Copper-mediated reductive dechlorination by green ...

Copper-mediated reductive dechlorination by green rust intercalated with dodecanoate Li-Zhi Huang, a,b,c Zhou Yin, b Nicola G.A.Cooper, b Weizhao Yin, d,* Emil Tveden Bjerglund,c Bjarne W. Strobel b and Hans Christian B. Hansen b a

School of Civil Engineering, Wuhan University, No. 8, East Lake South Road, Wuhan, P.R. China

b

Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-

1871 Frederiksberg C, Denmark c

Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark

d

School of Environment, Guangzhou Key Laboratory of Environmental Exposure and Health,

Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, China ∗ Corresponding author at: School of Environment, Guangzhou Key Laboratory of Environmental Exposure and Health, Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, China. E-mail addresses: [email protected], [email protected], [email protected] (W. Yin).

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ABSTRACT A layered FeII-FeIII hydroxide (green rust, GR) was intercalated with dodecanoate (known as GRC12) and then amended with CuII (GRC12(Cu)) before reaction with chloroform (CF), carbon tetrachloride (CT), trichloroethylene (TCE) or tetrachloroethylene (PCE). Reduction of CT by GRC12(Cu) was 37 times faster than with GRC12 alone before the active Cu species was consumed. The Cu mediated reaction followed the dichloroelimination pathway as observed for GRC12 alone, with carbon monoxide (82.5%) and formate (26.6%) as main degradation products. Also, CF was reduced by GRC12(Cu), which is not seen with GRC12. Neither GRC12(Cu) nor GRC12 reacted with PCE or TCE. The chlorinated solvents can partition into dodecanoate interlayer but only small CS molecules (CF, CT) can transport through the dodecanoate interlayer. Copper(II) added to GRC12 was reduced to CuI by FeII in GR, but CuI was not regenerated during the dechlorination. High resolution TEM showed that Cu was evenly distributed in the GR without formation of Cu nanoparticles on edges of GR. The active CuI sites are most likely located between the iron hydroxide layer and the hydrated negatively charged carboxylate groups in the interlayer of GR. This work shines new light on the Cu accelerated dechlorination by GR. KEYWORDS: metal catalysis; remediation; chlorinated solvents; copper(I); layered double hydroxide (LDH)

1. INTRODUCTION Chlorinated solvents (CSs) represent a group of toxic compounds, which are more or less recalcitrant under natural environmental conditions.1, 2 They often accumulate as dense non-aqueous phase liquids (DNAPL) at the bottom of aquifers and are therefore difficult to remediate.3 They are of particular concern to countries, which rely on aquifers as their source of drinking water.4, 5 Moreover, evaporation of CS into buildings located on contaminated sites is a serious problem.6 Commonly found CSs

include

carbon

tetrachloride

(CT),

chloroform

(CF),

tetrachloroethylene

(PCE)

and

trichloroethylene (TCE). Some of the current remediation technologies involving reduced iron (e.g. Fe0

2

(zero-valent iron, ZVI) and FeII) lead to incomplete dechlorination of CT (one chlorine substitution reaction) with CF as one of the main end products.7-10 Biological reduction of CSs is less active in case of a high concentration of the pollutant and low temperatures. Moreover, halorespiring microorganisms form hazardous compounds with only a minor part of the microbial species being able to entirely dehalogenate the CSs.11 Therefore, attention has been paid to use of inorganic reductants to degrade CSs, with reduced iron compounds being the cheapest among them – e.g. ZVI or FeII compounds.12, 13 Zero valent iron combined with different admixed sorbents (surfactant, sepiolite, filtralite, activated carbon) are reported as promising hybrid systems for reductive remediation.14, 15 Surfactants included in the reactive sorbent systems may both promote sorption of CS at ZVI surfaces or the opposite - insulate the ZVI surface from contact with the CS - thus either enhancing or suppressing reductive dechlorination.16 Cationic surfactants are usually seen to enhance the dechlorination by ZVI as it makes the particle surface less charged and more hydrophobic.16, 17 On the other hand, anionic and nonionic surfactants inhibits or have no effect on dechlorination at the surface of ZVI.18, 19 Green rust (GR) is a layered double hydroxide (LDH) consisting of positively charged FeII-FeIII hydroxide sheets with a negatively charged interlayer consisting of charge balancing anions and water. The structure is [FeII(1-x)FeIIIx(OH)2]x+[Ay-x/y·mH2O]x-, where x is the molar fraction of FeIII and Ay- is the charge balancing anion.20 Many different anions can accommodate in the interlayer with Cl-, SO42- and CO32- as typical examples of simple inorganic anions. Green rusts are hexagonal crystals that vary in lateral size depending on synthesis, ranging from nanoscale up to 13 µm.21-23 Green rust has a strong reducing strength probably due to the high density of FeII-O-FeIII in the metal hydroxide sheets. These properties make GRs interesting materials for soil and groundwater remediation, and various forms of GR are known to reduce chlorinated solvents,24, 25 as well as other oxidizing pollutants like antimony,26 nitrate,27 chromium,23 neptunyl (NpVI),28 uranium(UVI),29 and technetium (TcVII).30 A novel form of GR intercalated with the surfactant dodecanoate (C12), namely GRC12, has been synthesized.31 The hydrophobic interlayer of the GRC12 enhances CT dechlorination in two possible ways: i) by partitioning

