Effects of Fe(III)-concentration, speciation, excitation-wavelength and light intensity on the quantum yield of iron(III)-oxalato complex photolysis

Iron(III)oxalato complexes do frequently occur in the environment, specifically in surface waters, in atmospheric waters (clouds, rain, fog) or in waste waters. Due to their high photo-reactivity and their absorption overlap with the actinic spectrum, Fe(III)oxalato complex photochemistry is widespread and of broad interest. Fe(III)oxalato complex photolysis in deaerated solutions using single excimer laser flash photolysis at 308 and 351 nm and continuous Hg(Xe)-lamp irradiation at 313, 366 and 436 nm was quantified via Fe(II) quantum yield measurements with phenanthroline complexometry and UV–vis detection. Measured Fe(II) quantum yields showed a dependence on initial Fe(III)ferrioxalate concentration and irradiation energy at below millimolar concentrations. Individual molar extinctions (in l mol−1 cm−1) and individual quantum yields (Φ) were determined for initial Fe(III) concentrations of 4.85 × 10−4 M for the 1:2 (FeOx2−) and 1:3 (FeOx33−) complexes applying a regression analysis for solutions containing variable ratios of 1:2 and 1:3 complexes: ɛ1:2, 308 nm = 2300 ± 90, ɛ1:3, 308 nm = 2890 ± 40, Φ1:3, 308 nm = 0.93 ± 0.09; ɛ1:2, 351 nm = 1040 ± 30, ɛ1:3, 351 nm = 1120 ± 20, Φ1:3, 351 nm =0.88 ± 0.08; ɛ1:2, 313 nm = 2055 ± 111, ɛ1:3, 313 nm = 2663 ± 37, Φ1:3, 313 nm = 0.12 ± 0.05; ɛ1:2, 366 nm = 753 ± 357, ɛ1:3, 366 nm = 709 ± 10, Φ1:2, 366 nm = 1.17 ± 1.46, Φ1:3, 366 nm = 0.91 ± 0.09; ɛ1:2, 436 nm = 55 ± 9, ɛ1:3, 436 nm = 22 ± 2, Φ1:2, 436 nm = 1.40 ± 0.40, Φ1:3, 436 nm = 1.00 ± 0.20. Individual quantum yields for the 1:2 complex could only be determined for the excitation wavelengths 366 and 436 nm due to non-linearity of the data for 308, 351 and 313 nm. The non-linearity is ascribed to complicated interactions of secondary reactions involving Fe(III)oxalato educt-complexes, carboxyl radicals and Fe(II)-radical complexes. The 1:2 complex has generally a higher quantum yield compared to the 1:3 complex at all considered wavelengths.

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► Aqueous solutions of Fe(III)-oxalato complexes were photolyzed.
► The Fe(II) quantum yield decreases at [Fe(III)] < 1 × 10−3 M.
► Excitation energy influences Fe(II) quantum yield of dilute ferrioxalate solutions.
► 1:2 Fe(III)-oxalato complexes show higher quantum yields than 1:3 complexes.