Boehmite Composites with Enhanced

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Easily Regenerative Carbon/Boehmite Composites with Enhanced Cyclic Adsorption Performance towards Methylene Blue in Batch and Continuous Aqueous Systems Bowen Han, Weiquan Cai, and Zhichao Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05747 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Industrial & Engineering Chemistry Research

Easily Regenerative Carbon/Boehmite Composites with Enhanced Cyclic Adsorption Performance towards Methylene Blue in Batch and Continuous Aqueous Systems

Bowen Han†,‡, Weiquan Cai†,‡,*, Zhichao Yang‡ †School

of Chemistry and Chemical Engineering, Guangzhou University, 230

GuangZhou University City Outer Ring Road, Guangzhou 510006, China ‡School

of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of

Technology, 205 Luoshi Road, Wuhan 430070, China

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Carbon/Boehmite composites (HCB) with highly enhanced cyclic adsorption performance toward methylene blue (MB) were prepared successfully via a one-step hydrothermal method from boehmite sol and glucose. XRD, BET, FTIR and zeta potential, etc. were carried out to characterize the prepared hydrothermal carbon sphere (HC), boehmite and HCB–3.2 (the composite with the optimal adsorption performance). Static adsorption studies showed that the maximum adsorption capacity of HCB–3.2 (303.0 mg g−1) was well above those of HC (208.0 mg g−1) and boehmite (210.0 mg g−1). Fixed–bed column studies revealed that the lower bed depth and flow rate resulted in the higher adsorption capacity for MB, and the process of the dynamic adsorption was well fitted by the Yoon−Nelson model. Importantly, persulfate advanced oxidation method could efficiently regenerate the saturated HCB–3.2 with MB, and the supernatant containing persulfate was reused during the regeneration step to reduce the production of secondary wastewater. Furthermore, HCB–3.2 maintained excellent removal efficiency for MB after five cycles, indicating its great potential for dye removal in wastewater treatment.


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1. INTRODUCTION In recent years, synthesis of novel inorganic composites has attracted a lot of attentions due to synergistically combined properties of different inorganic components, and they have versatile applications including adsorbents, chemical sensors, drug carrier, and catalysts.1-6 Boehmite (γ-AlOOH) is an important precursor of transition alumina, and tends to interact with heavy–metal ions or dye molecules because of rich hydroxyl groups on the surface of its crystalline structure and large surface area.3 Similar attention is focused on synthesizing carbonaceous materials with abundant micro–/mesoporous structures and organic functional groups via hydrothermal carbonization of glucose, and they are widely used to functionalize the inorganic adsorbents to improve their adsorption performance.7 It was reported that, in comparsion with the single boehmite and carbon, their composite has better uptake ability due to their synergistic effect. For example, the adsorption capacity (85 mg g-1) of the biochar/AlOOH nanocomposite prepared from AlCl3 pretreated biomass via gradual pyrolysis in a nitrogen environment at 600 °C is much higher than those of AlOOH (10 mg g-1) and biochar (8 mg g-1) for Methyl blue (MB) removal;8 the adsorption capacity (59.17 mg g-1) of Al2O3/CSC is higher than those of CSC (11.81 mg g-1) and Al2O3 (40.01 mg g-1) for As(V) removal.1 MB is one of the most widely investigated pollutant with deep color, and mainly results from the dyeing and printing industries. MB solution with high concentration is poisonous and seriously affect the growth of aquatic plant.9 Thus, several methods including adsorption, ultrafiltration and sedimentation are used for MB removal.10-12



Especially, adsorption with simple operation process and high efficiency has drawn widely research interest.13 Various adsorbents such as polymer microspheres or networks,4 attapulgite and grapheme oxide (APT+GO),14 and alumina15 have been explored, while their poor adsorption performance, secondary pollution caused by regeneration and the multi-step preparation processes limit their applications.16,17 Therefore, novel composite materials with excellent recyclable adsorption stability and simple preparation process are still need to be developed. The main aim of this study is to introduce a one-step hydrothermal method to prepare an efficient carbon/boehmite composite (HCB) for MB removal. Although carbon or boehmite for MB removal is available in a batch system, MB removal using HCB as adsorbent in a fixed bed column was not reported. The fixed bed column system has the superiority of flexible operation, low cost, and easy from laboratory procedure to scale up. Its specific objectives based on static adsorption and continuous-flow experiments18,19 are as follows: (i) to study the adsorption performance and the adsorption mechanism of MB on HCB in a batch system, (ii) to study the influence of the flow rate and bed depth on the adsorption performance of HCB in a continuous-flow system, and (iii) to explore the recyclable adsorption stability of the spent HCB regenerated by the persulfate advanced oxidation method.

