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Colloids and Surfaces A: Physicochem. Eng. Aspects 446 (2014) 172–178

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa

nteractions between sulfa drug sulfadiazine and hydrophobicalc surfaces

uocheng Lva, Zhaohui Lia,b,c,∗, Nicole Hoeppnerc, Limei Wua, Libing Liaoa

School of Materials Science and Technology, China University of Geosciences, Beijing 100083, ChinaDepartment of Earth Science, National Cheng Kung University, 1 University Road, Tainan 70101, TaiwanGeosciences Department, University of Wisconsin–Parkside, Kenosha, WI 53141-2000, USA

i g h l i g h t s

Adsorption of sulfadiazine (SDZ) ontalc followed a linear type isotherm.Higher SDZ removal was found athigh and low solution pH.Simulation showed interactions ofbenzene ring and O of SDZ with Mgof talc.Hydrogen bonding between N of SDZand broken bond of O of talc alsoplayed a role.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 26 July 2013eceived in revised form0 December 2013ccepted 15 January 2014vailable online 29 January 2014

a b s t r a c t

Interactions between sulfadiazine (SDZ), a sulfa drug antibiotic, and talc, a low charge 2:1 phyllosilicate,were investigated under batch experimental conditions. The SDZ adsorption on talc was instantaneous,with a very large rate constant and a fast rate, although with a low amount. SDZ adsorption followed alinear sorption isotherm, suggesting that the hydrophobic interact instead of cation exchange was morelike responsible for the uptake of SDZ. Solution pH had a unique influence on SDZ adsorption. The solute

eywords:dsorptionydrogen bondingimulationulfadiazinealc

distribution coefficient was low in pH 3–7 range and increase at even lower and higher pH conditions.Molecular simulation suggested that the interactions between the benzene ring as well as the O of SDZand the Mg in the octahedral site of talc was partially responsible for SDZ uptake. In addition, the hydrogenbonding between the N in the amine as well as in the hetero ring and the broken bond of O on the 010plane of talc also contributed to SDZ uptake by talc.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

With the recent advancement in analytical methods and instru-

entations, many monitoring studies were conducted to document

he presence and distribution of pharmaceuticals and personal careroducts (PPCPs) in the environment. Among the PPCPs, the widely

∗ Corresponding author at: Geosciences Department, University ofisconsin–Parkside 900 Wood Road, Kenosha, WI 53141-2000, USA.

el.: +1 262 595 2487; fax: +1 262 595 2056.E-mail address: li@uwp.edu (Z. Li).

927-7757/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2014.01.014

distribution of antibiotics, such as tetracycline (TC), ciprofloxacin(CIP), and sulfa drugs, even at the effluent after the wastewatertreatment was reported in numerous cases [1–3]. In surface water,half of the 139 rivers surveyed in the United States had detectionof antibiotics [4]. Extensive use of antibiotics in animal feedingoperation also resulted in their significant accumulation in soil.

As soil is a major repository for antibiotics in the environment,the transport and fate of antibiotics in soil can be strongly influ-

enced by their adsorption process to soil components [2]. Mostof the studies on interactions between antibiotics and soil com-ponents were focused on clay minerals [5–8], metal oxides orhydroxides [9], quartz [10], as well as soil organic matters [11–13].

ysicoc

CmagIcbc[ap

butasmwflprbamtsthfimho[

G. Lv et al. / Colloids and Surfaces A: Ph

ation exchange interaction was attributed to the most importantechanism for the uptake of cationic antibiotics onto clay miner-

ls, thus, minerals with high cation exchange capacities (CEC) wereood candidates for the removal of cationic drugs from water [6,7].n addition to the study of interactions between antibiotics and soilomponents, the effect of adsorbed antibiotics on the antimicro-ial activities was also conducted. The antibacterial activity of TCould be maintained even after being adsorbed on montmorillonite14]. Thus, the presence of antibiotics in the environment wouldlso increase bacterial resistances to antibiotics acclaimed to theersistent low dose conditions [15].

