Hierarchical Porous Magnesium Oxide (Hr-MgO) Microspheres for

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Hierarchical porous magnesium oxide (Hr-MgO) microspheres for adsorption of an organophosphate pesticide: Kinetics, isotherm, thermodynamics and DFT studies Lekha Sharma, and Rita Kakkar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14370 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Hierarchical porous magnesium oxide (Hr-MgO) microspheres for adsorption of an organophosphate pesticide: Kinetics, isotherm, thermodynamics and DFT studies Lekha Sharma and Rita Kakkar* Department of Chemistry, University of Delhi, Delhi-110007, India; Tel.: +91-11-27666313;*E-mail: [email protected]

KEYWORDS: Hierarchical magnesium oxide (Hr-MgO); hydromagnesite (Hr-HM); organophosphate pesticide; chlorpyrifos (CPF); adsorption isotherm; thermodynamics; DFT ABSTRACT: In this study, hierarchical porous magnesium oxide (Hr-MgO) microspheres have been fabricated from hydromagnesite precursor via a facile precipitation method followed by calcination. The Hr-MgO microspheres consist of several nanosheet building blocks to generate a flower-like architecture. Chlorpyrifos (CPF), a persistent organic pollutant, has been chosen as a model organophosphate pesticide to determine the adsorptive capacities of the fabricated Hr-MgO. The equilibrium adsorption data fits well with the Langmuir isotherm model, having a maximum adsorption capacity of 3974 mg g-1, which is the highest value till date. Both kinetic as well as thermodynamic parameters reveal the spontaneous, exothermic and pseudo-second order nature of the adsorption process due to chemisorption between the pesticide and the adsorbent. DFT studies suggest the importance of hydroxylation on the MgO surface for the successful destructive adsorption, that takes place via the cleavage of S=P and Cl─C bonds resulting in the fragmentation of chlorpyrifos, in good agreement with FTIR and mass spectrometric studies. The present study shows the potential use of hierarchically structured porous MgO microspheres as an efficient adsorbent for the removal of chlorpyrifos pollutant.

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INTRODUCTION In recent years, the use of pesticides has radically increased due to growing population demands. This has resulted in serious water pollution, making living beings vulnerable to various diseases. Chlorpyrifos (C9H11Cl3NO3PS) (Scheme 1), is a wide-spectrum organophosphate pesticide which is used commonly in developing countries. Application of chlorpyrifos (CPF) for prolonged periods results in serious damage to the environment and the living. Its chronic exposure through drinking water has resulted in neuro-behavioral disorders, especially in children.1 Therefore, its remediation from water systems is an utmost challenge that needs to be addressed immediately.

Scheme 1. Structure of chlorpyrifos (CPF) (IUPAC name: O,O-Diethyl O-3,5,6-trichloropyridin-2-yl phosphorothioate)

Hierarchically porous structures are ubiquitous in nature and have exceptional properties and functions due to a high order of organized and interconnected porous nanostructures.2 They have gained tremendous attention in the field of adsorption and separation,3-6 energy conversion and storage,7-8 catalysis,9-11 photocatalysis12 and sensing13 in recent years. Among the various techniques used for environmental remediation, adsorption is the most conventional and widely accepted method for the elimination of water pollutants.14-15 Hierarchical materials are a standout choice as adsorbents among various classes of nanomaterials due to their extraordinary features like (1) highly ordered and interconnected network of porous nanostructures at different length scales which promotes fast diffusion and efficient mass transfer i.e. easy and quick delivery of pollutants to the surface, reducing mass transfer resistance,16-17 (2) high specific surface area due to the synergistic effect of porous sites at micro and mesoscale to increase the effective contact area and facilitating plenty of active sites for adsorption,18 resulting in fast and excellent removal capacities,19 (3) easy separation due to overall size range in micrometers, and (4) superior regeneration capacities due to robustness of the structural morphology, preventing the undesirable release of


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nanoparticles in the environment.12,13 These remarkable features provide them superior adsorption activities over other counterparts. Hierarchical metal oxides have recently garnered significant attention in water decontamination studies.2, 20-29 MgO has been extensively studied and applied in the field of adsorption and catalysis30-32 due to its special Lewis acid-base bifunctional activities, environmentally benign properties, low-cost owing to abundance in nature, and minimum environmental impact as per the Clean Water Act basic limits.33 Different morphologies of MgO, derived from hydromagnesite (Mg5(CO3)4(OH)2·4H2O), have been investigated as adsorbents in a few previous studies.34-37 Out of all the morphologies, the flower-like or the nest-like morphology is found to be superior for adsorption in comparison to sheets, flakes or rods due to enhanced surface area.38 These reports open up new avenues in the area of adsorption process for water treatment. Previously, Pradeep and group have reported studies on the treatment and removal of CPF using noble metal nanoparticles39-40 and graphene.41 Although these reports have been a valuable addition to the information regarding the process of pesticide uptake by nanosurfaces like gold/silver and graphene, there is an urgent need to develop environmentally benign materials which show faster and higher adsorption rates than the current reported materials for pesticides, and to get insights into the chemistry of the mechanism of the adsorption. With this inspiration, we herein report a facile synthesis of hierarchical porous magnesium oxide (Hr-MgO) microspheres as efficient adsorbents for the removal of CPF. To the best of our knowledge, no other study on adsorption of chlorpyrifos over hierarchical material with excellent adsorption capacities has been reported till date.

