Insights into high-efficient removal of tetracycline by a codoped mesoporous carbon adsorbent

Xinyu Chen,Shuo Shi,Ximei Han,Min Li,Ying Nian,Jing Sun,Wentao Zhang,*,Tianli Yue,Jianlong Wang

1 College of Food Science and Engineering,Northwest A&F University,Yangling 712100,China

2 Qinghai Provincial Key Laboratory of Qinghai-Tibet Plateau Biological Resources,Northwest Institute of Plateau Biology,Chinese Academy of Sciences,Xining 810008,China

Keywords:Mechanism Carbon Cobalt/nitrogen codoping Tetracycline Adsorption

ABSTRACT Adsorbents with simple preparation and high surface area have become increasingly prevalent for the removal of organic contaminants.Herein,a carbon nanoplate codoped by Co and N elements with abundant ordered mesoporous(Co/N-MCs)was applied as an adsorbent for tetracycline removal.Taking integrated advantages of ordered mesopores on carbon-based structures and N-doping inducing the strengthened π–π dispersion and generation of pyridinic N,as well as cobaltic nanoparticles embedded in carbon nanoplates,the Co/N-MCs was tailored for high efficiently absorbing tetracycline via π-π interaction,Lewis acid-base interaction,metal complexation and electrostatic attraction.The Co/N-MCs had the advantages of high surface area,porous structure,plenty adsorption sites,and easy separation.As such,the as-prepared Co/N-MCs adsorbents significantly enhanced tetracycline removal performance with a maximum adsorption capacity of 344.83 mg?g-1 at pH 6 and good reusability,which was finally applied to removal tetracycline from tap water sample.Furthermore,the adsorption process towards tetracycline hydrochloride could be well attributed to the pseudo-second-order kinetic and Langmuir isotherm models.Compared with traditional carbon-based adsorbents,it owns a simpler synthesis method and a higher adsorption capacity,as well as it is a promising candidate for water purification.

1.Introduction

Antibiotic drugs have been widely used as therapeutic drugs for human,animal diseases and feed additives in animal husbandry.The antibiotics which are ingested by animals or humans cannot be completely absorbed and are largely discharged into the environment.However,due to their ineffective biodegradation,they are considered as a worldwide pollutant[1].Residues of antibiotics in water can accumulate in the organism through the enrichment of the food chain,resulting in the spread of resistance genes.With the widespread use and abuse of traditional antibiotics,more and more antibiotic resistance has emerged in various pathogens worldwide [2].In particular,tetracycline hydrochloride (TC-HCl),a common antibiotic concentrated in soil and water,is extremely difficult to be completely removed [3].Therefore,the search for a rapid and effective removal method of antibiotics in water is a problem that needs to be solved urgently.So far,many techniques have been proposed to solve the problem of antibiotics removal from aqueous solutions,including membrane filtration,catalytic degradation,chemical oxidation,and adsorption.Therein,adsorption has been considered to be the most simple and costeffective method,which has currently been the most common and promising method for toxic and harmful organics removal form polluted waters [2,4–7].

Over the past few years,functionalized two-dimensional nanosheet materials have been used to fabricate a variety of complementary composite materials to solve this problem [1,8].However,the disadvantage of using such composite materials as adsorbents is their incapability of having excellent dispersibility in an aqueous solution which is caused by the polymerization effect produced between themselves [9].Additionally,the synthesis process of these hybrid composites usually requires a very large number of steps,which is a rather cumbersome and expensive process[10].As a novel pore material,two-dimensional porous carbon matrix material with excellent conductivity,high surface area exposure,a uniform pore network,a large pore volume,short channels,favorable for large-scale transfer,low cost and environmental friendly features,has attracted wide attention in the fields of catalysis,drug delivery,energy conversion,environment,and adsorption [11–13].Among various porous carbon matrix materials,thin porous carbon nanoplates are an extremely promising candidate for the removal of organic pollutants from water and further improving the safety of drinking water due to their highly porous walls,huge interior space,low-cost,and easy-separation[14].Compared with other adsorbents on the market,thinlayered porous carbon nanoplates are characterized by their excellent properties,such as high adsorption capacity,simple synthesis process,non-toxic and harmless to the environment as well as large-scale production,and they have caused widespread concern[15,16].

