New Insight into Adsorption Mechanism of Ionizable Compounds on

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New Insight into Adsorption Mechanism of Ionizable Compounds on Carbon Nanotubes XIAOYUN LI, Joseph J. Pignatello, Yiquan Wang, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es4011042 • Publication Date (Web): 25 Jun 2013 Downloaded from http://pubs.acs.org on July 2, 2013

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Environmental Science & Technology

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New Insight into Adsorption Mechanism of Ionizable Compounds on

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Carbon Nanotubes

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Xiaoyun Li, †, ‡ Joseph J, Pignatello, §,* Yiquan Wang, † and Baoshan Xing ‡,*

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†

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‡

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§

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College of Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA Department of Environmental Sciences, Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504-1106, USA

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*Corresponding authors

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Dr. Baoshan Xing

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Tel.: +1 413 545 5212

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Fax: +1 413 545 3958

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Email address: [email protected]

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Dr. Joseph J, Pignatello

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Tel.: 1 203 974 8518

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Fax: 1 203 974 8502

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Email address: [email protected]

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ACS Paragon Plus Environment


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TOC Art: ⇌

(a)

⇌

(b)

PA ⁄ ⇌ / ⋯ CNTs (c)

Site type I (-) CAHB

Site type II

(-) CAHB

C

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O

H

Schematic diagram for sorption of ionizable organic compounds on CNTs

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ABSTRACT: We studied the pH-dependent adsorption of benzoic acid (BA), phthalic

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acid (PA) and 2,6-dichloro-4-nitrophenol (DCNP) by hydroxylated, carboxylated, and

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graphitized carbon nanotubes (CNTs). Adsorption is contributed by formation of a

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charge-assisted H-bond (-)CAHB between a carboxyl group on the solute and a

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phenolate or carboxylate group on the surface having a comparable pKa. This

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exceptionally strong H-bond is depicted as (RCO2⋅⋅⋅H⋅⋅⋅O-CNTs)−. Over a limited pH

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range the free anion undergoes proton exchange with water concurrent with adsorption,

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releasing hydroxide ion in a stoichiometry of up to 1.0 for BA, 1.7 for PA, and 0.5 for

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DCNP. Little hydroxide is released upon adsorption by the O-sparse graphitized CNTs.

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Anion exchange and ligand exchange reactions as a source of hydroxide release were

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ruled out. The higher stoichiometry for PA indicates involvement of both carboxyl

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groups with adjacent surface oxyl groups. The lower stoichiometry for DCNP is

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consistent with steric inhibition of H-bonding by the ortho chlorines. Formation of

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(-)CAHB helps overcome the unfavorable free energy of proton exchange with water,

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and results in an upward shift in the pKa in the adsorbed state compared to the

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dissolved state from 0.9 to 3.1 units. The proposed mechanism is further supported by

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additional structure-activity considerations. The findings provide new understanding of

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the interactions between ionizable organic compounds and carbonaceous surfaces,

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which has implications for non-covalent derivatization of CNTs, fate of ionizable

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pollutants, and associations of natural organic matter with CNTs and other

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carbonaceous materials in the environment.

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INTRODUCTION

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Many pesticides, antibiotics and endocrine disrupting chemicals are ionizable in the

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environmentally relevant pHs. Therefore, it is essential to more fully understand the

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forces that drive adsorption of ionizable organic compounds (IOCs) from water to

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carbonaceous substances including carbon nanotubes (CNTs), activated carbon (AC)

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and natural and synthetic charcoals. These forces are important in the context of

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environmental fate of IOCs, use of carbonaceous materials as adsorbents in sensing

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and remediation of IOCs, synthesis and use of derivatized CNTs in materials and

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technological applications, and interactions of carbonaceous substances released to the

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environment with natural organic matter, which has abundant ionizable functional

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groups.