Sulfadiazine (SDZ) or 2-sulfanilamidopyrimidine is an antibioticelonging to the group of sulfonamides (SAs), which were widelysed in human therapy and veterinary medicine, especially in poul-ry, pig, and calf livestock [12,16]. It was the second highest soldntibiotics in UK with a usage of 14 t in 2000 [15]. Most of thetudies on adsorption and desorption of SDZ were on soils or soil-anure systems [16,17]. Adsorption of SAs to mineral soil colloidsas weaker and resulted in a stronger desorption from clay-size

ractions [17]. Neutral and cationic SA species did not interca-ate into swelling clay mineral montmorillonite, but interactedrimarily with external surfaces [18]. Like other SAs, sulfachloropy-idazine has a low adsorption potential and was, thus, likely toe highly mobile [19]. Adsorption of the SA antibiotics sulfanil-mide, sulfadimethoxine, and sulfapyridine was also conducted onodel soil organic matters using Fluka humic acid [13]. Adsorp-

ion of the amphoteric SAs was nonlinear and pH dependent andtronger adsorption to humic acid than to the oligomer was relatedo the more complex structure and functional group diversity ofumic acid [13]. On a mass basis, organic matter had higher affinity

or neutral SA antimicrobials than do smectite clays [12]. Model-ng and molecular mechanics calculations of antibiotic–soil organic

atter complexes showed preferred site-specific adsorption viaydrogen bonds and van der Waals interactions [17]. The presencef manure increased the adsorption tendency of SDZ significantly16].

N

N

N

NH S

O

NH2

N

N

NH S

O

O

N

N

N

N S

O

pKa1

Kt

+

+

Fig. 1. Speciatio

hem. Eng. Aspects 446 (2014) 172–178 173

Talc is a trioctahedral 2:1 phyllosilicate. Due to limited isomor-phic substitution, talc surface is hydrophobic with minimal CEC.Talc was commonly used as additives to many drug tablets, pre-sumably due to its inert surface. However, studies on interactionsbetween drugs and talc surfaces were limited. Adsorption of CIP ontalc resulted in a capacity of 740 �g/g, corresponding to 2.2 �mol/g[20] in comparison to 1.0 mmol/g on a Ca-montmorillonite [6]. Theamount of cimetidine adsorbed on talc was even less, at 290 �g/g,equivalent to only 1.2 �mol/g [21]. And the adsorption of chlor-pheniramine (CP) on talc resulted in a capacity of 0.06 mmol/g andwas attributed to hydrophobic interaction [22].

This goal of this study was to investigate the interactionsbetween SDZ and hydrophobic talc, a phyllosilicate with limitedsurface charge, in order to better understand the fate and trans-port of SDZ in the environment on one hand, and to understand theeffect of hydrophobicity and solution chemistry on the removal ofSDZ by low-charge clay minerals on the other hand.

2. Materials and methods

The talc used was purchased from Acros. It has a chemical for-mula of Mg3Si4O10(OH)2, a mean particle size less than 200 mesh(<0.075 �m), and an external surface area of 2.3 m2/g measuredby a multi-point BET method. Due to low isometric substitution intetrahedral and octahedral sites, talc had essential no permanentsurface charge [23] and its extremely low CEC was originated fromits pH dependent surface charges.

The sulfadiazine (CAS no.: 68-35-9), also called 2-sulfanilamidopyrimidine, was purchased from Alfa Aesar (WardHill, MA, USA). It has a molar mass of 250.28 g/mol and a log KOWvalue of −0.09 [17]. It has two dissociation constants: pKa1 at2.28–2.71 and refers to the protonation of the NH2-group and pKa2

at 6.45–6.52 and refers to the deprotonation of the sulfonamidogroup [24,25]. Its speciation under different pH conditions isillustrated in Fig. 1. The equilibrium constant for the speciesdistribution between a neutral and zwitterionic (Kt) form of SDZ

H2

O

NH2

N −

N

N S

O

O

H2

O

pKa2

−

n of SDZ.

1 ysicochem. Eng. Aspects 446 (2014) 172–178

iSrtw

ffmcctp3ffi

tmwoi

tacTabmc

ess

obcsˇu1

pUmAwd

3

3

afitg

q

w(q

0.23

0.24

0.25

0.26

0.27

0 5 10 15 20 25

y = 3.82x + 0.07 r2=1.000

0 20 40 60 80

100

0 5 10 15 20 25

Time (h)

Time (h)

Am

ount

SD

Z ad

sorb

ed (m

mol

/kg)

t/qt (

h-kg

/mm

ol)

74 G. Lv et al. / Colloids and Surfaces A: Ph

s 102.17 [24], suggesting a dominant species of SDZ0 instead ofDZ±, when solution pH was between 2.7 and 6.4 (Fig. 1). Itseported water solubility varied from 265 mg/L at pH 5.5 and 25 ◦Co 5690 mg/L at pH 8 and 37 ◦C [25]. A solubility as low as 77 mg/Las also reported [17].

