PVA composite membrane with enhanced adsorption performance towards Cr(VI)

Accepted Manuscript Title: Facile synthesis of boehmite/PVA composite membrane with enhanced adsorption performance towards Cr(VI) Author: Lei Luo Weiquan Cai Jiabin Zhou Yuanzhi Li PII: DOI: Reference:

S0304-3894(16)30640-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.07.019 HAZMAT 17875

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

9-4-2016 25-6-2016 6-7-2016

Please cite this article as: Lei Luo, Weiquan Cai, Jiabin Zhou, Yuanzhi Li, Facile synthesis of boehmite/PVA composite membrane with enhanced adsorption performance towards Cr(VI), Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.07.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of boehmite/PVA composite membrane with enhanced adsorption performance towards Cr(VI)

Lei Luoa, Weiquan Caia,*, Jiabin Zhoub, Yuanzhi Lia a

School of Chemistry, Chemical Engineering and Life Sciences, State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 205 Luoshi Road, Wuhan 430070, P. R. China. b

School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China

*

Corresponding author. Tel: +86-27-87156827; Fax: +86-27-87749379. E-mail: [email protected] (W. Q. Cai)

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Graphical abstract

A boehmite/PVA composite membrane (BPCM) was successfully synthesized from Al(NO3)3•9H2O using HAc as the peptizing agent via a facile sol-gel method. The BPCM shows excellent adsorption performance towards Cr(VI), and can be easily separated after adsorption.

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HIGHLIGHTS ●

Synthesis of boehmite/PVA membrane via a facile sol-gel method from

Al(NO3)3•9H2O. ●

Physicochemical properties of the easy-separated membrane were firstly described.

●

The membrane shows much higher adsorption capacity of Cr(VI) than the bohemite.

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Coexisting anions have a notable effect on the adsorption capacity of the membrane.

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A three step action mechanism of the membrane for Cr(VI) adsorption was proposed.

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Abstract A novel boehmite/PVA composite membrane (BPCM) with remarkably enhanced adsorption performance towards Cr(VI) was successfully synthesized from Al(NO3)3•9H2O using HAc as the peptizing agent via a facile sol-gel method. The physicochemical properties of the BPCM, the boehmite powder (BP) without PVA and a commercial boehmite powder (CBP) were comparatively characterized by XRD, TGA-DSC, FT-IR and XPS. Batch adsorption experiments showed that the adsorption performance of the BPCM is much better than those of BP and CBP. Its adsorption process was well described by the pseudo-second-order kinetic model, and its equilibrium data fit the Langmuir isotherm well with a maximum adsorption capacity of 36.41 mg g-1. Its interference adsorption experiment in presence of coexisting anions showed that SO42- and HPO42- have greater effect than those of the Cl-, F-, C2O42- and HCO3-. A three step action mechanism including adsorption of Cr(VI) anions, complexation between Cr(VI) anions and the functional groups on the surface of BPCM, and the reduction of Cr(VI) to Cr(III) was proposed to illustrate the adsorption process. This efficient film could be easily separated after adsorption, exhibiting great potential for the removal of Cr(VI) from aqueous solution, and other fields of environmental remediation. Keywords: boehmite/PVA composite membrane; sol-gel method; Cr(VI) adsorption; coexisting anions; adsorption mechanism

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1. Introduction Due to the carcinogenicity to human and high toxicity to living organisms, Cr(VI) compounds is classified as a serious environmental pollutant [1]. In recent years, Albased adsorbents, including aluminum oxide, aluminum hydroxide and aluminum oxyhydroxide have been widely studied owing to their excellent physicochemical properties for Cr(VI) removal [2-4]. However, these powder-based adsorbents have some drawbacks of agglomeration of fine particles, complicated separation process and expensive recovery equipment. Till now, alternative materials including biochar [5], triethylenetetramine modified graphene oxide/chitosan composite [6] and Fe2O3@AlO(OH) nanomaterial [7] with steady adsorption property for Cr(VI) have been reported to eliminate these problems. Nevertheless, the design of an adsorbent with common advantages of simple preparation process, efficient separation and enhanced adsorption performance is still a challenge. Recently, the organic/inorganic composite with good chemical and mechanical stability for adsorption application has attracted increasing attention. For example, the PAN/FeCl2 porous nanofibers prepared from electrospinning method shows excellent performance for Cr-removal in one step [8]; cellulose acetate phthalate-alumina nanoparticle mixed matrix membrane exhibits a high removal efficiency of 91% towards catechol [9], etc [10-14]. However, using cheap and nontoxic inorganic aluminum salts to prepare organic/inorganic composite membrane has not been reported for adsorption of toxic metals. Boehmite is an important precursor for transition aluminas, which are widely used

