In early 2018, my research for an upcoming presentation at the Reinhold APC Conference, led me to a technical paper released in 2017 but two young Japanese Geochemists; Kohei Tokunaga and Yoshio Takahashi. They were affiliated with the Department of Earth and Planetary Science at the University of Tokyo and the Advanced Science Research Center, Japan Atomic Energy Agency. Graphs from the abstract of that paper are in the headline above. Since then, I have affectionately referred to them as the T&T brothers.
At the time, I was unaware of the term Barite. I quickly learned, it is the mineral form of barium sulfate. Barium sulfate finds widespread use as drilling mud. What intrigued me about these two graphs was the suggestion that higher sulfate ion concentration improved selenium capture.
The y-axis on the graph to the right, above, plots the distribution coefficient for the selenium T&T captured in the barite versus what remained in the water during their experimentation. The x-axis plots experimental conditions such as pH, ionic strength, coexistent cations [Ca++], and sulfate [SO4–]. Could barite somehow be used to capture aqueous selenium in the WFGD absorber slurry liquid?
In their batch experiments, T&T precipitated BaSO4 from a mixture of sodium sulfate Na2SO4 and barium chloride BaCl2.2H2O. There were many other particulars to their test, but I’ll only add here that they ran their batch tests on MQ water, and another set of tests on Artificial Sea Water ASW. It got me to thinking…Could I get a utility interested in running some beaker tests at their plant, using real WFGD slurry from a LSFO (Limestone Forced Oxidation) WFGD?
At that time, the industry was especially focused on trying to minimize ORP (Oxidation Reduction Potential) in the WFGD absorber tank, to minimize selenate Se(VI). Se(VI) was problematic in the wastewater phys/chem systems, and substantial effort and capital was employed to find a consistent way for the absorber ORP to be minimized, without going negative, to keep the Se(VI) fraction low. To say the control of ORP in WFGD has been difficult, is an understatement. Something else was needed.
Could barium save the day?
A number of people informed me injection of soluble barium in the WFGD “will never work.” With ~5,000 ppm sulfate in absorbing solutions, and three orders of magnitude less selenium in these solutions, the sulfate would overwhelm the barium, and no appreciable selenium would be removed.
Fortunately, that is not what happened.