Limitaciones

The environmental impact of coal ash production has at least two aspects: (a) emission and deposition of enormous amounts of coal ash, polluting air, water and soil with ash particles (including the problem of huge ash dumps); (b) leaching of microelements (including toxic heavy metals), but also major cations and anions from ash by atmospheric and surface waters (Popovic et al., 2001). The content of trace elements in coal fly ash is clearly relevant to any environmental aspect during beneficiation or usage and disposal. The heavy metals found in greatest concentration include antimony, cadmium, lead, nickel, thallium, and zinc; other toxic elements include arsenic and selenium (Wadge and Hutton, 1987; Querol et al., 2001). There is existence of either easily exchangeable or adsorbed molecules on the surface of the spheres of fly ash which become dissolved when fly ash comes in contact with water. This mechanism produces leachate (Jo et al., 2008) and the process is called leaching. Environmentally sensitive trace elements can be concentrated at or near the surface of fly ash particles during combustion. The presence of a non-porous continuous outer surface and a dense particle interior can restrict heavy metal leachability from residues. Leaching of trace elements from combustion residues is a very slow process and the solid and liquid phase equilibrium may not be attained even with long leaching times (Fishman et al. 1999; Saikia et al., 2006). Dudas (1981) found rapid dissolution of inorganic salts dominate compositional trends during the early stages of leaching

and the slow dissolution of glassy ash particles which becomes partially evident only in the later stages of leaching after most or all of the inorganic salts have dissolved.

As noted above, fly ash contains many potentially toxic trace elements; leaching test has shown that these are stable within aluminosilicate matrix (Hower et al., 1996). In view of that fly ash is not classified as a hazardous waste in America (Hower et al., 1996). The ministry of Environment in Israel, however considers the use of fly ash as landfill potentially harmful, and forbids its use as landfill (Foner et al., 1999), maybe in response to the greater leaching test results of Nathan et al. (1999). The only element that might pose a problem is hexavalent chromium (Foner et al., 1999). The major environmental concern with fly ash disposal is the possible leaching of heavy metals and toxic element to the underground water underneath the disposal site. Therefore, leaching characteristics is one of the major environmental concerns of fly ash (Yan and Neretnieks, 1995). The leaching behaviour is influenced by several factors (Yan and Neretnieks, 1995) therefore results can be expected to vary for fly ash samples from different sources. The composition of the spherical portion of fly ash is immune to dissolution due to its glassy structure. The elemental composition and leaching properties of this spherical portion is quite similar to glass, and is relatively inert. The reactivity of fly ash is determined by the particle size. The smaller particle has a larger specific surface area, making a large area susceptible to hydrolysis. According to studies, only about 1-3 % fly ash material is soluble in water with lignite fly ashes having a higher proportion of water soluble constituents (Keyser et al., 1978; Iyer, 2002). The particle size distribution being constant after the leaching process proves that the surface of fly ash particle, a few microns in thickness is wholly involved in leaching. Therefore the charge on the surface of fly ash particle and formation of the diffuse double layer plays a significant role in leaching (lyer, 2002).

The elements Mn, Ba, V, Co, Cr, Ni, Ln, Ga, Nd, As, Sb, Sn, Br, Zn, Se, Pb, Hg and S are usually volatile to a significant extent in combustion process. The elements Mn, Ba, V, Co, Cr, Ni, Ln, Ga, Nd, As, Sb, Sn, Br, Zn, Se, Pb, Hg and S are usually volatile to a significant extent in combustion process. The volatility for these elements is inversely proportional to particle size. Elements like Mg, Na, K, Mo, Ce, Rb, Cs and Nb possess a smaller volatilized fraction during coal combustion (Lyer, 2002). The volatility is directly proportional to particle size. The elements Si, As, Fe, Ca, Sr, La, Sm, Eu, Tb, Py, Yb, Y, Se, Zr, Ta, Na, Th, Ag and Zn are either

not volatilize or may show minor trends related to geochemistry of mineral matter. The volatility of trace elements increased from larger to smaller particle size and establishes an inverse relationship of volatility and particle size (Fischer et al., 1979). Saikia et al. (2006) recognised that the leaching behaviour of coal ash is controlled by the mineralogical composition of the combustion residues. The alkalinity and acidity controlled extractability of elements, such as, As, B, Be, Cd, Cr, Cu, F, Mo, Se, V and Zn. Eisenberg et al. (1986) found that aqueous extracts of an acidic fly ash contained concentrations of Cd, Co, Cu, Mn, Ni, Zn, As, B, Be, Cd, F, Mo, Se and V. The aqueous extracts of an acidic fly ash contain concentrations of most of the elements mentioned above. Depending on the reaction time and water/solid ratio in batch equilibration or with column length and flow rate in a dynamic leaching test, small sample can show marked differences in leachate water chemistry (Iyer, 2002).