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as catalyst support [15], adsorbent [16], membrane [17,18] and drug adjuvant [19]. Its application performance depends on its crystallinity, textural properties, surface basicacidic characteristic, etc [20]. It is well known that some polymers with good hydrophilic nature and film-forming characteristics can be used to prepare freestanding membrane for environmental remediation. For example, nontoxic Polyvinyl alcohol (PVA) with favourable price/performance ratio has been developed for nanofiltration [21], biomedical application [22] due to its unique mechanical strength, biocompatibility and toughness. It was found that PVA can reduce tension in the gel, and its functional groups can interact with inorganic materials [23,24]. Herein, a freestanding boehmite/PVA composite membrane (BPCM) was firstly prepared via a facile sol-gel method from nontoxic and cheap inorganic aluminum salts instead of toxic and expensive organic aluminium alkoxide. Importantly, the BPCM with an easily separated characteristic after adsorption shows much higher adsorption capacity towards Cr(VI) than those of the boehmite powder (BP) without PVA and a commercial boehmite powder (CBP). Moreover, the residual concentration of Cr(VI) meets the wastewater discharge Standard GB 8978-1996 (0.5 mg L-1) of China.

2. Experimental 2.1. Sample preparation All the reagents from Shanghai Chemical Reagent Ltd. (China) are of analytical grade and used as received without further purification. In a typical synthesis, 35 mL

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of 5 wt% ammonia solution was dropwise added into 25 ml of 0.96 M Al(NO3)3·9H2O solution under vigorous stirring at 25 °C. The precipitate was collected by centrifugation, and then washed several times with deionized water until pH of the filtrate nearly reached 7.0. The filter cake was dispersed into 50 ml deionized water under ultrasound for 10 min to make it uniformly disperse. 1.24 g PVA was dissolved in 15 ml deionized water at 90 °C, and then 0.44 ml glacial acetic acid (HAc) with molar ratio of n[H+]/n[Al3+]=0.3 was added to the mixture. After stirring for 6 h at 25 °C, the mixed sol covered with PE film was transferred into a thermostatic water bath, sealed and aged at 90 °C for 24 h. The freestanding BPCM was formed by pouring about 5 ml of aged alumina sol into a glass slide (2 cm×4 cm) and dried at 60 °C for 12 h before it was torn off. 2.2. Characterization The BPCM and CBP (supplied by Zibo Santi Chi Fine Chemical Co., Ltd., Shandong, China) were characterized by X-Ray diffraction (XRD) on a Rigaku D/MAX-RB diffractometer using Cu Kα radiation at a scanning speed (2θ) of 10°/min. The accelerating voltage and applied current were 40 kV and 80 mA, respectively. Fourier transform infrared spectra (FT-IR) were performed using a Nicolet 6700 spectrometer with KBr as background. The spectra were collected after 32 scans between 4000-400 cm-1 with a resolution of 4 cm-1. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed by SDT Q600 V5.0 Build 63 from 25 °C to 700 °C. The morphologies of BPCM and pure PVA were observed by an S-4800 Field Emission Scanning electron microscopy (FESEM,

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Hitachi, Japan) at an accelerating voltage of 5.0 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a VG Multilab 2000 spectrometer with Al mono Kα X-ray source (1486.71 eV photons). All binding energies (BEs) were referenced to the C 1s neutral carbon peak at 284.6 eV. 2.3. Adsorption experiments The Cr(VI) solution of 1000 mg/L was prepared by dissolving 2.829 g K2Cr2O7 in 1000 ml deionized water. The working concentration was obtained by diluting it on demand, and its pH was maintained at near 5.5 by adding 0.01 M NaOH or 0.01 M HNO3 as pH adjusting agent. The batch adsorption experiment was carried out in a beaker containing 0.2 g adsorbent and the Cr(VI) solution in a rotary shaker (25 °C, 150 r/min). The solution at defined time intervals was sampled to analyze the residual Cr(VI) concentration. The filtrate was analyzed by a UV-Visible spectrophotometer (UV-1240, Shimadzu, Japan) at a maximum wavelength of 540 nm after complexation with 1,5diphenylcarbazide following a standard method [25]. The adsorption isotherm was measured by varying the initial Cr(VI) concentration in the range of 20-100 mg/L. The removal efficiency (%) and adsorption capacity (mg g-1) of Cr(VI) were calculated as follows: Removal efficiency (%)= Adsorption capacity q e 

