Study of the paracetamol degradation pathway that generates color and turbidity in oxidized wastewaters by photo-Fenton technology

Article abstract of DOI:10.1016/j.jphotochem.2016.06.024

This study aims determining the effect that certain kind of water contaminants have on the changes of turbidity during their oxidation. Phenol is considered by its frequent presence in industrial discharges; meanwhile paracetamol is representative of emerging pollutants of pharmaceutical origin. Quite different results are observed in the turbidity changes during the oxidation of both pollutants that evolve following the kinetics of a reaction intermediate. The analysis of paracetamol and phenol degradation pathways reveals that operating conditions are important in the formation of intermediates that cause turbidity. The maximum turbidity levels are achieved operating at the ratios 12?mol HO^? per 100?mg contaminant. However the turbidity generated during the paracetamol oxidation only reaches a third of the intensity achieved with phenol. During the paracetamol degradation, the intermediates causing turbidity are similar to the ones found during the phenol decomposition. These species are generated during the initial minutes of oxidation and possess structures of large size and molecular weight. At the máximum turbidity point, muconic acid and hydroquinone are identified and found to coexist with other compounds such as pyrogallol and resorcinol. Therefore, the path involving metasubstitution would be the main originator of turbidity. It is noteworthy the rapid formation of muconic acid that coexists with resorcinol-like species. These compounds enable the establishment of hydrogen bond interactions that yield supramolecular structures.

Full text of DOI:10.1016/j.jphotochem.2016.06.024

Accepted Manuscript
Title: Study of the paracetamol degradation pathway that
generates color and turbidity in oxidized wastewaters by
photo-Fenton technology
Author: Natalia Villota Jose M. Lomas Luis M. Camarero
PII:
S1010-6030(16)30113-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.06.024
JPC 10271
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Journal of Photochemistry and Photobiology A: Chemistry
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Accepted date:
16-2-2016
17-6-2016
23-6-2016
Please cite this article as: Natalia Villota, Jose M.Lomas, Luis M.Camarero, Study of
the paracetamol degradation pathway that generates color and turbidity in oxidized
wastewaters by photo-Fenton technology, Journal of Photochemistry and Photobiology
A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2016.06.024
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Study of the paracetamol degradation pathway that generates color and turbidity in oxidized
wastewaters by photo-Fenton technology
Natalia Villota, Jose M. Lomas, Luis M. Camarero
Department of Chemical and Environmental Engineering, Escuela Universitaria de Ingeniería de
Vitoria-Gasteiz, University of the Basque Country UPV/EHU, Spain. E-mail:
natalia.villota@yahoo.es

HIGHLIGHTS
●
Propose a more completed pathway for aqueous paracetamol degradation.
●
Delve into the origin of the turbidity and color produced during the oxidation of phenol and
paracetamol by the photo-Fenton treatment.
●
Comparare the selectivity of the oxidation mechanism leading to the formation of
intermediates that cause turbidity and color when degrading contaminants with bencenic ring
structures.
●
●
Determine that the stoichiometric ratio favoring the turbidity formation corresponds to 12
mmol HO● 100 mg contaminant.
Establish that the main intermediates causing turbidity are the meta-substituted bencene rings
and some dihydroxylated benzoquinones, since the resulting species would establish
interactions by hydrogen bonds leading to supramolecular structures.

ABSTRACT
This study aims determining the effect that certain kind of water contaminants have on the changes
of turbidity during their oxidation. Phenol is considered by its frequent presence in industrial
discharges; meanwhile paracetamol is representative of emerging pollutants of pharmaceutical
origin. Quite different results are observed in the turbidity changes during the oxidation of both
pollutants that evolve following the kinetics of a reaction intermediate. The analysis of paracetamol
and phenol degradation pathways reveals that operating conditions are important in the formation of
intermediates that cause turbidity. The maximum turbidity levels are achieved operating at the ratios
12 mol HO● per 100 mg contaminant. However the turbidity generated during the paracetamol
oxidation only reaches a third of the intensity achieved with phenol.
During the paracetamol degradation, the intermediates causing turbidity are similar to the ones
found during the phenol decomposition. These species are generated during the initial minutes of
oxidation and possess structures of large size and molecular weight. At the máximum turbidity
point, muconic acid and hydroquinone are identified and found to coexist with other compounds
such as pyrogallol and resorcinol. Therefore, the path involving metasubstitution would be the main
originator of turbidity. It is noteworthy the rapid formation of muconic acid that coexists with
resorcinol-like species. These compounds enable the establishment of hydrogen bond interactions
that yield supramolecular structures.
Keywords: Color; paracetamol; photo-Fenton; turbidity
Corresponding author:
Natalia Villota
Department of Chemical and Environmental Engineering
Escuela Universitaria de Ingeniería Vitoria-Gasteiz
University of the Basque Country UPV/EHU
Nieves Cano, 12, 01006 Vitoria-Gasteiz, Spain
email: natalia.villota@yahoo.es
phone: +34 646261951
fax. +34 945013270

