Partitioning and speciation of selenium in wet limestone flue gas desulphurisation systems: A review

This paper presents a general review of the selenium chemistry and fate in wet limestone flue gas desulphurisation (FGD) systems and major factors associated to the operation of wet limestone FGDs affecting Se oxidation. Selenium chemistry in FGD waters is more than the interconversion between selenite and selenate; selenium can form a complex array of chemical aqueous complexes some of them analogous to those of sulphur as well as interact with polyoxosulfur anions. However, most of Se-aqueous complexes that may encounter in FGD waters are still unidentified. FGD water re-circulation, fly ashes (FAs) accumulated in the FGD scrubber, oxi-reduction potential (ORP), water flow quantity, flow variability, and pressure are operating FGD conditions that may affect the implementation of effective strategies in management and control of Se in FGD waters. These operating FGD conditions may have a strong influence in the reaction kinetics and the mass transfer of water and constituents through the water treatment processes for Se. Other influences that will affect selenium removal in FGDs can be the oxidizing and reducing agents complexing or chelating agents and/or competing oxyanions. Development of effective strategies in management and control of Se is also complicated by difficulties in measuring and analysing selenium speciation in FGD wastewaters. The effect of sample handling and preservation methods on selenium speciation is not well understood yet and analytical methods may not achieve accurate total selenium concentrations in complex FGD wastewater matrices.