C0  Ce 100 C0

(C0  Ce )V m

(1) (2)

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Where C0 and Ce (mg/L) are the initial concentration and equilibrium concentration of Cr(VI), respectively; m (g) is the adsorbent mass, and V (L) is the solution volume. The interference tests of coexisting anions were similar with the above adsorption experiments. Various 1 mmol/L and 5 mmol/L coexisting anions solutions were prepared by dissolving sodium salts of Cl-, F-, C2O42-, SO42-, HPO42- and HCO32respectively, with the Cr(VI) concentration of 30 mg/L and the pH of 5.5.

3. Results and discussion 3.1. Physicochemical properties of the samples The XRD pattern of the BPCM and CBP were shown in Fig. 1. It shows that their XRD peaks well match the boehmite (JCPDS File No. 21-1307), and thus the BPCM is indexed to this orthorhombic crystal structure with amam space group (63) and lattice constants of about a=3.700 Å, b=12.227 Å and c=2.868 Å. Its higher diffraction intensity shows that it has a higher crystallinity than that of the CBP. Their average crystallite sizes were quantitatively calculated by Scherrer formula D=kλ/(βcosθ), where D is the average crystallite size, k=0.89 is the Scherrer constant, λ is the Cu Kα wavelength of 0.15418 nm, β is the full width at half maximum intensity of the isolated (020) strong peak in radians, and θ is the Bragg’s diffraction angle, respectively) to be about 7.6 nm and 2.5 nm, respectively. Furthermore, there is no characteristic peak of other impurities, indicating that the BPCM has a high purity. Their decomposition characteristics upon heat treatment were measured to

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understand their thermal behavior, transformation temperatures and the interaction among HAc, inorganic precursor and PVA. The TGA-DSC curves of as-prepared samples and pure PVA were shown in Fig. 2 and Fig. S3, respectively. For BP, there exist two degradation steps. The 1st step is an endothermic peak around 69 °C due to the elimination of abundant physically adsorbed water. The 2nd step between 280~550 °C with an endothermic peak at 339 °C due to the decomposition of HAc molecules coordinated with boehmite [26,27], and its dehydroxylation is described as follows [28]: 2AlO(OH) →Al2O3+H2O

(3)

For BPCM, about 60.2 % total weight loss is reasonable agreement with the sum of PVA, HAc additives and water used to prepare it. Besides, its TGA-DSC curve could be divided into three steps. At step I up to 159 °C, the endothermic peak around 105 °C is higher than that of BP, and the weight loss was slowly decreased by 9.2 %, indicating that there is less physically adsorbed water in BPCM. At step II between 159-235 °C, in comparison with BP, one weak endothermic peak appears at 214 °C and the weight loss decreases by 4.4 %, resulting from the elimination of water adsorbed in the membrane, dehydration of polymer chain and the melting of PVA. However, the melting temperature of PVA is higher than that of the pure PVA [29], which is possibly due to the connection bond of Al-O-C. At step III between 235-550 °C with an endothermic peak at 354 °C was approximately 46.6 %. Accordingly, the broad exothermic peak could be attributed to the weakened dehydroxylation of boehmite, and the production of

