Chemosphere 111 (2014) 283–290

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Modeling of levofloxacin adsorption to goethite and the competitionwith phosphate

http://dx.doi.org/10.1016/j.chemosphere.2014.04.0320045-6535/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 010 82321027; fax: +86 010 82321081.E-mail address: feiliu@cugb.edu.cn (F. Liu).

Xiaopeng Qin a, Fei Liu a,⇑, Guangcai Wang a, Lu Li a, Yang Wang a, Liping Weng b

a School of Water Resources and Environment, and Beijing Key Laboratory of Water Resources and Environmental Engineering, China University of Geosciences (Beijing),Beijing 100083, PR Chinab Department of Soil Quality, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands

h i g h l i g h t s

�We investigate adsorption oflevofloxacin (LEV) to goethite.� The presence of phosphate decreases

the adsorption of LEV.� Eight types of LEV–goethite

complexes were proposed andmodeled.� Adsorption of LEV (or phosphate) is

well predicted using the CD-MUSICmodel.

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

a r t i c l e i n f o

Article history:Received 8 November 2013Received in revised form 18 March 2014Accepted 12 April 2014Available online 13 May 2014

Handling Editor: X. Cao

Keywords:AdsorptionGoethiteLevofloxacinFluoroquinolonePhosphateCD-MUSIC model

a b s t r a c t

Interaction between various compounds in natural systems may influence the adsorption of these speciesand their environmental fate. In this work, we studied the interactions between a widely used antibioticlevofloxacin (LEV) and phosphate at the surface of goethite (a-FeOOH), which was important to betterunderstand the competitive adsorption of antibiotics and oxyanions in natural systems. The presenceof phosphate decreased LEV adsorption to goethite significantly over the whole pH range. The otherway around, LEV had a little influence on phosphate adsorption. Eight types of LEV–goethite complexeswere proposed and modeled in our study. Electrostatic competition was the main reason for the compe-tition of binary components (LEV and phosphate) to goethite surface. Adsorption of single component(LEV or phosphate) to goethite was well predicted using the CD-MUSIC (Charge Distribution Multi-SiteComplexation) model. In competition experiments, phosphate adsorption was still predicted well, butLEV adsorption was overestimated in model calculations. This is because less negative charge of LEV islocated at outer electrostatic plane in our study, which decreases their electrostatic competition to goe-thite surface.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction (Sarmah et al., 2006). Residuals of antibiotics have been detected

Fluoroquinolones are a group of broad spectrum antibacterialagents that are widely used in human and veterinary treatments

in surface waters, wastewaters, sediments, soils, and groundwaters worldwide (Golet et al., 2003; Fatta-Kassinos et al., 2011;Senta et al., 2013). The presence of incompletely metabolized anti-biotics in the ecosystem could alter the microbial communities andlead to the occurrence of antibiotic resistance in natural systems

284 X. Qin et al. / Chemosphere 111 (2014) 283–290

(Córdova-Kreylos and Scow, 2007; Girardi et al., 2011). Chemicalspeciation of antibiotics in the environment influences their mobil-ity, bioavailability and degradability. Therefore, the interactionsbetween antibiotics and soils/sediments play a key role in deter-mining their environmental fate.

Adsorption to natural particle surfaces is one important mech-anism of interactions between antibiotics and soils/sediments.Natural particle surfaces reactive to antibiotics include both natu-ral organic particles and minerals. Adsorption of antibiotics to min-erals have been studied previously, using goethite (Figueroa andMackay, 2005; Zhang and Huang, 2007; MacKay and Seremet,2008), hematite (Figueroa and Mackay, 2005), alumina (Goyneet al., 2005; Peterson et al., 2010), montmorillonite (Nowaraet al., 1997; Figueroa et al., 2004), and kaolinite (MacKay andSeremet, 2008; Li et al., 2011). In these studies, the adsorption datawere usually fitted using Langmuir or Freundlich models. Underthe experimental conditions, the fitting is relatively straightfor-ward due to the simplicity of these models. However, antibioticsmostly contain several functional groups (i.e. carboxyl, amine),and they are present as various species in the solution phase underdifferent pH conditions, and probably also on the mineral surfacewhen adsorbed. Therefore, empirically derived models are notapplicable to conditions beyond the calibration range (i.e. otherpH, concentration). The presence of both anionic and cationicgroups on the adsorbed antibiotics further complicates descriptionof their electrostatic interactions with both the surface and otheradsorbate. For the purpose of both understanding and prediction,mechanistic-based surface complexation models are more power-ful compared to the empirical adsorption models.

