ArticleKinetics of zinc and arsenate co-sorption at the goethite–water interface
Introduction
Little or no information is available in the literature about reaction processes and mechanisms of co-sorbing metals and arsenate (As(V)) on variable-charged surfaces or factors influencing these reactions. Arsenic and metal contamination are, however, a common co-occurrence in many contaminated environments (Carlson et al 2002, Williams 2001), and it is reasonable to assume that precipitated metal–arsenate phases may exist in co-contaminated soils and sediments. Little or no research has been conducted on the formation and stability of such precipitates. Understanding the mobility and fate of two or more co-occurring contaminants of differing chemical properties is of great importance to make appropriate decisions concerning the stabilization and remediation of such sites.
Waychunas et al. (1993) investigated the effects of arsenate (As(V) = H3-nAsO40-n) on ferrihydrite precipitation using extended X-ray absorption fine structure (EXAFS) spectroscopy and concluded that As(V) inhibited ferrihydrite precipitation by binding to growth sites on the precipitate. Tournassat et al. (2002) observed the precipitation of a manganese(II)–arsenate precipitate at the birnessite–water interface after the oxidation of an 11 mM arsenite (As(III)) solution at the birnessite–water interface. In natural environments, Langner et al. (2001) observed ferric–arsenate precipitation following the oxidation of As(III) in hot sulfur springs. Sadiq (1997) concluded from thermodynamic data that the solubility of arsenic in acidic environments was controlled by iron and aluminum–arsenates and at alkaline pH by calcium–arsenates. Other metal–arsenates such as copper, zinc, nickel, or cadmium–arsenates were considered less soluble and could accumulate in the environment. First row transition metals form thermodynamically very stable arsenate complexes (Kso of ≥1019, Gustafsson, 2004). In mineralogy, several phosphate/arsenate mineral classes are recognized (Gaines et al., 1997). The formation of a solid phase that contains environmentally critical elements, that has low solubility, and that increases in stability over time is favorable from an environmental remediation standpoint. Reaction processes/mechanisms and factors that contribute and inhibit the formation of such solid phases should be understood well to make appropriate decisions for contaminated sites.
Recently, we reported on As(V) and Zn(II) co-sorption at the goethite–water interface as a function of pH (4 and 7). Arsenate and Zn(II) sorbing on goethite above site saturation resulted in the formation of an adamite-like (Zn2(AsO4)OH) surface precipitate at pH 7 (Gräfe et al., 2004). Extended XAFS spectroscopy analyses showed that an adamite-like precipitate formed on the goethite surface at pH 7, while at pH 4, As(V) and Zn(II) existed as co-sorbed species on the goethite surface. The study suggested that the amount of surface area (and therefore the number of reactive surface sites) may control the precipitation of a zinc–arsenate solid phase. In the current study, we hypothesize that for a given aqueous, undersaturated (Zn3(AsO4)2: log ion activity product (IAP)/Kso<0) concentration of Zn(II) and As(V) at a favorable pH (e.g., pH ∼ 7), a greater solid–solution ratio will result in the formation of mostly two dimensional or inner-sphere adsorbed species. In lower solid–solution ratios, a precipitation reaction may occur facilitated by the presence of the goethite surface. The objectives of this study were (i) to investigate the co-sorption kinetics of zinc and arsenate at the goethite–water interface at pH 7 as a function of the solid–solution ratio, (ii) to determine the bonding environment of As(V) and Zn(II) on goethite for certain periods of reaction time using EXAFS spectroscopy, and (iii) to evaluate the stability of the solid phases against background electrolyte adjusted to pH 5.5 and 4.0.
Information gleaned from this study may be useful in predicting the fate and mobility of co-occurring metals and oxyanions and in devising remediation strategies to lower their bioavailability at co-contaminated sites, in ground and surface-waters, and in other applicable situations.
Section snippets
Materials
The preparation and characterization of goethite (α-FeOOH) was reported elsewhere (Gräfe et al., 2004). Briefly, the specific surface area of the goethite is ∼70 m2 g−1, with 2.4% porosity. The average particle size is ∼ 30–200 nm. All reagents used in the study were ACS grade. A 799 GPT Titrino automated titrator (Metrohm, Herisau, Switzerland) was used to control the pH of the kinetic reactions for the first 8 h. The setup of the kinetic studies is similar to the one reported earlier (Gräfe
Sorption Kinetics
Common, biphasic sorption reactions were observed in both single and co-sorption experiments of As(V) and Zn(II) in 1000-ppm goethite suspensions (Gräfe et al 2001, Grossl and Sparks 1995, McBride 1994, O’Reilly et al 2001, Sparks 2002, Strawn et al 1998, Xue and Huang 1995). Approximately 95% As(V) and 72% Zn(II) sorbed on goethite in the first 8 h (Fig. 1A,B). The initial sorption rates (0–8 h) increased by one order of magnitude with every order of magnitude increase in goethite suspension
Heterogeneous Nucleation
The literature recognizes two types of nucleation reactions: homogeneous and heterogeneous (Stumm, 1992). In the absence of a sorbent, a homogeneous nucleation reaction may occur when the solution is saturated with the precipitating ions (i.e., log IAP/ Kso = 1), but is kinetically limited until a critical oversaturation (log IAP/Kso >1) of the solution has occurred. The Gibbs free energy of the precipitation reaction (ΔGrxn) is dependent on the energy gained from making bonds (ΔGbulk) and the
Conclusions
Heterogeneous nucleation reactions and possibly the formation of poly-nuclear zinc–arsenate solution species near the goethite surface are likely responsible for the reactions that are presented in this study. Four different zinc–arsenate solid phases formed depending on the solid–solution ratio of goethite: koettigite-like precipitates (0-ppm goethite, log (IAP/Ks) = 6.49), koritnigite-like precipitates (10-ppm goethite, log (IAP/Ks) = −0.92), adamite-like precipitates (100-ppm goethite, log
Acknowledgments
The authors would like to thank the Environmental Soil Chemistry Research Group for useful discussions about the project. Specifically, we would like to thank Cathy Dowding, Jennifer Seiter, Ryan Tappero, Gerald Hendricks, and Dr. Peltier for their help during EXAFS data and macroscopic data collection. We are grateful for the assistance from Dr. K. Pandya during the EXAFS data collection at beamline X-11A and to Kirk Czymek and Deborah Powell from the Delaware Biotechnology Institute for field
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Associate editor: U. Becker
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Present address: Faculty of Agriculture, Food, and Natural Resources, Ross Street Building, Rm. #322, The University of Sydney, NSW 2006, Sydney, Australia.