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This study focuses on adsorption of weak acids such as carboxylic acids and

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phenols to CNTs. Adsorption of weak acids by carbonaceous adsorbents is generally

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pH-dependent due to ionization of the acid in solution and the variable charge on the

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surface governed by ionization of dissociable groups, typically phenolic and carboxylic

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acid groups. 1-5 From the existing literature it is clear that adsorption of weak acids is

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controlled, at a minimum, by a combination of hydrophobic effects, van der Waals

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forces, and coulombic attraction or repulsion, depending on pH. Combining

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electrochemical, diffuse-double-layer, and adsorption models, Müller et al.6, 7 modeled

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adsorption of benzoic acid and 4-nitrophenol on AC by assuming that affinity for the

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surface of the molecular and ionized forms are identical except for charge

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attraction/repulsion acting on the ionized form. Hydrogen (H-) bonding has also been 4


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implicated in the adsorption of weak acids.

However, until now there is no

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consensus on the role of H-bonding in adsorption of weak acids onto O-enriched

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carbon materials. The influence of surface oxygen on adsorption to AC and charcoal is

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difficult to interpret because of the effects that oppose H-bonding, such as crowding

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due to water clustering around polar surface groups10-12 and possibly electron

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withdrawing effects on the polyaromatic basil surface that alter van der Waals

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interactions.13 When water is abundant, solute-water and water-surface H-bonds exist

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in strong competition with solute-surface H-bonds. Only solute-surface H-bonds that

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are much stronger than solute-water and water-surface H-bonds can contribute

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substantially to overall adsorption.

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Recently, it was suggested that adsorption of weak acids to synthesized biomass

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charcoal (biochar) is enhanced by formation of exceptionally strong H-bonds at the

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surface. Ni et al.,14 working with cinnamic and coumaric acids, and Teixido et al.15,

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working with the sulfonamide antibiotic sulfamethazine, postulated formation of a

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negative charge-assisted H-bond, (-)CAHB between the carboxyl or sulfonamide group

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of the adsorbate and a surface carboxyl or hydroxyl group, denoted (X···H···Y)−. The

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(-)CAHB, is a subset of the Low Barrier Hydrogen Bond (LBHB), which is

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distinguished by a nearly symmetric double-well potential with a low barrier

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separating the wells.16-18 The driving force of the LBHBs increases with decreasing

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∆pKa = pKa,DH – pKa,AH+ ( C-CNT > G-CNT,

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trending in the same way as the N2 B.E.T. surface area (228, 164 and 117 m2/g,

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respectively). With two strong π-electron withdrawing substituents (-CO2H), PA may

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interact through π-π electron donor-acceptor interaction with the presumed

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π-electron-rich CNT surface; however, the limited data here provide no support for or

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against it.

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< BA (1.85)

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< DCNP (2.94)

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. This result

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Adsorption as a function of solution pH.

With increasing pH, the surface

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negative charge increases due to dissociation of –OH and –CO2H groups.24 The point

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of zero charge determined by zeta potential measurement occurs around pH 4 (Figure

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S6). Meanwhile, dissolved acid is converted to anionic forms that are more hydrophilic.

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The ratio Kow(acid)/Kow(anion) is 102.97 for BA−, 101.46 for PA−, 105.36 for PA2− and 10


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101.39 for DCNP−, according to SPARC calculator (http://archemcalc.com/sparc/test/;

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accessed January 5, 2013). The increase in surface charge and solute dissociation in

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solution both act to reduce adsorption. Experimentally (Figure 1), the effect of

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increasing pH varies with the acid: adsorption generally decreases with pH for PA and

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DCNP, whereas BA shows a maximum at pH 4.

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Noteworthy from Figure 1 are the following results: i) Significant adsorption occurs

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at pH 7 where the acids are almost completely dissociated in solution. This means the

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anion has appreciable affinity for the surface, even when, as is the case for BA and PA,

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the calculated log Kow is less than zero—i.e., when the anion favors the aqueous phase

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over a mixed polar-nonpolar phase of octanol. ii) Between pH 4 and pH 7, adsorption

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of BA and PA falls more gradually for the oxygen-rich H-CNTs and C-CNTs than for

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the oxygen-poor G-CNTs. Adsorption of DCNP to G-CNTs is almost pH-independent.

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iii) Adsorption of BA (pKa = 4.17) rises between pH 2 and pH 4, despite 48%

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conversion to its anion over that pH range. Such behavior has been interpreted to

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reflect anion exchange in the pH region where residual positive charge still exists on

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the surface while the acid begins to ionize6, 7— roughly pH 3-4 in the present cases.

