Lead ion adsorption on montmorillonite-Al hydroxide polymer systems

Clay¿Al hydroxide polymer systems (CAlHO) can bind heavy metals effectively. Their adsorption behaviour depends on the type of metal. We studied the dependence of Al-loading and pH on the adsorption of Pb to Na-saturated montmorillonite¿Al hydroxide polymer systems. The available binding sites on the Al hydroxide polymers (AlHO) had a strong affinity for Pb ions, whereas a minor amount of Pb was bound to the clay surface. The pH had a pronounced effect on the Pb binding to the AlHO. At pH 6.0, AlHO effectively adsorbed Pb. The amount of Pb adsorbed to the AlHO, expressed per mole Al, increased with increasing amount of AlHO fixed. Lead bound to the AlHO could easily be removed by exchange and is therefore reversibly bound to the AlHO. Probably, Pb is outer-sphere bound to the AlHO and an electrostatic bonding mechanism is involved. At pH 5.0 there was almost no adsorption of Pb to the AlHO, and the adsorption of Pb to the clay surface was limited because Al3+ ions competed with Pb2+ for exchangeable sites. At pH 6.6 and relatively large Pb concentrations, separate precipitates of Pb(OH)2 and Al(OH)3 were formed. Results from experiments in pure clay systems suggest that the Pb(OH)2 precipitate was present as positively charged Pb polymers