The Charge Distribution Multi Site Complexation (CD-MUSIC)model of Hiemstra and Van Riemsdijk (1996) is a mechanistic sur-face complexation model developed for mineral surfaces. The CD-MUSIC model aims to base the surface species used in the modelas much as possible on the spectroscopic evidence. Even for inor-ganic ions, the charge of the ion adsorbed is not considered as pointcharge, but distributed over different electrostatic planes (ChargeDistribution, CD). The CD-MUSIC model has been successfullyapplied to describe adsorption of various anions and cations(Venema et al., 1997; Rahnemaie et al., 2007a,b; Weng et al.,2008, 2012), small organic acids (Geelhoed et al., 1998; Filius,2001), and humic substances (Filius, 2001; Weng et al., 2005) tominerals. Application of advanced surface complexation model todescribe adsorption of antibiotics has just started. Paul et al.(2012) investigated the adsorption of ofloxacin, one fluoroquino-lone antibiotic, to anatase (TiO2), and applied the CD-MUSIC mod-eling approach for the first time to describe the effects of pH,concentration and ionic strength on ofloxacin adsorption to ana-tase. The model is constrained by spectroscopic observations andconsiders formation of tridentate species involving bridging biden-tate inner-sphere coordination of the deprotonated carboxylategroup and hydrogen bonding through the adjacent carbonyl group.

To become applicable to natural samples, a surface complexa-tion model has to be able to describe the adsorption behavior inmulti-component systems. Phosphate is a major anion in environ-mental samples and has a high affinity to metal (hydr)oxides insoils and sediments. It has been shown that presence of phosphatesignificantly affected the adsorption of other compounds (i.e.humic substances, citrate, Ca, and As) (Geelhoed et al., 1998;Rietra et al., 2001; Weng et al., 2008; Qin et al., 2012; Sø et al.,2012). So far, the influence of phosphate on antibiotics adsorptionto minerals has been seldom reported and modeled. Description ofmulti-component adsorption is a strong test of the robustness ofsurface complexation models.

In this study, a widely used antibiotic levofloxacin (LEV) wasstudied for its adsorption and competition with phosphate to goe-thite. The CD-MUSIC model was firstly used to describe LEV

adsorption to goethite under different pH conditions and concen-trations of LEV. Secondly, the model with the same parametersderived from the single component adsorption experiment wasused to predict the competitive adsorption of LEV and phosphate.To our knowledge, this is the first modeling exercise to describecompetitive adsorption of antibiotics and oxyanions on minerals.

2. Materials and methods

2.1. Materials

Goethite was prepared by hydrolyzing iron salt followed byaging (Hiemstra et al., 1989a; Antelo et al., 2005). It was confirmedto be well-crystallized a-FeOOH by the X-ray diffraction measure-ment (Rigaku, D/MAX 2500). The pristine point of zero charge(PZC) of goethite prepared following the same protocol is between9.0 and 9.3 (Antelo et al., 2005; Weng et al., 2008). Its specific sur-face area is 98.2 ± 0.3 m2 g�1 determined with N2 BET measure-ments (Micromeritics, ASAP 2010, USA). Stock suspension ofgoethite was prepared under the acidic condition (pH 5) andflushed with N2.

