Morphology Controlled Fabrication of Highly Permeable Carbon

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Morphology Controlled Fabrication of Highly Permeable Carbon Coated Rod-Shaped Magnesium Oxide as a Sustainable Arsenite Adsorbent Ananya Ghosh,† Sourav Biswas,† Sayanta Sikdar,†,‡ and Rajnarayan Saha*,† †

Department of Chemistry and ‡Department of Earth and Environmental Studies, National Institute of Technology, Durgapur, West Bengal, India 713209

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S Supporting Information *

ABSTRACT: Arsenic poisoning from drinking water has been causing distress to millions of people worldwide due to its toxicity and carcinogenicity. Regardless of numerous long stretches of research, manageable arsenic treatment innovations which are cost-effective in implementation seem to be sporadic. Herein, we demonstrate the facile synthesis of financially savvy rod-shaped carbon coated magnesium oxide (C-MgO) and its characteristic capacity toward arsenite adsorption. With a specific surface area of 117.6 m2/g and a controlled mesoporous shell, this material is a better adsorbent than its synthetic counterpart, N-MgO, integrated without a dextrose intervened calcination procedure of precursor. Results show that C-MgO has an adsorption capacity of 142 mg/g at pH 7 after 4 h with an initial As(III) concentration of 80 mg/L. In situ formed easily accessible hydroxyl groups on the surface and porous channels articulate its multilayer chemisorption ability which is empirically well-fitted to the Freundlich isotherm model with an R2 value of 0.996, while the adsorption kinetics data follows pseudo-second-order kinetics (R2 = 0.999). In addition, C-MgO has efficient reusability with almost 67% removal efficiency after four cycles. As far as real-time applicability is concerned, 90% of As(III) was adsorbed within 20 min for groundwater As(III) concentrations up to 165 ppb. These qualities give noteworthy innovative perceptions to the utilization of tailored MgO nanoparticles in groundwater purification.

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INTRODUCTION The proliferating population of the earth has impelled people to extensively utilize numerous drinking water resources. Groundwater is one of the most common wellsprings of drinking water, and almost half of the global populace depends on this resource.1 However, groundwater has become exceedingly polluted due to various man-made and natural causes.2−4 The categories of pollutants include heavy metals (As, Cd, Hg, Pb, etc.), 5−10 inorganic anions (F −, NO 3− , etc.),11,12 pathogens,13 and different organic compounds (pesticides, drugs, volatile organic compounds, etc.).14 Among these pollutants, arsenic is one of WHO’s 10 chemicals of significant general wellbeing concern.15 A list of nations all over the world that are affected by this toxic metalloid includes Bangladesh, India, Thailand, Inner Mongolia, Argentina, China, Brazil, Mexico, etc.16,17 In India, West Bengal, Bihar, Jharkhand, parts of Uttar Pradesh, Assam, and Chhattisgarh are among some of the most contaminated areas.17,18 Arsenic is used for different industrial purposes, and in pesticides and pharmaceuticals.19,20 The arsenic loaded effluents from these industries, with or without a primary treatment, are discharged into local water bodies. Besides these, dissolution of arsenic due to weathering of sulfide ores of arsenic and iron oxides and hydroxides also leads to groundwater contamination.21 Arsenite has caused severe health problems to a lot of people around the world as ingesting arsenic contaminated water could lead to its accumulation in the body causing lesions on the skin, © XXXX American Chemical Society

hyperpigmentation, and keratosis, and in the long run, it can cause cancer.22 Several research groups have come up with different techniques for the removal of arsenite (H3AsO3), which include the oxidation of arsenite to the less toxic arsenate (H3AsO4) form and the adsorption of the ions from the medium.23,24 These methods of removal also include membrane filtration, ion exchange, electrochemical removal, and reverse osmosis.25 Each of these strategies has certain favorable circumstances as well as some disservices related to them. Hybrid ion exchangers can retain almost 80 mg/g of its weight, but the exchanging resins get saturated after a certain time and the discharge of the arsenite-rich effluent would revive the problem.26 Membrane filtration, reverse osmosis, and electrochemical removal can remove 90% of arsenic but are too expensive to be afforded by the common people.27,28 Among all these methods, adsorption by different materials seems to be a good option for removal as it is affordable, is easy to use, and generates less amount of secondary toxicants.29−31 Different materials have been in use for the adsorption of arsenite from groundwater which includes sand,32 red soil,33 different clays, metallic compounds,34 hollow organic frameworks,35 and hydrogels,36 to name a few. Over the past decade, Received: Revised: Accepted: Published: A

