Preparation of thiourea-modified magnetic chitosan composite with efficient removal efficiency for Cr(VI)

Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 150–158

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Preparation of thiourea-modified magnetic chitosan composite with efficient removal efficiency for Cr(VI) Weiquan Cai a,b,∗ , Feng Zhu b , Hong Liang a,∗ , Yihong Jiang b , Wenjun Tu b , Zhijun Cai c , Junrong Wu a , Jiabin Zhou d a

School of Chemistry and Chemical Engineering, Guangzhou University, 230 Guangzhou University City Outer Ring Road, Guangzhou, 510006, China b School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, 205 Luoshi Road, Wuhan, 430070, China c International School of Materials and Engineering, Wuhan University of Technology, Luoshi Road 205#, Wuhan, 430070, China d School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, China

a r t i c l e

i n f o

a b s t r a c t

Article history:

The thiourea-modified magnetic chitosan (CS) Fe3 O4 @Al2 O3 -CS with high adsorption capac-

Received 17 November 2018

ity and separation efficiency for Cr(VI) was prepared successfully, and its physicochemical

Received in revised form 25 January

properties were characterized via XRD, SEM, TEM, N2 adsorption-desorption, FT-IR, TGA-

2019

DSC, Zeta-potential and VSM, respectively. The batch adsorption results show that its kinetic

Accepted 31 January 2019

data match the pseudo-second order well, and the equilibrium data are well depicted by

Available online 11 February 2019

the Langmuir adsorption isotherm with the maximum adsorption capacity of 327.8 mg/g.

Keywords:

endothermic characteristic. Most importantly, the Fe3 O4 @Al2 O3 -CS shows good selective

Magnetic chitosan

and recyclable performance for Cr(VI), indicating that it has a good application prospect to

Thiourea-modification

remove heavy metals ions from wastewater.

The thermodynamic parameters of the adsorption process reveal its spontaneous and

Cr(VI) adsorption

© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Coexisting ions Cyclic adsorption/desorption performance

1.

Introduction

Heavy metal Cr(VI) is widely used in electroplating, leather tanning, pigment manufacture, textile dying and metal polishing, and it also causes a serious harm to the ecological system and the public health due to its mutagenic and carcinogenic properties (Oladoja et al., 2013; Jiang et al., 2018). Up to now, the main methods for Cr(VI) and other heavy metal removal from wastewater include chemical precipitation, solvent extraction, electrodialysis (Huang et al., 2018), ion exchange (Xing et al., 2018), membrane filtration (Li et al., 2016), reverse osmosis (Cimen, 2015) and adsorption (Cai et al., 2018a, 2018b; Delavar et al., 2017). However, most of them have some certain limitations such as

∗

high capital investment and operation cost, and accordingly adsorption is considered as a simple, effective and low-cost method. Chitosan (CS) as a biological adsorbent, has attracted more and more attention because of its renewability, biocompatibility, biodegradation and the large amount of hydroxyl and amino groups (Liu et al., 2018). However, its recovery from wastewater using conventional methods such as filtration and centrifugation leads to high cost, waste of time, loss of mass and secondary pollution (Luo et al., 2016). Therefore, magnetic CSs with magnetic cores were developed for treating Cr(VI) and other heavy metal ions wastewater, and it could be recovered easily via an outside magnetic field (Wang et al., 2010; Lin et al., 2017). The ethylenediamine-modified cross-linked magnetic CS resin,

Corresponding authors at: School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, 510006, China. E-mail addresses: [email protected] (W. Cai), [email protected] (H. Liang). https://doi.org/10.1016/j.cherd.2019.01.031 0263-8762/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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the anthranilic acid-modified magnetic CS, and the magnetic CS coprecipitated by Ethanol/NH3 ·H2 O with the maximum adsorption capacities of 51.8 mg/g, 58.5 mg/g and 242.1 mg/g, respectively have been successfully synthesized to adsorb Cr(VI) (Hu et al., 2011; Elreash et al., 2011; Cai et al., 2018a, 2018b). However, it is still necessary to develop a magnetic CS with higher adsorption capacity and separation efficiency. The iron oxides are widely used as the magnetic cores of the magnetic adsorbents, and the coprecipitation method is usually used to prepare magnetic Fe3 O4 (Oladipo and Lfebajo, 2018; Heydaripour et al., 2018). However, the as prepared magnetic cores tend to be corroded in an acidic environment and slowly oxidized in the air. Herein,

