Amine-Functionalized and Hydroxyl-Functionalized Magnesium

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Amine-Functionalized and Hydroxyl-Functionalized Magnesium Ferrite Nanoparticles for Congo Red Adsorption Chattharika Aoopngan,†,‡ Jeeranan Nonkumwong,† Santi Phumying,§ Wilasinee Promjantuek,∥ Santi Maensiri,§ Parinya Noisa,∥ Supree Pinitsoontorn,⊥ Supon Ananta,# and Laongnuan Srisombat*,†,∇

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†

Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand ‡ Graduate School, Chiang Mai University, Chiang Mai 50200, Thailand § Advanced Materials Physics Laboratory (Amp.), School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand ∥ Laboratory of Cell-Based Assays and Innovations, School of Biotechnology, Institute of Agricultural Technology, Suranaree University of Technology, 111 University Avenue, Nakhon Ratchasima 30000, Thailand ⊥ Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), Khon Kaen University, Khon Kaen 40002, Thailand # Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand ∇ Center of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand S Supporting Information *

ABSTRACT: In this paper, the potential use of either aminefunctionalized or hydroxyl-functionalized magnesium ferrite (MgFe2O4) nanoparticles (NPs) as Congo red nanoadsorbents is explored and compared. The amine-functionalized MgFe2O4 NPs (denoted as MgFe2O4−NH2 NPs) were synthesized by a one-pot coprecipitation method using ethanolamine as a surface modifier, while the hydroxylfunctionalized MgFe2O4 NPs (denoted as MgFe2O4−OH NPs) were prepared by a hydrothermal method. In general, both nanoadsorbents can be successfully produced without calcination and were found to possess superparamagnetic properties with high saturation magnetization (Ms). In particular, MgFe2O4−OH NPs exhibit a higher Ms value of ∼53 emu g−1, promoting the rapid separation ability of the NPs from the treated solution using an external permanent magnet. The Congo red removal performance of these nanoadsorbents was investigated as a function of the pH of the aqueous solution and contact time. The removal efficiency of Congo red by MgFe2O4−NH2 NPs was found to be ∼96% within 180 min at pH 6, while MgFe2O4−OH NPs provided a removal efficiency at ∼88% within 420 min at pH 8. In addition, the maximum adsorption capacities (qm) calculated using the Langmuir isotherm equation were found to be 71.4 and 67.6 mg g−1 for MgFe2O4−NH2 and MgFe2O4−OH NPs, respectively. The higher qm value of MgFe2O4−NH2 NPs could be attributed to stronger electrostatic interactions with the sulfonate groups of Congo red formed by larger numbers of protonated amine groups than protonated hydroxyl groups of the adsorbents under the performed conditions. Moreover, reusability experiments also revealed that MgFe2O4−NH2 NPs offered a higher removal efficiency than MgFe2O4−OH NPs for the same cycles tested. Therefore, this study demonstrates that MgFe2O4−NH2 NPs synthesized by a simple one-pot synthetic method are applicable as reusable magnetic nanoadsorbents for Congo red removal in current practice. KEYWORDS: Congo red, MgFe2O4 nanoparticles, adsorption, magnetic nanoadsorbents, amine-functionalized nanoparticles, hydroxyl-functionalized nanoparticles

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INTRODUCTION Environmental problems, particularly water pollution caused by industrial growth, are a matter of great concern.1 Congo red is an azo dye that is widely used in the manufacturing of textiles, papers, plastics, etc.2 A more serious problem has arisen in connection with Congo red contamination in water due to improper processing, resulting in both oxygen reduction and poor penetration of sunlight.2 These circumstances can © XXXX American Chemical Society

significantly affect the biological activities of marine life, especially plants and aquatic organisms. In addition, Congo red also causes toxicity and diseases such as cancer and respiratory tract irritation for water consumers. Therefore, treatment of Received: July 10, 2019 Accepted: July 23, 2019

A

DOI: 10.1021/acsanm.9b01305 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Synthesis of (a) MgFe2O4−NH2 NPs by Coprecipitation Method and (b) MgFe2O4−OH NPs by Hydrothermal Method

respectively, could adsorb Congo red most effectively at pH 6. The discrepancy could be due to their different adsorption characteristics generated via different processing histories. Apart from the popular Fe3O4, magnesium ferrite (MgFe2O4) materials could be another option as a Congo red adsorbent since they are also composed of nontoxic elements that are safe to use and environmentally friendly. To support this claim, these materials have been reported as noncytotoxic materials.15 However, some ferrites, such as cobalt ferrite (CoFe2O4), may cause toxicity to the skin and respiratory, reproductive, nervous, and immune systems according to the literature.16 More interestingly, when using MgFe2O4 as an adsorbent, the presence of Mg in MgFe2O4 shows enhanced adsorption capacity of metal ions compared with Fe2O3.17 In the literature to date, there are few reports on the utilization of MgFe2O4 for Congo red adsorption except for He et al.18 However, a twostep synthetic method consisting of solution combustion and gel calcination processes was employed to synthesize MgFe2O4 in their work. Therefore, the simple preparation of MgFe2O4 NPs without calcination is an interesting issue for further study. Apart from the synthetic method, surface modification of the adsorbent is also a key issue to be considered. It has been known that the adsorption behavior of dyes strongly depends on the surface functional group of adsorbents, which provides binding affinity to adsorbates. The amine group has been used to modify the surface of adsorbents and shows great binding affinity and selectivity to anionic dyes.19,20 However, the surface functionalization processes of adsorbents are complex and cost-effective. Recently, our group reported a successful one-pot synthesis of amine-functionalized MgFe2O4 nanoparticles via a coprecipitation method. In addition, these nanoparticles provided effective removal of Pb2+ from wastewater.11 To this end, we are interested in the utilization of amine-functionalized MgFe2O4 NPs synthesized via a one-

the dye effluent before discharging wastewater into any natural source is very important.3 Several techniques have been developed to remove Congo red from wastewater, such as photolysis,4 coagulation,5 electrochemical processes,6 and adsorption.1 Among these methods, adsorption is a popular solution owing to its effectiveness, low cost, lack of secondary pollution, and easy implementation.7 A number of adsorbents have been reported for the removal of Congo red.3,7,8 A wellknown adsorbent for many types of adsorbates is activated carbon,7 which is expensive and difficult to regenerate.2 Nanomagnetic materials are very attractive due to their superior specific surface areas and are suitable for use as an adsorbent.3 Moreover, they can be separated from the medium by simply applying an external magnetic field after the adsorption process, which enables their further regeneration and reuse.9 One of the most popular nanomagnetic materials is ferrite, which has been widely used to remove several dyes and heavy metals from aqueous solutions owing to their excellent physical and chemical properties, such as high saturation magnetization, high chemical stability, and easy surface modification or functionalization,10 including their large specific surface area characteristics, which are generally required for adsorption processes.11 Several reports have been published on employing magnetic ferrite (MFe2O4,8 where M represents divalent metal cation) for Congo red removal.12−14 These materials were synthesized by several methods promoting different surface properties (e.g., charge, functional group, and surface area). These differences in turn influence the adsorption mechanism between Congo red and nanoadsorbents under optimized conditions. For example, the adsorption process between magnetite (Fe3O4) nanoparticles synthesized via the hydrothermal method and Congo red occurred effectively at pH 2.15 However, the same material provided by Chaudhary et al.12 and Paşka et al.14 synthesized via simple chemical precipitation and combustion methods, B


