Does Adsorption of Cd, Cu and Pb on Polymeric Silicic Acid Occur Under Acidic Conditions?

Contamination with Cd, Cu and Pb is a major environmental issue. Cations of those metals may adsorb on negatively charged surfaces of polymeric silicic acid (pSi), altering their environmental fate. Aiming to elucidate the underlying mechanisms and the extent of adsorption, we conducted batch adsorption experiments at pH 4 to 6 and concentrations that excluded precipitation of solid phases. Zeta-potential measurements were conducted to monitor surface charge changes. In addition, isothermal titration calorimetry (ITC) was used to derive thermodynamic parameters of the interaction between the metals and pSi. Surprisingly, neither did batch-adsorption experiments reveal any metal adsorption on pSi after 24 h reaction time, nor did ITC experiments show any evidence for chemical adsorption of the metals, as no heat was released or absorbed during the experiments. However, zeta-potential measurements indicated weak electrostatic interactions between the negatively charged silanol groups and the metals. These electrostatic interactions may be the initial step of metal incorporation into the matrix of polymerizing silicic acid, which were spectroscopically proven in long-term experiments.