Adsorption and Molecular Fractionation of ... - ACS Publications

Subscriber access provided by READING UNIV

Article

Adsorption and molecular fractionation of dissolved organic matter on iron-bearing mineral matrices of varying crystallinity Elizabeth K Coward, Tsutomu Ohno, and Alain F. Plante Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04953 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

Environmental Science & Technology

1

Adsorption and molecular fractionation of dissolved organic matter on

2

iron-bearing mineral matrices of varying crystallinity

3

4

Elizabeth K. Coward1,2*, Tsutomu Ohno3, Alain F. Plante2

5

1

Delaware Environmental Institute, University of Delaware, Newark DE 19716-7310, USA.

6

2

Department of Earth & Environmental Science, University of Pennsylvania, Philadelphia PA

7

19104-6316, USA.

8

3

School of Food & Agriculture, University of Maine, Orono ME 04469-5763, USA.

ACS Paragon Plus Environment

1


Page 2 of 33

9

Abstract. Iron (Fe)-bearing mineral phases contribute disproportionately to adsorption of soil

10

organic matter (SOM), due to their elevated chemical reactivity and specific surface area (SSA).

11

However, the spectrum of Fe solid-phase speciation present in oxidation-reduction-active soils

12

challenges analysis of SOM-mineral interactions and may induce differential molecular

13

fractionation of dissolved organic matter (DOM). This work used paired selective dissolution

14

experiments and batch sorption of post-extraction residues to 1) quantify the contributions of Fe-

15

bearing minerals of varying crystallinity to DOM sorption, and 2) characterize molecular

16

fractionation using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS).

17

A substantial proportion of soil SSA was derived from extracted Fe-bearing phases, and FT-ICR-

18

MS analysis of extracted DOM revealed distinct chemical signatures across Fe-OM associations.

19

Sorbed carbon (C) was highly correlated with Fe concentrations, suggesting that Fe-bearing

20

phases are strong drivers of sorption in these soils. Molecular fractionation was observed across

21

treatments, particularly those dominated by short-range-order (SRO) mineral phases, which

22

preferentially adsorbed aromatic and lignin-like formulae, and higher-crystallinity phases,

23

associated with aliphatic DOM. These findings suggest Fe speciation-mediated complexation

24

acts as a physicochemical filter of DOM moving through the critical zone, an important

25

observation as predicted changes in precipitation may dynamically alter Fe crystallinity and C

26

stability.

27 ACS Paragon Plus Environment

2

Page 3 of 33

28


INTRODUCTION

29

Humid tropical forest soils contain a substantial portion (~500 Pg) of the global terrestrial

30

carbon (C) pool1, yet the persistence of soil organic matter (SOM) in these systems remains

31

unclear, largely due to mechanistic uncertainty in the biogeochemical controls on soil C.

32

Organomineral complexation, driven by high concentrations of iron (Fe)-bearing mineral phases

33

accumulating in surface horizons, has been framed as a potential C stabilization mechanism2-4.

34

However, humid tropical soils often contain a diverse array of Fe-bearing minerals produced by

35

pedogenesis5,

36

confounding our understanding of Fe contributions to C accumulation and stabilization.

37

Particularly, the solid-phase speciation of Fe minerals in soils, such as their size, structure, and

38

surface area, can regulate the dynamics of C sorption reactions8-11, which have been linked to

39

long-term retention of C in soils12,

40

volcanic soils underlying many humid tropical forests14, are exceptionally reactive due to their

41

high specific surface area (SSA) and large numbers of hydroxyl groups present on mineral

42

surfaces15. As such, even small quantities of these minerals may provide an important sorbent for

43

organic compounds.

6

and frequent oxidation-reduction (redox) oscillations in surface horizons7,

13

. Short-range-order (SRO) minerals, abundant in the

44

Tropical systems are also characterized by large dissolved organic matter (DOM) input to

45

surface soils, a highly mobile and reactive pool of SOM 16. In many tropical forests, fine litterfall

46

accounts for ~60% of aboveground net primary productivity17 and rainfall is both frequent and

47

abundant. As such, the movement of litter-leached DOM represents an important, near-constant

48

input of C that fuels substantial heterotrophic respiration and could account for a large proportion

49

of soil CO2 fluxes18. This DOM is thought to be comprised of both largely plant-derived

50

compounds, such as lignin-derived phenols, mobilized through leaching, root tissue and


3


Page 4 of 33

51

exudates19, 20, and char21. Recent evidence has shown DOM is also highly processed by microbial

52

assimilation and turnover21-23.

