Lead [Pb, 82]
169
During weathering, Pb sulfides are slowly oxidized, and in soils are fixed by
clay minerals, hydroxides, and soluble organic matter (SOM), which increased
with increasing pH (Mboringong et al. 2013). Lead reveals a strong affinity for SOM,
and the formation of inner-sphere metal complexes (Basta et al. 2005). Lead adsorption on Al–Fe and Mn oxides may be increased due to the presence of sulfate and
phosphate compounds (Violante 2013). The geochemical characteristics of Pb 2+
resemble the divalent alkaline-earth group elements, thus it has an ability to replace
K, Ba, Ca, and Sr, in both minerals and sorption sites (Kothe and Ajit 2012).
Pb distribution in soil profiles is not uniform and reveals a great association
with hydroxides, especially of Fe and Mn. Its concentration in Fe–Mn nodules may
be very high, up to 20,000 mg/kg. It may also be accumulated in calcium carbonate, phosphate, and some feldspar particles, especially at pH > 6. The formation of
hydrocerussite, Pb 3 (CO 3 ) 2 (OH) 2 , and pyromorphite, Pb 3 Cl(PO 4 ) 3 , in soils contaminated with Pb, has also been observed (Kabata-Pendias 2011).
Usually Pb is accumulated in surface-soil horizons, mainly due to its sorption by
SOM. However, in some soils, mainly podzolic, it may be concentrated in deeper
soil layers. The Pb fixation by clay minerals also plays a significant role in its distribution. Lead species in soils are slightly mobile, but some Pb–OM complexes and
acidity increase its mobility.
Lead concentration in soil solution is relatively low, within the range of
<1–60 μg/L, and is highly depending on methods used for obtaining of soil solution.
It occurs in soil solutions as cationic species such as Pb 2+ , PbCl + , and PbOH + , and as
anionic species such as PbCl 3 and Pb(CO ) 2
3
2 (Kabata-Pendias and Sadurski 2004).
Due to microbial activities, Pb adsorbed at the surface of Fe minerals may be dissolved in the soil solution (Perelomov and Kandeler 2006). According to Pampura
et al. (2007), the predictability of free metal ion values for Pb in soil solution is high
and may be useful for the critical load calculation.
The background Pb content of soils is inherited from parent rocks. However, due to
the widespread Pb pollution, most soils, especially top horizons, are likely to be enriched
in this metal. Sometimes, it is difficult to separate the data for background Pb levels in
soils from those of anthropogenic sources. Low Pb levels in soils from remote regions
suggest that the baseline value of this metal in the most of worldwide soils should not be
much higher than lithogenic concentration, estimated at 20 mg/kg. However, its higher
levels are noticed in all soils, also from uncontaminated regions (Table 24.1). Very often
Pb level is elevated in rhizospheric soils (up to above 10,000 mg/kg), which resulted in its
higher contents of some plants. Some microorganisms (Rhizopus arrhizus) accumulate
33 mg Pb/g, within the broad range of pH 3–7 (Perelomov et al. 2013).
Lead pools in mountain forest soils of the National Park in Poland range between
0.16 and 15.6 mg/kg, and is concentrated in forest litter, often over 100 mg/kg. Its significantly higher levels are in the lower altitudinal zones as compared to the higher zones
(Szopka et al. 2013). Lead is likely to be concentrated in the upper soil layer, and in all
soil layers, lead predominates its residual fraction over other species (Makuch 2012).
Lead contamination of soils from mining and industrial activities is an old problem, and began when our ancestors learned to use fire. The estimation, using Pb
isotopes, informed that between 40% and 100% of the total Pb in contaminated soils
in France come from the Medieval workshops (Baron et al. 2006).