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Efficient and rapid removal of environmental malignant Arsenic (III) and industrial dyes using re-usable, recoverable ternary Iron Oxide – ORMOSIL - Graphene Oxide composite Tushar Kanta Sahu, Sonia Arora, Avishek Banik, Parameswar Krishnan Iyer, and Mohammad Qureshi ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 5, 2017

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ACS Sustainable Chemistry & Engineering

Efficient and rapid removal of environmental malignant Arsenic (III) and industrial dyes using reusable, recoverable ternary Iron Oxide – ORMOSIL - Graphene Oxide composite Tushar Kanta Sahu, Sonia Arora, Avishek Banik, Parameswar Krishnan Iyer, Mohammad Qureshi* Material Science Laboratory, Department of Chemistry, Indian Institute Technology, Guwahati – 781039, Assam, India. E-mail: [email protected]; Fax: +91-361-2582349; Tel: +91-361-2582320 KEYWORDS: Organically modified silica, reduced graphene oxide, magnetite, arsenic removal, dye removal 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 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 huge impact in developing/ underdeveloped

ACS Paragon Plus Environment

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countries in south Asian and some of the American regions. Structural, physical and chemical properties of this composite was investigated through various characterization techniques like Powder X-ray 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). Analysis using Langmuir isotherms, adsorption capacity of ORMOSIL-Fe3O4-RGO composite for Rhodamine B is calculated to be ~1339 mg/g which is much higher as compared to Fe3O4RGO composite (~342 mg/g). Furthermore, the capacity of arsenic adsorption of this novel composite material is ~25% higher than Fe3O4-RGO according to Langmuir adsorption isotherm.

INTRODUCTION Reliable accessibility and safety of drinking water is a basic human need. 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 this natural water sources due to industrial effluents


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and human activities makes it unhealthy for consumption. Among various contaminants, heavy metals, which are due to both industrial as well as geographical concentration in the earth’s crust and organic pollutants, mainly due to the industries such as leather tanneries, textile industries etc., has become a matter of global concern. Arsenic is one among the ten most toxic heavy metals which is the cause of chronic health disorders like cancer of bladder, kidney, liver, lungs, skin and uterus, respiratory diseases, peripheral vascular diseases and cardiac vascular diseases in the affected countries like USA, Taiwan, Poland, New Zealand, Mexico, Japan, India, Hungary, China, Chile, Canada, Bangladesh and Argentina.2-6Among these countries a large number of population in Indian state of West Bengal and Bangladesh are at high risk with groundwater arsenic contamination.7 According to guidelines set by World Health Organization (WHO), the arsenic content limit in water supplies is 10 parts per billion, but over 150 million people worldwide drink water with higher than that level.8 Over the last decades, various techniques were introduced for removal of Arsenic, such as co-precipitation,9 membrane filtration,10 photocatalysis,11 ion exchange,12 and flocculation.13However, beside these conventional techniques adsorption technique offer a promising substitute due to its easy handling, cost-effectiveness and toxic free nature.14-16 Besides heavy metals, organic dyes are another kind of pollutants 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.


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Over these years, iron oxide nanoparticles have been employed to eliminate arsenic from water due to their properties such as magnetic separation and their ability to functionalize with numerous chemical groups.19-20But, due to their small particle size and highly oxidizing nature on exposure to atmosphere, magnetite based adsorbents are not suitable to use in a continuous flow systems.21To increase the stability of iron oxides many researchers have made composites with carbon,22-23 carbon nanotubes,24-25 and carbon flakes.26 Due to 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, 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 shortly ORMOSIL, a class of unique materials produced by chemical modification of silica gels by organic precursor. It can host diversities of inorganic and organic substrates. Simple synthesis procedure and high specific surface area make it a potential material for different applications. The novelty of modified silica based materials are that they are environmental friendly and shows 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 ternary composite of Fe3O4ORMOSIL-RGO where ORMOSIL is coated over Fe3O4-RGO as a functional and protective layer. Materials which can remove multiple contaminants from water is said to be an ideal and economic for water treatment. These types of multifunctional materials are of urgent needs for economic treatment of polluted water. ORMOSIL has potential in the elimination of both As(III)


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and Rhodamine B but the addition of magnetic material make it easy for the recovery 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 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 Sulphuric acid were purchased from Merck. Ferrous chloride and Vinyltrimethoxysilane (VTMOS) were purchased from Sigma Aldrich. 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 Fe3O4-RGO composites, a previously reported method was adopted.27 0.2g graphene oxide was dispersed in 130 ml water. An aqua solution of FeCl3 (1.54 g/10 mL) and FeCl2 (0.566 g/10mL) 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 solution was


