Heavy Metal Adsorption by Functionalized Clays

TLDR

Functionalized clays aim to overcome porosity congestion, a key barrier to heavy‑metal adsorption. The study grafted thiol‑containing ligands onto sepiolite and montmorillonite to evaluate their adsorption of Hg(II), Pb(II), and Zn(II) ions. Sepiolite was covalently grafted with 3‑mercaptopropyltrimethoxysilane, while montmorillonite was ion‑exchanged with 2‑mercaptoethylammonium cations. The modified clays retained accessible surface area, adsorbed Hg(II) to saturation, captured Pb(II) at low concentrations, but were less effective for Pb(II) and Zn(II) at high loads; electrolyte presence did not affect Hg(II) uptake, showing that matching ligand type to clay structure enhances heavy‑metal adsorption.

Abstract

Organic ligands containing the thiol (−SH) metal-chelating functionality were either grafted to the external surface silanol groups of sepiolite or introduced in the interlayers of montmorillonite, and the resulting functionalized 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 groups 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 congestion of the internal porosity of the clays, which has recently been shown to be the main obstacle to heavy metal adsorption by functionalized clays. Infrared spectroscopy and elemental analyses demonstrated the presence of the organic ligands in the modified clays. X-ray diffraction analysis indicated the organic cations (MEA) occupied the interlayers of montmorillonite. N2 specific surface area measurements suggested that much of the surface area of montmorillonite and sepiolite remained accessible upon functionalization and that the organic ligand kept the montmorillonite interlayers open. The functionalized clays adsorbed most of the Hg(II) ions present in solution up to saturation and were also good adsorbents of Pb(II) at low metal ion concentrations (i.e., <0.02 mM). They were, however, less effective toward Pb(II) and Zn(II) at high metal ion concentrations. The presence of NaNO3 or Ca(NO3)2 as background electrolytes at concentra tions 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 functionalization can be optimized by matching clay structure with a suitable reactive (i.e., fibrous clay with a graftable ligand or expandable clay with an exchangeable 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.