In-Situ Investigation of Interactions between Magnesium Ion and

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In-Situ Investigation of Interactions between Magnesium Ion and Natural Organic Matter Mingquan Yan, Yujuan Lu, Yuan Gao, Marc F. Benedetti, and Gregory V. Korshin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00003 • Publication Date (Web): 19 Jun 2015 Downloaded from http://pubs.acs.org on June 21, 2015

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Environmental Science & Technology

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In-Situ Investigation of Interactions between Magnesium Ion and Natural

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Organic Matter 1

3

Mingquan Yan #, Yujuan Lu ## , Yuan Gao ###, Marc F. Benedetti & and Gregory V. Korshin ###

4

5

#

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of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China

7

##

8

518060, China

9

###

Department of Environmental Engineering, Peking University, The Key Laboratory

College of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen,

Department of Civil and Environmental Engineering, University of Washington,

10

Box 352700, Seattle, WA 98195-2700, United States

11

& Institut de Physique du Globe de Paris – Sorbonne Paris Cité - Université

12

Paris-Diderot , UMR CNRS 7154, Paris, France

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Keywords: Carboxylic; Complexation; Differential absorbance spectroscopy;

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Magnesium; Natural Organic matter; Phenolic

1

Corresponding author. Address: Department of Environmental Engineering, College of

Environmental Sciences and Engineering, Peking University, Beijing 100871, China; Tel: +86 10 62755914-81, Fax: +86 10 62756526. E-mail: [email protected] 1

ACS Paragon Plus Environment


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Abstract: Natural organic matter (NOM) generated in all niches of the environment

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constitutes a large fraction of the global pool of organic carbon while magnesium is

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one of the most abundant elements that has multiple roles in both biotic and abiotic

18

processes. Although interactions between Mg2+ and NOM have been recognized to

19

affect many environmental processes, little is understood about relevant mechanisms

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and equilibria. This study addressed this deficiency and quantified Mg2+-NOM

21

interactions using differential absorbance spectroscopy (DAS) in combination with

22

the NICA-Donnan speciation model. DAS data were obtained for varying total Mg

23

concentrations, pHs from 5.0 to 11.0 and ionic strengths from 0.001 to 0.3 mol L-1.

24

DAS results demonstrated the existence of strong interactions between magnesium

25

and NOM at all examined conditions and demonstrated that the binding of Mg2+ by

26

NOM was accompanied by the replacement of protons in the protonation-active

27

phenolic and carboxylic groups. The slope of the log-transformed absorbance spectra

28

of NOM in the range of wavelength 350-400 nm was found to be indicative of the

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extent of Mg2+-NOM binding. The differential and absolute values of the spectral

30

slopes were strongly correlated with the amount of NOM-bound Mg2+ ions and with

31

the concentrations of NOM-bound protons.

2


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Introduction

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Magnesium has many important roles in abiotic and biochemical environmental

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processes and is an essential nutrient affecting many physiological functions 1-3. In the

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environment, interactions between magnesium, other hardness cations, notably

36

calcium and, on the other hand, dissolved or soil organic matter and mineral surfaces

37

are accompanied by the formation of soluble and sorbed complexes and a variety of

38

solid phases. These interactions also affect the speciation of many heavy metals and

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other processes, for instance the formation of fouling scales on membrane surfaces,

40

removal of natural organic matter (NOM) in water softening 4-11.

41

Due to its importance in biological processes, interactions of Mg2+ with

42

macromolecules involved in biotic processes have been examined in considerable

43

detail

44

achieved using the structure-sensitive methods of NMR or X-Ray Absorbance

45

Spectroscopy (XAS) that can provide important details concerning the number and

46

chemical nature of the atoms surrounding a magnesium atom

47

methods require using relatively high concentrations of the target metal and the

48

interpretation of their results may depend on the selection of representative model

49

compounds

50

complicated by the fact that NOM is a site-specific polydisperse entity

51

multiple types of proton- and metal-binding groups.