The initial SDZ concentration varied from 0.01 to 0.08 mmol/Lor the adsorption isotherm study and was fixed at 0.08 mmol/Lor kinetic, pH dependent, and temperature studies. To each 50-

L centrifuge tube, 10 mL of SDZ solution and 1.0 g of talc wereombined and shaken at 150 rpm for 24 h at which equilibriumould be reached based on the kinetic study. For the kinetic studyhe equilibration time varied from 0.25 to 24 h, and for the tem-erature study, the equilibrium temperature was set at 296, 306,16, and 326 K. After the mixtures were centrifuged at 3800 rpmor 10 min, the supernatants were filtered through 0.45 �m syringelters before being analyzed for equilibrium SDZ concentrations.

The SDZ concentration was measured using a UV-Vis spec-rophotometer at the wavelength of 275 nm. Calibrations were

ade using standards of 0.002, 0.01, 0.02, 0.05, and 0.08 mmol/Lith regression coefficients all greater than 0.998. The amount

f SDZ adsorbed was calculated from the difference between thenitial and equilibrium concentrations.

The point of zero charge (pzc) was determined using a poten-iometric titration method. The pzc is the pH at which the netdsorption of potential-determining ions, H+ and OH−, on variable-harged surfaces is independent of electrolyte concentration [26].o each 50 mL beaker, one gram of talc and 20 mL of NaCl solutiont concentrations of 0.01, 0.1, and 1.0 M were mixed and the pH sta-ilized. Aliquots of 0.2 mL of 0.01 M NaOH or HCl were added to theixture and the pH recorded. The pH at the intercept of titration

urves under different ionic strengths indicates the pzc [27].The FTIR spectra were acquired on a Jasco FT/IR-4100 Spectrom-

ter equipped with a ZnSe attenuated total reflection accessory. Thepectra were obtained from 650 to 4000 cm−1 by accumulating 256cans at a resolution 4 cm−1.

Molecular simulation was performed under the module Forcitef Materials Studio 5.0 software to investigate the interactionsetween SDZ and talc in aqueous solution. The talc model wasonstructed and the atomic coordinates were derived from thepace group of C2/c with a = 5.27 A, b = 9.12 A, c = 18.85 A, = � = 90◦,

= 100.016◦ [28]. The supercell of the model was made of 12 talcnit cells at 3a × 2b × 2c. On the surface of XOZ, the area was5.81 A × 37.70 A.

The established model was optimized geometrically. The tem-erature was set at 298 K and time was 1 ns with a time step of 1 fs.niversal force field was used during simulation. The Ewald sum-ation method was used to calculate the electrostatic interaction.fter the system reached equilibrium, the NVT kinetic simulationas performed under the same time constant and temperature con-itions. The data were collected on the last 500 ps for later analyses.

. Results and discussion

.1. Kinetics of SDZ adsorption

The kinetic study showed that the adsorption of SDZ on talc waslmost instantaneous (Fig. 2). The data were fitted to the pseudo-rst-order, pseudo-second-order, and Elovich kinetic models. Onlyhe pseudo-second-order kinetics fitted the data well. The inte-rated rate law of the pseudo-second-order kinetics is:

t = kq2e t

(1)

1 + kqet

here k (kg/mmol-h) is the rate constant of adsorption, qe

mmol/kg) is the amount of solute adsorbed at equilibrium, andt (mmol/kg) is the amount of solute adsorbed on the surface of

Fig. 2. Kinetic study of SDZ adsorption on talc. The line is the pseudo-second-orderfit to the observed data and the inset is the plot of Eq. (2).

the adsorbent at any time, t. Eq. (1) can be re-arranged into a linearform

t

qt= 1

kq2e

+ 1qe

t (2)

where kqe2 is the initial rate (mmol/kg-h). Fitting the experimental

data to the pseudo-second order kinetic model resulted in a coeffi-cient of determination r2 of 0.9999, an initial rate of 14 mmol/kg-h,a rate constant of 205 kg/mmol-h, and a qe of 0.26 mmol/kg.The initial rate and qe are much smaller than the initial rate of700 mmol/kg-h and the qe of 50 mmol/kg for CP adsorption on talc[22]. But the rate constant of 205 kg/mmol-h for SDZ adsorption ontalc is much larger than the rate constant of 0.23 kg/mmol-h for CPadsorption on the same talc [22]. The large rate constant confirmedinstantaneous SDZ adsorption on talc. The initial rate was slightlyless than 53 mmol/kg-h for TC adsorption on kaolinite [29].

