The influence of crystal structure and formation path of precursor on phosphate adsorption during oxidation-hydrolysis phase transition of siderite

Fe(III)-(hydr)oxides can be formed by the oxidation-hydrolysis of Fe(II) minerals, its initial state is usually amorphous, which can form dense flocs quickly with a high specific surface area, resulting in widely used in the field of water treatment. However, the rate and path of oxidation-hydrolysis lead to the difference in the crystal structure of the precursor, which are directly determined by the crystallinity and adsorption activity of the final oxidized hydrolysate. Therefore, this study investigated the phase transition of siderite under different oxidation-hydrolysis paths. The results suggested KMnO4 could first oxidize the surface layer Fe(II) of siderite; and then Mn(II), hidden in the crystal lattice of siderite, was continuously exposed to the surface of siderite; after that, Mn(II) was oxidized by KMnO4 to form MnO2, which acts as an ion channel to allow internal Fe(II) of siderite further hydrolysis to form crystalline Fe(OH)2 and then further oxidation to form crystalline two-line ferrihydrite (δ-Keggin). Although Fe(OH)2 as transient precursor will disappear with the continuation of oxidation, its presence will greatly reduce the nucleation barrier of two-line ferrihydrite. These mineral phase transitions resulted in the low concentration of KMnO4 (0.03 mmol/L) could substantially enhance the ability of siderite to remove phosphate, with the maximum adsorption capacity (13.04 mg/g, Langmuir). However, H2O2 could only oxidize Fe(II) on the surface of siderite to form amorphous Fe(OH)3, while Mn(II) in the siderite lattice could not be oxidized. The surface coverage of amorphous Fe(OH)3 and exposed Mn(II) formed a dense passive film, resulting in the termination of the oxidation and showed a low adsorption activity (3.28 mg/g)