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Analyses of Levofloxacin Adsorption on Pretreated Barley Straw with Respect to Temperature: Kinetics, #-# ElectronDonor-Acceptor Interaction and Site Energy Distribution Bei Yan, Catherine Hui Niu, and Jian Wang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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Environmental Science & Technology

1 2

Analyses of Levofloxacin Adsorption on Pretreated Barley Straw with Respect to

3

Temperature: Kinetics, π-π Electron-Donor-Acceptor Interaction and Site Energy

4

Distribution

5

Bei Yan a, Catherine Hui Niu a,b,* and Jian Wang c

6

7 8

9 10

11 12

a

School of Environment and Sustainability, University of Saskatchewan, 117 Science

Place, Saskatoon, Saskatchewan, Canada S7N 5C8 b

Department of Chemical and Biological Engineering, University of Saskatchewan, 57

Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A9 c

Canadian Light Source, 44 Innovation Boulevard, Saskatoon, Saskatchewan, Canada

S7N 2V3

13

1 ACS Paragon Plus Environment


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Abstract: Levofloxacin, representative of an important class of fluoroquinolone antibiotics, has

15

been considered to be one of the emerging pollutants in various water sources. In this paper,

16

adsorption of levofloxacin from artificial contaminated water was done by pretreated barley

17

straw adsorbent obtained from raw barley straw after modification by H3PO4 impregnation and

18

microwave heating. The adsorption kinetics was investigated at various temperatures and

19

levofloxacin concentrations, and the activation energy was determined. In addition, site energy

20

distribution of the pretreated barley straw for levofloxacin adsorption was estimated based on the

21

equilibrium adsorption data. The average site energy and standard deviation of the distribution

22

were determined and applied to analyze the interaction strength between the adsorbent and

23

adsorbate, and adsorption site heterogeneity. The π-π electron-donor-acceptor interactions

24

between the π* aromatic C=C of pretreated barley straw adsorbent and π* carbon atom in benzene

25

ring attached to fluorine of levofloxacin was investigated by C K-edge X-ray absorption near-

26

edge structure spectroscopy. The results and methodologies in this work could be transferrable to

27

investigate extended systems of water treatment.

28


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TOC Art

30



31

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■ INTRODUCTION

32

Fluoroquinolones (FQs) is an important class of antibiotic compounds commonly used in

33

both human and veterinary medicine worldwide. FQs acts via inhibiting the activity of DNA

34

gyrase enzyme for bacterial DNA replication.1 Particularly, levofloxacin (LEV) is a new kind but

35

already widely used FQs antibiotic. In 2011, the total amount of LEV used in human medicine

36

was more than 55 tons, and it ranked second in the FQs consumption in the USA.2 However, FQs

37

cannot be completely metabolized in both humans and animals, neither can they be removed

38

effectively during wastewater treatment using current technologies, e.g. activated sludge3 and

39

advanced oxidation technologies.4 According to a case study in Japan, the concentration of LEV

40

presented in sources of sewage before treatment was 552 ng/L, though it was treated by the

41

process of activated sludge, only 49% of LEV was removed.5 As such, they were discharged into

42

the environment. For example, the level of LEV of one of the downstream rivers close to a drug

43

formulation facility in Pakistan was up to 8000 ng/L.6 The water resources contaminated by LEV

44

may present a risk to human health.

45

Concerns about the environmental and health risks associated with the water resources

46

contaminated by FQs and other aromatic compounds have prompted research on effective and

47

efficient treatment technologies. Adsorption has been proposed as one of the most effective

48

technologies for removing contaminants from aquatic systems. Recently, preliminary research

49

demonstrated that LEV was able to be effectively adsorbed by pretreated barley straw (PBS) in a

50

wide range of solution pHs (4.0-9.6), and the optimum solution pH for LEV adsorption was

51

determined to be 6.88.7 PBS demonstrated much higher adsorption capacity (403±15 mg LEV/g

52

PBS at pH 6.88) than adsorbents such as goethite (1.03 mg/g),8 iron-pillared montmorillonite

53

(48.61 mg/g),9 charcoal (87 mg/g)10 and graphene oxide (256.6 mg/g).11 However, systematic 4 ACS Paragon Plus Environment

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study on the characteristics of LEV adsorption process including kinetics, isotherms, and

55

adsorption energy with respect to temperature has not been done yet. The respective knowledge

56

is important for understanding the mechanisms of LEV and FQs adsorption, and application of

57

this technology in treating the water contaminated by LEV, FQs, or the like.

58

The π-π electron-donor-acceptor (EDA) interactions have been proposed as one of the

59

predominant driving forces for the adsorption of adsorbates with benzene rings.12, 13 The benzene

60

ring of norfloxacin was proposed as π-electron-acceptor due to the strong electron withdrawing

61

ability of the fluorine group,13 while the aromatic groups on a heterogeneous adsorbent can be π-

62

electron-donors.12, 14 LEV contains a benzene ring in its molecular structure. Whether the π-π

63

electron-donor-acceptor interactions serve as an important role in LEV adsorption needs further

64

investigation. In addition, analysis of site energy distribution of adsorbents is helpful for a better

65

understanding of the adsorption mechanism for target molecules. Site energy distribution curve

66

provides information of the distribution of high and low energy binding sites on adsorbents for

67

the target molecules, which assists of elucidating adsorption mechanisms.15, 16 However, analysis

68

of LEV adsorption site energy with respect to temperature has not been done yet.

69

In this work, barley straw, an abundantly generated agricultural byproduct with the main

70

organic constitutions of cellulose, hemicelluloses and lignin, was modified by H3PO4

71

impregnation and microwave heating. Lignin contains benzene rings. The pretreated barley straw

72

(PBS) was used as adsorbent for LEV removal from artificial wastewater at various temperatures.

