Use of Nanostructured Layered Double Hydroxides as Nanofilters in the Removal of Fe 2 + and Ca 2 + Ions from Oil Wells

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 coprecipitationmethod followed by mild hydrothermal processing at 60°C. Mg and Ca in solution with Al were titrated with NaOH over 3–5 h to yield Mg-Al and Ca-Al layered double hydroxides, respectively, incorporating PO4, PO4PF6, and SO4 anions in the interlamellar spaces. (e 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). (e SEM micrographs of Mg-O-Al-OH and Ca-O-Al-OH layers intercalated with PO4 and/or [PO4PF6] 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 SO4 anions, consists of hexagonal nanoplate crystals. In the infrared spectra, the characteristic absorption band for water molecules was observed in all the compounds. (e 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 PO4 anion. (e XRD pattern for Ca-Al-SO4 LDHs was quite different, showing the presence of lowangle peaks at 2θ�11.68 and 14.72°. (e results of the column adsorption studies showed that there was a significant removal of Ca by all the compounds under investigation with an efficiency of 84–99%. However, compounds 1 and 2 remove Fe effectively with the efficiency of 98.73 and 99.77%, respectively; compounds 3 and 4 were shown to have little or no effect.


Introduction
Recently, it has been reported that as gas and oil production progresses, in many oil fields, the ratio of produced brine water to hydrocarbon often increases.
ese brines are corrosive and tend to produce calcite or sulphate scales [1,2].Scaling, which stems from supersaturation of mineral ions in the process fluid, is the deposition of a mineral salt on processing equipment.A natural gas well will, besides producing natural gas, also produce water and carbon dioxide (CO 2 ).
e produced water can come from two sources: water vapor in the gas that condenses into liquid water and formation water containing salts [3]. is water is the source of hydrate formation, and in combination with CO 2 , it forms a weak carbonic acid (H 2 CO 3 ), as shown in (1).
Scaling is caused by salts and can occur when the produced water contains formation water [4]: is carbonic acid will continue to dissociate hydrogen, creating new deprotonated species of carbonic acid, as seen in the following equation: In the water mixture, there will be a mixture of H 2 CO 3 , HCO 3 − , and CO 3 2− .Finally, in the presence of calcium and carbonic acid, calcium carbonate will precipitate out [5,6]: e composition of the formation water varies greatly with the reservoirs, but the usual constituents are Na + , Ca 2+ , K + , Mg 2+ , Fe 2+ , Cl , and HCO 3 − .Long pipelines are constructed of carbon steel [6].Carbonic acid will corrode the iron in the pipeline wall, producing iron carbonate.is iron carbonate can precipitate in the production fluid and follow the gas and liquid flow, causing problems downstream: e formation of scale blockage inside oil wells constricts the flow of geothermal fluids in these wells thus significantly reducing their output [7].Furthermore, fresh water and marine sediments are contaminated by oil spills along with urban runoffs and industrial and domestic effluents.is poses a serious concern as the presence of contaminants affects aquatic organisms.Inorganic nanomaterials with well-defined cavities and surfaces have great potential for remediating today's environmental and industrial problems [8].Nanostructured layered double hydroxides (LDHs) are promising materials in wastewater treatment due to their ability to capture organic and inorganic anions [9,10].Layered double hydroxides (LDHs) are a class of anionic clays with the structure based on brucite-(Mg(OH) 2 −) like layers [11][12][13][14][15].
e lattice structure of LDHs, with the general formula , has a positively charged brucite-shaped layers, consisting of a divalent metal ion M 2+ (e.g., Ca 2+ , Zn 2+ , Mg 2+ , and Ni 2+ ) octahedrally surrounded by six OH − hydroxyl groups [16][17][18].e substitution of the M 2+ metal with a trivalent M 3+ cation gives rise to the periodic repetition of positively charged sheets (lamellas) alternating with charge-counter balancing A n-ions.
During the course of our investigation of LDHs as a scale inhibitor, we were interested in using Mg and Ca as divalent metal ions to prepare Mg-Al-and Ca-Al layered double hydroxides intercalated with PO 4 3− , PO  (4).In this work, the synthesized LDH nanostructures are being investigated as nanofiltration materials to remove scale forming ions like Fe 2+ and Ca 2+ .e synthesis, characterization, and adsorption properties of these four new compounds have been reported.

Methods.
e standard procedure of coprecipitating a divalent with trivalent metal ions in the presence of a base was followed in the present studies [27][28][29][30].e coprecipitation was carried out at room temperature, and the gel was continuously and magnetically stirred.e mixture was kept under magnetic stirring for 3 h.e precipitate was heated in the mother liquor for 18 h at 60 °C, and then it was washed with distilled water and separated by centrifugation.
e resulting material was dried overnight at 70 °C in the incubator.For compound 1, orthophoshoric acid was used as the source of the PO 4 3− anion.For compounds 2 and 4, 1-ethyl-3-methylimidazolium hexafluorophosphate and orthophosphoric acid were used to introduce PF 6 − and PO 4 3− anions, respectively.For compound 3, the source of the SO 4 2− anion was H 2 SO 4 .In a typical synthesis of 1, a 250 mL solution containing Mg(NO 3 ) 2 (0.0375 mmol), Al (OH) 3 (0.0125 mmol) (with the Mg-Al ratio of 3 : 1), and H 3 PO 4 (0.5 mmol) was added dropwise from burette into a 15 mL solution of NaOH (2 M) under constant stirring at room temperature for 3 h.e mixture with a pH of 10 was heated at 60 °C for 18 h, and the resulting slurry was collected via centrifugation (10 min, 500 min −1 ).
e product was washed thrice with deionized water before drying.A similar procedure was used for compounds 2-4, the difference being in the metal and SO 4 2− ions for compounds 3 and 4.

