Aluminum chloride induced ferrate activation for p-arsanilic acid removal: The significant synergism mechanism and enhanced role of reactive Fe(IV)/Fe(V)

The practical applications of potassium ferrate (K₂FeO₄) are limited due to self-decomposition at nearly neutral pH and low removal efficiency for electron deficient pollutants. That is, K₂FeO₄ was often insufficient to produce sufficient oxidative material, so the lack of oxidative reactivity remained a pressing problem. This study proposed an effective induction of K₂FeO₄ activation by aluminum chloride (AlCl₃) in a novel highly concentrated simultaneous oxidation and coagulation (HOC) system of K₂FeO₄/AlCl₃. It innovatively focused on the interaction between K₂FeO₄ and AlCl₃ for the removal of p-arsanilic acid (ASA) in a HOC system. The HOC system exhibited higher ASA removal (62.6%) in a shorter period of time compared to K₂FeO₄ oxidation (28.0%) and pre-oxidation-coagulation (POC) system (51.6%) with the addition of 1.5 mgAl/L of AlCl₃ at pH 7. AlCl₃ could induce the self-decomposition of K₂FeO₄, and more importantly, it could facilitated the conversion of Fe(VI) into the more reactive Fe(V)/Fe(IV) species for ASA removal. Moreover, AlCl₃ hydrolysis promoted the in-situ H₂O₂ generation from K₂FeO₄ decomposition, enhancing Fenton-like reaction and increasing [rad]OH and [rad]O₂^– for ASA removal. K₂FeO₄ greatly influenced AlCl₃ hydrolysis and promoted rapid transformation of small polyaluminum Al_a and medium polyaluminum Al_b to colloidal high polyaluminum Al_c. The content of Fe elements in the flocs produced by the HOC-ASA floc system was 19.27%, which was much higher than the 4.58% of the POC-ASA system, which enhanced the interfacial reaction and electron transfer and facilitated the removal of ASA.