2. EXPERIMENTAL 2.1. Materials and Experiments All reagents used are analytical grade and purchased from Shanghai Chemical Reagent Ltd (China). In a typical procedure, boehmite was prepared through


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hydrolysis and condensation of aluminium isopropoxide.20 Firstly, 8.18 g aluminium isopropoxide was hydrolyzed in 72 mL deionized water at 80 °C. The resulting slurry was peptized with 1 M HNO3 and refluxed for 12 h at 90 °C to obtain boehmite sol. Then, 4.0 g glucose was dissolved in 40 mL deoinized water and desired amount of above sol was added along with 10 mL of 99.5 +% anhydrous ethanol (Acros Organics) under magnetic stirring. The mixture was placed in an 80 mL Teflon-lined stainless-steel autoclave, and heated at 180 °C for 12 h. Finally, the dark brown product was washed twice with deionized water and once with ethanol, then put in vacuum oven at 80 °C for 5 h. The synthetic process was also depicted in Figure S1. The resulting samples were labeled as HCB–x, where HCB represented the composites based on glucose and boehmite sol, “x” denoted the mass ratio of boehmite sol and glucose and varied from 0.4, 0.8, 1.6, 3.2 to 4.0. Furthermore, the sample HC was prepared by glucose in this hydrothermal process without adding boehmite sol; the boehmite was obtained without adding glucose. 2.2. Characterization The XRD measurements of the samples were performed with a Rigaku D/MAX-RB diffractometer using Cu Kα radiation (1.5406 Å) in the 2θ range from 10° to 80° to identify their phase structures of the samples. FT-IR spectra between 4000 cm−1 and 400 cm−1 were characterized by a Nicolet 6700 spectrometer to analyze the surface chemical characteristics before and after adsorption for MB. The TG measurements were performed on a Simultaneous Thermal Analysis (STA449F3) with a heating rate of 10 °C min-1 up to 800 °C in flowing air. The C, H and O



contents of the composite was measured using an Elementar Vario EL cube elemental analyzer. The content of Al in solid product was detected with inductively coupled plasma atomic emission spectrometry (ICP-AES, Prodigy 7). N2 adsorption isotherms were performed at -196 °C using Tristar II 3020 surface area and porosity manufactured by Micromeritics (USA). All the samples were degassed at 150 °C for 4 h before BET analysis. The surface areas of the samples were obtained by the Brunauer-Emmett-Teller (BET) method, and desorption branch of the isotherm was used to analyze the pore size distribution (PSD) and pore volume by the Barret– Joyner–Halenda method. Morphological analysis was performed at an accelerating voltage of 20 kV by a JSM-IT300 scanning electron microscope (JEOL, Japan). Zeta potentials of the samples at the pH value range from 1 to 12 were measured with a zeta voltmeter (Zetasizer Nano ZS90). 100 mg sample was introduced into 200 mL of 10-4 M NaCl solution, and the initial pH value of the solution was adjusted with 0.1 M HCl or 0.1 M NaOH solution.3 2.3. Adsorption Experiments The static adsorption experiment was performed in a stable temperature rotary shaker at 28 ºC under 180 r min-1. For the adsorption isotherm and kinetic study, 100 mg adsorbent samples was introduced into 100 mL MB solution of 25~500 mg L-1. At different time intervals, 2 mL suspension was withdrawn and filtered via a microporous filtering membrane with pore size of 0.45 μm and diameter of 25 mm for subsequent analysis. The concentration of MB was measured by an ultraviolet spectrophotometer (UV-1240, Shimadzu, Japan). Each adsorption experiment was


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duplicated, and the error line was presented in each curves. The dynamic adsorption experiment was carried out in a U glass tube as a fixed-bed column with the maximum height of 20 cm and an internal diameter of 2.0 cm. The U glass tube was sustained and sealed by glass bead which effectively immobilize the adsorbent. The bed depth of 2.5~7.5 cm and the inlet flow rate of 1.5~3.5 mL min-1 were used. 2000 mL MB solution with a concentration of 25 mg L-1 was pumped into the column in a right–left mode by a peristaltic pump. Figure S2 showed the schematic diagram of the dynamic adsorption and regeneration system for HCB–3.2. The residual amount of MB on glass bead was negligible. The effluent was collected and analyzed for the residual MB concentration at different time interval. 2.4. Regeneration and Recycling The recyclability of the adsorbent was explored in static adsorption (Figure S3) and dynamic adsorption (Figure S2). For the dynamic adsorption and regeneration system, pinched flatjaw pinchcock III and IV were applied to stop the progress of adsorption, and released flatjaw pinchcock I and II were used for regeneration of the adsorbents. The rollers of peristaltic pump should be in a clockwise direction at this time. Otherwise, the progress of adsorption would be promoted. When the effluent concentration of MB tended to be equal to the initial concentration, the residual solution in the column was dumped for the next run. Then, 100 mL ammonium persulfate solution with concentration of 50 mg L-1 was fed into the system, and the water bath was maintained at 60 ºC for 15 min. Finally, the supernatant was stored, and the fixed–bed column was dried in an oven at 80 ºC for 4 h. To reduce the



production of secondary wastewater, the collected supernatant contained persulfate and the regenerated fixed–bed column could be taken for next cycle. It should be noted that the supernatant containing persulfate was reused for regenerating MB loaded HCB–3.2, and the oxidation products are environmental friendly.

