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Zn isotope fractionation during sorption onto kaolinite Damien Guinoiseau, Alexandre Gelabert, Julien Moureau, Pascale Louvat, and Marc F. Benedetti Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05347 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016

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Environmental Science & Technology

1

Zn isotope fractionation during sorption onto

2

kaolinite

3

Damien Guinoiseau, Alexandre Gélabert, Julien Moureau, Pascale Louvat and Marc F.

4

Benedetti

*

5

INSTITUT DE PHYSIQUE DU GLOBE DE PARIS – SORBONNE PARIS CITE –

6

UNIVERSITE PARIS DIDEROT – CNRS UMR 7154, PARIS, FRANCE

7 8

* corresponding author : [email protected]

Abstract

9

In this study, we quantify zinc isotope fractionation during its sorption onto kaolinite, by

10

performing experiments under various pH, ionic strength and total Zn concentrations. A

11

systematic enrichment in heavy Zn isotopes on the surface of kaolinite was measured, with

12

∆66Znadsorbed-solution ranging from 0.11 ‰ at low pH and low ionic strength to 0.49 ‰ at high pH

13

and high ionic strength. Both the measured Zn concentration and its isotopic ratio are correctly

14

described using a thermodynamic sorption model that considers two binding sites: external basal

15

surfaces and edge sites. Based on this modelling approach, two distinct Zn isotopic fractionation

16

factors were calculated: ∆66Znadsorbed-solution = 0.18 ± 0.06 ‰ for ion exchange onto basal sites, and

17

∆66Znadsorbed-solution = 0.49 ± 0.06 ‰ for specific complexation onto edge sites. These two distinct

1 ACS Paragon Plus Environment


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factors indicate that Zn isotope fractionation is dominantly controlled by the chemical

19

composition of the solution (pH, ionic strength).

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Introduction

21

As bioessential element, Zinc is necessary for growth and reproduction of living organisms1

22

and Zn deficiency in oceanic or continental environments as well as in human diet is of a

23

growing concern. By contrast, toxic levels of Zn are frequently observed in highly polluted sites

24

affected by mining or smelting activities. An accurate understanding of Zn biogeochemical cycle

25

is thus required at the Earth’s surface. Over the last two decades, Zn stable isotope signatures2

26

have proved effective in tracking Zn sources in polluted environments3-8. But in ecosystems like

27

soils or rivers, interactions between Zn in solution and in minerals can lead to Zn isotope

28

fractionation that may hamper Zn source identification. Besides its environmental toxicity, Zn

29

availability or mobility is clearly dependent on Zn speciation, which is difficult to determine. Zn

30

isotope measurements can help identify the ways in which Zn interacts between solution and

31

solids. In the environment, Zn isotope signatures reflect both Zn sources and/or in-situ processes.

32

In order to distinguish between these two factors, isotopic fractionation during Zn complexation

33

by reactive phases within the aquatic environment has to be addressed. Among the three main

34

groups of reactive phases, metal oxides, organic matter, and phyllosilicates, Zn isotopic

35

fractionation factors during adsorption are only known for the first two.

36

Enrichment in heavy Zn isotopes at the surface of iron oxides with respect to coexisting solution

37

was reported, with ∆66Znsolid-solution of 0.29 ± 0.07 ‰9 for goethite and 0.53 ± 0.07 ‰9,

38

ferrihydrite. This systematic enrichment is due to stronger Zn-O bonds at the mineral surface

39

than in solution, as evidenced by EXAFS spectroscopy9. For Zn adsorption on birnessite

10

for


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40

(manganese oxides), Zn isotopes show only limited fractionation at low ionic strength, while

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heavy Zn isotopes are sorbed preferentially at high ionic strength (∆66Znsolid-solution from 0.52 to

42

0.77 ‰)11. The impact of surface loading was also noted, with substantial fractionation occurring

43

at low surface coverage and high ionic strength (∆66Znsolid-solution up to 2.74 ‰)11. Such variable

44

Zn fractionation factors had previously been reported for sorption on Fe, Al and Mn oxides12,

45

with preferential sorption of light Zn isotopes onto goethite and of heavy ones onto birnessite, a

46

result inconsistent with other studies9, 11.

47

Regarding organic matter, Zn binds to the phenolic sites of purified humic acid with a ∆66Znsolid-

48

solution

49

heavy Zn isotope enrichment onto diatom surfaces (∆66Zndiatom-solution = 0.35 ± 0.10 ‰)14, but

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preferential light Zn isotope internalization by cells (∆66Zncell-solution -0.2 to -0.8 ‰)15. Zn isotope

51

fractionation in higher plants is complex: despite isotope equilibrium between free, organic and

52

inorganic Zn complexes in soil solutions, heavy Zn isotopes rather sorb onto the root surface,

53

while lighter δ66Zn values in the aerial parts result from preferential light Zn isotope transport

54

into stems and leaves16-18.

55

Given the large range of Zn isotopic fractionation associated with these mineral and biological

56

sorption effects, most of the observed δ66Zn range in geological records on Earth (-0.4 to 1.4

57

‰)19 could be explained by such processes. However, Zn isotope fractionation during Zn

58

sorption onto one of the most important mineralogical groups, the phyllosilicates, is still missing

59

and needs to be investigated.

