Sorption of Heterocyclic Organic Compounds to Multiwalled Carbon

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Sorption of heterocyclic organic compounds to multi-walled carbon nanotubes Florian Metzelder, Matin Funck, and Torsten Claus Schmidt Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05205 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Environmental Science & Technology

Table of Contents/Abstract Graphic 83x40mm (120 x 120 DPI)

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Sorption of heterocyclic organic compounds to

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multi-walled carbon nanotubes

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Florian Metzeldera; Matin Funcka; Torsten C. Schmidta,b,c*

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a

Instrumental Analytical Chemistry, University of Duisburg-Essen, Universitätsstrasse 5, 45141 Essen, Germany

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b

Centre for Water and Environmental Research (ZWU), University of Duisburg-Essen, Universitätsstrasse 2, 45141 Essen, Germany

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c

IWW Water Centre, Moritzstraße 26, 45476 Mülheim an der Ruhr, Germany

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

*Corresponding author (current address):

25 26 27 28 29 30 31 32

Torsten C. Schmidt Instrumental Analytical Chemistry University of Duisburg-Essen Universitätsstrasse 5 45141 Essen, Germany phone: +49-201-1836774 fax: +49-201-1836773 e-mail: [email protected] ACS Paragon Plus Environment

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Abstract

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Sorption is an important natural and technical process. Sorption coefficients are typically

35

determined in batch experiments but this may be challenging for weakly sorbing compounds.

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An alternative method enabling analysis of those compounds is column chromatography. A

37

column packed with the sorbent is used and sorption data are determined by relating sorbate

38

retention to that of a non-retarded tracer. In this study, column chromatography was applied

39

for the first time to study sorption of previously hardly investigated heterocyclic organic

40

compounds to multi-walled carbon nanotubes (MWCNTs). Sorption data for these

41

compounds are very limited in literature and weak sorption is expected from predictions.

42

Deuterium oxide was used as non-retarded tracer. Sorption isotherms were well described by

43

the Freundlich model and data showed reasonable agreement with predicted values. Sorption

44

was exothermic and physisorption was observed. H-bonding may contribute to overall

45

sorption, which is supported by reduced sorption with increasing ionic strength due to

46

blocking of functional groups. Lowering pH reduced sorption of ionizable compounds, due to

47

electrostatic repulsion at pH 3 where sorbent as well as sorbates were positively charged.

48

Overall, column chromatography was successfully used to study sorption of heterocyclic

49

compounds to MWCNTs and could be applied for other carbon-based sorbents.

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1. Introduction

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In the field of chemistry, research concerning carbon-based nanomaterials (CNMs)

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and carbon nanotubes (CNTs) as one class of CNMs often takes advantage of their strong

54

sorption properties.1-5 Strong sorption of different compound classes led to the application of

55

for example multi-walled carbon nanotubes (MWCNTs) as solid phase extraction material for

56

enrichment of organic1, 4, 5 and inorganic compounds2, 3 prior to analysis.

57

To characterize these sorption properties and study the influence of environmental

58

conditions on sorption, batch experiments are typically conducted for organic6-17 and

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inorganic compounds.18-22 Batch experiments are simple to perform, but are accompanied by

60

several drawbacks. Firstly, the long equilibration time from hours23 up to several weeks6, 7

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applied in literature in case of organic compounds makes experiments time intensive. This

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especially increases the time expenses to study the influence of environmental conditions on

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sorption if preliminary tests are necessary to adjust sorbent and sorbate amount needed to

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yield sufficient but not too strong / weak sorption. Secondly, the limited sorbent to solution

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ratio in batch experiments may reveal difficulties in the determination of reliable sorption data

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for highly soluble and weakly sorbing compounds (e.g., heterocyclic organic compounds like

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pyridine).24 Thirdly, a high amount of sorbent is necessary in case of weak sorption to

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investigate sorption, especially, when investigating the influence of environmental conditions

69

on sorption in batch experiments. As an example, for sorption of perchlorate to MWCNTs in

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batch experiments, 37 mg sorbent were necessary when studying sorption at one pH value.18

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Using ten different pH values in triplicates would require more than one gram of sorbent. In

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case of more expensive sorbents like single-walled carbon nanotubes or functionalized CNTs,

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one gram can cost up to 1000 euro. Finally, separation of CNMs from the water phase may be

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a challenging step, especially in case of dispersed materials25, 26. In this case, typically used

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techniques like filtration or sedimentation may be insufficient.6 Additionally, the

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quantification of low concentrations of CNTs in matrices is challenging so that the efficiency

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of phase separation processes can be hardly quantified.27 As consequence of these drawbacks,

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sorption models like poly-parameter linear free-energy relationships (ppLFERs) were

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developed for MWCNTs allowing the prediction of distribution coefficients (Kd) for organic

80

compounds,28,

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previously unknown compounds. However, these models are only valid for one setup of

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environmental conditions, which limits their applicability for varying environmental

29

without the necessity to perform additional sorption experiments for


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conditions. It has been previously shown that pH13, 15, ionic strength30, 31 and temperature32, 33

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influence sorption properties of sorbates and CNMs.34

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As alternative to batch experiments, column experiments can be applied to study

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sorption, by determination of Kd values as well as complete isotherms using the retardation of

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the sorbate in relation to a non-retarded tracer in a sorbent packed column. This experimental

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approach has been used successfully for sorption studies to soot particles,35 soils24,

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mineral surfaces,46, 47. Recently, sorption of inorganic anions to MWCNTs was also studied in

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column experiments showing the general applicability of this approach for MWCNTs.48

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Drawbacks of batch experiments can potentially be overcome by column experiments. The

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analysis of weakly sorbing and highly soluble compounds is possible because of higher

93

sorbent to solution ratios without a simultaneous need of large amounts of sorbent material.