3. RESULTS AND DISCUSSION 3.1. Characterization of the samples The XRD patterns of the as-prepared samples were depicted in Figure 1. The broad diffraction peak around 23º appeared in Figure 1a, indicating that the HC is amorphous carbon.21 Figure 1b showed the diffraction peaks at 14º, 28º, 38º, 49º and 60º which correspond to the (020), (120), (031), (200) and (231) planes of boehmite (JCPDS File No. 21–1307), respectively.3 Basal peaks of the boehmite were clearly seen in Figure 1c of HCB–3.2. However, in comparsion with the diffraction peaks of the boehmite, these peaks intensity decreased apparently due to a great influence of the amorphous carbon. The FT-IR spectra were further depicted in Figure 2. For HC in Figure 2a, the band at 873 cm-1 ascribed to the in–plane benzene ring; the peaks at 1695 and 2915 cm-1 attributed to –CHO vibration and –COOH stretching vibration, respectively. For boehmite in Figure 2b, the two peaks at 635 and 1069 cm-1 attributed to the typical (OH)–Al=O angle bending and (HO)–Al=O asymmetric stretching, respectively22,23; the wide peak at 3415 cm-1 ascribed to the symmetric and asymmetric stretching vibrations of –OH groups.22 For HCB–3.2, the peaks at 635 and 1069 cm-1 also


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appeared in Figure 2c, and their intensity increased significantly. Furthermore, the ether stretching at 1013 cm-1 was hidden; the intensity of in–plane benzene ring at 873 cm-1 decreased obviously, due to that the in–plane benzene ring of HC reacted with (OH)–Al=O resulting in C–O–Al=O.3 Moreover, other absorption peaks of functional groups in boehmite and HC particles appeared in HCB–3.2, indicating that HCB had the dual characteristics of boehmite and HC. The TG-DSC curve of HCB–3.2 could be divided into three parts in Figure S4. For part I up to 190 °C, the corresponding 6.1 % weight loss resulted from the physically adsorbed water. For part II between 190 °C and 388 °C, its weight loss decreased by 24.2 %, resulting from the decomposition of carbohydrates and lipids, dehydration reactions and the decomposition of crystal water in boehmite.3,52 For part III between 388 °C and 601 °C, the 42.7 % weight loss resulted from the conversion of boehmite to γ-Al2O3.53 Furthermore, there was no obvious weight loss above 601 °C. The results showed that the boehmite sol was not combined with carbon sphere completely. This is consistent with the result of elementary analysis (Table S1) about real Al/C ratio of HCB–3.2. N2 adsorption−desorption isotherms and the corresponding PSD curves of the samples were depicted in Figure S5. All the samples in Figure S5a present type IV isotherms with a hysteresis loop (H2 type for boehmite and H3 type for the others). In comparison with HC and boehmite, HCB–3.2 showed higher N2 uptake. Figure S5b showed that a relatively narrow PSD of boehmite was around 6.1 nm, PSD of HC was around 18.7 nm, and the wide PSD of HCB–3.2 with bimodal PSD centered around



3.42 nm and 8.27 nm, respectively. This may be due to that boehmite nanoparticles deposited on the surface of HC sphere, resulting in covering the most of its surface large pores. The similar result was reported for porous magnetic [email protected] The corresponding pore structure parameters were provided in Table S2. HCB–3.2 showed a specific surface area of 167.0 m2 g-1 and a pore volume of 0.48 cm3 g-1, which were higher than those of HC (4.2 m2 g-1, 0.02 cm3 g-1) and was close to boehmite (170.0 m2 g-1, 0.38 cm3 g-1), respectively. The bimodal PSD structure for HCB–3.2 may be beneficial for achieving a higher diffusion rate, and the high specific surface area may bring more abundant organic functional groups, which is beneficial for MB adsorption.51 The physicochemical properties of other HCB–x (Figure S5 and Table S2) confirmed that the suitable mass ratio of boehmite sol and glucose resulted in large surface area and bimodal PSD structure, which are critical for MB removal. Figure S6 showed the SEM images of HC and HCB–3.2. HC in Figure S6a and Figure S6b presented uniform spheres with coarse surface. Most HCB–3.2 in Figure S6c presented a spherical morphology with a diameter of about 8 μm which is beneficial for its separation after adsorption. Figure S6d confirmed that the boehmite nanoparticles deposited on the surface of HC spheres. 3.2. Static adsorption Preliminary experiments were carried out to decide the optimal ratio for preparing HCB–x with the largest adsorption capacity for MB. The relation between mass ratio and adsorption capacity of HCB–x was shown in Figure S7, and the optimal mass


