Insights into tetracycline adsorption onto goethite: Experiments and modeling

o ro jing H I G H L I G H T S h does not. eTCH20.5 e ionizable functional diketone group and a rotonation–deproton- ending on the solution Science of the Total Environment 470–471 (2014) 19–25 Contents lists available at ScienceDirect Science of the Tot l se rates of bioactive compounds and the potential of promoting the evolu- tion of microbial populations resistant to TCs, the fate and behavior of TCs in the environment have become a source of growing concern in pH (Parolo et al., 2008). Dissolved TC species may have net charges that are positive (H3TC+, pH b 3.3), neutral (H2TC0, 3.3 b pH b 7.68), one negative (HTC−, 7.68 b pH b 9.68) or two negative (TC2−, pH N 9.68) incorporated into animal feeds to enhance growth and feed efficiency in healthy livestock (Hu et al., 2010; Luo et al., 2011; Sarmah et al., 2006). Most TCs administered to livestock are excreted unmetabolized (Ji et al., 2009; Pils and Laird, 2007). Considering the high excretion TC is an amphoteric molecule with multipl groups: a tricarbonylamide group, a phenolic dimethylamino group (Fig. S1a). It can undergo p ation reactions and present different species dep Adsorption Complexation Model pH, ionic strength and surface coverage. However, since the model did not fully consider the molecular size of TC, the model might overestimate the adsorption when TC surface coverage is higher than 1.42 μmol m−2. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Tetracyclines (TCs), one of themostwidely used antibiotic families in theworld, are often used to treat and prevent diseases in animals and are major process affecting the transport and degradation of TCs in the en- vironment (Kulshrestha et al., 2004). Hence, the assessment of the sorp- tion and mobility of TCs in soils is important regarding their risk to the environment and human health. Goethite diketone groups. The model could well predict the adsorption behavior of TC under a relatively wide range of recent years (Thiele-Bruhn and Beck, 2005). W cation of livestock wastes to agricultural fiel ⁎ Corresponding author at: 163, Xianlin Ave., Nanjin School of the Environment, Nanjing, 210023, China. Tel./f E-mail address: [email protected] (X. Gu). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All ri http://dx.doi.org/10.1016/j.scitotenv.2013.09.059 complexation species could successfully describe observed adsorption trends: under acidic condition TCmay in- teract with the surface of goethite, forming a monodentate complex through the tricarbonylamide group, while under alkaline condition forming a more stable bidentate complex via the tricarbonylamide and phenolic Keywords: Tetracycline • The tricarbonylamide group and the pho • Two inner-sphere surface complexes `F a r t i c l e i n f o Article history: Received 26 July 2013 Received in revised form 18 September 2013 Accepted 20 September 2013 Available online 10 October 2013 Editor: Eddy Y. Zeng oup are involved in adsorption. + and `Fe2TC are proposed. a b s t r a c t The surface adsorption behavior of tetracycline (TC), a zwitterionic antibiotic, to goethite was investigated as a function of pH, ionic strength and TC concentration using batch adsorption experiments and structural informa- tionwas derived fromattenuated total reflectance Fourier transform infrared spectrumobservations. The spectro- scopic results suggested that the tricarbonylamide group and the phenolic diketone group of the TC molecule were involved in interacting with the goethite surface depending on the pH level. A charge distribution surface complexation model was developed to describe the macroscopic adsorption trends. Two inner-sphere surface • The adsorption was successfully predicted by a ba netic gr sic Stern model with CD approach. • pH greatly influences tetracycline adsorption on goethite, while ionic strengt Insights into tetracycline adsorption onto g Experiments and modeling Yanping Zhao, Fei Tong, Xueyuan Gu ⁎, Cheng