Sorption and desorption behavior of lead on a Chinese kaolin

The sorption and desorption of Pb on kaolin from Suzhou, China were investigated. The sorption experiment was conducted in a 0.01 M NaNO₃ aqueous solution with respect to the effects of initial Pb concentration, pH, solid/solution ratio, and temperature. Langmuir and Freundlich models well described equilibrium isotherms of Pb sorption. Less than 5% of Pb sorbed was desorbed with 0.01 M NaNO₃ extraction, suggesting a dominant specific sorption of Pb. As the initial pH increased, the sorption of Pb on the kaolin increased. More than 60% of Pb was sorbed at pH 2.69 far below the point of zero net charge of the kaolin at 4.2, indicating a mechanism of Pb inner-sphere complex or by chemisorption. Lead sorption decreased with increasing temperature, indicating a process of exothermic sorption.