Surface Complexation Modeling Approach for Aluminum-Substituted Ferrihydrites

Abstract

Surface complexation models (SCMs) have been successfully used to describe and predict adsorption phenomena for a large number of surfaces. SCMs have benefited from advancements in the microscopic characterization of interfacial structure which has aided in constraining their parametrization. Yet, SCMs have not adequately addressed the role of defects and substitutions in surface reactivity despite the fact that such surfaces are more environmentally prevalent than their ideal counterparts. In this study, we address this challenge by investigating the aluminum-doped variant of ferrihydrite (Fh). We consider the placement of Al within the Fh structure and develop a basic Stern SCM accommodating that placement which we apply to a set of previously published ζ potential data. Our hypothesis is that Al primarily replaces Fe1 surface sites within the Fh structure forming singly coordinated AlOH surface groups possessing higher protonation constants than their FeOH counterparts. The SCM fits the ζ potential data adequately, including a close match to the isoelectric pH values (pHiep, ζ potential = 0) of 8.0, 8.5, and 8.9 for pure, 12% Al-substituted, and 24% Al-substituted Fh, respectively. Moreover, the fit slipping plane distance, where the ζ potential is assumed to be expressed, is consistent with previous modeling efforts. Although a wider variety of experimental data will be necessary to determine the true extent and location of Al substitution within the Fh structure, and to thoroughly characterize the charging and adsorptive properties of variously substituted Al-Fhs, our SCM represents a vital stepping stone toward developing more environmentally relevant SCMs.

Interfacial Properties of Al-Ferrihydrites: Surface Complexation Modeling as a Probe of Surface Structure

“…At 2 decimal points in MSC (or 6 in MSE), a log K HAlOH + > 10− 10.5 would give equivalent solutions, which is a value similar to the protonation constants reported in the literature, which typically average around log K H + ∼10. 27,33,52,54 As shown in Figure 1, the model fits with a log K HAlOH + of 10.5 yield visually acceptable result and are also physically closer to what is expected for the protonation constant of this surface moiety.…”

“…Potentiometric titrations at different ionic strengths showed a shift of the PZC with increasing Al content (Figure 1): 8.1 for 0 Al-Fh, 8.3 for 6 Al-Fh, 8.5 for 12 Al-Fh, and 8.6−8.7 for 18 Al-Fh, similar to the PZC values reported in the literature. 17,20,33 Plots of potentiometric titrations with respect to pH PZC -pH are also shown in Figure S7. The PZC shift was attributed to the higher protonation constant of SC AlOH compared to SC FeOH groups.…”

“…Recently, Hsu et al 35 showed Al segregation near the surface of Algoethites utilizing iron K-edge X-ray absorption (Fe-XAS) and X-ray photoelectron spectroscopy (XPS). Adams et al 33 modeled ζ-potential titrations with the 15-site model assuming a uniform distribution between the bulk and surface structure. This study investigates this question through modeling simulations of the potentiometric titrations.…”

“…Hiemstra et al 52 determined a log K of 9.0 for SC AlOH groups on the surface of pure Al oxyhydroxides, and subsequent studies adopted an approach of a small increase in the AlOH protonation constant with respect to FeOH. Specifically, Xu et al 27 utilized a log K HAlOH + of 10, and Adams et al, 33 two values of 9 and 11, to represent an increase in the log K of the substituted FeOH sites, which were 8 and 10.4, respectively. While fits were acceptable in both cases, the sensitivity of the models to those values was not tested.…”

“…21 Al dopants tend to increase the surface charge density and point of zero charge (PZC) of iron oxides as Al content increases. 17,20,27,33 Ion uptake is affected to a smaller extent; Adra et al 34 and Liang et al 28 showed an increase of As(VI) and decrease of As(III) and Pb(II) adsorption with increasing Al content, nevertheless, other studies did not observe any significant changes. 17,20 Al substitution has shown to increase outer-sphere complexation of oxyanions, 22,34 while cation surface speciation was observed to be impacted.…”