Efficient and Rapid Removal of Environmental Malignant Arsenic(III

Research Article pubs.acs.org/journal/ascecg

Efficient and Rapid Removal of Environmental Malignant Arsenic(III) and Industrial Dyes Using Reusable, Recoverable Ternary Iron Oxide - ORMOSIL - Reduced Graphene Oxide Composite Tushar Kanta Sahu, Sonia Arora, Avishek Banik, Parameswar Krishnan Iyer, and Mohammad Qureshi* Material Science Laboratory, Department of Chemistry, Indian Institute Technology, Guwahati-781039, Assam, India S Supporting Information *

ABSTRACT: In this work, we have demonstrated an efficient and simple reusable catalyst, which can be operated on site for water remediation. In the present report, we have proposed a near 100% dye adsorption and the effective removal of arsenic(III) using a ternary composite consisting of ORMOSIL-Fe3O4-RGO. A simple and low-temperature synthesis to prepare an ORMOSIL-Fe3O4-RGO composite has been developed as a one stop solution for water remediation. Particularly, this composite was employed for the elimination of arsenite (III) ions and Rhodamine B dye from water, which has a huge impact in developing/underdeveloped countries in South Asian and some of the American regions. The structural, physical, and chemical properties of this composite were investigated through various characterization techniques like powder Xray diffraction (PXRD), fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy dispersion X-ray (EDS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). Using Langmuir isotherms, we calculated the adsorption capacity of the ORMOSIL-Fe3O4-RGO composite for Rhodamine B to be ∼1339 mg/g, which is much higher as compared to that of the Fe3O4-RGO composite (∼342 mg/g). Furthermore, the capacity of arsenic adsorption of this novel composite material is ∼25% higher than that of Fe3O4-RGO according to the Langmuir adsorption isotherm. KEYWORDS: Organically modified silica, Reduced graphene oxide, Magnetite, Arsenic removal, Dye removal

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worldwide drink water with higher than that level of arsenic.8 Over the last decades, various techniques were introduced for the removal of arsenic, such as coprecipitation,9 membrane filtration,10 photocatalysis,11 ion exchange,12 and flocculation.13However, besides these conventional techniques, the adsorption technique offers a promising substitute due to its ease of handling, cost-effectiveness, and toxic free nature.14−16 Besides heavy metals, organic dyes are another kind of pollutant commonly used in textile, paper, food, and cosmetic industries. Many of these dyes are toxic as well as carcinogenic, and their discharge to water bodies would contaminate water and become harmful for aquatic marine life.17 Most of the dyes are stable to light and oxidation, generally not degradable due to their complex polymeric structure.18 Among different physical techniques, adsorption is most effective for the removal of dye from water because of cost effectiveness, simple operation, and higher efficiency. Over these years, iron oxide nanoparticles have been employed to eliminate arsenic from water due to their

INTRODUCTION The reliable accessibility and safety of drinking water are basic human needs. Less than 3% of the total water is freshwater on the surface of the Earth, and only one-third of this freshwater is available for drinking.1 Contamination of these natural water sources due to industrial effluents and human activities makes it unhealthy for consumption. Among various contaminants, heavy metals, which are present due to both industrial as well as geographical concentration in the earth’s crust and organic pollutants, mainly due to industries such as leather tanneries, textile industries, etc., have become a matter of global concern. Arsenic is one among the ten most toxic heavy metals and is the cause of chronic health disorders like cancer of the bladder, kidney, liver, lungs, skin, and uterus, respiratory diseases, peripheral vascular diseases, and cardiac vascular diseases in affected countries like the USA, Taiwan, Poland, New Zealand, Mexico, Japan, India, Hungary, China, Chile, Canada, Bangladesh, and Argentina.2−6Among these countries, a large number of the population in the Indian state of West Bengal and Bangladesh are at high risk with groundwater arsenic contamination.7 According to guidelines set by the World Health Organization (WHO), the arsenic content limit in water supplies is 10 parts per billion, but over 150 million people © 2017 American Chemical Society

Received: February 28, 2017 Revised: May 24, 2017 Published: June 5, 2017 5912

DOI: 10.1021/acssuschemeng.7b00632 ACS Sustainable Chem. Eng. 2017, 5, 5912−5921

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of ORMOSIL-Fe3O4-RGO Compositea

a

A typical modified Hummers method is used to produce RGO, followed by the in-situ synthesis Fe3O4. Base hydrolysis using TEOS and VTMOS resulted in a cage-like matrix incorporated with Fe3O4-RGO. The as-synthesized composite is magnetic in nature, which can be easily separated from the solution with the help of a permanent magnet as shown in the scheme.

