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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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In-Situ Investigation of Interactions between Magnesium Ion and Natural

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Organic Matter 1

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

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#

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

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##

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518060, China

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###

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,

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

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

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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

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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

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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

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interactions using differential absorbance spectroscopy (DAS) in combination with

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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.

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DAS results demonstrated the existence of strong interactions between magnesium

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

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

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

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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

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

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the concentrations of NOM-bound protons.

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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

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

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

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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,

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

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

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macromolecules involved in biotic processes have been examined in considerable

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detail

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

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

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chemical nature of the atoms surrounding a magnesium atom

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

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

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compounds

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

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multiple types of proton- and metal-binding groups.

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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

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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

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not be as important

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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

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

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be negligible compared with that of Ca2+

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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.

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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.

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

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

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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

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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

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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

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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

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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

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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

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

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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

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experiments, except when incremental amounts of magnesium stock solution or

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pH-controlling agent were added.

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

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

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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

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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:

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∆Aλ = Aλ, i - Aλ, ref

(1)

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DLnAλ= LnAλ,i - LnAλ,ref

(2)

data

processing

Linear

differential

11

absorbance

. Data for total

spectra

and

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In these equations, Aλ, i and Aλ,

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

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

ref

are, respectively, absorbance measured at any

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

d ln A(λ ) dλ 350− to − 400nm

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S350− 400 =

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

(3)

(4)

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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

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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),

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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

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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

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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

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system was examined.

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Figure 1

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

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

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slopes. Increasing magnesium concentrations caused consistent changes of the slopes

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

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

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

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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

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Mg2+-NOM binding.

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

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

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strength are shown in Figure 2.

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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

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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

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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

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

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

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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

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

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

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experiments for SRHA at pH 11.0.

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Figure 4

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The data demonstrate that DAS intensities determined for varying total Mg

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concentrations decrease consistently as IS values increase but the relative prominence

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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.

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Figure 5

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

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

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

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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

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the general trend were observed to exist for pH 5.0.

Figure 6

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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

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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.

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

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

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

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

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unambiguously determined but preceding studies of NOM interactions with the proton

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and metal cations other than Mg2+

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

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

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

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

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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

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ascertained.

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

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deprotonation of carboxylic and phenolic functionalities in NOM caused by the

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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

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

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

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

35, 36

have demonstrated that the band at 250 nm

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Figure 7

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The observations presented above demonstrate the applicability and a reasonable

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

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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

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

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

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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

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

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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

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

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

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Shenzhen Science and Technology Program (Grant JC201005250054A) and Organic

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

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

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

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

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publication do not necessarily represent those of the funding agencies.

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Supporting Information

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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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sites in aquatic humic substances. Environ Sci Technol 2000, 34, (11), 2138-2142.

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23. Korshin, G. V.; Frenkel, A. I.; Stern, E. A., EXAFS study of the inner shell

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structure in copper(II) complexes with humic substances. Environ Sci Technol 1998,

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32, (18), 2699-2705.

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24. Li, Q.; Elimelech, M., Organic fouling and chemical cleaning of nanofiltration

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membranes:  Measurements and mechanisms. Environ Sci Technol, 2004, 38,

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4683-4693.

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25. Müller, M.; Rouilly, M.; Rusterholz, B.; Maj-Żurawska, M.; Hu, Z.; Simon, W.,

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Magnesium selective electrodes for blood serum studies and water hardness

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measurement. Microchim Acta 1988, 96, (1-6), 283-290.

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26. Chandra, S.; Sharma, K.; Kumar, A., Mg(II) selective PVC membrane electrode

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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.

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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.

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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

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

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

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

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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.

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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.

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

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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

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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.

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

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

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

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

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

369 370

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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

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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).

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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)

Mg concentration (mol L-1)

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

Mg concentration (mol L-1)

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;

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(d)

differential

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Environmental Science & Technology

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.

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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.

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Differential absorbance (a.u.)

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

Differential absorbance (a.u.)

0.10 0.09

(b) 0.01M

0.08 0.07

Mg concentration (mol L-1)

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

Differential absorbance (a.u.)

0.10 0.09

(c) 0.1M

0.08 0.07 0.06

Mg concentration (mol L-1)

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

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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.

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

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

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

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

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

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)

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0.0005

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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.

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

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

Concentration of Mg bound onto SRHA

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

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

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

Concentration of Mg bound onto SRHA

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)

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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.

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-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).

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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)

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