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Furthermore, changing the composition of the mobile phase regarding ionic strength, type of

95

ions and pH or by changing the column temperature using a thermostat facilitates

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investigations of the influence of environmental conditions on sorption. Additionally,

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equilibration times can be significantly reduced. Additional equipment like centrifuges can be

98

avoided, as the sorbent remains fixed in the column.

36-45

or

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However, column experiments also may have disadvantages. Column clogging and too

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strong sorption can be serious problems, which lead to very high backpressure or cause long

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retention times and deterioration of the signal by peak broadening, respectively. Concomitant,

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backpressure and sorption can both be reduced by the dilution of the sorbent material with

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inert material like quartz24,

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with the sorbate. The dilution with inert material further reduces the sorbent demand and,

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consequently, costs to purchase those materials. An important disadvantage, potentially

106

preventing the applicability of column experiments, is sorption hysteresis. Sorption hysteresis

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describes the incomplete desorption of a sorbate from the sorbent. Using a Dirac input (i.e.,

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injection of a sorbate pulse instead of a continuous sorbate input) desorption is a prerequisite

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in column experiments because otherwise no signal can be recorded. If sorbate molecules

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remain sorbed to the surface, changed interactions between the sorbent surface and sorbate

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molecules will be the consequence in consecutive measurements and this will result in

112

potentially misleading sorption data. For the measurement of breakthrough curves, sorption

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hysteresis is less problematic, but each column could only be used for one compound and one

114

introduced concentration. For further consecutive measurements the sorbent surface would

115

not be pristine as sorbate molecules remain sorbed on the surface. Sorption hysteresis has

40

or silicium carbide37, which should not show any interaction



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already been shown for sorption of some organic compounds to CNMs in batch

117

experiments49-51. However, for inorganic anions, sorption hysteresis to MWCNTs was not

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observed in a previous study using column experiments.48

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Thus, the aim of this study was to investigate sorption of weakly sorbing organic

120

compounds to MWCNTs using column chromatography and to identify the influence of

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environmental conditions like temperature, ionic strength and pH on sorption. Heterocyclic

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organic compounds were chosen as sorbates for this purpose because only very limited

123

sorption data are available in literature for this compound class and sorption data predicted

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using ppLFERs from literature28 indicated only weak sorption that may preferably be

125

analyzed using column chromatography instead of classical batch experiments.

126 127

2. Materials and Methods

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2.1 Chemicals and Packing Materials

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Chemicals used as sorbates in this study were heterocyclic organic compounds (furan,

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imidazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrimidine, pyrrole, thiazole and

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thiophene). Detailed information on manufacturer and purity are given in section SI.1 in the

132

supporting information. Additional chemicals used in this study were hydrochloric acid (HCl)

133

(37 %, analytical grade, Fisher Scientific), sodium hydroxide (NaOH) (1 mol L-1, Bernd

134

Kraft), calcium chloride (CaCl2 × 2 H2O) (≥ 99.5%, AppliChem Panreac) and deuterium

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oxide (D2O) (≥ 99.9atom-%, Sigma-Aldrich). Eluents for high performance liquid

136

chromatography (HPLC) were prepared with ultrapure water (18.2 MΩ cm-1 resistance,

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PURELAB Ultra, ELGA LabWater, Celle, Germany) and addition of appropriate amounts of

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salts, acid or base to reach the desired concentrations or pH value. Individual stock solutions

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of each sorbate and tracer were prepared in the respective eluent used for measurements and

140

stored at 4°C in the dark.

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MWCNTs (C150HP) were purchased from Bayer Material Science (Leverkusen,

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Germany). General properties of MWCNTs used are given elsewhere.1, 48 Amorphous carbon

143

was removed prior to use by heating MWCNTs to 350°C for 1 h.52 Removal of amorphous

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carbon by this treatment was confirmed previously using transmission electron microscopy.52

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Quartz, used as inert material, with a particle size < 63 µm was purchased from Fluka (purity

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≥ 99.7%). 5 ACS Paragon Plus Environment

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2.2 Apparatus

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Empty HPLC-columns (length: 14; diameter: 3 mm) and supplementary equipment

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used for column packing (precolumns, connecting nuts, stainless steel sieves (3 µm pore size),

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glass fiber filters and PTFE seals) were purchased from Bischoff Chromatography (Bischoff

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Analysentechnik u. –geräte GmbH, Leonberg, Germany).

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The HPLC-system (Knauer Wissenschaftliche Geräte GmbH, Berlin, Germany) used

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for column packing and sorption studies consisted the following parts: HPLC-pump (K-1001),

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degasser, autosampler used for sample injection (Triathlon Spark Holland), column oven

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(Jetstream 2 Plus), diode-array-detector (DAD) with deuterium lamp (K-2700 and K-2701)

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connected to a refractive index (RI) detector (K-2301) in serial connection. A more detailed

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description and a sketch of the experimental setup is given elsewhere.48 Dirac input of sorbate

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solutions was done by an injection-loop-valve using 5 µL injection volume.

