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Predicting the sorption of aromatic acids to non-carbonized and carbonized sorbents Gabriel Sigmund, Huichao Sun, Thilo Hofmann, and Melanie Kah Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b06033 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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Environmental Science & Technology

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Predicting the sorption of aromatic acids to non-

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carbonized and carbonized sorbents

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Gabriel Sigmund, Huichao Sun, Thilo Hofmann*, Melanie Kah*

5 6

University of Vienna

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Department of Environmental Geosciences

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Althanstrasse 14 UZA II, 1090 Vienna, Austria

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* Corresponding authors:

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Melanie Kah, e-mail: [email protected]; Tel.: (0043)-1-4277-53381.

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Thilo Hofmann, e-mail: [email protected]; Tel.: (0043)-1-4277-53320.

ACS Paragon Plus Environment


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Abstract Art

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Abstract

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Approaches based on the octanol-water partition coefficient are commonly used to describe

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sorption of neutral organic compounds in environmental systems, but they are not suitable for

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organic acids, which can dissociate to form anions. We here investigate the applicability of an

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alternative approach based on the pH-dependent distribution ratio (DOW) to describe sorption of

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aromatic acids to sorbents representing different degrees of carbonization.

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Sorption isotherms for four structurally similar acids (2,4-D, MCPA, 2,4-DB, and triclosan) was

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measured for 15 sorbents: fresh and carbonized wood shavings, pig manure, and sewage sludge;

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carbon nanotubes and activated carbon. Dissociation greatly affected the sorption of all acids.

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Sorption coefficients measured in the high pH range indicated that sorption of the anions ranged

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over several orders of magnitude and should not be neglected. Sorption trends for all sorbates

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and carbonized sorbents could be very well described by a single regression equation that

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included DOW of the sorbate and the specific surface area of the sorbent (R2 > 0.89).

29 30

Keywords: biochar, activated carbon, carbon nanotubes, sewage sludge, manure, DOW, surface

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area, carbonization



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1. Introduction

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The sorption of nonpolar organic compounds to naturally occurring sorbents such as soils or

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sediments is largely driven by the sorbent's total organic carbon content (TOC%) and takes place

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mainly through non-specific interactions, including dispersive and solvophobic interactions.

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Predictive approaches based on the sorption coefficient normalized to TOC% (KOC) and the

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octanol-water partition coefficient (KOW) are therefore extensively used in the context of

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chemical exposure assessment, remediation, and water treatment.

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Carbonized materials such as, for example, soot or black carbon, typically comprise only a small

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proportion of the soil or sediment matrix. They have, however, been shown to have a large

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influence on the sorption of neutral compounds, even at small concentrations. Sorption isotherm

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non-linearity typically increases with increasing degree of carbonization 1. Although KOW-based

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approaches are useful for describing sorption to carbonized sorbents, additional parameters (e.g.,

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sorbent specific surface area) may be necessary to account for sites available for adsorption 2,3.

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Many contaminants, as well as molecules with important biological functions (e.g., info-

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chemicals), are organic acids that dissociate to form anions within naturally occurring pH ranges.

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Dissociation affects sorption and KOW-based approaches, which were initially developed for

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neutral compounds, are thus inadequate for describing the sorption of organic acids 4. Previous

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investigations into sorption of organic acids have demonstrated that the distribution ratio DOW is

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a useful alternative to KOW to describe sorption to soils, as it takes into account the dissociation

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of a compound

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predict the sorption of organic acids to carbonized sorbents. As was previously noted for neutral

53

compounds, additional parameters may need to be considered in order to account for specific

54

adsorption mechanisms and provide a satisfactory description of sorption.

5,6

. It is not known, however, whether a DOW-based approach can be used to


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The sorption of dissociated organic acids (i.e., anions) to naturally occurring sorbents is typically

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weaker than that of the respective neutral species 7, and is driven by more complex processes that

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are not yet fully understood. For instance, electrostatic interactions are expected to occur and

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sorption may be strongly influenced by factors that play only a minor role in the sorption of

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neutral compounds, such as ionic strength and pH

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increase sorption through charge screening and cation bridging 4,9, or reduce sorption through the

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agglomeration of sorbent particles and consequent reduction in the surface area available for

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adsorption 10. The sorption of anions to negatively charged surfaces is expected to be weak due

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to electrostatic repulsion, but it may not necessarily be negligible due to the occurrence of cation

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bridging and/or the strong, charge-assisted H-bonding that has recently been demonstrated to

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occur between dissociated organic acids and O-containing functional groups on the surfaces of

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carbonized sorbents 11.

