Stabilization of Natural Organic Matter by Short-Range-Order Iron

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Stabilization of Natural Organic Matter by Short-Range-Order Fe Hydroxides Kai-Yue Chen, Tsan-Yao Chen, Ya-Ting Chan, ChingYun Cheng, Yu-Min Tzou, Yu-Ting Liu, and Heng-Yi Teah Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02793 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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Environmental Science & Technology

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Stabilization of Natural Organic Matter by Short-Range-Order Fe Hydroxides

4 5 6

Kai-Yue Chen,a Tsan-Yao Chen,b Ya-Ting Chan,a Ching-Yun Cheng,a Yu-Min Tzou,*,a Yu-Ting Liu,*,a and Heng-Yi Teahc

7 8 9

a

Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung 40227, Taiwan, R.O.C.

10 11

b

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

c

Department of Engineering and System Sciences, National Tsing Hua University, Hsinchu 30043, Taiwan, R.O.C.

Division of Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, 332 Building of Environmental Studies, 5-1-5 Kashiwanoha, Kashiwa City, Chiba 277-8563, Japan

Corresponding author's information: Yu-Ting Liu Department of Soil and Environmental Sciences, National Chung Hsing University, 145 Xingda Rd., Taichung 40227, Taiwan, R.O.C. Tel: +886-4-22840373 Ext. 3402 Fax: +886-4-22855167 E-mail address: [email protected] Yu-Min Tzou Department of Soil and Environmental Sciences,

28 29 30 31

National Chung Hsing University, 145 Xingda Rd., Taichung 40227, Taiwan, R.O.C. Tel: +886-4-22840373 Ext. 4206 Fax: +886-4-22855167

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E-mail address: [email protected]

1

ACS Paragon Plus Environment


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Abstract

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Dissolved organic matter (DOM) is capable of modifying the surfaces of soil

35

minerals (e.g., Fe hydroxides) or even forming stable coprecipitates with Fe(III) in a

36

neutral environment. The DOM/Fe coprecipitation may alter biogeochemical carbon

37

cycling in soils if the relatively mobile DOM is sorbed by soil minerals against

38

leaching, runoff, and biodegradation. In this study, we aimed to determine the

39

structural development of DOM/Fe coprecipitates in relation to changes in pH and

40

C/(C+Fe) ratios using XRD, XPS, Fe K-edge XAS, FTIR, and C-NEXAFS

41

techniques. The results showed that in the system with bulk C/(C+Fe) molar ratios ≤

42

0.65, the ferrihydrite-like Fe domains were precipitated as the core and covered by the

43

C shells. When the C/(C+Fe) molar ratio ranged between 0.71 and 0.89, the emerging

44

Fe-C bonding suggested a more substantial association between Fe domains including

45

edge- and corner-sharing FeO6 octahedra and DOM. With C/(C+Fe) bulk molar ratios

46

≥ 0.92, only corner-sharing FeO6 octahedra along with Fe-C bonding was found. The

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homogeneously distributed C and Fe domains caused the enhancement of Fe and C

48

solubilisation from coprecipitates. The C/(C+Fe) ratios dominated structural

49

compositions and stabilities of C/Fe coprecipitates and may directly affect the Fe and

50

C cycles in soils.

51 2


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Introduction

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The total carbon (C) present in terrestrial ecosystems is approximately 3170 GT,

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and it is estimated that nearly 80 % of the inventory, including 1550 GT of organic

55

and 950 GT of inorganic carbon, is preserved in soils.

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major source of soil organic carbon, consists of non-humic substances such as

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biochemical materials3 and dissolved organic matter (DOM), and humic substances

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such as humic acid, fulvic acid, and humin.4 Generally, non-humic substances are

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more biodegradable than humic substances. Any changes in the turnover rates of soil

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carbon pools potentially alters the atmospheric CO2 concentration as a result of the

61

significant correlation between the global carbon cycle and the soil carbon pool.

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Therefore, stabilizing SOM and preventing its physicochemical and/or biological

63

mineralization may partially alleviate global warming caused by an elevation of the

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carbon content in the atmosphere. Soil minerals play a relatively important and key

65

role in preserving SOM.5

1,2

Soil organic matter (SOM), a

66

The SOM could be stabilized by forming humic substances, black carbon-like

67

materials, and aggregates with soil minerals.6-16 By means of cation bridges, SOM/

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DOM could complex/precipitate with metal cations, particularly the polyvalent metals,

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and subsequently stabilize C against bio-degradations and transportations.5, 15, 17-20

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Complexation or precipitation between trivalent Fe/Al cations and SOM/DOM has 3



71

been commonly found in the leached soils, mine drainage waters, and aquifers.21-29

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Along with the fluctuation of redox conditions, organic-metal precipitates could also

73

form via a strong covalent bond between DOM and Fe(III),27 which was derived from

74

the oxidation of soluble Fe(II) as a result of an intrusion of dissolved oxygen.30

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Although partition/adsorption of organic molecules may occur on the Fe hydroxides,

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carboxyl groups that dominate DOM structures provide abundant binding sites to

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coprecipitate with Fe(III).31-33 Because coprecipitation showed a relatively higher C

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stabilization than partition/adsorption,31,

79

coprecipitation between DOM and Fe was an important factor contributing to soil C

80

storages.34

33

previous studies evidenced that

81

The pH values and C/(C+Fe) molar ratios affected both the structures and the

82

electro-affinity of DOM and Fe-containing minerals during the coprecipitation

83

processes of Fe and DOM.29, 35-37 For instance, the positive charge density on the

