The biogeochemical cycle of Iron: The role of Natural Organic Matter
12 / 2008 - 12 / 2012
The biogeochemical iron (Fe) cycle affects many important environmental processes related to water and soil quality. In many parts of the world, phytoplankton and plants suffer from an insufficient iron uptake. There is a large body of literature, which proposes that practically all dissolved iron in the world s oceans is bound by strong organic ligands. Recent work (Hiemstra and van Riemsdijk, 2006) suggests that the behaviour of dissolved iron in the ocean may be explained by the interaction of iron with marine dissolved organic matter that has fulvic acid like properties. For soils, a current study shows Fe deficiency in case of low concentrations of dissolved organic carbon (DOC) and a high pH, suggesting that humics may act as a Fe carrier. Humic and fulvic acids are also important in many terrestrial and aquatic environments in relation to the mobility and bioavailability of other heavy metal ions like Cu2+ and Pb2+. These metals may be in competition with adsorbed Fe(III) and with ions like Al3+ and Ca2+.
At present, the nature of the interactions between iron and natural organic matter (NOM) is poorly understood. In surface water (red circles) and soil solutions (open triangles), the amount of iron carried by DOM may be much larger than found for Humic Acid (orange squares) and marine DOM (blue squares). The reason may be the formation of nano-iron oxide particles that are associated with DOM. Such particles may for instance form in acid streams from swamps by gradual neutralization or by oxidation of Fe(II) from a sediment. The DOM-Fe associates may interact with anions like phosphate, arsenate, chromate etcetera, which may affect the environmental mobility of these ions. Understanding of the interaction processes of Fe(III) with DOM will provide insight in factors that determine overall ecosystem quality and the impact on the primary production in both terrestrial and aquatic ecosystems. log Fe / kg DOM-6-4-203579pHSoil & Surface watersOcean waterlog waterHumic Acid At present, it is very difficult to measure the speciation of soluble iron in natural samples in a reliable manner and this currently forms a major handicap in getting a better quantitative understanding of the dynamics of the biogeochemical iron cycle and the interaction of iron with competing ions. For this reason, development and testing of analytical techniques is a prerequisite to measure the iron speciation and better understand the iron cycle in different environmental situations. In this project, a unique combination of techniques will be used to determine the Fe speciation in the presence of natural organic matter, i.e. voltammetry (e.g. CLE-AdSV), the Donnan Membrane Technique (DMT), and liquid helium 57Fe Mössbauer spectroscopy. With help of such techniques, we will study the Fe speciation for environmentally relevant situations over a large pH range that involves the competition with/from other ions like calcium, magnesium and heavy metal ions. The Fe speciation at interaction with oxyanions like phosphate, arsenate with nano-sized iron-DOM entities will also be characterized. As part of the project, the data sets that are collected will be modelled using a combination of models including the NICA model, the CD & MUSIC model.