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Environmental Pollution 158 (2010) 3642e3649

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Environmental Pollution

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The pH-dependent adsorption of tributyltin to charcoals and soot

Liping Fang*, Ole K. Borggaard, Helle Marcussen, Peter E. Holm, Hans Christian Bruun HansenDepartment of Basic Sciences and Environment, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark

Tributyltin adsorption to black carbon increases at increasing pH butonly adsorbs hydrophobically.

charcoal exhibits electrostatic and hydrophobic adsorption, whereas soot

a r t i c l e i n f o

Article history:Received 9 February 2010Received in revised form23 July 2010Accepted 3 August 2010

Keywords:Langmuir modelHydrophobic adsorptionElectrostatic bondingOrganotinBlack carbon

* Corresponding author.E-mail address: fang@life.ku.dk (L. Fang).

0269-7491/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.envpol.2010.08.003

a b s t r a c t

Widespread use of tributyltin (TBT) poses a serious environmental problem. Adsorption by black carbon(BC) may strongly affect its behavior. The adsorption of TBT to well characterized soot and two charcoalswith specific surface area in the range of 62e111 m2 g�1 have been investigated with main focus on pHeffects. The charcoals but not soot possess acidic functional groups. TBT adsorption reaches maximum atpH 6e7 for charcoals, and at pH> 6 for soot. Soot has between 1.5 and 15 times higher adsorptiondensity (0.09e1.77 mmolm�2) than charcoals, but charcoals show up to 17 times higher sorption affin-ities than soot. TBT adsorption is successfully described by a new pH-dependent dual Langmuir modelconsidering electrostatic and hydrophobic adsorption, and pH effects on TBT speciation and BC surfacecharge. It is inferred that strong sorption of the TBTOH species to BC may affect TBT toxicity.

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1. Introduction

Tributyltin (TBT) is a unique organotin biocide that has beenwidely used for different purposes starting in the 1950s, e.g. asantifouling paints on ships, wood preservative and agriculturefungicides (e.g., Huggett et al., 1992; Hoch, 2001). However, TBT hasbeen found to exhibit acute and chronic toxicity towards aquaticorganisms and humans (Blaber, 1970; Boyer, 1988; Fent, 1996; Lauet al., 2007; Nicklin and Robson, 1988). For example, for the verysensitive mollusc Nucella lapillus (dog whelk), the no observedeffect concentration (NOEC) is only 2 ng L�1 (ECHA, 2008). Due topublic concern about TBT toxicity, its use in shipyards wascompletely banned worldwide in 2008 (Brandli et al., 2009; IMO,2003). Accordingly, decreasing TBT concentrations have beenreported at various places, e.g. declining from 81 to 7 mmol kg�1 insediments, and from 1.3 to 0.2 nmol L�1 in seawater (Choi et al.,2009). However, high concentrations of TBT can still be found incertain aquatic organisms (0.03e3.2 mmol kg�1) (Sousa et al., 2009),in seawater (w5 nmol L�1) (Berto et al., 2007) and in sediments(w74 mmol kg�1) (Burton et al., 2005). The relatively high concen-trations of TBT in organisms and sediments compared to water,reflect the high bioconcentration and strong adsorption of TBT(Choi et al., 2009; Burton et al., 2004), which may retard

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degradation of adsorbed compared to dissolved TBT (Craiq, 2002;Hoch, 2001).

Black carbon (BC) is ubiquitous and originates from incompletecombustion of biomass and fossil fuels (Goldberg, 1985; Kuhlbuschand Crutzen, 1995; Novakov, 1984; Penner et al., 1993), which hasbeen interpreted to consist of two continua formed in differentways: (i) soot formed by condensation of volatiles at hightemperature, and (ii) solid residues of char (charcoal) formed at lowtemperature combustion (Elmquist et al., 2006; Hedges et al., 2000;Masiello, 2004). Themode of formation affects the physicochemicalcharacteristics of BC such as size, structure, element ratios andfunctionalities (Baldock and Smernik, 2002; Masiello, 2004;Preston and Schmidt, 2006). Owing to their composition, BCexhibits stronger adsorption affinity for hydrophobic organiccontaminants (HOCs) than natural organic matter (NOM), espe-cially at low aqueous concentrations (Bucheli and Gustafsson,2000; Cornelissen et al., 2004; Lohmann et al., 2005; Nguyenet al., 2004), indicating the environmental importance of BC.Accordingly, adsorption to BC contributes significantly to the totaladsorption of HOCs in soils and sediments (Bucheli and Gustafsson,2003; Gustafsson et al., 1997). Furthermore, oxygen-rich charcoal-BC with abundant carboxylic and phenolic groups resembling NOMadsorbs heavy metal cations (Chen et al., 2007), demonstrating thatadsorption of cationic pollutant species should be accounted forwhen quantifying adsorption to charcoals.

