Photolytic Destruction Of Halogeneated Pyridines In Wastewaters
a
Maria Papadakia*, David R. Stapletona and Dionyssios Mantzavinosb
Chemical Engineering, School of Process Environmental and Materials Engineering, University of Leeds
Clarendon road, Leeds LS2 9JT, UK
b
Department of Environmental Engineering, Technical University of Crete
GR-73100 Chania, Greece
Abstract
The degradation of 2-chloropyridine and 2-fluoropyridine, compounds typically found in effluents of
pharmaceutical processing, was studied by means of ultraviolet irradiation. Experiments were conducted at
temperatures of 293, 303, 313 and 323 K for 2-chloropydine and 313K for 2-fluoropyridine, liquid volumes of
0.25, 0.40 and 0.55 L and initial substrate concentrations of 300 and 500ppm in a flow-through reactor with
internal recycle. Both compounds were readily susceptible to photolytic degradation with complete conversion
being achieved within 2-4 hr of reaction depending on the operating conditions employed; however, during that
period mineralization was generally low with the extent of organic carbon not exceeding 20%. However
complete mineralization was achieved after approximately 50 hr of irradiation. At 313 K, 2-fluoropyridine was
more readily degradable than 2-chloropyridine. Degradation was found to be first order in substrate
concentration with the rate increasing with increasing temperature and decreasing liquid volume and initial
concentration. 2-pyridinol was found to be a major degradation by-product.
Keywords: photolysis, kinetics, 2-chloropyridine, 2-fluoropyridine
1. Introduction
Advances in chemical water and wastewater treatment have led to a range of processes termed advanced
oxidation processes (AOPs) being developed. These processes include, amongst others, ultraviolet irradiation,
ozonation, the Fenton reagent, electrochemical oxidation as well as various combinations of them, and more
recently the use of ultrasound irradiation. AOPs have shown great potential in treating pollutants at low and high
concentrations and have found applications as diverse as groundwater treatment, municipal wastewater treatment
sludge destruction, VOCs control and water disinfection (Suty et al., 2004). Nowadays, complete remediation of
water and wastewaters by AOPs from persistent organic pollutants that cannot be treated by conventional
biological technologies becomes increasingly prominent given (i) the increased public concern about the
potential toxic effects of contaminants in water and (ii) the progressively more restrictive requirements regarding
the admissible levels of such contaminants in the aquatic environment imposed by the regulatory agencies. As an
alternative to complete chemical oxidation treatment, AOPs may be employed as a pre-treatment stage followed
by biological post-treatment (Mantzavinos ans Psillakis, 2004).
Pyridine and its derivatives constitute an important class of compounds with several applications in the
pharmaceuticals and cosmetics sector (Paune et al., 1998) as well as being used as pesticides (Abramovic et al.,
2004). Chloropyridines, in particular, are known to be hardly biodegradable and highly toxic to fresh water and
marine bacteria (Liu et al., 1998) and special attention is needed as these compounds can be found not only in
the effluents of certain industrial activities but also in raw and drinking water due to spills or fugitive emissions.
For instance, concentrations of 2-chloropyridine as high as about 60 and 75 mg/L in raw and drinking water
respectively have been reported in the literature (Paune et al., 1998, Malato et al., 2002).
Ultraviolet (UV) light-induced degradation processes constitute a well-established practice in water and
wastewater treatment and numerous research studies have been undertaken over the past few years. UV-driven
AOPs are primarily based on the generation of powerful oxidizing species, such as OH radicals, through the
direct photolysis of H2O or through photo-induced processes as in the photo-Fenton reactions and semiconductor
photocatalysis (Parsons, 2004). In addition to degrading organic contaminants, UV processes serve as an
excellent means of disinfection (Lawryshyn and Cairns, 2003).
In this work, we investigate the treatment of 2-chloropyridine and 2-fluoropyridine in water by means of UV
irradiation with emphasis on the effect of various operating conditions (i.e. reaction temperature, liquid volume,
initial substrate concentration) on degradation rates. In our previous work (Stapleton et al, in press) we reported
on a comparative study of the degradation of pyridines using different AOPs; photolysis and photo-Fenton
oxidation were found to be the most effective methods for pyridine degradation.
2. Materials and Methods
2.1 Materials
2-Chloropyridine (2CP), 2-fluoropyridine (2FP) (all 98% purity) were supplied by Fluka and used without
further purification.
