(nZVI) PARTICLES PRODUCED BY BOROHYDRIDE

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NANO: Brief Reports and Reviews Vol. 3, No. 5 (2008) 341–349 c World Scientific Publishing Company !

COMPARISON OF THE REACTIVITY OF NANOSIZED ZERO-VALENT IRON (nZVI) PARTICLES PRODUCED BY BOROHYDRIDE AND DITHIONITE REDUCTION OF IRON SALTS QUAN SUN, ANDREW J. FEITZ, JING GUAN and T. DAVID WAITE∗ School of Civil and Environmental Engineering The University of New South Wales Sydney NSW 2052, Australia ∗[email protected]

Dithionite can be used to reduce Fe(II) and produce nanoscale zero-valent iron (nZVI) under conditions of high pH and in the absence of oxygen. The nZVI is coprecipitated with a sulfite hydrate in a thin platelet. The nanoparticles formed are not pure iron but this feature does not appear to affect their degradation performance under air or N2 gas conditions. The efficiency of trichloroethylene (TCE) degradation, when one is employing nanoparticles manufactured using dithionite (nZVIS2 O4 ), is similar to if not slightly better than that of the more conventional borohydride procedure (nZVIBH4 ). The other advantages of the dithionite method are that (i) it uses a less expensive and widely available reducing agent, and (ii) there is no production of potentially explosive hydrogen gas. Oxidation of benzoic acid using the nZVIS2 O4 particles results in different byproducts than those produced when nZVIBH4 particles are used. The low oxidant yield based on hydroxybenzoic acid generation is offset by the production of higher concentrations of phenol. The high concentration of phenol compared to hydroxybenzoic acids suggests that OH• addition is not the primary oxidation pathway when one is using the nZVIS2 O4 particles. It is proposed that sulfate radicals (SO•− 4 ) are produced as a result of hydroxyl radical attack on the sulfite matrix surrounding the nZVIS2 O4 particles, with these radicals oxidizing benzoic acid via electron transfer reactions rather than addition reactions. Keywords: Zero-valent iron; dithionate; oxidation; benzoic acid; oxone.

in contaminated groundwater.1,3,4 The reduction process in ZVI systems is an abiotic redox reaction where the metal serves as an electron donor for the reduction of oxidized species. Under anaerobic conditions, and in the absence of any competitors, iron can slowly reduce water, resulting in the formation of hydrogen gas,5 i.e.

1. Introduction Zero-valent metals are surprisingly effective agents for the remediation of contaminated groundwaters.1,2 Zero-valent iron (ZVI, or Fe0 ), in particular, has been the subject of numerous studies over the last 10 years and has proven effective for the reduction of a diverse range of contaminants, including the dechlorination of chlorinated solvents

Fe0 + 2H2 O → Fe2+ + H2 + 2OH− .

∗

Corresponding author. 341

(1)

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Other reactants, even if present at trace concentrations, may also be reduced by iron. For example, the overall surface-controlled hydrogenolysis of alkyl chlorides (R–Cl) by Fe0 is likely to occur as follows3,5,6 : Fe0 + R−Cl + H+ → Fe2+ + R−H + Cl− .

(2)

In addition to reducing toxic contaminants, ZVI also can reduce O2 : 2Fe0 + O2 + 2H2 O → 2Fe2+ + 4OH− .

(3)

In remedial applications in which the reductive properties of ZVI are exploited, such as in ZVI permeable reactive barriers,7–9 the reaction of ZVI with O2 is considered undesirable because O2 competes with contaminants for ZVI. Furthermore, the Fe(II) and Fe(III) precipitates produced by these reactions can decrease the reactivity and permeability of the ZVI permeable reactive barriers.10 However, these reactions also may have some beneficial uses, since recent studies have shown that nanoscale ZVI (nZVI) in the presence of O2 can lead to the oxidation of contaminants.11 Results show that the nZVI/O2 process degrades contaminants via a process involving the formation of hydroxyl radicals arising from the rapid oxidation of nanoscale iron on exposure to air.12 As the iron corrodes and reacts with O2 , it forms hydrogen peroxide: Fe0 + O2 + 2H+ → Fe2+ + H2 O2 .

