18
Gold [Au, 79]
18.1 INTRODUCTION
Gold (Au), a chemical element of the group 11 in the periodic table of elements, is
one of the metals of the so-called group noble metals. Its abundance in the Earth’s
crust averages 3–4 μg/kg. Its content in ultramafic igneous rocks is up to 5 μg/kg,
and in sedimentary rocks, its content varies from 2–6 and in calcareous rocks and
sandstones, its content is in the range of 3–7 μg/kg. In coal, Au may be concentrated
within the range of 10–50 μg/kg.
The main oxidation state of Au is +1, but may also exist from 0 to +5. Gold
reveals a tendency to occur in the free oxidation state and remains in this form, as
it has a great chemical resistance. Most often, it occurs in the form of the native
metal, and may be a minor constituent of some types of ores. Recently, Au became
an important by-product from some nonferrous Cu and Pb ores. Relatively common
Au minerals are associated with Te such as calverite, AuTe 2 ; montbrayite, Au 2 Te 3 ;
petzite, Au,AgTe 2 ; and sylvanite, Au,AgTe 4 .
Gold was one of the first metals to be recognized by humans, and due to its
chemical and physical properties, it became very important throughout the human
civilization.
World mine Au production (rounded) in 2010 was 2500 kt, of which Australia
produced 255; China produced 345; Russia produced 190; Uzbekistan produced 190;
Peru produced 170; and Ghana produced 100 (USGS 2011). In some countries, Au
production is partly under reported. There are several Au-mining methods, depending on the type of its deposits. Flotation methods are common, based on the amalgamation with Hg, and bioextraction methods, using some microorganisms, mainly
Thiobacillus ferrooxidans (Reith and McPhail 2007). Gold is also obtained from
some ore materials by extractions, using Na-cyanide and Ca-hydroxide. Both methods cause health risks, because the workers contact with toxic chemicals (Veiga and
Baker 2004).
Gold is widely used in jewelry, coinage, dental alloys, electroplating electronics,
and antiarthritic drugs.
18.2 SOILS
The average Au contents of soils is estimated at 3 μg/kg. In organic soils, it may be
elevated up to 8 μg/kg (Table 18.1). However, some alluvial soils, especially soils
with postmining Au deposits, and soils with Au-enriched sludge may contain much
higher Au levels, within the range from 43 to 473 μg/kg. Sewage sludge contains Au
at about 800 μg/kg. Forest soil mull fractions and clay mineral fractions, especially
in soils from Au-mineralized areas, may contain Au up to 5000 μg/kg. Soil bacterial
communities (e.g., Bacillus cereus) may accumulate Au in spores, up to 1100 μg/kg.
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