52

Mechanisms of binding of Mg2+ by NOM have not been unambiguously ascertained.

3, 12-15

. Elucidation of the microscopic aspects of such interactions can be

16-20

. However, these

8, 21-23

. Investigation of interactions between Mg2+ and NOM is also

3


10, 11

that has


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Selected prior studies have suggested that Ca2+ and Mg2+ are likely to be coordinated

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primarily the carboxyl groups of NOM and contributions of the phenolic groups may

55

not be as important

56

negatively charged carboxyl groups of NOM than Mg2+

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Mg2+ by NOM was deemed to be mediated primarily by non-specific Donnan

58

electrostatic interactions, the concentration of Mg2+ bound onto NOM was expected to

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be negligible compared with that of Ca2+

60

geochemistry demonstrate the occurrence of pronounced fractionation among it

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isotopes and these observations require that a more detailed understanding of Mg

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interactions with NOM and suspended materials be available to interpret the field

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fractionation data 6.

64

Some of the challenges in elucidation the nature and extent of the binding of Mg2+ by

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NOM are associated with the limitations of methods available for probing these

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interactions, preferably at environmentally-relevant concentrations of Mg2+ and NOM.

67

As opposed to the extensive data obtained using Ca2+-selective electrodes (ISE)

68

Mg2+–specific ISE have not been successfully used to examine NOM-Mg2+

69

interactions

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constants derived to account for Mg2+-complexation in geochemical models, e.g.

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NICA-Donnan Model and Model V have been derived via the analysis of the

72

acid-base titration curves generated in the absence and presence of MgCl2 9-11,

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These and related experiments were carried out using solutions in which the

74

concentrations of Mg2+ and humic substances were several orders of magnitude

8, 11

. Ca2+ has been posited to interact more strongly with the 24

. Because the binding of

8, 24

. Recent developments in Mg isotope

10

,

25-27

, apparently due to the limitations of their performance. Stability

4


28-30

.

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higher than those in environmental systems 28-30. The data obtained in these studies for

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Mg2+-NOM interactions are limited, and the model parameters suggested in prior

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research remain largely untested 9-11.

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This issue of the nature of Mg2+-NOM interactions can be explored in more detail via

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the use of an in situ approach that tracks the behavior of NOM chromophores as a

80

function of metal concentrations and other solution parameters. This approach uses

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the principle of differential spectroscopy

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sensitivity of NOM chomophores to interactions with many metal cations (e.g., Cu2+,

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Al3+, Cd2+)

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features resulting from interactions between metal ions and discrete NOM

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functionalities or possibly from effects of metal binding on charge-transfer transitions

86

in NOM molecules 34. Effects of Donnan-type interactions between metal cations and

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NOM on its chromophores have not been examined in sufficient detail.

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In this study, we used this approach to carry out an examination of interactions of

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magnesium and the chromophores in standard aquatic NOM. These experiments were

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performed for a wide range of pHs, ionic strengths and total metal loads at

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environmentally relevant concentrations of the system components. The objectives of

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this study were i) to ascertain effects of Mg2+ on NOM chromophores, ii) explore

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possibilities to assign such effects to specific NOM functionalities, iii) examine

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relationships between the intensity of features observed in the differential spectra of

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NOM and the extent of Mg2+-NOM binding and iv) to determine whether results of in

31-33

. Prior studies have demonstrated the

31-33

. The differential spectra of metal-NOM systems exhibit prominent

5



96

situ measurements of Mg2+-NOM interactions agree with the data of currently

97

available models of Mg2+-NOM complexation.

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Material and Methods

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Reagents and chemicals All chemicals were AR grade unless otherwise mentioned.

