Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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(a) about 0°C to about 
10°C for psychrophiles, (b) about 10°C to about 45°C for mesophiles, and 
(c) about 45°C to about 75°C and above for thermophiles. The lower and 
71Nature of Soils
upper bounds of the temperature ranges are not absolute. It is commonly 
accepted that a 10°C increase in temperature will double the growth rate. 
Since normal ground temperatures in most habitable regions range from 
slightly below 0°C to about 45°C, psychrophiles and mesophiles are the 
most common types of microorganisms found in the ground. Capsule for-
mation enables microorganisms to survive at reduced temperatures and 
grow as the temperature increases to more favourable conditions (Sims 
et al., 1990).
Water availability is essential for the survival of microorganisms, with 
moisture levels of 50% to 75% being considered to be ideal conditions. Water 
constitutes (a) the principal component of cell protoplasm and (b) the carrier 
for nutrient transport into the cell. An excess of water could result in oxygen 
limitations, because the low solubility of oxygen in water (9 mg/L at 20°C) 
makes it difficult for oxygen to be transported to bacteria in saturated soil 
pore spaces. Sufficient oxygen must be available for aerobic and facultative 
anaerobic bacteria, with a concentration of approximately 2.0 mg/L being 
considered to be a good value (Reynolds and Richards, 1996). This means 
that partly saturated soils are better conditions for optimal microorganism 
growth. Microbial activities in reduction of electron acceptors will affect pH 
levels through production of H+.
Microbial flora in groundwater include (a) aerobic and microaero-
philic heterotrophic microorganisms and (b) anaerobic iron-reducing and 
sulphate-reducing bacteria such as Desulfovibrio and Desulfotomaculum. 
Chemoautotrophs such as T. thiooxidans and T. ferrooxidans play significant 
roles in the subsurface environment, where organic matter is in short supply, 
because of their ability to (a) obtain energy required to sustain life by oxida-
tion of inorganic matter, and (b) obtain carbon required for the formation 
of bodies by decomposing carbon dioxide. Microorganisms react with clay 
minerals by producing extracellular polysaccharides that coat the microor-
ganisms through the various forms of interaction. Indigenous bacteria in 
soils, together with extraneous bacteria transported into soils by groundwa-
ter, will grow on the surfaces of soil particles in large pores or fractures with 
widths larger than 5 \u3bcm, provided that water and nutrients are available for 
growth in the microenvironment. Microbial flora will grow and induce bio-
logical dissolution/transformation of clays or metals following their adher-
ence on the surfaces of clay particles, or following migration into fissures 
and cracks in clay buffer/barriers of more than 0.25 \u3bcm in width (the mini-
mum size of bacteria).
2.8.4 bacterial growth Kinetics
For bacterial growth, the maximum rate of growth and metabolism 
occurs during the log growth phase, in which the log of the number of 
cells versus time is linear according to Monod (1949). As the substrate oxy-
gen or nutrient becomes limiting or is depleted, pH shifts result, or toxic 
72 Environmental Soil Properties and Behaviour
components start to accumulate as the declining growth phase occurs 
(Figure  2.18). The rate of growth decreases, and microorganisms begin 
to die. The stationary phase occurs as the numbers of cells produced and 
dying are equal. This occurs usually over a 12- to 36-h period. The death 
phase occurs when the number of microorganisms dying exceeds the 
amount growing, and the cells may either be inactivated or may start to 
degrade and break apart.