160 161

2.3 Column chromatography

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A procedure used in literature to pack columns with reference soils was adapted for

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this study.53 A detailed description of the procedure used in this study was reported

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previously.48 MWCNTs were diluted with quartz as inert material by a factor of 20 for all

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sorbates except thiophene, i.e., a MWCNT content of 5% (wt/wt). Dilution with quartz

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increased both the stability of the packing material inside the column and reduced the

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backpressure. Even with this dilution, sorption was strong leading to measurement times of up

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to 90 minutes per measurement run in case of pyridine. For thiophene a dilution by a factor of

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200, i.e., a MWCNT content of 0.5% (wt/wt), was necessary as retention was too strong in

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columns containing 5% MWCNTs. In 5% MWCNT columns, measurement times of more

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than 150 minutes and a maximum absorption of only 60 mAU even for an almost saturated

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solution of thiophene were recorded because of a pronounced tailing. Maximum intensity in

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the chromatogram was much lower compared to the other sorbates which was typically

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around 1500 mAU for the highest injected concentration. To reach a similar range in

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absorption, dilution of MWCNTs had to be increased.

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The conservative tracer used in this study was D2O. D2O was diluted 1:9 with the

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respective eluent (vol.-%/vol.-%) and detected using the RI-detector. Other tracers typically 6 ACS Paragon Plus Environment


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used in column experiments detectable using a DAD like inorganic anions showed, depending

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on the environmental conditions, strong sorption to MWCNTs.48 Retention times of sorbates

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and tracer used for calculation of the retardation factor (R) (equation 1) and Kd values

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(equation 2) were determined based on the half-mass point, i.e., the point at which the peak

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area is divided in two equal parts.54 Peaks of sorbates in this study showed a pronounced

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tailing (e.g. asymmetry factors of peaks of pyridine determined at 10% of the maximum

184

intensity were up to 7.9). Standard deviations of R and Kd, determined by error propagation,

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were typically smallest for the half-mass method compared to other typically used approaches

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like the apex point of the eluting peak or the first moment approach36, 37 (data not shown).

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Furthermore, the half-mass point has previously shown its applicability in sorption studies on

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soil or minerals24, 39, 40, 46 but also for MWCNTs.48

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R = (t – t*) / (t0 – t´)

(1)

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R = 1 + ρb / θ x Kd

(2)

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t is the retention time of the sorbate while t0 is the retention time of the tracer. t* is the

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traveling time through injector and capillaries between autosampler and DAD without

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column, while t´ is the traveling time from autosampler to the RI-detector without column.

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Both detectors were connected in serial connection, which required different corrections for

195

the retention time of sorbate and tracer. t* and t´ were determined by repeated injections of

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sorbate and tracer without column installed in the system (n = 10). For measurements without

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column, capillaries were connected using a low dead volume stainless steel union (0.25mm

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bore, VICI Jour.). θ [-] and ρb [kg L-1] are the porosity and the bulk density of the column

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used, respectively. Both were calculated from repeated measurements of D2O with and

200

without column installed in the system (n = 10). Determined values for both characteristics of

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columns used in this study are given in the supporting information (section SI.2). Recovery of

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D2O was determined from peak areas with and without column and was found to be

203

quantitative (90 to 105%). Regular measurements of D2O during sorption studies showed no

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significant shift of the retention time of D2O for more than 200 injections (data not shown).

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Wavelengths used to record chromatograms of the different sorbates are given in

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section SI.1 in the supporting information. Interaction of the different sorbates with the inert

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material was checked in a column packed with exclusively quartz and was negligible (Kd

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calculated from the quartz column < 0.1% of that calculated from the MWCNT column). Kd

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values for pyrazole, which was used as reference compound for each MWCNT-column, 7 ACS Paragon Plus Environment

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varied with a relative standard deviation of 6.2% (ten columns), when injecting the same

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concentration in each column. This indicates an acceptable reproducibility for self-packed

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columns. Mass recovery of sorbates was calculated by comparison of peak areas of each

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sorbate from MWCNT-columns with those from measurements without column or a quartz

214

column. Recoveries ranged between 80 and 120% for all sorbates with exceptions for very

215

low injected concentrations. In these cases, the measured absorption was very low and

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baseline noise became an issue because of peak broadening. Conclusively, sorption hysteresis

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was not observed in this study as recovery was quantitative. Error bars depicted in figures

218

indicate the maximum error, which was derived from the individual standard deviations of

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retention times of sorbate and tracer with and without column via error propagation.

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Columns were equilibrated for at least 12 hours by flushing at 0.1 mL min-1 with the

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respective eluent. Preliminary measurements were performed for each sorbate at 10 mM

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CaCl2, pH 6 and 25 °C for different flow rates / pore water velocities to test the degree of non-

223

equilibrium in the experiments. One concentration of each sorbate was separately injected

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(concentrations are mentioned in section SI.3 in the supporting information). Sorption was at

225

or very close to equilibrium, because determined R values varied only slightly with maximum

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8% at different flow rates / velocities ranging from 0.015 – 0.1 mL min-1 or 0.33 – 2.2 cm

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min-1, respectively (shown for selected compounds in section SI.3 in the supporting

228

information). Because of the small influence of the flow rate, 0.1 mL min-1 was used as flow

229

rate for all experiments.

230 231

2.4 Isotherm determination

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Isotherms of sorbates were determined using an eluent with 10 mM CaCl2 at pH 6 and

233

25 °C controlled by the column thermostat, as these conditions were also applied in previous

234

studies using the same type of MWCNTs.28, 48 Sorbates were injected separately in MWCNT

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columns with increasing concentration and three replicates for each concentration with

236

exception of pyridine, where two replicates were used due to long measurement times. At

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least nine different concentrations were injected and used for isotherm generation of each

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sorbate. The injected concentrations of each sorbate are given in section SI.4 in the supporting

239

information.