EXPERIMENTALAND COMPUTATIONAL DETAILS Materials. CPF was taken from Sigma-Aldrich under the name Dursban. All other chemicals were taken from Merck and used as received. Triply distilled water was used throughout the experiments. Typical procedure for the synthesis of Hr-MgO. Hierarchical porous magnesium oxide (Hr-MgO) was synthesized by modifying a previously reported method.37 Briefly, 50 mL of 1M Na2CO3 solution was dripped into 40 mL of 1M MgCl2.6H2O at room temperature with 3 ACS Paragon Plus Environment


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occasional swirling of the reaction contents. The appeared white precipitate was aged without stirring at 80 °C for 2 h to generate the hierarchical hydromagnesite precursor (Hr-HM), Mg5(CO3)4(OH)2·4H2O. It was washed thrice with triply distilled water and acetone and then dried at 100 °C. The as-synthesized Hr-HM was calcined at 500 °C for two hours to obtain Hr-MgO. Characterization of synthesized Hr-HM and Hr-MgO. Powder X-ray diffraction (PXRD) of the as-synthesized Hr-HM and Hr-MgO was obtained using a Bruker D8 Advance diffractometer with graphite monochromatized Cu/Kα radiation at a scanning rate of 4º min−1 in the 2θ range of 10-80º (λ = 0.15405 nm, 40 kV, 40 mA). Fourier transform infrared (FTIR) spectra were recorded through a Perkin Elmer Spectrum 2000 FT-IR Spectrometer in the range 4000−400 cm−1 with a resolution of 1 cm−1 under atmospheric conditions. KBr disc method was used to record the spectrum. Size and morphology analysis of the prepared nanomaterials was performed using a Jeol scanning electron microscope (SEM). Samples were prepared by placing the sample powder on double-sided tape held to metal stubs. The sample was then placed onto the tape. Excess sample was removed using pressurized air. Transmission Electron Microscopy (TEM) was fetched by a Technai G2T30 electron microscope from FEI, Netherlands with a resolution of 0.19 nm operative in the voltage range of 50-300 kV. To prepare TEM grids, a drop of diluted ultra-sonicated solution of nanoparticles in water was dropped onto a carbon-coated 200 mesh copper grid purchased from Ted Pella Inc., USA. Energy dispersive X-ray spectroscopy (EDX) was carried out with a SEM instrument at an accelerating voltage of 300 keV and beam current of 1 nA. UV-Vis absorption spectra were recorded in the spectral range of 190-400 nm with the help of a Cary 100 Spectrometer from Agilent technologies, California, USA. The Brunaur-Emmett-Teller (BET) specific surface area of Hr-MgO was analyzed by nitrogen (N2) adsorption using a Quantachrome NovaWin apparatus using a multipoint BET method with the adsorption data at the relative pressure (P/P0) range of 0.05─0.3. LCMS/MS analysis of the degradation products was performed with the help of an Agilent G6530AA high resolution Liquid Chromatography mass spectrometer in the positive electrospray ionization (ESI) mode. The mixture of degraded products was extracted with ethyl acetate from the supernatant aqueous solution after the reaction, and the solid mass was obtained after evaporating the solvent. Xray photoelectron spectroscopy with Auger electron spectroscopy (AES) module PHI/5000 Versa Prob II, FEI Inc. and C60 sputter gun was used for the characterization of the pure and pesticide adsorbed samples scanning the C1s, Mg2s, O1s, S2p, Cl2p and P2p regions. Al Kα 4 ACS Paragon Plus Environment

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X-ray radiation was used as the source for excitation (1486.8 eV, 500 mm). Atomic absorption spectrometer graphite furnace was used with the ZEEnite 65 AAS by Analytic Jena, Germany for the determination of Mg2+ ions. Adsorption Experiments. A 10 mL stock solution of 1000 mg L−1 CPF was prepared in methanol and stored in a capped glass vial for further use. Dilutions were carried out with triply distilled water. The working pH for all the experiments was in the range of 6.8 to 7.1. The CPF uptake capacities of Hr-MgO were studied by batch adsorption studies. The working volume was maintained at 10 mL. 20 mg L−1 of CPF solution and different Hr-MgO loadings (1000, 500, 100, 50, 20, 10, 5, 3 mg L−1) were taken in vials and sonicated for two minutes to disperse the adsorbent uniformly. The contents of the vials were magnetically stirred till the equilibrium time of two hours. The effect of pH was studied by adding adequate amounts of 0.1 M NaOH and HCl solutions to the batch. For isotherm studies, different concentrations of CPF (10, 20, 30, 40, 50, 60 and 70 mg L−1) and 0.1 mg adsorbent were taken in vials and stirred till equilibrium. Kinetic experiments were conducted by taking 50 mL of 1, 2 and 10 mg L−1 CPF solution and adding 0.5 mg of powdered adsorbent. The mixture was stirred and aliquots of 3 mL were taken out at intervals of 5 min. For thermodynamic experiments, 50 mL of 2 mg L−1 CPF solution was taken and 0.5 mg of powdered Hr-MgO was added. The mixture was stirred in a thermostat at 295, 311 and 323 K and aliquots of 3 mL were taken out at fixed time intervals. The contents of the vials were centrifuged at 10000 rpm for 5 min to remove the powdered adsorbent. UV-Visible spectroscopic measurements were made to analyze the contents of the centrifuged solution. DFT Calculations. First-principles density functional (DF) calculations were performed using the DMol3 code available from Accelrys Inc. in the Materials Studio 5.5 package. DMol3 uses numerical functions on an atom centered grid as its atomic basis, which were constructed specifically for use in DFT calculations.42-48 Their high quality minimizes superposition (BSSE) effects.42,47 The long-range tail of the basis set exhibits correct charge distribution and allows an improved description of molecular polarizabilities.49 The calculations employed numerical basis sets of double zeta quality plus polarization function (DNP). The DNP basis set is the numerical equivalent of the Gaussian basis, 6-31G**, but is much more accurate. The cores were treated using DFT Semilocal Pseudo-Potentials (DSPP), specifically designed for DFT calculations.



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The geometries of various structures were fully optimized, without restrictions, using delocalized internal coordinates. A comparison of geometries from the various LDA and GGA functionals50 had revealed that GGA-PBE51,52 results give best agreement with experimental geometries. The energy values reported here are the Gibbs energies, obtained after making the necessary thermal and vibrational corrections. RESULTS AND DISCUSSION Synthesis of Hr-HM and Hr-MgO microspheres. Hierarchically porous MgO microspheres were fabricated in four simple steps. The first step involved the nucleation of Mg2+, CO32− and OH− from a mixture of a homogeneous solution of MgCl2 and Na2CO3 under slow stirring to afford a white precipitate of Mg(OH)2, which is formed initially due to its low solubility product.53 Using control experiments, the formation of Mg(OH)2 was confirmed by conducting the powder XRD analysis of the as-obtained precipitates from different samples (Table S1, See SI for description). The XRD showed a slight hump and a prominent peak near 18° corresponding to plane (001) in sample I and II respectively, due to the initially formed brucite, Mg(OH)2 (Figure S1a and b, SI).54 As the concentration of Na2CO3 was substantially increased from sample II to IV, the peak at 18° disappeared due to a parallel competing reaction in which nesquehonite formation takes place over the nuclei of brucite (Figure S1c and d). The peaks at 30° and 42° indicate the formation of nesquehonite, MgCO3.3H2O.54-55 The observed peaks are broad due to the small crystallite size. The nesquehonite phase becomes unstable at the ageing temperature of 80 °C and is converted to hydromagnesite, Mg5(CO3)4(OH)2.4H2O with an in situ evolution of CO2. Hydromagnesite particles self-assemble onto the existing nuclei to afford flower-like hierarchical hydromagnesite (Hr-HM) structures. The chemical reactions involved are: CO32−+ H2O → OH−+HCO3−