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All solutions were prepared using Milli-Q water (18.2 MΩ cm-1, Millipore Corp.,

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MA, USA). Suwannee River humic acid (SRHA, 1R101H) and fulvic acid (SRFA,

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1S101F) obtained from the International Humic Substances Society (IHSS). SRHA

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and SRFA solution were prepared by Milli-Q water with a concentration of 5.0 mg L-1

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as dissolved organic carbon, DOC. DOC concentrations were determined with a

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Shimadzu TOC-Vcsh carbon analyzer. Background ionic strength was controlled by

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adding requisite amount of NaClO4. Stock solutions of Mg(II) were prepared using

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Mg(ClO4)2 salt.

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Titrations Mg(II) titrations were performed by adding desired increments of

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Mg(ClO4)2 stock solution to a 100 mL volume of NOM solution with a DOC

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concentration of 5 mg L-1. pH of the solutions was controlled by adding HEPES

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buffer and, as necessary, HClO4 or NaOH. Containers were closed during the

112

experiments, except when incremental amounts of magnesium stock solution or

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pH-controlling agent were added.

114

After each addition of Mg(II) stock, the solution was allowed to equilibrate for about

115

5 min prior to the removal of 5 mL aliquots for absorbance spectra acquisition. The

116

aliquots were then returned back to the solutions prior to the next addition of Mg(II), 6


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HClO4 or NaOH titration. The overall duration of a titration for any given pH or ionic

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strength was about one hour. The absorbance spectra were recorded using a

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Perkin-Elmer Lambda 18 or Shimadzu UV-2700 UV/Vis spectrophotometer (with a 5

120

cm cell) at wavelengths from 200 to 600 nm. Effects of dilution associated with the

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addition of requisite amounts of Mg2+ stock on the absorbance of NOM were

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corrected by correcting the measured absorbance by the ratio of the initial solution

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volume plus the titrant added to it normalized by the initial solution volume.

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NOM-Mg ions complexation was modeled using the NICA-Donnan approach and

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complexation constants available in the Visual MINTEQ database

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Mg2+ concentrations exceeding the precipitation levels of its solid phases determined

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based on Visual MINTEQ calculations were excluded. Complexation parameters used

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in these calculations are compiled in Table S1.

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Absorbance

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log-transformed differential absorbance spectra were calculated using the equations (1)

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and (2), respectively:

132

∆Aλ = Aλ, i - Aλ, ref

(1)

133

DLnAλ= LnAλ,i - LnAλ,ref

(2)

data

processing

Linear

differential

11

absorbance

. Data for total

spectra

and

134

In these equations, Aλ, i and Aλ,

135

selected magnesium concentration, pH and IS and the applicable reference that had

136

the same pH and IS in the absence of magnesium. The slopes and differentials slopes

ref

are, respectively, absorbance measured at any

7



137

of the log-transformed spectra of NOM were calculated and as defined below:

d ln A(λ ) dλ 350− to − 400nm

138

S350− 400 =

139

DS350 − 400 = S350 − 400,i − S350 − 400, ref

(3)

(4)

140

In the above equations, S350-400 is the slope of the linear correlation that fits the

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log-transformed absorbance spectra in the range between 350 and 400 nm. S350− 400 ,i

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and S 350 − 400,ref are the spectral slopes determined for any selected condition and an

143

applicable reference, respectively.

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Results and Discussion

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Effects of magnesium on NOM ultra-visible absorbance spectra

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The absorbance spectra of SRHA and SRFA do not have conspicuous features and

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their intensity decreases near-exponentially with the observation wavelength.

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Addition of increasing magnesium concentration resulted in subtle but consistent

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changes of absorbance of NOM for all examined pH values and ionic strengths (IS),

150

as illustrated in in Figure 1 (a)) for SRHA at pH 7.0 and ionic strength 0.01 mol L-1.

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This was consistent with observations made in previous studies of metal-NOM

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interactions 31-33.

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To discern effects of magnesium on NOM chromophores in more detail, differential

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absorbance spectra (DAS) were calculated using eq (1). Results of this data

155

processing are presented in Figure 1(b). The shown spectra have three distinct bands 8


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with peaks at wavelength 250, 310 and 390 nm. The band with the 250 nm maximum

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is relatively less intense and its sign is negative indicating a decrease rather than

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increase of absorbance of NOM in this range. The sign of the other two bands is

159

positive and they are more intense, especially the band with the 390 nm maximum.