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Different approaches have been used in literature to construct sorption isotherms from

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column studies.35-37 In this study, the approach used by Schenzel et al.37 was adapted. The 8 ACS Paragon Plus Environment


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eluted peak of the respective sorbate was used to derive the representative dissolved

243

concentration at equilibrium (cw,eq). Using an external calibration on a spectrophotometer

244

(Shimadzu UV-1650PC, Shimadzu Europe GmbH, Duisburg, Germany), the absorption in the

245

chromatogram at the half-mass point was converted into cw,eq. Calibrations used for isotherm

246

calculation are given in section SI.5 in the supporting information. cw,eq was multiplied with

247

the corresponding Kd value to finally yield the corresponding sorbed concentration at

248

equilibrium (cs,eq). This approach is later referred as the “classical approach”.

249

As alternative method to derive sorption isotherms, the approach by Schenzel et al.37

250

was adapted and combined with the approach by Bürgisser et al..55 For this method, the whole

251

tailing of the peak in the chromatogram was used to construct the sorption isotherm from one

252

injected concentration of the sorbate instead of the injection of several concentrations.

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Bürgisser et al.55 derived the sorption isotherm directly from the chromatogram. The highest

254

concentration injected of each sorbate was used for this purpose. The absorption in the

255

chromatogram was transferred into the dissolved concentration at equilibrium (cw,eq) using the

256

external calibration starting at the determined half-mass point in the chromatogram. For each

257

data point in the chromatogram following the half-mass point, a Kd value was calculated using

258

equation 1 and 2, which was multiplied with the corresponding cw,eq in order to yield cs,eq.

259

This approach is later referred as the “alternative approach”. Both approaches are explained

260

stepwise in the supporting information (section SI.4) The similarity between both approaches

261

was evaluated because applying the alternative approach reduces the time necessary for

262

determination of an isotherm almost by a factor of ten compared to the classical approach in

263

case of nine different concentrations used for isotherm construction.

264

2.5 Influence of environmental conditions on sorption

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To reduce the time expense in studying the influence of environmental conditions on

266

sorption, a selection of heterocyclic compounds was chosen and only one concentration but

267

still three replicates per sorbate were injected. Evaluation was done based on the “alternative

268

approach”. Compound selection was done according to the sorption strength / retention inside

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the column represented by the Freundlich-coefficient (Kf) of isotherms previously determined

270

(see chapter 2.4), water-air distribution by the Kwa-value and the pKa-value of the

271

corresponding acid of the compound (section SI.7 in the supporting information). A broad

272

compound range was represented by the selected compounds, as weakly to strongly sorbing

273

compounds, compounds with low to high Kwa-values were present and compounds of which

274

speciation is changed or remains the same by changing the pH value were used. Mean values 9 ACS Paragon Plus Environment

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of the three replicates of Kd as well as Kf and the Freundlich exponent (n) derived from

276

chromatograms at varied environmental conditions are reported. Injected concentrations for

277

the different conditions are given in the supporting information (section SI.7).

278

1) Temperature: Column CNT52 and CNT53; flow rate of 0.1 mL min-1; 10 mM CaCl2

279

in the mobile phase; pH 6; 25, 40 and 55 °C. CNT52 was used for the high

280

concentrations of each compound, while CNT53 was used for the low concentrations.

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2) Ionic strength: Column CNT54; flow rate of 0.1 mL min-1; 25°C; pH 6; 1, 10 or 100 mM CaCl2 in the mobile phase.

283

3) pH value: Column CNT55; flow rate of 0.1 mL min-1; 25°C; pH 3, 6 or 9 using 10

284

mM CaCl2. Ultrapure water with 10 mM CaCl2 had the desired pH value of 6. NaOH

285

was used to change the pH to 9. At pH 3, a concentration of 1 mM HCl was added to

286

reach the desired pH value. For pH 3 and 9 the ionic strength was chanced by less than

287

5% compared to pH 6 and was assumed to be constant.

288

Variation of ionic strength at pH 3: Column CNT56; flow rate of 0.1 mL min-1; 25°C;

289

pH 3; 1 mM CaCl2 + 1 mM HCl, 10 mM CaCl2 + 1 mM HCl or 100 mM CaCl2 + 1

290

mM HCl in the mobile phase.

291

3. Results and discussion

292

3.1 Determination of sorption isotherms

293

Different heterocyclic organic compounds were analyzed to determine sorption

294

isotherms. With increasing injected concentration, the apex point of the peak increases in

295

intensity and decreases in retention time. Selected chromatograms of pyrazole for different

296

injected concentrations are shown in figure S4 in the supporting information (section SI.4).

297

The same behavior is observed for the determined half-mass point and for all compounds

298

investigated in this study.

299

Consequently, increasing injected concentration resulted in decreasing Kd values. In

300

case of pyrazole, Kd decreased by more than a factor of five (111.8 L kg-1 for 15 mg L-1

301

injected concentration and 19.5 L kg-1 for 1000 mg L-1). For the other compounds, the

302

decrease was similar relative to the respective values. Decrease in Kd was much stronger

303

compared to a previous study dealing with sorption of inorganic anions to MWCNTs in

304

column experiments.48 For inorganic anions, like iodide, the decrease was at maximum about

305

50% but in most cases only about 20% or less, when increasing the injected concentration

306

from 0.6 to 63 mg L-1. 10 ACS Paragon Plus Environment


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Strongly decreasing Kd values with increasing sorbate concentrations are a strong

308

indicator for non-linear sorption. Sorption isotherms derived from chromatograms were

309

calculated according to both methods mentioned in section 2.4 and fitted using the Freundlich

310

model (cs,eq = Kf x cw,eqn), which is often used in batch experiments on sorption to CNTs.10, 13,

311

15, 16, 28, 33, 56-58

312

information. values for Kf, n and correlation coefficients (R2) of all sorbates are summarized

313

in table 1.