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of CT into the GR interlayer facilitating access to internal FeII sites,13 and ii) by stimulating the dicholoroelimination pathway compared to hydrogenolysis because of the hydrophobic interlayer that provides a poor hydrogen donating environment and hence causing less CF formation.13 Transition metal cations can act as redox mediators and enhance the reductive reactivity of GR. Green rust can reduce metal cations such as Ag+, Au3+, Cu2+, Hg2+ to their zero-valent forms.32, 33 One generally accepted mechanism is that electron transfer from GR to the oxidant (e.g. CS) can be catalyzed via cycling of the doped metal between reduced and oxidized states, and the reductive transformation then takes place on the particle surface of the doped metal instead of GR which is oxidized typically to magnetite during the reaction.33-38 Standard reduction potentials for some CSs and Cu redox couples at 25 °C and pH 7 are shown in Table S1. The redox potentials (E0’) of Cu0 and CuI are sufficiently negative compared with those for reduction of CSs, indicating that dechlorination is thermodynamically feasible. However, the exact oxidation state and the regeneration of reduced Cu forms in the metal redox cycling and the exact location of the Cu in CuII doped GRs is not well understood. Cu0 nano particles alone can also reduce CT very slowly at neutral or slightly alkaline pH.33, 39

Large particle size (~20 µm), inactivation of the metal particle surfaces and agglomeration could be

the reasons for the slow reaction.40 The objectives of this work are i) to investigate the enhanced reactivity of GRC12(Cu) towards reduction of CT and CF, ii) to test if TCE and PCE follows the same reduction pattern as CT, and iii) to further explore the interplay between the intercalated surfactant, chlorinated solvents and Cu in reductive dechlorination. The oxidation state and regeneration of active Cu species, and the exact location of Cu in doped GRs have had specific focus.

2. MATERIALS AND METHODS Chemicals. All chemicals were of analytical grade and used without further purification. Chemicals were supplied by Sigma-Aldrich. Triple-deionized water (resistivity of 20 MΩ cm–1) and argon gas 4

(>99.9995 %), supplied by Air Liquide (Denmark) were used throughout. A CuII dodecanoate (CuII-C12) precipitate was synthesized by mixing 14 mL sodium dodecanoate solution (25 mM, 40% ethanol) with 1 mL of 1 mM CuSO4. The precipitate was washed with ethanol, air-dried and collected for characterization. Synthesis of GRC12/GRC12(Cu), GRSO4 and GRCl. GRC12, GRSO4 and GRCl were synthesized using a one-pot method comprising the controlled partial oxidation of a dodecanoic acid-containing FeSO4, FeSO4, and FeCl2 solution by air at a constant pH of 8 at room temperature.31 Once synthesized, the GRC12 was transferred to an O2 free glovebox (Coy Laboratory Products, USA; 95% N2 and 5% H2) for further handling. The supernatant was removed by careful decanting, followed by centrifugation (7440 g), triple washing and resuspension in oxygen-free triple deionized water to achieve a final GR concentration corresponding to 22 mM FeII in GR. For the synthesis of GRC12(Cu), 11.8 mL of the GRC12 suspension was transferred to 12.1 mL glass vials. 100 µL of a solution containing 133 mM oxygen-free CuSO4 and 62.5 mM H2SO4 was added to each vial using a 1 mL gas tight syringe, giving a final Cu total concentration of 1.1 mM (around 5% of the total FeII concentration). 100 µL of 125 mM NaOH was added at the same time to keep the pH constant (around 7.5). Vials were crimped, wrapped in foil and placed on an end-over-end shaker at a speed of 15 rpm for 24 hours. The vials with the so formed GRC12(Cu) was subsequently used for dechlorination experiments. Dechlorination experiments. 100 µL of oxygen-free CSs (CT, CF, PCE and TCE) in methanol were injected into the vials containing GRC12(Cu). The experimental set up was such that a vial was sacrificed each time a sample was taken. The dechlorination experiments using sulphate interlayered GR (GRSO4) was included for comparison. Control experiments were performed under the same conditions by injecting 100 µL of CS into vials containing water, C12 or aqueous FeII. The CSs were determined using a gas chromatograph equipped with an electron capture detector (GC-ECD). At the end of the CT experiments samples were taken for ion chromatography (IC) to measure chloride and formate and for a gas reduction detector to measure CO, as detailed below. Vials were placed on a shaking table or endover-end rotator (depending on the length of the trial) and sampled periodically. 5

