Antimony (Sb) Recycling from Wastewater to Supercapacitor Electrode Using Ordered Porous Amidoxime-Polyacrylonitrile: Chemical Evolution and Transformation Mechanism

Exhausted adsorbents from Sb-laden wastewater are hazardous and generally landfilled. This study presents that exhausted Sb adsorbents can be thermally reduced and recycled as supercapacitor electrode. Specifically, ordered hierarchical porous amidoxime-polyacrylonitrile (OHPAPAN) is synthesized for highly efficient Sb(V) ions adsorption and then thermal conversion of exhausted OHPAPAN to Sb-contained char, rendering excellent electrochemical activity. Sb 0 atom clusters are found to form during recycling process by X-ray absorption spectroscopy (XAFS), along with Sb oxide with average valence of +2.63. The 4d and 4p orbital of Sb cluster exhibits abundant electron state in Fermi level and valence band, making a pseudocapacitive contribution. Moreover, ordered hierarchical pore is confirmed to be inherited in the char from OHPAPAN, further enhancing electrical double layer capacitance. Coordination configuration of Sb and N/O becomes more stable from sexadentate (coordinate atom distance of 1.98 and 2.66 Å) in OHPAPAN to ≈2 N/O ligands in Sb-contained char (1.97 Å), underlining the shorter atom distance and higher orbital energy for Sb immobilization. Consequently, the capacitance maintains 96.1% after 6000 cycles, indicating that its excellent stability can rival relevant commercial electrode. This study offers a novel insight that Sb mineral sources can be derived from industrial wastewater, not just mining activity.