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However, the same trend is not observed for PA (pKa1 = 2.98, pKa2 = 5.28) nor DCNP

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(pKa = 3.55), despite being more ionized at pH 4 (88% and 73%, respectively) than BA

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and, therefore, more prone to undergoing anion-exchange. iv) Anion exchange and

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metal bridging interactions do not appear to play a role in adsorption of the acid anions.

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Ionic strength should suppress anion exchange by charge screening,

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multivalent cations may enhance anion adsorption also by bridging with negatively 11


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while


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charged surface groups. 31-33 However, at pH 7 neither ionic strength (up to 0.3 M) nor

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the presence of Ca2+ (up to 0.1 M) impacted adsorption of BA, PA or DCNP on

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H-CNTs (Figure S7). Zhang et al.

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NaCl or CaCl2 (0.001–0.1 M) had negligible effect on the adsorption of

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2-phenylphenol by CNTs. Ni et al.14 reported that neither Ca2+ nor Mg2+ up to 0.1 M

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(0.3 ionic strength) in pH 7 buffer effected cinnamate and coumarate adsorption to

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charcoals. Méndez-Díaz et al.

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influence on PA adsorption to activated carbons at pH ~ 4.

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lower at pH 4 than pH 2, despite nearly complete dissociation to PA− from pH 2 to pH

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4. Adsorption of PA is only a factor of ~3 times smaller at pH 7 than at pH 2 despite its

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full conversion to the dianion, PA2− in solution. vi) At pH 7, where PA2− and BA−

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predominate in solution, PA2− adsorbs more strongly than BA− even though PA2− is

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more hydrophilic.

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also found that increasing ionic strength with

indicated that NaCl (0~0.01 M) had no major v) Adsorption of PA is C-CNTs >

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G-CNTs, the same as the order in oxygen content. It is noteworthy that on H-CNTs and

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C-CNTs the stoichiometry for BA declines from 1.0 to 0.1 (mol·mol-1) with loading,

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while the stoichiometry for PA declines from 1.7 to 0.1. The higher initial ratio for PA

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is explained by assuming simultaneous engagement of both -COOH groups of PA in

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(-)CAHB with adjacent surface oxyl groups (equations 7-10), the second one limited 13



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by the less-than-full availability of proximal groups (see Figure 3). The stoichiometry

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for DCNP achieves a maximum of only ~0.45. The experiments were repeated at a

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lower initial pH (Figure S10) and verify the results in Figure 2. As will be discussed

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later, reaction 2 for DCNP is likely inhibited by steric effects and intra-molecular

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H-bonding. The stoichiometry in all cases decreases with rising pH due to reduction of

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proton concentration and development of charge on the surface.14 Comparing

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adsorption on the O-rich CNTs with adsorption on G-CNTs lends further support for

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the (-)CAHB. Like H-CNTs and C-CNTs, adsorption of all acids on G-CNTs is

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accompanied by hydroxide release, but the OH−/adsorbed acid stoichiometry is much

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smaller, never exceeding 0.08. The much smaller OH−/adsorbed acid stoichiometry

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( PA, which trends with hydrophobicity (Kow)

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of the acid or its corresponding anion. However, on the relatively oxygen-rich surfaces

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of H-CNT and C-CNT, the order in adsorption shifts from BA > PA at pH 2 and pH 4,

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where hydrophobic effects are relatively more important, to PA > BA at pH 7, where

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hydrophobic effects are relatively less important. The switch in order may be due to the

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ability of PA to engage in dual (-)CAHBs with neighboring oxyl groups on the

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oxygen-rich surfaces. The fact that BA and PA adsorb significantly at pH 7, even

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though in solution they are completely dissociated and hydrophilic (log Kow < 0), is

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consistent with involvement of a special interaction like the (-)CAHB. At pH 7,

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adsorption for each acid follows the same order as the acidic group content of the

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CNTs (Table S2): G-CNTs < C-CNTs < H-CNTs. For DCNP the hydroxide release

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stoichiometry is greater for C-CNTs than H-CNTs. This is in keeping with the

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expectation that surface carboxylate groups will be closer in pKa to DCNP than will

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surface hydroxyl groups, resulting in a stronger H-bond. Lastly, the order in the

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apparent pKa shift (i.e., pKa,ads – pKa) for BA and PA is C-CNTs > H-CNTs. This 19



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indicates a stronger or more abundant (-)CAHB interaction of the acid on C-CNTs than

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on H-CNTs, which makes intuitive sense for the same reason.