Gu, Xiao State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nan j ourna l homepage: www.e ith the land-based appli- ds, sorption to soils is a g University Xianlin Campus, ax: +86 25 89680595. ghts reserved. ethite: ng Wang, Yan Zhang University, Nanjing, Jiangsu 210023, China al Environment v ie r .com/ locate /sc i totenv (Fig. S1b). It has been reported that TC can be highly adsorbed on clay minerals (Figueroa et al., 2004; Pils and Laird, 2007; Wang et al., 2008), oxide minerals (Chen and Huang, 2010; Figueroa and Mackay, 2005), humic substances (Gu et al., 2007; MacKay and Canterbury, 2005), soils (Wan et al., 2010; Zhang et al., 2011) and sediments (Wang et al., 2010; Xu and Li, 2010). The mechanisms of TC adsorption to clay minerals 20 Y. Zhao et al. / Science of the Total Environment 470–471 (2014) 19–25 normally involve cation exchange reactions for cationic species (H3TC+) and surface complexation for zwitterions (H2TC0) (Figueroa et al., 2004). Unlike the well-characterized interaction of TC with phyllosilicate clay minerals, its sorption on oxide surfaces has received less attention. The adsorption of oxytetracycline onto goethite and hematite was inves- tigated at various pH values and an adsorption maximum at pH 8 was observed, and a surface complexation mechanism was proposed (Figueroa and Mackay, 2005). Similar adsorption behavior was also re- ported between TC and goethite or ferrihydrite-coated quartz (Tanis et al., 2008), and iron hydrous oxide (Gu and Karthikeyan, 2005). It was suggested that the strong sorption was dominated by inner- sphere surface complexation. Moreover, FTIR spectroscopic evidence showed that tricarbonylamide and carbonyl groups might be responsi- ble for the adsorption (Gu and Karthikeyan, 2005). Recently, one of our studies showed that different background cations (Li+, Na+, K+, Ca2+, and Mg2+) showed little effect on the TC adsorption under pH range 3–10, also indicating a surface complexation mechanism involved (Zhao et al., 2011). However, the surface complexation species and elec- trostatic effect of the interaction between TC molecular and goethite surface remain unclear. Models of the solid–solution interface are applied to understand and predict the reactivity of the surface. Surface complexation models (SCMs), a thermodynamic approach to describe the formation of surface complexes between dissolved solutes and surface functional groups, have beenwidely used to describe surface adsorption behavior, particu- larly the adsorption of various heavy metals onto minerals (Gu and Evans, 2008; Lund et al., 2008; Marcussen et al., 2009; Wang et al., 2006). Over the past decade, SCMs have also been successfully applied to adsorption of ionic organics to oxide minerals. A two-layer model was used to predict the adsorption of a series of organic acids onto goethite (Evanko and Dzombak, 1999). In recent years, SCM was successfully developed to describe the adsorption of glyphosate onto goethite surface (Jonsson et al., 2008), L-aspartate to rutile (Jonsson et al., 2010) and ofloxacin onto nano-anatase titanium dioxide (Paul et al., 2012). However, to our knowledge, there are only two studies that reported the modeling of TC adsorption onto iron oxide minerals so far. Figueroa andMackay (2005)modeled the adsorption of oxytetra- cycline on goethite and hematite with two surface species and Tanis et al. (2008) used one binuclear surface species to model the surface complexation of TC onto goethite-coated quartz. These two studies did not clearly define the oxide surface-site heterogeneity and surface acid–base reactions. Meanwhile, they considered the stoichiometry of surface reactions, but ignored the possible electrostatic effect of the sur- face interaction. To describe the surface complexation behavior of an ionizable molecule, like TC, to a variable charged surface, like goethite, one of the most common and stable crystalline iron (hydr)oxides in natural environments, the surface electrostatic effect needs to be