The work presents results of studies on 2,4-dichlorophenol (2,4-DCP) degradation in aqueous solutions using photochemically initiated processes by simulated and natural sunlight. A number of possible substrate photodegradation routes were investigated, by both direct photolysis and photosensitized oxidation process. The major role of singlet oxygen in 2,4-DCP photodegradation was proved. Rose Bengal and derivatives of porphine and phthalocyanine were used as sensitizers. The influences of various process parameters on the reaction rate were investigated. On the basis of experimental data reaction rate constants of 2,4-DCP photosensitized oxidation were determined. The possibility of using natural sunlight to degrade 2,4-DCP in water in the middle latitudes was stated. The acute toxicity bioassay was conducted with the marine bacterium
The pollution of aquatic environment by trace amounts of anthropogenic chemical substances has a hazardous impact on the regular development of plants and animals as well as on the human health [
Recently the use of advanced oxidation processes using different chemical and photochemical processes for the purification of wastewater or water treatment has become very popular. The main mechanism of compound degradation by these methods is the hydroxyl radical generation and utilization. Thus, they are very effective in removing micropollutants (also microorganisms) present in the wastewater, especially when these are found in low concentrations [
An alternative process seems to be the photosensitized oxidation process, particularly using molecular oxygen. It involves the absorption of the visible light energy by the photosensitizer to bring it to a reactive excitation state, transferring its excitation energy to the surrounding molecules. It can occur according to two different photochemical mechanisms: type I (as electron or hydrogen atom transfer), type II photooxidation (as energy transfer to the oxygen molecule and generating reactive singlet oxygen form), or a combination of both. The domination of any of these mechanisms is dependent on many factors including the nature of the substrate and of the sensitizer and the properties of their microenvironment. Thus, the sensitizer acts here, in principle, as a specific kind of catalyst which enables the reaction to occur while remaining unchanged itself.
It is important to note that in this process the photosensitizer may be used in homogeneous solution or may be immobilized on various kinds of carriers [
Herein we present results of photooxidation of 2,4-dichlorophenol under simulated and natural sunlight via singlet oxygen generated by various excited photosensitizers in homogeneous solutions. The objective is to define the conditions that maximize the rate of photosensitized degradation of 2,4-DCP in the aquatic solution, to determine the kinetic constants, and to compare the photodegradation rate under simulated and natural sunlight.
All chemicals were commercially available products. The model pollutant 2,4-dichlorophenol (≥99%) was purchased from Aldrich. Sodium azide (NaN3) and
The pH of mixtures during experiments was maintained stable using a buffer solution (according to Britton-Robinson). All buffer reagents H3PO4, H3BO3, CH3COOH, and NaOH were p.a. quality (POCH, Poland). Samples for chromatographic analysis were acidified with 0.5 M phosphoric(V) acid (p.a. POCH, Gliwice).
All reaction solutions were prepared in distilled water (Millipore Milli-Q Plus System, 18.2 MΩ).
The following dyes were used as photosensitizers: Rose Bengal, RB, was purchased from Fluka; aluminum(III) phthalocyanine chloride tetrasulfonic acid, AlPcS4, was purchased from Frontier Scientific Inc. (Logan, USA); mesotetrasulphonato phenyl porphyrin, TPPS4, was purchased from Fluka (Steinheim, Germany); zinc(II) phthalocyanine tetrasulfonate tetrasodium salt, ZnPcS4, was purchased from Frontier Scientific Inc. (Logan, USA).