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raised and maintained at 90°C followed by addition of 3 mL of hydrazine hydrate with constant stirring, resulting in black color solution. The solution was stirred rapidly for 4 hour 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 Organically Modified Silica-magnetite-reduced graphene oxide (ORMOSILFe3O4-RGO) composite A solution of Tetraethylorthosilicate (TEOS) and Vinyltrimethoxysilane (VTMOS) was prepared in 1:1 molar ratio and hydrolyzed for 5 hrs. To this solution specific weight percentage of Fe3O4-RGO (5 wt%, 10 wt%, 15 wt%) was added and stirred for 1 hr. Then 1M NaOH solution was added for base hydrolysis. The reaction mixture was kept at 60oC in hot air oven for 24 hrs. The obtained samples were named as OFG-X (X=5, 10, 15) according to the addition of Fe3O4-RGO at 5, 10 and 15 weight %, respectively. The graphical of this synthetic protocol is represented in Scheme 1. Material Characterization Powder XRD measurements were performed using Rigaku TTRAX III and Bruker D2 PHASER X-ray diffractometer where Copper Kα (λ = 1.54 Å) was used as the source with operating current of 10 mA and operating voltage of 30 kV. The XRD patterns for the 2θ range of 10°−80° was recorded at the scan rate 0.3o/s. For the measurement of UV-vis diffuse reflectance absorption (DRS) spectra JASCO (Model V-650) spectrophotometer was used. Perkin Elmer Spectrum Two instrument was used to record the Fourier transform infrared (FTIR) spectra in KBr pellets. The FESEM of all the samples were 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.


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Magnetic measurements were performed in a Lakeshore 7410 series VSM with an applied magnetic field of 300 Oe at room temperature. The concentration of the Rhodamine B after adsorption was determined by Perkin Elmer Lamda 750 Uv-vis spectrophotometer. The concentration of arsenic after adsorption was determined by VARIAN SpectrAA 55B Atomic Adsorption Spectrometer and Perkin Elmer Lamda 750 Uv-vis spectrophotometer. BET surface area was analyzed with Beckman-Coulter SA 3100 nitrogen adsorption apparatus by adsorptiondesorption at liquid nitrogen temperature. Prior to the nitrogen gas adsorption measurements all the samples were degassed at 150°C for 2h. 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 minutes to equilibrate and the adsorbent was separated either through external magnetic field or centrifugation. To obtain the kinetic parameters of the adsorption, the adsorption experiments were performed at different contact times (from 2 minutes to 60 minutes). 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: =

− 01

where q represent the amount of dye adsorbed onto the adsorbents, C0 represent initial dye concentration, Ce represents the equilibrium dye concentration, V represent the volume of dye solution and W represent the mass of the adsorbents. As (III) adsorption Experiment


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Sodium arsenite (NaAsO2) was used as a source of arsenic (III). A calculated amount of sodium arsenite was dissolved to prepare 100 mL stock solution of arsenic with 1000 ppm concentration. The desired solutions were prepared by diluting arsenic stock solution with MilliQ water. The adsorption of Arsenic was performed in a round bottomed flask of 100 mL at room temperature with pH~7 as from 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 hours to equilibrate and then separated by using external magnetic field. To know the effect of contact times, the samples were collected in different times ranging from 20 minutes to 120 minutes. The concentrations of arsenic before and after adsorption were measured by UV-vis spectrophotometer by a previously reported method with detection limit 0.04 ppm to 0.4 ppm.36 In this method the samples containing arsenic was 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: 2 + 5 + 2 → + 5 +

RESULTS AND DISCUSSIONS The structural properties of the GO, RGO, ORMOSIL, composites Fe3O4-RGO, OFG-5, OFG10 and OFG-15 were studied by powder X-ray diffraction, as shown in Figure 1(A). Graphene