Previous study has demonstrated that nitrogen-doped carbons or cobaltic nanomaterials possesses high performance for tetracycline or other organic pollutant removal [17,18].Therefore,this research aims to investigate the adsorption behavior and mechanism of cobalt/nitrogen-codoped ordered mesoporous carbon nanoplate (Co/N-MCs) towards tetracycline.As expected,the assynthesized Co/N-MCs exhibited efficient adsorption ability towards TC.The TC adsorption capacity of Co/N-MCs was comparable with those of some state-of-art TC adsorbents.The effects of initial pH,reaction temperature,and ionic strength of coexisting anion on TC adsorption were investigated.Then,the reusability of materials was also identified in order to evaluate the potential application of the material.Finally,the mechanism for the enhancement of tetracycline adsorption was explored through a series of detailed descriptions of the adsorption process and structural features.

2.Experimental Section

2.1.Synthesis of Co/N-MCs

All reagents and solvents were used as received without further purification.In a typical synthesis[19],triblock copolymer Pluronic F127(2 g),3-aminophenol(1 g),HMTA(0.80 g)and Co(CH3COO)2-?4H2O (0.25 g) were mixed into deionized water (60 ml) to form a homogeneous pink solution in a flask at room temperature.Then,28% ammonia (2 ml) was added into the above mixed solution drop by drop.Next,the reaction mixture was transferred into a closed glass container and heated at 80°C for 24 h under continuous stirring until form cobalt-containing nitrided carbon polymer nanoplates (Co/N-CPNS).After the reaction,the successfully synthesized brown Co/N-CPNS product was collected by centrifugation at 10000 r?min-1for 10 min,washed for three times,and then dried at 60 °C for 12 h under vacuum.Ultimately,the asprepared the Co/N-CPNS was then calcined at 800 °C for 2 h with a ramp rate of 3°C?min-1in argon atmosphere,followed by natural cooling.After that,Co/N-MCs were obtained.

2.2.Physical characterization

The morphologies of the obtained Co/N-MCs were explored using a Hitachi S-4800 field emission scanning electron microscope(FE-SEM) equipped with an EDAX energy dispersive detector.Transmission electron microscopy (TEM) was performed with a transmission electron microscope (JEOL 2010),with a field emission gun operating at 200 kV under transmission mode.UV–vis spectra were operated by using a UV-2550 spectrophotometer(Shimadzu,Japan).X-ray photoelectron spectroscopy(XPS)spectra were recorded by using an Axis Ultra DLD X-ray photoelectron spectrometer equipped with an Al Kα X-ray source (1486.6 eV).Powder X-ray diffraction (XRD) patterns were recorded with a powder diffractometer,a Bruker D8 Advance diffractometer system,equipped with a Cu Kα (0.15418 nm) source.The Fouriertransform infrared (FT-IR) spectra were collect by a Vetex70 spectrometer (Bruker Corp,Germany) in KBr pellets at room temperature.The nitrogen adsorption experiment was operated in N2atmosphere with a Gemini VII2390 instrument.

