Heavy metal adsorption by functionalized clays

Organic ligands containing the thiol (-SH) metal-chelating functionality we re either grafted to the external surface silanol groups of sepiolite or in troduced in the interlayers of montmorillonite, and the resulting functiona lized clays were characterized and assayed as adsorbents for Hg(II), Pb(II) , and Zn(II) ions from solution. Sepiolite was functionalized by covalently grafting 3-mercapto-pro-pyltrimethoxysilane (MPS) to the surface =SI-OH gr oups of the clay, whereas montmorillonite was functionalized by replacement of the interlayer inorganic cation (Na+) by 2-mercaptoethylammonium (MEA) cations. These clay-organic ligand systems were selected to minimize the co ngestion of the internal porosity of the clays, which has recently been sho wn to be the main obstacle to heavy metal adsorption by functionalized clay s. Infrared spectroscopy and elemental analyses demonstrated the presence o f the organic ligands in the modified clays. X-ray diffraction analysis ind icated the organic cations (MEA) occupied the interlayers of montmorillonit e. N-2 specific surface area measurements suggested that much of the surfac e area of montmorillonite and sepiolite remained accessible upon functional ization and that the organic ligand kept the montmorillonite interlayers op en. The functionalized clays adsorbed most of the Hg(II) ions present in so lution up to saturation and were also good adsorbents of Ph(II) at low meta l ion concentrations (i.e., <0.02 mM). They were, however, less effective t oward Pb(II) and Zn(II) at high metal ion concentrations. The presence of N aNO3 or Ca(NO3)(2)<bullet>(.) as background electrolytes at concentrations ranging from 0.001 to 0.1 M did not alter the great adsorption capacity of functionalized sepiolite for Hg(II). The results show that clay functionali zation can be optimized by matching clay structure with a suitable reactive (i.e.,fibrous clay with a graftable ligand or expandable clay with an exch angeable cationic ligand) and minimizing the gallery volume taken up by the organic ligand, thus improving the performance of the functionalized clay as adsorbent of heavy metals from solution.