Abiotic Bromination of Soil Organic Matter - Environmental Science

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Abiotic bromination of soil organic matter Alessandra C. Leri, and Bruce Ravel Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03937 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

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Environmental Science & Technology

1 2

Abiotic bromination of soil organic matter

3 4 5 6 7 8 9 10 11 12 13

Alessandra C. Leri1,* and Bruce Ravel2 1

Department of Natural Sciences, Marymount Manhattan College, 221 E 71st St., New York, NY 10021, USA, (212) 517-0661, [email protected] 2

National Institute of Standards and Technology, 100 Bureau Drive MS 8520, Gaithersburg, MD 20899, USA, (631) 344-3613, [email protected]

*Corresponding author.

ACS Paragon Plus Environment


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Leri & Ravel, 2 14

Abstract

15 16

Biogeochemical transformations of plant-derived soil organic matter (SOM) involve

17

complex abiotic and microbially mediated reactions. One such reaction is halogenation,

18

which occurs naturally in the soil environment and has been associated with enzymatic

19

activity of decomposer organisms. Building on a recent finding that naturally produced

20

organobromine is ubiquitous in SOM, we hypothesized that inorganic bromide could be

21

subject to abiotic oxidations resulting in bromination of SOM. Through lab-based

22

degradation treatments of plant material and soil humus, we have shown that abiotic

23

bromination of particulate organic matter occurs in the presence of a range of inorganic

24

oxidants, including hydrogen peroxide and assorted forms of ferric iron, producing

25

both aliphatic and aromatic forms of organobromine. Bromination of oak and pine

26

litter is limited primarily by bromide concentration. Fresh plant material is more

27

susceptible to bromination than decayed litter and soil humus, due to a labile pool of

28

mainly aliphatic compounds that break down during early stages of SOM formation. As

29

the first evidence of abiotic bromination of particulate SOM, this study identifies a

30

mechanistic source of the natural organobromine in humic substances and the soil

31

organic horizon. Formation of organobromine through oxidative treatments of plant

32

material also provides insights into the relative stability of aromatic and aliphatic

33

components of SOM.


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Leri & Ravel, 3 34 35

1. Introduction The geochemical behavior of bromine (Br) in soils has received little attention, in part

36

because terrestrial concentrations tend to be low and in part because inorganic bromide (Br-)

37

is widely considered unreactive in the soil environment. The latter perception is rooted in the

38

low sorption affinity of Br- for anionic mineral surfaces,1 a characteristic which, along with

39

its solubility and general scarcity, makes Br- useful as a tracer in soil hydrology.2-4 While Br-

40

may not be prone to adsorption, it does appear reactive towards terrestrial organic matter;

41

recent studies have demonstrated fractionation of Br into organic and inorganic pools in

42

lakes,5-7 peat bogs,8 and forest soil.9

43

This ubiquity of brominated SOM provided a molecular explanation for the indirect

44

evidence of Br fractionation that had been accumulating since Yamada’s 1968 measurements

45

of extraordinarily high Br levels in humic-rich andosols.10 Globally, soil Br concentrations

46

average around 5 mg·kg-1 11 but show a wide range, tending to increase with two distinct

47

factors: 1) proximity to the sea shore; and 2) organic matter content.12 As a result, total soil

48

Br does not necessarily correlate with inorganic Br-. In the organic fraction of highly leached

49

forest soils, all detectable Br is bonded to carbon.9 In peats, organobromine content is

50

proportional to total Br, which is not the case for Cl.13 These findings suggest that Br- may

51

function as a limiting reactant in soil bromination processes.

52

In principle, bromination of SOM could proceed through an assortment of

53

mechanisms. There exist several biological pathways involving various brominating

54

enzymes, including haloperoxidases and other halogenases.14-17 Vanadium-based

55

bromoperoxidases, perhaps the best characterized of the brominating enzymes, catalyze the

56

oxidation of inorganic Br- by hydrogen peroxide (H2O2) to a “Br+-like” intermediate that goes

57

on to brominate electron-rich organic substrates non-specifically.18, 19 Haloperoxidase-like

58

activity has been detected in forest soil,20 and application of the exogenous enzyme



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Leri & Ravel, 4 59

incorporates Br into plant litter,9 implicating haloperoxidase catalysis as one possible

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mechanistic explanation for the natural organobromine observed in SOM. Another pathway

61

for enzymatic bromination involves methyl transferases, which are implicated in the

62

production of volatile halocarbons like the methyl bromide emitted to the atmosphere by

63

higher plants.21

64

Several abiotic bromination pathways have also been identified in natural systems.

65

Volatile bromocarbons like methyl bromide (CH3Br) form abiotically through alkylation of

66

Br- ions during the oxidation of SOM by ferric iron (Fe3+).22 Combustion of biomass is

67

another abiotic source of CH3Br.23 In aqueous systems, abiotic bromination may occur

68

through photooxidation reactions. For example, phenol can be brominated under UV-Vis

69

irradiation in the presence of oxidants like Fe3+ or nitrate (NO3-) through a suspected

70

dibromide radical (Br2-·) intermediate.24 Formation of Br2-· in natural waters is thought to

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happen through radical mechanisms involving oxidation of Br- by hydroxyl radicals and/or

72

the triplet state of dissolved organic matter.25 Irradiation of natural seawater spiked with

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phenol generates para-bromophenol, among other by-products;26 goethite and Fe(II) may act

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as photosensitizers towards this bromination of phenol.27 Salicylic acid can also be

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brominated through photochemical mechanisms in seawater.28 In these studies, phenol and

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salicylic acid function as models for common aromatic moieties in natural organic matter. A

77

recent study has shown more generally that dissolved organic matter reacts with

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photochemically generated reactive Br species in seawater to produce organobromines.29

79

Bromination of marine particulate organic matter has also been shown to occur through

80

various peroxidative, photochemical, and Fenton-like mechanisms, creating both aliphatic

81

and aromatic forms of organobromine.30

82 83

The abiotic natural bromination reactions that have been documented thus far22-29 are associated with production of volatile and dissolved forms of organobromine. It remains


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Leri & Ravel, 5 84

unknown whether abiotic mechanisms play a role in the incorporation of Br into humus and

85

other soil particulates. In this study, we performed laboratory-based reactions designed to

86

model abiotic oxidations of decaying organic material by H2O2 and/or Fe3+ salts in the soil

87

environment. Our models explore the effects of decay stage, H2O2 concentration gradients,

88

light vs. dark reactions, and pH, among other environmental variables, on abiotic bromination

89

of particulate SOM. Organobromine concentrations in treated particulates were measured

90

using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy,

91

according to our rigorous quantification protocol.31 By differentiating aliphatic from aromatic

92

forms of organobromine, we were able to assess the relative reactivity of different SOM

93

components towards oxidative bromination.

