Removal of Typical Organic Contaminants with a Recyclable Calcined

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Removal of Typical Organic Contaminants with a Recyclable Calcined Chitosan-Supported Layered Double Hydroxide Adsorbent: Kinetics and Equilibrium Isotherms Hanjun Wu,† Hongyu Gao,‡ Qinxue Yang,† Huali Zhang,§ Dongsheng Wang,†,∥ Weijun Zhang,*,⊥ and Xiaofang Yang*,∥ †

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, Hubei China Institute of Resources and Environment Engineering, Shanxi University, Taiyuan 030006, Shanxi China § School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430074, Hubei China ∥ State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ⊥ School of Environmental Studies, China University of Geosciences, Wuhan 430074, Hubei China ‡

ABSTRACT: In this work, the calcined chitosan (CS)-supported layered double hydroxides were prepared, characterized, and further applied to remove organic contaminants in aqueous solution. The chitosan-supported layered double hydroxide calcined at 400 °C (CSLDO400) was found to be effective for the adsorption of azo dye and antibiotic. CSLDO400 showed excellent adsorption performance at a wide pH range from 5 to 9 for methyl orange (MO) while the adsorption of tetracycline (TC) was most efficient at a pH of 9, and the optimal CSLDO400 dosages were 0.175 g L−1 for MO and 0.375 g L−1 for TC. The divalent and trivalent anions have a great effect on removal efficiency of MO and TC, while the effect of monovalent anions adsorption can be neglected. The adsorption kinetics indicated that the MO and TC removal followed the pseudo-second-order model. In addition, it can be fitted well with both the Langmuir and Freundlich model for MO and TC at 298 K, respectively. The adsorptions of MO and TC on CSLDO400 were both spontaneous and endothermic. After five adsorption−desorption cycles, CSLDO400 still showed high efficiency with an adsorption capacity of 60.72 mg g−1 for MO and 21.92 mg g−1 for TC. Therefore, the CSLDO400 are promising for use in the treatment of drinking water and wastewater contaminanted by organic matters. Fe(III) reduction,9 nitrification,10 and microbial soil respiration.11 Various methods concerning these organic pollutants removal from aqueous solutions have been reported, such as photocatalytic degradation, adsorption, electrochemical oxidation, coagulation, and biological technology.12 Among these methods, adsorption is most promising and widely used because of its low cost, friendly environment, high efficiency and simple operation.13 Abatement of these noxious impurities from aqueous solutions has been tried using a wide variety of adsorbents, such as montmorillonite,14 chitosan particles,15 the modified activated carbon,16 and aluminum oxide17 etc. However, the greatest drawback to traditional adsorption materials is the fact that these have a low removal performance in actual water treatment and limited recycling ability. In view of this, there is considerable interest in developing novel materials for the removal of such pollutants.

1. INTRODUCTION The contamination caused by organic pollutants poses a great threat to the safety of water ecosystems and human health.1 In recent years, many studies have been carried out on the removal of organic pollutants because of the increasing concentration in aqueous solutions caused by sewage discharge from industry, household, and agriculture.2 The organic pollutants consist of various species such as dyes, antibiotics, and phenols, etc. Among them, azo dyes represent a major group of all the dyes produced worldwide.3,4 Azo dyes contain an −NN− group, which results in the formation of aromatic amines. In industry, approximately 10−15% of dyes stuff is released into the environment.5 Those dyes are always carcinogenic and seriously harm human health and the environment. Also, antibiotics are also widely used in the farming industry and for human therapy,6 and comprehensive attention has been focused on the antibiotics in recent years because of the great risk related to their existence in the environment, which increases their potential to cause antibiotic resistance among bacteria.7,8 In particular, as a commonly used antibiotics, tetracycline antibiotics have been proven to affect © XXXX American Chemical Society