fully considered. Among the SCMs developed for ion binding to oxides, the CD-MUSIC (charge distribution multi site complexation) model (Hiemstra and VanRiemsdijk, 1996) has the advantage that it is based on the structure of the mineral surface, the structure of the adsorbed species and the structure of the electrostatic potential profile in the vicinity of the mineral–water interface (Weng et al., 2006). The CD-MUSIC model has been successfully applied in modeling boron (Goli et al., 2011), arsenate (Salazar-Camacho and Villalobos, 2010), MCPA (4-chloro-2-methylphenoxyacetic acid) (Iglesias et al., 2010a), paraquat (Iglesias et al., 2010b) and many other substances on goethite surfaces. The aim of this work was to characterize the surface complexa- tion behavior of TC to goethite surfaces and use the CD-MUSIC model to predict its adsorption over relatively wide pH, ionic strength and surface coverage conditions. Such a model could help to predict the distribution of TC in natural waters, and this contributes to a better understanding of its mobility and bioavailability in the environment. 2. Materials and methods 2.1. Materials and chemicals Tetracycline (96% purity) was purchased from Sigma-Aldrich Co. (St. Louis, MO). Other chemicals, including NaNO3 and Fe (NO3)3·9H2O, were of A.R. grade andwere obtained from SinopharmChemical Reagent Co. (Shanghai, China). Deionized water (resistivity = 18.2 MΩ cm−1) was used throughout the study. Goethite samplewas synthesized according to themethod described in our other study (Zhao et al., 2011). X-ray diffraction (XRD) was used to confirm the formation of goethite (D/MAX-RA, RigaKa, Japan). The specific surface area measured by N2-BET analysis was 63.53 m2 g−1 (ASAP 2020, Micromeritics, USA), and the mean particle size was 3.04 μm, as measured by a laser particle analyzer (Mastersizer 2000, Malvern Co., UK). 2.2. FTIR measurements FTIR spectra were collected using a Vertex 70 V FTIR spectrometer (Bruker, Germany) equipped with a liquid-nitrogen-cooled MCT detec- tor and a 022-2020 ZnSe crystal fitted in a horizontal attenuated total reflectance (ATR) cell (Pike Technologies). A total of 256 scans with a spectral resolution of 4 cm−1 were performed. TC solutions (5 mM) within 2 g L−1 goethite suspension were pre- pared at different pH values (pH 3, 5, 7, 8, 10) adjusted with NaOH/ HNO3. NaNO3 solution (0.01 M) was used as background electrolyte. The samples were wrapped in aluminum foil to prevent exposure to light and shaken at 150 rpm at 25 ± 0.5 °C for 24 h. After equilibrium, the tubes were centrifuged for 30 min at 2880 g, and the solid and su- pernatant were collected separately and immediately spread on the ZnSe crystal surface to obtain a thin layer; FTIR spectra were then mea- sured. Spectra of goethite in 0.01 M NaNO3 were also obtained at five different pHs in the absence of TC. Difference spectra attributable only to the TC–goethite surface complexes were obtained by subtracting the supernatant and goethite-NaNO3 suspension signals from the corre- sponding spectra of the solid. For the spectra of aqueous TC at different pH values (pH 3, 5, 7, 8, 10), the spectral contributions fromwater were removed by subtraction of the spectrum of the background electrolyte solution (at the same pH) from the TC spectrum. 2.3. Batch adsorption experiments The effects of pH and ionic strength on the adsorption were examined using batch experiments in 0.001 M, 0.01 M and 0.1 M NaNO3 background electrolyte at pH 3–10. All of the adsorption experi- ments were conducted using 22 mL glass vials equipped with polytetrafluoroethylene-lined screw caps. The solid-to-water ratio was 2 g L−1, the initial TC concentration was 0.1125 mM and the samples were mixed on a reciprocal shaker at 150 rpm at 25 ± 0.5 °C for 24 h. The solutions were purged with N2 to remove dissolved oxygen to pre- vent the possible oxygen-mediated degradation of TC (Ji et al., 2009). The final pH values were measured immediately following equilibration using an