314 315 316 317 318 319

Table 1: Determined Freundlich coefficients (Kf) [(mg kg-1) (mg L-1)-n], Freundlich exponents (n) [-] and correlation coefficients (R2) [-] of isotherms of sorbates calculated based on the classical and the alternative approach. Uncertainties given in the table are the standard errors of the regression with Kf as y-intercept and n as slope of the regression for the classical approach and the standard deviation of the three replicates for the alternative approach.

Selected isotherms are also shown in an diagram in figure S5 in the supporting

Classical approach sorbate furan imidazole oxazole pyrazine pyrazole pyridazine pyridine pyrimidine pyrrole thiazole thiophenea

Kf

n

132.2 ± 2.3 39.3 ± 0.3 36.5 ± 0.8 222.2 ± 5.2 57.6 ± 2.9 137.9 ± 0.4 243.1 ± 3.0 154.0 ± 4.9 65.2 ± 2.2 158.4 ± 3.3 127.5 ± 0.8

0.60 ± 0.01 0.69 ± 0.01 0.77 ± 0.01 0.58 ± 0.01 0.71 ± 0.02 0.64 ± 0.01 0.53 ± 0.01 0.67 ±0.01 0.71 ± 0.02 0.59 ± 0.01 0.74 ± 0.01

Alternative approach

R2 0.9995 0.9997 0.9978 0.9967 0.9920 0.9999 0.9985 0.9961 0.9963 0.9978 0.9999

Kf

n

R2

154.8 ± 0.7 48.8 ± 0.6 47.9 ± 0.2 231.7 ± 3.8 73.3 ± 0.2 144.9 ± 0.1 261.1 ± 2.9 177.0 ± 2.9 82.2 ± 0.1 187.9 ± 0.3 234.2 ± 1.3

0.54 ± 0.01 0.60 ± 0.01 0.68 ± 0.01 0.57 ± 0.01 0.65 ± 0.01 0.62 ± 0.01 0.50 ± 0.01 0.62 ± 0.01 0.62 ± 0.01 0.53 ± 0.01 0.61 ± 0.01

0.9998 ± 0.0001 0.9998 ± 0.0001 0.9994 ± 0.0001 0.9987 ± 0.0007 0.9965 ± 0.0001 0.9999 ± 0.0001 0.9994 ± 0.0001 0.9996 ± 0.0001 0.9997 ± 0.0001 0.9997 ± 0.0001 0.9991 ± 0.0001

320

a: determined with a column containing only 0.5% MWCNTs, which may result in slightly smaller values

321

compared to the other compounds.


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For both evaluation approaches, data were well fit by the Freundlich model as

324

indicated by the high R2 (R2 > 0.992). Determined Kf values differed strongly between the

325

different sorbates. The smallest value was determined for oxazole while an about six times

326

higher value was determined for pyridine. The alternative approach gave generally higher

327

values for Kf, but results were in all cases very similar to the classical approach with less than

328

30% higher values for the alternative approach with exception of thiophene (84% higher). The

329

MWCNT content inside the column was lower for thiophene compared to the other sorbates,

330

which led to a stronger increase in signal intensity with increasing injected concentration and

331

a generally lower retention inside the column compared to the other sorbates (figure S6 and

332

S7 in the supporting information). This results in a larger deviation between both approaches

333

as the absorption used to calculate cw,eq in case of the alternative approach is much higher

334

compared to the absorption used for the classical approach. Determined n values indicate that

335

sorption of all sorbates was non-linear (0.50 – 0.75). Non-linearity was in all cases larger for

336

the alternative approach, but deviation was less than 20% for all compounds, which again

337

shows the similarity of results obtained by both evaluation approaches. To further compare

338

sorption isotherms, Kd values calculated based on the classical and alternative approach for

339

1%, 0.1% and 0.01% of the maximum solubility in water of the respective compound (figures

340

S5 – S6 in the supporting information (section SI.6)) agree well with each other with a

341

deviation of less than 0.3 log units. Consequently, the alternative approach proved its

342

suitability as Kf, n and Kd values are similar to those of the classical approach but requires

343

significantly reduced time. Non-linear sorption to CNTs is commonly found in literature for organic10, 13, 15, 16, 28,

344 345

33, 56-58

and inorganic compounds18,

20, 21

346

inorganic anions in a previous study48 or in a very low concentration range for PAHs6,

347

sorption to MWCNTs was found to be linear. When extending the concentration range also

348

for PAHs6 the sorption isotherms became non-linear. The non-linearity is attributed to the

349

heterogeneous energy distribution of sorption sites becoming relevant when reaching higher

350

concentrations. At low concentrations only high energy sorption sites like defects or

351

functional groups are occupied, while at higher concentrations also sorption sites exhibiting a

352

lower energy become relevant and sorption affinity decreases.34

in batch experiments. Only in rare cases, like for

353

Only for pyridine a sorption isotherm using MWCNTs as sorbent was found in

354

literature, but experimental conditions were different.59 Sorption experiments were done in 12 ACS Paragon Plus Environment


355

batch experiments at pH 7 and 10, i.e. higher than the pH of 6 used in this study, which

356

influences the speciation of pyridine. The fraction of positively charged pyridine drops from

357

15% at pH 6 to 1.5% at pH 7, which may also result in changing interactions between sorbent

358

and sorbate due to charged sorbate molecules and sorbent surface. Sorption was found to be

359

almost linear as n was with 0.94 close to unity.59 Reason may be the small concentration

360

range investigated. In a narrow concentration range, linearity may always approach one

361

compared to a wider concentration range as already observed for PAHs.6 Kf was found to be

