One-pot synthesis of layered double hydroxide hollow nanospheres with ultrafast removal efficiency for heavy metal ions and organic contaminants

One-pot synthesis of layered double hydroxide hollow nanospheres with ultrafast removal efficiency for heavy metal ions and organic contaminants

Chemosphere 201 (2018) 676e686 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere One-pot ...

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Chemosphere 201 (2018) 676e686

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

One-pot synthesis of layered double hydroxide hollow nanospheres with ultrafast removal efficiency for heavy metal ions and organic contaminants Mahfuza Mubarak a, b, Hyokyung Jeon a, Md. Shahinul Islam b, Cheolho Yoon c, Jong-Seong Bae d, Seong-Ju Hwang b, Won San Choi e, Ha-Jin Lee a, b, * a

Western Seoul Center, Korea Basic Science Institute, 150 Bugahyun-ro, Seoudaemun-gu, Seoul, 03759, Republic of Korea Department of Chemistry and Nano Science, Ewha Womans University, 52 Ewhayeodae-gil, Seoudaemun-gu, Seoul, 03760, Republic of Korea Seoul Center, Korea Basic Science Institute, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea d Busan Center, Korea Basic Science Institute, 30 Gwahaksandan 1-ro 60beon-gil, Gangseo-gu, Busan, 46742, Republic of Korea e Department of Chemical and Biological Engineering, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon, 34158, Republic of Korea b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 MF-LDH and MF-LDO hollow nanospheres were prepared by one-step thermal method.  MF-LDH and MF-LDO showed ultrafast removal efficiency for As(V) and Cr(VI).  MF-LDO purified contaminated water up to drinking water level within 20 min.  Gold nanoparticles were successfully introduced into MF-LDO hollow nanosphere structure.  Au-MF-LDO completely reduced 4nitrophenol to 4-aminophenol within 5 min.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 September 2017 Received in revised form 2 March 2018 Accepted 5 March 2018 Available online 6 March 2018

Herein, Mg/Fe layered double hydroxide (MF-LDH) hollow nanospheres were successfully prepared by a one-step thermal method. After the thermal treatment of MF-LDH nanospheres at 400  C, the MF-LDH was converted into the corresponding oxide, Mg/Fe layered double oxide (MF-LDO), which maintained the hollow nanosphere structure. The MF-LDO hollow nanospheres exhibited excellent adsorption efficiency for both As(V) and Cr(VI), showing 99% removal within 5 min and providing maximum removal capacities of 178.6 mg g1 [As(VI)] and 148.7 mg g1 [Cr(VI)]. Moreover, it met the maximum contaminant level requirements recommended by World Health Organization (WHO); 10 ppm for As(V) and 50 ppm for Cr(VI) in 10 and 20 min, respectively. Furthermore, Au nanoparticles were successfully introduced in the MF-LDO hollow nanospheres, and the products showed a conversion rate of 100% for the reduction of 4-nitrophenol into 4-aminophenol within 5 min. It is believed that these excellent and versatile abilities integrated with a facile synthetic strategy will facilitate the practical application of this material in cost-effective wastewater purification. © 2018 Elsevier Ltd. All rights reserved.

Handling Editor: Shane Snyder Keywords: Layered double hydroxides Nanosphere Adsorption Heavy metal ions Catalyst

* Corresponding author. Western Seoul Center, Korea Basic Science Institute, 150 Bugahyun-ro, Seoudaemun-gu, Seoul, 03759, Republic of Korea. E-mail address: [email protected] (H.-J. Lee). https://doi.org/10.1016/j.chemosphere.2018.03.046 0045-6535/© 2018 Elsevier Ltd. All rights reserved.

M. Mubarak et al. / Chemosphere 201 (2018) 676e686

1. Introduction The supply of safe and clean drinking water is essential to humans and other lifeforms. Water pollution mainly occurs when pollutants are directly or indirectly discharged into bodies of water without adequate treatment. The contaminants accumulated from industrial effluents and agricultural wastes contain health hazardous chemicals, such as heavy metals and organic pollutants, which pose serious risks to human health and ecological systems. In particular, arsenic and chromium are heavy metal ions that are highly toxic to the human body, and long-term exposure to arsenic and chromium are responsible for skin, liver and lung cancer, kidney damage and anaemia (Naujokas et al., 2013; Kieber et al., 2002; Islam et al., 2017). Due to their serious virulence, the WHO (World Health Organization) defines the acceptable level as 10 ppb for the maximum concentration of arsenic and 50 ppb for chromium in safe drinking water (USEPA, 2003; WHO, 2008). In recent years, many conventional techniques have been developed for the removal of heavy metal ions from wastewater, including adsorption, chemical precipitation, chemical redox reactions, electrochemical treatments, membrane processes, and ion exchange (Islam et al., 2017; Tchobanoglous et al., 2003; AlShannag et al., 2015; Admassie et al., 2015; Sounthararajah et al., 2015; Vijayakumar et al., 2015). Among the aforementioned methods, adsorption is considered to be one of the most economical and effective techniques owing to its simplicity, ease of operation and cost effectiveness (Wen et al., 2017). This technique is easy to operate and equally effective in the removal of toxic pollutants, even at low concentrations. Since the effectiveness and efficiency is the core of the adsorption technique, high surface area and active adsorption sites are necessary for the adsorbents (Zeng et al., 2015; Guo et al., 2014). Layered double hydroxides (LDHs, [M2þ1xM3þx(OH)2]xþ[(An)x/ x n] $mH2O) in the form of anionic clays have attracted increasing attention for the adsorption of anionic inorganic and organic pollutants thanks to their layered structure, high surface area, porous structure and interlayer ion exchange (Abellan et al., 2015; Zubair et al., 2017). Easily prepared LDHs, such as MgAl-LDH, CaFe-LDH, and ZnAl-LDH, have been widely applied as adsorbents for various organic dyes and heavy metal ions due to their high adsorption capacity, low-cost and non-toxicity (Shan et al., 2015; Wu et al., 2012; Li et al., 2014). In particular, spherical LDH microparticles with porous structures have attracted significant attention for their structural stability and high surface area, which are essential factors enhancing their removal capabilities for water pollutants (Li et al., 2014; Sun et al., 2015; Lei et al., 2017a, 2017b; Lin et al., 2015). To control the morphology and porosity in the LDH structure, hydrothermal methods have been employed in aqueous media using surfactants (Sun et al., 2015), sacrificial templates or urea as a precipitating agent (Li et al., 2014; Lei et al., 2017a, 2017b; Lin et al., 2015). The porous LDH particles have been applied as adsorbents for anionic organic dyes and heavy metal ions. Despite enhancing the adsorption efficiency for pollutants, several considerable problems remain, such as the toxicity of the surfactants, multiple steps for the synthesis and the performance limit for heavy metal removal. Therefore, developing a simple, nontoxic, low-cost synthetic strategy with some unique features towards excellent remediation performance is still a great challenge. In this work, Mg/Fe-LDH hollow nanospheres with high specific surface area were synthesized by a simple ethylene glycolmediated thermal method using only two metal precursors, Mg2þ and Fe3þ. After calcination at 400  C for 1 h, the Mg/Fe-LDH was oxidized to Mg/Fe layered double oxide (LDO) retaining the hollow sphere shape, which could rapidly purify water contaminated by heavy metals to drinking water standards. In addition to the heavy