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oxide shows a broad peak of (001) plane approximately at 10.50. XRD pattern of reduced graphene oxide show two broad peaks at 24.70 and 42.750 corresponding to (002) and (100) plane, respectively. The broadness of these two peaks indicates the amorphous nature of RGO. ORMOSIL with a broad peak at 230 indicate 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 Fe3O4-RGO.37 There is no significant peak observed for RGO due to its weak crystallinity of carbon based materials as compared to metal oxide, Fe3O4. The average crystallite size calculated by Debye-Scherer equation for Fe3O4 nanoparticles is found to be ~7 nm. The XRD pattern of ORMOSIL-Fe3O4- RGO composite has similar pattern as of Fe3O4RGO, because of the amorphous nature of ORMOSIL although it has high concentration in the composites. The red circle in Figure 1(A) shows the corresponding ORMOSIL peak in the composites, whose magnified peaks are shown in Figure 1(B). All 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 absorption peak of C-O at 1199 cm-1 arises due to Fe-O-C vibration, which affirm the interaction between Fe3O4 and RGO in the composite. The strong band around 570 cm-1 is attributed to vibrational mode of Fe-O in Fe3O4.39 IR spectra of ORMOSIL shows abroad absorption band around 1135 cm-1, attributed to the stretching vibration of Si-O-Si and a peak at 770 cm-1 corresponds to Si-O stretching vibration. Two low


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intensity bands at 3065 cm-1 and 2957 cm-1 corresponds to the stretching vibrations of –CH and CH2=, respectively. The band at 1602 cm-1 is for 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 cm-1 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 Fe-O stretching vibration and the redshift in absorption 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 S1(A). Figure S1(B) shows the Fe3O4 nanoparticles and Figure S1(C) shows the bare ORMOSIL structure. Figure 3(A-B) shows the morphology of Fe3O4-RGO where the Fe3O4 nanoparticles are distributed over the RGO sheets and Figure 3(C-D) shows the morphology of best performing adsorbent OFG-10 which shows that the ORMOSIL matrix is coated over 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 the elements are homogeneously distributed over entire range. The TEM image of OFG-10 is shown in Figure 5. Figure 5(A) 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 ORMOSIL matrix. The HRTEM image in Figure 5(B) shows the lattice fringes from Fe3O4 nanoparticle in the surrounding of RGO matrix.


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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. The magnetic behavior of the Fe3O4-RGO and ORMOSIL-Fe3O4-RGO composites were analyzed by VSM, as shown in Figure S2. The magnetic hysteresis curves show little or no coercivity with no saturation, indicate 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 Fe3O4-RGO because of the addition of nonmagnetic material (ORMOSIL), as a result the magnetic separation takes longer time in case of the latter. Inset to Figure S2 shows the dispersion of best performing adsorbent OFG-10 in water, containing 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 adsorbent after the completion of the process. BET surface area analysis of ORMOSIL, Fe3O4-RGO, OFG-05, OFG-10 and OFG-15 are shown in Figure S3. All the compositess have Type IV isotherm and H3 hysteresis loops, which indicates 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 0.450 cm3/g and surface area 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 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


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volume and surface area of OFG-15 are 0.429 cm3/g and 165.255 m2/g. The decrease in surface area with increase in the Fe3O4-RGO concentration can be attributed to the fact that, higher loading of Fe3O4-RGO blocking 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 OFG-15. 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 the composites including Fe3O4-RGO, OFG-05, OFG-10 and OFG-15 absorbs in the visible region.45 With the increase in the concentration of ORMOSIL there is a slight decrease in the absorbance. All 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 dye adsorption experiment in the presence of ORMOSIL, Fe3O4-RGO, OFG-5, OFG-10 and OFG15. Within two minutes of stirring at room temperature, above 97 % of Rhodamine B is adsorbed in case of OFG-10 and around 92 % for OFG-5 which is much higher than ORMOSIL (~75 %), Fe3O4-RGO (~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, increase in concentration of Fe3O4RGO from 10 wt% to 15 wt% presumably blocks the voids of ORMOSIL cage, which results in decrease in the active sites. The efficient adsorption capacity of this composite is mostly due to hydrophobic moieties present in ORMOSIL as well as RGO which easily entrap both organic


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and inorganic pollutants. The hydrophobic interactions between hydrophobic moieties of ORMOSIL and RGO with aromatic rings of the Rhodamine B results 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 pseudo second order equation: 1 1 = + 02 where qt represent adsorption capacity of the adsorbent at time t, qe represent adsorption capacity at equilibrium and k2 (g/(mg min)) represent 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 were shown in Fig. 7. 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 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 nearly 20 times higher than only Fe3O4-RGO. To predict the relationship between the adsorbent and adsorbate, the adsorption equilibrium data were fitted to Langmuir and Freundlich isotherm models. Linearized form was applied to obtain all the parameters of Langmuir and Freundlich isotherm (equation 3 and 4) as shown in Figure S5. The experimental adsorption data of Rhodamine B fitted well to Langmuir as well as Freundlich isotherm. All the isotherm parameters are shown in Table S2. The value of n greater than 1 conclude that it is favorable for adsorption. Analysis using Langmuir isotherms,


