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Mesoporous Magnesium Oxide Hollow Spheres as Superior Arsenite Adsorbent: Synthesis and Adsorption Behaviour Swasmi Purwajanti, Hongwei Zhang, Xiaodan Huang, Hao Song, Yannan Yang, Jun Zhang, Yuting Niu, Anand Kumar Meka, Owen Noonan, and Chengzhong Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08322 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Applied Materials & Interfaces

Mesoporous Magnesium Oxide Hollow Spheres as Superior Arsenite Adsorbent: Synthesis and Adsorption Behaviour Swasmi Purwajantia,b, Hongwei Zhanga, Xiaodan Huanga, Hao Songa, Yannan Yanga, Jun Zhanga, Yuting Niua, Anand Kumar Mekaa, Owen Noonana and Chengzhong Yua,* a

Australian Institute for Bioengineering and Nanotechnology, The University of Queensland,

Brisbane, QLD 4072, Australia. b

The Agency for Assessment and Application of Technology (BPPT), Indonesian Ministry of

Research, Technology and Higher Education, Jl. M.H. Thamrin No.8, Jakarta 10340, Indonesia KEYWORDS: arsenic removal, mesoporous, magnesium oxide, hollow structure, adsorption

ABSTRACT: Arsenic contamination in natural water has posed significant threat to global health due to its toxicity and carcinogenity. Adsorption technology is an easy and flexible method for arsenic removal with high efficiency. In this paper, we demonstrated the synthesis of mesoporous MgO hollow spheres (MgO-HS) and their application as high performance arsenite (As(III)) adsorbent. MgO-HS with uniform particle size (~180 nm), high specific surface area (175 m2 g-1), and distinguished mesopores (9.5 nm in size) have been prepared by hard-

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templating approach using mesoporous hollow carbon spheres as templates. An ultra-high maximum As(III) adsorption capacity (Qmax) of 892 mg g-1 was achieved in batch As(III) removal study. Adsorption kinetic study demonstrated that MgO-HS could enable As(III) adsorption 6 times faster commercial MgO adsorbent. The ultra-high adsorption capacity and faster adsorption kinetic were attributed to the unique structure and morphology of MgO-HS that enabled fast transformation into flower-like porous structure composed of ultrathin Mg(OH)2 nanosheets. This in situ formed structure provided abundant and highly accessible hydroxyl groups which enhanced the adsorption performance towards As(III). The outstanding As(III) removal capability of MgO-HS showed their great promise as high efficient adsorbent for As(III) sequestration from contaminated water.

Introduction Arsenic is one of the heavy metal pollutants which affects millions of people across the world. It may enter to groundwater through a combination of natural process such as weathering reactions, mineral dissolutions and biological activity as well as through anthropogenic activities such as mining, agriculture and manufacturing.1-2 Long-term exposure of arsenic-contaminated water may result in severe diseases such as lungs, bladder, kidney and skin cancer.3 Of the many approaches to remove arsenic, adsorption is considered to be an economically favorable, technically feasible and also socially acceptable technique.4 In natural water, inorganic arsenic is mostly found as trivalent arsenite As(III) or pentavalent arsenate As(V). Compared to As(V), As (III) is more toxic than As(V) since it can form a strong bond with the building block of protein which results in biological disruption.5 As(III) is also more difficult to remove owing to its weaker affinity to the surface of adsorbent.


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A wide range of nanostructured adsorbents have been developed for arsenite adsorption such as alumina,6 ZrO2,7

CuO,8 carbon nanospheres as well as various composites, including

Fe3O4/graphene, lanthanum-alumina,9 Fe/cerium alkoxide,10 γ-Fe2O3/SiO2,11 and Fe2O3/sepiolite fibers.12 However these adsorbents show relatively low performance towards As(III) adsorption. MgO nanomaterials have recently been identified as a new class of arsenite adsorbents with enhanced performances over aforementioned materials.13 Various MgO nanomaterials have been prepared for arsenite adsorption, including nanoporous microspheres,14 nanocrystal,15 nanoflowers,16 and nanoflakes,17 generally showing high As(III) adsorption capacities in the range of ~ 500-800 mg g-1. It has been reported that the As(III) removal mechanism of MgO is dependent on the entire or partial in situ formation of Mg(OH)2 during interacting with water.1517