2.2 Experimental setup
Figure 1 shows the arrangement employed for the photodegradation experiments. It consists of a tubular reactor,
(R), inside which an Upland Pen Ray UV lamp (254 nm, 110 W) with a diameter of approximately 10 mm and a
length of 200mm was submerged. The reactor diameter was 30 mm. A cooling system facility was also
available, thus maintaining a constant liquid temperature. A schematic of the arrangement employed is shown in
Figure 1.
R
P
10mm
300mm
UV lamp
V
es
se
l
Temperature bath
30mm
Figure 1. Equipment schematic
A vessel (V) of an appropriate volume was submerged into the constant temperature bath. A pump (P) capable of
circulating 200 ml/min was used to ensure isothermal operation via continuous recirculation of the solution.
It should be noted that substrate degradation other than in the illuminated area, ie inside the tubular reactor was
found to be negligible.
2.3. Analytical
Samples periodically drawn from the reactor were analyzed by means of HPLC (Hitachi/Merk LaChrom L7400
UV detector, L-7100 pump) to follow the concentration profiles of the substrate. Separation was carried out on a
Symmetry Shield RP8 column (Waters) using 20:80 acetonitrile:water as an isocratic mobile phase at 1 ml/min
or gradient elution from 20:80 to 0:100 acetonitrile:water at 1 ml/min. UV detection was set at 265 nm.
The total organic carbon (TOC) content of the liquid phase was measured using a Shimadzu 5050A analyzer
equipped with an automatic sampler. A Corning – pH/ion meter 135 was used to measure the pH of the liquid
phase. All runs were carried out at the solution ambient pH, with an initial value of about 8 for 2CP for instance.
Solution pH was left uncontrolled during the experiments although it was periodically monitored.
3. Results and Discussion
Experiments were conducted at 293, 303, 314 and 323 K at initial substrate concentrations of 300 and 500ppm in
aqueous solutions of 0.25, 0.40 and 0.55 L. Complete degradation of both substances was achieved within a few
hours of irradiation, depending on the experimental conditions employed. Moreover, the decomposition of the
original substrate was accompanied by the release of inorganic Cl - and F-, which was manifested by the rapid
drop in the solution pH, as reported elsewhere (Stapleton et al, in press). 2-Pyridinol has been identified as one
of the major intermediates formed during the degradation of both substances. TOC removal was very small
during this period, but continuing irradiation over a period of two to three days resulted in complete
mineralisation. Figures 2 and 3 show concentration-time profiles during the photodegradation of 2CPy (2chloropyridine) as a function of liquid volume and reaction temperature at initial concentrations of 300 and
500ppm respectively.
1
0.9
0.8
0.7
313K_300PPM_0.25L
303K_300PPM_0.25L
313K_300PPM_0.4L
313K_300PPM_0.55L
293K_300PPM_0.25L
303K_300PPM_0.4L
293K_300PPM_0.4L
303K_300PPM_0.55L
C/C0
0.6
0.5
0.4
0.3
0.2
0.1
0
0
20
40
60
80
100
120
140
160
180
200
Time (min)
Figure 2. Normalised concentration reduction during the photolytic degradation of 2-chloropyridine in aqueous
solution with an initial concentration of 300ppm.
As can be seen, photolysis proceeds faster at higher temperatures and smaller liquid volumes. It can be seen that
measurements performed at the same temperature and initial concentration reach almost complete degradation in
approximately proportional lengths of time. So, at 313K and 300 ppm an 80% conversion is achieved in 20 min
in a volume of 0.25L, 40 min in a volume of 0.4 L and 60 min in a volume of 0.55L. Accordingly, a 70%
conversion is achieved in 20, 40 and 60 min if a 500ppm solution of volumes 0.25, 0.4 and 0.55L, respectively,
is irradiated at 313 K. Measurements on solutions of different initial concentrations but of the same volume and
temperature also display a similar behaviour as can be seen by a comparison of figures 2 and 3. For instance, for
measurements on 0.4 L solution volume at 313K, a 70% conversion is achieved in 50 and 28min for samples of
initial concentration of 500 and 300 ppm respectively. Similarly, for samples of 500ppm and 300ppm of a 0.55L
volume treated under the same temperatures the process time for a 70% conversion is 60 and 40 min,
respectively. It is worth noticing, however, that at very low volumes (0.25L) and high temperatures (40 oC) the
substrate degradation is very fast and there is very little difference in the time needed to achieve complete
decomposition between the two selected initial concentrations.
1
0.9
313K_500PPM_0.25L
323K_500PPM_0.4L
0.8
313K_500ppm_0.4L
313K_500PPM_0.55L
293K_500PPM_0.25L
293K_500PPM_0.4L
0.7
C/C0
0.6
0.5
0.4
0.3
0.2
0.1
0
0
50
100
150
200
250
Time (min)
Figure 3. Normalised concentration reduction during the photolytic degradation of 2-chloropyridine in aqueous
solution with an initial concentration of 500ppm.
