Money doesn’t grow on trees…but does gold?
Gold (Au) is a familiar precious metal used for jewellery, in electronics, coins and some of you might have it in your teeth. The demand for gold is always on the rise due to ongoing economic growth and desire for these products. Today, it’s mined from deposits, such as the Grasberg deposit in Papua, Indonesia. However, these resources will come to an end. So exploration companies are always looking for new deposits and new possibilities. One of the most curious, unconventional places scientists have looked for gold is in…plants.
The precious metal isn’t essential for a plant’s metabolism, hence the concentrations are very low, and background levels are approximately 10 parts per billion (ppb). Nonetheless, researchers around the world have observed enrichment of gold particles (Fig. 1) in trees and plants of up to 610 parts per million (ppm) in concentration. What causes these differences? And why do plants uptake gold at all?
In the 1980s, researchers from the United States Geological Survey, followed by scientists in the UK at Westfield College London, investigated the uptake of elements into plants in more detail. Generally, plants need essential elements such as nitrogen, phosphorus, potassium, iron and copper, which they take up through their roots from water and accumulate in their tissue. However, depending on the plant species, growth rate, soil and environmental factors, further elements – for instance chromium, nickel, arsenic, gold and silver – can be identified in plant tissues. Both research groups agreed that the presence of cyanide increases the uptake of gold into plants. This cyanide is produced by some plants, fungi, bacteria and algae themselves, meaning that not all plants have the same ability to accumulate gold.
Using this knowledge, the study group of Anderson at Massey University in New Zealand, aimed to use plants to mine for gold, so-called “phytomining”. The idea is to plant mustard (Fig. 2), sunflower or tobacco – plants that grow rapidly and have high biomass – in gold-bearing soils, such as waste mining areas. During modern ore extraction processes, gold can never be removed completely, thus there is potential that small quantities of the precious metal survive in mining waste. As soon as the crop reaches its full height, ammonium thiocyanate is artificially introduced into the soils, making the gold turn into soluble nanoparticles that can be taken up by the plant. These plants can then be harvested and burned. As a result, 57 µg can be extracted per 1 g dry weight.
This process does not only lead to the uptake of gold, it also mobilizes the hazardous elements of the mine waste, such as mercury, arsenic and copper, to accumulate in the plant tissues. Thus, once contaminated soil can be remediated. However, although theoretically this technique is promising, the extracted gold concentrations are still too low in terms of mining potential and, furthermore, there is a potential hazardous effect to the environment caused by the addition of cyanides.
Haverkamps research group at Massey University, New Zealand, developed other ideas associated with gold uptake by plants. Gold nanoparticles are good catalysts in chemical reactions – a feature that can be exploited in many new technologies. For example, these catalysts are in high-tech portable fuel cells used by the military. These nanoparticles had only been synthesized in the laboratory until the research group was able to extract gold-silver-copper alloys as nanoparticles from leaves. However, there are still issues in controlling the size of these nanoparticles within the plant material.
Harvesting gold or alloys from plants is a great idea, but still in its early stages of development. Fortunately, the attribute of gold uptake in plants can help to locate mineral gold deposits. Geochemists from Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) discovered gold in eucalyptus tree foliage (Fig 3), near a region of gold mineralization. By using x-rays, researchers identified gold grains just 1/5 of human hair in size. The roots of eucalyptus trees can extend several tens of meters into the ground, from where they draw groundwater up. If this groundwater is gold bearing, for instance due to a nearby gold deposit, the metal migrates through the roots into leaves and branches.
Harvesting gold remains challenging but using plants as an indicator for the presence of mineral deposits may get more attention in the future. Being a geologist is more than just looking at rocks…you should never underestimate the power of biology!