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

Authors: Emmanuel K. Ephraim; Chinyere A. Anyama; Ayi A. Ayi; Jude C. Onwuka
Journal: Advances in Materials Science and Engineering
 (2018)

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## 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.


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## Body

## 1. 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. These 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 (CO2). The produced water can come from two sources: water vapor in the gas that condenses into liquid water and formation water containing salts [3]. This water is the source of hydrate formation, and in combination with CO2, it forms a weak carbonic acid (H2CO3), as shown in (1). Scaling is caused by salts and can occur when the produced water contains formation water [4]:(1)CO2g+H2Ol=H2CO3aqThis carbonic acid will continue to dissociate hydrogen, creating new deprotonated species of carbonic acid, as seen in the following equation:(2)H2CO3aq+H2Ol=H3O+aq+HCO3−aq(3)HCO3−aq+H2Ol=H3O+aq+CO32−aqIn the water mixture, there will be a mixture ofH2CO3, HCO3−, and CO32−. Finally, in the presence of calcium and carbonic acid, calcium carbonate will precipitate out [5, 6]:(4)CO32−aq+Ca2+aq=CaCO3sThe composition of the formation water varies greatly with the reservoirs, but the usual constituents are Na+, Ca2+, K+, Mg2+, Fe2+, Cl−, SO42−, and HCO3−. Long pipelines are constructed of carbon steel [6]. Carbonic acid will corrode the iron in the pipeline wall, producing iron carbonate. This iron carbonate can precipitate in the production fluid and follow the gas and liquid flow, causing problems downstream:(5)Fes+H2CO3aq=FeCO3s+H2gThe 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. This 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 [11–15]. The lattice structure of LDHs, with the general formula [M2+1–xM3+x(OH)2]x+(An–)x/n·yH2O, has a positively charged brucite-shaped layers, consisting of a divalent metal ion M2+ (e.g., Ca2+, Zn2+, Mg2+, and Ni2+) octahedrally surrounded by six OH− hydroxyl groups [16–18]. The substitution of the M2+ metal with a trivalent M3+ cation gives rise to the periodic repetition of positively charged sheets (lamellas) alternating with charge-counter balancing An– ions. These LDHs have found widespread applications in diverse areas as sensors [19, 20], adsorbents in wastewater treatment [21–23], catalytic removal of soot and NOx in vehicle engine exhausts [10], and as drug and gene carriers [24–26].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 PO43−, PO43−/PF6−, and SO42− anions, and we have been successful in synthesizing four new LDHs with nanostructures, namely, Mg-Al(OH)2PO4 (1), Mg-Al(OH)2PO4PF6 (2), Ca-Al(OH)2SO4 (3), and Ca-Al(OH)2PO4PF6 (4). In this work, the synthesized LDH nanostructures are being investigated as nanofiltration materials to remove scale forming ions like Fe2+ and Ca2+. The synthesis, characterization, and adsorption properties of these four new compounds have been reported.


## 2. Materials and Methods

### 2.1. Materials

Orthophosphoric acid (H3PO4), magnesium nitrate (Mg(NO3)2), aluminum sulphate (Al2(SO4)3), sodium hydroxide (NaOH), sulphuric acid (H2SO4), aluminum hydroxide (Al(OH)3), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF6) were used. Analytical grades of all the chemicals were obtained from commercial sources and used without further purification.


### 2.2. Methods

The standard procedure of coprecipitating a divalent with trivalent metal ions in the presence of a base was followed in the present studies [27–30]. The coprecipitation was carried out at room temperature, and the gel was continuously and magnetically stirred. The mixture was kept under magnetic stirring for 3 h. The 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. The resulting material was dried overnight at 70°C in the incubator. For compound 1, orthophoshoric acid was used as the source of the PO43− anion. For compounds 2 and 4, 1-ethyl-3-methylimidazolium hexafluorophosphate and orthophosphoric acid were used to introduce PF6− and PO43− anions, respectively. For compound 3, the source of the SO42− anion was H2SO4. In a typical synthesis of 1, a 250 mL solution containing Mg(NO3)2 (0.0375 mmol), Al(OH)3 (0.0125 mmol) (with the Mg-Al ratio of 3 : 1), and H3PO4 (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. The 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). The 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 SO42− ions for compounds 3 and 4.