Received: August 23, 2017 Accepted: November 16, 2017

A

DOI: 10.1021/acs.jced.7b00752 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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resistance and chelation capacity of transition metal, but poor acid resistance. The chitosan support layered double hydroxide (CSLHH) was synthesized by the coprecipitation method. CS solution with a concentration of 4 wt % was prepared by dissolving 3 g of chitosan in glacial acetic acid (5 wt %). The wet mass was stirred continuously, to which Mg(NO3)2· 6H2O−Fe(NO3)3·9H2O solution (mole ratio: 3) and CS solution were added simultaneously using two constant pressure funnels. After the above mixture solution was stirred for 1 h, the NaOH and Na2CO3 mixture solution was then slowly added dropwise to the mixture above with vigorous stirring at 60 °C for 0.5 h, and the solution pH was held in the range of 11−12. The precipitated mass was aged at 65 °C for 18 h, filtered, washed with distilled water until the filtrate was neutral to litmus, and freeze-dried, and the CSLHH was obtained. Three CSLDOs, calcined at 300, 400, and 500 °C (measured from the increase in calcination temperature) under N2 protection for 2 h, CSLDO300, CSLDO400, CSLDO500, respectively, were obtained by varying the calcination temperature of CSLDHs. Unsupport LDHs were calcined at 400 °C (LDO400) as a comparison. 2.3. Sample Characterization. The CSLDH, unsupported LDO400, and CSLDO were characterized for their mineralogical phases, morphology, specific surface area, and crystallinity. The morphology of the microspheres was investigated by using FEI Sirion 200 (FEI Co., Netherlands) FEI-SEM with an accelerating voltage of 20 Kv. The microstructure of the samples was examined with transmission electron microscopy (TEM) (HITACHI, H-7500). BET surface area of these samples were determined by N2 adsorption−desorption technique on a Micrometrics 2020HD88 (Micrometrics Instrument Co., USA) apparatus at 77 K. The surface area was calculated by Brunauer−Emmett− Teller method. The Fourier transform infrared spectroscopy (FT-IR) was measured using Vertex 70 (Bruker, USA). The mineralogical phases and crystallinity were studied by X-ray diffraction (Rigaku Miniflex X-ray diffractometer) using Cu Ka radiation. Thermogravimetric analysis (TGA) was measured using Pyris 1 TGA (TA Instruments) thermal analyzer system at the heating rate of 10 °C min−1. 2.4. Adsorption Experiments. A contrast experiment about effects of time was made between CSLDH, LDO400, CSLDO. The adsorptions of MO and TC were optimized by considering CSLDO dosage, solution pH, adsorption kinetics, isotherms, and thermodynamics. The adsorption isotherms were finished in the range of 25−45 °C at initial concentration of 10−600 mg L−1 for MO and 5−300 mg L−1 for TC. To optimize calcination temperature, CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 with different dosages (MO, 7 mg; TC, 15 mg) were added respectively to the 40 mL aqueous solutions of MO and TC (initial concentration was 20 mg L−1) in a 50 mL polypropylene (PP) bottle in an thermostatic shaker (Changzhou Guoyu Instrument Manufacturing Co., Ltd. CHA-SA) at 170 rpm at 298 K for 4 h. All the following experiments were done by using the 50 mL polypropylene (PP) bottle and shaking at a constant rate. The initial and final concentration (Ce) of MO and TC was determined using a UV spectrophotometer at 464 and 362 nm wavelength for MO and TC, respectively. The adsorption capacity (qe, mg g−1) of CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 were calculated using eq 1.

Layered double hydroxides (LDHs) is a class of synthetic anionic clay consisting of positively charged hydroxy layers of bivalent and trivalent metal ions with exchangeable interlayer anions,18 which have raised considerable concerns in the recent decade for their application in water pollution control.19−21 The most prominent structure of LDHs is the flat twodimensional structure network which is composed of trivalent metal ions and different hydroxyl layers of bivalent ions.22 The positive residual charges generated by the isomorphic substitution of some bivalent metal ions of LDHs with trivalent ions will help offset water molecules and anions located interstitially.23,24 The general formula of LDHs can be described as [Mx2+My3+(OH)2(x+y)]Ay/nn‑·mH2O (M3+, trivalent metal ions; M2+, bivalent metal ions; A, exchangeable anion).23 The metal hydroxide can adsorb some anions through electrostatic interactions because they have the positive surplus charges.25 LDHs have been investigated as potential adsorbents for removing organic pollutants from aqueous systems. However, most of these LDHs focus on aluminum-based compounds and long-time exposure to Al has been pointed out as a potential risk factor for human health and environment.26,27 In light of that, it is necessary to modify the LDHs prior to practical application. The calcined chitosan-supported Mg−Fe LDHs are environment-friendly materials and may be an effective adsorbent in organic contaminants removal. Chitosan (CS) is an environmentally friendly polymer material which is composed mostly of a glucosamine unit. It exhibits excellent adsorption performance on various organic pollutants attributed to the presence of active hydroxyl (−OH) and amino (−NH2) functional groups.28 Furthermore, CS has strong chelating ability because the transition metal ions and active amino or hydroxyl functional groups of the molecular structure of CS form a coordinate covalent bond. It has been reported that chitosan−Fe3+ was used as the adsorbent to remove alkaline dyes from aqueous solution.29 Thus, it is possible to promote the crystallization of Mg−Fe LDHs on the surface of chitosan based on the chitosan-Fe3+ precursor by the coprecipitation method. The calcined chitosan-supported layered double hydroxides (CSLDOs) may result in a small amount of particles aggregation of LDHs and large specific surface areas, which probably contribute to more adsorption sites and enhance its adsorption capacity. Herein, we prepared a low-cost and biodegradable material that can act as a support for LDH and also enhance its azo dyes and antibiotics adsorption capacity. Batch adsorption experiments were carried out to optimize the adsorption parameters, kinetics, isotherms, and thermodynamics to understand the adsorption mechanisms, and the used CSLDO was also regenerated to test the reusability.

2. EXPERIMENTAL SECTION 2.1. Materials. Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and iron(III) nitrate nonahydrate (Fe(NO3)3· 9H2O) used for Mg−Fe LDH syntheses by the coprecipitation were synthesis grade, purchased all from Sinopharm Chemical Reagent Co., Ltd. The CS support for the preparation of Mg− Fe LDH (CSLDH) was prepared from CS solution, CS, MO, and TC were purchased from Aladdin. Simulated wastewater of different concentrations in the adsorption experiments were prepared by dissolving appropriate quantities of MO and TC in deionized water. 2.2. Synthesis of Calcined Chitosan Support Layered Double Hydroxide (CSLDO). CS has excellent alkali B