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surfaces of Fe hydroxides varied along with changes in the solution pH and

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essentially affected bonding abilities of Fe cations with DOM.35 While being

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dissociated, O-containing functional groups including carboxyl and phenolic groups

87

of DOM provide abundant binding sites for Fe.38,

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precipitation between DOM and Fe was favorable due to an inhibition of the

89

formation of Fe hydroxides.27 In terms of C/(C+Fe) ratios, previous studies indicated

39

4


Under acidic conditions,

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that the C/Fe coprecipitates could only be produced in an acidic environment when

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the C/metal molar ratios ranged from 10 to 33.3.35,

92

examined the productive conditions of the metal-organic coprecipitates, the mechanic

93

mechanisms of structural development at the molecular scale and stabilizations of

94

coprecipitates as influenced by pH and C/(C+Fe) ratios during the coprecipitation

95

processes remain unclear.

96

Understanding

the

structures

and

40

Even if these studies had

stabilization/persistence

of

DOM/Fe

97

coprecipitates is an essential component of evaluating their contributions to carbon

98

conservation in terrestrial systems. Structural and surface properties of DOM/Fe

99

coprecipitates have been previously determined using several methods such as X-ray

100

diffraction (XRD),41 Fourier-transform infrared spectroscopy (FTIR),31 and X-ray

101

photoelectron spectroscopy (XPS).33 Nonetheless, the effects of Fe proportion on the

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molecular-level structural development and changes of DOM/Fe coprecipitates should

103

also be investigated to determine changes in carbon stabilization as the function of

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compositions for DOM/Fe coprecipitates.

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Lately, synchrotron-based techniques such as X-ray absorption spectroscopy (XAS),

106

including X-ray absorption near edge structure (XANES) and extended X-ray

107

absorption fine structure (EXAFS) spectroscopy, have received significant scientific

108

attention because of their capability to determine the local coordination environments 5



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on the molecular scale. For example, while using synchrotron-based techniques,

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Karlsson et al.42 indicated that a mononuclear Fe/C bond was formed in the Fe/OM

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complex with a carboxyl/Fe ratio ≥ 22.5. However, mixtures of mononuclear Fe/C

112

bonding and Fe hydroxides were found with a carboxyl/Fe ratio < 22.5. Previous

113

studies focused mainly on the effects of organic molecules on the crystalline Fe

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hydroxides and the changes in the lengths and coordination numbers of the Fe-Fe

115

bond with increasing C dose.42 However, the local structural coordination including

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Fe-C and Fe-Fe bonds in DOM/Fe coprecipitates and whether such structural

117

attributes affect C stabilization as functions of C/(C+Fe) ratios in an acidic solution

118

still remained unexplored. In this study, we aimed to determine the structural

119

stabilization of DOM/Fe coprecipitates in relation to changes in pH and C/(C+Fe)

120

ratios using Fe K-edge XAS, C (1s) near edge X-ray absorption fine structure

121

(C-NEXAFS), and other conventional spectroscopic techniques. The C-stabilization

122

and Fe-C bonds associated with structural changes of Fe-DOM coprecipitates due to

123

Fe intrusion were systematically examined.

124 125

Materials and Methods

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Extraction of DOM. The DOM was extracted from the Changhua (CHA) peat soil

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collected from central Taiwan. The vegetation type is weed. Briefly, the CHA soil was 6


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mixed with de-ionized (DI) water in a reaction vessel with a solid to liquid ratio of 20

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g L-1. After shaking for 96 h, the suspension was passed through a 0.45 µm membrane

130

filter,16 and the filtrate that contained the DOM was used for coprecipitation

131

experiments. In total, 200 mg L-1 of NaN3 was added into the DOM solutions to

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inhibit microbe growth. The C concentrations in the DOM were measured by a total

133

organic carbon (TOC) analyzer (multi N/C 2100, analytikjena s). The DOM was also

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determined by an elemental analyzer (Heraeus CHN-O-S Rapid Analyzer), NMR

135

(nuclear magnetic resonance, Bruker DSX400WB NMR), and FTIR (Thermo Nicolet

136

Nexus).

137

,

Coprecipitation

and

solubilisation

of

DOM/Fe(III).

The

DOM/Fe(III)

138

coprecipitates (DFC) were synthesized by adding 0.5, 1, 2.5, or 5 mM Fe(III) to a

139

DOM solution containing 100 mg L-1 of DOC (8.33 mM C). The C/(C+Fe) molar

140

ratios of initial addition were 0.62, 0.77, 0.89, and 0.94, which were denoted as

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DFC62, DFC77, DFC89 and DFC94, respectively. During the coprecipitation, the

142

suspension pH was maintained at 3, 4.5, or 6 individually using 0.1 M KOH/HCl with

143

constant stirring at 150 rpm for 12 h. Hereafter, the coprecipitation pH is denoted in

144

front of the abbreviation of DFC. After 12 h, suspensions were centrifuged at 7155 g

145

for 20 min, and concentrations of C and Fe(III) in the supernatants were measured by

146

the TOC analyzer and inductively coupled plasma - atomic emission spectrometer 7



147

(ICP-AES, Spectro Genesis), respectively. The coprecipitates were washed 3-5 times

148

using DI water and then freeze-dried for elemental and structural analyses. The

149

solubilisation of C and Fe from DFC samples was examined at a solid concentration

150

of 60 mg C L-1 at the same pH used in the coprecipitation. The concentration of 60 mg

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C L-1 was chosen as the naturally dissolved OM from the peat soil generally ranges

152

from 20 to 60 mg C L-1.43 After 24 h, the suspensions were centrifuged at 7155 g for

153

20 min and then passed through 0.45 µm filter membranes. The concentrations of C

154

and Fe in the filtrate were determined using TOC and ICP-AES.

