Environ. Sci. Technol. 1994, 28, 204-289
Mechanisms of Chromium( I I I ) Sorption on Silica. 1. Cr( 1I I ) Surface Structure Derived by Extended X-ray Absorption Fine Structure Spectroscopy Scott E. Fendorf,’lt Gerry M. Lamble,* Michael G. Stapleton,$ Michael J. Kelley,ii and Donald L. Sparks5 Soil Science Division, University of Idaho, Moscow, Idaho 83844, Brookhaven National Laboratory, Upton, New York 11973, Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 197 17, and Engineering Technology Laboratory, DuPont Company, P.O. Box 80304, Wilmington, Delaware 19880-0304
Metal ion reactions at the solid/solution interface are important in an array of disciplines and are of environmental significance as such reactions can greatly affect the risk imposed by metals. The structural environment of metals at the solid/water interface determines their potential for remobilization to the aqueous environment and the physical/chemical modifications of the sorbent. In this study, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to discern the local structural environment of Cr(II1) sorbed on silica. Chromium(II1)formed a monodentate surface complexon silica, with a Cr-Si distance of 3.39 A. At the surface coverages investigated, a polynuclear chromium hydroxide surface phase occurred with Cr-Cr distances of 2.99 A, indicative of edge-sharingCr octahedra. Crystallographicparameters resulting from the measured atomic distances dictate that the surface phase was most likely of the y-CrOOH-type local structure. Environmental considerations of Cr(II1) remobilization must therefore consider the chemical/ physical properties of the monodentate surface-complexed Cr(II1) and surface-nucleated chromium hydroxide.
Introduction Inorganic compounds are potential pollutants that can be particularly problematic due to their stability in the environment (i.e., lack of degradation). Chromium is an environmentally significant metal used in various industrial processes: tanneries, plating and alloying industries, and as a cooling water anticorrosion agent. Chromium primarily enters soils and waters at hazardous levels from industrial wastes or spills. Two oxidation states of Cr are stable under surficial conditions, Cr(II1) and Cr(V1). Chromium(V1) is much more hazardous than Cr(II1) because it is mobile through plant and animal membranes and is toxic to cells due to its strong oxidizing nature. In addition, anionic Cr(V1) species are very mobile in the environment. Due to the toxicity and mobility of Cr (VI), the maximum allowable concentration of Cr in drinking water is M (1). It is therefore important to determine reactions that affect the oxidation state of Cr. Chromium(111)oxidation is an important process as the rather benign trivalent species is transformed into the hazardous Cr(VI) species. Fortunately, the only known naturally occurring oxidants of Cr(II1) are manganese oxides (2). The oxidation of Cr(II1) by manganese oxides is dependent upon the formation of a Cr(III)-Mn02 complex;
* To whom correspondence should be addressed. University of Idaho. Brookhaven National Laboratory. f University of Delaware. 11 DuPont Company.
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thus, reactions which limit the formation of this complex may decrease the potential for Cr(V1) production. One possible mechanism for retarding Cr(II1) oxidation is its retention by other nonredox reactive sorbents present in soils or waters. This process prevents Cr(II1)complexation with manganese oxides and, consequently, limits its oxidation. An array of studies have investigated the retention of metals by soils and soil components. Although permanently charged aluminosilicate clay minerals often are the dominant sorbents in soils, hydrous oxides can play an important role in the environmental behavior of metal ions. The influence of hydrous oxides, e.g., silica (SiOz), is enhanced by the formation of high surface area particles and surface coatings on other soil materials. Moreover, sorption reactions regulate the risk of metal ions in the environment since the metals are removed from the aqueous phase. Hence, to determine the hazard and risk of Cr in the environment, knowledgeof Cr(II1)sorption on nonredox reactive solids is necessary. Mechanistic interpretations and models have been developed for metal sorption reactions at the oxidelwater interface; however, they have largely been based on macroscopic data without direct atomic-level evidence. Although the heterogeneous nature of hydrous oxide surfaces has been recognized (31, surface complexation models have predominantly been based on the conception of a homogeneous surface with a single surface site, which is composed of a hydroxyl species bound to a central cation, that can undergo protonation (forming a surface water group) or deprotonation (forming an oxo group) reactions. Additionally, the sorbing species is