Degradation of polyphenolic contaminants using a natural polymeric dye of riboflavin-5´phosphate cross-linked chitosan as photosensitizer

Abstract

Generally, water treatment plants fail to completely eliminate pollutants, thus resulting in some contaminants passing into water bodies, producing harmful effects on the environment. One of the main environmental pollutants degradation mechanisms are chemical pathway, which involve several transformations, some of them take place in the presence of solar radiation known as direct or sensitized photoreactions. The emphasis will be on a sensitized photolysis reaction, involving the presence of another chemical, a dye that absorbs light radiation and produce excited states capable of inducing a photoprocess cascade, in which highly reactive species are formed. Besides, the dyes used in the photodegradation processes have the disadvantage that these dyes remain in the environment. The main issue with these methods lies in extracting the dye once the reaction is complete. This problem may be reduced, or fully solved with the use of polymeric dyes (PD), whose water solubility varieswith the pH of the solution. In this case, the photosensitized contaminant degradation occurs while the PD is dissolved in acid media.The PD consists of two natural compounds, specifically the sensitizer riboflavin 5´-phosphate (FMN) and chitosan(CH), which makes PD fully aquatic-friendly. 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) was used for PD synthesis as a crosslinker to create a phosphoramidate bond via EDC, which is formed by the FMN phosphate group and the CH amino group. A polymer of the same colour and similar photophysical properties of free sensitizer was obtained. Another important feature of this PD is that it is more photostable than the free dye and is readily removed by pH shift from the medium since it is soluble in solution of acetic acid and precipitates in solution of pH>5.9. Polyphenols (THB) were selected to test the photocatalytic activity of the synthesized polymer. THBs are known pollutants that are reactive to singlet oxygen (1O2) in homogeneous media. Oxygen uptake and UV-Vis spectroscopy monitored the photosensitised oxidation of THBs by PD. These results are comparing with those obtained using the free sensitizer. Different scavengers were used to evaluate the reactive oxygen species (ROS) involvement in the degradation process. The participation of 1O2, H2O2 and HO· in the degradation of THBs were studied in these experiments. In addition, laser flash photolysis studies have shown that CP?s ectronically excited states are also involved in THB degradation. The use of the CP in the photosensitized process represents a great advantage over the free dye and heterogeneous sytems. The CP has a higher photostability, which allows it to be used for longer periods of times. It does not present the diffusion problems intrinsic to heterogeneous media. Finally, one of the most promising characteristics of these CP, is that it can be extracted after photosensitized reaction, leaving the aquatic environments free of pollutants or pigments. This environmental friendly CP, provides a practical solution for those who are immersed in the subject of environment remediation or water treatment technologies