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adsorption capacity of ORMOSIL-Fe3O4-RGO composite for Rhodamine B is found to be ~1339 mg/g which is much higher as compared to Fe3O4-RGO composite (~342 mg/g). In addition, the experimental data were analysed by different methods of least-squares regression.47-49 Three different linearized form of Langmuir isotherm model were analysed for Rhodamine B adsorption and the results were provided in the Table S5. From the analysis it is found that the linearized models are fitted well to the experimental data, with Langmuir III having least error. Isotherm

Nonlinear form

Langmuir I

!" #$ %!" #$

Langmuir II

Linear form

Plot

1 1 1 = + & & '(

1 1 )

= −

1 + & '(

1 = + & '( &

Langmuir III

)

)

Table 1. Linear forms of Langmuir isotherm.

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. Fig. 8(A) 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 8(B) 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


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conditions, which validate the use of UV-visible spectrophotometer for Arsenic detection in the range 0.04 ppm to 0.4 ppm. All the experiments were repeated and checked by atomic absorption spectrophotometer. Lower concentrations of Arsenic were detected with the help of an atomic absorption spectrophotometer. Figure S6(A) 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 Langmuir adsorption model, for which the mathematical linear expressions can be written as: 1 = + 03 & * Where Ce (mg/L) represent the dye/arsenic concentration at equilibrium, qe (mg/g) represent the adsorption capacity at equilibrium, qm (mg/g) represent the maximum adsorption capacity, and b (L/mg) represent the constant related to free energy of adsorption. The mathematical linearized form of Freundlich adsorption isotherm can be expressed by the following equation: 1 log = log '/ + log 04 0 Where Ce (mg/L) represent the concentration of dye/arsenic at equilibrium, qe (mg/g) represent the equilibrium adsorption capacity, and Kf (mg/g) represent the Freundlich experimental constants related to adsorption capacity and n represent the adsorption intensity.

The linear forms of Langmuir and Freundlich isotherm were fitted to obtain all the parameters (Figure S6(B) and Figure S7). At the higher concentration range all the adsorption data are fitted well to both Langmuir and Freundlich models. At the lower concentration range the experimental


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data are fitted well only to 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 Fe3O4-RGO (~31 mg/g) according to Langmuir isotherm. The n value from 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 expressed as follows: 20

− = −% 05

Where qe is the capacity of adsorption of adsorbent at equilibrium, qt is the capacity of adsorption of 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 equation 2. The plot of t/qt versus t gives the values of k2 and qe. Different kinetic parameters after calculation are summarized on 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 correlation coefficients of pseudo-first order (R2= 0.983-0.890). Moreover, the theoretical and experimental values of qe, for pseudo-second 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


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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 cycle for adsorption of Rhodamine B onto OFG-10. Due to the magnetic nature of the sample, recovery can be achieved by the help a permanent magnet followed by centrifugation and washing with ethanol. Recovered powder sample is dried at 80oC. 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 Fig. 10 and the recovered adsorbent is found to be chemically 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 95oC for further use. Successive five cycles adsorption –desorption experiments were repeated and the adsorption efficiency for arsenic adsorption was found to be 81% as shown in Fig. 11. The adsorption in cycle 3 was found to be slightly lower (less than 1%) than cycle 4, which may be attributed to the random error caused by experimental uncertainties due to slight differences in temperature or time during desorption process.

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.

A near 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 Fe3O4-RGO and ORMOSIL. A remarkable adsorption capability for the


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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.

Supporting Information The 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 are given in the Electronic Supporting Information. The Supporting Information is available free of charge via ACS Publications website http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Fax: +91-361-2582349; Tel: +91-361-2582320, Material Science Laboratory, Department of Chemistry, Indian Institute Technology, Guwahati – 781039, Assam, India.

ACKNOWLEDGMENT 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/2014–15 and Council of Scientific and Industrial Research (CSIR), India for financial support via Grant no.


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CSIR/01 (2704)/12/EMR-II. The help from IIT Guwahati, Central Instruments Facility – IIT Guwahati are highly appreciated for providing infrastructure and instrumentation. TKS thanks Shraban Kumar Sahoo, Mohammad Shaad Ansari and Gaurangi Gogoi for fruitful discussions and assistance in characterizations.

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49. Conney, D.O. Adsorption Design for Wastewater Treatment, Lewis Publishers, Boca Raton, FL, 1998.

