Soil, Wastewater, Air
Twenty years ago, numerous researchers turned to well-known sulfide precipitation agents as Wet Flue Gas Desulfurization WFGD additives for the control of trace amounts of mercury from the stacks of coal-fired power plants.
Many different sulfides were used, and are still used – whether they are inorganic, poly- or organic. Most are sufficient in providing mercury control in FGD systems such that the majority of customers, who were asked to meet an emissions limit of ~1 ppb due to the Mercury and Air Toxics Standard MATS rule, could meet the target.
The idea for MSA was spawned in 2018, when we read a technical paper titled “Effective Removal of Selenite and Selenate Ions from Aqueous Solution by Barite”. This initial impetus led to the ultimate issue of our US Patent 11,319,233.
The adoption of MSA within the WFGD industry has been slowed, unfortunately, by an undue focus on what turned out to be the wrong operating characteristic – MSA molar ratio. Get the right molar ratio, we thought, and all would be well. The initial full-scale test run in 2019 which showed a ~50% phase partitioning of selenium from the aqueous phase to the solid phase, could not be repeated. Yet, we learned that under certain conditions, not only can mercury be phase-partitioned to the solid phase within the FGD, but the aqueous selenium can also be phase-partitioned to the solid phase, certainly by the time the wastewater is about to be discharged at the plant boundary. The journey from Absorber bleed slurry to secondary clarifier outlet leads to substantial selenium reductions, and more than is possible without MSA. There are logical reasons for this.
We feel MSA is a unique sulfide. It falls within the realm of chemical intermediary, helping to keep its price low. It is an inorganic sulfide, not a poly-sulfide or an organic sulfide of ever higher molecular weight, with the concomitant increasing chemical costs. In addition, MSA is delivered as a solid, and can be added as a solid (soil), or it can be slurried (WFGD) and pumped.
We continue to develop our selenium ion immobilization pathway, as a way to minimize, or eliminate, the need for a bioreactor (WFGD systems) or provide a lower cost sulfide (wastewater and soil). We know Se can be incorporated into the Barite lattice due to the strong crystal stability of the Ba-SeO3-SeO4-SO4 for Se immobilization. This is confirmed by the full-scale WFGD testing that has been completed between 2019 and 2022. We will continue to optimize selenium immobilization.
Currently, MSA is being considered in a number of soil remediation projects where its full scale efficacy can be properly evaluated, but initial testing appears promising. Bench scale testing shows metals and the metalloid Arsenic can be effectively stabilized. Work continues on improving conditions for selenium precipitation.