Studies on TBT adsorption to soils (Huang and Matzner, 2004),sediments (Burton et al., 2004; Dowson et al., 1993), sand (Behra et al.,

L. Fang et al. / Environmental Pollution 158 (2010) 3642e3649 3643

2003; Bueno et al., 1998, 2001), metal oxides (Weidenhaupt et al.,1997), clay minerals (Behra et al., 2003; Hoch and Schwesig, 2004;Weidenhaupt et al., 1997), humic acid (Arnold et al., 1998) and algae(StLouis et al.,1997) demonstrate that TBTadsorption is pHdependent.Although a study of TBT adsorption by a non-characterized BC hasbeen performed (Brandli et al., 2009), it does not reveal the mecha-nism behind the adsorption process. Besides, information about theinfluence of pH on TBT adsorption to BC is still not available.

The aim of this investigation was to quantify and model theinfluence of TBT concentration and pH on the adsorption of TBT tothree different BC samples including beech charcoal, wheat char-coal and soot. To avoid confusing, the terminology “TBT” is used fordescribing the total tributyltin, including ionic (TBTþ) and neutral(TBTOH) species in this paper.

2. The pH-dependent dual Langmuir model

Based on the results of preliminary investigations, a new simplemodel addressing different aspects of TBTeBC interactions wasdeveloped. The model is shortly described below; more details maybe found in the Supplementary data (S1). TBT is a weak acid withpKa¼ 6.3 (Arnold et al., 1997) composed of a cationic form (TBTþ,(C4H9)3Snþ) in equilibrium with a neutral form (TBTOH,(C4H9)3SnOH) according to Eq. (1):

TBTþ þ H2O$TBTOHþHþ (1)

Hydrophobic (van der Waals) forces are considered to deter-mine TBTOH adsorption by uncharged BC sites, whereas electro-static forces govern adsorption of TBTþ by negatively chargedsurface sites. Both adsorption reactions can be described bya Langmuir equation. The charcoals, but not the soot, possess pH-dependent negative surface sites due to dissociation of carboxylicand phenolic groups (Buffle, 1988). Consequently, modeling of TBTadsorption by BC includes Langmuir adsorption of the two TBTspecies and accounts for the influence of pH on the distributionbetween TBTþ and TBTOH as well as the pH dependency of thesurface charge of the BC. This is the case for the new pH-dependentdual Langmuir (pH-DL) model (Eq. (2)):

½TBT�BC ¼aTBTOH$GhmaxK

hL ½TBT�aq

1þ�KhLaTBTOH½TBT�aq

�

þ aTBTþ$achargedGemaxK

eL½TBT�aq

1þ�KeLaTBT½TBT�aq

� ð2Þ

where [TBT]BC represents the adsorbed amount of TBT (in units ofmmolm�2), [TBT]aq (mmol L�1) is the total aqueous concentration ofTBT. Gmax

h and Gmaxe (mmolm�2) are maxima of TBTOH and TBTþ

adsorption density, respectively, while KLh and KL

e (L mmol�1) areLangmuir binding coefficients for TBTOH and TBTþ, respectively.acharged is the percentage of the negatively charged surfaces on BCobtained by means of titration, aTBTOH and aTBTþ are the fractions ofTBTOH and TBTþ, respectively.

In the development of Eq. (2), TBTþ was considered to interactonly with the negatively charged BC sites, i.e. interactions betweenTBTþ and relevant dissolved anions (nitrate and acetate, cf. Section3.4.2) are considered negligible in accordance with the results ofArnold et al. (1997) and Shoukry (1993).

All adsorption data of each BC sample at three pHs were usedto fit with the pH-DL model. In addition, the model was validatedvia predicting the adsorption edges of the BC samples andcomparing with measured data obtained from adsorption edgesexperiments. The modeling was performed by using R version2.10.0 (R Development Core Team, 2009).

3. Materials and methods

3.1. Reagents

TBTwas purchased as liquid tributyltin chloride ((C4H9)3SnCl; density: 1.2 gmL�1)(96%) from Sigma Aldrich (Denmark). Palladiumnitrate (2% Pd(NO3)2 in 5% HNO3) andTriton X-100 (C14H22O(C2H4O)n) were obtained from Perkin Elmer (Denmark). NaOHandHCl solutions used for titration, and concentratedHNO3 (69%)were obtained fromJ.T. Baker (Netherlands), and HPLC-grademethanolwas purchased fromSigma Aldrich(Denmark). All chemicals were pro analysis or better and triple deionized water(conductivity below 0.1 mS cm�1) was used throughout the experiments.

An 8.8 mmol L�1 TBT stock solution was prepared by dissolving 250 mL TBT in100 mL methanol, and stored in the dark at 5 �C.