60

In this study, and for the first time, we experimentally determine Zn isotope fractionation during

61

its sorption onto kaolinite, the dominant clay mineral in highly weathered soils, e.g. in tropical

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and subtropical regions. Cu isotope fractionation during sorption on kaolinite was recently

= 0.24 ± 0.06 ‰13. Additionally, biological activity impacts Zn cycling, with preferential



63 64

reported (∆65Cusolid-solution = -0.29 ‰)20;

65

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Cu enrichment being rather attributed to isotopic

fractionation between Cu(H 0) and Cu(H 0) in solution and the preferential sorption of the

65

latter than to water-mineral surface processes. These results contradict previous observations of a

66

systematic enrichment of heavy Cu isotopes on Al and Fe oxide surfaces (∆65Cusolid-solution from

67

0.6 to 1.3 ‰)10,

68

solution.

69

Zn sorption on kaolinite has been extensively studied22-25 and is invariably described by a two-

70

site model involving; (1) an ion exchange reaction at low pH on permanent negatively charged

71

sites located on basal sheets and (2) a specific binding at higher pH on pH-dependent-charge

72

sites located on the clay edges.

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The objective of this study is twofold: (1) define and correctly model zinc sorption onto kaolinite

74

under multiple pH, ionic strength and zinc concentration conditions, (2) measure the

75

corresponding Zn isotope signatures in the same experimental conditions in both dissolved and

76

particulate phases, to determine Zn isotope fractionation factors during its sorption onto

77

kaolinite.

21

, due to stronger Cu-O bonds formed at the mineral surface compared to

78

Materials and methods

79

Starting material

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20 grams of Source Clay KGa-2 kaolinite (Warren County, Georgia) from the Clay Minerals

81

Society were decanted in a sodium hexametaphosphate (dispersant) - milli-Q water (mQ)

82

mixture for 16 h, and the fraction < 2 µm was recovered. The residual metal oxides were

83

removed by DCB extraction using Holmgren’s protocol26. The clay was washed several times

84

with mQ water, conditioned under Na form (in NaNO3 solution) and washed again three times to 4 ACS Paragon Plus Environment

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85

remove the salt excess. The clay was then freeze-dried, and stored at room temperature. Based on

86

XRD measurements treated with Rietveld refinement, the starting sample is composed of around

87

99 % kaolinite and 1 % anatase (TiO2). The specific surface area (SSA) determined by N2-BET27

88

measurement is equal to 20.7 m2/g, similar to a previous study23.

89

All reagents were prepared from distilled acids, Titrinorm bases and mQ water to avoid Zn

90

contamination. Zn stock solution used in all experiments was a 1000 ppm AAS specpure solution

91

(Alfa Aesar) in 5 % HNO3.

92

Sorption experiments

93

Three sets of experiments were performed:

94

Experiment set 1: acid-base titrations of kaolinite at 20 g/L in 0.1 M, 0.01 M and 0.005 M

95

NaNO3 (background electrolyte) were conducted to determine protonation parameters and

96

reactive site densities. Forward and back titrations were performed twice for each experiment

97

with a Titrando unit (Metrohm) under continuous N2 flux (details in SI.1)

98

Experiment set 2: sorption edge experiments at constant initial [Zn] and variable pH were

99

conducted in batch in a glove box (Jacomex) under a N2 atmosphere to avoid the presence of

100

carbonate species and minimize any possible Zn isotopic fractionation between Zn2+ and ZnCO3

101

aqueous species. A suspension of kaolinite at 5 g/L was equilibrated overnight in a N2-saturated-

102

NaNO3 solution (0.1 M in a first series and 0.01 M in a second one) prior to the addition of a Zn

103

solution (final [Zn] = 50 ± 3 µM). In each tube, the fixed pH value (between 3 and 9) was

104

adjusted daily by addition of small volumes of 1N HNO3 and/or 1N NaOH. Experiment

105

equilibration time was 48 h, enough to reach sorption equilibrium and short enough to avoid clay

106

dissolution and/or precipitation of Zn-Al-Layer-Double-Hydroxide (LDH)28. After the final pH



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107

was measured, the suspensions were centrifuged, supernatants filtered at 0.22 µm and decanted

108

kaolinite samples freeze-dried.

109

Experiment set 3: Sorption isotherms at constant pH and variable total [Zn] were performed

110

under conditions similar to set 2. A suspension of kaolinite at 5 g/L was equilibrated overnight in

111

0.01 M N2-saturated-NaNO3. Total Zn concentration in each batch ranged from 5 to 1500 µM.

112

pH was fixed at 4.00 ± 0.10 for a first series, and at 6.00 ± 0.10 for a second one. After two days,

113

samples were treated as in set 2.

114

In sets 2 and 3, sample solutions were acidified to 0.5 M HNO3 and analyzed for Zn and Al

115

concentrations with ICP-OES (Thermo, ICAP 6000 series). As Al concentration remained lower

116

than 4 µM, kaolinite dissolution was therefore negligible, even at acidic pH. Suspended fractions

117

were digested with HNO3-HF at 100°C and Zn concentrations were determined with ICP-OES.