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much heat corresponding to the structural decomposition of the residual PVA. Furthermore, both of them have no obvious weight loss above 550 °C. The bond configuration of PVA, BP and BPCM were presented in Fig. 3a, b and c, respectively. Fig. 3 shows that the strong absorption peaks at about 3440 and 1622 cm−1 can be assigned to the stretching vibration and bending vibration of the -OH bond, indicating that the exist of significant amount of adsorbed water and coordinated water, respectively [30,31]. The bands at about 2933 cm−1 and 1094 cm−1 in Fig.3a are attributed to the asymmetric stretching vibration of –CH2– and the C–O stretching vibration of acetyl groups in PVA, respectively [32]. For BP in Fig. 3b, the band at about 1070cm -1 is attributed to the bending vibration of deprotonated hydroxyl groups of Al-O, suggesting the tetrahedral symmetry structure of boehmite [33] in Fig. 1. Besides, the band at 1496 cm -1 can be assigned to the stretching vibrations of the bridging monodentate or bidentate coordinated carboxylate groups which are binded with the Al atoms of the boehmite surface to form ≡Al-Ac complex [26]. This result reveals that HAc acts not only as a peptizing agent but also as a complexing agent for aluminum hydroxide [34]. As show in Fig. 3c, the adsorption bands of BPCM in the presence of PVA is quite different from those of pure PVA membrane and BP. The Al-O stretching at 1070 cm −1 of BP is hidden within 1094 cm-1 of the C-O vibration. The absorption peaks intensity of 2933 cm−1 and 1094 cm -1 of PVA membrane decrease significantly as well as the bands at 1469 cm -1 disappear, which can be explained by the reason that C-OH group of PVA reacts with ≡Al-Ac and ≡Al-OH,

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and then transforms into ≡Al-O-C [35,36]. The SEM image of BPCM was also shown in Fig. 4. From the figure, BPCM presents a very rough surface with wrinkled structure which is similar to a wave to rise and fall. However, pure PVA presents a smooth surface without regular morphology (see Fig. S4). 3.2 Adsorption behaviour of Cr(VI) ions 3.2.1 Effect of PVA content Effects of PVA content on the adsorption and mechanical properties of BPCM were shown in Fig. 5 and Fig. S5, respectively. Fig. 5 shows that with increasing its content to 50.0 % PVA, the BPCM reaches the maximum adsorption amount of 31.0 mg/g. Fig. S5 also shows that its elongation at break and tensile breaking strength reach the maximum values of 23.2 % and 2.63 MP, respectively. Accordingly, the BPCM with a PVA content of 50.0 % was selected in the further experiments. The optical images of the Cr(VI) solution, the BPCM before adsorption, the system after adsorption and the BPCM with adsorbed Cr(VI) were illustrated in Fig. 4a, c, b and d, respectively. Obviously, the solution totally became colorless, and the BPCM turned yellow due to Cr(VI) adsorption, indicating that the BPCM exhibits excellent adsorption performance. Furthermore, in comparison with powder adsorbent, it is easy to separate BPCM from the treated waste water. These excellent characteristics showed that the as-prepared BPCM is an effective and easy-separated adsorbent for Cr(VI) removal. 3.2.2 Adsorption kinetic Effects of the contact time on the adsorption of 50 mg/L Cr(VI) solution for

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pure PVA, CBP, BP and BPCM were comparatively carried out in Fig. 6. It was shown that the maximum adsorption capacity (30.97 mg g-1) of the BPCM is significantly higher than the BP (11.19 mg g-1) and the CBP (5.66 mg g-1). This may be related to the addition of PVA; as a film-forming agent, it can effectively avoid the agglomeration of boehmite particles and further promote the uniform distribution of their active sites on the surface of BPCM. Furthermore, the adsorption rates of BP and BPCM were rapid initially, then slowed down and finally reached equilibrium within 360 min. Their crystallinity may also play an important role for the adsorption difference due to BPCM with a higher crystallinity than that of the CBP (see Fig. 1). However, pure PVA can not adsorb Cr(VI). The pseudo-first-order model and the pseudo-second-order model which are beneficial to obtain essential information of the adsorption process are as follows, respectively [37]:

ln(qe  qt )  ln qe  k1t

t 1 t   2 qt k2 qe qe

(4) (5)

where qe and qt (mg/g) represent adsorption amount at equilibrium and a time t (min), respectively. k1 (min-1) is obtained from the slope of the linear plot of ln(qe-qt) against t, and k2 (g mg-1min-1) is the rate constant [38-39]. The corresponding kinetic parameters were summarized in Table 1. It was shown that the pseudo-second-order model fits the data much better than the pseudo-firstorder model based on the correlation coefficient (R2). In addition, the theoretical calculated qe,cal values (31.49, 11.31 and 5.57 mg g-1 for BPCM, BP and CBP, 13