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energy dispersive X-ray spectroscopy (EDS) analyzer equipped with a field emission scanning electron microscope (SEM; JEOL JSM6335F) was employed for chemical composition analysis. The magnetic properties of the samples were measured by a vibrating sample magnetometer (VSM; Quantum Design Versalab) at room temperature in an applied field range of ±10 kOe. The specific surface area and pore size of the nanoadsorbents were analyzed via N2 adsorption−desorption isotherms with surface area and pore size analyzers (Quantachrome model Autosorp 1 MP) using the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively. The samples were dehydrated prior to the experiment at 100 °C for 12 h. The functionalization of the powders was characterized by Fourier transform infrared (FT-IR) spectroscopy with a Bruker TENSOR 27 in the range of 400−4000 cm−1 using the KBr method. The surface chemical composition data of nanoadsorbents were examined by X-ray photoelectron spectrometry (XPS; Kratos AXIS Ultra DLD) with a monochromatic Al Kα X-ray source (1486.6 eV). C 1s with a peak at 285.0 eV was used as the reference peak. All XPS peaks were deconvoluted with respect to spin−orbit splitting using Vision2 software. Zeta potential measurement was performed using a Zetasizer Nano ZS (Brookhaven, ZetaPALS) at room temperature, in which the measured particle velocity is due to an applied electric field that is related to the surface charge.25 The measurements were taken by dispersing the sonicated samples at a concentration of 0.1 mg mL−1. The ionic strength was adjusted to 1 × 10−3 mol L−1 NaCl. The pH was adjusted using 0.1 mol L−1 hydrochloric acid and 0.1 mol L−1 sodium hydroxide. Adsorption Studies. Congo red adsorption was conducted by batch experiments. In a typical adsorption experiment, an amount of 42 mg of MgFe2O4−NH2 and MgFe2O4−OH powders was added into a series of flasks, each containing 15 mL of 30 mg L−1 aqueous solution (the concentration where the highest removal percentage was obtained, data not shown) of Congo red (structure is shown in Figure S1). Then, the mixture was shaken using a digital precise shaking water bath (DAIHAN Scientific, WSB-45) with a speed of 160 rpm at 25 °C. The effects of pH and contact time on Congo red removal efficiency were examined. The pH of the Congo red solution was adjusted to range from 2 to 12 using 0.1 mol L−1 HNO3 or NaOH solutions for the effect of the pH experiment, using 180 and 420 min as contact times for MgFe2O4−NH2 and MgFe2O4−OH nanoadsorbents, respectively. To study the effect of contact time, the optimized pH values were fixed at pH 6 and pH 8 for MgFe2O4−NH2 and MgFe2O4−OH nanoadsorbents, respectively. The flasks were removed from the shaker at different time intervals from 0 to 540 min. At the end of the adsorption experiments, the adsorbents were separated by a magnetic separator. The absorbance of Congo red in the solution, corresponding to adsorption efficiencies of the adsorbents, was measured by UV−visible spectroscopy (PG Instruments T80 UV/vis spectrophotometer) at the maximum absorbance wavelength of 499 nm for both before and after adsorption processes. The removal efficiency (%) and amount of adsorbed Congo red at equilibrium (mg g−1) were calculated as the equations given below:

pot coprecipitation method, no calcination step, and prompt surface modification in the adsorbent preparation step as an adsorbent for the removal of Congo red. The adsorption behavior of the MgFe2O4−NH2 NPs will be investigated and then compared with that of MgFe2O4 NPs synthesized by the hydrothermal method, which generally provided MgFe2O4containing hydroxyl (−OH) groups on the surface (MgFe2O4−OH NPs).17,21 The hydrothermal method, a widely used method to synthesize metal oxide nanoparticles, is also employed here due to the lack of calcination steps required to form MgFe2O4 17,21,22 and as a nonfunctionalized surface. Generally, the surface of MFe2O4 nanoparticles is in hydroxyl form in air (moisture existence) or when dispersed in water.23,24 Moreover, an attempt to synthesize bare MgFe2O4 via a coprecipitation method was unsuccessful. After the synthesis of MgFe2O4 nanoparticles from both methods, significant characteristics of the products were carefully determined using a combination of several techniques. In particular, the Congo red removal performance of these nanoadsorbents was investigated as a function of the pH of aqueous solution and contact time to obtain optimized conditions for each product. The Congo red adsorption kinetics, isotherms, adsorption capacity, and reusability of these nanoadsorbents were also examined and compared.

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EXPERIMENTAL SECTION

Materials. Mg(NO3)2·6H2O (Loba Chemie, 99% purity), Fe(NO3)3·9H2O (Carlo Erba, 98% purity), ethylene glycol (EG) (Fisher Scientific, 99.5% purity), CH3COONa (Loba Chemie, 99.5% purity), ethanolamine (QRëC 99% purity), HNO3 (Merck, 65% ACS reagent), NaOH (Ajax, 97.0% purity), and Congo red (Loba Chemie, 95% purity) were used as the starting chemicals, without further purification. Synthesis of Magnesium Ferrite Nanoadsorbents. Coprecipitation Method. Amine-functionalized magnesium ferrite (MgFe2O4−NH2) nanoadsorbent was synthesized by the coprecipitation method, which was slightly modified from a procedure advocated by Nonkumwong et al.11 The synthesis scheme is provided in Scheme 1. Generally, 15 mmol of CH3COONa was dissolved in 10.0 mL of EG and heated to 100 °C under a magnetic stirring and refluxing system for 15 min. A solution of 1 mmol of Mg(NO3)2· 6H2O and 2 mmol of Fe(NO3)3·9H2O dissolved in 5.0 mL of EG was poured into the preheated solution rapidly. The mixture was stirred for 30 min, and then 3.5 mL of ethanolamine was added. The mixture was heated to 200 °C and maintained for 12 h. After completion of the reaction, the system was cooled to room temperature naturally. The black precipitate was collected by a magnetic separator, washed with deionized (DI) water excessively, and then ethyl alcohol. The final product was dried at 70 °C for 12 h. Hydrothermal Method. The preparation process of hydroxylfunctionalized magnesium ferrite (MgFe2O4−OH) nanoadsorbent was performed via the hydrothermal method, which was slightly modified from a procedure advocated by Nonkumwong et al.22 In a typical experiment, 1 mmol of Mg(NO3)2·6H2O and 2 mmol Fe(NO3)3·9H2O were dissolved in 30 mL of EG, followed by the addition of 15 mmol of CH3COONa. The solution was transferred into a Teflon-lined stainless steel autoclave (series 5500 HP compact reactor), heated to 200 °C, maintained for 12 h, and finally cooled to room temperature. The black precipitate was collected by a magnetic separator and washed with DI water and ethyl alcohol. The final product was dried at 70 °C for 12 h. Characterization. Phase formation of the as-synthesized products was characterized by X-ray diffraction (XRD; Rigaku SmartLab) using Cu Kα radiation (λ = 1.5406 Å). The diffraction patterns were collected in the region of 2θ from 15° to 85°, at scan steps of 0.02°. The morphology was examined by transmission electron microscopy (TEM; JEOL JEM-2010) using an accelerating voltage of 200 kV. An

