Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien

Feb 19, 2016 - The Wien effect method enables evaluation of the mean Gibbs free adsorption and binding energies of metal cations on soil particles.(15...
0 downloads 0 Views 1MB Size
Subscriber access provided by Western Michigan University

Article

Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien Effect Measurements and EXAFS Spectroscopy Tingting Fan, Yu-Jun Wang, Cheng-Bao Li, Jian-Zhou He, Juan Gao, Dongmei Zhou, Shmulik P. Friedman, and Donald L. Sparks Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05281 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 19, 2016

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 29

Environmental Science & Technology

1

Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien

2

Effect Measurements and EXAFS Spectroscopy

3

Ting-Ting Fan1, 2, Yu-Jun Wang1*, Cheng-Bao Li1, Jian-Zhou He1, 2, Juan Gao1, Dong-Mei

4

Zhou1*, Shmulik P. Friedman3, and Donald Sparks4

5

1

6

the Chinese Academy of Sciences, Nanjing 210008, China

7

2

University of Chinese Academy of Sciences, Beijing 100049, China

8

3

Institute of Soil, Water, and Environmental Sciences, Agricultural Research Organization,

9

Volcani Center, Bet Dagan 50250, Israel

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science,

10

4

11

and Soil Sciences, University of Delaware, Newark, Delaware 19717-1303, USA

Environmental Soil Chemistry Group, Delaware Environmental Institute and Dept of Plant

12 13 14

*Corresponding

author,

Tel:

0086-25-86881182,

Fax:

15

[email protected] (Yu-Jun Wang), [email protected] (Dong-Mei Zhou)

1

ACS Paragon Plus Environment

0086-25-86881000,

E-mail:

Environmental Science & Technology

16

Abstract

17

Soil organic matter (SOM) is the major factor affecting sequestration of heavy metals in

18

soil. The mean free binding energy and the mean free adsorption energy and speciation of Zn

19

in soil, as affected by SOM, were determined by employing Wien effect measurements. The

20

presence of SOM markedly decreased the Zn binding energy in soils in the order: Top (5.86

21

kJ mol-1) < Bottom (8.66 kJ mol-1) < Top OM-free (9.44 kJ mol-1) ≈ Bottom OM-free (9.50 kJ

22

mol-1). The SOM also significantly decreased the adsorption energy of Zn on black soil

23

particles by reducing non-specific adsorption of Zn on their surfaces. The speciation of Zn in

24

soils was elucidated by extended X-ray absorption fine structure (EXAFS) spectroscopy and micro-focus

25

X-ray fluorescence (µ-XRF). The results obtained by linear combination fitting (LCF) of EXAFS

26

spectra revealed that the main forms of Zn in soil were outer-sphere Zn, Zn-illite, Zn-kaolinite,

27

and HA-Zn. As the SOM content increased, the proportion of HA-Zn among the total

28

immobilized Zn increased and the proportion of non-specific adsorbed Zn decreased. The

29

present results implied that SOM is an important controlling factor for the environmental

30

behavior of Zn in soils.

31 32 33

Key words: Zn, organic matter, adsorption energy, binding energy, EXAFS, µ-XRF

34

2

ACS Paragon Plus Environment

Page 2 of 29

Page 3 of 29

35

Environmental Science & Technology

Introduction

36

Zinc (Zn) is a key element for plant growth and human health; it is essential for the

37

normal activity of DNA polymerase and protein synthesis, and it plays a vital role in the

38

healthy development of many life forms.1, 2 Excessive amounts of Zn, however, can be toxic,

39

not only to plants and animals but also to humans.3 Plants obtain their Zn from the soil

40

solution mainly in the form of ions or chelates.2 Adsorption controls the sequestration and

41

mobility of heavy metals in soils, and it is well known that variations in soil properties such as

42

pH, cation exchange capacity (CEC), texture, and soil organic matter (SOM) content

43

significantly influence the adsorption of heavy metals.

44

The term SOM refers to all natural and biologically- derived organic materials found in

45

the soil,4 which includes organic matter associated with soil particles and soluble or dissolved

46

organic matter (DOM). SOM has a high specific surface area (approximately 800-900 m2 g-1)

47

and a CEC that ranges from 150 to 300 cmol kg-1.5 The SOM includes some functional groups

48

such as carboxylates and phenolics, therefore, it can be complexed with metals, which affects

49

their retention and mobility in soils. 5, 6 DOM enhances the solubility of metals and organic

50

matter associated with soil particles adsorbs metals, thus reduces their solubility and mobility.