### 2.3. Characterization

The 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°.The 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 analyzer (EDX). The specimens were Au coated (sputtering) to make them conductive. The SEM acceleration voltage was 10 kV.The 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. The powdered samples were mixed with KBr (in a 1 : 200 ratio of their weight) and pressed in the form of pellets for measurement.


### 2.4. Adsorption Column Experiments

Aqueous solutions of Fe2+ and Ca2+ prepared from their salts FeSO4·7H2O and CaCl2, respectively, were analyzed by atomic absorption spectroscopy. The 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 filter (Scheme 1). The metal ion solutions were collected at the outlet of the LDH column, and the eluded portion (i.e., the filtrate) was also analyzed by AAS. The degree of surface covered (θ) by the ions was computed using the following equation:(6)θ=1−CeC0.Scheme 1
The experimental setup for the column adsorption studies.The effectiveness of the LDHs as adsorbent was calculated with the help of the following equation:(7)Kd=C0Ca,where Kd is the partition coefficient, C0 is the concentration of Fe2+ or Ca2+ ions in solution, and Ca is the concentration of the ions adsorbed in the LDHs.The removal efficiency of Fe2+ and Ca2+ by the LDHs in percentage was calculated using the following equation:(8)Efficiency%=CaC0×100,where C0 is the initial concentration of the ions and Ca is the concentration of the ions adsorbed in LDHs.


## 2.1. Materials

Orthophosphoric acid (H3PO4), magnesium nitrate (Mg(NO3)2), aluminum sulphate (Al2(SO4)3), sodium hydroxide (NaOH), sulphuric acid (H2SO4), aluminum hydroxide (Al(OH)3), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF6) were used. Analytical grades of all the chemicals were obtained from commercial sources and used without further purification.


## 2.2. Methods

The standard procedure of coprecipitating a divalent with trivalent metal ions in the presence of a base was followed in the present studies [27–30]. The coprecipitation was carried out at room temperature, and the gel was continuously and magnetically stirred. The mixture was kept under magnetic stirring for 3 h. The 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. The resulting material was dried overnight at 70°C in the incubator. For compound 1, orthophoshoric acid was used as the source of the PO43− anion. For compounds 2 and 4, 1-ethyl-3-methylimidazolium hexafluorophosphate and orthophosphoric acid were used to introduce PF6− and PO43− anions, respectively. For compound 3, the source of the SO42− anion was H2SO4. In a typical synthesis of 1, a 250 mL solution containing Mg(NO3)2 (0.0375 mmol), Al(OH)3 (0.0125 mmol) (with the Mg-Al ratio of 3 : 1), and H3PO4 (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. The 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). The 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 SO42− ions for compounds 3 and 4.


## 2.3. Characterization

The 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°.The 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 analyzer (EDX). The specimens were Au coated (sputtering) to make them conductive. The SEM acceleration voltage was 10 kV.The 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. The powdered samples were mixed with KBr (in a 1 : 200 ratio of their weight) and pressed in the form of pellets for measurement.


## 2.4. Adsorption Column Experiments

Aqueous solutions of Fe2+ and Ca2+ prepared from their salts FeSO4·7H2O and CaCl2, respectively, were analyzed by atomic absorption spectroscopy. The 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 filter (Scheme 1). The metal ion solutions were collected at the outlet of the LDH column, and the eluded portion (i.e., the filtrate) was also analyzed by AAS. The degree of surface covered (θ) by the ions was computed using the following equation:(6)θ=1−CeC0.Scheme 1
The experimental setup for the column adsorption studies.The effectiveness of the LDHs as adsorbent was calculated with the help of the following equation:(7)Kd=C0Ca,where Kd is the partition coefficient, C0 is the concentration of Fe2+ or Ca2+ ions in solution, and Ca is the concentration of the ions adsorbed in the LDHs.The removal efficiency of Fe2+ and Ca2+ by the LDHs in percentage was calculated using the following equation:(8)Efficiency%=CaC0×100,where C0 is the initial concentration of the ions and Ca is the concentration of the ions adsorbed in LDHs.