1. Introduction
This paper focuses on the study of turbidity changes that are produced when waters containing
organic contaminants are treated with Advanced Oxidation Processes (AOPs) technologies. The
treatment used was the Fenton reagent enhanced by UV radiation. This technology was selected due
to ease of operation and efficiency [1]. Moreover, only one kind of treatment was applied because
the degradation byproducts generating turbidity with different AOPs were alike, regardless of the
techniques employed. Main current research on oxidation of industrial effluents by Fenton
technologies focuses on the wastewater treatment from the petrochemical, paper, chemical and
pharmaceutical (emerging contaminants) sectors [2].
The ultimate objective in this work is to analyze the origin of the turbidity of the treated water
and the factors that affect it. The interest in studying this parameter lies in the determination of
treated waters quality, since it is restricted by the environmental legislation in the EU Water
Framework Directive (Directive 2000/60/EC). In this way, the characteristics of the reactions that
contribute to the turbidity formation and the quality of the water thus treated are identified and
discussed.
Two representative contaminants of common industrial and urban effluents were chosen for
the oxidation study. Hence, the turbidity induced by the oxidation treatment of phenolic compounds
was duly investigated. Additionally, the oxidation of a well known emerging contaminant of
pharmaceutical origin, acetaminophen (paracetamol in Europe) was also characterized. These two
compounds were selected based on their characteristic concentration levels and the effects they may
bear on natural systems.
Phenolic contaminants in water may significantly impact on aquatic echosystems thus
affecting rural communities and urban areas [3]. Usually, concentration, temperature, structure, pH
of the compound and media determine the degree to which these compounds affect living organisms
[4-5]. The operating conditions performed by many authors in studies on phenol oxidation have
been applied to other matrices, in order to compare their effectiveness. The considered variables in
this work were the catalyst and oxidant dosages, because they determine the reaction rate and the
extent of oxidation.
Emerging contaminants and their characterization and study are presently raising high interest
because of their increasing consumption. Several pharmaceutical products such as analgesics or
antibiotics can be recognized in this group of substances since they have been increasingly detected
in wastewaters and even potable water. These drugs generate metabolites in the body that are
excreted together with their residual active ingredients. As a result, they often reach the wastewater
treatment plants [6-7]. Even in low concentrations, these substances can be cause of concern, as they
accumulate in the receiving environment [8]. These emerging contaminants are not fully degradated
in wastewater treatment plants, so that they end up being dumped into the rivers [9].
To degrade this kind of contaminants, the viability of AOPs are currently applied. In general,
these technologies use the following control parameters: decontamination performance achieved
dosage of oxidant and catalyst employed, initial charge of contaminant in the water, pH and
temperature [10]. Studies reported in the literature show that some organoleptic parameters can be
regarded as indicators of water quality [11]. Thus, this paper considers the turbidity and determines
its suitability as an efficiency indicator of water treatment. Turbidity analysis can usually be fast,
cheap and simple. However, its practical application depends on a number of operating parameters
such as nature of contaminating substances, acidity of the medium, temperature, and concentration
of dissolved species. Then, it is essential to keep these conditions constant to obtain reliable
measurements.
The ultimate objective is to elucidate the origin of the turbidity of the treated water and its
control factors. In this way, the characteristics of the reactions contributing to turbidity are
discussed, as well as if organic matter affects to it [12-13]. In this way, the work aims to refocus the
understanding of AOPs to clarify the relationship between turbidity and oxidation intermediates [14]
as well as to study the effect of the substituted groups and their position on the benzene ring.

