| Power plants that use coal as their primary source of energy require the installation of Flue Gas Desulfurization (FGD) units to control SO x emissions to the environment. While FGD units are successful in reducing the SOx emissions, wastewater generated through the use of the units requires treatment to reduce the concentration of toxic cations to meet the permissible discharge limits. The new effluent limitation guidelines and standards for the steam electric power generating industry that were finalized in September 2015 set stricter limits for the discharge of wastewater, particularly for mercury, arsenic, and selenium. In order to address this issue, simultaneous removal of several heavy metal ions was investigated by employing high-surface area amorphous silica grafted with chelating ligands containing -N, -S or mercapto (-SH) groups that demonstrated high affinity towards these toxic cations.;Adsorption tests were conducted at higher than relevant concentrations of toxic cations in simulated FGD matrix to predict the kinetics and competitive adsorption between the toxic cations. The concentration of heavy metal ions was determined using the Inductively Coupled Plasma -- Mass Spectroscopy (ICP-MS). The behavior of the toxic cations towards the surface functional groups was correlated to the stability constants of the formed complexes to explain the observed adsorption pattern. The adsorption capacity of the functionalized materials for Hg2+ was estimated using the Langmuir isotherm model. The best adsorbents from the kinetic and equilibrium studies were tested on simulated FGD wastewater containing realistic concentrations of toxic cations. Regeneration studies on spent sorbents were performed using dilute HCl and it was found that disposal of sorbents was preferred over regeneration. The best adsorbents and conventional sulfide precipitation method were also tested on real FGD wastewater to address deeper removal of toxic cations. A preliminary cost estimate is provided for the use of the proposed technology in coal utility power plants. Ultimately, based on the results of this study, comparison between the proposed adsorbents and sulfide precipitation methods, and cost analysis, it was decided that MPTMS-silica was a better candidate to treat FGD wastewater. Future research is required to reduce the cost of MPTMS-silica through optimization of operational parameters. |
