Effective Regeneration of Flue Gas Wastewater by One-Step Hydrothermal Removal of High Concentrations of Nitrate and Sulfate

Wet flue-gas purification in non-ferrous smelting facilities consumes large volumes of fresh water and generates wastewater rich in sulfate and nitrate. On-site recirculation of this wastewater is currently limited by progressive nitrate accumulation, because no conventional single-step technology can simultaneously remove sulfate and nitrate; the available routes are separate, energy- or reagent-intensive steps that either transfer nitrate into hazardous mixed salts or generate secondary sludge. The raw wastewater investigated here contained 53.3–57.5 g/L nitrate, 10.5–12.5 g/L sulfate, approximately 1.35 g/L TOC, and trace Cr, Cu, and Al. Herein, we report a one-step hydrothermal process in which an organic iron reagent, ferric citrate, simultaneously removes sulfate and nitrate from flue-gas wastewater at a moderate temperature of 120 °C—a temperature attainable with low-grade waste heat that is otherwise unrecovered on site. The advancement of knowledge lies in the underlying chemistry: thermally activated electron transfer from the citrate ligand to Fe3+ generates Fe2+, which catalytically reduces nitrate (>99% removal), while Fe3+ concurrently precipitates sulfate (and co-removes Cr and Al) as schwertmannite, an iron oxyhydroxysulfate that itself becomes a potentially recyclable iron resource. Subsequent neutralization with Ca(OH)2 further reduced residual sulfate to 4.1 g/L, making the water suitable for return to the flue-gas purification loop after optional polishing. For plant operators and decision-makers, the implication is that the current evaporation-to-hazardous-salt practice can, in principle, be replaced by a closed water cycle driven by the plant’s own waste heat, thereby reducing freshwater withdrawal, avoiding the disposal of a nitrate-bearing mixed salt, and recovering an iron-rich by-product.