Heavy-metal uptake by a high cation-exchange-capacity montmorillonite: the role of permanent charge sites

Abstract

A High Cation Exchange Capacity (HCM) montmorillonite clay has been prepared by acetate treatment of Zenith clay. The HCM has been evaluated for metal-uptake from aqueous solutions. The present data show that the cation exchange sites can play a significant role in the adsorption of metals in smectite clays. A theoretical analysis scheme has been developed which shows that permanent-charge sites can become dominant in metal-uptake by clays. In addition, it was shown that the permanent charge can influence the ionic-strength sensitivity of the Point of Zero Charge of the clay. KEYWORDS:Montmorillonite; CEC; Adsorption; Heavy metals; Surface Complexation Models; Permanent charge. ΙNTRODUCTIONUnder certain environmental conditions, heavy metals might accumulate up to toxic concentrations and cause ecological damage. Heavy metals are often introduced to the environment through modern human activities. Montmorillonites are a class of layered aluminosilicate minerals with unique combinations of swelling and ion exchange properties that make them valuable nanostructures in diverse fields. Their structures consist of an octahedral alumina layer fused between two tetrahedral silica layers. Smectite clays have a cation exhange capacity, which depends on the substitution of low-valent atoms such as Mg 2+ or Al 3+ in the octahedral sheet, and Al 3+ or Si 4+ in the tetrahedral sites. As a consequence, the layers are negatively charged (Sposito, 1989;Sposito, 2004). Soluble heavy metals can be removed from the environment, by adsorption. Clay minerals are low cost sorbents and are highly valued for their adsorptive properties. Therefore, a number of studies performed using clays for heavy metal adsorption showed effectiveness in removing heavy metals or organic molecules from aqueous solutions (Ikhsan et al., 2005;Barbier et al., 2000;Bradbury and Bayens, 1999;Bayens and Bradbury, 1997). Adsorption of ions on clay minerals is controlled by two different mechanisms: (i) pH-independent adsorption, usually attributed to cation exchange in the intrer-layer and resulting from the electrostatic interaction between the ions and the permanent charge. (ii) a pH dependent adsorption, thought to result from the surface complexation reaction similar to those on oxides. Cation exchange has been fount to be an important sorption mechanism for many clay minerals (Ikhsan et al., 2005;Kraepiel et al., 1999;Avena and De Pauli, 1998) with a number of studies suggesting that exchange sites are responsible for cation uptake at low pH. Adsorption models for clays based on surface complexation modeling (SCM) usually involving two distinct types of surface groups (1) X -groups bearing a permanent charge with account to the cation exchange part of adsorption (2) amphoteric sites of SOH groups. Recently we have shown that the cation exchange capacity (CEC) of a Zn-containing montmorillonite (code name Zenith) can be increased by selective removal of Zn atoms from the clay lattice (Stathi et al., 2009). According to this methodology treatment ...

Biosorption of Lead (II) by Natural Bentonite: Adsorbent Characterization and Performance Assessment

“…where A0 is amount (mmol) of Ca 2+ ions in 50 ml of saturated calcium solution, and A1 is amount (mmol) of Ca 2+ ions in 50 ml of calcium solution after stirring 1 gram of bentonite at 30 °C for 24 h. Determined CEC for 1 gram of bentonite is 0.85 mmol Ca 2+ , and for 100 grams of bentonite is 85 mmol Ca 2+ , which is most often used. This value of CEC confirms that used natural bentonite is a type of clay mostly consists of the mineral montmorillonite [8,14,15,16]. Montmorillonite mineral is characterized by the presence of exchangeable cations (Na, Ca, and Mg) [17].…”