Environ. Sci. Technol. 1993, 27, 970-975
Effect of Component Oxide Interaction on the Adsorption Properties of Mixed Oxides Xlaoguang Mengt and Raymond D. Letterman' Department of Civil and Environmental Engineering, Syracuse University, Syracuse, New York 13244-1190
Aqueous suspension of two mixed oxides [A1(OH)3/Si02 and Fe(OH)~/Si021were used to study the effect of the nature of the interaction between the hydroxide and silica on the uptake of Cd2+and Ca2+. The combined results of transmission electron microscopy, deuterium exchange experiments for surface-site concentration determination, Si02 dissolution, and electrokinetic measurements showed that the interaction between Al(OH)3 and Si02 was relatively strong, while a discrete Fe(OH)3 phase was formed in the Fe(OH)3/SiOzsystem. Enhanced Cd2+and Ca2+ adsorption occurred on Si02 partially covered by Al(OHl3compared to adsorption on the component oxides. The adsorption on SiOz completely covered by Al(OH)3 was essentially identical to uptake by Al(OH)3 without silica. In contrast, it appeared that Cd2+was adsorbed on the independent Fe(OH)3and Si02 phases in the Fe(OH)3/ Si02 suspension. Introduction
Evidence has shown that the retention of trace elements by mixed solids, such as soils and sediments, is directly related to their A1 and Fe content (1-5). Other studies have investigated the relationship between the adsorption features of mixed solids and their composition (6-10). In the typical study, the amount of trace elements adsorbed by a synthesized solid mixture is compared with the amount calculated using the adsorption properties of the individual components. When the experimental adsorption is different than the calculated uptake, certain kinds of interactions between the component solids are typically used to explain the discrepancies. However, only a few authors (11,12)have examined the surface properties of mixed solids. Our limited knowledge of the physicalchemical characteristics of mixed solids has restricted our ability to predict trace element distribution in aquatic environments and soils (9, 13-15). Various kinds of interactions are possible between the component solids in a mixed system, including heterocoagulation (16), surface precipitation (17, 18), and coprecipitation (19). While certain reactions, such as the formation of a uniform surface coating, can significantly change the surface properties of the base particles (17, 20), other interactions, such as heterocoagulation, may only slightly reduce the surface sites available for adsorbates. Therefore, it is essential to know the surface characteristics of mixed oxides in order to predict the adsorption properties of the solid. Recently, Anderson and Benjamin (12) studied the surface characteristics of mixed oxides of Al(OH)3, Fe(OH)3, and Si02 and their influence on ion adsorption. The information they obtained from particle size distributions, surface area, X-ray photoelectron spectroscopy, and the point of zero charge (pHpzc) measurements enabled them to explain ion + Present address: Center for Environmental Engineering,Stevens Institute of Technology, Hoboken, N J 07030.
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Environ. Sci. Technol., Vol. 27, No. 5, 1993
adsorption in the mixed oxide system (12,21). However, the enhanced Cd2+uptake observed by Meng and Letterman (22) at low AI(OH)3 to Si02 ratios cannot be explained by the particle heterocoagulation argument used by Anderson and Benjamin (12) for the Al(OH)3/Si02 system. This study examined the physical-chemical characteristics of the mixed oxides Al(OH)3/Si02 and Fe(OH)3/ Si02 and their effects on ion adsorption. Particle morphology was determined by transmission electron microscopy (TEM). The total concentration of surface sites was measured by deuterium isotopic exchange and deuterium nuclear magnetic resonance (2H NMR) spectroscopy. The specific surface area, determined by nitrogen adsorption, and the surface OH concentration were used to estimate the surface-site density. Electrokinetic measurements provided information on the relative amounts of surface sites from each component. Cd2+ and Ca2+ adsorption were studied using a range of Al(OH)3[or Fe(0H)al to Si02 ratios. Experimental Section
Materials. The silica @ioz) used in this study was Cab-0-Si1 M5 (Cabot Corp., Tuscola, IL), a fumed silica with a BET surface area of 200 f 25 m2/g. Cab-0-Si1 is believed to have a high proportion of free or isolated silanol groups and an essentially nonporous surface (23). Preparation of Mixed Oxide Suspensions. The suspensions containing the mixed oxides were prepared by the addition of acidified (0.001 M "OB) 0.25 M Al(N03)3or Fe(N03)3 solution to the Si02 suspension. The pH of the suspension was raised slowly to 8.0 by the addition of 1N KOH solution. The suspension was aged at room temperature (22 f 3 "C) and under a N2 atmosphere for 2 h. The suspension was stirred during the base titration and aging, and the final KN03 concentration was 0.04 M. Deuterium Exchange. The deuterium exchange method is similar to the tritium exchange method of Yates and Healy (24). Davydov et al. (25) used deuterium exchange to study the hydroxyl groups on silica. In this study the suspension was ultracentrifuged, and the solid was washed once with 99.9 atom D % DzO. A volume (1722 mL) of D20 was added to the centrifuge tube containing the solid. The sealed tubes were then shaken for 1 h to exchange surface protons with D2O. Davydov et al. (25) demonstrated that 1 h is sufficient for this exchange to reach equilibrium. The suspension was ultracentrifuged, and the centrifuge tubes containing the wet oxides were degassed in a vacuum (