51

Sauve et al. demonstrated that higher soil organic matter content was associated with also

52

increased DOM, and thus promoted the formation of organo-Pb complexes, which enhanced

53

Pb solubility. 7 Mesquita and Carranca found that the presence of DOM decreased adsorption

54

of Cu and Zn to soil particles.8 We also found that some low-molecular-weight organic acids,

55

such as malic, succinic, citric, acetic, oxalic, and tartaric acids, significantly decreased the

56

adsorbed amount of Cu(II) on hydroxyapatite (HAP) particles in the clay-size fraction.9 3

ACS Paragon Plus Environment

Environmental Science & Technology

Page 4 of 29

57

Comparison of the effects of various organic acids showed that the maximum quantity of

58

Cu(II) adsorbed onto the HAP particles increased exponentially with the cumulative

59

formation constants of Cu(II) and the organic acids.9 In addition to experimental results, Weng

60

et al. found that DOM could increase the concentration of dissolved metals, although

61

metal-DOM complexes comprises only a small fraction of metals sorbed on soils by the

62

theoretical calculations with a NICA-Donnan model.10

63

Some of SOM is present as a coating or a thin layer on the mineral surfaces,11 and its

64

removal changes the sorption of heavy metals in the soils, i.e., the sorption capacity increases

65

or decreases. Shuman, using batch adsorption experiments, found that the removal of SOM by

66

sodium hypochlorite lowered Zn sorption capacity and decreased Zn binding energy.

67

However, Hinz and Selim, who used the thin-disk flow method, reported that the removal of

68

OM resulted in a two- to four- fold increase in Zn retention. 13 Trehan and Sekhon, using

69

traditional batch experiments, also found that the removal of SOM significantly increased

70

sorption of Zn on soil. 14 As indicated above, sorption of Zn is largely controlled by SOM,

71

which thereby determines Zn bioavailability in soils. Thus, some contradictory results have

72

been published regarding SOM effects on Zn sorption capacity of soil particles, which

73

indicates that the effect of SOM on Zn sorption in soils is not well understood.

12

74

In the last decade, the increase with applied electrical field (E), of the electrical

75

conductivity (EC) of soil/water/electrolyte systems, termed as the Wien effect, has been

76

measured successfully by our research group. The Wien effect method enables evaluation of

77

the mean Gibbs free adsorption and binding energies of metal cations on soil particles.15-20 In

78

a previous Wien effect study we found that the removal of SOM slightly decreased the 4

ACS Paragon Plus Environment

Page 5 of 29

Environmental Science & Technology

79

binding energies, which represent the whole spectrum of adsorption energies, of K+, Ca2+, and

80

Cd2+ on a paddy soil, but that the adsorption energies, which represent the fraction of low

81

adsorption energies of these metals on this soil, significantly increased.21 In contrast to these

82

findings, the removal of SOM from a black soil, richer in organic matter than the paddy soil,

83

increased the binding energies of Cd2+, Cu2+, Pb2+, and Ca2+.22

84

How does SOM affect the sorption of metals on soils? In the present study, the effect of

85

SOM (both DOM and OM associated with the solid particles) on the interaction between Zn

86

and clay-fraction particles was elucidated via the Wien effect method. This method provides

87

quantitative information on the distribution (spectrum) of adsorption energies and direct

88

insights on the effect of SOM on the adsorption mechanism. Complementary extended X-ray

89

absorption fine structure (EXAFS) spectroscopy and micro-focus X-ray fluorescence (µ-XRF)

90

were employed to determine the speciation and distribution of Zn in soils. The main

91

objectives were to explore the effects of SOM on the molecular mechanisms of Zn sorption on

92

soil particles and on the speciation of Zn immobilized in soil, and thereby to enhance and

93

deepen the characterization, quantification and understanding of the environmental behavior

94

of Zn in soils.

95 96

MATERIALS AND METHODS

97

Soil Samples

98

Black soil samples from Hailun, Heilongjiang Province, China were collected from depths

99

of 0 – 20 cm (designated as top soil) and 100 – 120 cm (designated as bottom soil). The basic

100

properties of the sampled soils were listed in Table S1. After the samples were dried, ground, 5

ACS Paragon Plus Environment

Environmental Science & Technology

101

and passed through a 60-mesh sieve, the clay-fraction particles, less than 2 µm in diameter,

102

were separated by sedimentation. Organic matter (OM) was removed from clay fractions of

103

the top and bottom soil samples by adding 6% H2O2,23 these samples labeled as “Top OM-free”

104

and “Bottom OM-free” (the clay-fraction particles without removal of OM labeled as “Top”

105

and “Bottom”). The mineral composition of the clay-fraction particles with and without

106

removal of OM was determined by X-ray diffraction (XRD) analysis and the results are listed

107

in Table S2. The top black soil-fraction particles contained mainly kaolinite and illite; the

108

bottom black soil-fraction particles were richer in 2:1 silicate minerals than the top soil (Table

109

S2). The removal of the OM did not alter the mineral composition of the soil clay fractions.