155

Characterizations of DOM/Fe coprecipitates. A subsample of each coprecipitates

156

was washed with deionized water to remove excess salts, and freeze-dried or

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preserved as the moist paste prior to analyses of elemental analyzer (EA) and

158

spectroscopic techniques, including FTIR, nuclear magnetic resonance (NMR), XRD,

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XPS, Fe-XAS, and C-NEXAFS [see “Chemical Compositions, Mineralogical,

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Structural, and Surface Analyses of DOM/Fe Coprecipitates” in the Supporting

161

Information (SI) for details]. Briefly, EA was used to determine the contents for C, H,

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N, and O in coprecipitates. The functional groups on DOM and DFC samples were

163

determined using FTIR, NMR, and C-NEXAFS analyses. Elemental distribution on

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near surfaces of DFC samples was determined using XPS analysis. The XRD patterns

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of DFC samples were collected to determine the mineralogical attributes. The Fe 8


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K-edge XAS Spectroscopy including XANES and EXAFS was used to determine the

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Fe species and local structures surrounding central Fe atoms, respectively. The

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synchrotron based analyses of C-NEXAFS and Fe-XAS were conducted at the

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National Synchrotron Radiation Research Center (NSRRC), Taiwan.

170 171

Results

172

Characterizations of DOM. The results of the elemental analysis indicated that

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the C content in the DOM was approximately 38 % (Table S1 in the supporting

174

information, SI). The C13 NMR result revealed that the DOM contains 29.4 % alkyl,

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36.7 % O-alkyl, 14.0 % aromatic, 5.0 % phenolic, 12.6 % carboxyl, and 2.2 %

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carbonyl functional groups (Figure S1 in SI). The FTIR spectrum of DOM showed –

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CH2 and –CH3 stretching in the aliphatic region at 2918 and 2849 cm–1, symmetric

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C=O stretch of esters and asymmetric –C=O stretching derived from the carboxyl and

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carbonyl groups at 1730 and 1610 cm-1, the symmetric COO- band of the carboxyl

180

group at 1400 cm-1, O–H stretching of the phenolic OH and/or OH deformation of

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COOH at 1270 cm-1, and the C–O stretch of carbohydrate group at 1050 cm-1 (Figure

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S2 in SI).29, 38, 44-48 Both the FTIR and NMR results indicated that the DOM consists

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mainly of aliphatic carbon enriched with O-containing groups.

184

Precipitation, solubilisation, crystallization, and distribution of C and Fe in 9



185

DFC samples. At each precipitation pH, the amounts of precipitated C in the DFC

186

samples tended to increase with increasing C/(C+Fe) ratios of initial addition,

187

whereas the precipitated Fe showed an opposite trend (Table 1 and Figure S3). With

188

the exception of the 3DFC62 and 3DFC77 samples, the final (bulk) C/(C+Fe) molar

189

ratios in the DFC samples were very close to the C/(C+Fe) ratios of their initial

190

counterparts (Table 1). The DFC samples precipitated at pH 3 seemed to have

191

relatively higher C/(C+Fe) ratios, which could be attributed to the flocculation of C

192

domains that facilitated the formation of C/Fe coprecipitates at acidic conditions.49, 50

193

However, such C enrichment at pH 3 was not found when the initial C/(C+Fe) ratios

194

were greater than 0.77.37 On the other hand, the relatively lower C/(C+Fe) ratios were

195

observed for the DFC samples prepared at pH 6 than those prepared at pH 3 and pH

196

4.5 (Table 1). Such results were likely due to the occurrence of Fe hydrolysis, forming

197

Fe hydroxides at a higher pH, that might hinder the association of C and Fe(III).

198

The stabilization of the DFC samples was determined based on the degree of C

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and Fe solubilisation upon incubation with 0.01 M KCl for 12 h at the same pH in

200

which the DFC was synthesized during the precipitation process. In general, the

201

solubilisation of both C and Fe was more pronounced for the DFC samples prepared

202

at pH 6 (Table 1). For instance, up to 24.9 and 8.5 % of C and Fe, respectively, were

203

dissolved from the 6DFC94 sample and the released C and Fe were 3.5 and 2.2 times 10


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greater than that of the 3DFC94 sample (Table 1). In addition, the solubilisation of C

205

and Fe tended to decrease with decreasing C/(C+Fe) molar ratios (Table 1), indicating

206

that the DFC samples with relatively lower C/(C+Fe) ratios were more stable. The

207

significant variations in the amounts of C and Fe solubilisation suggested the presence

208

of structural and compositional differences among these DFC samples.

209

The X-ray diffraction patterns demonstrated a variation in the degree of

210

crystallinity for the DFC samples (Figure 1). For the sample precipitated at pH 3 with

211

a C/(C+Fe) molar ratio of 0.71 (i.e., 3DFC62), two broad peaks centered at 2.6 and

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1.5 Å indicated the presence of two-line ferrihydrite.51 With increasing precipitation

213

pH, these two peaks could be observed in the samples with a relatively higher

214

C/(C+Fe) ratio. For instance, the ferrihydrite structure could be observed in the

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sample with a C/(C+Fe) ratio up to 0.89 at pH 6 (6DFC89) (Figure 1). That is, in a

216

system with a higher proportion of impurities, e.g., C, the poorly crystalline Fe

217

hydroxides were readily formed at relatively alkaline conditions.52

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Given that the surface compositions might affect the physicochemical properties