(a)

Mg2++ CO32-+ H2O→ MgCO3.3H2O

(b)

HCO3−

(c)

CO2+ OH−

Mg2++ CO2 + OH−→ Mg5(CO3)4(OH)2.4H2O

(d)

Finally, Hr-HM microspheres, upon calcination at 500 °C, release excessive amounts of CO2 and H2O, which results in shrinking of the nanosheets due to a considerable loss of mass and 6 ACS Paragon Plus Environment

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volume, to form Hr-MgO microspheres (Scheme 2). Hence, the formation of Hr-MgO microspheres is accompanied by reduction in size of the microspheres. Mg5(CO3)4(OH)2.4H2O → 5MgO + 4CO2 + 5H2O

(e)

Scheme 2. Schematic representation of the synthesis of Hr-MgO microspheres

Characterization of Hr-HM and Hr-MgO. The PXRD of hierarchical hydromagnesite (HrHM) revealed as a monoclinic structure, which is well matched with the JCPDS file no. 250513 (Figure 1, trace a). The phase structure of Hr-MgO (Figure 1, trace b) is in good agreement with the standard data (JCPDS file no. 45-0946). The crystallite size of Hr-MgO was calculated from the most intense peak at (200) and was measured to be 15 nm.



200

022

211

110

113

210 121 002 022

221

011

012

Intensity (a.u.)

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023 230

123

033

400

220

322 143

052 422

(a) Hr-HM

111

311

222

(b) Hr-MgO 10

20

30

40

50

60

70

80

Two theta/ degree

Figure 1. Powder XRD pattern for as-synthesized (a) Hr-HM and (b) Hr-MgO

The presence of various functionalities on the surface of Hr-HM and Hr-MgO was examined by FTIR spectroscopy (Figure 2). The black trace is composed of a broad band at 3500 cm−1 attributed to O─H stretching vibrations due to the presence of crystalline water molecules in the hydromagnesite phase. Furthermore, bands split into 802, 856 and 884 cm-1 due to the bending vibration of surface-bridged carbonate groups, are characteristic features of hydromagnesite, in addition to CO32- absorption bands at 1428 and 1486 cm-1.56 Absorption bands at 1160, 1434 and 1628 cm-1 can be attributed to CO32- groups chemisorbed over the MgO surface (red trace). The reduced intensity of these bands in Hr-MgO suggests the removal of a major percentage of carbonate in the form of CO2 during calcination. While a broad band at 3444 cm-1 is responsible for the presence of multi-coordinated hydroxyl groups, a shoulder band at 3704 cm-1 confirms the presence of three coordinated O2- ions.57 The bands at 422, 658 and 870 cm-1 could be inferred to be Mg─O bond stretching frequencies.58


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1160 870

Hr-MgO 2858 1628

2928

Transmittance (a.u.)

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


1434

Hr-HM

1118

3444

2858

3638

884

712

856

802

596

3444

1486

4000

422

2924

3506

422

658

3500

3000

2500

2000

1428

1500

1000

500

Wavenumber/ cm-1

Figure 2. FT-IR spectra of the prepared Hr-HM and Hr-MgO



The chemical composition and the surface environment of the as-prepared Hr-MgO sample were further confirmed by X-ray photoelectron spectroscopy (XPS) (Figure 3, A-D). The XPS survey spectrum shows the presence of Mg 2p, Mg 2s, O 1s and C 1s photoelectrons. The high-resolution spectrum of Mg 2s shows distinct peaks at 86.98 and 88.6 eV, which can be attributed to the Mg-OH and Mg-O linkage, respectively. As for the O 1s spectra, the deconvoluted peak observed at binding energy 528.33 eV is due to the lattice oxide (O2-), the one at 530.15 eV corresponds to the surface hydroxyl groups present on the Hr-MgO, and that at 531.28 eV is due to carbonate groups (CO32-) present, which might be due to carbon dioxide adsorbed from the atmosphere.30, 32 Weak C 1s peaks with binding energy of ~284 eV and 288 eV indicate the presence of trace amounts of organic amorphous carbon and surface adsorbed carbonate groups.32 Interestingly, the percentage ratio of O2- to −OH groups based on the area of the peak is found to be 40:60.

Mg 2s

C 1s 1200

1000

800

600

400

200

Mg 2p

O 1s

A

c/s

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

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0

Binding Energy (eV)

B

90

Mg2s

88

86

C

84 525

O1s

528

531

D

C1s

534 291 288 285 282

Figure 3. (A) XPS survey spectra and high-resolution spectra of (B) Mg 2s, (C) O 1s and (D) C 1s of as-prepared Hr-MgO.