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Similar features were observed in practically all conditions at which the Mg2+/NOM

161

system was examined.

162

Figure 1

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Figure 1(c) demonstrates that the logarithms of absorbance of NOM decrease linearly

164

with the wavelength but the log-transformed spectra have several regions (e.g., 450 nm) with somewhat different

166

slopes. Increasing magnesium concentrations caused consistent changes of the slopes

167

of the log-transformed spectra, especially for wavelengths > 350 nm (Figure 1(d)).

168

Because the slope of log-transformed absorbance of NOM in the range of

169

wavelengths 350-400 nm is most sensitive to variations of Mg2+ concentrations, the

170

absolute values of the slopes of log-transformed spectra in this wavelength range and

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their changes (denoted as S350-400 and DS350-400, respectively) were used to estimate

172

Mg2+-NOM binding.

173

Results of these calculations for SRHA and SRFA in the presence of varying

174

magnesium concentrations at pH values ranging from 5 to 11 and a 0.01 mol L-1 ionic

175

strength are shown in Figure 2.

176

Figure 2 9



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The datasets of DS350-400 values shown in the latter figure were compared with the

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concentrations of Mg2+-SRHA or Mg2+-SRFA complexes estimated using the

179

NICA-Donnan model, in which metal binding accounts for specific interactions with

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the operationally defined carboxylic and phenolic groups in DOM and non-specific

181

Donnan electrostatic interactions

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constants included in the Visual MINTEQ database (Table S1)

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between the MINTEQ-based estimates of the concentrations of magnesium bound by

184

NOM and corresponding changes of the parameter DS350-400 are shown in Figure 3. It

185

demonstrates that for all examined conditions, DS350-400 values determined are linearly

186

correlated with the modeled concentrations of NOM-bound Mg2+ ions.

10, 11

. These calculations used the complexation 11

. Relationships

Figure 3

187

188

Effects of ionic strength on magnesium –SRHA interaction

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To further explore effects of solution chemistry on Mg2+-NOM interactions, responses

190

of NOM chromophores to their interactions with magnesium were examined for IS

191

values varying from 0.001 to 0.3 mol L-1. Figure 4 presents DAS results for these

192

experiments for SRHA at pH 11.0.

193

Figure 4

194

The data demonstrate that DAS intensities determined for varying total Mg

195

concentrations decrease consistently as IS values increase but the relative prominence

196

of each band in the differential spectra remains largely the same. The behavior of the 10


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spectral slope S350-400 and its differential in these conditions were similar as shown in

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Figure 5.

199

Figure 5

200

The DS350-400 data shown in the latter figure were compared with the corresponding

201

concentrations Mg2+-SRHA complexes estimated using Visual MINTEQ. Results of

202

this comparison shown in Figure 6 demonstrate the existence of a nearly-linear

203

correlation between DS350-400 and the extent of Mg2+-SRHA complexation for the

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entire range of IS values and pH 9.0 and 11.0. However, significant deviations from

205

the general trend were observed to exist for pH 5.0.

Figure 6

206

207

Interpretation of Magnesium-NOM interactions based on spectroscopic data

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The results presented above demonstrate that the DAS method and quantitation of its

209

data using, for instance, the spectral slope S350-400 and its differential DS350-400 allow

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ascertaining interactions between magnesium and NOM at varying pHs, IS,

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magnesium concentrations, different NOM samples and at environmentally relevant

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DOC concentrations.

213

In all cases, the differential spectra have three distinct bands with peaks at wavelength

214

250, 310 and 390 nm. The intensities of these bands are affected by such system

215

parameters as pH, IS and Mg2+ concentrations, as demonstrated in Figure S1-S3.