The buffer solutions used contained 0.01 MNaNO3 and 0.01 M CH3COONa for pH4 and pH 6, and 0.01 M CH3COONH4 for pH 8. The pH values were adjusted by addingsmall amounts of 1 M HNO3 or 1 M NaOH solutions. The ionic strength (I) of thebuffer solutions was equivalent to 0.02 M NaNO3.

3.2. BC adsorbents

Wheat charcoal (WBC) was produced as described by Bruun et al. (2008). Briefly,portions of 10 g finely ground wheat straw in a steel dish (30 cm� 20 cm� 2 cm)covered with a steel lid was heated in a muffle furnace at 300 �C for 24 h. Theresulting BC (w4.0 g) was stored in a dessicator over silica gel. Beech charcoal (BBC)produced by a method similar to Hammes et al. (2006) was donated by Institute ofForest and Landscape Center (University of Copenhagen, Denmark). Soot blackcarbon (SBC) produced via combustion of acetylene was donated by TIMCAL Ltd.(Switzerland).

The two charcoals were finely ground in an agate ball mill, and washed with 1 MHCl (2�100 mL), 1 M HF (2�100 mL) and triple deionized water (4�100 mL) inpolyethylene bottles to remove silica and salts (Yang et al., 2004). Finally, thecharcoal was dried at 100 �C until constant weight was achieved.

3.3. Characterization of the BC samples

The elemental composition of the three BC samples was determined on a FlashEA 1112 CHN elemental analyzer (Thermo Fisher Scientific, Italy) with a combustiontemperature around 1800 �C.

The concentrations of carboxylic acid (eCOOH) and phenolic groups (Ar-OH) inthe BC samples were determined by potentiometric titration of BC suspensions.Briefly, 0.1 g BC was added to 40 mL 0.01 M NaNO3 solution. The mixture was ultra-sonically dispersed for 1 min using Branson sonifier 150 (Branson Ultrasonics Co. Ltd,USA) with 60Woutput power and shaken over night before titration. To get an initialpH> 11, 0.5 mL of NaOH (1 M) was added to the suspension and then the BCsuspension was titrated with 0.02 M HCl using a Metrohm 665 Dosimate (MetrohmInc, USA) under argon flushing and aMetrohm 691 pHmeter (Metrohm Inc, USA). Theconcentrations of carboxylic and phenolic groups were determined as titratableacidity between pH 3 and 7, and between pH 7 and 11, respectively (Buffle, 1988).

Fourier-Transform Infrared (FTIR) absorption spectra were obtained using the KBrpellet method with 0.1e0.3 mg of BC mixed with 300 mg of KBr compressed at150 MPa under vacuum in an evacuated die to produce a pellet (13 mm diameter).Spectra were recorded as the averages of 10 scans measured against a blind over therange 4000e400 cm�1 using a Perkin Elmer FTIR 2000 spectrometer (Perkin Elmer,Denmark).

The specific surface area (SSA) of the BC samples wasmeasured by water sorption(Pyman and Posner, 1978). Briefly, 1.0 g of the BC sample was dried over P2O5 in anevacuated dessicator (22�1 �C) until constant weight was achieved, then the BCsample was placed in an evacuated dessicator over saturated CaBr2 solution until itagain reached constantweight. The increment of theweight of the BC sample betweenthe two treatments is considered to correspond to the weight of a monolayer of watermolecules (Quirk, 1955). Taking the area of a H2O molecule as 0.108 nm2, the SSA canthen be calculated. The SSAs of the BC sampleswere alsomeasured by conventional N2

sorption procedure using a Gemini 2375 V4.01 surface area analyzer (MicromeriticsInc., USA). Prior to N2-BET measurement, the BC samples were degassed over nightunder N2 flow at 105 �C. The SSAs were calculated from a BET plot of N2 sorption atrelative N2 gas pressures (P/Po) ranging from 0.05 to 0.35.

X-ray diffraction (XRD) diffractograms of the three BC samples were recorded inthe range of 5e90� 2q with a speed of 0.3� (2q) min�1 using a Siemens D5000Diffractometer (Siemens, Germany) and Co Ka radiation at 40 kV and 40 mA.

Microscopic analysis was conducted using a Quanta 200 scanning electron micro-scope (FEI, USA) at ambient temperature and 15 kV. Samples were prepared on 12mmcarbon adhesive tabs and sputter-coated with 5 nm of gold using a Polaron SC 7640(Quorum Tech., UK).