February 7, 2019 April 24, 2019 May 28, 2019 May 28, 2019 DOI: 10.1021/acs.iecr.9b00709 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Schematic Representation of N-MgO and C-MgO Synthesis

Figure 1. SEM micrographs of (a) N-MgO and (b) C-MgO. TEM images of (c) N-MgO and (d) C-MgO. (inset) High resolution TEM images and SAED patterns of corresponding particles.

ques such as chemical vapor deposition.49 Our aim here is to synthesize MgO by a minimal effort strategy which can be employed for the remediation of arsenic-contaminated water. In light of the above-mentioned facts, we concentrate on synthesizing MgO microstructures by a simple precipitation technique and apply it toward the adsorption of As(III) from real groundwater. We have introduced a carbon support in the form of dextrose as a template amid the calcination process so as to see the change in the properties of the particles orchestrated by this course. To date little work has been done on groundwater arsenic decontamination keeping in mind to make the technology viable, and this work is a step toward that initiative. The use of low-cost chemicals and easy appliances

micro- and nanostructures of different inorganic materials have emerged as strong contestants for different applications and the removal of heavy metals is one of them. Among all, MgO nano- and micromaterials have been used by several research groups for the removal of arsenite from aqueous medium.37−40 MgO is nontoxic, is thermally stable, is abundant, has high surface area, and has unsaturated surface sites that allow for better adsorption of pollutants.41 The versatile nature of MgO allows it to react with various other pollutants as well which include heavy metals,42,43 fluoride,44 and bacterial species.45 Previously MgO has been synthesized by several methods which involve common techniques such as the sol−gel method,46 solvothermal synthesis,47 chemical precipitation,48 and solution combustion technique,44 and expensive techniB

DOI: 10.1021/acs.iecr.9b00709 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. (a) XRD and (b) FT-IR spectra of N-MgO and C-MgO.

procedures and analytical methods for As(III) remediation is in the Supporting Information.

makes this a worthy technique for arsenic adsorption from groundwater.

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MATERIALS AND METHODS Chemicals. Magnesium chloride hexahydrate (MgCl2· 6H2O), anhydrous sodium carbonate, dextrose, and absolute ethanol were procured from Merck (India). A 1000 mg/L stock solution of As(III) was procured from Merck (Germany) which was diluted to prepare the experimental solutions. All the experimental solutions were maintained below pH 2 to avoid metal precipitation. Milli-Q H2O (18.2 MΩ) was utilized for the preparation of all the solutions. Synthesis of Particles. MgO particles were prepared by modifying the method of Purwajanti et al.37 Briefly, 100 mL of freshly prepared 1 M sodium carbonate solution was added dropwise into 100 mL of 1 M solution of MgCl2·6H2O with continuous stirring for 2 h. A white precipitate was obtained which was centrifuged and washed with water followed by absolute ethanol. Finally, the obtained precipitate was dried and used as the precursor. Further, the precursor was ground and mixed with 3.0 g of dextrose and calcined in a muffle furnace for 2 h at a fixed temperature of 550 °C. To carry out the comparative study, a similar preparation of MgO was carried out in the absence of dextrose and the process is schematically depicted in Scheme 1. Particle Characterization. A transmisssion electron microscope (TEM; FEI Technai T20) working at 200 kV was used to assess the microstructure of synthesized nanoparticles. A Bruker XFlash 6160 field emission scanning electron microscope (FESEM) was used to evaluate the microstructure of synthesized nanoparticles. Energy dispersive spectroscopic (EDS) analysis was also performed by using the attachment of the same SEM instrument. An X’Pert Pro (PANalytical) X-ray diffraction (XRD) unit was utilized to record the crystal structure data (with Cu Kα radiation source, λ = 1.5406 Å, 40 kV, and 30 mA). A Thermo-Nicolet iS10 spectrometer was used here for Fourier transform infrared (FT-IR) analysis where all samples were pelletized using KBr. Surface topography measurements were performed using a Quantachrome Instrument NOVA 1000e surface analyzer (USA; NOVA 1000e). Thermogravimetric analysis (TGA) was carried out in N2 atmosphere using the TA Instruments. A Hitachi F-2500 spectrofluorimeter was used for the fluorescence study. A Shimadzu UV-1800 was utilized for UV−vis absorption study. A detailed explanation of batch experimental