219

of the DFC, we used the XPS technique to determine distributions of C and Fe on

220

near surfaces of the individual DFC samples. Figure 2 displays the C/(C+Fe) molar

221

ratio on near surfaces as a function of that of the bulk precipitate. Regardless of the

222

bulk C/(C+Fe) molar ratios, C instead of Fe seemed to dominate near surfaces of each 11



223

synthesized DFC sample (Figure 2). When bulk C/(C+Fe) molar ratios increased from

224

0.62 to 0.95, the ratios on near surfaces were essentially greater than 0.94 for all DFC

225

particles (Figure 2). That is, particularly at relatively lower bulk C/(C+Fe) ratios, C

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tended to precipitate on particle surfaces. On the contrary, a nearly 1:1 relationship

227

between bulk and surface C/(C+Fe) ratios was found when the bulk C/(C+Fe) ratio

228

was > 0.92. Such results implied a relatively more homogeneous distribution of C and

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Fe in the DFC samples with a greater C/(C+Fe) molar ratio. The significant

230

discrepancies in the correlation between bulk and surface C/(C+Fe) ratios for

231

individual DFC samples suggested the essential changes in the structural association

232

between C and Fe at different C/(C+Fe) ratios.

233

Speciation and local structures of Fe in DFC samples. The Fe speciation in

234

the DFC samples was determined using linear combination fitting (LCF) of Fe K-edge

235

XANES spectra.53 The LCF results presented in Table 2 revealed that the Fe species

236

in all DFC samples could be fit as a combination of 2-line ferrihydrite and Fe(III)

237

complexed with citrate.54 Figure S4 in the SI contains examples of the LCF fitting for

238

the DFC samples coprecipitated at pH 3. While ferrihydrite dominated the Fe species

239

in the DFC sample with a bulk C/(C+Fe) molar ratio ≤ 0.65, the proportion of Fe(III)

240

complexed with organic groups increased as the C/(C+Fe) molar ratio increased. For

241

instance, the percentages of Fe(III) bound to organic groups in the DFC samples 12


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increased from 18.7 to 96.3 when the bulk C/(C+Fe) molar ratios increased from 0.71

243

to 0.95 at pH 3. These results were in line with the previous study31, which reported a

244

mixture of 2-line ferrihydrite and insoluble Fe(III)-organic complexes with a C/(C+Fe)

245

molar ratio ≥ 0.74.

246

We subsequently used Fe-EXAFS analysis to investigate the local structures

247

surrounding the central Fe atom in the DFC samples. The Fe K-edge EXAFS spectra

248

in Figure S5a in the SI showed that oscillation amplitudes at k ≈ 5.0 and 7.5 Å-1

249

generally decreased with an increasing proportion of C, corresponding to a diminish

250

in high-shell backscattering signals between 2.3 and 3.4 Å in the Fourier transformed

251

(FT) spectra (Figure S5b in the SI).51 However, oscillation amplitudes at 6.3 and 8.5

252

Å-1 showed the opposite trend, suggesting high C proportions affected the

253

wave-beating patterns of ferrihydrite featured structures and the Fe coordination

254

environment.55 The EXAFS data were fit with an EXAFS model derived from

255

ferrihydrite56 that generated the single-scattering Fe-O and Fe-Fe paths out to 3.45 Å

256

and Fe(III)-NOM complexes57 that generated the single-scattering Fe-C paths in the

257

range 2.82-2.98 Å (Table 3). Coordination numbers of the first-shell Fe-O paths that

258

were fit at 1.96-1.99 Å across all DFC samples were fixed at six (Table 3) because

259

each Fe atom was expected to be surrounded by six oxygen atoms under the moist

260

paste conditions of the prepared EXAFS samples, and the surface defects of Fe 13



261

hydroxides were presumed to be covered by water molecules.58 The σ2 values of the

262

Fe-C path as well as the edge- and corner-sharing Fe-Fe paths for DFC samples were

263

floated according to the precipitated pH due to the structural ordering difference.51, 59

264

With the exception of the DFC62 samples precipitated at pH 4.5 and 6, whose

265

C/(C+Fe) ratios were ≤ 0.65, the second-shell coordination surrounding the central

266

Fe atom for the other DFC samples were designated as the Fe-C bonds with an

267

average distance of 2.97 Å, and there were no significant differences in the

268

coordination number (CN) for these Fe-C bonds (Table 3). For the Fe-Fe paths, we

269

found two average distances at 3.04 (3.01~3.07) and 3.42 (3.40~3.45) Å,

270

corresponding to edge- and corner-shared FeO6 octahedral linkages, respectively.60, 61

271

While the corner-shared (CS) FeO6 octahedra were found across all DFC samples, the

272

edge-shared (ES) FeO6 octahedra might only occur in samples with a C/(C+Fe) molar

273

ratio ≤ 0.89. The CN for ES and CS Fe-Fe paths varied in the range of 0.82-1.77 and

274

0.39-1.87, respectively, and no regular changes were found along with either

275

coprecipitation pH or C/(C+Fe) molar ratios (Table 3). The CN for both ES and CS

276

FeO6 octahedra in our DFC samples was essentially less than that of two-line

277

ferrihydrite.41 Based on a structural model proposed elsewhere,61, 62 the decrease in Fe

278

octahedral linkages could be translated as a decrease in the domain size of Fe

279

polyhedra. That is, the Fe domain in our DFC samples is possibly smaller than ∼2 nm, 14


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which is the general domain size of two-line ferrihydrite.56 According to the EXAFS

281

results, the local coordination of Fe in the DFC samples could be plausibly grouped

282

into the following three types: (1) the ferrihydrite-like Fe domain with C/(C+Fe) ≤

283

0.65; (2) the mixture of ferrihydrite-like Fe domains or ES/CS FeO6 octahedra with

284

Fe-C bonds when C/(C+Fe) ranged between 0.71 and 0.89; and (3) the CS FeO6

285

octahedra associated with Fe-C bonds when C/(C+Fe) ≥ 0.92. The vicissitude of the

286

ferrihydrite-like structure as a function of C/(C+Fe) molar ratios in DFC samples

287

obtained from EXAFS analyses was consistent with LCF and XRD results.

