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 (RCa/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 (RCa/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.