Porphyrin as photosensitizer of irrigation water, photostability and efficiency
The process of photosensitization (phototreatment) of water is gaining much interest as it involves three components that are individually harmless to the biological environment, namely, the photosensitizer, light, and molecular oxygen. Laboratory experiments on study factors affecting photostability of T4MPyP (meso-tetra [4-N-methylpyridyl] porphyrin) in water show that wavelength and intensity of incident light, concentration of photosensitizer and water quality (presence of organics contaminants, turbidity, pH, dissolved oxygen, and thickness of the water sheet) all affects the rate of photostability and phototreatment. The experimental study consisted in bringing i) secondary treated (by activated sludge) wastewater, ii) the same type of wastewater but photosensitized with 5 μM∙L-1 of T4MPyP during 6h of sunshine, and iii) well water to a sandy soil. Phototreated wastewater met Tunisian standards (NT 106.03) indicating that the porphyrin was efficient in removing indicator bacteria (fecal coliforms removal averaged 99.99%) but still contained a residual concentration of porphyrin (about 35% of the initial concentration). At the end of the experiment, the saline configuration of the soil depended on the irrigation water quality. In the case of the secondary wastewater, the surface layer (0-5 cm) was characterized by high concentration of soluble salts and by proliferation of some algae which resulted in surface clogging and lack of macropore continuity. In contrast, biological, chemical and physical clogging processes were reduced by using phototreated wastewater with a residual photosensitizer. The absence of algae in the surface layer of soil and the decrease of nutrients (nitrogen and phosphorus) and bacteria could reduce clogging of irrigated water. The higher salinity was observed in the deep layer (5-15 cm) due to a high mobility of the sodium and chloride ions followed by sulfates and calcium from the layer 0-5 cm to the layer 5-15 cm which can be attributed to the higher oxidation of organic matter induced by the presence of porphyrin. The results revealed that using wastewater with micromolar concentrations of T4MPyP in agriculture would inhibit the development of algae at the soil surface and lead to a better infiltration of salts in depths, which would prevent surface clogging. In addition, better retention of ammonium and orthophosphate ions was observed in the sandy soil during percolation of phototreated wastewater.