288

Changes of organic functional groups in DFC samples. The changes in

289

organic functional groups in DFC samples were first examined using FTIR analysis

290

(Figure 3). Generally, each major absorption peak of the DOM became broad or even

291

disappeared with decreasing C/(C+Fe) molar ratios. Disappearance of the symmetric

292

–C=O stretch at 1730 and 1400 cm-1 for DFC62 and DFC77 samples at all pH values

293

indicated that the carboxyl groups of the DOM might strongly associate with Fe

294

during coprecipitation processes. However, the association seemed incomplete for the

295

DFC89 and DFC94 samples at all pH values, probably due to the presence of a higher

296

number of carboxyl groups in these samples. Upon Fe complexation, the

297

characteristic absorption peak of the carboxyl group (–C=O band) was shifted to a

298

lower frequency, broadening the asymmetric –C–O stretch of the carbonyl group in 15



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299

the region from 1610 to 1645 cm-1.16 In addition, the O–H stretching vibration of the

300

phenolic OH at 1270 cm-1 disappeared across all DFC samples with the addition of Fe,

301

suggesting the phenolic groups might also participate in the coprecipitation reactions.

302

The variation in the C–O stretching of carbohydrates at 1050 cm-1 was relatively

303

insignificant with increasing C/(C+Fe) ratios, demonstrating low affinity between this

304

functional group and Fe(III).31, 39

305

The C NEXAFS analysis was used to further examine the changes in C

306

functional groups of the DFC samples (Figure 4). Three major peaks of carbon

307

materials in the DFC samples indicated the bands of H and C-substituted aromatic

308

carbon (π*C=C, 285.1 ± 0.2 eV), O-substituted aromatic carbon (phenolic carbon, π

309

*C=C-O, 286.5 ± 0.2 eV), and carboxyl carbon (π*C=O, 288.5 ± 0.3 eV).31,

310

Compared with the original DOM, the carboxyl C peak and aromatic C peak for DFC

311

samples became broader and less intense, suggesting the interaction between Fe(III)

312

and these two groups was significant in all DFC samples.38, 39 The phenolic carbon

313

disappeared for all of the 6DFC samples due to their strong associations with Fe(III)

314

at pH 6.65 A slight decrease in the intensity of the phenolic peak was observed for the

315

3DFC and 4.5DFC samples, which may be the collective result of low affinity or

316

lesser amounts of bonding between Fe(III) and the phenolic groups and flocculation

317

of some unreacted phenolic groups under acidic conditions, i.e., pH 3 and 4.5. 16


63, 64

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318

However, subtle changes in the phenolic groups upon the coprecipitation of DOM and

319

Fe at various pH values were not observed in the FTIR spectra with a lower energy.

320

Accordingly, the current results were consistent with the previous studies in that the

321

carboxyl groups and aromatic moieties of DOM exhibited various degrees of high

322

affinity with Fe(III).31, 32

323 324

Discussion

325

Interactions with Fe(III) control the stabilization and biogeochemical cycling of

326

C. A significant decrease in Fe(III) solubilisation was observed with increasing Fe

327

proportion in the DFC samples among all tested pH values (Table 1). We presumed

328

that the Fe domains became unstable due to the loss of edge-sharing Fe octahedra

329

with an increase in the C proportion, leading to an elevation of Fe solubilisation

330

(Table 1). According to Fe K-edge XAS results contained in the supplementary

331

information provided by other spectroscopic analyses, structural developments of

332

DOM/Fe(III) coprecipitates could be generally categorized into three types as a

333

function of the C/(C+Fe) ratio.

334

In the system with C/(C+Fe) molar ratios ≤ 0.65, XRD and Fe-XAS analyses

335

(Figure 1, Table 2 and 3) implied that such DFC samples precipitated as a core-shell

336

structure. The ferrihydrite-like Fe domains precipitated as the cores, and these 17



337

Fe-cores were covered with DOM molecules as indicated by XPS analysis, which

338

showed an approximate 1.5-fold enrichment of near-surface C proportion relative to

339

bulk C proportion (Figure 2, Table S2). The association between the Fe cores and the

340

C shells was evidenced by the significantly decreased peak intensity of the carboxyl

341

functional groups as shown in the FTIR and C-NEXAFS results (Figure 3 and 4).

342

These samples also had the least amounts of C and Fe solubilisation (Table 1),

343

suggesting a relatively stable structure in the Fe-core/C-shell type.

344

When the C/(C+Fe) molar ratio ranges between 0.71 and 0.89, the formation of

345

Fe-C bonds suggested a more substantial association between Fe domains including

346

ES and CS FeO6 octahedra and DOM (Table 3). In accordance with the variation in

347

the local structure of the Fe domains, the LCF results of Fe-XANES spectra showed

348

the proportion of ferrihydrite in the Fe inventory decreased concomitantly with the

349

increasing proportion of Fe(III) complexed with organic functional groups (Table 2).