depicted as one that binds only at isolated sites (3-7). Only a few models have considered surface precipitation reactions (8, 9). Recent experimental evidence has indicated that surface nucleation of metal hydroxides occurs much more frequently than previously believed ( I 0-1 5 ) . In this paper, we use the term surface nucleation to denote the general process that marks the onset of a three-dimensional growth mechanism. Polymerization is used to denote the formation of small multinuclear species, such as dimers or trimers, which may lead to nucleation. When a precipitate grows away from the surface before distributing over it, we use the connotation ‘surfacecluster’or ‘island structure’; those which are distributed across the surface are simply termed ‘surface precipitates’. Multinuclear metal hydroxides of Pb, Co, and Cr(II1) on oxides and aluminosilicate minerals have been discerned with extended X-ray absorption fine structure (EXAFS) spectroscopy (10-13, 16). The number of cations present in the polymerized moiety was observed to vary depending on the sorbent. At surface loadings of 1.1-1.2 pmol m-2, Co-hydroxide nucleation was greater on y-Al203 than on rutile or kaolinite (11, 12). Copper(I1) sorption on aluminum oxides was 0013-936X/94/0928-0284$04.50/0
0 1994 American Chemical Society
investigated with electron paramagnetic resonance (EPR) spectroscopy (14,15) and was found to progress from an isolated site-binding mechanism to Cu(OH)2 surface nucleation at fractional surface coverage. Only surface functional groups singly coordinated to A1 were observed to be reactive with Cu2+. Advances in understanding metal ion retention mechanisms have been made with the employment of atomic resolution experimentation, but many sorption mechanisms and reaction factors influencing these mechanisms (e.g., pH, ionic strength, competing ions, etc.) remain unresolved. It is important for researchers to utilize techniques, or preferably a multitude of techniques, which give direct evidence for reaction mechanisms. The results of such studies should enhance the development of physically accurate surface complexation models. EXAFS spectroscopy can yield direct information on the local chemical and structural environment of an element. This makes EXAFS a useful method for studying metals ions sorbed on oxides. One should recognize that EXAFS probes the average state of an element in a sample, and thus if the sorbed ion resides in a spectrum of structural environments within a sample it may not be possible to resolve these states. The objective of this study was to determine the structural environment of Cr(II1) on silica using EXAFS spectroscopy.
Materials and Methods
Batch Studies. The silica used in this study was a Huber Zeothix 265 amorphous Si02 colloid, synthesized as described by Wason (17). The oxide was washed in pH 3.5 HN03and then dialyzed in doubly-distilled deionized water unit a stable conductivity resulted for 24 h. The surface area was 221 m2/g, as determined by the ethylene glycolmonoethyl ether (EGME)method (18). The particle size of the oxide was less than 2.0 pm. Batch studies were performed to determine the amount of Cr(II1) sorbed on Si02 as a function of pH and initial Cr(II1) concentration ([Crl,). A pH range of 3-7 was investigated with initial concentrations of 100, 200,400, and 5 X 103 pM Cr(II1). For the batch studies, 0.5 g of Si02 was dispersed in reaction vessels with 2 L of 0.1 M NaNO3. A 10mM Cr(II1)stock solution was used to obtain the desired Cr(II1) concentrations. The stock solutions were made from ACS reagent-grade Cr(N03)~9H20, with acidified deionized water (pH I2) and were never allowed to age more than 5 days to limit potential polymerization. The oxide was allowed to hydrate for 48 h prior to reaction. After the hydration period, the pH was adjusted, the desired amount of Cr(II1) was added, and the final volume was brought to 2 L, yielding a suspension density of 0.25 g/L. The pH was held constant with a pH-stat system; upon reaching a steady pH, the vessels were placed in a water bath reciprocating shaker. After 48 h, the samples were filtered through a 0.22-pm pore membrane, and the solution was analyzed for Cr with a JY-70 ICP spectrophotometer. The solids were also digested with 12 N "03 and analyzed for total Cr. The sorbed quantities are reported from the loss of Cr(II1)in solution (i.e., changes in Cr solution concentration induced by the silica). The sorption data and structural information gleaned in this study are representative of the reaction after a 48-h time period and, therefore, may not be indicative of sorption at equilibrium (19).
Table 1. Solution Speciation and Saturation Indices for 50 pM Cr(II1) at pH 6, As Predicted by the Computer Program MINTEQA2 (23) Using Thermodynamic Values from Rai et al. (24)
species
%"
SI*b
Cr3+ 3 Cr(OH)2+ 68 Cr(OH)Z+ 17 Cr(OH)3(aq) 6 . Crz(OH)z4+