118

Zn isotopes analysis

119

Prior to Zn isotope analysis, sample solutions were evaporated and kaolinite residues were

120

digested. In a HEPA 13 air-filtered and over-pressurized cleanroom, samples were then dissolved

121

in HCl 6 M for chromatographic elution. Zn was isolated following a protocol adapted from

122

Marechal et al.2 using polypropylene columns (0.8 I.D. x 4 cm length) and AG-MP1 200-400

123

mesh resin (Biorad). Yields were measured after Zn extraction and were always in the range 100

124

± 5 %. Total procedural blanks were 7 ± 4 ng Zn (n=13), which represented less than 1 % of the

125

sample Zn amount. This low blank and the equilibrated isotopic mass balance between Zn in

126

solution and in kaolinite for almost all samples (Fig. S7) means that the procedural blank had a

127

negligible impact on measured Zn isotope ratio measurement, contrary to other experiments11. A

128

Cu Alfa Aesar stock solution (65/63Cu = 0.44574)29 was added to each sample as an internal

129

spike, with a Zn/Cu ratio of 2, in order to correct for instrumental mass bias (details in SI.2).


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130

Samples were analyzed on a MC-ICP-MS Neptune Plus (ThermoFinnigan) using a sample-

131

standard bracketing technique (SSB)30 with the Zn and Cu Alfa Aesar solutions (66/64Zn =

132

0.565055 ± 0.000004 (2σ) and

133

referred to in the text as ZnIPG or CuIPG, respectively. Zn isotope composition was expressed as

65/63

Cu = 0.44574 ± 0.000004 (2σ))29 as in-house standards,

Zn Zn % δ

Zn =

− 1$ ∗ 1000 (1) Zn Zn #

134 135

As samples obeyed a mass dependent fractionation law (Fig. S2), only δ66Zn is reported in this

136

study. The in-house ZnIPG solution was calibrated with respect to IRMM 3702 and JMC Lyon2

137

international standards, giving δ66ZnIPG/IRMM = -0.23 ± 0.04 ‰ (2σ, n=82) and δ66ZnIPG/JMC =

138

0.04 ± 0.03 ‰ (2σ, n=55), respectively. Additionally, independent cross calibration of ZnIRMM

139

over ZnJMC gave δ66ZnIRMM/JMC = 0.28 ± 0.03 ‰ (2σ, n=14) in the range of previously reported

140

signatures4, 31-33. Repeated Zn chemical purification from the certified BCR-2 basalt gave δ66Zn

141

of 0.24 ± 0.03 ‰ (2σ, n=11) relative to JMC, in excellent agreement with former published

142

values34,

143

solution. For kaolinite samples, measured δ66Znsolid was corrected from the structural Zn initially

144

present in the mineral lattice ([Zn]struct..= 37 ± 5 ppm (n=2) and δ66Znstruct. = 0.47 ± 0.02 ‰ (n=2)

145

determined after acid digestion) following:

35

. All sample δ66Zn values are expressed relative to our in-house Zn IPG standard

δ Zn'()*' =

+Zn,(-' ∗ δ

Zn(-' − +Zn,.)/0.. ∗ δ

Zn.)/0.. +Zn,'()*'

(2)

146

The isotopic fractionation between Zn adsorbed on kaolinite and Zn in solution, ∆66Znadsorbed-

147

solution was

defined as:



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∆

Zn'()*'4(/.-( = δ

Zn'()*' − δ

Zn(/.-( (3) 148

Two standard deviations (2σ) reported in table S1 and in figures were calculated from the three

149

replicates of each sample δ66Zn measurement. If the resulting 2σ was lower than 0.04 ‰, the

150

long-term reproducibility of IRMM 3702 (0.04 ‰, 2σ) was reported instead.

151

Sorption modeling

152

Previous studies22,

23, 25, 36

revealed that trace metal sorption onto kaolinite occurs at two

153

different types of sites. The first are permanent negatively charged sites, X- sites, allowing ion

154

exchange with the solution through electrostatic interactions. They are located at the surface of

155

clay sheets and result from substitution in Al or Si sheets. The second ones correspond to pH-

156

dependent edge sites, S-OH0.5-sites that favor specific Zn-solid complexes.

157

The sorption of cations (H+, Na+ and Zn2+) was modeled with ECOSAT v.4.7 (2001)37 software

158

using Gaines-Thomas ion exchange formalism for basal site binding. According to previous

159

models22, 24, 25, the X- sites are fully saturated with Na+, and Zn2+ is exchanged with a Zn:Na ratio

160

of 0.5. The input equations are:

XNa + H ⇋ XH + Na logK ?@ (4)

2XNa + Zn ⇋ X Zn + 2Na logK ?B (5) 161

1-pK Triple Layer complexation Model (1-pK TLM) was chosen for edge site sorption with a

162

single deprotonation step rather than the 2-pK TLM model because of the reduced number of

163

adjustable parameters38. Heidmann et al23 also used a unique deprotonation step but with a 1-pK

164

Basic Stern Model. Edge sites are expressed as SOH0.5- in their deprotonated form due to the

165

incomplete neutralization of O2- charge by the octahedrally coordinated Al3+ comprising the

166

dominant aluminol edge sites. AFM studies39,

167

represents only 30 % of the KGa-2 kaolinite. Furthermore, Zn complexation is expected to occur

40

reported that the edge site surface area


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168

mainly as an inner-sphere complex (ISC), in agreement with EXAFS study28 results. The binding

169

model also takes into account the possible effect of Na+ and NO3- competition on these sites: S − OH F.4 + H ⇋ S − OH F. logK GHIB J.KL (6) S − OH F.4 + Na ⇋ S − OH − NaF. logK GHI4@J.KL (7) S − OH F.4 + H + N0O 4 ⇋ S − OH − N0O F.4 logK

GHIB 4@FP J.KQ

(8)

S − OH F.4 + Zn ⇋ S − OHZnS. logK GHIT.KL (9) 170

In the 1-pK TLM model, ISC compensations of H+ and Zn2+ charges are assumed onto the 0-

171

plane at the kaolinite surface. The non-specific ions, Na+ and NO3-, are compensated on the

172

inner-diffuse-layer-plane instead.

