Calcium hydroxide coating on highly reactive nanoscale zero-valent iron for in situ remediation application

Nano scale zero-valent iron (nZVI), a promising engineering technology for in situ remediation, has been greatly limited by quick self-corrosion and low mobility in porous media. Highly reactive nZVI particles produced from the borohydride reduction method were enclosed in a releasable Ca(OH)₂ layer by the chemical deposition method. The amount of Ca(OH)₂ coated on nZVI surface were well controlled by the precursor dosage. At moderate Ca(OH)₂ dosage (R_Ca/TFe = 0.25) condition, the increment of Fe⁰ content for the obtained nZVI/Ca-0.25 sample was observed. The interfacial reactions between the iron oxide shell and the Ca(OH)₂ saturated environment were delicately elucidated by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) spectrum. And the coverage of Ca(OH)₂ shell on spherical nZVI surface was found more complete and uniform for the nZVI/Ca sample obtained from the moderate precursor dosage condition (R_Ca/TFe = 0.25). The Ca(OH)₂ shell before dissolution was demonstrated owning the anti-corrosion capability to slow down the oxidation of Fe⁰ core in air, during ethanol storage and in aqueous environment. The mechanism of anti-corrosion capability for nZVI/Ca-0.25 particle was interestingly found to be attributed to the Ca(OH)₂ shell isolation and also be potentially due to the iron oxide shell phase transformation mediated by the outer Ca(OH)₂ shell. An improved trichloroethylene reduction performance was observed for nZVI/Ca-0.25 than bare nZVI. The mobility of nZVI/Ca particles in water-saturated porous media was moderately improved before shell dissolution.