The N2 adsorption-desorption studies and BJH pore distribution studies were conducted to investigate the surface area and pore structure of the as-prepared samples. Both Hr-HM and Hr-MgO shows type IV isotherm with H1 hysteresis loop with pore diameter in the range of 10 ACS Paragon Plus Environment

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2-18 nm, confirming the co-existence of small number of micropores (< 2 nm) and mainly mesopores (2-50 nm) to form a hierarchical morphology (Figure 4). The BET surface area of precursor Hr-HM and its calcined product, Hr-MgO were found to be 21.3 and 65.3 m2g-1, which suggests the presence of sufficient number of active sites for the efficient uptake of the pesticide. The rise in surface area from precursor to oxide is due to CO2 evolution from carbonate precursors at high temperature to form porous hierarchical oxide. 60 -1 -1

(b)

3

Differential pore volume/ cm nm g

3

-1

Volume of N2 adsorbed/ (cm g )

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45

30

(b) (a) 2

4

6 8 10 12 14 16 18 Pore diameter/ nm

15

(a)

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

Figure 4. N2 adsorption/ desorption isotherms and pore size distribution (inset) of the (a) Hr-HM and (b) Hr-MgO samples

The surface morphology of the as-prepared Hr-HM and Hr-MgO is shown in SEM images (Figure 5a,c). Each microsphere is composed of several nanosheets of thickness ~10 nm (see SI, Figure S2a) arranged in a spherical flower-like morphology. This well-defined hierarchical architecture is achieved by synthesizing the precursor in static conditions (i.e. without stirring the contents of the reaction mixture). Even after calcining the precursor HrHM at high temperature, the morphology remains intact, suggesting that the hierarchical morphology is stable and well-preserved. In situ evolution of excessive amount of CO2 results in shrinking and significant loss in the volume of the nanosheets, as evident from Figure S2b (SI). The EDX elemental analysis of Hr-HM and Hr-MgO was carried out (SI, Figures S3 and S4). It is evident that oxygen is in excess (52.6 at %) over magnesium (47.4 at%) in Hr-MgO, which may be due to the presence of OH groups attached to the surface of MgO. Transmission electron microscopy (TEM) images revealed the internal morphology of the synthesized Hr-HM and Hr-MgO microspheres. Clearly, Hr-HM is composed of thin smooth sheets which are too dense to reveal the porosity (Figure 5b). However, at higher 11 ACS Paragon Plus Environment


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magnification, the surface porosity of sheets is clearly visible where several nanoparticles are embedded into the sheet (Figure S5a, SI). The calcined product, Hr-MgO, consists of very thin nanosheets curled and attached together to form a network, as evidenced by the faint and slightly dark contrast of the sheets (Figure 5d). The porosity of the sheets after calcination is considerably increased, as evident from Figure S5b (SI) due to the evolution of large amounts of CO2 and water vapor. The sheet is composed of MgO nanoparticles in the diameter range of 10-30 nm, having acquired rectangular or hexagonal shape. It can be stated that these nanoparticles act as building blocks in the formation of a porous hierarchical system where three different levels of hierarchy from microstructure to nanosheets to nanoparticles is observed.

50 µm

1 µm

20 µm

200 nm

Figure 5.(a, c) SEM images and (b, d) TEM images of as-synthesized Hr-HM and Hr-MgO, respectively

Adsorption Capacity. The variation of pesticide adsorption capacity and removal percentage corresponding to different Hr-MgO loading concentrations, is depicted in Figure 6. At high adsorbent dosage, the removal percentage is maximum due to increase in sorptive surface area and availability of active binding sites to facilitate complete removal. In contrast, high adsorbent dosage is indicated to be unfavorable for better adsorptive capacity of Hr-MgO. This is due to poor separation between the nanosheets, resulting in formation of aggregation 12 ACS Paragon Plus Environment

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of adsorption sites, thereby decreasing the total effective adsorption surface area available to CPF.59 Decreasing Hr-MgO loading facilitates increased mass transfer at the adsorbateadsorbent interface, resulting in higher adsorbed amount of CPF onto unit mass of Hr-MgO, causing a rise in qe.41 The maximum adsorption capacity, qm, was found to be as high as 3970 mg g–1. This value is exceptionally high, as compared to other nanomaterials reported in the literature.41,57,60

71.6%

4000

Removal% q e (mg g-1 )

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


qe

3000

73%

2000 77%

1000 80%

0

100% 99.6%

94%

88%

1000ppm 500ppm 100ppm 50ppm

20ppm

10ppm

5ppm

3ppm

HMO Loading

Figure 6. Adsorption capacities and removal percentage of CPF w.r.t. different Hr-MgO loadings

Adsorption Kinetics. Adsorption kinetics provides valuable information regarding uptake rate of the pollutant by the adsorbent and offers critical insights into the uptake mechanism. The controlling mechanism of adsorption was investigated using thepseudo-first-order61 and pseudo-second-order62 kinetic models. The linear forms of these models are respectively as follows: log( q e − q t ) = log q e −

k1 t 2 . 303

1 t t = + 2 qt k 2 qe qe

(1)

(2)

where qt (mg g–1) is the amount of CPF adsorbed at time t, qe (mg g–1) is the amount of CPF adsorbed at equilibrium, t (min) is the adsorption time, k1 (min–1) and k2 (g mg–1 min–1) are the pseudo-first-order and pseudo-second-order rate constants, respectively. The calculated kinetic model parameters of 2 mg L-1 CPF solution at different temperatures in the range of 13 ACS Paragon Plus Environment


20 to 50 °C (Table 1 and Figures7a,b) indicate that the experimental data fit best with the pseudo-second-order kinetic model. A pseudo-second-order reaction infers that the rate determining step may be chemisorption between chlorpyrifos and Hr-MgO. This interpretation is validated in the further sections. The effect of CPF concentration on its removal rate was also studied (Figure 8). It can be seen that after 5 minutes, more than 80% and 50% of the pesticide is removed for a concentration of 1 and 10 mg L-1, respectively. Complete removal of 1 mg L-1 CPF occurs within 60 minutes, suggesting that the synthesized Hr-MgO microspheres are efficient adsorbents for waste-water systems. Table 1: Experimental and calculated parameters of the kinetic models Model

Pseudo-first-order -1

-1

k2(g mg-1 min-1)

R2

0.929

185.18

2.41×10-3

0.966

0.734

166.60

6.63×10-3

0.958

153.84

-2

0.988

qe(exp) (mg g )

k1(min )

R

295

184.6

0.0407

311

169.2

0.0457

323

160.4

0.0757

Pseudo-second-order qe(cal) (mg g-1)

T/ K

2

0.986

8.45×10

0.16

2.2

295 K 311K 323 K

0.12

-1

t/q t (min g mg )

2.0 1.8

ln (q e-q t)

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

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1.6 1.4

0.08

311 K 295 K 323 K

0.04

1.2 0.00

1.0 0

5

10

15

20

0

5

Time/ min

10

15

Time/ min

Figure 7.(a) Pseudo-first-order and (b) Pseudo-second-order plot of adsorption of 2 mg L-1 CPF on Hr-MgO adsorbent at different temperatures.