removal efficiency (%) =

qe =

(C0 − Ce) V m

C0 − Ce × 100 C0

(1)

(2)

where qe is the equilibrium adsorption capacity per gram dry weight of the adsorbent (mg g−1), C0 is the initial concentration of Congo red in the solution (mg L−1), Ce is the concentration of Congo red at equilibrium (mg L−1), V is the volume of the solution (L), and m is the dry weight of the adsorbents (g). Next, the adsorption kinetics and isotherms were investigated. To study the adsorption kinetics, an amount of 42 mg of nanoadsorbent powders was mixed with 15 mL of Congo red solution (30 mg L−1) under predetermined conditions of pH and different contact times of MgFe2O4−NH2 and MgFe2O4−OH NPs. The adsorption isotherms were carried out with initial concentrations of Congo red solution C


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ACS Applied Nano Materials ranging from 30 to 300 mg L−1 (15 mL) for 24 h, and the amount of nanoadsorbents was kept constant at 42 mg. The amount of adsorbed Congo red at time t (qt) was calculated using the equation given below: qt =

(C0 − C t) V m

1.5−11.8 nm ranges (Figure S4 inset), respectively. The SBET and pore volume values of nanoadsorbents are then used to calculate the average pore radii, which are found to be approximately 14 nm (MgFe2O4−NH2 NPs) and 10 nm (MgFe2O4−OH NPs). Although the size of MgFe2O4−NH2 particles is smaller than that of the MgFe2O4−OH particles (Figure S3), their SBET values are lower. This is probably due to the variation of their porous characteristics. Unlike nonporous materials, the tendency of SBET values according to their particle sizes could not be directly predicted. In this case, the smaller SBET of the MgFe2O4−NH2 NPs could be explained because their pore sizes are larger than that of the MgFe2O4−OH NPs. The larger pore size of MgFe2O4−NH2 NPs then causes smaller numbers of pores compared to MgFe2O4−OH NPs with the same mass of powders (i.e., MgFe2O4−NH2 NPs, 1 g), consequently resulting in less pore volume, which is consistent with another report.27 These determined characteristics will be useful afterward for supplementary discussion on Congo red adsorption because the larger surface area of the MgFe2O4−OH NPs might not be the only hint predicting whether these NPs could provide greater adsorption performance over the MgFe2O4−NH2 NPs. Since the magnetic properties of these adsorbents directly influence the separation efficiency, excellent saturation magnetization (Ms) is one of the key requirements for the magnetic adsorbent. Figure S5 shows the magnetization curves of both the MgFe2O4−NH2 and MgFe2O4−OH NP samples measured using VSM at room temperature. In general, there is no significant difference between the cases before and after Congo red adsorption. Their magnetic hysteresis curves exhibit nearzero remanence and coercivity, indicating superparamagnetic properties,28,29 which could prevent the NPs from aggregating in water. MgFe2O4−OH NPs exhibit higher Ms values (∼53 emu g−1) than MgFe2O4−NH2 NPs (∼37 emu g−1). This could be due to the higher atomic ratio of Fe/Mg in the MgFe2O4−OH sample (consistent with the obtained EDS results, Figures S6 and S7), since Mg ions possess no magnetic moment.30 The absence of ethanolamine in the hydrothermal synthesis method causes a lower pH solution than that in the coprecipitation method, consistent with our previous work.31 Consequently, a smaller amount of Mg2+ coprecipitates with Fe3+, as attributed to a higher solubility product constant of Mg(OH)2 than that of Fe(OH)3,32,33 which in turn causes nonstoichiometric magnesium ferrite with higher Fe/Mg atomic ratio. Additionally, smaller particle sizes of MgFe2O4−NH2 NPs (revealed in Figure S3) might also result in lower Ms values, as reported in the literature.34 To this point, it should be noted that a smaller particle size causes a larger surface-to-volume ratio, which in turn causes larger thermal fluctuations and a magnetically disordered surface, causing a lower Ms value.34 On the other hand, the coverage of an ethanolamine modifier on the surface of MgFe2O4 NPs synthesized by the coprecipitation method is also expected to cause a slightly smaller value of Ms due to the contribution of the nonmagnetic amino group layer on the surface, as previously noted by Ren et al.35 However, in the present study, the observed Ms values for both MgFe2O4−NH2 and MgFe2O4−OH NPs are strong enough for our separation purposes (insets of Figure S5a and Figure S5b) because the dispersed NPs can be completely collected by applying an external magnet field within only 3 min. Similar findings were also reported by several works using magnetic-based materials.8,35

(3) −1

where Ct is the concentration at time t (mg L ). Regeneration and Reusability. To determine the reusability of the adsorbents, the Congo red adsorption−desorption potential was examined by considering the removal efficiencies of the adsorbents after each cycle. Desorption experiments were carried out using EtOH and pH-adjusted EtOH as eluting solvents.26 First, the magnetic MgFe2O4−NH2 NPs loaded with Congo red were dried at 60 °C overnight. Then, 50 mg of the dried MgFe2O4−NH2 NPs loaded with Congo red was added into 50 mL each of EtOH, and EtOH adjusted the pH to 9.0 using 0.1 M NaOH (aq). The mixture was shaken for 2 h. The adsorbent was collected by an external magnetic and then dried at 60 °C for 3 h. For MgFe2O4−OH NPs, the EtOH adjusted the pH to 6.0 using 0.1 M HNO3 (aq) as an eluent. The recovered MgFe2O4−NH2 and MgFe2O4−OH NPs were then used to adsorb Congo red to determine their removal efficiencies compared with fresh adsorbent (i.e., the first cycle).