2.3.TC-HCl adsorption on Co/N-MCs

With deionized water as the experimental diluent,different concentrations of TC-HCl were prepared to test the adsorption activities of Co/N-MCs towards TC-HCl in aqueous solution.At the same time,in order to explore the possible influence of pH on the adsorption activity of Co/N-MCs towards TC-HCl,the initial pH of the TC-HCl solution (1000 mg?L-1) was adjusted to 1–6 with 0.1 mol?L-1NaOH or 0.1 mol?L-1HCl.Due to the deepening of the color of the tetracycline hydrochloride solution,the adsorption of tetracycline hydrochloride under alkaline conditions was not discussed in this experiment.Adsorption isotherm experiments were performed in centrifugal tubes by adding 10 mg Co/N-MCs with 8 ml TC-HCl of different concentrations from 5 to 1000 mg?L-1,and then the reaction systems were wrapped in aluminum foil to avoid light irradiation and shaken at 200 r?min-1at different temperatures for 1 h.After centrifuging Co/N-MCs,the concentrations of residual TC-HCl in the supernatant at different time intervals between the initial time concentration (C0) and the equilibrium time concentration (Ce) were detected at a fixed wavelength of 357 nm.The adsorption capacity of TC-HCl per gram of adsorbentqe(mg?g-1) was calculated from Eq.(1),whereV(L) referred to the volume in liter of TC-HCl andm/g denoted the mass in gram of the Co/N-MCs adsorbent used.The experiments were conducted under the same conditions with parallel three times to ensure the feasibility and accuracy of the experimental results.

In adsorption kinetics experiments,the reaction conditions and equilibrium studies were identical.A solution of 100 mg?L-1TC-HCl was added 10 mg Co/N-MCs adsorbent for typical adsorption experiments,in which the initial pH was controlled to be 6.0±0.2,and the TC-HCl concentration of all the samples at different time intervals was examined.The specific amounts of TC-HCl adsorbed at various times can be calculated from Eq.(2)

In this equation,qt(mg?g-1) referred to the amount of TC-HCl adsorbed at timet.In Eq.(2),C0(mg?L-1)denoted the initial concentration of TC-HCl,andCt(mg?L-1) represented the instant concentration of TC-HCl at timet,whereV(L) referred to the volume in liter of the initial TC-HCl solution andm(g) denoted the mass in gram of the Co/N-MCs adsorbent consumed,respectively.

Moreover,to explore interference effects caused by other ions competition,several completely different salt ions including KCl,NaCl,CaCl2and MgCl2of different concentrations were respectively mixed with TC-HCl,and TC-HCl adsorption efficiency was measured at different molar ratios of ions/TC-HCl.In addition,for the reuse of materials,the recovered Co/N-MCs adsorbent was washed with deionized water for several times and the added into 0.1 mol?L-1NaOH solution for the desorption of TC-HCl from adsorbent at 200 r?min-1for 2 h to attain sufficient desorption.The reborn adsorbent was then washed by deionized water until a neutral pH was acquired.The adsorption efficiency of the Co/N-MCs adsorbent for 1000 mg?L-1TC-HCl solutions was operated at least five times to estimate its reutilization.

2.4.Applicability of method in tap water

The applicability of the method was attested through analysis of tap water.For TC removal from tap water,0.5 mg TC/100.0 ml of each sample were incubated with 10 mg Co/N-MCs.The concentration of residual TC species in tap water was determined by UV–Vis spectrophotometer.

3.Results and Discussion

3.1.Characterization

The manufacturing process of Co/N-MCs was divided into two steps,including the self-assembly process of cobalt ion coordination and the subsequent carbonization process.After the 800 °C thermal cracking process,the synthesized product Co/N-MCs showed a lamellar structure(Fig.1a).It was worth mentioning that the lamellar carbon matrix of Co/N-MCs was interconnected with the mesopores that describe the exact ordering pore size of approximately 4 nm (Fig.1(b)).The microstructures of Co/N-MCs were further explored via TEM.As could be seen from Fig.1(c),a very large number of tightly packed pores marked in white circles were uniformly distributed on the lamellar carbon matrix of Co/NMCs.The TEM images of Co/N-MCs has also successfully demonstrated that cobalt nanoparticles marked in red circles has been completely doped into carbon nanoplates (Fig.1(c)).The results from EDAX analysis further confirmed the synthesis of Co/N-MCs after high-temperature pyrolysis process,and cobalt and nitrogen were successfully incorporated into carbon nanoplates (Fig.1(d)).

