Silica-coating-mediated depassivation of nanosized zero-valent iron by Fe(II) at circumneutral pH

Surface passivation of materials based on nanosized zero-valent iron (nZVI) limits the reductive reactivities of such materials at near-neutral pH. To overcome this, a reactivation strategy was developed involving adding aqueous Fe(II) to a silica-coated nZVI (nZVI@SiO₂) system, which markedly increased the nitrobenzene reduction efficiency. The Fe(0) core in nZVI@SiO₂ acted as the main reductant, and Fe(II) sorbed to the SiO₂ coating acted as a reactivating agent that facilitated electron conduction mainly through the 'chemi-conductor' mechanism. The SiO₂ coating had several properties that promoted sorption and was essential for facilitating Fe(II) sorption and therefore increasing the reactivation efficiency. The [Si]/[Fe] ratio was found to be a critical factor for regulating the thickness of the SiO₂ layer and therefore strongly influenced the reactivity of the material. An optimal [Si]/[Fe] ratio of 0.2 was proposed because it gave a complete but thin SiO₂ coating layer that promoted Fe(II) sorption while preventing inhibition of electron conduction and nitrobenzene diffusion. The Fe(II)-induced reactivation efficiency positively correlated with the extent of Fe(II) coverage on the nZVI@SiO₂ surfaces. The results indicated that nZVI@SiO₂ is extremely compatible with the Fe(II) reactivation process to achieve reductive degradation at near-neutral pH.