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The relative contribution of different sorption mechanisms is strongly dependent on the

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particular combination of sorbate and sorbent-suspension, and is always challenging to

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determine. For neutral aromatic sorbate it is nevertheless well accepted that non-specific

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(ab)sorption dominates for non-carbonized organic carbon, and that adsorption interactions

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significantly enhance interactions with carbonized sorbents (graphite-like structures allowing

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specific interactions such as π-π electron interactions) 12. A dual-mode model combining a linear

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partition model and a non-linear Langmuir model has been successfully applied to describe

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sorption of neutral sorbates to mixtures of non-carbonized and carbonized sorbents

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of dissociated organic acids may be significantly influenced by factors independent from degree

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of carbonization (e.g., ionic strength), and it is not known whether the distinction between non-

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carbonized and carbonized sorbents is relevant to the sorption of organic acids.

4,7,8

. An increase in ionic strength can either


13

. Sorption


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The overall aim of these investigations was to test the applicability of approaches previously

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developed to describe the sorption of neutral compounds, and to adapt the approaches for organic

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acids and carbonized sorbents, as necessary.

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With this objective in mind, the sorption of a series of organic acids was measured on a carefully

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selected series of sorbents covering a wide range of properties that would be expected to affect

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the sorption of organic acids. Sorption data were combined with extensive characterization of the

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sorbents in a comprehensive statistical analysis. The specific objectives were to (i) systematically

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investigate the influence that inherent sorbent properties, including the pH of suspension and the

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degree of carbonization, have on sorption, (ii) test whether the approaches used to describe

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sorption of neutral sorbates may be adapted to describe the sorption of organic acids, and (iii)

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identify key parameters that can be used for prediction purposes.

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The sorbents considered represent a range in degree of carbonization, and include three fresh

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feedstocks, a series of biochar (BC), carbon nanotubes (CNTs) and activated carbon (AC). The

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effect of pH was investigated using the intrinsic pH of the sorbents, without artificial

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modifications of pH, thereby avoiding changes of the sorbents composition and surface

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chemistry.

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Sorption batch experiments were performed with four structurally similar aromatic acids that

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cover a range of dissociation constants (pKa). The selected sorbates are contaminants of concern

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that are used in plant protection and personal care products: (2,4-dichlorophenoxy)acetic acid

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(2,4-D), (4-chloro-2-methylphenoxy)acetic acid (MCPA), 4-(2,4-dichlorophenoxy)butanoic acid

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(2,4-DB), and 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan). The results obtained are thus

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of direct environmental significance.


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2. Experimental

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2.1 Chemicals

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Standards of 2,4-D, MCPA, 2,4-DB, and triclosan with purities ≥ 97% were purchased from

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Sigma Aldrich (Germany). Selected sorbate properties are presented in Table 1, including several

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indices of hydrophobicity. By definition, the KOW concept only considers the partitioning of

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neutral species between octanol and water; when dealing with ionizable compounds, DOW

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provides a useful alternative to represent the change in hydrophobicity upon dissociation. When

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P (octanol-water partition coefficient for the neutral species) and Pi (octanol-water partition

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coefficient for the ionized species) are known, DOW can be calculated for a given pH 6:

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‫ܦ‬ைௐ =

௉ା௉௜∗ଵ଴೛ಹష೛಼ೌ ଵାଵ଴೛ಹష೛಼ೌ

.

(1)

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It should be noted, that Pi is not a constant value, as ion partitioning strongly depends on co-

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existing ions, which makes the experimental determination of Pi challenging 6. In the absence of

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experimental data, P and Pi can be estimated, for example using the SPARC online calculator

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(ARChem, USA, see Table 1 and Table S-2 in the supporting information). The SPARC

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calculator is based on linear solvation energy relationships to calculate P and Pi, but information

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regarding the underlying theoretical model is limited and was previously identified as one of the

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weaknesses of the software

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(i.e., 0.03 M) was used for Pi calculations in SPARC. Higher ionic strength are expected in the

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sorbent suspensions, especially for sorbents with high ash content, but there was no significant

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difference between DOW values calculated for 0.03 M and for 0.20 M, and the DOW values

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derived should thus well apply to the conditions of the batch experiments. The discrepancies

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between experimental and predicted values for P (PEXP and PPRE values in Table 1) illustrate the

14

. In this study, the ionic strength corresponding to 0.01 M CaCl2



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degree of uncertainty typically associated with such descriptors. Further development of both

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experimental and predictive approaches have been identified as urgent objectives for further

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research 5.

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HPLC grade acetonitrile, acetic acid, and CaCl2 were purchased from Merck (Germany). Ultra-

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filtrated water (Elix 5 Ultrapure Water Purification System) was used in the preparation of all

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solutions. Table 1: Selected sorbate properties 2,4 – D

MCPA

2,4 – DB

triclosan

94-75-7

94-74-6

94-82-6

3380-34-5

Log PEXP 15

2.81

3.25

3.53

4.76

pKa 15

2.73

3.13

4.95

7.90

Log PPRE 16

2.94

2.77

3.47

5.41

Log Pi 16

-0.77

-0.96

-0.62

-0.57

Polarizability 16

19.31

19.29

22.97

27.40

CAS

Structure

CAS: Chemical Abstracts Service Registry Number, PEXP: experimentally determined octanol-water partition coefficient, pKa: dissociation constant, PPRE, Pi: predicted octanolwater partition coefficient of the neutral species and the anionic species, respectively. 128

