Fly ash degree of reaction in hypersaline NaCl and CaCl2 brines: Effects of calcium-based additives.

The pozzolanic reaction of fly ashes with calcium-based additives can be effectively used to solidify and chemically stabilize (S&S process) highly concentrated brines inside a cementitious matrix. However, complex interactions between the fly ash, the additive, and the brine typically affect the phases formed at equilibrium, and the resulting solid capacity to successfully encapsulate the brine and its contaminants. Here, the performances of two types of fly ash (a Class C and Class F fly ash) are assessed when combined with different additives (two types of cement, or lime with and without NaAlO2), and two types of brine (NaCl or CaCl2) over a range of concentrations (0 ≤ [Cl-] ≤ 2 M). The best performing matrices - i.e., the matrices with the highest Cl-containing phases content - were identified using XRD and TGA. The experimental results were then combined with thermodynamic modeling to dissociate the contribution of the fly ash from that of the additives. All results were implemented in a machine learning model that showed good accuracy at predicting the fly ash degree of reaction, allowing for the robust prediction of extended systems performance when combined with thermodynamic modeling.

Atomic Dislocations and Bond Rupture Govern Dissolution Enhancement under Acoustic Stimulation.

By focusing the power of sound, acoustic stimulation (i.e., often referred to as sonication) enables numerous "green chemistry" pathways to enhance chemical reaction rates, for instance, of mineral dissolution in aqueous environments. However, a clear understanding of the atomistic mechanism(s) by which acoustic stimulation promotes mineral dissolution remains unclear. Herein, by combining nanoscale observations of dissolving surface topographies using vertical scanning interferometry, quantifications of mineral dissolution rates via analysis of solution compositions using inductively coupled plasma optical emission spectrometry, and classical molecular dynamics simulations, we reveal how acoustic stimulation induces dissolution enhancement. Across a wide range of minerals (Mohs hardness ranging from 3 to 7, surface energy ranging from 0.3 to 7.3 J/m2, and stacking fault energy ranging from 0.8 to 10.0 J/m2), we show that acoustic fields enhance mineral dissolution rates (reactivity) by inducing atomic dislocations and/or atomic bond rupture. The relative contributions of these mechanisms depend on the mineral's underlying mechanical properties. Based on this new understanding, we create a unifying model that comprehensively describes how cavitation and acoustic stimulation processes affect mineral dissolution rates.

Solidification/stabilization of flue gas desulfurization brine and coal fly ash for heavy metals and chloride immobilization: Effects of S/S conditions and zero-valent-iron pretreatment.

Effective management of flue-gas-desulfurization (FGD) wastewater and coal-combustion-residues (CCRs) are major challenges in the coal-fired power industry. The zero-liquid-discharge (ZLD) method of combining FGD brines and CCRs in solidification/stabilization (S/S) is promising due to its potential of treating both wastes in the same process. This study evaluated the performance of such a ZLD method for immobilizing heavy metals (Se, As, Cd and Cr) and chloride in FGD wastewater and/or CCRs. Effects of different coal fly ash (bituminous (BCFA) and sub-bituminous (SCFA)), activating agent (Portland cement (PC) and lime) and pretreatment of brines by zero valent iron (ZVI) on the S/S process were evaluated. Short-term and long-term leaching tests were conducted to evaluate performance of the S/S solids in pollutant retainment. The pre-treatment of FGD brine by ZVI enhanced the retainment of heavy metals when BCFA was used, but not when SCFA was used since it already performed quite well without ZVI pretreatment. Quantitative X-ray diffraction and scanning electron microscopy analyses strongly indicated the formation of Friedel's salt, Ca2Al(OH)6(Cl,OH)·2H2O, is critical in the retainment of heavy metals and chloride. SCFA contained higher lime and reactive aluminate contents than BCFA; thus, S/S solids made with SCFA contained higher amounts of Friedel's salt.

Removal of heavy metals by aged zero-valent iron from flue-gas-desulfurization brine under high salt and temperature conditions.

To achieve zero liquid discharge, the flue-gas-desulfurization (FGD) wastewater at coal-fired power plants can be concentrated into brine through thermal evaporation to maximize water reuse; however, the hot brine generated requires further treatment prior to disposal. To address this need, this study investigates the performance of aged, micron-sized zero-valent iron (ZVI) for heavy metal removal in simulated and real FGD hot brines, which was scarcely studied previously. The effects of temperature, pH, total dissolved solids, ZVI dosage, major cations, nitrate and sulfate on the reactivity of ZVI in the brines were evaluated. Among many factors, higher temperature and Mg2+ exert the dominant influence. At 80 °C, almost 100% of arsenate (1 mg/L) and chromate (1 mg/L) can be removed in <5 min using 4.17 g/L of ZVI in simulated brines, while selenate (25 mg/L) and cadmium (5 mg/L) can be completely removed within 30 min. Mg2+ ions naturally present in FGD brines account for the depassivation of aged ZVI. X-ray diffraction results suggest that green rust is the reactive intermediate for selenate and cadmium removal. Overall, this study demonstrates that ZVI is an effective material for removing heavy metals in hot FGD brines generated through thermal evaporation at power plants.