RESULTS AND DISCUSSION Characterization of Synthesized Particles. As discussed earlier, two types of MgO have been prepared which are indicated here as N-MgO and C-MgO, starting from a single precursor. SEM and TEM images in Figure 1 clearly depict the imperative part of dextrose in the calcination process which significantly alters the clustered structure of N-MgO to rodshaped C-MgO. The presence of dextrose initially helps in assembling the cluster and extends the anisotropic growth by selective capping of specific facets in the case of C-MgO. Figure 1a,c confirms the irregular aggregates of N-MgO particles, while Figure 1b,d confirms the regular arrangement of rod-shaped C-MgO particles with an average length of ca. 7−8 μm. Interestingly, the high resolution TEM (HRTEM) image inset of Figure 1d shows that, due to the presence of excess dextrose in the calcination process, a 250 nm thick amorphous coating has been produced on the surface of the CMgO, whereas no such layer was obtained in the case of the NMgO cluster. The formation of this remarkable 250 nm thick amorphous shell can be attributed to the interaction of the CMgO surface with the generated water vapor and CO2 during the calcination process. In addition, the diffuse selected area electron diffraction (SAED) pattern of C-MgO in comparison with N-MgO supports the porous nature of C-MgO particles. Additionally, for analyzing and quantifying the pore structures of these two synthesized MgO particles, N2 adsorption−desorption isotherm data were collected (Figure S1a). The data reveal that C-MgO has a higher surface area than N-MgO. In the case of C-MgO it is 117.579 m2/g, whereas for N-MgO it is 68.449 m2/g. Pore size and average pore volume are also high in the case of C-MgO and were recorded for both materials (Table S1). Therefore, it is clear that the shell around the rod-shaped C-MgO particles formed during the calcination process due to the interaction of the generated CO2 and water vapor with MgO is mainly porous in nature. This highlights the role of dextrose in enhancing the porosity of C-MgO. Again, in both cases N2 adsorption− desorption plots show a type IV isotherm pattern which indicates the existence of mesopores on the surface of both types of particles. Mesopores are the aftereffect of significant deformities in the structure of a solid and serve as passages, giving a vehicle framework, to the micropores. These C


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Figure 3. Time dependent concentration variation studies for removal of As(III) of various concentrations using (a) N-MgO and (b) C-MgO. Variation in As(III) removal efficiencies by (c) N-MgO and (d) C-MgO.