94

2. Materials and Methods

95 96

2.1. Field sample details Plant material was collected from the Brendan Byrne State Forest in New Jersey,

97

USA (DMS 39° 53’ 27.66 ” N 74° 34' 46.63" W), an area of the Pine Barrens lying ~15 km

98

from the Atlantic coast. This forest features a mix of coniferous and deciduous vegetation

99

above sandy, acidic soil (pH 3-5). Fresh, healthy pine needles and oak leaves were harvested

100

from pitch pine (Pinus rigida) and white oak (Quercus alba) trees in late summer. Mixed,

101

weathered plant litter was grab-sampled from the forest floor below the tree canopy, along

102

with samples of dark-colored, fine-grained humus from the top of the soil organic horizon.

103

Plant material (both fresh and decayed) was air-dried for one week in the laboratory, then

104

pulverized in a mechanical mill. Humus was passed through a 5-mm sieve. The resulting

105

powders were stored at -20˚C until treatment or analysis.

106 107 108

2.2. Abiotic bromination treatments Treatments were carried out on pulverized material (1.0 g of pine powder, oak powder, litter powder, or humus) in thick suspensions buffered at pH 3.0 (phosphate) or 5.0



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Leri & Ravel, 6 109

(acetate) with various combinations of the following reagents: potassium bromide (KBr; 40

110

mM), hydrogen peroxide (H2O2; 2.0 mM except where otherwise specified), ferric nitrate

111

nonahydrate (Fe(NO3)3 9H2O; 20 mM), ammonium ferric citrate (C6H8O7 xFe3+ yNH3;

112

20 mM), ferric sulfate (Fe2(SO4)3; 20 mM), and ferrous sulfate (FeSO4; 100 µM). Specified

113

concentrations refer to final solution volume, which was 10.0 mL for all treatments. The

114

mixture of 1.0 g particulates and 10.0 mL aqueous solution created a thick heterogeneous

115

suspension intended to resemble conditions of moist plant litter decomposing on the forest

116

floor. The levels of added Br- are similar to those used in a previous lab-based study showing

117

production of halomethanes from halide-treated soil.22 These millimolar Br- levels are high

118

relative to typical concentrations in natural environments. The intention of using large

119

excesses of Br- was to promote bromination on relatively short experimental timescales.

120

Reagents were combined with plant material in 50-mL centrifuge tubes, vortexed, and

121

placed on a shaker for 24, 48, or 96 hours of reaction time at room temperature. For the

122

“dark” reactions, centrifuge tubes were wrapped in aluminum foil. Following the treatments,

123

solids were rinsed using six successive rinse/vortex/centrifuge/decant cycles with 50-mL

124

portions of deionized water to remove unreacted Br-. Thorough rinsing is crucial to prevent

125

excess Br- from dominating the Br XANES signal. Centrifugation was performed at 10˚C and

126

3000 rpm for 20 min per cycle. Rinsed solids were spread on watch glasses to dry overnight

127

at 35˚C before being compressed into 13-mm pellets in a hydraulic laboratory press at 15,000

128

lbs.

129

2.3. Br K-edge X-ray absorption near-edge structure (XANES) spectroscopy

130

2.3.1. Beamline and data collection specifics

131

Br K-edge XANES spectra were collected at the National Synchrotron Light Source

132

(Upton, NY, USA) at beamline X23A2, an unfocused bend magnet beamline with a Si(311)

133

fixed-exit monochromator of a Golovchenko-Cowan design.32 Sample pellets were mounted


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Leri & Ravel, 7 134

on halogen-free XRF tape and exposed at a 45° angle to the incoming X-ray beam, with a

135

spot size of 4.5 mm (H) x 0.5 mm (V) centered on the pellet. XANES scans were measured

136

from 13,374 to 13,674 eV using a 0.25 eV step size around the Br K-edge (13,474 eV) and

137

0.5-5.0 eV step sizes above and below the edge, with 1.0-sec dwell times. Spectra were

138

collected in fluorescence mode using a four-element Si-drift detector with the algorithm of

139

Woicik et al.33 used to correct detector deadtime. Between four and twelve XANES scans

140

were collected per sample, depending on spectral noise.

141

Samples were measured using two internal standards: an energy calibration

142

transmission standard mounted behind the sample and a [Br] quantification standard mounted

143

alongside the samples. The transmission standard consisted of bromophenol blue, which has a

144

discrete absorption maximum well suited to energy alignment. The quantification standards

145

were pellets containing specific concentrations of KBr homogenized via dissolution in a

146

matrix of sodium polyacrylate. Energy alignment and quantification were achieved as

147

described below.

148

2.3.2. Analysis of XANES data for Br concentration and speciation

149

XANES spectra were processed using the ATHENA program in the Demeter software

150

package.34 Individual XANES scans were aligned using the absorption maximum of the

151

bromophenol blue transmission spectrum, which was calibrated to 13,473.4 eV. After energy

152

alignment, XANES scans were averaged. Each averaged spectrum was background-

153

subtracted and normalized with polynomials fit through the pre-edge (13,412 eV to 13,442

154

eV) and post-edge (13,552 eV to 13,652 eV) regions. The “edge step” of each XANES

155

spectrum was recorded as the difference at 13,472.0 eV between these two regression lines.

156

The edge step of a Br XANES spectrum is linearly related to the absolute

157

concentration of Br in the sample.31 A series of KBr pellet standards were used to produce a

158

calibration line from 0-500 mgkg-1 total Br concentration. Beamline X23A2 featured a



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Leri & Ravel, 8 159

moveable sample stage on which several pellets were mounted at once. Once the calibration

160

line was established, an internal KBr standard was included adjacent to sample pellets on

161

each mount, and the edge step of this quantification standard was used to position the detector

162

for subsequent measurements of all pellets on the mount. Edge steps of the sample spectra

163

could therefore be converted into total Br concentrations using the calibration line. This could

164

be achieved directly provided the pellets were of uniform mass (335 ± 5 mg). If sample

165

pellets were underweight, a thickness correction factor was applied as we have described in

166

detail previously.31 Uncertainties in the edge step measurement were computed as 1σ error.

167

To determine Br speciation in the samples, normalized XANES spectra were fit via

168

linear combination (LC) with spectra of Br-containing model compounds, several examples

169

of which are shown in Fig. 1A. The vertical lines on the chart intersect the characteristic

170

absorption maxima of aliphatic organobromine (represented by tert-butylbromide and 1-

171

bromoadamantane, Fig. 1A, a-b) and aromatic organobromine (represented by para-

172

bromophenol and –dibromobenzene, Fig. 1A, c-d). These sharp, low-energy features

173

correspond to transitions of the 1s electron to σ* and π* orbitals associated with the C-Br

174

bond. The typical absorption maximum, or “white line,” for an aromatic organobromine

175

appears almost a full eV higher in energy than that of an aliphatic compound, due to

176

differences in bond energy imparted by partial π character of the aromatic C-Br bond. When

177

there is no covalent bond present, XANES spectral features appear broader and at higher

178

energies, as exemplified by the spectrum of aqueous KBr (Fig. 1A, e). The LC fitting routine

179

provides quantitative estimates of the relative proportions of inorganic Br- and

180

aromatic/aliphatic organobromine in sample XANES spectra with 10-15% error. Spectral

181

fitting was carried out in ATHENA on the first derivative of the near-edge region of each

182

spectrum, from 13,465 to 13,500 eV.

