Page 204 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 193

the important role of organic matter in solubilizing heavy metals in sewage sludges
and in agricultural soils amended with liquid manure can be clearly shown.
Aluminum mobilization in Spodosols was demonstrated to be largely determined
by the formation, solubilization, migration, and subsequent precipitation of organic
complexes. Organic matter plays an important role not in forming complexes
and also in retaining heavy metals in an exchangeable form. Copper is bound and

rendered unavailable chiefly through the formation of complexes, while Cd is
retained in an exchangeable form and is more readily available. When mobility
increases, the quantities of heavy metals available to plants increase, resulting in the
appearance of toxicity phenomena.

Arsenic

Arsenic is an element in group VA and period IV of the periodic table. It is a metalloid
−3
having atomic number 33, atomic mass 75, density 5.72 g cm , melting point 817 °C,
and boiling point 613 °C and can be present in several oxidation states (−III, 0, III, V)
(Smith et al. 1995 ). It naturally occurs in a wide variety of minerals, mainly as
arsenolite As 2 O 3 . It occurs in ores containing Cu, Pb, Zn, Ag, and Au. In aerobic
3−
environments, As (V) is dominant, usually in the form of arsenate (AsO 4 ) in various
3−
−
2−
protonation states: H 3 AsO 4 , H 2 AsO 4 , HAsO 4 , and AsO 4 . Arsenate and other
anionic forms of arsenic behave as chelates and can precipitate when metal cations
are present (Bodek et al. 1988 ). Arsenic (V) is adsorbed on iron oxyhydroxides and
can coprecipitate with them under acidic and moderately reducing conditions.
Coprecipitates are immobile under these conditions, but arsenic mobility increases
as pH increases (Smith et al. 1995 ). Under reducing conditions, As(III) dominates,
2−
3−
−
existing as arsenite (AsO 3 ) and its protonated forms H 3 AsO 3 , H 2 AsO 3 , and HAsO 3 .
Arsenite has a high affinity for sulfur compounds and can adsorb or coprecipitate

with metal sulfides. Elemental arsenic and arsine, AsH 3 , may be present under
extreme reducing conditions. Methylation of arsenic forms methylated derivatives
of arsine, including highly volatile dimethyl arsine HAs(CH 3 ) 2 and trimethylarsine
As(CH 3 ) 3 . Arsenic may be present in organometallic forms such as methylarsinic
acid (CH 3 )AsO 2 H 2 and dimethylarsinic acid (CH 3 ) 2 AsO 2 H.
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All rocks contain some arsenic, typically 1–5 mg kg . Some igneous and
sedimentary rocks may contain higher As concentrations. Principal arsenic bearing
minerals are arsenopyrite (AsFeS), realgar (AsS), and orpiment (As 2 S 3 ). The sources
of As in soils are both geochemical and anthropogenic. Metallic arsenic is mainly
used for strengthening alloys of copper and lead. Arsenic is used in semiconductor
electronic devices. Arsenic compounds are used for production of pesticides,
herbicides, and insecticides. Several organoarsenic compounds were developed as
chemical warfare agents during World War I, including vesicants such as lewisite
and vomiting agents such as adamsite. Residues of these materials have found their
way to the soil (Smith et al. 1998a , b ). Arsenic is a notorious pollutant of groundwater
which is used for irrigation of extensive rice fields in Southeast Asia.

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−1
Soils may contain 1–40 mg As kg , with an average of 5–6 mg kg . Indiscriminate
use of arsenical pesticides during the early to mid-1900s has led to extensive