Thermochimica Acta 421 (2004) 211–215
Thermal characterization during dehydration of
nitrifying and denitrifying microbiological
mud encapsulated in silica gel
G. Aguirre a , G. Arriola a , J. Gómez-Hernández a , T. López a , M. Picquart a,∗ ,
D.H. Aguilar b , P. Quintana b , J.J. Alvarado-Gil b
a
b
Universidad Autónoma Metropolitana-Iztapalapa, A.P. 55-534, Mexico 09340, D.F., Mexico
Cinvestav-IPN, Unidad Mérida, Departamento de Fı́sica Aplicada, A.P. 73. Cordemex, Mérida, Yucatán, Mexico 97310, Mexico
Received 3 December 2003; received in revised form 12 February 2004; accepted 5 April 2004
Available online 21 July 2004
Abstract
An alternative method to diminish the nitrogen pollutant levels in waste waters is to encapsulate microorganisms whitin ceramic materials.
The sol–gel method has been extensively used for the preparation of such kind of materials permitting a higher stability and viability of useful
organisms. In this work, the thermal characterization during dehydration of nitrifying and denitrifying aqueous emulsions of mud encapsulated
in sol–gel silica is presented during the process of dehydration in ambient conditions and as a function of temperature. The characterization
was performed by a photopyroelectric (PPE) technique, whose detector was made with a 110 mm polyvinyldifluoride (PVDF). The cell was
constructed in such a way that the sample was inside the cell, and the bottom of the cell was closed by the PVDF foil. Thermal effusivity as
a function of temperature was obtained illuminating the PVDF directly by a modulated 1W tungsten lamp. The sample is enclosed inside a
chamber, using a Peltier cell that controls temperature in a range from 40 to 27 ◦ C. The sample is on top of the PVDF, which is illuminated
by the modulated tungsten lamp.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Nitrifying; Denitrifying; Muds; Sol–gel; Thermal properties; Photopyroelectric technique
1. Introduction
Nitrate is an important contaminant of water sources due
to its high mobility and stability [1]. Several studies reveal
that high concentrations of this pollutant (more than 45 mg/l)
produce infant diseases as methemoglobinemia, commonly
known as “infant cyanosis” and human cancer.
The problem for the elimination of nitrogenous compounds has not yet been completely solved. The physical–
chemical elimination of ammoniac compounds became too
expensive in waste waters with high concentration of these
products. For these reasons, several research groups have focused their studies in the biological elimination of nitrogen,
using microbial consortiums to reduce the nitrate concentration in waste water [2].
∗ Corresponding author. Tel.: +52-5-558-04-46-21;
fax: +52-5-558-04-46-11.
E-mail address: mp@xanum.uam.mx (M. Picquart).
0040-6031/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2004.04.010
Biological nitrate elimination can occur by denitrifying
or nitrifying processes. During denitrification, nitrate is reduced to N2 by a wide number of heterotrophic bacteria,
such as Aerobacter, Klebsiella, and E. coli, and few fungi,
e.g., Neurospora. The nitrification is a process made by
Gram-negative litoautotrophic unsporulated microorganisms
from the family Nitrobacteriaceae.
The use of microbial muds to degrade nitrates presents
some limitations, such as the contact surface area and
the instability of microbial species. However, it has been
proved that employing the sol–gel method to encapsulate
biomolecules and microorganisms, significantly reduce
restrictions [3–6]. The sol–gel process permits the encapsulations of several organic compounds and living cells due
to its preparation features, such as: low temperature, high
stability, pH regulation, and contact surface area. Encapsulation allows the permanent interaction between the microorganisms and the substrate, and does not harm viability
and cell reproduction [3,5,7].
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G. Aguirre et al. / Thermochimica Acta 421 (2004) 211–215
Fig. 2. Cross-section of the photoacoustic cell used: (a) sample, (b) 1 cm
acrylic ring, (c) aluminum reference material, (d) quartz window, (e)
modulated light, and (f) microphone.
Fig. 1. Sol–gel encapsulation of microbiological muds.
2. Experimental
2.1. Mud encapsulation
Two kinds of microbial consortiums were used—denitrifying mud and nitrifying mud. Both were obtained by continuous cultures as reported by Gomez et al. [2].
The sol–gel method was employed to encapsulate the
muds within a silica network, as shown in Fig. 1. Tetraethyl
orthosilicate (TEOS, 99.999+%) was used as precursor and
a water relationship of 1:8 was used. The mixture was kept
with low stirring and ambient temperature (27 ◦ C), 5 ml of
nitrifying mud was added to the prehydrolyzed solution drop
by drop until gellation. The stirring continued until gellation. The gel was aged during 168 h and then dried into a
rotary evaporator system (Eseve D402-2) at ambient temperature until eliminate residues of water and alkoxide. Nitrifying mud was synthesized at pH 6, whereas denitrifying
mud at pH 7.