Orion 8272 PerpHect Ross Sure-Flow electrode. The suspen- sions were then centrifuged, and TC in the supernatant was analyzed using HPLC. For detailed measurement methods, readers can refer to Zhao et al. (2011) and Zhao et al. (2012). To take account for solute loss from processes other than adsorbent sorption, calibration curves were built separately at the same treatment and conditions (pH, temper- ature, equilibration time, etc.) as the adsorption samples without adsor- bent. Before detected by HPLC, the pH values of both adsorption samples and calibration curve samples were adjusted to 2.5 using diluted HNO3 solution. The method used to investigate the effect of surface coverage was similar to the method described previously, except with respect to the initial TC concentrations. 2.4. Surface complexation modeling The basic Sternmodel combinedwith the charge distributionmulti- site complexation (CD-MUSIC) model (Hiemstra and VanRiemsdijk, 1996) was used to quantify experimental data and to provide a micro- scopic interpretation of adsorption interactions. The CD-MUSIC cal- culations were done with the ECOSAT 4.9 program (Keizer and Van Riemsdijk, 1998) in combination with a recent version of FIT 2.581 (Kinniburgh, 1993). The solution species, surface species of goethite and the corresponding formation constants were given in Table. 1. For more details about the CD-MUSIC approach, readers can refer to Hiemstra and VanRiemsdijk (1996). The charging behavior of the goe- thite in absence of TC can be described by assuming the protonation of singly and triply coordinated surface groups according to Eqs. (4) and (7) in Table 1. The charging behavior of goethite can be described using the basic Stern model. Protons are located in the surface plane (0-plane), whereas the ion pair formers are present at the head end of the diffuse double layer (β-plane) (Filius et al., 2000; Hiemstra and VanRiemsdijk, 1996). 3. Results and discussion 3.1. ATR–FTIR analysis No. Deprotonation of TC in solutiona log K0b 21Y. Zhao et al. / Science of the Total Environment 470–471 (2014) 19–25 1 H3TC+ = H2TC± + H+ −3.30 2 H2TC± = HTC− + H+ −7.68 3 HTC− = TC2− + H+ −9.68 Protonation of goethite surfacec Δz0 Δzβ 4 `FeOH0.5− + H+ = `FeOH20.5+ 1 0 9.4 5 `FeOH0.5− + Na+ = `FeOH0.5−·Na+ 0 1 −0.69 6 `FeOH0.5− + H+ + NO3− = `FeOH20.5+·NO3− 1 −1 8.51 7 `Fe3O0.5− + H+ = `Fe3OH0.5+ 1 0 9.4 8 `Fe3O0.5− + Na+ = `FeO0.5−·Na+ 0 1 −0.69 9 `Fe3O0.5− + H+ + NO3− = `Fe3OH0.5+·NO3− 1 −1 8.51 Adsorption of TC onto Goethited 10 `FeOH0.5− + TC2− + 3H+ = [`FeTCH2]0.5+ + H2O 1 0 29.76 ± 0.02 11 2`FeOH0.5− + TC2− + 2H+ = [`Fe2TC]− + 2H2O 0 0 22.81 ± 0.02 a Constants from Figueroa et al. (2004). b Constants at zero ionic strength and constants for surface reactions are intrinsic. c Constants from Jonsson et al. (2008). The site densities of`FeOH−0.5 and`Fe3O−0.5 are 3.64 and 2.73 sites nm−2, respectively. d Surface reaction constants and the charge distribution factor were optimized in this work. The errors are one standard deviation. The capacitance of the inner-layer is −2 ATR–FTIR spectroscopy was used as a sensitive technique to charac- terize IR active vibrations of the TC molecule and to clearly identify the interaction between TC and the surface of goethite that modifies these vibrations. The spectra of both TC in electrolyte solution at different pHs (3, 5, 7, 8, 10) and the spectra of TC adsorbed onto goethite at iden- tical pHs were obtained and shown in Fig. 1a and Fig. 1b, respectively. Since the most characteristic region of the TC spectrum occurred be- tween 1150 and 1750 cm−1, only this region is interpreted in detail. The ATR–FTIR spectra of TC show a clear pH-dependent behavior (Fig. 1a). Firstly, at pH 2.9, an absorption peak at 1705 cm−1 (a) corre- sponds to the CfO stretch of the amide group (Li et al., 2010a; Li et al., 2010c). With increasing pH, deprotonation of the tricarbonylamide group results in the diminution of the absorbance in the CfO stretching region (Aristilde et al., 2010). Consequently, the peak corresponding to CfO in amide group disappeared at higher pHs (Fig. 1a). Secondly, a Table 1 Proposed SCM model parameters for TC adsorption onto Goethite. 