Figure 4 shows a comparison between the photolytic decomposition of 2-chloropyridine and 2-fluoropyridine at
different conditions. As can be seen the rate of decomposition of 2-FPy(2-fluoropyridine) at 313 is higher than
that of 2CPy in all volumes and/or initial concentrations employed. The electronegativity of fluorine is greater
than that of chlorine, which should result in stronger bonding. In fact C-F bond energy is reported as 488 kJ/mol
and the C-Cl bond is 330 kJ/mol. The F-C bond should therefore be less susceptible to radical attack. The UV
absorbance at 254nm of 2-FPy is weaker than that of 2-CPy, (3.43 and 3.63 respectively, units in logarithm
epsilon of absorbance) indicating that 2-CPy should be more susceptible to absorption and therefore
decomposition. However there are more major intermediates formed in the decomposition of 2-FPy indicating a
more complex reaction mechanism. The role played by inorganic fluorine in solution is not known, the
behaviour of this may explain the different photoproducts and reaction rates.
1
FPY_313K_300PPM_0.25L
FPY_313K_500_0.25L
FPY_313K_300PPM_0.4L
CPY_313K_300PPM_0.25L
FPY_313K_500PPM_0.4L
CPY_313K_500PPM_0.25L
CPY_313K_300PPM_0.4L
CPY_313K_500ppm_0.4L
CPY_313K_500PPM_0.55L
0.9
0.8
0.7
C/C0
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
70
80
90
100
Time (min)
Figure 4. Comparison of normalised concentration reduction during the photolytic degradation of 2chloropyridine and 2-fluoropyridine in aqueous solution at different conditions.
Although it is believed and it is currently under investigation that the reaction follows a more complex
mechanism, especially during the initial stages of UV irradiation, the data can be fitted adequately well to a
pseudo-first order rate. In figure 5 there are typical results of the linear fit of –ln(C/C0) versus time for a number
of measurements at different conditions.
3.5
3
CPY_313K_300PPM_0.25L
CPY_313K_500ppm_0.4L
CPY_293K_500PPM_0.4L
CPY_313K_500PPM_0.25L
CPY_303K_300PPM_0.25L
CPY_303K_500PPM_0.25L
CPY_293K_300PPM_0.55L
FPY_313K_500_0.25L
FPY_313k_500PPM_0.4L
- ln (C/C0)
2.5
2
1.5
1
0.5
0
0
20
40
60
80
100
120
140
160
180
200
Time (min)
Figure 5. Pseudo-first order kinetics of 2-chloropyridine degradation at different conditions.
The rate constants for all measurements of 2CP were plotted in an Arrhenius type graph shown in Figure 6. It
can be seen that different volumes and initial concentrations form nearly parallel straight lines, which implies
that the the activation energy is approximately the same. The shift of the lines (different frequency factors) as
the initial concentration or the solution volume changes indicates that the reaction follows a different type
kinetics as suggested by Jirkovsky et al 2004. This is currently being investigated.
0.00305
-2
-2.5
0.0031
0.00315
0.0032
0.00325
0.0033
ln(k)
0.0034
y1 = -5563.7x + 12.034
5
y2 = -6004.3x + 16.355
1
-4
2
1: 300ppm_0.25L
-4.5
-5
-6
y4 = -7105.6x + 18.93
3
y5 = -6651.4x + 17.351
2: 500ppm_0.25L
3: 300ppm_0.4L
4: 300ppm_0.55L
-5.5
0.00345
y3 = -5374.7x + 13.845
-3
-3.5
0.00335
y6 = -7394.9x + 19.324
4
5: 500ppm_0.4L
6: 500ppm_0.55L
6
1/T (K-1)
Figure 6. Arrhenius dependency of pseudo-first order rate coefficient k on absolute temperature of 2chloropyridine degradation at different conditions.
4. Conclusions
The photolytic degradation of 2-chloropyridine and 2-fluoropyridine has been studied at different temperatures,
volumes and initial concentrations with 2-chloropyridine being more stable than fluoropyridine at the conditions
under consideration. Their rate of photolytic decomposition follows a pheudo-first order kinetics. The
decomposition is faster at lower initial concentrations and solution volumes. 2-pyridinol is one of the major
intermediates formed during the photolysis of either substance. Complete TOC removal is achieved at prolonged
irradiation times.
5. References
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Acknowledgments
The financial support of this work from the Engineering and Physical Sciences Research Council, UK is greatly
acknowledged.