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properties such as magnetic separation and their ability to functionalize with numerous chemical groups.19,20 However, due to their small particle size and highly oxidizing nature on exposure to atmosphere, magnetite based adsorbents are not suitable for use in continuous flow systems.21 To increase the stability of iron oxides, many researchers have made composites with carbon,22,23 carbon nanotubes,24,25 and carbon flakes.26 Because of the high surface area and good chemical stability, graphene-based materials, for instance, graphene, graphene oxide (GO), and reduced graphene oxide (RGO) with Fe3O4 nanoparticles, have been used for not only water remediation but also for magnetic resonance imaging and targeted drug delivery.27−30 The high surface area, thermal stability, and efficient carrier mobility properties of RGO over GO motivate us to choose the former as a component for the composite.27,31 Organically modified silica or ORMOSIL is a class of unique materials produced by chemical modification of silica gels by organic precursor. It can host diverse inorganic and organic substrates. The simple synthesis procedure and high specific surface area make it a potential material for different applications. The novelty of modified silica based materials is that they are environmental friendly and show long-term stability. It has many more potential applications in catalysis, gene delivery, solar cells, and matrix material for lasers.32 ORMOSIL is known for its high surface area, but it has very limited application in water treatment.33 In our work, we present a low temperature synthesis of a ternary composite of Fe3O4−ORMOSIL-RGO where ORMOSIL is coated over Fe3O4-RGO as a functional and protective layer. Materials which can remove multiple contaminants from water are said to be ideal and economic for water treatment. These types of multifunctional materials are urgently needed for the economic treatment of polluted water. ORMOSIL has potential in the elimination of both As(III) and Rhodamine B, but the addition of magnetic material makes it easy to recover using a permanent magnet with more efficient adsorption. The as-prepared adsorbent exhibits excellent capabilities to remove As(III) and Rhodamine B.

EXPERIMENTAL SECTION

Materials. All of the chemicals used for experiments were of analytical grade. Milli-Q water (18.2 MΩ) was used for solution preparation and synthesis. Graphite powder (purity 99%) and hydrazine hydrate (80%) were obtained from Loba Chemie. Anhydrous ferric chloride, potassium permanganate, sodium hydroxide, sodium nitrate, hydrogen peroxide, tetraethylorthosilicate (TEOS), sodium arsenite, potassium iodide, Rhodamine B, and sulfuric acid were purchased from Merck. Ferrous chloride and vinyltrimethoxysilane (VTMOS) were purchased from Sigma-Aldrich. A stoichiometric amount of NaAsO2 was dissolved in Milli-Q water to prepare the stock solution of As (III). Synthesis of Graphene Oxide. Modified Hummer’s method was followed to prepare grapheme oxide (GO) by oxidation of graphite powder reported earlier.34,35 Synthesis of Fe3O4-RGO Composite. For the synthesis of Fe3O4RGO composites, a previously reported method was adopted.27 0.2 g of graphene oxide was dispersed in 130 mL of water. An aqua solution of FeCl3 (1.54 g/10 mL) and FeCl2 (0.566 g/10 mL) was prepared in 2:1 ratio followed by its slow addition to grapheme oxide solution at room temperature. Ammonia solution (25%) was added to maintain the pH of the solution around 10. After the addition of ammonia solution, the temperature of the solution was raised and maintained at 90 °C followed by the addition of 3 mL of hydrazine hydrate with constant stirring, resulting in a black solution. The solution was stirred rapidly for 4 h and then cooled at room temperature. The black precipitate from the solution was filtered, washed with water and ethanol several times, and finally dried in vacuum oven at 80 °C. Synthesis of an Organically Modified Silica-MagnetiteReduced Graphene Oxide (ORMOSIL-Fe3O4-RGO) Composite. A solution of tetraethylorthosilicate (TEOS) and vinyltrimethoxysilane (VTMOS) was prepared in 1:1 molar ratio and hydrolyzed for 5 h. To this solution, a specific weight percentage of Fe3O4-RGO (5 wt %, 10 wt %, 15 wt %) was added and stirred for 1 h. Then 1 M NaOH solution was added for base hydrolysis. The reaction mixture was kept at 60 °C in a hot air oven for 24 h. The obtained samples were named as OFG-X (X = 5, 10, 15) according to the addition of Fe3O4-RGO at 5, 10, and 15 wt %, respectively. The graphic of this synthetic protocol is represented in Scheme 1. Material Characterization. Powder XRD measurements were performed using a Rigaku TTRAX III and Bruker D2 PHASER X-ray diffractometer where Copper Kα (λ = 1.54 Å) was used as the source with an operating current of 10 mA and an operating voltage of 30 kV. 5913