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metal adsorption tests, excellent catalytic performance for the reduction of 4-nitophenol by introducing Au nanoparticles into the LDO nanosphere structure was also observed. 2. Materials and methods 2.1. Materials Magnesium acetate tetrahydrate (Mg(OAc)2$4H2O, 98%), iron (III) chloride hexahydrate (FeCl3$6H2O, 98%), sodium arsenate dibasic heptahydrate (Na2HasO4$7H2O, 99.99%), potassium dichromate (K2Cr2O7, 99.99%), ethylene glycol (C2H6O2, 99.9%), iron (III) sulfate hydrate (Fe2(SO3)3$xH2O, 97%), chloroauric acid (HAuCl4, 99.99%), sodium borohydrate (NaBH4), 4-nitrophenol, sodium chloride (NaCl, 99.5%), sodium carbonate monohydrate (Na2CO3$H2O, 99.5%), potassium phosphate (KH2PO4, 99%), sodium sulfate (Na2SO4, 99%), and calcium nitrate tetrahydrate (Ca(NO3)2$4H2O, 99%) were purchased from Sigma-Aldrich. All chemicals were of analytical grade and used without further purification. 2.2. Synthesis Mg/Fe-LDH hollow nanospheres were synthesized by a simple one-step thermal method. First, Mg(OAc)2 (1 g, 4.7 mmol) was dissolved in 40 mL of ethylene glycol. Separately, FeCl3 (0.095 g, 0.35 mmol) was dissolved in 10 mL of ethylene glycol and added dropwise to the Mg(OAc)2 solution under stirring at room temperature. After stirring for 4 h, the solution was transferred to a Teflon-lined autoclave bomb and heated at 200  C in an electric oven for 8 h. The final product was cooled to room temperature and then washed three times with ethanol and DI water. The white MFLDH hollow nanospheres were obtained after drying overnight at 80  C. The MF-LDO hollow nanospheres were obtained by heat treatment of MF-LDH at 400  C for 1 h in a muffle furnace. Au nanoparticle (AuNP) embedded-MF-LDO (Au-MF-LDO) was prepared to test the catalytic ability for the reduction of 4nitrophenol to 4-aminophenol. MF-LDO nanospheres (20 g) were dispersed in 20 mL of DI water by ultrasonic treatment for 5 min. An aqueous HAuCl4 solution (10 mL, 1 mg mL1) was added dropwise to the MF-LDO suspension under stirring, and then NaBH4 (0.1 mL, 10 mM) was added to the solution. After further stirring for 10 min, the Au-MF-LDO was collected by centrifugation and washed with DI water three times. 2.3. Characterization The MF-LDH and MF-LDO hollow nanospheres were comprehensively characterized using field emission scanning electron microscopy (FE-SEM, JSM-6700F, JEOL) and field emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL). Powder Xray diffraction (XRD) analysis was carried out using a Rigaku D/ Max-2000/PC diffractometer (Cu Ka radiation, 298K). The BrunauerEmmetTeller (BET) surface areas and the BarretJoynerHalanda (BJH) pore-size distributions were measured using an accelerated surface area and porosimetry system (Micromeritics ASAP 2010, USA). Fourier transform infrared (FT-IR) spectra were recorded on a Varian 800 FT-IR instrument (Scimitar Series), and X-ray photoelectron spectroscopy (XPS) studies were performed using a K-ALPHAþ (Thermo Scientific. UK) system with an aluminum anode (Al Ka, 1486.6 eV) at 12 kV and 72 W. Thermogravimetric analysis (TGA) was performed using a TGA N-1000 (Scinco, Korea) at a heating rate of 5  C/min under flowing nitrogen, and z-potential measurements were performed on a Zeta-potential & particle size Analyzer ELSZ-2000 (Otsuka Electronics) at room temperature in water. UVevis absorption spectra were recorded on

