Magnetite as a catalyst for chemical oxidation of hydrocarbons spiked on sand under flow through conditions

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Mixed FeII-FeIII oxides (magnetite and green rust)

The green rust (GR) is a group of mixed FeII-FeIII hydroxides salts known as GR due to its intense bluish-green colour. Structurally, GR is a member of layered double hydroxides family (LDHs), sometimes also called anionic clays. LDHs are composed of divalent-trivalent ions minerals which are characterized by a crystal structure that consists of the stacking of brucite-like layers carrying a positive charge and layers constituted of anions and water molecules. The positive charge of metal hydroxide sheets is due to the partial replacement of divalent by trivalent metal cations. Our paper with detailed description about iron-based LDH (Ruby et al., 2010) is presented in annex (Annex 1). Their structure and composition depend upon the specific anions they incorporate. The general formula of LDHs is: [MII(1−x)MIIIx(OH)2]x+.[(x/n) An−, m H2O]x− where MII and MIII are metallic cations present in brucite-type layers Mg(OH)2, An− is an intercalated anion or a negatively charged molecule and also the electrostatic charge of both the brucite-type layers and the anionic interlayers and m is the number of intercalated water molecules. Most commonly, the values of x are found in the range [0.2-0.33] (Khan & O’Hare, 2002).
GR is a particular type of LDHs containing Fe as cation in the brucite type layers i.e. the LDH[FeII-FeIII] with common intercalated anions are Cl−, SO24- , CO32- . On the basis of X-ray diffraction (XRD) main features, GR GR1 and green rust two, GR2 (Bernal were initially classified in two types: green rust one, et al., 1959). GR1 was obtained with the incorporation of spherical and planar anions (Cl−, I−, Br− and CO32−) and it is characterized by a rhombohedral unit cell consisting of 3 repeat units (Fig. 2a) (Refait et al., 1998). The interlayer spaces are composed of a single plane of compensating anions and water. GR2 incorporates three-dimensional tetrahedral anions (SO42− and SeO42−) and contains a hexagonal unit cell composed of 2 repeat units (Fig.2b) (Simon et al., 2003). In GR2, interlayer spaces are composed of two adjacent planes of anions and water molecules (Simon et al., 2003).

Synthesis of green rust and magnetite

For the first time, the synthesis of GR was reported by Girard & Chaudron (Girard & Chaudron, 1935). Initially GRs had been identified and studied as corrosion products of iron-based materials (Stampfl, 1969). Its first existence as a natural mineral in soil was evidenced by Trolard and coworkers from our lab (Trolard et al., 1997). It was christened fougerite (IMA 2003-057) since the first extracted samples came from the hydromorphic gley soil located in forest of Fougères (Brittany-France). Another occurrence of GR in nature was recently observed in ground water (Christiansen et al., 2009). Various synthetic procedures for GR or magnetite formation have been developed that are believed to mimic those operative in the environment, including biotic and abiotic pathways. Brief discussion of these synthesis routes is provided in following sections.

Co-precipitation of soluble FeII and FeIII species

Co-precipitation of FeII and FeIII species is a simple method that consists of adding a basic solution to a mixture of both FeII and FeIII dissolved species. Synthesis of FeII-FeIII mixed compounds in different kinds of aqueous solutions by this method has been widely reported in literature (Arden, 1950; Hansen, 1989; Jolivet et al., 1992; Géhin et al., 2002; Refait et al., 2003; Ruby et al., 2003; Aissa et al., 2006; Ruby et al., 2006a; Ruby et al., 2006b). An illustration of synthesis routes of magnetite or GR by co-precipitation and oxidation in the mass balance diagram is presented in figure 4.