X-ray photoelectron spectroscopy (XPS) characterization was used to detect the chemical composition,and Fig.S1(a) illustrated the existence of C,O,N,and Co elements in the as-prepared Co/NMCs (Detailed analysis could be found in the part of ‘‘3.7.Adsorption mechanism”).The crystal properties of Co/N-MCs were then studied by XRD characterization (Fig.S1(b)).The XRD results showed diffraction peaks belonging to metallic cobalt.In addition,a diffraction peak of approximately 26.2°should be associated with the(002)reflection of graphitic carbon[20].Furthermore,nitrogen gas adsorption–desorption isotherms were used to test porous structural properties and specific surface area (Figs.S1(c) and S2).The specific surface area and total pore volume of the Co/NMCs adsorbent were measured to be 374.43 m2?g-1and 0.44 cm3-?g-1.Fig.S1(d)displayed the pore size distribution of Co/N-MCs at about 3.68 nm,which was in agreement with the SEM results(Fig.1(b)).In general,large surface areas and high mesoporosity have proved to facilitate the introduction of more active adsorption sites and more efficient adsorption,allowing us to expect that it could provide a large number of exposed adsorption sites and promote material diffusion [21].

3.2.Batch TC-HCl adsorption performance studies

Fig.1.Characterizations of Co/N-MCs.(a),(b) SEM images of Co/N-MCs.(c) TEM image of Co/N-MCs.(d) SEM image of high concentration Co/N-MCs and related EDX elemental mappings of Co,N,and C.

Through previous reports,we could understand the role of pH value of TC-HCl in the adsorption process and the surface properties of absorbents changed with different pH values.Therefore,necessarily,the adsorption performance of Co/N-MCs at different pH was explored within a pH range from 1 to 6(Fig.S3).According to the literature,TC molecules exhibit the aqueous dissociation constants as pK1=3.3,pK2=7.68,pK3=9.69.When the solution pH below 3.3 (pK1),TC is mainly present in the solution as TCH3+.With increasing the pH values of solution from 3.3 to 7.68 (pK2),the predominant tetracycline species are TCH2and TCH-.At higher pH values ranging from 7.68 to 9.69 (pK3),TC abundant existing species are TCH-and TC2-[22,23].As displayed in Fig.S3,when the pH was less than 3.0,Co/N-MCs showed low adsorption capacity.This phenomenon might occur due to the electrostatic repulsion between the surface of Co/N-MCs and TCH3+,as well as because of potential ionic group competition between the TCH3+and H+.With gradually increasing the pH,the adsorption capacity of Co/N-MCs increased and raised to the maximum value at pH 6.Moreover,we could also try to infer the possible electrostatic attraction of the positively charged surface of the Co/N-MCs adsorbent and negatively charged species such as TCH-/TC2-in the TC-HCl solution,which also greatly promoted the TC-HCl adsorption capacity.It must be mentioned that Co2+is a toxic heavy metal ion,so after the adsorption process we calculated the leaching of cobalt ion at different pH values.The average value of leaching concentration test results was 0.05 μg?L-1.The test results clarified that the concentration of Co2+was far below the amount allowed by the World Health Organization for drinking water standards within the experimental pH range,implying that the Co/N-MCs adsorbent remained chemically stable throughout the adsorption process.

3.3.Adsorption kinetics

In order to explore the adsorption equilibrium time and the dynamic interaction between the Co/N-MCs adsorbent and the TC-HCl adsorbate,experiments were operated at 35 °C with 100 mg?L-1TC-HCl at pH 6.As shown in Fig.S4,the adsorption amounts of Co/N-MCs towards TC-HCl rapidly increased before initial 15 min and almost reached equilibrium after 40 min.In the early stage of the adsorption process,the rapid increase of the adsorption rate might be caused by the abundant adsorption sites on the surface of Co/N-MCs adsorbent.On the basis of these data,the kinetic model of TC-HCl adsorption by Co/N-MCs adsorbent was fitted by using three kinetic models,including pseudo-firstorder kinetic,pseudo-second-order kinetic,and intra-particle diffusion model.The pseudo-first-order kinetic and pseudo-secondorder kinetic models were determined by using the formulas as following Eq.(3) and Eq.(4),respectively.

