Speciation of arsenic and selenium during leaching of fly ash

The leaching (release) of large amounts of oxyanions, such as those of arsenic and selenium, is an major environmental problem when it comes to the disposal or use of coal fly ash. To predict environmentally safe conditions for the disposal or use of fly ash in, for example, construction materials, it is necessary to know the underlying leaching processes. The processes which cause leaching of arsenic and selenium from coal fly ash are studied in this thesis. Although combustion residues, such as coal fly ash, show a high variability in elemental composition, the residues show a systematic leaching behaviour. It is, therefore, also possible to use the results of this thesis to obtain a better understanding of the leaching of other combustion residues. Arsenic and selenium can be present in different valency states in the environment. The valency state influences the toxicity and mobility of the oxyanions in the natural environment. Studies concerning the valency state of As and Se in fly ashes are reviewed in Chapter 2. Mter testing determination methods, the valency state was determined in leachates of five different coal fly ashes at different pH values. The results showed that As and Se leach preferentially as the oxyanions arsenate, As(V), and selenite, Se(N). However, a relatively large amount of selenate, Se(Vl), was found in the leachates of brown coal fly ash originating from a fluidized bed boiler. The leaching process is not only is controlled by the speciation in solution, but also by the speciation on the solid (e.g. adsorbed species and minerals). The leaching of major elements can be generally described by precipitation/dissolution reactions. Trace element leaching is generally not controlled by these reactions but is expected to be controlled by sorption reactions. The possible controlling sorbents for As and Se are studied in Chapter 3 by comparing the leaching of As and Se from an acidic fly ash and an alkaline fly ash with the sorption on their oxyanions the minerals hematite, mullite, portlandite and ettringite, which are generally present in the fly ash matrix. Hematite is one of the principal iron bearing minerals in fly ash. Mullite is an important crystalline aluminium silicate. Portlandite (calcium hydroxide) and ettringite (calcium-aluminium hydroxysulphate) may be formed during leaching of alkaline fly ashes. It was shown that the leaching of As and Se from acidic fly ash can be described by sorption on an iron (hydr)oxide, such as hematite, and the leaching from a fly ash by sorption on an alkaline calcium phase. Following the results of Chapter 3, an attempt is made to model the leaching of As and Se from acidic fly ash by sorption on iron (hydr)oxide in Chapter 4. Extractions of the crystalline and amorphous iron (hydr)oxide show that latter mineral is likely to dominate in controlling the leaching of As and Se. The amount of As, Se and Fe involved in the sorption process, are obtained from hydroxylamine extraction of amorphous iron (hydr)oxide an from isotopic exchange. It is shown that the leaching of As and Se can be described with an overall surface complexation model for the sorption of As and Se on amorphous iron (hydr)oxide. When sorption on iron (hydr)oxides controls the leaching of As and Se from acidic fly ash the actual release of these elements is also dependent on the reversibility of the sorption process. The sorption reversibility is studied on different mineral phases in chapter 2. In Chapter 5, the reversibility of arsenate, selenite and molybdate on amorphous iron (hydr)oxide and hematite is investigated. Arsenate and molybdate sorb less reversible on the iron (hydr)oxides while selenite sorbs reversible with respect to changes in the pH. The pH at which the oxyanion is sorbed is crucial in controlling its sorption reversibility on iron (hydr)oxides. Arsenate is less reversibly sorbed than selenite on all the minerals studied. This behaviour may well be the reason for the general observation that the availability of As for leaching is less than that of Se. Chapter 6 describes an investigation of the processes controlling As and Se leaching from fly ash under natural conditions using mechanistic insights developed in the earlier chapters of this thesis. For this purpose samples of solids and pore water were taken form an 11 year old, naturally weathered, fly ash at different depth in a lysimeter and were studied both experimentally and by geochemical modelling. The potentially important sorbents for As and Se, amorphous iron (hydr)oxide and alkaline Camineral ettringite, are shown to be present and likely to be in equilibrium with the pore waters. Lysimeter profiles, pH dependent leaching experiments and extraction of As, Se, and Fe in the weathered fly ash, as well as the modelling results, confirm the findings in the earlier chapters; As and Se leaching from the fly ash, especially in the upper section of the lysimeters, is likely to be controlled by sorption on amorphous iron (hydr)oxide. In the lower section of the lysimeter interaction with alkaline Ca-minerals such as ettringite may contribute in controlling the leaching process