Iron Speciation of In-Situ Solutions from an Episaturated Spodosol

Abstract

Complexation of iron (Fe) by organic compounds as well as reductive dissolution are potentially important Fe dissolution processes in organic-rich soils such as Aquods. The objectives of this study were to develop a method to sample Aquod water table solutions and determine solution Eh and pH values without introducing oxygen from the atmosphere, and to determine the quantities of reduced and organically-complexed Fe in the soil solutions. Two soil profiles of a Typic Epiaquod in New Hampshire were studied. Zero tension lysimeters were installed in each of 3 horizons of the profiles. Iron (II) and total Fe were determined with the FerroZine method. Complexed (including organically-complexed) Fe was assessed using anion exchange and C18 columns. The solution samples were maintained in a closed system throughout sampling and determination of Eh and pH. In general, the redox potentials in the soil decreased with time during the spring, while the pH values increased with time. Samples obtained just before the Bs horizon drained had high levels of total Fe(II) and C18-extractable Fe. Median percents of anionic Fe were 43, 12, and 0% in OA, Bhs, and Bs horizon solutions, respectively, while those of C18-extracted Fe were 28, 66, and 0%. C18-extracted Fe, total Fe and Fe(II) were all strongly related with linear r 2 ranging from 0.92 to 1.00. This study indicates that Fe reduction can occur in spodic horizons, and organically-complexed Fe(II) can be a significant form of Fe in the soil solutions prior to draining of the profile. The latter form of Fe should be considered in predicting Fe dissolution reactions in Aquods.