The Comparative Photodegradation Activities of Pentachlorophenol (PCP) and Polychlorinated Biphenyls (PCBs) Using UV Alone and TiO2-Derived Photocatalysts in Methanol Soil Washing Solution
Lu W (2014) The Comparative Photodegradation Activities of Pentachlorophenol (PCP) and Polychlorinated
Biphenyls (PCBs) Using UV Alone and TiO2-Derived Photocatalysts in Methanol Soil Washing Solution. PLoS ONE 9(9): e108765. doi:10.1371/journal.pone.0108765
The Comparative Photodegradation Activities of Pentachlorophenol (PCP) and Polychlorinated Biphenyls (PCBs) Using UV Alone and TiO2-Derived Photocatalysts in Methanol Soil Washing Solution
Zeyu Zhou 0
Yaxin Zhang 0
Hongtao Wang 0
Tan Chen 0
Wenjing Lu 0
Hans-Joachim Lehmler, The University of Iowa, United States of America
0 1 Department of Environmental Science and Engineering, Tsinghua University , Beijing , P.R. China , 2 College of Environmental Science and Engineering, Hunan University , Hunan , P.R. China
Photochemical treatment is increasingly being applied to remedy environmental problems. TiO2-derived catalysts are efficiently and widely used in photodegradation applications. The efficiency of various photochemical treatments, namely, the use of UV irradiation without catalyst or with TiO2/graphene-TiO2 photodegradation methods was determined by comparing the photodegadation of two main types of hydrophobic chlorinated aromatic pollutants, namely, pentachlorophenol (PCP) and polychlorinated biphenyls (PCBs). Results show that photodegradation in methanol solution under pure UV irradiation was more efficient than that with either one of the catalysts tested, contrary to previous results in which photodegradation rates were enhanced using TiO2-derived catalysts. The effects of various factors, such as UV light illumination, addition of methanol to the solution, catalyst dosage, and the pH of the reaction mixture, were examined. The degradation pathway was deduced. The photochemical treatment in methanol soil washing solution did not benefit from the use of the catalysts tested. Pure UV irradiation was sufficient for the dechlorination and degradation of the PCP and PCBs.
Funding: The authors appreciate the generous financial support for this work from the National Natural Science Foundation of China (No. 41371472) and the
Major Science and Technology Program for Water Pollution Control and Treatment of the Ministry of Environmental Protection of China (No. 2011ZX07317-001).
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Hydrophobic chlorinated aromatic pollutants, such as
pentachlorophenol (PCP) and polychlorinated biphenyls (PCBs), are
among the most important environmental pollutants in the
twentieth century. For several decades after their commercial
production, these compounds had been widely used for numerous
applications, such as in wood protection, pesticides, and dielectric
fluids in capacitors and transformers, before their global biota
accumulation and genotoxic activity were gradually noticed .
Given their potential health hazard for humans and wildlife, the
Stockholm Convention in 2001 classified PCP and PCBs as
Persistent Organic Pollutants. Despite a comprehensive
production ban since that time, millions of tons of these compounds
continue to circulate in the environment [2,3]. Thus, the
dechlorination and degradation of PCP and PCBs have emerged
as major issues.
The cleanup of hydrophobic chlorinated aromatic pollutants is
a challenging task. Various remediation technologies have been
developed because of the extremely slow natural degradation of
these compounds . Some of these techniques are based on a
dig and dump approach, such as landfilling or capping. This
method immobilizes the contaminant to prevent it from entering
the aqueous phase . Other methods are based on a dig and
incinerate approach, such as thermal treatment in which heat is
used to remove or destroy the contaminants . Among all the
remediation technologies, soil washing combined with
photochemical treatments has become increasingly advantageous
because this method does not release toxic by-products into the
environment and the cost is reasonable . Soil washing is a
welldeveloped technology that removes contaminants from polluted
solid phase. The target contaminant can be extracted from the soil
by a soil washing solvent, and the following photocatalytic
degradation can be altered with different soil washing solvents
[8,9]. Among the various solvents used to extract the target
contaminant from the soil matrix, alcohol, such as methanol and
ethanol, has been successfully used to remove PCP and other
contaminants [10,11]. The solvent contains target contaminants
after soil washing, which are usually treated under ultraviolet light
photolysis, and the organic contaminants inside can be
decomposed by photodegradation . During UV irradiation, NOH can
be formed from water molecules or other highly reactive solvents,
initiating the decomposition of the pollutants .