173

For the model adjustment, first, the number of sites, capacitance, protonation/deprotonation

174

constants and influence of NaNO3 electrolyte were fitted, using acid/base titrations (experiment

175

set 1, fig. S1). Then, Zn binding constants were estimated from sorption edge and isotherm

176

experiments (sets 2 and 3, Figure 1 and Table 1). Model parameters were adjusted to obtain the

177

best R2 value between measured and modeled [Zn] (Fig. S3, R2 = 0.98 and p < 0.00001).

178

Results

179

Titration and sorption experiments

180

Potentiometric titrations: Acid-base titrations of KGa-2 in the absence of Zn were performed at

181

0.005, 0.01 and 0.1 M NaNO3. The adsorbed H+ density is lower at high ionic strength, due to



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182

stronger competition between Na+ and H+. A common intersection point around pH = 5.3 defines

183

the point of zero charge and agrees with published data22.

184

Sorption edges: The two Zn sorption edges performed at pH from 3 to 9 in 0.01 and 0.1 M

185

NaNO3 (Figure 1A and 1B respectively) differ from each other. At low ionic strength (Figure

186

1A), with lower competition between Zn2+ and Na+, Zn sorption begins below pH 3 and is

187

complete by pH 7.5. At high ionic strength (Figure 1B), due to strong competition with Na+ on

188

negatively charged basal sites, Zn sorption is lower than 0.05 µmol Zn/m2 below pH 6 and

189

increases to reach complete sorption with 0.49 µmol Zn/m2 at pH 7.5.

190

Sorption isotherms: At pH 4, Zn sorption increases from 0.02 µmol Zn/m2, to a plateau at 0.48

191

µmol Zn/m2 when free [Zn2+] increases from 3 to 1400 µM (Figure 1C). At pH 6, (Figure 1D),

192

[Zn]adsorbed increases from 0.04 to 0.96 µmol Zn/m2, with free [Zn2+] raising from 3 to 1500 µM.

193

Conversely to the pH 4 isotherm, no plateau is reached at pH 6.

194 195

Zn isotope ratio changes during sorption

196

Sorption edges: Five and six samples from the 0.1 M and 0.01 M sorption edge experiments,

197

respectively, were analyzed (supernatant and solid fraction) (Figure 2). Zn isotopes behave in a

198

similar manner at both ionic strengths: continual enrichment in heavy isotopes in the solid phase

199

and in light isotopes in the solution when pH increases. Mass balance calculations for almost all

200

samples yield an isotope composition indistinguishable from that of the starting solution (ie. 0.00

201

± 0.04 ‰, fig. S7).

202

At high ionic strength (Figure 2A), δ66Znsolution evolves towards a lighter isotopic signature, from

203

0.00 ‰ when Zn sorption is minimal to -0.49 ‰ at 95 % of adsorbed Zn. Simultaneously,


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204

positive δ66Znadsorbed on kaolinite varies from 0.31 ‰ at low coverage to 0.04 ‰ at 95 %

205

sorption, with a maximum of 0.35 ‰ for approximately 25 % of sorption.

206

At low ionic strength (Figure 2B), δ66Znadsorbed decreases almost continuously from 0.18 to 0.07

207

‰. δ66Znsolution is slightly negative (0.00 to -0.07 ‰) up to 50 % sorption then sharply decreases

208

up to 100 % sorption (-0.07 to -0.41 ‰). At 30 % of sorption, the low δ66Znadsorbed (0.05 ‰) is

209

considered as an outlier, as will be discussed later.

210

For each experiment, the corresponding ∆66Znadsorbed-solution is high and evolves with Zn surface

211

coverage from 0.32 to 0.52 ‰ at 0.1 M NaNO3, and from 0.18 to 0.49 ‰ at 0.01 M NaNO3

212

(Figure 2 and table S1). The difference of ∆66Znadsorbed-solution between the two ionic strengths

213

indicates that at least two different sorption processes implying that two different mechanisms

214

driving Zn isotope fractionation are operating.

215

Sorption isotherms: Five and six samples were analyzed from the experiments at pH 4 and 6,

216

respectively (both the supernatant and the solid fraction). As for sorption edges, heavy Zn

217

isotopes are preferentially adsorbed on kaolinite surfaces compared to solution (Figure 3).

218

Isotopic mass balances for solid and solution give the Zn isotope signature of the stock solution

219

used (0.00 ± 0.04 ‰, Fig. S7), except for the first point of each isotherm where δ66Znadsorbed is

220

the highest.

221

At pH 4 (Figure 3A), δ66Znsolution (0.02 ± 0.02 ‰) remains close to the initial δ66Znstock-solution,

222

while δ66Znadsorbed increases from 0.10 to 0.34 ‰. At 500 µM Zn2+eq., as for sorption edge, an

223

outlier is observed with unexpectedly low δ66Znadsorbed. Excluding the high ∆66Znadsorbed-solution

224

(0.32 ‰) determined for the first isotherm point at low [Zn2+], the ∆66Znadsorbed-solution remains

225

stable at 0.11 ± 0.01 ‰ for [Zn2+] between 160 to 980 µM. 11 ACS Paragon Plus Environment


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226

At pH 6 (Figure 3B), δ66Znsolution is enriched in light isotopes for low [Zn]eq. compared to

227

δ66Znstock-solution. Adsorbed Zn is enriched in heavy isotopes (from 0.12 to 0.22 ‰); δ66Znadsorbed

228

first decreases for [Zn] between 0 and 120 µM down to 0.12 ‰ and then increases up to 0.2 ‰.

