Speciation of Iron in Rain Water by Size Fractionation, Acid Decomposition, Acid Extraction and Catalytic Determination

For the speciation of trace iron in rain water, the concentration (CL) of labile iron was determined by a catalytic spectrophotometric method based on the oxidation of o-phenylenediamine with hydrogen peroxide. The total concentration (CT) of iron was determined by the same method after acid decomposition to obtain the concentration (CT-CL) of non-labile iron. For the particle fraction (particle size ≥0.45 µm), 0.1 M HCl-soluble iron was determined by atomic absorption spectrometry. Coexisting humic substances were spectrofluorometrically determined as humic acid. Labile iron was found in the fraction of molecular weight (MW) < 103 and estimated as FeOH2+ and Fe(OH)2+ from the chemical equilibria of hydroxo iron(III) ions. Non-labile iron was found in the particle fraction and characterized as acid-soluble iron hydroxide and humic iron aggregates as well as acid-insoluble compounds, like oxides and silicates. Labile iron was also found in the particle fraction of initial rainfall samples and characterized as iron complexed on humic iron aggregates. The above speciation results were confirmed by the analysis of synthetic samples.