(4)

The H2 O2 produced could oxidize another Fe0 : Fe + H2 O2 → Fe 0

2+

−

+ 2OH .

Fe3+ + 3BH− 4 + 3H2 O → Fe(s) + 3B(OH)3 + 10.5H2 .

(7)

As a result, if nZVI technology is to be widely used, alternate, less expensive methods of producing nZVI are required. A new synthesis method that uses dithionite (nZVIS2 O4 ) as a reductant for nZVI production has recently been reported.14 nZVIS2 O4 is a substantially less expensive reducing agent than borohydride and, at high pH, is sufficiently powerful to reduce Fe(0) to Fe(II), i.e. − Fe2+ + S2O2− 4 + 4OH

→ Fe(s) + 2SO2− 3 + 2H2 O .

(8)

At high pH, however, Fe(II) quickly oxidizes to Fe(III) and it is therefore necessary to conduct the reaction under anoxic conditions (e.g. under N2 ). Additional insight into the nZVIS2 O4 -mediated process has been obtained from comparative studies of degradation of benzoic acid and TCE using nZVIS2 O4 particles and nZVIBH4 particles.

2. Materials and Methods (5)

This process results in a four-electron transfer and an overall stoichiometry of two moles of Fe0 oxidized per mole of O2 . Alternatively, H2 O2 could react with species such as Fe(II) via the Fenton reaction: Fe2+ + H2 O2 → Fe3+ + OH• + OH− .

iron particles, especially when one is using iron nanoparticles. The most-widely-used method of producing nZVI (nZVIBH4 ) involves the reduction of Fe(III) by sodium borohydride; however, this method is expensive and potentially dangerous because of the H2 gas produced.13

(6)

These reactions could occur on the Fe0 surface or could involve transfer of electrons through an iron oxide layer, depending upon the reaction rates and affinity of the species for surfaces. Although the four-electron transfer process is usually the dominant mechanism of O2 -mediated corrosion, the twoelectron transfer process can result in production of significant amounts of O2− 2 or H2 O2 , especially after an oxide coating has been formed on the surface. The discovery of the oxidation degradation pathway dramatically widens the applicability of

2.1. Regents All chemicals were of high purity and were used as received. Reagents were prepared using 18 MΩcm Milli-Q water (Millipore). The reactions of nZVI were studied in pH-buffered solutions with an ionic strength adjusted to 0.03 M with NaCl. When necessary, the pH of the solutions was adjusted using 0.1 N HCl or 0.1 N NaOH. Benzoic acid, trichloroethylene (TCE), ethylendiaminetetraacetic acid (EDTA), hydroxybenzoic acids, dihydroxybenzoic acids, phenol, ferric chloride, ferrous chloride, sodium borohydride and sodium dithionate were obtained from SigmaAldrich. All glassware and containers used were soaked in 5% HCl overnight and rinsed thoroughly with Milli-Q water prior to use. pH measurements were made on a Metrohm 692 pH/ion meter calibrated against standard buffers.

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Comparison of the Reactivity of Nanosized Zero-Valent Iron (nZVI ) Particles

2.2. Manufacture 2.2.1. Method for ZVI synthesis using sodium dithionite and ferrous chloride Nanoscale ZVI particles were produced by adding 40 mL of 0.1 M FeCl2 (FeCl2 · 4H2 O, 99%, Sigma) aqueous solution dropwise to a 50 mL, 0.2 M Na2 S2 O4 (85% purity, Sigma) aqueous solution at ambient temperature with magnetic stirring. NaOH was added to maintain the pH at 10 or above. Fe2+ is reduced and Fe0 is precipitated under these conditions according to the following reactions14 : Reaction

EH (volts)

− S2 O2− 4 + 4OH 2− → 2SO3 + 2H2 O + 2e−

Fe2+ + 2e− → Fe(s)

S2 O2− 4

2+

−

+ Fe + 4OH → Fe(s) + 2SO2− 3 + 2H2 O

+ 1.12 − 0.44 + 0.68

The first 5 min of the reaction was carried out under nitrogen gas and the container was then sealed for the remainder of the reaction time (4 h). The formation of black particulates occurred during the first 5 min. Dry metal particles were obtained by washing the wet precipitates with 10−4 M HCl solution and Milli-Q water and drying using a vacuum drier for 2 days. It was essential to store the dry particles under an inert atmosphere (e.g. argon or nitrogen) in order to prevent rapid oxidation.