110

The positive and negative charge densities of the clay fraction of the soil samples at various

111

pH values were determined by the modified Schofield method and were presented in Figure

112

S1. The samples carried almost no positive charges, which met the requirements of the

113

applied Wien effect method.15 The OM removal increased the negative charge densities on the

114

particles at near-neutral pH values, but did not significantly affect the negative charge

115

densities under the experimental pH conditions, i.e., pH 5 (Table 1).

116

Preparations of Homoionic Soil Samples and Suspensions

117

25 mL of 0.5 mol L-1 ZnCl2 solution was added into a 0.5 g sample of the original and

118

OM-removed clay fraction particles; the resultant mixture was shaken for 5 h at 25 °C and

119

centrifuged, then the supernatants were discarded. This procedure was repeated three times.

120

Subsequently, deionized (DI) water was added into the tubes to thoroughly wash the clay

121

paste separated from centrifugation until Cl- couldn’t be detected in the discarded

122

supernatants. Finally, the samples (hereafter referred to as Zn-saturated samples) were dried 6

ACS Paragon Plus Environment

Page 6 of 29

Page 7 of 29

Environmental Science & Technology

123

and ground to prepare the suspensions for the Wien effect measurements and the

124

powder-based tablets for the EXAFS and µ-XRF measurements. The Zn content of

125

Zn-saturated samples was shown in Figure S2.

126

0.25 g Zn-saturated samples and 25 mL DI water were added into 50-mL plastic

127

centrifuge tubes to achieve a solid concentration of 10 g L-1. The centrifuge tubes were sealed

128

and mixed followed by their ultrasonically dispersing for 45 min and oscillating for 1 h.

129

Hereafter, the suspensions were shaken for 1 h daily for 7 to 10 days to achieve equilibration

130

of ion reactions. During the equilibration period, the ECs of suspensions were regularly

131

monitored. When little or no change of ECs was observed, equilibration was considered

132

acceptable and the suspensions were ready to be used to conduct the Wien effect

133

measurements. All suspensions were prepared in duplicate.

134

Wien Effect Measurements

135

The electrical conductivity under strong electrical fields was measured with the SHP-2

136

(short high-voltage pulse) apparatus. The structure of the SHP-2 apparatus and the measuring

137

procedure were outlined in detail in previous publications.15-17, 21, 24 Before measuring the

138

Wien effect with the apparatus, the weak-field EC of the sample was determined with a

139

regular conductivity meter (DDS-310, Shanghai Kangyi instrument co.) at a constant room

140

temperature of 25 ºC to ensure that the resistance of the test suspension was within the

141

measurement range of 200 Ω to 20 kΩ. The strong-field ECs of the suspensions were

142

measured by applying a voltage drop that increased from 1.0 kV up to the occurrence of

143

sparking (dielectric breakdown) in the suspensions, which were held in a thermostatic

144

chamber at 25ºC. The electrode spacing was kept constant at 1 mm. After a phase of 7

ACS Paragon Plus Environment

Environmental Science & Technology

145

increasing electrical field strength, the measurements were repeated in reverse, with

146

decreasing field strength, in order to eliminate possible effects of long-term heating and other

147

irreversible phenomena.

148

Evaluation of Mean Gibbs Free Binding and Adsorption Energies

149

An analogy between ion activity and electrical conductivity was assumed for evaluating

150

the mean Gibbs free binding and adsorption energies. The measured pH values of the tested

151

suspensions and the charge density curves of the soil particles (Figure S1) were used to

152

calculate the CEC at the specific suspension pH. The measured weak-field electrical

153

conductivity (EC0) was used to calculate the mean Gibbs free binding energy according to Li

154

et al.15:

155

∆Gbi = RT ln

2CEC × C p × λ

[1]

EC0

156

in which R is the universal gas constant (8.315 J mol–1 K–1), T is the thermodynamic

157

temperature (K), CEC is the negative charge density (mol kg–1), Cp is the solid concentration

158

of the suspension (g L-1), and λ is the equivalent conductivity of Zn (52.8 mS cm–1 mol–1 L).