Here,k1andk2denoted the equilibrium rate constants of the pseudo-first-order and pseudo-second-order kinetic models equations,respectively.qeandqtreferred to the adsorption capacity of Co/N-MCs towards TC-HCl at the equilibrium and the instant timet,respectively.Eq.(5) and Eq.(6) showed the linear deformation fitting of the above two kinetics formulas as follows:

Herein,k1,k2,andqecould be obtained via the formulas and the specific values were listed in Table S1.From the Figs.2(a),S5 and Table S1,it could be seen that the pseudo-second-order kinetic model (Fig.2(a)) had greater correlation coefficients (R2>0.99)than pseudo-first-order kinetic model (Fig.S5),indicating the former kinetic model fitted to these data better,indicating that chemisorption might be the main rate-limiting step during the adsorption of TC-HCl by Co/N-MCs[9,24].The second-order kinetic equation is a partial differential equation obtained by mechanism reasoning assumptions and setting boundary conditions.It refers to the linear relationship between the reaction rate and the concentration of the two reactants.

The intra-particle diffusion model,Eq.(7),was then put into use to explain the diffusion mechanism in process of Co/N-MCs absorbing TC-HCl (Fig.2(b)).

In this formula,kpandCdenoted the constant of intra-particle diffusion rate and the boundary layer thickness,respectively.The specific values of the related parameterskp,C,andR2were calculated and summarized in Table S1.A drawing ofqtaboutt1/2in Fig.2(b) illustrated the multi-linearity relationship could be divided into 2 stages.The first stage denoted the transient external mass transfer of TC-HCl from aqueous solution to the Co/N-MCs.The second stage occurred because of the rate limitation of internal diffusion of TC-HCl.What should be particularly mentioned was that internal diffusion in Co/N-MCs was not the main speedlimiting step because the fitted line in the picture did not pass through the origin of coordinates [24].

3.4.Adsorption isotherm

Adsorption isotherms could elucidate the surface properties and affinity of the Co/N-MCs adsorbent,so the experimental data obtained were then fitted by using different isothermal models.Also,the adsorption performance of Co/N-MCs could be measured systematically and comprehensively.In our current research,the adsorption isotherms of TC-HCl (5–1000 mg?L-1) by Co/N-MCs adsorbent were explored at 298 K,308 K and 318 K for 1 h.The measured experimental data was fitted and compared by two isothermal models,including Langmuir and Freundlich isotherm models (Fig.2(c) and (e),and the equations of the two isothermal models were listed as Eq.(8) and Eq.(9),respectively.

Herein,qe(mg?g-1)andCereferred to the adsorption capacity of Co/N-MCs and the concentration of TC-HCl at equilibrium time,respectively.The coefficientqmax(mg?g-1) represented the maximum adsorption capacity of Co/N-MCs in Langmuir isotherm.KLdenoted the Langmuir adsorption constant.Langmuir isotherm was a hypothesis that monolayer adsorption took place in the adsorbed TC molecules.In contrast,Freundlich isotherm put forward the hypothesis that multilayer adsorption on a heterogeneous Co/N-MCs surface occurs.The Freundlich isotherm parametersCeandqealso denoted as above.TheKFandnreferred to the Freundlich isotherm constant and Freundlich linearity index,respectively.The Table S2 summarized the fitted parameters and correlation coefficients (R2) of these two adsorption isotherm.