5.0 F m . strong absorption peak at 1579 cm−1 (c) is assigned to the CfO group (ring C) (Li et al., 2010b), and its width increased with increasing pH due to thepossible impact of thedeprotonation of the adjacent hydroxyl groups. Thirdly, the absorption peaks at 1400 cm−1 (f) which repre- sents the \CH3 deformation vibration (Parolo et al., 2010) at pH 10.2 were obviously different from those at pH 2.9, 5.3, 7.0 and 8.1 due to the deprotonation of the dimethylamino group under an extremely alkaline condition. Besides, for reference comparison, the vibration bands at 1659 cm−1 (b), 1515 cm−1 (d), 1453 cm−1 (e), 1255 cm−1 (g), 1229 cm−1 (h) and 1186 cm−1 (i) in Fig. 1a (pH 2.9) are associated with the CfO group at ring A, the\N\H (or amide II), C\C stretching, the amino C\N in the amide group, the C\N in the dimethylamino group and the phenolic C\O group (Aristilde et al., 2010; Chang et al., 2009; Kulshrestha et al., 2004; Li et al., 2010b; Parolo et al., 2010), respectively. The interaction between TC and goethite was investigated by com- paring the spectra of adsorbed TC with those of free molecules in solu- tion. The IR spectra of TC adsorbed on goethite at different pH values are shown in Fig. 1b and present some difference from the spectra of free TC in solution (Fig. 1a). A distinct difference is that the CfO amide I stretching band (a) even at pH 3.0 was significantly suppressed compared to free TC (Fig. 1a), consistent with the previous studies (Gu and Karthikeyan, 2005; Li et al., 2010b), indicative of the involve- ment of CfO amide I group during sorption to goethite. The vibration band (a) disappears as pH increases like free TC spectra because of the deprotonation of the tricarbonylamide group. Meanwhile, the absorp- tion peak of the\N\H (or amide II) (g) disappeared at five pH values compared to free TC (Fig. 1a) due to the impact of possible complexa- tion with goethite that occurred at the amide group (C13). In addition, compared to the spectrum for free TC in solution (Fig. 1a), the vibration frequencies of the CfO group (ring C) (c) shifted from 1579 cm−1 to 1618 cm−1 after adsorbed on goethite at pH 10.1 (Fig. 1b). This assignment is consistent with that in Li et al. (2010b). It was proposed that the adjacent \OH at C12 may be involved in the complexation with the goethite surface under alkaline condition, which reduced the conjugation effect of the CfO (ring C) with the CfC and consequently led to its high frequency shift. There was no obvious shift for other bands after TC adsorbed onto goethite. By combining the ATR–FTIR spectra results and goethite surface property, it may be concluded that the tricarbonylamide group (C13) and the phenolic diketone group (C11–C12) of TC are the major func- tional groups to complex with goethite surface to form monodentate and/or bidentate surface species depending on the pH level. 3.2. Adsorption edges of TC to goethite The effect of ionic strength (0.001 M, 0.01 M, 0.1 MNaNO3) on TC ad- sorption onto goethite over the pH range from 3 to 10 was investigated by batch adsorption experiments, and the results are shown in Fig. 2. In general, the adsorption edges of TC onto goethite look like a non- typical ligand adsorption envelope. The adsorption is greatly influenced by pH, while less sensitive to ionic strength. The adsorption edges show two stages: at pH range 3–6, TC adsorption increased with increasing pH and reached a small plateau at pH range 4–6; then adsorption in- creased to a maximum at approximately pH 8. The two stage adsorption behavior suggests that there are probably two surface complexation reactions involved in the studied pH range. The adsorption decreased at higher pH, which reflects the unfavorable electrostatic conditions for negatively charged TC molecules and negatively charged goethite surfaces. Ionic