Levofloxacin (LEV) (>98.0%) was obtained from Tokyo ChemicalIndustry Co. (Japan), and used as received. Acetonitrile (HPLCgrade) was purchased from Honeywell Burdick & Jackson (USA).Other chemicals were analytical reagent grade or higher. MilliQwater was used throughout the experiments.

2.2. Single component adsorption to goethite

Adsorption edges of LEV in the absence of phosphate wereobtained from batch experiments. 50 mL polyethylene centrifugetubes were used, and they were flushed with ultrapure N2 duringsuspension preparations. 10 mL LEV stock solution and 10 mL goe-thite suspension of 1.6 g L�1 (or MilliQ water) were added into thetubes under close to dark conditions to prevent the possible photo-degradation. Both the LEV and goethite stock solutions were pre-pared in 0.01 M NaCl. The final total concentrations of LEV werefrom 1 to 10 lM. The final goethite concentration was 0.8 g L�1

by weighing three subsamples after drying. Acid or base (0.01 MHCl and NaOH) was used to adjust solution pH to a chosen valuein the range of 3–10.

All suspensions were shaken at 175 rpm for 24 h in the dark.After equilibration, the end pH value of each sample was measuredwith the calibration method using a pH meter (Sartorius PB-10,Germany). Then the samples were centrifuged at 12000 rpm for20 min, and the supernatant was stored at 4 �C in the dark for lateranalyses. All experiments were conducted in triplicates.

2.3. Binary components adsorption to goethite

Competitive adsorption experiments of phosphate and LEV togoethite were measured following a similar procedure. Each batchcontained three series of treatments, i.e. LEV-goethite, phosphate-goethite, and LEV-phosphate-goethite, adjusted to a pH range of3–10. In all treatments, the final total concentrations of LEV andgoethite were 10 lM and 0.8 g L�1, respectively. Three levels ofphosphate 20100, and 200 lM) were studied.

2.4. Chemical analysis

The concentration of LEV in solutions was determined usingthe high performance liquid chromatography (HPLC) method aspreviously described (Qin et al., 2014), and was detected at thepeak wavelength of 293 nm. The phosphate concentrationwas measured with the molybdenum blue method using an

X. Qin et al. / Chemosphere 111 (2014) 283–290 285

ultraviolet–visible (UV–Vis) spectrophotometer (HP 8453, USA).The adsorbed amounts of LEV (or phosphate) were calculated bydifferences in their initial and equilibrium concentrations in solu-tions. The results of control samples (without goethite) indicatedthat there were no significant losses in the concentration of LEVor phosphate, and no interferences during their measurements incompetition experiments.

2.5. Model approach

The LEV molecule contains two variable charge groups withinenvironmental relevant pH range: one carboxylic group and onetertiary amine group on the piperazine ring (Fig. S1 in the Supple-mentary material). Their protonation constants (pKa1 and pKa2) are6.02 and 8.15 respectively (Sousa et al., 2012). Using capillary zoneelectrophoresis or 1H NMR-pH titration method (Lin et al., 2004;Rusu et al., 2012), some authors also indicated that the secondamine group in the piperazinyl ring of fluoroquinolone antibioticscan become protonated under very acidic conditions. The pKa forthe second amine group is found to be around�0.4 for several fluo-roquinolone antibiotics, but it is not available for LEV. In the solu-tion, around neutral pH, the zwitterion is the major species,whereas at lower pH, the cationic species, at higher pH the anionicspecies, dominates. The neutral species is not important over thewhole pH range (Fig. S2). The presence of the second amine grouphas negligible effect on species distribution of LEV at pH > 3.