In order to test the catalytic role of Cu in the dechlorination process, CT reduction with i) Cu (13.3 μM) in excess of CT (0.248 μM), and ii) CT in excess (14.5 μM) of Cu (13.3 μM) were compared. In both cases, the concentration of FeII in GRC12 was 266 μM, which is in stoichiometric excess to both Cu and CT. The degradation of CT was monitored by GC-ECD as described above. The oxidation of FeII in GRC12 by CuII in aqueous suspension was investigated by titrating GRC12 with 1 M CuSO4 in 0.05 M HEPES buffer (pH 7); vials containing GRC12 suspensions was added with different concentration of Cu. Each reaction vial was crimped, wrapped in foil and placed on a table shaker in the glovebox at a speed of 15 rpm for 30 min. In a separate trial the full oxidation of GRC12 by CuII or CT was carried out with the oxidants in excess, i.e. [CuII]/[FeII] =10, [CT]/[FeII] =1630, and the resulting samples were designated as oxGRC12(Cu) and oxGRC12(CT). The addition of excess CuII resulted in a full oxidation of GRC12 immediately with the color changing from bluish-green to yellow. On the other hand, the oxidation of GRC12 by CT toke much longer time, although the excess of the oxidant (CT) was much higher. Sorption isotherms. For evaluating the sorption of CSs to GRC12 the oxidized form of GRC12 (oxGRC12) was used in order to exclude redox reactivity, and hence to exclude reduction of CSs as part of sorption. Freshly synthesized GRC12 was washed and oxidized in water by air to form oxGRC12. It has the same layer structure as GRC12 except that all FeII has been converted to FeIII as shown in our previous study.41 The oxGRC12 suspensions were centrifuged and redispersed in HEPES buffer (0.05 M, pH 7). The concentration of oxGRC12 was 64 mM in terms of FeIII. Sorption isotherms were determined using batch equilibration (25 °C) of 10 mL oxGRC12 suspensions at a concentration of 11 g L-1 in 20 mL headspace amber vials. Methanolic stock solutions of CF, CT, TCE or PCE were added using a Hamilton microliter syringe at concentrations below their water solubility ranging from 0 to 50 µM. The vials were immediately closed with Teflon-lined caps and shaken on an end-over-end rotator at 15 rpm for 24 h. The CSs concentrations were then determined using gas chromatography–mass spectrometry (GC-MS). Controls without added solids were run in parallel for each concentration of CS investigated.

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Analytical Methods. Determination of FeII/FeIII ratios in the GRs, CSs by GC-ECD and GC-MS, chloride and formate by IC, CO gas by gas reduction detector, iron and copper in solution and in solids, and solid state characterization including powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microcopy (SEM), transmission electron microscopy (TEM), Xray photoelectron spectra (XPS) analysis were performed according to our previous publications.31, 42 The detailed information is described in supporting information.

