Mercury Removal

mercury_removalThe main sources of mercury emissions to land, water and air are the processes of ores mining and smelting (in particular Cu and Zn smelting), burning of fossil fuels (mainly coal), industrial production processes (Hg cell chlor-alkali processes for the production of Cl and caustic soda) and consumption related discharges (including waste incineration) (Haidouti, 1997). It was reported that zeolites are materials that can be used for mercury removal from soils (Haidouti, 1997), flue gases (Morency et al., 2002; Jurng et al., 2002) and solutions.

Zeolites are considered to be, next to clays, iron­oxide-coated sands and activated carbons, low-cost sorbents (Bailey et al., 1999). They are naturally occur­ring silicate minerals, which can also be produced syn­thetically. Clinoptilolite is probably the mostly abundant out of more than 40 natural zeolites (Bailey et al., 1999; Ming and Dixon, 1987), making it readily available and inexpensive (Bailey et al., 1999).

Zeolites offer a potential for a variety of industrial uses including molecular sieves, ion-exchangers, ad­sorbers, catalysts, detergent builders (Haidouti, 1997; Ouki and Kavannagh, 1997; Panayotowa, 2003), the removal of cations from acid mine drainage and indus­trial waste waters (Mondale et al., 1995). Leppert (1990) reported that zeolites, clinoptilolite in particular, have strong affinity for heavy metal ions. The mechanism of adsorption by zeolites was found to be ion-exchange. In the three dimensional structure there are large channels containing negatively charged sites resulting from Al3þ replacement of Si4þ in the O4 tetrahedral–linked by sharing oxygen atoms in rings and cages-cavities occupied by cations which are weakly held in the structure to compensate the charge imbal­ance (Bailey et al., 1999; Mondale et al., 1995; Sid­heswaran and Bhat, 1997). Zeolites contain various types of cationic sites (Abusafa and Yucel, 2002). The overall negative charge of the anions is balanced by cations that occupy the channels within the structure, and can be replaced with heavy metal ions (Bailey et al., 1999; Sidheswaran and Bhat, 1997).

A wide variation in the cation-exchange capacity of zeolites was reported because of different nature of various cage structures of zeolites, natural structural defects, adsorbed ions and their associated gangue minerals (Mondale et al., 1995). Structural imperfec­tions, a variety of dimensions, degree of hydration, and the presence of clays and other slime particles may lead to differences in properties between zeolites (Mondale et al., 1995). Reported sorption capacities of zeolites are (mg/g): Cd 84.3, Cr(III) 26.0, Hg 150.4, Pb 155.4 (Lep­pert, 1990). The overall sorption capacity was evaluated as ca. 1.5 meq/g