Use of Nanostructured Layered Double Hydroxides as Nanofilters in the Removal of Fe2+ and Ca2+ Ions from Oil Wells

Abstract

Four new metal-aluminum layered double hydroxides (LDHs), Mg-Al(OH)2PO4 (1), Mg-Al(OH)2PO4PF6 (2), Ca-Al(OH)2SO4 (3), and Ca-Al(OH)2PO4PF6 (4), were prepared by the coprecipitation method followed by mild hydrothermal processing at 60°C. Mg2+ and Ca2+ in solution with Al3+ were titrated with NaOH over 3–5 h to yield Mg-Al and Ca-Al layered double hydroxides, respectively, incorporating PO43−, PO43−PF6−, and SO42− anions in the interlamellar spaces. The isolated compounds were characterized with the help of XRD, IR, and SEM/EDAX, and their ability to remove scale-forming ions from the aqueous system was studied with the help of atomic absorption spectroscopy (AAS). The SEM micrographs of Mg-O-Al-OH and Ca-O-Al-OH layers intercalated with PO43− and/or [PO4PF6]4− anions are similar consisting of uniform nanospheres with an average size of 100 nm, while the M-O-Al-OH layer of compound 3, intercalated with SO42− anions, consists of hexagonal nanoplate crystals. In the infrared spectra, the characteristic absorption band for water molecules was observed in all the compounds. The XRD pattern showed that d012 and d104 peaks of M-Al-PO4 LDHs corresponding to interplanar spacing of 3.4804 and 2.5504 Å, respectively, shifted to higher 2θ values for the M-Al-PO4PF6 system, which indicates a decrease in the interlamellar spacing as PF6− was incorporated along with PO43− anion. The XRD pattern for Ca-Al-SO4 LDHs was quite different, showing the presence of low-angle peaks at 2θ = 11.68 and 14.72°. The results of the column adsorption studies showed that there was a significant removal of Ca2+ by all the compounds under investigation with an efficiency of 84–99%. However, compounds 1 and 2 remove Fe2+ effectively with the efficiency of 98.73 and 99.77%, respectively; compounds 3 and 4 were shown to have little or no effect.