IOA IUVA World Congress & Exhibition, Paris, France
– May 23-27, 2011
O3/UV treatment of a wastewater consisting of an organic compound (phenol, xylene
or t-butanol) and sodium acetate
Wim Van de Moortel1, Jan Degrève1, Jan Luyten2
1. K.U.Leuven, Department of Chemical Engineering, Willem de Croylaan 46, 3001 Heverlee, Belgium
2. Associated Faculty of Industrial and Biological Sciences, Campus De Nayer, J. De Nayerlaan 5,
2860 Sint-Katelijne-Waver, Belgium
Abstract
In the present work, the effect of the addition of sodium acetate on the COD-degradation of an
organic compound (phenol, xylene or t-butanol) is presented. The combination of O3 and UV was
used to treat the synthetic wastewater. Combining sodium acetate with an organic compound leads
to a faster COD-degradation compared to individual treatment.
Key-words: Ozone, UV, matrix-effect, phenol, xylene, sodium acetate, t-butanol
Introduction
Increased industrialisation and urbanization have lead to the presence of many harmful chemicals
in the wastewater streams [1]. In the past, wastewater treatment objectives were concerned with
the removal of suspended solids and floatable materials; the treatment of biodegradable organics
and the elimination of pathogenic organisms via conventional methods (physical, chemical and
biological). These methods however failed to meet the demands of recent environmental
regulations. Due to an increased scientific knowledge and an expanded information base,
wastewater treatment has begun to focus on the newer objectives with more stringent
environmental regulations, prompting research towards newer techniques such as advanced
oxidation processes [2]. Advanced oxidation processes are characterised by the generation of
highly reactive radicals, namely hydroxyl radicals (OH∙) [3]. The AOP studied in this research is,
ozone in combination with (O3/UV).
The process to obtain these radicals by O3/UV is:
The energy supplied by UV-radiation interacts with O3 [4], the global reaction being:
h
O3  H 2O 
 2OH  O2
Phenol and its derivates in aqueous solutions cause severe environmental problems [5]. Phenol is,
however, an important industrial product for synthesis of drugs, weed killers, and synthetic resins.
It can also be found in wastewater of pulp mills, paint and dyes manufactories, wine distilleries, oil
and gasoline industries, synthetic rubber manufactories, textile industries [6]. Xylene and t-butanol
(paint and varnish industry [7])are also present in many industrial wastewaters. Sodium acetate is
an easy biodegradable compound, used in wastewater treatment [8]
The purpose of this study was to investigate the effect of sodium acetate on the degradation of
three organic compounds, namely phenol, xylene and t-butanol by O3/UV.
Ozone has the main advantage that it is very unstable in water, and so no residual concentrations
after treatment will be present in the water. The decay of ozone in natural waters is characterized
by a fast initial decrease of ozone, followed by a second phase in which ozone decreases with firstorder kinetics. This degradation mechanism of elementary reactions involved in ozone
decomposition have been yet investigated in different studies [9].
IOA IUVA World Congress & Exhibition, Paris, France
– May 23-27, 2011
Material and methods
In this study, phenol, xylene, t-butanol and sodium acetate were used as model components. The
phenol solution was prepared from pure standard (Acros Organics, 99 % of purity), just as the
sodium acetate (VWR, Prolabo, 98 % of purity), t-butanol (Acros Organics, 99.5 % of purity) and
xylene (MERCK, 98 % of purity). The experiments are performed in a labscale reactor with an
internal volume of approximately 1 l, but only 900 ml was used during tests (figure 1). The labscale
reactor has a Low Pressure mercury UV-lamp of 11 W. Ozone was dosed into the fluid by bubbles,
with a flow of 16 gO3/h (Pacific Ozone Model SGC-21, Aqua Purification Systems, Inc.).
O3-inlet
UV-lamp
Figure 1: Labscale reactor
Measurements of COD are done by Nanocolor tubes with different indeces (CSB 160, CSB 1500).
BOD was measured by BODTrack measurement device, produced by HachLange , according to
the standard methods [10]. TC was analysed by Shimadzu TOC-analyser (TC 4110).
Ozone may react in two distinct ways with organic compounds:
i. Direct reaction of molecular ozone, and
ii. Indirect reaction though the formation of hydroxyl radicals.
The generation of hydroxyl readicals is promoted by higher pH values. Therefore, as pH is the
predominant process variable, two different pH values will be evaluated, namely pH 3 and 9.
The relative importance of direct and indirect oxidation can be evaluated by the respective reaction
rate constants kOH,i and kOzone,i. Vandersmissen [11] used the DOP-concept (Dominant Oxidation
Path) to express the relative importance of both pathways. A DOP-value is calculated for an
organic compound (DOPOH) which incorporates both concentrations of ozone and the OH-radical
and the respective rate constants both species with the organic compound:
DOPOH 
kO3 Org [O3 ]
kOH Org [OH ]
With [O3] the ozone concentration in solution, [OH] the OH-radical concentration in solution and
kO3-Org and kOH-Org the rate constants for respectively ozone and radical reaction.
Changes in pH are made by sulphuric acid and by sodium hydroxide. The pH was not kept
constant through the experiments.
Results and discussion
Phenol degradation
In the first part of this research, phenol degradation by O 3/UV was investigated. Tests were run at
two different pH’s. As said before, at lower pH, direct attack of ozone will be investigated, while a
higher pH, the decomposition of ozone for the generation of hydroxyl radicals will be most
IOA IUVA World Congress & Exhibition, Paris, France
– May 23-27, 2011
important. Figure 2 shows the evolution of COD and TC during ozonation reactions at the two
different pH’s. The trend of COD and TC are not the same, while COD-degradation at high pH is
faster, TC-degradation at this pH is much slower. These tests give small proof of the mineralisation
effect of direct ozone attack and the partial oxidation at high pH.
(a)
(b)
Figure 2: The degradation of phenol by O3/UV ( a: COD , b: TC)
T-butanol degradation
T-butanol reacts quickly with OH-radicals and very slowly with molecular ozone [12]. Therefore, tbutanol is used to study the direct reaction kinetics of ozone with organic compounds[13]. Figure 3
shows the COD and TC degradation of t-butanol. At low pH, with predominantly direct reaction by
ozone, almost no COD and TC degradation occurs. At high pH, a fast degradation is observed.
(a)
(b)
Figure 3: The degradation of T-butanol by O3/UV (a: COD, b: TC)
Xylene degradation
Xylene is a chemical compound that occurs naturally in crude oil, and is used in many common
products, such as paints, rubbers, adhesives, plastics and clothing [14]. It is not only toxic to the
liver and the kidneys, but also hazardous for the central nervous system when it enters the body by
skin contact or breathing [15].
Figure 4 (a en b) shows de COD and TC degradation of xylene in synthetic wastewater. Compared
to phenol, the COD and TC of xylene degrades much faster. A possible explanation is the volatile
properties of xylene. Further research is necessary.
IOA IUVA World Congress & Exhibition, Paris, France
– May 23-27, 2011
(a)
(b)
Figure 4: The degradation of xylene by O3/UV ( a: COD , b: TC)
Acetate degradation
Next, the degradation of (sodium) acetate in aqueous solutions was investigated using O 3 and UV.
At low values for the pH (pKa(HAc) = 4.74 [16]), acetic acid will be the dominant form, while at
higher pH values, acetate remains in the unprotonated form.
At a pH value of 3, the main reaction is expected to be direct ozonation of acetic acid. A second
test was run at a pH value of 9. Under these circumstances, indirect ozonation is expected upon
the formation of reactive hydroxyl radicals.
As shown in figure 4, the degradation rate of NaAc at pH 9 is much higher than at pH 3. At pH 3 no
significant degradation can be observed. This can be attributed to the slow nucleofilic reaction of
ozone with organic acids, as acetic acid. On the other hand, at a pH of 9, the degradation rate is
much faster. After 1 hour of reaction time, 60 % is mineralised to CO2 and H2O.
(a)
(b)
Figure 5: The degradation of sodium acetate by O3/UV (a: COD, b: TC)
Water matrix effect
An important issue, using advanced oxidation processes, is the interaction in a water matrix. For
that reason, sodium acetate is combined with three different organic compounds, namely phenol, tbutanol and xylene. Figure 6 showes the combined degradation compared to the individual
treatment at the same treatment conditions. A synergistic effect is shown, at both pH. Presumably,
the formation of reaction intermediates initiates a faster degradation of the present compounds.
IOA IUVA World Congress & Exhibition, Paris, France
– May 23-27, 2011
(a)
(b)
(c)
(d)
(e)
(f)
Figure 6 : Comparison between individual treatment (♦) and combined degradation (■) for different combinations: a: t-butanol
and sodium acetate at pH3; b: t-butanol and sodium acetate at pH9; c: phenol and sodium acetate at pH3; d: phenol and sodium
acetate at pH9; e: xylene and sodium acetate at pH3; f: xylene and sodium acetate at pH9
IOA IUVA World Congress & Exhibition, Paris, France
– May 23-27, 2011
A synergistic factor can be defined as:
 dCOD 