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RESULTS AND DISCUSSION Characterization of Surface-Functionalized MgFe2O4 NPs. To determine the phase formation in each product, XRD and SAED techniques were employed. Figure S2 shows the XRD and SAED patterns of the as-synthesized MgFe2O4−NH2 and MgFe2O4−OH NPs before and after Congo red adsorption. The pure phase of cubic spinel MgFe2O4, matched with JCPDS file no. 88-1935, is observed in all diffraction patterns without coexistence of any impurity phase.11 No phase changes after Congo red adsorption indicate that the nanoadsorbents were not collapsed, showing their reusability, which will be discussed later. Therefore, it could be summarized that the phase composition of MgFe2O4 nanoadsorbents was stable throughout the removal process of Congo red from wastewater. The morphologies of the nanoadsorbents before and after Congo red adsorption were explored using TEM. As shown in Figure S3, porous structures of the as-synthesized MgFe2O4− NH2 and MgFe2O4−OH NPs formed by spherical aggregation of primary particles are revealed. Porous materials serve a large specific surface area, which in turn rationally exhibit excellent adsorption performance. Thus, ferrite nanomaterials have been chosen as adsorbents for several works.8−10,17 In addition, the adsorption processes did not affect the morphologies of either MgFe2O4−NH2 or MgFe2O4−OH NPs (Figure S3ii-a and S3ii-b), with average diameters of approximately 70 ± 13 nm and 82 ± 15 nm, compared to Figure S3i-a and S3i-b, respectively. In other words, no collapse in particles is observed after adsorption, supporting no changes in phase evolution revealed by XRD patterns (Figure S2), as mentioned earlier. According to the TEM results (Figure S3), as-synthesized NPs are porous in nature. Thus, surface area and porosity, which are unique in each material, must be investigated utilizing a multipoint BET method via N2 adsorption− desorption isotherm analysis (Figure S4). The calculated BET specific surface area (SBET) and specific pore volume of the as-synthesized MgFe2O4−NH2 NPs are approximately 47 m2 g−1 and 0.3 cm3 g−1, respectively, whereas those values are approximately 79 m2 g−1 and 0.4 cm3 g−1, respectively, for the MgFe2O4−OH NPs. The pore radii of MgFe2O4−NH2 NPs and MgFe2O4−OH NPs mainly fall in the 1.5−28.4 nm and D


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For the FT-IR of the as-synthesized MgFe2O4−OH NPs before Congo red adsorption (Figure 1b(i)), the broad band observed at approximately 3452 cm−1 indicates stretching vibration of the −OH group functionalized on the surface.15,38 Moreover, there are two peaks observed at ∼2923 and ∼2851 cm−1, the C−H stretching vibrations, which might be from the ethylene glycol constituent.36 Similar to the as-synthesized MgFe2O4−NH2 NPs, the strong peaks in the range of 500− 850 cm−1 are also assigned to the metal oxide stretching mode from MgFe2O4. For the FT-IR spectrum of MgFe2O4−OH NPs after Congo red adsorption (Figure 1b(ii)), it shows the new bands at ∼3466 and ∼3417 cm−1, which might be attributed to the nitrogen atom of −NH2 in Congo red molecules.40 Moreover, this spectrum also shows the new band of SO stretching mode at 1054 cm−1.39 Thus, these results confirmed that the Congo red molecules were adsorbed on the surface of MgFe2O4−OH NPs. XPS was also performed to further confirm the surface functionalization of the adsorbent. Moreover, information derived from this technique might provide possible interaction behaviors between adsorbents and adsorbates in the present work. As shown in the XPS survey spectra (Figures S8a and S9a), the elementary Mg, Fe, and O can be observed from both MgFe2O4−NH2 and MgFe2O4−OH NPs. In addition, an N signal is observed for the as-synthesized MgFe2O4−NH2 NPs due to the ethanolamine molecules immobilized on their surfaces. C signals might be attributed to hydrocarbons (i.e., ethanolamine37 and/or adventitious hydrocarbon41) in both as-synthesized adsorbents. For more insight comparisons before and after Congo red adsorption, high-resolution XPS spectra of N and S, the key elements of the Congo red molecule, collected from both adsorbents and pure Congo red powders (Figure 2) were carefully recorded. For the MgFe2O4−NH2 adsorbent (Figure 2a), the N 1s signal at ∼399.8 eV could correspond to −NH2 37,42 from either its own surface functional group or the adsorbed Congo red molecules. Therefore, considering the S (Figure 2b) signal instead is more appropriate to confirm the adsorption of Congo red. S 2p of Congo red can be deconvoluted into two main peaks at 168.2 and 169.4 eV, which are assigned as S 2p3/2 and S 2p1/2, corresponding to −SO32− of Congo red molecules.43 After Congo red adsorption onto the surface of MgFe2O4−NH2 NPs, a slight shift of the S 2p peaks to lower binding energies (at 167.8 and 169.1 eV) was observed, suggesting that electrostatic attraction occurred via the −SO32− groups of Congo red.44 However, for the MgFe2O4−OH adsorbent (Figure 2c,d), the signal from the N element, which is not a part of this adsorbent surface, is considered in this case because the S signal is undetectable here, which might be due to the limitation of the instrument. Further investigation using synchrotron radiation as a source for XPS is an alternative for obtaining better information in the future. After Congo red adsorption, the presence of new peaks of N 1s at 400.0 and 401.8 eV ascribed to amine (−NH2) and protonated amine (−NH2+− or −NH3+) groups, respectively, of Congo red molecules45 is clearly seen. These results strongly suggest that Congo red is adsorbed onto the surface of both adsorbents. As expected, the intensity of N signal from MgFe2O4−NH2 NPs after adsorption is slightly higher than that from MgFe2O4−OH NPs after adsorption since the MgFe2O4−NH2 adsorbent itself contains −NH2 on the surface. Apart from these two elements, other elements were also recorded and fully peak assigned as provided in the

The FT-IR spectra of the MgFe2O4 NPs before Congo red adsorption were taken to verify the details of the functionalization of amine and hydroxyl groups on their surfaces of MgFe2O4 NPs. In addition, the interactions between the MgFe2O4 NPs and Congo red were also included for consideration by taking FT-IR spectra for the NPs after Congo red adsorption. In the case of the as-synthesized MgFe2O4−NH2 NPs (Figure 1a(i)), the existence of doublets

