Sequestration of Selenium in Flue Gas Desulfurization Wastewater Using Constructed Wetlands: A Synchrotron Based Investigation.

Flue gas desulfurization (FGD) technology is commonly used in coal-fired power plants to minimize environmental impact caused by sulfur dioxide emission to the atmosphere. The quality of wastewater generated by FGD systems can be compromised due to enrichment of trace metals such as selenium (Se), arsenic (As), mercury (Hg), as well as many other constituents. Therefore, constructed wetland treatment systems (CWTS) are being used to improve the quality of FGD wastewater. Our objectives were to: (1) study fate and transport of Se in CWTS soil materials using a continuous flow-through column system, (2) understand the role of drying and re-wetting conditions on Se mobility, and (3) to gather information on sequestered-Se speciation in CWTS soil materials using sequential chemical extraction (SCE), and synchrotron based techniques. There was no Se detected in effluent samples collected during 100 days of feeding (bottom-up) with FGD wastewater and an additional 100 days of flushing with raw water (river water). This suggests that Se was strongly retained by these soil materials. Soil analysis data showed Se was only sequestrated in the bottom part of the soil columns indicating that Se mobility in these soil materials is limited. However, effluent samples from columns that were dried and re-wetted did contain some Se indicating that some retained Se can be remobilized by changing redox conditions. Bulk-XANES data of sequestrated Se in soils showed that most Se were in reduced forms (Se(IV), Se(0), and Se(organic)). In addition, bulk-XANSE data showed that flushing with raw water for an additional 100 days resulted in further reduction of Se. These results were also supported by Se fractionation results obtained from the SCE. Micro-scale-XRF and -XANES data will also be presented.