362

almost one order of magnitude higher in literature (2.5 x 103 (mg kg-1) (mg L-1)-n (changed

363

from unit (mmol kg-1) (mmol L-1)-n)).59 These observations show substantial differences

364

between both isotherms, which may be related to differences in the investigated concentration

365

range, environmental conditions (pH, ionic strength), type of MWCNTs and sorbent to

366

solution ratio limiting the comparability and resulting in large deviations. As further

367

experimental sorption data for the other sorbates are missing, Kd values for all compounds

368

were calculated from the respective isotherms and had to be compared to Kd values predicted

369

using ppLFERs.28

370

ppLFERs were derived from batch isotherms using the exact same environmental

371

conditions and sorbent material so that the experimental conditions are well comparable.28

372

Solute descriptors are mentioned in section SI.6 in the supporting information. The

373

comparison of Kd values is shown for 1%, 0.1% and 0.01% of the maximum solubility in

374

water in figures S8 – S10 in the supporting information (section SI.6). For oxazole,

375

pyrimidine, pyrrole and thiazole agreement of data calculated based on the isotherms of the

376

classical and alternative approach with ppLFER data was good. Deviation was less than 0.3

377

log-units at all three concentrations levels, which is typically an acceptable deviation for

378

predicted values. Higher deviations were observed for the other compounds of more than 0.5

379

log-units with exception of furan and pyrazole at 0.01%. Maximum deviation was 0.9 log-

380

units. For those compounds no general tendency could be observed, as predicted values were

381

larger than measured values for furan, pyridine and thiophene, while for imidazole, pyrazole

382

and pyridazine the opposite behavior was found. Regarding the relationship between structure

383

and sorption of heterocyclic compounds it could be observed that sulfur atoms may enhance

384

sorption. Thiazole has the heteroatoms in the same positions than imidazole and oxazole, but

385

has a sulfur atom instead of oxygen or nitrogen. Thiazole showed much stronger sorption at

386

the same relative concentration with regard to the calculated Kd values. Equally, thiophene

387

containing sulfur had higher Kd values compared to furan containing oxygen and pyrrole

388

containing nitrogen. For the ppLFER predictions it should be noted here, that predicted values 13 ACS Paragon Plus Environment

Page 14 of 27

Page 15 of 27


389

are also connected with an uncertainty. Firstly, solute descriptors used to set up the ppLFERs

390

and predict Kd values are connected with an uncertainty. Secondly, mainly non-heterocyclic

391

compounds were used to set up the ppLFERs with the exception of larger heterocyclic

392

compounds indole and benzofuran, which may result in deviations for the compounds

393

analyzed in this study. Thirdly, only non-ionic species were used to set up the ppLFERs.28

394

Thus, pyridine and imidazole, which were partially charged at the environmental conditions

395

used in this study, are compounds with limited suitability of the ppLFERs. This may also

396

explain to some extent the larger deviations to predicted values for these two compounds.

397

In summary, column experiments proved suitable as fast technique to determine

398

sorption isotherms for heterocyclic organic compounds. Additionally, much smaller amounts

399

of approximately 8 mg sorbent material per sorbate and column were necessary because of the

400

dilution with quartz and the possibility to use columns for several consecutive measurements

401

applying different concentrations. Using batch experiments, the sorbent demand would have

402

been much higher. More than one gram of the sorbent per isotherm of a sorbate would be

403

necessary assuming triplicates and the sorbent demand used in case of sorption of

404

trihalomethanes to MWCNTs.33 The alternative approach in isotherm evaluation showed its

405

applicability and reduces the time demand for isotherm measurement by almost a factor of

406

ten. Consequently, for further experiments to evaluate the influence of environmental

407

conditions on sorption the alternative approach was used. Thus, besides Kd values also

408

isotherms could be derived using only one injected concentration measured in triplicates.

409 410

3.2 Influence of temperature

411

The influence of temperature on sorption is shown in figure 1. Sorption coefficients

412

decreased with increasing temperature for all analytes. The sorption enthalpy (∆H) was

413

calculated using the van´t Hoff equation from the slope of the regression line in figure 1

414

according to equation 3.

415

ln Kd = ∆S/R - ∆H/R x 1/T

416 417

(3)

Where ∆H is the sorption enthalpy [kJ mol-1], ∆S the entropy change [kJ mol-1 K-1], R the gas constant [8.3145 × 10-3 kJ mol-1 K-1] and T the temperature [K].



ln Kd [L/kg]

148.4

54.6

Page 16 of 27

furan imidazole oxazole pyrazole pyridine pyrrole

20.1

7.4

0.0030

0.0031

0.0032

0.0033

0.0034

1/T [1/K]

418 419 420 421 422

Figure 1: Determined Kd values of the analytes plotted against the reciprocal temperature at 10mM CaCl2 in the eluent at pH 6. Error bars indicate the maximum error estimated from standard deviation of retention times of sorbate and tracer via error propagation (n=3).