229

The corresponding ∆66Znadsorbed-solution ranges from 0.17 to 0.28 ‰ with an almost constant value

230

of 0.18 ‰ for Zn addition between 96 and 394 µM (Table S1).

231

Discussion

232

Evolution of Zn-kaolinite binding with pH: a two-site model

233

Results from sorption of H+ and Zn2+ were well fitted using our two-site model. Adjusted

234

parameters for the model are given in Table 1. The concentration of SOH0.5- sites (171.9

235

mmol/kg), representing 16.6 sites/nm2 of edge surface area, is nine times that of X- sites (19

236

mmol/kg). The former value is slightly higher than the 12.2 sites/nm2 previously reported for the

237

same KGa-2 clay23. The high uncertainties concerning edge surface area determinations for

238

KGa-2 and other poorly ordered kaolinites (between 12 and over 30 %)39-41 may explain this

239

small discrepancy between the two studies. The low amount of X- sites, which arise from rare

240

substitutions in the kaolinite lattice, is slightly higher than in previous estimations (13.6

241

mmol/kg)42. The complexation constants logK ?B (Eq. (5)) and logK GHIT.KL (Eq. (9)), are

242

fitted at 0.8 and 1.2 (Table 1). For non-specific ionic exchange, this logK ?B value agrees with

243

previous determination using a similar formalism22,

244

protonation for edge sites22,

245

single protonation step for SOH0.5- sites. However, Heidmann et al.23 assumed a bidendate

246

complex involving the hydrolyzed-metal forms of Cu and Pb. Such Zn hydrolyzed species were

247

implemented in our model for Zn sorption but did not improve the fit.

24, 25

23

. For ISC, all studies used a two-step-

, except for Heidmann et al.23 and in this study who adopt a


Page 13 of 33


248

According to our model, at low pH, Zn is bound exclusively onto pH-independent basal sites

249

(Figure 1) and the strong influence of ionic strength during sorption edge experiments at this pH

250

(3 to 5) indicates an outer-sphere complexation (OSC). At higher pH, binding to edge sites

251

becomes dominant (Figure 1B), and the negligible effect of Na+ and NO3- concentrations on Zn

252

sorption suggests that zinc is adsorbed as an ISC. This corroborates the findings of Nachtegaal et

253

al.28 who find a Zn monodendate ISC on kaolinite at pH 5 and 1 mM [Zn]tot. During Zn sorption,

254

precipitation of Zn-Al LDH was observed for kaolinite28, pyrophyllite43 or montmorillonite44 by

255

EXAFS spectroscopy. The degree of precipitation is proportional to [Zn], pH and aging time.

256

Our low Zn concentrations (50µM) and equilibration time (2 days) for sorption edges mean that

257

LDH precipitations is not widespread. However, higher [Zn] were introduced for the final points

258

of the sorption isotherms. The local environment of Zn once sorbed on kaolinite at pH 4, 6 and 8,

259

with a [Zn] of 800 µM but an aging time of 29 days has been checked by X-ray absorption

260

spectroscopy. Preliminary data treatment (not shown here) seems to indicate an absence of Zn-

261

Al-LDH precipitates at pH 4, traces of LDH at pH 6 and a significant amount of Zn into the

262

newly-formed Zn-Al-LDH phase at pH 8. The presence of Zn-Al-LDH after only 2 days of

263

reaction, in our pH 4 and 6 sorption isotherms is thus highly improbable.

264

As in the sorption edge experiments, our model faithfully reproduces the Zn sorption isotherms.

265

All Zn is complexed onto basal sites at pH 4 because edge sites are protonated. At pH 6, this

266

ionic exchange also dominates at low Zn concentrations until saturation. The specific binding

267

onto edge sites prevails at high [Zn] (>0.7 mM). The successive involvement of the different

268

binding sites was also previously reported for Cu sorption onto kaolinite23.

269

Zinc isotope fractionation during binding



Page 14 of 33

270

δ66Zn measurements in the sorption edge experiments (Figure 2A and 2B) reveal distinct Zn

271

isotope behavior according to the ionic strength. In both cases, enrichment of heavy Zn isotopes

272

onto the kaolinite surface is measured, with ∆66Znadsorbed-solution between 0.18 and 0.52 ‰. The

273

two patterns observed (Figure 2A and 2B) are explained by an equilibrium fractionation (a

274

Rayleigh fractionation process is rejected on the basis of details in Fig. S4) with two distinct

275

fractionation factors: one for low coverage when Zn sorption occurs predominantly onto

276

exchange sites, and one for higher coverage during Zn sorption onto edge sites. For each

277

coverage value, the adsorbed δ66Zn signature results from the relative proportions of each

278

binding site type. The whole δ66Znadsorbed vs. Zn coverage curves allow for the determination of

279

the Zn isotope fractionation factor of each site.