thiourea was dissolved in 100 mL deionized water, followed by adding 6 mL, 25% glutaraldehyde solution, and the solution was stirred vigorously in a water bath at 50 ◦ C for 2 h. Thirdly, 0.5 g Fe3 O4 @Al2 O3 was added into the solution to form a uniform mixture under stirring. Finally, the CS solution was added into the mixture under magnetic stirring, and the new mixture was heated up to 70 ◦ C for 6 h to form gel; the gel was rinsed with deionized water for three times, and then dried at 60 ◦ C for 12 h in a vacuum oven to obtain Fe3 O4 @Al2 O3 CS. Its synthesis procedure was also illustrated in Fig. S1 in Supplementary material.

alumina with stable physicochemical properties was chosen as the protective layer to protect the inner magnetic core from corrosion and oxidation. The magnetic Fe3 O4 @Al2 O3 -CS with efficient removal efficiency for Cr(VI) was successfully prepared by the cross-linking reaction of glutaraldehyde with CS, and following modification with thiourea simultaneously to simplify the preparation process.

2.

Experimental

2.1.

Materials

CS with a degree of deacetylation of 95% was purchased from Aladin Inc. FeCl3 ·6H2 O, NaAc, polyethylene glycol 2000 (PEG 2000), ethylene glycol, acetic acid, ethanol, aluminum isopropoxide, 1,5-diphenylcarbazide, thiourea, glutaraldehyde, Mg(NO3 )2 ·6H2 O, Zn(NO3 )2 ·6H2 O, Cu(NO3 )2 ·3H2 O, Cd(NO3 )2 ·4H2 O, Ni(NO3 )2 ·6H2 O, K2 Cr2 O7 , NaOH, HCl and H2 SO4 were supplied by Sinopharm Chemical Reagent Co., Ltd. All the reagents are of analytical grade, and deionized water was used all through the experiment.

2.2. Preparation of thiourea-modified magnetic CS composite 2.2.1.

Preparation of magnetic Fe3 O4

Magnetic Fe3 O4 was prepared via a modified solvothermal method (Deng et al., 2005). In a typical experiment, 2.5 g FeCl3 ·6H2 O was dissolved with 60 mL ethylene glycol under magnetic stirring, following by adding 4 g NaAc and 1 g PEG 2000, successively. Then the solution was transferred to a Teflon-lined autoclave with 100 mL, and kept at 200 ◦ C for 24 h. After being cooled to ambient temperature naturally, the black precipitate was separated by a magnet, then rinsed three times with ethanol, and finally dried overnight in a vacuum drying oven at 60 ◦ C.

2.2.2.

Preparation of Fe3 O4 @Al2 O3 microspheres

The core–shell Fe3 O4 @Al2 O3 microspheres were prepared via a modified sol gel method (Peng et al., 2011). Firstly, 0.2 g Fe3 O4 particles were dispersed in an aluminum isopropoxide solution of ethanol (120 mL, 0.033 mol/L) under ultrasonication. Secondly, a mixture of deionized water and ethanol with volume ratio of 1:5 was put dropwise into the solution under energetic stirring. After stirring 6 h, the precipitate was separated with a permanent magnet, rinsed with deionized water and ethanol for three times, and dried in vacuum at 50 ◦ C for 12 h successively. The resulted sample was calcined at 500 ◦ C for 4 h in N2 to obtain Fe3 O4 @Al2 O3 .

2.2.3.

2.3.