3.4. Adsorption experiments

3.4.1. Preparation of BC stock suspensionsStock suspensions of the two charcoals were prepared by adding known amounts

of WBC or BBC to 1 L pH 4, pH 6 or pH 8 buffer solutions, respectively. Then the

L. Fang et al. / Environmental Pollution 158 (2010) 3642e36493644

suspensions were ultrasonicated using a Branson Sonifier 150 equipped witha microtip (Branson Ultrasonics Co. Ltd, USA) for 1 min at 60W to disperse the BCparticles. A total 3 L of suspension at each pHwere prepared, mixed and stored in twoglass bottles (2 L). Test of the homogeneity of suspensionwas performed by pipetting5�100 mL of suspension from the two bottles with stock solution and evaporating todryness at 100 �C (RSD< 6%) followedbyweighing. For each individual TBTadsorptionexperiment 50 mL of the charcoal stock suspensions were used. In order to obtaina similar range of final TBT aqueous concentrations in the adsorption experiments,different concentrations of theBCsuspensionswereusedat thedifferent pH levels. Theconcentration of the WBC in the suspensions at pH 4, pH 6 and pH 8 was 240 mg L�1,140 mg L�1 and 60 mg L�1, respectively, while for the BBC suspensions it was280 mg L�1, 220 mg L�1 and 140 mgL�1, respectively. However, the attempt to makehomogenous SBC stock suspension failed (RSD> 15%, n¼ 5) probably due to floccu-lation of the particles. Therefore, individual SBC suspensions were prepared foradsorption experiments by adding known amounts of SBC particles to buffer solutionsin Pyrex glass tubes, and then, dispersed ultrasonically as described above. Final SBCconcentrations of 320e400 mg L�1, 80e260 mg L�1 or 60e100 mg L�1 were used forthe adsorption experiments at pH 4, pH 6 and pH 8, respectively.

For the adsorption edge experiment, different amounts of 1 M HNO3 or NaOHwere added to 50 mL suspensions prepared as described above with concentrationsof 60 mg L�1 WBC, 140 mg L�1 BBC or 120 mg L�1 SBC in Pyrex centrifuge tubes(60 mL) to cover the pH range of 3e10.

All BC suspensions were allowed to equilibrate with the buffer over night beforeinitiating the adsorption experiments.

3.4.2. Adsorption isotherms and adsorption edgeAdsorption isotherms at pH 4, pH 6 and pH 8 were determined by spiking

appropriate amounts of TBT stock solution or its diluents (in methanol) to the50 mL BC suspensions in pH 4, pH 6 and pH 8 buffers to obtain initial TBTconcentrations in the range 0.42e8.42 mmol L�1. After TBT addition, the suspen-sions were shaken horizontally (50 rpm) for 24 h in the dark at room temperature(22�1 �C). A preliminary test with different shaking times of 1 he15 days, showedthat equilibrium can be reached within a few hours at pH 4 and 6, and within 24 hfor pH 8 (t-test, P> 0.05). All adsorption experiments were carried out in triplicate.

The pH-dependent adsorption (adsorption edge) of TBT to the three BC sampleswas determined for the pH range 3e10. All 50-mL suspensions were spiked withdiluted TBT stock solution (in methanol) to achieve an initial TBT concentration of0.42 mmol L�1. Subsequently, the samples were shaken in the dark for 24 h at roomtemperature (22�1 �C). In addition, pilot tests showed that methanol content (lessthan 0.15% (V/V)) used in this study did not interfere with TBT adsorption to the BCsamples.

To test the possible effect of the acetate buffer, adsorptions were also performedin 0.02 M NaNO3 with pH adjusted to 6 or 8 with 1 M NaOH or 1 M HNO3. Nosignificant differences were obtained with and without acetate buffer (t-test,P> 0.1).

3.5. TBT analysis

After equilibrating for 24 h, 10 mL of each suspension was filtered througha mixed cellulose ester syringe filter (0.22 mm pore size, 25 mm diameter, acrylichousing, Advantec MFC Inc, Japan) into a 10 mL volumetric flask containing 30 mL69% HNO3. This acidification avoided adsorption of TBT to the glass walls of thevolumetric flasks, stabilized TBT in solution and improved the sensitivity during theinstrumental analysis. Losses of TBT during all adsorption and pretreatment stepswere estimated by performing experiments both with and without adding BC andevery experiment was done in triplicates. About 5% and 10% of TBT losses to Pyrextubes and syringe filters were found, respectively. The adsorption data were cor-rected according to the losses.

The TBT concentrations in the filtrates were determined by measuring total TBTas Sn by means of a Zeeman-5100 graphite furnace atomic absorption spectrometer(GF-AAS) equipped with an electrodeless tin discharge lamp (Perkin Elmer,

Table 1Selected physical and chemical properties of the BC materials used in this study.

Elemental composition (Wt %) Atom

C H Oc N H/C

SBC 99.22� 0.2a <LODb 0.78� 0.24 <LOD 0WBC 69.43� 0.6 3.59� 0 25.06� 0.20 1.00� 0.01 0.62BBC 77.92� 0.3 3.37� 0.01 17.70� 0.26 0.28� 0 0.52

a Average� standard deviation (n¼ 3).b Below limit of detection (<LOD).c Oxygen contents were calculated by difference.d The SSAs of SBC, WBC, and BBC measured by using the N2-BET method are 63� 1 me The functional groups were determined by potentiometric titration.