decreased in the case of C-MgO which represents the stability. A new peak is generated for C-MgO at 2921 cm−1; this corresponds to the sp3 C−H stretching frequency and confirms the presence of carbon. Room temperature UV−vis absorption spectra and fluorescence study also ensure the shell formation around C-MgO (Figure S3). The presence of a broad peak in absorption spectra is mostly because of larger particle size; however, the peak intensity is slightly decreased due to the shell formation which reduces the probability of generation of excited O2− surface ions in C-MgO. A similar type of observation is depicted in fluorescence study after exciting at 280 nm which is mainly due to the various structural defects and lesser availability of surface oxygen vacancies. Arsenic Adsorption Studies. The effectiveness of any nano- or micromaterial to react with any chemical substance relies on its size and available exposed surface area.52,53 This work mainly focuses on an economical synthesis of MgO with porous carbon as the template, so we have carried out experiments to compare the adsorption abilities of N-MgO and C-MgO. Figure 3 exhibits the time-dependent studies of different As(III) concentrations made to react with 0.5 g/L of both N-MgO and C-MgO. While Figure 3a,b shows the reduction in initial As(III) concentration with time, Figure 3c,d shows the changes in removal efficiency with time. It can be observed from Figure 3 that As(III) was adsorbed much better by C-MgO than by N-MgO. Figure 3 reveals that, after 2 h of reaction, C-MgO is able to remove about 80% of As(III) while N-MgO can remove only 70% of As(III) by that time from a solution containing 80 mg/L As(III). The uptake capacities were found to be 141.9 mg/g for C-MgO and 126 mg/g for N-MgO as calculated after 6 h of reaction (Figure

mesopores lead to good adsorption of the arsenite anions on the surface of the materials. The crystal structure of MgO particles was analyzed from their X-ray diffractograms (Figure 2a), and identified peaks were matched with cubic crystalline MgO structure corresponding to JCPDS No. 00-045-0946.50 The characteristic broad signal corresponds to the Bragg angle for the (111), (200), (220), (311), and (222) planes. The obtained XRD patterns of both particles are fully consistent with the previously shown SAED patterns (Figure 1). Thermogravimetric analysis shows a minimum weight loss of 10% in the case of N-MgO particles due to its highly hygroscopic nature (Figure S1b). However, in the case of C-MgO, the weight loss is controlled and it shows better thermal stability than the previous one due to the shell formation. However, to quantify the impregnation of carbon atoms in C-MgO which facilitates the controlled growth of the clusters, EDS spectra were recorded (Figure S2). The average atomic percentage ratio of Mg to O in N-MgO is 49:51. Incorporation of dextrose prior to the calcination process changes the overall composition and the observed atomic percentage ratio of Mg, O, and C becomes 42:47:11. Further, the FT-IR analysis confirms the presence of C−H bond in the C-MgO structure (Figure 2b). In both cases, the wide-ranging band from 3000 to 3700 cm−1 resembles the O−H stretching frequency. The peak at 1636 cm−1 corresponds to the CO stretching, and that at 1430 cm−1 corresponds to the bending vibrations of H−O−H which are readily adsorbed in the porous network.51 Interestingly, a sharp peak for N-MgO at 3700 cm −1 corresponds to the A2u(OH) lattice vibration of Mg(OH)2 which is generated due to hydration.38 But this peak is notably D


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Figure 4. (a) Langmuir isotherm plot for As(III) adsorption by N-MgO and C-MgO and linearized form of the Langmuir isotherm for (b) N-MgO and (c) C-MgO. (d) Freundlich Isotherm plot for adsorption by N-MgO and C-MgO and linearized form of the isotherm for (e) N-MgO and (f) C-MgO.

adsorption process occurs when the value of 1/n is below 1; otherwise, a cooperative adsorption process might occur.57 The adsorption data from the experimental studies were plotted using both types of isotherms (Figure 4). The parameters obtained from the plots were also recorded (Table S2). It is clear from Figure 4 that the adsorption by both types of particles follows the Freundlich adsorption isotherm model, as confirmed from the linear regression coefficient values obtained from the linearized plot of both types of isotherms. In the case of the Freundlich isotherm, R2 = 0.989 and 0.998 for adsorption by N-MgO and C-MgO, respectively. The value of the heterogeneity parameter (1/n) derived from the Freundlich Isotherm model is 0.7 in the case of N-MgO and 0.383 in the case of C-MgO, which further supports the fact that both types of particles follow the Freundlich isotherm model very well. This indicates multilayer adsorption of the arsenite anions on the surface of both types of particles.55 As the value of the heterogeneity factor gets nearer to zero, the chemisorption process becomes more heterogeneous. Here it can be pointed out that a higher value of the Freundlich adsorption coefficient (KF) indicates greater adsorption.58 Hence C-MgO having a KF value equal to 67.37 has better adsorption capacity in comparison to N-MgO having an adsorption capacity equal to 21.47. Arsenic Kinetic Studies Data. The adsorption kinetics data were analyzed using two very well-known kinetic models: the pseudo-first-order kinetic model and the pseudo-secondorder kinetic model. The pseudo-first-order kinetic model, which was at first presented by Lagergren to define the kinetic