The adsorption of LEV or/and phosphate to goethite was simu-lated using the CD-MUSIC model (Hiemstra et al., 1989b; Hiemstraand Van Riemsdijk, 1996, 2006), in combination with the extendedStern model for the compact part of the electric double layer,which has two Stern layers, one surface electrostatic plane(0-plane) and two outer-electrostatic planes (1- and 2-plane)(Hiemstra et al., 1989b). Two types of surface groups ongoethite are proton reactive, i.e. the singly (�FeOH�0.5) and triply(�Fe3O�0.5) coordinated surface groups (Hiemstra and VanRiemsdijk, 1996). The basic CD-MUSIC parameters have beenderived previously (Weng et al., 2007; Hiemstra et al., 2010).Following Hiemstra and Van Riemsdijk (1996), the intrinsic protonaffinities (logKH,Fe1 and logKH,Fe3) are assumed to be the same forthese two types of sites and are equated to the PZC of the goethite,i.e. logKH,Fe1 = logKH,Fe3 = PZC. Indifferent electrolyte ions (Na+, Cl�)can form ion pairs with both the singly and triply coordinatedsurface sites. The reaction constants for the ion pair formationhave been set equal for the two types of sites (Table 1).

As identified by Fourier transform infrared spectroscopy (FTIR)and molecular orbital calculations (Tejedor-Tejedor andAnderson, 1990; Rahnemaie et al., 2007a), phosphate predomi-nantly formed nonprotonated bidentate (�(Fe1O)2PO2

�2, B)complexes at low loading, nonprotonated monodentate(�Fe1OPO3

�2.5, M) at high pH, and the single protonated monodentate

Table 1Surface species and CD-MUSIC model parameters of goethitea.

Basic parameters Sites PZC Site density

�Fe1OH�0.5

9.0 3.45Charge distribution

Surface species �Fe1OH�0.5 �Fe3O�0.5 Dz0 Dz1

� FeOH2+0.5 1 0 1 0

� Fe3OH+0.5 0 1 1 0� FeOH� � �Na+0.5 1 0 0 1� Fe3O� � �Na+0.5 0 1 0 1� FeOH� � �Cl–0.5 1 0 1 �1� FeOH� � �Cl�0.5 1 0 1 �1�(FeO)2PO2

�2 1 0 0.46 �1.46� FeOPO3

�2.5 1 0 0.22 �2.22

a Dz0, Dz1, and Dz2 are charge attributed to 0-plane, 1-plane, and 2-plane. R2 = 0.996,

(�Fe1OPO2OH�1.5, MH) complexes at high loading and low pHconditions, with the charge distributed between the surface plane(0-plane) and the middle plane (1-plane). During the model calcu-lations, the MH complex was excluded, because the loading ofphosphate was relatively low (<1.5 lmol m�2) and the MH specieshad no influence on fitting. The surface species (B and M) and CDvalues for phosphate adsorption from Rahnemaie et al. (2007a)were used in the current modeling without change (Table 1),whereas the adsorption affinity constants (logK) were optimizedusing the data of phosphate adsorption.

Spectroscopic data on LEV adsorption to goethite were notavailable. Some authors investigated the interactions betweenother fluoroquinolone antibiotics (i.e. ciprofloxacin and ofloxacin),which have a similar structure and adsorption behavior to LEV,with various oxides (hydrous ferric oxides (HFO), goethite, magne-tite, hydrous aluminum oxides (HAO), alumina, quartz, and ana-tase). The spectroscopic techniques used include ATR-FTIR(attenuated total reflectance FTIR) and UV–Vis spectroscopy. Inall the studies, ligand exchange mechanisms were proposed, butthe exact surface configurations derived from the spectroscopicdata were different. These surface structures proposed include:the mononuclear monodentate complexes (Fig. 1B1) onto HAOthrough the involvement of one carboxylate oxygen along with aweak hydrogen bonding association of the ketone group (Gu andKarthikeyan, 2005), the six-ring mononuclear bidentate complexes(Fig. 1A2) onto HFO or alumina via the involvement of one carbox-ylate oxygen and the ketone group (Goyne et al., 2005; Gu andKarthikeyan, 2005), the cation bridge onto quartz via the proton-ated amine group (Goyne et al., 2005), the mononuclear bidentatecomplexes (Fig. 1A3) onto goethite surface via two oxygen atomson the carboxylate group (Trivedi and Vasudevan, 2007), and thebridging binuclear bidentate complexes (Fig. 1C1, and D) involvingtwo surface groups and two oxygen atoms of the carboxylate groupon colloidal iron oxide, anatase and magnetite with or without ahydrogen bonding involving the ketone group (Trivedi andVasudevan, 2007; Paul et al., 2012; Rakshit et al., 2013). Similarreaction mechanisms were also proposed in the adsorption ofsmall organic acids to minerals (Yost et al., 1990; Biber andStumm, 1994; Filius, 2001).

