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

Abstract

The competitive sorption of Pb 2+, Cd 2+, and H + ions onto a low iron Texas bentonite (BL) has been studied under conditions where the quantity of Pb 2+ and Cd 2+ 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 [M 2+] ranged from 0.02 to 0.04 mol dm −3. The adsorption of both Pb 2+ and Cd 2+ 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 Pb 2+ to the clay either by increasing the aqueous Pb 2+ concentration or reducing the clay content in the suspension had little influence on the shape of the uptake curves for Pb 2+ and H +. Cd 2+ ions were less able to compete with aqueous H + ions for exchange sites on Na +-BL than Pb 2+ ions. Pb 2+ competed more effectively with H + for the exchange sites on Cd 2+-BL than Cd 2+ was able to compete with protons for the sites on Pb 2+-BL. Protons were less able to compete for exchange sites on both Pb 2+- and Cd 2+-BL when the aqueous phase contained 1 CEC of Pb 2+or Cd 2+, respectively.