73

The adsorption kinetics, and isotherms involved in the adsorption processes were investigated at

74

different initial LEV concentrations and solution temperatures. The Langmuir-Freundlich model

75

was applied to simulate the equilibrium LEV adsorption data. Site energy and its distribution

76

function were determined to analyze the interaction between the adsorbent and adsorbate, and 5 ACS Paragon Plus Environment


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adsorption site energetical heterogeneity. The EDA interactions between the aromatic groups

78

(electron-donor) of PBS and benzene rings (electron-acceptor) of LEV, were investigated with

79

the aid of C K-edge X-ray absorption near-edge structure spectroscopy (XANES) and scanning

80

transmission X-ray microscopy (STXM).

81

■ THEORY

82

Kinetics Analysis. Kinetics models based on chemical reactions were used to describe

83

adsorption. The pseudo-second-order kinetics model assumes that the adsorption capacity is

84

proportional to the number of active sites occupied on the adsorbent.17 The adsorption of LEV on

85

iron-pillared montmorillonite was well fitted by the pseudo-second-order kinetic model and was

86

proposed as chemical adsorption.9 In this work, this model was also applied to simulate the LEV

87

adsorption on PBS. The basic equation is

88 89 90

=( − )

(1)

Integration with the initial condition qt=0 at t=0 gives

=

+

(2)

91

where k is the pseudo-second-order rate constant (g/(mg·h)), qe denotes the equilibrium

92

adsorption capacity of the adsorbate (mg/g).

93

Once the kinetic data of LEV adsorption was obtained, the rate constant at different

94

temperatures could be determined. According to the linearized Arrhenius equation (Eq. (3)),

95

activation energy can be determined by plotting lnk against 1/T (in Kelvin),

96

= −

+

(3)


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97

where Ea and R are the activation energy of adsorption (kJ/mol) and the universal gas constant

98

8.314 J/(mol·K), respectively. The magnitude of the activation energy was commonly used as

99

one of the criteria for differentiating physisorption and chemisorption. Physical adsorption is

100

readily reversible, and equilibrium is attained rapidly, thus energy requirements are small. The

101

energy of activation for physical adsorption is usually no more than 1 kcal/mol (equivalent to 4.2

102

kJ/mol). While the chemical adsorption is specific, involves stronger forces, thus requires larger

103

activation energies.18, 19

104

Isotherm Model. Considering that PBS was prepared from a lignocellulosic biomass and

105

contains various components, the Langmuir-Freundlich model20 was selected for simulating the

106

experimental data in this work and presented below,

107

=

(4)

108

where Ce represents the equilibrium concentration of adsorbate in liquid phase (mg/L); qm is the

109

maximum adsorption capacity of the adsorbent (mg/g); b is the adsorption equilibrium constant

110

relating to the binding energy of the adsorption system (L/mg); and n is indicative of the surface

111

site heterogeneity of the adsorbent.

112

Approximate Site Energy Distribution. Equilibrium adsorption capacity is associated with

113

the adsorption site energy distribution of the adsorbent. The relationship underlying the theory of

114

heterogeneous surfaces can be represented by the following Eq. (5),14, 21

115

(

(! ) = ") # ($, ! )&($)d$

(5)

116

where # ($, ! ) is the homogeneous isotherm over local adsorption sites with adsorption energy

117

E, and F(E) is the site energy frequency distribution over a range of sites with homogeneous



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energies. Adsorption energy E refers to the difference of adsorption energy between the solute

119

(adsorbate, LEV) and solvent (water) for a given adsorption site. The limits on the integral are

120

most appropriately based on the minimum and maximum adsorption energies, and generally

121

assumed from zero to infinity.22

122 123

According to the Cerofolini approximation,23, 24 the equilibrium liquid phase concentration (Ce) of adsorbate is related to the energy of adsorption (E) given by /0

124

! = !* +,-[−

125

! = !* +,- 1− 345

]

(6)

$∗

(7)

126

where Cs is the maximum solubility of adsorbate. In this work, the Cs value of LEV in water is

127

50 mg/mL at 298.15 K and pH 6.88,25 and that at 308.15 K and 318.15 K were estimated to be

128

58 mg/mL and 66 mg/mL, respectively, by the method from the references.26,

129

adsorption energy corresponding to Ce = Cs,28

27

Es is the

130

$ ∗ = $ − $*

131

The calculated E* refers to the difference of adsorption energies between the adsorbate and

132

solvent to the adsorbent surfaces based on the reference point Es. Thus E* can be calculated by

133

incorporation of the known values of Cs and Ce into Eq. (7). Assuming that the Langmuir-

134

Freundlich isotherm model is applicable to the adsorption system in this work, by incorporating

135

Eq. (7) into Eq. (4), the isotherm qe(Ce) can be written as a function of E*, expressed as qe(E*),

136

=

(8)

∗ −$ 34 6 7!8 + ∗ −$ 1+7!8 + 34

(9)

137

Then differentiating the isotherm, qe(E*) with respect to E*, an approximate site energy

138

distribution function F(E*) is obtained as follows,14, 21


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139


&($

∗)

=

/ ( ∗ )

(10)

∗

; ; )

140

&($ ∗ ) =

141

Eq. (11) was used to determine the site energy distribution of the adsorbent PBS for LEV

142

adsorption in this work after the parameters of the Langmuir-Freundlich model were determined

143

with the equilibrium adsorption data. Because the resulting site energy distributions are not

144

normalized, the area under the distribution equals to the maximum adsorption capacity qm:

145

146

")

(

&($ ∗ )?$ ∗ = @

(11)

(12)

■ EXPERIMENTAL SECTION

147

Materials. The raw barley straw (RBS) was provided by the Poultry Center of the University

148

of Saskatchewan, Saskatoon, Canada. It was sun dried, crushed and sieved to achieve the particle

149

sizes of 0.425-1.18 mm, and then the straw was dried in an oven at 378.15 K and kept in

150

desiccators.