respectively) obtained from the pseudo-second-order model are closer to the experimental ones (qe,exp) than those (21.73, 7.68 and 5.31 mg g-1 for BPCM, BP and CBP, respectively) from the pseudo-first-order model. Thus, based on the assumption of the pseudo-second-order model, their adsorption rate for Cr(VI) was controlled by the chemical interaction [40]. During the adsorption process, the mass transport of metal ions can be classified into several regimes in terms of the diffusional mass transfer that limits the reaction rate at each step. Thus, the kinetic of the Cr(VI) adsorption on them were also evaluated by the intra-particle diffusion model as follows [41]: qt=kit1/2+Ci

(6)

Where qt (mg g-1) is the adsorption amount at a time t (min), ki (mg g-1 min-1/2) is the intra-particle diffusion rate constant calculated from the slope of the straight line of qt vs. t1/2. Ci is the intercept of stage i reflecting the boundary layer effect (the larger of the Ci, the greater the contribution of the surface adsorption in the rate-controlling step). If the rate limiting process only depends on the intra-particle diffusion, the straight line will pass through the origin. Furthermore, other mechanisms (e.g., boundary layer or film diffusion) is also involved [40]. Fig. S6 shows the intra-particle diffusion plots of Cr(VI) on them. The multi-linearity represents that two or more steps take place during the adsorption process [42]. The first linear region indicates that the adsorption is rapid because a large number of Cr(VI) from bulk easily transport to the external active sites of the adsorbents. With the occupation of the exterior active sites and the decrease of the Cr(VI)

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concentration, the pore diffusion and the adsorption of Cr(VI) on the interior surface of the adsorbent arise in the second linear region. The last linear region is the final equilibrium step controlled by extremely low remnant adsorbate concentration in the solution. The corresponding values of ki, Ci and coefficient R2 caculated from the plots were listed in Table 2. For the three adsorbents, the external transport ki1 is higher than both of the internal transport ki2 and ki3, indicating that the primary rate is governed by intra-particle diffusion [43]. The diffusion rate constants ki1 and ki2 on BPCM are much higher than those of BP and CBP, resulting from more active sites on the surface or inside the pores of BPCM [44] due to addition of PVA. 3.2.3 Adsorption isotherm Effects of initial concentration of Cr(VI) on the adsorption capacities of BPCM, BP and CBP were shown in Fig. 7. It was shown that with increasing concentration of Cr(VI), their adsorption amount rise sharply at low concentrations, and then slowly reach a plateau, indicating that their adsorption can be completely saturated at an enough high initial concentration of Cr(VI). Furthermore, the adsorption equilibrium concentration (Ce) of BPCM is less than 0.5 mg L-1 when the initial concentration of Cr(VI) is lower than 35 mg L-1. Adsorption isotherm is used to investigate the optimizing dosage of an adsorbent, since it can not only assess its adsorption capacity, but also describe how adsorbate interacts with the adsorbent [45]. Thus, the Langmuir and Freundlich adsorption isotherms were adopted to simulate the adsorption data. The corresponding adsorption

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isotherms can be expressed as follows [46,47]: qe 

k LCe qm 1  k LCe

qe  k f Ce1/ n

(7) (8)

Where Ce (mg L-1) and qe (mg g-1) are the equilibrium concentration and equilibrium amount of Cr(VI) adsorbed, respectively; qm (mg g-1) is the maximum adsorption capacity of an adsorbent, and kL (L mg-1) is the Langmuir constant related to the adsorption bond energy. The kf and n are the Freundlich constants related to the adsorption capacity and intensity of an adsorbent, respectively. The corresponding parameters of the simulated adsorption isotherms for Cr(VI) on BPCM, BP and CBP were listed in Table 3. In comparison to the Freundlich model, the Langmuir model fits better with the experimental data of them due to the higher R2. This may result from the homogenous distribution of active sites on the adsorbent surfaces [48,49]. The qm of BPCM (36.41 mg g-1) from Langmuir equation is much higher than those of BP (12.26 mg g-1) and CBP (7.44 mg g-1), and higher Kf from Freundlich equation indicates that BPCM has an enhanced affinity for Cr(VI). Furthermore, it is striking that the maximum adsorption capacity of BPCM is much higher than most reported adsorbents in Table S1, and thus it can be employed as a highly efficient adsorbent to remove the trace Cr(VI). 3.3 Effect of the coexisting anions Industrial effluents usually contain more than one kind of coexisting anions, which compete with Cr(VI) for the active adsorption sites on adsorbent. Therefore, a potential adsorbent should have a high selectivity toward the adsorbate. For this 16