To enhance the degradation rate, different types of
photocatalysts such as TiO2 and ZnO have been investigated for
photodegradation . In these studies, the photocatalytic
degradations of PCP have been enhanced by various catalysts
compared with TiO2 or pure UV irradiation. Although
remediation systems using a photocatalyst combined with UV irradiation
have been successfully demonstrated to treat heterogeneous
polluted soil or water [18,19], whether the addition of
photocatalyst provideds more efficient photodegradation in any condition
than using pure UV irradiation has not been proved. Considering
that TiO2 has been used extensively for water treatment and
control of organic contaminants, the heterogeneous modifications
of TiO2 have been evaluated . To demonstrate the
improvements of modified TiO2 catalysts, numerous studies have
conducted photodegradation competitions between new catalysts
and the original TiO2 catalyst. Degradations using pure UV
irradiation under analogous conditions were rarely investigated
The different methods for treating PCP and PCBs in methanol
soil washing solvent under UV irradiation with or without a
TiO2derived catalyst were compared in this study. TiO2 coupled with
graphene was used as modified TiO2 catalyst in this study.
Graphene has high electrical conductivity and efficient electron
storage and shuttling capabilities. In this study, the degradation
rates under UV irradiation with and without a TiO2-derived
catalyst are presented. Although the degradation rates using
modified TiO2 catalysts were all remarkably higher compared
with the original TiO2 catalyst, they were not as high as that when
only UV irradiation was used. The main factors influencing the
degradation that were taken into consideration included UV light
illumination, addition of methanol to the solution, catalyst dosage,
and the pH value of the solution. The beneficial effect of adding a
catalyst for the contaminant photodegradation in methanol soil
washing solvent was not confirmed.
Materials and Methods
Reagents and materials
Commercial P25 TiO2 (80% anatase, 20% rutile) was supplied
by the Degussa (Germany). Hexane and methanol (HPLC grade)
were purchased from Fisher Scientific (USA). Graphene oxide was
purchased from XFNano (China). Water, which was used as a
solvent, was obtained using a Milli-Q Water Purification System.
Pentachlorophenol (PCP, purity .98.0%) and pentachlorophenol
sodium salt (PCP-Na) were purchased from the Sigma-Aldrich
(USA). PCBs were obtained directly from a waste transformer
factory in China. Methyl Orange (MO, analytical grade) was
purchased from Sino Chemical Reagent (China).
Graphene-TiO2 was prepared via a hydrothermal method.
25 mg of graphene oxide were dispersed in 50 mL water via
sonication for 1 h to form a stable solution. Then, 1 g of TiO2 was
added into the graphene oxide suspension, which was stirred to
mix the solution thoroughly. The mixture was moved into a
Teflon-lined autoclave and heated at 180uC for 6 h. The resulting
grey slurry was filtered and dried prior to use [24,25].
All UV irradiation parts of the experiments were conducted in a
500 mL glass reactor (length 310 mm, diameter 70 mm), which
contained a UV lighting system, as shown in Fig. 1. The lighting
system included a high-pressure mercury lamp (GGZ300, Phillips,
maximum wavelengths at 254, 292, 313, 334, 365, 436 and
546 nm) for 100 W and 300 W UV irradiation; a low-pressure
mercury lamp (Hagende, maximum wavelength at 254 nm) for
9 W UV irradiation; a quartz well (length 300 mm, diameter
55 mm) equipped with a circulating water unit to maintain the
system at 20uC; and a magnetic stirrer to promote uniform mixing
of the catalyst in the solution. The UV light intensities at the
reaction point were 2.0 * 103 Lx for the 9 W mercury lamp, 7.2 *
104 Lx for the 100 W mercury lamp, and approximately 3 * 105
Lx for the 300 W mercury lamp, as measured by a Lux tester
(Hioki, Japan). The radiation powers of UV light at the reaction
point were 5.2 * 102 mW cm22 for the 9 W mercury lamp, 14.7 *
103 mW cm22 for the 100 W mercury lamp, and 72.3 * 103 mW
cm22 for the 300 W mercury lamp, as measured by a radiometer
Typically the reactor contained 500 mL of the soil washing
solution with a concentration of 10 mg L21 of target contaminant
(e.g., 0.0375 mM PCP solution) with or without 200 mg catalyst
prior to photodegradation . After 2 h of equilibrating the
solutions in the dark, UV light irradiation was initiated to start the
To follow the degradation process, 1 mL of mixture was
extracted from the soil washing solution at the beginning, after
equilibration, and after UV irradiation at predetermined
timepoints (5, 10, and 20 min for the 300, 100, and 9 W mercury
lamps, respectively). Every mixture was adequately shaken with
the same volume of methanol and then filtered through a 0.45 mm
Figure 3. Percentage remained of PCBs after 2 hours photodegradation under UV light with TiO2, graphene-TiO2 and without
catalyst: (a) congener 8; (b) congener 28; (c) congener 30; (d) congener 31; (e) congener 33; (f) congener 74.