151

Levofloxacin (LEV, C18H20FN3O4, ≥98 wt%, 361.37 g/mol) and acetonitrile anhydrous (99.8

152

wt%) were obtained from Sigma-Aldrich. Formic acid (88 wt%) and phosphoric acid (H3PO4, 85

153

wt%) were purchased from Fisher Scientific. Deionized water was used in all procedures.



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Pretreatment Method. PBS was prepared according to the previous work.7, 29 20 g dried

155

barley straws were impregnated with 400 mL 5 wt% H3PO4 solution, stirred by magnetic force at

156

100 rpm for 24 h, then filtered. The wet samples were transferred into a microwave furnace

157

(Rival, 700 W) for 9 min. During the heating process, temperature increased from 298.15 K to

158

the maximum value 844.15 K. After radiation, the samples were mixed with deionized water and

159

heated to 353.15-363.15 K for 30 min to remove residual H3PO4 and other salts until the filtrate

160

pH became constant at about 4. The wet samples were dried at 378.15 K. The particle size of

161

PBS was measured by Mastersizer 2000 (Malvern Instruments). The volume mean diameter was

162

316.1 µm, and median was 250.7 µm. The diameter where 10% of the population lies below this

163

value was 82.5 µm, and that where 90% of the population lies below this value was 650.9 µm.

164

The characterizations of PBS, such as Fourier transform infrared spectroscopy (FTIR), scanning

165

electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface area, were done in the

166

author’s previous works.7, 29


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Adsorption Experiments. All adsorption experiments were carried out in a batch mode. 5

168

mg PBS was mixed with 50.0 mL LEV solution. The initial concentrations of LEV ranged from

169

10 mg/L to 100 mg/L. The value of initial solution pH was adjusted to 7.03 ± 0.05 by 0.1 mol/L

170

NaOH. The value of final solution pH was 6.88 ± 0.03, which was chosen as the equilibrium

171

solution pH. During the adsorption process, no acid or base solution was added. The pH change

172

with a reference to the initial pH was lower than 2.1%. In the kinetics experiments, 5 mg PBS

173

particles were mixed with 50 mL LEV solution varying with three different initial concentrations

174

(10 mg/L, 40 mg/L and 80 mg/L) for 1, 2, 4, 6, 12, 24, 30, 36, 48, 72, 96, 120, 144 and 168 h.

175

The LEV solution temperatures were 298.15, 308.15 and 318.15 K, respectively. To reach

176

equilibrium, the contact time of PBS particles and LEV solution was set as 168 h in the isotherm

177

studies.

178

All suspensions were shaken at 150 rpm in the dark to avoid light. The supernatant of the

179

suspensions was filtered using 0.20 µm Acrodisc Syringe Filter (Pall Corporation) with 1 mL

180

syringe, and concentration of LEV in the filtrate was analyzed by high performance liquid

181

chromatography (HPLC). All experiments were conducted in triplicates. The same concentration

182

series of LEV solution without PBS were run at the same condition as the controls, showing that

183

the initially added amounts of LEV remained unchanged and no degradation of LEV in the

184

solution was observed. In addition, the t-test demonstrated that there was no evidence at the 1%

185

level of confidence that the original LEV concentration without filtration was higher than the

186

LEV concentration of the respective solution after filtration by the 0.20 µm Acrodisc Syringe

187

Filter. Thus, adsorption of LEV by the Acrodisc Syringe Filter was negligible. As such, the

188

amount of LEV adsorbed per unit mass of adsorbents was calculated by the mass difference of

189

LEV at the initial and final stage of adsorption divided the dry net weight of PBS.



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LEV concentration in the aqueous solution was determined by HPLC (Agilent Technologies

191

1260 Infinity Quaternary LC) equipped with a Poroshell 120, EC-C18 column (2.7 µm, 4.6×100

192

mm) and a UV detector at 293 nm. The mobile phase was 60:40 (v/v) of acetonitrile anhydrous

193

and formic acid at 0.1% in deionized water with a flow rate of 0.75 mL/min. The retention time

194

of LEV in the HPLC was 1.23 min, similar to the literatures.30, 31

195

STXM and XANES Spectroscopy. 5 mg PBS was mixed with 50.0 mL LEV solution (C0=100

196

mg/L). The value of initial solution pH was adjusted to 7.03 ± 0.05 by 0.1 mol/L NaOH. The

197

solution temperature was 298.15 K. All suspensions were shaken at 150 rpm in the dark to avoid

198

light. 168 hours were required to reach the adsorption equilibrium. The value of final solution pH

199

was 6.88 ± 0.03. The adsorbent (PBS) was collected by filtration and washed by deionized water

200

for 3 times, then freeze-dried. To obtain adequate samples for analysis, the experiments were

201

repeated for 3 times.

202

In addition, the particle samples of LEV-loaded PBS were dispersed in ethanol (95%, the

203

solubility of LEV in ethanol is 0.018 M32) before deposition on a Si3N4 window for measurement

204

of scanning transmission X-ray microscopy (STXM). Thus, the freely dissolved LEV was

205

removed from the LEV-loaded PBS samples. Then the STXM measurement was conducted at

206

the soft X-ray beamline 10ID-1 of the Canadian Light Source which is a 2.9 GeV third-

207

generation synchrotron facility. To provide a spatial resolution of 30 nm, a 25 nm outermost-

208

zone plate (CXRO, Berkeley Lab) was used. The in-plane polarization dependence of the sample

209

was averaged out by a circularly polarized soft X-ray beam generated from the SM elliptically

210

polarized undulator (EPU). Over a range of photon energies across the elemental edges of

211

interest, the samples were raster-scanned with the synchronized detection of transmitted X-rays

212

to generate image sequences (stacks). Nanospectroscopic data of X-ray absorption near-edge 12 ACS Paragon Plus Environment

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structure spectroscopy (XANES) were obtained from any physical location on the image stacks,

