Abstract
Selenium is an essential trace element but is increasingly becoming a contaminant of concern in the electric power industry due to the challenges of removing solubilized selenate anions, particularly in the presence of sulfate. In this work, we evaluate granulated layered double hydroxide (LDH) materials as sorbents for selenium removal from wastewaters obtained from a natural gas power plant with the aim to elucidate the effect of competing ions on the sorption capacities for selenium removal. We first present jar test data, followed by small-scale column testing in 0.43 inch (1.1 cm) and 2 inch (5.08 cm) diameter testbed columns for the treatment of as-obtained cooling tower blowdown waters and plant wastewaters. Finally, we present field results from a pilot-scale study evaluating the LDH media for treatment of cooling tower blowdown water. We find that despite the high levels of total dissolved solids and competing sulfate ions, the selenium oxoanions and other regulated metals such as chromium and arsenic are successfully removed using LDH media without needing any pre-treatment or pH adjustment of the wastewater.
Highlights
Selenium removal from electric power plant wastewaters has received much attention recently due to the revised effluent limitations guidelines (ELG) for steam electric power plants published by the U.S Environmental Protection Agency in 2015 [1]
The physical properties of the granular layered double hydroxide (LDH) used in the column tests of this study were already investigated in our prior work [44,46], which established that the as-obtained, mixed metal oxide material exhibited the cubic periclase structure of a magnesium/aluminum solid solution, which is typically obtained when LDH is calcined at high temperatures to remove interlayer water and anions [47,48,49]
Upon re-introduction of the granules to DI water, the layered double hydroxide structure was successfully recovered [44] in what is known as the “memory effect” of LDHs [38]
Summary
Selenium removal from electric power plant wastewaters has received much attention recently due to the revised effluent limitations guidelines (ELG) for steam electric power plants published by the U.S Environmental Protection Agency in 2015 [1] The purpose of these ELGs was to set new, more stringent federal limits on the levels of pollutants that can be discharged in wastewaters from power plants, notably toxic and bioaccumulative pollutants such as arsenic, mercury, selenium, chromium, and cadmium. The fundamental challenges and treatment technologies for selenium have been highlighted in several reviews [2,7,8,9,10] and include chemical, physical, and biological methods, with bioreactors containing selenium-reducing microorganisms being one of the most mature technologies that have been evaluated for pilot and full-scale wastewater treatment [11,12]. This, along with its chemical and physical similarities to sulfate, make selectivity of common sorbents to selenate a challenge
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