53

This abundance of DOM entering humid tropical soils may undergo molecular

54

fractionation at the mineral interface, driven by the molecular diversity of DOM and the

55

spectrum of reactive minerals present. DOM is comprised of numerous small molecular

56

components bound by hydrophobic interactions and hydrogen bonds, which may be broken when

57

external forces from mineral surfaces are greater than the intermolecular forces between the

58

molecular components of DOM24. Molecular components with greater affinity to mineral

59

surfaces, often hydrophobic-, aromatic-, and carboxylic-rich fractions, will likely be bound while

60

DOM characterize by lower affinity functional groups will remain in solution, leading to the

61

molecular fractionation of DOM25, 26. The study of such molecular fractionation has, however,

62

been historically limited to spectroscopic methods, which provide moiety-scale assessment of

63

DOM chemistry, but are not capable of resolving molecular formulae. The chemical

64

characterization of DOM composition has recently been enhanced by the ultrahigh resolution

65

capabilities of electrospray ionization Fourier transform ion cyclotron resonance mass

66

spectrometry (FT-ICR-MS), capable of resolving thousands of individual OM components27.

67

This work uses FT-ICR-MS analysis to investigate the role of Fe-bearing mineral

68

crystallinity in adsorbing and fractionating DOM in humid tropical soil matrices. Such chemical

69

fractionation of DOM by the physicochemical filter of Fe mineralogy may modulate the

70

composition of DOM reaching subsoils and the critical zone at large. Thus, characterizing the

71

contribution of mineral matrices with varying Fe-bearing solid-phase speciation may aid in

72

understanding the fate of DOM at the watershed or ecosystem level, in reconciling the wide


4

Page 5 of 33


73

range of C turnover times observed in tropical systems, and in disentangling the complex

74

response of a substantial, but understudied, C reservoir to projected changes in climate.

75

We aim to assess the contribution of Fe content and crystallinity to these organomineral

76

complexes via two methodologies: 1) molecular characterization of DOM extracted with various

77

Fe-bearing mineral phases, and 2) assessment of the contribution of Fe-bearing mineral phases to

78

adsorptive molecular fractionation of DOM. Several recent studies have used FT-ICR-MS

79

analysis to provide strong evidence of adsorptive fractionation of organic acids and DOM by

80

synthetic metal oxides 24, 28, 29 and a temperate soil matrix 30. However, the role of Fe solid-phase

81

speciation in governing molecular adsorptive fractionation of complex, litter-derived DOM

82

remains uncertain Inorganic selective dissolution of distinct Fe-bearing mineral phases from

83

humid tropical Oxisol soils and associated OM were followed by FT-ICR-MS analysis of the

84

extracts to detect in-situ molecular fractionation of OM. Fe-induced fractionation was also

85

investigated in a laboratory batch sorption experiment. Soil residues post-dissolution, a

86

Ferrihydrite-coated (Fh+) soil and untreated control soil, comprising five distinct matrices

87

varying in iron content, crystallinity and SSA, were then reacted with DOM in a batch sorption

88

experiment. Unsorbed and post-sorption DOM solutions were analyzed for C concentrations to

89

assess bulk sorption effects and via FT-ICR-MS to determine preferential adsorption of OM

90

components.

91

MATERIALS AND METHODS

92

Soil Sampling and Selective Dissolution.

93

Soil samples originated from an extensive sampling campaign completed in 2010 31 spanning

94

the tropical montane forests of the Luquillo Critical Zone Observatory (LCZO). For the present

95

study, one surface (0 - 20 cm) horizon from a quantitative soil pit of Oxisol soil order was


5


Page 6 of 33

96

selected to assess the contribution of matrix Fe-bearing mineralogy to OM sorption and

97

molecular fractionation. Detailed site information and soil characteristics may be found in the SI.