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a UVeviseNIR spectrophotometer (Shimadzu UV-3600). The heavy metal ion concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS, ICP-MS 7700, Agilent). 2.4. Adsorption of heavy metal ions As(V) and Cr(VI) adsorption performance was studied by batchtype adsorption experiments. Aqueous solutions with different concentrations (10e300 ppm) of As(V) and Cr(VI) were prepared using Na2HAsO4$7H2O and K2Cr2O7, respectively. For the adsorption kinetics study, 20 mg of the hollow nanosphere adsorbents were added to a heavy metal solution (10 ppm, 20 mL), and the mixture was sonicated for a specified amount of time at room temperature. After a given time interval, a 2 mL aliquot was collected, and the adsorbent was collected by centrifugation at 14,000 rpm for 4 min. The concentration of heavy metal ions in the supernatant was determined using ICP-MS spectroscopy. The heavy metal adsorption capacity (qe, mg g1) and efficiency (%) was calculated using the following equations: qe (mg g1) ¼ (CoCe) V/m

(1)

efficiency (%) ¼ (CoCe)/Co  100%

(2)

where Co and Ce are the initial and equilibrium concentrations of the heavy metal ion (mg mL1), respectively. V is the solution volume (mL), and m is the mass of adsorbent (g). 2.5. Heavy metal ion adsorption from simulated wastewater (SWW) SWW was prepared by dissolving NaCl, Na2CO3, KH2PO4, Na2SO4 and Ca(NO3)2 in 100 mL of DI water. The concentrations of these salts were 10 ppm. The heavy metal ions (As(V) or Cr(VI): 10e40 ppm) and MF-LDO hollow nanospheres (100 mg) were sequentially added to the SWW solution, and the resulting mixture was sonicated for 20 min at room temperature. The adsorbent was collected by centrifugation, and the supernatant was analysed using ICP-MS to determine the heavy metal concentration. 2.6. Catalytic reduction of 4-nitrophenol The catalytic reduction of 4-NPh by Au-MF-LDOs was studied by UVevis absorption spectroscopy; 2 mL of NaBH4 solution (10 mM), 0.5 mL of 4-nitrophenol (0.4 mM) and 0.5 mL of DI water were mixed in a UV quartz cell (1  1 cm2). Au-MF-LDO hollow nanospheres (1 mg) were added to the mixture in the cell and then mixed well using a micropipette. The absorption spectra were recorded at constant time intervals in the spectral range of 200e600 nm at room temperature. 3. Results and discussion 3.1. Synthesis of MF-LDH and MF-LDO hollow nanosphere The preparation strategy for constructing the Mg/Fe-LDH (MFLDH) and the thermally oxidized MF-LDH (MF-LDO) nanospheres with hollow cores is shown in Fig. S1, which involves simple onestep thermal reaction between Mg(OAc)2 and FeCl3 in ethylene glycol as the solvent. The morphologies of the nanospheres were characterized by SEM and TEM (Fig. 1aed). As-synthesized MF-LDH nanospheres have a hollow structure with a 500 nm flower shape (Fig. 1a and b). The hollow flower structure is retained after calcination (Fig. 1c and d), which means that the MF-LDH nanospheres were thermally stable. Energy-dispersive X-ray (EDX)

measurements of the MF-LDO hollow nanospheres confirmed the existence of both the Mg and Fe in 25.68 at% and 2.19 at%, respectively, and the inset elemental mapping image shows that the Mg, Fe and O are homogeneously distributed in the hollow nanosphere structure (Fig. 1e). Large-scale electron micrographs of MF-LDH and MF-LDO samples exhibit homogeneous size distributions (Fig. S2), and the indicated sizes are much smaller compared to other LDH spheres, expecting high specific surface areas (Li et al., 2014; Sun et al., 2015; Lei et al., 2017a, 2017b; Lin et al., 2015). According to the control experiments, the solvent and the anions of the metal precursors have important roles in the formation of the hollow nanosphere shape of the LDH structure. When the ethanol and Fe2(SO3)3 were used as an alternative solvent and metal precursor, respectively, instead of ethylene glycol and FeCl3 under the same other reaction conditions, only irregular plates or sheets could be observed (Fig. S3). Meanwhile, when MgCl2 was used instead of Mg(OAc)2, no solid products were observed (data are not shown). In general, ethylene glycol is known as strong reducing agent, and metal acetate is known as both electrostatic stabilizer and alkali (Zhang et al., 2011; Deng et al., 2005). During the reaction, the former reduces Fe3þ to Fe2þ, which can be used as a dication composing LDH structure, and the latter contributes to the formation of spherical product acting both as an alkaline and as an electrostatic stabilizing agent to prevent particle agglomeration (Cheng et al., 2010; Wang et al., 2009). Time-dependent morphological evolution during the thermal reaction was monitored by SEM (Fig. S4). Aggregated nanoflakes were formed in the initial stage (1e2 h), and the nanoflakes started to formulate flat templates with hollow structures in the process of the reaction time, as other reported LDH spheres were formed (Li et al., 2014; Lin et al., 2015). Finally, well-defined hollow nanostructures could be formed when the reaction time was extended up to 8 h. 3.2. Characterization of MF-LDH and MF-LDO hollow nanosphere The phase structures of the hollow nanospheres were characterized by X-ray diffraction (XRD), and the diffraction patterns are displayed in Fig. 2a. The MF-LDH has 5 major peaks at 10.0 and 20.1, 35.8 , 42.2 and 60.0 indexed as the (003), (006), (012), (015) and (110) planes (Fig. 2aei). Relatively sharp and symmetrical peaks at lower 2q values compared to those at higher 2q values indicate that the MF-LDH hollow nanosphere has a hydrotalcitelike LDH crystal structure (Li et al., 2016). The interlayer distance (d003) of the MF-LDH sample was calculated to be 0.87 nm using Bragg's equation (d ¼ l/2sinq, l ¼ 1.54 nm). After calcination, the layered structure was destroyed and changed into metal oxide MFLDO (Fig. 2a-ii). The peaks at 36.8 , 42.9 and 62.2 were indexed to the (111), (200) and (220) planes of MgO (ICSD 98-010-1007), respectively. The characteristic peaks of iron oxide in the MF-LDO sample did not appear due to a low crystallinity. After the adsorption of As(V) and Cr(VI) ions, MF-LDO nanocrystals with a restored hydrotalcite crystal structure occurred by the memory effect, which is a distinctive phenomenon in LDH (Fig. 2a-iii and iv) (Cherepanova et al., 2015). The d003 spacing values of MF-LDO with adsorbed As(V) and Cr(VI) were increased to 0.98 nm and 0.99 nm, respectively. The FT-IR spectra of the MF-LDH and MF-LDO hollow nanospheres are displayed in Fig. 2b with the MF-LDO hollow nanospheres after the adsorption of As(V) and Cr(VI) ions. The broad adsorption bands at approximately 3426 cm1 and weak peaks at approximately 571 cm1 in all spectra were associated with the stretching vibration of the eOH groups in the surface or interlayer water molecules and metal oxide (M-O) vibration, respectively (Shan et al., 2015; Lei et al., 2017a). The MF-LDH hollow nanospheres showed further characteristic bands at 2854 cm1