Characterization techniques

X-ray diffraction (XRD) spectra of the samples were gotten on a Rigaku D/MAX-RB diffractometer using Cu-K␣ radiation at a scanning speed (2␪) of 10◦ /min from 10 to 80◦ . Their morphologies were observed by a field emission scanning electron microscopy (SEM) on Zeiss Ultra 55 and a transmission electron microscopy (TEM) on FEI tecnai G2 F30 with acceleration voltage of 200 kV. Their data of texture properties were obtained with a TriStar II 3020 instrument. Fourier transform infrared spectra (FT-IR) were measured via a Nicolet 6700 spectrometer with KBr as back ground in the wave numbers ranging from 4000 to 400 cm−1 . Their thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted via SDT Q600 V5.0 Build 63 under flowing O2 with a heating rate of 10 ◦ C min−1 between ambient temperature and 1000 ◦ C. Their Zeta-potentials were obtained on a Malvern Zetasizer Nano ZS. Contents of heavy metals ions in the simulated wastewater were measured via inductively coupled plasma (ICP/MS) on an ICP-MS Elan 6000 PerkinElmer Sciex. Their magnetic properties were evaluated by a vibrating sample magnetometer (VSM, PPMS-9T) at 25 ◦ C, and an applied magnetic field ranging from −20,000 to 20,000 Oe was used.

2.4.

Adsorption experiment

The static adsorption was performed in a beaker containing 25 mL Cr(VI) solution and 25 mg adsorbent in a rotary shaker under 180 r/min as a function of initial pH of 1–7, contact time of 0–480 min, original Cr(VI) concentration of 50–1000 mg/L, temperature of 30–50 ◦ C and coexisting cations. The original pH of Cr(VI) solution was adjusted via adding 1 mol/L NaOH solution or 1 mol/L HCl; Cr(VI)-adsorbed adsorbent was regenerated by 0.01 mol/L NaOH solution. After complexation with 1,5-diphenylcarbazide, the residual Cr(VI) concentration was measured via a UV–vis spectrophotometer (UV-1240, Shimadzu, Japan) with the maximum wavelength of 540 nm (Ren et al., 2014). The adsorption amount (mg/g) and the removal efficiency (%) of Cr(VI) were computed, respectively as below:

Adsorption amount qe =

Removal

(C0 − Ce )V m

efficiency (%) =

C0 − Ce × 100 C0

(1)

(2)

Preparation of Fe3 O4 @Al2 O3 -CS

Fe3 O4 @Al2 O3 -CS was prepared as described previously (Donia et al., 2008). Firstly, CS solution was obtained by dissolving 0.5 g CS in 25% acetic acid solution with 50 mL. Secondly, 2 g

where C0 and Ce are the initial and equilibrium concentration of Cr(VI), respectively, mg/L; m is the addition dosage of the adsorbent, mg; V is the volume of the solution, mL.

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Fig. 1 – XRD patterns of Fe3 O4 (a), Al2 O3 (b), Fe3 O4 @Al2 O3 (c) and Fe3 O4 @Al2 O3 -CS (d).

3.

Results and discussions

3.1.

Characterization

3.1.1.

The phase structures and morphologies analysis

The typical phase structures of the samples were identified by XRD in Fig. 1. Fig. 1a shows that there appears six characteristic peaks at 30.1◦ , 35.5◦ , 43.1◦ , 53.4◦ , 57.0◦ and 62.6◦ , corresponding to (220), (311), (400), (422), (511) and (400) of the Fe3 O4 , respectively (JCPDS card No. 65-3107) (Sun et al., 2014), and no impurity peak was observed, confirming its high purity. Fig. 1b shows that no characteristic peak of alumina was observed because of its weak crystallization, revealing its amorphous structure (Cai et al., 2011). For Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS, the characteristic peaks of Fe3 O4 in Fig. 1c, d appear, suggesting that Fe3 O4 nanoparticles were inserted in alumina and CS, and its phase structure was unchanged. Furthermore, the broad peak in Fig. 1d at about 22◦ shows no crystalline phase of CS. The morphologies of the typical samples were further examined via SEM and TEM in Fig. 2. As displayed in Fig. 2a, b, the Fe3 O4 presents uniformly spherical particles, and their average diameters approximately vary from 250 to 600 nm. The Fe3 O4 @Al2 O3 in Fig. 2c, d shows irregular spherical ultrafine particles with an obvious contrast between the pale edge and the dark central part, indicating that the Fe3 O4 particles were successfully coated with Al2 O3 , and its shell thickness is about 50 nm. The Fe3 O4 @Al2 O3 -CS in Fig. 2e, f shows irregular microspheres with a rough surface, and its particles embed into CS randomly. Besides, according to the results of N2 adsorption–desorption in Fig. 3 and Table S1 in Supplementary material, all the Fe3 O4, Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS show mesoporous structures with specific surface area of 28.9,113.8 and 6.9 m2 /g, successively. Therefore,