Oxidation of hydroxylated FeII solution

Partial chemical oxidation of soluble FeII or Fe(OH)2 precipitates is another common method to synthesize the mixed FeII-FeIII oxides. Oxidation can be carried out by O2 bubbling in controlled environment, addition of an oxidant or by aerial oxidation. To achieve the desired end product i.e. GR or magnetite, constant monitoring of pH or electrode potential is required during synthesis. Synthesis of magnetite was conducted by aerial oxidation of FeII solution at basic pH (Ishikawa et al., 2002) and oxidizing an aqueous solution containing FeII and the anion to intercalate yielded GR (Schwertmann & Fechter, 1994). The mechanism of formation of GR starts with initial oxidation of FeII into FeIII followed by the precipitation of FeIII as poorly ordered FeIII oxide which ultimately reacts with FeII and OH− forming GR (Schwertmann & Fechter, 1994).
Mixed FeII-FeIII compounds were also synthesized by the partial oxidation of Fe(OH)2 at neutral pH (Kiyama, 1974; Tamaura et al., 1981; Olowe & Génin, 1991; Refait & Génin, 1993; Génin et al., 1996; Génin et al., 2006a). This method involves the precipitation/oxidation route (Fig. 4) with two steps: (i) base addition for initial precipitation of FeII solution into FeII(OH)2 or [FeII(OH)2, FeIIaq) (Fig. 4. Path BC) and (ii) oxidation of divalent species to from GR (Path CD) or magnetite (Path GF). Nature of the oxidant determines the slope of line CD (Fig. 4).
The easiest way of oxidation is agitating the FeII(OH)2 suspension in contact with air and controlling the redox potential (Eh) of the suspension to follow the oxidation steps. Other oxidants such as hydrogen peroxide H2O2, iodine I2 or persulfate S2O82− can also be used. The advantage is to know the oxidation state of the product by controlling the amount of added oxidant. In sulfate and chloride solutions (Génin et al., 2006a), only suspensions containing a specific amount of soluble FeII species led to the formation of pure GR. For example in sulfated medium, this excess of FeIIaq is described by the following chemical reaction: 5 FeII(OH)2 + Fe2+aq + SO4 2− + 1/2 O2 + H2O <=> FeII4 FeIII2 (OH)12 SO4 (3).

Role of Mixed FeII-FeIII oxides in environment

Both magnetite and green rust are considered as more reactive compounds than ferric oxides due to the presence of structural FeII. They play an important role in environmental fate and transport of various organic and inorganic pollutants. The use of GR has been extensively reported for the reductive tranformations of inorganic pollutants like nitrate, UVI, CrVI, SeIV and TcO4- (Erbs et al., 1999; Fredrickson et al., 2004; O’Loughlin & Burris, 2004) as well as organic contaminants (Elsner et al., 2004; Larese-Casanova & Scherer, 2008; Kone et al., 2009). Considerable interest has been given to reduction of NO3-, which is widely known as an agricultural pollutant (Hansen et al., 1996; Hansen & Koch, 1998; Hansen et al., 2001). Its reduction helps to protect groundwater from high NO3- concentrations. Magnetite was also found involved in contaminant reduction (Gorski et al., 2009; Gorski & Scherer, 2009).
Due to the presence of FeII, they can serve as an iron source to catalyze Fenton oxidation of various pollutants. The role of magnetite has been widely investigated in oxidation through Fenton reaction for various pollutant like pentachlorphenol (Xue et al., 2009a; Xue et al., 2009c), trinitrotoluene (Matta et al., 2007), rhodmadine B (Xue et al., 2009b), azo dye (Hanna et al., 2008). Only few studies have reported the use of GR in Fenton oxidation like phenol (Matta et al., 2008) and azo dye etc. (Kone et al., 2009). Till date, the reactivity of magnetite or GR has not been tested for the oxidation (Fenton or persulfate) of polycyclic aromatic hydrocarbons or aliphatic hydrocarbons which is briefly explained in next sections.