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ratio was confirmed to be 3.2. The adsorption behavior of HCB–3.2 in different pH conditions were provided in Figure S8. The adsorption behavior of HCB–3.2 did not change so much with pH varying from 5.0 to 7.0, and thus the solution pH of the original MB solution was chosen as the adsorption pH. Adsorption kinetics in Figure 3a presented that their adsorption amounts for MB increased with increase of adsorption time. The adsorption capacity of HCB–3.2 was 230.0 mg g−1 which was significantly more than those of HC (93.8 mg g−1) and boehmite (107.0 mg g−1), respectively. This may be due to the synergistic effect of boehmite with large surface area and HC with abundant organic functional groups which is beneficial for dye removal. The pseudo-first-order model and pseudo-second-order model are adopted to obtain crucial information of the adsorption process as follows: 𝑙𝑛 (𝑞𝑒 ― 𝑞𝑡) = 𝑙𝑛𝑞𝑒 ― 𝑘1𝑡 𝑡 𝑞𝑡

1

𝑡

= 𝑘 𝑞2 + 𝑞𝑒

(1) (2)

2 𝑒

where 𝑞𝑒 and 𝑞𝑡 (mg g-1) refer to adsorption amount at equilibrium and a certain time (min), respectively; 𝑘1 (min−1) and 𝑘2 (g mg−1 min−1) are the rate constants. Furthermore, as in the kinetics experiments, the adequacy of the models adjustment and the goodness-of-fit was determined by the Residual Root Mean Square Error (RMSE) selection criteria and the chi-square test (also see Supporting Information)26. The corresponding parameters of the two models were listed in Table S3 (also see Figure 3b and Figure 3c). The values of the pseudo-second order model with RMSE and the chi-square test were smaller than two other models, respectively, indicating



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that this model fitted the curve better. The theoretical values 𝑞𝑒 (108.2, 94.6 and 229.4 mg g-1 for boehmite, HC and HCB–3.2, respectively) of the pseudo-second order model were more close to the experimental adsorption capacities, indicating that the adsorption process was controlled by chemisorption through sharing or exchanging electrons between the adsorbate and adsorbent.24 The mass transport of MB molecules during the adsorption process could affect the adsorption rate. Thus, the intra-particle diffusion model should be evaluated as follows:27 𝑞𝑡 = 𝑘𝑖𝑡1/2 + 𝐶𝑖

(3)

where 𝑘𝑖 (mg g−1 min−1/2) is the rate constant, and 𝐶𝑖 is the intercept of intraparticle diffusion at different stages. The adsorption process was also controlled by pore diffusion due to that the straight lines were not from the origin in Figure 3d. The related parameters were listed in Table S3, and the intercept of second stage is larger than the first stage suggesting that the diffusion is mainly on the inner surface of the adsorbent.3 The diffusion rate constants of HCB–3.2 were much higher than those of HC and boehmite, respectively, resulting from more active sites on the surface of HCB–3.2.28,29 It could be concluded that this adsorption process was controlled by the chemisorption and the intraparticle diffusion which were similar to some other systems.5,11 The adsorption isotherms of HC, boehmite and HCB–3.2 were shown in Figure 4. It showed that HCB–3.2 adsorbed MB with very high affinity (L-type isotherm), while HC and boehmite showed lower affinity (S-type isotherm). Therefore, the


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adsorption behavior of HCB-3.2 is better than the other two materials, and HCB–3.2 is suitable for removing MB with low concentration in a short time. The Langmuir model is used to describe monolayer adsorption on a homogeneous surface, and the Freundlich model describes the process of adsorption on a heterogeneous surface. They are applied to simulate the data of adsorption isotherms as follows, respectively: 𝑘𝐿𝑞𝑚𝐶𝑒

𝑞𝑒 = 1 + 𝑘𝐿𝐶𝑒

(4)