S4). The above data can be validated by the fact that the mesoporous shell formed during the calcination, due to the charring of dextrose, increases the porosity and results in better adsorption of As(III). But the same thing does not apply for NMgO, which leads to less porosity and lesser As(III) adsorption ability. The adsorption data were plotted by using the Langmuir and Freundlich isotherm models. The Langmuir adsorption isotherm can be expressed by the following equation:53,54 Ce C 1 = e + qe qm qmKL

(1)

where qe is the adsorption capacity of the adsorbent per gram, Ce is the adsorbate concentration at equilibrium, KL is the Langmuir adsorption constant, and qm is the maximum adsorption capacity of adsorbent per gram. qm and KL were evaluated from the slope and intercept of the Ce vs Ce plot. qe

The Freundlich adsorption isotherm can be expressed by55 log qe = log KF +

1 log Ce n

(2)

where Ce is the equilibrium adsorbate concentration, KF is the Freundlich adsorption constant, and n is adsorption intensity.56 Here, KF and n were obtained from the plot of log qe vs log Ce. The value of n determines a favorable adsorption process when it has a value between 1 and 10. In addition, a normal E


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Figure 5. Kinetics plots for As(III) Adsorption of N-MgO and C-MgO: (a and c) pseudo-first-order model; (b and d) pseudo-second-order model.

Figure 6. (a) Variation in removal efficiency of nanoparticles with time. (b) Variation in uptake capacity of nanoparticles with dose variation.

based on the concentration dependent adsorption capacity.60 It can be written as

process of liquid−solid phase adsorption, depends on the adsorption capacity, which is nothing but the amount of adsorbate adsorbed per gram of adsorbent.55,59 The equation can be expressed as log(qe − qt ) = log qe −

k1 t 2.303

t 1 t = + 2 qt qe k 2qe

(4)

where the rate constant (k2) can be determined from a plot of t and time (t); qe and qt have the same meanings as described

(3)

where qe and qt are the quantities adsorbed at equilibrium and time t (min) respectively and the rate constant (k1) was evaluated from the slope of the plot of log(qe − qt) versus time. On the other hand, the pseudo-second-order kinetic model was presented by Ho for distinguishing the kinetic equation

qe

earlier. Figure 5 shows the kinetics plots for As(III) adsorption by N-MgO and C-MgO. The slope and the intercept from each plot provide the adsorption rate constants and the amount adsorbed at equilibrium (qe) for both types of F


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Figure 7. (a) Variation of removal efficiency of C-MgO in 50 mg/L As(III) at various pH values. (b) Variation of pH of aqueous solution containing C-MgO with time.

Figure 8. (a) Leached concentration of Mg and (b) As(III) holding capacity of C-MgO in time variables.

variation on As(III) removal was examined employing C-MgO only. On increasing the nanoparticle dose, the removal efficiency also increases and is highest for 1.0 g/L C-MgO particles (Figure 6a). However, the uptake capacity of the nanomaterials decreases with increasing dose value (Figure 6b) and is maximum for 0.25 g/L sorbent. It was calculated to be 119.08 mg of arsenite/g of sorbent because the adsorption capacity has an inverse relationship with the weight of the adsorbent. Effect of pH of the Medium. The pH of the reaction medium has a remarkable influence on the rate of adsorption of any contaminant from the reaction medium. The pH variation studies were conducted at five different pH values ranging from 3 to 11, for fixed C-MgO dose and As(III) concentration. The results of the experiments have been presented in Figure 7a and recorded in Table S5. Figure 7a depicts that varying the pH of the reaction medium does not appear to have much influence on the removal efficiency. As(III) is extracted from the reaction medium with more or less the same efficiency at all the pH values involved in the study. This fact can be clarified from Figure 7b. It is observed that, within a few minutes of commencement of the reaction, the pH of the medium becomes 11. This behavior might be attributed to the mildly basic nature of magnesium oxide. When water is added to MgO, it is adsorbed on the surface of MgO and this results in the formation of Mg(OH)+ and (OH)− ions.62 At pH 7, As(III) is present in its neutral form,

