Chemical and Surface Transformations of Bituminous Coal Fly Ash Used in Israel Following Treatments with Acidic and Neutral Aqueous Solutions

Abstract

Fly ashes are produced in Israel via the combustion of bituminous coals and are defined as Class F (i.e., very basic because of the high Ca content). Most fly ashes produced in Israel are from South African and Colombian coals and, therefore, were chosen for the present study. It has been shown that fly ash can be used as a scrubber and fixation reagent for acidic wastes (from the phosphate or regeneration of used motor oil industries). Furthermore, the scrubbed product can serve as a partial substitute to sand and cement in concrete, while the produced bricks have proven to be strong enough for concrete standards. To explore the fixation mechanism, the fly ashes have been treated with acidic (0.1 M HCl) and neutral (ultrapure deionized water, denoted as UPDI) aqueous solutions. Chemical compositions and surface analysis before and after treatment were conducted for assessing changes in the coal fly ash particles. The treated fly ash structure has been changed appreciably. For example, dissolution of Ca resulted in exposure of the outer surface. Hence, the treatment may change the modes of interaction of trace elements with the surface of the fly ash particles. Three possible modes of interaction between the fly ash and waste are suggested: cation exchange, chemical bonding, and electrostatic adsorption of very fine precipitate at the fly ash surface. Probably, the silicate and aluminate groups (or aluminosilicates) at the surface of the fly ash particles are involved in these interactions.