3.2. SDZ adsorption isotherm

The Langmuir, Fruendlich, and linear isotherms were used tocharacterize SDZ adsorption on talc. The linear form of the Freund-lich model is expressed as:

log CS = log Kf + 1n

CL (3)

where CS and CL are the solute concentrations on solid (mmol/kg)and in solution (mmol/L), Kf is the Freundlich constant (mmol/kg), ameasure of the adsorption capacity of the adsorbent, and 1/n is theheterogeneity factor, a constant relating to adsorption intensity orsurface heterogeneity. The linear adsorption isotherm is expressedas:

CS = KdCL (4)

where Kd is the solute distribution coefficient between the solidand the solution.

Freundlich model was used to fit the adsorption of sul-fachloropyridazine on a clay loam and a sandy loam with the 1/nclose to unity, suggesting that a linear isotherm might be an alter-native model for its adsorption [19]. In this study, the adsorption ofSDZ followed a similar trend. The Langmuir fit resulted in an r2 of0.68 and a negative sorption maximum. The Langmuir model wasbased on charge-limited or surface-limited monolayer adsorption.The low r2 value suggested that neither surface charge nor sur-

face area should be the limiting factor for SDZ adsorption on talc.The Freundlich fit ended with an r2 of 0.98, but the best fit to theobserved data was the linear sorption isotherm with an r2 of 0.99and a Kd value of 5.6 L/kg (Fig. 3), in comparison to a Kd values of

G. Lv et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 446 (2014) 172–178 175

0.0

0.1

0.2

0.3

0.00 0.01 0.02 0.03 0.04 0.05 0.06

y = 5.62x - 0.04 r2=0.99

Equilibrium SDZ concentration (mmol/L)

Am

ount

SD

Z ad

sorb

ed (m

mol

/kg)

FF

1ssatae

owlotrmtw1c

3

tsafaOavimKtwr[sdh

t

K

wt

Fig. 4. SDZ adsorption on talc as affected by equilibrium solution pH (♦ with left

ig. 3. Adsorption of SDZ on talc. The solid and dashed lines are the linear andreundlich fits to the observed data.

.4 to 2.8 L/kg for SDZ adsorption on soil [17], 1.8 and 0.9 L/kg forulfachloropyridazine adsorption on a clay loam and sandy loamoil, respectively [19], and to 0.07 to 0.91 L/kg for sulfadimethoxinend sulfamethoxazole adsorption on soils [5]. The larger Kd value inhis study suggested that the hydrophobic surface of talc resulted

higher SDZ uptake, an indication of hydrophobic interaction inffect.

The adsorption of other ionizable antibiotics such as TC and CIPnto swelling clay minerals such as montmorillonite and rectoriteas fitted well to the Langmuir isotherm, suggesting a surface-

imited or charge-limited adsorption [6,7]. Similarly, the adsorptionf CP on montmorillonite and talc also fitted to the Langmuir sorp-ion isotherm well [22]. Fitting to the linear sorption isothermather than the Langmuir isotherm suggested a different sorptionechanism for SDZ adsorption on talc. At the equilibrium solu-

ion concentration of 0.055 mmol/L, the amount of SDZ adsorbedas 0.25 mmol/kg (Fig. 3) in comparison to the CEC of talc at about

0 mmol/kg [30], again suggesting that cation exchange may notontribute to SDZ adsorption on talc.