183


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Leri & Ravel, 9 184 185 186

3. Results and Discussion 3.1. Background Br in untreated plant material and soil humus Tree-harvested white oak leaves and pitch pine needles have very little Br—

187

undetectable in the case of the former and at the detection limit (~1 mg·kg-1) in the latter, with

188

a weak aromatic organobromine signal. Compared with fresh plant materials, soil humus is

189

enriched in Br, with 5 mg·kg-1 aromatic organobromine revealed in a noisy XANES spectrum

190

(Fig. 1B, a). These low concentrations are consistent with the low background Br in the

191

highly leached soil environment of the Pine Barrens.

192

3.2. Rationale for treatments of plant material and soil humus

193

Treatments of SOM particulates with Br- and H2O2 at acidic pH were designed to

194

create hypobromous acid (HOBr) or a similar reactive Br species through a peroxidative

195

reaction:

196

Br- + H2O2 + H+ HOBr + H2O

197

Oxidized forms of Br, including HOBr, hypobromite (OBr-), and elemental bromine (Br2),

198

are highly reactive towards olefins and phenolic molecules,35 and plant material contains

199

myriad potential substrates for electrophilic bromination, including unsaturated lipids (fatty

200

acids, waxes, terpenoids), aromatic amino acids, polyphenolic molecules, and lignin.

201

Hydrogen peroxide is ubiquitous in the soil environment. Substantial quantities of H2O2 are

202

introduced to soil from wet and dry deposition, and decomposition of H2O2 to the hydroxyl

203

radical may have far-reaching effects on the oxidative chemistry of soils.36

(1)

204

Some treatments incorporated oxidants other than H2O2, including various forms of

205

ferric iron, which could function as a potential metal catalyst for Eq. 1 or be involved in an

206

alternative bromination mechanism, such as the previously demonstrated oxidation of SOM

207

with concomitant emission of CH3Br.22 We also investigated the effects of several other



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Leri & Ravel, 10 208

variables on bromination, including pH, H2O2 concentration, treatment time, and substrate

209

characteristics (oak vs. pine and fresh vs. decayed plant material).

210

The objectives of this study are twofold: 1) certain model experiments are intended to

211

elucidate abiotic bromination pathways that can reasonably be expected to occur in real soil

212

environments; and 2) models using powerful oxidants not likely to occur in nature are useful

213

for providing insight into the comparative reactivity of different SOM components to

214

bromination.

215

3.3. Br speciation in treated plant material and soil humus

216

Following treatment with aqueous Br- and H2O2 for 48 h at pH 3.0, oak, pine, and

217

humus particulates display strong organobromine signals in their XANES spectra (Fig. 1B, b-

218

d). The absorption maxima of the spectra differs by substrate—while treated humus shows a

219

white line position corresponding to aromatic organobromine (Fig. 1B, b), the white line of

220

treated pine needles falls slightly below this energy (Fig. 1B, c), and that of treated oak leaves

221

falls closer to the characteristic energy of aliphatic organobromine (Fig. 1B, d). LC fitting of

222

the spectra confirmed higher proportions of aliphatic organobromine in treated oak leaves,

223

with approximately 3:2 aliphatic:aromatic compared with 1:4 aliphatic:aromatic for treated

224

pine needles and 100% aromatic in treated humus. Br speciation will be presented

225

quantitatively for various treatments in the sections that follow.

226 227

3.4. Br enrichment in treated humus Br enrichment of treated particulates was revealed by their absolute fluorescence

228

intensity, as exemplified in the unnormalized spectra of treated humus in Fig. 2. Beyond the

229

oscillations of the near-edge region that reveal Br speciation, the post-edge rise in spectral

230

intensity is linearly proportional to Br concentration in the sample. Br concentrations are

231

computed from the edge step value with high precision following our previously described


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quantification procedure.31 Total [Br] values are then combined with LC fitting results to give

233

absolute concentrations of aliphatic/aromatic and inorganic Br species in the sample.

234

Treatment of soil humus with H2O2 and aqueous Br- at pH 3.0 for 48 h resulted in

235

dramatic Br enrichment to 64 mg·kg-1 (from 5 mg·kg-1 in untreated humus), all in the form of

236

aromatic organobromine (Fig. 2). Combination of H2O2 and Br- with ferric citrate or nitrate

237

produced less organobromine, 36 mg·kg-1 and 52 mg·kg-1, respectively, with ferric nitrate

238

producing 6 mg·kg-1 aliphatic organobromine in addition to 46 mg·kg-1 aromatic. However,

239

each of the ferric oxidants combined with Br- in the absence of H2O2 increased the

240

organobromine content compared with the control, with the ferric citrate treatment producing

241

14 mg·kg-1 aromatic organobromine and ferric nitrate 13 mg·kg-1 aromatic and 5 mg·kg-1

242

aliphatic. Application of aqueous Br- to humus in the absence of oxidants did not

243

significantly increase organobromine concentrations compared with the untreated control

244

(Fig. 2).

245

These results demonstrate that the brominating effects of H2O2 and the ferric salts are

246

not additive. On the contrary, ferric oxidants act as brominating agents on their own but

247

impede bromination in the presence of H2O2. Combination of H2O2 with ferric salts may have

248

caused catalytic H2O2 decomposition37 or created such strongly oxidizing conditions that

249

organobromine was broken down as well as produced. The latter possibility is supported by

250

the production of aliphatic organobromine in the ferric nitrate treatments, as these aliphatics

251

could represent breakdown products from oxidation of brominated aromatics.

252 253 254

All concentrations reported subsequently were obtained from unnormalized XANES spectra similar to those of the humus treatments in the Fig. 2 example. 3.5. Bromination of various substrates

255

To compare the susceptibility of assorted SOM precursors to abiotic bromination,

256

treatments at pH 3.0 were carried out on fresh plant material, partially decayed plant litter



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from the forest floor, and highly degraded soil humus (Fig. 3). For this comparison, “oak”

258

and “pine” refer to green, tree-harvested plant material. “Mixed litter” consists of brown,

259

decayed oak leaves and pine needles of various species after approximately nine months of

260

weathering on the forest floor, an intermediate decay stage compared with the dark-colored,

261

oxidized “humus” material.

262

Results for treated oak and pine substrates contrast sharply with those of soil humus,

263

with more organobromine produced in fresh material than in humus. Even in the absence of

264

added oxidants, application of 40 mM Br- caused substantial production of both aromatic and

265

aliphatic forms of organobromine in oak and pine substrates compared with untreated

266

controls (Fig. 3). Aliphatic organobromine production was greater in oak than pine material.

