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

• A Sn(II) phosphate adsorbent ensured Cr(VI) reductive-adsorption up to 15 mg·g −1. • Adsorbent characterization revealed Sn(II) presence mainly as hydroromarchite phase. • Adsorption mechanism implied reduction, electrostatic interactions and coordination. • Upcycling of the spent adsorbent into catalyst promoted 90% NO conversion to NO 2. 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 NO 2 . 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 SnO 2 ) and hydroromarchite (tin oxide-hydroxide Sn 6 O 4 (OH) 4 ) crystalline phases characterized the material, along with the presence of an amorphous Ca 3 (PO 4 ) 2 phase. Notably, about 15 mg Cr(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.