Fig. 2 – SEM and TEM images of Fe3 O4 (a–b), Fe3 O4 @Al2 O3 (c–d) and Fe3 O4 @Al2 O3 -CS (e–f). surface area of Fe3 O4 can be remarkably improved via coating Al2 O3 . In comparison with Fe3 O4 @Al2 O3 , specific surface area of Fe3 O4 @Al2 O3 -CS significantly decreases. This attributes that a large number of magnetic microspheres are wrapped together by CS to form a monolithic magnetic CS resin, resulting in reduced particle dispersion and porosity.

3.1.2.

The FT-IR and TGA-DSC analysis

The FT-IR spectra for Fe3 O4 , Fe3 O4 @Al2 O3 , Fe3 O4 @Al2 O3 -CS and CS are shown in Fig. 4. Their absorption peaks at 1619 and 3396 cm−1 correspond to the bending vibration and stretching vibration of the O H bond, respectively (Jin et al., 2017). For the spectra of Fig. 4a–c, the sharp peak at 579 cm−1 attributes to the Fe O stretching of Fe3 O4 (Yamaura et al., 2004). Compared with Fe3 O4 , the vibration absorption peak of the Al O bond at about 900 cm−1 for Fe3 O4 @Al2 O3 appears (Adamczyk and Dlugon, 2012), due to the existence of alumina on the Fe3 O4 surface which also implies successful coating of Al2 O3 . For the CS, the peaks at 1662, 1603 and 1068 cm−1 attribute to the C O, N H and C–O vibration absorption peaks, respectively (Pawlak

Fig. 3 – N2 adsorption–desorption isotherms (a) and the corresponding pore size distribution curves (b) of Fe3 O4 , Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS.

Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 150–158

Fig. 4 – FT-IR spectra of Fe3 O4 (a), Fe3 O4 @Al2 O3 (b), Fe3 O4 @Al2 O3 -CS (c) and CS (d). and Mucha, 2003; Monier et al., 2010). In addition, the spectrum of Fe3 O4 @Al2 O3 -CS is similar to that of CS, revealing the presence of CS in Fe3 O4 @Al2 O3 . Furthermore, the new small peaks for the C N and C S bands of Fe3 O4 @Al2 O3 -CS appear at about 1642 and 1400 cm−1 , respectively (Pavithra et al., 2017; Li et al., 2017). This may be due to that the CHO group in glutaraldehyde reacts with the NH2 groups in the CS, indicating its cross-linkage through glutaraldehyde (Lei et al., 2009) and successfully thiourea modification. TGA-DSC analysis of Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS was further used to determine the amount of residual organic species in Fig. 5a, b, respectively. The TGA curve of Fe3 O4 @Al2 O3 shows that its weight loss from 25 to 1000 ◦ C is about 6% for escape of physically adsorbed water and/or structure water. Moreover, the weight percent of Fe3 O4 @Al2 O3 increases slightly between 200–550 ◦ C resulting from the slight oxidation of Fe3 O4 (Ge et al., 2012). However, for Fe3 O4 @Al2 O3 CS, its weight loss below 180 ◦ C is about 8.8%, due to the release of moisture on its surface; while its weight loss between 180 to 400 ◦ C is associated with the escape of the structure water (Wang et al., 2010), which is about 39.2%. Apart from water loss, CS on the surface of Fe3 O4 @Al2 O3 begins to degrade rapidly more than 400 ◦ C, and the organic polymers are almost completely decomposed at 700 ◦ C (Monier et al., 2010). Therefore, a weight loss of 35.2% for Fe3 O4 @Al2 O3 -CS from 400 to 700 ◦ C can be used to estimate the weight proportions of the cross-linked CS in Fe3 O4 @Al2 O3 -CS.

3.1.3.

The Zeta-potential and VSM analysis

The Zeta-potentials of Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS were measured in a 0.001 mol/L NaCl solution at pH values of 2.0–12.0. As displayed in Fig. 6, the isoelectric point of Fe3 O4 @Al2 O3 is about 7.8. After bonding CS, the isoelectric point of Fe3 O4 @Al2 O3 -CS shifts to 11.8, resulting from

153

Fig. 6 – Zeta-potential of Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS at different pH values.

