Cadmium Sorption on Specimen and Soil Smectites in Sodium and Calcium Electrolytes

Abstract

Although specimen smectities (e.g., SWy‐1) are often used as analogues of the exchanger phase in smectitic soils, few comparisons of metal ion sorption on specimen and soil smectites have been made. In this study, the sorption of Cd was measured on SWy‐1 and on clay‐sized separates from two smectitic subsoils to evaluate the selectivity of specimen and soil‐derived smectites for Cd. Sorption was measured in clay suspensions (≈1 mmolc L−1 equivalent charge concentration at pH 6.0) in Na+, Ca2+, and Na+‐Ca2+ perchlorate solutions across pH 4.5 to 8.5 and at ionic strengths (I) ranging from 0.005 to 0.1. Ionic strength and electrolyte cation valence strongly influenced Cd sorption by SWy‐1 and the soil smectites. Ion exchange dominated Cd sorption at low ionic strength in Na+ electrolyte (I = 0.005–0.014). Increasing Na+ concentrations to I = 0.1 or changing the electrolyte cation to Ca2+ at I = 0.003 to 0.006 suppressed ion exchange. When ion exchange was suppressed, Cd sorption to both specimen and soil smectites showed little dependence on ionic strength and increased with pH. Except at the lowest Na+ concentration (I = 0.005), conditional equilibrium constants (Kv) for Cd2+ exchange increased with increases in both ionic strength and pH. These increases were ascribed to Cd complexation reactions to edge sites on the layer silicates whose effects became evident only under conditions that suppressed ion exchange. At pH 6 and I = 0.05–0.01, SWy‐1 did not exhibit any preference for Na+, Ca2+, or Cd2+. The smectitic soil separates, in contrast, showed (i) sorption behavior that increased sharply with pH, (ii) preference for Cd in Na+ and Ca2+ electrolytes, and (iii) variation in Kv with ionic strength, pH, and surface coverage. The contrasting sorption behavior of the soil smectites was hypothesized to result from (i) a greater edge surface area, which increased the contribution of oxidelike complexation reactions to Cd sorption, and (ii) the presence of minor associated organic material and Fe oxides that functioned as co‐complexants for Cd.

A Model for Metal Adsorption on Montmorillonite

“…As expected from a simple Donnan equilibrium, the ratio of adsorbed metal to metal in solution is multiplied by 100 when the ionic strength is divided by 10 (see Eqs. [7] and [13]), and the percentage of adsorbed Ni at low pH increases from 10% at I ϭ 0.1 M to 90% at I ϭ 0.01 M. The success of the model is an a posteriori justification of the treatment of cation exchange as a purely electrostatic process in this system. …”

The Influence of Uranyl Hydrolysis and Multiple Site-Binding Reactions on Adsorption of U(VI) to Montmorillonite

“…These hydroxylated sites exhibit acid/base behavior and coordinative properties analogous to those of their component oxides [i.e., SiO2, AI(OH)3]; recent experimental and modeling studies have attempted to quantify the contributions of edge sites to metal ion retention by smectites and other layer silicates (Fletcher and Sposito 1989, Zachara et al 1993, Stadler and Schindler 1993, Zachara and Smith 1994.…”

Reconsideration of theoretical basis of Freundlich adsorption isotherm equation

“…43 as the Freundlich isotherm equation: It is important to define the range of application of this model. As described in the preceding report, the Freundlich isotherm equation was successfully applied to heavy metal adsorption (Zachara et al 1993), oxoanion adsorption (Parfitt 1989;Sakagami et al 1993), and cation adsorption on allophanic Andisols (Okamura and Wada 1983;Nakahara 1994). This model is the most suitable for heavy metal adsorption on the edge surface of layer silicate minerals because many experiments on heavy metal adsorption on soils were conducted using the constant ionic strength and constant pH solutions (Ziper et al 1988;Zachara et al 1992Zachara et al , 1993.…”

“…For this purpose, let us consider the adsorption processes of heavy metals and oxoanions on soil materials because these adsorption processes were often described by the Freundlich isotherms (Ziper et al 1988;Parfitt 1989;Zachara et al 1993). The common characteristics of such adsorption processes are likely to give clues for the derivation of the Freundlich isotherm equation.…”

“…Many reports showed that Cd 2+ adsorption was strongly influenced by the ionic strength and ionic composition of the bathing solutions, suggesting that the outer-sphere surface complexation or diffuse ion swarm formation is involved in the mechanism of Cd 2 § adsorption (Zachara et al 1992(Zachara et al , 1993. Recently Ziper et al (1988) have shown that Cd 2+ adsorption on the layer silicates mainly occurs in the edge surface, and the contribution of the external planar and interlayer surfaces is negligible.…”