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100

80

Removal %

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


1 mg/L 10 mg/L

60

40

20

0 0

20

40

60

80

100

120

140

time (min)

Figure 8. Chlorpyrifos removal percentage w.r.t. time for 1 and 10 mg L-1 CPF solutions.

Adsorption Isotherms. In order to understand the interaction of CPF with Hr-MgO, two well-known isotherm models, the Freundlich and Langmuir isotherms, were selected to analyze the equilibrium adsorption data. The Freundlich isotherm describes the non-ideal adsorption of a heterogeneous system and reversible adsorption, and can be expressed linearly as equation (3). On the other hand, the Langmuir isotherm is based on the assumption of a structurally homogeneous adsorbent and a monolayer coverage, and the expression is given in equation (4). log q e = log K

f

+

1 log C e n

1 1 1 = + q e q m q m K L Ce

(3)

(4)

Linear regression of the two adsorption isotherms was employed to investigate the pesticideadsorbent interaction and the results of fitting the Freundlich and Langmuir equations are summarized in Table 2. Figure 9 shows that the adsorption of CPF onto Hr-MgO can be better described by the Langmuir model (Figure 9a) for the entire concentration range, whereas the Freundlich model does not fit well (Figure 9b). This is further confirmed by the R2 values. Hence, it can be concluded that the adsorption of CPF on the porous surface of HrMgO is monolayer in nature. Notably, the value of qm is observed to be 3974 mg g-1 (Figure S6). 15 ACS Paragon Plus Environment


The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor, or equilibrium parameter, RL, which is defined as:

RL =

1 1 + KLC0

(5)

The value of RL indicates that the isotherm is either irreversible (RL= 0), favorable (0 1), where C0 is the initial concentration of CPF. The value of RL is given in Table 2 at the initial CPF concentration of 2 mg L-1, suggesting a linear isotherm. Table 2: Langmuir and Freundlich isotherm parameters for the adsorption of CPF on Hr-MgO Model

Langmuir qm (mg g )

KL (L mg )

RL

R

n

Kf

R2

Hr-MgO─CPF system

3974

8.00×10-4

0.99

0.999

4.74

28.8

0.644

0.009

-1

Freundlich

Parameter

-1

a

5

2

b

4 0.006

ln q e

-1

Ce/qe (L g )

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

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3 2

0.003

1 0 -1

0.000 0

10

-1

20

-1.0

30

-0.5

0.0

0.5

1.0

1.5

2.0

ln Ce

Ce (mg L )

Figure 9.(a) Linearized Langmuir and (b) Freundlich plot for 2 mg L-1 CPF adsorption onto Hr-MgO at 22 °C

Adsorption Thermodynamics. To gather information about the thermodynamic parameters of the reaction, i.e. change in Gibbs energy (∆GΘ, kJ mol-1), enthalpy (∆HΘ, kJ mol-1) and entropy (∆SΘ, JK-1mol-1) associated with the adsorption process, thermodynamic experiments were performed at three different temperatures, viz. 295, 311 and 323 K. These were evaluated by using the following equations:


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∆GΘ = −RT ln Kd

Kd =

(6)

qe Ce

ln Kd =

(7)

∆S Θ ∆H Θ − R RT

(8)

where T is the absolute temperature (K), R is the universal gas constant (8.314 J K-1 mol-1) and Kd is the thermodynamic equilibrium constant. The value of Kd at different temperatures was calculated using the values of qe and Ce known to us. The values of ∆HΘ and ∆SΘ can be calculated from the slope and intercept of the van’t Hoff plot between lnKd and 1/T (R2=0.987) (Figure10). All the thermodynamic parameters are given in Table 3. 3.2 3.1 3.0 2.9

ln K d

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


2.8 2.7 2.6 2.5 0.0031

0.0032

0.0033

0.0034

-1

1/T (K

)

Figure 10.van’t Hoff plot for the adsorption of CPF by Hr-MgO Table 3: Thermodynamic parameters for the adsorption of CPF on the Hr-MgO surface

Θ

-1

-1

Θ

∆GΘ(kJ mol-1)

-1

∆S (J K mol )

∆H (kJ mol )

-19.03

-13.19

295 K

311 K

323 K

-7.552

-7.323

-7.008

The negative value of ∆HΘ, as calculated from the van’t Hoff plot, implies exothermic nature of the adsorption process. This is further supported by the observation that, with rising 17 ACS Paragon Plus Environment


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temperature, the rate of the adsorption process decreases. Furthermore, the negative value of ∆SΘ suggests decreasing randomness at the solid/liquid interface during the adsorption of CPF on Hr-MgO microspheres, leading to the inference that the pesticide molecules were initially orderly adsorbed on the surface of Hr-MgO. The negative values of ∆GΘ indicate the feasibility of the process, but, with rising temperature, there is a decrease in the magnitude of ∆GΘ, which indicates that the adsorption becomes less favorable at higher temperatures. The Arrhenius equation was applied to measure the activation energy of the adsorption process. The activation energy represents the minimum energy that the reactants must have for the reaction to proceed, as shown in the equation below. ln k 2 = ln A −

Ea RT

(9)

where Ea is the Arrhenius activation energy (kJ mol-1), k2 is the rate constant of the pseudosecond-order adsorption and A is the Arrhenius factor. When lnk2 was plotted against 1/T, a straight line with slope −Ea/R was obtained (Figure 11). The magnitude of the activation energy gives an idea about the type of adsorption, i.e. chemical or physical adsorption. The physisorption processes typically have activation energies in the range of 0-40 kJ mol-1, while higher activation energies (40-800 kJ mol-1) imply chemisorption. The value of the activation energy, calculated to be 42.0 kJ mol-1, indicates that CPF is both physisorbed and chemisorbed onto the pores and the surface of Hr-MgO. This is also evident from the adsorption kinetics studies as discussed above. Effect of pH. The effect of solution pH on the removal of CPF is marginal, as shown in Figure 12. At very low and very high pH, the removal percent is maximum; however, removal is slightly lower in the pH range 5-6. Therefore, pH has a small role in facilitating the adsorption of CPF over the surface and pores of the adsorbent. The heterogeneous nature of the adsorbent over the whole pH range was checked by performing the leaching test. During the test, the adsorbent was taken out from the reaction mixture via centrifugation after 5 min when 50% of the pesticide was removed as determined by the UV-Visible spectrophotometer. The filtrate was analyzed further by UV-Vis spectrophotometry and no change in the remaining pesticide concentration was observed. To authenticate the leaching 18 ACS Paragon Plus Environment