Fig. 7 – Magnetization hysteresis loops of Fe3 O4 (a), Fe3 O4 @Al2 O3 (b) and Fe3 O4 @Al2 O3 -CS (c).

protonation of the large number of surface amino groups. This phenomenon is beneficial for the electrostatic attraction between anions and cations, revealing that the Fe3 O4 @Al2 O3 CS is positively charged at a pH value less than 11.8. The magnetic hysteresis loops of Fe3 O4 , Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS are shown in Fig. 7. As shown in Fig. 7, they all show low remanence and coercivity behaviors, and their corresponding saturation magnetization values are 81.5, 30.2 and 9.2 emu/g, successively. Though the saturation magnetization value of Fe3 O4 @Al2 O3 -CS is less than those of Fe3 O4 and Fe3 O4 @Al2 O3 due to the coated CS, it still can be separated from the treated suspension by a permanent magnet within 40 s (see the inset in Fig. 7). Therefore, the Fe3 O4 @Al2 O3 CS presents adequate magnetic response to meet the need of magnetic separation (Fan et al., 2011).

Fig. 5 – TGA-DSC curves of Fe3 O4 @Al2 O3 (a) and Fe3 O4 @Al2 O3 -CS (b).

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3.2.

Adsorption experiments

3.2.1.

Effect of pH

pH is an important parameter which affects Cr(VI) adsorption, and determines conversion of Cr species and the surface charge density of the adsorbent simultaneously. With pH changes, aqueous Cr(VI) mainly exists in five forms (Saha and Orvig, 2010): H2 CrO4 , HCrO4 − , CrO4 2− , HCr2 O7 − and Cr2 O7 2− . At a pH above 6.0, CrO4 2− is the main specie. However, HCrO4 − and Cr2 O7 2− are the dominant species at a pH between 2.0 and 6.0, and H2 CrO4 is the main specie at a pH less than 1.0. The influence of pH between 1.0 and 7.0 on the adsorption capacity of Fe3 O4 @Al2 O3 -CS for Cr(VI) is depicted in Fig. S2 in Supplementary material. It shows that its adsorption capacity decreases with increasing pH, and its adsorption capacity of 48.7 mg/g appears at the pH = 1. The above phenomena could be elucidated by the electrostatic attraction between the cationic basic groups and the anionic species of Cr(VI including HCrO4 − , Cr2 O7 2− and CrO4 2− in the adsorbent (Sengupta and Cllfford, 1986). According to the Zeta-potential results at low pH, the amino groups are generally protonated and the adsorbent is positively charged, resulting in electrostatic attraction between the adsorbate and the adsorbent. With increasing pH, the amino groups experience deprotonation resulting in decrease of the adsorption capacity of the adsorbent. Thus, pH of 1 was chosen to perform the following experiment.

3.2.2.

Adsorption kinetic

Fig. 8a shows the influence of contact time on adsorption of the adsorbents with initial Cr(VI) concentration of 100 mg/L. It shows that their adsorption rate is quite high initially, and gradually reaches equilibrium within 120 min. However, the maximum adsorption capacity of Fe3 O4 @Al2 O3 CS is 95.5 mg/g which is significantly more than 20.1 mg/g of Fe3 O4 @Al2 O3 . The adsorption kinetics data were analyzed by the pseudo-first-order model, the pseudo-second order model and the intra-particle diffusion model to investigate the ratecontrolling step (Ren et al., 2013; Konicki et al., 2018) as below, respectively: lg (qe − qt ) = lgqe −

k1 t 2.303

(3)

t 1 t = + qt qe k2 q2e

(4)

qt = kid t1/2 + cid

(5)

where qt and qe represent the adsorption capacity at a time t and the equilibrium, respectively, mg/g; k1 (min−1 ); k2 [g/(mg min)] and kid [mg/(g min1/2 )] are the rate constants of the models, respectively; cid (mg/g) is the intercept, reflecting the boundary layer effect. Their plots are depicted in Fig. S3 in Supplementary material, and the kinetic parameters were collected in Tables S2 and S3 in Supplementary material. It was shown that the correlation coefficients of 0.995 for Fe3 O4 @Al2 O3 and 0.999 for Fe3 O4 @Al2 O3 -CS of the pseudo-second-order kinetic are much higher than those (0.963 and 0.852, respectively) of their pseudo-first-order kinetic. Furthermore, their corresponding equilibrium adsorption capacities qe,cal (20.5 and 95.2 mg/g, successively) calculated from the pseudo-second-order are much closer to the experimental adsorption capacities qe,exp