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Fig. 1. SEM and TEM images of (a and b) MF-LDH and (c and d) MF-LDO hollow nanospheres. (e) EDX spectrum of MF-LDOs. Inset: EDX data and elemental mapping images of MFLDOs.

(stretching vibration of eCH3 groups), 1635 cm1 (CO 2 stretching vibration), 1373 cm1 (deformation vibration of eCH3 groups) and 1096 cm1 (CeO stretching vibration) from acetate anions (Fig. 2bei). After the calcination, the four acetate peaks disappeared, and the strong peaks at approximately 1451 cm1 appeared correspond to the stretching vibration of CO2 3 groups which might be generated by introducing CO2 into the layer structure during the air drying process (Fig. 2beii) (Yao et al., 2017). The weak peaks at approximately 1654 cm1 in MF-LDO after the adsorption of As(V) and Cr(VI) ions were attributed to the eOH bending vibration of water molecules which were introduced in the adsorption procedure (Fig. 2b-iii and iv) (Extremera et al., 2012). The N2 adsorptiondesorption isotherms of the resulting MFLDH and MF-LDO samples exhibited typical type-IV hysteresis, indicating a mesoporous structure (Fig. 2c). The BrunauerEmmettTeller (BET) specific surface area and BarrettJoynerHalenda (BJH) pore size of the MF-LDH sample were determined to be 154.03 m2 g1 and 14.46 nm, respectively. After calcination, both the BET surface area and BJH pore size were increased to 205.82 m2 g1 and 17.08 nm, respectively, in the MFLDO sample, indicating removal of organic acetate anion groups and solvent present in the internal structure of MF-LDH. The removal of organic groups in MF-LDO hollow nanosphere structures was further confirmed by thermogravimetric analyses

(TGA), and the result is displayed in Fig. 2d for comparison with that of the MF-LDH hollow nanosphere. Both the nanospheres were thermally stable up to 300  C, showing little weight loss at approximately 100  C and 200  C due to the elimination of interparticle pore water and the liberation of external surface water or gallery water, respectively (Constantino and Pinnavaia, 1995). The MF-LDH sample showed a dramatic weight loss at approximately 400  C, corresponding to the elimination of the acetate anion group incorporated in the layer structure to form the metal oxide. In contrast, the MF-LDO sample maintained the metal oxide structures after a little weight loss at approximately 350  C corresponding to dihydroxylation and decarbonation (Zhu et al., 2017). The surface composition and chemical status of MF-LDH and MF-LDO hollow nanospheres were investigated by XPS. The survey spectra in Fig. 3a illustrate that the hollow nanosphere adsorbents are mainly composed of C, O, Mg and Fe. The high resolution XPS spectra of Mg 2p, Fe 2p and C 1s for both compounds are displayed in Fig. 3bed. The Mg 2p spectra show two peaks at approximately 49 and 50 eV, which are ascribed to MgeOH and MgeO bonding, respectively (Fig. 3b) (Ardizzone et al., 1997). Based on the result of calculating the peak area, the relative MgeO content of the MF-LDO sample increased from 9.7% to 14.9%, confirming the formation of metal oxide after calcination. The Fe 2p spectra show two major peak groups at approximately 710 and 723 eV, which are assigned

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Fig. 2. (a) XRD patterns and (b) FT-IR spectra of MF-LDH (black line), MF-LDO before (red line) and after adsorption of As(V) (blue line) and Cr(VI) (green line). (c) N2 adsorptiondesorption isotherms and the corresponding pore size distribution (inset) of MF-LDH (black line) and MF-LDO (red line). (d) TGA curves of MF-LDH (black line) and MF-LDO (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

