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.