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Using two-dimensional correlation size exclusion chromatography (2D-CoSEC) and EEM-PARAFAC to explore the heterogeneous adsorption behavior of humic substances on nanoparticles with respect to molecular sizes Diep Dinh Phong, and Jin Hur Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04311 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017
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Using two-dimensional correlation size exclusion chromatography (2D-
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CoSEC) and EEM-PARAFAC to explore the heterogeneous adsorption
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behavior of humic substances on nanoparticles with respect to molecular
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sizes
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Diep Dinh Phong a,b and Jin Hura,*
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a
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gu, Seoul, 05006, South Korea
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b
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Hanoi, 100000, Vietnam
Department of Environment and Energy, Sejong University, 209 Neungdong-ro, Gwangjin-
Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay City,
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Revised and Re-submitted to Environmental Science & Technology, December, 2017
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* Corresponding author: Tel. +82-2-3408-3826.
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E-mail:
[email protected] Fax +82-2-3408-4320.
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Abstract
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The adsorption behaviors of different constituents within bulk humic substances (HS) on two
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nanoparticles, TiO2 and ZnO, were examined by using two-dimensional correlation size
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exclusion chromatography (2D-CoSEC) and excitation emission matrix – parallel factor
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analysis (EEM-PARAFAC), which separated bulk HS into different size fractions and
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fluorescent components, respectively. Subtle changes in the size distributions of HS with
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increasing adsorbents were successfully identified and tracked via the 2D-CoSEC. From
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adsorption isotherm experiments, three different HS constituent groups with respect to sizes
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and fluorescence features were identified by the 2D-CoSEC and EEM-PARAFAC,
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respectively. The chromatographically separated HS size groups presented dissimilar
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adsorption behaviors in terms of adsorption affinity and isotherm non-linearity. The sequence
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orders of adsorption, interpreted from the 2D-CoSEC, was consistent with those of the
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isotherm model parameters individually calculated for different HS size sub-fractions,
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signifying the promising application of 2D-CoSEC in obtaining an insight into the
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heterogeneous adsorption of HS in terms of molecular sizes. EEM-PARAFAC results also
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supported the major finding of the 2D-CoSEC as shown by the preferential adsorption of the
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fluorescent components associated with large molecular sizes.
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Keywords: EEM-PARAFAC; two-dimensional correlation spectroscopy (2D-COS); size
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exclusion chromatography (SEC); preferential adsorption; photocatalysts
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1. Introduction
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Nano-oxides, such as TiO2 and ZnO, have been widely used for a number of
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applications including catalysts, industrial filters, ceramics, coatings, semiconductors, and
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microelectronics,1,2 which renders the nanoparticles frequently exposed to aquatic
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environments, even potentially causing harm to the ecosystem.3-5 Meanwhile, dissolved
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organic matter (DOM), ubiquitously present in aquatic environments, is known to be a
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heterogeneous mixture of acidic, randomly polymerized, and polydisperse macromolecules
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with a broad spectrum of sizes and chemical functional groups.6 Humic substances (HS), a
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major DOM constituent with an operational definition of hydrophobic acids, can strongly
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interact with inorganic particles.7
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In aquatic environments, adsorption of HS onto nanoparticles is considered a primary
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process that affects the fate, stability, surface properties, and environmental functions of the
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particles.8,9 The major mechanisms by which HS adsorb onto inorganic surfaces are
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documented to be: (i) hydrophobic interaction, (ii) electrostatic interaction, (iii) van der
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Waals interaction, (iv) ligand exchange, (v) hydrogen bonding, and (vi) cation bridging.8,9
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Both the structural heterogeneity of HS and a range of surface properties of nanoparticles
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make the interactions very complicated even under a fixed solution chemistry. Adsorption
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behavior of HS has traditionally been described by using bulk HS parameters such as UV
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absorbance and dissolved organic carbon (DOC),10,11 which often failed to fully address the
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complex adsorption phenomenon caused by the heterogeneous nature of HS. For example,
