In situ ATR-FTIR spectroscopic analysis of the co-adsorption of orthophosphate and Cd(II) onto hematite

Abstract

We used in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to study the impact of Cd(II) on the coordination of orthophosphate to the surface of hematite in the pH range 4.5–9.0, and at aqueous reactant concentrations below saturation with respect to Cd(II)–phosphate precipitates. In the absence of Cd(II), the orthophosphate surface speciation was pH dependent and dominated by two surface species assigned as monodentate monoprotonated complexes dominating at alkaline pH and additional formation of bidentate monoprotonated complexes at pH <8.0. Addition of aqueous Cd(II) raised the amount of orthophosphate adsorbed across the pH range, with promotive effects increasing with increasing pH. We observe the formation of two structurally distinct ternary Cd(II)–orthophosphate surface complexes which change proportion with pH. The IR spectra suggest stronger distortion of the orthophosphate tetrahedra involved in the ternary complexes formed at low pH relative to those formed at high pH, indicating differences in protonation state, surface coordination, and/or coordination to surface Cd(II) between the two ternary complexes. Over most of the pH range covered, the two ternary complexes are present simultaneously at the hematite surface, and co-exist with the two binary orthophosphate surface species, with the relative proportions of the various complexes varying with pH. The presence of Cd(II) thus not only raises the extent of orthophosphate adsorption but also the level of complexity of the orthophosphate surface speciation. These results imply that the simultaneous presence of divalent metals and orthophosphate significantly influences the solubility and speciation of these compounds in environmental settings even under conditions where precipitation of metal–phosphates does not occur.

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 (H_xPO₄^x−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.