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of the adsorbent, the filtrate was analyzed by Atomic absorption spectrophotometry (AAS). It was observed that in acidic medium (pH 3 to 5), 10-12% Hr-MgO leaches into the solution as Mg2+ ions. However, in neutral and basic medium, partial or no leaching is observed (~0.1%). However, leaving the reaction for longer time intervals results in more than 80% leaching at pH 3 to 4, 10% leaching at pH 5-6 and none at pH 7 to 9. The explanation for the observed marginal effect of pH on CPF removal may be due to the lower dosage of MgO at lower pH due to which the mass transfer ratio of adsorbate to adsorbent is higher, resulting in greater removal percentage. A similar higher removal percentage at higher pH can be explained on the basis of excess ─OH functional groups present on the MgO surface, resulting in better removal of CPF. 0

-1

ln k 2

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

-3

-4 0.0031

0.0032

0.0033

0.0034

-1

1/T (K

)

Figure 11. Plot of lnk2 versus 1/T for CPF adsorption



Removal%

100 80 60 40 20 0 3

4

5

6

7

8

9

pH Figure 12. Effect of pH on the adsorption of CPF by Hr-MgO

Regeneration studies. The effect of regenerated Hr-MgO on the adsorption of CPF was studied in five cycles, as shown in Figure 13. After the experiment, the adsorbent was separated from the reaction mixture by centrifugation, and washed with ethanol thrice to remove adsorbed organic impurities from its surface. The recovered adsorbent was dried at 100 °C for 2 h under vacuum and reused for the next cycles. It can be seen that, up to the fifth cycle, about 85% of the pesticide was removed. PXRD analysis was performed for Hr-MgO after the fifth cycle (Figure S7). The drastic difference of peak positions compared to the synthesized pure Hr-MgO suggests the adsorption to be chemical in nature, indicating the chemical changes that occur in the crystal structure of the adsorbent.

100

99%

97% 91%

89% 85%

80

Removal%

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

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60

40

20

0 1

2

3

4

5

Cycle

Figure 13. Removal percentage of CPF with regeneration cycles


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Characterization of Hr-MgO after CPF adsorption. To investigate the changes occurring in the chemical environment of each element of the adsorbent as well as pesticide after adsorption, XPS studies were conducted. The high resolution spectra of Mg, O, C, P, S and Cl are given in Figure14 (A to F). The surface of Hr-MgO displays predominant changes in aqueous medium after adsorption. The O1s peaks are shifted to higher binding energies of 529.76, 530.02 and 531.86 eV in comparison with pure Hr-MgO. These peaks correspond to bound lattice oxide (O2-), surface hydroxyl oxygens (-OH) and carbonate groups present over the surface of adsorbent, respectively. Interestingly, the ratio of the area under the peaks due to O2- and OH- increases from 1: 1.65 to 1: 4. This striking observation can be explained by the fact that in the presence of water, the lattice bound Mg-O undergoes hydrolysis to form surface bound hydroxyl groups. This is in accord with the basic nature of MgO. After adsorption, Mg 2s peaks shift slightly to 87.06 and 88.8 eV, implying the possible interactions of sulfur and chlorine with magnesium. This is further revealed by the S 2p and Cl 2p XPS spectra, shown in Figure 14E-F. The S 2p spectrum shows a doublet peak between 159 to 164 eV, which could be due to the metal-thiol group. Free thiol groups show an intense peak near 164 eV; however, the reduction in binding energy is due to the binding of sulfur to the surface of Hr-MgO via magnesium. Lower values of both 2P1/2 and 2P3/2 peaks indicate the metal bound state of sulfur. The chlorine shows similar behavior with a low binding energy of 197.10 and 198.72 eV for the deconvoluted peaks 2P3/2 and 2P1/2 respectively. For phosphorus 2p, the spin coupling peaks 2P3/2 and 2P1/2 are observed at 132.53 and 131.27 eV respectively. These values are on the low binding energies side than the P 2p values for pure chlorpyrifos and some other organophosphate molecules,40,63-64 indicating the chemisorption of the pesticide chlorpyrifos over the surface of Hr-MgO.



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

A

90

89

Mg2s

88

87

86

D

85 P2p

B

84 533

E

C

O1s

532

531

530

529

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528 280

C1s

282

F

S2p 2p3/2

286

288 Cl2p

161.48

2p3/2

2p1/2

2p3/2

284

160.41 2p1/2

136

134

132

130

2p1/2

168 166 164 162 160 158 156

204 202 200 198 196 194

Figure 14. High resolution XPS spectra of CPF adsorbed Hr-MgO sample (A) Mg 2s, (B) O 1s, (C) C 1s, (D) P 2p, (E) S 2p and (F) Cl 2p

The surface functional groups of Hr-MgO after CPF adsorption were investigated by FTIR spectroscopy. The FTIR spectrum of CPF (trace a) and that of CPF adsorbed on the Hr-MgO surface (trace b) are shown in Figure 15. The characteristic IR bands of CPF are seen at 530, 632, 676, 746, 852, 968, 1022, 1088, 1170, 1412 and 1550 cm-1. In the case of CPF adsorbed Hr-MgO, major bands can be seen at 446, 596, 722, 798, 804, 880,1014 cm-1and in the range of 1600-1100 cm-1. The P─O (aromatic) stretching of CPF at 968 cm-1 vanishes in the case of CPF adsorbed Hr-MgO, indicating the dissociation of CPF from P─O (Ar) bond. However, P─O(─C2H5) stretching of CPF in the range of 1064 and 1032 cm-1 is intact with much less intensity after adsorption. Hence, it can be suggested that these bonds are unbroken during the adsorption. Furthermore, stretching frequencies at 676 cm-1 (P=S) and 720 cm-1 (C─Cl) are broadened and shifted, along with the appearance of a new peak at 602 cm-1, indicating the possible interaction of (O─)Mg+←Cl. A slight hump near 2600 cm-1 can be assigned to the S-H stretching mode, after the possible formation of Mg─SH over the surface of Hr-


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MgO. These results are consistent with the XPS observations. Therefore, it may be inferred that CPF has undergone bond breaking and bond forming during the adsorption process.