2.2.2. Method for ZVI synthesis using sodium borohydride and ferric chloride Nanoscale ZVI particles were produced by adding 0.16 M NaBH4 (98%, Aldrich) in 0.1 M NaOH solution dropwise to a 0.1 M FeCl3 · 6H2 O (98%, Aldrich) aqueous solution at ambient temperature with magnetic stirring. Fe3+ is reduced and precipitated according to the following reaction: Reaction

EH (volts)

3BH− 4 + 9H2 O → 3B(OH)3 + 10.5H2 + 3e−

+ 0.481

Fe3+ + 3BH− 4

+ 0.444

Fe3+ + 3e− → Fe(s)

+ 9H2 O → Fe(s) + 3B(OH)3 + 10.5H2

− 0.037

343

Metal particles were obtained by washing the wet precipitates with 10−4 M HCl 3–4 times and storing in 10−4 M HCl at a concentration of 200 mg Fe/mL. For dry particle characterization, the particle suspension was freeze-dried under vacuum. In some instances, particles were prepared in a glovebox under a N2 atmosphere in order to limit oxidation losses during particle manufacture.

2.3. Analysis techniques 2.3.1. Analysis of TCE and byproducts of benzoic acid Hydroxybenzoic acid (HBA) and dihydroxybenzoic acid (DHBA) concentrations were determined by high-performance liquid chromatography (HPLC) using a Hewlett-Packard 1100 series HPLC system equipped with a 250 × 4.6 mm Waters Spherisorb ODS-2 5 µm column (Alltech, IL). A two-solvent gradient elution, consisting of water (pH 3) and acetonitrile (85:15, v/v %) at a flow rate of 1.0 mL/min, was used to separate benzoic acid (BA) and isomers of HBA. The p-HBA isomer was quantified at 255 nm; o-HBA and m-HBA isomers were quantified at 290 nm; phenol and BA were quantified at 270 nm. All standard curves were linear with regression coefficients of > 0.999 in all cases. The method detection limits for BA and HBA were 2.5 µM and 0.1 µM respectively. The TCE concentrations were measured using solid phase microextraction (SPME) followed by a GC (HP-6890)/MS (HP5973) equipped with an electron capture detector (ECD) and an HP-5MS capillary column (30 m × 250 µm × 0.25 µm). For the SPME process, 4 mL vials were filled with 2 mL of aqueous samples containing TCE and extracted onto the fiber for 15 min under rapid stirring conditions. The fiber was removed from the sample and introduced into the GC/MS injector, where TCE was thermally desorbed for 3 min and injected onto the column in splitless mode. The temperature of the detector was set at 280◦ C, helium (pure carrier gas grade) was used as the carrier gas at a flow rate of 1.0 mL/min and the electron impact (EI) ionization energy was set to 70 eV.

2.4. Batch experimental setup An aqueous suspension of particles prepared by the borohydride method and particles prepared by the dithionite method were added to the respective aqueous solutions containing TCE. The

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concentration of TCE was measured over a period of 40 h in the absence of atmospheric oxygen. All results were compared against a control solution of TCE (to which no ZVI was added). BA experiments were conducted at pH 3 and 4.5 (controlled through HCl addition) under aerobic conditions with a BA concentration of 10 mM. TCE experiments were carried out under nitrogen atmospheres without pH control. All experiments were carried out at room temperature (20 ± 2◦ C) and conducted in 100 mL serum bottles using a total suspension volume of 50 mL. nZVI suspensions were prepared from 5 mg Fe/mL stock suspensions, and BA and ligands were added using a micropipette. Each bottle was continuously shaken at 175 rpm using an orbital shaker (Hybritech Incorporated) for the duration of the experiment.

