Surface Acidity and As(V) Complexation of Iron Oxyhydroxides: Insights from First-Principles Molecular Dynamics Simulations

Abstract

Iron hydroxides are ubiquitous in soils and aquifers and have been adopted as adsorbents for As­(V) removal. However, the complexation mechanisms of As­(V) have not been well understood due to the lack of information on the reactive sites and acidities of iron hydroxides. In this work, we first calculated the acidity constants (pK as) of surface groups on lepidocrocite (010), (001), and (100) surfaces by using the first-principles molecular dynamics (FPMD)-based vertical energy gap method. Then, the desorption free energies of As­(V) on goethite (110) and lepidocrocite (001) surfaces were calculated by using constrained FPMD simulations. The point of zero charges and reactive sites of individual surfaces were obtained based on the calculated pK as. The structures, thermodynamics, and pH dependence for As­(V) complexation were derived by integrating the pK as and desorption free energies. The pK a data sets obtained are fundamental parameters that control the charging and adsorption behavior of iron oxyhydroxides and will be very useful in investigating the adsorption processes on these minerals. The pH-dependent complexation mechanisms of As­(V) derived in this study would be helpful for the development of effective adsorbent materials and the prediction of the long-term behavior of As­(V) in natural environments.

Deciphering the Anomalous Acidic Tendency of Terminal Water at Rutile(110)–Water Interfaces

“…Unexpectedly, the pK a of Ti 5c OH − is 5.61, smaller than that of Ti 5c OH 2 , which contradicts the general understanding that the second pK a is larger than first pK a for the same surface species. 30 By converting the pK a values into the free energy of reaction 12, we obtain the ΔG PT = −0.16 eV, which confirms our speculation and thereby rationalizes the stability of Ti 5c O 2− . Moreover, we verify this conclusion using PBE-D3, SCAN, and HSE06 functional with simplified model, as shown in Table 3 (see the Supporting Information for computational details).…”

“…Unexpectedly, the p K a of Ti 5c OH – is 5.61, smaller than that of Ti 5c OH 2 , which contradicts the general understanding that the second p K a is larger than first p K a for the same surface species . By converting the p K a values into the free energy of reaction , we obtain the Δ G PT = −0.16 eV, which confirms our speculation and thereby rationalizes the stability of Ti 5c O 2– .…”

“…In summary, both the bond lengths and the coordination of Ti 5c contribute to the reversed order of the acidity constants. In most surface species, 30 the bond lengths of M−OH − tend to be larger than those of M−O 2− . However, the first pK a values are still lower than the second pK a for the same surface species.…”

“…The main reason is that the intermediate Ti 5c O 2– in the PT–ET pathway is thought to be unstable. Previous AIMD calculations have predicted the acidity constant of Ti 5c OH 2 to be 6.3. , Typically, the second acidity constant is larger than the first acidity constant by 8–13 p K a units for the same surface species . Therefore, the acidity constant of Ti 5c OH – should be larger than 14, indicating that Ti 5c O 2– is not stable within the common pH range (0–14).…”

“…28,29 Typically, the second acidity constant is larger than the first acidity constant by 8−13 pK a units for the same surface species. 30 Therefore, the acidity constant of Ti 5c OH − should be larger than 14, indicating that Ti 5c O 2− is not stable within the common pH range (0−14). Although previous studies under ultrahigh vacuum and aqueous conditions 31−33 have suggested the existence of terminal oxygen adatoms (Ti 5c O), the oxidation states of these adatoms are ambiguous.…”