But the as-prepared Mg(OH)2 nanomaterial was found to be six times less effective than its

oxide counterpart in terms of adsorption capacity, because of it low surface area and less activity comparing to in situ formation of Mg(OH)2 nanostructures.17 Hollow structured materials have been identified as one type of promising materials for adsorption application since the unique structure provides an enhanced surface-to-volume ratio and reduced transport length.18-19 It is expected that such morphology is beneficial in further enhancing the arsenite adsorption performance. Compared to other methods for hollow/porous structure preparation, hard-templating method offers several advantages such as easy structure control and straightforward morphology replication.20-21 Previously hard-templating approach has been reported to synthesize MgO hollow structures, using porous hollow carbon microspheres or solid carbon microspheres as templates.22-23 However, in those preparations, expensive metal alkoxide precursor and precipitating agents were involved and only micron size hollow spheres with very low surface area were obtained.


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In this study, we report a hard-templating approach using mesoporous hollow carbon spheres (MCHS) as templates to synthesize mesoporous MgO hollow spheres (MgO-HS). The carbon spheres are advantageous when used as templates for MgO-HS since they can be easily removed by combustion in air during which the hollow structures of MgO are formed simultaneously.24 The synthesized mesoporous MgO hollow spheres were applied as adsorbent to remove As(III) in model aqueous solutions, and demonstrated high maximum adsorption capacity, fast removal rate, and high efficiency at low arsenite concentrations. A new understanding of the relationship between the mesoporous hollow morphology, structure changing during adsorption process and the adsorption performance has been revealed, which is useful for the rational design of highly efficient adsorbents for As(III) removal.

Scheme 1. A schematic illustration of synthesis of MgO-HS using MCHS as templates (Step I) and the proposed structure change during adsorption process (Step II). Experimental Section Materials All chemicals were used as received without further purification. Milli-Q water (18.2 MΩ) was used for solution preparation and synthesis. Magnesium nitrate hexahydrates (Mg(NO3)2.6H2O) is supplied from Chem-supply chemicals. Iron nitrate nonahydrates (Fe(NO3)3.9H2O), Aluminum nitrate hexahydrates (Al(NO3)3.6H2O), resorcinol, formaldehyde


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37% weight, tetraethyl orthosilicate (TEOS) and tetrapropyl orthosilicate (TPOS), NH4OH 25% weight and NaAsO2 were obtained from Sigma Aldrich. The 400 mg L-1 As (III) stock solution was prepared by dissolving a proper amount of NaAsO2 in Milli-Q water. Synthesis of particles Mesoporous carbon hollow spheres (MCHS) were prepared via synthesized via a one-pot surfactant-free synthesis under the Stöber conditions, as reported by Zhang et.al.25 The typical procedure for the synthesis of MCHS was as follow. First, TPOS (3.46 ml, 12 mmol) was added to the solution containing ethanol (70 ml), H2O (10 ml) and NH3H2O (3 ml, 25 wt%) under stirring at room temperature. Fifteen minutes later, resorcinol (0.4 g) and formaldehyde (0.56 ml, 37 wt %) were added to the solution and the system was kept stirring for 24 hours. The obtained solid precipitates were separated by centrifugation, washed with water and ethanol, and dried at 50 °C overnight. Final MCHS was obtained after carbonization at 700 °C under N2 for 5 hours and removal of silica by hydrofluoric acid (HF, 5 wt %). Meanwhile microporous carbon hollow spheres (MCHS-micropores) was prepared using TEOS with total silicon amount of 12 mmol while the other conditions were kept unchanged. Mesoporous metal oxides hollow spheres were prepared through a wet impregnation approach using MCHS as templates. For the synthesis of MgO-HS, specifically, Mg(NO3)2.6H2O was dissolved in milli-Q water to form precursors solution with a concentration of 1.5 M. MCHS (100 mg) was added to the precursor solution (10 ml) and the mixture was sonicated for 1h and kept stirring for 24 h at room temperature. The MCHS-Mg(NO3)2 samples were then separated by centrifugation and dried overnight at 50 °C, followed by calcination at 500 °C in air for 3 h to obtain MgO-HS. Other metal oxides hollow spheres such as Fe2O3 and Al2O3 were synthesized through the similar route, except that Fe(NO3)3.9H2O and Al(NO3)3.6H2O were employed as the metal salts precursors.