Journal of Chemical & Engineering Data qe =

(C0 − Ce) × V m

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CSLDH and LDO400, the layered structure of CSLDO300, CSLDO400, and CSLDO500 were more unordered, indicating the pitted holes, which originated from the chitosan carbonization. As can be seen from Figure 1 panels h−j, the microstructure of the calcined samples changed as the calcination temperature increased. This may be due to the different carbonization degree of the supported CS and magnesium and iron mixed oxides. The magnesuium and iron mixed oxides could not be formed when calcined at 300 °C. However, MgFe2O4 and MgO will be produced at 500 °C.26 This result also confirmed the BET analysis, which showed that CSLDO400 had the largest specific surface areas. The layer structure and charge property might play an important role in MO and TC adsorption. 3.1.2. BET Analysis. To the best of our knowledge, the extremely reactive mixed oxides, which were produced from the calcination of LDHs, have been reported previously.30,31 The porous properties of CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 were characterized, and the results are summarized in Table 1. As shown in Figure 2a, a

(1)

And the removal rates (R: %) were calculated using the formula R(%) =

C0 − Ce × 100 C0

(2) −1

where C0 and Ce (mg L ) are the initial and residual concentrations of MO and TC, respectively; V (L) is the total test solution volume and m (mg) is CSLDO dosage. The effect of CSLDO400 dosages were investigated by mixing CSLDO400 (MO, 0.075−0.375 g L−1; TC, 0.125−0.75 g L−1) with 40 mL aqueous solutions of MO and TC (20 mg L−1), respectively. The initial values for pH were set in the range of 3.0 to 13.0. The effect of coanions was carried out in the presence of various competing anions. To study adsorption rate and dynamic characteristics, CSLDO400 (MO, 0.175 g L−1; TC, 0.375 g L−1) was mixed with MO and TC solution (20 mg L−1) for varying times. The adsorption isotherms were finished in the range of 25−45 °C at an initial concentration of 10−600 mg L−1 for MO and 5−300 mg L−1 for TC. To study the thermodynamic features, isothermal experiments were done at 298, 308, and 318 K, respectively. The reusability of CSLDO400 was studied. In detail, the used CSLDO400 was eluted by sodium carbonate solution (0.5 mol L−1), and then filtered, washed with distilled water until the filtrate was neutral to litmus, and calcined at 400 °C for 2 h.

Table 1. Specific Surface Area, and Pore Volume Parameters of CSLDH, LDO400, CSLDO300, CSLDO400, CSLDO500

3. RESULTS AND DISCUSSION 3.1. Characterization of the Adsorbent. 3.1.1. FE-SEM Analysis. FE-SEM and TEM were carried out to obtain the microstructure of the samples prepared by different methods, and the images are shown in Figure 1. The representative

SBETa

Smicb

Vmicc

Vmesod

Vt e

Dpf

material

m2 g−1

m2 g−1

cm3 g−1

cm3 g−1

cm3 g−1

nm

CSLDH LDO400 CSLDO300 CSLDO400 CSLDO500

16.38 80.73 47.55 116.98 94.35

2.03 7.59 4.05 4.37 2.21

0.0003 0.0056 0.0012 0.0014 0.0005

0.0557 0.3174 0.1538 0.4106 0.3175

0.056 0.323 0.155 0.412 0.318

13.28 15.83 12.01 8.84 10.99

a Determined by N2 adsorption using the Brunauer−Emmett−Teller (BET) method. bMicropore area, determined by DFT. cMicropore volume, calculated using the Dubinin−Astakhov method. dMesopore volume, calculated by Vt − Vmic. eTotal pore volume, determined at P/ P0 = 0.9923. fAdsorption average pore width (4 V/A by BET).

high uptake of nitrogen was observed at low relative pressures, representing the microporous nature of the layered oxides and carbide networks. However, the isotherms appeared to have a steep upward-loping trend at higher pressures (P/P0 > 0.9) because of the presence of larger pores.32 It can be seen from Figure 2b that the five samples showed a predominant presence of mesopores. As shown in Table 1, the pore volume and surface area of CSLDO400 were larger than that of CSLDH, LDO400, CSLDO300, and CSLDO500, while the pore size was smaller, revealing that CSLDO400 had more plentiful pores than the other samples. During the calcination process, the carbonated chitosan support layer formed a porous carrier; the layered structure of hydrotalcite was destroyed due to breakup of the crystal structure, resulting in the formation of the porous structure in the interlayer of CSLDO400. Then, it caused a considerable increase of the surface area of CSLDO400, which provided more surface active sites that might result in better adsorption performance. The total pore volumes of CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 were calculated to be 0.056, 0.323, 0.155, 0.412, and 0.318 cm3 g−1, respectively, and the pore diameters were 13.28, 15.83, 12.01, 8.839, and 10.99 nm (the so-called “mesoporous” size), indicating the contaminants were likely to be able to enter into the interlayers of LDO400.

Figure 1. FE-SEM and TEM images of CSLDHs (a,f), LDO400 (b,g), CSLDHO300 (c,h), CSLDO400 (d,i) and CSLDO500 (e,j).

micrographs of CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 are presented in Figure 1a,f, 1b,g, 1c,h, 1d,i, and 1e,j, respectively. As shown in Figure 1a and 1f, CSLDH appeared to be obvious cross-linked lamellar structures. This phenomenon indicated that the LDHs crystal successfully grew on the surface of chitosan-Fe3+. It can be seen in Figure 1b and 1g that the layered structure and surfaces of LDO400 were evident and smooth. The layered structure was clear with visible edges. The micrographs of the calcined chitosansupported LDHs (As shown in Figure 1c, 1d and 1e) were observed that the surfaces and layered structure were pitted and unordered, respectively. Compared to the micrographs of C



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Figure 2. (a) N2 adsorption−desorption isotherms of CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 at 77 K and (b) pore size distribution.

