In-situ formation of green rust during Fe(II) coagulation: Dual reductive and adsorptive pathways for dyeing wastewater treatment

Green rust (GR), a reductive iron-based mineral pivotal in soil and groundwater remediation, remains unexplored for wastewater treatment. This study reveals that high-pH-Fe(II) coagulation enables in-situ generation of stabilized GR, thereby enhancing the coagulation efficiency (> 90 % removal for diverse dyes). The formation mechanism includes three sequential stages: Initially, alkaline conditions facilitate abundant Fe(OH)₂; Subsequently, partial oxidation of Fe(II) by oxidants produces anoxic zones, where Fe(II)/Fe(III) co-precipitation initiates GR crystallization; Simultaneously, multivalent anions intercalate into GR interlayers, further enhancing GR stability through strong electrostatic interactions. GR-enhanced-Fe(II) coagulation demonstrates dual dyes removal mechanisms: direct reductive degradation of electrophilic moieties (e.g., azo bond) and adsorption-coprecipitation of macromolecules. Based on the mechanism insights, it is found that GR-enhanced-Fe(II) coagulation achieves unparalleled removal of both anionic and cationic dyes (nearly 100 % decolorization for 50 mg/L dye solutions) under anoxic conditions. HPLC-MS and DFT confirm GR-driven reductive cleavage of Reactive Red 2 into low-molecular-weight byproducts. Pilot and full-scale trials at a dyeing wastewater treatment plant concluded that GR-enhanced-Fe(II) coagulation achieves 53.5 % COD removal, outperforming conventional Fe(II) processes (46.5 %) under actual hydraulic and pollutant load fluctuations. This work expands coagulation theory and offers a cost-effective strategy for wastewater treatment, adaptable to diverse effluents while retaining operational simplicity.