The important physicochemical properties of the dyes which allow them to act as efficient photosensitizers (PS) are presented in Table
The physicochemical properties of the dyes used in experiments [
PS | Structure |
|
|
---|---|---|---|
RB |
|
0.76 | 0.98 |
|
|||
TPPS4 |
|
0.62 | 0.78 |
|
|||
AlPcS4 |
|
0.34 | 0.44 |
|
|||
ZnPcS4 |
|
0.32 | 0.56 |
The experiments were conducted in a semicontinuous system in flat reactors (0.06 × 0.10 m) of the volume of 0.01 dm3 each one. Five reactors were symmetrically positioned around the xenon lamp (Osram 100 W,
The photochemical experimental setup with natural sunlight consisted of a glass reactor with a cooling jacket, equipped with a porous plate to deliver air into the reaction solution. Experiments were carried out in a semibatch mode. The volume of mixture was equal to 0.5 dm3 and it was buffered to maintain constant pH during irradiation. Experiments were performed always at the same time of the day (between 10 a.m. and 2 p.m.) at various atmospheric conditions. The spectrum of light was collected with an Oceans Optics USB 4000 fiber optic spectrometer with an approximate resolution of 0.4 nm.
The 2,4-DCP decay was monitored by HPLC (Waters) equipped with a UV diode array detector and a Nova-Pak 150/C18 column. The wavelength for the UV detection of 2,4-DCP was equal to 284 nm. A mixture of methanol and acidified water (0.01% H3PO4) was used as an eluent (70 : 30 vol./vol.) at the constant flow rate equal to 0.7 mL min−1.
The acute toxicity bioassay was conducted using a Microtox® Model 500 analyzer (Modern Water, New Castle, DE, USA) with the marine bacterium
In our previous experiments a large dependence of the sensitized oxidation reaction rate on pH of reaction mixture has been found. It is probably connected with pH-dependent degree of compound dissociation [
The dependence of 2,4-DCP relative concentrations decay on pH of reaction solution and presence of various ROS scavengers (
Following irradiation of 2,4-DCP by xenon lamp in the absence of the dye compound we recognized that the direct photolysis process is responsible for the disappearance of the substrate in about 20% (Figure
To fully characterize the photosensitized oxidation of 2,4-DCP we have to take into account as much as possible reactions which can occur during irradiation of the solution in the presence of sensitizer and oxygen. It is well known that during photosensitized oxidation process also other
Three factors must coexist in solution for the occurrence of photosensitized oxidation of a substrate: light, oxygen, and sensitizer. In the next series of experiments we tested whether the oxygen presence/concentration in the solution and type of photosensitizer really affect the reaction progress. As can be seen in Figure
The dependence of 2,4-DCP relative concentrations decay on the concentrations of oxygen in the system (
As can be seen in Figure
In Figure
The dependence of 2,4-DCP relative concentrations decay on the type of used photosensitizer (
We have tried to establish the kinetics of the 2,4-DCP photodegradation process. We have assumed that the following reactions occur during photosensitized oxidation of 2,4-DCP using RB as photosensitizer:
The results shown in Figure
To investigate the effect of the initial concentration of substrate on the rate of photosensitized oxidation process, the test was carried out for six initial concentrations of 2,4-DCP in different pH of the solution (for undissociated and dissociated form) (Figure
The dependence of the reaction rate photosensitized oxidation of the initial substrate concentration of 2,4-DCP using RB as a photosensitizer (
The application of (
Determined constant values (mean) of pH dependence of the reaction.
