The influence of metal concentration, solution pH and exposure time on the phytoextraction (i.e. separation
using vascular plants) of Au was investigated for the known metallophytes Brassica juncea (BJ)
and Medicago sativa (MS). Metal uptake was inferred using Inductively Coupled Plasma Optical Emission
Spectroscopy (ICP-OES) and in vivo localisation and distribution using proton induced X-ray emission
spectroscopy (-PIXE). MS roots accumulated a maximum of 287mgAu g−1 (dry biomass) and BJ roots
a maximum of 227mgAu g−1 (dry biomass), both when exposed to a 10,000ppm aqueous solution of
KAuCl4. MS was found to accumulate comparatively greater quantities of Au than BJ across higher substrate
concentrations (40–10,000ppm Au) whereas BJ was found to be a better accumulator of Au at
lower concentrations (5–20ppm Au). In general MS showed an increase in Au uptake with an increase
in Au substrate concentration and the time exposed, whereas for BJ the maximum uptake was observed
after 48 h of exposure at higher concentrations (100–10,000 ppm), and then decreased at longer exposure
times. The uptake ratio (UR), defined as the ratio of Au concentration in plant tissues to the concentration
in the substrate, increased with increasing concentration and exposure time, to a maximum of 995
for MS roots after 72 h exposure. Metal translocation from roots to shoots in BJ increased with increasing
substrate concentration, however in the shoots, metal uptake increased from 24 to 48 h and then
decreased at 72 h, indicating some threshold level had been reached and metal was then being excluded
from the cells, possibly through the phloem to the Au solution. Elemental distribution maps of plant
tissues measured using -PIXE, show Au present across the entire sample, ranging from the epidermis
and cortex, with the greatest concentration occurring within the central stele. This result is suggestive
of xylem loading. These results collectively suggest that the separation of Au using vascular plants for
applications in mining (phytomining) and remediation (phytoremediation) are viable technologies