Effect of mineralogical characteristics evolution of vermiculite upon thermal and chemical expansions on its adsorption behavior for aqueous Pb(II) removal

Abstract

Vermiculite undergoes significant changes in mineralogical characteristics upon chemical (with H2O2) and thermal (roasting) expansions, resulting in a marked impact on its adsorption behavior toward Pb(II) removal. Both chemical and thermal expansions exfoliated the tightly-stacked bulk structure of raw vermiculite into ultra-thin sheets and built abundantly nanoporous channels in the lamellar region, offering sufficient adsorption sites. Meanwhile, the surface properties of hydrophilicity and negative charge in raw vermiculite remain unaltered during expansions, thus enabling an enhanced adsorption capacity through electrostatic attraction. However, severe loss of cation exchange capacity and structural hydroxyl groups due to thermal expansion damaged mixed crystal phases in vermiculite and hydrophlogopite, leading to the poorer Pb(II) removal capability. The results demonstrate that the favorable influence of chemical expansion in raw vermiculite for Pb(II) removal is attributed to enhanced cation exchange within the interlayer, functional groups of structural hydroxyl and siloxane, and negative surface charge on hydrophilic surface.