Denmark). The acidified filtrates were transferred into 2 mL polystyrene samplecups (VWR, Denmark), and placed on an AS-60 Autosampler (Perkin Elmer,Denmark). Finally, 50 mL filtrate and 5 mL matrix modifier (0.1% Pd(NO3)2 in 1%HNO3) were injected into the graphite tube for analysis. After each injection, theinjector tip was rinsed with 0.03 (v/v) % Triton X-100 in 0.1% HNO3 solution. Theoptimized temperature program was as follows: a) drying: ramp from 20 to 120 �Cin 10 s, and held at 120 �C for 45 s; b) pyrolysis: ramp from 120 to 1200 �C in 10 s,and held at 1200 �C for 25 s; c) atomization: 2500 �C for 3 s. The argon flow wasstabilized at 300 mLmin�1 except during atomization. The wavelength was set at246.8 nm, and the slit width at 0.7 nm. Each sample measurement was performed induplicate. The limit of detection (LOD) and limit of quantification (LOQ) for TBTdetermined as Sn was 0.004, and 0.01 mmol L�1, respectively, corresponding to 3times and 10 times the standard deviation of 10 measurements of blanks.

A preliminary test of TBT losses to sample cups was performed by using a TBTstandard solution (in HNO3 acidified buffer solution) at a low TBT concentration(0.042 mmol L�1). The standardwas pipetted into a GF-AAS sample cup and analyzedafter 0, 1, 2, and 4 h, respectively. No significant decrease was detected between theresults analyzed after 0 and 4 h (t-test, P> 0.1).

4. Results and discussion

4.1. Characterization of BC samples

Selected physical and chemical properties of the three BC samplesare presented in Table 1. According to elemental analysis, the carboncontent ofWBC (w69%) is lower than that of BBC (w78%), but oppositefor oxygen content (e.g.,w25% ofWBC>w17% of BBC) (Table 1). Thisis in good agreementwith the contents of carboxylic acid andphenolicgroups in the two charcoals. The SBC shows relatively higher carboncontent (w99%) compared with previously studied soot samples, e.g.,hexane soot and diesel sootNISTstandard referencematerial 2975 (Imet al., 2008; Nguyen et al., 2004). As SBC is almost pure carbon thecontents of oxygen and hence carboxylic or phenolic groups arenegligible (Table 1).

The FTIR spectrum of SBC is almost devoid of absorbance bands(Fig. S3). In contrast, both charcoals exhibited bands between 3700and 2500 cm�1 attributed to eOH, peak at 1700 cm�1 attributed toC]O in carboxylic groups and a peak at 1610 cm�1 correspondingto C]C stretching in aromatic groups (Geng et al., 2009).

The XRD diffractograms of the three BC samples are presented inFig. S4, showing that all BC samples contain no crystalline impurities.SBC shows two distinct diffraction peaks at diffraction angels 2Q of30� and 50�, which is similar to soot produced from acetylene (Ebertet al., 1988). BBC exhibits two broad peaks at 2Q of 25� and 50�, butonly one peak at 25� is seen forWBC. The peaks located between 30�

and 25�, and 2Q of 50� is assigned to the 002 plane, and overlapping101 and 100 planes, respectively of graphene sheets within turbos-tratic crystallites (Kercher and Nagle, 2003; Kumar et al., 1993).These diffractograms indicate thatWBC is the least crystalline, whileSBC is less disordered. This supports the general view that moregraphite-like structures are produced at higher combustiontemperatures (Freitas et al., 1999), asWBC has been generated at thelowest temperature and SBC at the highest temperature.

ic ratio Ash (Wt %) SSAd (m2 g�1) Functional groupe

(mmol g�1)

O/C Carboxylic Phenolic

0.01 0 62� 4 0 00.27 0.93 111� 7 0.70� 0.05 0.75� 0.040.17 0.74 76� 5 0.11� 0.01 0.14� 0.01

2 g�1, 4.0� 0.4 m2 g�1, and 18.4� 0.6 m2 g�1, respectively.

Table 2Surface acidity constants obtained from fitting titration data with the modifiedHendersoneHasselbach equation (Eq. (S7) in Supplementary data) according toRitchie and Perdue (2003).a

Sample log K1c n log K2

c m R2

WBC 5.43� 0.23b 0.58� 0.01 8.57� 0.22 0.55� 0.01 0.992BBC 4.85� 0.33 0.55� 0.04 9.32� 0.28 0.47� 0.01 0.997

a The fitted curves for WBC and BBC are given in Fig. S2.b Average� standard deviation (n¼ 3).c K1 and K2 are mean proton binding coefficients for carboxylic and phenolic

groups.