267

This general pattern continues in treatments incorporating H2O2, ferric nitrate, or both in

268

addition to Br-, with some interesting distinctions among the oxidants. Combination of H2O2

269

with Br- produced no more organobromine in fresh oak than Br- alone and only slightly more

270

in fresh pine (Fig. 3). This held true for treatments with varying H2O2 concentrations, from

271

0.1 to 2.0 mM, and in the dark as much as the light (Fig. S1 in Supporting Information). By

272

contrast, addition of ferric nitrate with Br- dramatically increased organobromine production

273

in oak and pine, in both aromatic and aliphatic fractions (Fig. 3). Using both ferric nitrate and

274

H2O2 with Br- brominated oak and pine somewhat less than ferric nitrate and Br- alone.

275

While application of Br- by itself did not enrich humus in organobromine compared

276

with the untreated control, combination of Br- and H2O2 increased the aromatic

277

organobromine content of the humus by an order of magnitude (Fig. 3). Addition of ferric

278

nitrate, H2O2, and Br- to humus produced less organobromine than H2O2 and Br- alone and

279

created a small aliphatic fraction in addition to aromatic organobromine. Even less

280

bromination was observed with the application of Br- and ferric nitrate to humus.


Page 13 of 32



The results of these treatments illuminate several phenomena. The straightforward

282

incorporation of Br into fresh oak and pine material suggests a limiting reagent role for Br- in

283

the bromination of fresh organic material in acidic soils. In contrast with the natural soil

284

environment, the addition of large excesses of Br- switched the limiting reagent to reactive

285

organic moieties in plant material, resulting in extensive bromination. The superfluity of

286

added H2O2 in the bromination of fresh plant material does not preclude Eq. 1 as a

287

bromination pathway, because mechanical pulverization of the plant material could cause

288

endogenous H2O2 release.38 Bromination is just as effective in the dark, implying the reaction

289

is not photochemical. More bromination in fresh vs. oxidized substrates is indicative of more

290

labile organics in fresh plant material. In all treatments, oak material has higher

291

organobromine concentrations than pine, and most of the additional organobromine is

292

aliphatic. For each treatment, results for mixed plant litter fall intermediate between those for

293

fresh oak/pine and humus, with the caveat that the mixed litter treatments were carried out for

294

24 h compared with 48 h for the other materials (Fig. 3). Partially decayed litter has more

295

brominatable aliphatics than humus, though not as many as fresh oak leaves and pine needles.

296 297

3.6. Effects of pH on abiotic bromination If Eq. 1 represents the dominant reaction at work in our models, lower pH should

298

enhance bromination. To assess the effects of pH, we treated oak and pine materials at pH

299

values of 3.0 and 5.0, bracketing a typical range for Pine Barrens soil. In pine material,

300

bromination occurred at both pH values, but each treatment produced less than half of the

301

organobromine at pH 5.0 than at pH 3.0 (Fig. 4A). This held true for treatments incorporating

302

H2O2 and ferric nitrate oxidants as well as those applying Br- by itself. Oak material also

303

showed less bromination at higher pH, with particular diminishment in the aliphatic

304

organobromine fraction (Fig. 4B).



Page 14 of 32


These results implicate a pH-dependent pathway like peroxidative bromination (Eq.

306

1) in abiotic organobromine production and further imply that such processes will be of

307

greater importance in more acidic soils. Ferric nitrate promotes bromination of oak and pine

308

at both pH 3.0 and 5.0 (Fig. 4). Iron-bearing soil minerals could potentially serve as Lewis

309

acid catalysts to facilitate abiotic Br- oxidation, as they do in certain brominations devised by

310

synthetic chemists. Inspired by the action of vanadium bromoperoxidases, chemists have

311

sought biomimetic transition metal catalysts for mild peroxidative bromination,39-43 mainly to

312

circumvent the need for the corrosive and toxic Br2 reagent in organic syntheses. However,

313

there has been some controversy over the efficacy of various transition metal catalysts in

314

biomimetic oxybromination reactions, with Rothenberg and Clark finding that reactions

315

ostensibly catalyzed by molybdenum and vanadium were dependent rather on stoichiometric

316

quantities of acid,44 in accordance with Eq. 1. The acidity of the reaction medium thus

317

appears to be a crucial variable in peroxidative bromination, with the need for activation of

318

peroxide either by Brønsted or Lewis acids. This translates into pH values below 3 or else

319

substantial amounts of vanadate or a similar Lewis acid to make this reaction efficient enough

320

for organic syntheses in aqueous solution.45 Our reactions were buffered to minimize pH

321

changes from added reagents. The role of ferric nitrate in enhancing bromination in our

322

treatments will be explored further in the subsequent section.

323

Since peroxidative bromination proceeds with good yields in aqueous solution at pH

324

2-3,45 then it likely occurs with reduced efficiency in soils. Soil pH values below 5 are

325

common, particularly in certain highly leached and/or organic-rich environments. An

326

important natural system for abiotic bromination is likely to be peat lands, which cover

327

approximately 2–3% of total land area and are one of the largest terrestrial reservoirs of

328

halogens.46 Peats bogs are acidic (sphagnum or “typical” bogs have pH < 5), and peats are


Page 15 of 32



highly enriched in Br.13 The dominant mechanisms of Br incorporation into peats have not

330

yet been established, but the abiotic reactions demonstrated here are probable contributors.

331 332

3.7. Effects of ferric salts on abiotic bromination The most effective treatments in brominating oak and pine material involve ferric

333

nitrate (Figs. 3-4). The maximum organobromine produced in any experiment was 550

334

mg·kg-1, in oak material treated with ferric nitrate and Br- for two days. Interestingly, similar

335

treatment of oak for four days produced only 360 mg·kg-1 organobromine (Fig. 5), suggesting

336

degradation upon extended exposure to ferric nitrate. Most of the loss comes from the

337

aliphatic organobromine fraction, which decreases from 385 mg·kg-1 after two days to 255

338

mg·kg-1 after four days. The lability of the aliphatic fraction in oaks is also supported by the

339

result that ferric nitrate yields substantially more aliphatic organobromine in the absence of

340

H2O2 (385 vs. 250 mg·kg-1–see Fig. 3). As posited in section 3.4, organobromines may break

341

down under the highly oxidizing conditions created by combining ferric nitrate and H2O2.

342

Likewise, more aliphatic organobromine is produced in pine material by ferric nitrate

343

and Br- than by ferric nitrate, Br-, and H2O2; however, the difference is less dramatic than for

344

oak, in which the concentrations of aliphatic organobromine produced are much higher (Fig.

345

3). The reaction time trend differs for pine; four days of treatment with ferric nitrate and Br-

346

resulted in slightly more aliphatic organobromine than two days, although still far less than in

347

oak (Fig. 5).

348

Unlike some of the milder treatments, the ferric nitrate experiments do not directly

349

model reaction conditions in the natural environment. A strong oxidant like ferric nitrate is

350

apt to carry out rapid oxidation of labile organics in plant material, much like potassium

351

permanganate, the reagent that operationally (and controversially) defines labile soil carbon.47

352

Application of ferric nitrate to fresh oak and pine had dramatic effects on their appearance,

353

darkening the material so that it took on the deep brown color of naturally oxidized soil



Page 16 of 32


humus. Like the production of aliphatic organobromine, the darkening effect was more

355

pronounced in oak (Fig. S2) than in pine (Fig. S3) material.