98

Six soil subsamples from the source pit were subjected to three parallel inorganic chemical

99

extractions (n=18) for quantification of extractable Fe-containing secondary weathering phases

100

and associated C, as previously detailed in Coward et al.3.

101

Preparation of Sorbent Matrices and Dissolved Organic Matter.

102

The iron oxyhydroxide Ferrihydrite (Fh) was synthesized according to methods described 32

103

by Herbel and Fendorf

104

NaOH until stable at pH 7.5. The suspension was allowed to equilibrate for at least 2 h, during

105

which time the pH was adjusted to maintain a value of 7.5. Synthesized Fe oxyhydroxides were

106

repeatedly washed (4×) by centrifugation, re-suspended in Milli-Q water and immediately added

107

to substrates at 25 °C: one of 30 g soil and another of 30 g 50-mesh quartz sand. Each slurry was

108

shaken horizontally for 48 hours until dry; this process was repeated three times to maximize

109

surficial coating. Ferrihydrite coatings were isolated from quartz sand samples via suspended

110

sonication and centrifugation for mineralogical confirmation. Mineral identity was established by

111

powder X-ray diffraction (pXRD) methods (Fig. S1).

. Briefly, a ferric chloride solution was rapidly titrated with 1.0 N

112

DOM was extracted from O horizon material collected during the 2010 sampling

113

campaign. The composition of the original litter was 31.0 % C and 1.1 % N31. The extract was

114

obtained by shaking a 1:10 soil:Milli-Q water solution for 24 hours. The supernatant was

115

obtained by centrifugation at 5300 × g and vacuum filtration through 0.22 µm nylon filters. The

116

DOM solution was kept in the dark at 4 °C until TOC analysis via a persulfate digestion and

117

spectrophotometric method (Method 10173, Hach Company, Loveland CO). Initial DOC

118

concentration was 200 ± 1.48 mg C L-1, which is relatively high but within the range of

119

concentrations used in previous sorption studies (e.g., Feng et al.33). The high DOC


6

Page 7 of 33


120

concentration was used in the aim of saturating the system and reaching maximum sorption

121

potential. pH of the final extract was 4.4 due to the acidic character of the water-soluble

122

compounds in the initial litter.

123

Sorption Experiments.

124

Batch sorption experiments were conducted by mixing 0.5 g aliquots of soil with 15 mL of

125

DOM in a 50 mL Erlenmeyer flask. Batch sorption experiments included four replicates of each

126

sorbent matrix and four control samples consisting solely of DOM solution (~200 mg C L-1) to

127

determine the extent of microbial DOM degradation during the experiment. Thus, a total of 24

128

Erlenmeyer flasks were shaken for 48 h on a reciprocal shaker (120 rpm). A biocide was not

129

used and experiments were performed at room temperature (25 °C). After sorption, suspensions

130

were centrifuged at 5,300 × g, filtered through 0.22 µm Whatman filters and stored in the dark at

131

4 °C. Supernatants from the batch sorption experiments were analyzed for DOC as described

132

above. The concentration of DOM adsorbed to the mineral matrices was calculated by the

133

difference between concentrations in the initial and post-sorption solutions. No detectable

134

differences in DOM control solutions were detected, and thus no correction was used in this

135

calculation.

136

FT-ICR-MS Analysis.

137

FT-ICR-MS analysis was used to characterize DOM of (i) selective dissolution extracts,

138

(ii) initial DOM, and (iii) post-sorption supernatants. Solid phase extraction (SPE) was

139

performed on all the samples to desalinate and concentrate the DOC solution according to

140

Dittmar et al.34. Briefly, extracts were prepared by diluting the extracts to 100 mg/L DOC with

141

mass spectrometry grade water and processing through Agilent PPL™ solid-phase extraction

142

cartridges. The adsorbed organic matter was eluted with mass spectrometry grade methanol for


7


Page 8 of 33

143

FT-ICR-MS analysis, which was conducted with a 12-T Bruker Daltonics Apex Qe FT-ICR-MS

144

housed at the College of Sciences Major Instrumentation Cluster at Old Dominion University.