280

In the sorption edge experiment at high pH (pH > 6.5) and high ionic strength (Figure 1B), 50

281

to 100 % of Zn is adsorbed, an average ∆66Znadsorbed-solution of 0.49 ‰ is observed (Figure 2A) and

282

only SOH0.5- sites contribute to the binding. Therefore, the isotope fractionation factor for Zn

283

sorption onto edge sites is ∆GHIT.KL = 0.49 ± 0.06 ‰. For sorption edge at low pH (pH < 5.5)

284

and low ionic strength (0.01 M NaNO3, Figure 1A), 20 to 40 % of zinc is adsorbed, a

285

∆66Znadsorbed-solution of 0.18 ‰ is observed for the first point (Figure 2B), and the model predicts

286

an ionic exchange binding exclusively on basal X- sites (Figure 1A). The isotope fractionation

287

factor for Zn sorption onto exchange sites is thus ∆?B = 0.18 ± 0.06 ‰.

288

To confirm that these fractionation factors correctly explain the δ66Znadsorbed and δ66Znsolution

289

when both binding sites are involved during sorption, we calculate the theoretical δ

Zn.X( (/.-( .

290

The calculation is based on the proportion of Zn adsorbed (f), the proportion of each site (pX2Zn

291

and pSOHZn1.5+), and their associated fractionation coefficient (Y?B and YGHIT.KL ):


Page 15 of 33


δ

Zn.X( (/.-( = − 1000f

292

p? ]α?B − 1_ pGHIS. (αGHIT.KL − 1) + (10) 1 − f + fα?B 1 − f + fαGHIT.KL

The theoretical δ

Zn.X( '()*' is obtained by:

.X( δ

Zn.X( '()*' = δ Zn'-. + p?B ∗ ∆?B + pGHIT.KL ∗ ∆GHIT.KL (11)

293

Theoretical calculations are reported in Figure 2 for sorption edges and show the δ

Zn.X( (/.-(

294

(red dotted line) and the δ

Zn.X( '()*' (red solid line). Additionally, the blue dotted line

295

separates the proportion of Zn linked to basal sites (green area) from those linked to edge sites

296

.X( (blue area). δ

Zn.X( (/.-( and δ Zn'()*' values are in good agreement with measured

297

δ66Znsolution and δ66Znadsorbed in sorption edges, validating: (1) the dominance of edge sites at high

298

pH or high ionic strength (Figure 2A), (2) the control by basal sites at low pH and low ionic

299

strength (Figure 2B), and (3) intermediate proportions in intermediate conditions. This good

300

agreement between our two-site complexation model used to describe [Zn] evolution and its

301

robustness to describe zinc isotope fractionation also validates the two zinc isotope fractionation

302

factors we propose for Zn sorption on kaolinite edge and basal sites.

303

A similar calculation is done for sorption isotherms at pH 4 and 6 (Figure 3). Even if the model

304

validates the enrichment of heavy isotopes onto the kaolinite surface, the δ

Zn.X( '()*' (red

305

solid line in Fig. 3) slightly overestimates measured δ66Znadsorbed, except for the lowest [Zn]eq.

306

where δ66Znadsorbed is underestimated. For these low [Zn] experiments and in the first point of

307

Figure 2A, the isotopic mass balances are not fulfilled (Fig.S7). The amount of Zn adsorbed is

308

very low and close to that initially present into the crystal lattice of kaolinite. The uncertainty on

309

[Zn]struct. within the kaolinite lattice (37 ± 5 ppm) combined with the correction of this crystalline

310

Zn pool (δ66Znstruct. = 0.47 ± 0.02 ‰) in the δ66Znsolid measured can explain this difference



Page 16 of 33

311

(details in Fig. S5). For higher [Zn], the uncertainty in crystalline Zn cannot be responsible for

312

the observed offset, however other issues may arise:

313

1) Suspended kaolinite was separated from the solution by centrifugation, however some

314

solution may remain in the kaolinite pellet porosity and contribute to lower the measured

315

δ66Znadsorbed. This hypothesis is applicable for the isotherm at pH 4 (Figure 3A), where dissolved

316

[Zn] is much higher than adsorbed [Zn]. For instance, 200 µL of solution would be enough to

317

shift the adsorbed signature by 0.1 ‰. But at pH 6, the volume of interstitial solution needed to

318

decrease δ66Znadsorbed is too large (> 2 mL) to account for the discrepancy. The same issue was

319

reported by Balistrieri et al.10 and Juillot et al.

320

dissolved Zn contamination. However, this may overprint previous fractionation by attempting to

321

establish a new equilibrium with the washing solution.

322

2) For the isotherm at pH 6 (Figure 3B), a slight overestimation of the SOH0.5- proportion (blue

323

9

who tested washing their oxides to remove

area in fig 3B) may be the cause for a δ

Zn.X( '()*' higher than measured. A 10 %

324

overestimation of the SOH0.5- proportion would shift the δ

Zn.X( '()*' by 0.05 ‰. Thus,

325

uncertainties on edge and basal sites proportions may explain most of the discrepancy reported

326

between theoretical and measured δ66Znadsorbed.

327

3) A change in sorbed Zn speciation at high surface coverage (high [Zn]) cannot be excluded.