11



216

The nature of interactions associated with the emergence of these bands is still to be

217

unambiguously determined but preceding studies of NOM interactions with the proton

218

and metal cations other than Mg2+

219

can be considered as a signature of the mode of binding that involves primarily the

220

carboxylic groups in NOM while the bands located in the 300-390 nm region may be

221

more reflective of the binding involving NOM phenolic groups. These features have

222

also been hypothesized to be a manifestation of a bathochromic shift of absorbance

223

sub-bands caused by the presence ligand-to-metal charge transfer transitions in NOM

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molecules 37, 38 but the occurrence of this phenomenon in the case of NOM remains to

225

ascertained.

226

Mechanistically, the features in the differential spectra appear to be a result of the

227

deprotonation of carboxylic and phenolic functionalities in NOM caused by the

228

replacement of NOM-bound protons by the Mg2+ ions interacting with NOM

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molecules. This point is confirmed by the data shown in Figure 7 demonstrating that

230

S350-400 values obtained for a wide range of pHs, total magnesium concentrations and

231

ionic strengths are correlated with the amount of protons bound onto the carboxylic

232

and phenolic functional groups of NOM predicted by NICA-Donnan model.

35, 36

have demonstrated that the band at 250 nm

233

Figure 7

234

The observations presented above demonstrate the applicability and a reasonable

235

albeit not perfect precision of the NICA-Donnan model in describing interactions of

236

the examined NOM samples with Mg2+ for a wide range of pHs, metal concentrations 12


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and ionic strengths. However, deviations from model predictions become more

238

obvious at acidic pH (pH 5) and high ionic strength (IS>0.1 mol L-1). This indicates

239

that the parameters describing Mg2+-NOM complexation in the NICA-Donnan and

240

related models need to be optimized. Information for such optimization can be

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obtained via the analysis of the correlations between Mg2+-NOM concentrations and

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DS350-400 values or related spectroscopic parameters that can be determined based on

243

in situ measurements. The latter aspect of the presented results will be explored in

244

more detail in future studies.

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Acknowledgements

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This study was partially supported by China NSF (Grant 50808001) and the New Star

247

of Science and Technology program of the Beijing Metropolis (Grant 2011009).

248

Yujuan Lu thanks the Shenzhen University Visiting Scholar Foundation Program, the

249

Shenzhen Science and Technology Program (Grant JC201005250054A) and Organic

250

Geochemistry National Key Laboratories Fund (Grant OGL-201105) for supporting

251

her work at University of Washington. Gregory Korshin thanks the Foreign Experts

252

Program of China for supporting his work at Peking University. Partial support by the

253

U.S. NSF is acknowledged as well (Grant 0931676). The views represented in this

254

publication do not necessarily represent those of the funding agencies.

255

Supporting Information

256

Additional table and figures. This information is available free of charge via the

257

Internet at http://pubs.acs.org. 13



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based on Methyl Phenyl Semicarbazone as an ionophore. J Chemistry 2013, 2013, 7.

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27. Gupta, V. K.; Chandra, S.; Mangla, R., Magnesium-selective electrodes. Sensor

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and Actuat B: Chem 2002, 86, (2–3), 235-241.

340

28. Van Dijk, H., Cation binding of humic acids. Geoderma 1971, 5, (1), 53-67.

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29. Lead, J. R.; Hamilton-Taylor, J.; Hesketh, N.; Jones, M. N.; Wilkinson, A. E.;

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Tipping, E., A comparative study of proton and alkaline earth metal binding by humic

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substances. Anal Chim Acta 1994, 294, (3), 319-327.

344

30. Schnitzer M; Skinner, S. I. M., Organo-metallic interactions in soils. 7. Stability

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constants of Pb++-, Ni++-, Mn++-, Co++-, Ca++-, and Mg++- fulvic acid complexes. 17



346

Soil Sci 1967, 103, (4), 247-&.

347

31. Yan, M.; Dryer, D.; Korshin, G. V.; Benedetti, M. F., In-situ study of binding of

348

copper by fulvic acid: Comparison of differential absorbance data and model

349

predictions. Water Res 2013, 47, (2), 588-596.

