44
Silver [Ag, 47]
44.1 INTRODUCTION
Silver (Ag) is a soft, lustrous transition metal of group 11 in the periodic table of
elements, called noble metals. Its average abundance in the Earth’s crust is estimated
to be about 0.08 mg/kg. In some sandstones and calcareous rocks, its contents may
be enriched up to about 0.2 mg/kg. It reveals chalcophilic properties and, after rock’s
weathering, easily precipitates in alkaline media. Silver may concentrate in some
biolits and may be accumulated in coal, up to 20  mg/kg, and in crude oil, up to
0.3 mg/kg.
The common oxidation state of Ag is +1, but may also be +2 and +3. Typical minerals of silver are argentite, Ag 2 S; cerargite, AgCl; arsenide, Ag 3 As; and proustite,
Ag 3 AsS 3 . It forms several other minerals, and its host minerals are mainly sulfides.
It may also occur as an alloy with other metals.
Global Ag production (rounded), excluding the United States, in 2010 was
22,200 t, of which 4,700 t was produced in Peru, 3,500 t in Mexico, 3,000 t in China,
and 1,700 t in Australia (USGS 2011). Most Ag is produced as a by-product of Au,
Cu, Pb, and Zn mining and refining.
Silver was discovered about 4000 BC, when it was used in jewelry and a medium
of exchange in bullion. It has very high electrical and thermal conductivity, and
antibacterial properties, especially in forms of nanoparticles. It is used in various
electrical equipments, mirrors, and catalytic chemistry. Among all metals, Ag is
unique in its behavior with oxygen and has a great catalytic power for the oxidation. In sanitation processes, it reveals catalytic oxidation of cell surface. Currently,
the major part of the Ag production is used in manufacturing and photographic
industries. It is also applied in some surgical equipments. Colloidal Ag is used as an
antibacterial agent in ceramic filters for household waters. Silver may also be used to
replace more costly metals in catalytic converters for off-road vehicles.
44.2 SOILS
The average Ag content in the worldwide soils is estimated to be 0.13 mg/kg, within
the range in soils of various countries from 0.05 to 0.4 mg/kg. Its higher contents
are noticed mainly in organic soils, which may contain up to 5 mg/kg. Also soils
from mineralized areas contain usually elevated Ag amounts; particularly soils from
the regions of old Pb mines (e.g., in Wales) contain increased Ag level, up to 9 mg/kg
(Jones  et al. vide Kabata-Pendias 2011). Elevated Ag amounts are also reported
for garden soils (0.4 mg/kg) and street dust (0.2  mg/kg) (Rasmussen et al. 2001).
However, garden soils, as well as vineyard soils, may contain much higher Ag levels,
up to about 1 mg/kg.
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