Surface complexation modeling of coadsorption of antibiotic ciprofloxacin and Cu(II) and onto goethite surfaces

Chemical Engineering Journal 269 (2015) 113–120

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Surface complexation modeling of coadsorption of antibiotic ciprofloxacin and Cu(II) and onto goethite surfaces Xueyuan Gu ⇑, Yinyue Tan, Fei Tong, Cheng Gu State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 163, Xianlin Ave., Nanjing 210023, PR China

h i g h l i g h t s

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

 Cip forms inner-sphere complex with

goethite via carboxylic group.  A goethite–Cu–Cip ternary surface

complex is formed to enhance Cip adsorption.  The carboxylic group and adjacent carbonyl group form a six-member ring with Cu.  A CD–MUSIC approach was developed to predict the adsorption.

a r t i c l e

i n f o

Article history: Received 31 October 2014 Received in revised form 29 December 2014 Accepted 31 December 2014 Available online 21 January 2015 Keywords: Fluoroquinolone Goethite Adsorption Modeling EXAFS FTIR

a b s t r a c t The coadsorption behavior of Cu(II) and ciprofloxacin (Cip), a zwitterionic fluoroquinolone (FQ) antibiotic, to goethite surfaces was characterized by means of batch adsorption experiments, attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The collective quantitative and spectroscopic results indicate that Cip was adsorbed onto goethite surfaces through a tridentate complex involving the bidentate inner-sphere coordination of the deprotonated carboxylate group and H-bonding through the adjacent carbonyl group on the quinoline ring. In contrast, in the presence of Cu(II), Cip adsorption is enhanced around pH 6. Spectroscopic results showed that a goethite–Cu–Cip ternary surface complex is formed through the bidentate coordination of Cu(II) with the oxygen from the deprotonated carboxylate group and the adjacent carbonyl oxygen. A charge distribution (CD) surface complexation model constrained by spectroscopic observations was developed to describe macroscopic adsorption trends. The model well described the adsorption of Cip and Cu(II) under various conditions with one set of parameters. These findings will help quantitatively predict the adsorption behavior of Cip and Cu(II) onto ferric oxides. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Fluoroquinolones (FQs) are a commonly used antibiotic family that inhibit bacterial growth by blocking DNA gyrase, an enzyme responsible for DNA molecular coiling [1]. Ciprofloxacin (Cip), a ⇑ Corresponding author. Tel./fax: +86 25 89680361. E-mail addresses: [email protected] (X. Gu), [email protected] (Y. Tan), [email protected] (F. Tong), [email protected] (C. Gu). http://dx.doi.org/10.1016/j.cej.2014.12.114 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

second-generation FQ, is currently the most widely prescribed FQ in the world [2]. In the last decade, antibiotics were widely detected in the aquatic and soil environment [2–4] and concerns about the development of antibiotic-resistant microbial populations and the health risks to animals and humans via the food chain are increasing [2,5,6]. Most FQs exhibit a high aqueous solubility under various pH and a large chemical stability in soil and wastewater system [3,7–10]. Conventional wastewater treatment processes are not

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effective at completely remove all antibiotics from wastewater [7,11,12]. Adsorption is a key process to understand the migration FQs in the environment and also an important technique to remove antibiotics in water treatment process [13]. In soils, Cip can be easily adsorbed with a high soil–water partition coefficient Kd of 210– 61,000 L kg1 depending on the soil properties [10,14]. A number of recent studies have investigated the adsorption of FQs onto soils and various minerals, such as amorphous aluminum and iron hydrous oxides [15,16], goethite [17,18], clay minerals [9,19–23], antatase [24] and soils [9,10]. Previous results suggested that the adsorption of FQs is greatly influenced by pH and occurs mainly through cation exchange reactions to clay minerals, surface complexation reactions to oxide minerals, and/or hydrogen bonding and columbic attraction to soil organic matter [15,17,19,20]. Paul et al. [24] successfully modeled the adsorption of ofloxacin onto a nano titanium oxide under various pH and electrolyte conditions and suggested a tridentate sorption mode involving the carboxylate group and the adjacent carbonyl group on the quinoline ring. It has long been known that FQs form stable complexes with metallic cations [25]. A number of studies revealed the crystal structures of FQs and some divalent metals, like Cu, Ni and Zn complexes using various spectroscopic techniques, in which metal was found to form complexes with FQs through the carbonyl and carboxylate group [25–28]. Recent studies demonstrated that the presence of divalent metals could enhance the retention of FQs in soils: Pei et al. [23] found that Cu(II) greatly increased the adsorption of Cip onto montmorillonite at pH >6.0 and suggested that it was due to a Cu(II) bridge effect; Perez-Guaita et al. [29] found that Cu(II) facilitated flumequine sorption onto an alkaline soil via the formation of a soil–Cu–flumequine ternary surface complex; and a recent study suggested that a high amount of Cu and Ca could decrease the Cip adsorption on sand media via competing adsorption under pH 5.6 condition [30]. However, so far, the adsorption of Cip in the Cip/metal/soil mineral ternary system has not been carefully characterized. In this work, the influence of Cu(II) on the adsorption of Cip onto goethite, a common crystalline iron oxide, was examined as a function of the pH, surface coverage and ionic strength using batch experiments and microscopic spectroscopy techniques. In addition, a surface complexation model was developed to predict the partition of Cip under the various conditions. The results will help better understand and predict the partition of FQs at ferric oxide/water interface. 2. Experimental 2.1. Materials Goethite was synthesized according to published methods [31]. The specific surface area of the sample as measured by N2-BET analysis was 63.53 m2 g1 (ASAP 2020, Micromeritics, USA), and the mean particle size was 3.04 lm (Mastersizer 2000, Malvern Co., UK). Cip (purity >98%) was obtained from the Sigma–Aldrich Corporation (St. Louis, MO) and stored at 20 °C prior to use. For the batch experiments, 200 mg L1 (e.g., 0.604 mM) aqueous Cip stock solution was prepared freshly by dissolving Cip in 2.4 mM HNO3. For the other chemical reagents that were used, please refer to the Supporting Information. 2.2. ATR–FTIR spectroscopy Fourier Transform Infrared (FTIR) spectra of aqueous Cip species and Cip adsorbed onto the goethite with and without Cu(II) were collected using a Vertex 70V FTIR spectrometer (Bruker, Germany) equipped with deuterated triglycine sulfate (DTGS) detector outfitted with a ZnSe crystal fitted in a horizontal attenuated total reflec-