2.2. Sample preparation
For the photoacoustic (PA) and photopyroelectric (PPE)
measurements, the encapsulated muds were grinded and
mixed with water in order to get a liquid suspension. The relationship, mud:water was of 1:1.5 for the denitrifying and
1:4 for the nitrifying mud.
2.3. Photoacoustic measurements
PA monitoring was carried out using the PA cell shown
in Fig. 2. In this configuration the PA cell is closed at
the bottom end, by a glass window and at the top end,
by a removable substrate. An electret microphone is used,
coupled to the cavity wall, to sense the pressure fluctuations in the PA chamber produced by the periodic heating
of the substrate due to the pumping beam. This substrate
is used as a reference material. The sample is deposited
on the external surface of this reference material. The experimental arrangement consists of a He–Ne 25 mW laser
beam, which is mechanically modulated by an optical
chopper (SR540) at constant frequency (f = 7 Hz), for
all experiments, and focused onto the reference. The microphone signal is fed into a lock-in amplifier (SR830),
from where the output signal amplitude is recorded, as a
function of time, in a personal computer. All measurements
were performed using an aluminum foil of 50 m thickness as the reference material, with a thermal diffusivity
of 0.9 cm2 /s.
According to the Rosencwaig and Gersho model [8,9],
the PA signal is determined by the temperature fluctuation,
θ, at the air–substrate interface (x = 0). Solving the thermal
diffusion equation for the configuration shown in Fig. 2, one
gets
b tanh(σl) + 1
θ = θ0
(1)
b coth(σl) + 1
here σ is the complex thermal diffusion coefficient, defined
by σ = (1 + i)a, with a defined by (π/fα)1/2 , b = εb /ε
the thermal coupling coefficient, with ε and εb defined as
thermal effusivities of the substrate and sample, respectively. θ 0 the expression for the temperature fluctuation
in the absence of the sample on the substrate, which is
given by θ0 = (I0 /2k)coth(σl). In arriving at Eq. (1), we
used the fact that the effusivity of air is much smaller
than the thermal effusivity of the substrate, which is optically opaque. Also, it has been considered that the optical
absorption coefficient, β is much larger, that the absolute value of the complex thermal diffusion coefficient,
|σ|, for the frequency is used here. Moreover, the backing thickness, lb , is supposed to be much larger than the
thermal diffusion length of the sample, which is given by
G. Aguirre et al. / Thermochimica Acta 421 (2004) 211–215
µb = (αb /πf)1/2 , being αb , the thermal diffusivity of the
sample. From Eq. (1) the amplitude of the ratio θ/θ 0 is
given by
θ
q =
θ0
√
cosh(2al) − cos(2al)
=√
cosh(2al) + cos(2al)
(b + 1)2 e2al + (b − 1)2 e−2al − 2(b2 − 1)cos(2al)
×
(b + 1)2 e2al + (b − 1)2 e−2al + 2(b2 − 1)cos(2al)
(2)
Solving Eq. (2) for the thermal coupling coefficient, one
gets the sample effusivity
√
−B + B2 − AC
εb = ε
(3)
A
where:
A = (q2 − 1) cosh2 (2x) + cos2 (2x)
+ 2(q2 + 1)cosh(2x)cos(2x)
B = sinh(2x) (q2 − 1)cosh(2x) + (q2 + 1)cos(2x)
C = (q2 − 1) cosh2 (2x) − cos2 (2x)
and x = al
Measuring the evolution of q = q(t), the values of εb (t)
can be obtained.
2.4. Photopyroelectric measurements
The system for determining the thermal effusivity consisted of a closed PPE cell as shown in Fig. 3. The pyroelectric film polyvinyldifluoride (PVDF) was directly
213
illuminated by a tungsten low power lamp (2 mW) operating in modulated form using a circuit coupled to a power
supply and at a given frequency fixed by the TTL output of
a SR830 lock-in amplifier.
The temperature control consisted of a homemade system
based on the use of a Peltier cell for temperature control coupled to a programmable power supply and a LM35DZ device for monitoring the temperature of the cell. The current
provided by the power supply is regulated by a PID control
system in LabView. The samples were positioned inside of
the cell at 40 ◦ C, and the measurements were performed by
lowering the temperature at a rate of 1 ◦ C/h. After each programmed change of temperature, a scan of the modulation
frequency of the lamp were performed for 0.1, 0.2, 0.3, 0.4,
and 0.5 Hz. The detection of the PPE signal and the control
of the modulation frequency of the lamp were done, using
the lock-in amplifier (SRS830), interfaced with a personal
computer via a GPIB bus.
Thermal effusivity was determined directly from the experimental data, using the fact that for samples and sensor
are thermally thick and thermally thin respectively; the PPE
signal is inversely proportional to the effusivity of the sample [10,11]:
√
f
V =K
(4)
em
where K is a factor depending on the sensor, em the thermal
effusivity of the sample, and f the modulation frequency of
the lamp. The cell was calibrated, filling it with water and
developing the temperature scan from 40 to 25 ◦ C. Afterwards, the cell was filled with mud, and a scan in the same
range of temperature is performed. The signal obtained for
the mud is normalized using the calibration values for the
water.
