Peipei Shi et al. Metallurgy andMetalurgia materials e materiais Geochemical assessment of platinum group metals http://dx.doi.org/10.1590/0370-44672019730038 for phytomining Peipei Shi1,3 Abstract https://orcid.org/0000-0003-3399-0154 Marcello Veiga1,4 Phytomining is suggested as a technology to obtain platinum group metals https://orcid.org/0000-0001-9261-738X (PGMs) nanoparticles from plants grown on the mineralized soils or tailings. Sam- Christopher Anderson2,5 ples from North American Palladium (Canada) and gossans from Broken Hill (BH) https://orcid.org/0000-0003-0935-1475 (Australia) were studied to assess the possibility of using these PGM-rich samples as substrates for phytomining. The bioavailability of PGMs was indirectly assessed using 1University of British Columbia, Mining Engineering, geochemical procedures. The selective extractions showed that the highest available Vancouver - British Columbia - Canada. concentration of Pd is 5.38 ppm in BH gossan 1. The extraction of PGMs by ammo- nium acetate, fulvic acid or citrate-dithionite indicates natural availability to plants. 2Massey University, School of Agriculture and The BH gossan 1 was the best of the five studied samples for phytomining of Pd due to Environment, Palmerston North, available Pd concentration (> 2 mg/kg), low Electric Conductivity (< 2dS/m), high CEC Manawatu-Wanganui - New Zealand. (Cation Exchange Capacity) (38.8 meq/100g), and proper pH (6.5). Cu-tolerant plant species should be chosen to grow on BH gossan 1. A criterium for choosing substrates E-mails: [email protected], [email protected], for phytomining of Pd was developed comprising various classical soil parameters plus [email protected] selective extraction procedures. Keywords: phytomining, platinum group metals, bioavailability, plant growth parameters. REM, Int. Eng. J., Ouro Preto, 73(1), 85-91, jan. mar. | 2020 85 Geochemical assessment of platinum group metals for phytomining 1. Introduction The concept of phytomining is an 2015). Parker et al. (2014) reported the first tion is bound to different minerals that are adaptation of the use of an old concept of use of living plants to recover palladium more difficult to be incorporated by plants using plants for prospecting metals (Chaney and the production of catalytically active but this can change gradually by chemical et al., 2018). The phytomining requires palladium nanoparticles with excellent weathering and biological process (Dairy selection of hyperaccumulating plants, as catalytic activity across a range of coupling Soils and Fertiliser Manual, 2013). The well understanding which metals species reactions that produced higher yields than primary determinant of metal uptake by a are bioavailable in the substrate and how commercial Pd catalyst. This process can plant is the bioavailability of the metal in toxic for the plants they are (Kramer, 2019). reduce the number of production steps the soil-plant system, i.e. how easily plants Plant species and methods for phytomining compared to traditional catalyst production can access the metal in a form they re- for platinum group metals (PGMs) involves methods. In automotive industries, for ex- quire. The type and nature of the substrate growing plants on PGM-rich substrates ample, platinum (Pt), along with palladium constituents, pH, organic matter, cation that are capable of selectively incorporating (Pd), and rhodium (Rh) are coated onto exchange capacity and competing ions have these metals into their cellular structures a substrate housed in the exhaust system all been shown to influence PGM bioavail- (Sheoran et al., 2009). Theoretically, the and act as catalysts to convert toxic vehicle ability (Ko et al., 2008; Wilson-Corral et plants can then be harvested and subjected emissions, such as carbon monoxide (CO), al., 2012). As such, the primary interest of to controlled pyrolysis in order to yield a hydrocarbons (HC) and oxides of nitrogen this study was to use geochemical methods, material with stabilized PGM nanoparticles (NOx), to less harmful substances (Saguru such as sequential-selective extractions, to that can later be used in catalytic reactions et al., 2018). In the soil, a small portion of assess the bioavailability of PGMs from (Siddiq, & Husen, 2016, Dodson et al., metals is available to plants as a large por- high grade PGM samples. 2. Materials and methods Samples from two sites were col- (NAP), Canada and from Broken Hill lected: North American Palladium (BH), Australia. 