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ACS Sustainable Chemistry & Engineering

Figure 1. (A) Powder X-ray diffraction pattern of graphene oxide (GO), ORMOSIL, Fe3O4-RGO, OFG-5, OFG-10, and OFG-15. The red circle portion including OFG-5, OFG-10, and OFG-15 shows a very weak and broad peak of ORMOSIL due to its amorphous nature. The low intensity peaks of Fe3O4 are due to weak crystallinity as a result of less reaction time. (B) The ORMOSIL peaks in OFG-5, OFG-10, and OFG-15 which are within the red circle portion in A. equilibrate and then separated by using an external magnetic field. To know the effect of contact times, the samples were collected in different times ranging from 20 to 120 min. The concentrations of arsenic before and after adsorption were measured by a UV−vis spectrophotometer by a previously reported method with a detection limit 0.04 ppm to 0.4 ppm.36 In this method, the samples containing arsenic were treated with potassium iodate to liberate iodine in acidic medium. The iodine which was liberated bleaches the color of Rhodamine-B, which was measured at 553 nm. The detection of arsenic concentration was based on the following reaction:

The XRD patterns for the 2θ range of 10°−80° was recorded at the scan rate 0.3°/s. For the measurement of UV−vis diffuse reflectance absorption (DRS) spectra, a JASCO (Model V-650) spectrophotometer was used. A PerkinElmer Spectrum Two instrument was used to record the Fourier transform infrared (FT-IR) spectra in KBr pellets. The FESEM of all of the samples was investigated on a Zeiss (Gemini) instrument operated at 5 kV to know the surface morphology. TEM measurements of the samples were carried out in a JEOL (JEM-2100) microscope with 200 kV operating voltage. Magnetic measurements were performed in a Lakeshore 7410 series VSM with an applied magnetic field of 300 Oe at room temperature. The concentration of Rhodamine B after adsorption was determined by a PerkinElmer Lamda 750 UV−vis spectrophotometer. The concentration of arsenic after adsorption was determined by a VARIAN SpectrAA 55B atomic adsorption spectrometer and a PerkinElmer Lamda 750 UV−vis spectrophotometer. BET surface area was analyzed with a Beckman-Coulter SA 3100 nitrogen adsorption apparatus by adsorption−desorption at liquid nitrogen temperature. Prior to the nitrogen gas adsorption measurements, all of the samples were degassed at 150 °C for 2 h. Dye Adsorption Experiment. The adsorption of Rhodamine B dye was performed in a round bottomed flask of 100 mL at room temperature. During the experiment, 50 mg of the adsorbent was added to 50 mL of Rhodamine B solution in the concentration range of 25−100 mg/L and stirred for 90 min to equilibrate, and the adsorbent was separated either through an external magnetic field or centrifugation. To obtain the kinetic parameters of the adsorption, the adsorption experiments were performed at different contact times (from 2 to 60 min). The residual concentrations of the dye were measured with the help of a UV−vis spectrophotometer at 554 nm. The adsorption capacity of the dye on the adsorbent was calculated using the following equation: q=