214

which was from a single pixel to a desired area of the specimen of interest. The C K-edge image

215

sequence (stack) covered an energy range of 280-320 eV, with energy steps as fine as 0.1 eV

216

around the XANES peaks, and 0.4-1.0 eV in the pre-edge and continuum. The XANES spectra

217

of PBS, LEV and PBS loading LEV were extracted from each pure region in the C K-edge image

218

stack. More details on STXM experimental and data analysis can be found in the publications.33-

219

35

220

■ RESULTS AND DISCUSSION

221

Adsorption Kinetics. As one of the important characteristics, adsorption kinetics affects the

222

adsorption efficiency which was illustrated in Figure S1. Figure 1a demonstrates the effect of

223

contact time on LEV adsorption by PBS at different solution temperatures. A rapid adsorption of

224

LEV was observed at the first 12 h, then followed by a slower adsorption process till reaching

225

equilibrium. At higher temperature, the LEV adsorption rate and equilibrium adsorption capacity

226

increased.

227

In addition, the results achieved in the previous work of the authors7 demonstrated that after

228

modification by H3PO4 impregnation and microwave heating, the LEV adsorption capacity of

229

PBS was much higher than that of RBS. For an example, 347 ± 12 mg LEV/g PBS was achieved

230

in comparison with 6.2 ± 0.5 mg LEV/g RBS at pH 6.80 ± 0.15 with the initial LEV

231

concentration being 40 mg/L. Furthermore, total organic carbon (TOC) released into suspensions

232

from the adsorbents significantly reduced from 34.4 ± 0.9 mg/g (RBS) to 0.9 ± 0.2 mg/g (PBS)

233

indicating enhanced stability of PBS. The detailed results in the regards can be found in the

234

previous publications of the authors.7, 29



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Simulation of the pseudo-second-order model for LEV adsorption on PBS at different

236

temperatures was displayed in Figure 1b. The fitting results along with corresponding values of

237

coefficient of determination (R2) and residual sum of square (RSS) were listed in Table 1. The

238

pseudo-second-order rate equation fitted the kinetics data well (R2>0.999). Furthermore, the

239

initial rate of LEV adsorption on PBS (calculated from kqe2) was 55.04 mg/(g·h) (40 mg/L, pH

240

6.88 and 298.15 K). This may be due to the well-developed porous structure (revealed by the

241

results of SEM and high specific surface area 1314 ± 10 m2/g) and modified functional groups of

242

PBS.7 Moreover, the initial rate of LEV adsorbed on PBS increased from 55.04 mg/(g·h) to

243

102.56 and 210.97 mg/(g·h) as temperature was increased from 298.15 K to 308.15 and 318.15

244

K. Temperature may have two effects on the adsorption kinetics: 1) the adsorption driving force

245

( − ) increased due to the increase of equilibrium adsorption capacity qe with increasing

246

temperature; 2) the adsorption rate constant k increased. The influence of temperature on

247

adsorption rate constant k can be quantified by the Arrhenius equation.

248

The values of LEV adsorption rate constant k at the tested temperatures determined by the

249

pseudo-second-order kinetics model were presented in Table 1. Then the linearized Arrhenius

250

equation (Eq. (3)) was used to determine the activation energy (Figure 1c), which was 45.9

251

kJ/mol. The obtained value of activation energy 45.9 kJ/mol in this work suggested that LEV

252

was primarily adsorbed on PBS via chemical adsorption.18, 19 In order to further elucidate the

253

adsorption mechanism, X-ray analyses were done as follows.


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254

π-π EDA Interaction Characterized by XANES Spectroscopy. The C 1s K-edge XANES

255

spectra of PBS, LEV and PBS loading LEV were displayed in Figure 2. There was a resonance

256

peak at an energy level of 285.52 eV observed on the spectrum of PBS. It represented 1s →π*

257

C=C of aromatic C.35 In addition, the peaks at 285.22 eV, 285.98 eV and 286.73 eV observed on

258

the LEV spectrum were attributed to carbon atoms within a benzene ring that are bound to

259

hydrogen (C=C*-H),36 nitrogen (C=C*-N)37 and fluorine (C=C*-F),38, 39 respectively. The strong

260

peak at 288.54 eV of LEV corresponded to 1s →π* C=O transition of carboxylic C.35 Compared

261

with the spectra of PBS, LEV, and PBS loading LEV, the peak at 285.52 eV of PBS blue shifted

262

to 285.82 eV of PBS loading LEV. This indicated the higher energy value of photon resonance

263

for excitation, and carbon atom of aromatic π* C=C on the adsorbent had a partial-positive

264

charge with less electron density and worked as electron donor.40 On the other hand, there was a

265

red shift from 286.73 eV of LEV to 286.58 eV of PBS loading LEV. According to the research

266

of norfloxacin adsorption on surface-modified carbon nanotubes,13

267

indicate that carbon atoms in the benzene ring attached to fluorine (C=C*-F) of LEV had partial-

268

negative charge with more electron density and served as electron acceptor due to the strong

269

electron withdrawing ability of F. The result of the C K-edge XANES spectroscopy was in

270

supportive of π-π EDA interactions between the aromatic π* C=C of PBS as the electron-donor

271

and π* carbon atoms in benzene ring attached to fluorine of LEV as the electron-acceptor.

this phenomenon may

272

The π-π EDA interactions have been reported as the predominant mechanism in the

273

adsorption of organic compounds (e.g. norfloxacin,13 naphthalene and atrazine14) on

274

heterogeneous adsorbents (e.g. lignin41 and humic substances42). PBS contains hydroxyl,

275

carboxyl and other polar groups,7 which tend to be polarizable. Surface sites close to polarized

276

edge sites or defects of graphene sheets were reported to be electron-rich-π-donors,43,

44

as 15

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277

addressed in adsorption of aromatic compounds (e.g. benzene and polycyclic aromatic