(20.1 and 95.5 mg/g, successively) than those obtained from the pseudo-first-order model, indicating that the their kinetics data better fit the pseudo-second-order. This shows that their adsorption processes might be dominated by chemical adsorption (Ho and Mckay, 1999). As for the intra-particle diffusion model, the multi-linearity stands for three steps which take place during the adsorption process (Hao et al., 2010). The 1st rapid step is due to the easy transportation of Cr(VI) to the outside active sites of the adsorbents due to that the external film resistance decreases under vigorous shaking. With the outside active sites of the adsorbents are occupied and Cr(VI) concentration decreases, pore diffusion and adsorption on the interior surface of the adsorbents appears in the 2nd step. The intra-particle diffusion rate in the 3rd step is almost zero, and the adsorption equilibrium is realized. Furthermore, the diffusion rate constants including ki1 and ki2 for Fe3 O4 @Al2 O3 -CS are higher than those of Fe3 O4 @Al2 O3 , respectively, due to more active sites inside the pores or on the surface of Fe3 O4 @Al2 O3 -CS to attract Cr(VI) and enhance its diffusion rate. Furthermore, no linear portions passes through the origin, suggesting that intra-particle diffusion is not the sole rate-limiting step, and the chemical chelating reaction and boundary layer diffusion also affect the adsorption process to a certain extent (Tan et al., 2009).

3.2.3.

Adsorption isotherm

Fig. 8b shows the adsorption isotherms of Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS. With the equilibrium concentration of Cr(VI) increases, firstly, their adsorption capacities increase remarkably, and then gently approach to the equilibrium, suggesting that their adsorption could be absolutely saturated at a highly original Cr(VI) concentration. The adsorption isotherm is used to evaluate the adsorption capacity of an adsorbent, and depict the interaction pathway between the adsorbent and the adsorbate. Thus, the Langmuir and Freundlich isotherms were adopted to analyze the data (see Fig. S4 in Supplementary material) as follows, respectively (Cai et al., 2014): Ce 1 Ce = + qe qm KL qm lnqe =

1 lnCe +lnKF n

(6)

(7)

where Ce (mg/L), qe (mg/g) and qm (mg/g) represent concentration of Cr(VI) at equilibrium, the adsorption capacity, and the maximum adsorption capacity, successively; KL is the Langmuir constant which relates to the adsorption energy; as the Freundlich constants, n and KF [(mg/g) (L/mg)1/n] represent the intensity and capacity of the adsorption, successively. The corresponding parameters of the adsorption isotherms were listed in Table S4 in Supplementary material. The data fit better with the Langmuir model than with the Freundlich model due to higher R2 values of the former, suggesting that a monolayer adsorption occurs. Furthermore, the adsorption capacity of 327.8 mg/g for Fe3 O4 @Al2 O3 -CS is significantly higher than that (43.4 mg/g) of Fe3 O4 @Al2 O3 , indicating that introduction of CS can improve its adsorption capacity by approximately 7.55 times. Furthermore, the qm value of Fe3 O4 @Al2 O3 -CS is much more than some magnetic CS (see Table 1). This may be due to the introduction of more nitrogen atoms from thiourea besides amino groups. Because of the free lone pair of electrons, the nitrogen atoms can be easily protonated, and the magnetic adsorbent offers more active sites.