Schematic

Scheme 1. Synthesis of ORMOSIL-Fe3O4-RGO composite. Typical modified Hummers method is used to produce RGO, followed by the in-situ synthesis Fe3O4. Base hydrolysis using TEOS and VTMOS resulted in 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 picture.


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Figures and Captions

(A)

(220) (220)

(311)

(440) (440)

(311)

OFG-15 OFG-10

(B)

OFG-15

OFG-5

Intensity (a.u.)

(440) (220) (311)

Intensity (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


(311) (220)

(400)

(422) (511)

(440)

Fe3O4-RGO

ORMOSIL (002)

RGO

(100)

OFG-10

OFG-05

GO

10

20

30

40

50

60

70

80

10

15

2θ (degree)

20

25

30

2θ (degree)

Figure 1. (A) Powder X-ray diffraction pattern of Graphene Oxide (GO), ORMOSIL, Fe3O4RGO, OFG-5, OFG-10 and OFG-15. The red circle portion including OFG-5, OFG-10 and OFG15 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 under red circle portion in (A).


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Figure 2. Infra-red spectra of RGO, Fe3O4-RGO, ORMOSIL and OFG-10. The solid line parallel to 570 cm-1, which corresponds to Fe-O band can only be observed for Fe3O4-RGO and ORMOSIL-Fe3O4-RGO (OFG-10) with a red shift. The dotted line parallel to 1465 cm-1 shows that this peak can only be seen for 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 interaction between iron oxide and reduced graphene oxide.


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


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Figure 4. Energy-dispersive X-ray spectroscopy (EDX) mapping of best performing adsorbent OFG-10 (A) and the corresponding elemental mappings of Silicon (B), Oxygen (C), Iron (D) and Carbon (E), suggests the homogeneous distribution of all the elements in the area (A).


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Figure 5. (A) TEM image of 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.


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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). Inset shows the amount of dye adsorbs after 2 minutes with respect to all the adsorbents.


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Figure 7. Pseudo-second order kinetics for Rhodamine B adsorption on adsorbents ORMOSIL, Fe3O4-RGO, OFG-5, OFG-10 and OFG-15.(adsorbent dose- 0.5 g/L, dye concentration- 50 mg/L, temperature- 298 K). The rate constant for best performing adsorbent OFG-10 is almost 20 times higher than Fe3O4-RGO.


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Amount of As(III) Adsorbed (mg/g)

100

A

80 60 40

ORMOSIL Fe3O4-RGO

20

OFG-05 OFG-10 OFG-15

0 0

20

40

60

80

100

100

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B

80 60 40

Fe3O4-RGO (Dark)

20

OFG-10 (Dark) Fe3O4-RGO (Light) OFG-10 (Light)

0 0

120

20

40

60

80

100

120

Time (min)

Time (min)

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, pH~7) and (B) detection of arsenic concentration in light and dark conditions.

1.8

1.8

A

3 ppm 4 ppm 5 ppm

1.6 1.4

1.6

B

3 ppm 4 ppm 5 ppm

t/q (min g/mg) t

2.0

t/q (min g/mg) 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

Amount of As(III) Adsorbed (mg/g)


1.4 1.2

1.2

1.0

1.0

0.8

0.8

0.6

0.6 0.4

0.4

0.2

0.2 0

20

40

60

80

100

0

Time (min)

20

40

60

80

100

Time (min)

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


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


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100

Percentage of Adsorption

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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80

60

40

20

0 1

2

3

4

5

No. of Cycles Figure 11. The adsorption of arsenic up to five successive cycles. After third cycle the adsorption efficiency is found to be almost constant (~81%).


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Table of Content:


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Graphical Abstract: Efficient and rapid removal of environmental malignant Arsenic (III) and industrial dyes using re-usable, recoverable ternary Iron Oxide – ORMOSIL - Graphene Oxide composite Tushar Kanta Sahu, Sonia Arora, Avishek Banik, Parameswar Krishnan Iyer, Mohammad Qureshi* Material Science Laboratory, Department of Chemistry, Indian Institute Technology, Guwahati – 781039, Assam, India. E-mail: [email protected]; Fax: +91-361-2582349; Tel: +91-3612582320

Modified Hummer’s Method GO

Graphite

0 Sec

30% NH3 N2H4, 90oC

240 Sec

0 Sec

Fe(III)

Fe(II)

30% H2O2

40 Sec Fe3O4-RGO VTMOS, TEOS

ADSORPTION

MAGNETIC SEPARATION


ORMOSIL-Fe3O4-RGO

Efficient and rapid removal of environmental ... - ACS Publications

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