1

𝑞𝑒 = 𝑘𝐹𝐶𝑒𝑛

(5)

where 𝐶𝑒 (mg L−1) and 𝑞𝑒 (mg g−1) are the equilibrium concentration and adsorption capacity of MB, respectively; 𝑞𝑚 (mg g−1) is the maximum adsorption capacity of MB, and 𝑘𝐿 (L mg−1) and 𝑘𝐹 (mg g−1) are the constants related to the two models, respectively. Their related fitting parameters of the adsorption isotherms were shown in Table 1 (also see Figure S9). It was found that the Langmuir model had the smaller value of RMSE and the chi-square test than Freundlich model, indicating the active sites homogenously distributed on the surface of the adsorbent,31,32 this is beneficial for explaining the mechanism of MB removal and the easily regeneration of MB exhausted HCB–3.2. The fitting values were both between 0 and 1, indicating that the adsorption conditions were beneficial for the interaction between the adsorbent and adsorbate.31 The 𝑞𝑚 of HCB–3.2 (303.0 mg g−1) was much higher than those of HC (208.0 mg g−1) and boehmite (210.0 mg g−1) for Langmuir model. Furthermore, HCB–3.2 showed much higher adsorption capacity than most similar adsorbents for



MB in Table 2. 3.3. Mechanism of MB removal As can be seen from Figure 5, it was found that the isoelectric points (IEP) of boehmite, HC and HCB–3.2 are 9.9, 1.1 and 2.6, respectively. When the value of solution pH is larger than that of IEP, a higher adsorption efficiency of cationic species is expected because of the electrostatic attraction forces between the negatively charged adsorbent surface and positively charged adsorbate. HCB–3.2 possesses a negative surface charge which is favorable for adsorption of cations. Though carbon spheres has a lower IEP, the adsorbed ability is poor due to the low degree of carbonization.29 The HCB-3.2 is negatively charged and the MB molecule is positively charged with dissociation of chloride ion in solution, respectively. Therefore, the interaction between HCB–3.2 and MB molecules ascribes to the electrostatic attraction. For FT-IR pattern of MB-loaded HCB–3.2 (Figure 2d), the peaks at 588 and 1593 cm-1 could be ascribed to the stretching vibration of –C–S and –C=N in MB; the weak peaks at 635 and 1069 cm−1 ascribed to boehmite; the peaks at 1153 and 1593 cm−1 appear due to that (OH)–Al=O groups of boehmite reacted with positively part of MB molecules. As shown in Figure 6, the bound effect of MB and HCB–3.2 could possibly form the hydrogen bonding between de-proton atom Al−OH group (adsorbent surface) and the N or S (dye molecule).4,24 It was shown that the adsorption of MB molecules on HCB–3.2 surface mainly resulted from H-bonding and electrostatic interaction, including the components of HC with deprotonated


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carboxyl groups, the components of boehmite with de-proton atom Al−OH group and the newly generated functional group with C–O–Al=O. In comparsion with the single boehmite and carbon, the composite has more functional groups due to their synergistic effect, although some of them may be inactive due to the interaction among the components. Because of its rich functional groups, high surface area and special bimodal PSD structure (Figure S5), HCB–3.2 showed the highest maximum adsorption capacity and the highest adsorption rate among the samples. 3.4. Fixed–bed column studies In view of the excellent performance of static adsorption and the adsorption interaction which occurred on the surface of the adsorbent, it is crucial to conduct the fixed-bed column studies for scale up from a laboratory procedure. The design of the breakthrough curve for the effluent is the premise of simulating the fixed–bed column system.39 The corresponding calculation formulas of theoretical background are given in the supporting information. 3.4.1. Modeling. The Adams−Bohart model is only applied for the first section of the breakthrough curve. It assumes that equilibrium is not instantaneous, and certain limitations exist. However, the Yoon–Nelson model is used for the whole breakthrough curve without physical properties of the adsorbent in the dynamic adsorption.41 To simulate the adsorption process mathematically, the adsorption breakthrough curves are fitted by the metioned two models, respectively as follows:40,42 𝐶 𝐶0

𝐻

= 𝑒𝑥𝑝 (𝑘𝐴𝐵𝐶0𝑡 ― 𝑘𝐴𝐵𝑁0𝑈0)


(6)


𝐶

𝑒𝑥𝑝 (𝑘𝑌𝑁𝑡 ― 𝜏𝑘𝑌𝑁)

𝐶0 = 1 + 𝑒𝑥𝑝(𝑘𝑌𝑁𝑡 ― 𝜏𝑘𝑌𝑁)