tance (ATR) cell (Pike Technologies). A total of 256 scans with a spectral resolution of 4 cm1 were performed. The samples were obtained following the same procedure used for the batch sorption experiments. Briefly, 0.9 mM Cip was added to 4 g L1 goethite suspension with 0.01 M NaNO3 as a background electrolyte. After 48 h shaking, the suspension was centrifuged and the wet paste was collected and deposited into the ZnSe cell evenly. The spectra were recorded repeatedly until the signal stabilized. For the Cip–Cu–goethite ternary system, 0.9 mM Cu(II) and Cip were added simultaneously. FTIR spectra of the aqueous Cip species (0.9 mM) alone were also collected using the same procedure. To characterize the effect of pH, all samples were prepared at pH 3, 6 and 9. The spectral contributions from water were removed by subtracting the spectrum of the same electrolyte at the same pH as the Cip spectrum.

2.3. EXAFS data collection and analysis The extended X-ray absorption fine structure (EXAFS) spectra of Cu(II) that adsorbed onto goethite surface alone and with Cip at pH 6 were collected at the SSRF Synchrotron (Shanghai, China) on the beamline 14W1. The spectra of a synthesized Cu–Cip complex were collected as a reference [27]. All of the samples were mounted in thin plastic sample holders sealed with Kapton tape. The ring energy was 3.5 GeV with an optimal storage beam current around 200–300 mA. The measurements were carried out at the Cu K-edge (8979 eV) and the incident beam was monochromatized with a Si(1 1 1) double crystal. The energy was calibrated using a copper foil. The spectra were measured at room temperature with a 32-element Ge detector. An Al filter was used to reduce the interference from Fe. Three scans were averaged for each sample. EXAFS data analysis was performed using the Demeter package 0.9.18 with Ifeffit 1.2.11d [32]. The v(k) function was Fourier transformed by k3 weighting using the program Athena, and the shell-by-shell fitting was done in R-space using the program Artemis. More details on the sample preparation and data analysis are provided in the Supporting Information.

2.4. Batch adsorption experiments For all of the batch sorption experiments, 0.01 g goethite was added to a 20 mL glass vial. After a aliquot of Cip was added, a 0.01 M NaNO3 background electrolyte solution was added to bring the final volume up to 10 mL. The pH of the suspension was adjusted using HNO3 or NaOH across a pH range of 3–9. The glass vials were then wrapped with aluminum foil to prevent lightinduced decomposition. Suspensions were shaken for 48 h at 25 ± 0.5 °C. A previous adsorption kinetics experiment showed that the adsorption equilibrium was achieved in 24 h (see Supporting information). Controls (no goethite) were used to explain losses from sorption to glass during the experiments. The pH of the suspension was measured immediately after equilibration using an Orion 8272 PerpHect Ross Sure-Flow electrode. Then, the tubes were centrifuged and the supernatant was filtered through 0.22 lm nylon membrane filters. The Cip concentration in the supernatant was quantified using high performance liquid chromatography (Waters 2695-2489) that was equipped with a UV detector operating at 280 nm. An XDB-C18 column (5 lM 4.6  150 mm, Agilent 1200) was used, and the samples were evaluated at flow rate of 0.9 mL min1 with 20/80% (v/v) acetonitrile/0.05 M phosphoric acid mobile phases. The metal concentrations in the supernatant were measured by an atomic adsorption spectrometer (AAS) (Z-8100, Hitachi, Japan). The adsorbed amounts of Cip were determined by the differences.

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2.5. Surface complexation modeling

3.2. Adsorption edges of Cip onto goethite surfaces

The adsorption experimental data were fit using Basic Stern Model (BSM) and calculations were performed using the ECOSAT 4.9 program [33] in combination with a recent version of FIT 2.581[34]. The heterogeneity of goethite surfaces is described using a 1-pKa model of the singly and triply coordinated O(H) groups, e.g., „FeOH0.5 and „Fe3O0.5 sites with the charge distribution multisite complexation (CD–MUSIC) model developed by Hiemstra and co-workers [35–37]. The site densities of the two sites are 3.64 and 2.73 sites nm2, respectively [35,38]. The intrinsic proton affinities of the two sites were set equal to the point of zero charge (PZC) value of goethite [37] (Table1). The surface reactions and intrinsic surface constants (log K) of Cu(II) onto goethite were from Weng et al. [39] (Table1). The surface complexation constants of Cip on the goethite surfaces were fit from the goethite–Cip binary system; then these parameters were held constant when fitting the goethite–Cu–Cip ternary adsorption data.

