Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 106

2.3.1.1 Acids and Bases
Acids and bases all are capable of dissociating in water and affecting the pH of the
soil. They are usually highly soluble in water, even if they are weak acids or bases,
and therefore they are mobile. By affecting the pH of the soil, they can change the
solubility and mobility of contaminants, particularly heavy metals and metalloids.
Soils with a significant cation exchange capacity (CEC) can adsorb a high amount
of the proton, H + , and other positively charged molecules. By the same token, high
CEC soils can buffer external acidifying and alkalifying input better.
Liquid acidic chemical wastes that may have been applied to the soil include
diluted HCl, H 2 SO 4 ,H 3 PO 4 and organic acids such as acetic acid. Alkaline liquid
wastes include Na(OH) solutions, other caustic solutions, hydrazobenzene solu-
tions, or solid phase burnt lime CaO. These compounds can have large effects on
the permeability of the soil and its shrinkage properties, as well as cause direct dis-
solution of the soil (Dragun 2007 ). Depending on the soil type, extreme changes in
soil pH also can destroy living organisms.
Soils may become very acid, anthropogenically, by long-term and copious use
of certain fertilizers like ammonium nitrate, ammonium sulphate, and monocalcium
phosphate (MCP) as autotrophic bacteria convert the ammonium ion to 2H + and
NO 3 − , and as the MCP hydrolyses to form dicalcium phosphate and phosphoric
acid. Mine wastes or dredged soils that include sulphide minerals, such as pyrite,
FeS 2 , can also create extreme acidity through oxidation of the S to SO 4 2 − .This
process is called “acid sulphate soil formation” and can lead to extremely low pH
values that are lethal to plants and soil organisms and can cause acid runoff that
might kill fish and other aquatic life forms. Such an oxidation process was already
identified in the eighteenth century by the Swedish botanist Linnaeus (1707-1778).
However, soil minerals such as carbonates, alumino-silicate clays and oxide and
hydroxide minerals can act as buffers to excessive acidity by reacting with the H +
ions, dissolving and forming water and very weak acids.
Soils affected by extreme acidity can be neutralised by a variety of liming mate-
rials such as calcium oxide, calcium hydroxide, calcium carbonate, and dolomite.
Extreme alkalinity can be neutralised by adding elemental sulphur, aluminium
sulphate, or calcium polysulphide.
2.3.1.2 Water-Immiscible Contaminants
Water-immiscible contaminants include petroleum hydrocarbons such as NAPL,
LNAPL, DNAPL. In an extensive discussion of this topic, Dragun ( 2007 ) states
that the molecular structure of these contaminants is the chief determinant of the
strength of adsorption as structure determines the molecular volume, often also the
water solubility, the octanol-water partition coefficient, and the vapour pressure. The
larger the molecule, the smaller will be its polarity, and the more important will be
the van der Waals forces' attraction to the solid phase surfaces. This is also the case
for very large water soluble organic molecules. Hydrophobicity is another important
factor governing adsorption to soil surfaces. Hydrophobic (lipophilic) contaminants