3.1.3. FT-IR Analysis. The FT-IR spectra of CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 were shown in Figure 3. As shown in the Figure 3a (CSLDHs), the

Figure 4. X-ray diffraction (XRD) patterns of chitason-supported LDHs, CSLDH(a), calcined LDH, LDO400(b), calcined chitasonsupported LDH, CSLDO300(c), CSLDO400(d), CSLDO500(e).

while, the diffraction pattern for the electrodeposited film had broad peaks at 11.68° and 23.59°, according to the 003 and 006 crystal planes, which meant that a typical structure of layered double hydroxides successfully formed.27 The patterns of calcined samples (Figure 4b−e) showed that the diffraction peaks of layered double hydroxides have disappeared, which meant that the layered structure and crystal structure were destroyed and changed, and indicated the presence of magnesium and iron oxide peaks. Yet compared to that of the CSLDH, it was noted that the intensity peaks of the calcined products decreased. The calcined LDHs will spontaneously restored to their original layered structure after entering into the aqueous solution, which was called the “memory effect”.30 Therefore, based on the memory effect, the calcined chitosan-supported LDHs were foreced to adsorb anionic contaminants from water. 3.1.5. TGA Analysis. Thermogravimetric analysis (TGA) was carried out to study the thermal stability of the adsorbents. The results were shown in Figure 5. Below 300 °C, the weight of CS was almost unchanged, and that of the LDHs and CSLDHs showed a smaller decrease. This process could contribute to the

Figure 3. FT-IR spectra of chitosan-supported LDHs, CSLDHs(a), calcined LDHs, LDO400(b), calcined chitosan-supported LDHs, CSLDO300(c), CSLDO400(d), CSLDO500(e), MO-adsorbed (f) and TC-adsorbed (g).

appearance of two adsorption peaks at 3700−3500 cm−1 was explained by the stretching N−H stretching. This result suggested that the LDHs were supported successfully by chitosan which contained an amino group. The absorptions bands of the interlayer NO3− of CSLDH can be observed at ∼1384 cm−1. The spectrum of the LDO400 (Figure 3b) and CSLDH were very similar, except for the bands at 3700−3500 cm−1. The disappearance of a band at ∼3664 cm−1 might be attributed to the peak overlapping of the carbonation of chitosan and the interlayer anions of CSLDH. The spectrum of the LDO400 (Figure 3b), CSLDO300 (Figure 3c), CSLDO400 (Figure 3d), and CSLDO500 (Figure 3e) were very similar, especially the bands at ∼1440 and 1384 cm−1 derived from the interlayer anions (CO32− and NO3−) were not found. It can be seen from Figure 3 panels f and g that both adsorption bands of MO and TC were present in the spectra of MO-adsorbed CSLDO400 and TC-adsorbed CSLDO400, which indicated that the organic contaminants were successfully adsorbed in the interlayer of CSLDO400. The adsorption sites of CSLDH were occupied by those interlayer anions, in particular, CO32− ions have a very strong affinity to hydrotalcite. 3.1.4. XRD Analysis. The X-ray diffraction (XRD) patterns of CSLDH, LDO400, CSLDO300, CSLDO400, and CSLDO500 were shown in Figure 4. As seen from the synthesized products of CSLDHs in Figure 4a, symmetric and sharp peaks appeared, indicating the high crystallinity. Mean-

Figure 5. TGA of CS, LDH, and CSLDH composites. D



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Figure 6. Effect of time on CSLDH, LDO400, CSLDO300, CSLDO400, CSLDO500, and CS on the adsorption of MO (a) and TC (b) (dosages, 0.175 g L−1 for MO and 0.375 g L−1 for TC; solutions, 20 mg L−1; pH = 9; 4 h; temperature, 298 K).

Figure 7. Effect of adsorption conditions: pH (a); CSLDO400 dosages for MO (b); and TC (c); coanions (d) (initial solutions, 20 mg L−1; adsorption time, 3 h; temperature, 298 K).

release of water vapor and carbon dioxide in the interlayers of LDHs. At the range of 300−400 °C, the weight of CS, LDH, and CSLDH reduced rapidly because of the formation of magnesium and iron mixed oxides, and the residual cross-liked degradation chitosan.33 CS, LDH, and CSLDH, had limited weight-loss until the temperature was above 400 °C, indicating their excellent thermal stability. 3.2. Evaluation of Organic Contaminants Removal Efficiency with Different Hydrotalcite-like Materials. 3.2.1. Effect of Calcination Treatment on MO and TC Removal. As shown in Figure 6, after reaching adsorption equilibrium, the samples without calcination pretreatment (CSLDHs) showed lower adsorption capacity qe (mg g−1) than the calcined adsorbents (LDO400, CSLDO300, CSLDO400, and CSLDO500). The adsorption capacity of CSLDH decreased when the calcination temperature was above or below 400 °C. Meanwhile, the adsorption capacity of calcined chitosan-supported layered double hydroxides was higher than that of the unsupported. Compared to CSLDO400, CS showed lower adsorption capacity for MO, and nearly no adsorption capacity for TC. Since the affinities of various anions toward interlayers of hydrotalcite (LDHs) follow the order CO32− > SO42− > OH− > F− > Cl− > Br− > NO3− > I−,34

