Minimizing Arsenic Mobility Using Ferrihydrite in a Small-Scale Constructed Wetland Treatment System Designed for Removal of Selenium in FGD Wastewater.

Flue gas desulfurization (FGD) technology is implemented in coal-fired power plants to minimize sulfur dioxide emission and meet new requirements comes under the Clean Air Act. The wastewater generated by FGD systems commonly fail to meet surface water quality standards due to high concentrations of trace elements such as selenium (Se), arsenic (As) and many other constituents. As a solution, constructed wetland treatment systems (CWTS) are being considered to remove Se and other trace elements from FGD wastewater before releasing into surface water. Under suboxic and anoxic conditions, Se is immobile and become less toxic in contrast to the behavior of As. We carried out laboratory based soil column experiments (small-scale CWTS) mimicking wetland conditions to understand the performance of a pilot-scale CWTS. The main objective of this study was to achieve maximum trace element retention in wetlands using ferrihydrite as an amendment (0.1% w/w). Background concentrations of Se and As in wetland soil material were 0.82 mg/kg and 5.78 mg/kg whereas concentrations of Se and As in the FGD wastewater were 135 μg/L and 1.18 μg/L, respectively. Experiment was conducted for 60 days of feeding with 1:1 mixture of FGD and river water. Concentration of Se in collected effluents was non-detectable in both non-amended and ferrihydrite-amended soil columns. However, As concentration of effluents collected from non-amended columns increased with time while that of ferrihydrite-amended columns remained comparatively low. Under the reduced conditions established in small-scale CWTS, strong retention of Se had occurred whereas reductive dissolution of soil minerals resulted in release of soil As into the soil solution. Dissolved As may have re-adsorbed onto the ferrihydrite surfaces resulting in less mobility. It is important to gather mechanistic information on As retention in CWTS material using synchrotron based techniques and micro-scale-XRF and -XANES data will also be presented.