(C0 − Ce)V W

2IO−3 + 5As(III ) + 2H+ → I 2 + 5As(V) + H 2O

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RESULTS AND DISCUSSION The structural properties of the GO, RGO, ORMOSIL, composites Fe3O4-RGO, OFG-5, OFG-10, and OFG-15 were studied by powder X-ray diffraction, as shown in Figure 1A. Graphene oxide shows a broad peak of the (001) plane approximately at 10.5°. The XRD pattern of the reduced graphene oxide shows two broad peaks at 24.7° and 42.75° corresponding to the (002) and (100) plane, respectively. The broadness of these two peaks indicates the amorphous nature of RGO. ORMOSIL with a broad peak at 23° indicates its amorphous nature. The relative intensity and position of diffraction peaks of Fe3O4 are consistent with magnetite (JCPDS card no-65-3107) in the XRD pattern of Fe3O4RGO.37 There is no significant peak observed for RGO due to the weak crystallinity of carbon based materials as compared to the metal oxide Fe3O4. The average crystallite size calculated by the Debye-Scherer equation for Fe3O4 nanoparticles is found to be ∼7 nm. The XRD pattern of the ORMOSIL-Fe3O4- RGO composite has a similar pattern as that of Fe3O4-RGO because of the amorphous nature of ORMOSIL, although it has high a concentration in the composites. The red circle in Figure 1A shows the corresponding ORMOSIL peak in the composites, whose magnified peaks are shown in Figure 1B. All of the peaks of Fe3O4 in the composites conclude that the magnetite phase was not destroyed during the composites preparation. The FT-IR spectra of as prepared compounds are shown in Figure 2. The IR spectrum of RGO shows the absorption stretching vibrations peaks at 1574 and 1203 cm−1 which can be attributed to aromatic CC and C−O groups, respectively. IR spectra of Fe3O4-RGO also shows an aromatic CC (1574 cm−1) and C−O (1199 cm−1) stretch, confirming the formation of RGO in the composites.38 The red shift of the

(1)

where q represents the amount of dye adsorbed onto the adsorbents, C0 represents initial dye concentration, Ce represents the equilibrium dye concentration, V represents the volume of dye solution, and W represents the mass of the adsorbents. As (III) Adsorption Experiment. Sodium arsenite (NaAsO2) was used as a source of arsenic(III). A calculated amount of sodium arsenite was dissolved to prepare 100 mL of stock solution of arsenic with 1000 ppm concentration. The desired solutions were prepared by diluting arsenic stock solution with Milli-Q water. The adsorption of arsenic was performed in a round bottomed flask of 100 mL at room temperature with pH ∼ 7 because in the earlier study, the efficient adsorption of As(III) was achieved at this particular pH.27 Initially, a calculated amount of adsorbent was dispersed in 50 mL of specific concentration of arsenic and stirred for 2 h to 5914


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respectively. The band at 1602 cm−1 is for the CC vibration and 1407 cm−1 for Si-CH−CH2 banding deformation. The broad and strong band at 3450 cm−1 is observed as a result of vibration of Si−OH groups.40 OFG-10 exhibits two new bands at 1058 and 1165 cm−1 as compared to ORMOSIL, which can be attributed to the stretching and deformation vibrations of Si−O−Fe due to ORMOSIL coatings over Fe3O4-RGO. The band at 550 cm−1 in OFG-10 is due to the Fe−O stretching vibration, and the redshift in absorption is attributed to the interaction between iron and ORMOSIL. The morphology of the synthesized adsorbent was analyzed by FESEM. The sheet like morphology of RGO can be clearly observed in Figure S1A. Figure S1B shows the Fe 3 O 4 nanoparticles, and Figure S1C shows the bare ORMOSIL structure. Figure 3A,B shows the morphology of Fe3O4-RGO where the Fe3O4 nanoparticles are distributed over the RGO sheets, and Figure 3C,D shows the morphology of the best performing adsorbent OFG-10, which shows that the ORMOSIL matrix is coated over the Fe3O4-RGO composite. To know the distribution of various components in the composite, elemental distribution of the composite was analyzed by energy dispersive X-ray spectroscopy as shown in Figure 4. It is observed that all of the elements are homogeneously distributed over the entire range. The TEM image of OFG-10 is shown in Figure 5. Figure 5A shows the presence of Fe3O4, RGO, and ORMOSIL in the composite OFG-10. The average particle size of Fe3O4 nanoparticles are around 12 nm, which are dispersed over RGO and ORMOSI matrix. It can be clearly seen that the Fe3O4 nanoparticles are enclosed inside RGO and the ORMOSIL matrix. The HRTEM image in Figure 5B shows the lattice fringes from the Fe3O4 nanoparticle in the surrounding of the RGO matrix. The lattice spacing of 0.215 nm corresponds to the 400 crystal plane.27 The selected area diffraction (SAED) pattern shows that the composite is polycrystalline in nature.

