**Abstract**

Iron oxides were synthesized in conditions similar to those that prevail in deeps of the Red Sea (2–5M NaCl, temperatures 60–80°C, and pH 6.5–10.4). The main phase that was crystallized was submicron magnetite. Additional phases of feroxyhyte, goethite, and akagenéite were also detected. Magnetite morphology observed through high-resolution scanning electron microscopy (HRSEM) varied between euhedral plates and octahedral or unhedral crystals. The euhedral plates were probably crystallized pseudomorphically after platy green rust or Fe(OH)2 due to its quick crystallization. Size of magnetite varied between 18 and 45 nm. The addition of Si retarded crystal growth, and at Si/Fe = 0.5, short-range ordered phases are formed and not magnetite. This finding is in line with earlier laboratory experiments in which Si was found to retard goethite and lepidocrocite crystallization.

**Keywords:** magnetite, HRSEM, hypersaline environment, feroxyhyte, goethite lepidocrocite, akagenéite

## **1. Introduction**

Synthesis of iron oxides in hypersaline solutions was performed to imitate conditions that prevail in the Dead Sea and in deeps of the Red Sea. Salinity in the Dead Sea reaches 340 g/l. In the Red Sea, hydrothermal hypersaline brine discharges into the Atlantis II Deep with Cl concentrations in the upper layer of the brine at 67 g of Cl per kg of water (67 g/kg) and at the lower layer of the brine, 158 g/kg. The pH value decreases from 8.13 at RSDW to 5.2 at the lower layer [1]. The temperature of the lower layer is ∼68°C measured in 2008 [2]. A narrow channel that connects the southern part of the Atlantis II Deep with the Chain Deeps enables overflowing of the brine of the lower convecting layer to Chain A, B, and C deeps. It was suggested that not only does the overflowing brine feed the adjacent deeps, but a fracture and fissure system enables a sub-bottom flow of the brine of the LCL from the Atlantis II Deep into the Chain and Discovery Deeps, leading to hydrothermal conditions in those deeps [1, 3]. The temperature in Chain A is 52.4–54°C; in Chain B, 46°C; in Chain C, 44.3°C; and in Discovery Deep, 44.7°C [4, 5]. There is no current hydrothermal activity in the Thetis Deep, yet it was active in the past but without a brine that filled the Deep [6].

Iron oxides (hematite-Fe2O3 and magnetite-Fe3O4), well-crystallized oxyhydroxides (goethite-αFeOOH, lepidocrocite-γFeOOH, feroxyhyte-δFeOOH, and akaganéiteβFeOOH), and short-range ordered oxyhydroxides (ferrihydrite-Fe5HO8·4H2O and singerite-SiFe4O6(OH)4H2O) precipitated in the hypersaline environment of the Dead

#### **Figure 1.**

*Pictures obtained by electron microscopy: Magnetite from the Thetis deep (scanning electron microscope (SEM)); lepidocrocite from the Thetis deep; lepidocrocite from the Dead Sea; mono- and multi-domain goethite from Atlantis II deep, Red Sea; mono-domain goethite from chain a deep, Red Sea; and cluster of akaganéite from the Dead Sea (high-resolution transmission electron microscope).*

Sea and in the deeps of the Red Sea (Atlantis II, Chain A, Chain B, Discovery, and Thetis) [7–10] (**Figure 1**). Precipitation of these phases occurs at hypersaline environment with elevated temperatures and varying pH. Morphology of the crystals and their size is observed by high-resolution transmission electron microscopy (HRTEM). Using electron diffraction enables identifying the crystallographic structure. Goethite and lepidocrocite have orthorhombic structure, akaganéite has monoclinic structure, and feroxyhyte has hexagonal structure [11].

Short-range ordered ferrihydrite recrystallizes into well-crystallized phase, yet its initial morphology of plates at the size of <10 nm is preserved. Singerite (100 nm) disintegrates into iron-rich clay mineral [12]. Magnetite has a cubic morphology of octahedron or cubes 2–4 μm as observed in the Thetis Deep of the Red Sea.

Goethite has acicular form and changes at elevated pH from mono-domain to multi-domain rods. Crystal size at the Dead Sea area and the Red Sea varied from few 100 nm to 3 μm.

