The distribution of iron oxidation states in a constructed wetland as an indicator of its redox properties.

Wastewater-treatment processes taking place inside constructed wetlands are closely connected with chemical properties of these systems. The aeration of a wetland via the roots of the vegetation (and a subsequent formation of redox-potential gradients) strongly influences the wastewater treatment efficiency, and thus it represents one of the most important characteristics of the wetland. The concentration ratios of individual iron oxidation states (Fe(II) and Fe(III)) were determined as the indicator of the redox properties of the constructed wetland reed bed during this study. Interstitial water from the wetland was sampled eleven times throughout the year 2005. The spectrophotometric method using 1,10-phenanthroline was properly optimized (limits of detection and quantification, sensitivity, linear dynamic range, repeatability, and accuracy values were assessed) and applied for iron determination. Most of iron, ca. 98%, is reduced to the Fe(II) form in raw wastewater and water from the inflow zone of the constructed wetland, however, at the outflow and in the vegetation bed both iron oxidation states are usually detected. The presence of Fe(III) in the reed bed (ca. 10-30% for some samples) demonstrates the aeration of the wetland by the vegetation (Phragmites australis) resulting in a re-oxidation of Fe(II).