350

32. Yan, M.; Korshin, G. V., Comparative examination of effects of binding of

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different metals on chromophores of dissolved organic matter. Environ Sci Technol

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2014, 48, 3177-3185.

353

33. Yan, M.; Wang, D.; Korshin, G. V.; Benedetti, M. F., Quantifying metal ions

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binding onto dissolved organic matter using log-transformed absorbance spectra.

355

Water Res 2013, 47, (7), 2603-2611.

356

34. Del Vecchio, R.; Blough, N. V., On the origin of the optical properties of humic

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substances. Environ Sci Technol 2004, 38, (14), 3885-3891.

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35. Dryer, D. J.; Korshin, G. V.; Fabbricino, M., In situ examination of the

359

protonation behavior of fulvic acids using differential absorbance spectroscopy.

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Environ Sci Technol 2008, 42, (17), 6644-6649.

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36. Yan, M.; Korshin, G. V.; Claret, F.; Croué, J.-P.; Fabbricino, M.; Gallard, H.;

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Schäfer, T.; Benedetti, M. F., Effects of charging on the chromophores of dissolved

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organic matter from the Rio Negro basin. Water Res 2014, 59, (0), 154-164.

364

37. Cornard, J. P.; Dangleterre, L.; Lapouge, C., Computational and spectroscopic

365

characterization of the molecular and electronic structure of the Pb(II)-quercetin

366

complex. J Phys Chem A 2005, 109, (44), 10044-10051.

367

38. Cornard, J. P.; Lapouge, C., Theoretical and spectroscopic investigations of a 18


Page 18 of 32

Page 19 of 32


368

complex of Al(III) with caffeic acid. J Phys Chem A 2004, 108, (20), 4470-4478.

369 370

19



Page 20 of 32

371

Captions of Figure

372

Figure 1 Absorbance spectra of SRHA recorded at varying concentrations of

373

magnesium at pH 7.0 and ionic strength 0.01 mol L-1. (a) zero-order spectra; (b)

374

differential

375

log-transformed spectra. DOC concentration 5.0 mg L-1; cell length 5 cm.

spectra;

(c)

log-transformed

spectra;

and

(d)

differential

376 377

Figure 2 Changes of the differential spectral slope DS350-400 as a function of Mg2+

378

concentrations at varying pH. Ionic strength 0.01 mol L-1.

379 380

Figure 3 Correlation between DS350-400 values and concentrations of Mg2+-NOM

381

complexes calculated using the NICA-Donnan model (parameters shown in Table

382

S1). Results for varying pHs, total magnesium concentrations and a 0.01 mol L-1

383

ionic strength.

384 385

Figure 4 Differential spectra obtained at varying total magnesium concentrations

386

for SRHA at pH 11.0 and ionic strengths varying from 0.001 to 0.1 mol L-1. DOC

387

concentration 5.0 mg L-1; cell length 5 cm.

388 389

Figure 5 Changes of the spectral slope DS350-400 as a function of total magnesium

390

concentrations at varying ionic strengths and (a) pH 5.0; (b) pH 7.0 and (c) pH

391

11.0.

392 20


Page 21 of 32


393

Figure 6 Correlations between differential spectral slope DS350-400 and

394

concentrations of Mg2+-SRHA complexes determined using the NICA-Donnan

395

model at varying ionic strengths and (a) pH 5.0; (b) pH 7.0 and (c) pH 11.0.

396 397

Figure 7 Correlation between concentrations of protons bound by carboxylic and

398

phenolic moieties in SRHA and the slopes of log-transformed spectra in the

399

wavelength 350-400 nm (S350-400) in the presence of varying magnesium

400

concentration, pHs and ionic strengths. (Unit of ionic strengths is mol L-1).

21



401

Page 22 of 32

Figures 0.045

(a)

Absorbance (A.U)

3.50 3.00 2.50

0.00E+00 4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 3.48E-04 4.35E-04

Mg concentration (M)

2.00 1.50 1.00 0.50

Differential absorbance (a.u.)