pore size membrane filter (for PCBs, every mixture was shaken
with 2 mL of hexane).
PCP and phenol were analyzed using high-performance liquid
chromatography (HPLC, Agilent 1260) equipped with an Agilent
TC-C18 reverse phase column. UV detection was performed at
249, 270, and 254 nm for PCP, phenol, and other by-products,
respectively . A mixture of methanol and water was used as the
mobile phase, with a gradient mixture and a flow rate of 1.0 mL
min21. The quantification of the target analytes was based on the
calibration curve in the HPLC analysis, and the linear range was
0 mg L21 to 10 mg L21. Analytical results were verified by
standard PCP and phenol solution with certain concentration.
PCBs were determined using a gas chromatography mass
spectrometry instrument (GC/MS-QP2010, Shimadzu, Japan)
equipped with an HP-5MS capillary column. The temperature of
the GC oven was held at 150uC for 1 min, increased to 185uC at a
rate of 20uC min21, followed by an increase to 245uC at a rate of
2uC min21, and then held at 245uC for 3 min prior to further
increase to 290uC at a rate of 6uC min21. The injector and
detector temperatures were 250 and 290uC, respectively. The
carrier gas was helium, which was utilized at a flow rate of 1.0 mL
min21. The MS ion source and interface temperatures were 200
and 220uC, respectively.
The identification results were confirmed by GC/MS with the
same HP-5MS capillary column and temperature control
procedure as the GC analysis. Quantification of different PCB
congeners was caculated based on the peak areas of their
respective response factors of an authentic standard .
Results and Discussion
Photodegradation of PCP under UV irradiation
To explore the photodegradation rate under UV illumination,
variations in the concentrations of PCP were investigated. The
PCP photoactivities under 100 W UV light with TiO2, with
graphene-TiO2, and without catalyst are compared in Fig. 2. The
initial PCP concentration in the solution was 10 mg L21
(0.0375 mM) and the solvent was a 1:100 methanolwater soil
washing solution. The photocatalytic activity using TiO2 was
considerably lower than those in the other two treatments, and
pure UV (without a catalyst) showed the highest activity. After
120 min of UV irradiation, the removal rates of PCP by pure UV
(without a catalyst), UV with graphene-TiO2, and UV with TiO2
were 94%, 92%, and 57%, respectively.
The logarithm of the ratio between the initial concentration (C0)
of PCP and its concentration (C) at a specific given time is shown
in the inset of Fig. 2. The slope of these straight lines provided the
apparent rate constant. All correlation coefficients (r2) obtained
were higher than 0.95. As shown in this figure, pure UV
irradiation of the PCP solution provided the highest photocatalytic
rate constant, namely, 0.0236 min21. The photodegradation rate
constant using graphene-TiO2 was slightly lower at 0.0191 min21.
