Leaching characteristics of selected South African fly ashes: Effect of pH on the release of major and trace species

Abstract

Fly ash samples from two South African coal-fired power stations were subjected to different leaching tests under alkaline and acidic conditions in an attempt to assess the effect of pH on the leachability of species from the fly ashes and also assess the potential impact of the fly ashes disposal on groundwater and the receiving environment. To achieve this, German Standard leaching (DIN-S4) and Acid Neutralization Capacity (ANC) tests were employed. Mineralogical characterization of the fresh fly ashes revealed mullite and quartz as the major mineral phases with minor peaks of CaO and calcite. Chemical characterization by X-ray fluorescence (XRF) spectrometry revealed that the two fly ashes are similar, and consist of SiO(2), Al(2)O(3), Fe(2)O(3) and CaO as the main components with Cr, Co, Ni, Cu, Zn, V and Pb as minor components. Ca, Mg, Na, K and SO(4) were significantly leached into solution under the two leaching conditions with the total amounts in ANC leachates higher than that of DIN-S4. This indicates that a large fraction of the soluble salts in unweathered fly ash are easily leached. These species represents the fraction that can be flushed off initially from the surface of ash particles on contacting the ash with water. Al and Si were only observed in the leachates of the ANC test. Results obtained from the total acid-digestion and DIN-S4 leaching test indicated some toxic elements in the fly ashes are not easily solubilized. The amounts of toxic trace elements such as As, Se, Cd, Cr and Pb leached out of the fly ashes when in contact with de-mineralized water (DIN-S4 test) were low and below the Target Water Quality Range (TWQR) of South Africa. This is explained by their low concentrations in the fly ashes and their solubility dependence on the pH of the leaching solution. However the amounts of some minor elements such as B, Mn, Fe, As and Se leached out at lower pH ranging between 10 to 4 (ANC test) were slightly higher than the TWQR, an indication that the pH of the leaching solution plays a significant role on the leaching of species in fly ash. The high concentrations of the toxic elements released from the fly ashes at lower pH gives an indication that the disposal of the fly ash could have adverse effects on the receiving environment if the pH of the solution contacting the ashes is not properly monitored. The study indicated that on contact with water in a disposal scenario fly ash will release high amounts of soluble species.

Long-term brine impacted fly ash, Part II: Mobility of major species in the ash residues

“…The low concentration of Mg at the initial high pH of the ash residues when in contact with ultrapure water at the start of the ANC tests could indicate that the probable Mg-rich mineral phases that precipitated during the long-term interactions experiments are not mobile at high pH (when in contact with ultrapure water). At high pH, the solubility of Mg is controlled by the formation of either Mg(OH) 2 (Gitari et al 2009;Dijkstra et al 2006;Vitkova et al 2013) or by the more stable Mg layered double hydroxide (LDH) phase (Mg 2 Al(OH) 6 (-CO 3 ) 0.5 ÁH 2 O (Johnson and Glasser 2003). Mg was found to be a less readily soluble component of fly ash due to its incorporation mostly in the glass phases of the fly ash (Ugurlu 2004).…”

“…The increase observed at pH below 6 indicates that Al could be released from the ash residues under low pH conditions, which might not be envisaged in the real disposal scenario. The rapid and continuous increase in the concentration of Al at pH \ 6 could be as a result of the dissolution of the aluminosilicate phases in the ash residues (Gitari et al 2009). The slow initial release of Al in the ANC systems could be governed by the precipitation of amorphous phases at high pH while the dissolution of the amorphous and crystalline mineral phases at lower pH could responsible for the increase thereafter.…”

“…The difference in the amounts of these oxides can be ascribed to the coal geology associated with the site from where the coal was mined. The oxides of Ca and Mg are the major alkalinity contributors in fly ash, and the concentrations of these basic oxides in the fly ash determine the pH of the fly ash when in contact with aqueous solution (Gitari et al 2009). The lower buffering capacity of the TFA sample series indicates that the ash source would acidify more rapidly during weathering than the SFA series of samples.…”

“…The leaching of species from fly ash or municipal solid waste incineration (MSWI) ash depends on several factors which include physical and chemical composition of the fly ash (Dijkstra et al 2006). The leaching of species from fly ash also depends on the pH of the leachant (Gitari et al 2009; Van der Sloot and Dijkstra 2004). Any aqueous flow that percolates or comes in contact with the fly ash dissolves or precipitates the species (Dijkstra et al 2006) depending on its pH.…”

Laboratory Study on the Mobility of Major Species in Fly Ash–Brine Co-disposal Systems: Up-flow Percolation Test

“…The disposal of brine is regulated due to its chemical composition thereby causing some power stations to co-dispose fly ash and brine with the aim of using fly ash to capture the species in brine. The leaching behaviour of major and minor species from coal fly ash and municipal solid waste incineration ash has been widely studied (Gitari et al 2009;Meima and Comans 1999;Hyks et al 2009) in order to assess the environmental implications of their utilization and disposal. Despite several studies on the release of species from fly ash when in contact with water and acid mine drainage (Baba and Kaya 2004;Polettini and Pomi 2004;Gitari et al 2006), adequate attention has not been given to understanding the mobility of some species when fly ash is in contact with brine solution.…”