strength showed little effect to the TC adsorption. Increasing ionic strength only caused a slight decrease in TC adsorption, which be- came more pronounced under alkaline conditions. It was reported that inner- and outer-sphere surface complexes could be distinguished by studying the effects of ionic strength on the adsorption process (Hayes and Leckie, 1987). Inner-sphere complexes are hardly influenced by Fig. 1.ATR–FTIR difference spectra of TC in aqueous solution (a) and of TC on goethite surface (b) a 22 Y. Zhao et al. / Science of the Total Environment 470–471 (2014) 19–25 changes in ionic strength, whereas outer-sphere complexes are greatly influenced. Hence, the adsorption edges of TC to goethite surfaces sug- gest that they may interact through inner-sphere surface complexes. 3.3. Modeling of TC adsorption to goethite SCM is used to describe the adsorption edges of TC on goethite surface. In this study, basic Stern model (BSM) was chosen to account for the electrostatic effect of the variable charged goethite surface. It de- scribes the surface potential by placing charges at the inner-sphere compact Stern layer (0-plane) and the diffuse layer (β-plane). The Fig. 2. Adsorption edges of TC on goethite at different ionic strengths at 25 °C. Suspension density: 2.0 g L−1. Initial concentration of TC: 0.113 mM(50 mg L−1). Equilibration time: 24 h. The dots are experimental data, and solid lines represent the model predicted using parameters in Table 1. BSM is the simplest model that can describe the ionic-strength depen- dence by introducing ion-pair formation between the ions of the elec- t different pH values (indicated). The TC structure shows the assigned IR vibration bands (a–i). trolyte and the oppositely charged surface functional groups (Jonsson et al., 2008). In the BSM, an apparent surface binding constant (Kapp) in- cludes an intrinsic constant (Kint) and the electrostatic corrections: Kapp ¼ K int exp −Δz0ψ0 F=RTð Þ exp −Δzβψβ F=RT � � ð1Þ where Δz0 and Δzβ are the differences in charge between the formed and reacting surface species at the 0- and β-planes, respectively. ψ0 and ψβ are the potentials at these planes. In this study, the charge- distribution (CD) approach which was firstly developed by Hiemstra and VanRiemsdijk (1996) was incorporated under the BSM framework. Thus, a fraction (f) of the charge of a surface complex is assumed to be located at the 0-plane and the others (1-f) located at the β-plane. It is a more realistic approach for treating the electrostatic contributions of bulky ligands with several functional groups, like TC molecule in this study. The heterogeneity of the surface of goethite is addressed using the MUSIC model developed by Hiemstra et al. (1989a), Hiemstra and VanRiemsdijk (1996) andHiemstra et al. (1989b). The acid–base proper- ties of goethite surfaces are described using the 1 pKmodel of the singly and triply coordinated O(H) groups, e.g. `FeOH0.5− and `Fe3O0.5− sites. The site densities of the two sites are 3.64 and 2.73 sites nm−2 (Hiemstra and VanRiemsdijk, 1996; Jonsson et al., 2008), respectively. The pKa values of the protonation reactions of the goethite surface, as well as the formation constants for the ion-pairing reactions with Na+ and NO3− are from literature (Jonsson et al., 2008) and summarized in Table 1. The adsorption edges of TC onto goethite surface suggested that there are probably at least two surface complexation reactions involved in the studied pH range. Figueroa andMackay (2005) also used two sur- face reactions tomodel the adsorption of oxytetracycline onto hematite. Fig. 3. Modeled surface speciation diagrams of TC adsorption onto goethite at three ionic strengths, at 25 °C. Symbols are experimental data, and lines are fitted using the SCM model parameters in Table 1. 