In this study, we take into account the formation of the mono-dentate, mononuclear bidentate, binuclear bidentate, and triden-tate species with or without the hydrogen bonding (Fig. 1). Thecharge of the carboxylate group on the LEV adsorbed to goethitesurface is distributed between 0- and 1-plane. The charge of theprotonated amine group is located on the 2-plane (Table 1). TheCD values and logK were derived by fitting. There are two optionsin the modeling exercise regarding the LEV adsorption. In option I,one protonated amine group was considered, whereas in option II,two LEV surface species including the unprotonated and proton-ated amine group were taken into account. In each scenario, one

(nm2) Capacitance (F/m2) logK

�Fe3O�0.5 C1 C2

2.70 0.93 0.75Ions or ligands

Dz2 H+ Na+ Cl� PO43�

0 1 0 0 0 9.000 1 0 0 0 9.000 0 1 0 0 �0.600 0 1 0 0 �0.600 1 0 1 0 8.550 1 0 1 0 8.550 2 0 0 1 26.410 2 0 0 1 20.71

RMSE = 0.024, n = 24.

Fig. 1. Proposed structures for LEV complexation on the surface of goethite. The surface species and model parameters of the complexes are listed in Table 1.

286 X. Qin et al. / Chemosphere 111 (2014) 283–290

type of surface configuration between the LEV and surface groupswas used.

The site density, capacitance and other basic properties of goe-thite were reported by Hiemstra et al. (2010). The surface speciesof phosphate and basic CD-MUSIC model parameters were listedin Table 1. Adsorption of LEV (or phosphate) on goethite in the com-petition experiment was calculated with CD-MUSIC model using thesame model parameters used for the single component adsorption.

The model calculations and optimization of the adsorptionparameters were performed using the software ECOSAT program(Keizer and Van Riemsdijk, 1998) combined with the FIT programfrom Kinniburgh (1993).

3. Results and discussion

3.1. LEV adsorption in the absence of phosphate (data)

The amounts of adsorbed LEV increase with initial concentra-tions of LEV under different pH conditions (Fig. 2). Similar pHdependent adsorption characteristics are observed at various ini-tial concentrations of LEV. The maximum amounts of adsorbedLEV appear at pH 5–6, and then decrease at lower or higher pH.The trend is also reported by others when studying the sorptionof antibiotics with similar structures to minerals (Zhang andHuang, 2007; MacKay and Seremet, 2008; Paul et al., 2012). Forinstance, MacKay and Seremet (2008) observed an adsorption

maximum of ciprofloxacin on goethite around pH 6.2, and themaximum amount of adsorbed ofloxacin to TiO2 in the backgroundsolution of NaCl was at pH 6.0 (Paul et al., 2012), which were bothclose to the pKa values (the carboxylic group) of these antibiotics.

Obviously, solution pH had a significant influence on the speciesdistribution of LEV and its adsorption onto goethite. Above the pHof maximum adsorption (>pH 6), the decrease of LEV adsorptionwith increasing pH is predominantly determined by the decreaseof the positive charge on goethite surface. At pH values abovethe PZC of goethite (pH 9), LEV and goethite surface are both neg-atively charged. Therefore, almost no LEV was adsorbed by goe-thite due to electrostatic repulsions. At pH below the adsorptionmaximum (<pH 5), the decrease of adsorption was largely attrib-uted to electrostatic repulsion resulted from the positively chargedamine group on LEV and the positive electrostatic potential on thegoethite surface. This phenomenon was quite different from theadsorption of small organic acids without the amine group orflumequine (another fluoroquinolone without the piperazinylgroup). The adsorption of small organic acids (Geelhoed et al.,1998; Filius, 2001) and flumequine (Zhang and Huang, 2007;MacKay and Seremet, 2008) to goethite did not show a significantdecrease at pH < 5, as LEV did. This comparison supports the con-clusion that the increased positive charge on the amine group withdecreasing pH results in the decrease adsorption at acidic pH ofLEV and similar antibiotics. The presence of the piperazinyl aminegroups in antibiotics inhibited their adsorption to minerals in gen-eral, especially under acidic conditions.

