Environmental application of millimetre-scale sponge iron (s-Fe⁰) particles (III): The effect of surface silver

Highlights: • Direct reductive deposition reaction achieves surfaced decoration of s-Fe⁰ particles. • Ag⁰-s-Fe⁰ displays similar removal efficiency of PCP as compared to bimetal of nZVI. • Ag⁰-s-Fe⁰ can be utilized under mild reaction condition compared to bimetal of nZVI. • The catalytic mechanism over Ag⁰-s-Fe⁰ under US condition is elucidated. - Abstract: To enhance the dechlorination reactivity of millimetric sponge iron (s-Fe⁰), a facile one-pot method was used to decorate s-Fe⁰ with Ag⁺ ions under ambient conditions. The results recorded by X-ray diffraction patterns, X-ray photoelectron spectra and high-resolution transmission electron microscopy demonstrated that the growth of Ag⁰ was dominated primarily by (1 1 1) plane with a mean length of ∼20 nm. The roles of Ag⁰ loading, catalyst dosage, particle size, initial pH and contaminant concentration were assessed during the removal of pentachlorophenol (PCP). Catalyst recyclability was also studied. The results revealed that 3–5 mm s-Fe⁰ particles with 5 wt% Ag⁰ loading exhibited the best performance with a dose of 3.0 g per 60 mL PCP solution. In addition, the dechlorination of PCP followed two-step, pseudo-first-order reaction kinetics, and Ag⁰-s-Fe⁰ was advantageous compared with bimetals of nanoscale zero-valent iron, iron power and iron flakes. The dechlorination mechanism of PCP over Ag⁰-s-Fe⁰ was attributed to the surface Ag⁰ decoration, which catalyzed the formation of reactive hydrogen atoms for indirect reaction, and the direct electron transfer via Fe–Ag⁰ galvanic cells for direct reaction. This suggests that Ag-based bimetals of s-Fe⁰ have great potential in the pretreatment of organic halogen compounds in aqueous solution.