The growth rate of living bacteria can be expressed as (Monod, 1949)
 d
dt
m
m
\u3c1 µ \u3ba \u3c1= \u2212( ) (2.3)
where \u3c1m is the biomass density, µ is the intrinsic growth rate, and \u3ba is the 
intrinsic decay rate coefficient. Nakano and Kawamura (2010) have sug-
gested that in view of the ageing and lifespan of bacteria, the intrinsic 
growth and decay rate coefficients can be considered as functions of time 
as follows:
 µ µ µ µ= \u2212 \u2212( )max .exp( )1 2a b t (2.4)
Stationary
phase
Log ph
ase
Death phase
De
clin
ing
gro
wt
h
Ex
po
ne
nt
ial
 or
lo
g p
ha
se
0
1
2
3
4
5
6
Time in Hours
Lo
g N
um
be
r o
f B
ac
te
ria
4 8 12 16 20 24
FIguRE 2.18
Bacterial growth as a function of time. (Adapted from Yong, R.N., and Mulligan, C.N., 2004, 
Natural Attenuation of Contaminants in Soils, CRC Press LLC, Boca Raton, FL, 319 pp.)
73Nature of Soils
 \u3ba \u3ba \u3ba \u3ba= \u2212 \u2212( )max .exp( )1 2a b t (2.5)
where \u3bcmax and \u3bamax are the maximum values of \u3bc and \u3ba, respectively; t is time; 
and a and b are constants.
We can observe the microbial growth kinetics under conditions that 
substrate and oxygen concentration control an increase in biomass, as for 
example: (a) under conditions where the microenvironment is isolated 
from the outside, such as those in simple laboratory culture experiments, 
or (b) where the inflow of substrate and oxygen into the system is smaller 
than the demand by microbes in natural soils. In such cases, we describe 
bacteria growth kinetics using the equation that explicitly includes the 
factors relating to the concentration of substrate and oxygen. The two 
types of equation describing this event are (a) those taking into consid-
eration the effects on the intrinsic growth rate only (Molz et al., 1986), 
and (b) those multiplying the intrinsic growth rate by the inhibition term 
(Thullner, 2003).
 \u2202
\u2202 = + + \u2212
\uf8eb
\uf8ed\uf8ec
\uf8f6
\uf8f8\uf8f7
\u3c1 µ \u3ba \u3c1m s
s s
o
o o
mt
c
K c
c
K cmax
 (2.6)
 \u2202
\u2202 = + + +
\u3c1 µ \u3c1m s
s s
o
o o
inh
inh inh
mt
c
K c
c
K c
k
k cmax
 (2.7)
where c is the concentration, K is the half-saturation constant in solution or 
gas, subscripts s and o refer to substrate and oxygen respectively, cinh is the 
concentration of substances inhibiting the growth, and kinh is the inhibition 
constant.
In the environment, following the expiration of a group of bacteria due to 
certain inhibition factors, a new group of bacteria may arise after an interval 
from bacteria originating from the outside or from indigenous bacteria. For 
the long term, they would repeat the growth and decay cycle as in process 
A shown in Figure 2.19. As another case, we can consider an event where the 
decay process overlaps with the next growth process that occurs in the new 
group of bacteria, resulting in a steady population of bacteria at any time as 
shown in process B in Figure 2.19. The changes in population of bacteria in 
soils will be influenced by the local environmental conditions in the pores 
of soil.
74 Environmental Soil Properties and Behaviour
2.9 Laboratory Determinations
2.9.1 Scope of Tests
In this section, we will be concerned with what should normally be con-
ducted in laboratory tests in respect to determination of soil type and its 
physical properties for a particular soil being considered for soil and geoen-
vironmental engineering applications. The scope of laboratory tests (extent 
and detailed scrutiny) for type of soil and physical properties is dependent 
on at least two main groups of factors. Group I is the most obvious group of 
factors, whilst the second (Group II) is perhaps somewhat hard to fully grasp. 
To a large extent, this is because the two factors in Group II are intertwined. 
Defining the various impacts and their consequences will depend on the 
kinds of expertise \u201cbrought to the table.\u201d
Group I:
\u2022	 Type of engineering application
\u2022	 Economic considerations
\u2022	 Time constraints
Po
pu
lat
io
n 
of
 M
icr
oo
rg
an
ism
s
Time
Time
Process A
Process B
1st 2nd 3rd 
1st 2nd 3rd