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Adsorption of DNCP. The hydroxide release stoichiometry for DCNP (up to ~0.5

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for C-CNTs, 0.25 for H-CNTs, and 0.02 for G-CNTs) is generally less than that of the

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other monobasic acid, BA (Figure 2 and S10). A likely explanation is steric hindrance

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to H-bond formation with the surface site by the ortho chlorines. Using DFT

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computations, Han et al.41 found that the ROH···OH2 or ROH···NH3 H-bond was

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significantly weakened and more bent for ortho dichloro- or ortho dibromo-substituted

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phenols compared to other halogenated phenols due to steric constraints. Nielsen42

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calculated solvent-independent homoconjugation constants Kconj for phenols in

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solution [ArOH + ArO− (ArOHOAr)−]. A clear steric effect was found for phenols

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ortho-substituted with methyl: phenol (6.8 × 106), 3-methylphenol (5.2 × 106),

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4-methylphenol (5.6×106), 2-methylphenol (2.2×106), and 2,6-dimethylphenol (6.4×

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105). Based on Bondi radii, the van der Waals volumes of -Cl and -CH3 groups (22.45

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Å3 and 24.54 Å3, respectively43) are comparable. Hence, it is reasonable to expect that

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the (-)CAHB between DCNP and an oxyl group on CNTs surface is sterically hindered

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by the bulky Cl atoms at the 2- and 6-positions.

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In the present work, we have demonstrated the important role of (-) CAHB in IOCs

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adsorption onto O-rich CNTs, particularly for the IOCs without steric hindrance. From

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the pH and concentration range of our experiments, adsorption accompanied by a

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(-)CAHB appears to contribute from ∼10% to ∼ 100% of total adsorption. Meanwhile,

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the apparent acidity constant of the adsorbed acid is shifted to a more positive value 20


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relative to its pKa in solution. These findings provide new insight into the interactions

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between weak acids and carbonaceous surfaces that has important implications for

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non-covalent derivatization of CNTs in technological applications, the fate of

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anthropogenic pollutants, and interactions of natural organic matter with carbonaceous

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materials in the environment. In addition, because most of pharmaceuticals and

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pesticides are IOCs and natural organic matter are rich in oxyl groups, formation of (-)

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CAHB could be important for the environmental fate and bioavailability of these weak

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acids. Also, soil and natural organic matter contain abundant IOCs, they may undergo

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(-) CAHB with soil organic matter, thereby affecting their allelopathic activity and

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mineral dissolution.

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Acknowledgments

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This work was supported in part by the National Science Foundation (CBET 1235459),

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USDA AFRI (MAS 00978), and BARD (IS-4353-10).

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Supporting Information Available.

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Selected characteristics of sorbates and CNTs; sorption model parameters; sorption

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equilibrium time; species distribution of sorbates as a function of solution pH; sorption

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isotherms of a given sobate on three CNTs or three IOCs on a given sorbent at pH 2.0;

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sorption isotherms of BA and PA on a given adsorbent as a function of pH; pHzpc

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results of CNTs; effects of metal ion concentration; titration curves of the CNTs;

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consumption of H+ as a function of pH for the CNTs; stoichiometry of OHˉrelease

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and pH records as a function of sorption of DCNP by three CNTs. This information is

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available free of charge via the Internet at http://pubs.acs.org/. 21



