Abstract
The sunlight-initiated photoreactions of Cu(II) complexes may have significant effects on the speciation and geochemical cycling of copper, which in turn can affect the bioavailability or toxicity of copper in the upper water column. However, there has not been a thorough investigation of the effect of molecular oxygen on the photolysis of Cu(II) complexes in aqueous solution. In this study, the photochemical formation of ammonia from the photolysis of Cu(II)-L-alanine complexes was measured under monochromatic radiation at 313 nm in 0.10M ionic strength aqueous solution at 25 °C and the results were compared in de-aerated solution and in oxygen-saturated solution. System parameters such as pH and the total initial concentrations of L-Alanine and Cu(II), which can vary the equilibrium speciation of Cu(II), were examined to quantify and characterize the effect of the Cu(II) complex stoichiometry on the photoformation of ammonia from ligand decomposition. The photoformation of ammonia from the photolysis of Cu(II)-L-alanine complexes in de-aerated solution was always less than that in oxygen-saturated solution. In de-aerated solution, the quantum yield of ammonia at 313 nm for CuL (0.050) was higher than that for CuL2 (0.036), which may be attributed to the stabilizing effect of the second ligand. In contrast, the quantum yield of ammonia at 313 nm for CuL (0.057) was slightly less than that for CuL2 (0.064) in oxygen-saturated solution. The different trend observed was ascribed to the formation of reactive oxygen radicals from molecular oxygen, which may directly decompose L-alanine ligand in addition to the ligand-to-metal charge transfer mechanism. The temperature dependence of the photochemical formation of ammonia was also investigated. The activation energy for the photoformation of ammonia can be easily determined by the following Arrhenius-type equation. The average activation energy in de-aerated solution (12 kJ/mol) was less than that in oxygen-saturated solution (39 kJ/mol), which could be implied the different pathways for the photoformation of ammonia from the photolysis of Cu(II)-L-alanine complexes.