Mechanism and formation process of schwertmannite under electrochemical deposition

Schwertmannite has recently attracted increasing attention due to its important geochemical role influencing the fate of toxic elements in acidic waterbodies, as well as its material properties for adsorbing or catalytic degrading environmental pollutants. However, the existing methods are either time-consuming (>30 d) or the obtained products possess small special surface areas (4−14 m2/g). The properties of products synthesized even by the same method are often different due to deviation of mineral forming conditions. In this study, a new method was developed by providing electrochemical energy to promote schwertmannite formation in electrochemical system. The method designed was focused on relatively precise control in pH and the oxidation rate of Fe2+, that is, pH buffering and multi-potential cycling redox for controlling Fe2+ oxidation rate. Adding CH3COOH to FeSO4 electrolyte reduced the pH drop and produced a more homogeneous schwertmannite mineral phase. Precipitative efficiency of 27.0 % for total Fe and 5.4 % for SO42− were obtained under multi-potential step (MPS) mode in 35 °C electrolyte. A sea urchin-like morphology schwertmnanite mineral with relatively bigger Fe/S ratio (5.3−6.2) and higher special surface area (BET of 104−178 m2/g) was synthesized in a relatively shorter time (electrolysis 6 h + aging 24 h) by the electrodeposition method.