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Vinu, A.; Hossain, K. Z.; Kumar, G. S.; Ariga, K. Adsorption of L-histidine over mesoporous carbon molecular sieves. Carbon 2006, 44 (3), 530-536. Cho, H. H.; Huang, H. O.; Schwab, K. Effects of solution chemistry on the adsorption of ibuprofen and triclosan onto carbon nanotubes. Langmuir 2011, 27 (21), 12960-12967. Zhang, S. J.; Shao, T.; Bekaroglu, S. S. K.; Karanfil, T. Adsorption of synthetic organic chemicals by carbon nanotubes: Effects of background solution chemistry. Water Reseach 2010, 44, 2067-2074. Chernyshova, I. V.; Ponnurangam, S.; Somasundaran, P. Adsorption of fatty acids on iron (hydr) oxides from aqueous solution. Langmuir 2011, 27, 10007-10018. Nore´n, K.; Persson, P. Adsorption of monocarboxylates at the water/goethite interface: The importance of hydrogen bonding. Geochim. Cosmochim. Acta 2007, 71, 5717-5730. Thomas, J. E.; Kelley, M. J. Interaction of mineral surfaces with simple organic molecules by diffuse reflectance IR spectroscopy (DRIFT). J. Colloid Interface Sci. 2008, 322, 516-526. Maity, N.; Payne, G.F. Adsorption from aqueous solutions based on a combination of hydrogen bonding and hydrophobic interactions." Langmuir 1991, 7: 1247-1254. Lide, D. R., Handbook of Chemistry and Physics. 85th ed.; CRC Press: Boca Raton, FL, 2004. Tolstoy, P. M.; Schah-Mohammedi, P.; Smirnov, S. N.; Golubev, N. S.; Denisov, G. S.; Limbach, H. H., Characterization of fluxional hydrogen-bonded complexes of acetic acid and acetate by NMR: Geometries and isotope and solvent effects. J. Am. Chem. Soc. 2004, 126 (17), 5621-5634. Han, J.; Deming, R. L.; Tao, F.M. Theoretical Study of Hydrogen-Bonded Complexes of Chlorophenols with Water or Ammonia: Correlations and Predictions of pKa Values. J. Phys. Chem. A 2005, 109, 1159-1167. Nielsen, M. F. A model for quantitative evaluation of equilibrium constants for hydrogen bonding in polar aprotic solvents based on incomplete data sets related to different solvents. Acta Chem. Scand. 1992, 46, 533-548. Zhao, Y. H.; Abraham, M. H.; Zissimos, A. M. Fast Calculation of van der Waals Volume as a Sum of Atomic and Bond Contributions and Its Application to Drug Compounds. J. Org. Chem. 2003, 68 (19), 7368-7373.

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Page 24 of 28

Page 25 of 28


Qe (mg/kg)

pH 2.0 pH 4.0 pH 7.0

BA

20000

20000

10000

10000

0 100

200

BA

pH 2.0 pH 4.0 pH 7.0

10000

B. C-CNTs 0

300

100

200

pH 2.0 pH 4.0 pH 7.0

pH 2.0 pH 4.0 pH 7.0

15000

15000 10000

5000

5000

5000

D. H-CNTs 100

200

0

E. C-CNTs

300

40000

0

100

200

200

300

PA

0

F. G-CNTs 0

300

100

200

300

30000

30000

DCNP Q (mg/kg) e

100

pH 2.0 pH 4.0 pH 7.0

PA

10000

0

DCNP

DCNP

30000

20000

20000

20000 pH 2.0 pH 4.0 pH 7.0

10000 0

G. H-CNTs 0

20 Ce (mg/L)

558 559 560

0

10000

0

C. C-CNTs

20000

PA

15000

0

300

20000

20000

BA

pH 2.0 pH 4.0 pH 7.0

20000

0

A. H-CNTs 0

Qe (mg/kg)

30000

30000

30000

40

60

10000

pH 2.0 pH 4.0 pH 7.0

0

H. C-CNTs 0

20

40

60

Ce (mg/L)

10000 pH 2.0 pH 4.0 pH 7.0

0

I. G-CNTs 0

20

40 Ce (mg/L)

Figure 1. Sorption isotherms of BA, PA, and DCNP onto three CNTs at different pHs. Qe (mg/kg) is the solid-phase concentration and Ce (mg/L) is the equilibrium aqueous concentration.