356

Although ferric nitrate contains two powerful oxidants, Fe3+ and NO3-, neither has

357

sufficient potential to oxidize aqueous Br- directly to Br2 or HOBr. To clarify the roles of

358

Fe3+ and NO3-, we varied the counteranion in a series of oak and pine treatments with ferric

359

salts, finding that treatments with ferric citrate or sulfate did not produce nearly as much

360

organobromine as those with ferric nitrate (Fig. 5). This implicates NO3- rather than Fe3+ as

361

the key oxidant leading to bromination in the ferric nitrate treatments. Moreover, the large

362

production of aliphatic organobromine in oak and pine is specific to the ferric nitrate

363

treatments.

364

These data suggest that the bromination mechanism at work in the ferric nitrate

365

treatments involves oxidation of SOM rather than the oxidation of Br- as in Eq. 1. Such a

366

pathway would be consistent with the findings of Keppler et al., who demonstrated

367

production of volatile bromocarbons resulting from oxidation of natural organics by Fe3+ and

368

concomitant methylation of Br-.22 Accordingly, in our experiments, NO3-, and to a lesser

369

extent Fe3+, could oxidize electron-rich organic substrates in plant material that proceed to

370

alkylate Br-. Such a pathway could account for the particularly large increases in brominated

371

aliphatics upon treatment of oak and pine with ferric nitrate. Alternatively, Fe3+ may promote

372

aromatic ring cleavage to produce low molecular weight bromoalkanes.48

373

Although ferric nitrate dramatically enhances bromination of fresh plant material, it

374

does not increase organobromine levels in humus (Fig. 3), presumably because the labile

375

carbon has already been consumed through natural oxidative processes. Thus, the substrates

376

susceptible to bromination through the treatment of fresh plant material with ferric nitrate

377

represent labile forms of carbon that break down during initial decay stages on the forest

378

floor.


Page 17 of 32


Leri & Ravel, 17 379 380

3.8. Effects of Fenton-like conditions on abiotic bromination The well-known Fenton reaction involves the Fe2+-catalyzed disproportionation of

381

H2O2 to produce hydroxyl radicals (OH), which may proceed to react with halides, forming

382

reactive halogen species.48 The combination of Fe2+ with H2O2 in seawater has been shown to

383

induce dramatic bromination of marine organic matter.30 To investigate whether Fenton-like

384

conditions would lead to abiotic bromination of SOM, we carried out experiments on plant

385

material and humus with Br-, H2O2, and Fe2+ (in the form of ferrous sulfate) at pH 3.0. For

386

oak and humus, the addition of Fe2+ made no significant difference in organobromine yield

387

(Fig. 6). For pine, Fe2+ increased the production of aromatic organobromine, although the

388

effect is within the margin of error (Fig. 6).

389

These Fenton-like conditions did not meaningfully change the yields of

390

organobromine, suggesting that hydroxyl radicals do not promote bromination. Furthermore,

391

performing treatments (Fenton-like or otherwise) in the dark did not diminish organobromine

392

yields (Fig. 6), discounting a photochemical pathway. The results of these experiments imply

393

that peroxidative bromination is more likely than a radical-based pathway.

394

3.9. Insights into relative stabilities of SOM components

395

In addition to illuminating environmentally feasible pathways of abiotic bromination,

396

our experiments have identified aliphatic and aromatic fractions of varying reactivity in SOM

397

and its plant-derived precursors. The chemical composition and reactivity of various SOM

398

fractions, which are crucial to understanding soil carbon turnover, have not been adequately

399

characterized.49, 50 One of the most striking results of our experiments is the existence of

400

readily brominatable aliphatics in fresh oak and pine material, a fraction that is no longer

401

present in humus. In the natural oxidative transformation of plant litter to soil humus, these

402

aliphatic compounds must be broken down or chemically altered in a way that diminishes

403

their susceptibility to bromination.



Page 18 of 32


Brominatable aliphatics in oak and pine litter probably make up part of the so-called

405

“labile soil carbon” pool. The quickest SOM fraction to turn over, labile soil carbon is

406

chemically heterogeneous and poorly defined, consisting of carbohydrates, polysaccharides,

407

proteins, amino acids, waxes, fatty acids, and other unspecified compounds.51 Unsaturated

408

moieties in this pool would be highly susceptible to bromination through a mechanism like

409

peroxidative bromination (Eq. 1). The labile fraction decomposes over a period between days

410

and few years,49 comparable to the timescale over which plant litter is converted to soil

411

humus. Our treatments produced substantially more aliphatic organobromine in oak leaves

412

than in pine needles, potentially signifying that deciduous leaves contribute more labile

413

aliphatics to SOM than do coniferous needles.

414

Our treatments were also effective in producing aromatic organobromine, in fresh oak

415

and pine as well as humus. Bromination of aromatics likely involves oxidation of Br- to

416

HOBr (Eq. 1) or a similarly reactive species that proceeds to brominate phenolic and anisolic

417

moieties. Such activated benzene rings are common in SOM, from plant polyphenolics to

418

lignin, the recalcitrant plant polymer. The brominated aromatic fraction, which is more stable

419

than the aliphatic fraction on the timescale of our experiments, could represent a reservoir of

420

less decomposable organic matter—part of the stable or inert fractions of SOM, with longer

421

turnover on the order of decades to centuries.49

422

This and other recent mechanistic explorations24, 28-30 have begun to illuminate the

423

geochemical dynamics of Br under various environmental conditions. Our results

424

demonstrate that peroxidative bromination can occur abiotically in the soil environment,

425

particularly under acidic conditions. The incorporation of Br- into fresh plant material seems

426

to be limited primarily by Br- concentration. This suggests that in natural environments where

427

total Br concentrations are low and organic matter concentrations are high (e.g. the soil

428

organic horizon), inorganic Br- is actively scavenged, effectively operating as a limiting


Page 19 of 32



reactant in abiotic oxidations. Thus, our study has revealed a natural, abiotic source of

430

particulate organobromine in the terrestrial environment. Furthermore, by assessing the

431

susceptibility of different substrates to bromination, we have identified a labile aliphatic

432

fraction of plant material that does not persist in humus as well as a more stable, recalcitrant

433

aromatic fraction--another small step in the chemical characterization of particulate SOM.

434

Supporting Information

435

Three supplementary figures (Figs. S1-S3) supplied as Supporting Information. This material

436

is available free of charge via the Internet at http://pubs.acs.org.

437 438

Acknowledgements

439

Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was

440

supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy

441

Sciences, under Contract No. DE-AC02-98CH10886. A.C.L. is supported by the Marymount

442

Manhattan College Distinguished Chair award.

443 444

References

445 446

1. Gilley, J. E.; Finkner, S. C.; Doran, J. W.; Kottwitz, E. R. Adsorption of bromide tracers onto sediment. Appl. Eng. Agric. 1990, 6 (1), 35-38.