3. RESULTS 3.1 Effect of Cu on CT reduction The rate of reduction of CT by GRC12 is substantially increased compared to reaction with sulphate interlayered GR (GRSO4), confirming previous findings (Figure 1).13 The doping of GRC12 with Cu further increases the reduction rate: complete degradation of 29 μM CT in presence of 22 mM FeIIGRC12(Cu) was achieved after 1 h compared to 12 h in presence of non-doped GRC12. The first-order rate constant kobs for reduction of CT by GRC12(Cu) was estimated to 9.94 h-1, 37 times faster than for GRC12 (0.27 h-1) and 250 times faster than for GRSO4 (0.04 h-1). It should be noted that GRC12 reduced CT faster than previously observed,13 which is attributed to the use of a different kind of agitation mechanism (table top shaker vs. head over rotator). The Cu mediated reaction followed the dichloroelimination pathway as observed for GRC12 alone, with carbon monoxide (82.5%) and formate (26.6%) as degradation products.13 3.2 Effect of Cu on CF reduction Doping of GRC12(Cu) causes CF to be reduced, a reaction that does not proceed with non-doped GRC12 (Figure 2). However, dechlorination was not complete for this reaction; the detected products were dichloromethane (DCM) and chloromethane (CM). The mass balance indicates that a product is missing,

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and methane may be formed as well. The first order rate constant for CF reduction by GRC12(Cu) (0.22 h-1) is much lower than that for CT reduction (9.94 h-1). 3.3 Reduction of PCE and TCE No significant degradation of TCE and PCE and reduction products by GRC12 and GRC12(Cu) was observed after 21 days of reaction (Figure S1). 3.4 Sorption of CSs by oxGRC12 Linear sorption isotherms are observed for sorption of CSs to oxGRC12, which has also been observed by Zhao and Nagy with dodecyl sulfate intercalated magnesium-aluminum LDH:43 Qs = Kd Ce

(1)

where Qs is the amount of CS sorbed (mg kg-1), Ce is the equilibrium CS concentration (mg L-1), and Kd is the sorption coefficient (L kg-1). The mass units are used here to allow direct comparison with literature data. The Kd values for sorption of CF, TCE, CT, and PCE are 33, 86, 95, and 217 L kg-1 which increase in the order of hydrophobicity of the CSs (logKow values are 2.88, 2.64, 2.53, and 1.97, respectively, Figure 3).43 The Kd values are within the range observed for sorption of CSs by dodecyl sulfate intercalated magnesium-aluminum LDH.43 3.5 The oxidation state of Cu in GRC12(Cu) The addition of CuII leads to the oxidation of FeII to FeIII in GRC12 (Figure S2). The reduction of CuII was revealed by Cu 2p spectra (Figure S3). Although the signal was noisy due to low contents of Cu ( ~ 0.5%) in the GRC12(Cu), the main Cu 2p peak and the spin split orbit peak 19.75 eV away, can be seen. The main Cu 2p peak is centered at ~933 eV, which is consistent with either Cu0 or CuI states. To further reveal the oxidation state of Cu, redox titration of FeII in GRC12 with CuII was done (Figure S2). Cu in solution increases with Cu addition, but keeps below 0.2 mM throughout. The slope of the regression line is 1.1 confirming that CuI was produced. 8

3.6 SEM and TEM of GRC12(Cu) SEM analysis showed that the morphology of GRC12 (Cu) was the same before and after reaction with CT (Figure 4 a, b). The hydrophobic interaction between C12 molecules causes strong particle aggregation as shown by both the SEM and TEM images (Figure 4 a-c). No Cu2O or metallic Cu particles were observed on the surface of GRC12 particles. In order to investigate the location of Cu species, high resolution TEM with elemental mapping of GRC12(Cu) was done. A homogenous and similar distribution of Cu, Fe, O, S, C elements was observed throughout the GR particles (Figure 4 d-i). The Fe and O distribution throughout the particles is in agreement with the composition of GR, whereas the C distribution indicates the homogenous intercalation of C12 in the interlayer. Sulphur mapping indicates that the SO42- impurities are homogenously distributed in the interlayer and hence no segregation has taken place.

