Mineralogy optimization for immobilization of potentially toxic elements and halides in Co-disposed flue gas desulfurization brines and bituminous coal fly ash

Abstract

• Co-disposal of flue gas desulfurization wastewater brine and bituminous coal fly ash. • Zero liquid discharge through solidification/stabilization. • Contaminants encapsulation through mineralogy optimization. • Aluminate addition enhances the formation of Friedel's salt. • Gypsum addition enhances the formation of ettringite. The flue gas desulfurization (FGD) wastewater and coal fly ash (CFA) are waste products containing potentially toxic elements (PTEs) and halides of environmental concerns. A novel co-disposal strategy for these two wastes is a zero-liquid-discharge (ZLD) method which treats the concentrated FGD brine via a solidification/stabilization (S/S) process using CFA and alkaline additives. The objective of this study was to improve PTE and halide immobilization of this ZLD method by optimizing the mineralogy of the S/S solids in co-disposed FGD brines and bituminous CFA. The formation of Friedel's salt, which is critical in binding oxyanions and halides, was specifically evaluated. Various mixtures of FGD brines, CFA, and alkaline additives were systematically tested and the formed S/S solids were evaluated by long-term leaching tests and quantitative X-ray diffraction (QXRD) analyses. Addition of reactive aluminate (2.5 % by weight) and lime (10% by weight) yielded S/S solids with enhanced formation of Friedel's salt by up to 23%, which were able to retain the majority of selenate (>90%), arsenate (>99%), chromate (>99%) and chloride (>50%) from the brine after nine weeks of leaching tests. Comparatively, addition of gypsum promoted the formation of ettringite, which only enhanced the immobilization of borate, but not selenate and chloride. The hydraulic conductivity of the S/S solids was another important factor for contaminant leaching, because it could affect the stability of Friedel's salt under the leaching conditions and governed the immobilization of halides since the halide concentration from the brine exceeded the binding capacity of formed Friedel's salt.