.3. Effect of solution pH on SDZ adsorption

Solution pH had a drastic effect on SDZ adsorption on talc. Usinghe pKa1 and pKa2 values of 2.7 and 6.4 for SDZ [24], the dominantpecies of SDZ was in a cationic form when solution pH was <2.7,

neutral molecular form between pH 2.7 and 6.4, and an anionicorm when pH > 6.4. When solution pH increased from 2 to 4, themount of SDZ adsorbed decreased from 0.4 to 0.2 mmol/kg (Fig. 4).n the other hand, as the solution pH increased from 5 to 10, the SDZdsorption increased back from 0.2 to 0.4 mmol/kg (Fig. 4). The Kdalues followed a similar trend, varying from 3 to 9 L/kg, suggest-ng that the minimal SDZ adsorption occurred when neutral SDZ

olecules were present in solution (Figs. 1–4). In comparison, thed of sulfamethazine on five soils decreased monotonically as solu-ion pH increased from 5.5 to 9.0 [31]. Literature data of Kd for SDZere scarce and were restricted to a narrow pH range (6.9–7.5) [32],

esulting in a small variation of the observed Kd of 1.4 to 2.8 L/kg17]. In general, the amphoteric SAs behave as weak acids and formalts in strongly acidic or basic solutions [2]. This salt-forming ten-ency might also be responsible for the higher Kd values at low andigh solution pH conditions.

Contributions of different species to the overall Kd can be relatedo the mass fraction (˛) and the Kd of each species by:

d = KSDZ+d

˛+ + KSDZ0d ˛0 + KSDZ−

d˛− (5)

here the superscripts SDZ+, SDZ0, and SDZ– refer to cationic, zwit-erionic or molecular, and anionic forms of SDZ, respectively. The

y-axis) and Kd as affected by solution pH (© right y-axis). The solid line is the fittedKd as a function of pH and the vertical dashed lines are the pKa values of SDZ; (b)speciation of SDZ under different pHs.

values are a function of solution pH and pKa values of the solute.A multi-variable regression was performed using ˛+, ˛0, and ˛–

against Kd under different pH conditions with the intercept forcedto zero. The fitted results are 8.7, 1.9, and 8.6 L/kg for Kd

SDZ+, KdSDZ0,

and KdSDZ–, respectively, with an r2 of 0.98 (Fig. 4). The calculated

KdSDZ0 is only 1/4 of that of Kd

SDZ+ or KdSDZ–, confirming that neutral

species of SDZ had less affinity to talc surfaces.

3.4. Effect of temperature on SDZ adsorption

The adsorption of SDZ on talc decreased as the equilibriumtemperature increased at initial SDZ concentrations of 0.04 and0.08 mmol/L (Fig. 5), suggesting an exothermic reaction. Therelationship between Kd and the thermodynamic parameters ofadsorption is expressed as

ln Kd = −�H

RT+ �S

R(6)

where �H is the change in enthalpy, �S is the change in entropy, Ris the gas constant, and T is the reaction temperature in K. The freeenergy (�G) of adsorption can be determined by

�G = �H − T�S (7)

The calculated thermodynamic parameters are listed in Table 1.The negative �G values indicated attractive interactions betweenSDZ and talc, thus a spontaneous adsorption of SDZ on talc. How-ever, the �G values were less negative compared to −8 kJ/mol

for TC adsorption on silica [10], and to −8 kJ/mol for promet-hazine hydrochloride adsorption on iron rich smectite [33]. Also,the small negative �G values suggested that the adsorption of SDZon talc was relatively weak, owing to physical adsorption such as

176 G. Lv et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 446 (2014) 172–178

Table 1Thermodynamic values of SDZ adsorption on talc at different temperatures and initial concentrations.

Cini (mM) Ln(Kd) (L/kg) �G (kJ/mol) �H (kJ/mol) �S (kJ/mol-K)

296 K 306 K 316 K 326 K 296 K 306 K 316 K 326 K

0.04 1.43 1.22 1.18 1.02 −3.7 −3.2 −3.1 −2.7 −10.1 −0.020.08 1.67 1.48 1.44 1.34 −4.1 −3.8 −3.7 −3.5 −5.7 −0.01

y = 1210x - 2.7 r2=0.83

y = 688x - 0.8 r2=0.95

0.8

1.0

1.2

1.4

1.6

0.0030 0.0031 0.0032 0.0033 0.0034

1/T

Ln (K

d) Initial conc. = 0.08 mmol/L

Initial conc. = 0.04 mmol/ L

Fa

hinatf

3

tmaa

wrffermt3H(pf

nbrbg

mT1z

70

80

90

100

110

120

2700 2900 3100 3300 3500 3700

20

40

60

80

100

120

700 900 1100 1300 1500 1700

SDZ

Raw talc

308 K

318 K

328 K

SDZ

Raw talc 308 K 318 K 328 K

(a)