278

hydrocarbons) on black carbon (e.g. charcoal and soot).44-46 As such, the proposed force, π-π

279

EDA interactions between the polarizable PBS (aromatic groups, π-donors) and LEV (benzene

280

ring attached to F, π-acceptor), became reasonable. It is also consistent to the result of activated

281

energy determined in this work which fell in the energy range of chemical adsorption. In addition,

282

preliminary results showed that less than 30% of the adsorbed LEV was desorbed from PBS at

283

pH 2 with ethylenediaminetetraacetic acid.7 This also demonstrated the interaction between LEV

284

and PBS was strong. For aqueous environmental systems, multiple bonding mechanisms are

285

expected to operate simultaneously. Revealed by the results of FTIR in the previous work of the

286

authors,7 PBS has oxygen-containing functional groups, such as -OH and -COOH. These groups

287

might form hydrogen bonds with C=O and O-H groups in LEV molecule. H-bonds was also

288

reported in the adsorption of norfloxacin13 and substituted aromatics by carbon nanotube.47

289

However, the role of H-bonds in adsorption of LEV by PBS may not be significant because the

290

desorption efficiency at various pHs was very low. The details could be found in the previous

291

work of the authors.7 Hydrophobic interaction could play a role in LEV adsorption. In this work,

292

progress was made on elucidating the π-π interactions. Clearly, further study is required to

293

evaluate the importance of the multiple adsorption mechanisms.


Page 17 of 36


294

Adsorption Site Energy Analysis. In order to determine the adsorption site energy, the

295

adsorption isotherms of LEV on PBS at temperatures of 298.15, 308.15 and 308.15 K, were first

296

obtained and presented in Figure 3. The equilibrium LEV adsorption capacity of PBS increased

297

as temperature was increased at the tested conditions, revealing that the adsorption of LEV on

298

PBS was an endothermic process. This is consistent with the adsorption of norfloxacin on carbon

299

nanotubes.13 The Langmuir-Freundlich isotherm model (Eq. (4)) successfully described the LEV

300

equilibrium data with high values of R2, as given in Table 2. The values of n obtained at different

301

temperatures were similar, indicating the heterogeneity of the surface was similar in the tested

302

temperature range.

303

Based on the isotherm modeling results, the site energy E* was determined by Eq. (7). It was

304

plotted as a function of equilibrium LEV adsorption capacity at different temperatures in Figure

305

4a. As the amount of LEV adsorbed on PBS increased, the E* values decreased dramatically,

306

revealing that the high-energy adsorption sites on PBS were first occupied by LEV, then the low-

307

energy adsorption sites. This is consistent to the adsorption of norfloxacin on carbon nanotube.13

308

The site energy distributions determined based on the Langmuir-Freundlich isotherm model (Eq.

309

(11)) for LEV adsorption on PBS at different temperatures with respect to qm, b and n were

310

illustrated in Figure 4b.

311

To help understand the adsorption affinity and surface energy heterogeneity of PBS at

312

various temperatures, the average site energy was used to depict the interaction strength between

313

the adsorbent and adsorbate, and the width of the site energy distribution was employed to

314

describe the surface energy heterogeneity of the adsorbent.21 To get the average site energy µ(E*)

315

of PBS for LEV, the mathematical expectation of E* based on the site energy distribution in the

316

range from zero to infinity was calculated below: 17 ACS Paragon Plus Environment


317

A($

∗)

DE

=

"F

∗ ∙C( ∗ ) ∗

DE

"F

Page 18 of 36

(13)

C( ∗ ) ∗

318

Incorporating Eq. (11) and Eq. (12) into Eq. (13) and integrating leads to Eq. (14) to determining

319

the average site energy.

320

A($ ∗ ) =

321

It was reported that the higher the value of the average site energy, the higher the adsorption

322

affinity.21 Calculated by Eq. (14), the average site energy at 318.15 K (30.29 kJ/mol) was slightly

323

higher than that at 298.15 K (27.05 kJ/mol) and 308.15 K (28.48 kJ/mol), which demonstrated

324

the slightly higher adsorption affinity at 318.15K. The average site energy diversification of PBS

325

induced by the change of solution temperature can be explained as follows. LEV adsorption on

326

PBS was mainly through EDA interactions. They are polar interactions and based on the

327

attractive forces between electron-rich (donors) and electron-deficient (acceptors) entities.12, 48 In

328

this work, electron-rich aromatic π-system (PBS) served as π-donors, and electron-deficient π-

329

system (LEV) worked as π-acceptors in the adsorption process of LEV on PBS. It is known that

330

the strength of electron-donor and -acceptor increases with increasing polarizability of the

331

compound or structure involved. Increasing temperature increased the static dipole

332

polarizability,49, 50 which made the adsorbent (PBS) and adsorbate (LEV) to be stronger π-donors

333

and π-acceptors, therefore the corresponding adsorption affinity (π-π EDA interaction) was

334

enhanced. This was reflected by the slight increase of the average site energy as temperature was

335

increased.

n

ln(1 + 7!*: )

(14)

336

It was also found that similar to the values of activation energy, the obtained values of

337

average site energy (27.05-30.29 kJ/mol) fell in the range of the heat of chemical reaction (21-

338

420 kJ/mol),18 again supported chemisorption is predominant in LEV adsorption on PBS. 18 ACS Paragon Plus Environment

Page 19 of 36


339

Evidenced by the site energy distribution (Figure 4b), site energy heterogeneity was revealed

340

for adsorption of LEV on PBS, which can be characterized by the standard deviation I∗ of the

341

distribution.14 The higher the value of I∗ , the stronger the heterogeneity. I∗ is quantified by the

342

following equations,

343

344

345

346

A($

∗ )

=

DE

"F

∗ ∙ C( ∗ ) ∗

DE

"F

(15)