The data for the photodegradation using TiO2 was not linear at
the beginning of the irradiation procedure. Thus, the slope was
measured using data corresponding to 20 min or more of
photoirradiation, which provided a photocatalytic rate constant
of 0.0031 min21. This value was almost 1/8 of that of the highest
measured rate constant. The result showed that the PCP can react
with the NOH in a solution formed by UV irradiation and undergo
degradation. The higher photodegradation rate using pure UV
light than those with catalyst could be attributed to two main
reasons. First, the CO bond energy of methanol compound was
lower than the CCl bond energy of the PCP compound, and the
methanol compound is much easier attached to the surface of
TiO2 than hydrophobic chlorinated aromatic pollutant. When
photo irradiation began, the methanol attached to the TiO2
surface was degraded first. Second, the UV light, which could act
on PCP was blocked and absorbed by TiO2 catalyst and led to an
energy loss. The high photocatalytic rate constant at the beginning
of the photodegradation procedure was also caused by two
reasons. First, soil washing solutions were equilibrated with the
photocatalyst prior to irradiation. The high concentration of the
target contaminant resulted in their adsorption on the catalyst
surface, which reacted easily with the photoexcited electrons when
UV irradiation began. Thus, the energy wasted was minimal.
After a period of reaction, the pollution on the catalyst surface was
degraded, and the photodegradation rate decreased. Second,
when mercury lamp was turned on briefly, the energy in UV
region was slightly higher, which also led a higher constant rate at
production of initial PCP
Photodegradation of PCBs under UV irradiation
The experiments with the mixture of PCBs were complex
because various PCB congeners were present in the sample, and
excessive by-products were created during photodegradation. The
experiments were simplified by monitoring the concentrations of
six major components to explore the photodegradation rate of
PCBs in methanol soil washing solution under 100 W UV
irradiation. The initial concentration of PCBs in the solution
was 10 mg L21, and the solvent was a 1:10 methanolwater soil
washing solution. The concentrations of the six major components
in the solution were 0.67 mg L21 2,49-dichlorobiphenyl (congener
8), 1.48 mg L21 2,4,49-trichlorobiphenyl (congener 28), 0.61 mg
L21 2,4,6-trichlorobiphenyl (congener 30), 0.83 mg L21
2,49,5trichlorobiphenyl (congener 31), 0.68 mg L21
29,3,4-trichlorobiphenyl (congener 33), and 0.58 mg L21
2,4,49,5-tetrachlorobiphenyl (congener 74). Figure 3 compares the percentage of
photodegradation for each PCB congener under UV irradiation
either with TiO2, with graphene-TiO2 or without catalyst.
The results in Fig. 3 indicated that all congeners obtained the
highest photodegradation rates using pure UV irradiation. The
half-life of congener 8 was the shortest among the six congeners.
Congener 8 in the mixture of PCBs was fully photodegraded in
20 min using pure UV irradiation but required 100 min if
graphene-TiO2 was present in the mixture. For reactions that
contained TiO2, congener 8 was degraded by 77% after 2 h of
reaction time. Congener 30 was completely removed after
110 min using pure UV irradiation. The degradation rates were
71% and 87% after 2 h of reaction for systems with extra TiO2
and graphene-TiO2, respectively. Congener 33 was eliminated
after 100 min, which was slightly faster than congener 30.
However, the degradation rates of congener 30 using TiO2 and
graphene-TiO2 as the catalysts were 53% and 78% after the
reaction, respectively. These values were lower than those
observed for congener 30. Congeners 28, 31, and 74 remained
in each of the mixtures after the photodegradation, and their
degradation rates under these experimental conditions were
similar. The degradation rates were 91%, 72%, and 81% for
congener 28, 77%, 46%, and 65% for congener 31, and 74%,
34%, and 58% for congener 74. The photodegradation of PCBs
did not quite follow the pseudo-first order kinetics as described in
previous articles [9,12]. This finding may be caused by the fact
that some PCB congeners may be parts of the pathway of another
reagent, such as congeners 28 and congener 31, which can be
formed from congener 74 via the dechlorination of one chlorine
atom. The concentration of biphenyl could not be tested for the
entire duration of all three procedures because its degradation rate
was higher than those of the PCBs . Chlorine atoms in the PCB
molecules break apart from biphenyl during UV irradiation.
When the number of the chlorine atoms in the PCB congeners
decreases, they separate easily. PCB molecules will be
dechlorinated gradually to biphenyl and then be decomposed to small
molecule. Similar to PCP degradation, the photocatalytic activity
of TiO2-derived catalysts was based on creating electron-hole pairs
when exposed to UV radiation. The photoexcited electrons and
holes were mainly reacting with the methanol in the solution, and
the decomposition of target pollutants was delayed. The
electronhole pairs created on the photocatalyst surface were barely acting
on the PCBs, and the block of UV light caused an energy loss,
especially for the original TiO2 catalyst.