Lepidocrocite usually precipitates at fast oxidation rate at the presence of chloride, and its morphology changes from plates formed at lower pH to multi-domain rods that are formed at elevated pH. Lepidocrocite crystals size varied from 100 nm in the Dead Sea to 300 nm in the Red Sea deeps.

Rods and multi-domain akaganéite were observed at the Dead Sea with crystal size that varied from 100 nm to 1 μm.

Feroxyhyte plates are formed at very high oxidation rate and had a plate morphology with crystals size that reached 300 nm.

In earlier study, synthesis was performed imitating the upper convecting layer of the Atlantis II Deep with lower temperatures and salinity [13]. In this research, iron oxides were synthesized at higher pH, elevated temperatures, and hypersaline brines.

**93**

*Iron Oxides Synthesized in Hypersaline Solutions DOI: http://dx.doi.org/10.5772/intechopen.88948*

Fe2+ solutions to yield a concentration of 0.06M.

process using "Celsiz" software did the unit cell refinement.

ture, pH, and oxidation rates.

Iron oxides were synthesized under changing conditions of salinity, tempera-

NaCl salt (Loba Chemie) was used to prepare NaCl solutions (2, 4, and 5M) that were used as a matrix and were kept in water bath at 40, 60, 70, and 80°C. Prior to synthesis, N2 was bubbled through the solutions for 20 minutes to remove dissolved oxygen. pH buffering of the solutions was accomplished by adding either NaOH (Daejung) or NaHCO3 (Carlo Erba) in small amounts, hence slightly changing Na concentration. pH of the solutions were 5.5, 7, 8.2, and 10.5. Cl concentration also changed due to dissolution of FeCl2·4H2O (Sigma-Aldrich) that was chosen for the

Fe oxidation was carried out by introducing air at flow rates of 25, 40, 110, and 200 ml/min which was monitored with a flow meter and was kept stable during the

In order to isolate Si effects on the crystallization of iron oxides, Na2SiO3 (Sigma) was added to some of the solutions. To avoid any side effects, all of the samples were synthesized using the same polyethylene ware and under constant stirring speed. The precipitates were slightly washed and freeze-dried immediately after their synthesis. Analyses of the precipitates were performed using X-ray diffraction (XRD). The fitting of the peak profiles was performed by using Pseudo-Voigt function with the APD computer program developed by Philips Export B.V. A least square

Sample morphology was observed through transmission electron microscopy (TEM) and high-resolution scanning electron microscopy (HRSEM). Transmission electron microscopy was carried out on a JEOL FasTEM 2010 electron microscope equipped with the Noran energy dispersive spectrometer (EDS) for microprobe elemental analyses. All chemical analyses were obtained by point analyses with beam width of 25 nm and are presented as atomic ratios. A NORAN Standardless Metallurgical Thin Films program based on the *Cliff-Lorimer* ratio technique with an accuracy of about 5% was used for calculations. CuKα line was used for spectrometer calibration. Crystalline phases were identified using selected area electron diffraction (SAED) in the TEM.

Initial identification of iron oxides was obtained by using X-ray diffractions (**Figure 2**). X-ray diffraction of all samples synthesized yielded the following results: At elevated temperatures, alkaline media and concentrated brine magnetite was the main phase that precipitated. At neutral to slightly acidic conditions, lepidocrocite, akaganéite, and goethite co-precipitated. Additional NaHCO3 caused

In this study, pictures of iron oxides obtained by high-resolution scanning electron

Unit cell parameters of magnetite vary from 0.8373 to 0.8396 nm with upper limit of 0.8396 nm, identical to that of well-crystallized magnetite [13]. Crystallite size changes from 18 to 45 nm. The smallest crystallites were obtained at pH 7.5 in 4 or 5M NaCl at all temperatures, whereas the largest crystallites were obtained at under elevated pH (9.5) and temperatures. The lowermost unit cell parameter was obtained for samples that crystallized at 60°C pH 8.5 and in 4M NaCl matrix. The largest unit

microscope and high-resolution transmission electron microscope are presented.

**2. Methods**

hours of synthesis.

**3. Results and discussion**

precipitation of siderite at 60°C.

**3.1 Magnetite FeO-Fe2O3**

*Iron Oxides Synthesized in Hypersaline Solutions DOI: http://dx.doi.org/10.5772/intechopen.88948*