4. DISCUSSION 4.1 Different reactivity of GRC12(Cu) towards CF, CT, TCE and PCE: CSs need to penetrate C12 surfactant layer to react with FeII and CuI active sites Linear isotherms (Figure 3) suggest a partitioning process for CS sorption in the C12 interlayer.43 Surfactant intercalation and Cu doping in GR both enhance the reductive dechlorination of CF and CT. These observations are in line with previous findings for surfactant-intercalated GRs and Cu amended inorganic GRs.13, 33 The CT reduction by GRC12(Cu) is much faster than reduction by GRC12, indicating that Cu is located in the C12 interlayer into which CT is sorbed.13 The CF reduction by GRC12(Cu) is much faster than reduction by Cu amended inorganic GRs,33 demonstrating that also the partitioning of CF into the C12 interlayer favors dechlorination. Formation of DCM and CM products for CF degradation show that the reaction follows a hydrogenolysis pathway and hence involves proton addition (Figure 2b). For CT however a dichloroelimination pathway is followed suggesting stabilization of intermediate radicals such as carbenes.13 Once DCM and CM had formed, they persisted 9

throughout the whole reaction which was also observed in O’Loughlin et al.’s experiments with Cu doped GRSO4.33 The fact that Cu amended GR cannot reduce DCM and CM indicate the reaction may proceed via formation of chloromethyl radicals and carbene (CH3, :CH) rather than sequential hydrogenolysis (Figure 2).33 DCM and CM are formed via a side reaction in which chloromethyl radicals accept an additional electron and a proton (Figure S4). Although strongly sorbed by oxGRC12, PCE and TCE are not reduced by GRC12 or GRC12(Cu) during the experimental period of 21 days which is in contrast with the fact that GRCl can dechlorinate PCE and TCE slowly and the addition of CuII to GRCl enhances the reaction rate by 4.7-7 times at the same pH and similar experimental period used in this work.37 The possible explanation is that the reduction of CS only takes place if they have direct contact with reactive iron hydroxide layer, as the long alkyl strands of C12 molecules are insulators and cannot transfer electrons. The strong sorption/immobilization of PCE by hydrophobic C12 layers can be seen from the large partitioning coefficient compared to the other CSs (Figure 3). The C12 layers show similar partitioning coefficients for CT and TCE but the influence of C12 intercalation on the CSs reduction by GR is dramatically different: enhanced CT reduction but blockage of TCE reduction. We hypothesize that larger molecules such as PCE and TCE may be immobilized once sorbed in the C12 interlayer, while smaller molecules such as CT and CF are mobile in the interlayer. A model for the interlayer configuration based on the charge density of host LDH layers and the cross-sectional area of C12 anions may be used to explain the partitioning of CSs into the C12 interlayer (Figure 6). The cross-sectional area perpendicular to the chain for C12 is around 23 Å2 (assuming that the carboxylate head and alkyl chain occupy the same area).43, 44 A high charge density of the metal hydroxide layer (low FeII/FeIII ratio) results in a small available area per monovalent anion (Å2 charge-1), and interlayer C12 anions are supposedly densely packed. The FeII/FeIII ratio in GRC12 is 2–3 which results in an available area per monovalent anion of 24.4–32.3 Å2 charge-1.43 The space left on iron hydroxide layers to accommodate CS molecules between the C12 anions is hence 1.4–19.3 Å2 (Figure 6a). This narrow space makes access to active sites on the iron

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hydroxide layer very difficult for larger OC molecules such as PCE and TCE molecules which have an estimated critical molecular dimension of 6.7 Å (length) × 6.5 Å (width) × 3.7 Å (thickness). 45 In contrast, smaller CF and CF molecules were estimated less than half of the length of PCE and TCE molecules which makes their access to active sites on the iron hydroxide easier. Thus, the poor mobility in the C12 interlayer may hinder the reduction of PCE and TCE. On the other hand, although the carboxyl head of the C12 molecule is coordinated to Fe in the metal hydroxide layer, the possible tilting of C12 alkyl strands is likely to establish space in middle of the bilayer (Figure 6b). Thus, the sorption capacity of C12 layers to CSs mainly depends on their hydrophobicity as shown in Figure 3, whereas the size of CSs plays a crucial role in reactivity. Increasing the FeII/FeIII ratio in GRC12 (e.g. by use of GRCl as precursor) and intercalating divalent anions (such as dicarboxylates) may increase the space between interlayer organic anions, enabling penetration and mobility of PCE and TCE. 4.2 Regeneration of Cu: does Cu act as a catalyst? 4.2.1