350

Moreover, the less discernible ferrihydrite patterns in the XRD analyses for samples

351

with C/(C+Fe) molar ratios from 0.71 to 0.89 also attested to the decreasing

352

proportion of ferrihydrite in the total Fe inventory and/or the more disrupted structural

353

ordering of ferrihydrite (Figure 1).31, 41

354

In DFC samples with C/(C+Fe) molar ratios ≥ 0.92, only CS FeO6 octahedra

355

along with Fe-C bonding was found in coprecipitates (Table 3). Such non-crystalline 18


Page 18 of 36

Page 19 of 36


356

Fe hydroxides were also evidenced by the loss of diagnostic ferrihydrite peaks

357

observed in XRD analysis (Table 3). Due to the inherent speciation uncertainty caused

358

by the fitting models that can only approximate reality and/or the limits in the

359

separation of spectral features in the LCF of XANES spectra,66 our LCF results still

360

indicate ferrihydrite in the DFC samples with C/(C+Fe) molar ratios ≥ 0.92 although

361

the proportions were essentially less than those in other DFC samples. Moreover, the

362

loss of ES Fe octahedral linkages in these DFC samples compared with other DFC

363

samples implied a decrease in the Fe domain size when the C/(C+Fe) molar ratio was

364

≥ 0.92. In contrast to DFC samples with limited C [C/(C+Fe) molar ratios ≤ 0.89],

365

wherein Fe hydroxides served as a heterogeneous nucleation template for surface

366

outgrowth of DOM molecules, Fe and C in samples with C/(C+Fe) molar ratios ≥

367

0.92 tended to be distributed homogeneously on the external portions of the DFC

368

structures, resulting in the nearly 1:1 relationship between bulk and surface C/(C+Fe)

369

ratios (Figure 2). Due to the associations between Fe and C, interferences of the

370

growths for C and Fe domains caused unstable structures for Fe hydroxides and

371

subsequently enhanced the Fe solubilisation (Table 1). Such Fe solubilisation might

372

result in the concurrent release of C from those Fe sites with bonded C.

373 374

Implications 19



375

Coprecipitation of NOM and Fe is an important process that stabilizes C and Fe

376

in the soil system. Many studies also indicated that the variation in the coprecipitate

377

structures is effected by the C/(C+Fe) ratios. Our collective results demonstrated the

378

C/(C+Fe) ratio-driven transformation of DFC coprecipitated structures. While the

379

outgrowth of the DOM on Fe hydroxides at C/(C+Fe) ratios between 0.71 and 0.89

380

stabilized the Fe domains, Fe hydroxides were subject to solubilisation at C/(C+Fe)

381

ratios ≥ 0.92 as a consequence of the homogeneously distributed unstable structures

382

between Fe and C domains. Once C/Fe coprecipitates formed in soil systems, such

383

association of organic matter to soil inorganic minerals increases the resistance of

384

organic matter to microbial decomposition In addition, C/Fe coprecipitates also serve

385

as carriers controlling the mobility and bioavailability of environmental inorganic

386

pollutants such as chromium and arsenic. Recognition of the structural composition

387

and stability of C/Fe coprecipitates could lead to a better quantification of the

388

dynamics and mass balances of Fe and C in soils, resulting in an improved prediction

389

of the geochemistry of nutrients such as phosphorus and contaminants such as arsenic

390

and heavy metals.

391 392 393

Acknowledgements The authors are grateful to Dr. Jyh-Fu Li for the assistance at beamline 17C and 20


Page 20 of 36

Page 21 of 36


394

Dr. Jin-Ming Chen for the assistance at beamline 20A, NSRRC. This work is

395

financially supported by the Ministry of Science and Technology, ROC under the

396

project

397

103-2112-M-007-022-MY3, and 104-2311-B-005-016-MY3.

number

of

101-2313-B-005-047-MY3,

102-2313-B-029-005-MY2,

398 399

Supporting information

400

Additional information includes elemental composition, NMR, and FTIR results

401

of original CHA DOM, chemical compositions, mineralogical, structural, and surface

402

analyses of DOM/Fe Coprecipitates, as well as XAS analysis including XANES-LCF

403

results and EXAFS fitting results. This information is available free of charge via the

404

Internet at http://pubs.acs.org.

405 406 407 408 409 410 411 412 21



413

References

414

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586 587 588 589 590 591 592 593 594 595 596 597 26


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598

Table captions Table 1. The initial added, precipitated, and dissolved C and Fe for DOM/Fe(III) coprecipitates containing various C/(C+Fe) molar ratios at pH 3, 4.5, and 6. Table 2. Results of Fe-XANES LCF analysis for DFC samples precipitated at pH 3, 4.5, and 6. The initial additions of C/(C+Fe) molar ratios are 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94). Table 3. Fits to Fourier transformed EXAFS data over the k-range 2.5-11.5 Å-1 for 2-line ferrihydrite and DFC samples precipitated at pH 3, 4.5, and 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94). Figure captions Figure 1. X-ray diffraction patterns for DFC samples precipitated at (a) pH 3 (b) pH 4.5 and (c) pH 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94). Data were collected using the incident X-ray wavelength of 0.77 Å. Figure 2. The C/(C+Fe) molar ratio on near surfaces as the function of that on bulk precipitates. The diamond, triangle, and circle symbols represent the DFC samples precipitates at pH 3, 4.5, and 6, respectively. The dashed line

27



shows the 1:1 relationship of C/(C+Fe) molar ratios on near surfaces vs. bulk particles of DFC samples. Figure 3. Fourier-transform infrared spectra for DFC samples precipitated at (a) pH 3 (b) pH 4.5 and (c) pH 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94). Figure 4. C 1s NEXAFS spectra for DFC samples precipitated at (a) pH 3 (b) pH 4.5 and (c) pH 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94).