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Arsenic adsorption test Batch adsorption of As(III) was carried out in a shaker incubator to disperse adsorbents to ensure them have a good contact with arsenic contamination. NaAsO2 was chosen as the source of As(III). In order to evaluate the adsorption properties of the adsorbent in practical water treatment applications, in the arsenite adsorption isotherm experiment and kinetics study, there was no pH adjustment prior to the adsorption process.26-28 Adsorbents loading of 0.4 g L-1 were used for both isotherm and kinetic study. Typically, 4 mg of MgO was dispersed in 10 mL of As(III) solution with variable concentrations (50 − 400 mg L-1), followed by shaking (21 rad s-1) at room temperature for 24 h to achieve equilibrium state. The supernatant was obtained after recovering the adsorbent by centrifugation at 492 rad s-1 for 10 min and proper dilution to the particular concentration range. Concentrated HNO3 was added into the supernatant (5% by volume) to preserve arsenic species. The As(III) concentration in the supernatant was analyzed by inductively coupled plasma-optical emission spectrophotometry (ICP-EOS) — Perkin Elmer Optima 7300DV — with arsenic detection limits between 1-10 µg L-1. Adsorption experiments were carried out in triplicates and the arsenite adsorption performance data shown in this study were the average results with relative standard deviations < 5%. The

As(III)

adsorption

capacity

was

ܳ݁ =

calculated

using

the

following

(஼బ ି஼೐ )௏ ௠

equation: (1)

where C0 and Ce represent the initial and the equilibrium As(III) concentration (mg L-1), respectively. V is the volume of the As(III) solution (L) and m is the amount of adsorbent (g). Qe is the adsorption capacity at equilibrium (mg g-1). The adsorption isotherm data were fitted with the Langmuir model. The equation is:


6

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஼೐ ொ೐

ଵ

஼

೐ = ொ௠௔௫௄ + ொ௠௔௫

(2)

ಽ

where Qmax and KL are the Langmuir constant, representing the maximum adsorption amount of adsorbents (mg g-1) and the energy of adsorption, respectively. The kinetic data can be fitted to a pseudo second-order kinetic model, the equation is: ௗொ೟ ௗ௧

= ݇(ܳ௘ − ܳ௧ )ଶ

(3)

where Qt is the amount adsorbed at a contact time t (mg g-1), and k is the pseudo second-order rate constant (g mg-1 min-1). Nonlinear least-squares regression analysis was applied to acquire the best estimation of all constants for all the models in arsenic adsorption test. Determination of point of zero charge Four milligram of MgO-HS was added into different vials containing 10 ml of 0.1 M NaCl solution with the initial pH ranging from 6-12. The solution initial pH was adjusted by addition of HCl and NaOH. Then these vials were placed on a platform shaker for 24h at 25 °C to reach equilibrium. After 24 h, the final pH of the suspension was measured. Point zero charge (pzc) curve was obtained by plotting the different between initial pH and final pH versus initial pH. The point at which the curve crossed pH axis was determined as the pH pzc of the MgO-HS.29 Characterizations Field emission scanning electron microscope (FESEM, JEOL 7001) operated at 15 kV and transmission electron microscope (TEM, JEOL 2100) at 200 kV were used to investigate the morphology and structure of the samples. X-ray diffraction (XRD) patterns were performed on Bruker D8 Advanced X-Ray Diffractometer with Cu Kα radiation (λ=0.154 nm). X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis ULTRA X-ray photoelectron