Figure 1. FT-IR spectra of (a) MgFe2O4−NH2 NPs and (b) MgFe2O4−OH NPs (i) before and (ii) after Congo red adsorption and (iii) Congo red.

at ∼3468 and ∼3410 cm−1 and doublets at ∼1636 and ∼1618 cm−1 shows the characteristics of the N−H stretching and bending modes, respectively. This clearly indicates successful amine functionalization from ethanolamine. Small peaks at ∼2927 and ∼2849 cm−1, indicating the stretching vibration of C−H bonds in either ethanolamine or ethylene glycol,11,36 are also observed. The broad peak at ∼1055 cm−1 could be assigned as C−O overlapping with C−N stretching vibrations,37,38 supporting the grafting of amine groups on the MgFe2O4 NPs. In addition, the peaks in the range of 500−850 cm−1, which are attributed to stretching modes of metal oxide (M−O, M = Fe and Mg), are responsible for the formation of spinel MgFe2O4.38 After Congo red adsorption, the FT-IR spectrum of MgFe2O4−NH2 NPs (Figure 1a(ii)) illustrates the new bands of SO stretching mode at ∼1220 and ∼1170 cm−1 39 with a slightly lower wavenumber in comparison with the FT-IR spectrum of the pristine Congo red (Figure 1a(iii)), confirming the adsorption of Congo red on MgFe2O4−NH2 NPs. E


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Figure 2. High-resolution scan XPS spectra of (a) N 1s and (b) S 2p of MgFe2O4−NH2 NPs and (c) N 1s and (d) S 2p of MgFe2O4−OH NPs before and after Congo red adsorption.

Figure 3. (a) Zeta potential of MgFe2O4−NH2 and MgFe2O4−OH NPs as a function of solution pH at room temperature. (b) Effect of pH on Congo red adsorption onto MgFe2O4−NH2 NPs for 180 min and onto MgFe2O4−OH NPs for 420 min. Effect of contact time on Congo red adsorption onto (c) MgFe2O4−NH2 NPs at pH 6 and (d) MgFe2O4−OH NPs at pH 8. Initial Congo red concentration = 30 mg L−1. Adsorbent dose = 2.8 g L−1 at room temperature.

Congo red adsorption for both adsorbents, indicating that this element of the adsorbents is not responsible for chemical bonding with the adsorbate. In contrast, a slight shifting of Fe

Supporting Information, Figures S8b−e and S9b−e. There are no noticeable differences in the peak shape and peak shifting of Mg 1s (Figures S8b and S9b) spectra between before and after F


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sulfonate group of adsorbate and adsorbent. At pH values between 4.0 and 6.0, the adsorption capacities are above 85% and then gradually decrease with further increases in pH from 6.0 to 12.0. The maximum removal efficiency at ∼93% could be obtained at pH 6.0 within 180 min. Congo red molecules are deprotonated at pH > 5.5.56 Therefore, it is believed that the adsorption process could occur through electrostatic interactions between −NH3+ on the MgFe2O4 surface and the negative charge of the −SO3− groups of Congo red, consistent with earlier work57 and XPS results (Figure 2b). Since the pKa of Congo red is ∼3 and the pI of MgFe2O4− NH2 NP adsorbent is ∼8.4, it is likely that the surfaces of amine-functionalized adsorbent are dominantly positive in acidic medium, causing good affinity with negatively charged Congo red.19 When the pH of the Congo red solution was increased above 6.0, the removal efficiency of Congo red was found to decrease continuously. This observation could be attributed to the fact that higher pH causes an increase of the hydroxyl ion (OH−). Therefore, it could compete with the anionic ion (−SO3−) of Congo red to adsorb on the MgFe2O4−NH3+ surface. Huang et al.58 suggested that a higher solution pH led to an increase in the −OH − concentration. The −OH− and Congo red molecules compete to adsorb onto the adsorbent surface, which inhibits the adsorption capacity. They also found that the removal efficiency largely decreased from 53.4% to 27.3% once the solution pH was increased from 6.0 to 10.0. Similar observations were also found by Rahimi et al.,13 where Congo red removal is believed to occur via electrostatic attraction between the positive charges of the adsorbent surface and the −SO3− groups of Congo red at pH 6. At higher pH (i.e., pH 7−9), the MgFe2O4−NH2 NPs provide more than 50% removal efficiency due to other adsorption mechanisms apart from electrostatic interactions, as described in the XPS results. For MgFe2O4−OH NPs, removal efficiencies drastically increase from pH 2.0 to 5.0 and then continuously increase in the pH range of 5.0−12.0. The results show that the removal efficiencies of Congo red on MgFe2O4−OH NPs increase with pH. It is possible that in acidic medium, the amino groups of Congo red are protonated, causing electrostatic repulsion between Congo red and positively charged MgFe2O4−OH NPs. The maximum removal efficiency of MgFe2O4−OH NPs, at 88% within 420 min, is observed at pH 8.0, while the pI of MgFe2O4−OH NPs is ∼7.8. Under this optimized condition, it is likely that electrostatic attraction might not be the dominant mechanism affecting the adsorption capacity of MgFe2O4−OH NPs for Congo red removal. Hou et al.59 proposed that the adsorption of Congo red on the hydroxyapatite/chitosan composite occurred via H-bonding between the −NH2 groups of Congo red and the −OH groups of the adsorbent. However, there is no strong evidence, such as shifting of O 1s XPS peaks (Figure S9e), supporting this statement. Therefore, it is possible that the coordination effect between Fe3+ of the adsorbent and −NH2 groups of Congo red, as proposed in the literature,1,47 is dominant here, consistent with the previously discussed XPS results (Figure S9c), where the shifting of the Fe 2p spectrum was found after adsorption. Effect of Contact Time. The effect of the adsorption time for Congo red removal by MgFe2O4−NH2 and MgFe2O4−OH NPs was examined to identify the equilibrium time for the maximum removal efficiency under certain conditions, i.e., the selected pH, adsorbent dose, operating temperature, etc. The