3. Results and Discussion 3.1. Characterization of nZVIS2 O4 particles The morphology of the nZVIS2 O4 particles is uniquely suited to oxidation applications as the particles are very thin platelets (< 10 nm thick) [Fig. 1(b)], unlike the larger spheres (50 nm) produced in the borohydride-mediated synthesis [Fig. 1(a)]. Particle size measurements have also shown that the aggregate size in aqueous suspension is also smaller than particles manufactured using the borohydride technique (Fig. 2). The particles manufactured using the dithionite technique form small, thin, black, platelet-like crystals [Fig. 1(b)] that appear to be a mixture of very small elemental iron particles imbedded in a

Fig. 2. Aggregate size at pH ∼ 6–7 for both dithionite (Na2 S2 O4 ) and borohydride (NaBH4) produced nZVI particles.

sulfite hydrate crystal matrix. The particles are too thin and too readily oxidized to be analyzed using conventional XRD analysis; however, XRD analysis of the brown oxidized particles, shortly after exposure to air, shows that the particles are consistent with an Fe(II)-HSO3 · H2 O precipitate and not with FeS or FeS2 (data not shown). Elemental analysis shows the presence of sulfur in the particle aggregates (Table 1). TEM energy-dispersive x-ray spectroscopy (EDAX) analysis further supports this assessment and indicates that the Fe:S ratio is not uniform throughout a platelet (Fig. 3). TEM diffraction analysis is averaged and the presence of rings indicates that the particles are crystalline, with at least two phases within the platelets over a small (10 nm) area (Fig. 4). A definitive method for determining the redox state of the iron could be achieved by stabilizing the particles in a new oxygen impervious matrix and then analyzing for the redox

Table 1. X-ray fluorescence (XRF) analysis of a freeze– dried sample produced from FeCl2 and sodium dithionite. The elemental analysis indicates that the bulk sample Fe:S ratio is approximately 3:1 wt%.

(a)

(b)

Fig. 1. TEM of nZVI spheres produced using the borohydride technique (a) and nZVI platelets produced using the dithionite technique (b).

Compound

Unit

Value

Na2 O SO3 Fe2 O3 Total Loss on ignition

% % % % %

0.82 32.5 59.6 94.5 5.5

∗

Major elemental contribution (g/g oxidized sample)∗ 0.006 0.128 0.413

Note: XRF analysis does not include the element O, H or C.

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state of the iron using X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS). This will be the subject of future studies.

3.2. Degradation of TCE under reducing conditions

(a) Low sulfur

The results of comparison experiments between particles prepared by the borohydride (nZVIBH4 ) and dithionite (nZVIS2 O4 ) methods are shown in Fig. 5, where C/C0 represents the ratio of the experimental concentration of TCE to the initial concentration of TCE. It can be seen from the figure that the two types of particles are similarly effective in degrading TCE under anoxic conditions. TCE concentration was decreased by about 50% after the reactions had proceeded for 40 h.

3.3. Degradation of BA under oxic conditions

(b) High sulfur

Fig. 3. TEM EDAX analysis of a freeze-dried sample produced from FeCl2 and sodium dithionite, illustrating the differing Fe:S ratios within a platelet. The copper present in the figure is interference from the copper-coated sample grid.

Fig. 4. TEM diffraction pattern of air-exposed nanosized particles manufactured from FeCl2 and sodium dithionite (scale bar = 10 nm).

It was surprising to discover after the good TCE degradation results that the apparent oxidizing ability of the dithionite-prepared nZVIS2O4 particles was considerably less than that of the nZVIBH4 particles. The amount of p-HBA produced using the nZVIS2 O4 particles was only 14% of the amount obtained using nZVIBH4 (Fig. 6). The reason for the low HBA concentrations could be related to the presence of the sulfite matrix encasing the nZVIS2 O4 particles. Sulfite (SO2− 3 ) and bisulfite (HSO− ) react rapidly with hydroxyl radicals at 3

Fig. 5. Comparison of trichloroethylene (TCE) degradation performance using ZVI particles prepared according to the dithionite and borohydride methods ([TCE]0 = 17.52 ppm; [nZVI] = 2 g/L).

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Fig. 6. Comparison of oxidation performance for the nZVIBH4 and nZVIS2 O4 particles by measuring p-HBA evolution (10 mM benzoic acid; pH 3, 0.9 mM nZVIBH4 or nZVIS2 O4 ).

almost diffusion-controlled rates: •− − OH• + SO2− 3 → OH + SO3 ,

k = 5.5 × 109 M−1 s−1 ,

(9)

k = 9.5 × 109 M−1 s−1 .

