There’s Gold (Au) in Them Thar Leaves! (with apologies to M. F. Stephenson)
In the previous post, we explored the notion that plants could be used as a sort of “biosensor” to detect the presence of dead bodies.
By the way, a “biosensor” may be loosely defined as an analytical device, used for the detection of a chemical substance, that has a living organism as a key component. For example, a canary in a coal mine, used to detect carbon monoxide and other toxic gases before they hurt humans, is essentially a “biosensor”.
Using plants as biosensors (or “detectorists” as I’m calling them) makes a lot of sense.
After all, solar-powered plants not only interact with the atmosphere around them, but also with the soil, often growing roots deep (meters) underground. And substances taken up by these roots, such as microscopic gold (Au) particles, may indeed be transported and accumulated in the leaves (e.g., see Refs. 1 & 2 below) of some plant species.
Indeed, there is a long (centuries) history of reports of using plants for gold (Au) prospecting (e.g., see Ref. 1 below). One problem with many of these reports, however, is that Au detected in the leaves probably wasn’t transported from the roots, but, more likely, was contaminated from dust in the air that settled on the leaves. (See Ref. 2 below)
Ah, but in 2013 along came Lintern, et al. (Ref. 2) who reported the following: “…although Au has been previously measured in plant samples, there has been doubt as to whether it was truly absorbed rather than merely adsorbed on the plant surface as aeolian contamination. Here we show the first evidence of particulate Au within natural specimens of living biological tissue (not from laboratory experimentation). This observation conclusively demonstrates active biogeochemical adsorption of Au and provides insight into its behaviour in natural samples. The confirmation of biogeochemical adsorption of Au, and of a link with abiotic processes, promotes confidence in an emerging technique that may lead to future exploration success and maintain continuity of supply.”
This report added legitimacy to the concept of using plants to prospect for underground gold deposits.
But the usefulness of the ability of some plants to take up gold particles from the soil and accumulate them in the leaves has gone beyond merely detecting Au deposits to actually using such plants to mine gold.
Phytomining of Gold
Some plants are able to extract heavy metals from the soil and accumulate them in aerial parts of the plants, such as leaves.
For example, “The term “hyperaccumulator” describes a number of plants that belong to distantly related families, but share the ability to grow on metalliferous soils and to accumulate extraordinarily high amounts of heavy metals in the aerial organs, far in excess of the levels found in the majority of species, without suffering phytotoxic effects.“(from Ref. 3 below)
Decades ago, such plants were seen as potentially able to help clean heavy metal contaminated soils via a concept called “phytoremediation”.
But the mining industry also took note of these “hyperaccumulators”. (For example, please see Refs. 4 & 5 below.)
As stated in Ref. 4 below: “The precious metal gold can be found at high concentration in tailings dumps and waste rock piles at many mining locations around the world. Conventional technology is generally unable to economically recover this residual gold, and, as a result, the potential resource is wasted, presenting environmental risk to the wider ecosystem through particulate and dissolved metal leaching and erosion. For the past 14 years, the idea of gold phytomining to recover this gold resource has been researched by various scientific groups worldwide. A number of plant species have been tested under laboratory, greenhouse, and field conditions to determine their potential for use in the phytoextraction of gold.”
Basically, the way this would work is: “Plants called hyperaccumulators absorb metals through their root system and accumulate them in their aerial organs, a process termed phytoextraction. Phytomining is an application of this process in which hyperaccumulators are grown on a substrate having sufficient metal concentration, and are then harvested and processed to extract the metals. Processing involves burning the plants after harvest to create ash from which the metal is recovered by smelting or by application of reagents.” (From Ref. 6 below)
Though promising, the jury is still out on the practicality of phytomining for gold.
Next Time: Phyto-detecting explosives!!!
1. Erdman, J. A. and J. C. Olson (1985) “The use of plants in prospecting for gold: A brief overview with a selected bibliography and topic index.” Journal of Geochemical Exploration, Vol. 24, pp. 281-304. (Abstract)
2. Lintern, M., et al. (2013) “Natural gold particles in Eucalyptus leaves and their
relevance to exploration for buried gold deposits.” Nature Communications, Vol. 4, 2614. DOI: 10.1038/ncomms3614 (Full Text)
3. Rascio, N. and F. Navari-Izzo (2011) “Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?” Plant Science, Vol. 180, pp. 169-181. (Abstract)
4. Wilson-Corral, V., C. W. N. Anderson and M. Rodriguez-Lopez (2012) “Gold phytomining. A review of the relevance of this technology to mineral extraction in the 21st century.” Journal of Environmental Management, Vol. 111, pp. 249-257. (Abstract)
5. V. Sheoran, A. S. Sheoran and P. Poonia (2013) “Phytomining of gold: A review.” Journal of Geochemical Exploration, Vol. 128, pp. 42-50. (Abstract)
6. Dunbar, W. S. (2016) “Biotechnology and the Mine of Tomorrow.” Trends in Biotechnology,
Vol. 35, pp. 79-89. (Abstract)