2p occurs, with two spin−orbit doublets that are specific to Fe 2p3/2 and Fe 2p1/2, which correspond to Fe3+ in MgFe2O4 46 with binding energies of ∼710.0 eV and ∼711.3 eV, after adsorption is found for the MgFe2O4−OH adsorbent (Figure S9c). This might suggest that the interaction between the MgFe2O4−OH adsorbent and Congo red occurs through the coordination effect between Fe3+ of the adsorbent and −NH2 groups of Congo red1,47 since the position of N 1s peaks from the Congo red-adsorbed adsorbent slightly shifts to a higher binding energy compared to pristine Congo red (Figure 2). Considering the O 1s spectra, the O 1s peak of MgFe2O4− NH2 NPs (Figure S8e) after Congo red adsorption shifts to a higher binding energy. The first peak (∼529.7 eV) can be assigned to oxygen in the lattice (Fe−O, Mg−O).48,49 The second peak (∼531.0 eV) can be assigned to surface adsorbed oxygen species (i.e., oxygen in −OH) due to moisture or organic compounds.48,50 From related literature, the peak at ∼532.0 eV can be assigned to defect sites with low oxygen coordination as synthesized,49,51 while the peak at ∼533.2 eV corresponds to chemically or physically adsorbed water or oxygen on the C−O bond.38,52 This may suggest other possible binding mechanisms between the MgFe2O4−NH2 adsorbent and Congo red. These possible interactions involved in the Congo red adsorption could be dipole−dipole hydrogen bonding (H-bonding) between the electronegative −NH2 group in Congo red and O atoms in the lattice53 or surface −OH groups54 of metal oxide adsorbents. In addition, during the adsorption process of Congo red, the dipole−dipole Hbonding interaction (between −NN− of Congo red and −OH of adsorbent surface) and Yoshida H-bonding interaction (between the aromatic ring of Congo red and −OH groups of adsorbent surface) might also play a role in Congo red adsorption.54 However, these assumptions might be only minor mechanisms for the Congo red adsorption onto the MgFe2O4−NH2 surface since the shift of the S 2p peaks is more pronounced, and there is a tiny shift of the −NH2 peak (from 399.8 to 399.6 eV). In contrast, the shifting of O 1s peaks is not obvious for MgFe2O4−OH NPs, which might be because the mentioned coordinative interaction is dominant in this case. On the basis of these XPS results, more pronounced changes observed from the MgFe2O4−NH2 NPs than MgFe2O4−OH NPs might lead to better adsorption ability, as discussed below. Influencing Factors on Congo Red Adsorption. Effect of pH. Since the pH of the solution is indicated to be a significant variable affecting the adsorption process between the nanoadsorbents and Congo red,19,55 the optimal pH value for the system investigated in this study was thus carefully determined. To explain the effect of pH on adsorption performance, zeta potential measurements of the adsorbents were then performed, with the results given in Figure 3a. Generally, the positive charges on adsorbent surfaces are formed at pH values less than the isoelectric point (pI) of each adsorbent. In contrast, the adsorbent surface is negatively charged at pH above pI.8,43 In this work, the pI values of MgFe2O4−NH2 and MgFe2O4−OH NPs are observed at ∼8.4 and ∼7.8, respectively. The removal efficiencies of Congo red by either MgFe2O4− NH2 or MgFe2O4−OH NPs at the initial pH ranging from 2.0 to 12.0 are shown in Figure 3b. As seen, the removal efficiency of Congo red by MgFe2O4−NH2 NPs at pH 2 is less than 80% since the nondeprotonated sulfonate group of Congo red leads to weakening of the electrostatic interaction between the G


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Figure 4. (a) Pseudo-first-order and (b) pseudo-second-order model of Congo red adsorption onto MgFe2O4−NH2 NPs at pH 6 for 180 min. (c) Pseudo-first-order and (d) pseudo-second-order model of Congo red adsorption onto MgFe2O4−OH NPs at pH 8 for 420 min. Initial Congo red concentration = 30 mg L−1. Adsorbent dose = 2.8 g L−1 at room temperature.

adsorbents.61 Additionally, different functional groups on their surfaces also seem to affect their different adsorption performance, similar to previous works that studied aminefunctionalized and nonfunctionalized (i.e., hydroxyl surface) oxide adsorbents for dye adsorption.62,63 Higher removal efficiencies were also found in the amine-functionalized adsorbents of these works, suggesting that higher efficiency of MgFe2O4−NH2 NPs might be attributed to stronger electrostatic interactions with sulfonate groups of Congo red formed by larger numbers of protonated amine groups than protonated hydroxyl groups of the adsorbents under the performed conditions. In another aspect, as mentioned in the previous XPS section, it is likely that H-bonding between the electronegative −NH2 or −NN− groups in Congo red and surface −OH groups on MgFe2O4−NH2 surface47,53,54 might also help to obtain higher removal efficiency in the MgFe2O4− NH2 case. Adsorption Kinetics. For practical applications, the adsorption kinetics of porous materials is an important requirement for the optimal design of adsorption and separation processes.64,65 To evaluate the adsorption kinetics, pseudo-first-order and pseudo-second-order models were adopted. The pseudo-first-order kinetic model of Lagergren was used to verify the adsorption of an adsorbate from an aqueous solution based upon the assumption of physisorption process.60,66 A linear form of the pseudo-first order equation is described by Lagergren as eq 4:60

adsorption experiment was carried out with various contact times (t). Figure 3c shows plotting of the amount of Congo red adsorbed onto 42 mg of MgFe2O4−NH2 NPs (qt) against contact time. A higher adsorption rate was detected at the initial stage and then gradually slowed down and reached equilibrium at 180 min with a Congo red adsorption capacity of ∼9.9 mg g−1, i.e., ∼96%. In the same way, plotting the amount of Congo red adsorbed onto MgFe2O4−OH NPs against contact time is presented in Figure 3d. As observed, the outstanding maximum adsorption capacity could be obtained as high as 9.0 mg g−1 at pH 8 within 420 min, i.e., ∼85%. Generally, the dye was adsorbed expeditiously in the first stage, probably due mainly to the availability of numerous free surface-active sites of the functionalized MgFe2O4 NPs for dye adsorption. Then, the adsorption rate became slower and finally approached equilibrium once the vacant surface active sites were fully occupied with dye ions, generating more repulsive forces to hinder other dye molecules from adsorption on the adsorbent surface.60 It seems that both adsorbents take time to reach equilibrium; however, the adsorption time is comparable with that of the other ferrite-based nanoparticles, as tabulated in Table S1. In particular, MgFe2O4−NH2 NPs take shorter time, which might indicate the advantage of MgFe2O4−NH2 NPs as an adsorbent. The present work demonstrated that higher removal efficiency and shorter adsorption equilibrium time are found from MgFe2O4−NH2 NPs even though MgFe2O4−OH NPs possess a larger specific surface area, as discussed earlier. However, the slightly larger average pore size of MgFe2O4− NH2 NPs might assist in better adsorption ability since adsorbates could more easily adsorb on larger pore-size

log(qe − qt ) = log qe − H

k1 t 2.303

(4)


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ACS Applied Nano Materials Table 1. Adsorption Kinetic Parameters Obtained from Pseudo-First-Order and Pseudo-Second-Order Models pseudo-first-order model

pseudo-second-order model

adsorbent

k1 (min−1)

qe (mg g−1)

R2

k2 (g mg−1 min−1)

qe (mg g−1)

R2

MgFe2O4−NH2 NPs MgFe2O4−OH NPs

1.7 × 10−2 8.3 × 10−3

8.08 5.69

0.969 0.932

1.2 × 10−2 1.1 × 10−2

9.78 8.98

0.982 0.990

Figure 5. (a) Langmuir isotherms and (b) Freundlich isotherms of Congo red adsorption onto MgFe2O4−NH2 NPs at pH 6. (c) Langmuir isotherms and (d) Freundlich isotherms of Congo red adsorption onto MgFe2O4−OH NPs at pH 8. Initial Congo red concentration = 30 mg L−1. Adsorbent dose = 2.8 g L−1 at room temperature.

