Influence of Mineralogy and Chemistry on Uptake of Pd by Hemp

In simple terms, the aim of the Phytocat project is to transfer palladium (Pd) and other precious metals from soil to plants, and to use these plants as catalysts for chemical reactions. The process to get the metal into plants has a couple of key steps, and exploring these has been the responsibility of a UBC, Massey University team (WorkPlan 1). Throughout the Phytocat project, WP1 has assessed the suitability of precious-metal rich rocks around the work for their suitability to phytomining. Precious metal mines are a good target, however, the likelihood of accessing high-grade ore or concentrate is uncertain and tailings are unlikely to have the necessary concentration of metal to yield enough metal in the plants for catalytic activity. Instead, WP1 has focused on small deposits of mineralised rock around the world that is not currently exploited by mining operations. One target is the Mulga Springs Gossan, near Broken Hill in Australia, with a Pd concentration up to 50 mg/kg, and significant concentration of Pt, Ir, Rh, Os, Ru. This rock is, by all accounts, ‘unique’ on the Earth. The Mulga Springs gossan is heavily weathered, and is dominated by iron hydrous oxide. Precious metals appear to be associated with Ba, and have been concentrated in the gossan (relative to the original rock) through weathering. Work on the Mulga Springs gossan has been supported by trials in NZ and the UK using synthetic ore materials.

A series of pot trials were conducted using this gossan. Precious metal phytomining depends on the use of chemicals to promote metal solubility for uptake, and cyanide showed the greatest potential to dissolve metals in the gossan. However, the amount of precious metal in hemp was inversely related to the extent of weathering; the iron hydrous oxides appeared to adsorb the precious metal before hemp could accumulate them. In other words, WP1 made the metals soluble in the lab, but in pot trials these metals were stolen from solution before plants could accumulate them. Copper in the gossan turned out to have a major effect on Pd uptake. The copper concentration of the rock is close to 1% and inhibited the germination of many plants during pot trials (hemp was surprisingly tolerant). Treatment of the ‘soil’ with cyanide induced significant copper solubility, increasing the concentration of metal in hemp from 50 mg/kg to over 5,000 mg/kg. This quickly killed the plants, limiting the potential for precious metal uptake. The maximum Pd concentration recorded in hemp was 123 mg/kg.

There are a couple of key lessons learned from WP1: 1) finding material to grow plants on is not easy; 2) once we have material, we need to consider the mineralogy of the rock, and how this will affect the chemistry of the soil-plant system; and 3) we have not yet got to the level of metal in plants growing on a ‘real’ substrate that we likely need for catalytic activity. 

Link to webpage of speaker: http://www.massey.ac.nz/massey/expertise/profile.cfm?stref=165530