23Y. Zhao et al. / Science of the Total Environment 470–471 (2014) 19–25 Meanwhile, ATR–FTIR spectra of TC adsorbed onto goethite also indicated that the tricarbonylamide group of TC probably is the main functional group to interact with the goethite surface over the whole pH range, and the phenolic diketone groupwould also be involved in surface com- plexation under alkaline condition. Thus, two TC binding surface spe- cies, a monodentate complex `FeTCH20.5+ and a bidentate complex `Fe2TC−, were used to fit the adsorption edges. All the adsorption edge data in different ionic strengths were used to fit the model simul- taneously. The formation intrinsic constants and the charge distribution of the two surface reactions were optimized using ECOSAT combined with FIT. The obtained parameters are summarized in Table 1 and pre- dicted adsorption results are shown in Fig. 2. The model is capable of fitting the experimental results well over the entire pH range at three ionic strengths. The distribution of TC surface species at three different ionic strengths (Fig. 3) showed that `FeTCH20.5+ and `Fe2TC− were the two major surface species under acidic and alkaline conditions, respectively. The amount of `FeTCH20.5+ increased as pH increased until a maximum occurred at approximately pH 5, while the species of `Fe2TC− occurred at pH 6 and increased to a maximum at pH 8.5. At pH N 9, the adsorption rapidly decreased due to the electrostatic repulsion from the negatively charged goethite surface. Considering the structure of the TCmolecule and FTIR spectra results, two coordinatemodes of TC–goethite complexes were proposed (Fig. 4). In acidic to neutral conditions, the TC molecule is mainly in zwitterion species H2TC± (Fig. S1b) and the deprotonated tricarbonylamide group (ring A) of TC can interact with the positively charged surface sites of goethite to form an inner-sphere monodentate complex `FeTCH20.5+ (Fig. 4a). Although the deprotonated O at C3 is expected to be the prior acting site to interact with the goethite surface, the steric restrain from (a) Fig. 4. Proposed stoichiometry and structure of TC–goethite surface complexes based on FTI (b) bidentate inner-sphere species`Fe2TC. the dimethylamino group might not favor this site to access goethite surface. The negative charge might be transferred to the O at C13 or C1 through the conjugate system and FTIR observation found the suppres- sion of CfO at C13, suggesting amide O might be the acting site. The model results showed that the surface charges are assigned in the 0-plane, which may be explained that the positively charged dimethylamino group at C4 locates close to the surface. Thus, it gave an overall charge difference at 0-plane Δz0 = +0.5 − (−0.5) = 1. As pH increases, the phenolic diketone group at C11–C12 starts to deprotonate and C10 or C12 may both serve as the complex sites with the goethite surface. Since the ABCD rings of TC do not locate at the same plain and the C12 hydroxyl group is near the deprotonated C13 amide O and both along the same side of the BCD rings, it is very possible that they complex with the goethite surface simultaneously to form a bidentate surface complex `Fe2TC− (Fig. 4b). Obviously, the bidentate complex is more stable than the monodentate complex, which may explain the maximum adsorption of TC on goethite that occurred at approximately pH 8 (Fig. 3). In mode b, the dimethylamino group is deprotonated; hence, overall the surface charge does not change after the reaction, e.g. Δz0 = Δzβ = 0. Although the log K value of the dimethylamino group of aqueous TC is 9.68, it is very likely that after TC being absorbed, the positively charged goethite surfaces would facilitate the deprotonation of the dimethylamino group of TC. 3.4. Model validation Two sets of batch adsorption experimental datawere used to test the applicability of the model established above. One is adsorption edges with different initial TC concentrations in a 0.01 M NaNO3 background (b) R spectroscopy andmodeling results. (a)Monodentate inner-sphere species`FeTCH20.5+; TC has a relatively large molecule (length 1.29 nm, height 0.62 nm and thickness 0.75 nm) (Gambinossi et al., 2004) and the site density e pa NO3 24 Y. Zhao