28


Page 28 of 36

Page 29 of 36


Table 1. The initial added, precipitated, and solubilisation C and Fe for DOM/Fe(III) coprecipitates containing various C/(C+Fe) molar ratios at pH 3, 4.5, and 6.a Copreci pitation pH

Sample

Initial added C/(C+Fe)

Precipitated C (mmole g-1)

Precipitated Fe (mmole g-1)

Bulk (precipitated) C/(C+Fe)

Bulk (precipitated) C/Fe

C solubilisation (%)

Fe solubilisation (%)

pH 3

DFC62

0.62

14.25 (0.08)

5.87 (0.02)

0.71 (0.002)

2.43 (0.023)

3.21 (1.83)

0.60 (0.10)

DFC77

0.77

23.88 (0.07)

3.49 (0.06)

0.87 (0.002)

6.84 (0.100)

2.38 (0.29)

1.95 (0.05)

DFC89

0.89

27.51 (0.01)

2.49 (0.03)

0.92 (0.001)

11.07 (0.115)

4.96 (1.16)

6.35 (0.05)

DFC94

0.94

30.08 (0.09)

1.54 (0.11)

0.95 (0.003)

19.59 (1.332)

7.00 (1.56)

3.90 (0.10)

DFC62

0.62

11.29 (0.01)

6.17 (0.01)

0.65 (0.001)

1.83 (0.005)

1.58 (0.01)

NDb

DFC77

0.77

16.75 (0.04)

5.33 (0.04)

0.76 (0.001)

3.14 (0.013)

2.69 (0.17)

0.05 (0.05)

DFC89

0.89

25.85 (0.05)

2.31 (0.02)

0.92 (0.001)

11.21 (0.055)

7.80 (2.22)

0.25 (0.25)

DFC94

0.94

28.60 (0.06)

1.47 (0.06)

0.95 (0.002)

19.43 (0.787)

9.91 (3.38)

0.60 (0.10)

DFC62

0.62

10.30 (0.03)

6.45 (0.02)

0.62 (0.001)

1.60 (0.001)

3.96 (0.44)

NDb

DFC77

0.77

16.04 (0.16)

5.75 (0.05)

0.74 (0.001)

2.79 (0.007)

12.96 (0.17)

2.85 (0.65)

DFC89

0.89

22.93 (0.05)

2.85 (0.03)

0.89 (0.001)

8.05 (0.051)

25.05 (4.23)

7.55 (1.85)

DFC94

0.94

27.90 (0.02)

2.34 (0.05)

0.92 (0.002)

11.93 (0.262)

24.90 (1.74)

8.45 (2.35)

pH 4.5

pH 6

a b

Numbers in parentheses represent standard deviations derived from two replicates. Not detected.



Page 30 of 36

Table 2. Results of Fe-XANES LCF analysis for DFC samples precipitated at pH 3, 4.5, and 6. The initial additions of C/(C+Fe) molar ratios are 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94). Bulk Ferrihydrite Fe-citrate R factor Sample (%)a C/(C+Fe) (%)a (×103)b pH 3 DFC62 0.71 81.3 (2.0) 18.7 (2.0) 0.40

pH 4.5

pH 6

DFC77

0.87

18.5 (3.6)

81.5 (3.6)

1.31

DFC89

0.92

3.7 (3.6)

96.3 (3.6)

1.28

DFC94

0.95

7.5 (2.5)

92.5 (2.5)

0.59

DFC62

0.65

100 (0.0)

0 (0.0)

4.58

DFC77

0.76

43.8 (2.0)

56.2 (2.0)

0.41

DFC89

0.92

16.5 (2.3)

83.5 (2.3)

0.56

DFC94

0.95

8.4 (1.5)

91.6 (1.5)

0.23

DFC62

0.62

100 (0.0)

0 (0.0)

2.17

DFC77

0.74

30.0 (4.9)

70.0 (4.9)

2.38

DFC89

0.89

26.5 (5.0)

73.5 (5.0)

2.34

DFC94

0.92

24.4 (1.8)

75.6 (1.8)

0.32

a

Numbers in parentheses represent standard deviation. The weighting factors on each fit were summed to 100 ± 1% and were normalized to 100%. Although five Fe species were used as end-members in the LCF analyses, the best fits for all samples were obtained using a combination of ferrihydrite and Fe(III) complexed with citrate. b Normalized sum of the squared residuals of the fit (R-factor = ∑(data-fit)2/∑data2).


Page 31 of 36


Table 3. Fits to Fourier transformed EXAFS data over the k-range 2.5-11.5 Å-1 for 2-line ferrihydrite and DFC samples precipitated at pH 3, 4.5, and 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94).a Fe-C (SS)

pH 3

pH 4.5

pH 6

Fe-Fe1 (SS)

Sample

CN

R (Å)

σ2 (Å2)b

CN

Ferrihydrite

-

-

-

2.84 (0.48)

DFC62

1.77 (0.14)

2.96 (0.011) 0.004 (0.001)

DFC77

1.65 (0.71)

DFC89

Fe-Fe2 (SS) CN

R (Å)

σ2 (Å2)b

3.03 (0.011) 0.014 (0.002)

0.79 (0.22)

3.45 (0.014)

0.005 (0.002)

1.77 (0.14)

3.07 (0.005) 0.008 (0.001)

1.35 (0.53)

