Reduction of mercury (II) by carbon dioxide radical, generated from the interaction of formate with the triplets states of 1,4-naphthoquinone.

ANDREA M. BERKOVIC, MÓNICA C. GONZALEZ, REINALDO PIS DIEZ, S.G. BERTOLOTTI, AND DANIEL O. MÁRTIRE

Menoza

Congreso; 21 st IAPS Inter -American Photochemical Society; 2011

Mercury is one of the most important environmental contaminants that need more attention from policy makers, industry, and general public. This contaminant is toxic, persistent, and long-lived in the atmosphere. Coal combustion is believed to be the main source of mercury emissions to the atmosphere. Because of mercury cycles, it can be carried over long distances away from its initial source location. Mercury is mainly deposited into the aquatic environments through precipitation. Once in the aqueous phase, some biological processes transform mercury into organic compounds (methylated mercury compounds), which are extremely toxic and can be accumulated in fish, and can present significant risk for the human health. For these reasons it is necessary to develop methods of water treatment based on the elimination of mercury from waters. In this work we investigated the photo-induced reduction of Hg(II), in aqueous solutions of HgCl2 by the carbon dioxide radical anion, CO2.-, which presents a suitable redox potential for the reduction of Hg (II) in solution (Eº= 0.85 V vs. NHE). The CO2.- radical anions were generated by photolysis of aqueous solutions of 1,4-naphthoquinone (NQ)in the presence of formate ion in excess . As the CO2.- is a one-electron reducing agent, it transforms Hg (II) into Hg (I), which is insoluble in the presence of chloride ion. Thus, the method allows the elimination of mercury ions from the solutions. The laser flash-photolysis technique with excitation at 355 nm was employed to investigate the reaction mechanism at room temperature. Due to the high absorbance of the solutions below 300 nm, the direct detection of the CO2.- was not possible. For this reason, methyl viologen (MV2+) was used as a probe to detect the formation of CO2.-. This radical is able to reduce the MV2+ to MV·+, a radical species that presents a characteristic absorption spectrum in the UV-Vis (lambda = 390 nm and other lambda max = 600nm), allowing therefore the indirect monitoring of the CO2.- radical.