et al. / Science of the Total Environment 470–471 (2014) 19–25 for the singly coordinated FeOH(H) group on goethite surface is 3.64 sites nm−2. Although themodel considered the surface charge dis- tribution of the adsorbed ligands in surface layer, themodel did not fully account for the possible steric restriction of a bulky ligand, like TC, under a high surface coverage condition,whichmight induce the overestimate of the adsorption. However, 1.42 μmol m−2 is a relatively high surface coverage. In the most environmental conditions, the concentration of TC will be much lower than that, which means the model is applicable under most environmental conditions. 4. Conclusions The present study employed a combination of experimental tech- niques and modeling to provide new insight into how TC molecules in- teract with goethite surface and what surface species are formed in this process. The ATR–FTIR spectroscopic evidence indicated that the electrolyte at pH range of 3–10 and the other is the adsorption isotherm with initial TC concentration from 0.08 mM to 0.225 mM (Fig. 5). The experimental data were predicted using the model parameters in Table 1. The results showed that the prediction was reasonably good except a little overestimated when the initial TC concentrations were higher than 0.18 mM (e.g., 1.42 μmol m−2). This is probably because Fig. 5.Model validation results. Symbols are experimental data and lines are fitted using th and 0.225 mM. Suspension density is 2.0 g L−1 and background electrolyte is 0.01 M Na electrolyte is 0.01 M NaNO3 solution (correlation coefficient = 0.95). tricarbonylamide and phenolic diketone groupsmight be the functional groups to complex with the goethite surfaces. A BSM incorporated with CD approach was developed to fit the macroscopic batch experimental data. Two surface species`FeTCH20.5+ and`Fe2TC−, which were struc- turally constrained by spectroscopic observations, could successfully fit all sets of data with different pHs, ionic strength and surface coverage. The binding constants (log K) for the two surface reactionswere estimat- ed and the surface complexationmodeswere proposed. Cautions need to be paidwhen themodel is applied to the relatively high surface coverage (N1.42 μmol m−2) since the model might overestimate the adsorption somehow. In general the model established in this study is relatively robust when predicting the adsorption of TC on goethite at a relatively wide range of ionic strengths, pH condition and surface coverage. Conflict of interest statement The manuscript (Insights into tetracycline adsorption onto goethite: Experiments and modeling) submitted to Science of The Total Environ- ment is an original work and all the results are not previously published or under consideration for publication, andwhichwill not be submitted for publication elsewhere while under consideration for Science of The Total Environment. The manuscript has no conflict of interest including any financial, personal or other relationships with other people or organizations. Acknowledgments Wewould like to thank Prof. Xinghua Xia and Dr.Wenjing Bao of the State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry & Chemical Engineering, Nanjing University, for their help in ATR–FTIR analysis. 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MacKay AA, Canterbury B. Oxytetracycline sorption to organic matter by metal-bridging. J Environ Qual 2005;34:1964–71. Marcussen H, Holm PE, Strobel BW, Hansen HCB. Nickel sorption to goethite and Insights into tetracycline adsorption onto goethite: Experiments and modeling 1. Introduction 2. Materials and methods 2.1. Materials and chemicals 2.2. FTIR measurements 2.3. Batch adsorption experiments 2.4. Surface complexation modeling 3. Results and discussion 3.1. ATR–FTIR analysis 3.2. Adsorption edges of TC to goethite 3.3. Modeling of TC adsorption to goethite 3.4. Model validation 4. Conclusions Conflict of interest statement Acknowledgments Appendix A. Supplementary data References