carbonate and hydroxyl ions were not easily replaced with MO and TC in the interlayers of CSLDH which were not calcined. With calcination temperature increasing, carbonate and hydroxyl ions reduced in the interlayers of CSLDH gradually, and disappeared almost entirely at 400 °C calcination. When CSLDHs were calcined at 500 °C, the double oxide structures were destroyed and their structure restoring capacitymemory effectwas lost, so MO and TC hardly penetrated into the CSLDO500 layers. As mentioned above, the CSLDO400 had more plentiful porous structure and larger specific surface area, so the adsorption equilibrium amounts qe (mg g−1) of CSLDO400 were higher than that of LDO400. In summary, CSLDO400 was carried out as the optimal adsorbent for the following adsorption experiments. 3.2.2. pH Effect. Figure 7a showed the effect of initial solution pH on MO and TC removal on CSLDO400, and it can be seen that it was important for the adsorption of MO and TC with optimal pH value. For MO, the adsorption equilibrium amounts of CSLDO400 processes an obvious increase when the initial pH values of the solutions ranged from 3 to 5, while it dropped abruptly when the initial pH values were increased from 9 to 13. The MO adsorption capability overall retained a relatively high situation when the initial pH values ranged from E



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TC, which indicated that CSLDO400 exhibited considerable potential for the removal of MO and TC in real water treatment. 3.3. Adsorption Theory Discussion. 3.3.1. Adsorption Kinetics. Kinetic studies of the removal of MO and TC by CSLDO400 were helpful to access to essential information, which was important to enable the adsorbent to be used in actual treatment systems. The adsorption kinetics were shown in Figure 8a. For MO, the adsorption was very rapid, occurring

5 to 9. It was very likely that the layered structures of the adsorbent had a wealth of surface properties. At high pH levels, a strong competitive adsorption between hydroxyl and MO was predominant in the interlayer of CSLDO400, while the double oxides structure was damaged by the reaction of excess H+ and double oxides at low pH values. Therefore, the pH of 7 (range from 5 to 9) was used in the following experiments. As shown in Figure 7a, the equilibrium adsorption amounts qe (mg g−1) for TC increased gradually by rasing the pH from 3 to 9, while it dropped sharply when the pH values were over 9. The phenomenon can be attributed to the existing form of TC and selective adsorption of adsorbent. Generally, TC is an amphiphathic molecule; it existed in the form of cationic form (H3TC+) at a pH less than 3.3, while the neutral molecule (H2TC) and anionic (HTC− and TC2−) form at a pH from 3.3 to 7.68 and higher than 7.68, respectively.35 To achieve the recovery of layered structures of CSLDO400, only the anionic form of TC could be forced to enter into the interlayer of CSLDO400 through the electrostatic attraction and anionic ions exchange when the initial pH values exceeded 7.68. However, hydroxyl showed a strong competitive adsorption effect to TC at higher pH levels; the adsorption equilibrium amounts of CSLDO400 decreased gradually. The neutral and cationic forms of TC were not able to enter into the interlayer of the CSLDO400, since the layered structure not only would be disrupted in acidic solution but also the cationic and neutral contaminants would not equal the positive surplus charges of the interlayers of CSLDO400 through electrostatic interactions. 3.2.3. CSLDO400 Dosage. As shown in Figure 7b,c, the equilibrium removal rates went up with the increase of dosage. On the contrary, the equilibrium adsorption amounts qe (mg g−1) of MO and TC decreased obviously. The enhancement of removal efficiency can be attributed to the high number of unsaturated adsorption sites, and an increase in adsorption surface area of CSLDO400. On the one hand, the absorbents had extra surface active sites because of more CSLDO400, on the other hand, the increase of the concentration of CSLDO400 particles improved the chance of collision and agglomeration of CSLDO400 particles. Considering the cost and efficiency of water treatment, the CSLDO400 dosages were decided accurately at 0.175 mg L−1 for MO and 0.375 mg L−1 for TC. 3.2.4. Effect of Co-anions. Anions exist universally in actual sewage water, which may influence the sorption of MO and TC. Therefore, the effect of NO3−, Cl−, CO32−, SO42−, and PO43− on the adsorption efficiency of MO and TC using CSLDO400 as adsorbent were investigated. It can be seen from Figure 7d that the presence of various competing anions caused different effects on removal of MO and TC obviously, and the adsorption capacity of both MO and TC decreased in the following order:

Figure 8. (a) Effect of contact time on the adsorption of MO and TC (solutions, 20 mg L−1, pH = 9, 0.175 g L−1, and 0,375 g L−1 dosages for MO and TC; temperature, 298 K); (b) pseudo-first-order kinetic plots for adsorption of MO and TC; (c) pseudo-second-order kinetic plots for adsorption of MO and TC.

within 90 min, and then slowed down. For TC, the previous 30 min belonged to rapid adsorption followed by a minor increase; no significant change was observed after 120 min. Pseudo-firstorder and pseudo-second-order kinetic models have been used to shed some light on the adsorption behavior and potential rate controlling steps.36,37