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previous studies demonstrated preferential adsorption of certain HS constituents occurred on
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mineral surfaces due to surface characteristics with preferable adsorption affinities for
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specific HS sizes/moieties, which evidenced the limitation of using bulk HS parameters in 2
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fully describing HS adsorption behavior.8,12 Fluorescence excitation emission matrix combined with parallel factor analysis (EEM-
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PARAFAC) and size exclusion chromatography (SEC), both recent advanced HS
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characterization tools, can separate bulk HS into several different components or groups
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with respect to the spectral features and molecular sizes, respectively. The two techniques
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are based on non-destructive, reliable, and sensitive measurements, which are advantageous
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for quickly probing and tracking different HS constituents.14-16 In our previous study, EEM-
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PARAFAC proved its successful application in individually tracing the changes of different
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fluorescent HS components in photo-oxidation systems containing TiO2.17 It has also been
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utilized to characterize DOM adsorption onto granular activated carbon or graphene
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oxide.18,19. Meanwhile, Mwaanga et al. (2014) and Drosos et al. (2015) used the SEC
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technique to demonstrate the preferential adsorption of certain sized HS molecules onto TiO2
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particles.20,21 Combined use of the two tools could advance the understanding of HS
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adsorption behavior on nano-particles because individual results are closely linked together
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and complementary to each other. 14,18
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Two dimensional correlation spectroscopy (2D-COS) is a powerful tool for exploring
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the dynamic features of the relationships between two different spectral variables (e.g.,
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wavelengths).22 Combined information of the synchronous and the asynchronous maps,
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which are generated from 2D-COS, reveals the sequential order of any subtle spectral
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change in response to external perturbations in a high resolution. It can also guide one to
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identify the exact bands (or the ranges of variables) responding to specified
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processes/reactions, presenting a clear picture of the dynamic changes occurring in a wide
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range of variables.22 There is no doubt that using 2D-COS is beneficial for HS studies 3
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considering the heterogeneous character of HS. The spectral variables used for 2D-COS
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maps were mostly based on UV-visible and fluorescence spectroscopy or Fourier-transform
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infrared spectroscopy (FT-IR).23-25 However, it should be noted that 2D-COS applications
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are not limited to such spectroscopic measurements because it is a mathematical
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representation for resolving the overlapping signals of chemical mixtures like HS.21 For
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example, He et al (2015) have shown its usefulness of 2D-COS and SEC chromatograms in
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combination in examining size-dependent changes of DOM with composting process.26 The
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applicability can be extended to other HS studies such as HS adsorption behavior onto
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minerals or nano-particles.
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Although large sized HS molecules tend to adsorb onto inorganic surfaces to a great
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extent, the exact preference of the adsorption for different sized HS molecules is dependent
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on the surface characteristics and solution pH.27-29 Considering the complexity of the size-
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dependent HS adsorption onto nanoparticles, 2D-COS combined with SEC (2D-CoSEC) can
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be suggested as a promising tool to obtain an insight into the interactions between HS and
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nanoparticles. Compared with the conventional SEC technique, 2D-CoSEC can be more
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advantageous in identifying any subtle changes or differences that might occur within a
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small range of HS molecular sizes upon the complex HS adsorption. The objectives of this
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study were (1) to track the changes of different constituents within bulk HS upon the
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adsorption onto two different nanoparticles (TiO2 and ZnO), and (2) to explore the
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applicability of 2D-CoSEC and EEM-PARAFAC to obtain new insights into the
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heterogeneous adsorption behavior.
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2. Materials and Methods 4
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2.1 Reagents
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Two types of nanoparticles with similar particle sizes, TiO2 and ZnO, were used for this
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study without further purification. The nano-particles have been widely used in a wide range
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of applications such as electronics, chemical sensing, antimicrobial agents, and photocatalysts.