328

For birnessite, a progressive change from tetrahedral to tetrahedral/octahedral coordination of

329

adsorbed complexes occurred at high [Zn]11. For kaolinite, such a modification from octahedral

330

to tetrahedral coordination at high [Zn] could have happened and would result, by strengthening

331

of Zn complex bonds, in an increase of δ66Znadsorbed values, which is not observed here. However,

332

Zn could alternatively be adsorbed on a third type of binding sites at high [Zn]. 16 ACS Paragon Plus Environment

Page 17 of 33


333

Zinc speciation impact on Zn isotope fractionation processes

334

The preferential sorption of heavier Zn isotopes on kaolinite relative to aqueous solution can be

335

reconciled with molecular complexation mechanisms. Two main processes can cause such an

336

equilibrium fractionation: (1) isotopic partitioning between Zn aqueous species in solution, and

337

(2) fractionation between dissolved and adsorbed Zn species at the mineral-solution interface.

338

The first step is to constrain the multiplicity of Zn species in solution (1). To this end, aqueous

339

Zn speciation calculated with ECOSAT v.4.7 (2001)37 for sorption edge solutions with pH < 8

340 341 342 343 344

shows that Zn(H 0)

is the major species, with a very small contribution of ZnNOO (H 0) and

of ZnOH(H 0) , whereas Zn(OH) (H 0) is negligible below pH 7 (Fig. S6). The formation of ZnNOO (H 0) and ZnOH(H 0) may induce isotope fractionation among Zn species in

solution. Several studies reported ab initio isotopic partition coefficients between Zn aqueous

species in solution45, 46, but those for ZnNOO (H 0) and ZnOH(H 0) are currently unknown.

345

Extension of such calculations to these species would be useful to see if such processes could

346

account for the measured δ66Znadsorbed.

347 348

66 Independent of the proportion of ZnNOO (H 0) , both [Zn] and δ Zn are well modeled by

sorption of Zn(H 0)

. ZnNOO (H 0) is not likely to be adsorbed on kaolinite edge sites.

349

ZnOH(H 0) is not needed to describe sorption experiments (see section Sorption modeling).

350

However, its appearance at around pH 6.0 (Fig. S6A & B) coincides with the steep increase of

351

Zn sorption on edge sites in Figure 1A and 1B. Under our experimental conditions, we cannot

352

completely rule out a possible sorption of this species on kaolinite. But its impact on Zn isotope

353

fractionation in solution is uncertain since its partition coefficient is unknown. Spectroscopic



Page 18 of 33

354

studies of the kaolinite surface have thus far been unable to identify the coordination of Zn

355

during edge site binding.

356

Adsorption of aqueous species onto the surface of minerals is frequently associated with bond

357

stiffening owing to the different complexing environment, and should thus also engender an

358

isotopic fractionation. Stable isotope theory47,

359

found in stronger bonding environments. All else being equal, low coordination number and/or

360

stronger-field ligands concentrate heavy isotopes. During sorption onto iron oxides (goethite and

361

ferrihydrite) enrichment in Zn heavy isotopes at the mineral surface results from stronger surface

362

complexes compared to dissolved Zn9. For Zn binding by organic matter, Jouvin et al.13 assumed

363

that carboxylate and phenolate complexes act as analogues of the low and high affinity sites of

364

humic acid, respectively. The length difference (2.00 Å for carboxylate and 1.91 Å for

365

phenolate) explains the lack of isotope fractionation onto low affinity sites, and the significant

366

fractionation for high affinity sites. Previous sorption studies14-18 onto diatoms or roots also

367

underlined this trend, with heavy isotope enrichment at diatom and root surfaces due to a change

368

in Zn coordination; from octahedral in solution to tetrahedral on the surfaces.

369

Our results confirm a higher intensity of isotopic fractionation during Zn sorption onto specific

370

edge sites as ISC (∆GHIT.KL = 0.49 ‰), compared to ionic exchange (∆?B = 0.18 ‰) where

371

OSC is foreseen. First, Zn sorption on edge sites (Figure 2A) induces an isotope fractionation

372

close to that resulting from zinc sorption on ferrihydrite surface (∆ = 0.53 ± 0.07 ‰)9. For

373

ferrihydrite, EXAFS data show a Zn-O bond length of 1.96 Å and a tetrahedral complex. A

374

longer Zn-O bond (2.06 Å) and an octahedral conformation for aqueous zinc in solution is

375

observed49, inducing a stronger Zn-O bond for ferrihydrite compared to Zn in solution9. In

376

kaolinite, Zn sorption on edge sites was previously observed on a unique sample as a

48

predicts that heavy isotopes are preferentially


Page 19 of 33


377

monodendate inner-sphere complex by X-ray absorption spectroscopy28. An octahedral complex

378

at the surface is suggested by a 6 fold coordination geometry28, and the measured Zn-O distance

379

(2.07 Å) is similar to that reported for Zn(H 0)

. Thus, bond length and coordination number

380

of Zn in solution seem to be preserved during sorption, however a significant zinc fractionation

381

occurs in our kaolinite experiment (∆GHIT.KL = 0.49 ± 0.06 ‰). Since Nachtegaal et al.28 study

382

reported only one measurement and at a much higher [Zn] than in our study, additional EXAFS

383

spectroscopic studies should be performed to constrain the molecular mechanism responsible for

384

this ∆GHIT.KL fractionation.

385

Moreover, as shown by the observed competition between Na+ and Zn2+ at high ionic strength,

386

the basal site complexation of Zn would correspond to an OSC, where Zn remains in a six-fold

387

coordination geometry with its hydration shell and would exhibit bond lengths similar to the

388

aqueous species, as was recorded for Cu50, 51. Since the mechanism leading to heavy Zn isotope

389

enrichment during OSC complexation formation (∆?B = 0.18 ± 0.06 ‰) remains unclear,

390

additional spectroscopic studies should again be performed to identify it. Nevertheless, several

391

possibilities may be proposed, such as a symmetry modification or a distortion of Zn hydration

392

sphere during its sorption, as was reported for Se complexing with maghemite52.