Contrasted the correlation coefficients in Table S2,it could be inferred that the obtained experimental data of Co/N-MCs absorbing TC was more suitable for the Langmuir isotherm with higherR2values near 1.0.It illustrated that the adsorption of TC-HCl on Co/N-MCs adsorbent occurred homogeneous by monolayer adsorption[25].In the Langmuir model,it is possible for the adsorbed particles to collide with the empty adsorption center to be adsorbed,and an adsorbed particle occupies only one adsorption center,so the adsorption is monolayer and localized.As shown in Fig.2(c)and (e),it could be inferred that as the temperature gradually increases,the adsorption capacity of both isothermal models increases,which meant that higher temperatures result in higher diffusion rates and adsorption rates.Based on the Langmuir isotherm data,the maximum adsorption capacity of Co/N-MCs towards TC was calculated to be 344.83 mg?g-1.In addition,through data analysis,the standard deviation of the adsorption isotherm was 0.82,which further proves the repeatability of the experiment.Also,the N-doped carbon without cobalt doping and mesoporous structure was synthesized under the same conditions for Co/N-MCs synthesis (Fig.S6).The N-doped carbon showed a adsorption capacity of 133.85 mg?g-1towards TC (Fig.S7),which was much lower than that of Co/N-MCs,preliminarily indicating the advantages of Co doping and mesoporous structure.Also,the adsorption capacity of Co/N-MCs was comparable with those of some state-of-art carbon-based TC adsorbents(Table S3),implying that the great potential of the Co/N-MCs adsorbent.

Fig.2.Adsorption kinetic and isotherm of Co/N-MCs towards TC.(a)The linear fit of pseudo-second-order kinetic model.(b)Intra-particle diffusion kinetic model for TC-HCl adsorption on Co/N-MCs adsorbent.(c) Adsorption Langmuir isotherm and (d) the linear fit of the model for TC-HCl on Co/N-MCs at different temperatures.(e) Adsorption Freundlich isotherm and (f) the linear fit of the model for TC-HCl on Co/N-MCs at different temperatures.

3.5.Effects of ionic strength

In view of various salt ions commonly found in water in the ecosystem environment,the effect of four common cations on the adsorption capacity of the Co/N-MCs adsorbent towards TCHCl was investigated,and the possible ions competition between TC-HCl and these common cations were successfully verified(Fig.3(a) and (b)).As displayed in Fig.3(a),the removal efficiency of TC-HCl slightly increased with the augment of Na+and K+concentrations.In the experimentally controlled pH range,the main form of tetracycline in TC-HCl solution was the TCH2and TCH-,and because they had a relatively small hydration radius,they possess the ability to complex with Na+and K+[25].In addition,during the adsorption of TC-HCl,surface complexes could be formed and the Co/N-MCs adsorbent removal effect could be improved due to the existence of Na+and K+.In contrast,as the Mg2+salt ion concentration increases,TC-HCl adsorption efficiency gradually decreased and stabilized,because its relatively large hydration radius in turn suppressed the removal of TC-HCl.As for Ca2+,as the salt ion concentration increases,the TC-HCl adsorption efficiency increases and then decreases (Fig.3(b)).This may be due to the low concentration of Ca2+forming complexes with TC-HCl,and when the concentration reaches a certain value will in turn hinder the adsorption process.Additionally,the adsorption process could form two completely different complexes,including a complex formed by bonding the weak bonds of the outer layer and a complex formed by the strong bonding of the inner layer [26].

3.6.Reusability and applicability of Co/N-MCs

Fig.3.Effect of interfering ions on the removal efficiency for TC-HCl by Co/N-MCs,inculding(a)Na+ion and K+ion,and(b)Mg2+ion and Ca2+ion of different concentrations.

