Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 49

ground and in the receiving waters can be traced to deposition of airborne lead obtained
from emissions such as burning of wastes and fuel and from transport of lead compounds
in the soil. Lead is considered to be a nonessential toxic element, and ingestion or inha-
lation of lead will result in consequences to the central nervous system and damages to
kidneys and reproductive system.
2.4.1.6 Nickel (Ni)
Nickel is quite widely found in nature in various soil deposits, e.g., laterite deposits, and
generally in mineral form in combination with oxygen or sulfur as oxides or sulides.
These include nickel sulide (NiS), nickel arsenide (NiAs), nickel diarsenide (NiAs 2 ), and
nickel thioarsenide (NiAsS). Some debate exists concerning whether nickel is, or is not, an
essential element. It is maintained that small amounts of nickel are essential for maintain-
ing the good health of animals, and to a lesser extent, humans.
Nickel found in the geoenvironment, other than from natural sources, can be traced
to fugitive atmospheric nickel and waste discharge associated with nickel mining activ-
ities, burning of waste, operation of oil- and coal-burning power plants, discharges
from manufacturing industries using nickel alloys and compounds. Nickel does not
precipitate but is sorbed onto clays, oxides of manganese and iron, and organic material
occurs. Mobility increases with the formation of complexes with organic and inorganic
ligands.
2.4.1.7 Zinc (Zn)
Similar to nickel, zinc is found in soil deposits and does not generally exist as a free ele-
ment. Instead, it is found in mineral form in combination with oxides, sulides, and carbon-
ates to form zinc compounds. The sulide form is perhaps the more common form for zinc
found naturally in the environment. Some of the naturally occurring zinc compounds are
zincite (zinc oxide, ZnO), hemimorphite (zinc silicate, 2ZnO·SiO 2 H 2 O), smithsonite (zinc
carbonate, ZnCO 3 ), and as sphalerite (zinc sulide, ZnS). Natural levels of zinc in soils are
30 to 150 ppm.
Typical uses and products for zinc in element form as oxide and sulide compounds
include alloys, batteries, paints, dyes, galvanized metals, pharmaceuticals, cosmetics, plas-
tics, electronics, and ointments. It follows that non-naturally occurring zinc found in the
geoenvironment would be the zinc compounds associated with the production and use
of industrial products. Deposition of airborne fugitive zinc from mining and extraction
of zinc, together with discharges (spills, wastes, and waste streams) from the processing
and production of products utilizing zinc compounds, account for the major sources of
non-naturally occurring zinc found in the geoenvironment. Some of these sources are
galvanizing plant efluents, coal and waste burning, leachates from galvanized structures,
natural ores, and municipal waste treatment plant discharge.
Although not as toxic as cadmium, zinc is quite often associated with this metal. Under
acidic conditions below its precipitation pH, zinc is usually divalent and quite mobile. In
the divalent state, sorption onto the surfaces of reactive soil particles includes ionic bond-
ing and sequestering by organic matter. At high pH, the solubility of its organic and min-
eral colloids can render zinc bioavailable. Zinc hydrolyzes at pH 7.0 to 7.5, forming Zn(OH) 2
at pH values higher than 8. Under anoxic conditions, ZnS can form upon precipitation,
whereas the unprecipitated zinc can form ZnOH + , ZnCO 3 , and ZnCl + .