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The TiO2 (AEROXIDE® P25) nanoparticles were purchased from Sigma-Aldrich, with a BET
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surface area of 50±15 m2/g and an average size of 21 nm. The ZnO nanoparticles, obtained
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from NanoAmor, have a BET surface area of 40 m2/g and an average size of 20 nm. The
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point zero of charge (PZC) values of TiO2 and ZnO were ~7.0 and ~10.0, respectively, which
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were determined by zeta potential measurements (Fig. S1). The measured PZC values were
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comparable to those previously reported.29-32 Each type of nanoparticle was mixed with
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distilled deionized water (DDW) to prepare a slurry before an amount of HS (15 mg C/L) was
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added. The HS-nanoparticle suspension solutions could simulate natural environments and/or
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industrial wastewater containing the nano-particles, in which HS can interact with suspended
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nano-particles.
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Elliott soil humic acid (ESHA) and Suwannee River humic acid (SRHA) were purchased
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from the International Humic Substance Society (IHSS). The materials represent two
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contrasting HS sources (i.e., soil-derived versus aquatic HS) with different structural and
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compositional features (www.humicsubstnaces.org).18
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2.2 Adsorption isotherm experiments
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Adsorption experiments were conducted in batch using an initial HS concentration of 15
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mgC/L, which fell within the range found in natural/engineered systems.14,33 The amounts of
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nanoparticles were varied from 0.0 to 1.0 g/L with stepwise increases of 0.025 g/L up to 0.1 5
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g/L and 0.1 g/L for the rest, which were determined based on preliminary tests to achieve the
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adsorption isotherm. The solution pH was fixed to 7.0 by adding 0.1 N NaOH or 0.1 N HCl
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solutions. The mixtures were shaken at room temperature (20±1 oC) on an orbit shaker at 150
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rpm for 24 hours in the dark. No substantial change of pH was observed during the shaking,
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which only deviated by ±0.3 from the original value. Dissolved HS (or the residual HS after
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adsorption) was separated by centrifugation at 5000 rpm for 30 min, and the supernatant was
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filtered through a pre-washed 0.2 µm membrane filter (cellulose acetate membrane, Advantec)
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to remove the remaining particles. Adsorption isotherm parameters were estimated based on a
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simple Freundlich model 1
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Qe =k F Cen
(1)
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Where Ce is the equilibrium HS concentration in solution. kF and 1/n represent the
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Freundlich model capacity factor and the Freundlich model site heterogeneity factor, an
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indicator of isotherm nonlinearity, respectively.
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2.3 Analytical methods
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DOC concentrations were determined using a TOC analyzer (Shimadzu V-CPH). UV–
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visible spectra from 200 to 700 nm were recorded for HS samples using a UV–visible
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spectrophotometer (UV-1800, Shimadzu) with a 1-cm quartz cuvette.
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Fluorescence EEMs were measured with a fluorescence spectrophotometer (F-7000,
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Hitachi) by scanning the emission spectra from 280 to 550 nm at 1 nm-increments and
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stepping through the excitation wavelengths (Ex) from 220 to 450 nm at 5 nm intervals.
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Excitation and emission slits were adjusted to 10 nm and 5 nm, respectively, and the scanning
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speed was fixed at 12,000 nm min–1. To limit second order Raleigh scattering, a 290 nm
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cutoff filter was used for all the fluorescence measurements. If the UV absorption coefficient 6
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of samples at 254 nm exceeded 0.05 cm–1, they were diluted to avoid the need for inner-filter
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correction.34 The background fluorescence EEM from a blank solution (DDW) was also taken
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into account by subtraction. Fluorescence intensity was normalized using quinine sulfate
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equivalent units (QSU), in which 1 QSU corresponded to the maximum fluorescence
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intensity for 1 µg/L of quinine in 0.1 N H2SO4 at Ex/Em of 350/450 nm. Relative precisions
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of intermediate MW > low MW
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