Dual-mode Mn– and Sn–Silica composites from millet husk: Photoreduction of Cr(VI) and sulphanilamide, adsorptive capture of basic fuchsin

Biogenic silica extracted from foxtail millet husk was used as a sustainable matrix for preparing Mn–SiO 2 and Sn–SiO 2 composites with pollutant-specific functionality. Mn incorporation narrows the band gap to ∼3.1 eV and anchors Mn 2+ within the amorphous network, enabling efficient UV-C-driven photoreduction of Cr(VI) and partial oxidative degradation of sulphanilamide. Sn remains surface-localised, generating Lewis-acidic sites that promote chemisorptive uptake of Basic Fuchsin, achieving 94.5% removal at near-neutral pH. Together, these materials address three pollutant classes, heavy metals, antibiotics and cationic dyes, through orthogonal pathways of redox catalysis (Mn–SiO 2 ) and adsorption (Sn–SiO 2 ), without relying on crystalline order or complex synthesis. Structural and spectroscopic analyses (BET, XRD, FTIR, XPS, DRS) confirm distinct bonding environments and functional divergence between the two dopants. Kinetic modelling shows pseudo-first-order behaviour for Mn-assisted Cr(VI) reduction and pseudo-second-order adsorption for Sn-assisted dye removal. Although competitive-ion effects, reusability and intermediate toxicity were not evaluated, the results establish a proof-of-concept for designing multifunctional, low-cost composites from agricultural waste. This approach highlights the potential for scalable, application-specific remediation using rural biomass as a silica progenitor. • Biogenic silica from foxtail millet husk was used to synthesize Mn– and Sn–doped composites without templates or high-complexity processing. • Mn–SiO2 shows band-gap narrowing (∼3.1 eV) and efficient Cr(VI) photoreduction with partial sulphanilamide degradation under UV-C irradiation. • Sn–SiO2 exhibits strong adsorption capacity, removing 94.5% Basic Fuchsin at near-neutral pH via Lewis-acidic surface sites. • Distinct functionalities arise from dopant identity, with Mn acting as a redox mediator and Sn as a chemisorptive adsorbent. • Kinetic analyses confirm pseudo-first-order Cr(VI) reduction (Mn) and pseudo-second-order adsorption (Sn). • A simple, low-cost platform demonstrates the feasibility of tuning pollutant selectivity using rural biomass-derived silica .