 dt i  NaAc
Ri 
  dCOD   dCOD 



 

  dt i  dt  NaAc 
Table 1: synergistic factor Ri for the different combinations with sodium acetate
pH3
phenol t-butanol
5 min
15 min
25 min
35 min
45 min
55 min
pH9
xylene
phenol t-butanol
1,216814 4,55
3,1547619 1,178475
4,55
0,90096 0,875 0,89747537 1,140694 0,875
/
2,566667 0,63095238 0,962121 2,566667
1,323285 -0,15556 1,24875992 0,980141 -0,15556
2,309735
1,4
2,4537037 1,240123
1,4
5,293142 1,507692 1,10416667 0,985695 1,507692
xylene
3,154762
0,897475
0,630952
1,24876
2,453704
1,104167
Table 1 shows the synergistic factor during the treatment of the different water matrices. In most
cases the factor is higher at the beginning of the treatment and lower at the end of the treatment.
Especially for t-butanol and xylene, and less for the combination with phenol.
Conclusions
First in this study, the COD-degradation rate of four different compounds was studied at pH3 and
9. At low pH, phenol and xylene will react predominantly with molecular ozone, while sodium
acetate and t-butanol degraded very slow. At high pH, all compounds degrade properly when
indirect reactions with hydroxyl radical is predominant.
However, the main aim of this study was to show the effect on COD-degradation by an O3/UVprocess, when sodium acetate was added to a dissolved organic compound. Different tests
showed the increasing COD-degradation of the water matrix compared to individual treatment. This
effect was higher in different matrices. Also the pH at the beginning of the experiment showed to
be important. The synergistic effect of adding sodium acetate is higher at the start of the treatment,
and degrades at the end of the treatment, especially for the combination between sodium acetate
and t-butanol and xylene.
Acknowledgment
“Research funded by a Ph.D. grant of the Agency for Innovation by Science and Technology (IWT)
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IOA IUVA World Congress & Exhibition, Paris, France
– May 23-27, 2011
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