5Photolytic degradation

of organic compounds of environmental concern in RTILs



Photolytic degradation of chlorinated phenols in room temperature ionic liquids

Introduction

Chlorinated aromatics, such as chlorophenols, can be treated using UV irradiation.
Studies suggest that photolytic transformation is one of the major mechanisms contribute to the degradation of chlorophenols in aquatic environments. Direct photolysis of chlorophenols involves both homolytic and heterolytic C–Cl bond cleavage followed by a series of subsequent reactions.
Although photolytic degradation of chlorophenols in water has been proven to be feasible, this technique may not be cost effective when the contaminant concentration in water is low and the volume of polluted water is large. For those hydrophobic compounds [i.e., polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs)] that are tightly chemisorbed on the solid matrices such as soils and dredged sediments, VOCs are usually used to wash them off. This may cause additional environmental problems.

Ionic liquids application

RTILs can be used for the extraction of chlorophenols, PCBs or PAHs from contaminated solid matrices. The contaminants transferred to the ionic liquid phase can be subsequently degraded in situ using UV radiation.
This two-step remediation technology avoids the use of VOCs and can also result in the regeneration of the ionic liquid solvent (step 2). Since ionic liquids are capable of solubilizing a wide variety of hydrophobic organic compounds to high concentrations, one batch of ionic liquid solvent can be repeatedly used for extraction in order to highly concentrate the organic compounds in the ionic liquid phase, while simultaneously achieving minimization in solvent use. For certain contaminants, such as PCP, PAHs, and PCBs, this approach is impractical when water is used as solvent due to their very low solubility in water.
In addition, a higher contaminant concentration in the solvent during the photodegradation process will result in lowering process cost. This is because the photodegradation rates will be higher and the reactor volume will be smaller.



Photolytic degradation of chlorinated phenols in room temperature ionic liquids

A recent study reports a new application of RTILs as solvent media for the photodegradation of organic compounds of environmental concern, certain chlorinated phenols from one to five chlorine atoms in two RTILs, namely, 1-butyl-3-methylimidazolium hexafluorophosphate {[bmim+]PF6} and 1-ethyl-3-methylimidazolium bis-(perfluoroethylsulfonyl)imide {[emim+]beti}. The stability of the ionic liquids, the influence of impurities present in these ionic liquids, the role of oxygen on the photodegradation reaction, and the recyclability of the solvents are also discussed in this work.

This study is the first demonstration that the destruction of chlorinated aromatics is feasible in room temperature ionic liquids using photolysis under 253.7 nm UV radiation.
In addition and after proper selection of the UV wavelength, this technology can be used to eliminate a variety of organic impurities and byproducts from contaminated ionic liquids during several chemical and engineering processes dealing with chemical synthesis, catalysis, and electrochemistry.

Results:

  • Stability of ionic liquids.Handling
    If this application involving the use of ionic liquids as both extractants and photodegradation reaction media is to be feasible, the reaction media must exhibit certain stability upon exposure to UV radiation. 1H NMR, 13C NMR and UV-Vis spectrometer were utilized to examine the stability of the ionic liquids under UV-C radiation.
    • Comparison of 1H NMR spectra of {[bmim+]PF6} before and after UV-C radiation for 6 h indicated no detectable destruction or any other alteration of the molecular structure and bonding.
    • Then UV-Vis spectrometer was utilized to examine the structure of the ionic liquids: 1 h of UV-C irradiation was sufficient enough to cause some changes to the ionic liquid solvents. The results suggest that 2% of impurities present in the ionic liquids utilized or even the ionic liquids themselves might undergo degradation to some extent. This resulted in the formation of UV-absorbing intermediates.

  • Impurities effects
    UV absorbing impurities present in less pure ionic liquid samples significantly lower the photo-degradation rate of chlorophenols. The intermediates and the UV-absorbing impurities present in the less pure ionic liquid could compete not only with the probe compounds but also with the ionic liquid for photon absorption. This competition could result in the reduction of degradation rate of the probe compounds as well as the reduction of possible degradation of the ionic liquid.
    On the other hand, the competition between these impurities and the ionic liquid for photon adsorption to some extent protect the ionic liquid from photolysis and enhanced the stability of the solvent.
    Although the ionic liquids tested in this study may also be affected by UV-C radiation, their stability is expected to be larger than that of volatile organic compounds.