Constant of reaction |
pH 5 | pH 10.8 |
---|---|---|
|
1.65 × 106 | 3.64 × 107 |
|
1.05 × 108 | 4.95 × 108 |
Determined constants of chemical and physical quenching and chemical reaction rate constants have the same order obtained in our earlier research for butylparaben [
Interesting results were obtained in experiments in which the solution containing 2,4-DCP, a sensitizer, and oxygen was exposed to natural sunlight. The rate of the sensitized oxidation of 2,4-DCP was unexpectedly high. In sunny days complete disappearance of the substrate in alkaline solution in less than 10 minutes was achieved despite the fact that the mixture was bubbled only atmospheric air instead of pure oxygen (Figure
The dependence of 2,4-DCP relative concentrations decay on the type of used photosensitizer, in the absence of any sensitizer and in the presence of singlet oxygen scavenger, sodium azide (
The other observations are in agreement with the previous study with the experiments under the xenon lamp. The major role of singlet oxygen in the degradation of 2,4-DCP was also confirmed (the addition of sodium azide completely stopped the reaction). Similar results were observed in our previous studies with other substrates such as parabens [
There were no marked differences in the substrate decay rate using TPPS4, AlPcS4, and RB (perhaps in such short reaction time these differences were difficult to notice) while the presence of ZnPcS4 caused a significant inhibition of the degradation rate of 2,4-DCP (Figure
The strong dependence of reaction progress on the pH of the environment in the range from 5 to 10.8 has also been proven. While in the alkaline solution (pH = 10.8) 2,4-DCP was almost completely removed after 10 minutes, at pH = 5 the photosensitized process resulted only in a 40% reduction of 2,4-DCP concentration after 2 hours of sunlight irradiation (Figure
The dependence of 2,4-DCP relative concentrations decay on pH of solution (
The long-term purpose of our work was also to assess the yield of the photosensitized oxidation of 2,4-DCP using natural sunlight in central Poland irradiation conditions. Conducting the experiments under various atmospheric conditions both in cloudless days and with semiclouded or completely clouded sky was therefore decided. These investigations were performed in a neutral pH in order to estimate theoretically the rate of this photoprocess which might occur in natural water due to sunlight in the presence of compounds which may act as sensitizers, for example, humic acids. Several experiments have been made under various weather conditions. The results are shown in Figure
The dependence of 2,4-DCP relative concentrations decay under various sun insolation (
It was observed that even in an “unfavorable” (not optimum) pH of the reaction medium the complete disappearance of 2,4-DCP was achieved after approx. 2 hours of exposure.
In view of obtained results we can state that photosensitized oxidation is effective method of xenobiotic degradation in water environments even in the middle latitude.
As can be seen in Figure
Variation in the concentration of 2,4-DCP during photodegradation experiments mediated by TPPS4 and without TPPS4 and induced by solar and lamp radiation. Insert: incident energy (
The toxicity assessment was conducted by a screening test and an EC50 test for the photosensitized oxidation of 2,4-DCP in homogenous aqueous solutions under lamp irradiation. The results of the screening test during treatment time are shown in Figure
Toxicity analysis of 2,4-DCP photosensitized oxidation.
The determined EC50 values are presented in Table
Determined values of EC50, the effective concentration of the sample that causes 50% decrease in the light output of the
Time (min) | EC50 (%) | 2,4-DCP conc. in reaction solution (mg L−1) |
---|---|---|
0 | 2.69 | 47.80 |
5 | 4.05 | 7.74 |
10 | 6.06 | 0.96 |
25 | 9.64 | 0.81 |
However, it should be noticed that despite a large decrease in the 2,4-DCP concentration (about 60 times) EC50 value has increased slightly (only about 3.5 times). That unfortunately shows that although the transformation products are less toxic than 2,4-DCP, they are still characterized by very high toxicity.
Degradation of 2,4-DCP in the aqueous solution by photosensitized oxidation in a homogeneous aqueous solution under visible light irradiation is effective. The degradation occurs mainly due to the reaction with a singlet oxygen and to a small extent due to photolysis. The experimental evidence shows that this pathway of 2,4-DCP decomposition can appear to have a higher rate in the alkaline medium. The efficiency of the 2,4-DCP photodegradation strongly depends on the type of photosensitizer, initial concentration of 2,4-DCP, and oxygen content in reaction mixture. The toxicity assessment showed that photosensitized oxidation of 2,4-DCP resulted in a toxicity decrease.
In our opinion the photooxidation of phenolic compounds by singlet oxygen appears to be an interesting option from both the scientific and technological points of view due to its advantage of using ubiquitous substrates: oxygen from air and solar radiation. The interaction between them frequently occurs in nature, for example, in surface water, where the humic acids could act as the sensitizers.
The possibility of use of air and sunlight creates great opportunities of practical applications. The application of this process for large scale wastewater treatment would require the use of optimum sensitizer concentration, pH adjustment, and optimization of all other reaction conditions.
The authors declare that there are no competing interests regarding the publication of this paper.
This research was supported by the National Science Centre (NCN) in Poland within research project 2012/07/B/ST8/03787. Marta Gmurek acknowledges the support from Foundation for Polish Science within the START scholarship.