(10)

•− OH• + HSO− 3 → H2 O + SO3 ,

The newly formed sulfite radicals can react with O2 to form peroxysulfate radicals: •− SO•− 3 + O2 → SO5 ,

k = 5.5 × 109 M−1 s−1 . (11)

The sulfite and peroxysulfate radicals are mild oxidants, especially when compared to hydroxyl radi2− cals. The redox potential for the SO•− 3 /SO3 couple •− is 0.97 V at pH 3 and 1.19 V for the SO5 /SO2− 5 cou16 ple at pH 3. The redox potential for the hydroxyl radicals is much greater, at 2.7 V, under acidic conditions.17 Sulfite radicals are very selective oxidants, reacting rapidly with hydroxybenzenes at high pH (e.g. ∼ 107 –109 M−1 s−1 ) but slowly or not at all at lower pH.16,18 While the peroxysulfate radical is a stronger oxidant than the sulfite radical,19 an important 2+ and the reaction is the quenching of SO•− 5 by Fe formation of the sulfate radical, SO•− 4 , i.e. 2+ Fe2+ + SO•− + HSO− 5 + H2 O → FeOH 5 ,

k = 6.5 × 105 M−1 s−1 ,

(12)

k = 3.1 × 104 M−1 s−1 .

(13)

2+ Fe2+ + HSO− + SO•− 5 → FeOH 4 ,

The sulfate radical is a very strong oxidant (between 2.5 and 3.1 V) and is similar in oxidation power

Fig. 7. Benzoic acid oxidation via hydroxyl radical attack (adapted from Ref. 21).

to the hydroxyl radical.19 Further complicating the reaction mechanism is the iron-catalyzed oxidation of sulfite, i.e. + FeOH2+ + HSO− 3 [FeOHSO3 H] , K0 = 600 dm3 /mol ,

(14)

[FeOHSO3 H]+ → Fe2+ + SO•− 3 + H2 O k = 0.065 s−1 .

(15)

The production of sulfite radicals via this pathway will also lead to the generation of sulfate radicals. Despite the presence of Fe2+ and H2 O2 in the above iron-catalyzed oxidation of the sulfite system, there is no evidence of a hydroxyl-radical-mediated degradation when one is using tertiary butanol as a scavenger.20 Therefore, if the hydroxyl radicals are initially scavenged by the sulfite matrix and form sulfate radicals, there is a possibly that oxidation with the nZVIS2 O4 particles is producing a sulfated adduct of BA as a byproduct in addition to HBAs. Another likely oxidation pathway is the formation initially

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Fig. 8.

347

Benzoic acid degradation using nZVIS2 O4 at different pH values.

of a radical cation with rapid decarboxylation to form the phenyl radical19 and ultimately phenol (Fig. 8). An important distinction between the sulfate and hydroxyl radicals is that hydroxyl radicals tend to undergo hydrogen abstraction and addition reactions with aromatic compounds whereas sulfate radicals preferentially undergo electron transfer reactions. This results in different reaction products and different yields than would be obtained in the case of hydroxyl radical attack.23–25 For example, oxidation of the dye Acid Yellow 9 with sulfate radicals leads to the formation of a sulfated adduct of Acid Yellow 9 whereas oxidation with hydroxyl radicals leads to the formation of hydroxylated dye products.23 The sulfated adduct is considered to form from the reaction between the dye radical cation (formed from electron transfer) and sulfate radicals. The extent of BA decarboxylation will be inhibited by protonation of the carboxylic group, whereas benzoates readily undergo decarboxylation.22 Therefore, increasing the pH to close to or above the pKa of BA (pKa = 4.2) should lead to an improved phenol yield if the primary route of oxidation is via electron transfer (Fig. 8). The net effect is that the total hydroxybenzoic yield could be considerably lower for the nZVIS2 O4 particles than for the nZVIBH4 particles if degradation is via an electron transfer pathway. To investigate the mechanism of nZVIS2 O4 versus nZVIBH4 mediated oxidation of BA, the number of oxidation products monitored was broadened to include all HBA isomers, phenol and all DHBA isomers. A summary of the results is included in Fig. 9. It is clear that the nZVIBH4 particles produce a greater total quantity of oxidation products than the nZVIS2 O4 particles, particularly HBAs. Interestingly, the two types of particles produce a similar