## 3. Results and Discussion

All the metal-aluminum layered double hydroxides were prepared by the coprecipitation method followed by mild hydrothermal treatment at 60°C. Divalent metal ions (Mg and Ca) in solution with Al3+ titrated with NaOH over 3–5 h yielded Mg-Al and Ca-Al layered double hydroxides, respectively. In the present studies, SO42−, PO43−, and PF6− anions were used in the interlamellar space to balance the layer positive charge. The conditions chosen for the synthesis provided an aqueous solution of Al3+ and M2+ (molar fraction Al3+/(Al3+ + M2+) = 0.25 and M2+/Al3+ = 3 molar ratio), prepared by dissolving metal salts in distilled water to give M2+-Al-LDH with x = 0.25 (i.e., the composition is more likely represented by M2+0.75Al0.25(OH)2An−x/n·yH2O. Where x/n for PO43− 0.23/3=0.083, [PO4PF6]4− 0.23/4=0.0625, and SO42− 0.25/2=0.125).Figure1 shows the FTIR spectra of the isolated layered double hydroxides. The characteristic absorption band for the water molecules was observed in all the compounds. The band observed in the region 3402–3455 cm−1 corresponds to the OH stretching vibration, while the band at 1640 cm−1 is attributed to the bending mode of the lattice water. The stretching vibrational modes of PO43− or SO42− units were observed in the region 1057–1130 cm−1. The bands in the region 600–430 cm−1 are attributable to the M–O vibrations. The various assignments are in good agreement with similar compounds in the literature [29–36].Figure 1
FTIR spectra of the as-synthesized layered double hydroxides: (a) Mg-Al(OH)2PO4 (1), (b) Mg-Al(OH)2PO4PF6 (2), (c) Ca-Al(OH)2SO4 (3), and (d) Ca-Al(OH)2PO4PF6 (4).Figure2 shows the representative XRD patterns of the M-Al-LDHs. The X-ray powder diffraction lines of Mg-Al(OH)2PO4 (1) were preliminarily indexed in the rhombohedral system space group R-3c (167) with the unit cell data: a = b = 4.7602 and c = 12.9930 Å (Ref. Code = 01-075-1862). The 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 angles for Mg-Al(OH)2PO4PF6 (2), which indicates a decrease in the interlamellar spacing as PF6− was incorporated along with PO43− anion. The XRD pattern for Ca-Al(OH)2SO4 (3) was quite different, showing the presence of low-angle peaks at 2θ = 11.68 and 14.72°, which are indicative of carbonate anions being incorporated into the interlamellar space of LDH. The presence of the carbonate is due to the fact that the synthesis was carried out in air at a pH value of 10. The result obtained is in agreement with similar compounds in the literature [28–30, 36]. Compound 3 crystallizes in monoclinic space group (Ref. Code = 00-047-0964) with the following cell parameters: a = 12.6700, b = 6.9270, and c = 12.0280 Å and volume = 1055.63 Å3. The d110 peak has a d-spacing of 6.027 Å, whereas the d-spacing for the d013 and d004 peaks are 4.474 and 3.005 Å, respectively.Figure 2
XRD pattern of as-synthesized compounds: (a) Mg-Al(OH)2PO4 (1), (b) Mg-Al(OH)2PO4PF6 (2), (c) Ca-Al(OH)2SO4 (3), and (d) Ca-Al(OH)2PO4PF6 (4).The surface structure and morphology of the metal-aluminum LDHs were studied with the help of scanning electron microscope (SEM). The representative SEM images of the as-synthesized layered double hydroxides are presented in Figure3. The representative 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 of M-Al layered double hydroxides, while the Ca-O-Al-OH layer of compound 3, intercalated with SO42− anions, consists of hexagonal nanoplates.Figure 3
