Extreme chromium transformation and immobilization via ferrous coupling surfactant-enhanced nanofiltration process

The efficient reduction and immobilization of highly toxic and migratory Cr(VI) present significant challenges in chromium-containing wastewater. This research demonstrates that the Fe(II)-coupled micelle-enhanced nanofiltration (MENF) system facilitated the rapid conversion and removal of Cr(VI). The co-complexation of cetyltrimethylammonium chloride (CTAC) and Fe(II) improved the molecular-level coordination and reduction capacity of Cr(VI), leading to its effective transformation to Cr(III) and a 41 % removal rate of Cr. The interaction between CTAC, Cr, and Fe generated larger aggregates, such as CTAC-Cr, (Fe_x, Cr_1−x)(OH)₃, and CTAC-Cr-Fe, which enhanced Cr immobilization. Moreover, during the Fe(II)-coupled MENF process, a large amount of CTAC-Cr(III)-Fe(II, III) precipitates were removed during the complexation phase, while a small portion of the complexes adhered to the membrane surface, forming a porous cake layer. Due to the increased hydrodynamic diameter of the aforementioned Cr complexes, they could be easily retained by the nanofiltration membrane. Even under interference from high concentrations of anions and humic acids, the Fe(II)-coupled MENF system could still completely remove Cr(VI). This investigation contributes to a holistic understanding of the utilization of Fe(II)-coupled MENF systems and proposes effective sustainable strategies for Cr(VI) removal.