Figure 2. Infrared spectra of RGO, Fe3O4-RGO, ORMOSIL, and OFG-10. The solid line parallel to 570 cm−1, which corresponds to the Fe−O band can only be observed for Fe3O4-RGO and ORMOSILFe3O4-RGO (OFG-10) with a red shift. The dotted line parallel to 1465 cm−1 shows that this peak can only be seen for the ternary composite, concluding the interaction between these materials which arises due to banding deformation. The dotted line parallel to 3440 cm−1(for RGO) shows a shift for Fe3O4-RGO (3390 cm−1) indicating the interaction between iron oxide and reduced graphene oxide.

absorption peak of C−O at 1199 cm−1 arises due to Fe−O−C vibration, which affirms the interaction between Fe3O4 and RGO in the composite. The strong band around 570 cm−1 is attributed to the vibrational mode of Fe−O in Fe3O4.39 The IR spectra of ORMOSIL shows a broad absorption band around 1135 cm−1, attributed to the stretching vibration of Si−O−Si, and a peak at 770 cm−1 corresponds to the Si−O stretching vibration. Two low intensity bands at 3065 and 2957 cm−1 correspond to the stretching vibrations of −CH and CH2,

Figure 3. Field emission scanning electron microscope (FESEM) images of Fe3O4-RGO (A,B) and OFG-10 (C,D). 5915


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Figure 4. Energy-dispersive X-ray spectroscopy (EDX) mapping of the best performing adsorbent OFG-10 (A) and the corresponding elemental mappings of silicon (B), oxygen (C), iron (D), and carbon (E) suggest the homogeneous distribution of all the elements in the area (A).

Figure 5. (A) TEM image of the best performing adsorbent OFG-10. (B) HRTEM image of OFG-10 which shows the lattice fringes of an interlayer distance of 0.215 nm corresponding to the (400) plane. (C) The SAED pattern of OFG-10 shows the polycrystalline nature of the composite.

The magnetic behavior of the Fe3O4-RGO and ORMOSILFe3O4-RGO composites were analyzed by VSM, as shown in Figure S2. The magnetic hysteresis curves show little or no coercivity with no saturation, indicating that all these magnetite based composites are superparamagnetic at room temperature. The superparamagnetic behavior is an indication for the formation of Fe3O4 nanoparticles with size around 10 nm.27,41 The saturation magnetization of Fe3O4-RGO and OFG-10 are 43.61 emu/g and 7.75 emu/g, respectively. The saturation magnetization value for OFG-10 is less than that of Fe3O4RGO because of the addition of nonmagnetic material (ORMOSIL); as a result, the magnetic separation takes a longer time in case of the latter. The inset to Figure S2 shows the dispersion of the best performing adsorbent OFG-10 in water, containing a few drops of methanol and its easy separation with the help of a small permanent magnet, which is

useful for the easy recovery of these adsorbents after the completion of the process. BET surface area analysis of ORMOSIL, Fe3O4-RGO, OFG05, OFG-10, and OFG-15 are shown in Figure S3. All the composites have a Type IV isotherm and H3 hysteresis loops, which indicate its mesoporous nature.42 The pore volume and specific surface area of Fe3O4-RGO are found to be 0.419 cm3/ g and 127.315 m2/g, respectively. The pore volume and surface area of bare ORMOSIL are 0.664 cm3/g and 150.98 m2/g, respectively. OFG-05 has a pore volume of 0.450 cm3/g and a surface area of 220.13 m2/g. The pore volume and surface area of OFG-10 are 0.558 cm3/g and 217.162 m2/g, respectively, with a surface area 170% higher than that of bare Fe3O4-RGO. Also, it is found that with the increase in loading of Fe3O4-RGO from 10 wt % to 15 wt %, both the pore volume and surface area of the adsorbent significantly decreased as the pore volume and surface area of OFG-15 are 0.429 cm3/g and 165.255 m2/g. 5916


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ACS Sustainable Chemistry & Engineering t 1 1 = + t 2 qt qe k 2qe