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spectrometer using a monochromatic Al Kα (1486.6 eV) X-ray source and a 165 mm hemispherical electron energy analyzer. Nitrogen adsorption-desorption isotherms were measured at 77 K using a TriStar II Surface Area and Porosity analyzer (Micromeritics). The samples were degassed under vacuum at 180 °C for 6 hours before analysis. Thermo gravimetric analysis (TGA) was performed on a TGA/DSC1 STARe System in nitrogen (40−800 °C, 2 °C min-1). The electron tomogram specimens were prepared by dispersing the samples in ethanol followed by ultrasonication and then deposited the suspension directly onto copper grids (2000×1000 slot, Proscitech) with Formvar supporting films. Colloidal gold particles (10 nm) were deposited on both surfaces of the grid as fiducial markers for the subsequent image alignment procedures. The tomographic tilt series were carried out by tilting the specimen inside the microscope around double axis from +70° to -70° at an increment of 1° under the electron beam. Alignment and 3D reconstructions of MCHS was carried out on IMOD software. The thickness of Mg(OH)2 plates was recorded on an a Cypher S Atomic Force Microscope (AFM, Oxford Instruments Company). The image was obtained a Cypher S AFM (Oxford Instruments Company). Results and Discussion Physicochemical properties of the synthesized MgO-HS MCHS templates were prepared according to a method recently reported.25 MCHS possess a surface area of 912 m2 g-1, a pore size of 6.9 nm, a shell thickness of 45 nm and a diameter of 275 nm (Figure S1a). Afterwards, magnesium nitrate was incorporated into MCHS through a wet impregnation method followed by calcination in air (Scheme 1, Step I). From SEM and TEM images in Figure 1a and b, uniform mesoporous MgO-HS with an average particle size of 180


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nm and a shell thickness of 20 nm are obtained. The particle size reduction compared to that of MCHS is attributed to the thermal shrinkage during the calcination process.30 One electron tomogram (ET) slice further proves that the MgO-HS have a hollow cavity and a porous shell (Figure 1c), confirming that mesoporous MgO-HS are successfully synthesized.

Figure 1. SEM (a) and TEM (b) images, ET slice (c) and Nitrogen adsorption-desorption isotherm and pore size (d) of MgO-HS The porous shell of MgO-HS has a polycrystalline nature as evidenced by wide-angle XRD pattern (Figure S2). The nanocrystal size is calculated to be ~7 nm according to Scherrer equation. The diffraction peaks of MgO-HS can be assigned to magnesia (JCPDS 65-0476). From nitrogen sorption analysis (Figure 1d, Table 1), it is shown that MgO-HS possess a surface area of 175 m2 g-1, a pore volume of 0.83 cm3 g-1, and a Barrett-Joyner-Halenda (BJH) pore size


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centered at 9.5 nm calculated from the adsorption branch. The mesopores are probably from the packing voids of MgO nanocrystals in the wall. The use of MCHS with large pores and relatively thick carbon shells is key to the successful preparation of MgO-HS. When MCHS with microporous and thin shells were used as the template (Figure S1b-d), only MgO fragments rather than hollow structures can be obtained (Figure S3a, b). From N2 sorption analysis (Figure S3c, d and Table 1), this MgO-fragment has a relatively low surface area of 55 m2 g-1 and no obvious mesopores. To understand the possible reason, characterizations on impregnated carbon materials were conducted. From TEM image in Figure S4a-b, it is observed that there is no obvious change in the physical appearance of impregnated MCHS and MCHS-micropores compared to bare carbon materials before impregnation (FigureS1a-b). However, reduction of both surface area and pore volume after impregnation indicates Mg(NO3)2 has been successfully infiltrated in both templates (Table S1, Fig S1c-d and Figure S4c). From thermogravimetric analysis (TGA) under nitrogen, the impregnated MgO weight ratios in MCHS and MCHS-micropores were determined to be 75.2 and 59.2%, respectively (Figure S5, Table S2 and calculation method in supplementary data). The small amount of MgO precursor impregnated in MCHS-micropores due to its microporous pore structure and thin wall is not sufficient to support the formation of a rigid hollow structure after template removal. In addition, the hard-templating approach to synthesize various porous metal oxide hollow spheres using MCHS as templates is general and versatile. We have extended this approach to prepare other nanoporous metal oxide hollow spheres, such as Fe2O3 and Al2O3. From TEM images (Figure S6a and b), well-defined hollow spheres with nanoporous shells can be observed for both Fe2O3 and Al2O3.