adsorption kinetics data and have been listed in Tables S3 and S4. The value of the regression coefficient data is 0.99 for all As(III) concentrations, which makes it clear that the kinetics data fit the pseudo-second-order model better than the other model. This indicates chemical interactions between the sorbate and the sorbent through interchanging of the charge carriers during the process of adsorption.60 The chemical sorption or “chemisorption” process removes As(III) from the medium due to physicochemical interactions between the sorbate and the sorbent which can also be confirmed from the FT-IR data of C-MgO collected after reaction with As(III).61 The adsorption capacity of the sorbent (qe) obtained from the pseudo-second-order kinetics plot also matches well with the calculated qe values, which further strengthens the reason for selecting this kinetic model. The regression coefficient values for the pseudo-second-order kinetic model are greater for CMgO than for N-MgO, so it can be said that C-MgO is a better choice for removing As(III) than N-MgO. Effect of C-MgO Dose Value. An essential part of any reaction is the surface area available for reaction.39 When the dose value of any nano- or micromaterial is increased, the available surface area of the nanoparticles is enhanced, and this improves the removal efficiency of the material. Figure 6 demonstrates the effect of increasing the dose value on the removal efficiency of As(III). As it can be already inferred from the previous experiments that C-MgO is superior in performance in comparison to N-MgO, the impact of the dose G


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Figure 9. (a) SEM micrograph of C-MgO with As(III), (b) magnified SEM micrograph of C-MgO with As(III), (c) TEM image of C-MgO with As(III), and (d) magnified TEM image of C-MgO with As(III).

but it becomes negatively charged with the increasing pH value which favors the adsorption process because of the electrostatic attraction forces from the positively charged particle surface.39 However, beyond pH 9, the efficiency decreases with increasing pH. This is because the adsorption of weak acid anions by metallic oxides reduces when the pKa1 of the acid is reached.27 As the pKa1 for arsenious acid is 9.1, the removal efficiency decreases after that. Table S5 presents the data for the pH variation in a summarized form. However, on a large scale the pH of the water samples should not change much due to the addition of a small amount of C-MgO, so the effluent after reaction should be within limits as per drinking water regulations. Magnesium and Arsenic Leaching Study. Here, we performed a leaching study of Mg and As(III) to verify the stability in the aqueous medium and the holding capacity of CMgO. Figure 8a shows the time profile of magnesium leaching when 0.5 g of C-MgO was added to 1 L of deionized water. The samples were withdrawn from the reaction medium at regular intervals and were preserved for analysis of magnesium ions by flame atomic absorption spectrophotometry. Figure 8a reveals that the synthesized compound is quite stable and very little magnesium was released during the experiment. Figure 8b indicates the As(III) holding capacity of C-MgO. In order to perform the As(III) leaching study, C-MgO particles were added to 100 mg/L test solutions of As(III) and then the arsenic loaded adsorbent was removed from the solution and dried in an oven at 100 °C. The adsorbent was then added to deionized water and agitated for 48 h inside an incubator cum shaker. The filtrates obtained after separation were preserved for As(III) analysis. The concentrations of As(III) in the