    • Effect of acid impurities in ionic liquids.
      During the preparation of water-saturated {[bmim+]PF6}, the pH of the aqueous phase decreased significantly from the initial value of 5.8 to 3.0, which suggests the presence of acids as impurities in the ionic liquid. However, acid impurities seem not to affect the initial reaction rates.

    • Importance of the purity grade of ionic liquids.
      Although the purity of the ionic liquid is enhanced only by 1% (i.e. {[bmim+]PF6} of 98% purity and {[bmim+]PF6} of 97% purity used in the experiment) the degradation rate of 2-CP was increased considerably.
      Therefore, the availability of spectroscopic grade ionic liquids for studying photochemical reactions in ionic liquids is of great importance.

  • Physical propertiesIL's color.Physical properties
    The color of less pure ionic liquids generally ranges from yellowish to orange. The formation of the color has been attributed to the use of raw materials with color or excessive heating during the synthesis of imidazolium salt. A number of precautions for synthesis of colorless ionic liquids have been described, and a procedure for removal of color from impure ionic liquids using acidic alumina and activated charcoal has also been proposed. Indeed, higher reaction rates could be achieved after purification of this ionic liquid using activated carbon.

  • Photodegradation of 2-CP.
    The initial concentration of 2-CP affects the initial reaction rate in water-saturated {[bmim+]PF6}.
    In the lower concentration region (less than 1.57 mM), the reaction followed a pseudo-first-order degradation process with respect to concentration. When the experiments were performed at initial contaminant concentration above 15.89 mM, the reaction kinetics shifted from the pseudo- first-order to zero-order, and a transition zone existed between the low and high concentration regions.
    In the low concentration region, the amount of photons incident in the photochemical reactor is adequate for the number of 2-CP molecules available to absorb photon energy. The degradation rate is limited by the amount of contaminant molecules available, and increasing the concentration of 2-CP molecules enhances the initial degradation rate. When the initial contaminant concentration is high enough, the degradation rate is limited by the amount of photons incident in the reaction solution. In this region, increasing the contaminant concentration has little effect on the degradation rate.
    It should also be emphasized that the use of high initial contaminant concentration may not only change the kinetics of the reaction but may also affect the reaction mechanism.

  • Mechanism of reaction
    Phenol was found to be among the stable photo-transformation products of 2-CP in {[bmim+]PF6}. The proposed mechanism of formation is:
    Proposed mechanism of reaction for the formation of phenol
    Both the formation of phenoxyl radical and carbene may lead to phenol, while the ionic liquid may act as H-donor.

  • Influence of water-saturation of ionic liquids.Physical propertiesHandling
    In this study, the increase of water content in the ionic liquid phase did not show significant effect on the initial reaction rates, although the water content has an influence on the viscosity of the ionic liquids (viscosity measurement indicated that ionic liquids became less viscous with increasing water content.

  • The role of oxygen.
    Oxygen was found to have no significant influence on the photo-transformation of 2-CP. The degradation rates of 2-CP under aerobic and anoxic conditions were approximately the same.

  • Photodegradation of other chlorophenols.
    Similar degradation trends to that of 2-CP were observed for 2,4-DCP, 2,4,6-TCP, 2,3,4,5-TeCP and PCP in {[bmim+]PF6}. The addition of chlorine atom has previously been found to reduce the degradation rate of chlorophenols, but, in some cases, the position of chlorine substituent exhibited dominant effect and a reverse order could be obtained. In this study, it seems that ortho-substituent on the phenolic ring enhanced the activity of chlorophenols towards UV radiation.

  • Recycling of {[bmim+]PF6}Green Aspects
    Compared to the ionic liquid sample used as obtained from the manufacturer, the degradation rate of 2-CP was slower in recycle samples. Moreover, the degradation rate decreased as the number of recycles increased. This could be attributed to the formation of degradation intermediates of 2-CP that are resistant to UV-C radiation.

  • Toxic aspectsIL's Toxicology
    The toxic aspect of {[bmim+]PF6} needs to be carefully investigated due to the formation of its decomposition product, 1-butyl-3-methylimidazolium fluoride hydrate, during the purification of the ionic liquid and possible formation of toxic product HF.
    In this study, although the photodegradation of chlorophenols in the ionic liquids is feasible, the application of this process for environmental remediation purpose certainly requires the use of ionic liquids that will give no harmful or acceptable impact on the environment.


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