4.00

4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 3.48E-04 4.35E-04

0.035 0.025 0.015

(b)

0.005 -0.005 225

275

325

375

425

475

525

Mg concentration (mol L-1)

(d)

-0.015

0.00 225

275

325

375

425

475

525

575

Wavelength (nm)

Wavelength (nm)

402 1.00

0.10

(c)


0.50 0.00 225

275

325

375

425

475

-0.50 -1.00 -1.50 -2.00 -2.50

0.00E+00 4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 3.48E-04 4.35E-04

525

Differential log-transformed absorbance

Log-transformed absorbance


0.08

4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 3.48E-04 4.35E-04

0.06

0.04

0.02

0.00 225

-3.00

275

325

375

425

475

525

-0.02

Wavelength (nm)

Wavelength (nm)

403 404

Figure 1 Absorbance spectra of SRHA recorded at varying concentrations of

405

magnesium at pH 7.0 and ionic strength 0.01 mol L-1. (a) zero-order spectra; (b)

406

differential

407

log-transformed spectra. DOC concentration 5.0 mg L-1; cell length 5 cm.

spectra;

(c)

log-transformed

spectra;

22


and

(d)

differential

Page 23 of 32


0.0018

-1 Differential slope DS350 350--400 (nm )

0.0016 0.0014

SRHA pH5.0

SRFA pH7.0

SRHA pH7.0

SRFA pH8.0

SRHA pH11.0

SRFA pH9.0

0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0000 0

0.0001

0.0002

0.0003

0.0004

0.0005

Total Mg concentration (mol L-1)

408 409

Figure 2 Changes of the differential spectral slope DS350-400 as a function of Mg2+

410

concentrations at varying pH. Ionic strength 0.01 mol L-1.

23



Page 24 of 32

411

NICA--donnan model (mol L-1) predicted by NICA

Concentration of Mg bound onto NOM

2.5E-05

R² = 0.93 2.0E-05

1.5E-05

SRHA pH5.0 SRHA pH7.0

1.0E-05

SRHA pH11.0 SRFA pH5.0

5.0E-06

SRFA pH8.0 SRFA pH9.5

0.0E+00 0.0000

0.0005

0.0010

0.0015

0.0020

-1 Differential slope DS350350-400 (nm )

412 413

Figure 3 Correlation between DS350-400 values and concentrations of Mg2+-NOM

414

complexes calculated using the NICA-Donnan model (parameters shown in Table

415

S1). Results for varying pHs, total magnesium concentrations and a 0.01 mol L-1

416

ionic strength.

24




Page 25 of 32

0.10 0.09

(a) 0.001M

0.08

Mg concentration (mol

L-1)

0.07 0.06 0.05 0.04 0.03

4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 3.48E-04 4.35E-04

0.02 0.01 0.00 -0.01225

275

325

375

425

475

525

-0.02 -0.03

Wavelength (nm) 417


0.10 0.09

(b) 0.01M

0.08 0.07


0.06 0.05 0.04 0.03 0.02 0.01

4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 4.35E-04

0.00 -0.01 225

275

325

375

425

475

525

-0.02 -0.03

Wavelength (nm) 418


0.10 0.09

(c) 0.1M

0.08 0.07 0.06


0.05 0.04 0.03 0.02

4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 3.48E-04 4.35E-04

0.01 0.00 -0.01225

275

325

375

425

475

-0.02 -0.03

Wavelength (nm) 419 25


525


420

Figure 4 Differential spectra obtained at varying total magnesium concentrations

421

for SRHA at pH 11.0 and ionic strengths varying from 0.001 to 0.1 mol L-1. DOC

422

concentration 5.0 mg L-1; cell length 5 cm.