393

Environmental implications

394

This study adds a new piece to the complex puzzle of Zn isotopic cycle at the Earth’s surface.

395

Even if simplified conditions were used in our experiments (absence of carbonate and a single

396

mineral phase), their results may be applied to natural systems such as soils where Zn availability

397

is high and directly linked to the clay and/or organic matter content given their high CEC

398

value53. Except in certain Zn deposits like smelter slags, most of the pedological δ66Zn values of 19 ACS Paragon Plus Environment


Page 20 of 33

399

both natural and anthropogenic origins span a range (-0.33 to 0.49 ‰)54 as large as that reported

400

in this study for Zn sorbed onto kaolinite surface (0.00 to 0.35 ‰). The identification of Zn

401

sources can be efficient, and particularly strengthened by combining Zn isotope tracer with

402

spectroscopic observations5. However, some δ66Zn fluctuations observed in mineral horizons55, 56

403

or sub-surficial layers5 of soils still remain misunderstood and may be attributed to in-situ

404

fractionating processes, such as Zn sorption on kaolinite clay. This observation is particularly

405

relevant in systems like in tropical soils, where the weathering reactions are intense and clays are

406

abundant.

407

Associated content: Additional information is presented as detailed in the text. This material is

408

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

409

Author information:

410

Corresponding author: e-mail: [email protected] Equipe de Géochimie des Eaux – IPGP-

411

SORBONNE PARIS CITE – UNIVERSITE PARIS DIDEROT – CNRS UMR 7154 – 1 rue

412

Jussieu, 75238 Paris Cedex 05

413

Acknowledgements: Authors greatly thank Jean-Yves Piquemal for BET measurements, Laure

414

Cordier for technical assistance with ICP-OES analyses and Sophie Nowak for XRD acquisition.

415

Farid Juillot and Marc Blanchard are thanked for fruitful discussions. The manuscript was deeply

416

improved by comments of P. Sossi, four anonymous reviewers and D. Giammar. This work was

417

partly funded by EC2CO FIZCAMO project (CNRS-INSU) and is part of CAPES-COFECUB

418

project n°713-2011.


Page 21 of 33


419 420

Figure 1: Zinc sorption evolution with pH for sorption edge experiments at 0.01 M NaNO3 (A)

421

and 0.1 M NaNO3 (B). Zn sorption evolution with Zn concentration in solution at 0.01 M NaNO3

422

for sorption isotherms at pH 4 (C) and pH 6 (D). Experimental points (red dots) are well fitted

423

(black straight line) with various proportions of ionic exchange (green dotted line), and specific

424

edge binding sites (blue dotted line). Samples analyzed for their Zn isotope composition are

425

marked by black squares.



Page 22 of 33

426 427

Figure 2: Zinc isotope compositions for sorption edge experiments ([Zn]ini.=50±3µM and varying

428

pH) plotted against the amount of Zn adsorbed onto kaolinite at high (A) and low (B) ionic

429

strength. White and black dots refer to dissolved and adsorbed δ66Zn, respectively, with an initial

430

δ66Znstock-solution at 0.00 ± 0.04 ‰. Red solid and dotted lines depict theoretical δ66Znadsorbed and

431

δ66Znsolution obtained from our two-site model. The blue dotted lines separate the proportion of 22 ACS Paragon Plus Environment

Page 23 of 33


432

∆66Znadsorbed-solution explained by a Zn sorption onto exchange basal sites (green area) or onto edge

433

sites (blue area).

434 435

Figure 3: Zinc isotope composition as a function of [Zn]aq. added at pH 4 (A) and pH 6 (B).

436

White and black dots refer to dissolved and adsorbed δ66Zn, respectively, with an initial 23 ACS Paragon Plus Environment


Page 24 of 33

437

δ66Znstock-solution at 0.00 ± 0.04 ‰. Red solid and dotted lines depict theoretical δ66Znadsorbed and

438

δ66Znsolution obtained from our two-site model. The blue dotted lines separate the proportion of

439

∆66Znadsorbed-solution explained by a Zn sorption onto exchange basal sites (green area) or onto edge

440

sites (blue area).

441

Table 1: Fitted parameters of the two-site model used to describe Zn evolution. Site density and

442

protonation constant for edge sites were derived from acid base titrations of kaolinite. Zn

443

complexation constants in both binding sites are fitted according to sorption experiments. General features [kaol.] (g/L) Specific Surface Area (m2/g) Site

-

SOH0.5

5 or 20 20.7 Parameter on Triple Layer Model Site density (sites/nm2) Inner capacitance (κ) (F/m2) Outer capacitance (κ) (F/m2) log KSOH20.5+ log KSOH-Zn1.5+

16.6 0.4 5.0 5.3 1.2

log KSOH-Na0.5+

-1.1

0.5-

log KSOH2-NO3

XNa

444

4.1

Parameter on Gaines-Thomas ionic exchange Site concentration (mmol/kg) 18.0 log KXH 1.3 log KX2-Zn

0.8

445

446

447

448


Page 25 of 33

449


TOC/Abstract Art

450 451 452 453 454 455 456 457 458 459 460 461



462 463 464

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Page 26 of 33

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