosphate, reflected as a correspondingly enhanced level of removal of Nd(III) (as NdPO4) in flow-through columns utilizing the biofilm on reticulated foam. The biofilms retained activity in the removal of Nd(III) for >1 year, losing activity exponentially with a half life of 3 months. The flow rate giving 50% removal (FA1/2) of Nd(III) by 3 month old biofilms at pH 5.5 was 272 and 275 mL h−1 using two independent biofilm preparations, equivalent to a FA1/2 of 34 column volumes per h for fresh biofilms. The removal of Nd(III) was sustained at pH values down to 3.5 with approx. 20% of the column activity lost upon return to pH 5.5. A similar result occurred in the presence of the common REE leaching agent ammonium sulfate (100 mM), but this did not affect the ability of Serratia sp. to recover REEs. With a view to the potential for future biomanufacturing of Nd(III)-catalysts, the deposited material was identified as NdPO4 by X-ray powder diffraction with a nanoparticle size of 14.5 nm, irrespective of the biofilm age.

Today’s Wastes, Tomorrow’s Materials for Environmental Protection

Over the past 30 years the literature has burgeoned with bioremediation approaches to heavy metal removal from wastes. The price of base and precious metals has dramatically increased. With the resurgence of nuclear energy uranium has become a strategic resource. Other ‘non-carbon energy’ technologies are driven by the need to reduce CO2 emissions. The ‘New Biohydrometallurgy’ we describe unites these drivers by the concept of conversion of wastes into new materials for environmental applications. The new materials, fashioned, bottom-up, into nanomaterials under biocontrol, can be termed ‘Functional Bionanomaterials’. This new discipline, encompassing waste treatment along with nanocatalysis or other applications, can be summarized as ‘Environmental Bionanotechnology’. Several case histories illustrate the scope and potential of this concept.

Biorefining of platinum group metals from model waste solutions into catalytically active bimetallic nanoparticles

Bacteria can fabricate platinum group metal (PGM) catalysts cheaply, a key consideration of industrial processes and waste decontaminations. Biorecovery of PGMs from wastes is promising but PGM leachates made from metallic scraps are acidic. A two‐step biosynthesis ‘pre‐seeds’ metallic deposits onto bacterial cells benignly; chemical reduction of subsequent metal from acidic solution via the seeds makes bioscaffolded nanoparticles (NPs). Cells of Escherichia coli were seeded using Pd(II) or Pt(IV) and exposed to a mixed Pd(II)/Pt(IV) model solution under H2 to make bimetallic catalyst. Its catalytic activity was assessed in the reduction of Cr(VI), with 2 wt% or 5 wt% preloading of Pd giving the best catalytic activity, while 1 wt% seeds gave a poorer catalyst. Use of Pt seeds gave less effective catalyst in the final bimetallic catalyst, attributed to fewer and larger initial seeds as shown by electron microscopy, which also showed a different pattern of Pd and Pt deposition. Bimetallic catalyst (using cells preloaded with 2 wt% Pd) was used in the hydrogenation of soybean oil which was enhanced by ~fourfold using the bimetallic catalyst made from a model waste solution as compared to 2 wt% Pd preloaded cells alone, with a similar selectivity to cis C18:1 product as found using a Pd‐Al2O3 commercial catalyst.