447 448

2. Nobles, M. M.; Wilding, L. P.; Lin, H. S. Flow pathways of bromide and Brilliant Blue FCF tracers in caliche soils. J. Hydrology 2010, 393 (1–2), 114-122.

449 450 451

3. Ma, R.; Zheng, C.; Zachara, J. M.; Tonkin, M. Utility of bromide and heat tracers for aquifer characterization affected by highly transient flow conditions. Water Resour. Res. 2012, 48 (8), W08523.

452 453 454

4. Mollerup, M.; Abrahamsen, P.; Petersen, C. T.; Hansen, S. Comparison of simulated water, nitrate, and bromide transport using a Hooghoudt-based and a dynamic drainage model. Water Resour. Res. 2014, 50 (2), 1080-1094.

455 456

5. Putschew, A.; Mania, M.; Jekel, M. Occurrence and source of brominated organic compounds in surface waters. Chemosphere 2003, 52 (2), 399-407.



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6. Gilfedder, B. S.; Petri, M.; Wessels, M.; Biester, H. Bromine species fluxes from Lake Constance’s catchment, and a preliminary lake mass balance. Geochim. Cosmochim. Acta 2011, 75 (12), 3385-3401.

460 461

7. Hütteroth, A.; Putschew, A.; Jekel, M. Natural production of organic bromine compounds in Berlin lakes. Environ. Sci. Technol. 2007, 41 (10), 3607-3612.

462 463 464

8. Biester, H.; Keppler, F.; Putschew, A.; Martinez-Cortizas, A.; Petri, M. Halogen retention, organohalogens, and the role of organic matter decomposition on halogen enrichment in two Chilean peat bogs. Environ. Sci. Technol. 2004, 38 (7), 1984-1991.

465 466

9. Leri, A. C.; Myneni, S. C. B. Natural organobromine in terrestrial ecosystems. Geochim. Cosmochim. Acta 2012, 77, 1-10.

467

10.

468 469

11. Yuita, K.; Nobusawa, Y.; Shibuya, M.; Aso, S. Iodine, bromine and chlorine contents in soils and plants of Japan. Soil Sci. Plant Nutr. 1982, 28 (3), 315-336.

470 471

12. Flury, M.; Papritz, A. Bromide in the natural environment: Occurrence and toxicity. J. Environ. Qual. 1993, 22 (4), 747-758.

472 473 474

13. Putschew, A.; Keppler, F.; Jekel, M. Differentiation of the halogen content of peat samples using ion chromatography after combustion (TX/TOX-IC). Anal. Bioanal. Chem. 2003, 375 (6), 781785.

475 476

14. Butler, A.; Sandy, M. Mechanistic considerations of halogenating enzymes. Nature 2009, 460 (7257), 848-854.

477 478

15. Winter, J. M.; Moore, B. S. Exploring the chemistry and biology of vanadium-dependent haloperoxidases. J. Biol. Chem. 2009, 284 (28), 18577-18581.

479 480

16. Blasiak, L. C.; Drennan, C. L. Structural perspective on enzymatic halogenation. Acc. Chem. Res. 2009, 42 (1), 147-155.

481

17.

482 483 484

18. Everett, R. R.; Butler, A. Bromide-assisted hydrogen peroxide disproportionation catalyzed by vanadium bromoperoxidase: Absence of direct catalase activity and implications for the catalytic mechanism. Inorg. Chem. 1989, 28 (3), 393-395.

485 486

19. de Boer, E.; Wever, R. The reaction mechanism of the novel vanadium-bromoperoxidase: A steady-state kinetic analysis. J. Biol. Chem. 1988, 263 (25), 12326-32.

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20. Laturnus, F.; Mehrtens, G.; Grøn, C. Haloperoxidase-like activity in spruce forest soil - a source of volatile halogenated organic compounds? Chemosphere 1995, 31, 3709-3719.

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21. Gan, J.; Yates, S. R.; Ohr, H. D.; Sims, J. J. Production of methyl bromide by terrestrial higher plants. Geophys. Res. Lett. 1998, 25, 3595-3598.

491 492

22. Keppler, F.; Eiden, R.; Niedan, V.; Pracht, J.; Schöler, H. F. Halocarbons produced by natural oxidation processes during degradation of organic matter. Nature 2000, 403, 298-301.

493 494

23. Mano, S.; Andreae, M. O. Emission of methyl bromide from biomass burning. Science 1994, 263 (5151), 1255-1257.

Yamada, Y. Occurrence of bromine in plants and soil. Talanta 1968, 15 (11), 1135-1141.

van Pee, K. H. Enzymatic chlorination and bromination. Meth. Enzymol. 2012, 516, 237-57.


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24. Vione, D.; Maurino, V.; Man, S. C.; Khanra, S.; Arsene, C.; Olariu, R.-I.; Minero, C. Formation of organobrominated compounds in the presence of bromide under simulated atmospheric aerosol conditions. ChemSusChem 2008, 1 (3), 197-204.

498 499 500

25. De Laurentiis, E.; Minella, M.; Maurino, V.; Minero, C.; Mailhot, G.; Sarakha, M.; Brigante, M.; Vione, D. Assessing the occurrence of the dibromide radical (Br2−) in natural waters: Measures of triplet-sensitised formation, reactivity, and modelling. Sci. Tot. Env. 2012, 439, 299-306.

501 502

26. Calza, P.; Massolino, C.; Pelizzetti, E.; Minero, C. Solar driven production of toxic halogenated and nitroaromatic compounds in natural seawater. Sci. Tot. Env. 2008, 398, 196-202.

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27. Calza, P.; Massolino, C.; Pelizzetti, E.; Minero, C. Role of iron species in the phototransformation of phenol in artificial and natural seawater. Sci. Tot. Env. 2012, 426, 281-288.

505 506

28. Tamtam, F.; Chiron, S. New insight into photo-bromination processes in saline surface waters: The case of salicylic acid. Sci. Tot. Env. 2012, 435–436, 345-350.

507 508 509 510

29. Méndez-Díaz, J. D.; Shimabuku, K. K.; Ma, J.; Enumah, Z. O.; Pignatello, J. J.; Mitch, W. A.; Dodd, M. C. Sunlight-driven photochemical halogenation of dissolved organic matter in seawater: A natural abiotic source of organobromine and organoiodine. Environ. Sci. Technol. 2014, 48, 74187427.

511 512

30. Leri, A. C.; Mayer, L. M.; Thornton, K. R.; Ravel, B. Bromination of marine particulate organic matter through oxidative mechanisms. Geochim. Cosmochim. Acta 2014, 142, 53-63.

513 514

31. Leri, A. C.; Ravel, B. Sample thickness and quantitative concentration measurements in Br Kedge XANES spectroscopy of organic materials. J. Synch. Rad. 2014, 21 (3), 623-626.

515 516

32. Golovchenko, J. A.; Levesque, R. A.; Cowan, P. L. X-ray monochromator system for use with synchrotron radiation sources. Rev. Sci. Instr. 1981, 52 (4), 509-516.

