Study On The Phototoxicity, Photodegradation And Application Of Xanthene Derivatives

Photosensitizers have great potentials in environmental engineering, such as wastewater treatment, environment friendly agrochemicals and algae bloom control. Because vegetation sourced photosensitizers are unstable, low concentration in plant and insignificant effect, we put our emphasis on a kind of chemical photosensitizers - xanthene.Xanthene chemicals have been used in production and life. However, the research of their photoactivity and photodegradation is not systemic and adequate. At the same time, the methods and results differ in thousands ways from different research. This restricts the full application of this kind of chemical. Thus, this study focuses on the rules of molecular structure, photo bioactivity and photodegradation, which will provide the theoretics basis for application. We also research on the effect of xanthene chemicals to the growth of Microcystis aeruginosa and the content of chlorophyll a, and discuss on the possibility for the xanthene chemicals to be developed as algaecide. The results of experiments are as per below.1. The rules of physiochemical characters are: In vitro physicochemical tests showed that the max absorption wavelength of the photosensitizers is between 490 to 539 nm, which coincides with the spectral output of the sunlight; The sequence of lipophilicity of xanthene derivatives is: FlI2(1.21)>FlBr2(1.01)>Na2FlBr4Cl4(-0.21)>Na2FlI4(-0.24)>Na2FlBr4(-0.25)>Na2Fl(-0.28); Each of the xanthene derivatives was able to photosensitize the production of singlet oxygen, in the order of Na2FlBr4Cl4(1)>Na2FlI4(0.92) > Na2FlBr4(0.81) > FlI2(0.69) > FlBr2(0.67) > Na2F(0.02). With the increasing number of halogen substituents, the singlet oxygen yields increased and the phototoxic activity increased too.2. The rules of photo bioactivities are: Without illumination, the tested xanthene derivatives showed almost the same level of inherent toxicity to the same organism, which showed the inherent toxicity of xanthene derivatives were primarily dependent on the structure of parent molecule. Upon illumination, the photosensitizers showed obvious phototoxicity to all organisms. The xanthene derivatives showed stronger phototoxicity to Gram-positive bacteria. With the increasing number of halogen substituents, the singlet oxygen yields increased and the phototoxic activity increased too. There was no obvious correlation between relative lipophilicity and activity in the current study. Na2Fl showed obvious synergetic action to other xanthene derivatives; it also show obvious synergetic action as combined lipophilic xanthene chemical and hydrophilic xanthene chemical. The synergetic action for xanthene derivatives is more effective to Escherichia coli that the other two microorganisms.3. The rules of photodegradation are: The degradation of xanthene derivatives in solution and on the surface of silica TLC plates is pseudo first-order kinetic reaction. The half life of xanthene derivatives in water is between 5 hours to 7 hours, and on the surface of silica TLC plates is between 1 hour to 2 hours. The half life of lipophilic xanthene derivatives is longer than that of hydrophilic xanthene derivatives.4. The research of the intermediate products and final products during photodegradation shows: The complete degradation of photosensitizers to CO2 is of great environmental significance. During photodegradation test, the TOC content of C20H6O5Br4Na2 decreased around 50% during the 8 hour irradiation period. Irradiation also led to a gradual increase in the number of microorganisms that could survive. This means, when subject to light, the photosensitizer solutions degraded rapidly and the phototoxicity of the residues disappeared quickly. Actually, the energy density in the natural environment is much stronger than that in the experiment. Thus, the photosensitizers are expected to degrade more rapidly in the environment.5. Na2Fl shows no obvious algaecidal activity with or without illumination, however, the other five xanthene derivatives shows approximately similar inherent toxicity and phototoxicity. Na2FlBr4Cl4 shows the maximum algaecidal activity, and the MLC value is 40μM with illumination and 90μM without illumination. From the research on the Na2FlBr4Cl4 against the growth of algae and the content of chlorophyll a, we obtains that xanthene derivatives have little effect on the growth of algae at low concentration and can restrain the growth of algae at middle and high concentration. Because xanthene derivatives can degrade rapidly in the natural environment, thus, they can be developed as algaecide. The mechanism to kill the algae needs further research.