Fig. 1 summarizes the adsorption the pH-edges of Cip alone and coadsorption with Cu(II) onto goethite surfaces as a function of the ionic strength and loading level. In general, the adsorption edges of Cip have two stages with increasing pH (Fig. 1a and b): at pH 3–6, the adsorption was little affected by pH; at pH >6, the adsorption rapidly decreased to minimal adsorption, which is consistent with previous studies for metal oxides [15,18,24]. Higher Cip loading level results in a higher adsorbed concentration, but a lower adsorption percentage at a fixed pH value and ionic strength (Fig. 1b). The most significant feature of Cip adsorption edges onto goethite is that increasing ionic strength leads to significant increases in Cip adsorption under acidic to neutral condition, which is opposite with knowledge for most organic ligands, which tend to either decrease or have little effect on adsorption with increasing ionic strength. Paul et al. [24] observed similar phenomenon when Cip adsorbed on TiO2. They suggested that this could be explained by the shielding effect of protonated amine group at the end of Cip molecular by electrolyte anions at higher ionic strength. Under acidic conditions, goethite surfaces carry positive charges and the amine group is protonated, resulting in electrostatic repulsion to goethite surfaces. A higher ionic strength will shield the electrostatic interactions between them, thus leading to higher adsorption. Adding Cu(II) to the system generally increased the Cip adsorption (Fig. 1c); however, the enhancement is pH-dependent: at pH 6, a maximum enhancement (50%) was observed at about 1:1 M ratio of Cu and Cip. It is probably because at pH 6, Cu(II) reached its maximum adsorption proportion onto goethite surface (Fig. 1d) and maximum complexation proportion with Cip as well (Fig. S2b). However, at pH greater than 6, deprotonated Cip starts to increase and induces desorption from the surfaces. Therefore, a maximum adsorption of Cip with Cu was observed at approximately pH 6. Results suggested that goethite might be a potential sorbent to remove Cip and Cu(II) at one time. In addition, phenomenon of higher ionic strength leads to higher Cip adsorption was also observed for the ternary system, indicating that Cu(II) did not affect the electrostatic shielding effect. On the other hand, the presence of Cip had almost no effect on the adsorption of Cu(II) (Fig. 1d) due to the strong surface complexation between Cu(II) and goethite surfaces.

3. Results and discussion 3.1. Aqueous speciation of Cip and complexation with Cu(II) Cip has multiple ionizable functional groups (Fig. S1), and can exist as a cation species (at pH <6), zwitterion (at pH 6–8), or anion species (at pH >8.5) depending on the solution pH (Fig. S2a) [15]. Molecular Cip contains a carboxylic group that can function as an efficient functional group to complex with metal cations [25,41]. Metals cations can form mono- or binary complex with FQs [29,41–44]. Turel and Bukoved [25] measured the complexation constants log K of Cip and Cu(II) using potentiometric titration technique (Table1) and suggested that three aqueous species, e.g. Cu(CipH±)2+, Cu(CipH±)2+ and Cu(CipH±/ 2  + Cip ) , were formed. The speciation distribution of Cip–Cu complexes shows that Cu(II) has strong complexation ability with Cip (Fig. S2b): when pH >4.5, approximately 50% Cip is complexed with Cu(II), and, when pH >5.5, more than 90% of Cip is bond with Cu(II). Under neutral condition the main species are Cu(CipH±)2+ and Cu(CipH±)2+ 2 , under alkaline condition is Cu(CipH±/Cip)+. Table 1 Summary of the surface complexation model parameters used in this study. No. 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. a

log Ka

Source

Cip + H = CipH CipH± + H+ = CipH+2 Cip + Cu2+ + H+ = Cu(CipH±)2+ 2Cip + Cu2+ + 2H+ = Cu(CipH±)2+ 2 2Cip + Cu2+ + H+ = Cu(CipH±/Cip)+2

8.54 6.17 14.73 28.53 22.18

Ref. Ref. Ref. Ref. Ref.

Surface reactionsb

log Ka

Dz 0

Dzs

9.3 9.3 0.6 0.6 8.62 8.62 9.18 3.6 3.65 3.1 38.82 24.88 15.64

1 1 0 0 1 1 0.84 0.84 0.84 0.84 1.95 0.8 1

0 0 1 1 1 1 1.16 0.16 1.16 0.16 1.05 1.2 0

Aqueous reactions 

+

±

+

„FeOH + H = „FeOH+0.5 2 „Fe3O0.5 + H+ = „Fe3OH+0.5 0.5 + +0.5 0.5

„FeOH + Na = „FeOHNa „Fe3O0.5 + Na+ = „Fe3ONa+0.5 0.5 0.5 „FeOH + NO 3 = „FeOH2NO3 0.5 „Fe3O0.5 + NO = „Fe OHNO 3 3 3 2„FeOH0.5 + Cu2+ = („FeOH)2Cu+1 0.5 2+ 2„FeOH + Cu + H2O = („FeOH)2CuOH0 + H+ + 2„FeOH0.5 + 2Cu2+ + 2H2O = („FeOH)2Cu2(OH)+1 2 + 2H 2„FeOH0.5 + 2Cu2+ + 2H2O = („FeOH)2Cu2(OH)03 + 3H+ 3„FeOH0.5 + Cip + 4H+ = („FeO)2(„FeOH2) CipH+1.5 + 2H2O 2„FeOH0.5 + Cip + Cu2+ + H+ = („FeOH)2CuCipH+1 2„FeOH0.5 + Cip + Cu2+ = („FeOH)2CuCip0

[21] [21] [21] [21] [21]

Ref. [43] Ref. [43] Ref. [43] Ref. [43] Ref. [43] Ref. [43] Ref. [43] Ref. [43] Ref. [43] Ref. [43] This studyc This studyc This studyc

All of the constants were at I = 0 calculated using the Davies equation [40]. The site densities of „FeOH0.5 and „Fe3O0.5 is 3.64 and 2.73 sites/nm2, respectively [35,38]. The specific surface area of goethite is 63.53 m2/g measured using N2-BET method. The Stern layer capacitance (j) is 0.1 F/m2 optimized in this study. c Surface reaction constants of Cip and the charge distribution factor were optimized in this work. The errors are one standard deviation. b

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-7

8.0x10

8.0x10 I = 0.1 M I = 0.01 M I = 0.001 M

-7

2

6.0x10

(b)Cip only Adsorbed CIP (mol/m )

2

Adsorbed CIP (mol/m )

(a)Cip only

-7

4.0x10

-7

2.0x10

-7

4.0x10

-7

2.0x10

(d)Cip+Cu

-7

8.0x10 2

-7

6.0x10

-7

6.0x10

0.0 -6 1.0x10

I=0.1M I=0.01M I=0.001M

(c)Cip+Cu

Adsorbed Cu (mol/m )

2

Adsorbed CIP (mol/m )

0.0 -7 8.0x10

0.034 mM CIP 0.060 mM CIP 0.139 mM CIP

-7

4.0x10

-7

2.0x10

-7

6.0x10

-7

4.0x10

-7

2.0x10

I=0.1M I=0.01M I=0.001M

0.0 0.0

2

3

4

5

6

7

8

9

pH

10

2

3

4

5

6

7

8

9

10

pH

Fig. 1. Adsorption edges of Cip onto goethite surfaces at (a) different ionic strengths and (b) different Cip concentrations. Co-adsorption edges of (c) Cip and (d) Cu(II) onto goethite surfaces in different ionic strengths. The suspension density of goethite was 1.0 g L1 and the background electrolyte was NaNO3. The initial concentration of Cip in (a), (c) and (d) was 0.060 mM and ionic strength in (b) was 0.01 M. Dots are experimental data and lines are model predication using parameters in Table 1.

