Acid/Base, Copper Binding, and Cu²⁺/H⁺ Exchange Properties of Goethite, an Experimental and Modeling Study

For aquatic systems, the movement, fate, and impact of trace metals strongly depend on the metal's tendency to partition to surfaces. An experimental and modeling study of proton and copper(II) sorption and Cu 2+ /H + exchange to goethite (α-FeOOH), an important oxide mineral, was conducted. Copper partitioning data covered three pHs, two ionic strengths, three orders of magnitude in copper surface coverage, and six orders of magnitude in solution copper(II) activity. Three surface complexation models [diffuse layer model (DLM), triple-layer model (TLM), and modified triple-layer model (Mod TL)] successfully replicated copper partitioning when at least two classes of copper binding sites, differing significantly in copper binding affinity, were included in the analysis. The (combination of) surface complex(es) that fit the data well depends on the model employed. Fits to the copper partitioning data do not permit selection of a “best” model/complex(es) combination. However, only the TLM and Mod TL reasonably replicate the proton partitioning (acid/base) properties of the goethite.