The decrease in surface area with increase in the Fe3O4-RGO concentration can be attributed to the fact that a higher loading of Fe3O4-RGO blocks the surface active sites of ORMOSIL matrices. Figure S4 shows the UV−visible diffuse reflectance spectra of ORMOSIL, Fe3O4, Fe3O4-RGO, OFG-5, OFG-10, and OFG15. It is observed from the figure that bare ORMOSIL absorbs in the UV region around ∼210 nm and bare Fe3O4 absorbs throughout the visible region.43,44 All of the composites including Fe3O4-RGO, OFG-05, OFG-10, and OFG-15 absorb in the visible region.45 With the increase in the concentration of ORMOSIL, there is a slight decrease in the absorbance. All of the changes in the absorption spectra reflect the changes in size and shape of the composites due to the addition of RGO and ORMOSIL to Fe3O4. Dye Adsorption Analysis. Figure 6 shows the decrease in Rhodamine B concentration with respect to time during the

(2)

where qt represents the adsorption capacity of the adsorbent at time t, qe represents the adsorption capacity at equilibrium, and k2 (g/(mg min)) represents the pseudo-second-order rate constant. The plot of t/qt versus t gives the values of k2 and qe. The linear fitting of pseudo-second order kinetics for all the adsorbents is shown in Figure 7.

Figure 7. Pseudo-second-order kinetics for Rhodamine B adsorption on adsorbents ORMOSIL, Fe3O4-RGO, OFG-5, OFG-10, and OFG15 (adsorbent dose = 0.5 g/L, dye concentration = 50 mg/L, and temperature = 298 K). The rate constant for best performing adsorbent OFG-10 is almost 20 times higher than that of Fe3O4-RGO.

Figure 6. Adsorption of Rhodamine B on different adsorbents at room temperature (concentration of Rhodamine B = 50 mg/L, and adsorbent dose = 0.5 g/L). The inset shows the amount of dye adsorbed after 2 min with respect to all of the adsorbents.

The calculated values of rate constants, adsorption capacities, correlation capacities, and the experimental values of adsorption capacities are listed in Table S1. From the correlation coefficients obtained from pseudo-second-order, it can be seen that pseudo-second-order kinetics fitted well for Rhodamine B adsorption. Besides, the calculated adsorption capacity (qe, cal) values are almost equivalent to the experimental values. In addition, the rate constant k2 for OFG-10 was found to be nearly 20 times higher than that of only Fe3O4-RGO. To predict the relationship between the adsorbent and adsorbate, the adsorption equilibrium data were fitted to Langmuir and Freundlich isotherm models. The linearized form was applied to obtain all the parameters of the Langmuir and Freundlich isotherm (eqs 3 and 4) as shown in Figure S5. The experimental adsorption data of Rhodamine B fitted well to the Langmuir as well as Freundlich isotherm. All the isotherm parameters are shown in Table S2. The value of n greater than 1 concludes that it is favorable for adsorption. Using Langmuir isotherms, the adsorption capacity of the ORMOSIL-Fe3O4RGO composite for Rhodamine B was found to be ∼1339 mg/ g, which is much higher as compared to that of the Fe3O4-RGO composite (∼342 mg/g). In addition, the experimental data were analyzed by different methods of least-squares regression.47−49 Three different linearized forms of the Langmuir isotherm model (Table 1) were analyzed for Rhodamine B adsorption, and the results are provided in the Table S5. From the analysis, it is found that the

dye adsorption experiment in the presence of ORMOSIL, Fe3O4-RGO, OFG-5, OFG-10, and OFG-15. Within 2 min of stirring at room temperature, above 97% of Rhodamine B is adsorbed in the case of OFG-10 and around 92% for OFG-5, which is much higher than that of ORMOSIL (∼75%), Fe3O4RGO (∼73%), and OFG-15 (∼64%). There is a significant decrease in the adsorption of Rhodamine B when the amount of Fe3O4-RGO in ORMOSIL is increasing from 10 wt % to 15 wt %. This could be attributed to the decrease in pore volume and surface area when the amount of Fe3O4-RGO increases from 10 wt % to 15 wt %. Further, the increase in concentration of Fe3O4-RGO from 10 wt % to 15 wt % presumably blocks the voids of the ORMOSIL cage, which results in a decrease in the active sites. The efficient adsorption capacity of this composite is mostly due to the hydrophobic moieties present in ORMOSIL as well as RGO which easily entrap both organic and inorganic pollutants. The hydrophobic interactions between the hydrophobic moieties of ORMOSIL and RGO with aromatic rings of the Rhodamine B result in strong π−π stacking interactions at unbiased conditions, which is the main reason behind the superior adsorption by the composite.46 The dye adsorption kinetics for all the samples were studied using a pseudo-second-order equation: 5917