3.3. ATR–FTIR spectroscopy The in situ ATR–FTIR spectra of Cip bonded on goethite surfaces alone and with Cu(II) at different pH condition are shown in Fig. 2. Spectra of aqueous Cip were also collected for reference. For the spectra of aqueous Cip, when pH increased from 3 to 6, the peak at 1720 cm1 assigned to carboxylic acid C@O stretch(mC=Ocarboxyl) [17] disappeared, indicating deprotonation of the carboxylic group when pH > pKa1. Concurrently, two bonds at 1580 cm1 and 1380 cm1 associated to asymmetric and symmetric stretching of the COO group, mCOOas and mCOOs, significantly strengthened in intensity with increasing pH, suggesting that deprotonation is enhanced with increasing pH. The peak of mCOOs downshifted from 1399 to 1375 cm1 with increasing pH, whereas mCOOas showed no obvious shift, which is consistent with previous results [17]. The intensity of the peak at 1620 cm1 associated to stretch of carbonyl group at quinoline ring (mC=Oketone) was consistent with the pH, whereas the position upshifted slightly, which might be induced by the neighboring deprotonated carboxyl group [15,17]. The spectra features of the adsorbed Cip with or without Cu(II) are relatively invariable with pH compared with aqueous Cip. The 1720 cm1 mC=Ocarboxyl bond was absent, and relatively strong mCOOas and mCOOs bonds were observed even at pH 3, indicating that the presence of goethite enhanced the deprotonation of the carboxylic group; therefore, complexation was expected between Cip and goethite surfaces. The most significant features of Cip adsorbed onto goethite are the position shift of mCOOas and mCOOs. It was observed that mCOOs remained invariable compared with aqueous Cip species, while mCOOas shifted to lower wavenumber. Nakamoto [45] introduced the use of the difference (Dm = mCOOas  mCOOs) to identify the binding mode of acetate groups with metals and suggested that a Dm of carboxylate–metal complex greater than the Dm of the free carboxylate species indicates a monodentate complex, otherwise

Fig. 2. ATR-FTIR spectra of Cip under aqueous conditions and adsorbed to goethite(GT) with and without Cu(II) at different pH. The total concentration is about 3.5 lmol Cip and 3.5 lmol Cu(II)/m2 goethite.

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a bidentate chelate. The Dm of goethite–Cip was less than that of free Cip at every pH value (Table S1), indicating a bidentate complex occurred between the carboxylic group of Cip and the goethite surface. Trivedi et al. [17] also found similar FTIR spectra feature for the goethite–Cip complex under different pH condition. However, with the presence of Cu(II), the Dm of goethite–Cu–Cip system remains less than those of free Cip at pH 3 and 9, but greater at pH 6. The results suggested that Cu(II) might induce a monodentate complex with the carboxylic group of Cip at about pH 6, which is consistent with previous report that Cu(II) complexes with FQs through the carboxylic and carbonyl O to form a six-member ring [25]. Additionally, the peak of quinoline ring carbonyl group (mC=Oketone) is widen and upshift upon Cip adsorption compared to aqueous Cip, suggesting the involvement of H-bonding or the coordination of the carbonyl group [24].

Table 2 Structure data of the first coordination shells obtained from R-space fits of the EXAFS spectra. Sample

Scattered

N

R (Å)

r2 (Å2)

R factor

CuCip22H2O K range: 3.5–10.8 Å1 R range: 1–4 Å

CuAO CuAO(H2O) CuAC1 CuAC2 CuAO(C@O) CuAC4

3.60 1.80 3.60 1.80 2.34 6.30

1.93 2.82 3.02 3.27 4.01 4.31

0.0044 0.0006 0.0045 0.0001 0.0019 0.0001

0.013

GT–Cu K range: 3.42–10.52 Å1 R range: 1–4 Å

CuAO CuAO CuACu CuAFe

3.60 0.9 0.9 2.7

1.95 2.23 2.93 3.47

0.0059 0.0025 0.0070 0.0208

0.0065

GT–Cu–Cip K range: 3.42–10.52 Å1 R range: 1–4 Å

CuAO CuAC1 CuAC2 CuAFe CuAO(C@O)

3.60 1.80 0.90 2.70 0.90

1.95 2.99 3.22 3.45 4.14

0.0041 0.0003 0.0002 0.0215 0.0005

0.015

3.4. EXAFS spectroscopy

Note: N number of neighbors; R absorber–neighbor distance; r2 Debye–Waller factor; R factor is the overall goodness of fit; GT refers to goethite. Uncertainties are estimated in coordination numbers to ±20%, in R to 0.02 Å, and in r2 to ±0.001 Å.