In order to avoid environmental damage caused by secondary pollution of Co/N-MCs,we have evaluated the actual recyclability of porous nanoplates and conducted research on regeneration.In this experiment,due to its high purity and stable chemical properties,0.1 mol?L-1NaOH was chosen as the desorbent and the adsorption–desorption cycle was repeated 5 times[27].The results showed that after the Co/N-MCs was recycled five times,the overall TC-HCl removal efficiency still reached over 86% (Fig.S8).Also,compared with other adsorbents,it owns better reusability,as reported in the literature(Table S4).Also,Co/N-MCs could removal 99.6% TC-HCl from tap water sample containing 5 mg?L-1TC-HCl,demonstrating good applicability of Co/N-MCs in tap water purification.Therefore,a simple synthesis method,extremely high adsorption capacity and excellent reusability prove that it has a very large application potential in the eco-environmental water purification system.

3.7.Adsorption mechanism

Firstly,based on the result of pH effect,we could infer that the electrostatic attraction between the positively charged surface of the Co/N-MCs adsorbent and the negatively charged ionic groups such as TCH-/TC2-in the TC-HCl solution,which also promoted TC-HCl adsorption reaction.The previous characterization results indicated that the higher adsorption capacity exhibited by Co/NMCs adsorbent was also partly due to the existence of a large number of tight mesoporous structures on the surface of Co/N-MCs,which enable plenty of active sites for effective TC-HCl removal.

In order to further grasped the adsorption mechanism of Co/NMCs adsorbent for TC-HCl,the XPS analysis was conducted.As shown in the XPS analysis of Fig.S1(a),Co/N-MCs contained large amount of nitrogen and cobalt doping.Fig.4(a) revealed that N 1s could be deconvoluted into three peaks,locating at 398.11,399.58 and 401.02 eV,which were correspond to pyridinic N,pyrrolic N and graphitic N,respectively [28].As reported,the pyrrolic and pyridinic nitrogen atoms on the Co/N-MCs supplied their pelectrons to π-conjugated rings;carbon-based material with many pyrrolic and pyridinic groups had higher electron density[29].And tetracycline adsorption could be considered as the interaction of πelectrons between aromatic rings and the graphitic structure of Co/N-MCs.Due to the higher electron density of the Co/N-MCs,more tetracycline molecules could be adsorbed through π-electrons dispersion between aromatic ring and basal carbon plane of Co/NMCs [30].Besides,as shown in Fig.4b,the high-resolution C 1s spectra of Co/N-MCs before adsorption could fall into three peaks at 284.80,286.51 and 288.55 eV,corresponding to C-C/C=C(sp2),C=N/C-O(sp2),and C-N/C=O(sp3)[31],respectively,further revealing that nitrogen element was successfully doped into the carbon network.After TC uptake,the peak belonged to C-C/C=C group of Co/N-MCs at 284.80 eV slightly shifted to 284.86 eV.The above results gave strong evidence that the π-π interaction participated in the adsorption,which was then enhanced by higher electron density induced by N-doping [32].As reported,the pyridinic N on carbon might serve as weak Lewis base to interact with the O-containing moieties of tetracycline via Lewis acid-base interactions,contributing to the shift of pyridinic N to 398.32 eV in the XPS spectra of used Co/N-MCs (Fig.4(a))[33,34].Also,the nitrogen doping enable the carbon plane of Co/N-MCs with a greater number of polar functional groups,resulting in a higher hydrophilicity,in turn promoting the contact of the carbons with the solution leading to an efficient process[35].Overall,the doped-N could promote the adsorption of Co/N-MCs towards TC-HCl.