C( ∗ ) ∗

Again, incorporating Eq. (11) and Eq. (12) in the above equation and integrating, give A($ ∗ ) =

() :

0

")

(1 + 7!+ )?(7!+ )

(16)

Then the standard deviation can be calculated,

347

I∗ = JA($ ∗ ) − A($ ∗ )

348

Determined by Eq. (17), the I∗ values of PBS at 298.15, 308.15 and 318.15 K were 4.31,

349

4.51 and 4.30 kJ/mol, respectively. Generally, heterogeneity of adsorption sites for carbonaceous

350

adsorbents originated from the defect structures, as well as the crosslinking and disordered

351

arrangement of various carbon structures. The heterogeneous adsorption sites of graphitized

352

carbons for organic pollutants have been attributed to these aspects.43,

353

heterogeneity of adsorption sites could also be derived from the grafted functional groups

354

(chemical composition heterogeneity), especially oxygen-containing functional groups.53 In this

355

work, PBS was obtained from barley straw with H3PO4 impregnation and microwave heating.

356

The specific structure and induced functional groups, e.g., carboxyl and hydroxyl groups,

357

contributed to the heterogeneity of PBS. The values of I∗ , obtained at the tested three

358

temperatures are very similar, so are the values of n obtained from the Langmuir-Freundlich

359

model, which demonstrated the consistent result that in the tested temperature range, the

360

heterogeneity of the PBS surface was similar.

(17)

51, 52

In addition,



Page 20 of 36

361

In summary, the activation energy 45.9 kJ/mol, site energy and X-ray analyses in this work

362

supported that chemisorption was an important mechanism in the adsorption of LEV on PBS. π-π

363

EDA interactions played an important role.

364 365 366 367 368 369

As a critical environment parameter, temperature influences the adsorption process as follows: 1) The adsorption rate constant was increased at higher temperature which resulted in the increase of LEV adsorption rate. 2) Increasing temperature enhanced LEV adsorption capacity of PBS, demonstrating LEV adsorption on PBS was an endothermic process.

370

3) Elevated temperature increased the static dipole polarizability which resulted in

371

enhancement of the electron-donor and -acceptor strength. Thus, the π-π EDA

372

interactions between the aromatic adsorbent PBS and adsorbate LEV with benzene rings

373

were strengthened. This was reflected by the increase of average site energy.

374 375

4) Temperature did not significantly affect the site energy heterogeneity in the tested range (298.15 K-318.15 K).


Page 21 of 36


376

Environmental Impactions. PBS made from barley straw which is representative of

377

cellulose based agricultural byproducts, had a much higher LEV adsorption capacity compared

378

with most of the reported adsorbents. Adsorbents made from barley straw or similar

379

lignocellulosic biomass may potentially affect the environmental fate and transport of the

380

emerging pharmaceutical contaminants, and their possible toxicity and risks. The adsorption

381

kinetics, site energy distribution and X-ray spectroscopy analysis in this work provided

382

important information on elucidating adsorption mechanism. The methodology used in this work

383

may be transferable to investigate removal of other pollutants using adsorption.

384

■ NOMENCLATURE

385

b adsorption equilibrium constant (L/mg)

386

C0 initial adsorbate concentration (mg/L)

387

Ce equilibrium adsorbate concentration (mg/L)

388

Cs maximum solubility of adsorbate in water (mg/L)

389

E

390

adsorption energy refers to the difference between the adsorbate and solvent (water) for a given adsorption site (kJ/mol)

391

E* difference of adsorption energy at Ce and Cs (kJ/mol)

392

Es value of the adsorption energy corresponding to Ce = Cs (kJ/mol)

393

F(E) site energy frequency distribution over a range of energies

394

F(E*) site energy distribution over a range of energies (mg⋅mol/(g⋅kJ))

395

k pseudo-second-order rate constant (g/(mg·h))

396

n indicator of the surface site heterogeneity of adsorbent (dimensionless)

397

qe experimental equilibrium adsorption capacity (mg/g)

398

qe,cal equilibrium adsorption capacity from kinetics model (mg/g) 21 ACS Paragon Plus Environment


Page 22 of 36

399

qh(E,Ce) energetically homogeneous isotherm

400

qm maximum adsorption capacity (mg/g )

401

qt adsorption amount of the adsorbate at time t (mg/g)

402

R gas constant (8.314 J/(mol·K))

403

R2 coefficient of determination

404

RSS residual sum of squares ((mg/g)2)

405

T temperature (K)

406

I∗ energetical heterogeneity (kJ/mol)

407

µ(E*) average site energy (kJ/mol)

408

■ ASSOCIATED CONTENT

409

Support Information. Levofloxacin removal ratio as a function of initial levofloxacin

410

concentration

411

■ AUTHOR INFORMATION

412

Corresponding Author

413

Catherine Hui Niu

414

*

415

Notes

416

The authors declare no competing financial interest.

417

■ ACKNOWLEDGMENTS

418

Financial support for this project was provided by the China Scholarship Council (No.

419

201408530054), Natural Science and Engineering Research Council of Canada (No. RGPIN

Tel: 1 306 966 2174. Fax: 1 306 966 4777. E-mail: [email protected]


Page 23 of 36


420

299061-2013), Canada Foundation for Innovation (No. 11357), and the Saskatchewan Ministry

421

of Agriculture through Agriculture Development Fund (No. 20130220).

422

The synchrotron X-ray measurements were performed at the Canadian Light Source, Saskatoon,

423

Saskatchewan. We sincerely thank Richard Blondin of the Department of Chemical and

424

Biological Engineering at the University of Saskatchewan for his help in HPLC analysis.

425

All the supports are highly appreciated.

426

■ REFERENCES

427

(1) Yamashita, N.; Yasojima, M.; Nakada, N.; Miyajima, K.; Komori, K.; Suzuki, Y.; Tanaka,

428

H. Effects of antibacterial agents, levofloxacin and clarithromycin, on aquatic organisms. Water

429

Sci. Technol. 2006, 53 (11), 65-72.