Fig. 2. The amounts (A) and species fractions (B) of adsorbed LEV on goethitesurface under different pH conditions at 0.01 M NaCl. The concentration of goethiteis 0.8 g L�1. In Fig. 2A, the solid lines (option A (II)) and dash lines (option B (II)) aremodel calculations in Table 2, and error bars (±1 standard deviation, n = 3) areshown. In B, the Fe2OLEV+ and Fe2OLEV0 are the protonated and unprotonatedspecies calculated with option (B). The total concentration of LEV is 10 lM, and thefractions at other concentrations are shown in Fig. S3.

X. Qin et al. / Chemosphere 111 (2014) 283–290 287

3.2. LEV adsorption in the absence of phosphate (modeling)

A reasonable fit with the data of LEV adsorption was obtainedusing different assumptions regarding the surface species formedbetween LEV and goethite. The fitting of CD values and logK of

Table 2Surface species and model parameters for LEV–goethite complexesa.

Surface species Sites Charge distribution

�Fe1OH�0.5 �Fe3O�0.5 Dz0 Dz1 Dz2

� FeOLH+0.5 1 0 0.10 �0.10 1� FeOLH+0.5 1 0 0.07 �0.07 1� FeOL�0.5 1 0 0.07 �0.07 0�(FeO)2LH2

+2 2 0 1.11 �0.11 1�(FeO)2LH2

+2 2 0 1.08 �0.08 1�(FeO)2LH+1 2 0 1.08 �0.08 0�(FeO)2LH2

+2 2 0 1.20 �0.20 1�(FeO)2LH2

+2 2 0 1.20 �0.20 1�(FeO)2LH+1 2 0 1.20 �0.20 0�(FeO)3LH2

+2.5 3 0 0.97 1.03 0�(FeO)3LH3

+3.5 3 0 0.99 1.01 1�(FeO)3LH2

+2.5 3 0 0.99 0.01 0

a Dz0, Dz1, and Dz2 are charge attributed to 0-plane, 1-plane, and 2-plane. LH is the z(n = 40).

b The unprotonated LEV molecules were used in option D (I).

LEV-goethite complexes was carried out using the five adsorptionedge data sets at 0.01 M NaCl. In the modeling, the charge of LEVis distributed into three electrostatic planes on the surface of goe-thite. The charge of the protonated amine group on the piperazinylring of LEV is located at the 2-plane, whereas the charge of the car-boxylate group is distributed between 0-plane and 1-plane(Table 2). As shown in Fig. 2A, the adsorption of LEV to goethiteis well described with the CD-MUSIC model, using the parametersin Table 2.

As shown in Fig. 2B, the protonated LEV is mainly adsorbed togoethite surface under acidic conditions (pH < 5). As mentionedabove, goethite surface becomes positive charged with thedecreasing pH, which will inhibit the adsorption of LEV. Theamounts of adsorbed unprotonated LEV increase with the increas-ing pH, which is the major attributor at pH > 8. The loading of LEVto goethite has no significant influence on the species distributionsof adsorbed LEV (Fig. S3). This is quite different from the adsorp-tion of phosphate.

In this study, the eight options of modeling LEV adsorption togoethite seemed to be reasonable. In the next section, we willuse the CD-MUSIC model with the same parameters to predict itsadsorption in the presence of phosphate.

