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Cadmium Sorption on Specimen and Soil Smectites in Sodium and Calcium Electrolytes
Author(s) -
Zachara J. M.,
Smith S. C.,
McKinley J. P.,
Resch C. T.
Publication year - 1993
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1993.03615995005700060017x
Subject(s) - sorption , chemistry , cation exchange capacity , ionic strength , electrolyte , ion exchange , inorganic chemistry , cadmium , sodium , ionic bonding , soil water , adsorption , ion , geology , soil science , aqueous solution , organic chemistry , electrode
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 mmol c L −1 equivalent charge concentration at pH 6.0) in Na + , Ca 2+ , and Na + ‐Ca 2+ 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 Ca 2+ 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 ( K v ) for Cd 2+ 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 + , Ca 2+ , or Cd 2+ . The smectitic soil separates, in contrast, showed (i) sorption behavior that increased sharply with pH, (ii) preference for Cd in Na + and Ca 2+ electrolytes, and (iii) variation in K v 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.