Speciation and calcium competition effects on cadmium sorption by sandy soil at various pHs

Summary The effects of Ca competition, ionic strength, inorganic complexation and pH on cadmium adsorption by a sandy soil were studied. Sorption of Cd was measured using four different electrolytes CaCl 2 , Ca(NO 3 ) 2 , NaNO 3 and NaCl at a constant ionic strength ( I ) of 0.003 M at three different pHs, at variable Ca/Na ratio with a constant ionic strength of 0.03 and at variable ionic strengths between 0.003 and 0.3 M for two different pHs for Ca(NO 3 ) 2 and NaNO 3 . The measured Cd sorption isotherms were non‐linear. In the case of Cl as electrolyte anion, 13% of the Cd in solution is complexed at I = 0.003 (0.002 M Cl) and 91% of Cd is complexed at I = 0.3 (0.2 M Cl). If NO 3 is the anion, none of Cd is complexed at I = 0.003 and 11% at I = 0.3. The Cd complexes do not adsorb significantly. Calcium competition, at an ionic strength of 0.03, reduced the Cd adsorption by 60–80% compared with the case that Na is the cation. Increasing the ionic strength from 0.003 to 0.3 decreased Cd sorption by 60% for Ca(NO 3 ) 2 and 25% for NaNO 3 due to a decrease of the activity coefficient, increase of inorganic complexation and increase of Ca competition. A decrease of one pH unit reduces Cd sorption of about 75%. Sorption of Cd by soil could be described adequately with the three‐species Freundlich (3SF) equation in which pH, complexation, Ca competition and ionic strength effects were taken into account.