3.43 (0.037)

0.018 (0.002)

2.96 (0.011) 0.004 (0.001)

0.82 (0.31)

3.07 (0.005) 0.008 (0.001)

1.77 (1.06)

3.43 (0.037)

0.018 (0.002)

1.64 (1.10)

2.98 (0.019) 0.004 (0.001)

-

-

-

1.87 (1.74)

3.40 (0.051)

0.018 (0.002)

DFC94

1.58 (0.69)

2.98 (0.019) 0.004 (0.001)

-

-

-

1.66 (0.96)

3.41 (0.041)

0.018 (0.002)

DFC62

-

DFC77

1.13 (0.42)

DFC89

-

σ2 (Å2)b

0.87 (0.21)

3.02 (0.025) 0.009 (0.001)

0.64 (0.32)

3.42 (0.024)

0.008 (0.002)

2.96 (0.033) 0.004 (0.002)

0.88 (0.21)

3.05 (0.008) 0.009 (0.001)

0.87 (0.21)

3.43 (0.008)

0.008 (0.002)

1.88 (1.62)

2.99 (0.026) 0.004 (0.002)

-

-

-

0.88 (0.87)

3.43 (0.033)

0.008 (0.002)

DFC94

1.33 (0.72)

2.99 (0.026) 0.004 (0.002)

-

-

-

0.47 (0.36)

3.43 (0.033)

0.008 (0.002)

DFC62

-

DFC77

1.32 (1.11)

DFC89 DFC94

-

-

R (Å)

-

1.02 (0.20)

3.01 (0.015) 0.010 (0.001)

0.55 (0.15)

3.44 (0.008)

0.005 (0.002)

2.95 (0.023) 0.006 (0.004)

1.66 (0.50)

3.06 (0.008) 0.010 (0.001)

0.47 (0.35)

3.44 (0.008)

0.005 (0.002)

1.30 (0.93)

2.95 (0.023) 0.006 (0.004)

1.16 (0.42)

3.06 (0.008) 0.010 (0.001)

0.39 (0.27)

3.44 (0.008)

0.005 (0.002)

1.55 (1.26)

2.95 (0.023) 0.006 (0.004)

-

0.39 (0.38)

3.43 (0.059)

0.005 (0.002)

-

-

Paths used in EXAFS fitting are all single scattering (SS) paths. S02 (fixed amplitude reduction factor based on first-shell fitting of hematite) = 0.83. ΔE (fitted energy shift parameter) = -2.44 (0.43), -3.74 (0.26), and -2.85 (0.31) eV for DFC samples precipitated at pH 3, 4.5, and 6. For the Fe-O paths among all samples, the interatomic distances were fitted around 1.96-1.99 Å and the fitted σ2 (mean-square displacements of interatomic distances) values ranged from 0.006-0.019 Å2. The CN (coordination number) was fixed at 6.0. Details for fitting parameters of Fe-O paths were tabulated in Table S3 in SI. b 2 σ values were grouped according to precipitated pH and fitted. a



(a) pH 3

2.6 Å

1.5 Å DFC62

DFC77

DFC89 DFC94

(b) pH 4.5 2.6 Å 1.5 Å

intensity (cps)

DFC62 DFC77 DFC89 DFC94

(c) pH 6

15000

2.6 Å 1.5 Å DFC62

10000 DFC77 DFC89

5000

DFC94

0 0

10

20

30

40

50

2theta

Figure 1. X-ray diffraction patterns for DFC samples precipitated at (a) pH 3 (b) pH 4.5 and (c) pH 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94). Data were collected using the incident X-ray wavelength of 0.77 Å.


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1.0

Surface C/(C+Fe) ratio

0.9

0.8 pH 3 pH 4.5

0.7

pH 6 0.6 0.6

0.7

0.8

0.9

1.0

Bulk C/(C+Fe) ratio Figure 2. The C/(C+Fe) molar ratio on near surfaces as the function of that on bulk precipitates. The diamond, triangle, and circle symbols represent the DFC samples precipitates at pH 3, 4.5, and 6, respectively. The dashed line shows the 1:1 relationship of C/(C+Fe) molar ratios on near surfaces vs. bulk particles of DFC samples.


(a) pH 3

1050

1270

1400

1610

1730


DFC62 DFC77 DFC89 DFC94 DOM

(b) pH 4.5

DFC62

Absorbance

DFC77 DFC89 DFC94

DOM

3

(c) pH 6

DFC62 DFC77

2 DFC89 DFC94

1 DOM

0 2000

1800

1600

1400

1200

1000

800

Wavenumber (cm-1)

Figure 3. Fourier-transform infrared spectra for DFC samples precipitated at (a) pH 3 (b) pH 4.5 and (c) pH 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94).


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phenolic C

(a) pH3 aromatic C carboxyl C DFC62 DFC77 DFC89 DFC94 DOM

(b) pH4.5 Normalized intensity (a.u.)

DFC62 DFC77 DFC89 DFC94 DOM

(c) pH6 1.0

DFC62 DFC77 DFC89

0.5

DFC94 DOM

0.0 280

285

290

295

Energy (eV)

Figure 4. C 1s NEXAFS spectra for DFC samples precipitated at (a) pH 3 (b) pH 4.5 and (c) pH 6. For each precipitation condition, the initial additions of C/(C+Fe) molar ratios were 0.62, 0.77, 0.89 and 0.94 (DFC62, DFC77, DFC89, and DFC94).



TOC/A Abstract A rt (4.5 cm m*5.88 cm m)


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Stabilization of Natural Organic Matter by Short-Range-Order Iron

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