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

Abstract

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 FeSO 4 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 SeO 4 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 SeO 4 2− substitution in the AFm phase because of higher stability under long-term landfill conditions.

Fly Ash–Ca(OH)2 Reactivity in Hypersaline NaCl and CaCl2 Brines

“…This monolith retains the dissolved salts originally present in the brine. 3,4 Such salts may be entrapped by precipitation as insoluble species, 5 by sorption on the surfaces of hydrated phases, 6 and/or by incorporation into the lattice (structure) of the hydrated phases. 7,8 Furthermore, the solution may also be entrapped within the pore spaces of the solidified mass.…”

“…9,10 In particular, there is considerable interest in the use of fly ashes, i.e., a coproduct of coal combustion on account of their ability to motivate so-called pozzolanic reactions upon reaction with water contained in the brine, in the presence of calcium-based additives including quicklime, hydrated lime, and/or ordinary portland cement (OPC). 3,4,9 Recently, Song et al suggested that typical ASTM classifications of fly ashes that are used to assess their use in construction applications (e.g., using ASTM C618) may have led to the exclusion of fly ashes whose reactivity was underestimated. 11 Lower-performance requirements (e.g., in terms of strength) of encapsulation materials would imply that a wide range of fly ashes, including currently produced and harvested and reclaimed ("historical production") fly ashes, may be suitable for use in S&S applications.…”

Calcium Sulfite Solids Activated by Iron for Enhancing As(III) Oxidation in Water

“…The wet desulfurization process has significant advantages of fast reaction and high efficiency, and has been widely used in China. However, desulfurized gypsum (DG) cannot be fully utilized, and its disposal constitutes a waste of resources [ 1 , 2 ]. Currently, DG is mainly used as a soil conditioner [ 3 ], serving to improve certain properties of soil, such as its pH, water absorption, water retention, and so on.…”

Cotreatment of MSWI Fly Ash and Granulated Lead Smelting Slag Using a Geopolymer System

“…According to the previous study, the anionic species is often difficult to solidify/stabilize in geopolymers, but Friedel’s salt seems to immobilize them effectively. Recently, Friedel’s salt has attracted increasing attention in the past several years because of its good ion exchange characteristics [33,34,35,36,37]. It was reported that Friedel’s salt could effectively adsorb Zn 2+ [38], AsO 4 3− and PbO 2 − from wastewater.…”