Environ. Sci. Technol. 2005, 39, 7111-7117
Effect of Alcohols on the Retention Mechanisms of Cd and Zn on Wyoming Bentonite and Illite RAFIF K. SROUR* AND LOUIS M. MCDONALD Division of Plant & Soil Sciences, West Virginia University, Morgantown, West Virginia 26506-6108
The effects of ethanol- and methanol-water mixtures on Zn and Cd sorption onto bentonite and illite were investigated at low initial metal concentration (e10-5 M) and low ionic strength (2.5 mM Ca(NO3)2). For all cosolvent fractions, the percent coverage of Zn and Cd to clay minerals was low ( 40) at low ionic strength and low initial metal concentrations (e10-5 M). Under these conditions, it is hypothesized that the effects of on the solution properties, i.e., ion activity, are minimal and that the net effect of cosolvents on the retention mechanisms of Cd and Zn to these clay minerals is driven by solvent-surface interactions. The solvents selected for this study (hereafter referred to as cosolvents), methanol and ethanol, represent neutral amphiprotic solvents. Illites are nonexpanding clays with low cation exchange capacity, high surface charge density, and high edge/interlayer ratio (16). Bentonites are highly expandable clays with high cation exchange capacity, lower edge/interlayer ratio, and lower surface charge density than illites. Clays in general and bentonites in particular are important sinks for metals either as clay lenses in aquifers or as part of landfill liners (8). The determination of pH in nonaqueous and aqueous mixtures of organic solvents is challenging and hence has not always been reported in cosolvent studies (17), making the interpretation of sorption studies difficult (18). We chose a spectrophotometric method to determine equilibrium pH, because our solutions were colorless and nonturbid, and because IUPAC primary reference value standards are available for these solvents. Detailed discussion of the relative advantages and disadvantages of potentiometric and spectrophotometric methods are available (19-25).
Experimental Section Clays. Wyoming bentonite was obtained from the American Colloid Co., at Upton, WY plant. Silver Hill Illite (IMt-2) was obtained from the Source Clay Minerals Repository, University of Missouri. Before clay size separation, soluble and insoluble salts, organic matter, and iron oxides were removed according to standard procedures (26). The F
source of variation
degrees of freedom
Cd2+
Zn2+
clay cosolvent clay × cosolvent % cosolvent clay × % cosolvent cosolvent × % cosolvent clay × cosolvent × % cosolvent pairing
1 1 1 3 3 2 2 1
