Two wet digestion methods were performed and compared to
determine the digestion procedure appropriate for both mine tailings
and biosolids. Mine tailings and biosolids samples were
digested in aqua regia (3:1 conc. HCl: conc. HNO3) and reverse aqua
regia (1:3 conc. HCl: conc. HNO3) as described by Lomonte et al.
(2008). Acid digests were analyzed for Hg by atomic fluorescence
spectrometry (PS Analytical Millenium) and for Au by graphite furnace
atomic absorption spectrometry (Hitachi Z-2000 Atomic
Absorption Spectrometer). All measurements were done in triplicate
and the Hg and Au concentrations are expressed on a dry mass
basis. Elemental composition (5 replicates) was determined by
inductively coupled optical emission spectrometry (ICP-OES)
(Optima 4400, Perkin Elmer).
The biosolids and mine tailings samples determined to have the
highest Hg and Au concentrations were used for plant growth studies.
The salinity of the substrates had to be decreased prior to
planting as high salinity was expected to affect adversely the
plants. A leaching experiment was done in order to find out how
much water was needed to decrease the salinity of each of the substrates.
This was done by stepwise addition of water to each of the
potted substrates, followed by collection of the leachate and measurement
of Total Dissolved Salts (TDS) by Multiline P4 universal
meter. The TDS of the leachate was measured after each leaching
increment until satisfactory soil salinity was achieved (i.e., 0–
200 lS cm1). The mine tailings required 1.1 L of water to achieve
a TDS of 172 lS cm1, while biosolids alone and the mine
tailings-biosolids mixtures described below required 5 L water
for their salinity to decrease to 147 and 177 lS cm1, respectively.
After leaching, the substrates were air-dried