Surface structure of ferrihydrite: Insights from modeling surface charge

Abstract

Ferrihydrite (FH) plays an important role in controlling the fate and transport of many compounds in nature due to its large surface area and high reactivity. This study is the first attempt to build a surface complexation model using the recently proposed surface structure that incorporates tetrahedrally coordinated Fe atoms (Hiemstra, 2013). The ability of the model to describe the surface charge curves of FH with different preparation methods and Points of Zero Net Proton Charge (PZNPC) is tested. In general, FH particles that have been subject to aging are larger and have lower specific surface area (SSA) and higher PZNPCs. The structural model includes 2 types of singly coordinated (SC) oxygens that are present only on the (1-11) and (1-10) faces and 5 types of triply coordinated (TC) oxygens that are also present on the basal planes (001) and (00-1), for a total of 11 sites. The 11 - site model was able to simulate fresh FH datasets with PZNPC lower than 8.5, but could only simulate higher PZNPCs when the contribution of the more acidic basal planes was minimized. The available microscopic observations do not support this condition, which suggests TC groups on the basal planes likely have log K values higher than the macroscopic PZNPC. We attempted to test this hypothesis through three versions of simplified 3-site models, using SC and one TC on (1-10) and (1-11), with log K 8.0 (equal to fresh FH PZNPC) and one TC group on the basal planes with log K 9.5. This enables fitting of the PZNPC of aged FH datasets by adjusting the face contributions. An unresolved issue is whether this model accurately describes the relative contribution of SC and TC sites to the overall charge, which has implications for accurate description of specific ion adsorption.