to Fe 2p3/2 and Fe 2p1/2, respectively, where each peak group can be deconvoluted into Fe3þ, Fe2þ and their corresponding satellites (Fig. 3c) (Yamashita and Hayes, 2008). The Fe 2p3/2 peak of the MFLDH sample can be deconvoluted into three peaks at 708.8 eV, 711.0 eV, and 714.6 eV, which correspond to the Fe2þ, Fe3þ, and Fe2þ satellite, respectively. The Fe 2p1/2 peak of the MF-LDH is also deconvoluted into three peaks at 722.6 eV, 726.9 eV, and 730.0 eV, which correspond to the Fe2þ, Fe3þ, and Fe2þ satellite, respectively. After calcination, intensities of both Fe3þ peaks of Fe 2p3/2 and Fe 2p1/2 increase, and the corresponding Fe3þ satellite peaks newly appear at 719.1 and 733.4 eV resulting from the formation of iron oxide species by the thermal oxidation reaction. The deconvoluted Fe 2p XPS data and the relative ratio of Fe3þ/Fe2þ calculated based on the peak areas are shown in Table S1. It clearly shows the ratio increasing from 0.34:1 to 1.9:1 for the MF-LDH and MF-LDO samples, respectively. The C 1s spectrum of the MF-LDH sample can be fitted to three peaks at 284.6, 286.1 and 288.0 eV. The main peak at 284.6 eV corresponds to CeC coordination of surface adventitious carbon. The peaks at 286.1 and 288.0 eV are assigned to CeO and C]O from the acetate anion (Cano et al., 2001). In the MF-LDO sample, the relatively strong peak at 286.2 and the broad peak at 289.3 eV correspond to the carbonate species generated by introducing CO2 into the layer structure during the air drying process, which agrees well with the result of the IR measurement (Lei et al., 2017a). The surface charge of the hollow nanospheres was investigated in DI water with various pH values using the zeta potential analyzer in order to determine the optimal heavy metal ion adsorption

conditions. Fig. S5 displays the results showing the surface charges of MF-LDOs under the various pH conditions along with the SEM images at the corresponding pH values. Based on the results, the maximum surface charge of the MF-LDO nanospheres was observed at pH ¼ 10 (31.69 mV), which was in the as-dispersed condition in water without adjusting the pH. Over the whole range of pH values, the surface charges showed positive charges except at pH ¼ 2. Meanwhile, the hollow nanosphere structure was most stably maintained at pH ¼ 10 (as-dispersed condition) compared to those under other pH conditions. Therefore, all the heavy metal adsorption tests were performed under the asdispersed pH condition by conventional batch-type experiments. 3.3. Adsorption kinetics Prior to performing the heavy metal adsorption tests, the mechanical stability of the MF-LDH adsorbents for the ultrasonic treatment was examined. According to the result, it was confirmed that the MF-LDH adsorbents well maintain their hollow flower structure even after sonication for 30 min, which means that the ultrasonic treatment does not affect the stability of the MF-LDH structure (Fig. S6). The adsorption of heavy metal ions, As(V) and Cr(VI), on both the hollow nanosphere adsorbents, MF-LDH and MF-LDO, as a function of contact time under the ultrasonic treatment was investigated using 10 ppm heavy metal ion solution and a 1 mg mL1 dosage of adsorbent, and the results are displayed in Fig. 4a, S7 and S8. The adsorption capacities and efficiencies were calculated by equations (1) and (2), respectively, based on the ICP-

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Fig. 3. (a) XPS survey spectra, and high resolution (b) Mg 2p, (c) Fe 2p and (d) C 1s spectra of MF-LDH (down) and MF-LDO (up).

MS results. The adsorption efficiencies of both the heavy metal ions reached more than 98% after 5 min, showing higher adsorption capacities for the Cr(VI) than for As(V). The MF-LDO adsorbent showed a higher adsorption rate than that of the MF-LDH adsorbent, which exhibited more than 97% removal of heavy metal ions by the adsorbent within 1 min, and it reached the drinking water level (WHO standards) after 20 min for As(V) and 10 min Cr(VI) (Fig. S7b and S8b). Observing adsorption kinetics is essential in adsorption studies because it provides a prediction of the adsorption rate and valuable data to understand the possible adsorption mechanism of the sorption reaction. The pseudo-first-order and pseudo-second-order models were applied to simulate the kinetic experimental parameters of the MF-LDH and MF-LDO adsorbents. The two kinetic rate equations can be described as follows (Ho et al., 2000):

lnðqe  qt Þ ¼ ln qe  k1 t

(3)

t 1 1 ¼ þ t qt k2 q2e qe

(4)

where qe (mg g1) and qt (mg g1) are the adsorption capacity at equilibrium and at contact time t (min), respectively, and k1 (min1) and k2 (g mg1 min1) are the equilibrium rate constants. The corresponding kinetic plots are presented in Fig. 4b and c, and the calculated kinetic model constants are summarized in Table 1. The calculated adsorption capacities (qe,cal) derived from the pseudo-

second-order kinetic model are closer to corresponding experimental values (qe,exp). Moreover, the correlation coefficient values (R2) close to 1 indicate that the adsorption for the heavy metal ions on two adsorbents can be well described with a pseudo-second order kinetic model, suggesting the adsorption involves chemisorption (Ma et al., 2016).

3.4. Adsorption isotherms Adsorption isotherms experiments were performed to investigate the adsorption behavior and determination of the maximum adsorption capacity of adsorbents. The isotherms of heavy metal ions on the MF-LDH and MF-LDO samples were determined using a 1 mg mL1 dosage of adsorbents in 10 mL of heavy metal ion solution with various concentrations (10e300 ppm) under ultrasonic treatment for 20 min at 25  C. The adsorption isotherm data were fitted by two equilibrium models, the Langmuir (1918) and Freundlich (1906) isothermal models, which are, respectively, given by the following linear equations:

Langmuir:

Ce 1 Ce ¼ þ qe qm KL qm

Freundlich: ln qe ¼

1 ln Ce þ ln KF n

(5)

(6)

where Ce (mg L1) is the equilibrium concentration of heavy metal

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Fig. 4. (a) Absorption capacities of heavy metal ions by MF-LDH (black) and MF-LDO (red) as a function of contact time. The linear kinetic plot for the heavy metal adsorption by (b) the pseudo-second-order kinetic model and (c) the pseudo-first-order kinetic model. Heavy metal ions: As(V) (solid lines and filled dots) and Cr(VI) (dashed lines and empty dots), Dose of adsorbents ¼ 1 mg mL1, Co (heavy metal ions) ¼ 10 ppm, reaction time ¼ 1e30 min, T ¼ 298 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Table 1 Kinetic parameters of the pseudo-second-order and pseudo-first-order models for heavy metal ion removal by the adsorbents (1 mg mL1). Co (heavy metal ions) ¼ 10 ppm, reaction time ¼ 1e30 min, T ¼ 298 K. Adsorbents