MgFe2O4−NH2 and MgFe2O4−OH NPs, respectively (Figure 4b and Figure 4d). In addition, the calculated equilibrium adsorption capacity (qe) of Congo red removal by MgFe2O4−NH2 NPs using the pseudo-second-order equation was found to be 9.78 mg g−1, which is closer to the experimental value (9.90 mg g−1) from the previous experiment (Effect of Contact Time section) than that obtained from the pseudo-first-order equation (8.08 mg g−1). The equilibrium adsorption capacity of Congo red removal by MgFe2O4−OH NPs obtained from the pseudosecond-order equation (8.98 mg g−1) was also more comparable to the experimental value (9.20 mg g−1) than that calculated from the pseudo-first-order equation (5.69 mg g−1). These observations demonstrate that the experimental values are not comparable with the calculated qe values obtained from the pseudo-first-order kinetic model, suggesting that the adsorption of Congo red by both adsorbents in this work does not follow the pseudo-first-order kinetic model. Therefore, it is likely that chemisorption (as proposed in the previous sections) is the rate-determination step during the adsorption of Congo red onto MgFe2O4−NH2 and MgFe2O4− OH NPs. As revealed by a combination of TEM and BET analyses, the as-synthesized porous adsorbents are most likely comparable in particle size and pore size distribution. Since the average

where qe and qt refer to the adsorption capacities at equilibrium and time t, respectively, in mg g−1, and k1 is the pseudo-firstorder rate constant. Plotting the log(qe − qt) against t provides the slope and the intercept as −k1/2.303 and log(qe), respectively. In addition, the pseudo-second-order model describes the adsorption process derived from a chemical reaction between adsorbate and adsorbent, indicating that the adsorption process occurs through chemisorption.60,66 The pseudosecond-order model can be expressed as eq 5: t 1 1 = + t 2 qt q k 2qe e

(5)

where k2 is the pseudo-second-order rate constant. Plotting the t/qt against t provides the slope and the intercept as 1/qe and 1/(k2qe2), respectively. The adsorption kinetic plots for Congo red adsorption by MgFe2O4−NH2 and MgFe2O4−OH NPs are presented in Figure 4 along with the calculated kinetic parameter values given in Table 1. The obtained correlation coefficients (R2) of both models suggest that the adsorption followed the pseudosecond-order model rather than the pseudo-first-order model, with R2 = 0.982 and R2 = 0.990 for Congo red removal by I


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Table 2. Langmuir and Freundlich Isotherm Parameters for Congo Red Adsorption onto MgFe2O4−NH2 and MgFe2O4−OH NPs Langmuir isotherm adsorbent

qm (mg g−1)

MgFe2O4−NH2 NPs MgFe2O4−OH NPs

71.4 67.6

Freundlich isotherm

KL (L mg−1) −2

8.5 × 10 6.8 × 10−2

particle size of MgFe2O4−OH NPs is only approximately 10 nm larger than that of MgFe2O4−NH2 NPs, the particle size effect on adsorption capacity is then relatively negligible, according to the literature.67,68 In addition, the pore size of the adsorbents plays an important role in adsorption kinetics when the pore size of the adsorbents is comparable with the size of the adsorbate molecules.64 However, if the pore size of adsorbents (particle size up to 0.1 mm) is much larger than the size of adsorbate molecules, the adsorption kinetics are independent of porous properties.69 Thus, in our work, it is not surprising that the adsorption rate constant (k2) values, tabulated in Table 1, of both adsorbents are not significantly different because their average pore diameter values are much larger than the size of the Congo red molecule (i.e., width 0.7 nm and length 2.5 nm). Adsorption Isotherms. To understand the interaction between adsorbents and adsorbates, the adsorption isotherms were also investigated by Langmuir and Freundlich isotherms. The Langmuir isotherm model is based on homogeneous systems and the monolayer adsorption of adsorbate molecules onto the adsorbent surface.55,70 The linear form of the Langmuir equation is given as eq 6: Ce C 1 = + e qe qmKL qm

R2

KF (mg (L/mg)1/n g−1)

n

R2

0.990 0.994

15.0 12.1

3.0 2.8

0.935 0.896

maximum adsorption capacities (qm) of the adsorbents calculated from the Langmuir equation are 71.4 mg g−1 and 67.6 mg g−1 for MgFe2O4−NH2 and MgFe2O4−OH NPs, respectively. These values are comparable with those obtained from other ferrite-based nanoparticles, as listed in Table S1. This finding highlighted the potential of MgFe2O4−NH2 NPs for Congo red adsorption above MgFe2O4−OH NPs. Reusability of Adsorbents. The excellent reusability is also an important factor for magnetic adsorbents.9 Therefore, the desorption of dye species and regeneration of the adsorbent were also investigated. Figure S10 shows that the regeneration of magnetic MgFe2O4−NH2 nanoadsorbents using EtOH adjusted pH to 9.0 as an eluting solvent has better reusability compared to using EtOH. Therefore, EtOH adjusted to pH 9.0 was chosen as an eluting solvent for desorbing Congo red from MgFe2O4−NH2 NPs. Alcohol was chosen here as the eluting solvent since preliminary experiments using water-based eluting solvent were performed and showed that the desorption was poor (data not shown). EtOH adjusted pH to 6.0 was used as an eluting solvent for MgFe2O4−OH NPs. The removal efficiencies of Congo red by the regenerated magnetic MgFe2O4−NH2 and MgFe2O4−OH NPs are given in Figure 6. The results show that after three

(6)

where qe is the equilibrium dye concentration on the adsorbent (mg g−1), Ce is the equilibrium dye concentration in the solution (mg L−1), qm is the maximum capacity of adsorbent (mg g−1), and KL is the Langmuir adsorption constant (L mg−1). A plot of Ce/qe versus Ce gives a straight line with qm and KL determined from the slope and intercept, respectively. The Freundlich isotherm model describes heterogeneous systems and multilayer adsorption.55,70 The linear form of the Freundlich equation is given as eq 7: log qe = log K F +

