Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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redox capacity of the 
soil. In a sense, this concept is similar to the buffering capacity concept of 
clays. Many chemical reactions occurring in a soil\u2013water system are depen-
dent on temperature, concentration of solutes in the porewater, ligands in 
the porewater, and on Eh and also pH, with the latter two being important 
because proton transfer is neutralized by electrons. The Nernst equation, 
which is similar to Equation (3.34), demonstrates this influence:
 
pE Eh E
RT
nF
A H O
B
a w
= = +
\uf8eb
\uf8ed\uf8ec
\uf8f6
\uf8f8\uf8f7
\uf8ee\uf8f0 \uf8f9\uf8fb \uf8ee\uf8f0 \uf8f9\uf8fb
\uf8ee\uf8f0
16 92 0 2. ln
\uf8f9\uf8f9\uf8fb \uf8ee\uf8f0 \uf8f9\uf8fb+
b h
H
 (3.35)
where the superscripts a, b, w, and h refer to the number of moles of reactant, 
product, water and hydrogen ions, respectively.
3.7 Physical Reactions and Hydration
The discussion in this section follows the discussion given in Yong et al. 
(2010). The character of the hydration layer surrounding the surfaces of soil 
particles is different from that of bulk water. The charged surfaces of soil 
particles attract water molecules, resulting in the formation of hydrated soil 
particles. The ions in porewater, which are also hydrated (with water mol-
ecules), are bound to hydrated clay minerals by such forces as Coulomb, 
128 Environmental Soil Properties and Behaviour
van der Waals, short-range repulsion forces, and covalent bonds, as described 
in Chapter 2. In general, the hydration state of ions or the association state of 
ions with clay mineral surfaces can be demonstrated by observing the sta-
tus of oxygen around the ions in porewater, using neutron diffraction (ND), 
the x-ray diffraction (XRD) method, and the extended x-ray absorption fine 
structure spectroscopy (EXAFS) method (Nakano et al., 2004).
3.7.1 Hydrated Cations and Clay Minerals\u2014From EXAFS Analyses
The results from EXAFS analyses are summarized in Table 3.1 (Nakano et al., 
2004; Nakano and Kawamura, 2006). The results show the coordination num-
ber of water molecules around Cs+, Ba2+, and Sr2+ in porewater and their bond 
distances, determined by extended x-ray adsorption fine structure (EXAFS) 
analyses using the two-shell fitting technique on oxygen around metal ions in 
porewater. In addition, the number of the clay mineral oxygens associated with 
the metals and the bond distances between the metals and the clay mineral 
oxygen are shown for air-dried samples in comparison with that for the paste 
samples. The bond distance between the metals and the clay mineral oxygen 
expresses the distance between metal and clay mineral surface. In other words, 
Table 3.1 shows the hydration state of Cs+, Ba2+, and Sr2+ in porewater and the 
mode that the ion is attracted to the hydrated clay mineral surface. The hydra-
tion numbers of Cs+, Ba2+, and Sr2+ of 6, 8, and 8 in bulk solution, respectively, 
have been measured using neutron diffraction (ND) or x-ray diffraction (XRD) 
methods (Ohtomo and Arakawa, 1979; Albrigh, 1972). In comparison with 
these results, the hydration number of metals in porewater is smaller for air-
dried clays, whilst the hydration number for the paste samples is nearly equal 
TAbLE 3.1
Hydration of ions and their association with clay minerals
Sample pH
Cs air-dried 
paste
Ba 
air-dried paste
Sr
air-dried paste
(a) Hydration of metal ion in 
porewater, number of water 
molecules coordinated
4.5
10
4.5
4.5
5.6
6.1
5.1
5.7
6.9
6.8
5.6
5.4
6.8
7.2
Distance between metal and 
water molecule (Å)
4.5
10
3.17
3.18
3.19
3.18
2.88
2.85
2.87
2.87
2.58
2.58
2.59
2.61
(b) Association of metal ion 
with clay mineral, number 
of mineral oxygens 
associated with metal
Distance between metal and 
oxygen on mineral (Å)
4.5
10
5.9
6.3
3.9
4.2
3.7
4.3
3.7
4.1
2.7
2.5
2.4
2.5
4.5
10
3.55
3.56
3.62
3.61
3.09
3.07
3.11
3.11
2.77
2.76
2.80
2.83
Note: The data have been obtained from the two-shell fitting technique of EXAFS spectra.
 (a) denotes the first shell of oxygen around the metal ion, (b) denotes the second shell of oxygen 
around the metal ion.
129Soil\u2013Water Systems
to that in bulk solution. The number of clay mineral oxygens associated with 
the metals is about 3 to 4 except for that of the Cs ion in the air-dried state. Cs 
ions specifically associate with the mineral oxygen of 6, indicating their strong 
bonding to mineral surfaces in the air-dried state. The evidence suggests that 
metals in the porewater hover over clay mineral surfaces due to the hydra-
tion of clay minerals. This conclusion arises from the observation that the dis-
tances between metal and mineral oxygen is considered to be slightly larger 
than the bond distances of metal-oxygen in the hydrated metals in porewater. 
Accordingly, the metals are assumed to be easy to partition in porewater when 
water will migrate to the air-dried clays from the surrounding region.
3.8 Concluding Remarks
There are some important observations that need to be highlighted:
\u2022	 The energy status of a soil\u2013water system is an important character-
istic of soils. Measurement of the energy state of water in a soil (soil 
water) requires techniques and procedures that produce results that 
are often operationally defined. Take, for example, the pressure mem-
brane test system, which is a common laboratory technique used to 
determine such a characteristic. For pressure membrane tests, it is 
difficult to obtain representative undisturbed samples in small sizes 
to fit the lucite collars. Because of vapour losses and difficulties in 
obtaining representative samples, measurements from pressure 
membrane are less satisfactory for clays than for loams or sands. Air 
bubbles almost always accumulate in the water below the ceramic 
porous stone or membrane, due possibly to leaks or movement of 
air through the water phase due to a pressure gradient. The air will 
interfere with the measurement of water being discharged, and will 
also carry away water vapour from the sample in the chamber.
\u2022	 The results of tests using the Haines-type (suction) system and a pres-
sure system using the Buchner-type apparatus described in Figure 3.15 
are shown in Figure 3.25. Note the importance of the initial state of the 
sample under test. The differences in results are due to two important 
factors: (a) pore geometry effects between sorption and desorption 
(wetting and drying) as illustrated in Figure 3.2 and (b) entrapped air.
\u2022	 Hysteretic phenomena observed in water characteristic curves pre-
dominate in sands rather than in clays. The water characteristic 
curves are important pieces of information that reflect the changing 
water content processes, and are useful in analyses of water uptake, 
drainage, and water flow in engineering projects.
130 Environmental Soil Properties and Behaviour
\u2022	 Water movement in partly saturated soils occurs along film 
boundaries in soil pore spaces that are not completely filled with 
water, and as pore channel flow for those pore spaces that are 
completely filled with water. Initially, water uptake is through 
hydration processes. Further water transport into the soil is from 
film boundary transport. As further water is drawn in, the air 
within the soil must escape to the surface. At full saturation, if 
trapped air in some of the voids cannot escape, it will remain in 
the microstructural unit.
\u2022	 Pore channel flow has been modelled as saturated flow, which is obvi-
ously inappropriate. Obviously, we cannot evaluate or analyze unsat-
urated flow using two separate and different analytical models; that 
is, it is not practical to perform film boundary flow analysis in con-
junction or combination with saturated flow analysis since we have no 
means to determine the proportion of film boundary or saturated flow 
contributing to the total flow. Instead, we have