L. Fang et al. / Environmental Pollution 158 (2010) 3642e3649 3645

The SSAs of the BC samples determined both by H2O and N2sorption are listed in Table 1. The order of SSAs is: WBC(w111 m2 g�1)> BBC (w76 m2 g�1)> SBC (w62 m2 g�1) determinedby the single-point H2O sorption method. The N2-BET methodresults in very low SSAs for the charcoals, resulting in unlikely highcharge densities. Previous studies of SSA determination of BCdemonstrate both successful uses (e.g. Freeman and McLeod, 1983)and the opposite (e.g. Denoyel et al.,1993; Lamond andMarsh,1964)when the N2-BET method is applied. As seen in Fig. 1, the charcoalsexhibit irregular surfaces and a number of micropores, whereas SBCis dense and consists of aggregated globuleswithmuchmore regularsurfaces. Hence, following the observation by Denoyel et al. (1993), itis expected that the SSAs of charcoals would be underestimatedcompared with the SSA of SBC when the N2-BET method is used.Normally, the N2-BETmeasurement is done at a temperature of 77 K,whichmay also cause the pores to get rigid and less accessible for N2.Finally, as the dipolar H2Omoleculemay be a better descriptor of theTBT-molecule than N2, the SSA values determined by the H2O sorp-tion method have been used in this work.

WBC has higher concentrations of carboxylic and phenolicgroups than BBC (Table 1). As the SSAs for the charcoals arecomparable, the charge densities due to carboxylic and phenolicgroups are considerably higher onWBC containing 7.9 (COOHþAr-OH) groups nm�2 comparedwith 2.0 (COOHþAr-OH) groups nm�2

on BBC. These values are comparable to those reported by Yanget al. (2004) for a wheat charcoal with a high SSA (310 m2 g�1;2.55 (COOHþAr-OH) groups per nm2).

The charge ratio on the charcoals is well described by Eq. (S7) (inSupplementary data), and the results of the fitting to the experi-mental data are shown in Table 2. The proton formation coefficientsof the two charcoals are comparable and resemble those for humic

Fig. 1. Scanning electron microscopy

substances with log value of K1 and K2 in the range of 3.8e4.8, and9.2e10.7, respectively (Ritchie and Perdue, 2003).

4.2. Adsorption edge

The adsorption of TBT strongly increases from pH 3 to pH 6 forSBC, and from pH 3 to pH 7 for WBC and BBC (Fig. 2), whereafter,TBT adsorption to SBC remains constant but decreases for the twocharcoals.

As shown in Fig. 2, the percentage of the neutral TBTOH inaqueous solution increases with increasing pH, and reaches approx.100% at pH> 8 (Weidenhaupt et al., 1997). Since hydrophobicadsorption is considered to be the only mechanism for TBT adsorp-tion to SBC (Brandli et al., 2009), the pH-dependent increase of TBTadsorption to non-charged SBC will be determined by the fraction ofTBTOH in solution. However, the pH-dependent increase of adsorbedTBT is shifted slightly compared with TBTOH versus pH distribution(Fig. 2). This change may be attributed to high affinity of SBC for

images of SBC, WBC and BBC.

Fig. 2. Adsorption edges of adsorbed TBT (%) by WBC (d d), BBC (d $$) and SBC (d)versus pH, respectively, curves are added as a guide to eye. Data points represent allthe measured values of TBT adsorption to WBC (-), BBC (:), and SBC (C) at differentpHs. The initial TBT concentration was 0.42 mmol L�1. The mass of the WBC, BBC andSBC in 50 mL suspension was 3 mg, 7 mg and 6 mg, respectively. The proportion ofTBTOH versus pH (//) has been computed by using Visual Minteq (Gustafsson, 2006;0.42 mmol L�1 TBTCl, I¼ 0.02 M NaNO3).

L. Fang et al. / Environmental Pollution 158 (2010) 3642e36493646

TBTOH shifting Eq. (1) in favor of TBTOH. A similar shift is often seenfor adsorption of heavy metal cations by soils (Sposito, 1989).

For the two charcoals, themaximumadsorptionof TBT takesplacein a similar pH range as found for TBTadsorption to other adsorbents,such as sediments and minerals (Hoch and Weerasooriya, 2005;Weidenhaupt et al., 1997), humic acid (Arnold et al., 1998;O’Loughlin et al., 2000), sediments (Burton et al., 2004; Hoch et al.,2002), and sand (Bueno et al., 1998; Behra et al., 2003). For thesecharged adsorbents, electrostatic bonding is the major adsorptionmechanism for TBT. Consequently, in addition to hydrophobicadsorption of TBTOH to uncharged sites, TBT adsorption by charcoalsdepends on: (i) the increasing negative charge on the adsorbentsurfaceswith increasing pH, and (ii) the fraction of positively chargedTBTþ that decreases with raising pH.