Fig. 3. Photopyroelectric cell.
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G. Aguirre et al. / Thermochimica Acta 421 (2004) 211–215
3. Results
The experimental results for the PA measurements as a
function of time are shown in Fig. 4. The processes begin
with a low and constant signal and after some time it increases, reaching a stable level, this behavior is due to the dehydration process of the muds. The lower level corresponds
to the high content of water, meanwhile the upper one is related to the dried stage. Due to the shape of the curves of
the obtained data, it can assumed that a second-order kinetics is present, therefore, the data can be analyzed using a
sigmoidal fitting functions of the following form:
B
1 + e(t−t0 )/td
S =A+
(5)
where S is the PA signal, td is related to the time interval
where the maximum variation in the signal occurs and t0
is identified as the time at which the concavity of the sig-
moidal curve changes. A and B are constants related with
the maxima and minima of the signals.
Using Eq. (5), the fitting values for nitrifying (Fig. 4a) and
denitrifying mud (Fig. 4b), for the time when the process
reaches the maximum velocity t0 , are 4.563 and 2481 s; and
for the time interval in which the increasing process occurs,
td are 110 and 277 s, respectively. Time t0 is 1.84 larger and
td is 0.4 times lower for nitrifying than for the denitrifying
mud, as can be seen in Fig. 4a and b. These results indicate,
the fact that the beginning of the dehydration process is
more delayed in the nitrifying mud case. However, when the
process develops, the dehydration occurs faster in the case
of the nitrifying mud. Given, that the nitrifying mud presents
a clay-like consistency, the results of these measurements
only show the first stages of dehydration, since the whole
process takes several days. Therefore, our results indicate,
that in the first stages of dehydration, water is released more
easily in nitrifying muds.
-3
1.1x10
PA Amplitude (mV)
-3
1.1x10
Nitrifying mud
encapsulated in SiO2
at pH=6
-3
1.1x10
-3
1.0x10
t 0= 4563 s
t d= 110 s
-3
1.0x10
-3
1.0x10
-3
1.0x10
0
1000
2000
PA Amplitude (mV)
6.0x10
-4
5.8x10
-4
4000
5000
6000
5.6x10
-4
5.4x10
-4
5.2x10
-4
5.0x10
-4
4.8x10
-4
4.6x10
-4
4.4x10
-4
4.2x10
-4
Denitrifying mud
encapsulated in SiO2
at pH=7
t0= 2481 s
td= 277 s
0
(b)
3000
time (s)
(a)
1000
2000
3000
4000
time (s)
Fig. 4. Photoacoustic monitoring of the dehydration process of muds
encapsulated in SiO2 as a function of time: (a) nitrifying mud and (b)
denitrifying mud.
Fig. 5. Behavior of the thermal effusivity effect of microbiologicals muds
when temperature diminishes: (a) nitrifying mud encapsulated in SiO2 at
pH 6 and (b) denitrifying mud encapsulated in SiO2 at pH 7. Arrows
indicate the temperature progression.
G. Aguirre et al. / Thermochimica Acta 421 (2004) 211–215
On the other hand, the measurements of the thermal effusivity, using the PPE technique, at controlled and variable
temperature, presents a very different behavior for the two
types of muds. In the case of nitrifying muds thermal effusivity grows when the temperature decreases (Fig. 5a), in
contrast, the denitrifying mud thermal effusivity decreases
for lower temperatures (Fig. 5b). This behavior can be understood, taking into account that the consistency of each
mud, denitrifying ones present a gel-like aspect and dissolves easily in water. Therefore, the behavior is similar to
the one found for pure water. In contrast, nitrifying mud
behaves as a clay, requires more water and agitation during its preparation, showing a higher viscosity. The slight
increase of thermal effusivity when the temperature is lowered can be explained by an increase in the viscosity of
the mud. This increase indicates that the structure of this
mud is becoming more compact and this could enhance
heat diffusion. In agreement with these results, dehydration
shows a delay and with a high initial velocity, indicating
that free water is easily evaporated and the real process of
dehydration takes much longer period of time (more than
1 day).
4. Conclusions
Dehydration process of nitrifying and denitrifying muds
has been analyzed by the PA technique. The behavior of
thermal effusivity, as a function of temperature has been
monitored using PPE technique. Our results, show that nitrifying muds retain water for longer period of time, but
release water faster in the initial stages of the dehydration process. Thermal properties are strongly influenced
215
by the physical characteristics of the muds, denitrifying
muds showing a water-like behavior, and nitrifying muds,
a clay-like behavior. Our results could be very useful in the
understanding of the mud with water when encapsulated in
a sol–gel ceramic matrix.
Acknowledgements
The authors wish to thank J. Bante and F. Kantun for
technical assistance. This work was partially supported by
Conacyt Project No. 38493-U.
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