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Publisher Summary This chapter focuses on mineral surfaces and some of the factors affecting their chemical reactivity with water and aqueous species. It presents a brief overview of the geochemistry of mineral surfaces, focusing on metal-oxide and metal-(oxy)hydroxide surfaces, including their dissolution mechanisms, development of electrical charge when in contact with aqueous solutions, and uptake of aqueous cations and anions. It also discusses some of the factors that control their chemical reactivity, including defect density, cooperative effects among adsorbates, intrinsic differences in surface properties such as isoelectric points, and differences in surface structure under hydrated conditions. It begins with a discussion of the most common minerals present in Earth's crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is it provides a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH; and the nature of the electrical double layer. It also describes how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid. Furthermore, it deals with dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. Finally, it explores some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces.

Arsenate and silicate are tetrahedral anions that strongly sorb to positive Fe oxide surfaces over the pH range 2–7. Both are important agents for modification of Fe oxide surface reactivity, and notably passivate against other sorption reactions. Arsenate is a significant health hazard as a sorbed pollutant associated with acid mine drainage, while silicate is a common anion in natural solutions. Our aim is to understand the types of sorption complexes that form with these anions on different crystal faces, and whether polymerization occurs with the silicate units. Silicate polymerization could dramatically alter Fe oxide surface reactivity. The structural characterization is conducted using both grazing incidence extended X-ray absorption fine structure (GIEXAFS) at Stanford Synchrotron Radiation Laboratory (SSRL) and the National Synchrotron Light Source (NSLS), and surface diffraction (using crystal truncation rod (CTR) analysis) at the Advanced Photon Source (APS). GIEXAFS yields interatomic distances from arsenic and silicon to their oxygen first neighbor shell and second Fe or other next neighbor shell, and thus allows identification of the local geometry of sorption. Polarized X-ray fine structure spectroscopy further allows determination of the orientation and density of the complexes on the various Fe surface planes. However, this information is incomplete as any response of the surface to sorption is not revealed, and hydrogen bonding and water molecule arrangement at the surface can be changed due to the sorption process. To access these we use CTR experiments and compare the results with samples without sorbed anions. GIEXAFS results for both hematite (0 0 0 1) and ( 1 1 ¯ 02 ) planes show arsenate sorbed in two ways: bidentate binuclear and bidentate mononuclear. Most of the latter type of sorption geometry appears to be present on surface step edges on the ( 1 1 ¯ 02 ) surface, while there is little or no such attachment to the ( 1 1 ¯ 02 ) surface terraces. These results appear consistent with preliminary structural models obtained by CTR work for the clean and sorbed wet hematite surfaces, and further analysis is in progress. In the case of silicate, strong changes in the CTR are observed indicating considerable surface interactions. CTR measurements of the silicate on the ( 1 1 ¯ 02 ) surface of hematite suggest that a monodentate sorption geometry is more dominant than other possible sorption geometries. However GIEXAFS analysis done on analogous samples at the Si K-edge shows second shell contributions that are similar to amorphous silica, and are largely independent of sorption density. These results suggest that there are at least two kinds of silicate on the hematite surface: a chemically bound fraction present as a silicate–Fe3+ octahedral complex, and an incoherent portion which is not detectable by the CTR measurements but appears to dominate the GIEXAFS results. The complexed silicate on the hematite ( 1 1 ¯ 02 ) surface is linked by a single oxygen to surface Fe, i.e., a monodentate connection, with an interatomic Si-Fe distance close to those observed in the nontronite and acmite structures. This is the first evidence to our knowledge that identifies silicate as a well-defined sorption complex rather than only as an amorphous surface precipitate. More significantly, the monodentate complex is apparently favored over a more strongly bound bidentate complexation geometry, suggesting that polymerization at the interface may originate synchronously with sorption and be mitigated by the position of surface sites consistent with a polymerized network. These findings have important significance for the nature of passivation processes on Fe oxides and other important reactive natural surfaces.

The surface structure and composition of the three distinct iron-(hydr)oxide systems, goethite (1 0 0), hematite ( 1 1 ¯ 02 ), and magnetite (1 1 1) were determined under hydrated conditions at room temperature using crystal truncation rod (CTR) analysis. The prediction of surface protonation states and the overall chemical plausibility of the experimental surface models are performed using a bond-valence (BV) analysis. Further analysis of the surface energetics is carried out using ab initio density functional theory (DFT). The analysis of three common iron-(hydr)oxide surface systems reveals the differences in interface structure and distribution of hydroxyl groups at substrate–water interfaces. The goethite (1 0 0) interface structure is determined to have a relaxed double hydroxyl termination with the presence of two semi-ordered water layers that expose a surface with A-type (Fe-OH2) and B-type (Fe2-OH) hydroxyl groups. The hydrated hematite ( 1 1 ¯ 02 ) interface structure has vacancies in the near surface metal sites, resulting in three types of surface functional groups: A type, B type, and C type (Fe3-O). The interface structure of magnetite (1 1 1) shows two chemically nonequivalent oxygen surface terminations in the surface ratio of 70 O4-Feoh-O4-Fetd1−oh−td2:30 O4-Fetd1−oh−td2-O4-Feoh suggesting that the octahedral irons are the principal irons involved at the environmental interfaces. In the above three systems, there also is evidence for multiple domains with fractional ordered unit cell steps determined by atomic force microscopy (AFM). Results obtained for the structure of the iron-(hydr)oxide–water interfaces from the CTR and DFT analyses are different from stoichiometric termination of the bulk structure or hydroxylation of the ultra high vacuum (UHV) determined surface structures.