Selective Manganese Precipitation via Neutralization and Ozone Oxidation under pH Conditions Similar to Steel Pickling Wastewater: Thermodynamic Assessment and Experimental XANES Evaluation

Steel pickling wastewater contains valuable iron. Nonetheless, coexisting elements such as Mn need to be separated before Fe recovery. This work studies Mn precipitation phenomena under a pH resembling steel pickling wastewater and compares it to that of Fe under the same conditions. A neutralization–oxidation approach was studied, whereby either NaOH or NH₃ were used as neutralizers and O₃ was the oxidizer. A thermodynamic assessment indicated that NaOH is more effective than NH₃ for precipitation because Mn can react freely with O₃ after NaOH addition, whereas NH₃ may react with O₃ instead. Experimental data showed that neutralization followed by oxidation results in the formation of different Mn oxides, with NaOH confirmed as the most effective neutralizer. Moreover, XRD and XANES analyses showed that the Mn oxidation state in the solids depends on the neutralizer used. Conversely, Fe precipitation was thermodynamically and experimentally observed to depend entirely on pH, with NaOH being a better neutralizer than NH₃, and pH = 1.5 being the maximum pH where Fe remains dissolved. These insights suggest that using a neutralization–oxidation method that increases the oxidation potential high enough for Mn oxidation while keeping the pH low enough for Fe to remain dissolved could be an effective approach for the selective precipitation of Mn from steel pickling wastewater.