To explore whether Co-doing influence the adsorption activity of Co/N-MCs towards tetracycline,the high-resolution Co 2p spectra of Co/N-MCs before and after TC adsorption were compared.As shown in Fig.4(c),before adsorption,Co 2p shows peaks at 780.74 and 796.67 eV,which were assigned to Co 2p2/3and Co 2p2/1for the metallic cobalt(0),respectively[36].And these were consistent with the results of XRD characterization mentioned above.The binding energies of 781.90 and 797.64 eV were accompanied by two strongly vibrating satellites (785.89 and 803.01 eV),suggesting the existence of cobalt-oxygen bonds,which could be attributed to the surface oxidation of metallic cobalt during preparation[37].And cobalt mainly distributed on laminated carbon structures with cobalt oxide exposed on surfaces [37,38].Absorbing tetracycline,the peaks for high-resolution Co 2p spectra of cobaltoxygen bond changed from 781.90 and 797.64 eV to 781.63 and 797.34 eV,respectively,suggesting the surface combination of CoO with TC.Noted that,the content of C,N,O and Co for Co/NMCs was 92.66%,2.73%,4.16%and 0.45%through XPS data analysis.As reported,Co(II)could complex with tetracycline though binding with the 1,2,3-tricarbonylmethane group [22].Therefore,it was reasonable that Co-doing could further enhance the adsorption activity of Co/N-MCs towards tetracycline.

Fig.4.The high-resolution XPS spectra of (a) N 1s,(b) C 1s and (c) Co 2p in Co/N-MCs before and after TC adsorption.

Fig.5.(a) FT-IR spectra of TC-HCl,Co/N-MCs and TC-HCl-loaded Co/N-MCs before and after TC-HCl adsorption.(b) A partial amplified view of (a).

Fig.6.Illustration of adsorption mechanism of TC-HCl on the Co/N-MCs adsorbent.

To confirm these interaction mechanisms obtained from XPS analysis,FT-IR characterization of Co/N-MCs before and after TC adsorption were performed and the results are presented in Fig.5.At 606 cm-1,the cobalt-oxide vibration bands of the pure Co/N-MCs functional group appeared.After the adsorption process,a fingerprint peak of TC-HCl appeared in the Fourier infrared spectrum of Co/N-MCs,from 1600 to 750 cm-1,which revealed that TCHCl had been successfully adsorbed(Fig.5)[39].Additionally,after adsorption of TC-HCl,the cobalt-oxide bonds vibration band was shifted to 592 cm-1,which was possibly due to the strong surface complexation between the adsorbents and was coincident with above XPS result [40].Also,after the adsorption of TC onto Co/NMCs,the peaks of C=C bonds presented a shift from 1636 to 1619 cm-1,suggesting that the π-π interaction might be responsible for this significant changes [32],which also supported the above XPS result.

In summary,Co and N codoped porous structure had a greater impact on the adsorption process of Co/N-MCs to TC-HCl,including π-π interaction,Lewis acid-base interaction,metal complexation and electrostatic attraction.Importantly,the high specific surface area and unique mesoporous structure also provided a large number of adsorption sites for TC-HCl adsorption.Fig.6 illustrated the overall adsorption mechanism of TC-HCl on the Co/N-MCs adsorbent.

4.Conclusions

In conclusion,we codoped carbon nanoplate with abundant ordered mesoporous with Co and N elements (Co/N-MCs),obtaining an excellent adsorbent for tetracycline removal.The Co/N-MCs had the advantages of high surface area,porous structure,plenty adsorption sites,and easy separation.Furthermore,adsorption experiments showed that the adsorption process of Co/N-MCs towards TC-HCl could be well attributed to the pseudo-secondorder kinetic and Langmuir isotherm models,and the adsorption capacity was 344.83 mg?g-1at pH 6.In addition,the dominant mechanism for the adsorption of TC-HCl by Co/N-MCs was proposed.With simple material synthesis procedure and excellent reproducibility,Co/N-MCs was more suitable for large-scale production and application than traditional adsorbents previously reported,showing great potential for water purification.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financed by Grants from National Science Foundation of China(21675127,31901794),Chinese Universities Scientific Fund (2452018083),the National Postdoctoral Program for Innovative Talents (BX20180263),the Tang Scholar by Cyrus Tang Foundation,the Young Talent Fund of University Association for Science and Technology in Shaanxi,China(2019-02-03),the Development Project of Qinghai Provincial Key Laboratory (2017-ZJY10),the Key Research and Development Program of Shaanxi Province (2019NY-111).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.07.023.