(a) CPF

Transmittance (a.u.)

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


1550

746 1170

1412

(b) Hr-MgO

852 968

1738 1028

1658

880 466

1490 4000

3500

3000

2500

2000

1435 1500

1000

500

Wavenumber/ cm-1

Figure 15. FTIR spectra of (a) pure CPF and (b) CPF-adsorbed Hr-MgO

In order to further reveal the adsorption mechanism, the identification of the fragments present in the supernatant solution was studied by LCMS/MS. The fragmentation pattern is shown in Figure 16. Major fragment ions corresponding to peaks at m/z 59, 81, 90, 99, 107, 117, 141, 143, 149, 179 and 198.12 were found. The peak at m/z 198 corresponds to the major metabolite of chlorpyrifos, 3,5,6-trichloropyridinol (C5H2Cl3NO, TCP). The subsequent step-wise replacement of chlorine with the ─OH group resulted in appearance of peaks at m/z 179 (C5H3Cl2NO2), 161.5 (C5H4ClNO3, structure not shown) and 143 (C5H5NO4). The structures of other possible fragment ions are also shown in Figure 16. Major peaks correspond to fragment ions with lower molar mass, suggesting that the pesticide molecule has been completely converted into the smaller fragments and relatively lesser toxic byproducts. The above proposed pattern of fragmentation is further discussed and confirmed by our DFT studies.



1.0 5

x10

0.8 Intensity

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

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59.05

0.6

81.03 78.12 99.05

0.4

107.04

72.60

0.2

179.06

198.12

143.0

90.03

0.0 60

80

100

120

140

160

180

200

m/z Figure 16. Fragmentation pattern of the products formed after degradation of chlorpyrifos

DFT Studies. Due to lack of existing literature on the detailed conformation of chlorpyrifos (CPF), we performed a thorough conformational analysis on the same. The conformational analysis of chlorpyrifos was performed by varying the six torsion angles mentioned in the Xray crystallographic studies.65 A systematic grid scan was performed using the Universal Force Field66 using an energy window of 10 kcal mol-1. Different conformers were generated by varying the torsion angles, and the four lowest energy conformers are displayed in Figure 17. Table S2 provides the standard and relative Gibbs energies at standard temperature and pressure (298.15K, 1 atm) of the four conformers. It can be concluded that there is marginal difference in their relative Gibbs energies, with Conformer (I) having the least energy.


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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


Figure 17. Different low energy conformers of chlorpyrifos (CPF) (Color codes used throughout this work: Phosphorus- Purple, Sulfur- Yellow, Oxygen- Red; Chlorine- Green; Nitrogen- Blue, CarbonGray, Hydrogen- White)

Table S2 also gives the optimized dihedral angles for the four conformers, and these are compared with those obtained in the crystal structure.65 It can be seen that Conformer I also bears the closest resemblance to the crystal structure in terms of torsion angles. Furthermore, the calculated bond parameters were compared with the experimentally observed data (Table S3), and it was found that there is good agreement between the two. Linear regression coefficients between the calculated and experimental values for bond lengths and bond angles are 0.974 and 0.948 respectively, showing fairly good correspondence between the observed X-ray crystallographic data and our DFT calculated values. The values marked with an asterisk are due to disordering65 and, therefore, we have excluded them from our regression calculation. A glaring observation is that the P-O1 bond (Scheme 3) is significantly longer (1.69 Å) than the other two P-O bonds (~1.60 Å). Thus, this bond is already weakened and prone to dissociation. On the other hand, the C1-O1 bond (pyridoxyl group) is shorter (1.36 Å) than the two C-O methoxy bonds (~1.47 Å). The computed P-O1 and C1-O1 Mayer bond orders are 0.761 and 1.083, respectively, indicating that the oxygen is in conjugation with the aromatic ring, resulting in weakening of its bond with phosphorus. This gives the C1-O1 bond a double bond character, compared to the other two C-O methoxy bonds which have bond orders of 0.868 (C6-O3) and 0.887 (C8-O2). Similarly, the two other P-O2 and P-O3 have bond orders of 0.988 and 1.003, respectively.



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

Scheme 3. Chlorpyrifos

Hence, because of its proximity to the crystal structure, further calculations on CPF in this work were done by taking Conformer (I). In order to model the MgO nanosheet of Hr-MgO, a 5×6 slab of MgO (111) having lattice parameters of 15.18, 18.83 and 17.42 Å in the a, b and c directions, respectively (including a vacuum slab of thickness 15 Å in the c direction) was carved out from an optimized MgO bulk structure. CPF was placed in the vacuum slab and allowed to adsorb on the surface. No appreciable interaction was found when the molecule was adsorbed on the pristine MgO surface. However, the FTIR, EDX and XPS studies of the MgO nanosurface had revealed the presence of -OH bonds when it was exposed to the atmosphere. We therefore hydroxylated the surface. Of the 40 Mg atoms, seven were hydroxylated [17.5% hydroxylation (Figure 18a)] and the computation repeated. The result was a drastic increase in the adsorption energy to 567.8 kJ/mol, calculated using the following equation: Eads = EMgO + ECPF - EMgO-CPF

(10)

Thus, the adsorption is highly exothermic, as revealed by our experiments.


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a

b

Figure 18. (a) Hydroxylated MgO(111) surface (b) Destructive decomposition of CPF on hydroxylated MgO(111) surface

Table 4. Structures of the major metabolites obtained from DFT and MS studies Primary metabolite from DFT

Structure

m/z

Corresponding stable metabolite from MS

m/z

V

90

143

VI

121

107.04

Further, Figure 18b reveals that the surface becomes highly puckered, the molecule dissociates completely, the P=S, P-O bond with the pyridoxyl ring and C-Cl bonds getting broken. The sulfur and Cl get adsorbed on the Mg atoms on the surface. The primary dissociation products are V and VI (Table 4 and Figure 18b), besides three Cl- and one S2-, all attached to the MgO surface. In the mass spectrometric studies, analogous fragmentation products are found corresponding to m/z 90 and 107 (Figure 16). Therefore, it can be concluded that the destructive adsorption of chlorpyrifos on the surface of MgO is facilitated via S and Cl atoms which chemisorb on the metal oxide surface, causing dissociation of P=Sand C-Cl bonds, besides the already weak P-O bond, resulting in the formation of the above said primary and various other unstable secondary fragments.



