Fatty acid double layer coated iron-based nanoparticles with stable mobility and selective reactivity to hydrophobic contaminants in subsurface

Abstract

Injection of reactive iron-based nanoparticles (Fe NPs) to the DNAP sources in the subsurface is one of the emerging efficient technologies as an alternative to predominant treatment technologies. The small size of reactive nanomaterials (e.g., 10 ~ 100 nm) can offer the potential of being derivable in the contaminated subsurface, and thus, potentially eliminate the need for excavation. However, due to their high surface energy, Fe NPs tend to rapidly agglomerate in water forming micro to millimeter scale aggregates or flocs, which weaken their mobility in groundwater. Moreover, commonly encountered groundwater solutes such as anions, cations, and dissolved organic matters can affect the reactivity of Fe NPs negatively. In this study, fatty acid double layer mimic lipid bilayer, constituent of cell membrane, was used as a stabilizer of Fe NPs to achieve stable mobility and selective reactivity toward chlorinated hydrocarbons and the method for coating the fatty acid double layer onto the surface of Fe NP was developed. Fatty acid double layer coated Fe NPs (PO-OFe) were prepared through two sequential steps: 1) primary hydrophobic layer preparation via thermal decomposition method with oleic acid

2) secondary hydrophilic layer by potassium oleate. The mas of coated fatty acid was approximately 50% of total particle mass and zeta potential of PO-OFe has reached the great extent of – 115 mV, indicated that aqueous stability and superior mobility of PO-OFe can be obtained by electrostatic and steric repulsive forces induced from the fatty acid double layer. PO-OFe remained stable in aqueous phase in sedimentation test although unmodified Fe NPs were deposited on the bottom of the test cell only in 2 h, indicating fatty acid double layer prohibit nanoparticle from agglomeration. Consistent with the sedimentation teste results, no deposition was detected in 1-D column tests, the mobility of which is even better than that of CMC-Fe which are the modified Fe NPs by one of the mostly used modification for Fe NP stabilization in remediation application. Due to non-polar zone in fatty acid double layer onto the surface of PO-OFe, hydrophilic dissolved groundwater solutes as well as a reducible contaminant (nitrate) rarely affected the reactivity of PO-OFe toward carbon tetrachloride, a model chlorinated solvent, at an insignificant level, suggesting that PO-OFe have selective reactivity. Nitrate, that is well known as a reducible hydrophilic contaminant by zero valent iron, was not transformed by the contact with PO-OFe and the reaction rate of PO-OFe was slower than those of unmodified or CMC modified Fe NPs. These are attributable to the presence of non-polar zone in fatty acid double layer that can block hydrophilic molecules from permeation through the layer and require hydrophobic contaminants to be diffused through the layer onto the surface of Fe NPs. The selective reactivity will keep the fatty acid double layer coated iron nanoparticles reactive. Therefore, the greater longevity of PO-OFe can be expected especially in the geochemical condition of high concentration of groundwater solutes. Remedial action using the reactants with the longevity and great mobility can offer the chance to clean-up the site where as-used technologies cannot be economically applied. Fatty acid double layer coated Fe NPs may be a suitable remediation alternative for these sites.