From swollen gels to dried films: Relating the IR spectra of ferrihydrite dried as a film on an ATRIR crystal to aqueous suspensions

In aquatic systems ferrihydrite forms as highly porous aggregates that resemble swollen gels. Much insight into the surface chemistry of ferrihydrite has been gleaned using a technique in which ferrihydrite is dried on to the surface of a crystal that acts as an Attenuated Total Reflectance (ATR) medium for in situ infrared (IR) studies of ligand sorption. In this paper BET N2 adsorption, acid-base titration, H4SiO4 isotherms and the relationship between H4SiO4 sorption and H4SiO4 interfacial oligomerization are used to systematically probe the influence of this drying step on ferrihydrite's surface chemistry. This means that insights into sorption reactions revealed from in situ ATR-IR are can be increasingly related to real environmental systems. Comparing the measured acid-base titration curves to the theoretical curve provide a specific surface area (SSA) estimation of ≈600 and 290 m2 g−1 for the fresh and film-dried ferrihydrite respectively, compared to the BET N2 adsorption values of 240 and 140 m2 g−1 where the fresh ferrihydrite was freeze dried prior to BET measurement. The H4SiO4 isotherms were identical on the two ferrihydrite samples when normalized to the titration derived surface areas. The extent of interfacial H4SiO4 oligomerization provides insights into surface structures. Within experimental uncertainty, the process of drying ferrihydrite into a film did not alter either the general trend in the relationship between sorption and oligomerization, or the extent of oligomerization as a function of surface coverage normalized to the titration derived surface areas. Therefore, in comparable laboratory studies where sorption densities for dried films are compared with fresh suspensions the SSA values of 290 and 600 m2 g−1 respectively should be a reasonable estimate.