MF-LDO MF-LDH

Heavy metals

As(V) Cr(VI) As(V) Cr(VI)

qe,

exp

6.823 8.317 6.812 8.251

(mg g1)

Pseudo-second-order k2 (g mg1 min1)

R2

qe,cal (mg g1)

k1 (min1)

R2

6.835 8.333 6.840 8.264

2.744 2.483 1.069 1.926

0.999 0.999 0.999 0.999

0.506 0.766 1.828 1.023

0.289 0.364 0.259 0.215

0.734 0.805 0.808 0.689

ions; qe (mg g1) is the amount of heavy metal ions adsorbed on the adsorbent; qm (mg g1) is the maximum adsorption capacity of adsorbents and KL is the Langmuir constant, which relates to the enthalpy of sorption. KF and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The adsorption isotherms of As(V) and Cr(VI) plotted by the two linear fitting models are presented in Fig. 5, and the calculated isothermal parameters are tabulated in Table 2. According to the correlation coefficient values (R2), the Langmuir isotherm model is more suitable to describe the adsorption of the heavy metals than the Freundlich isotherm model, indicating that the surface of the adsorbents is homogeneous for the adsorption mechanism of monolayer uptake. In addition, for the Freundlich constants, a value of n higher than 1 suggests the good affinity of the adsorbents for both As(V) and Cr(VI). The essential feature of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, RL which is used to characterize the favorability of the adsorption process (Malkoc and Nuhoglu, 2007). The RL is defined by the following equation:

RL ¼

1 1 þ KL Co

Pseudo-first-order

qe,cal (mg g1)

(7)

where KL is the Langmuir constant and Co is the initial concentration of the heavy metal ions. The value of RL indicates the type of the isotherm: favorable (0 < RL < 1), linear (RL ¼ 1), unfavorable (RL > 1), or irreversible (RL ¼ 0). According to the calculation, RL for As(V) and Cr(VI) adsorption on the MF-LDH ranged from 0.0066 to 0.1617 and 0.0121 to 0.2256, respectively, and on the MF-LDO ranged from 0.0025 to 0.0694 and 0.0079 to 0.1589, respectively. This is for initial concentration of 10e300 ppm of the heavy metal ions. Therefore, the adsorption processes belong to favorable adsorption. According to the Langmuir equation fitting data, the maximum adsorption capacity (qmax) values of As(V) and Cr(VI) on MF-LDO

were 181.82 and 136.99 mg g1, respectively, which were slightly higher values than those on MF-LDH at the same equilibrium concentration of heavy metal ions. These values are in accordance with the experimental values. The adsorption capacities for heavy metal ions, As(V) and Cr(VI), were compared with another LDHbased adsorbent in Table 3. It clearly shows that the MF-LDH or eLDO hollow nanospheres have higher adsorption capacities than other LDH or LDO materials reported so far (Lei et al., 2017a; Li et al., 2016; Deng et al., 2015, 2017; Lu et al., 2016; Hong et al., 2014; Guo et al., 2012; Wen et al., 2013; Koilraj et al., 2016). Meanwhile, it was observed that the powdery MF-LDH adsorbent turned into a sticky gel after the heavy metal adsorption reaction, obstructing their reuse. In contrast, the MF-LDO adsorbent was stable in ambient conditions and well-maintained the hollow nanosphere shape even after the adsorption of As(V) and Cr(VI). Fig. S9 presents the morphologies of MF-LDOs observed by SEM, showing the preservation of the hollow nanosphere shape even after adsorption of heavy metal ions, and the corresponding EDX spectra clearly exhibited the presence of the heavy metal ions in the MF-LDO adsorbent. The adsorption of heavy metal ions was further confirmed by the XPS measurement. The XPS peaks at 44.8 eV in MF-LDO after the adsorption of As(V) (Fig. S10a), and at 576.3 and 586.2 eV after adsorption of Cr(VI) (Fig. S10b) were assigned to As 3d, Cr 2p3/2 and Cr 2p1/2, respectively, indicating the successful adsorption of heavy metal ions on the MF-LDO hollow nanosphere adsorbent. Due to the chemical instability of MF-LDH and the limitation for purification up to the drinking water level, only the MF-LDO hollow nanospheres were employed for the further environmental applications. 3.5. Heavy metal ion adsorption from simulated wastewater (SWW) To investigate the potential of MF-LDO hollow nanospheres for purifying wastewater to produce drinking water, simulated wastewater (SWW) was prepared by dissolving various salts and

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683

Fig. 5. Adsorption isotherms of heavy metal ions on MF-LDH (black) and MF-LDO (red) fitted by (a) the Langmuir model and (b) the Freundlich model. Dose of adsorbents ¼ 1 mg mL1, Co (heavy metal ions) ¼ 10e300 ppm, reaction time ¼ 20 min, T ¼ 298 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Table 2 The parameters for the Langmuir and Freundlich models of heavy metal ion adsorption on the adsorbents. Dose of adsorbents ¼ 1 mg mL1, Co (heavy metal ions) ¼ 10e300 ppm, reaction time ¼ 20 min, T ¼ 298 K. Adsorbents

MF-LDO MF-LDH

Heavy metals

As(V) Cr(VI) As(V) Cr(VI)

qe,

exp

(mg g1)

Langmuir

178.6 148.7 167.7 128.7

Freundlich

qm,cal (mg g1)

kL (L mg1)