purpose, various coexisting anions including Cl-, SO42-, HCO3-, HPO42-, F- and C2O42were chosen for interference study at the certain concentrations of 1 mmol/L and 5 mmol/L, respectively. As shown in Fig. 7, the adsorption rate of Cr(VI) drops to 3.518.4 % when the concentration of coexisting anions increases from 1 mmol/L to 5 mmol/L. The F-, Cl- and C2O42- hardly interfere with adsorption of Cr(VI) on BPCM. In contrast, HCO3-, HPO42- and SO42- have much more significant influence. This is due to the fact that their closer radii with HCrO4- lead to similar molecular dimensions and hydration energy, and they can compete more effectively against Cr(VI) anions for occupying the adsorbent surface [50,51]. The result also indicates that the divalent anions (SO42-, HPO42-) have stronger affinity to BPCM than the monovalent anions (HCO3-). When these anions have similar ionic radii, the stronger competing force for adsorption sites appears due to the higher density of the ionic charge [52]. Cr(VI) removal in the presence of coexisting anions have also been reported [45,53] for other types of adsorbents. This distinct performances mainly originate from the different adsorption mechanism of Cr(VI) on BPCM, which will be discussed in next section. 3.4 Mechanism of Cr(VI) removal As illustrated in previous studies [52,54], Cr(VI) ions exist predominately as HCrO4- and CrO42- ions in solution with a pH∼6, and the surface of BPCM is positively charged because of the protonation of surface oxygen-containing groups (Fig. S7). Therefore, the electrostatic attraction between the surface of the positively charged BPCM and the negatively charged Cr(VI) species presumably plays an initial driving force to bind the Cr(VI) anions, and the formation of electrostatic attraction

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can be described as follows [55]: AlOH + H+ →AlOH2+

(9)

AlOH + H+ + CrO42- →AlOH2+_CrO42-

(10)

AlOH + 2H+ + CrO42- →AlOH2+_HCrO42-

(11)

The chemical attachment of Cr(VI) onto the BPCM was examined by FT-IR. Fig. 3d shows the FT-IR spectrum of the BPCM after adsorption. In comparison with the sample before adsorption, the new peak at 727 cm-1 appears, the -OH peak centering at 1622 cm−1 shifts to 1647 cm−1 and its peak intensity becomes weaker for the BPCM due to the chromium adsorption. The two new peaks of 916 cm−1 and 850 cm−1 may be attributed to the inter-sphere complex of HCrO4- and CrO42- ions with BPCM by monodentate and bidentate [20]. These comparable oxyanions exhibit similar intermediate strength adsorption properties as chromate, such as adsorbed selenite [56] and selenium [57] on alumina, and they have similar complexes configuration. Thus, the possible formation can be speculated for complexation of chromate on BPCM as follows: AlOH + H+ + CrO42- → AlOCrO3- +H2O (monodentate)

(12)

2(AlOH) + 2H+ + CrO42- → 2(AlO)CrO2 + 2H2O (bidentate)

(13)

The corresponding XPS spectra of BPCM before and after adsorption for Cr(VI) (see Fig. S8) were adopted to gain more insights into the adsorption mechanism. The results confirm the existence of chromium on adsorbent surface. But the Cr(VI) peaks are run of the mill with a content of 0.28 which is far below the practical content of about 2.07 (adsorption amount 20.65 mg/g). This