25


60


Page 26 of 28

561

5.5

pKa1, ads

0.18

7.0

0.12

6.8

0.06

6.5

0.0 5.0 0.000 0.003 0.006 0.009 0.012 0.015 0.018

0.0 5.4 0.000 0.003 0.006 0.009 0.012 0.015 0.018

0.00 0.000

1.0

2.0

0.50

0.4

7.5

0.2

7.0

0.00 6.5 0.000 0.003 0.006 0.009 0.012 0.015 0.018

Moles BA sorbed (moles/kg)

pH

7.1

7.1 0.04 6.9 0.02

0.00 0.000

6.7

0.002

0.004

0.006

6.5 0.008

Moles PA sorbed (moles/kg)

Moles of H+ released /moles DCNP sorbed

0.02

7.3

0.06

7.1 0.30 0.20 6.9 0.10

0.007

0.014

0.021


0.028

6.7 0.035

7.2

G-CNTs

0.006 7.1 0.004 7.0 0.002

0.000 0.000

0.007

0.014

0.021

0.028

6.9 0.035

Moles of DCNP sorbed (moles/kg)

Figure 2. Stoichiometry of hydroxide release (closed diamonds) and pH (open circles, dotted line) as a function of sorption of BA, PA and DCNP by three CNTs. The cross-hatch and dotted line represent the controls (BA, PA or DCNP and background solution only) corresponding to the samples. The pH records of the controls manifest that adding sorbates does not contribute appreciably to the increase in [OH–] in solution in the absence of CNTs. The pKa,ads (red and blue dotted line) represents the raising pKa of the acid on the surface. The pKa1,ads and pKa2,ads represent the raising pKa1 and pKa2 of PA on the surface, respectively.

26

6.3 0.035

0.40

562 563 564 565 566 567 568 569 570

0.028

7.3

0.008

7.5

G-CNTs

0.021

C-CNTs

0.00 0.000

pH

7.3

6.7

7.0

0.08

7.5

6.9

7.4

0.0 6.6 0.000 0.003 0.006 0.009 0.012 0.015 0.018

G-CNTs

0.01

1.0

0.5

Moles OH- released / moles PA sorbed

0.03

7.8

0.014

pH

8.0

Moles of released / moles DCNP sorbed

0.6

8.2

1.5

pH

8.5

8.6

C-CNTs

H+

0.8

Moles

released / moles PA sorbed

9.0

OH-

C-CNTs

0.007

pH

6.4

5.9

0.5

0.0 6.5 0.000 0.003 0.006 0.009 0.012 0.015 0.018

Moles OH- released / moles BA sorbed

pKa2, ads

1.0

7.3

H-CNTs

pH

pKa,ads

Moles

0.2

6.0

OH-

0.4

6.9

Moles of released / moles DCNP sorped

6.5

1.5

pH

0.6

released / moles PA sorbed

7.0

pH

0.8

0.24

7.4

H-CNTs

H+

2.0

7.5

H-CNTs

pH



1.0

Page 27 of 28


571 O HO

⇌

HO

C

C

⇌

O

⁄

⇌

/

(a) (b)

⋯ CNTs (c)

pKa1 =2.98

pKa2 = 5.27

Site type I (-) CAHB

(-) CAHB

Site type II

Atom C

572 573 574 575 576 577

Atom O

Atom H

Figure 3. Theoretical illustration of the structure of PA molecule or anion adsorbed on the functionalized CNTs surface (red, oxygen; white, hydrogen; gray, carbon). The proton exchange step is described by reaction (a) and (b), and adsorption step is – indicated by reaction (c). Site type I and II represent adsorption of PA0 and PA on the CNTs, respectively.

27



Page 28 of 28

578

900

Kd (L/kg)

BA

600 pKa pKa, abs-H

300

pKa, ads-C H-CNTs C-CNTs

0 1

2

3

4

5

6

7

8

9

10

pH

579 580 581 582

Figure 4. Adsorption distribution coefficient (Kd) of BA on H-CNTs and C-CNTs as a function of pH. The experiment was conducted at a constant BA initial concentration of Ce / Sw = 0.01. pKa is the acidity coefficient of BA in solution, and pKa, ads-H and pKa, ads-C are the acidity coefficient of BA adsorbed on H-CNTs and C-CNTs surface.

28


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