R2

kF (mg g1)

n

R2

181.82 136.99 166.67 129.87

1.964 0.636 0.760 0.412

0.9972 0.9952 0.9881 0.9600

67.46 39.89 44.72 32.04

2.612 3.600 2.449 2.654

0.9408 0.9199 0.9767 0.9562

Table 3 Comparison of the maximum heavy metal ion removal capacities with other LDH-based adsorbents. Adsorbents

MF-LDH nanosphere MF-LDO nanosphere NiMgAl-LDO NiFe2O4/ZnAl-EDTA LDH CoFeO4/MgAl-LDH NiFe-LDH MgAl-LDH nanosheet MgFe-Alanine-LDH Cu/Mg/Fe/La-LDH Graphene oxide-Mg/Al-LDH Colloidal Mg2Al-LDH nanosheet

qmax (mg g1)

Adsorption conditions

References

As (V)

Cr (VI)

Contact time

Dose of adsorbent

T (K)

167.7 178.6 e e e e e 23.6 43.5 180.3 90.6

128.7 148.7 103.4 64.28 72.4 26.78 63.8 e e e e

20 min 20 min 24 h 2h e e 2h 24 h 8h 24 h 5h

1 g L1 1 g L1 0.5 g L1 2 g L1 3 g L1 0.2 g L1 1.6 g L1 0.2 g L1 0.2 g L1 0.5 g L1 0.78 g L1

298

heavy metal ions in deionized water (Islam et al., 2017; Bansal et al., 2014). In general, the ions that commonly exist in natural waters and wastewater might compete with heavy metal ions for available adsorbent binding sites, affecting the adsorption process (Wang et al., 2014). Therefore, the influence of common ions in water, 2 2  such as Naþ, Kþ, Ca2þ, Cl, CO2 3 , PO4 , SO4 , and NO3 , was evaluated for heavy metal removal by the MF-LDO adsorbent. Fig. 6 shows the results that the adsorption efficiencies of the MF-LDO adsorbent were not influenced by the presence of other ions, regardless of the heavy metal ion concentration. Therefore, is the adsorbent exhibited excellent adsorption performance, with more than 99% removal for both heavy metal ions, except for 40 ppm of Cr(VI), even in the presence of other ions. In particular, it was observed that the remaining concentration of the heavy metal ions using Co ¼ 10 ppm could be used to meet the maximum contaminant level requirements recommended by WHO (Table S2). Thus, MF-LDO hollow nanospheres are favorable adsorbents for

303 298 283 e e 298 e e 298

This work This work (Lei et al., 2017a) (Deng et al., 2017) (Deng et al., 2015) (Lu et al., 2016) (Li et al., 2016) (Hong et al., 2014) (Guo et al., 2012) (Wen et al., 2013) (Koilraj et al., 2016)

wastewater treatment and drinking water purification. 3.6. Recycle test of MF-LDOs material From an economic point of view, reuse of an adsorbent is very important. The reuse test of MF-LDOs was investigated using a 10 ppm solution of As(V) and Cr(VI). The heavy metal-adsorbed MFLDOs were gently dispersed in aqueous solution, washed with DI water three times, and then dried at 120  C for the next use. The adsorption-desorption cycles were repeated five times. Although the removal efficiency decreased by 7.2% over the five cycles for Cr(VI), it was still greater than 99% for As(V), indicating that the adsorbent showed good reusability and regeneration (Fig. 7). Most of all, the remaining concentration of the As(V) after five uses was 8 ppm, which is the drinking water level recommended by WHO. An SEM image of the adsorbent after the fifth cycle shows that the structure retains the hollow nanosphere shape with slight

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M. Mubarak et al. / Chemosphere 201 (2018) 676e686

deformation due to multiple uses (Fig. S11). 3.7. Catalytic test In another approach for environmental remediation, we also tested the hollow nanosphere adsorbents as catalysts for the efficient decomposition of toxic organic dyes. As a model system, the reduction of 4-nitrophenol (4-NPh), which is a toxic material generated from industrial wastewater, to 4-aminophenol (4-APh) was chosen. Since MF-LDO hollow nanospheres exhibit a highly positive charge in aqueous solution, a precursor of Au nanoparticles, [AuCl4], could easily bound to its surfaces. After reduction using NaBH4, an Au nanoparticle (AuNP) coated MF-LDO (AuMF-LDO) catalyst was successfully synthesized. Fig. S12 displays a TEM image and EDX spectrum confirming that the AuNPs are well distributed in the hollow nanosphere structure of the MF-LDOs. After addition of the catalyst into a 4-NPh solution, the conversion rates of the reaction were monitored using UVevis absorption spectroscopy. For comparison, the MF-LDO and MF-LDH samples were also used for the catalytic reduction tests. It is known that 4NPh in an aqueous solution containing NaBH4, shows a maximum absorption peak at approximately 400 nm resulting from the formation of the 4-nitrophenolate ion (Liu et al., 2006; Rahim et al., 2012; Sunkari et al., 2017). In the presence of proper catalysts such as AuNPs and AgNPs, the 4-NPh is reduced to 4-APh and the absorption peak at 400 nm disappears, while a new peak appears at around 300 nm corresponding to the formation of 4-APh (Fig. S13a). It has been proposed that the catalytic reduction of 4NPh with NaBH4 proceeded in two steps: (1) diffusion and adsorption of 4-NPh to the AuNP surface and (2) electron transfer mediated by the AuNP surface from BH 4 to 4-NPh (Hayakawa et al., 2003). A strong nucleophile such as BH 4 because of its diffusive nature and high electron injection capability transfers electrons to the substrates via metal particles. This helps to overcome the kinetic barrier of the reaction (Layek et al., 2012). In case of the AuMF-LDO hollow nanospheres, the reduction of 4-NPh was completed with the complete disappearance of the 4nitrophenolate peak at 400 nm in 5 min which was the highest

