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

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 SiO 2 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-Fe 2 O 3 , Fe 3 O 4 and α-FeOOH with some Fe 8 O 8 (OH) 6 SO 4 , which was identified by Raman spectroscopy, X-ray diffraction (XRD) and Fourier transform infrared (FTIR). With the presence of SiO 2 , surface complexes of ≡FeH 3 SiO 4 or ≡ Fe H 2 SiO 4 - $\equiv \text{Fe}{\text{H}}_2{\text{SiO}}_4^{-}$ for binding of silicates were formed. These observations indicated that the dissolved SiO 2 in the ZVI activated persulfate system affected the corrosion of iron, altered the surface area with the adsorption of SiO 2 , and appeared to influence the decomposition of persulfate and AO7.