517 518 519 520

33. Woicik, J. C.; Ravel, B.; Fischer, D. A.; Newburgh, W. J. Performance of a four-element Si drift detector for X-ray absorption fine-structure spectroscopy: Resolution, maximum count rate, and dead-time correction with incorporation into the ATHENA data analysis software. J. Synch. Rad. 2010, 17 (3), 409-413.

521 522

34. Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synch. Rad. 2005, 12 (4), 537-541.

523 524 525

35. Heeb, M. B.; Criquet, J.; Zimmermann-Steffens, S. G.; von Gunten, U. Oxidative treatment of bromide-containing waters: Formation of bromine and its reactions with inorganic and organic compounds--a critical review. Water Res. 2014, 48, 15-42.

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36. Petigara, B. R.; Blough, N. V.; Mignerey, A. C. Mechanisms of hydrogen peroxide decomposition in soils. Environ. Sci. Technol. 2002, 36 (4), 639-645.

528 529

37. Lin, S.-S.; Gurol, M. D. Catalytic decomposition of hydrogen peroxide on iron oxide: Kinetics, mechanism, and implications. Environ. Sci. Technol. 1998, 32 (10), 1417-1423.

530

38.

531 532 533

39. Sels, B.; Vos, D. D.; Buntinx, M.; Pierard, F.; Kirsch-De Mesmaeker, A.; Jacobs, P. Layered double hydroxides exchanged with tungstate as biomimetic catalysts for mild oxidative bromination. Nature 1999, 400 (6747), 855-857.

Olson, P. D.; Varner, J. E. Hydrogen peroxide and lignification. Plant J. 1993, 4 (5), 887-892.



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40. Conte, V.; Di Furia, F.; Moro, S. Mimicking the vanadium bromoperoxidases reactions: Mild and selective bromination of arenes and alkenes in a two-phase system. Tet. Lett. 1994, 35 (40), 74297432.

537 538

41. Meister, G. E.; Butler, A. Molybdenum(VI)-mediated and tungsten(VI)-mediated biomimetic chemistry of vanadium bromoperoxidase. Inorg. Chem. 1994, 33 (15), 3269-3275.

539 540 541

42. Hamstra, B. J.; Colpas, G. J.; Pecoraro, V. L. Reactivity of dioxovanadium(V) complexes with hydrogen peroxide: Implications for vanadium haloperoxidase. Inorg. Chem. 1998, 37 (5), 949955.

542 543

43. Podgorsek, A.; Zupan, M.; Iskra, J. Oxidative halogenation with "green" oxidants: Oxygen and hydrogen peroxide. Angew. Chem. 2009, 48 (45), 8424-50.

544 545

44. Rothenberg, G.; Clark, J. H. On oxyhalogenation, acids, and non-mimics of bromoperoxidase enzymes. Green Chem. 2000, 2 (5), 248-251.

546 547 548

45. Wischang, D.; Brücher, O.; Hartung, J. Bromoperoxidases and functional enzyme mimics as catalysts for oxidative bromination—A sustainable synthetic approach. Coord. Chem. Rev. 2011, 255 (19–20), 2204-2217.

549 550

46. Biester, H.; Selimović, D.; Hemmerich, S.; Petri, M. Halogens in pore water of peat bogs – the role of peat decomposition and dissolved organic matter. Biogeosciences 2006, 3 (1), 53-64.

551 552

47. Tirol-Padre, A.; Ladha, J. K. Assessing the reliability of permanganate-oxidizable carbon as an index of soil labile carbon. Soil Sci. Am. J. 2004, 68 (3), 969-978.

553 554

48. Comba, P.; Kerscher, M.; Krause, T.; Schöler, H. F. Iron-catalysed oxidation and halogenation of organic matter in nature. Environ. Chem. 2015, 12 (4), 381-395.

555 556

49. Strosser, E. Methods for determination of labile soil organic matter: An overview. J. Agrobiol. 2010, 27 (2), 49-60.

557 558

50. Nichols, K. A.; Wright, S. F. Carbon and nitrogen in operationally defined soil organic matter pools. Biol. Fertil. Soils 2006, 43 (2), 215-220.

559 560 561 562 563 564 565 566 567

51. Poirier, N.; Sohi, S. P.; Gaunt, J. L.; Mahieu, N.; Randall, E. W.; Powlson, D. S.; Evershed, R. P. The chemical composition of measurable soil organic matter pools. Org. Geochem. 2005, 36 (8), 1174-1189.

Figure Captions Figure 1. Normalized Br K-edge X-ray absorption near-edge structure (XANES) spectra.

568

A) Br-containing model compounds: a-b, aliphatic organobromine standards; c-d, aromatic

569

organobromine standards; e, inorganic bromide standard. B) Particulate SOM samples: a,

570

untreated soil humus; b-d, particulates treated with 40 mM Br- and 2.0 mM H2O2 for 48 h at


Page 23 of 32



pH 3.0. Vertical lines intersect the characteristic absorption energies of aliphatic and aromatic

572

organobromine.

573 574

Figure 2. Unnormalized Br K-edge XANES spectra of soil humus after various treatments,

575

all for 48 h at pH 3.0. The post-edge fluorescence intensities (the rise of the “edge step”) are

576

linearly proportional to total Br concentrations.31 The near-edge features around 13,473 eV

577

reveal Br speciation in the samples and can be resolved into inorganic, aliphatic, and aromatic

578

proportions via linear combination fitting of normalized spectra. The information from

579

XANES spectra can thus be transformed into the data in the table at right. All concentrations

580

reported in subsequent figures were obtained similarly from Br K-edge XANES spectra.

581 582

Figure 3. Aliphatic and aromatic organobromine concentrations in fresh oak leaves and pine

583

needles, mixed plant litter, and humus from the soil organic horizon, before and after

584

oxidative treatments. Treatments were carried out in phosphate buffer (pH 3.0) with 40 mM

585

KBr, 2 mM H2O2, and 20 mM Fe(NO3)3. Oak leaves, pine needles, and soil humus were

586

treated for 48 h; mixed plant litter was treated for 24 h.

587 588

Figure 4. Effect of pH on bromination of fresh: A) oak leaf; and B) pine needle material.

589

Treatments were carried out in phosphate (pH 3.0) or acetate (pH 5.0) buffer for 48 h with 40

590

mM KBr, 2 mM H2O2, and 20 mM Fe(NO3)3.

591 592

Figure 5. Effect of different ferric salts on bromination of fresh oak and pine material, with

593

treatment times as indicated.

594



Page 24 of 32


Figure 6. Effect of Fenton-like reaction conditions on bromination of fresh oak, fresh pine,

596

and humus material. Indicated treatments were performed in the dark.