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Table 1 Textural Properties and As(III) adsorption capacity of prepared MgO and commercial MgO adsorbent Surface Area (m2 g-1)

Pore Volume (cm3 g-1)

Pore Size (nm)

As(III) adsorption Capacity (mg g-1)

MgO-HS

175

0.83

9.5

732

MgO fragments

55

0.41

--

439

Commercial MgO

7

0.04

--

75

Sample

Arsenite adsorption The As(III) adsorption property of the prepared MgO-HS, MgO fragment and commercially available MgO powder were evaluated at an initial concentration of As(III) 400 mg L-1. MgO-HS show an adsorption capacity of 732 mg g-1 towards As(III), higher compared to MgO fragments (439 mg g-1), commercial MgO adsorbent (75 mg g-1, Table 1), and Fe2O3 and Al2O3 hollow spheres (54.7 and 66.5 mg g-1, respectively). We further used MgO-HS to carry out the equilibrium adsorption isotherm study by varying initial As(III) concentrations from 50 to 400 mg L-1. The Langmuir isotherm, derived with the assumption of monolayer adsorption, was used to model the adsorption isotherm of MgO-HS. The result shows that the Langmuir isotherm fitted the experimental data (R2 ≈ 0.98, Figure 2a, Table S3). The maximum As(III) adsorption capacity (Qmax) from the Langmuir model for MgO-HS was calculated to be 892 mg g-1, the highest among all previously reported As(III) adsorbents (Table S4).10,12,16-17,31-34 The adsorption kinetics of MgO-HS was also studied to investigate the adsorption efficiency. Figure 2b shows the time dependent adsorption profile of As(III) by MgO-HS, indicating that 80% As(III) was removed within 3 h. As comparison, at the similar period of time commercial


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MgO powder can only remove 40% of As(III) in the water model (Figure S7a). In Figure 2b inset, a pseudo second-order kinetic modelling was used to fit the experimental data (R2 ≈ 0.99, k = 0.031 g mg-1 min-1 (Table S3), indicating that the adsorption process is driven by chemisorption.35 At the same initial concentration of As(III) of 1.5 mg L-1, this value is threetimes and six times higher than those previous reported 16 and commercial MgO adsorbent (with k value ≈ 0.0046 g mg-1 min-1, Figure S7b), respectively, suggesting a faster As(III) removal rate.

Figure 2. Langmuir adsorption isotherm (a) and linearized Langmuir isotherm (insert) for As(III) by MgO-HS. As(III) removal percentage (b) during 24 h adsorption and pseudo-secondorder kinetics plot (insert) of MgO-HS for the adsorption of As(III) with an adsorbent dose of 0.5 g L-1 and an initial As(III) concentration of 1.5 mg L-1. MgO-HS adsorption performance in the presence of various coexisting anions that are usually present in groundwater and at different pH condition was evaluated to investigate its potential application in treating contaminated groundwater. Referring to concentration of coexisting anions that may present in groundwater, in this study we used concentration of 1 mg L-1 for each anions.36 We found that with the presence of 1 mg L-1 of chloride, carbonate, phosphate and silicate in the 400 mg L-1 initial As(III) concentration in solution, the adsorption capacity of


12

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MgO-HS is supressed by 28.3%, 27.9%, 28.2% and 23.96%, respectively (Figure 3a). The results in Figure 3a showed that the As(III) removal capability was supressed by each anion to a certain extent (< 30%), which can be attributed to the competition of anions for the surface sites. Similar suppression effect on the presence of coexisting anions also has been reported by other researchers.37-38

Figure 3. The Effect of (a) competing anions present in the model water and (b) pH of the solution on the adsorption capacity of MgO-HS. (c) MgO-HS adsorption performance at a low concentration of As(III) As shown in Figure 3b, with the increase of the pH value, the As(III) adsorption capacity increases from 376 to 732 mg g-1 (by 48.6%). This phenomenon can be attributed to the arsenic speciation variation and the adsorbent surface charge change, as the solution pH changes. At pH value between 6.0-9.0, the predominant As(III) species present are in the form of H3AsO30 and H2AsO3-.39 As pH value increases to 9, the amount of negatively charged As(III) species rises, while the surface of the adsorbent is positively charged (pH

Mesoporous Magnesium Oxide Hollow Spheres as ... - ACS Publications

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