Metal contaminant availability in freshly dredged sediments and controlling reactive mineral and organic surfaces

In this paper we evaluate the environmental and geochemical properties of freshly dewatered dredged sediment at the start of a multiyear field-scale ripening experiment designed to obtain suitable material for construction. The solid-solution partitioning of metal(loid) contaminants was determined over a wide pH-range and related to the binding properties of natural reactive mineral and organic surfaces in the sediment. The results were interpreted using a multisurface geochemical model, comparing partitioning with the ‘generalised two layer model’ (GTLM) and ‘charge distribution and multisite ion complexation’ (CD-MUSIC) submodels, which differ in the level of detail for adsorption of major and minor ions to Fe- and Al-(hydr)oxides. The measured and modelled solubility of metal(loid)s varied by several orders of magnitude across the pH-range, often with a minimum around neutral pH. Surface characterisation showed a high reactive surface area for the metal (hydr)oxides and a low organic matter reactivity for metal binding, in contrast to terrestrial soils. Consequently, the adsorption to metal (hydr)oxides was dominant in the modelled speciation of several metal(loid) contaminants. Despite this importance of metal (hydr)oxides, only minor differences were found between the solid-solution partitioning predicted using the GTLM and the CD-MUSIC submodels. Therefore, the predicted solid-solution partitioning did not fully reflect the benefits offered by the more mechanistic CD-MUSIC model for contaminant adsorption to metal (hydr)oxides in the sediment. We conclude it is rather the conceptual framework of the CD-MUSIC model which, when combined with independent information from surface characterisation and pH-dependent extractions, contributes to understanding contaminant behaviour in the freshly dredged sediment. • The pH-dependent metal(loid) solubility was assessed in freshly dewatered sediment. • Characterisation of reactive surfaces showed clear differences from aerobic soils. • Metal (hydr)oxides were important in the solubility of many metal(loid)s. • The modelled solid-solution partitioning was similar with GTLM and CD-MUSIC. • The added value of more mechanistic models is in adding conceptual understanding.