26


Page 26 of 32

Page 27 of 32



0.0004

IS=0.001 IS=0.01

0.0003

IS=0.03 0.0002

0.0001

0.0000 0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

Total Mg concentration (mol L-1)

423


0.0020

IS=0.001 IS=0.003 IS=0.01 IS=0.03 IS=0.1 IS=0.3

0.0015

0.0010

(b) pH7.0

0.0005

0.0000 0.0000

0.0001

0.0002

0.0003

0.0004

Total Mg concentration (mol


424

IS=0.001 IS=0.01 IS=0.1

0.0025 0.0020

IS=0.003 IS=0.03 IS=0.3

0.0005

L- 1 )

(c) pH11.0

0.0015 0.0010 0.0005 0.0000 0.0000

425

(a) pH5.0

IS=0.003

0.0001

0.0002

0.0003

Total Mg concentration (mol

0.0004 L-1)

27


0.0005


426

Figure 5 Changes of the spectral slope DS350-400 as a function of total magnesium

427

concentrations at varying ionic strengths and (a) pH 5.0; (b) pH 7.0 and (c) pH

428

11.0.

28


Page 28 of 32

Page 29 of 32



Concentration of Mg bound onto SRHA

3.0E-05 2.5E-05

(a) pH5.0

2.0E-05 1.5E-05

IS=0.001

1.0E-05

IS=0.003

5.0E-06

IS=0.03

IS=0.01

0.0E+00 0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

-1 Differential slope DS350350-400 (nm )

429



3.0E-05 (b) pH7.0

2.5E-05 2.0E-05

IS=0.001

1.5E-05

IS=0.003 IS=0.01

1.0E-05

IS=0.03 IS=0.1

5.0E-06

IS=0.3

0.0E+00 0.0000

0.0005

0.0010

0.0015

Differential slope DS350350-400

430

0.0020

0.0025

(nm-1)

431



3.0E-05 (c) pH11.0

2.5E-05 2.0E-05

IS=0.001 IS=0.003

1.5E-05

IS=0.01

1.0E-05

IS=0.03 IS=0.1

5.0E-06 0.0E+00 0.0000

0.0005

0.0010

0.0015

Differential slope DS350350-400

0.0020 (nm-1)

29


0.0025


432

Figure 6 Correlations between differential spectral slope DS350-400 and

433

concentrations of Mg2+-SRHA complexes determined using the NICA-Donnan

434

model at varying ionic strengths and (a) pH 5.0; (b) pH 7.0 and (c) pH 11.0.

30


Page 30 of 32

Page 31 of 32


-1 Slope S350 350--400 (nm )

-0.008

IS0.001 pH7.0

IS0.003 pH5.0

IS0.001 pH11.0

-0.009

IS0.003 pH7.0

IS0.01 pH5.0

IS0.003 pH11.0

IS0.01 pH7.0

IS0.03 pH5.0

IS0.01 pH11.0

-0.010

IS0.03 pH7.0

IS0.1 pH5.0

IS0.03 pH11.0

IS0.1 pH7.0

IS0.3 pH5.0

IS0.1M pH11.0

-0.011

IS0.3 pH7.0

-0.012 -0.013 -0.014 -0.015 -0.016 0.0E+00

1.0E-05

2.0E-05

3.0E-05

4.0E-05

Concentration of protons bound by carboxylic 435

and phenolic groups in SRHA (mol L-1)

436

Figure 7 Correlation between concentrations of protons bound by carboxylic and

437

phenolic moieties in SRHA and the slopes of log-transformed spectra in the

438

wavelength 350-400 nm (S350-400) in the presence of varying magnesium

439

concentration, pHs and ionic strengths. (Unit of ionic strengths is mol L-1).

31



Page 32 of 32

Graphical abstract

Differential log-transformed absorbance

0.10 Mg concentration (M)

0.08

4.35E-06 8.70E-06 1.74E-05 3.48E-05 4.35E-05 8.70E-05 1.74E-04 4.35E-04

0.06 0.04 0.02 0.00 225

275

325

375

425

-0.02 Wavelength (nm)


475

In-Situ Investigation of Interactions between Magnesium Ion and

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