Photodegradation of Methyl orange under UV irradiation
To certify that TiO2 could increase the photodegradation rate
in other conditions under UV irradiation, Methyl orange (MO), a
commonly used dye, was photodegraded. This dye could be easily
trapped by the holes on the catalyst surface . The reactor
contained 500 mL of 50 mg/L MO (0.15 mM) with/without 1 g
L21 of TiO2 before photo degradation. This system was
illuminated with 300 W UV lamp after adsorption-desorption
equilibrium. To evaluate the discoloration rate of MO, the
mixture withdrawn from the MO solution at every predetermined
time-point was filtered through 0.45 mm pore size membrane filter
and analyzed by UV-Vis spectroscopy at 463 nm (UV-2401PC,
The degradation of MO solution fitted the pseudo first-order
kinetic as reported. The significant degradation rates between pure
UV light and UV with TiO2 were consistent with previously
reported results . Photo degradation using TiO2 as catalyst
obtained a photocatalytic rate constant of 0.0759 min21, which
was significantly higher than that of pure UV irradiation which
was 0.0010 min21. After 60 min UV irradiation, the removal
efficiency of MO using TiO2 was 99%, whereas that of MO using
pure UV was only 6%. This result proved that TiO2 is actually
useful in another condition.
Effect of different UV illumination sources
To investigate the relationship between the photodegradation
rate and the UV illumination type, two groups of experiments
were conducted to degrade PCP using mercury lamps of different
intensities, as shown in Fig. 4. The same 300 W high-pressure
mercury lamp was used. The power was increased from 100 W to
300 W. The results were compared with those obtained using a
9 W low-pressure mercury lamp. The apparent photocatalytic rate
constant for each UV irradiation procedure is listed in Table 1.
Pure UV photodegradation resulted in the highest photocatalytic
rate constant at each of the different mercury lamp power
intensities. Systems with TiO2 as a catalyst resulted in the lowest
rate constants. The photocatalytic rate constants measured using
the 9 W UV lamp were almost similar to those using the 100 W
UV lamp. This result was ascribed to the fact that the low-pressure
mercury lamp offered a maximum wavelength at 254 nm, which
concentrated all the energy in the UV region. By contrast, the
energy from the high-pressure mercury lamp was separated both
in the UV and the visible areas. In other words, the energy from a
low-pressure mercury lamp (in this case, the 9 W UV lamp) was
fully utilized in the photodegradation procedure.
Effect of adding methanol and the mineralization
Methanol water mixture was used as soil washing solvent to
extract PCP or PCBs from contaminated soil for former
photodegradation experiments. To estimate the influence of
methanol and evaluate the mineralization of the pollution,
PCPNa was used for UV irradiation because it is soluble in water. A
solution that contained 10 mg L21 PCP-Na (0.0347 mM) was
degraded without additional methanol using a 100 W UV light
source. Following the aforementioned method for irradiation and
data analysis, a photocatalytic rate constant of 0.0429 min21 was
measured, which was higher than that (0.0236 min21) when 10 g
L21 methanol was included in the solution. The photodegradation
rate of 10 mg L21 PCP in pure methanol soil washing solution was
also tested. After the same photo irradiation method, the
photocatalytic rate constant was measured at 0.0143 min21. This
result indicated that methanol does not participate in the
degradation pathway of PCP. The reaction of methanol consumes
energy from the UV irradiation, which reduces the photocatalytic
rate constant for target contaminant. The total organic carbon
(TOC) of the solution was tested. TOC was 2.0 mg L21 at the
beginning while there is only PCP-Na in the solution. TOC
decreased to approximately 80% when the PCP-Na in the solution
was almost fully degraded, which indicated that dechlorination,
degradation and mineralization were coexistent. After 2 h of UV
irradiation, PCP-Na was fully removed and TOC decreased a half
to 1.0 mg L21, which showed that mineralization go along after
was lower than those of other types of chlorophenols on its
degradation pathway [29,30]. Chlorophenols produced in the
degradation pathway of PCP were dechlorinated or decomposed
because of their comparatively high photodegradation rate
constant, and their concentration was undetectable by HPLC in
the overall mixture. During UV irradiation, PCP lost chlorine
atoms one by one until all Cl atoms were separated from the
benzene ring at the beginning of the process. The dechlorination
product of chlorophenols was phenol before further conversion,
which resulted in the splitting of the benzene ring. The final
concentration of phenol was investigated after each UV
irradiation, as shown in Table 2. The results indicated that the final
concentration of phenol in solution were similar for the three
treatments. Given that the concentrations of other chlorophenols
were negligible in the solution, PCP degradation did not end after
dechlorination, whereas benzene ring continued to split. The
photodegradation with pure UV irradiation was similar to the
treatments using a catalyst, which did not simply terminate after
Effect of catalyst dosage
The effect of photocatalyst loading was investigated to
determine the relationship between the absorption of photons
and UV light energy blocked by the excess amount of catalyst.
