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

Xian Zhao; Jia-Yi Wang; Yu-Qian Wang; Chen-Hao Cui; Xin Yan; Wei Dai; An-Ran Zhang; Zhen-Ying Zhang; Ding-Ding Tang; Yan Zhou; Qiong Qin; Ji-Wei Pang; Lu-Yan Zhang; Nan-Qi Ren; Jie Ding; Shan-Shan Yang

doi:10.1016/j.watres.2025.124174

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

Water Res. 2025 Jul 6:285:124174. doi: 10.1016/j.watres.2025.124174. Online ahead of print.

Authors

Xian Zhao¹, Jia-Yi Wang¹, Yu-Qian Wang¹, Chen-Hao Cui¹, Xin Yan¹, Wei Dai¹, An-Ran Zhang¹, Zhen-Ying Zhang¹, Ding-Ding Tang², Yan Zhou², Qiong Qin², Ji-Wei Pang³, Lu-Yan Zhang⁴, Nan-Qi Ren¹, Jie Ding¹, Shan-Shan Yang⁵

Affiliations

¹ State Key Laboratory of Urban-rural Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China.
² China construction third bureau green industry investment Co., Ltd, Wuhan, 430056, China.
³ Harbin Corner Science & Technology Inc., Harbin 150023, China.
⁴ School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng, 224051, China.
⁵ State Key Laboratory of Urban-rural Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China. Electronic address: shanshanyang@hit.edu.cn.

PMID: 40644846
DOI: 10.1016/j.watres.2025.124174

Abstract

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.

Keywords: Adsorption; Dyeing wastewater; Fe(II) coagulation; Green rust; Pilot-scale; Reductive degradation.