Impact of organic matter types on surface charge and aggregation of goethite

In order to assess the mobility and function of Fe oxihydroxides in terrestrial and aquatic environments, knowledge of the parameters and conditions determining aggregation and the size of formed aggregates is crucial. Here we study the impact of different organic matter (OM) types on the aggregation of goethite (alpha-FeOOH) with particular focus on the relevance of surface charge (SC). Synthetic goethite was reacted with galacturonic acid (GA), polygalacturonic acid (PGA), and tannic acid (TA) as model substances as well as with natural dissolved OM (DOM) from a litter (Oi-DOM) and a humified horizon (Oa-DOM). The SC of goethite was adjusted at pH 4 and 6 by the adsorption of organic acids and DOM to equal positive and negative SC as well as point of zero charge (pzc). Aggregation was traced by laser light scattering and sedimentation experiments. Aggregation of all goethite-OM associations depended on OM type and could well be explained by SC. Associations of goethite with OM rich in acidic groups (PGA, Oi-DOM, and Oa-DOM) followed the aggregation behavior of pure goethite. Largest aggregates with diameters up to 7 mu m formed at pzc, whereas smaller ones (similar to 0.4 mu m) developed at positive or negative SC. Organic substances rich in acidic functional groups interacted strongly with goethite at high additions, thus favoring charge reversal and limiting aggregate growth. For OM with low acidity (TA and GA), adsorption on goethite was incomplete even at high additions. These associations remained close to pzc and, hence, were susceptible to aggregation with maximum diameters at 6 Aggregation was possibly also promoted by the exposure of less polar moieties exposed at the goethite-OM interface. Our data suggest that aggregation in environmental systems such as soils is driven by the nature and acidity of ubiquitous OM, which determines SC by the extent of adsorption to mineral surfaces.