Heterogeneous Oxidation of Phenolic Compounds with Photosensitizing Catalysts Incorporated into Chitosan

Abstract

The increasing amount of hazardous micropollutants in the aqueous environment has recently become a concern, especially because they are not usually included in environmental monitoring programs. There is also limited knowledge regarding their behavior in the environment and their toxicity. This paper presents results regarding the heterogeneous photosensitized oxidation of 10 phenolic compounds under visible light. All of the selected compounds are classified as pollutants of emerging concern. For the first time, the application of photosensitizing catalysts incorporated into a chitosan carrier was investigated from several points of view, namely, structure characterization, singlet oxygen generation potential, photodegradation ability, biodegradability, and toxicity assessment. It was found that compounds of different origins were degraded with high effectivity. Photoactive chitosan was stable and could be reused for at least 12 cycles without losing its photocatalytic activity. The Hammett constants for all of the degraded compounds were determined. Improved biodegradability after the treatment was achieved for almost all compounds, apart from 4-hydroxybenzoic acid, and only slightly for 2-phenylphenol. The acute toxicity was assessed using bioluminescent Vibrio fischeri bacteria, indicating lower toxicity than the parent compounds.Photosensitized oxidation is undoubtedly one of the most important photochemical methods, since it does not generate any additional pollutants [5]. Moreover, the advantage of photosensitized oxidation is the photooxidation of hard-degradable compounds by singlet oxygen generated in the presence of air and solar radiation [5][6][7], therefore, photosensitized oxidation is classified as a green chemistry process. Moreover, because the heterogeneous process is considered to leave no residue, no sludge is produced from the process. Furthermore, if the immobilized photosensitizer remains unchanged throughout the process, the photocatalyst can easily be reused.Two general mechanisms of photosensitized oxidation for such reactions are well-known as Type I, also called the radical-involving mechanism, and Type II, the singlet oxygen mechanism [8,9]. Our previous research showed that singlet oxygen could be used for water purification. For this kind of area, the immobilization of photosensitizers onto various materials (heterogeneous system) is a desirable approach due to the easy separation from the reaction mixture and the preparation for reuse. Moreover, in homogenous systems (where the photosensitizer is dissolved in a water solution), the photosensitizer undergoes photobleaching, resulting in a rapid decrease in yield. Immobilized photosensitizers exhibit the advantages of good activity, stability against photobleaching, and repeated use [10]. Immobilization of a photosensitizer is associated with a decrease in the quantum yield of singlet oxygen [8,9]. This may be due to limitations in oxygen diffusion into and from the carrier material [9]. These disadvantages can be overcome by reusi...

Optimization of Heterogeneous Photosensitized Oxidation for Winery Effluent Treatment

“…On the other hand, results for toxicity levels show an improvement in these levels for experiments carried out at pH 8.7 and 4.0 when compared with the initial effluent, agreeing with the study by Foszpa ńczyk et al [40], where toxicity levels decreased after 180 min of photosensitized oxidation treatment. In another work, Foszpa et al [16] studied the removal and toxicity levels of 10 aqueous contaminants, including phenolic compounds and parabens. Using chitosan carriers and sunlight, they removed over 50% of all pollutants and lowered the toxicity levels of 7 of those 10, as they presented lower toxicity than their parent compounds.…”

“…Photosensitized oxidation is a promising photochemical method for the removal of compounds that are difficult to degrade. Its application only requires three components: a radiation source, air or oxygen, and a photosensitizer (PS) [16]. Photosensitizers are chemical agents that are easily excited by the action of visible light and, when reacting with molecular oxygen, form singlets of oxygen ( 1 O 2 ), a strongly reactive species with the ability to oxidize resistant compounds [17].…”

“…To our knowledge, this is the first paper to deal with the application of heterogenous photosensitized oxidation for effluent purification. Up to now, this process has been employed mainly for the model aqueous environment [16,25]. However, there is a lack of information about its applicability in a highly polluted environment.…”

Emerging pollutants—Part II: Treatment

“…From their results, using 0.2-2.0 g/L catalyst concentration and H 2 O 2 stoichiometric dose, the target pollutants and their aromatic intermediates can be completely removed in 60-90 min. Foszpańczyk, Bilińska, Gmurek, and Ledakowicz (2019) reported the removal of 10 phenolic compounds in water by heterogeneous oxidation under visible light with photosensitizing catalysts incorporated into a chitosan carrier (PCICC). The 10 phenolic compounds included phenol, 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol (2,4,6-TCP), methyl paraben (MeP), benzyl paraben, 4-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, 2-phenylphenol, and 3-phenylphenol.…”

Sonophotocatalytic Degradation of Malachite Green by Nanocrystalline Chitosan-Ascorbic Acid@NiFe2O4 Spinel Ferrite

“…It is composed of D-glucosamine and N-acetyl-D-glucosamine linked by β- (1,4)-glycosidic bonds associated with a large number of hydroxyl and amino groups [29]. These functional groups provide a large number of chelation sites for the number of metal ions to reduce them in nanoparticles and demineralization of organic molecules under UV-Vis radiation [30]. Furthermore, to increase the rate of hydroxyl radical generation in the presence of light, chitosan was grafted with L-ascorbic acid (vitamin C) [31].…”