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

The pyridone ring structure (V) is highly unstable and, in aqueous medium, it may get converted to the more stable 2,3,5,6-tetrahydroxypyridine, C5H5NO4 (m/z 143, Table 4) by reaction with water molecules. It is heartening to note that the fragmentation pattern of the supernatant mixture shows a peak at m/z 143, corroborating the theoretical predictions. This peak is shown in Figure 16. Based on the results obtained from adsorption kinetics and thermodynamics and characterization techniques. such as XPS, FTIR, LCMS/MS and DFT analysis, we proposed a plausible mechanism for the degradation of chlorpyrifos, as shown in Scheme 4. The sulfur and chlorine atoms of CPF are responsible for chemisorption on the hydroxylated surface of Hr-MgO, as confirmed by XPS, FT-IR and DFT studies. Due to this polarization effect, the P─O(Ar) bond and the aromatic C─Cl bonds are weakened and subsequently the phosphorus and aromatic ring are attacked by the nucleophilic water molecules. As a result, the hydroxylsubstituted pyridine and phosphane species are formed. The metabolites of CPF are further disintegrated into smaller fragments, as observed in the mass spectrometric studies.

Scheme 4. Plausible reaction mechanism for the degradation of chlorpyrifos on the surface of hydroxylated Hr-MgO

CONCLUSIONS The present study establishes the potential application of hierarchical porous magnesium oxide microspheres in the field of water decontamination. The adsorption capacity of 28 ACS Paragon Plus Environment

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hierarchical MgO is observed to be as high as 3970 mg g-1 for chlorpyrifos in aqueous medium. This value is extraordinarily higher than any of the materials reported for this purpose. Fast adsorption rates, insensitivity to the solution pH and good regeneration and reusability are the other merits. Theoretical insights match well with the experimental outcomes that chlorpyrifos is chemisorbed on MgO via sulfur and chlorine atoms and the molecule disintegrates into smaller fragments. Our findings suggest that the hierarchical MgO can be a promising adsorbent for the adsorption of pesticides and other organic pollutants from wastewaters. ASSOCIATED CONTENT Supporting Information The supporting information is available [Procedure for control experiment for XRD studies, SEM, EDX and TEM images of synthesized Hr-HM and Hr-MgO; Adsorption isotherm plot for adsorption of CPF on Hr-MgO; PXRD of regenerated Hr-MgO; Data for lowest energy conformers of CPF; Bond parameters of optimized and X-ray crystallographic chlorpyrifos] AUTHOR INFORMATION Corresponding Author *Tel.: +91-11-27666313. E-mail: [email protected].

ORCID Rita Kakkar: 0000-0002-8666-2288 Lekha Sharma: 0000-0001-8890-4378 Author Contributions R.K. conceived the project. L.S. performed the experiments under the supervision of R.K. R.K. and L.S. wrote the manuscript. All authors have given approval to the final version of the manuscript. Notes 29 ACS Paragon Plus Environment


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Page 30 of 36

The authors declare no competing financial interest. ACKNOWLEDGEMENTS L.S. acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi for providing financial assistance in the form of a Senior Research Fellowship. The authors thank the University Science Instrumentation Centre, Central Instrumentation Facility (USIC-CIF) for facilitating XRD, FTIR, SEM and TEM analyses. The authors thank University of Delhi’s “Scheme to Strengthen Doctoral Research by Providing Funds to Faculty”. The authors acknowledge

the

surface

characterization

laboratory,

ACMS, IIT Kanpur,

India,

for providing the XPS facility. The authors gratefully acknowledge the anonymous referees for carefully reading the manuscript and providing their valuable and insightful comments. REFERENCES 1. Pradeep, T.; Anshup. Noble metal nanoparticles for water purification: A critical review. Thin Solid Films 2009, 517, 6441-6478. 2. Hartmann, M.; Schwieger, W. Hierarchically-structured porous materials: From basic understanding to applications. Chem. Soc. Rev. 2016, 45, 3311−3312. 3. Hu, J. S.; Zhong, L. S.; Song, W. G.; Wan, L. J. Synthesis of hierarchically structured metal oxides and their application in heavy metal ion removal. Adv. Mater. 2008, 20, 2977−2982. 4. Tang, H.; Huang, H.; Wang, X.; Wu, K.; Tang, G.; Li, C. Hydrothermal synthesis of 3D hierarchical flower-like MoSe2 microspheres and their adsorption performances for methyl orange. Appl. Surf. Sci. 2016, 379, 296-303. 5. Zhang, M.; Wang, Y.; Zhang, Y.; Ding, L.; Zheng, J.; Xu, L. Preparation of magnetic carbon nanotubes with hierarchical copper silicate nanostructure for efficient adsorption and removal of hemoglobin. Appl. Surf. Sci. 2016, 375, 154-161. 6. Li, P.; Liu, W.; Dennis, J. S.; Zeng, H. C. Synthetic architecture of MgO/C nanocomposite from hierarchical-structured coordination polymer toward enhanced CO2 capture. ACS Appl. Mater. Interfaces 2017, 9, 9592-9602. 7. Butt, F. K.; Tahir, M.; Cao, C.; Idrees, F.; Ahmed, R.; Khan, W. S.; Ali, Z.; Mahmood, N.; Tanveer, M.; Mahmood, A.; Aslam, I. Synthesis of novel ZnV2O4 hierarchical nanospheres and their applications as electrochemical supercapacitor and hydrogen storage material. ACS Appl. Mater. Interfaces 2014, 6, 13635−13641. 8. Li, Y.; Fu, Z. Y.; Su, B.-L. Hierarchically structured porous materials for energy conversion and storage. Adv. Funct. Mater. 2012, 22, 4634−4667.


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Graphical Abstract 86x50mm (300 x 300 DPI)

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Hierarchical Porous Magnesium Oxide (Hr-MgO) Microspheres for

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