samples were measured by an atomic absorption spectrophotometer working in the graphite furnace mode, as measured previously. It can be seen from Figure 8b that, even after 75 days, only 29% of As(III) has been released by the adsorbent, which proves the efficiency of this adsorbent. On the contrary, when similar studies were conducted employing N-MgO, it was seen that over 37% of As(III) was released over a span of 75 days, which also proves the superiority of C-MgO over NMgO and shows that the former has better sorption capacity as compared to the latter. Characterization of Particles after As(III) Adsorption and Mechanism of the Adsorption Process. It can be inferred from the above studies that C-MgO exhibits better As(III) adsorption efficiency than N-MgO. Therefore, to find out the actual mechanism of the As(III) adsorption on the mesoporous extended shell of the synthesized particles, we investigate the structural variation of C-MgO after the As(III) adsorption process by SEM and TEM studies. A noteworthy modification is observed in the morphology of the C-MgO after the adsorption process which is depicted in Figure 9. After 2 h immersion of C-MgO in the 80 mg/L As(III) solution, a coarse surface morphology is seen from SEM micrographs which are depicted in Figure 9a,b. Interestingly the TEM images in Figure 9c,d show a sharp 100 nm escalation in the thickness of the amorphous layers at the outer surface of C-MgO after the adsorption process. This observation clearly supports the multilayer adsorption of As(III) on the surface of the C-MgO particles as it follows the Freundlich adsorption isotherm. Further, FT-IR analysis results also support the adsorption of As(III) on the surface of C-MgO (Figure 10a). We compare H


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Figure 10. (a) FT-IR, (b) XRD, and (c) EDS analysis of C-MgO after As(III) adsorption

partially due to the conversion of MgO to Mg(OH)2; however, as the transformation proceeds toward completion, the adsorption process is controlled by the highly accessible hydroxyl groups. Interestingly, if we look at the uptake capacity of C-MgO, it is rapid at the initial stage. This observation is mainly attributed to the initial adsorption of As(III) on the exterior surface of the material. The adsorption on the external surface proceeds until saturation; however, As(III) then starts to enter into the porous shell to pronounce the adsorption process internally. During the diffusion of As(III) through the porous channel, diffusion resistance is enlarged which in turn creates the plateau in the uptake capacity curve. Therefore, it can be said that due to the multilayer chemisorption process the mesoporous carbon shell coated rod-shaped C-MgO shows high adsorption capacity and fast removal rate for its potential application. Reusability of Spent C-MgO Particles. Reusability is an important aspect of any good adsorbent other than being nontoxic and easily synthesizable. To test the reusability of the spent C-MgO nanoparticles, they were treated with a 0.05 M solution of NaOH for 8 h. Thereafter, the spent adsorbent was recovered and washed multiple times with deionized water and dried in the oven for 24 h. The regenerated adsorbent obtained in such way was again subjected to the arsenic extraction process of concentration 50 mg/L. This process of adsorption and desorption was continued up to four cycles which is

the FT-IR spectra of C-MgO, in water with and without the presence of As(III). We find that, in the presence of water, Mg(OH)2 is formed which is confirmed by the O−H stretching vibration peak at 3700 cm−1. But interestingly in the presence of As(III), the intensity of the O−H stretching vibration decreases whereas a fresh peak is detected at 779 cm−1 which correspond to the As−O bond frequency. Therefore, it is clear that the whole adsorption process involves several reactions. XRD (JCPDS No. 44-1482) (Figure 10b) and TGA also confirm the complete formation of Mg(OH)2 during the As(III) adsorption process (Figure S5). Room temperature UV−vis spectra and fluorescence study also approve of the adsorption on the surface of the synthesized particles (Figure S6). Due to the 150 nm thick shell of As(III) on the surface of the C-MgO, there is a complete loss of available oxygen ions for excitation which results in a deviation in the UV−vis absorption spectra. In the case of fluorescence study, after excitation at 280 nm wavelength, a quenching is observed in fluorescence intensity which clearly suggests the adsorption process. In order to quantify for the adsorbed arsenic on the surface of C-MgO, we have done an EDS study on the As(III) adsorbed surface which is depicted in Figure 10c. Due to the mesoporous surface morphology, C-MgO readily allows hydration on its surface and enhanced adsorption kinetics with time. Initially, the adsorption of As(III) occurs I


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among the samples with time. Within 20 min of reaction 90% removal was achieved and after 2 h more than 99% of the As(III) was adsorbed from the reaction medium in the presence of coexisting anions in the system (Figure 12c). These results simply justify the usability and applicability of the synthesized C-MgO particles for the fast removal of arsenite from groundwater samples.

depicted in Figure 11. Although decreased, the adsorption limit of recovered C-MgO is as yet higher for its handy appropriateness for long-term removal of As(III).