Characterization.
e samples were analyzed by the Rigaku MiniFlex II X-ray diffractometer using monochromatic Cu κα radiation (λ � 0.1541 nm) at the speed of 3 s in 2θ range between 5 and 75 °and step size of 0.03 °.  e scanning electron microscopy (SEM) studies of the LDHs were made with a field emission electron microscope (FESEM JSM-6700 F), coupled with an energy dispersion 2 Advances in Materials Science and Engineering analyzer (EDX).e specimens were Au coated (sputtering) to make them conductive.e SEM acceleration voltage was 10 kV. e Fourier transform infrared (FTIR) spectra for the synthesized LDHs were recorded over the wave number range of 400-4000 cm −1 using the Perkin Elmer FTIR spectrometer.e powdered samples were mixed with KBr (in a 1 : 200 ratio of their weight) and pressed in the form of pellets for measurement.

Adsorption Column Experiments. Aqueous solutions of
Fe 2+ and Ca 2+ prepared from their salts FeSO 4 •7H 2 O and CaCl 2 , respectively, were analyzed by atomic absorption spectroscopy.e column was set up by packing 5 g of LDHs in 20 mL syringes, and the metal ion solution was poured through the packed samples as lter (Scheme 1).e metal ion solutions were collected at the outlet of the LDH column, and the eluded portion (i.e., the ltrate) was also analyzed by AAS.
e degree of surface covered (θ) by the ions was computed using the following equation: e e ectiveness of the LDHs as adsorbent was calculated with the help of the following equation: where K d is the partition coe cient, C 0 is the concentration of Fe 2+ or Ca 2+ ions in solution, and C a is the concentration of the ions adsorbed in the LDHs.
e removal e ciency of Fe 2+ and Ca 2+ by the LDHs in percentage was calculated using the following equation: where C 0 is the initial concentration of the ions and C a is the concentration of the ions adsorbed in LDHs.

Results and Discussion
All the metal-aluminum layered double hydroxides were prepared by the coprecipitation method followed by mild hydrothermal treatment at 60 2− (0.25/2) 0.125).Figure 1 shows the FTIR spectra of the isolated layered double hydroxides.e characteristic absorption band for the water molecules was observed in all the compounds.e band observed in the region 3402-3455 cm −1 corresponds to  e various assignments are in good agreement with similar compounds in the literature [29][30][31][32][33][34][35][36].
Figure 2 shows the representative XRD patterns of the M-Al-LDHs.e X-ray powder diffraction lines of Mg-Al (OH) 2 PO 4 (1) were preliminarily indexed in the rhombohedral system space group R-3c (167) with the unit cell data:  1 and 2, respectively.e analyses of the results are shown in Figure 4. e results present in Table 1 show that there was a signi cant removal of Fe 2+ by compounds 1 and 2 with the e ciency of 98.73 and 99.77%, respectively, while compounds 3 and 4 have little or no e ect in removing Fe 2+ .e reduction in the adsorption e ciency of compound 3 for Fe 2+ is due to the presence of SO 4 2− in the interlamellar space.e literature reports have shown that the presence of SO 4 2− in the interlamellar space reduces the adsorption e ciency.When both SO 42− and CO 3 2− coexist, they had a signi cant e ect upon the adsorption e ciency [31].It has also been demonstrated that the interlayer CO 3 2− ions in LDHs are di cult to be exchanged by other anions [32][33][34][35][36][37].Furthermore, the reduction in the removal eciency of Fe 2+ by compounds 3 and 4 can be attributed to the larger size of Ca 2+ (100 pm) over Mg 2+ (72 pm), in addition to the plate-like morphology of 3 with 1 μm average size.e results for Ca 2+ removal present in Table 2 and Figure 4 show that all the synthesized LDHs under investigation were effective in the cleanup with 84-99% removal e ciency.e e ectiveness of compound 3 in removing Ca 2+ is attributed to its a nity for both SO 4 2− and CO 3 2− anions, which are both intercalated in the interlamellar space.

Conclusion
In conclusion, four metal-aluminum layered double hydroxides were prepared by coprecipitating Mg or Ca and Al metal salts with a base under controlled conditions.e SEM revealed that compounds 1, 2, and 4 consist of nanoparticles with an average size of 100 nm, whereas compound 3 consists of hexagonal plates with an average size of 1 μm.e XRD studies showed the crystalline nature of the nano-LDHs.e results of the column adsorption studies have shown that there is signi cant potential for using nanostructured LDHs as nano lters to remove ions responsible for scale formation in oil wells.However, compounds 1 and

Table 1 :
Results of the adsorption study of Fe 2+ .

Table 2 :
Results of the adsorption study of Ca 2+ .Advances in Materials Science and EngineeringResearch, in the XRD and SEM analyses is gratefully acknowledged.