2. Materials and methods
2.1. Reaction system
The experimental assays of this work were carried out with respectively aqueous samples of phenol
(C6H6O Panreac, 99%) and commercial paracetamol (C8H9NO2 Efferalgan, sorbitol excipient) of
100.0 mg/L prepared in the laboratory. In this way, 500.0 mL samples were poured into a simple
immersion-type photochemical reactor, 600 mL working volume with the inserted cooling sheath
[12-13]. The irradiation is effected by means of a medium pressure UV lamp (Heraeus TQ-150, 95%
transmission between 300-570 nm, 150 Watt, 87.4V, 167.3W, 2.20A), operated by utilizing a
vertically arranged immersion tube as well as a separate cooling sheath, immersed into the reaction
mixture.
Next, the catalyst dose of Fe (II) established in each set of experiments was added to the
reactor in each set of experiments, being the rank Fe=0-50.0 mg/L (FeSO4 2H2O Panreac, 80%).
The reaction mixture was homogenized by a magnetic stirrer at 500 rpm. Finally, hydrogen
peroxide was added to the reactant mixture in the interval H2O2=0-15.0 mM (H2O2 30%, Foret).
Therafter, the reaction started. Acidity was adjusted to pH=3.0 by pouring NaOH and HCl 0.2M
(Probus, 99.9%) with an automatic burette (Dosimat 665-Metrohm) and temperature was stabilized
around 25.0°C using a cryo-thermostat bath of 1150 watts (Selecta Frigiterm-10), and pumping
refrigeration water through the cooled reactor sheath, where the UV lamp is inserted.
2.2. Analysis methods
The concentration of organic species was analyzed by High Performance Liquid Chromatography,
attached to a spectrophotometer UV/Vis (Agilent Technologies 1200 Series). Common
concentrations of emerging pollutants in urban wastewaters are around micrograms per liter.
However, this work deals with 100.0 mg/L solutions, due to limitations in the detection range of the
chromatography equipment.
Analysis was performed injecting manually 20 μL samples, dragged by a carrier current flow
of 1.0 mL/min, consisting of a mixture of methanol and distilled water MeOH/H2O:20/80, through a
column C18, XBridge Phenyl 5 μm 4.6 x 250 mm (Bridge Waters), with detection limit (LOD) 0.1
mg/L. The concentrations of organic species were analyzed at 280 nm: phenol (Panreac, 99%),
hydroquinone (Panreac, 99%), catechol (Baker Organic Chemical, 99%), resorcinol (Panreac, 99%)
and phloroglucinol (Acros Organics, 99%); ZZ-muconic acid (Acros Organics, 98%) and EE-
muconic acid (Acros Organics, 97%) at 242 nm; p-benzoquinone (Alfa Aesar, 98%),
hydroxyhydroquinone (Aldrich, 99%) and pyrogallol (Sigma, 98%); paracetamol (Efferalgan,
sorbitol excipients) at 254 nm and 2,5-dihydroxy-1,4-benzoquinone (Acros Organics, 98%) at 210
nm. The solution turbidity was measured with a nephelometric turbidimeter (2100Qis Hach).
3. Results and discussion
3.1. Effect of operating pH on turbidity
Several pollutant solutions were oxidized, all containing Co=100.0 mg/L, at different pH in the
pH=2.0-5.0 range. Essays were performed dosing with H2O2=14.8 mM and Fe=20.0 mg/L catalyst,
and keeping temperature constant at T=25.0ºC. Figure 1 displays turbidity at different pH of phenol
and paracetamol aqueous samples after oxidation, when the maximum turbidity is generated
(NTUmax), as well as the turbidity that remains at steady state (NTUoo).
FIGURE 1

