“Characterization of triplet states of humic acids by laser flash-photolysis”

MARTIN, MARCELA; GONZALEZ, MÓNICA; BORSARELLI, CLAUDIO; MÁRTIRE, DANIEL

Cubatão, São Paulo, Brazil

Congreso; IX ELAFOT; 2008

ELAFOT

Characterization of triplet states of humic acids by laser flash-photolysis Marcela Martin1†, Mónica Gonzalez1, Claudio Borsarelli2, Daniel Mártire1 1INIFTA, UNLP. Diagonal 113 y 64 (CP. 1900) La Plata - Argentina. 2 Facultad de Agronomía y Agroindustrias, UNSE. Av. Belgrano (S) 1912 (CP. 4200) Santiago del Estero. † E-mail: marcemartin84@gmail.com Humic substances (HS) can act as photosensitizers and promote the degradation of organic pollutants in the environment. The time-resolved studies on the photolysis of HS are restricted. In this communication we investigate the spectral characteristics of the intermediate species formed by photolysis of humic acids in aqueous solution. The experiments were performed using the laser flash-photolysis technique (LFP). The excitation source was a Litron Nd:YAG laser (2 ns FWHM y 6 mJ per pulse at 266 nm). Previously purified commercial Aldrich humic acids (HA) were employed in the experiments. Traces were obtained with solutions of different pH values and saturated with Ar, air or oxygen. Since the system is complex, a bilinear regression program was used to analyze the minimum number of intermediate species involved in the decay traces. Data obtained at pH 4.0 Laser photolysis of HA at pH=4.0 in deoxygenated solutions results in the formation of two transients species: A and B. The decay kinetics of A is described by an exponential law, Equation 1. The formation-decay kinetics of B is described by a biexponential law, Equation 2. Equation 1 Equation 2 The introduction of air or oxygen into the solutions is accompanied by the shortening of the transient lifetimes (see Table 1). Table 1: Lifetimes , and  in solutions saturated in Ar- and Air. Solution of AH ms k1/ M-1s-1 ms k2/ M-1s-1 ms k3/ M-1s-1 Saturated in Ar 0.340  0.004 3.14•107 0.450  0.014 1.97•107 0.480  0.015 0.56•107 Saturated in Air 0.100 0.006 0.100 0.080 0.300 0.020 Figure 1- Absorption spectra and kinetic behavior of the short-lived intermediates formed from solutions of HA saturated in Ar at pH= 4.0(Fig. a), pH= 7.0(Fig. b) and pH= 11.7(Fig. c). Data obtained at pH 7.0 Experiments at pH=7.0 in deoxygenated solutions show the formation of three transients species: C, D and E. The decay kinetics of C is described by an exponential law (Equation 1) with the rate constants coincident with the rate of formation of D. The intermediate E is formed from the decay of D. Conclusion From the oxygen effect on the decay kinetics of the transients (Table 1), these species are assigned to triplet states. The observed mechanism agrees with the T-T energy transfer proposed for humic acids.