Immobilization of Heavy Metals by Solidification/Stabilization of Co-Disposed Flue Gas Desulfurization Brine and Coal Fly Ash

The coal-fired power industry in the United States faces growing needs to improve wastewater treatment and disposal practices, especially for wet flue gas desulfurization (FGD) systems. Zero liquid discharge (ZLD) treatment systems may be implemented, and one ZLD option is the coupling of brine concentration with solidification/stabilization (S/S). 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 As^V, Cd^II, Hg^II, and Se^IV in the toxicity characteristic leaching procedure (TCLP); however, poor retainment was observed for Cr^VI and Se^VI. Separate experiments showed good sorption of As^V, Cd^II, Hg^II, and Se^IV (average 56–100%) but poor sorption of Cr^VI and Se^VI to S/S solids. Meanwhile, Cr^VI and Se^VI retainment could be enhanced by addition of FeSO₄ to the S/S mixture, likely due to reduction of these metals to lower oxidation states. Compared to BCFA, S/S using sub-bituminous CFA (SCFA) resulted in higher pH S/S solids and final TCLP leachate, which increased retainment of As^V, Cd^II, and Se^VI. Apart from the pH impact on the process, As^V retainment was likely improved by the high Ca content of SCFA and Se^VI retainment was improved by the incorporation of SeO₄^2– in Friedel’s salt (AFm-Cl) by exchange with Cl^–. Friedel’s salt was positively identified by X-ray diffraction in the SCFA S/S solids but not for the BCFA S/S solids. Even so, reduction of Se^VI plus S/S is likely a better immobilization strategy than SeO₄^2– substitution in the AFm phase because of higher stability under long-term landfill conditions