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(7)

where 𝑘𝐴𝐵 is the kinetic constant (L mg−1 min−1); 𝐻 is the bed height of HCB– 3.2 (cm); 𝑁0 and 𝑈0 are the saturation concentration of MB (mg L−1) and the superficial velocity (cm min−1), respectively; 𝜏 is the time (min) and 𝑘𝑌𝑁 is a rate constant (min-1). 3.4.2. Effect of flow rate of MB solution. The corresponding breakthrough curves of the flow rate and adsorption amount of HCB-3.2 for MB were shown in Figure 7a. With the flow rate increasing from 1.5 mL min−1 to 3.5 mL min−1, the adsorption amount decreased from 20.0 mg g−1 to 14.7 mg g−1. The residence time of the adsorbate in the column was not enough for the adequate interaction between the adsorbent and adsorbate. The breakthrough time (tb), bed exhaustion time (te) and adsorption capacity were shown in Table S4. Likewise results were got for Cr(VI) and phosphate removal in a fixed–bed.38 This result showed that lower flow rate is favorable for MB removal in a column of HCB–3.2. 3.4.3. Effect of bed depth. The corresponding breakthrough curves of bed depth and the adsorption amounts of HCB–3.2 for MB were shown in Figure 7b. An earlier saturated time was obtained with a lower bed depth. With the bed depth increasing from 2.5 cm to 7.5 cm at a flow rate of 2.5 mL min−1, the adsorption amount of HCB– 3.2 decreased from 17.5 mg g−1 to 14.6 mg g−1. The value of tb and te was not significantly decreased due to the active sites of the adsorbent combined adequately with adsorbate at low bed depths. It is evident that lower bed depth is beneficial for MB removal in a column of HCB–3.2.


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In contrast, the adsorption capacity of HCB–3.2 in dynamic adsorption was lower than that in batch systems, because the influent constantly met a new part of adsorbent and new equilibrium of adsorption trended to be established. Furthermore, as can be observed in Figure 7, the breakthrough curves were described by the Yoon–Nelson model accurately. 3.5. Regeneration and recycling of the spent HCB–3.2. Dye exhausted adsorbents can be regenerated by surfactant assisted desorption,43 chemical oxidation,44,45 electrochemical oxidation,46 etc. Herein, persulphate advanced oxidation processes was chosen due to its fast reaction kinetics, high efficiency and low cost.47 The previous reports show that oxidation method generates mainly free sulfate radicals in acidic and neutral conditions, and the sulfate radicals have much higher oxidation capacity than hydroxyl radicals.48,49 When the persulfate is activated in the water bath, it mainly generates free radicals of sulfuric,47 which will undergo a series of radical chain reactions as follows: 𝑆2𝑂28 ―

∆ 𝑜𝑟 ℎ𝑣

2𝑆𝑂4∙ ―

𝑆𝑂4∙ ― + 𝐻2𝑂⟶ 𝑆𝑂24 ― + 𝐻𝑂 ∙ 𝑆𝑂4∙ ― + 𝑂𝐻 ∙ ⟶𝑆𝑂24 ― + 𝐻𝑂 ∙

(8) (9) (10)

The free radicals of sulfuric eventually form the state of coexistence of free sulfuric radicals and hydroxyl radicals. The MB attached on HCB–3.2 may be degraded completely with sustainably produced 𝑆𝑂4∙ ― in the process of persulfate advanced oxidation.30 The regeneration efficiency of HCB–3.2 was studied for five adsorption/desorption cycles, and the breakthrough curves were shown in Figure 8.



The MB removal rate slightly decreases from 89.6 % for the first cycle to 85.9 % for the fifth cycle. To illustrate the difference before and after adsorption, the regenerated HCB–3.2 was also characterized by FT-IR spectra in Figure 2e. Its FT-IR pattern is almost the same as that of the fresh HCB–3.2. The controlled results of MB degradation with HC, Boehmite, HCB–3.2 and without HCB–3.2 were given in Figure S3. It was shown that the oxidation of persulfate mainly occurred on the surface of the carbon spheres of HCB–3.2 instead of the boehmite of HCB. Moreover, the regeneration process had little effect on the adsorption performance of the HCB– 3.2, and it was benefited from a monolayer adsorption process on a homogeneous surface. Similar result was reported that persulfate oxidation of Azo dye acid orange 7 was catalyzed by activated carbon, and the carbon spheres had slightly effect in situ recovery performance.54 The mechanism could be explained that carbon may act as an activator of the electron-transfer mediator to activate persulfate anion (S2O82-) resulting in sulfate radical (𝑆𝑂4∙ ― ) for contaminant destruction, suggesting an alternative route for removing organic contaminants.55 The Co2+ was used to catalyze oxidant and oxone for degrading MB molecules in previous report; the residual metal ions caused secondary pollution and the ionic strength affected the adsorption performance.16 In this fixed–bed column study, it should be noted that the supernatant containing persulfate was reused for regeneration of HCB–3.2 to reduce the generation of secondary wastewater, and the oxidation products are environmental friendly. The results show that the persulfate oxidation method is effective for recycling MB saturated HCB–3.2.


