Influence of pH, ionic strength and humic acid on adsorption of Cd(II) and Pb(II) onto vermiculite

Adsorption of Cd(II) and Pb(II) onto a suspension of vermiculite was studied as a function of ionic medium (0.002 or 0.02 mol L−1 KNO3) and pH (5.0, 6.0 and 7.0). In order to evaluate the influence of natural organic matter on the adsorption properties of vermiculite, the clay mineral was treated with a sedimentary humic acid under the above mentioned conditions of pH and ionic strength, followed by the addition of Cd(II) and Pb(II). Adsorption was studied by the batch approach using a continuous flow-anodic stripping voltammetry technique for determination of the equilibrium concentrations of the metal cations. With this technique, the determination of metal concentrations higher than 2.0 × 10−8 mol L−1 was possible, a feature that allowed us to perform the adsorption studies using a range of initial concentrations between 2.0 × 10−7 and 1.0 × 10−5 mol L−1, which is at least one order of magnitude lower than usually reported in adsorption studies. Adsorption of Cd(II) was strongly pH dependent, with maximum removal occurring at pH 7.0 in ionic medium of 0.002 mol L−1 KNO3. Adsorption of Pb(II) was much more intense than Cd(II), and less pH dependent, with the maximum removal occurring at pH 5 and ionic strength 0.002. Adsorption of Cd(II) was enhanced onto the humic acid modified vermiculite in medium of 0.020 mol L−1 KNO3 in the three pH studied, but in 0.002 mol L−1 KNO3, only at pH 5.0 the presence of humic acid enhanced the adsorption of Cd(II), demonstrating that at this pH the predominant adsorption mechanism is ion exchange. At pH 6.0 and 7.0 a combination of ion exchange and complexation seems to govern adsorption of Cd(II) onto the humic acid modified vermiculite. For Pb(II) a similar behavior was observed at pH 5.0 and 6.0, but at pH 7.0 in 0.002 mol L−1 KNO3 the presence of humic acid decreased the adsorption as a consequence of the formation of stable complexes in solution, which desorbed the humic acid molecules previously adsorbed on the vermiculite surface.