PO4 3 − > CO32 − > SO4 2 − > HCO3− > Cl− ≈ NO3−

It was clear that the divalent and trivalent anions had more impact on removal efficiency of MO and TC than monovalent anions. The adsorption sites and capacity for MO and TC tended to decrease mostly because the competitive anions were introduced on the surface of CSLDO400. Simultaneously, it indicated that monovalent anions had a subtle effect on the removal of MO and TC, and the high valence anions can be used for regeneration of CSLDO400. The results in Figure 7d show that the real water (such as lake water, groundwater, and tap water) only had a subtle effect on the removal of MO and

ln(qe − qt ) = ln qe − k1t

(3)

t t 1 = + qt qe k 2qe 2

(4)

where qt and qe are the amount of adsorbed MO and TC (mg g−1) at time (t) and reach equilibrium, and k1 and k2 are the pseudo-first-order and the pseudo-second-order rate constants, F



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Table 2. Fitting Parameters by Pseudo-First-Order and Pseudo-Second-Order Models pseudo-first-order kinetic model qe(exp) adsorbates MO TC

mg g

k1

−1

min

75.96 30.51

−1

0.06081 0.02664

pseudo-second-order kinetic model k2(×10−4)

qe1(cal) mg g

−1

R

79.62 29.28

2

g mg

0.9908 0.9926

−1

min

14.35 48.54

−1

qe2(cal) mg g−1

R2

72.31 31.74

0.9966 0.99966

Figure 9. Adsorption isotherms for MO (a) and TC (b) (solutions = 5, 10, 20, 50, 100, 150, 200, 250, 300 mg L−1; pH = 9; 0.175 g L−1 and 0.375 dosages for MO and TC; 3 h), Langmuir plots of the isotherms for MO (c) and TC (d), Freundlich plots of the isotherms for MO (e) and TC (f).

Table 3. Parameters Fitted by Langmuir and Freundlich Models for MO and TC Langmuir model

Freundlich model

T (K)/adsorbates

b (L mg−1)

qmax (mg g−1)

R2

n

Kf

R2

298/MO 308/MO 318/MO 298/TC 308/TC 318/TC

0.005946 0.005286 0.004518 0.030212 0.029963 0.027702

1266.42 1277.61 1314.92 195.31 209.64 273.97

0.9765 0.97029 0.96026 0.99535 0.99464 0.99223

1.49939 1.46501 1.43796 1.72286 1.6984 1.58702

18.2082 15.89439 14.0687 9.8184 10.1982 11.3061

0.99624 0.99792 0.98853 0.97574 0.97919 0.98229

respectively. The model fttings are shown in Figure 8b and 8c, and the parameters qe, k1, and correlation coefficient (R2) were provided in Table 2. It was found that the values of correlation coefficient described the pseudo-second-order model very well, and that was a bit better than the pseudo-first-order model for both MO and TC. Thus, it could be concluded that the MO and TC

adsorption reaction with CSLDO400 took place more properly and obeyed the pseudo-second-order kinetics. 3.3.2. Adsorption Isotherm. Adsorption isotherms of MO and TC are shown in Figure 9a and 9b, respectively, which indicated that the adsorption of MO and TC were both promoted by raising the temperature. Langmuir and Freundlich models were used with an aim to fit the adsorption experimental data. The models were represented as follows:38 G


Journal of Chemical & Engineering Data Ce C 1 = e + qe qm bqm

CSLDO400 was able to restore its original layered structure when it was exposed to a water environment. Then Mg2+ ions in the interlayer of the structure of CSLDO400 were replaced by Fe3+, which resulted in a large number of negative charges in the interlayer of CSLDO400. Thus, based on electrostatic interaction between MO, TC, and the positive charge of the CSLDO400 host layers, CSLDO400 might be forced to adsorb anionic contaminants from the aqueous solution to matain a balance between charges in the interlayer of CSLDO400. In aqueous solution, hydroxylate surfaces were created through the coordination reaction between OH− and the iron hydroxides (oxide). The exchange between the anionic form (HTC− and TC2−) and the OH− of FeOH takes place, and then the surface complexes (>Fe−HTC− or Fe−TC2−) are formed. 3.4. Regeneration and Reuse. For actual application, the used adsorbent was necessary to be regenerated to study the reusability. During the adsorption of MO and TC, the layered structure of the CSLDO400 was not changed, and therefore high valence anions may be used for regeneration of CSLDO400. Thus, in order to recyle the saturated CSLDO400, desorption needed to take place. The results of adsorption−desorption were shown in Figure 10. The regenerated CSLDO400 experienced a slight decrease

(5)

1 ln Ce + ln K f n

ln qe =

Article

(6)

−1

where Ce (mg L ) is equilibrium liquid phase concentrations, qm (mg g−1) is maximum adsorption amounts, qe (mg g−1) is equilibrium adsorption amounts, b is a constant, and n and Kf are the Freundlich constants. The isotherm fttings were shown in Figure 9c−f, and the parameters b, n, qm, Kf, and correlation coefficient (R2) were provided in Table 3. The results showed that the Freundlich model gave a better fit to the experimental isotherm than the Langmuir isotherm model on the basis of correlation coefficients (R2) for MO, while the experimental data of TC could be fitted better using the Langmuir isotherm model. From the Langmuir isotherm model, the maximum adsorption capacity (qm) of MO and TC were 1266.42 mg g−1 and 195.31 at room temperature, respectively. 3.3.3. Adsorption Thermodynamics. To study the spontaneity and heat change of the adsorption reactions, the values of entropy change (ΔS), enthalpy change (ΔH), and the Gibbs free energy changes (ΔG) of adsorption can be calculated from the following formula: ΔS ΔH − R RT