The EXAFS experimental and fitting results for three samples are shown in Fig. 3 and Table 2. To characterize the coordination property of Cu(II) and Cip, a Cu–Cip complex was synthesized according to Perez-Guaita et al. [27]. It was expected that the Cu(II) could form the binary complex CuCip22H2O with two FQs molecules [27,28,44,46]. EXAFS fitting results identified the first strongest peak in R-space corresponding to the coordinated first-shell of Cu with four CuAO with a distance of 1.93 ± 0.002 Å and two other CuAO contributions with a distance of 2.82 ± 0.002 Å (Table 2). In the second-shell, four CuAC and two CuAC contributions with a distance of 3.02 ± 0.002 Å and 3.27 ± 0.002 Å, respectively, were identified which is closed to the theoretic mean distance of C1 and C3 2.88 Å and C2 3.26 Å at quinoline ring to Cu (Fig. S3) [28]. Two CuAO with a distance of 4.01 ± 0.002 Å may correspond to the theoretical distance 4.05 Å of the oxygen atom at C@O of the carboxylic group to Cu [28]. The results suggest that Cu can complex with two Cip molecular and form a slightly elongated octahedral geometry with the carboxylic and adjacent carbonyl oxygen atoms in the equatorial plane and two water moleculars in axial plane (Fig. S3) which is consistent with previous reports [27,28]. Because of the disturbance of Fe, the signals in the K-space of Cu adsorbed onto goethite surface are relatively jumpy compared with

the Cu–Cip complex (Fig. 3). Previous reports suggested that the first coordination sphere of Cu(II) adsorbed onto oxide surfaces would fill with O atoms due to the Jahn–Teller effect [47]. The first peak in Rspace of the adsorbed Cu(II) samples had a similar position with Cu–Cip complex, indicating that the first-shell coordination environment in the three cases was similar. The best fits for Cu on goethite surfaces coordinated with four equatorial O atoms (1.95 ± 0.002 Å) and one more distant O atom (2.23 ± 0.002 Å) in the axial plane, completing the first coordination sphere. A second shell of cation at 2.93 ± 0.002 Å was also found which may be result from CuACu in polymerized (CuO4Hn)n6 complexes [47,48]. Further shells at a mean 3.47 ± 0.002 Å were found and they maybe contributed by Fe atoms from the goethite surface [48]. Although the Debye–waller factor r2 for the Cu–Fe path is relatively high, it might due the interference of Fe. The results suggest that Cu(II) form an inner-sphere bidentate complex with goethite surface and that under high surface loading, polymerization may occur. The spectra of goethite–Cu–Cip ternary system is similar to the first strong peak in R-space, which was fitted by four CuAO bond

data fit

25 15 20

Cu co-adsorbed with Cip

|X(R)| (A )

3

10

-4

-3

K ⋅X(K) (A )

15 10 5

adsorbed Cu

5

0

CuCip2.2H2O

0

-5 0

2

4

6

8 -1

K (A )

10

12

0

1

2

3

4

5

R (Å)

Fig. 3. Experimental data (line), fitted (dot) k3-weighted v spectra (left) and corresponding Fourier transforms (right, uncorrected for phase shift) for the Cu–Cip complexes, Cu(II) adsorbed onto goethite and coadsorbed with Cip on goethite at pH 6. Cu–Cip complex was synthesized according to Ref. [27]. The total concentration of Cip and Cu(II) adsorbed on goethite is about 3.9 lmol/m2 goethite.

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with a distance of 1.95 ± 0.002 Å. However, adding one or two distant CuAO did not significantly improve the fitting. The small peaks after the first strong one were slightly different from those of goethite–Cu system, indicating a different coordination environment in the further shells. For the ternary system, two CuAC at 2.99 ± 0.002 Å, one CuAC at 3.22 ± 0.002 Å and one CuAO at 4.14 ± 0.002 Å were characterized, which may be contributed by the carbon atoms at the quinoline ring and the oxygen atom at carboxylic group of the Cip molecular. In addition, CuAFe scatters at 3.45 ± 0.002 Å were also identified. While, no CuACu scatter was found, indicating no polymerization under such condition. These parameters suggested that Cu(II) may work as a bridge ion between goethite surfaces and Cip. Based on the ATR–FTIR and EXAFS spectra results, we believe that when Cip is adsorbed alone, it is mainly bound onto goethite surface through the formation a bidentate complex via carboxylic group and the binding is also facilitated by H-bonding of adjacent carbonyl oxygen (Fig. 4a). With the presence of Cu(II), a goethite– Cu–Cip ternary surface species tends to form and Cip binds to Cu(II) on surfaces through the carboxylic and carbonyl group to form a six-member ring (Fig. 4b).

carbonyl oxygen to facilitate the adsorption. The surface complexation reaction can be described by the following equation: 

þ1:5

3BFeOH0:5 þ Cip þ 4Hþ ¼ ðBFeOÞ2 ðBFeOH2 ÞCipH

ð1Þ 0.5

The triply coordinated site („Fe3O ) was assumed not participate in the Cip binding reaction. Experimental data in Fig. 1a and b were used to optimize the model parameters: intrinsic surface complexation constants (log K), charge distribution value (Dz0, Dzs) and the Stern layer capacitance (j). The model fitting results are summarized in Table 1. Results showed that the suggested surface reaction could well describe the Cip adsorption behavior as a function of pH, ionic strength and loading level (R2 = 0.95) (Fig. 1). The model also successfully predicted the unexpected ionic strength effect, e.g. the adsorption increased with the ionic strength. The surface reaction product bears +1.5 charges (Eq. (1)), indicating the surface reaction brings +3.0 charge difference (Dz) to the surfaces. At the goethite surfaces, the third protonated „FeOH+0.5 site will contribute about +0.5 2 valence, and the protonated amine group @NH+2 at the end of Cip molecular gives another +1.0 valence. The best fitting showed the surface charge distributed at the inner-layer and the Stern layer with +0.45 and +1.05 valence, respectively (e.g. Dz0 +1.95 and Dzd +1.05), which is close to the charge contribution values expected according to the proposed surface species structure. Paul et al. [24] suggested two surface species to model ofloxacin onto nano-anatase: „Ti3OH2(H-ofx)+1.5 and „Ti3OH2(H-ofx)+1.5ClO 4. However, we found that the contribution of the latter species was negligible and including it did not improve the fitting. Thus, Eq. (1) was used in this study to describe the Cip adsorption behaviors. The optimized Stern layer capacitance (j) is 0.1 F/m2 and is fixed in the ternary system. To characterize the coadsorption behavior of Cu(II) and Cip, one and more ternary surface species were used to fit the experimental data in Fig. 1c and d. The ternary surface species were structurally constrained by the ATR–FTIR and EXAFS spectroscopic results (Fig. 4b). The proposed surface species involves two surface sites and Cu works as a bridge ion to form a six-member ring with the carboxylic group and carbonyl oxygen of Cip. The surface complexation reaction is described by the following equation:

3.5. Surface complexation modeling To better characterize the adsorption behavior of Cip onto goethite surfaces with and without interaction with Cu(II), the SCM of Cip adsorption was developed in this study. Because Cip has a relatively large molecular size, it is unrealistic to simplify as point charges, such as metal ions. Thus the adsorption of Cip was treated using the CD (charge distribution) approach [35– 37] in this study, which considers the surface electrostatic charges to be spatially distributed across the interfacial region and enable a more realistic depiction of the surface charges for oxyanions or organic ligands. The adsorption experimental data in Fig. 1a and b were fit by considering the formation of goethite–Cip surface complexes that were structurally constrained by in situ ATR–FTIR spectroscopy observation. Fig. 4a shows the proposed main surface species of Cip onto goethite. In the proposed surface complexes, three surface sites were involved in adsorption: two sites form bidentate surface complexes with both of the oxygen atoms in the carboxylic group and a third „FeOH+0.5 site forms H-bonding with the adjacent 2



þ1

2BFeOH0:5 þ Cip þ Cu2þ þ Hþ ¼ ðBFeOHÞ2 CuCipH

H2+ N

N

NH2+

F

N O

N

O Cu

+0.5

O

O

H H O

O

O

Fe

Fe

Fe

Fe

Fe

(a)

N

F O

O

þ 2H2 O

(b)

Fig. 4. Proposed surface complexation species of Cip onto goethite surfaces (a) alone and (b) with Cu(II).

ð2Þ

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X. Gu et al. / Chemical Engineering Journal 269 (2015) 113–120 -7

-6

1.0x10

(a) CIP I=0.01M

-7

8.0x10

(b) Cu I=0.01M

2

Adsorbed Cu (mol/m )

2

Adsorbed CIP (mol/m )

6.0x10

-7

4.0x10

(FeOH)2CuCipH Total

-7

2.0x10

Fe3OH2CipH (FeOH)2CuCip 0.0

3

4

5

6 pH

7

Total FeOHCuOH

-7

6.0x10

-7

4.0x10

(FeOH)2CuCipH -7

2.0x10

(FeOH)2CuCip

0.0 8

9

3

4

5

6

7

8

9

pH

Fig. 5. The modeled surface speciation diagram of (a) Cip and (b) Cu co-adsorbed onto goethite surfaces at 0.01 M ionic strength. The dots are experimental data, and the lines represent the model predicted using parameters in Table 1.

The previous fitting showed that only Eq. (2) could not describe the adsorption edges in the ternary system. Because the presence of Cu(II) generally enhanced the Cip binding, the deprotonation of the amine group need to be considered at pH > pKa2. Therefore, the deprotonation of amine group was added to the model: 

0

2BFeOH0:5 þ Cip þ Cu2þ ¼ ðBFeOHÞ2 CuCip

ð3Þ

In addition to the two surfaces reactions, the complexation of Cu(II) with Cip under aqueous conditions, and the Cu(II) and Cip adsorption reactions were also included in the model matrix (Table 1). Because the surface charge of („FeOH)2CuCip0 is zero, the Dz0 and Dzs were set as 1 and 0 at inner and Stern layers, respectively. Thus, a total of three parameters were optimized to fit the experimental data in Fig. 1c: the log K of reactions (2) and (3) and the charge distribution value of reaction (2). The best fitting results are shown in Table 1 and Fig. 1c and d. The suggested two surface reactions generally well described the coadsorption behavior of Cu(II) and Cip at different ionic strengths (R2 = 0.87). The model only slightly underestimated the Cip adsorption at the lowest ionic strength (Fig. 1c). The optimized charge difference of reaction (2) is Dz0 +0.8 and Dzs +1.2, indicating the surface charge is mainly located at the Stern layer, which might come from protonated amine group. The surface speciation distribution of Cip and Cu is shown in Fig. 5. The model prediction shows that although Cip has strong complexation ability with Cu(II), it was not bound to goethite through Cu(II) throughout the pH range. When pH <4.5, Cip is adsorbed through goethite–Cip complex. When pH = 4.5–6, ternary species („FeOH)2CuCipH+1 becomes the main surface species and enhances Cip adsorption to maximum at about pH 6. When pH >6, the amine group of Cip begins to deprotonate, and the adsorption of Cip quickly decreases to minimum at pH 9. On the other hand, the adsorption capacity of Cu(II) was not influenced by Cip, and the surface species distribution varied with Cip (Fig. 5b). The results showed that the main surface species of Cu(II) is („FeOH)2CuOH, whereas at pH 6, the formation of ternary species occupies about 1/3 of the adsorbed Cu(II). In summary, the three surface reactions we proposed based on spectroscopic observation can well describe the adsorption of Cip with and without Cu(II) as a function of pH, ionic strength and loading concentration. 4. Conclusion Cip can be strongly adsorbed onto goethite surface with a reversed adsorption edge along pH and higher adsorption was found with higher ionic strength condition. While with the presence of Cu(II), adsorption of Cip was significantly enhanced at pH