3.3. Competition between LEV and phosphate (Data)

The competitive adsorption of LEV and phosphate onto goethiteis shown in Fig. 3. The maximum amounts of adsorbed LEV are stillaround pH 6. The presence of phosphate decreased the adsorptionof LEV significantly over the whole pH range, especially at low pH.The inhibitions of LEV adsorption enhanced with increasing con-centrations of phosphate. These results were quite different fromthat of Rakshit et al. (2013), who reported that phosphate had anegligible effect on ciprofloxacin adsorption to nano-sized magne-tite. This difference can be mainly attributed to a weaker phos-phate adsorption on magnetite compared to on goethite, and alower loading of phosphate (<0.25 lmol m�2) and higher loadingof ciprofloxacin (0.20 lmol m�2, pH 6) in their study. When a rel-atively low concentration of phosphate compared to that of antibi-otic is present in the experiment, the competition effect will be lesssignificant.

There are several possible mechanisms for the inhibition of LEVadsorption by phosphate. Firstly, competition for binding sites wasconsidered to be an important factor during their adsorption ontogoethite. As shown in Fig. 3, phosphate has a much higher affinity(>50 times) to goethite and a smaller molecular size than LEV. Inthe binary systems, phosphate was easily adsorbed to goethite sur-face and occupied a substantial amount of the sites, leading to the

Ions or ligands logK Option R2

H+ Na+ Cl� LEV

1 0 0 1 12.87 A (I) 0.8201 0 0 1 12.75 A (II) 0.8930 0 0 1 4.66 A (II)2 0 0 1 22.17 B (I) 0.8222 0 0 1 22.06 B (II) 0.8911 0 0 1 13.94 B (II)2 0 0 1 22.33 C (I) 0.7852 0 0 1 22.31 C (II) 0.7961 0 0 1 13.61 C (II)2 0 0 1 22.76 D (I)b 0.8323 0 0 1 27.00 D (II) 0.8472 0 0 1 22.80 D (II)

witterion or neutral species of LEV. The RMSE values of all the fittings were < 0.003

Fig. 3. Competitive adsorption of LEV (A) and phosphate (B) onto goethite underdifferent pH conditions at 0.01 M NaCl. The concentration of LEV is 10 lM. Theconcentration of goethite is 0.8 g L�1. The solid lines (option A (II)) and dash lines(option B (II)) are model calculations (Table 1 for phosphate, and Table 2 for LEV). InFig. 2B, the dot lines are model calculations (Table 1) for phosphate adsorption inthe same conditions compared with the LEV-phosphate-goethite system, but in theabsence of LEV. Error bars (±1 standard deviation, n = 3) are shown in the figures.

Fig. 4. Phosphate adsorption on goethite surface under different pH conditions at0.01 M NaCl. The concentration of goethite is 0.8 g L�1. Solid lines are modelcalculations using the parameters shown in Table 1, and error bars (±1 standarddeviation, n = 3) are shown.

288 X. Qin et al. / Chemosphere 111 (2014) 283–290

decrease in LEV adsorption. A second mechanism of inhibition iselectrostatic competition between the adsorbed carboxylate groupon LEV and the phosphate ion, both of which are negativelycharged and located close to the surface. And thirdly, the interac-tion between the amine group of LEV and phosphate in solutionsmight also decrease LEV adsorption to goethite. However, flumeq-uine adsorption onto goethite was also inhibited by phosphateaccording to the study of Li (2013). As far as we know, no reactionconstants of phosphate and LEV complexation in solution are avail-able. From the fact that phosphate buffer was usually used as efflu-ent of HPLC in the measurements of fluoroquinolone antibiotics(Córdova-Kreylos and Scow, 2007; Zhang and Huang, 2007;MacKay and Seremet, 2008), it can be speculated that the reactionsinvolving the LEV amine group and phosphate could be negligible.This leaves the competition for the adsorption, via site and electro-static competition as the main mechanism.