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phenomena could be due to the intrinsic surface sensitivity of monochromatic XPS to Cr with low kinetic energy [58], and the complexation of Cr(VI) with BPCM further reduces the energy. This result is consistent with the FT-IR spectra. The high resolution XPS Cr 2p spectra were shown in Fig. 9. The peaks of Cr 2p 3/2 is broad and distorted. The optimum fitting could be obtained by disassemble the Cr 2p 3/2 spectrum into peak at 578.8 eV ascribed to Cr(III) 2p 3/2. The peak at 576.2 eV assigned to Cr(VI) 2p 3/2 indicats the presence of more than one Cr specie on the surface of BPCM [51]. This result reveals that a small part of Cr(VI) anions are reduced to less toxic Cr(III) during the sorption process, and BPCM also has the adsorption capacity for Cr(III). Oxygen-containing groups such as hydroxyl, carboxylic and unsaturated C=C double bond are Lewis base, which may be played as electron donor for the reduction of Cr(VI) to Cr(III) [59,60], and thus, the reduced process could be described as follows: HCrO4− + 7H+ + 3e− ↔ Cr3+ + 4H2O

(14)

CrO42− + 4H2O + 3e− ↔ Cr(OH)3 + 5OH−

(15)

Based on the above results, the adsorption process could be described by the following three steps: (1) firstly, Cr(VI) anions are adsorbed onto positively charged functional groups of the protonated BPCM surface; (2) secondly, the absorbed Cr(VI) anions form complexes with the functional groups on the surface of BPCM; (3) thirdly, the slightly adsorbed Cr(VI) anions are reduced to Cr(III) by adjacent electron-donor groups or dissolved organic carbon. The corresponding interaction between Cr(VI) and BPCM was depicted in Fig. 9. Electrostatic attraction, ligand exchange as well as

19

redox reaction may act main role, and other kinds of interactions including hydrogen bonds and physical adsorption may jointly contribute to the adsorption process. Also see the deductive formation process of the BPCM in Fig. S9.

4 Conclusions The BPCM with enhanced adsorption performance towards Cr(VI) was successfully synthesized via a facile sol-gel method. Its adsorption fits the pseudosecond-order model and intra-particle diffusion mode, indicating that the chemical adsorption and intra-particle diffusion were the control process. Its maximum adsorption capacity can reach 36.41 mg g-1, which is much higher than BP, CBP and some reported adsorbents. Furthermore, the BPCM exhibits relatively higher adsorption selectivity towards Cr(VI) in presence of some co-existing anions, and the interference effect of the anions can be summarized in this order: SO42->HPO42>HCO3->C2O42->F->Cl-. The interactions including the electrostatic attraction, the ligand exchange and the redox reaction between Cr(VI) and BPCM are responsible for its excellent adsorption performance. This composite membrane could be served as an efficient and easily separated adsorbent for Cr(VI) removal, and has potential application in the field of environmental remedy.

Acknowledgements: This work was financially supported by the National Natural Science Foundation of China (21476179 and 21277108), Program for New Century Excellent Talents in University of the Ministry of Education (NCET-13-0942),

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Program for the basic Research Project of Wuhan (2015060101010065), and Fundamental Research Funds for the Central Universities of Wuhan University of Technology (2014-Ⅶ-038). The authors gratefully acknowledge help of Dr. Wei Jin from Institute of Process Engineering, Chinese Academy of Sciences for his English improvement of the manuscript.

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Table 1 The correlated kinetic parameters of Cr(VI) adsorption on BPCM, BP and CBP according to the pseudo-first-order and pseudo-second-order equation Pseudo-first-order model Pseudo-second-order model qe,exp -3 Adsorbent qe,cal k1×10 qe,cal k2×10-3 2 (mg g-1) R R2 (mg g-1) (min-1) (mg g-1) (g min-1 mg-1) BPCM 30.97 21.73 9.40 0.966 31.49 1.52 1.000 BP 11.19 7.68 6.70 0.911 11.31 3.25 0.999 CBP 5.66 5.31 12.01 0.979 5.57 6.15 0.997 Table 2 Intra-particle diffusion model applied to Cr(VI) adsorption on BPCM, BP and CBP Intra-particle diffusion model Adsorbent ki1 ki2 ki3 C1 C2 C3 R12 R22 R33 mg g-1 min-1/2 BPCM 2.79 0.96 0.04 0.11 19.47 29.51 0.994 0.952 0.53 BP 0.97 0.26 0 -0.10 6.30 9.94 0.997 0.986 0.01 CBP 0.43 0.16 -0.014 -0.19 2.53 5.93 0.981 0.945 0.981 Table 3 Parameters of adsorption isotherms for Cr(VI) of BPCM, BP and CBP. Langmuir model Freundlich model Adsorbent qm kL R2 kf (mg(1-1/n)L1/n/g) n (mg/g) (L/mg) BPCM 36.41 5.95 0.995 25.93 9.12 BP 12.26 0.13 0.982 4.35 4.43 CBP 7.44 0.05 0.887 1.50 3.11