range around 6 by formation of a goethite–Cu–Cip ternary complex, indicating goethite might be a potential sorbent to remove Cip and Cu(II) simultaneously. The ATR–FTIR and EXAFS spectroscopic evidence indicated Cip can directly interact with goethite surface via formation of bidentate complex of carboxylic group. With the presence of Cu(II), a six-member ring is formed between Cu and the carboxylic group and adjacent carbonyl group of Cip. A CD–MUSIC model was developed to fit the macroscopic batch experimental data. Three surface species, („FeO)2(„FeOH2)CipH+1.5, („FeOH)2CuCipH+1 and („FeOH)2CuCip0, which were structurally constrained by spectroscopic observations, could successfully fit all sets of data with different pH, ionic strength and surface coverage. The results can help better understand and quantitatively predict the partition of Cip and Cu(II) at ferric oxide/ water interface. Acknowledgment The authors would like to thank the Natural Science Foundation of China (Nos. 21237001, 21277068) and Tianjin Municipal Science and Technology Commission (Grant 13JCZDJC35900) for financial support.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.12.114. References [1] D.C. Hooper, J.S. Wolfson, E.Y. Ng, M. Swartz, Mechanisms of action of and resistance to ciprofloxacin, Am. J. Med. 82 (1987) 12–20. [2] Y. Picó, V. Andreu, Fluoroquinolones in soil—risks and challenges, Anal. Bioanal. Chem. 387 (2006) 1287–1299. [3] S. Thiele-Bruhn, Pharmaceutical antibiotic compounds in soils – a review, J. Plant Nutr. Soil Sci. 166 (2003) 145–167. [4] D.W. Kolpin, E.T. Furlong, M.T. Meyer, E.M. Thurman, S.D. Zaugg, L.B. Barber, H.T. Buxton, Pharmaceuticals, hormones, and other organic wastewater contaminants in US streams, 1999–2000: a national reconnaissance, Environ. Sci. Technol. 36 (2002) 1202–1211. [5] D.G. Kennedy, A. Cannavan, R.J. McCracken, Regulatory problems caused by contamination, a frequently overlooked cause of veterinary drug residues, J. Chromatogr. A 882 (2000) 37–52. [6] S. Thiele-Bruhn, I.-C. Beck, Effects of sulfonamide and tetracycline antibiotics on soil microbial activity and microbial biomass, Chemosphere 59 (2005) 457– 465. [7] X. Yang, R.C. Flowers, H.S. Weinberg, P.C. Singer, Occurrence and removal of pharmaceuticals and personal care products (PPCPs) in an advanced wastewater reclamation plant, Water Res. 45 (2011) 5218–5228. [8] E.M. Golet, I. Xifra, H. Siegrist, A.C. Alder, W. Giger, Environmental exposure assessment of fluoroquinolone antibacterial agents from sewage to soil, Environ. Sci. Technol. 37 (2003) 3243–3249.

120

X. Gu et al. / Chemical Engineering Journal 269 (2015) 113–120

[9] A.J. Carrasquillo, G.L. Bruland, A.A. MacKay, D. Vasudevan, Sorption of ciprofloxacin and oxytetracycline zwitterions to soils and soil minerals: influence of compound structure, Environ. Sci. Technol. 42 (2008) 7634–7642. [10] D. Vasudevan, G.L. Bruland, B.S. Torrance, V.G. Upchurch, A.A. MacKay, pHdependent ciprofloxacin sorption to soils: interaction mechanisms and soil factors influencing sorption, Geoderma 151 (2009) 68–76. [11] M.R. Awual, M. Ismael, T. Yaita, S.A. El-Safty, H. Shiwaku, Y. Okamoto, S. Suzuki, Trace copper(II) ions detection and removal from water using novel ligand modified composite adsorbent, Chem. Eng. J. 222 (2013) 67–76. [12] M.R. Awual, T. Yaita, S.A. El-Safty, H. Shiwaku, S. Suzuki, Y. Okamoto, Copper(II) ions capturing from water using ligand modified a new type mesoporous adsorbent, Chem. Eng. J. 221 (2013) 322–330. [13] P. Drillia, K. Stamatelatou, G. Lyberatos, Fate and mobility of pharmaceuticals in solid matrices, Chemosphere 60 (2005) 1034–1044. [14] J. Tolls, Sorption of veterinary pharmaceuticals in soils – a review, Environ. Sci. Technol. 35 (2001) 3397–3406. [15] C. Gu, K.G. Karthikeyan, Sorption of the antimicrobial ciprofloxacin to aluminum and iron hydrous oxides, Environ. Sci. Technol. 39 (2005) 9166– 9173. [16] K.W. Goyne, J. Chorover, J.D. Kubicki, A.R. Zimmerman, S.L. Brantley, Sorption of the antibiotic ofloxacin to mesoporous and nonporous alumina and silica, J. Colloid Interface Sci. 283 (2005) 160–170. [17] P. Trivedi, D. Vasudevan, Spectroscopic investigation of ciprofloxacin speciation at the goethitewater interface, Environ. Sci. Technol. 41 (2007) 3153–3158. [18] H. Zhang, C.-H. Huang, Adsorption and oxidation of fluoroquinolone antibacterial agents and structurally related amines with goethite, Chemosphere 66 (2007) 1502–1512. [19] C.-J. Wang, Z. Li, W.-T. Jiang, J.-S. Jean, C.-C. Liu, Cation exchange interaction between antibiotic ciprofloxacin and montmorillonite, J. Hazard. Mater. 183 (2010) 309–314. [20] Q. Wu, Z. Li, H. Hong, K. Yin, L. Tie, Adsorption and intercalation of ciprofloxacin on montmorillonite, Appl. Clay Sci. 50 (2010) 204–211. [21] C.-J. Wang, Z. Li, W.-T. Jiang, Adsorption of ciprofloxacin on 2:1 dioctahedral clay minerals, Appl. Clay Sci. 53 (2011) 723–728. [22] A. Nowara, J. Burhenne, M. Spiteller, Binding of fluoroquinolone carboxylic acid derivatives to clay minerals, J. Agric. Food Chem. 45 (1997) 1459–1463. [23] Z.-G. Pei, X.-Q. Shan, J.-J. Kong, B. Wen, G. Owens, Coadsorption of ciprofloxacin and Cu(II) on montmorillonite and kaolinite as affected by solution pH, Environ. Sci. Technol. 44 (2010). [24] T. Paul, M.L. Machesky, T.J. Strathmann, Surface complexation of the zwitterionic fluoroquinolone antibiotic ofloxacin to nano-anatase TiO2 photocatalyst surfaces, Environ. Sci. Technol. 46 (2012) 11896–11904. [25] I. Turel, N. Bukovec, Complex formation between some metals and a quinolone family member (ciprofloxacin), Polyhedron 15 (1996) 269–275. [26] H. Ftouni, S. Sayen, S. Boudesocque, I. Dechamps-Olivier, E. Guillon, Structural study of the copper(II) – enrofloxacin metallo-antibiotic, Inorg. Chim. Acta 382 (2012) 186–190. [27] D. Perez-Guaita, S. Boudesocque, S. Sayen, E. Guillon, Cu(II) and Zn(II) complexes with a fluoroquinolone antibiotic: spectroscopic and X-ray absorption characterization, Polyhedron 30 (2011) 438–443. [28] J. Hernández-Gil, L. Perelló, R. Ortiz, G. Alzuet, M. González-Álvarez, M. LiuGonzález, Synthesis, structure and biological properties of several binary and ternary complexes of copper(II) with ciprofloxacin and 1,10 phenanthroline, Polyhedron 28 (2009) 138–144. [29] D.P. Guaita, S. Sayen, S. Boudesocque, E. Guillon, Copper(II) influence on flumequine retention in soils: macroscopic and molecular investigations, J. Colloid Interface Sci. 357 (2011) 453–459.

