Tea saponin co-ball milled commercial micro zero-valent iron for boosting Cr(VI) removal

Tea saponins (TS), a natural biosurfactant extracted from tea trees, were co-ball milled with commercial micro zero-valent iron (mZVI) to produce TS modified mZVI (TS-BZVI) for efficient hexavalent chromium (Cr(VI)) removal. The findings demonstrated that TS-BZVI could nearly remove 100% of Cr(VI) within 2 h, which was 1.43 times higher than that by ball milled mZVI (BZVI) (70%). Kinetics analysis demonstrated a high degree of compatibility with the pseudo-second-order (PSO), revealing that TS-BZVI exhibited a 2.83 times faster Cr(VI) removal rate involved primarily chemisorption. Further, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) measurements indicated that the TS co-ball milling process improved the exposure of Fe(II) and Fe(0) on mZVI, which further promoted the Cr(VI) reduction process. Impressively, the introduction of TS increased the hydrophobicity of ZVI, effectively inhibiting the H2 evolution by 95%, thus improved electron selectivity for efficient Cr(VI) removal. Ultimately, after operating for 10 days, a simulated permeable reactive barrier (PRB) column experiment revealed that TS-BZVI had a higher Cr(VI) elimination efficiency than BZVI, indicating that TS-BZVI was promising for practical environment remediation.