Moderate activation and mutual promotion of aluminum-iron covalent-bond coagulant with K2FeO4 for enhanced organic arsenic removal: Selective truncation-repolymerization of Fe(Ⅳ)/Fe(Ⅴ) and enhanced coagulation.

Ferrate (K2FeO4) enhanced organic arsenic removal when combined with Al/Fe coagulants. However, coagulants induced K2FeO4 activation was often suboptimal (too fast/slow), and hydrolysis products underutilized complexation sites. Optimizing this synergy was key for efficient organic arsenic oxidation and real-time capture of products, but its moderate activation mechanism remained unclear. This study explored moderate activation mechanism of covalent-bond coagulants (better stability and bridging than ionic Al/Fe salts) with K2FeO4 for enhanced organic arsenic removal. The results demonstrated that covalent-bonded aluminum (CSA), covalent-bonded iron (CSF), and covalent-bonded aluminum-iron coagulants (CSAF) coupled with K2FeO4 significantly improved organic arsenic removal. Compared to CSA (k = 865.55 M-1s-1, 36.46 %) and CSF (k = 474.91 M-1s-1, 29.62 %), CSAF 8:2 (Al:Fe = 8:2) exhibited distinct moderate mutual activation with K2FeO4, which could enhance the removal efficiency of organic arsenic at a relatively moderate reaction rate (k = 503.84 M-1s-1) and improved the utilization efficiency of tetravalent iron/pentavalent iron Fe(IV)/Fe(V). K2FeO4 enhanced coagulation by delaying the polymerization process of coagulant hydrolysis products, promoting the formation of monomeric polymers with more active groups. The moderate activation and mutual promotion mechanism was that the selective truncation and re-polymerization of Fe(IV) and Fe(V) from K2FeO4 with the hydrolysis products (Alhs and Fehs) of the covalent-bond aluminum and iron coagulants. CSAF improved CSF's rapid aggregation and provided more Alhs/Fehs sites to complex Fe(Ⅳ)/Fe(Ⅴ). Fe(Ⅴ) substituted Fe(Ⅳ) in Fehs-Fe(Ⅳ), achieving moderate K2FeO4 activation. The Fehs-Fe(Ⅴ) complex delayed Fe(Ⅴ) decay while exposing active groups for enhanced coagulation. This study innovatively elucidated the aluminum-iron covalent-bond coagulant moderate activation mechanisms from a coordination chemistry perspective.