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Soil Remediation by Chemical oxidation

The term ‘soil remediation’ refers to actions designed to eliminate or minimize the risk associated with contaminated soil. This goal may be achieved in several different ways and the selected method depends on factors such as the contaminants present, the site conditions and the cost. The ultimate goal of any degradation process is complete mineralization of the organic contaminants, resulting in carbon dioxide, water and other inorganic compounds. Many types of remediation technologies have been explored for the removal of PAHs or AHs from soils, using chemical (Ferguson et al., 2004; Ferrarese et al., 2008; Do et al., 2010; Lu et al., 2010a; Yap et al., 2011; Yen et al., 2011), thermal (Haapea & Tuhkanen, 2006; Biache et al., 2008), biological (Taylor & Jones, 2001; Chaîneau et al., 2003; Franco et al., 2004; Cai et al., 2007; Silva et al., 2009) or combined approaches (Allen & Reardon, 2000; Piskonen & Itävaara, 2004; Kulik et al., 2006; Lu et al., 2010b). Although, bioremediation techniques have become popular in recent years for treatment of PAHs contaminated soils (Wilson & Jones, 1993; Boopathy, 2000), these methods have shown limited capacity to degrade recalcitrant high molecular weight PAHs (Wilson & Jones, 1993; Henner et al., 1997; Andersson et al., 2003; Lundstedt et al., 2003). On the other hand, bioremediation has proven successful for AHs in oil contaminated soils as the majority of molecules in crude oil spills and refined products are biodegradable (Prince, 1993). However, also bioremediation is often unable to reduce the level of contamination and long term soil toxicity below the stringent environmental cleanup standards (Płaza et al., 2005). Moreover, a complete mineralization of oil to CO2 and H2O cannot be achieved by soil microorganisms and always leaves more or less complex residues (Atlas, 1995) like PAHs. Also bioremediation has limited application to biorefractory materials especially asphaltenes (Gough & Rowland, 1990; Chaîneau et al., 2003; Chaillan et al., 2006).
Faster and more efficient degradation of recalcitrant PAHs, oil hydrocarbons and biorefractory materials can be achieved using advanced oxidation processes (AOPs) (Kawahara et al., 1995; Watts & Dilly, 1996; Kong et al., 1998a; Nam et al., 2001; Watts et al., 2002; Ferrarese et al., 2008; Lu et al., 2010a; Yap et al., 2011; Yen et al., 2011). The AOPs are chemical methods which use various combinations of reactants to enhance the formation of highly reactive radicals, which can mineralize even the most recalcitrant organic compounds. The oxidants that are most commonly used for environmental purposes are ozone, hydrogen peroxide, permanganate and persulfate (Rivas, 2006; Ferrarese et al., 2008). This study deals with the two most common techniques among AOPs; Fenton based oxidation and activated persulfate which are briefly described in the following sections.

Table of contents :