Results in Figure 1 reveal that there are differences in water turbidity that depend on the type
of solution contaminant. Thus, paracetamol shows no major effect of pH on turbidity. However,
when the contaminant is phenol, a trend is evidenced that shows a minimum at pH=3.0-3.5. To
understand this behavior it is important to consider the diagram of iron species since this element is
added as a catalyst.
Consequently, the turbidity of oxidized phenol solutions manifests the trend established by
iron species in solution. Within the pH=2.0-3.5 turbidity diminishes with increasing pH. This could
be attributed to the decrease of ferric acid concentration in solution that contributes to water
turbidity. As well, the turbidity values keep decreasing along the oxidation process until they reach
steady state. This could also be associated with the precipitation reactions of ferric species in
solution which would decrease the turbidity as well.
Operating in the pH interval pH=3.5-5.0, the máximum turbidity undergoes an upsurge in
intensity when the reaction medium acidity diminishes. This trend may be caused by the formation
of ferric hydroxide which remains suspended in solution under these conditions. However, when the
oxidized samples evolve to steady state, turbidity experiments a variation as given by the NTUmax
-
NTUoo factor, which decreases when the acidity of the reaction medium is reduced. This
phenomenon may be a consequence of the precipitation reactions of the existing ferric hydroxides
in suspension.
However, the oxidation tests that were carried out with aqueous solutions of paracetamol
revealed no significant turbidity changes, after oxidation, with varying pH. Thus, results seem to
indicate that during paracetamol oxidation the iron species in solution do not bear a significant
effect on the turbidity.
3.2. Chemical pathway of paracetamol degradation
The results of this study indicate the importance of contaminant molecular structures on the
resulting turbidity that waters treated with this type of technologies acquire. Therefore, it is
necessary to study first the degradation mechanisms to establish the behavior of turbidity in
oxidation systems with contaminants of different type. Hence, the nature of the reaction
intermediates that contribute to the oxidation paths that cause turbidity can be suitably ascertained.
For this reason a study of the degradation paths of paracetamol and phenol [15] was carried out to
analyze the different behavior of oxidized aqueous solutions as a function of the contaminant. This
approach allows relating turbidity changes to the nature of the reaction intermediates that are
generated with the oxidation.
The selectivity of the reaction mechanism of paracetamol (Figure 2) is favored by the
hydroxylation of the benzene ring, leading to the formation of 3-hydroxyacetaminophen and
releasing acetamide as reaction byproduct. These species are characterized by being toxic and
refractory [16], which can be decomposed in later stages to oxalic and oxamic acids. In turn, these
substances can give rise to complexation reactions with ferric ions added as catalyst. In this way,
the complexes of iron (III)-oxalate can be decarboxylated with the corresponding loss of ferrous
ions, while the complex of iron (III)-oxamate becomes completely mineralized [17]. The formation
of these metal species during the degradation of paracetamol could be responsible for the absence of
important turbidity variations with the operating pH since ferric ions would be forming complexes
that would eventually become fully mineralized. On the other hand, the breakdown of the bond
between the amino group and the substituted methanol in the paracetamol molecule could lead to
degradation byproducts with substituted nitrogens, like 4-aminophenol and nitrophenol [14].
Moreover, this work considers a parallel via to this oxidation process, although as a minor
route. The rupture of the paracetamol molecule would occur in the bond linking the benzene ring
with the amino group, resulting in a mechanism similar to that of phenol oxidation. The
mineralization of paracetamol occurs through ortho, meta and para substituted oxidation pathways,
leading to the formation of dihydroxy aromatic rings: catechol, resorcinol and hydroquinone; and

trihydroxylated rings: pyrogallol, phloroglucinol and hydroxyhydroquinone [18-19]. These species
are in equilibrium with their corresponding para and ortho benzoquinones [20-21].
Next, if the oxidation conditions are hard enough, these compounds are decomposed to
biodegradable acids of cyclic chain: muconic and maleic acids; and linear chain: formic and acetic
acids [22-23]. Furthermore, there is another alternative via of contribution of carboxylic acid to the
system by direct oxidation of paracetamol to acetic and formic acids. These species are
characterized by being highly refractory, so they would persist as remaining byproducts of
oxidation.
FIGURE 2
Several studies on phenol oxidation in aqueous solutions show that the main degradation
routes that cause turbidity take place through aromatic compounds metasubstituted with hydroxyl
groups. This pathway leads to the formation of muconic acid [24]. Moreover, there is a minor
pathway in the degradation mechanism of paracetamol that occurs in a similar manner to that of
phenol oxidation. Thus, in the paracetamol oxidation tests, turbidity changes were studied under the
same conditions that favor the oxidation of phenol.
3.3. Changes of turbidity in the water during the oxidation of phenol
Tests were performed changing the oxidant dosing within the operating interval H2O2=0-15.0 mM,
and at pH=3.0, T=25.0ºC, and catalyst concentration Fe=20.0 mg/L (Figure 3). The importance of
controlling pH along the whole process is highlighted since, as Figure 1 shows, acidity is a key
variable in turbidity behaviour. In this way, significant pH changes occur during the first 30 min,
when hydroxyl radicals, which are energetic oxidizing species, are generated. However, the acidity
during the first 120 min of reaction hardly varies when reaching the steady state. Furthermore, the
possible buffer effect of paracetamol has not been considered.
The results reveal that turbidity changes are produced during phenol oxidations that depend on
the operating conditions of the treatment. The kinetics shown describes the evolution corresponding
to the formation of reaction intermediates whereby turbidity increases in the initial 15 minutes until
reaching maximum intensity. Ensuing this period, turbidity slowly decreases with time until a
steady value of the reacting media compounds.
FIGURE 3
It is noteworthy that operating conditions exert some influence on the selectivity of the
mechanism of phenol degradation that generates turbidity. In this manner, when the treatment is
performed with H2O2=6.4 mM, turbidity changes are manifested at its maximum intensity whereby
when 100.0 mg of phenol are degraded turbidity reaches 12 NTU after 15 minutes of reaction time.
This concentration may indicate that the stoichiometric relation of oxidant that is required to
degrade one mole of phenol through the formation of species that generate turbidity corresponds to
12 moles of hydroxyl radicals HO●.
At oxidant dosage H2O2>6.4 mM, the intensity of the turbidity diminishes, as well as the
residual turbidity, to steady values. This behavior may be caused by the growing hydroxyl radical
concentration that is generated by the larger dose of oxidant supplied to the system. These radicals
will degrade the intermediate that generate turbidity to acidic species. Hence, and within this
operation range, the phenol decomposition pathway to turbidity-causing intermediates tends to be
less favored at increasing oxidant concentration.
At oxidant doses lower than H2O2<6.4 mM, the turbidity of phenol oxidation displays an
increase in the initial minutes of reaction until it reaches its maximum value, staying there until it
finally reaches steady state. This evolution corresponds to the formation kinetics of an oxidation
intermediate when the operating conditions involve a reactant deficit which, in this case, is the