430

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clarithromycin and azithromycin in wastewater treatment plant in Japan. Water Sci. Technol.

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(6) Khan, G. A.; Berglund, B.; Khan, K. M.; Lindgren, P. E.; Fick, J. Occurrence and

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449

(9) Liu, Y. N.; Dong, C.; Wei, H.; Yuan, W. H.; Li, K. B. Adsorption of levofloxacin onto an

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iron-pillared montmorillonite (clay mineral): Kinetics, equilibrium and mechanism. Appl. Clay

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Sci. 2015, 118, 301-307.

452 453 454 455 456 457 458 459

(10) Hattab, A. M. Adsorption of some fluoroquinolones on selected adsorbents. Master Dissertation, An-Najah National University, Nablus, Palestine, 2010. (11) Dong, S.; Sun, Y.; Wu, J.; Wu, B.; Creamer, A. E.; Gao, B. Graphene oxide as filter media to remove levofloxacin and lead from aqueous solution. Chemosphere 2016, 150, 759-64. (12) Keiluweit, M.; Kleber, M. Molecular-level interactions in soils and sediments: The role of aromatic π-systems. Environ. Sci. Technol. 2009, 43 (10), 3421-3429. (13) Wang, Z.; Yu, X.; Pan, B.; Xing, B. S. Norfloxacin sorption and its thermodynamics on surface-modified carbon nanotubes. Environ. Sci. Technol. 2010, 44 (3), 978-984.

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(14) Shen, X.; Guo, X.; Zhang, M.; Tao, S.; Wang, X. Sorption mechanisms of organic

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compounds by carbonaceous materials: Site energy distribution consideration. Environ. Sci.

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(15) Kumar, K. V.; Monteiro de Castro, M.; Martinez-Escandell, M.; Molina-Sabio, M.;

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Rodriguez-Reinoso, F. A site energy distribution function from Toth isotherm for adsorption of

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gases on heterogeneous surfaces. Phys. Chem. Chem. Phys. 2011, 13 (13), 5753-5759.

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(16) Jin, Q.; Yang, Y.; Dong, X.; Fang, J. Site energy distribution analysis of Cu(II) adsorption

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on sediments and residues by sequential extraction method. Environ. Pollut., Ser. B 2016, 208,

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(17) Ho, Y. S.; McKay, G. Pseudo-second order model for sorption processes. Process Biochem. 1999, 34 (5), 451-465. (18) Smith, J. M. Chemical Engineering Kinetics, 2nd ed.; McGraw-Hill Book Company: New York, 1970.

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studies of the adsorption of lead(II) ions onto phosphate-modified kaolinite clay. J. Hazard.

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(20) Sips, R. On the structure of a catalyst surface. J. Chem. Phys. 1948, 16 (5), 490-495.

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(21) Carter, M. C.; Kilduff, J. E.; Weber, W. J. Site energy distribution analysis of preloaded

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adsorbents. Environ. Sci. Technol. 1995, 29 (7), 1773-1780. (22) Jaroniec, M. Adsorption on heterogeneous surfaces: The exponential equation for the overall adsorption isotherm. Surf. Sci. 1975, 50 (2), 553-564. (23) Cerofolini, G. F. Localized adsorption on heterogeneous surfaces. Thin Solid Films 1974, 23 (2), 129-152. (24) Seidel, A.; Carl, P. S. The concentration dependence of surface diffusion for adsorption on energetically heterogeneous adsorbents. Chem. Eng. J. 1989, 44 (1), 189-194.



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U.S.

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M133684.pdf.

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and

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Administration:

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Spring,

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2008;

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(27) Blokhina, S. V.; Sharapova, A. V.; Ol'khovich, M. V.; Volkova, Т. V.; Perlovich, G. L.

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Solubility, lipophilicity and membrane permeability of some fluoroquinolone antimicrobials. Eur.

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J. Pharm. Sci. 2016, 93, 29-37.

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(28) Derylo-Marczewska, A.; Jaroniec, M.; Gelbin, D.; Seidel, A. Heterogeneity effects in single-solute adsorption from dilute solutions on solids. Chem. Scr. 1984, 24 (4-5), 239-246. (29) Yan, B.; Niu, C. H. Pre-trteating biosorbents for purification of ethanol from aqueous solution. Int. J. Green Energy 2017, 14 (3), 245-252.

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(30) Locatelli, M.; Ciavarella, M. T.; Paolino, D.; Celia, C.; Fiscarelli, E.; Ricciotti, G.;

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Pompilio, A.; Di Bonaventura, G.; Grande, R.; Zengin, G.; Di Marzio, L. Determination of

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ciprofloxacin and levofloxacin in human sputum collected from cystic fibrosis patients using

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microextraction by packed sorbent-high performance liquid chromatography photodiode array

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detector. J. Chromatogr. A 2015, 1419, 58-66.

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(31) Dafale, N. A.; Semwal, U. P.; Agarwal, P. K.; Sharma, P.; Singh, G. N. Development and

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validation of microbial bioassay for quantification of levofloxacin in pharmaceutical preparations.

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J. Pharm. Anal. 2015, 5 (1), 18-26.


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(32) Zhang, J.; Yang, X.; Han, Y.; Li, W.; Wang, J. Measurement and correlation for solubility

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of levofloxacin in six solvents at temperatures from 288.15 to 328.15 K. Fluid Phase Equilib.

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2012, 335, 1-7.

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Microwave-assisted synthesis of a core-shell MWCNT/GONR heterostructure for the

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electrochemical detection of ascorbic acid, dopamine, and uric acid. ACS Nano 2011, 5 (10),

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7788-7795.

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(35) Chen, C.; Dynes, J. J.; Wang, J.; Karunakaran, C.; Sparks, D. L. Soft X-ray

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spectromicroscopy study of mineral-organic matter associations in pasture soil clay fractions.