Page 25 of 32


lized fluorescence yield (arbitrary units) Normalized fluorescence yield (arbitrary units)

A

13460

Fig. 1

B

Hum

t-butyl bromide a) humus (untreated)

a) t-butyl bromide

Hum b) humus + Br- + H2O2

1-bromoadamantane

b) 1-bromoadamantane

c) pine + Br- + H2O2

p-bromophenol

c) p-bromophenol

d) oak +

t-butyl bromide p-dibromobenzene 1-bromoadamantane

d) p-dibromobenzene

e) KBr (aq)

13460

p-bromophenol KBr (aq) p-dibromobenzene 13470 13480

Br-

13490

+ H2O2

13500

Photon KBr (aq) Energy (eV)

x = 13472.6 eV (aliphatic C-Br) x = 13472.6 eV (aliphatic C-Br)

13470

13480

13490

13500

x = 13473.5 eV (aromatic C-Br) x = 13473.5 eV (aromatic C-Br)

Photon Energy (eV) ACS Paragon Plus Environment

Pine

Oak

x= C-B x= C-B


Page 26 of 32

9.0

Aliphatic organo-Br

Absolute fluorescence yield (FF/I0)

8.0

Humus + Br- + H2O2

ud

Humus + Br- + H2O2 + Fe(III)-nitrate

6±1

6.0

5.0

64 ± 10 ud humus + H2O2 + Brhumus + Fe-nitrate + H2O2 + Br46 ± 7 ud humus + Fe-citrate + H2O2 + Br-

Humus + Br- + H2O2 + Fe(III)-citrate

4.0

ud

36 ± 5 ud humus + Fe-nitrate + Br-

Humus + Br- + Fe(III)-citrate

ud

humus + Fe-citrate + Br14 ± 2 4±1

Humus + Br- + Fe(III)-nitrate

5±1

3.0

2.0

1.0

Fig. 2

Inorganic bromide

All concentrations in mg·kg-1 dry mass (“ud” = undetectable).

7.0

0.0 13440

Aromatic organo-Br

13480

13520

Humus + Br-

ud

Humus (untreated)

ud

13560

13600

Photon Energy (eV) ACS Paragon Plus Environment

13 ±+2Brhumus

ud

6±1 2±1 untreated humus 5±1 ud

Page 27 of 32


600

[aliphatic organo-Br] [aromatic organo-Br]

-1) Concentration(mg (mg·kg Concentration kg-1)

500 400 300 200 100

No No BrBr-

BrBr-

BrBr- ++ H2O2 H2O2

Fig. 3 ACS Paragon Plus Environment

- +H2O2 BrBr+ H2O2 + Fe(III)-nitrate + Fe(NO3)3

soil humus

fresh pine

fresh oak

soil humus

mixed litter

fresh pine

fresh oak

soil humus

mixed litter

fresh pine

fresh oak

soil humus

mixed litter

fresh pine

fresh oak

soil humus

fresh pine

fresh oak

0

- + Br- +BrFe(III)nitrate Fe(NO 3 )3


160 160

Page 28 of 32

[aliphatic organo-Br] organo-Br] [aliphatic [aromatic organo-Br] organo-Br] [aromatic

-1 Concentration(mg (mg·kg Concentration kg Concentration (mg kg-1-1)))

140 140 120 120 100 100 80 80

60 60 40 40 20 20 0 0

Fig. 4A

H2O2 H2O2 H2O2

H2O2 ++ H2O2 H2O2 Fe(III)-+ Fe-nitrate Fe-nitrate nitrate

BrBrBr-

Br Br- ++ + BrH O 2 2 H2O2 H2O2

Pine Treatments Treatments pH pH 3 3 Pine

Br Br- ++ + BrH O 2 2 ++ H2O2 H2O2 Fe(III)-+ Fe-nitrate Fe-nitrate nitrate


Br BrBr-

Br Br- ++ + BrH O 2 2 H2O2 H2O2

Br Br- ++ + BrH O 2 2 ++ H2O2 H2O2 Fe(III)-+ Fe-nitrate Fe-nitrate nitrate Pine Treatments Treatments pH pH 5 5 Pine

Page 29 of 32


500 500 450 450

[aliphatic organo-Br] organo-Br] [aliphatic [aromatic organo-Br] organo-Br] [aromatic

-1 Concentration(mg (mg·kg Concentration kg Concentration (mg kg-1-1)))

400 400 350 350 300 300 250 250 200 200 150 150 100 100 50 50 0 0

Fig. 4B

H2O2 H2O2 H2O2

H2O2 ++ H2O2 H2O2 Fe(III)-+ Fe-nitrate Fe-nitrate nitrate

BrBrBr-

- + Br Br+ Br-O+ H 2 2 H2O2 H2O2

Oak Treatments Treatments pH pH 3 3 Oak

- + Br Br+ Br+ H O 2 2 H2O2++ + H2O2 Fe(III)Fe-nitrate Fe-nitrate nitrate


BrBrBr-

- + Br Br+ Br-O+ H 2 2 H2O2 H2O2

- + Br Br+ Br+ H O 2 2 H2O2++ + H2O2 Fe(III)Fe-nitrate Fe-nitrate nitrate Oak Treatments pH 5 Oak Treatments pH 5


600

Page 30 of 32

[aliphatic organo-Br] [aromatic organo-Br]

-1) Concentration(mg (mg·kg Concentration kg-1)

500

400

300

200

100

0

Fig. 5

Oak Pine

Oak Pine

Oak Pine

Oak Pine

Oak Pine

Fe-nitrate Fe(III)(4 d) nitrate (4 d)

Br- Br- (2 (2d)d)

Fe-citrate Br- + + Br-Fe(III)- (2 d) citrate (2 d)

Fe-nitrate Br- + + Br-Fe(III)- (2 d) nitrate (2 d)

Fe-nitrate Br- + + Br-Fe(III)- (4 d) nitrate (4 d)


Pine - + FeBr sulfFe(III)ate sulfate + Br(2 d) (2 d)

0

Fig. 6 Oak Treatments

Oak Treatments Pine Treatments Pine Treatments

ACS Paragon Plus Environment Humus Treatments

Humus Treatments

H2O2 + Fe(II)-sulfate Br- +Br-H+2O FeSO4 (dark) 2 + (dark)

Br+-H2O2 + 2Fe(II)-sulfate Br + H 2O + FeSO4

+2H2O2 (dark) Br-Br+H O2 (dark)

- ++H H2O2 BrBr2O2

BrBr-

NoBr BrNo

H2O2 + Fe(II)-sulfate Br- +BrH+2O FeSO4 (dark) 2 + (dark)

Br-Br + -H2O2 + Fe(II)-sulfate + H 2O 2 + FeSO4

+ 2H2O2 (dark) Br-Br+H O2 (dark)

80

- ++HH2O2 BrBr2O2

90

BrBr-

NoBr BrNo

Br- + H2O2 +(dark) FeSO4 (dark)

Br- + H2O2 + Fe(II)-sulfate

Br+ -H2O2 + 2Fe(II)-sulfate Br + H 2O + FeSO4

+ 2H2O2 (dark) Br-Br+H O2 (dark)

- ++H H2O2 BrBr2O2

BrBr-

NoBr BrNo

Concentration (mg kg -1)

Page 31 of 32 Environmental Science & Technology

[aliphatic organo-Br]

[aromatic organo-Br]

70

60

50

40

30

20

10



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Abiotic Bromination of Soil Organic Matter - Environmental Science

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