(7)

ΔG = ΔH − T ΔS

(8)

ln b =

where b is the Langmuir constant. Values of ΔS, ΔH, ΔG, and correlation coefficient (R2) are represented in Table 4. The Table 4. Thermodynamic Parameters for the Adsorption of MO and TC T (K)/ adsorbates

ΔS (J mol−1 K−1)

ΔH (kJ mol−1)

ΔG (kJ mol−1)

298/MO 308/MO 318/MO 298/TC 308/TC 318/TC

5.782

1.397

6.13

1.493

−0.326 −0.384 −0.442 −0.334 −0.395 −0.456

R2

Figure 10. Effect of recycling times on the adsorption removal of MO and TC.

0.9901

0.9373

of qe and showed excellent reusability compared to the virgin CSLDO400. In detail, 60.72 mg g−1 and 21.92 mg g−1 of qe for MO and TC were measured after five times recycles, respectively. CSLDO400 did not have pore channels but also layered structure, which effectively eliminates pore blockage. Because of the affinities of various anions toward the interlayers of hydrotalcite and electrostatic interaction between carbonate ions from NaCO3 solution, the MO or TC adsorbed in the interlayers of CSLDO400 were desorbed. Consequently, CSLDO400 was restored to its original adsorption property and showed excellent stability, which indicated a great possibility for large-scale water treatment. 3.5. Comparison of Organic Contaminants with Other Adsorbents. A comparison has been made between CSLDO400 and other previously reported adsorbents for the removal of MO and TC. CSLDO400 as adsorbent showed a wider range of pH to MO and better adsorption property to both of the two pollutants than many other adsorbent as reported in terms of Langmuir MO and TC adsorption capacity at 298 K (shown in Table 5). The high adsorption capacity obtained in this work was attributed to interconnecting pore channels, layered structure, and high affinity between Mg/Fe oxides and MO or TC. Therefore, CSLDO400 adsorbent

enthalpy changes of MO and TC were calculated to be 1.397 kJ mol−1 and 1.493 kJ mol−1 respectively, implying that the MO and TC adsorption process was endothermic. The positive entropy values of MO and TC indicated an increase in randomness of the irreversible adsorption processes and hence a good affinity of MO and TC toward the CSLDO400 surface. The ΔG was also found at 298, 308, and 318 K to be −0.326, −0.384, −0.442 kJ mol−1 and −0.334, −0.395, −0.456 kJ mol−1 for MO and TC, respectively. The decrease in negative values of ΔG indicated that the adsorption of MO and TC was of a spontaneous nature of the endothermic process. 3.3.4. Adsorption Mechanism. The possible mechanism of the adsorption process was controlled by the memory effect. MO and TC could be adsorbed onto CSLDO400 by chemisorption and surface coordination. The CSLDH translated into CSLDHO400 along with the removal of water molecules, interlayer anions, and generated bubble after being calcined at a certain temperature, which contribute to the increase of specific surface area and adsorption sites. H