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Fig. 8 – Kinetic adsorption curves (a), adsorption isotherms (b) of Cr(VI) on Fe3 O4 @Al2 O3 and Fe3 O4 @Al2 O3 -CS, plots of qe versus Ce at various temperatures for Fe3 O4 @Al2 O3 -CS (c). Table 1 – Comparison of adsorption capacities of Fe3 O4 @Al2 O3 -CS and related materials for Cr(VI) under certain conditions reported in the literatures. Adsorbent

qm (mg/g)

Conditions

Reference

DMCP PDFMC C4 GTMAC Fe3 O4 −CS3 Fe3 O4 @Zr-CTS MFSC CS-PDA Fe3 O4 @Al2 O3 -CS

153.9 176 182 233.1 242.1 280.97 336.7 374.4 327.8

30 ◦ C, pH 3 25 ◦ C, pH 2 25 ◦ C, pH 2 25 ◦ C, pH 2.5 25 ◦ C, pH 2 25 ◦ C, pH 2 25 ◦ C, pH 2 25 ◦ C, pH 2 30 ◦ C, pH 1

Zheng et al. (2018) Sakti et al. (2015) Moreira et al. (2018) Sun et al. (2016a, 2016b) Cai et al. (2018b) Chen et al. (2017) Jiang et al. (2019) Guo et al. (2018) This work

3.2.4.

Adsorption thermodynamics

Plots of qe versus Ce at various temperatures for Fe3 O4 @Al2 O3 are depicted in Fig. 8c. It shows that the adsorption capacity increases with increasing the temperature. The data were used to calculate the thermodynamic parameters including Gibbs free energy change (G◦ ), enthalpy change (H◦ ) and entropy change (S◦ ) as follows (Cai et al., 2014): Go = −RTlnK0 lnK0 =

Ho So − R RT

(8) (9)

where R is the gas constant of 8.314 J/(mol K); T is the absolute temperature, K; K0 is the thermodynamic equilibrium constant obtained by plotting ln(Cs /Ce ) versus Ce and extrapolating Ce to zero (Fig. S5a in Supplementary material). The S◦ (J/mol K) and H◦ (kJ/mol) are calculated from intercept and slope of the linear plot of ln K0 versus 1/T (Fig. S5b in Supplementary material).

The corresponding results were summarized in Table S5. The negative value of G◦ confirms the feasibility and spontaneous nature of its adsorption. The estimated H◦ and S◦ at 30 ◦ C are 9.39 kJ/mol and 95.6 J/(mol K), successively. The positive H◦ indicates its endothermal adsorption process, and can be promoted by a higher temperature; the positive S◦ reflects a raise in the randomness at the solid/solution interface (Ngah and Fatinathan, 2008).

3.2.5.

Effect of the coexisting cations

The composition of industrial wastewater is often complicated (Song et al., 2018), and it also contains other heavy metal ions which compete for the active adsorption sites in an adsorbent with Cr(VI). Thus, a mixed cationic solution containing 100 mg/L for Cr(VI), Zn(II), Cu(II), Mg(II), Cd(II) and Ni(II) simultaneously was prepared to simulate the industrial wastewater and test the selectivity towards Cr(VI) of the Fe3 O4 @Al2 O3 -CS. As shown in Fig. 9a, its removal efficiency of Cr(VI) can achieve 99.6%, while its removal efficiency of the other five metal cations is just about 5%, indicating that the mixed cations nearly have no significant effect on its adsorption towards Cr(VI). Its high selectivity towards Cr(VI) may be due to its highly protonated surface and the positive charge density at the pH of 1. Therefore, the adsorbent has strong electrostatic attraction for Cr(VI) which exist in anionic form, and electrostatic repulsion for the other metal cations. Influence of the coexisting anions on the removal efficiency of 100 mg/L Cr(VI) solution containing 5 mmol/L Cl− , F− , HCO3 − or SO4 2− , respectively is shown in Fig. 9b. It showed that Cl− and F− have slightly effects on Cr(VI) adsorption of Fe3 O4 @Al2 O3 -CS. However, compared with Cl− and F− on the effects of its adsorption, its removal efficiency for Cr(VI) was reduced by 20% in the presence of HCO3 − or SO4 2− due to their similar molecular size and closer ionic radius with HCrO4 -

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Fig. 9 – Influence of the coexisting heavy metal ions on the removal efficiency (a) and coexisting anions on the removal efficiency (b).

Fig. 10 – Effect of recycle times on the adsorption capacity of Fe3 O4 @Al2 O3 -CS for Cr(VI) (a), and the desorption kinetic of Fe3 O4 @Al2 O3 -CS-loaded Cr(VI) (b). which result in a competitive adsorption on the active sites of Fe3 O4 @Al2 O3 -CS.

3.2.6.

