Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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proposed by Arrhenius in 1881 and refined in 1887, has been shown 
to be useful in application to aqueous solutions, but faces difficulties in 
application to soil\u2013water systems, for example, application of the Arrhenius 
concept to solvents and to the specific requirement for the presence of OH\u2212 
ions in a base.
For soil\u2013water systems, because of the presence of reactive surfaces of soil 
particles, it is more convenient to consider the definitions of acids and bases 
in terms of proton donation or acceptance. By releasing one from consid-
ering acids and bases as aqueous substances, this allows one to categorize 
nonaqueous substances in terms of acids and bases. The Brønsted\u2013Lowry 
definition of acids and bases expands the concepts of Arrhenius (Brønsted, 
1923). In the Brønsted\u2013Lowry definition, an acid is a substance that can 
donate a proton (H+), and a base is a substance that can accept a proton. This 
(Brønsted\u2013Lowry) acid-base concept considers an acid as a proton donor (pro-
togenic substance) and a base as a proton acceptor (protophillic substance). Since 
water has the capability to both donate and accept protons (i.e., both proto-
genic and protophillic), it is called an amphiprotic substance.
125Soil\u2013Water Systems
In the Lewis (1923) concept, an acid is a substance that is capable of accept-
ing an electron pair from a base, and a base is a substance that is capable of 
donating an electron pair. This means that substances with unshared pairs 
of electrons are bases, and substances lacking an octet are acids. With this 
set of definitions, metal ions Mn+ are Lewis acids, and theoretically, all cat-
ions are Lewis acids. Water is both a Lewis base and a Brønsted base. Whilst 
Lewis bases are also Brønsted bases, it does not follow that Lewis acids are 
Brønsted acids. This is because Lewis acids include substances that are not 
proton donors. The Lewis concept of acids and bases allows one to treat 
metal\u2013ligand bonding as an acid-base reaction.
Pearson (1963) classified a whole range of atoms, ions, and molecules into 
three groups of acids and bases as hard, borderline, and soft Lewis acids and 
Lewis bases. This classification is known by the acronym HSAB (hard, soft, 
acid base) classification. With this classification, Pearson noted that hard acids 
prefer to bind with hard bases, and soft acids prefer to bind with soft bases. 
This classification scheme is useful when one considers examines and analyzes 
contaminant\u2013soil interactions and transport in soils. The following summary 
list from Yong et al. (2010) shows some of the more common contaminants 
found in soils as a result of facilities and activities associated with humans:
\u2022	 Lewis hard acids are generally small in size, with high positive 
charge, high electronegativity, low polarizability, and do not have 
unshared pairs of electrons in their valence shells. The acids include 
aluminium chloride, arsenic (III) ion, chloride ion, iron (III) ion, 
magnesium ion, manganese (II) ion to uranium (IV) ion, and zirco-
nium ion.
\u2022	 Lewis borderline acids range from antimony (III) ion, copper (II) ion, 
iron (II) ion, to sulphur dioxide and zinc ion.
\u2022	 Lewis soft acids are generally large in size with low positive charge 
and low electronegativity, in contrast to Lewis hard acids. They have 
unshared pairs of electrons in their valence shells. The acids range 
from borane, cadmium ion, hydroxyl cation, and iodine, to thallium 
(III) ion, and 1,3,5-trinitrobenzene.
\u2022	 Lewis hard bases usually have high electronegativity, low polariz-
ability, and are difficult to oxidize. The bases range from acetate ion, 
ammonia, carbonate ion, chloride ion, to hydroxide ion, nitrate ion, 
and water.
\u2022	 Lewis borderline bases include aniline, bromide ion, nitrogen, and 
sulphide ion.
\u2022	 Lewis soft bases usually have low electronegativity, high polariz-
ability, and are easy to oxidize, in contrast to the hard bases. The 
bases range from benzene, ethylene, hydride ion, and iodide ion 
to trimethylphosphite.
126 Environmental Soil Properties and Behaviour
3.6.2 Acid-base Reactions, Hydrolysis
The chemical reactions in porewater are acid-base reactions. Acid-base reac-
tions are defined as the proton transfer reactions between a proton donor 
(acid) and a proton acceptor (base). Proton loss is called protolysis, and the 
proton transfer reaction is a protolytic reaction. Hydrolysis is a neutralization 
process, and technically speaking classifies as an acid-base reaction inas-
much as it (hydrolysis) refers to the reaction of H+ and OH\u2212 ions of water with 
the dissolved solutes and other constituents present in the porewater. For 
example, the hydrolysis of iron in porewater is shown as
 Fe0 \u21d4 Fe2+ + 2e\u2012
 2H2O + 2e\u2012 \u21d4 2OH\u2012 + 2H+
Then, Fe0 + 2H2O \u21d4 Fe2+ + 2OH\u2012 + 2H+
or Fe2+ + 2H2O \u21d4 Fe(OH)2 + 2H+
The presence of ionized cations and anions associated with the soil par-
ticles in a soil\u2013water system results in pH levels in the soil\u2013water system 
that vary from below neutral to above neutral pH values dependent on the 
strength of ionization of the ions. Hydrolysis reactions in a soil\u2013water system 
will continue so long as the reaction products are removed from the system 
through processes associated with precipitation, complexation, and sorption 
by the soil particles. Hydrolysis reactions of metal ions in the porewater are 
influenced by (a) pH of the active system, (b) type, concentration, and oxida-
tion state of the metal cations, (c) redox environment, and (d) temperature. 
Favourable circumstances for hydrolysis reactions include high tempera-
tures, low organic contents, low pH environment, and low redox potentials.
3.6.3 Oxidation-Reduction (Redox) Reactions
The presence of solutes and microorganisms in the porewater of soils mean 
that abiotic and biotic oxidation-reduction (redox) reactions will occur in 
the porewater. Biotic redox reactions are of greater significance than abiotic 
redox reactions. Oxidation-reduction reactions involve the transfer of elec-
trons between the reactants. The activity of the electron e\u2212 in the chemical 
system typified by the reactive soil particles and solutes in the porewater is 
of particular importance. Generally speaking, the transfer of electrons in a 
redox reaction is accompanied by proton transfer. In the case of inorganic sol-
utes in the porewater, redox reactions result in the decrease or increase of the 
oxidation state of an atom\u2014a matter of some significance for those ions that 
have multiple oxidation states. A measure of the electron activity is the redox 
potential Eh. It allows one to determine the potential for oxidation-reduction 
127Soil\u2013Water Systems
reactions in a clay\u2013water system and is given as Eh= pE(2.3RT/F), where E is 
defined as the electrode potential, R = universal gas constant, T = absolute 
temperature, and F is the Faraday constant. The mathematical term pE is the 
negative logarithm of the electron activity e\u2212. The relationship between Eh 
and pE at a standard temperature of 25°C is
 
Eh pE E
RT
nF
a
a
i ox
i red
= = +
\uf8eb
\uf8ed\uf8ec
\uf8f6
\uf8f8\uf8f70 0591 0. ln
,
,
 (3.34)
where Eo refers to the standard reference potential, n is the number of elec-
trons, a is the activity, and the subscripts for the activity a refer to the activity 
of the ith species in the oxidized (ox) or reduced (red) states.
The redox capacity is a measure of the amount of electrons that can be 
added or removed from the soil\u2013water system without a measurable change 
in the Eh or pE, and is comparable to the buffering capacity, which measures 
the amount of acid or base that can be added to a soil\u2013water system without 
any measurable change in the system pH. The maximum amount of acid 
or base that can be added to a soil\u2013water system without any measurable 
change in the Eh or PE of the system will establish the