Article

multilayer membranes for wastewater effluent treatment. J. Membr. Sci. 2016, 512, 21−28. (3) Carliell, C. M.; Barclay, S. J.; Naidoo, N.; Buckley, C. A.; Mulholland, D. A.; Senior, E. Microbial decolourisation of a reactive azo dye under anaerobic conditions. Water SA 1995, 21, 61−69. (4) Kalyuzhnyi, S.; Sklyar, V. Biomineralisation of azo dyes and their breakown products in anaerobic-aerobic hybrid and UASB reactors. Water. Sci. Technol. 2000, 41, 23−30. (5) Tan, N. C.; Borger, A.; Slenders, P.; Svitelskaya, A.; Lettinga, G.; Field, J. A. Degradation of azo dye Mordant Yellow 10 in a sequential anaerobic and bioaugmented aerobic bioreactor. Water. Sci. Technol. 2000, 42, 337−344. (6) Ji, L.; Liu, F. L.; Xu, Z. Y.; Zheng, S. R.; Zhu, D. Q. Adsorption of pharmaceutical antibiotics on template-synthesized ordered micro-and mesoporous carbons. Environ. Sci. Technol. 2010, 44, 3116−3122. (7) Akiyama, T.; Savin, M. C. Populations of antibiotic-resistant coliform bacteria change rapidly in a wastewater effluent dominated stream. Sci. Total Environ. 2010, 408, 6192−6201. (8) Novo, A.; Manaia, C. M. Factors influencing antibiotic resistance burden in municipal wastewater treatment plants. Appl. Microbiol. Biotechnol. 2010, 87, 1157−1166. (9) Thiele-Bruhn, S.; Beck, I. C. Effects of sulfonamide and tetracycline antibiotics on soil microbial activity and microbial biomass. Chemosphere 2005, 59, 457−465. (10) Halling-Sørensen, B. Inhibition of aerobic growth and nitrification of bacteria in sewage sludge by antibacterial agents. Arch. Environ. Contam. Toxicol. 2001, 40, 451−460. (11) Boleas, S.; Alonso, C.; Pro, J.; Fernandez, C.; Carbonell, G.; Tarazona, J. V. Toxicity of the antimicrobial oxytetracycline to soil organisms in a multi-species-soil system (MS·3) and influence of manure co-addition. J. Hazard. Mater. 2005, 122, 233−241. (12) Mittal, A.; Malviya, A.; Kaur, D.; Mittal, J.; Kurup, L. Studies on the adsorption kinetics and isotherms for the removal and recovery of Methyl Orange from wastewaters using waste materials. J. Hazard. Mater. 2007, 148, 229−240. (13) Ou, H. J.; You, Q. L.; Li, J.; Liao, G. Y.; Xia, H.; Wang, D. S. A rich-amine porous organic polymer: an efficient and recyclable adsorbent for removal of azo dye and chlorophenol. RSC Adv. 2016, 6, 98487−98497. (14) Anggraini, M.; Kurniawan, A.; Ong, L. K.; Martin, M. A.; Liu, J. C.; Soetaredjo, F. E.; Indraswati, N.; Ismadji, S. Antibiotic detoxification from synthetic and real effluents using a novel MTAB surfactant-montmorillonite (organoclay) sorbent. RSC Adv. 2014, 4, 16298−16311. (15) Caroni, A.; de Lima, C. R. M.; Pereira, M. R.; Fonseca, J. L. C. The kinetics of adsorption of tetracycline on chitosan particles. J. Colloid Interface Sci. 2009, 340, 182−191. (16) Benredouane, S.; Berrama, T.; Doufene, N. Strategy of screening and optimization of process parameters using experimental design: application to amoxicillin elimination by adsorption on activated carbon. Chemom. Intell. Lab. Syst. 2016, 155, 128−137. (17) Chen, W. R.; Huang, C. H. Adsorption and transformation of tetracycline antibiotics with aluminum oxide. Chemosphere 2010, 79, 779−785. (18) Mandal, S.; Mayadevi, S. Cellulose supported layered double hydroxides for the adsorption of fluoride from aqueous solution. Chemosphere 2008, 72, 995−998. (19) He, S.; Zhao, Y. F.; Wei, M.; Evans, D. G.; Duan, X. Fabrication of hierarchical layered double hydroxide framework on aluminum foam as a structured adsorbent for water treatment. Ind. Eng. Chem. Res. 2012, 51, 285−291. (20) Zhao, Q.; Chang, Z.; Lei, X. D.; Sun, X. M. Adsorption behavior of thiophene from aqueous solution on carbonate-and dodecylsulfateintercalated ZnAl layered double hydroxides. Ind. Eng. Chem. Res. 2011, 50, 10253−10258. (21) Dadwhal, M.; Sahimi, M.; Tsotsis, T. T. Adsorption isotherms of arsenic on conditioned layered double hydroxides in the presence of various competing ions. Ind. Eng. Chem. Res. 2011, 50, 2220−2226.

Table 5. Comparison of Adsorption Capacity of CSLDO400 with Different Adsorbents adsorbents

adsorbates

qmax (mg g−1)

Lapindo volcanic mud MgNiAl-Ca mesoporous TiO2 RAPOPb CSLDO400 montmorillonite activated sludge palygorskite rectorite CSLDO400

MO MO MO MO MO TC TC TC TC TC

333.3 375.4 454.5 454.545 1266.42 54 72 99 140 195.31

pH

refs

3 6−9 5 3 5−9 5.5

38 39 40 13 present study 41 42 43 44 present study

8.7 9

a

Abbreviation: calcined MgNiAl layered double hydroxides (MgNiAlC). bAbbreviation: rich-amine porous organic polymer (RAPOP).

showed considerable potential for the removal of organic contaminants from an aqueous environment.

4. CONCLUSIONS In general, CSLDO400 was successfully synthesized by the coprecipitation method. The adsorption results showed that the optimal calcination temperature was 400 °C. The as-prepared CSLDO400 was characterized by a porous and layered structure and the highest surface area, resulting in superb adsorption performance toward organic contaminants (the maximum adsorption capacity and the adsorption equilibrium times at 298 K are about 1266.42 mg g−1 and 90 min for MO, and 195.31 mg g−1 and 120 min for TC). The action pH value was in a range from 5 to 9 for MO and 9 for TC; 0.175 mg L−1 and 0.375 mg L−1 CSLDO400 dosages were carried out for MO and TC, respectively. The adsorption process for both MO and TC could be described better with the pseudo-secondorder model. The Langmuir isotherm is correlated better with the experimental data than the Freundlich isotherm for MO, while it was the other way around for TC. The adsorptions of MO and TC were both spontaneous in nature of the endothermic process. CSLDO400 showed excellent reusability. Consequently, CSLDO400 is a promising functional material for purification of drinking water and wastewater contaminated by antibiotics and dyes.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel./Fax: (86)-10-62849138. *E-mail: [email protected]. Tel./Fax: (86)-10-62849138. ORCID

Hanjun Wu: 0000-0003-2686-2824 Funding

This work was supported by the National Nature Science Foundation of China (No.5167082583), Chinese Universities Scientific Fund (CUG160824), and China Postdoctoral Science Foundation (2016M590733). Notes

The authors declare no competing financial interest.

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J


Removal of Typical Organic Contaminants with a Recyclable Calcined

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