(b)

Wavenumber (cm–1)

Tran

smitt

ance

(%)

Tran

smitt

ance

(%)

ig. 5. SDZ adsorption on talc as affected by equilibrium temperature. The solid linesre the fit to Eq. (6).

ydrophobic or electrostatic interaction. The small negative changen �S indicated that spontaneous adsorption due to entropy wasot favored. While, the negative �H value suggested that SDZdsorption on talc was an exothermic process, agreeing well withhe �G value (Table 1). Therefore, a decrease in temperature shouldacilitate the adsorption of SDZ onto talc.

.5. FTIR analyses

The FTIR spectra of raw talc and SDZ-adsorbed talc from an ini-ial concentration of 0.08 mmol/L at different temperatures were

ore or less identical. Only minute peaks at 1093 and 2978 cm−1

ppeared after SDZ adsorption on talc (Fig. 6), due to the lowermount of SDZ uptake.

The –NH2 vibration absorption peaks are at 3424 and 3354 cm−1

ith N–H stretching of sulfonamide at 3269 cm−1 and its cor-esponding bending band at 1652 cm−1 [34]. The stretchingrequency of the sulfonamide N–H was reported at 3379 cm−1

or its acidic character of the molecules and the calculated andxperimental in-plane N–H mode was at 1499 and 1454 cm−1,espectively [35]. The absence of 3424 and 3354 cm−1 for SDZonomer indicated that the –NH2 was not existent [36]. On

he contrary, the broadening and shifted stretching N–H peak at345 cm−1 was a strong evidence of the presence of –NH3

+ [34].owever, in this study, the broader peak was located at 3390 cm−1

Fig. 6a). For raw talc, this peak is at 3440 cm−1. Thus, this broadereak may be attributed to the interaction of –NH3

+ with talc sur-aces.

In a study of interactions between SDZ and poly dendrimer aew band at 1633 cm−1 appeared and was probably due to theending mode of SDZ–NH2 group [34]. In this study, a weak bandemained at 1652 cm−1, which could be resulted from the N–H3

+

ending of sulfonamide rather than the bending mode of SDZ–NH2roup.

The vibrations at 1324 and 1150 cm−1 were attributed to sym-

etric and asymmetric vibration absorption peaks of –SO2– [36].

hey were absent after adsorbed on talc. The bands at 1150 and092 cm−1 were assigned to symmetric SO2N and substituted ben-ene ring [37]. After SDZ adsorption on talc, two smaller peaks at

Fig. 6. FTIR spectra of raw talc, solid SDZ and SDZ-adsorbed talc under different equi-librium solution temperature. The vertical lines are new peaks after SDZ adsorptionon talc.

1093 and 1053 cm−1 appeared (Fig. 6b). These two bands may beoriginated from the bands at 1150 and 1092 cm−1. If this assump-tion was valid, the adsorption of SDZ on talc may involve in theinteractions between the N of the sulfonamide and the substitutedbenzene ring with the surface of talc. Similarly, the interactionbetween SDZ and high silica zeolite Y was found involving in mul-tiple weak H-bond and van der Waals type interactions betweenthe amine protons of SDZ and lattice oxygen atoms and hydropho-bic interactions between the aromatic rings and zeolite cage walls[35].

3.6. Discussion on SDZ adsorption sites and mechanism

The pzc determined in this study is about 7.7 (Fig. 7), agree-ing well with the 7.7 ± 0.45 [38]. The surface charge of talc wasdetermined to be −3 meq/kg [39], about the same magnitude of5.3 mmol/kg as determined by benzyltrimethylammonium adsorp-tion [40]. The surface potential of talc basal plane was found to

be largely independent of solution pH, while the surface poten-tial of the edge surfaces reversed from positive to negative at pHaround 8 [41]. Still, significant amount of SDZ adsorption was foundeven when solution pH was much higher than 7.7 (Fig. 4). Thus,

G. Lv et al. / Colloids and Surfaces A: Physicoc

-200

-150

-100

-50

0

50

100

150

200 3 4 5 6 7 8 9 10

pH

mm

olc/k

g

F0

tfto

tbalta

Tzotbdthmkg

fagmi

FaSmmoi

ig. 7. Titration curves for determination of pzc of talc at NaCl concentrations of.01 M (♦), 0.1 M (�), and 1.0 M (©).

he surface charge cannot be accounted for SDZ adsorption on talc,urther confirming that hydrophobic interaction, rather than elec-rostatic interaction, was more responsible for the uptake of SDZn talc.