oxidant. Hence, phenol degrades to intermediate species that produce turbidity. However, when
operating with a shortfall of oxidant, the decomposition of hydrogen peroxide cannot supply a
sufficient amount of oxidizing radicals to further degrade the intermediate species to carboxylic
acids. In view of these results it is evidenced that the lower the oxidant dose used the greater the
residual turbidity which remains in the fully oxidized samples, because the oxidation level achieved
is less intense.
3.4. Changes of wastewater turbidity during paracetamol oxidation
Figure 4 shows the changes of turbidity during the first 120 minutes of reaction that paracetamol
samples exhibit when oxidized at the same conditions as phenol (Figure 3). The graph in the inside
box in Figure 4 shows the turbidity changes of paracetamol solutions at catalyst dosing Fe=0-50.0
mg/L, keeping the remaining control parameters of the process constant at pH=3.0, H2O2=14.8 mM
and T=25.0ºC.
FIGURE 4
In general, turbidity changes during paracetamol oxidation manifest a similar evolution
when either the oxidant or the catalyst dosing is varied. In either dosing change, turbidity exhibits a
slight linear increase with reaction time, the change not being significant. Nevertheless, the results
in Figure 5 reveal that when the tests are carried out at oxidant dosings: H2O2=6.4 mM, 8.5 mM and
10.6 mM (Figure 5), in combinación with Fe=20.0 mg/L as ferrous ions, pH=3.0 and T=25.0ºC,
turbidity changes are evidenced and follow similar trends to the formation kinetics of an oxidation
intermediate.
FIGURE 5
Figure 5 reveals turbidity changes during paracetamol oxidation using an oxidant
concentration of H2O2=6.4 mM, that induces the greatest turbidity variations, although its value is
only a third of that produced in phenol oxidation (Small figure in Figure 5). In this case, when a
paracetamol concentration of Pao=0.66 mM is oxidized, a turbidity value of 4.0 NTU is generated
after 3.5 minutes of reaction time. Consequently, it can be surmised that the intermediates that
contribute to turbidity in both cases are species of similar nature. Hence, these species in the case of
paracetamol degradation are characterized by being phenol degradation subproducts. This result
indicates that the dose of 12 HO● hydroxyl radicals establishes some selectivity in the mechanism
towards the formation of intermediates that generate turbidity in an independent mode from the
contaminant that is treated.
In the case of paracetamol, the turbidity generated is lower than that obtained in oxidized
phenolic solutions because other major oxidation pathways coexist and develop in parallel to the
formation of high turbidity intermediates. For this reason, the compounds that produce any turbidity
in the wastewater are minor species that negligibly contribute to it. However, when phenol is
oxidized, the intermediates are produced at much higher concentration and consequently yield
higher turbidity. When these results are quantified, based on the mass ratio experimentally obtained,
it is inferred that the production of high turbidity intermediates in the paracetamol oxidation is only
a third of that of phenol.
3.5. Oxidative pathways through the formation of intermediates causing turbidity
Figure 6 reveals the formation kinetics of some paracetamol oxidation intermediates when the
treatment applied involves H2O2=6.4 mM of oxidant. This condition steers the oxidation
mechanism towards the formation of high turbidity intermediates. The intermediates analyzed are
species that are common in either the paracetamol or the phenol mechanism since it can be

