There are a number of competing physical and chemical processes occurring during the active leaching of ash during utilisation or disposal. These include dissolution, advection, diffusion, adsorption and mineral precipitation, depicted in Figure 2.7. As highlighted in previous sections, the extent of many of these processes are site specific to a particular power station or coal source. Leaching of ash takes place through dissolution of constituents inside or on the surface of the ash and and transport through the pore structure to the surrounding pore waters (Côté et al., 1986). These processes can be categorised as chemical or physical (transport) phenomena (Côté et al., 1986). The most common progression for leaching many different waste materials is a large initial leachate plug, known as "initial washoff", which decreases rapidly to a much lower steady state value, controlled by a diffusive leaching flux (Côté, 1986). This "plug flow" behaviour is demonstrated by the column leaching tests of Black (1990a), shown earlier in Figure 2.5. An important distinction to make when comparing the results of different column leaching tests is whether the leachate concentrations are plotted versus time or versus pore volume of the ash in the column. Farquhar (1989) represented this declining rate of leaching as shown in Figure 2.8. As time progresses, the leachate concentration approaches a steady state value and the leached mass approaches a maximum.
Figure 2.7 - Schematic of Conceptual Leaching Processes in Solid Wastes (Côté, 1986) where z - distance from the ash particle surface; t - time;
C - concentration of specific solute or constituent (subscript n); L - leachant concentration subscript (influent water) [C(L)n (t)];
w - leachate concentration near the surface of the ash matrix [C(w)n (t)];
b - ash matrix concentration (ie. bulk waste concentration) [C(b)n / im(t) / C(b)n / mo(t)]; im / mo - immobile / mobile concentration within the ash matrix.
A constituent inside the ash matrix may be in an immobile form (C(b)n / im), such as
precipitated or sorbed, or a dissolved and therefore mobile form (C(b)n / mo). The
proportion of a constituent in the mobile and immobile phase may be described by equilibrium chemistry or kinetic (rate limited) chemistry (Côté et al., 1986). The position of this equilibrium may be disturbed by diffusion of the mobile phase outside of the ash matrix or advective flow, surface transfer phenomena (sorption) or diffusion of external species (such as acid, H+, or chemical complexing agents) into the ash matrix (Côté et al., 1986). The chemical concentration at the interface of the ash matrix and the pore space (C(w)n ) provides the driving force for exchange between the ash matrix and leachant inflows (Côté et al., 1986).
On the basis of the above discussion and these distinctions proposed by Côté et al. (1986), the exponential decline in leachate concentration for major elements such as SO4, Na and Cl, is controlled by rapid dissolution of available minerals and advective flow of leachate removing these constituents from the pore volume of the ash. The overall driving force for leaching is the concentration difference between C(w)n and
(b) mo n /
C . The influent solution (at concentration C(L)n ) mixes with the aqueous solution at
the ash-water interface (at C(w)n ) and transports the mass of the constituent away, altering the local equilibrium for the ash matrix and forcing more constituent into solution.
As reviewed earlier, a significant proportion of ash contains more readily soluble minerals, such as halite (NaCl), thenardite (Na2SO4) and gypsum (CaSO4.2H2O), which are predominantly controlled by equilibrium dissolution - an instantaneous process. The migration of fresh leachant into and out of the ash, therefore, leads to a rapid decline in the presence of these minerals as further leachant flows through the ash. This process leads to the exponential decline in leachate concentration described by Farquhar (1989) and typical column leaching tests. For trace and other elements such as Al, Ca and Fe, the controls on leaching would also include the pH of the interface and influent water, sorption ("surface") phenomena and redox state and their behaviour would therefore be different to the "initial washoff" observed for major elements.
An important distinction in the approach described above is the use of pore volumes, or the volume of porosity in a given ash or waste material. By calculating the volume of leachate flow through an ash disposal site or column test and dividing this by the pore volume of the ash material, a standardised curve can be adopted and leachate concentrations plotted versus pore volumes. The pore volumes essentially gives an indication of the liquid-to-solid ratio passed through an ash or waste material. The principal advantage is that this is a non-dimensional approach, facilitating comparison of different ashes and tests.
An alternative approach to leaching processes was presented by Kosson et al. (1996). They described leaching phenomena according to three fundamentals (factors) : (i) availability, (ii) solubility and (iii) mass transfer controlled release. The availability is defined as the maximum quantity or soluble fraction of a constituent that can be released into solution under aggressive leaching conditions. Such conditions, in theory, should provide worst case environmental release scenarios for 1,000 to 10,000 years, particularly for trace elements although the more soluble salts may reach this point in a matter of years. The second factor, the solubility of different minerals and trace elements, is also critical to leaching phenomenon, as the low liquid-to-solid ratios generally found in field disposal sites can often lead to a geochemically saturated leachate concentration with respect to particular constituents, minerals and trace elements. As noted previously, the solubility of most trace elements is strongly correlated with leachate pH, and can also be influenced by the presence of chemical complexing agents and redox conditions. The third factor, mass transfer-controlled release, occurs through the slow release of a constituent from the solid matrix into solution. Geochemical saturation or equilibrium with the leachate is often not achieved. This is typically due to control by diffusion, a slow process inside ash grains, although chemical sorption (retardation) and precipitation processes can also be important.
Based on the approach of Kosson et al. (1996), major constituents such as SO4, Na and Cl, are both soluble and available, whereas species such as Fe or Pb are available but not readily soluble. For example, the total percentage of thenardite and gypsum in ash sample would represent the available SO4 for leaching, but the solubility of SO4 would be controlled by leachate chemistry and site specific conditions. Trace elements, controlled by complex chemical reactions and sorption phenomena, are therefore limited in leachate by their lower mass-transfer rates.
In summary, by considering the fundamental physical and chemical processes occurring inside ash, leaching phenomena can be described and quantified. To assess these processes, a variety of leaching tests have been developed. These will now be reviewed.