Simultaneous Determination of Iron(III), Iron(II), and Manganese(II) in Environmental Samples by Ion Chromatography

A chromatographic method was developed for simultaneous determination of Fe(III), Fe(II), Mn(II), and other transition metals. A high-performance polymer-coated silica-based cation exchange column was used for the separation of metals from hydrochloric acid-extracted environmental samples. After separation, the metals were mixed with PAR [4-(2-pyridylazo)resorcinol] and passed through a supercoiled Teflon postcolumn reactor. The absorbance of the colored complexes was recorded on line at 520 nm. Detection limits for Fe(III), Fe(II), and Mn(II) were 19, 9, and 25 fmol, respectively. Linear detector response was observed up to concentrations of 5-20 nmol. At concentrations of 2 nmol, the analyses were reproducible with 0.4% for Fe(III), 0.2% for Fe(II), and 0.5% for Mn(II). The method was compared with a photometric assay using ferrozine for Fe(II) determinations and hydroxylamine as reducing agent for Fe(III). Concentrations of Fe(III) were calculated from ferrozine determinations prior to and after the reduction of samples. Good agreement of both methods was obtained for various applications. Fe(III) and Fe(II) concentrations were determined in a depth profile of a flooded rice paddy soil. The profile showed increasing Fe(II) concentrations from the upper 1 mm soil layer to a depth of 10 mm, indicating the highest iron-reducing activity in a soil depth of 3 mm. A growth experiment with the metalreducing bacterium Geobacter metallireducens showed Fe(III) reduction concomitant with the production of Fe(II) and consumption of acetate as a carbon and energy source. Production of Mn(II) from manganese oxide was followed with an enrichment culture of Mn(IV)-reducing bacteria.