conjectured that they are the main turbidity precursors. Hence, the study herein focuses on the
species that are present in solution when the oxidized samples exhibit the maximum turbidity.
FIGURE 6
The kinetics in Figure 6 denotes that during the initial minutes of paracetamol oxidation
turbidity increases until reaching its maximum value at 4 minutes. Ensuing this, turbidity plummets
so that after 8 minutes it attains a practically constant value that slightly increases following a linear
trend until reaching steady state. This behavior indicates that the species that produce the initial
turbidity peak are generated during the early minutes of reaction time. Therefore, in this short time
interval the formation of structures that generate large turbidity increments takes place, implying
that they must possess notable size and molecular weight.
However, turbidity variations show no trend that would parallel the evolution of the
intermediates that were analyzed. Results indicate that the maximum turbidity is produced when
38% of the starting load of paracetamol contained in solution is degraded. At this point in the
process, the reaction intermediates are muconic acid and hydroquinone and both present their
maximum generation when the solution acquires its maximum turbidity. Alternatively, these species
coexist with pyrogallol and resorcinol [25], which show their maximum concentration at 18 minutes
of reaction time. From these results it can be established that turbidity is not a parameter that is
produced by the formation of a unique species, but rather the result of the presence in solution of
several intermediate compounds, and/or the interaction among several species that would lead to the
formation of stable supramolecular structures [26-27]. When the principal intermediates of the
reaction period with maximum turbidity are identified, the oxidation pathway along the meta-
substitute alternative can be inferred as the foremost cause of turbidity. At this point, it is
remarkable the rapid formation of muconic acid, that occurs even before the formation of resorcinol
and pyrogallol. This circumstance can be attributed to the formation in parallel of muconic acid
through the ortho-substituted pathway. This occurrence makes it possible that meta-substitute rings,
like resorcinol and muconic acid, simultaneously coexist. The importance of this relies upon the
capacity that both species possess to establish interactions through hydrogen bond which would
lead to the formation of substances of large molecular structure.
If the resonant forms of the dihydroxilated rings were considered, the ortho and para
substituted rings would be oxidized to form quinoidal species (ortho and para benzoquinones).
However, the oxidation of meta substituted rings with two hydroxil groups (resorcinol) may lead to
the formation of species of the diresorcinol type [28-29]. Besides, the interactions between
molecules of the characteristics of resorcinol and muconic acid could establish hydrogen bonds [30].
These structures could lead to the formation of molecular adducts that would hinder the passage of
light through the solution, thus causing peaking turbidity during the paracetamol oxidation.
As per this discussion, it is noteworthy that the species that cause turbidity are of different
nature from the intermediates that impart color to the water. Figure 7 reveals the oxidation kinetics
of paracetamol, Pao=100.0 mg/L, treated with oxidant, H2O2=6.4 mM. Results show that
paracetamol degradation proceeds to almost completion after 40 minutes of reaction time. As well,
the formation of intermediates that bring turbidity and color to the solution occurs before the
degradation process is complete. However, maximum turbidity is evidenced after approximately 4
minutes of reaction time, whereas the emergence of species that bring in color happens after 16
minutes. These results demonstrate that water color and turbidity are organoleptic parameters
caused by species of different nature.
FIGURE 7
With respect to the color of solution standards used for intermediate species that are included
in this study (Figure 2), it was observed that the substances that showed colored solutions were
pyrogallol, hydroxyquinone, and benzoquinones. This characteristic can be attributed to the