[30] H. Chen, L.Q. Ma, B. Gao, C. Gu, Effects of Cu and Ca cations and Fe/Al coating on ciprofloxacin sorption onto sand media, J. Hazard. Mater. 252–253 (2013) 375–381. [31] M.L. Machevsky, M.A. Anderson, Calorimetric acid–base titrations of aqueous goethite and rutile suspensions, Langmuir 2 (1986) 583–587. [32] B. Ravel, M. Newville, ATHENA and ARTEMIS: data analysis for X-ray absorption spectroscopy using IFEFFIT, J. Synchrotron. Radiat. 12 (2005). [33] M.G. Keizer, W.H. van Riemsdijk, ECOSAT: A Computer Program for the Calculation of Speciation and Transport in Soil–Water Systems, Version 4.9 User’s Manual, Wageningen University, The Netherlands, 2009. [34] D.G. Kinniburgh, Fit, Technical Report WD/93/23, British Geological Survey, Keyworth, Nottinghamshire, 1993. [35] T. Hiemstra, W.H. van Riemsdijk, A surface structural approach to ion adsorption: the charge distribution (CD) model, J. Colloid Interface Sci. 179 (1996) 488–509. [36] T. Hiemstra, W.H. van Riemsdijk, G.H. Bolt, Multisite proton adsorption modeling at the solid-solution interface of (hydr)oxides: a new approach. 1. Model description and evaluation of intrinsic reaction constants, J. Colloid Interface Sci. 133 (1989) 91–104. [37] T. Hiemstra, J.C.M. Dewit, Multisite proton adsorption modeling at the solidsolution interface of (hydr)oxides: a new approach. 2. Application to various important (hydr)oxides, J. Colloid Interface Sci. 133 (1989) 105–177. [38] C.M. Jonsson, P. Persson, S. Sjöberg, J.S. Loring, Adsorption of glyphosate on goethite (a-FeOOH): surface complexation modeling combining spectroscopic and adsorption data, Environ. Sci. Technol. 42 (2008) 2464–2469. [39] L. Weng, W.H. Van Riemsdijk, T. Hiemstra, Cu2+ and Ca2+ adsorption to goethite in the presence of fulvic acids, Geochim. Cosmochim. Acta 72 (2008) 5857– 5870. [40] C.W. Davies, Ion Association, Butterworths, London, 1962. [41] I. Turel, The interactions of metal ions with quinolone antibacterial agents, Coord. Chem. Rev. 232 (2002) 27–47. [42] I. Turel, I. Leban, N. Bukovec, Synthesis, characterization, and crystal-structure of a copper(II) complex with quinolone family member (ciprofloxacin) – bis(1cyclopropyl-6-fluro-1,4-dihydro-4-oxo-7-piperazin-1-ylquinoline-3carboxylate) copper(II) chloride hexahydrate, J. Inorg. Biochem. 56 (1994) 273–282. [43] I. Turel, L. Golic, O.L.R. Ramirez, Crystal structure and characterization of a new copper(II)-ciprofloxacin (cf) complex Cu(cf)(H2O)(3) SO4 center dot 2H(2)O, Acta Chim. Slov. 46 (1999) 203–211. [44] E.K. Efthimiadou, Y. Sanakis, C.P. Raptopoulou, A. Karaliota, N. Katsaros, G. Psomas, Crystal structure, spectroscopic, and biological study of the copper(II) complex with third-generation quinolone antibiotic sparfloxacin, Bioorg. Med. Chem. Lett. 16 (2006) 3864–3867. [45] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part B: Applications in Coordination, Organometallic, and Biorganic Chemistry, 5th ed., Wiley-Interscience, New York, 1997. [46] I. Turel, L. Golic, P. Bukovec, M. Gubina, Antibacterial tests of bismuth(III)– quinolone (ciprofloxacin, cf) compounds against Helicobacter pylori and some other bacteria. Crystal structure of (cfH(2))(2) Bi2Cl104H(2)O, J. Inorg. Biochem. 71 (1998) 53–60. [47] R.H. Parkman, J.M. Charnock, N.D. Bryan, F.R. Livens, D.J. Vaughan, Reactions of copper and cadmium ions in aqueous solution with goethite, lepidocrocite, mackinawite, and pyrite, Am. Miner. 84 (1999) 407–419. [48] C.L. Peacock, D.M. Sherman, Copper(II) sorption onto goethite, hematite and lepidocrocite: a surface complexation model based on ab initio molecular geometries and EXAFS spectroscopy, Geochim. Cosmochim. Acta 68 (2004) 2623–2637.