Therefore, a series of experiments with different amounts of TiO2
was conducted. The same 1:100 methanol water soil washing
solution that contained 10 mg L21 PCP (0.0375 mM) was
degraded using a 100 W UV light source. All experiments
followed pseudo-first order kinetics. The photocatalytic rate
constant of each experiment was compared with that of pure
UV irradiation, as shown in Fig. 5. Photodegradation rate
decreased with increasing TiO2 loading. TiO2 catalyst almost
blocked the whole photon energy when loading was up to 10 g
L21. The result indicated that the reaction with NOH in methanol
soil washing solution is the main source of PCP degradation under
UV light, and TiO2 added reduced the degradation.
Effect of pH
The pH values of the soil washing solution remained at
approximately 4. To investigate the effect of pH on the
photodegradation rate, PCP solutions with four different pH
levels ranging from pH 1 to 13 were tested in the photoirradiation
experiments using a 100 W UV light source, as shown in Fig. 6.
The pH values were adjusted using either HCl or NaOH. The
photodegradation rate increased as the pH value increased in each
of the solutions. At high initial pH, the elevated concentration of
the hydroxide ions (OH2) is assumed to result in increased NOH
production that could accelerate the photodegradation rate. The
pure UV irradiation and graphene-TiO2 catalyzed systems
provided significantly higher degradation rates compared with
those measured using TiO2 at each of the different pH values. The
degradation rate of PCP under pure UV irradiation was slightly
higher than that of graphene-TiO2 catalyzed systems at not so
high pH level, whereas their rates became almost similar under
alkaline conditions. When pH was too low, NOH was difficult to
form, especially when considerable photon energy was blocked by
TiO2. Thus, the photocatalytic activity of TiO2 was lost when the
pH of the solution was adjusted to 1 using HCl.
Phenol production after UV irradiation of PCP
The main contaminant in the solution was PCP during the UV
irradiation procedure, because the photodegradation rate of PCP
TiO2-derived catalysts are increasingly being used to treat
samples that contain environmental pollutants, However, they are
not well suited for the photodegradation of PCP or PCBs in
methanol soil washing solution. Using graphene-TiO2 as a
photocatalyst evidently enhanced the photodegradation rate under
UV light irradiation compared with using TiO2 as the
photocatalyst. However, pure UV irradiation showed the highest
photocatalytic rate among the three treatment conditions. PCP and
PCBs exhibited photoreactivities via NOH in the solution and were
decomposed directly under UV irradiation. Addition of
TiO2derived catalysts led to a loss of energy in this condition, thereby
decelerating photodegradation. The photodegradation abilities
were similar under UV irradiation regardless of whether a
photocatalyst was added because all treatments completed the
dechlorination and degradation, which were similar to published
results [9,12]. The fact that biphenyl in the PCB solutions was
undetectable during the entire reaction progress and the TOC test
of PCP also illustrates that photodegradation under UV
irradiation without a catalyst allows both dechlorination and degradation
for the contaminants. Prior degradation of methanol and UV light
blocked by the photocatalyst suspension caused photoenergy loss
and a decrease in the photodegradation rate.
Conceived and designed the experiments: ZZ YZ. Performed the
experiments: ZZ. Analyzed the data: ZZ TC. Contributed reagents/
materials/analysis tools: HW WL. Contributed to the writing of the
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