Surface complexation modeling of Hg(II) adsorption at the goethite/water interface using the Charge Distribution Multi-Site Complexation (CD-MUSIC) model

Hypothesis: Previous attempts to describe Hg(II) adsorption onto mineral surfaces using surface complexation models (SCMs) have proven unsuccessful and/or require the use of hypothetical surface species. Given that metal ion adsorption at the mineral-water interface is greatly influenced by mineral surface heterogeneity and the presence of competing adsorbates in solution, it stands to reason that estimating the crystal face composition (CFC) of the mineral surface and the extent of carbonate contamination in the experimental system will improve SCM predictions. Experiments: The Charge Distribution Multi-Site Complexation (CD-MUSIC) model was used to simulate experimental Hg(II) adsorption data, collected on the iron hydroxide mineral goethite, in the presence and absence of competing adsorbates and complexing ligands as a function of pH and ionic strength. The CFC of each goethite sample studied was predicted using a newly discovered relationship between goethite's proton reactive site density (N_H) and specific surface area (SSA). Carbonate's presence in the experimental systems was determined utilizing a novel methodology developed in this work. Findings: The CD-MUSIC model developed in this study accurately predicted Hg(II) adsorption onto goethite over the entire range of experimental conditions investigated while only employing surface species consistent with spectroscopic evidence.