Fly ash from the exhaust flow of a coal when in contact with water can pass through a range of chemical alteration pathway. The alteration pathway of fly ash and the composition of the water/fluid in contact are a function of the initial chemical and mineralogy composition of the fly ash. The main components of fly ash are anhydrous phases such as aluminosilicates and salts such as sulphates, oxides and chlorides formed at high temperatures in coal-fired power generating stations (Mattigod et al., 1990; Gitari et al., 2006). Some of these phases (alkali metal oxides, sulphates, and chlorides) are highly unstable at room temperature and pressure and in presence of water. When fly ash contacts water these phases will dissolve completely, more stable and less soluble mineral phases will thereafter precipitate. Hence, the concentration of some constituent species in the leachate will be controlled by the solubility of the precipitating secondary mineral phases and concentration of other species will be controlled by their availability to the leachate solutions and by their diffusive flux into solution from the leaching of the primary phases with time (Eary et al., 1990; Prasad et al., 1996; Spears and Lee, 2004).

Coal fly ash alteration may also depend on the mobility or retention of metals when constituents of the ash react with water. The mobility or retention of metals when constituents of fly ash react with water can be affected by high pH and adsorption or co-precipitation phenomena. The rate of alteration of ash can influence initial and long-term solution composition (Fishman et al., 1999). These studies showed that leachates from coal ash usually have high pH and an excess of elements such as As, Cd, Cu, Pb, Zn, and Se. The chemical interaction of coal fly ash with

ingressed CO2, ingressed O2 and percolating rain water would lower initial pH of fly ash and

have significant impact on the leaching of soluble constituents of fly ash. In our studies, analysis of the extracted interstitial pore water showed Mg, Ca, Fe, K+, Na+, B, Cr, As, Mo, and Se are progressively leaching through the column of the ash. A significant decrease in the level of Se, Mo, B, and As was observed at the point of contact with the water level under the ash dump suggesting lateral diffusion of the these contaminants into the groundwater system (Akinyemi et al., 2011). The authors also found that the 1-year-old and 20-year-old coal fly ash cores showed a lower pH and greater leaching/flushing of the soluble buffering constituents than the 2-week- old placed ash (Akinyemi et al. 2011).

Several studies showed that leachates from coal ash usually have high pH and an excess of elements such as As, Cd, Cu, Pb, Zn, and Se. The leachability of these elements is generally affected by their solubility and adsorption capacity, composition of coal ash, and the chemistry of the extracting water (e.g. pH and ionic strength) (Fytianos et al., 1998; Eisenberg et al., 1986; Gutierrez et al., 1993). Prasad et al. (1996) found that leachability of heavy metals from the coal fly ash is relatively low and leaching extent is dependent on the conditions of water system. Trace metal concentration in the leachate depends on fly ash weight/solution, pH, and concentration of elements, temperature, pressure, and time. In water, rapid leaching of most of the trace metals (except Cu), takes place from the surface of ash particles in lower pH range; all trace elements lie within acceptable limits.

The composition of ash can influence the constituent released during leaching. Wan et al. (2006) also reported that the leaching behavior of heavy metals such as zinc, lead, cadmium and copper in MSWI fly ash have a dependency relationship with the components of calcium, such as aphthitalite, calcite, anhydrite and calcium aluminate or calcium aluminosilicate. Steenari et al. (1999a) through leaching tests found that upon reaction of fly ash with moderate amount of water secondary mineral phases such as ettringite and calcite were formed. These compounds were shown to affect the leaching rate for calcium and sulphate as well as pH of leachates (Steenari et al. 1999a; Steenari et al., 199b). In the case of ash pond leachate, Theis and Ritcher (1979) showed that adsorption onto hydrous Fe and Mn oxides is the major solubility control for cadmium, nickel, and zinc while precipitation of discrete phases controls for chromium, copper, and lead. Complexing agents strongly influence the leachability of metals from fly ashes (solid

wastes, in general) and, in most cases, increase significantly the amounts of pollutants released into the environment. This holds true also for naturally occurring complexing agents such as humic substances. The presence of humic substances usually increases the mobility of metals in environment (Janos et al., 2002).

Ding et al. (1998) identify Ca2+, K+ and Na+ as the soluble constituents of fly ash and find that a suspension of fly ash in water gives a pH of 12.2, in agreement with the present study as well as other work of Foner et al. (1999). The principal cations in water extracts are calcium and sodium, whereas anions are dominated by OH-, CO3 with aqueous extracts of ash saturated with

Ca(OH)2.(Elseewi et al., 1980; Menon et al., 1980). Bayat (1998) further determines, through

leaching experiments that Na and K are almost entirely in their free ionic states, whereas Ca and Mg are only predominantly in their free ionic states. Hydroxides and sulphates are also common in the fly ash suspensions (Bayat, 1998). A number of elements, notably Ca, B, Sr, and, to a lesser extent, V, were preferentially leached so that their concentrations in the weathered ash residue are substantially lower than in fresh ash. This preferential removal leads to negative enrichment such that concentrations of other elements like Al, Ba, Fe, K, Na, Mn, Pb, and Zn are higher in leached ash than in fresh ash (Dudas, 1981).