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CONCLUSION This work emphasized on a synthesis method of carbon coated MgO which is cost-effective and can be readily applied to groundwater decontamination. We have successfully synthesized magnesium oxide supported by a carbon matrix derived from calcination of a natural carbon source, dextrose, and applied it for arsenite removal present in synthetic and real groundwater samples. Microstructural characterization of the synthesized C-MgO particles showed that they had a rodlike morphology with a 250 nm thick mesoporous carbon coating on them. The adsorption data best fit the Freundlich adsorption isotherm showing that the adsorption process involves chemical interactions between the sorbent and the sorbate. Such an observation was also verified from the detailed characterization of the C-MgO particles collected after reaction. The maximum adsorption capacity of C-MgO is 142.85 mg/g as obtained from the pseudo-second-order kinetics model. The variation in the pH of the medium seemed to have little effect on the removal of As(III). Higher or alkaline pH values lower the removal efficiency of the particles, and the removal was highest at pH 5. On analyzing the real groundwater samples containing arsenic at concentrations higher than the permissible limit, it was seen that after 2 h of reaction time more than 99% of As(III) was adsorbed from the reaction medium in the presence of various other substances present in the medium. The appealing recovery and reusability test additionally ensure the applicability of these highly permeable rod-shaped C-MgO particles.

Figure 11. Reusability of C-MgO up to four cycles with respect to removal efficiency.

Real-Time Application of C-MgO Nanoparticles for Groundwater Treatment. Groundwater samples were collected from localities near the Karimpur-II block of Nadia district, West Bengal, India. This place is one of the most contaminated regions in West Bengal. The water quality parameters of the aqueous samples collected from that region have been recorded in Table S6. It can be seen from the table that all other parameters, besides the level of arsenic, meet the safe drinking water quality parameters. It has been shown in many studies that chronic exposure to even low concentrations of As(III) can lead to adverse health effects.63 Hence the realtime sampled solutions were treated by C-MgO particles. Table S7 represents the data of the aqueous samples which were treated with C-MgO particles (0.5 g/L). Figure 12a displays the variation in the As(III) concentration of three different As(III) solutions. It can be seen from Figure 12a that, within 20 min, there is a significant reduction in the amount of As(III) present in the solutions. The lowest concentration of As(III) in the samples is that of 21.75 ppb, which decreases to 0.07 ppb within a span of 2 h. Figure 12b shows the variation of the removal efficiency of the highest As(III) concentration

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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/acs.iecr.9b00709. Batch experimental procedures for As(III) remediation; N2 adsorption−desorption isotherm and TGA of NMgO and C-MgO; EDS analysis of N-MgO and C-

Figure 12. (a) Removal concentration of As(III) with time, (b) removal efficiency with time with 169.55 ppb As(III) present, and (c) bar plot of As(III) removal with time of real groundwater samples on treatment with 0.5 g/L C-MgO nanoparticles. J


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MgO; UV−vis absorption and fluorescence spectra of NMgO, C-MgO, and C-MgO after As(III) adsorption; As(III) uptake capacities of N-MgO and C-MgO; TGA analysis of As(III) adsorbed C-MgO particles; textural properties of synthesized particles; As(III) adsorption isotherm parameters of N-MgO and C-MgO; As(III) adsorption kinetic parameters by N-MgO and C-MgO; pH variation of 10 mg/L As(III) solution; water quality parameters of collected aqueous samples for real groundwater analysis; properties of real groundwater samples containing As(III) when treated with 0.5 g/L CMgO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 9434788063. ORCID

Ananya Ghosh: 0000-0003-1545-5916 Rajnarayan Saha: 0000-0001-6167-2271 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from NIT Durgapur. REFERENCES

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