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The mathematical linearized form of the Freundlich adsorption isotherm can be expressed by the following equation:

Table 1. Linear Forms of the Langmuir Isotherm isotherm Langmuir I

nonlinear form

qe = qmKLCe/1 + KLCe

Langmuir II

linear form

1 Ce

Ce = − K + qm q e

Ce vs

Ce qe

Ce qe

Ce qe

=

1 qm

+

1 qmKLCe C

1

L

Langmuir III

plot

vs

1 qe

=

1 qmKL

e

+

Ce qm

1 qe

log qe = log K f +

(4)

where Ce (mg/L) represents the concentration of dye/arsenic at equilibrium, qe (mg/g) represents the equilibrium adsorption capacity, Kf (mg/g) represents the Freundlich experimental constants related to adsorption capacity, and n represents the adsorption intensity. The linear forms of the Langmuir and Freundlich isotherm were fitted to obtain all the parameters (Figure S6B and Figure S7). At the higher concentration range, all the adsorption data fitted well to both the Langmuir and Freundlich models. At the lower concentration range, the experimental data fitted well only to the Freundlich model. All the parameters obtained are presented in Table S3. The maximum adsorption capacity (qm) of OFG-10 for As(III) is found to be 38 mg/g, which is higher than that of Fe3O4-RGO (∼31 mg/g) according to the Langmuir isotherm. The n value from the Freundlich isotherm is found to be greater than 1, which is in conclusion a favorable condition for adsorption. Pseudo-first-order and pseudo-second-order kinetics were applied to study the adsorption kinetics of arsenic from the experimental data (Figure 9 and Figure S8). The linear equation of pseudo-first order kinetics is expressed as follows:

vs Ce

linearized models are fitted well to the experimental data, with Langmuir III having the least error. Arsenic Adsorption Analysis. Arsenic adsorption analysis was performed for different concentrations of arsenic in water starting from parts per billion (ppb) ranges to parts per million (ppm) ranges. Figure 8A shows the adsorption of As (III) with time on ORMOSIL, Fe3O4-RGO, OFG-05, OFG-10, and OFG-15, where the initial arsenic concentrations were in the range of 2 to 4 ppm. As the maximum adsorption capacity was shown by OFG-10, further it was compared with Fe3O4-RGO for adsorption isotherm models. Figure 8B shows the detection of arsenic in both light and dark conditions. It can be observed that there is no change in the detection of arsenic for both conditions, which validates the use of a UV−visible spectrophotometer for arsenic detection in the range 0.04 ppm to 0.4 ppm. All the experiments were repeated and checked by an atomic absorption spectrophotometer. Lower concentrations of arsenic were detected with the help of an atomic absorption spectrophotometer. Figure S6A shows the adsorption of arsenic with different initial concentrations in the range 200 to 500 ppb. The experimental data of arsenic adsorption were fitted to Langmuir and Freundlich isotherm models. The adsorption data were simulated with the Langmuir adsorption model, for which the mathematical linear expressions can be written as

Ce C 1 = e + qe qm bqe

1 log Ce n

ln

(qe − qt) qe

= −k1t

(5)

where qe is the capacity of adsorption of adsorbent at equilibrium, qt is the capacity of adsorption of the adsorbent at time t, and k1 represents the pseudo-first-order rate constant. The plot of ln (qe − qt) versus t gives the values of k1 and qe. The linear equation of pseudo-second-order kinetics can be represented as given in eq 2. The plot of t/qt versus t gives the values of k2 and qe. Different kinetic parameters after calculation are summarized in Table S4. From the correlation coefficients listed on Table S4, it was found that pseudo-second-order correlation coefficients (R2 > 0.97) were better than pseudo-first-order correlation coefficients (R2= 0.983−0.890). Moreover, the theoretical and experimental values of qe for pseudo-second-

(3)

where Ce (mg/L) represents the dye/arsenic concentration at equilibrium, qe (mg/g) represents the adsorption capacity at equilibrium, qm (mg/g) represents the maximum adsorption capacity, and b (L/mg) represents the constant related to free energy of adsorption.

