A comparative study of the photoinactivation of bacteria by meso-substituted cationic porphyrin, rose Bengal and methylene blue

Advances in wastewater treatment technology have led many to predict that planned wastewater reuse in agriculture will soon become more common in some regions of the world which face acute problems of water quality and quantity. The use of ecologically friendly wastewater disinfection techniques could be one of the most exciting advances in this field. The combined action of a photosensitizer (meso-substituted cationic porphyrin, TMPyP; rose Bengal, RB; methylene blue, MB) and visible light, particularly sunlight, seem to be a promising approach to microbial inactivation, potentially applicable for disinfection of domestic effluents. In the present work, photosensitization was either performed on Gram-positive and Gram-negative bacteria in pure culture (Enterococcus hirae and Escherichia coli), or carried out with wild strains in secondary wastewater effluent (enterococci and E. coli). The results described in this paper show that TMPyP is the most effective for photoinactivation of the bacterial models studied here. The relative effectiveness of RB and MB was found to be tightly linked to bacteria Gram type. Whatever the sensitizer used, Gram-negative bacteria were more resistant to photosensitization than Gram-positive strains. The order of increasing effectiveness of the photosensitizers for photoinactivation of Gram-positive bacteria (TMPyP ≥ RB > MB) and Gram-negative bacteria (TMPyP > MB > RB) remains unchanged for either pure culture or wild strains (bacterial communities in wastewater). The effectiveness of the photochemical process depends primarily on the type of microorganisms as well as the type of photosensitizers (concentration, singlet oxygen quantum yield, ionic charge), and the reaction medium.

Irradiation with simulated sunlight in the presence of TMPyP, RB or MB produces effective photoinactivation of the Gram-positive and Gram-negative bacteria studied here. The results described in this paper show that TMPyP is more efficient than RB or MB. A concentration of 0.73 µmol/L of TMPyP allows one to yield a satisfactory log reduction in Gram-positive and Gramnegative bacteria. The findings reported herein further support the conclusion that porphyrins represent a class of photosensitizers that are potentially useful in environmental management. The efficiency of the combined action of a photosensitizer with visible light to eliminate microbial pollutants appears to be an ideal scenario for sunny countries in the arid and semi-arid zones. This work demonstrated the great potentials and the broad development perspectives offered to the photosensitization process for unrestricted wastewater reuse in agriculture. Photochemical disinfection of wastewater using TMPyP is possible in the future provided that we resolve the problems arising from the crippling cost of porphyrin and its dispersion in the environment. Using porphyrin-linked solid support for example would make it possible both to recycle/reuse the photosensitizer and to get rid of the problems of porphyrin elimination in dissolved phase. Thus, feasibility of this approach at laboratory scale will be considered in future research, as well as cost evaluation when reaching a stage at which it can be used on an industrial scale. In addition further attention will also be given to the regrowth (photoreactivation) of microorganisms submitted to photosensitization.