Article pubs.acs.org/EF
Chemical and Surface Transformations of Bituminous Coal Fly Ash Used in Israel Following Treatments with Acidic and Neutral Aqueous Solutions Roy Nir Lieberman,*,†,‡ Nadya Teutsch,§ and Haim Cohen†,∥ †
Department of Biological Chemistry, Ariel University at Samaria, Ariel 40700, Israel Department of Chemistry and the Institute of Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel § Geological Survey of Israel, Jerusalem 95501, Israel ∥ Ben-Gurion University of the Negev, Beer Sheva 84105, Israel ‡
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.
1. INTRODUCTION A major part of the electricity production in Israel (>63% in 20121) is by coal-fired power stations burning pulverized bituminous coal. The coals are imported from five main sources: South Africa, Colombia, Indonesia, Russia, and Australia, with the former two being the major suppliers.2 Coal from these sources contains low contents of sulfur (S) and phosphorus (P) because of Israeli strict environmental regulations. Consequently, most of the fly ash (FA) produced in Israel is highly basic (pH > 10.5 when exposed to water), mainly because of the high content of lime (CaO), and is defined as Class F. Therefore, the FA can act as a natural pozzolan (i.e., a material that, when combined with calcium hydroxide, exhibits cementitious properties). Currently, annual coal consumption in Israel is ∼12−13 × 109 kg, which yields ∼1.3 × 109 kg of coal FA per year.2 In Israel, around 50% of the generated FA is used as a cement additive (up to 10% weight content), and the rest is used for concrete production and structural filling in road construction and other minor applications.3−11 The FA has a large surface area for a nonporous material (0.97−1.2 m2/g),2 which enables it to act as a potential fixation reagent. The possibility of FA acting as an effective neutralization and fixation agent for dangerous acidic wastes has been previously examined.12−16 Recently, two types of wastes have been tested: (i) acidic organic waste produced during regeneration processes of used motor oil with oleum [this waste contained acid (−SO3H groups) with a strength of more than 10 M and also contained high concentrations of heavy and toxic metals17] and © 2014 American Chemical Society
(ii) acidic waste from the phosphate industry, which is a byproduct of phosphate rock treatment with sulfuric or hydrochloric acids.12 The results of these tests have shown that FA is very effective as a neutralization and fixation reagent for these types of wastes.13,15,17 The scrubbed product is a gray aggregate (sand like), which fixates the heavy metals effectively. In these studies, the fixation quality was tested by two types of leaching procedures: the United States Enviromental Protection Agency (U.S. EPA) toxicity characteristic leaching procedure (TCLP) 131118 and California waste extraction test (CAL-WET)19 methods. These procedures evaluate the quantities of metal leaching and have been used for regulation in Israel. All leached heavy metals resulted in concentrations that were well under TCLP toxicity criteria20 and Israeli drinking water limits,21 thus proving that the metal ions are trapped at the surface of the FA particles, and therefore, FA particles can serve as efficient fixation reagents. Three modes of fixation mechanism between the metal ions of the acidic waste solutions and the FA surface have been suggested: 1. Cation-Exchange Mechanism. The surface of the FA particles contains several anionic functional groups,22 mainly aluminates −AlO2− and silicates −SiO3−: Received: August 29, 2013 Revised: May 28, 2014 Published: June 4, 2014 4657
dx.doi.org/10.1021/ef500564k | Energy Fuels 2014, 28, 4657−4665
Energy & Fuels
Article
Scheme 1. Ion-Exchange (Cation-Exchange) Mechanism23 between Metal Ions and the Anionic Groups (Mainly Aluminates and Silicates) on the FA Surface
groups at the surface of the glassy FA particles with the species trapped by the FA particles. The objectives of this study were (a) to examine the interactions between the FA surface and trace elements in solution, (b) to investigate the effects of treatment of coal FA with water and dilute hydrochloric acid on the surface properties of the FAs, and (c) to study the different types of interaction (such as cation exchange or coordination bonding) between the FA surface and the metal ions in the solution or waste to shed light on the scrubbing and fixation qualities of wastes.
Thus, FA will behave as a cation-exchange material, as shown in Scheme 1. Typical metal cations that can undergo fixation to the FA surface are mono- or divalent metal cations (e.g., Cs+, Cd2+, Cu2+, and Sr2+). 2. Coordinative Bonding. Coordinative bonding is formed between the cation and non-bonding electrons of functional groups located at the surface of the FA particles. The cation behaves as a Lewis acid,24 and the FA surface behaves as a Lewis base. This is a mechanism in which a Lewis base donating the lone pair of electrons forms the bond with the metal cation, which is equivalent to the formation of a complex where the surface serves as the ligand. Energetically, it is a relatively strong bond, which can reach a strength of >150 kJ/ mol.25 This type of chemical bond most likely forms with oxide ions, as shown in Scheme 2.