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4. CONCLUSIONS The carbon/boehmite composites (HCB) based on glucose and boehmite sol was prepared successfully via a one-step hydrothermal method. It showed a highly enhanced cyclic adsorption capacity for MB in batch and continuous adsorption processes. The maximum adsorption capacity of HCB–3.2 obtained with the mass ratio of 3.2 for boehmite sol and glucose was 303.0 mg g−1 which was well above those of HC (208.0 mg g−1) and boehmite (210.0 mg g−1), and the data of adsorption kinetics and isotherms supported the chemisorption nature of the processes. Fixed– bed column studies of HCB–3.2 revealed that the lower bed depth and the flow rate resulted in its higher adsorption capacity for MB; the adsorption process in the column could be well depicted by the Yoon−Nelson model. Importantly, HCB–3.2 maintained good MB removal efficiency after five cycles. Moreover, the persulfate advanced oxidation method efficiently regenerated the MB saturated HCB–3.2, and the supernatant containing persulfate was reused during the regeneration step to reduce the production of secondary wastewater. Overall, HCB–3.2 showed a great potential for treating MB polluted wastewater in actual conditions.

■ ASSOCIATED CONTENT Supporting Information Available Theoretical background of calculation formulas for fixed–bed column study, Figure S1~Figure S9, Tables S1~Table S4, were given in the Supporting Information. These materials are available free of charge via the Internet at http://pubs.aca.org/.



■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86-20-39366905. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (21476179), one hundred talents project of Guangzhou University (69-18ZX10016) and 2016 Wuhan Yellow Crane Talents (Science) Program.

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Table captions Table 1. Parameters of the adsorption isotherms for MB Adsorption on HC, boehmite and HCB–3.2. Table 2. Adsorption capacities and equilibrium time of similar adsorbents for MB.


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Table 1. Parameters of the adsorption isotherms for MB adsorption on HC, boehmite and HCB–3.2. Langmuir model Sample

qe.max

Freundlich model

kL

kF RMSE

2

1/n

χ

RMSE

χ2

(mg g−1)

(L mg−1)

(mg g−1)

HC

208.0

0.006

0.009

0.010

0.336

3.63

0.403

0.163

Boehmite

210.0

0.090

0.104

0.156

0.757

6.36

0.569

0.357

HCB–3.2

303.0

0.160

0.095

0.109

0.656

72.6

0.378

0.114

Table 2. Adsorption capacities and equilibrium time of similar adsorbents for MB. Adsorbent dosage

Equilibrium time

Qmax

Adsorbent

Reference (g L-1)

(min)

(mg g-1)

HCB–3.2

1.0

300

303.0

This work

AC/OHa

2.5

1440

185.0

16

Wheat Straw

2.0

550

54.1

33

Magnetic graphene oxide

1.0

1000

64.9

34

BN hollow spheres

0.4

300

116.5

23

LDH−carbon dotsb

0.5

20

192.3

35

MCGOc

0.15

100

98.5

36

Graphene

1.0

450

300.0

37

aAC/OH,

activated carbon treated by NaOH solution; bLDH−carbon dots, Layered Double

Hydroxide−carbon dot composite; cMCGO, magnetic chitosan grafted with graphene oxide.



Figure captions Figure 1. XRD patterns of (a) HC, (b) boehmite and (c) HCB–3.2. Figure 2. FT-IR patterns of (a) HC, (b) boehmite, (c) HCB–3.2, (d) MB–loaded HCB–3.2 and (e) regenerated HCB–3.2. Figure 3. Kinetic adsorption curves (a), the pseudo-first-order kinetic model (b), the pseudo-second-order kinetic model (c) and the intra-particle diffusion model (d) of the samples (C0(MB) = 250 mg L-1, T = 28 ± 0.2 °C, solution pH = 6.8 ± 0.2, adsorbent dosage = 1 g L-1). Figure 4. Adsorption isotherms of HC, boehmite and HCB–3.2. (Contact time = 7 h, solution pH = 6.8 ± 0.2, T = 28 ± 0.2 °C, adsorbent dosage = 1 g L-1. The symbols represent the averages of three independent experimental replicates, and the error bars represent the standard deviations of the experimental replicates). Figure 5. Zeta potential analysis results of HC, boehmite and HCB–3.2 (The symbols represent the averages of three independent experimental replicates, and the error bars represent the standard deviations of the experimental replicates). Figure 6. Adsorption of MB molecules on HCB–3.2 surface through H-bonding and electrostatic interaction. Figure 7. Comparison between the influence of operational parameters of MB adsorption on HCB–3.2 with the Yoon−Nelson model and the Adams−Bohart model: (a) Flow rate, (b) Bed height. (C0(MB) = 25 mg L-1, solution pH = 6.8 ± 0.2, T = 28 ± 0.2 °C. The symbols represent the averages of three independent experimental replicates, the error bars represent the standard deviations of the experimental


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replicates, solid and dotted lines represent values calculated from models). Figure 8. The breakthrough curves of MB adsorption in recycle runs of spent HCB– 3.2 (C0(MB) = 25 mg L-1, solution pH = 6.8 ± 0.2, T = 28 ± 0.2 °C, F = 2.5 mL min-1, H = 2.5 cm. The symbols represent the averages of three independent experimental replicates, the error bars represent the standard deviations of the experimental replicates, solid lines represent values calculated from Yoon−Nelson model).


