Synthesis of Layered Double Hydroxides to Sequester Halides from Flue Gas Desulfurization Wastewater Concentrate
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Abstract
An emerging strategy for the management of flue gas desulfurization wastewater involves volume reduction of the wastewater by concentrating to brine and then mixing the brine with fly ash and other chemical additives to form a paste suitable for landfill disposal. This process, termed as paste encapsulation technology, has not achieved the same level of success in immobilizing halides as achieved in sequestering heavy metals, because of the high solubility and elevated concentrations of halides in the brines. Hypersaline brines like flue gas desulfurization wastewater concentrate are also difficult to treat because of high halide concentration and currently, there are a lack of cost-effective and energy-efficient technologies for removal of halides. This research explored the feasibility of removing halides from high-salinity brines through the precipitation of layered double hydroxides. Parameters that affect the reduction of concentration of halides including, the initial molar concentration of halides, the calcium to aluminum ratio, and reaction temperature were evaluated. The stability of the layered double hydroxide products at various prevailing solutions and environmental conditions were investigated and an optimal calcium to aluminum ratio for the effective removal of the halides were established. Experimental results confirmed that the initial halide concentration, rather than the calcium to aluminum ratio, highly influenced the mass of halides sequestered in the structure of the layered double hydroxide samples. It was also observed that the layered double hydroxide samples undergo congruent dissolution when mixed in water or solutions at basic pH, while dissolution rate of all the samples in acidic solution was high with little residues remaining. Based on the analyses from characterization using various instrumentation and also the statistical analyses, it could be hypothesized that the synthesis of layered double hydroxides using a calcium to aluminum ratio of 3:1 will be optimal for effective removal of halides from hypersaline solutions. The optimal condition of Ca:Al ratio of 3:1 was employed to remove halides from simulated FGD wastewater brines. X-ray diffraction and thermogravimetric analysis showed that the structure of the precipitate after treatment of the simulated brine is similar to a layered double hydroxide, especially Friedel’s salt, with chloride ions mainly in the interlayers. The optimal conditions that were determined in this research will provide a framework to treat field samples such as an actual FGD wastewater concentrate. These optimal conditions can also be employed to treat other sources of wastewater with high salinity and TDS, including RO reject water from desalination, produced water from oil and gas fracking, and rejects from industrial cooling towers. In addition, the results suggest that this process is a simple and effective method to treat high-salinity brines. This research will lead to a better understanding of the formation of layered double hydroxides while contributing to the evaluation of variables that influence the long-term sequestration of halides in the cementitious systems like fly ash-brine paste.