An effective cascade strategy over a Sn-enriched phosphate material: Upcycling a reductive adsorbent into an environmental catalyst

A calcium phosphate-based material enriched with Sn (8.3 at.%, as revealed by X-ray photoelectron spectroscopy) was successfully used in the reductive adsorption of Cr(VI) to Cr(III) for water remediation and, after use, repurposed into a catalyst for NO oxidation into NO2. The use of multiple characterization techniques (Mӧssbauer spectroscopy, synchrotron X-ray diffraction, TEM, and HAADF-STEM/EDS) provided information on Sn-speciation (Sn(II)/Sn(IV) = 1.2), structural composition, and morphological features. Cassiterite (rutile-type SnO2) and hydroromarchite (tin oxide-hydroxide Sn6O4(OH)4) crystalline phases characterized the material, along with the presence of an amorphous Ca3(PO4)2 phase. Notably, about 15 mgCr(VI)∙g−1 could be effectively reduced over a broad pH range (2–11), even in the simultaneous presence of various anions and cations, with concurrent adsorption of the resulting Cr(III), thanks to the adsorption properties of the phosphate phase. The kinetic constant of Cr(VI) removal was as high as 0.0058 g∙mg−1∙min−1 and increased in the presence of a pool of cations (0.0147 g∙mg−1∙min−1). The catalytic performances of the adsorbent after use in Cr(VI) reductive adsorption were studied in the 150–450 °C interval at 15,000–80,000 h−1 space velocity. About 90 % of NO conversion at 350 °C was observed. This study highlights the adaptability and efficiency of the developed material across different environmental remediation processes.