423

The determined values for ∆H ranged between -20.6 and -27.1 kJ mol-1 (all values are

424

shown in section SI.8 in the supporting information). Consequently, sorption of these organic

425

compounds is an exothermic process. In literature, sorption of different organic compounds

426

like

427

ciprofloxacin50 or toluene, m-xylene, ethylbenzene23 to different CNTs was also found to be

428

exothermic. Conversely, sorption of 1,2-dichlorobenzene,13 sulfamethazine,64 or dyes14 to

429

different CNTs was found to be endothermic or showed in case of dissolved organic matter to

430

SWCNTs65 no temperature dependency. Consequently, there is no coherent thermodynamic

431

basis for all compound classes reported in literature.

trihalomethanes,33

atrazine,32,

60

nitroaromatics,61

phenols,15,

62

oxaquindox,63

432

Covalent bonding like in case of chemisorption is absent in this study as determined

433

values for ∆H were smaller than typically determined values in the range from -60 to -80 kJ

434

mol-1 for organic compounds.66 Consequently, physisorption seems to be the underlying

435

mechanism. This was also concluded in other studies dealing with sorption to CNTs.50, 61, 63

436

Additionally, this is supported by quantitative recovery and, consequently, fully reversible

437

sorption, which is characteristic for physisorption. H-bonding (-10 – -40 kJ mol-1)67 with

438

functional groups located on the MWCNTs surface may be responsible for interactions

439

between the analytes and sorbent. Functional groups offering this kind of interactions like -

440

COOH, C=O and C-OH were present on the surface of MWCNTs as already shown

441

previously.48 Additionally, also van-der-Waals (vdW) forces between sorbent and sorbates

442

may be contributing due to the molecular structure of the sorbates, although typical ∆H-values 15 ACS Paragon Plus Environment

Page 17 of 27


443

are much smaller (vdW: -1 – -2 kJ mol-1)68 when exclusively these forces contribute to

444

sorption.

445

When the injected concentration of each analyte was reduced, the temperature effect

446

on sorption was more pronounced for all analytes with exception of oxazole. Values for ∆H

447

ranged at the lower injected concentration between -18.2 and -33.8 kJ mol-1 (all values are

448

shown in section SI.8 in the supporting information). Although the change in ∆H due to the

449

lower concentration was relatively small, it was higher than the measurement uncertainty. A

450

similar trend regarding ∆H with increasing concentration was also observed for sorption of

451

ciprofloxacin to SWCNTs50 or various heterocyclic compounds including pyridine to

452

reference soils.40 The more pronounced temperature dependency is related to a reduced

453

loading of the surface of MWCNTs with the consequence that energy distribution of sorption

454

sites is less spread compared to the higher injected concentration. At the high concentration,

455

energetically favorable and non-favorable sites are occupied, while at the lower concentration

456

a similar number of favorable sites and less non-favorable sites are occupied.34 This leads to

457

more negative values for ∆H. This was also concluded in case of sorption to reference soils.39

458

Sorption isotherms derived from measurements at various temperatures showed that Kf

459

decreased with increasing temperature while n only slightly increased (section SI.8 in the

460

supporting information). Decreasing Kf could be expected as sorption was generally reduced

461

with increasing temperature. Sorption linearity expressed by n was not influenced by

462

temperature. Similar observations regarding Kf and n were reported by Yan et al.60.

463 464

3.3 Influence of ionic strength

465

The influence of ionic strength on sorption of the selected compounds is shown in

466

figure 2. Determined Kd values reduced steadily but in total only slightly with increasing ionic

467

strength of the eluent for all compounds by 6 – 42% with exception of imidazole and pyridine.

468

For imidazole and pyridine, an overall tendency of reduced sorption with increasing ionic

469

strength was also observed, but Kd at 10 mM CaCl2 was even lower than those at 1 and

470

100 mM CaCl2. Imidazole and pyridine are the only compounds that are partially positively

471

charged (imidazole: ≈ 91%; pyridine: 15%) at the tested environmental conditions (pH 6),

472

while the sorbent was negatively charged at these conditions.48 Therefore, increased ionic

473

strength would constantly and more pronounced reduce sorption if electrostatic attraction

474

between charged sorbate molecules and the oppositely charged sorbent surface was 16 ACS Paragon Plus Environment


475

significantly contributing to overall sorption. Consequently, electrostatic attraction seems to

476

be less important in this case and is contributing at the most slightly to sorption of these two

477

compounds, which may be related to the competition of positively charged sorbate molecules

478

with Ca2+ present in solution. Ca2+ may interact preferably with the negatively charged

479

sorbent surface shielding the negative charge of the surface visible to positively charged

480

sorbate molecules so that electrostatic attraction of those is prevented. 120

1 mM 10 mM 100 mM

100

Kd [L/kg]

80 60 40 20

py rr ol e

in e py rid

py ra zo le

id az ol e

ox az ol e

481

im

fu ra n

0

482 483 484 485

Figure 2: Determined Kd values of the different analytes for three different concentrations of CaCl2 in the eluent (1, 10 and 100 mM). Error bars indicate the maximum error estimated from standard deviation of retention times of sorbate and tracer via error propagation (n=3).

486

Sorption isotherms derived from measurements showed that Kf and n remained almost

487

constant or only slightly changed (section SI.9 in the supporting information). This could be

488

expected as Kd values were also only slightly influenced. Kf of pyridine reduced stronger but

489

this was a consequence of a spectral interference with the increasing chloride concentration,

490

which is further discussed in the supporting information (section SI.9).

491

Reduced sorption to CNTs of chlorophenols with increasing concentration of sodium

492

chloride (NaCl) was reported in literature, while in the same study increasing concentration of

493

potassium chloride (KCl) showed no or only minimal reduction.31 The authors assigned the

494

difference to deviations in pi-cation interactions of Na+ and K+ between sorbents and sorbate.

495

Also in case of roxarsone30 increased ionic strength reduced sorption. Decreasing sorption

496

was assigned to changes in CNT-aggregates reducing the available surface area and cations

497

that interact with functional groups and therefore inhibit interactions between functional

498

groups and sorbates. Also for inorganic anions48 or cations69 sorption decreased with

499

increasing ionic strength of the eluent. Similar effects are expected in this study as increased 17 ACS Paragon Plus Environment

Page 18 of 27

Page 19 of 27


500

concentrations of ions in solution may interfere H-bonding interactions of polar heterocyclic

501

organic compounds with the sorbent as surface functional groups like COOH, C=O and C-OH

502

may be blocked by ions from the liquid matrix accumulating at these locations.

