Understanding Transport and Transformations of Selenium in Flue-Gas Desulfurization Waste Water Using Continuous Flow Column Systems.

Abstract    

Selenium (Se) is an essential element for animals, plants and microorganisms. However, it is toxic to biological organisms in naturally rich or polluted environments with Se. Among major Se species in soil solution, reduced Se species such as selenite (SeO₃^2-) and selenide (Se^2-) are strongly adsorbed by soil constituents forming inner-sphere complexes. Main objectives of this laboratory based column study was to mimic flue gas desulphurization (FGD) wastewater-wetland treatment system to investigate fate and transport of Se. FGD wastewater contained concerned constituents namely, Se, S, As, Hg, B, Na, F and Cl, with Se concentration of 128 µg/L. Contaminant breakthrough curves (BTCs) were developed to determine the capacity of constructed wetland cells to retain Se and other contaminants. Columns were filled with pre-wetted top and engineered soils (three each) at a uniform bulk density. Up-flow (bottom up) column study was conducted at the flow rate of 1.42 mL/hr. Effluent water was collected continuously and analyzed for Se and other constituents. We found no detectable Se (< 0.5 µg/L) in the 10 PVs of effluent water (~100 days) collected in this study. Boron and F^- were also retained partially while other constituents’ retention was very poor. Top soil consistently showed superior or equal retention capacity compared to the engineered soil. End of the study, soil analysis showed that Se was accumulated in the bottom 1/3 of the soil columns and there was no elevation of Se concentration in upper 2/3 of the columns. After 100 days of flushing with raw water, there was no retained Se released suggesting strong retention of Se had occurred under these conditions. Sequential extraction data of Se will also be presented.