3.4. Phosphate adsorption in the absence of LEV (data and modeling)

In the absence of LEV, phosphate adsorption on goethitedecreases slightly with increasing pH (Fig. 4). The logK values wereoptimized by fitting (Table 1). The adsorption processes are wellpredicated by CD-MUSIC model (R2 > 0.995, n = 24). Most of theadded phosphate is still adsorbed at pH values above PZC of goe-

thite, which differs from that of LEV, and shows a much strongerspecific adsorption of phosphate than LEV. The logK values derivedfor B and M complexes are 26.41 and 20.71, respectively.

As shown in Fig. S4, the M complex is the major phosphate sur-face species at intermediate and high pH, and its fraction increaseswith the loading of phosphate. At low pH conditions, the B complexalso contributes to the adsorbed phosphate, but is still much lowerthan the M species. As a consequence, the M complex is the majorcontributor, and the adjustment in logK value of the B species isreasonable.

3.5. Competition between LEV and phosphate (modeling)

It is much more challenging to get a good model descriptionof LEV adsorption in the presence of phosphate than in theabsence of phosphate. The difficulty arises from getting both suf-ficient prediction of the competition effect of phosphate on LEVadsorption, whereas maintaining the shape of the adsorptioncurve close to the data. From the model exercise, it shows thatcompetition for the surface site is not the most important mech-anism explaining the competition, because changing the LEV spe-ciation from with one, two, to three surface groups (Fig. 1) didnot increase the competition effect predicted (results not shown).However, on the other hand, the competition predicted is verysensitive to the charge of LEV located at the 1-plane. A more neg-ative charge distribution of LEV at the 1-plane will increase thecompetition effect predicted. This shows that electrostatic com-petition is the major mechanisms in the LEV and phosphateinteractions. However, to maintain enough adsorption of LEVunder acidic pH range, 2–3 protons need to be involved perLEV adsorbed. Only the charge of one proton can be locatedbetween the 0- and 1-plane, to obtain a reasonable shape ofthe adsorption curve. This leaves location of the charge of theremaining 1 to 2 protons to the 2-plane as the only option. Thepresence of one proton at the 2-plane can be explained relativelystraightforward by attributing it to the amine group on the pip-erazinyl ring. However, the presence of the second proton is lessdefendable. There are evidence show that the second aminegroup on the piperazinyl ring can also become protonated (Linet al., 2004; Rusu et al., 2012), but this happens in solutions onlyat rather low pH (Fig. S2B).

In the binary systems, model results for LEV and phosphateadsorption to goethite are quite different (Fig. 3). The model can

X. Qin et al. / Chemosphere 111 (2014) 283–290 289

well describe the adsorption of phosphate to goethite with theaddition of LEV (Fig. 3B). The presence of LEV has a little influenceon phosphate adsorption. The amounts of adsorbed LEV are overes-timated according to model calculations. This is because less neg-ative charge of LEV is located at 1-plane in our study. Otherwisethe calculated adsorbed amounts of LEV at acidic conditions willbe much lower than the observed data. As a result, modeling ofLEV adsorption to goethite in the presence of phosphate still needsfurther study.

4. Conclusions

Our study indicated that phosphate decreased LEV adsorptionto goethite significantly over the whole pH range, and LEV had alittle influence on phosphate adsorption. Electrostatic competitionwas the main reason for the competition of LEV and phosphate togoethite surface in competition experiments. Adsorption of LEV (orphosphate) to goethite was well predicted using the CD-MUSICmodel. In the natural environment, adsorption of phosphate bysoils/minerals will decrease the adsorption of antibiotics, whichmay lead to the leaching of antibiotics from soils into groundwaters.

Acknowledgments

This work was financed by the Fundamental Research Funds forthe National Natural Science Foundation of China (41172226), theFundamental Research Funds for the Central Universities(2652013024 and 2652013089), and the Special Fund for PublicInterest Research support by the Ministry of environmental protec-tion (201309001-3). We thank Juan Xiong for her help in advice onECOSAT.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.chemosphere.2014.04.032.

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