Fig. 3. Langmuir adsorption isotherms for binding of SBC, WBC, and BBC at threeconstant pH values (C: pH 4, B: pH 6,:: pH 8). The adsorption isotherms were fittedusing all raw data obtained in triplicates. Data points refer to averages of triplicates,

4.3. TBT adsorption isotherms

All TBT adsorption isotherms show convex shapes, resulting inadsorption saturation with increasing TBT solution concentration(Fig. 3). Moreover, the TBT adsorption to the three BC samplesdramatically increases from pH 4 to pH 8, which is in line with theobservations made from the adsorption edges (Fig. 2). The adsorp-tion isotherms could be successfully fitted (R2 ranging from 0.766 to0.975) with the Langmuir equation (Fig. 3, Table 3). For SBC theadsorption maximum (Cs,max) increases from 5.7 to 110 mmol kg�1

when pH increases from 4 to 8. A much lower adsorption maximumis seen for the charcoals; e.g. for BBC the adsorption maximumvaries between 4.2 mmol kg�1 (pH 4) and 13mmol kg�1 (pH 8). Thisemphasizes the importance of the BC composition and origin. Thecapacity of the BC samples to adsorb TBT is in accordance withearlier studies by Brandli et al. (2009) who unfortunately did notdistinguish between charcoal and soot.

vertical bars represent standard deviation (n¼ 3). The inserts show the adsorptionisotherms of the three BC samples at low TBT concentrations (Cw) scale. Note, thatdifferent ordinate axes have been used for SBC, WBC, and BBC, respectively.

4.4. Modeling TBT adsorption to BC

The adsorption isotherm data for TBT adsorption to the three BCsamples at three pHs were fitted to the pH-DL model (Eq. (2)); theresulting parameters are listed in Table 4. As expected themaximum of TBTOH adsorption density, Gmax

h , for SBC of1.57 mmolm�2 is significantly higher than the maxima for charcoals

(WBC: 0.65 mmolm�2, BBC 0.05 mmolm�2), reflecting that SBC hashigher capacity to adsorb TBTOH than the charcoals. This is in linewith previous studies on HOCs adsorption to oxidized surfaces ofcharcoal (Yang et al., 2004) and carbon nanotubes (Cho et al., 2008),

Table 3Langmuir adsorption parameters for bonding of TBT to the three BC samples.a

pH KL (L mmol�1) Cs,max (mmol kg�1) R2

WBC 4 4.2� 0.5b 5.4� 0.2 0.9456 1.7� 0.2 47.0� 2.2 0.9758 0.6� 0.1 67.0� 8.9 0.932

BBC 4 5.7� 1.3 4.2 � 0.2 0.8436 10.2� 1.7 9.2� 0.3 0.9248 1.9� 0.5 13.0� 0.8 0.844

SBC 4 0.7� 0.2 5.7� 0.9 0.7666 0.6� 0.1 110.0� 14.0 0.9508 1.5� 0.2 96.0� 4.8 0.968

a The parameters of the models were obtained by fitting of all raw data.b Average� standard error (n¼ 64).

Fig. 4. Prediction of adsorption edges of TBT (d), TBTþ (d $), and TBTOH (//) forSBC (the adsorption edges of TBT and TBTOH overlap, while no TBTþ adsorbs to SBC)and BBC using the pH-dependent dual Langmuir model (Eq. (2)), and the parameterslisted in Table 4. The dashed lines (d d) depict the 95% prediction interval, the datapoints (C) refer to the measured values for SBC and BBC obtained from adsorptionedge experiments. For the initial concentration of TBT and the concentrations of SBCand BBC refer to the caption of Fig. 2.

L. Fang et al. / Environmental Pollution 158 (2010) 3642e3649 3647

indicating that the hydrophilic property of functional groups couldattenuate hydrophobic adsorption. The maxima of TBTþ adsorptiondensity (Gmax

e ) for WBC (0.61 mmolm�2) and BBC (0.22 mmolm�2)are much lower than the surface charge densities calculated fromTable 1 data (WBC: 13.1 mmolm�2, BBC: 3.3 mmolm�2). Assumingmonolayer coverage, the adsorbed TBT density (TBTþþ TBTOH) forSBC and WBC corresponds to 0.94 and 0.76 nm2 per TBT-molecule,respectively, which is close to the TBTþ molecule size of 0.63 nm2

(Weidenhaupt et al., 1997), indicating monolayer coverage.However, for BBC, adsorption corresponds to only 0.16 nm2 per TBT-molecule, which may indicate steric hindrance (Fig. 1) or bondingsites with less adsorption capacity.