1 log Ce n

(7) Figure 6. Reusability of magnetic MgFe2O4−NH2 and MgFe2O4−OH NPs for Congo red adsorption.

where KF and n are the Freundlich’s empirical constants related to the maximum sorption capacity (mg g−1). Plotting log(qe) against log Ce provides the slope and the intercept as 1/n and log KF, respectively. Figure 5 shows plots of adsorption isotherm data collected under pH 6 and pH 8 for Congo red adsorption onto MgFe2O4−NH2 and MgFe2O4−OH NPs with the same initial Congo red concentration range (30−300 mg L−1) for 24 h at 25 °C. The Langmuir and Freundlich model parameters calculated from the linear plots are shown in Table 2. The R2 values fitted using the Langmuir model are 0.990 and 0.994 for MgFe2O4−NH2 and MgFe2O4−OH NPs, respectively, while the R2 values from the Freundlich model are 0.935 and 0.896, respectively. These values further indicate that the adsorption fitted better with the Langmuir model, implying that adsorption between the Congo red and both MgFe2O4− NH2 and MgFe2O4−OH adsorbent surfaces occurred as a monolayer coverage on the adsorbent surface. Significantly, the

cycles, the removal efficiencies of Congo red by MgFe2O4− NH2 and MgFe2O4−OH NPs remain at approximately 85% and 67%, respectively. Therefore, this study demonstrates that MgFe2O4−NH2 NPs substantiate the reusability potential above MgFe2O4−OH NPs for Congo red removal using the EtOH-adjusted pH to 9.0 as the eluting solvent. In addition, this study indicates that our experiments easily regenerated the adsorbent effectiveness by using its magnetic separation ability. Application of MgFe2O4−NH2 and MgFe2O4−OH NPs for Congo Red Removal from Wastewater. After the optimal adsorption conditions (adsorption time and pH of Congo red solution) were obtained for each type of adsorbent, these adsorption conditions were fixed for Congo red removal from real wastewater obtained from local textile dying business J


Article

ACS Applied Nano Materials using MgFe2O4−NH2 and MgFe2O4−OH NPs as adsorbents. To determine the adsorption capacities of both adsorbents, the solutions obtained after treating 15 mL of ∼30 mg L−1 Congo red wastewater using 42 mg of MgFe2O4−NH2 NPs for 180 min at pH 6 and 42 mg of MgFe2O4−OH NPs for 420 min at pH 8 were analyzed. It was found that even Congo red from real wastewater could also be adsorbed onto MgFe2O4−NH2 and MgFe2O4−OH surfaces with removal efficiencies of approximately 93% and 82%, respectively. The results obtained in this work suggest that the MgFe2O4−NH2 NPs could be regarded as a potential candidate for Congo red adsorption in real wastewater with slightly higher adsorption capacity than the MgFe2O4−OH NPs. Cytotoxicity Evaluation of Congo Red Solution before and after Dye Removal. Since the toxicity of dyecontaminated water before release to natural sources is of concern, cytotoxicity tests were compared between the treated solution and the solution before dye removal. The cytotoxicity experiments were carried out using human dermal fibroblasts as a target cell. The experimental details are provided in the Supporting Information. Figure S11 illustrates that a 15% initial concentration of Congo red solution (i.e., ∼27 mg L−1, approximately 30 mg L−1 used in the adsorption experiments) was not toxic to human dermal fibroblasts. On the other hand, by increase of the Congo red concentration to 50% of initial concentration, the survival rate of human dermal fibroblasts was drastically reduced to 60% compared to the untreated control cells. Interestingly, the removal of dye by both MgFe2O4−NH2 and MgFe2O4−OH NPs could significantly rescue cell survival to 70% and 90%, respectively. These results indicate that under the optimized conditions performed in previous adsorption studies (concentration of Congo red solution of ∼30 ppm), both types of adsorbents improved the quality of dye-contaminated water.

guideline for framing experiments for other researchers who are interested in developing Congo red removal from wastewater.

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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/acsanm.9b01305.

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Structure of Congo red, XRD patterns, SAED patterns, TEM images, N2 adsorption−desorption isotherms, pore size distribution with pore volume, and magnetization curves of MgFe2O4−NH2 NPs and MgFe2O4−OH NPs, EDS and XPS spectra of synthesized MgFe2O4 NPs before and after Congo red adsorption, removal efficiency of Congo red from MgFe2O4−NH2 NPs using different eluting solvents, and viability of human dermal fibroblasts (MTT assays) in the presence of Congo red solution and water after Congo red removal using MgFe2O4−NH2 and MgFe2O4−OH NPs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Parinya Noisa: 0000-0002-1474-7792 Supree Pinitsoontorn: 0000-0002-4921-1541 Laongnuan Srisombat: 0000-0003-3886-2637 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Ministry of Higher Education, Science, Research and Innovation is gratefully acknowledged. This research work was partially supported by Chiang Mai University. Finally, the authors thank Somchai Pornchai for the Congo red effluent to carry out the research study.

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CONCLUSIONS In summary, the potential of either amine-functionalized or hydroxyl-functionalized MgFe 2 O 4 (MgFe 2 O 4 −NH 2 or MgFe2O4−OH) nanoparticles (NPs) was comparatively investigated as nanoadsorbent candidates for the removal of Congo red from aqueous solution. Both MgFe2O4−NH2 and MgFe2O4−OH NPs were successfully prepared by employing coprecipitation and hydrothermal methods, respectively. Magnetic property measurements show that the saturation magnetization of MgFe2O4−OH NPs (53 emu g−1) is greater than that of MgFe2O4−NH2 NPs (37 emu g−1). However, these values can be easily separated from aqueous solution after the adsorption process. The adsorption of 15 mL of 30 mg g−1 Congo red, initial Congo red concentration, onto 42 mg of MgFe2O4−NH2 NPs for 180 min at pH 6 reached 96% removal efficiency, while MgFe2O4−OH NPs provided the removal efficiency at 88% within 420 min at pH 8. The adsorption isotherms of Congo red using MgFe2O4−NH2 and MgFe2O4−OH NPs as adsorbents followed the Langmuir isotherm model with maximum adsorption capacities of 71.4 mg g−1 and 67.6 mg g−1, respectively. The regeneration of the adsorbents was carried out using ethanol. After three cycles, the MgFe2O4−NH2 NPs showed a removal efficiency of ∼85%, while the MgFe2O4−OH NPs showed a removal efficiency of ∼67%. From the view of environmental impact, the MgFe2O4−NH2 NPs prepared by a simple one-pot synthetic method are therefore interesting for use as Congo red adsorbents, and this research can be used as a preliminary

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