145

The solid-phase extraction processed samples (eluted in methanol) were diluted by a factor of 5

146

with LCMS grade methanol and water to give a final sample composition of 50:50 (v/v)

147

methanol:water. Further details on SPE and FT-ICR-MS parameters are included in the

148

Supplementary Information.

149

RESULTS & DISCUSSION

150

Selective Dissolution of Fe-associated OM.

151

Chemical properties of soil samples and selective extractants post-dissolution (Table 1)

152

were consistent with previous applications of this selective dissolution method3. Briefly,

153

inorganic dithionite-HCl (DH) extracted crystalline and SRO Fe oxides, hydroxides, and

154

oxyhydroxides derived largely from pedogenesis. HCl-hydroxylamine (HH) extracted SRO Fe-

155

bearing mineral phases, and sodium pyrophosphate (PP) extracted chelated and dispersable Fe

156

phases. Hydroxylamine- and dithionite-extracted Fe were found to be the dominant extractable

157

Fe phases, suggesting crystalline and SRO Fe phases are prevalent in these soils. The highest

158

concentrations of C were solubilized by pyrophosphate extraction, however this extractant

159

solubilized the lowest observed concentrations of Fe. A maximum molar ratio of adsorption C to

160

Fe oxides is 1.0 for the co-extracted species35-37. Co-precipitation, chelation or aggregation of

161

organic ligands generates low- density, organic-rich structures with higher (> 1) C:Fe values Fe-

162

bearing mineral phases, largely via available surface area.36,

163

indicate that hydroxylamine (C:Fe = 0.88 ± 0.054) and dithionite (C:Fe = 0.54 ± 0.038) extracts

164

likely consist of Fe-C bound via inner-sphere adsorptive mechanisms, while pyrophosphate-

38

Resultant molar C:Fe ratios


8

Page 9 of 33


165

extractable C (C:Fe = 7.44 ± 0.32) may be associated via OM-rich mechanisms, such as co-

166

precipitation or aggregation.

167

Ultrahigh resolution mass spectrometry of the extracts was used to probe differences in

168

OM chemical composition liberated by each extract (Fig. 1). After post-spectral processing to

169

identify peaks present in at least two out of three analytical replicates, > 3,800 mass spectral

170

peaks were present in all three extractants, and elemental compositions were assigned in the

171

range 300 < m/z < 800. van Krevelen diagrams and relative distributions (Fig. 1A-C) of chemical

172

classifications reveal substantial differences in classification class abundances among extracts,

173

while highlighting the predominance of lignin-like compounds across all extracts. The strong

174

adsorption of lignin-like compounds to mineral surfaces has been well documented, and is likely

175

a function of both ligand exchange reactions through prevalent carboxyl groups20,

176

efficiency of microbial degradation of lignin and lignin-like compounds40,

177

persistence of such compounds in DOM.

41

39

and low

, leading to the

178

Beyond the abundance of lignin-like molecular formulae in all three extractant types, we

179

observed strong chemical fractionation among the three extractants, indicating OM associated

180

with Fe phases of varying crystallinity differs at the molecular level. Most notably,

181

pyrophosphate-solubilized DOM (Fig. 1A) differed substantially from DOM solubilized in other

182

extracts (Figs. 1B, 1C, S2). The relative abundances of compounds in each group reveal

183

pyrophosphate-extracted OM contained greater unsaturated aliphatic and saturated carbohydrate-

184

like contributions and fewer condensed lignin and aromatic contributions. DOM molecular

185

formulae in the aliphatic region (H/C > 1.5) of van Krevelen space are likely to be microbially-

186

derived, but may also may be sourced from root necromass or other root inputs to soils, such as

187

root exudates42. Carbohydrates in DOM are derived from decaying microorganisms, and are


9


Page 10 of 33

188

mainly composed of large aggregates of carbohydrates such as polysaccharides as well as

189

proteins and peptides43. The predominance of carbohydrate-based compounds associated to

190

chelated or colloidal Fe is in accordance to previous findings43, 44 that these inherently polar and

191

highly soluble compounds may not be adsorbed directly to mineral surfaces. More broadly,

192

chelated or colloidal OM appears to be largely microbial in origin, and, due to the weaker

193

electrostatic interactions stabilizing these structures, may represent a more rapidly cycling pool

194

of C. In accordance with the “cytoplasmic solute” hypothesis proposed by Fierer and Schimel45,

195

such hydrophilic compounds may be easily dissolved, and subsequently mineralized, during the

196

frequent precipitation (i.e., Fe-reducing) events in humid forest soils.

