It is known that power station fly ash can act as a sorbent for mercury in aqueous solutions, and that it appears to bind strongly to mercury present in soil. Fly ash itself is a multi-component system, however, composed of a glass, amorphous, crystalline and carbon phases. In addition, mercury appears in many forms, varying significantly in their stability and the degree of threat posed to human health and the environment. The overall behaviour of the ash-mercury system is thus extremely complex, and may involve both physical and chemical bonding, precipitation and entrapment. For fly ash to be considered as a potential remediation material, detailed knowledge of the processes by which it removes mercury is needed.
The project objectives are thus:
- To determine kinetic characteristics of the sorption/desorption reactions, including those at high sorbate concentrations, of different mercury species with power station fly ash and its sub-components;
- To add to the understanding of which components of power station fly ash are responsible for mercury immobilisation and the mechanisms through which this takes place;
- To identify, characterise and quantify the different forms of mercury that result from its interaction with power station fly ash and its sub-components;
- To assess the availability of mercury sorbed on power station fly ash to biological methylation;
- To provide fundamental kinetic data for future use in the estimation of the suitability of power station fly ash for the stabilisation and remediation of mercury contaminated sites.
The project will use two distinct ash types: the first with high alkalinity, the second type characteristic of coals with a low calcium content which form a lower alkalinity ash. As well as unmodified ash, the study will look at ash sorted to give fractions rich in the major constituents: glass cenospheres, isotropic and anisotropic coke, and intertinite. The sorption behaviour of the total ash and its fractions will be investigated with respect to the most important mercury species: inorganic (cationic, anionic in chloride complexes, oxidic), elementary (mercury vapour) and organic (CH3HgCl).
Sorption and desorption characteristics and equilibrium constants will be established using isotherm techniques for mercury species in solution (and mercury vapour - at different partial pressures) exposed to ash and its components at their inherent pH which may range from 4 to 10.5. The rates of adsorption and desorption will be established, with time sequencing analysis of the sorbate solutions. The equilibrium and kinetic parameters of dissolution of elementary mercury and mercury oxides in weakly mineralised water (0.5 g/l) will be experimentally determined to establish the relationship with pH.
Oxidation-reduction transformations of sorbed mercury will be carried out on ash samples containing various concentrations of inorganic or elementary mercury, with the help of weak redox reagents (oxygen from air, aldehydes) and at a pH close to neutral.
The ability of various forms of the mercury, bound to ash, to methylate in a methylating environment will be determined for ash with high, average and low levels of inorganic mercury in neutral and alkaline conditions.
Outputs will include:
i) Adsorption isotherms and graphs establishing the sorption rate. The sorption capacity and intensity established for different ashes and their enriched components can be used for prediction in future modelling studies. These results will be of significant practical use for assessment of the potential of ash-based sorbents, in both engineering and economic terms, and in estimation of environmental impact in mercury polluted areas;
ii) Improved understanding of the processes of desorption of mercury from ash materials. The results will allow estimation of the mercury concentration in waste and natural waters and the risk to the environment, when soluble forms of mercury are hydrolysed or reduced. Data on desorption capacities and rates will allow assessment of the risk of secondary mercury pollution during long-term storage of a contaminated sorbent, and prediction of the character of the sorption interaction between different forms of mercury and the components of ashes;
iii) Improved understanding of the level of risk of release of sorbed mercury under methylating conditions. The results obtained will be of both fundamental and applied interest: in the case of a water body containing contaminated sediments, for example, the level of methyl mercury may be controlled by rates of methylation rather than the total amount of mercury present, and this may therefore become the determining factor governing the choice of clean-up strategy;
iv) A database on properties of alkaline power station ashes from Kazakhstan and Siberia, and a review of current knowledge on properties of the phases of ash and their ability to sorb salts of mercury, elementary mercury and organic-mercury compounds.
188902 St. Petersburg
SO15 5JF Southampton