LIGHT-INDUCED PHOTOREDUCTION OF Cr(VI) IN WASTEWATER BY VALUABLE BACTERIAL PIGMENTS

1. Introduction Cr(VI) is known to be a highly toxic, carcinogenic and non-biodegradable pollutant, that is massively used in different industrial processes and can be commonly found in industrial effluents. Different technologies have been investigated for Cr(VI) remediation and, in the last years, large interest has been demonstrated towards the photoreduction/photocatalysis of Cr(VI) species to environmentally friendly Cr(III) ions due to its efficiency, low cost and no dangerous chemicals production. In this context, microbial biosynthesis processes constitute a cost effective and environmentally friendly alternative to produce biomolecules of technological interest. This work shows new insight about UV-light driven photochemical degradation of fluorescent pigments produced by an Antarctic bacterium that, under UV-irradiation, was able to photo-reduce Cr(VI) to Cr(III), which is less toxic and easier to remove by precipitation. These pigments have been spectroscopically characterized and identified by HPLC-DAD-FLD and HPLC-MS-MS analyses. The Cr(VI) photoreduction and the pigments photo-oxidation were investigated by UV-Vis and photoluminescence spectroscopy, respectively. 2. Results and Discussion A mix of fluorescent pigments, biosynthesized by an Antarctic salt tolerant Pseudomonas bacterium, were characterized as siderophores, that are typical bacterial metabolites produced during iron deficiency, due to their strong ability to chelate iron4 . These biosynthesized pigments, in aqueous solution, showed a pale-yellow color and the pH was at around 6. The characterization by UV-Vis measurements showed the typical features of pyoverdine compounds with different spectral changes as function of the pH that can be attributed to the protonation and deprotonation of -OH groups of the dicarboxylic acid, and of the catecholate group present in the di-hydroxyquinoline structure, the fluorescent chromophore4,5 . The fluorescence of the pigments solution changed as function of the pH showing different emission behavior at acidic, neutral and basic pHs. Specific HPLC-DAD-FLD and HPLC-MS-MS methods were developed, and the best separation of the compounds was achieved by using the mobile phase: 90% (H2O, 0.1% Formic acid) and 10% (MeOH, 0,1% Formic acid) with C-18 column; the DAD wavelengths were set at 306, 366 and 374 nm. The DAD chromatogram showed two main peaks at around 12 and 14 min, while the signal in FLD evidenced the high fluorescence of the compounds. The HPLC-MS/MS analysis allowed to detect the presence of three different pyoverdine molecules in the sample, in fact two pyoverdine coelute together, but by MS-spectrometer can be easily detected. The properties of these pigments were therefore largely examined; in particular, the siderophores solutions were irradiated under UV-light and sunlight, and the ability to absorb photon was largely investigated and confirmed by UV-Vis spectral changes, demonstrating the photooxidation resulting in a fast degradation of the molecules. The important perspective of the present finding was to use these pigments as potential photo-reducing molecules for the Cr(VI) under the presence of UV light. In that case, the mix of these pyoverdines was added to a synthetic contaminated Cr(VI) aqueous solution (15 ppm) and irradiated by UV-light (365nm). In these experimental conditions, Cr(VI) was present as HCrO4 – as the predominant form; a decrease in absorbance in the range between 350 and 400 nm during time, combined with diphenylcarbazide test6 for the Cr(VI) detection, demonstrated the photodegradation/oxidation of the pyoverdines and the consequent reduction of Cr(VI) to Cr(III), confirming the ability of these molecules to photo-reduce the hexavalent form of chromium. In addition, the further decrease of fluorescence demonstrated the pigment’s photooxidation; similar results were obtained when the same procedure was performed under natural sunlight exposure. 3. Conclusions The present study investigated the features of a mix of bacterial pigments. A newer and green perspective has been reported regarding the use of these compounds that are able to perform the photoreduction of toxic Cr(VI) to Cr(III), with their complete photodegradation under UV-light or solar irradiation. In fact, under UV-radiation or natural sunlight, a simple photocatalytic experiment revealed that very small quantities of these pigments are highly efficient for the quantitative reduction of Cr(VI) to Cr(III) in synthetic contaminated water containing 15 ppm of Cr(VI). These results highlight the remarkable performance of these natural compounds and therefore represent a promising potential for purifying contaminated water resources.