2.1 North American Palladium (NAP) North American Palladium is a plati- of platinum-group minerals occur either are: palladium tellurides > palladium anti- num group metals producer that has oper- interstitially to sulfides in association monides > palladium sulfides > sperrylite ated Lac des Iles (LDI) Mine in northern with gangue minerals such as plagioclase, (platinum arsenide) > gold-silver alloys. Ontario, Canada since 1993. The mine lies amphibole, chlorite, orthopyroxene, and PGM grades show varying degrees of cor- in the southern end of the Lac des Iles, 106 talc, or at sulfide-silicate boundaries. relation with nickel and copper concentra- km northwest of Thunder Bay, Ontario, The relative abundance of PGM-bearing tions (Yu et al., 2010). Feed, concentrate Canada, in an elliptical mafic-ultramafic minerals in the mill feed and concentrates and tailings samples were collected from intrusive complex (NAP, 2018). A majority from recently mined zones on the property the mineral processing plant. 2.2 Broken Hill (BH) The Broken Hill deposit in western by quartz- and garnet-rich host rocks. formation in the Broken Hill District. New South Wales, Australia has one In the upper parts of the mine, many Two types of gossans were selected to of the richest reserves of lead, zinc and of the sulfide minerals were converted be studied (gossan 1 and 2) with no silver in the world. The Broken Hill by weathering into a large suite of ox- visible lithological differences except ore body consists of a series of closely- ide minerals, some of which were first for the fact that gossan 2 seems denser spaced sulfides-rich deposits separated recognized as new minerals from their than gossan 1. 2.3 Analytical procedures Inductively coupled plasma mass samples were analyzed by a Philips XL- exchangeable metals in soil are the most spectrometry (ICP-MS) and inductively 30 scanning electron microscope (SEM) available to plants (Castilho and Rix, coupled plasma atomic emission spectros- equipped with a Bruker Quantax 200 1993). Citrate-dithionite was used by copy (ICP-AES) analyses for whole-rock energy-dispersion X-ray (EDX) micro- Gray et al. (1996) to selectively dissolve chemistry, and Instrumental Neutron analysis system. amorphous and crystalline Hydrous Activation Analysis (INAA) for PGMs The mineral association and bio- Ferric Oxides (HFO) such as limonite (Ir, Os, Pd, Pt, Rh, Ru) and Au were con- availability of PGMs and Au in all and goethite as well as crystalline iron ducted by Acme Analytical Labs Ltd. in samples were investigated by selective oxide like hematite. This work may be Vancouver. The mineralogy was examined extraction methods of samples ground the first trial to use fulvic acid in selective through quantitative X-ray powder dif- below 0.074 mm. Cu and Ni were also extraction of PGMs and Au. It was ex- fractometry (XRD) at the department of analyzed due to their phytotoxicity that pected that this humic substance could Earth, Ocean & Atmospheric Sciences at may limit plant growth. Ammonium ac- provide additional information about the University of British Columbia. etate (NH4C2H3O2) was used to extract the fraction of the metals that would be Due to the high grade of PGMs readily exchangeable metal species (Fer- bioavailable and possibly incorporated in the BH gossan samples only these reira and Veiga, 1995). It is believed that by plants. Reverse aqua regia was used 86 REM, Int. Eng. J., Ouro Preto, 73(1), 85-91, jan. mar. | 2020 Peipei Shi et al. to dissolve sulfides. This is a mixture tion of nitric acid increases dissolution acid provides complexing ligands for Pt of nitric acid and hydrochloric acid in of sulfides (Tessier et al., 1979). When and Pd that are associated with sulfides a molar ratio of 3:1. Higher concentra- sulfides are dissolved, hydrochloric (Colombo et al., 2008). 2.4 First selective extraction scheme Two sequences of selective extrac- twice in a shaker with 50 mL of lab- due was washed and leached twice for tions were employed. The first extraction grade fulvic acid for 5 h, at 500C. 16 h, heated at 500C with 30 ml 65% consisted of the following steps: 3. Dissolution of amorphous and HNO3, 10 mL 30% HCl (inverse aqua- 1. PGM associated with exchange- crystalline HFO and hematite: The resi- regia) and 10 mL H2O2. able cations: 3 g of sample were agitated due from the previous step was washed 5. Residual PGM associated with for 1 h with 50 mL 1 M NH4C2H3O2. and leached twice with 50 mL of other minerals: the final solid residues 2. PGM associated with extract- 0.3 M ammonium citrate and 1 g so- were melted with borate and dissolved able cations extracted by humic sub- dium dithionite for 16 hours, heated with aqua regia. stances: the residue from the previous at 500C. All solutions from each extraction extraction was washed and leached 4. Dissolution of sulfides: the resi- step were analyzed by ICP-MS. 2.5 Second selective extraction scheme The objective of the second extrac- used a citrate-dithionite leaching process above (step 3). After 4 hours, the pulp tion sequence was to remove more ferric as described by Veiga et al., (1991). The was filtered and the solution analyzed for oxides and associated metals from the method leached 3 g of sample ground Pt,Pd and Au. The procedure was repeated gossan samples since the NAP samples below 0.074 in a shaker at 50 oC with a 3 more times to guarantee that all ferric had low ferric oxides.