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Environ. Sci. Technol. 2014, 48 (12), 6678-6686.

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(36) Kuznetsova, A.; Popova, I.; Yates, J. T.; Bronikowski, M. J.; Huffman, C. B.; Liu, J.;

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Smalley, R. E.; Hwu, H. H.; Chen, J. G. Oxygen-containing functional groups on single-wall

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carbon nanotubes: NEXAFS and vibrational spectroscopic studies. J. Am. Chem. Soc. 2001, 123

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W.; Vinogradov, A. S. Electronic structure of fluorinated multiwalled carbon nanotubes studied

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absorption fine structure (NEXAFS) of model compounds for the humic acid/actinide ion

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(40) Okbinoğlu, T. N. Electronic structure of sulfur-nitrogen containing compounds:

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Correlations with theory and chemical reactivity. Ph.D. Dissertation, University of British

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Columbia, Vancouver, Canada, 2014.

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biopolymers: Role of polarity, structure and domain spatial arrangement. Chemosphere 2007, 66

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(42) Zhu, D.; Hyun, S.; Pignatello, J. J.; Lee, L. S. Evidence for π-π electron donor-acceptor

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interactions between π-donor aromatic compounds and π-acceptor sites in soil organic matter

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through ph effects on sorption. Environ. Sci. Technol. 2004, 38 (16), 4361-4368.

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(43) McDermott, M. T.; McCreery, R. L. Scanning tunneling microscopy of ordered graphite

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and glassy carbon surfaces: Electronic control of quinone adsorption. Langmuir 1994, 10 (11),

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black carbon (charcoal) assisted by graphite as a model. Environ. Sci. Technol. 2005, 39 (7),

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2033-2041.

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(45) Zhu, D.; Kwon, S.; Pignatello, J. J. Adsorption of single-ring organic compounds to wood

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charcoals prepared under different thermochemical conditions. Environ. Sci. Technol. 2005, 39

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aromaticity and substitution of hydroxyl groups. Environ. Sci. Technol. 2008, 42 (19), 7254-7259.


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(48) Foster, R.; Fyfe, C. A. Electron-donor-acceptor complex formation by compounds of

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biological interest II. The association constants of various 1,4-dinitrobenzene-phenothiazine drug

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complexes. Biochim. Biophys. Acta, Biophys. Incl. Photosynth. 1966, 112 (3), 490-495.

554 555

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the reaction mechanism of charge-transfer complexes between narcotic drugs and electronic

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acceptors. Int. J. Electrochem. Sci. 2013, 8 (1), 1274-1294.

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carbon surfaces: Relationships between electrode kinetics, capacitance, and morphology for

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glassy carbon electrodes. Anal. Chem. 1993, 65 (7), 937-944.

562 563

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564

(53) Yoon, T. H.; Benzerara, K.; Ahn, S.; Luthy, R. G.; Tyliszczak, T.; Brown, G. E.

565

Nanometer-scale chemical heterogeneities of black carbon materials and their impacts on PCB

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567

5923-5929.

568



Page 30 of 36

569

■ FIGURE CAPTIONS

570

Figure 1. Kinetics analysis of LEV adsorption on PBS at various temperatures, a) effect of

571

contact time on LEV adsorption on PBS; b) pseudo-second-order fitting of LEV adsorption on

572

PBS; c) analysis of activation energy of LEV adsorption on PBS. Error bars represent standard

573

deviation.

574

50.0 ± 0.5 mL LEV solution, 5.0 ± 0.1 mg PBS and pH 6.88 ± 0.03.

575

Figure 2. C K-edge XANES (C 1s) of PBS, LEV and PBS loading LEV. Spectra peaks

576

correspond to aromatic (C=C, 285.52 eV), carbon atom in benzene ring attached to hydrogen

577

(C=C*-H, 285.22 eV), carbon atom in benzene ring attached to amine group (C=C*-N, 285.98

578

eV), carbon atom in benzene ring attached to fluorine (C=C*-F, 286.73 eV) and carboxylic (-

579

COOH, 288.54 eV).

580

Figure 3. Adsorption isotherms of LEV on PBS at different temperatures. Error bars represent

581

standard deviation.

582

50.0 ± 0.5 mL LEV solution, 5.0 ± 0.1 mg PBS and pH 6.88 ± 0.03.

583

Figure 4. Site energy and its distribution of LEV adsorption on PBS at various temperatures, a)

584

dependence of site energy E* on LEV loading; b) site energy distribution.

585


Page 31 of 36


586 587

Figure 1



Page 32 of 36

588 589

Figure 2


Page 33 of 36


590 591

Figure 3

592



Page 34 of 36

593 594

Figure 4


Page 35 of 36

595


Table 1. Kinetics Parameters for the Adsorption of LEV on PBS Experimental C0

Pseudo-second-order qe

qe,cal

k

(mg/L) (mg/g)

(mg/g)

(g/(mg·h))

596

-

RSS R2

T (K)

(mg/g)2

298.15

10

96

98

2.43×10-3

0.999

335

298.15

40

347

357

4.31×10-4

0.999

5249

298.15

80

413

433

6.29×10-4

0.999

3477

308.15

40

358

365

7.70×10-4

0.999

5784

318.15

40

369

390

1.38×10-3

0.999

77961

RSS residual sum of squares ((mg/g)2)

597 598 599



600

Page 36 of 36

Table 2. Fitting Results of the Langmuir-Freundlich Model for LEV Adsorption on PBS qm

b

mg/g

(L/mg)

298.15

417

1.10

1.04

0.974

2525

308.15

437

1.17

1.03

0.977

2351

318.15

448

1.50

1.12

0.982

1970

RSS n

T (K)

R

2

(mg/g)2

601 602 603


Analyses of Levofloxacin Adsorption on Pretreated ... - ACS Publications

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