Effective Separation and Recovery of Manganese and Potassium from Biomass Ash by Solvent Extraction

Manganese (Mn) is considered an important, energy-critical metal due to its leading role in the production of electrochemical energy storage devices. One valuable source of Mn is hyperaccumulator plants used for the phytoremediation of contaminated soil. In this study, stems and leaves of ginger (Zingiber officinale), which accumulate Mn at moderate levels (∼0.2 wt %) and potassium (K) at high levels (>5 wt %), were analyzed to assess the potential of recovering metals from this plant. The extraction behaviors of Mn and K were studied using raw and ash samples (100-600 °C). It was crucial to set an appropriate incineration temperature (300 °C) to selectively extract K (∼96%) and Mn (∼90%) using water and nitric acid over two consecutive steps. Additionally, citric acid, a cost-effective and environmentally friendly solvent, was just as effective (∼85%) as nitric acid in extracting Mn. X-ray absorbance near-edge spectroscopy and X-ray diffraction analysis of the ash before and after extractions were applied to elucidate the extraction mechanism. The results revealed that selective extraction of both compounds was possible due to the change in the oxidative state of Mn(II) (soluble in water) into Mn(III) and Mn(IV) (insoluble in water) during sample incineration. Simultaneously, there were complex reactions associated with the changes within potassium carbonate compounds; however, these did not affect the K extraction efficiency.