Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 107

tend to adsorb strongly to the soil's organic matter. Molecular charge and hydro-
gen bonding are given as the third and fourth major factors in adsorption. Organic
molecules possessing an intrinsic positive or negative charge are adsorbed to cation
or anion exchange sites on the soil particles. The hydrogen bond is where a hydrogen
atom acts as a bridge between two electronegative atoms, one on the contaminant
and the other on the adsorptive surface. The strength of adsorption of a contaminant
is inversely related to its availability and mobility in the soil.
The majority of cases of soil contamination with these contaminants are the
result of industrial land use, such as petrol stations, fuel and oil depots, et cetera,
where tanks were leaking or spills occurred. However, river deltas are the natural
environment where buried organic materials often are transformed into petroleum
hydrocarbons so that low concentrations of these contaminants and their derivatives
can occur naturally in the soil. Marine hydrocarbon seeps that are the main source
of floating tar and tar in sediments are mentioned by Dragun ( 2007 ) and are not
uncommon.
Organic lipophilic contaminants that enter the soil in much smaller quantities,
but nevertheless are of great importance, include a wide range of pharmaceutical
products, some of which are used in bulk in animal husbandry and are applied to the
soil in manure (Kümmerer 2001 , Kümmerer 2004 ).
2.3.1.3 Metals and Metalloids - Trace Elements
Metals and metalloids that are used as catalysts and pigments and for electrolytic
plating and in other objects that are composed of metals are mostly in ionic form,
but can also be in elemental form, as contaminants in the soil. The most common
ones are: silver (Ag), arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu),
nickel (Ni), lead (Pb), tin (Sn), (vanadium) V and zinc (Zn). Their mobility and
bioavailability depend on their speciation which in turn depends on the soil pH
and on the concentration in the pore water of anions like chloride. Chloride ions
are capable of complexing some metals to form highly soluble negatively charged
anions, for example PbCl 6 2- . Thus, the presence of sodium chloride in pore water
affects the mobility of lead. Copper can be complexed to dissolved soil organic
carbon (DOC) and is then as mobile as the DOC. The solubility of the DOC, in turn,
is affected by the calcium concentration in the pore water (Römkens 1994 ).
However, several metals and metalloids also occur naturally in all geological
materials, rocks as well as soils, to varying degree, depending on their chemical
affinity to various minerals, varying presence in magmas, varying enrichment by
hydrothermal processes and varying history of weathering. Man-related soil con-
tamination with most of the metals mentioned above generally poses a much greater
environmental risk than natural geochemical accumulation.
2.3.1.4 Salts and Bases of Metal Alkaloids and Boron
Apart from trace elements, consideration must be given to other inorganic contami-
nants, particularly bases of metal-alkaloids and nutrients.