2.2 Sorbents

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A series of BCs were produced from softwood shavings (WS), pig manure (PM), and sewage

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sludge (SS) obtained from the surroundings of Vienna (Austria). The feedstock materials were

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dried at 50°C for 24 hours, and then sieved < 2 mm. The feedstocks were then placed in a closed


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ceramic crucible in a muffle furnace and heated under restricted O conditions, with an initial

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heating rate of 25°C/min up to 200°C, 350°C, or 500°C, maintained for 4 hours. Each BC is

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herein designated by its feedstock (e.g., WS for wood shavings) followed by the pyrolysis

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temperature (e.g., WS500 for WS pyrolyzed at 500°C). Three additional sorbents were

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considered: a commercially available BC produced from cellulose fibers and grain husks at

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600°C (refBC, Sonnenerde, Austria), an AC (Merck, Ref. no. 1.02518 Germany), and CNTs

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(Baytubes, ref. nr. C150HP, Germany). All sorbents except CNTs were crushed, sieved, and the

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fraction < 0.250 mm was used for experimentation. Hence a total of 15 sorbents comprising three

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feedstock materials, ten BCs, AC and CNTs, were investigated and compared.

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2.3 Sorbent characterization

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The ash content was determined by weighing samples before and after heating at 750°C for 6

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hours. The total C, H and N contents were determined using an elemental analyzer (Elementar

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VarioMacro), the O content calculated by mass balance: O% = 100 - (C + H + N + ash). The

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total organic carbon content (TOC%) was determined using a carbon analyzer equipped with a

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solid-state infrared detector (LECO RC-612). Metal contents (i.e., Ca, Mg, Na, K, Al, Fe, Mn,

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Si, Ti, P, Zn, Cu, Pb, Cr, Cd, and Ni contents) were measured using inductively coupled plasma

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optical emission spectrometry (Perkin-Elmer ICP-OES Optima 5300 DV) following microwave-

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assisted wet acid digestion with HNO3 and H2O2 17. Sorbent suspensions (2.5 g/L) in 0.01 M

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CaCl2 were used for the pH measurement, in order to emulate the conditions for the sorption

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experiments. The point of zero charge (PZC) was determined by an adaptation of the pH-drift

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method described by Moreno-Castilla et al. 2000

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Fourier transform infrared spectra (FTIR - Brunker Tensor 27) were measured with a resolution

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of 4 /cm and 128 scans. BET specific surface areas and pore size distributions were derived from

18

(see ST1 in the supporting information).



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N2 adsorption and desorption isotherms following outgassing overnight at 105°C. The N2 BET

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specific surface area was calculated on the basis of 6 points in the relative pressure region P/P0 =

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0.066 - 0.298 (Quantachrome Nova 2000 analyzer).

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2.4 Sorption experiments

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The standard batch equilibrium method

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sorbents. Suspensions containing 1 - 50 mg sorbents in 20 - 80 mL 0.01 M CaCl2 were pre-

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equilibrated overnight and spiked with one of the selected sorbates at concentrations 0.5 - 30

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mg/L. Stock solutions were prepared in acetonitrile but added volumes were small enough (
80%) for different organic acids

410

and across a wide range of sorbents can be well described by an adaptation of the approaches

411

previously developed for neutral compounds, using log DOW instead of log KOW in order to take

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into account the effect that dissociation has on sorption. Trends in sorption for a wide range of

, R² = 0.48 and differences between measured and predicted values exceeded five orders of

25,26

25

and triclosan

26

. A plot of measured

. Discrepancies could be partly, but not fully, explained by


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carbonized sorbents covering a broad range in pH (4.9 - 11.3) could be well described using a

414

single regression equation with just two parameters: log DOW and the specific surface area.

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Understanding sorption behavior is key to assess the environmental fate of organic acids, and to

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the development of new sorbents; further research will therefore be required to evaluate the

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applicability of the presented approach (i) to other organic acids (e.g., aliphatic or aromatic), (ii)

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to other carbonized sorbents, and (iii) under more complex water chemistries (e.g., different

419

types of ions and natural organic matter).

420

Acknowledgements

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The reference biochar was kindly provided by Sonnenerde Gerald Dunst Kulturerden GmbH and

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the CNTs by Baytubes. Access to the SPARC online calculator was provided by ARChem, USA.

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We are also grateful to Mikhail Borisover (Volcani Center, Israel) for constructive discussions.

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Supporting Information Available

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Parameters on sorbent characterization, predicted octanol-water partition coefficients and

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isotherm fits with the Freundlich model. Measured isotherms, FTIR spectra, effect of ionic

427

strength and comparison of different models to predict sorption. This information is available

428

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

429

Author information

430

Corresponding authors:

431

Melanie Kah, e-mail: [email protected]; Tel.: (0043)-1-4277-53381.

432

Thilo Hofmann, e-mail: [email protected]; Tel.: (0043)-1-4277-53320.

433

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