SEM images of as-synthesized compounds: (a) Mg-Al(OH)2PO4 (1), (b) Mg-Al(OH)2PO4PF6 (2), (c) Ca-Al(OH)2SO4 (3), and (d) Ca-Al(OH)2PO4PF6 (4).
(a)
(b)
(c)
(d)The adsorption efficiency of the LDHs in the removal of Fe2+ and Ca2+ scale-forming ions from aqueous solutions is presented in Tables 1 and 2, respectively. The analyses of the results are shown in Figure 4. The results present in Table 1 show that there was a significant removal of Fe2+ by compounds 1 and 2 with the efficiency of 98.73 and 99.77%, respectively, while compounds 3 and 4 have little or no effect in removing Fe2+. The reduction in the adsorption efficiency of compound 3 for Fe2+ is due to the presence of SO42− in the interlamellar space. The literature reports have shown that the presence of SO42− in the interlamellar space reduces the adsorption efficiency. When both SO42− and CO32− coexist, they had a significant effect upon the adsorption efficiency [31]. It has also been demonstrated that the interlayer CO32− ions in LDHs are difficult to be exchanged by other anions [32–37]. Furthermore, the reduction in the removal efficiency of Fe2+ by compounds 3 and 4 can be attributed to the larger size of Ca2+ (100 pm) over Mg2+ (72 pm), in addition to the plate-like morphology of 3 with 1 μm average size. The results for Ca2+ 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 efficiency. The effectiveness of compound 3 in removing Ca2+ is attributed to its affinity for both SO42− and CO32− anions, which are both intercalated in the interlamellar space.Table 1
Results of the adsorption study of Fe2+.
LDHs
Actual amount,C0 (ppm)
Eluded amount,Ce (ppm)
Absorbed amount,Ca (ppm)
Degree of surface coverage,θ
Distribution partition coefficient,Kd
Absorption efficiency (%)
1
9.8526
0.1247
9.7279
0.99
1.01
98.73
2
9.8526
0.0222
9.8304
1.00
1.00
99.77
3
9.8526
8.0994
1.7532
0.18
5.62
17.79
4
9.8526
7.1938
2.6588
0.27
3.71
26.99Table 2
Results of the adsorption study of Ca2+.
LDHs
Actual amount,C0 (ppm)
Eluded amount,Ce (ppm)
Absorbed amount,Ca (ppm)
Degree of surface coverage,θ
Distribution partition coefficient,Kd
Absorption efficiency (%)
1
7.7780
0.7165
7.7064
0.99
1.01
99.08
2
7.7780
0.7563
7.0217
0.90
1.11
90.28
3
7.7780
0.7937
6.9843
0.90
1.11
89.80
4
7.7780
1.2238
6.5542
0.84
1.19
84.27Figure 4
Results for the removal of Fe2+ and Ca2+ ions by compounds 1–4.


## 4. 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. The SEM revealed that compounds1, 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. The XRD studies showed the crystalline nature of the nano-LDHs. The results of the column adsorption studies have shown that there is significant potential for using nanostructured LDHs as nanofilters to remove ions responsible for scale formation in oil wells. However, compounds 1 and 2 can remove Fe2+ with greater efficiency, and all the synthesized LDHs nanostructures have been demonstrated to effectively remove Ca2+ from the oil wells. However, in moving from laboratory conditions to the complicated conditions of high salinity, low permeability, and heterogeneous rock properties in oilfield core materials [38], the transport and retention properties of the nanostructured LDHs should be investigated.



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*Source: 5306825-2018-04-24.xml*