2. MATERIALS AND METHODS 2.1. FA Samples. Two coal FA, South African (SAFA) and Columbian (COFA), supplied by the Israeli Electricity Company were used for leaching. The ash content of SA coal is 13.9 wt %, and that of CO coal is 8.7 wt %. The densities are 0.98 and 0.85 g/cm3, respectively, and the densities of untreated FA are 1.25 and 1.14 g/ cm3, respectively. Because strict Israeli environmental regulations prohibit high amounts of S and P in the bituminous coal, the ashes have a low amount of S and P (also much of the S is emitted from the stack via SOx). As a result of these regulations, the ash is enriched with alkali and alkali earth elements, such as Ca, which leads to its characterization as Class F FA (18 MΩ/cm. The chemicals used in the treatments were of analytical grade. Major element composition of the FAs (pre- and post-treatments) was determined by inductively coupled plasma−optical emission spectroscopy (ICP−OES, PerkinElmer, OPTIMA 3300) after lithium metaborate (LiBO2) fusion. This procedure yields whole sample concentrations in terms of major oxides, including SiO2. For data quality assurance, the international standards SO-3 (soil) and SRM 1633a (FA) as well as an internal laboratory standard of FA were analyzed. The overall errors of whole ash concentration analyses were less than 5% for all components and better than 2% for most of them. Trace element composition of the FAs (pre- and post-treatments) was determined by inductively coupled plasma−mass spectrometry (ICP− MS, DRC II Sciex, PerkinElmer) after fusion with Na2O2. Accuracy and precision were monitored by use of standard reference materials (SRMs). The overall errors of trace element concentration analyses were less than 10% for all components. Densities of the FAs were measured by measuring the weight of a determined volume of the FA sample. The particle composition and morphology of samples were investigated by a scanning electron microscope with energy-dispersive X-ray analyzer (SEM−EDX, MK2 Quanta 200). 2.3. pH Experiment. Chemical changes because of the treatment with water or dilute hydrochloric acid can affect the pH, such as dissolution of alkaline groups (such as CaO) at the FA surface. To examine the nature of the various functional groups left on the surface of the FA particles after treatment, the pH of the treated FAs has been studied. A total of 5 g of pre- and post-treatment FAs was shaken with 50 mL of UPDI [solid/liquid (S/L) = 1:10] for 30 min up to 48 h on an orbital shaker at 250 rpm. The pH values were measured yielding pH changes as a function of the shaking time.
3. RESULTS AND DISCUSSION 3.1. FA Characteristics. Both SAFA and COFA are composed mainly (>70%) of aluminosilicates (Table 1), which form the glassy phase. Moreover, the ash contains smaller quantities of Fe, Mg, Ca, and many other trace elements, including toxic elements. The two studied FAs, COFA and SAFA, have different compositions, resulting from different areas of the coalification process (the South African coal is low-volatile bituminous coal and high mineral content, whereas the Colombian coal is high-volatile bituminous coal with relatively low mineral content). Thus, the COFA has a higher concentration of Fe, K, and Si, while the SAFA has higher concentration of Al, Ca, and P. Probably, the different compositions of the FA influence the post-treatment chemical and physical characteristics. The spatial distribution of the various components in the FA particles is heterogeneous. For example, lime occurs mainly on the solid surface of the particle as it condenses during the cooling of the flue gas after char combustion at relatively low temperatures (melting point of the glassy particle is 2572 °C30). Trace elements can exhibit different leaching potentials when FA particles come in contact with water, resulting from the chemical structure and mode of occurrence in the FA particle.13,31−35 Overall, both FAs show similar properties, such as high surface area (for a non-porous material) of 2000−6800 cm2/g, alkaline pH level (>10.5), lightweight, and hardness.2 3.2. Leaching of Components from the FAs. Leaching properties depend upon the nature and charge of the dissolved species. The leaching potential of divalent trace (e.g., Cu and Pb) and minor (e.g., Ba and Mn) elements are low, whereas oxyanion trace elements, such as Se, Mo, and B, have a very high leaching potential.34 Also, the base cations (mainly Ca)
Figure 1. SEM images of FA: (A) non-treated (general appearance at 1400×) (SAFA), (B) cenosphere (10000×) (COFA), and (C) pleurosphere (5000×) (SAFA). 4659
dx.doi.org/10.1021/ef500564k | Energy Fuels 2014, 28, 4657−4665
Energy & Fuels
Article
Figure 2. SEM images taken for the SAFA: (A and B) untreated, (C and D) UPDI treated, and (E and F) HCl treated. (A−C) Magnification of 2000× and (D−F) magnification of 20000×.
Table 1. Average Chemical Composition of Major (wt %) and Trace (mg/kg) Elements of SAFA and COFA SiO2
a
SAFA COFA
41 56 Ag
SAFA COFA
14 9.5
Al2O3
TiO2
Fe2O3
CaO
MgO
K2O
31 23
1.6 1.0
3.1 7.1
9.5 3.4
2.2 1.7
0.8 1.6
Na2O
As
Ba
Be
Cd
Co
0.2 3.1 Cr
