Testing the component additivity approach for modelling Cu and Zn adsorption to a natural sediment

Surface complexation modelling (SCM) is a powerful tool for predicting metal adsorption to minerals and natural organic matter. However, the ability of SCM to predict metal adsorption to natural sediments, characterized by a great variety of co-existing inorganic and organic components, has not been fully investigated. The objective of this research was to study the effects of soil organic matter (SOM) and Fe/Al-oxyhydroxides depletion and grain-size fractionation on Cu and Zn adsorption to a natural sediment and examine the applicability of component additivity approach for modelling metal adsorption to natural sediments. A natural sediment sample was separated into six portions, one of which was unmodified, and the others chemically treated to remove SOM, Fe/Al-oxyhydroxides, or both SOM and Fe/Al-oxyhydroxides, or separated into coarse and fine grain-size fractions. Each of the six variations of the sediment was characterized and used in adsorption experiments. Experimental results showed that Cu and Zn adsorption decreased substantially when SOM and Fe/Al-oxyhydroxides in the sediment were removed, and that the coarse grain-size fraction adsorbed lower amounts of Cu and Zn, whereas the fine fraction adsorbed similar amount of Cu but higher amount of Zn compared to the unmodified sediment. A surface complexation model, in which only SOM, Fe/Al-oxyhydroxides, and clays were considered as active adsorbents, with their metal adsorption quantified using models independently developed by previous studies, successfully simulated Cu and Zn adsorption to the sediments using the component additivity approach. The percentage of active organic carbon (f(o)) and percentage of active Fe/Al-oxyhydroxides (f(Fe/Al-oxides)) were found to play critical roles in determining the accuracy of the model and had to be adjusted based on sediment type to reflect the distinct metal binding properties of SOM and Fe/Al-oxyhydroxides in different types of sediments.