In situ ATR-FTIR spectroscopic analysis of the co-adsorption of orthophosphate and Cd(II) onto hematite
Introduction
Adsorption reactions at mineral–water interfaces play a central role in determining the solubility and mobility trace elements and pollutants in aqueous geochemical environments. Much has been learnt about the mechanisms involved in the complexation of metal(loid) species at mineral–water interfaces through the application of in situ spectroscopic techniques such as X-ray absorption spectroscopy (XAS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, which have provided molecular-level information on the coordination of adsorbed metal(loid) species at aqueous mineral surfaces in a wide variety of systems (e.g. Strawn et al., 1998, Elzinga et al., 2001, Bostick and Fendorf, 2003, Peak et al., 2003, Rouff et al., 2005, Borer et al., 2007, Tang et al., 2007, Arai, 2008, Langner et al., 2011, Brechbühl et al., 2012; also see overview and references provided in Brown and Parks, 2001 and Lefèvre, 2004). Detailed mechanistic information is available in particular for binary model systems, where experiments involving adsorption of a single metal(loid) species to a pure, well-characterized mineral sorbent phase under well-controlled experimental conditions facilitate systematic characterization of the influence of major geochemical parameters on operational retention mechanisms. Much less spectroscopic work has been done on multi-component systems containing multiple sorbate species, where interactions in solution and at the mineral surface between the various species present may impact trace element partitioning between the aqueous and solid phase. Such chemically complex systems represent more closely the geochemical conditions in natural systems such as soils and sediments, which makes it important to obtain mechanistic information on the interactions that occur during co-adsorption of common environmental species for a more comprehensive understanding of the factors controlling their solubility and mobility.
There have been quite a few macroscopic studies addressing the co-sorption of metal cations and anionic ligands onto variable-charge metal-oxide surfaces, and these have shown that anions may strongly influence the adsorption of metals to mineral surfaces (e.g., Benjamin and Leckie, 1982, Clark and McBride, 1985, Ali and Dzombak, 1996, Webster et al., 1998, Ostergren et al., 2000a, Ostergren et al., 2000b, Boyle-Wight et al., 2002, Swedlund et al., 2003, Swedlund et al., 2009, Zaman et al., 2009), and that, in turn, metal cations may significantly impact the adsorption behavior of anions (e.g., Balistrieri and Murray, 1981, Marcano-Martinez and McBride, 1989, Bolan et al., 1993, Ali and Dzombak, 1996, Juang and Wu, 2002). The results of these studies are not readily generalized, as co-sorbing anions in some cases promote metal sorption (e.g., Marcano-Martinez and McBride, 1989, Gunneriusson et al., 1994) whereas they inhibit metal sorption in other cases (e.g., Davis and Leckie, 1978, Elrashdi and O’Connor, 1982, Boekhold et al., 1993), which indicates that specific effects are system-dependent and vary with sorbent type, pH range, and the type and concentration of the metals and ligands being studied. Decreased metal sorption is generally explained by competition between metal–ligand complexation in solution and metal complexation to the surface, or by direct competition between ligand anions and metal cations for coordination at mineral surface sites. Promotive effects of co-sorption are commonly attributed to three factors: (1) surface electrostatic effects, where changes in the mineral surface charge through inner-sphere ion adsorption are partially offset by (inner-sphere) co-adsorption of ions of the opposite charge, which promotes additional sorption (relative to binary systems) by lowering the electrostatic barrier for ion approach to the surface; (2) formation of ternary metal–ligand ternary complexes at the mineral surface, where physical or chemical bonds between metal and ligand surface species increase the stability of ion adsorption complexes; and (3) precipitation metal–ligand precipitates. The operation of these various mechanisms in ternary sorption systems has been inferred from results of surface complexation modeling (e.g. Hoins et al., 1993, Ali and Dzombak, 1996, Venema et al., 1997, Rietra et al., 2001, Christl and Kretzschmar, 2001, Swedlund et al., 2003, Swedlund et al., 2009) and confirmed by spectroscopic studies (e.g. Bargar et al., 1998, Weesner and Bleam, 1998, Ostergren et al., 2000a, Ostergren et al., 2000b, Elzinga et al., 2001, Zhang and Peak, 2007, Tang et al., 2009).
The work presented here deals with the mechanisms involved in the co-adsorption of orthophosphate (HxPO4x−3) and divalent cadmium (Cd(II)) to the surface of hematite (α-Fe2O3), as studied by in situ ATR-FTIR spectroscopy. Both orthophosphate and Cd(II) coordinate as inner-sphere adsorption complexes at iron-oxide surfaces (Tejedor-Tejedor and Anderson, 1990, Spadini et al., 1994, Persson et al., 1996, Collins et al., 1999, Randall et al., 1999, Arai and Sparks, 2001, Kwon and Kubicki, 2004, Elzinga and Sparks, 2007), suggesting the potential for interactions between the surface species of these compounds. ATR-FTIR spectroscopy has been successfully used to investigate the mechanisms of oxyanion adsorption at iron-oxide surfaces in both binary and ternary systems under in situ conditions (Hug, 1997, Peak et al., 1999, Wijnja and Schulthess, 2000, Elzinga et al., 2001, Peak et al., 2003, Lefèvre, 2004, Elzinga and Sparks, 2007, Beattie et al., 2008, Roonasi and Holmgren, 2009, Lindegren and Persson, 2010, Brechbühl et al., 2012). The aim of this study was to probe interactions between orthophosphate and Cd(II) surface ions across a wide pH range (4.5–9.0) and as a function of the Cd(II) solution concentration under aqueous chemical conditions undersaturated with respect to Cd(II)–orthophosphate precipitate phases.
Section snippets
Hematite preparation
The hematite sorbent used for the experiments was synthesized based on the procedure of Sugimoto et al. (1993). A 500 mL volume of a 2 M FeCl3 solution was added slowly over the course of 5 min to 500 mL of a stirred 5.4 M NaOH solution. The resulting gel was aged in a sealed Pyrex glass bottle at 101 °C for 8 days. After cooling to room temperature, the product was washed several times with doubly deionized (DDI) water (18.2 MΩ cm, Milli-Q, Millipore) until the electrical conductivity was <5 μS cm−1, and
Orthophosphate–hematite pH edge spectra
The influence of pH on the adsorption of orthophosphate anions to the surface of hematite is shown in Fig. 1a, which presents the ATR-FTIR data of the binary pH edge experiment collected in the frequency range of 1200–800 cm−1, which contains the PO, P–O, P–OH and P–OFe υ3 stretching bands of orthophosphate adsorption complexes at iron oxide surfaces. Two notable observations can be made from these data. The first is that the intensities of the IR absorption bands of the orthophosphate surface
Conclusions
We report substantial impacts of the co-sorption of Cd(II) and orthophosphate on both the extent and the mechanism of retention at the hematite surface. Increases in orthophosphate adsorption are seen upon addition of aqueous Cd(II) across the pH range, with promotive effects becoming increasingly pronounced with increasing pH. ATR-FTIR analysis of the ternary sorption systems reveals mechanistically complex surface chemistry, with formation of two distinct Cd(II)–orthophosphate ternary surface
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