Highly efficient and selective nitrate and Hg(II) removal from wet oxidation flue gas purification wastewater using bifunctional MXene nanofiltration membrane

The wet oxidation process is a promising method for simultaneously removing SO₂, NO, and Hg⁰ from coal-fired flue gas. However, the resource utilization of the resulting wastewater, which is enriched with complex ions such as Hg²⁺, NH₄⁺, NO₃⁻, and SO₄²⁻, remains a significant challenge. To achieve the simultaneous goals of concentrating NH₄⁺/NO₃⁻/SO₄²⁻ ions, adsorbing and separating Hg²⁺ ions, and reusing wastewater, a bifunctional MXene composite nanofiltration membrane with large interlayer spacing and excellent electrostatic repulsion was developed. The membrane was prepared by co-depositing MXene nanosheets, carboxylated multi-walled carbon nanotubes (MWCNTs), and sodium dodecyl sulfate (SDS) onto an NF-90 membrane. The resulting MXene-MWCNTs-SDS (MMS-COOH) composite membrane demonstrated exceptional salt rejection of 84.4 % for NO₃⁻, 89.0 % for NH₄⁺, 99.8 % for SO₄²⁻, and 99.7 % for Hg²⁺. After 30 cycles, the membrane structure remained stable, maintaining good reusability, with NH₄⁺ and SO₄²⁻ rejection still reaching 90.0 %. Moreover, the maximum adsorption capacity for Hg²⁺ was 2869.6 mg g⁻¹, while the capacities for Cr⁶⁺ and Pb²⁺ were 577.6 and 316.8 mg g⁻¹, respectively. The MMS-COOH composite membrane introduces an innovative method for the benign treatment of wet oxidation flue gas purification wastewater and facilitates the recovery of (NH₄)₂SO₄ and NH₄NO₃ as fertilizers, holding a promising application prospect.