Cu 2p XPS spectra: CuI is not regenerated by GR

The high-resolution Cu 2p XPS spectrum and the titration experiment demonstrate that CuI was produced in the synthesis of GRC12(Cu) (Figure S2, S3). In order to reveal the role of CuI played during CT reduction, the high resolution Cu XPS spectrum for GRC12(Cu) was compared before and after the reaction. The Cu 2p XPS signal becomes sharper and shifts to lower binding energy after reaction with higher concentration of CT (1 mM) (Figure S3). Although CT is added in less than stoichiometric consumption of FeII in GR, and hence allows for remaining FeII in GR to further reduce CuII formed during CT reduction, the changed Cu XPS spectrum after CT reaction suggests that Cu is not regenerated during GRC12(Cu) reduction of CT. Thus, Cu acts as a one-time electron mediator for CT reduction. Considering CuI is produced after addition of CuII to GRC12 (see above), and CO was the main product from the reduction of CT by GR C12(Cu), the following reactions can be proposed for the reactivity of the Cu component: CuII + FeII(GRC12) → CuI + FeIII(GRC12)

(2) 11

CCl4 + H2O + 2CuI → CO + 4Cl- + 2CuII +2H+ (3) The FeIII(GRC12) denotes that the FeII in GRC12 has been oxidized to FeIII in GR (similar to oxGRC12 described above). 4.2.2

Reactivity decreases after active Cu is consumed

To further examine if CuI is regenerated after oxidation to CuII by CT, reactions with Cu in excess of CT and CT in excess of Cu were compared while FeII was in stoichiometric excess of Cu and CT in both cases. For the experiments where CT is in excess of Cu, a dramatic decrease in CT reduction rate was seen after reaction for 0.7 h indicating a change in reaction mechanism (Figure 5 and Figure S5). At this time about 5 μM CT had been reduced which corresponds to oxidation of 10 μM CuII to CuI close to the 13.3 μM Cu added in total (eq. 3). This indicates that after CuI has been oxidized to CuII fast dechlorination by CuI switch to slower reaction with FeII in GR. Apparently, CuI is not regenerated by reduction of remaining FeII in GR, thus questioning the catalytic role of Cu in these systems. The noncatalytic role of Cu is in line with the findings of Maithreepala and Doong,37 although these authors did not specifically address this issue. However, in their study the reactivity of GRCl(Cu) towards reduction of TCE and PCE comes to a complete stop after about 20 days for the low Cu concentrations, which indicates CuI or Cu0 is not regenerated. In addition, in studies of nitrate reduction by fluoride interlayered GR(Cu) a decrease in the nitrate reduction rate was also observed when nitrate concentration was in excess of Cu,34 but the authors attributed this to surface saturation reaction. 4.3 Location of Cu Metal cations added to LDHs usually can be bound in four different ways: i) precipitated as metal hydroxides or other solids at the LDH particle surface, ii) complexed via surface hydroxyl groups, iii) isomorphically substituted with constitutive metal cations in the metal hydroxide layers, and iv) chelated/complexed

with ligands in the interlayers.46 Larger composite redox sensitive cationic

molecules like uranyl (UO22+) and neptunyl (NpO22+) are reduced by GR but precipitated at the edges of 12

GR crystal platelets.28, 29 Smaller redox sensitive cations like Ag+, Cu2+, Hg2+ and Au3+ are expected to be more mobile in the GR phase, but they are also reported to be reduced to metallic nanoparticles on the outer surfaces of GR.32 A rapid uptake of CuII into the interlayer region of LDH in the simultaneous presence of EDTA has been attributed to intercalation of anionic Cu EDTA-metal complexes in the interlayer.47 Copper(II) may form non-charged metal complexes such as CuSO40 and Cu(OH)20 which favor their diffusion into the C12 interlayer of GRC12. It has been reported that a local hydrophilic region exists between the metal hydroxide layer and the hydrated negatively charged carboxylate groups in the interlayer of hydrophobic organo-LDHs.48 Also, hydrated Na+ cations are reported to exist in the interlayer of GRSO4.49 Similarly, hydrated Cu2+ cations could be sitting in this hydrophilic region. Thus, CuII may diffuse into the same region, and subsequently get reduced to CuI in the close proximity of the FeII-FeIII hydroxide layer. 4.3.1

Evidence based on SEM, TEM and XRD

Copper appears evenly distributed throughout the GRC12(Cu) particles demonstrating that Cu has diffused into the interlayers and that it does not exist as separate Cu particles e.g. on the edges of the GRs (Figure 4h). No Cu nanoparticles could be observed with the resolution of the TEM indicating that CuI particles are very small (