The external surface area of talc was 2.3 m2/g. At SDZ adsorp-ion of 0.25 mmol/kg, the calculated SDZ adsorption density woulde about 1500 A2 per SDZ molecule on a monolayer coverage. Inddition, the amount of SDZ adsorbed at 0.25 mmol/kg was muchower than the CEC of talc at about 10 mmol/kg [30]. Thus, neitherhe surface area nor the CEC of talc was the limiting factor for SDZdsorption.

In comparison to other phyllosilicates, talc is quite hydrophobic.he hydrophilic part of the surface area estimated by ben-yltrimethylammonium adsorption was 2 m2/g, accounting for 20%f the total surface area of talc [40]. Adsorption of naphthalene onalc in aqueous solution was strongly dominated by the hydropho-ic surface of the talc and the major mechanism was due to vaner Waals force [42]. Hydrophobic interaction was attributed tohe adsorption of guar, a natural nonionic polysaccharide, onto theydrophobic sites of talc [43]. Thus, the hydrophobic interactionay play an important role in SDZ adsorption. Similarly, for pain-

iller drug acetaminophen, it adsorption on activated carbon wasreatly reduced as the surface became more hydrophilic [44].

Hydrogen bonding rather than electrostatic or hydrophobicorce was proposed as one of the main driving forces for guardsorption on talc [45]. Due to the presence of N in SDZ, hydro-

en bonding between N of the molecule and OH on the talc surfaceay be attributed more to adsorption of SDZ on talc. Simulation of

nteractions between SDZ and talc in aqueous solution showed that

ig. 8. Molecular dynamic simulation showing attractive interactions between SDZnd talc (0 1 0) surface. The N is indicated by blue, O by red, C by gray, H by white, and

by yellow dots. The N on the benzene ring and N of the sulfonamide of the top SDZolecule interacted with OH on talc (0 1 0) via hydrogen bonding. The bottom SDZolecule showed the interactions between octahedral Mg of talc and –NH2 and O

f SDZ. (For interpretation of the references to color in this figure legend, the readers referred to the web version of this article.)

[

[

[

[

hem. Eng. Aspects 446 (2014) 172–178 177

hydrogen bonding between N on the N-substituted benzene ring aswell as the N of the sulfonamide and the OH of the broken bond onthe 010 surface could be responsible partly for the SDZ adsorption(Fig. 8). Moreover, the interactions between the benzene ring aswell as the partially negatively charged O of SDZ and the Mg inthe octahedral sites of talc may also contribute to SDZ adsorption(Fig. 8). Under alkaline conditions, polysaccharide could directlyinteract with the hydroxiated talc surface by hydrogen bonding;thus, it was not the hydrophobicity of the surface of talc, but themetallic sites of talc that contributed to enhanced adsorption [46].Sorption of SAs by humic substances was mainly driven by thearomatic amino group which also formed covalent bonds with phe-nolic humic substances [11]. The lack of humic substances for thepure phase talc resulted in a much lower SDZ adsorption.

4. Conclusions

The adsorption of SDZ on talc was almost instantaneous andfollowed a linear adsorption isotherm with a Kd values between 3and 9 L/kg under neutral and high or low pH conditions. Althoughtalc has small cation exchange capacity and specific surface area,the amount of SDZ adsorbed was not limited by these factors. Dueto the fact that higher Kd values were found at higher and lowerpH conditions, it was anticipated that hydrogen bonding played amajor role on SDZ adsorption on talc. The interactions between thebenzene ring as well as O of SDZ and the Mg in the octahedral sitesof talc were also speculated to account for SDZ adsorption on talc.Hydrophobic interaction may also contribute to SDZ adsorption ontalc, but to a lesser degree.

Acknowledgments

The research was support by a grant from Wisconsin Ground-water Research Council. Sam Leick helped with the experiments.

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