Na/Pb, Na/Cd and Pb/Cd exchange on a low iron Texas bentonite in the presence of competing H+ ion

The competitive sorption of Pb2+, Cd2+, and H+ ions onto a low iron Texas bentonite (BL) has been studied under conditions where the quantity of Pb2+ and Cd2+ ions was equivalent to the cation exchange capacity (CEC) of the clay and the amount of added acid ranged from 0 to 1 mmol H+ (g clay)(-1). Clay concentrations were in the range 25-50 g dm(-3) and [M2+] ranged from 0.02 to 0.04 mol dm(-3). The adsorption of both Pb2+ and Cd2+ ions onto Na+-BL was reduced as the amount of H+ was increased to values equivalent to the CEC (1 mmol H+ (g clay)(-1)). Increasing the quantity of Pb2+ to the clay either by increasing the aqueous Pb2+ concentration or reducing the clay content in the suspension had little influence on the shape of the uptake curves for Pb2+ and H+ Cd2+ ions were less able to compete with aqueous H+ ions for exchange sites on Na+-BL than Pb2+ ions. Pb2+ competed more effectively with H+ for the exchange sites on Cd2+-BL than Cd2+ was able to compete with protons for the sites on Pb2+-BL. Protons were less able to compete for exchange sites on both Pb2+- and Cd2+-BL when the aqueous phase contained 1 CEC of Pb2+ or Cd2+, respectively. (C) 1999 Elsevier Science B.V. All rights reserved