I- BACKGROUND 
Iron oxides
1. Ferric (FeIII) oxides
1.1. Ferrihydrite
1.2. Goethite
1.3. Lepidocrocite
1.4. Hematite
2. Mixed FeII-FeIII oxides (magnetite and green rust)
3. Synthesis of green rust and magnetite
3.1. Abiotic synthesis
3.1.1. Co-precipitation of soluble FeII and FeIII species
3.1.2. Oxidation of hydroxylated FeII solution
3.1.3. FeII induced mineralogical transformations of FeIII oxides
3.2. Biotic formation of mixed FeII-FeIII oxides
4. Role of Mixed FeII-FeIII oxides in environment
Fenton and Persulfate based chemical oxidation for hydrocarbon remediation 
1. Soil Pollution
1.1. Polycyclic Aromatic Hydrocarbons
1.2. Aliphatic Hydrocarbons
2. Soil Remediation by Chemical oxidation
2.1. Fenton based oxidation
2.1.1. Conventional Fenton oxidation
2.1.2. Fenton-like oxidation
2.2. Persulfate oxidation
2.2.1. Background and reaction chemistry
2.2.2. Mechanism and reactivity
II- RESULTS 
SECTION 1 FORMATION OF MIXED FeII-FeIII OXIDES: MAGNETITE AND GREEN RUST 
Summary
FeII induced mineralogical transformation of ferric oxyhydroxides into magnetite under various experimental conditions
Abstract
1. Introduction
2. Materials and methods
2.1. Chemicals
2.2. Sample preparation
2.2.1. Initial ferric oxyhydroxides substrates
2.2.2. Batch experiments
2.2.3. Sample characterization
3. Results
3.1. Characterization of initial ferric oxyhydroxides
3.2. Transformation of ferrihydrite, goethite and lepidocrocite into magnetite
3.3. Transformation of three goethites into magnetite
4. Discussion
5. Conclusion
Formation of green rust via mineralogical transformation of ferric oxides (ferrihydrite, goethite and hematite)
Abstract
1. Introduction
2. Experimental section
2.1. Sample preparation
2.1.1. Initial ferric oxides/hydroxides substrates
2.1.2. Transformations of ferric oxyhydroxides in batch experiments
2.2. Sample characterization
2.2.1. Mössbauer Spectroscopy
2.2.2. Transmission electron microscopy
2.2.3. Analyses of soluble iron species by UV-Visible spectroscopy
3. Results
3.1. Initial ferric oxyhydroxides
3.2. Transformation products
3.2.1. Mössbauer spectroscopy
3.2.2. Transmission electron microscopy (TEM)
3.2.3. Concentration of soluble iron and mass balance diagram
4. Discussion
4.1. Order of reactivity of the various ferric oxides
4.2. Formation of green rust versus magnetite
5. Conclusion
Reactivity of FeIII oxyhydroxides with FeII in batch and dynamic flow systems 
Abstract
1. Introduction
2. Methods
3. Results
3.1. Static batch conditions
3.2. Saturated column test
4. Conclusions
SECTION 2  REACTIVITY OF MAGNETITE TO CATALYZE CHEMICAL OXIDATION FOR HYDROCARBON REMEDIATION IN SOILS
Summary
Application of magnetite catalyzed chemical oxidation (Fenton-like and persulfate) for the remediation of oil hydrocarbon contamination
Abstract
1. Introduction
2. Experimental Section
2.1. Chemicals
2.2. Synthesis and characterization of magnetite rich sandy soil (MRS)
2.3. Iron mineral characterization
2.4. Sample preparation
2.5. Oxidation experiments
2.6. Instrumental analysis
3. Results and Discussion
3.1. Characterization of magnetite rich sand (MRS)
3.2. Kinetic degradation of oil hydrocarbons
3.2.1. Extractable organic matter (EOM) and hydrocarbon index (HI)
3.2.2. GC-MS characterization
3.2.3. μFTIR characterization
Conclusion
Remediation of PAH-contaminated soils by magnetite catalyzed Fenton-like oxidation 
Abstract
1. Introduction
2. Experimental section
2.1. Chemicals
2.2. Soil samples
2.3. Oxidation procedures
2.4. Instrumental analysis
3. Results and Discussion
3.1. Degradation of fluorenone
3.2. Oxidation of two PAHs contaminated soils
3.3. PAHs degradation in organic extracts spiked on sand
3.4. PAHs degradation in pretreated soils
4. Conclusions
Application of magnetite-activated persulfate oxidation for the degradation of PAHs in contaminated soils
Abstract
1. Introduction
2. Experimental section
2.1. Chemical reactants
2.2. Soil samples
2.3. Oxidation experiments
2.4. Extraction and analysis
3. Results and Discussion
3.1. Oxidation of fluorenone
3.2. PAHs degradation in organic extracts spiked on sand
3.3. PAHs degradation in soils
4. Conclusion
Magnetite as a catalyst for chemical oxidation of hydrocarbons spiked on sand under flow through conditions
Abstract
1. Introduction
2. Experimental Section
2.1. Sample preparation
2.2. Oxidation under flow through conditions
2.3. Extraction and analysis
3. Results and Discussion
3.1. Degradation of oil hydrocarbons
3.2. Degradation of PAHs
3.3. μFTIR characterization
4. Conclusion
CONCLUSIONS AND PERSPECTIVES
References

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