Desorption and reusability

For more valuable in practical application, the adsorbent should have good characteristics of regeneration and reusability. The desorption and reusability experiment of Fe3 O4 @Al2 O3 -CS was evaluated as follows: the pH of 50 ml, 100 mg/L Cr(VI) solution was adjusted to 3 via 1 mol/L HCl solution, followed by adding 50 mg Fe3 O4 @Al2 O3 -CS. The adsorption process was conducted in a rotary shaker at 30 ◦ C for 4 h under 180 r/min, and then the residual Cr(VI) concentration was detected. After that, the adsorbent was separated by a permanent magnet, and then was regenerated in 100 ml of 0.01 mol/L NaOH solution for 12 h. After separation and wash, the adsorbent was added into the next round of adsorption process of Cr(VI) until its adsorption capacity obviously decreased. Effect of recycle times on the adsorption capacity of Fe3 O4 @Al2 O3 -CS is depicted in Fig. 10a. Its initial capacity is 87.1 mg/g, and its adsorption capacity after 5 cycles still maintains above 0% of its 7 initial adsorption capacity, indicating that the sample has a good cyclic adsorption performance after regeneration. Fig. 10b shows the change of Cr(VI) concentration with time during the first desorption of the Fe3 O4 @Al2 O3 -CS-loaded Cr(VI). It was shown that the desorption process nearly completed in the first 5 h, and reached desorption equilibrium after 10 h. A small amount of Cr cannot be desorbed completely due to the reduction of Cr(VI) to Cr(III) on the adsorbent.

3.2.7.

Adsorption mechanism

To investigate the adsorption mechanism of Fe3 O4 @Al2 O3 -CS, its FT-IR spectra and XPS spectra before and after adsorption are shown in Figs. S6 in Supplementary material and 11 , respectively. Fig. S6 in Supplementary material shows that the intensities of O H groups and N H groups become

weaker after adsorption suggesting that they are involved in the adsorption process. As can be seen from Fig. 11a, Cr 2p appears after adsorption suggesting that Fe3 O4 @Al2 O3 -CS successfully adsorbed Cr(VI). Furthermore, clear peaks belonging to Cr(VI) and Cr(III) appear in Fig. 11b indicating that the reduction of Cr(VI) to Cr(III) reacted. In the acidic conditions, Cr(VI) with strong oxidability could be reduced to Cr(III) by the –OH groups with reductive property in the adsorbent as follows: HCrO4 − + 7H+ + 3e− → Cr3+ + 4H2 O

(10)

Cr2 O7 2− + 14H+ + 6e− → 2Cr3+ + 7H2 O

(11)

Fig. 11c shows that the peak of N 1s shifts slightly to the left, and the intensity of NH2 peak decreases while the intensity of NH2 -metal increases. The NH2 groups could be protonated to NH3 + which can adsorb large amounts of Cr(VI) and form complex with the resulted Cr(III), and attract Cr(VI) as negative ions in the solution at a pH of about 3. Therefore, they are consumed in large quantities and more complexes are formed. Anyway, Cr(VI) adsorption of Fe3 O4 @Al2 O3 -CS is a complex process involving electrostatic attraction, ion complexation and reduction of Cr(VI). The specific mechanism needs to be further elucidated in the future.

4.

Conclusion

The thiourea-modified magnetic Fe3 O4 @Al2 O3 -CS with highly efficient removal efficiency for Cr(VI) was prepared successfully, and its maximum adsorption capacity is as high as 327.8 mg/g, resulting from the electrostatic attraction and redox reaction between Fe3 O4 @Al2 O3 -CS and Cr(VI). Its adsorption fits the pseudo-second-order model and the Langmuir adsorption isotherm well, indicating that the adsorption is a spontaneous and endothermic process in nature. More-

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Fig. 11 – XPS spectra: full range (a), Cr 2p (b) and N 1 s (c) of Fe3 O4 @Al2 O3 -CS before and after adsorption. over, its high adsorption selectivity, good recyclability and easily magnetic separation characteristics provide a high application potential for Cr(VI) removal from wastewater.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21476179), one hundred talents project of Guangzhou University (69-18ZX0016) and 2016 Wuhan Yellow Crane Talents (Science) Program.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/ j.cherd.2019.01.031.

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