Immobilization of heavy metals in solidified/stabilized co-disposed bituminous coal fly ash and concentrated flue gas desulfurization wastewater
Renew, Jay Earl
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The needs to reduce environmental discharges from the coal-fired power plants have prompted the industry to improve wastewater treatment and disposal practices, especially for wet flue gas desulfurization systems. One option is the implementation of zero liquid discharge (ZLD) treatment systems for FGD wastewater. ZLD can be achieved through the coupling of brine concentrator with a solidification/stabilization (S/S) process. This S/S process could be achieved by co-disposing the concentrated FGD brines with coal fly ash (CFA) and Portland cement. S/S using bituminous CFA (BCFA) achieved good retainment (average 68−90%) of AsV, CdII, HgII and SeIV, in the toxicity characteristic leaching procedure (TCLP). Meanwhile, CrVI and SeVI retainment could be enhanced by addition of FeSO4 (FS) to the S/S mixture, likely due to reduction of these metals to lower oxidation, less mobile species. Semi-dynamic tank leaching tests also showed the positive impact of FS on retainment of the toxic oxyanions AsV, CrVI, and SeVI. FS addition could reduce the cumulative release of these oxyanions over the long term, although the impact does decrease for longer leaching times. AsV, SeVI, and several of the major components releases from the S/S solids were modeled utilizing LeachXSTM software which accounts for elemental liquid-solid partitioning and mass transport release. Introduction of FGD wastewater ZLD materials to industry landfills could increase exposure of conventional coal combustion residuals (CCRs) to high salinity leachate which could increase metal leaching. Hence, this work also evaluated salt impact on metal leaching from CFA. Metal (As, Cd, Cr, Mn, Pb, Se, and Zn) leaching from CFA was evaluated through varying pH and salt (CaCl2, MgCl2, and NaCl) addition to the leachant in batch-equilibrium tests. As and Cd had the highest leaching increases with salt addition under conditions (medium to high pH) anticipated for landfill leachate at coal-fired power plants. Cationic metals were generally less soluble at higher pH and were impacted by salt addition through Cl- complexation and competitive cation exchange. Oxyanion leaching patterns influenced by salt addition differed based on pH for As and Se. PHREEQC modeling showed that increased As leaching with salt addition at medium pH could be modeled based on: (1) competition between Cl- and AsO43- for hydrous FeIII oxide sorption sites; and (2) complexation and sorption of the cations (Ca2+, Mg2+, and Na+) with AsO43- to CFA surface sites. Modeling of Cd leaching with increasing salt addition at neutral pH indicated that Cd leaching was significantly controlled by Cl- complexation, followed by competitive cation exchange for active sites with Ca2+, Mg2+, and Na+.