presence of chromophore groups in their internal moledular structure. Moreover, it is significant
that during paracetamol oxidation, the maximum color formation occurs when the concentration of
pyrogallol is the highest. Finally, the color that is generated in aqueous solutions with paracetamol
is less intense than in solutions with phenol, because color is produced via minor degradation
pathways.
4. Conclusions
This work presents a study on the effect of the nature of wastewater pollutants on the turbidity
generated during their degradation. Phenol and paracetamol are considered as model compounds.
The study analyzes the effect of the main control parameters of the Fenton reaction when activated
by UV light. Paracetamol oxidation shows no significant changes in turbidity when the solution pH
is varied. Nevertheless, when phenol is oxidized, changes in turbidity which evolve following the
kinetics of reaction intermediates are evident. This behavior is established by the iron species in
solution. Hence, turbidity is influenced by the concentration of ferric ions in the pH range pH=2.0-
3.5 whereas at a higher range, pH=3.5-5.0, it is related to the generation of ferric hydroxide.
Turbidity is the lowest in the range pH=3.0-3.5.
Paracetamol and phenol decomposition pathways are analyzed to correlate turbidity changes
with the nature of reaction intermediates. Operating conditions affect the selectivity of the
formation mechanism of intermediates that cause turbidity. The stoichiometric ratio 12 HO●
mol:100.0 mg of contaminant induces the maximum turbidity generation. However, during the
paracetamol oxidation the turbidity generated around one third of that in phenol oxidation. In both
cases, substances that contribute to the turbidity are of similar nature and are characterized by being
byproducts of phenol decomposition. These species are generated in the initial minutes of reaction
time and show large size and molecular weight.
About paracetamol oxidation, applying the conditions that enhance turbidity (H2O2=6.5 mM,
Fe=20.0 mg/L, pH=3.0), maximum turbidity (4.0 NTU) is achieved after five minutes of reaction,
being degraded 38% of the initial load. The reaction intermediates generated are muconic acid
(6.5%) and hydroquinone (4%) that coexist with species such as pyrogallol (5.5%) and resorcinol
(5.5%). Subsequently, the maximum color formation occurs at 15 min, with 75% degradation.
Finally, paracetamol is totally degraded after one hour, being the turbidity at steady state 1.1 NTU
and color 0.25 AU. On the other hand, about phenol oxidation, maximum turbidity (12.0 NTU)
occurs at 10 min of reaction corresponding to 95% degradation. Afterwards, phenol is completely
decomposed at 30 min, being turbidity at steady state 5.5 NTU.
These results indicate that the turbidity is not only the result of the formation of a unique
substance, but rather the interaction of several species in solution. The oxidation via meta-
substituted species would be one of the main sources of turbidity in which the rapid formation of
muconic acid is significant. This substance coexists with meta-substituted species such as
resorcinol. The molecular characteristics of these compounds permit the establishment of hydrogen
bond interactions which lead to the formation of supramolecular structures. The oxidation of
resorcinol would generate diresorcinol-like compounds that could lead to the formation of
molecular adducts.
Finally, color and turbidity are organoleptic parameters generated by species of different
nature. The species that provide turbidity are pyrogallol, hydroxyhydroquinone and benzoquinone.
In the oxidation of paracetamol, the maximum creation of color coincides with the largest
concentration of pyrogallol.
Acknowledgments
This work was supported by the Department of Environmental and Chemical Engineering from the
University of the Basque Country UPV/EHU.

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FIGURE 1
14
12
10
8
1,0
0,8
0,6
0,4
0,2
0,0
NTUmax paracetamol
NTUoo paracetamol
NTUmax phenol
NTUoo phenol
Fe+3
Fe(OH)3
6
4
2
0
2,0
2,5
3,0
3,5
pH
4,0
4,5
5,0
Fig. 1. pH effect on the wastewater turbidity during the oxidation of several contaminants in a
photo-Fenton system. Experimental conditions: Pho=100.0 mg/L; Pao=100.0 mg/L; Fe=20.0 mg/L;
H2O2=14.8 mM; T=25.0ºC.

FIGURE 2
OH
O
O
O
NH2
O
O
CO2
NH4
NO3
H
C
C
C
C
C
+
OH
OH
OH
-
OXALYC ACID
C2H2O4
FORMIC ACID
C2H5NO
OXAMATO
C2H3O3N
NITROPHENOL
C6H5NO3
NH2
CH3
OH
OH
ACETIC ACID
C2H2O4
OH
O
OH
O
ACETAMIDE
C2H5NO
H3C
C
OH
NH
NH2
NO
2
O
OH
CH3
4-AMINOPHENOL
C6H7NO
3-HYDROXYACETAMINOPHEN
C8H9NO3
OH
OH
OH
NH
O
CH3
PARACETAMOL
C8H9NO2
HYDROQUINONE
C6H6O2
CATECHOL
C6H6O2
OH
OH
OH
OH
OH
OH
OH
OH
OH
PYROGALLOL
C6H6O3
RESORCINOL
C6H6O2
HYDROXYHYDROQUINONE
C6H6O3
O
O
O
O
OH
PHLOROGLUCINOL
C6H6O3
OH
OH
o-BENZOQUINONE
C6H4O2
p-BENZOQUINONE
C6H4O2
COOH
COOH
H
O
C
H3C
OH
H
H
O
MUCONIC ACID
C6H6O4
ACETIC ACID
C2H2O4
H
FORMIC ACID
C2H5NO
H
C
OH
Fig. 2. Oxidation pathway proposed for the paracetamol oxidation.

