Influence of particle size of zero-valent iron and dissolved silica on the reactivity of activated persulfate for degradation of acid orange 7

• Different ZVI types exhibited respective level of ability to activate persulfate with an order of nano-ZVI > micro-ZVI > milli-ZVI.
• The presence of SiO2 decreased the AO7 degradation ratio and prolonged the persulfate half-life.
• The iron oxides of α-Fe2O3, Fe3O4, α-FeOOH and Fe8O8(OH)6SO4 was found on the ZVI surface after reaction.
• The complexes of ≡FeH3SiO4 or ≡FeH2SiO4- were formed on the surface of nano-ZVI in the presence of SiO2.

The environmental impact caused by azo dyes such as acid orange 7 (AO7) is currently a major concern, mainly due to their toxicity, potential carcinogenic nature and difficult degradation. This research described the heterogeneous reactions of persulfate with milli-ZVI (1 mm), micro-ZVI (150 μm) and nano-ZVI (50 nm) under various experimental conditions. The results demonstrated that the combination of persulfate with ZVI was efficient in promoting the degradation of AO7 with the pseudo first-order and second-order kinetics reaction, and different ZVI types exhibited respective level of ability to activate persulfate to degrade acid orange 7, which was ranged as nano-ZVI > micro-ZVI > milli-ZVI. However, the presence of dissolved SiO2 decreased the AO7 degradation ratio and prolonged the persulfate half-life. The iron corrosion products coating on the ZVI after reaction was comprised of α-Fe2O3, Fe3O4 and α-FeOOH with some Fe8O8(OH)6SO4, which was identified by Raman spectroscopy, X-ray diffraction (XRD) and Fourier transform infrared (FTIR). With the presence of SiO2, surface complexes of ≡FeH3SiO4 or ≡FeH2SiO4- for binding of silicates were formed. These observations indicated that the dissolved SiO2 in the ZVI activated persulfate system affected the corrosion of iron, altered the surface area with the adsorption of SiO2, and appeared to influence the decomposition of persulfate and AO7.