197

Hydroxylamine-extracted (Fig. 1B) and dithionite-extracted (Fig. 1C) OM occupied less

198

aliphatic space (H/C > 1.5), and were instead comprised of formulae with higher O/C and higher

199

double-bond equivalence (DBE), a measure of the number of double bonds and rings in a

200

molecule (Fig. S2, modified from analysis presented by Riedel et al., 201246), comprised of

201

polycyclic aromatic, aromatic and lignin components. Preferential adsorption of aromatic

202

structures to hydrous Fe phases is well-established25,

203

solubilized by these two extractants were expected, due to the likely extraction of SRO Fe phases

204

by both, differences between the Hydroxylamine-extracted (Fig. 1B) and dithionite-extracted

205

(Fig. 1C) OM were observed. Hydroxylamine-solubilized OM contained relatively lower

206

contributions of lignin-like components than those observed in dithionite-solubilized OM,

207

accompanied by greater polycyclic aromatic contributions. The increase in dithionite-extracted

208

lignin-like components is likely due to overall increased mineral dissolution and subsequent

209

breakage of ligand-exchange bonds. We suggest that polycyclic aromatic compounds may be

210

preferentially bound to those SRO mineral phases solubilized by hydroxylamine extraction.

47

. While similarities in the formulae


10

Page 11 of 33

211


DOM sorption by mineral matrices.

212

Post-extraction solid-phases residues, Fh+ soil, and untreated soils, were exposed to O

213

horizon-derived DOM in a batch sorption experiment to assess the contribution of Fe mineral

214

crystallinity and reactivity to matrix adsorption capacity. All sorbent matrices were characterized

215

for total C and Fe content and SSA prior to sorption (Table 1, Figure 2). The mineral matrix

216

treatments represent a wide range of Fe concentrations, which are strongly correlated (R2 = 0.90)

217

with measured SSA (m2 g-1) (Fig. 2A). The strength of the observed correlation confirms the

218

hypothesis that Fe-bearing mineral phases are a critical driver of available surface area in these

219

highly-weathered tropical soils.

220

Measurements of total C and SSA were obtained post-sorption to determine the

221

concentration of C sorbed and C loadings pre- and post-sorption. Notably, sorbed C

222

concentrations, derived from initial and post-sorption DOC measurements, were well-correlated

223

(R2 = 0.76) with Fe content (Fig. 2B). Such robust correlations suggest that Fe-bearing mineral

224

phases, largely via available surface area, are strong controls on C accumulation in these soils. It

225

is important to note untreated soils adsorbed anomalously low C concentrations, which we

226

suggest may be due to desorption or exchange of native SOM or to the existing occlusion of Fe-

227

bearing minerals by OM, as the other treatments either removed Fe but also C (selective

228

dissolution), or added Fe phases without increasing OM content (Fh coating). Comparison of C

229

loadings (mg C m-2) pre- and post-sorption (Fig. 2C) revealed an expected increase in loadings

230

due to sorption across all treatments, consistently correlated with Fe content (R2 = 0.79, R2 =

231

0.80).

232

Molecular fractionation of matrix-sorbed OM.


11


Page 12 of 33

233

FT-ICR-MS characterization of pre- and post-sorption DOM were used to identify

234

compounds that were selectively removed from the solution due to adsorptive molecular

235

fractionation of the bulk DOM (Fig. 3). After post-spectral processing to identify peaks present

236

in at least two out of three analytical replicates, each treatment contained > 2,000 mass spectral

237

peaks that were assigned elemental compositions in the range 220

Adsorption and Molecular Fractionation of ... - ACS Publications

Recommend Documents