503 504

3.4 Influence of pH

505

Finally, the influence of the pH value on sorption is shown in figure 3. Determined Kd

506

values were almost constant for pyrazole, pyrrole, furan and oxazole, while Kd decreased

507

strongly with decreasing pH for imidazole (54%) and pyridine (79%). In case of pyridine,

508

decrease in Kd is sharp between pH 6 and 3. Speciation of the sorbates imidazole and pyridine

509

is changed due to the pH value, while for the other compounds speciation remained

510

unchanged (oxazole, pyrrole, furan) or was only changed very slightly (pyrazole). While at

511

pH 3 more than 99% of imidazole and pyridine are protonated and positively charged, only

512

15% in case of pyridine and 92% in case of imidazole are positively charged at pH 6. At pH 9

513

only 1% or less of both compounds is present with a positive charge. Besides the speciation of

514

the sorbates also the speciation of the sorbent is changed. At pH 3 the surface is positively

515

charged, while at pH 6 and 9 the surface is negatively charged.48 Consequently, sorption of

516

imidazole and pyridine may be reduced at pH 3 due to electrostatic repulsion occurring

517

between the positively charged sorbent surface and the equally charged sorbate molecules.

518

Contribution of electrostatic repulsion could be confirmed due to measurements at increased

519

ionic strength at pH 3. At 100 mM CaCl2 sorption of pyridine and imidazole increased

520

compared to 10 mM CaCl2 by 90% and 50%, respectively (figure S12 in the supporting

521

information). Increased ionic strength results in reduction of the electrostatic repulsion of

522

equal charges by shielding those with oppositely charged ions from the liquid matrix. 100

pH 3 pH 6 pH 9

Kd [L/kg]

80 60 40 20

e py rr ol

in e py rid

le py ra zo

le

le id az o

ox az o

523

im

fu r

an

0



524 525 526

Figure 3: Determined Kd values of the different analytes for three different pH values of the eluent (3, 6 and 9). Error bars indicate the maximum error estimated from standard deviation of retention times of sorbate and tracer via error propagation (n=3).

527

Sorption isotherms derived from measurements showed that Kf of pyridine decreased

528

strongly with decreasing pH as it was observed for Kd (section SI.10 in the supporting

529

information). In case of imidazole the change was less pronounced but still visible. For the

530

other compounds, no clear trend could be observed regarding Kf and sorption linearity

531

expressed by n as it was also found for Kd.

532

Decreasing sorption to CNTs as consequence of electrostatic repulsion was also

533

described in literature. In case of different anilines sorption decreased with decreasing pH due

534

to electrostatic repulsion.17 As an example, the pKa-value of 4-chloroaniline is 4.15 indicating

535

an increasing amount of the positively charged species with decreasing pH. Sorption reduced

536

strongly when pH decreased below 6 when the fraction of the positively charged species

537

started to increase. Similar observations were made for different phenol derivates10, 16, 17, 31, 57,

538

62, 70

539

sorbates were negatively charged, electrostatic repulsion occurred. No influence of pH on

540

sorption for compounds of which speciation is not changed with changing pH was also

541

reported in literature for naphthalene or 1,2-dichlorobenzene,10 phenanthrene,16 atrazine58, 60

542

and m-xylene, toluene or ethylbenzene.23 This fits well to the results presented in this study.

, when pH was increased above the pKa of the respective compound. As sorbents and

543

Overall, column chromatography was used successfully for the first time to study

544

sorption of heterocyclic organic compounds to MWCNTs and the influence of varying

545

environmental conditions on their sorption. A fortitude of the column method is the fast and

546

simple investigation of the influence of environmental conditions on sorption while

547

concomitantly reducing the sorbent demand strongly. Trends and observations made with

548

classical batch experiments regarding the influence of temperature, pH and ionic strength on

549

sorption could also be observed using the column method presented here so that this method

550

represents a promising alternative also for sorption studies regarding CNMs with the aim to

551

investigate the influence of environmental conditions on sorption. This method is applicable

552

for all UV-active organic compounds. Coupling to other detection techniques (e.g. mass

553

spectrometry (MS)) would also allow the analysis of compounds with insufficient UV

554

absorption. Depending on the type of compound, dilution of MWCNTs may have to be

555

increased or decreased to be able to get sufficient but also not too strong retention inside the

556

column. The applicability of the method for other types of carbon-based sorbents is currently 19 ACS Paragon Plus Environment

Page 20 of 27

Page 21 of 27


557

investigated to compare sorption properties of these materials under similar conditions using

558

column experiments.

559 560

Supporting Information

561

Additional information about details of chemicals used in this study, column characteristics,

562

equilibrium and isotherm concentrations, UV-calibrations, solute descriptors for ppLFER and

563

details on isotherms derived at different environmental conditions are added.

564 565 566

Acknowledgements

567

This project was supported by the German Research Foundation (SCHM 1372/10-2) and the

568

Professor Werdelmann Foundation.

569 570

References

571

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572

walled carbon nanotubes as sorptive material for solventless in-tube microextraction (ITEX2)

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multiwalled carbon nanotubes as solid phase extraction adsorbent. J. Hazard. Mater. 2009,

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adsorption of PAHs to carbon nanotubes over a six order of magnitude wide concentration

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substituted aromatics to carbon nanotubes. Environ. Sci. Technol. 2008, 42, (18), 6862-6868.

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Sorption of Heterocyclic Organic Compounds to Multiwalled Carbon

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