Water pollutants are often very stable in environmental conditions. One of the reasons for that is that they do not absorb solar light, which could otherwise provide energy for their degradation. The solution to this problem may be the application of photosensitizers - organic compounds which absorb (solar) light and transfer its energy to the molecules of the pollutants which then undergo degradation. To be applicable for the purification of water the photosensitizer should be removable from water. This may be achieved by supporting the photosensitizing chromophores on a solid substrate, preferably also able to adsorb the pollutant molecules.

Goals of the research

The goal of the study was to synthesize the hybrid photosensitizers based on organic chromophores, including those of natural origin, supported on layered nanoclays such as bentonite or montmorillonite. We have studied the applicability of such hybrid photosensitizers for the degradation of (micro)pollutants of special interest such as cyanobacterial toxins (microcystin), azo dyes and phenolic compounds.

Results of the research

A series of hybrid photosensitizers was obtained using both low-molecular-weight and polymeric photosensitizers (i.e., obtained by covalent attachment of a chromophore to the chain of a synthetic polymer) supported on hydrophobically-modified layered nanoclays such as bentionite or montmorillonite. The chromophores used were both synthetic (naphthalene, rose bengal) and natural ones (porphyrin) and they could absorb visible and/or near ultraviolet light. The photocatalysts when dispersed in water could efficiently degrade real and model pollutants upon illumination by generation of singlet oxygen. Importantly, they could be removed upon photoreaction completion. For example, a photosensitizer obtained by adsorbing rose bengal between the layers of hydrophobically-modified nanoclay decreased the concentration of microcystin-LR, a peptide which appears in the water reservoirs as a result of cyanobacterial blooms and is strongly hepatotoxic. This 500-fold decrease was both due to chemical photooxidation and physical adsorbtion of the toxin onto the photosensitizer.