FIGURE 3
24
22
20
18
16
14
12
10
8
14
12
10
8
Fe=2.0 mg/L
Fe=20.0 mg/L
Fe=5.0 mg/L
Fe=40.0 mg/L
Fe=10.0 mg/L
Fe=50.0 mg/L
6
4
2
0
0
10
20
30
40
50
60
70
80
90
100
110
120
Time (min)
6
4
2
0
0
10 20 30 40 50 60 70 80 90 100 110 120
Time (min)
Fig. 3. Effect of oxidant dosage on turbidity changes during the degradation of phenol in the photo-
Fenton system. Experimental conditions: Pho=100.0 mg/L; Fe=20.0 mg/L; pH=3.0; T=25.0ºC;
H2O2= □ 1.1 mM, ● 2.1 mM, ■ 4.2 mM,  6.4 mM, ▲ 8.5 mM, ○ 14.8 mM. Small Figure: Effect
of catalyst dosage on turbidity changes during the paracetamol degradation in a photo-Fenton
system. Experimental conditions: Pho=100.0 mg/L; pH=3.0; H2O2=14.8 mM; T=25.0ºC.

FIGURE 4
9
8
7
6
5
4
3
2
1
0
5.0
4.0
3.0
2.0
1.0
0.0
Fe= 2.0 mg/L
Fe=20.0 mg/L
Fe= 5.0 mg/L
Fe=40.0 mg/L
Fe=10.0 mg/L
Fe=50.0 mg/L
c
0
10
20
30
40
50
60
70
80
90
100
110
120
Time (min)
0
10 20 30 40 50 60 70 80 90 100 110 120
Time (min)
Fig. 4. Effect of oxidant dosage on turbidity changes during the degradation of paracetamol in the
photo-Fenton system. Experimental conditions: Pao=100 mg/L; Fe=20.0 mg/L; pH=3.0; T=25.0ºC;
H2O2= □ 1.1 mM, ● 2.1 mM, ■ 4.2 mM,  6.4 mM, ▲ 8.5 mM, ◊ 10.6 mM, ○ 14.8 mM. Small
Figure: Effect of catalyst dosage on turbidity changes during the paracetamol degradation in a
photo-Fenton system. Experimental conditions: Pao=100 mg/L; pH=3.0; H2O2=14.8 mM;
T=25.0ºC.

FIGURE 5
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
14
12
10
8
Paracetamol
Phenol
6
4
2
0
0
10 20 30 40 50 60 70 80 90 100 110 120
Time (min)
0
5
10 15 20 25 30 35 40 45 50 55 60
Time (min)
Fig. 5. Turbidity changes operating at different dosages of oxidant during the oxidation of
paracetamol. Experimental conditions: Pao=100.0 mg/L; Fe=20.0 mg/L; pH=3.0; T=25.0ºC: H2O2=
▲ 6.4 mM, ■ 8.5 mM, ○ 10.6 mM. Small Figure: Kinetics of turbidity during the oxidation of
phenol and paracetamol in a photo-Fenton system. Experimental conditions: Pao=100.0 mg/L;
Co=100.0 mg/L; H2O2=6.4 mM; Fe=20.0 mg/L; pH=3.0; T=25.0ºC.

FIGURE 6
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
4.5
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
Hydroquinone
Resorcinol
Paracetamol
NTU
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Hydroxyhydroquinone
Pyrogallol
Muconic acid
0
15 30 45 60 75 90 105 120
0
15 30 45 60 75 90 105 120
Time (min)
Time (min)
a)
b)
Fig. 6. Changes of turbidity and reaction kinetics of intermediate species generated during the
oxidation of paracetamol. Experimental conditions: Pao=100.0 mg/L; pH=3.0; H2O2=6.4 mM;
Fe=20.0 mg/L; T=25.0ºC.

FIGURE 7
100
80
60
40
20
0
3.5
3.0
2.5
2.0
1.5
1.0
Paracetamol
Color
Turbidity
0
10 20 30 40 50 60 70 80 90 100
Time (min)
Fig. 7. Kinetics of paracetamol oxidation, color and turbidity changes during the paracetamol
degradation by photo-Fenton reagent. Experimental conditions: Pao=100.0 mg/L, pH=3.0, H2O2=6.4
mM, Fe=20.0 mg/L, T=25.0ºC.