Fig. 6. Comparative As(V) (black) and Cr(VI) (red) adsorption efficiencies by MF-LDO for different heavy metal concentrations in water (filled bars) and simulated wastewater (patterned bars). The wastewater contained certain amounts of Kþ, Naþ, Ca2þ, 1 2 3 Cl, CO2 3 , SO4 , and PO4 . Dose of MF-LDO ¼ 1 mg mL , Co (heavy metal ions) ¼ 10e40 ppm, concentration of ionic salts ¼ 10 ppm, reaction time ¼ 20 min, T ¼ 298 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7. Recycle ability of MF-LDO for As(V) (black bars) and Cr(VI) (red bars). Dose of adsorbents ¼ 1 mg mL1, Co (heavy metal ions) ¼ 10 ppm, each reaction time ¼ 20 min, T ¼ 298 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

value (Fig. 8 and S13a). While, when LDH materials not containing AuNPs was used as a catalyst, the reaction did not proceed within the optimized time frame (Fig. S13b-d). This clearly proves that AuNPs deposited on MF-LDO are an efficient catalyst for 4-NPh reduction; the surface of the AuNPs is used as the location where the catalytic reduction process occurs. The BH 4 binds to the surface of the catalyst and 4-NPh also adsorbs to the surface. Consequently, the 4-NPh is reduced by BH 4 to 4-APh. Interestingly, the MF-LDO sample also exhibits a good catalytic reduction property, showing a >75% conversion rate after 20 min (Fig. S13b). This could be explained by the iron oxide generated during heat treatment for the synthesis of MF-LDO from MF-LDH taking part in the catalytic activity (Naruse et al., 1980). On the other hand, the MF-LDH sample did not show any catalytic activity until 1 h (Fig. S12c). To confirm that the iron oxide present in the MF-LDO sample was involved in the catalytic reaction, magnesium oxide (MgO) was prepared following the same procedure with the synthetic method for MFLDO in the absence of FeCl3, the precursor of iron oxide, and used to test 4-NPh reduction. The MgO also did not show any catalytic activity until 1 h (Fig. S12d). In order to investigate the effect of catalytic reduction of 4-NPh on the content of AuNPs loaded on MF-LDO, we prepared Au0.5-MFLDO and Au2.0-MF-LDO using 0.5 mg mL1 and 2.0 mg mL1 of AuCl 4 , respectively, and compared their catalytic performances to that of Au1.0-MF-LDO using 1.0 mg mL1 of AuCl 4 (Fig. S14). According the result, in case of Au0.5-MF-LDO the initial catalytic reduction rate was faster than that of Au1.0-MF-LDO, however the catalytic reduction was not completed until 10 min. On the other hand, Au2.0-MF-LDO completed the reduction within 5 min similar to Au1.0-MF-LDO. It indicates that 1.0 mg mL1 of AuCl 4 is sufficient concentration to prepare Au-MF-LDO catalyst showing maximum catalytic activity for reduction of 4-NPh to 4-APh. From the overall results, it can be concluded that the Au-MF-LDO sample has an advantage to significantly enhance the conversion rate due to the synergetic effect between iron oxide and the AuNPs present in the MF-LDO nanosphere network. 4. Conclusion In summary, Mg/Fe layered double hydroxide (MF-LDH) hollow nanospheres were prepared by a simple thermal method. After the

M. Mubarak et al. / Chemosphere 201 (2018) 676e686

Fig. 8. Catalytic reduction rate of 4-NPh to 4-APh by MF-LDO with (red line) and without (blue line) AuNPs, MF-LDH (black line) and MgO (green line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

calcination at 400  C, the MF-LDH was converted into the corresponding oxide, Mg/Fe layered double oxide (MF-LDO), retaining the hollow nanosphere structure. The MF-LDO nanospheres showed excellent removal efficiency for both As(V) and Cr(VI) ions, with maximum adsorption capacities of 178.6 mg g1 [As(VI)] and 148.7 mg g1 [Cr(VI)], and complete heavy metal removal (~99.9%) from wastewater up to the drinking water level (WHO standards) was achieved in 20 min [As(V)] and 10 min [Cr(VI)]. Furthermore, the MF-LDO samples could be used as catalytic templates to introduce Au nanoparticles into the hollow sphere-structure and exhibited 100% conversion rate for the reduction of 4-nitrophenol into 4-aminophenol within 5 min. Considering the excellent and versatile properties integrated with a facile synthetic strategy to produce the MF-LDH-based nanospheres, this material should facilitate practical applications in cost-effective wastewater purification. Acknowledgement This research was supported by the National Research Council of Science and Technology through the Degree and Research Center Program (DRC-14-1-KBSI). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.chemosphere.2018.03.046. References Abellan, G., Marti-Gastaldo, C., Ribera, A., Coronado, E., 2015. Hybrid materials based on magnetic layered double hydroxides: a molecular perspective. Acc. Chem. Res. 48, 1601e1611. Admassie, S., Elfwing, A., Skallberg, A., Ingan€ as, O., 2015. Extracting metal ions from water with redox active biopolymer electrodes. Environ. Sci.: Water Res. Technol. 1, 326e331. Al-Shannag, M., Al-Qodah, Z., Bani-Melhem, K., Qtaishat, M.R., Alkasrawi, M., 2015. Heavy metal ions removal from metal plating wastewater using electro coagulation: kinetic study and process performance. Chem. Eng. J. 260, 749e756. Ardizzone, S., Bianchi, C.L., Fadoni, M., Vercelli, B., 1997. Magnesium salts and oxide: an XPS overview. Appl. Surf. Sci. 119, 253e259. Bansal, M., Mudhoo, A., Garg, V.K., Singh, D., 2014. Preparation and characterization of biosorbents and copper sequestration from simulated wastewater. Int. J.

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