R2 0.800 0.861 0.749

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Figure Captions Fig. 1. XRD patterns of the typical (a) BPCM and (b) CBP samples. Fig. 2. TGA-DSC curves of (a) BP and (b) BPCM. Fig. 3. FT-IR patterns of (a) pure PVA, (b) BP, (c) BPCM and (d) Cr(VI)-loaded BPCM. Fig. 4. SEM image of BPCM. Fig. 5. Cr(VI) adsorption on BPCM by adding different amount of PVA; the inset is the optical photographs for solution of Cr(VI) (a), the system after adsorption (b), BPCM (c) and the BPCM with adsorbed Cr(VI) after adsorption (d), respectively. Fig. 6. Kinetic adsorption curves of BP, BPCM, CBP and pure PVA. Fig. 7. Adsorption isotherms of Cr(VI) onto BPCM, BP and CBP. Fig. 8. Effect of coexisting anions on the Cr(VI) adsorption. Fig. 9. High resolution XPS spectra of Cr 2p3/2. Fig. 10. Tentative illustration for the interaction between BPCM and Cr(VI).

32

(251)

(151)

(231)

(200)

(031)

(120)

(020) Relative intensity (a.u.)

a

b 10

20

30

40 50 2 Theta (degree)

60

70

80

Fig. 1. XRD patterns of the typical (a) BPCM and (b) CBP samples.

33

100

（ a）

Heat Flow (W/g)

Weight (%)

90 0 339

80 69 70

-1 60 100

200

300 400 500 Temperature (°C)

100

600

700

（ b） 2

Weight (%)

80

1

354

70 0

60 50

105

Heat Flow (W/g)

90

214 -1

40 100

200

300 400 500 Temperature (°C)

600

700

Fig. 2. TGA-DSC curves of (a) BP and (b) BPCM.

34

Transmittance %

d

2925

c b

1647 1622

1093

3440

a

1469 1070

2933 4000

916850727

3500

3000

1094

2500 2000 1500 Wavenumbers (cm-1)

1000

500

Fig. 3. FT-IR patterns of (a) pure PVA, (b) BP, (c) BPCM and (d) Cr(VI)-loaded BPCM.

35

Fig. 4. SEM image of BPCM.

36

30

qt(mg Cr/g Al2O3)

28 26 24 22 20 18 0.1

0.2

0.3

0.4 0.5 m(PVA)/m(Al2O3)

0.6

0.7

Fig. 5. Cr(VI) adsorption on BPCM by adding different amount of PVA; the inset is the optical photographs for solution of Cr(VI) (a), the system after adsorption (b), BPCM (c) and the BPCM with adsorbed Cr(VI) after adsorption (d), respectively.

37

30 BPCM BP CBP pure PVA

25

qt (mg/g)

20 15 10 5 0 0

200

400

600

800

t(min)

1000

1200

1400

Fig. 6. Kinetic adsorption curves of BP, BPCM, CBP and pure PVA.

38

40 35 BPCM BP CBP Langmuir model Freundlich model

30

qe(mg/g)

25 20 15 10 5 0

20

40

Ce(mg/L)

60

80

100

Fig. 7. Adsorption isotherms of Cr(VI) onto BPCM, BP and CBP.

39

100

1mmol/L 5mmol/L

Removal efficiency (%)

80

60

40

20

0

H2O HCO3

HPO42-

C2O42-

F-

Cl-

SO42-

Fig. 8. Effect of coexisting anions on the Cr(VI) adsorption.

40

Cr(VI)

Intensity (a.u.)

Cr(III)

582

580

578

576

574

Binding energy (eV)

Fig. 9. High resolution XPS spectra of Cr 2p3/2.

41

Fig. 10. Tentative illustration for the interaction between BPCM and Cr(VI).

42