Page 32 of 38

(020)

20

50

2 (degree)

60

(251)

(231)

40

(151)

30

(200)

(031)

a b c

(120)

10

70

80

Figure 1. XRD patterns of HC (a), boehmite (b) and HCB–3.2 (c).

1000

2000

3000 -1

Wavenumber (cm )

3415

2915

1593 1695

1069

635

b d

873

a a c e

588

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Relative intensity(a.u.)


4000

Figure 2. FT-IR patterns of HC (a), boehmite (b), HCB–3.2 (c), MB-loaded HCB–3.2 (d) and regenerated HCB–3.2 (e).


Page 33 of 38

(a)

250

(b) 2.5

200

1.5

lg(qe-qt)

-1

qt/(mg g )

150

HC Boehmite HCB-3.2

2.0

HC Boehmite HCB-3.2

1.0

100

0.5 50

0.0 -0.5

0 0

(c)

100

5

200

t (min)

300

0

400

(d)

HC Boehmite HCB-3.2

200

t (min)

300

400

200 HC Boehmite HCB-3.2

qt/(mg g )

4

100

250

-1

3

t/qt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

150 100

1

50

0 0

100

200

t (min)

300

400

0 5

10

1/2

1/2

t (min )

15

20

Figure 3. Kinetic adsorption curves (a), the pseudo-first-order kinetic model (b), the pseudo-second-order kinetic model (c) and the intra-particle diffusion model (d) of the samples (C0 = 250 mg L-1, T = 28 ± 0.2 °C, solution pH = 6.8 ± 0.2, adsorbent dosage = 1 g L-1. The symbols represent the averages of three independent experimental replicates, the error bars represent the standard deviations of the experimental replicates).



300 HC Boehmite HCB-3.2

250 -1

qe (mg g )

200 150 100 50 0 0

50

100

150

200 -1

250

300

ce (mg L ) Figure 4. Adsorption isotherms of HC, boehmite and HCB–3.2. (contact time = 7 h, solution pH = 6.8 ± 0.2, T = 28 ± 0.2 °C, adsorbent dosage = 1 g L-1. The symbols represent the averages of three independent experimental replicates, the error bars represent the standard deviations of the experimental replicates). 20

Zeta potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

HC Boehmite HCB-3.2

10 0 -10 -20 0

2

4

6

8

10

12

pH Figure 5. Zeta potential analysis results of HC, boehmite and HCB–3.2. (The symbols represent the averages of three independent experimental replicates, the error bars represent the standard deviations of the experimental replicates)


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Figure 6. Adsorption of MB molecules on the surface of HCB–3.2 through H-bonding and electrostatic interaction.


1.0


Page 36 of 38 0.8

0.6

1.0

(a)

-1

0.4

1.5 mL min -1 2.5 mL min -1 3.5 mL min Adams-Bohart model Yoon-Nelson model

0.8

0.0

Ct/C0

0.6

0.2

0

0.4

200

400

0.2 0.0 0

200

400

600

800

1000

Time (min) 1.0

(b)

1.0

0.8

0.8

0.6

Ct/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6

0.4 2.5 cm 5.0 cm 7.5 cm Adams-Bohart model Yoon-Nelson model

0.2 0.0 0

400

800

1200

1600

Time (min)

2000

Figure 7. Comparison between the influence of operational parameters of MB adsorption on HCB–3.2 with the Yoon−Nelson model and the Adams−Bohart model: (a) flow rate, (b) bed height. (C0 = 25 mg L-1, solution pH = 6.8 ± 0.2, T = 28 ± 0.2 °C. The symbols represent the averages of three independent experimental replicates, the error bars represent the standard deviations of the experimental replicates, solid and dotted lines represent values calculated from models.)


0.4

0.2

0.0

Page 37 of 38

1.0

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

0.8 0.6

Ct/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.2 0.0 0

200

400

Time (min)

600

800

Figure 8. The breakthrough curves of MB adsorption in recycle runs of spent HCB– 3.2 (C0 = 25 mg L-1, solution pH = 6.8 ± 0.2, T = 28 ± 0.2 °C, F = 2.5 mL min-1, H = 2.5 cm. The symbols represent the averages of three independent experimental replicates, the error bars represent the standard deviations of the experimental replicates, solid lines represent values calculated from Yoon−Nelson model).



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Boehmite Composites with Enhanced

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