| Iron interaction with NOM plays a key role in many processes, both in soils and in the aquatic environment. The vast majority (>99%) of dissolved iron is often complexed with NOM. However, the free ion concentration is the important parameter in equilibrium applications, determining the extent of the adsorption to NOM and its competitive effects on other ions. The measurement of the iron speciation is demanding because the free iron concentrations are low to very low (nM-pM) in a major part of the natural pH range (Millero 1998, Liu and Millero, 1999, 2002). A relatively high total iron concentration may be accompanied by the presence of nano-sized iron (hydr)oxide particles in the system of study. If not properly detected, this iron is treated incorrectly as being complexed by NOM. This feature may largely invalidate experimental data and their interpretation. It is highly relevant to predict the loading conditions of humics in cases where formation of nano-iron (hydr)oxides colloids is to be expected and in cases where it is not. The first step in tackling the problem is to develop analytical techniques that enable a correct measurement of the free inorganic Fe concentrations if dissolved organic matter is present. In the next step, the focus can shift to the measurement and interpretation of the free Fe concentration in systems with a relatively high loading of Fe. Another aspect of the interaction between Fe and DOM, which requires further characterization, is the prediction and experimental verification of adsorption by other ions that strongly bind to humics. The last aspect regarded here is the measurement and interpretation of the adsorption/desorption kinetics of metal ions like iron and copper to/from humic substances. |