Figure 8. (A) Effect of contact time for the adsorption of As(III) (conditions: As(III) concentration = 3 ppm, adsorbent dose = 0.5 g/L, temperature = 298 K, and pH ∼ 7) and (B) detection of arsenic concentration in light and dark conditions. 5918


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Figure 9. Pseudo-second-order kinetics for (A) Fe3O4-RGO and (B) OFG-10. The experimental data of arsenic adsorption fitted well to pseudosecond-order kinetics with R2> 0.97.

order were almost similar, whereas it shows a large deviation for pseudo-first-order. The above results concluded that the adsorption process followed pseudo-second-order kinetics very well. Desorption and Recycling Experiments. The reusability of an adsorbent even after several adsorption−desorption cycles is an important requirement. The constancy of the removal efficiency was tested by repeated cycles for adsorption of Rhodamine B onto OFG-10. Because of the magnetic nature of the sample, recovery can be achieved by the help of a permanent magnet followed by centrifugation and washing with ethanol. The recovered powder sample was dried at 80 °C. The regenerated powder was further utilized for the adsorption of Rhodamine B. Successive five cycles of adsorption−desorption experiments were carried out. After the fifth cycle, the adsorption efficiency was found to be 95% as shown in Figure 10, and the recovered adsorbent is found to be chemically

Figure 11. Adsorption of arsenic up to five successive cycles. After the third cycle, the adsorption efficiency is found to be almost constant (∼81%).

cycle 3 was found to be slightly lower (less than 1%) than that of cycle 4, which may be attributed to the random error caused by experimental uncertainties due to slight differences in temperature or time during the desorption process.

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CONCLUSION We report a low temperature synthesis of bifunctional ORMOSIL coated Fe3O4-RGO composite for water remediation. This new composite material shows a high adsorption capability for the removal of arsenic and Rhodamine B, which was evaluated by adsorption isotherms and kinetics. Close to 100% of the dye is adsorbed within half an hour of the equilibrium. The adsorption capacity of adsorbent OFG-10 is found to be 1338 mg/g, which is much higher than that of Fe 3 O 4-RGO and ORMOSIL. A remarkable adsorption capability for the removal of arsenic (∼38 mg/g) has been observed using our best performing catalyst having 10 wt % of Fe3O4-RGO in the ORMOSIL. The recovery and reusability of the catalyst has been demonstrated, owing to its magnetic property and chemical stability.

Figure 10. C/C0 vs time plot for the adsorption of Rhodamine B by the best performing adsorbent OFG-10 for five successive cycles. Very little change in efficiency has been observed after the fifth cycle, i.e., 97% to 95%, showing the stability and recyclability of the adsorbent.

stable. Also, the reusability of the adsorbent was tested for arsenic adsorption onto OFG-10. The adsorbent was recovered with the help of a permanent magnet and washed with 0.1 M HNO3 and water. It was dried at 95 °C for further use. Successive five cycle adsorption−desorption experiments were repeated, and the adsorption efficiency for arsenic adsorption was found to be 81% as shown in Figure 11. The adsorption in

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00632. 5919


Research Article


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FESEM, magnetic hysteresis curve, BET surface area, UV−visible absorption spectra, Langmuir and Freundlich adsorption isotherm, pseudo-first-order kinetics along with kinetic and isotherm parameters (PDF) Adsorption of Rhodamine B by the best performing adsorbent OFG-10 and its separation from the solution using a magnetic stick (AVI)

AUTHOR INFORMATION

Corresponding Author

*Tel: +91-361-2582320. Fax: +91-361-2582349. E-mail: mq@ iitg.ernet.in. ORCID

Parameswar Krishnan Iyer: 0000-0003-4126-3774 Mohammad Qureshi: 0000-0003-0970-6870 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge Department of Science and Technology (DST), India for a Women Scientist fellowship (WOS-A) Grant no. SR/WOS − A/CS-33/2013 (G): C/503/IFD/201415 and Council of Scientific and Industrial Research (CSIR), India for financial support via Grant no. CSIR/01 (2704)/12/ EMR-II. IIT Guwahati and Central Instruments Facility−IIT Guwahati are highly appreciated for providing infrastructure and instrumentation. T.K.S. thanks Shraban Kumar Sahoo, Mohammad Shaad Ansari, and Gaurangi Gogoi for fruitful discussions and assistance in characterizations.

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Efficient and Rapid Removal of Environmental Malignant Arsenic(III

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