Fig. 9. Phenol, hydroxybenzoic acid and dihydroxybenzoic acid formation under different conditions. [nZVIS2 O4 ] = 0.9 mM; [nZVIBH4 ] = 0.9 mM; [oxone] = 0.9 mM; [Fe(II)] = 0.1 mM.

amount of phenol at pH 3 and at pH 4.5. No DHBAs were detected using the nZVIS2 O4 particles. The results suggest that a different oxidation mechanism is dominant for the nZVIS2 O4 particles compared to the nZVIBH4 particles. One explanation is that the nZVIS2 O4 particles are facilitating primarily electron-transfer-mediated oxidation rather than hydroxyl-mediated oxidation as discussed above. This could be via the proposed sulfate radical pathway. In contrast to the formation of phenol through reaction with SO•− and 4 decarboxylation of BA in the nZVIS2 O4 system, the pathway for phenol formation from reaction with hydroxyl radicals is typically via HBA intermediates (Fig. 7). The detection of trace amounts of DHBA in the nZVIBH4 system is further evidence that the HBAs are being oxidized by hydroxyl radicals.

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In order to determine whether sulfate radicals could be involved, we conducted experiments with oxone. Oxone is the commercial name of the triple salt 2KHSO5 · KHSO4 · K2 SO4 , which is the source of the strong oxidant peroxymonosulfate (KHSO5 ) and is also known as peroxomonosulfate (PMS). As mentioned above, when Fe2+ interacts with PMS, freely diffusible sulfate radicals are formed. 2+ Fe2+ + HSO•− + SO•− 5 → FeOH 4 .

primary oxidation pathway when one is using the nZVIS2 O4 particles, with the possibility that the coprecipitated sulfite matrix is influencing radical production. There is a possibility that oxidation by the nZVIS2 O4 particles occurs via generation of sulfate radicals (SO•− 4 ) which primarily oxidize benzoic acid via electron transfer reactions rather than addition reactions.

(16)

The presence of oxone alone at pH 3 and pH 4.5 did not result in the formation of oxidation products, ruling out KHSD5 as an oxidant. Addition of Fe2+ to oxone did, however, result in the formation of oxidation byproducts. Low levels of HBAs were observed at both pH 3 and pH 4.5 but phenol was observed only at pH 4.5. This is consistent with oxidation via sulfate radicals, where decarboxylation of BA will be inhibited by protonation of the carboxylic group (i.e. pKa = 4.2) (Fig. 8). The Fe(II)-oxone results are more comparable to the oxidation products observed using nZVIS2 O4 rather than nZVIBH4 particles, i.e. a higher relative concentration of phenol compared to the HBA and an increase in phenol concentration with an increase in pH above the pKa of BAs. The results support the dominance of an electron-transfer-mechanism for the nZVIS2 O4 mediated oxidation of BA rather than a hydroxyl radical pathway, although whether this is via sulfate radicals is at this stage unclear.

4. Conclusion A new method for nanoscale zero-valent iron synthesis using sodium dithionite to reduce ferrous chloride under conditions of high pH and in the absence of oxygen is described here. The advantages of the dithionite method are that (i) it uses less expensive and widely available reducing agents, and that (ii) dithionite reduction of metal ions does not produce explosive hydrogen gas. The reductive performance of the nZVIS2 O4 particles for TCE degradation is similar to that when one is using conventionally produced nZVIBH4 particles. Oxidation of benzoic acid using the nZVIS2 O4 particles results in byproducts different from those obtained with the nZVIBH4 particles. The low oxidant yield based on hydroxybenzoic acid concentration produced is offset to some degree by the production of higher concentrations of phenol. The high concentration of phenol compared to hydroxybenzoic acids suggests that OH• addition is not a

Acknowledgments Financial support from the Australia Research Council (ARC) and industry partner Orica Australia via ARC Linkage Grant LP0348062 is gratefully acknowledged.

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