Introduction to Soil Chemistry

cations 201
Manganese does, however, have some unusual characteristics. It is very
unusual to find soil situations where iron is toxic, whereas manganese toxicity
is known.As noted above, iron is found in only two oxidation states while manganese
can have three oxidation states. However, the situation is found to be
much more complex than this when soil is analyzed for the species of manganese
present. A simple analysis for manganese might indicate an oxidation
state of +3.5, indicating that the material analyzed contains an unknown
mixture of the common oxidation states of soil manganese. This then creates
problems in understanding the chemistry of manganese and thus reactions
leading to its deficiency, toxicity, biological availability, and movement in the
environment.This is an issue and problem which has received significant attention
and research [10].
10.1.2.3 Chromium
Chromium has numerous oxidation states, some of which are strongly oxidizing.
The most highly oxidized species is Cr 6+ , and the reduced ion is Cr 3+ .In
soil, because of its strong oxidizing characteristics and oxidizable species
present, Cr 6+ is rapidly reduced to Cr 3+ . Chromium species of intermediate oxidation
states can exist in soil; however, the +3 state is the most common. As
with the other metals, all the possible combinations of species with other components
are possible and must be kept in mind when carrying out an analysis
for chromium species in soil [11–13].
10.1.2.4 Mercury
Mercury is unusual in that it is found in the environment as both oxidized
mercury ions and reduced methyl mercury. The mercurous (Hg + ) ion is unstable
and not likely to be found in soil, while mercuric (Hg 2+ ) ions and methyl
mercury compounds are. All forms of mercury are of environmental concern,
and mercury ions can form the same types of interactions with soil constituents
as those described for other multi-oxidation-state metals. Mercury in all its
forms is toxic and thus of concern; however, methyl mercury, which can form
in soil under anaerobic conditions, is particularly dangerous because of its
extreme toxicity.
Mercury has several other characteristics that render it of particular environmental
concern and make it likely to be found as many different species.
It is a natural constituent of soil, although it occurs at low concentrations. It
is widely used in both industry and in the laboratory, making it a common contaminant
of reference materials. Metallic mercury has a relatively high vapor
pressure, which means that it can occur in measurable amounts in the soil
atmosphere.
Analysis of mercury is difficult, and specialized sampling and instrumental
techniques are generally required to carry out an accurate analysis. Although