159

In accordance with the principle of thermodynamic equilibrium, and with defining states (1)

160

and (2) as the ECs under the weak and strong electrical fields, the mean Gibbs free adsorption

161

energy was evaluated from15

162

∆Gad = RT ln(EC/EC0)

[2]

163

in which EC is the strong-field electrical conductivity obtained from the Wien effect

164

measurement results, i.e., the EC(E) curves.

165

EXAFS and µ-XRF Measurements

166

Zn K-edge (9659 eV) EXAFS measurements were conducted at beamline BL14W at the 8

ACS Paragon Plus Environment

Page 8 of 29

Page 9 of 29

Environmental Science & Technology

167

Shanghai Synchrotron Radiation Facility (SSRF) in fluorescence modes. The spatial

168

distributions of elements (Zn, Fe, Mn and K) in the samples were recorded at the beamline

169

BL15U at the SSRF. Details of the EXAFS data and µ-XRF data collection procedures and

170

data analysis were described in the supporting information (SI) following published

171

documentation.25, 26

172 173

RESULTS AND DISCUSSION

174

EC-E Curves of Suspensions of Black Soil Particles Saturated with Zn2+

175

The repeated (increasing-/decreasing-E phases) EC-E curves of suspensions of

176

Zn-saturated soil particles (original and OM-free) were similar, therefore only one of the two

177

sets was presented in Figure 1. It was evident that the electrical conductivity of all samples

178

increased nonlinearly with electrical field strength. The suspensions of Top samples had

179

higher EC than the OM poorer and the OM depleted samples. It was interesting that the EC-E

180

curve of Top OM-free samples was almost identical to that of Bottom OM-free samples.

181

Though top and bottom black soil particles had differing clay-mineral compositions because

182

they were sampled from different horizons, they elicited similar EC-E curves after the OM

183

was removed. This indicated that the SOM was the major factor determining the EC-E curves

184

of the different soil horizons. The rate of EC increase of the original top black soil particles

185

was the lowest, the order of EC increase being Top < Bottom < Top OM-free ≈ Bottom

186

OM-free. This indicated that, within the experimentally observed range of field strengths, the

187

tested OM-free suspensions released more zinc ions and thereby contributed more to the

188

suspension EC than the OM-containing suspensions. Figure 1 also showed that the weak field 9

ACS Paragon Plus Environment

Environmental Science & Technology

189

(< 15 kV cm-1) conductivities (EC0) of the suspensions increased with increasing OM content

190

in the order: OM-free (0.0073–0.0076 mS cm-1) < Bottom (0.0124 mS cm-1) < Top (0.0321

191

mS cm-1). The observed order of the EC0 values of the original and OM-free samples was

192

consistent with previous findings.21 Soil samples with more OM also were richer in dissolved

193

organic matter (DOM) (Table S1). Because the point of zero charge (PZC) of the soil organic

194

matter was low, at about pH 3, in the studied higher pH range it was a variable-charge soil

195

component.5 The SOM contained a number of oxygen-containing functional groups,27 some

196

of which were carboxyls (pKa < 5) and quinones, which dissociated readily to form ions that

197

contributed to the soil-suspension EC0. In contrast, the organic matter dissolved in the soil

198

solution comprised mainly low-molecular-weight and low-aromatization-state organic

199

molecules that could form soluble complexes with metals. These complexes could be ionized

200

to form central cations and ligands with electroconducting functional groups, thereby

201

increasing the electrical conductivity of soil suspensions.6 Cabaniss demonstrated this

202

phenomenon and found that the affinity of Zn(II) and Cd(II) to DOM was in general weaker

203

than that of Cu(II), Pb(II), and Ni(II). 28 It was also confirmed by Pandey et al. employing

204

anion exchange equilibrium method that the order of stabilities of complexes formed between

205

metals and humic acid isolated from a natural soil is Cu (5.28) > Fe (5.03) > Pb (3.66) > Ni

206

(3.20) > Co (2.82) > Ca (2.78) > Cd (2.78) > Zn (2.74) > Mn (2.62) > Mg (2.35).29 Bai et al.

207

also found that the lower was the aromatization of humic acid, the smaller were its

208

coordination number and its complexation constant with Zn or Cd. 30

209

Effect of OM on the Mean Free Binding Energy of Zn2+ to Soil Particles

210

The mean free binding energies of the soil suspensions were evaluated by using Eq. (1) 10

ACS Paragon Plus Environment

Page 10 of 29

Page 11 of 29

Environmental Science & Technology

211

with the parameters listed in Table 1. The mean free binding energy of Zn2+ to the soil

212

particles decreased with increasing SOM content (Table 1) in the order: Top (5.86 kJ mol-1)