Validation of the pH-DL model was achieved by applying Eq. (2)to the adsorption edge data for the three sorbents (Fig. 4; Fig. S5).As expected TBT adsorption to SBC is governed exclusively by thehydrophobic adsorption of TBTOH. For the charcoals, the predictedTBT adsorption edges show slightly displacement which may bedue to uncertainty of the model. The model substantiates themagnitude of adsorption of TBTþ and TBTOH species as a functionof pH. It shows TBT adsorption by charcoals is exclusively due toTBTOH adsorption at pH> 9. This seems in agreement withprevious observations (Brandli et al., 2009). In contrast, Arnold et al.(1998) found that only about 40% of total adsorbed TBT by NOMwasattributed to TBTOH adsorption at pH 9, showing notably smallerhydrophobic adsorption capacity of NOM than of BC.

To our knowledge, this is the first model to estimate thecombined hydrophobic and electrostatic adsorption of TBT to BCmaterials with variable surface properties. Although not beinga universal model, it provides a simple approach to describe theinterplay between BC properties, pH and adsorption mechanism(electrostatic, hydrophobic) for TBT adsorption to BC.

4.5. Environmental implications

The results of this study have demonstrated that BC samplesboth have high adsorption capacities for TBT, especially at pH> 6,

Table 4Parameters of pH-dependent dual Langmuir model (Eq. (2)) for TBT adsorption tothe three BC materials.a,b

Gmaxh (mmolm�2) KL

h (L mmol�1) Gmaxe (mmolm�2) KL

e (L mmol�1) R2

SBC 1.57� 0.06c 1.83� 0.19 NAd NA 0.962WBC 0.65� 0.12 0.38� 0.07 0.61� 0.05 6.83� 1.09 0.974BBC 0.05� 0.04 14.93� 6.69 0.22� 0.08 38.85� 9.30 0.862

a All the parameters obtained by fitting all raw data from adsorption experiments.b Gmax

h and Gmaxe represent the TBT maximum adsorption capacities due to

hydrophobic (TBTOH) and electrostatic (TBTþ) adsorption, respectively. KLh and KL

e

(L mmol�1) are the hydrophobic (TBTOH) and electrostatic (TBTþ) Langmuir bindingcoefficients.

c Average� standard error.d Electrostatic adsorption is not applicable (NA) for SBC.

but also that different BC samples may have widely differentadsorption properties. Hence, adsorption of TBT to BC may stronglyinfluence the environmental distribution of TBT, and its bio-accumulation and ecotoxicity. To illustrate the significance of BC toTBT adsorption in sediments, we carry out a calculation where TBTis adsorbed at pH 8 by sediment. The sediment consists of 95.4%sand (>90% quartz), 2% clay (kaolinite), and 2.6% C in NOM and BCwith a BC: NOM ratio of 0.25 using adsorption data from varioussources (Table 3; Arnold et al., 1998; Burton et al., 2004; Hammeset al., 2007; Weidenhaupt et al., 1997). According to this calcula-tion, which is described in more detail in the Supplementary dataS6, 25% of total adsorbed TBT is adsorbed by BC and 45% by NOM.However, looking specifically at the TBTOH species and assumingthat only NOM including BC contributes to hydrophobic adsorption,approx. 69% of adsorbed TBTOH is contributed by BC and only 31%by NOM. Although the composition of natural sediments and theenvironmental conditions are often more complex, this exampleillustrates that BC present even in small amounts, strongly affectsTBT adsorption, and especially TBTOH adsorption. As the averagemortality of TBTOH to organisms (e.g., Daphnia magna) is about 3times higher than that for TBTþ (Fent and Looser, 1995), BC isexpected to affect the ecotoxicity of TBT in soils and sediments.

L. Fang et al. / Environmental Pollution 158 (2010) 3642e36493648

5. Conclusions

All three BC samples have high SSAs ranging between 62 and111 m2 g�1. The concentration of carboxylic and phenolic groups inWBC is 1.45 mmol g�2, which is four times higher than that of BBC.The SBC carries no functional groups. The TBT adsorption edgesreveal that TBT adsorption reaches maxima at pH 6e7 for the twocharcoals, while for the SBC the adsorption reaches a maximumplateau at pH> 6. All adsorption isotherms were successfully fittedusing the Langmuir equation (R2 ranging between 0.766 and 0.975),showing that the SBC has the highest adsorption capacities(5.7e110 mmol kg�1) and beech charcoal has the lowest (4.2e13mmol kg�1) at all three pHs. The new pH-dependent dual Langmuirmodel demonstrates that TBT adsorption to BC can be modeled asthe sum of TBTOH adsorption to hydrophobic sites and TBTþ

adsorption by the anionic sites on the BC particle surfaces, thedensity of which depends on the BC and pH. Consequently, BCsubstantially contributes to the total adsorption of TBT in theenvironment, especially to adsorption of very toxic TBTOH species.

Acknowledgements

The authors appreciate the very useful comments andimprovements made by four anonymous reviewers.

Appendix. Supplementary data

Supplementary data associated with this article can be found inthe online version, at doi:10.1016/j.envpol.2010.08.003.

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