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Original article
12 (
8
); 3704-3722
doi:
10.1016/j.arabjc.2015.12.012

Single and simultaneous adsorption of methyl orange and phenol onto magnetic iron oxide/carbon nanocomposites

, , ,
a Politehnica University Timişoara, Faculty of Industrial Chemistry and Environmental Engineering, 6 Pîrvan Blv., 300223 Timişoara, Romania
b Politehnica University Timişoara, Research Institute for Renewable Energy, P-ta Victoriei No. 2, 300006 Timisoara, Romania

⁎Corresponding author at: Politehnica University Timişoara, Faculty of Industrial Chemistry and Environmental Engineering, 6 Pîrvan Blv., 300223 Timişoara, Romania. Tel.: +40 256404158; fax: +40 256403060. marcela.stoia@upt.ro (Marcela Stoia)

Received: , Accepted: ,
© The Author(s) 2016
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Magnetic iron oxide/carbon nanocomposites were synthesized by a facile, one-step solvothermal method. The magnetic nanopowders were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermal analysis (DSC–TG), scanning electron microscopy (SEM), specific surface area and particle size measurements, pore size distributions and magnetic properties. The magnetic nanopowders were tested as adsorbents for the removal of methyl orange (MO) and phenol (Ph) from aqueous solutions. The effects of solution pH, contact time, adsorbent dose and initial pollutants concentration on the adsorption of MO and phenol onto the investigated adsorbents were studied. A significant increase in the removal efficiency, both for MO and phenol, with the increase in the carbon content of the magnetic nanopowder was evidenced. New experimental data were provided regarding the bicomponent adsorption of MO and phenol. Pseudo-second order equation was fitted to the kinetic data and four isotherm models, namely Langmuir, Freundlich, Redlich–Peterson and Sips were used to analyze the equilibrium data in both single and binary-component solutions. The investigated adsorbents showed a higher adsorption capacity toward MO than phenol. The simultaneous adsorption of the two pollutants in bicomponent solutions indicated that the MO adsorption is practically not affected by the presence of phenol while the adsorption of phenol is significantly reduced in the presence of MO. The benefits of obtaining low-cost nanocomposites with adsorption capacity and magnetic separation tailored, effective in single and bicomponent adsorption of MO and phenol, represent strong arguments regarding their great potential for practical applications.

Keywords

Magnetic iron oxides
Carbon
Nanocomposites
Simultaneous adsorption
Phenol
Methyl orange
1

1 Introduction

The Industrial wastewater contamination with dyes is a global problem. Their widespread use generates large amounts of wastewater producing toxicological problems related to environmental protection (Pillaia et al., 2015). Their presence, even in small quantities, causes changes in terms of water color and besides the cumulative effect, reduces the amount of light that reaches the aquatic environment, complicating the process of photosynthesis (Kyzas et al., 2014). The main industry branches that generate wastewater contaminated with dyes are as follows: textile industry, paint industry, paper industry, pharmaceutical industry, printing, leather industry, etc. (Kyzas et al., 2014; Fan et al., 2012). However, the wastewaters from these industries do not contain only dyes as pollutants; the phenolic compounds can be also found in significant quantities (Rehmana et al., 2015; Makrigianni et al., 2015). The phenols were considered as priority pollutants by the US Environmental Protection Agency (EPA) and by the EU, due to their high toxicity even at very small concentrations. Besides their tastes and odors, they are also potential carcinogens (Yang et al., 2015).

Both phenols and dyes have a complex chemical structure which makes them very resistant in terms of natural biodegradation (Fu et al., 2015). For these reasons, these pollutants must be removed from industrial effluents before discharging into the environment. Over time many techniques have been developed for the disposal of both dyes and phenols from wastewater such as coagulation (Khayet et al., 2011; Szygułaa et al., 2009), precipitation (Zhu et al., 2007), separation trough membranes (Alventosa-deLara et al., 2012; Zagklis et al., 2015), electrolysis (Wang, 2009), photolysis (Guo et al., 2006; Fatimah and Narsito, 2015; Ruzimuradov et al., 2015), extraction (Abbassiana et al., 2015; El-Ashtoukhy and Fouad, 2015), oxidation (Duana et al., 2015; Gomes et al., 2012; Wu et al., 2000), biological treatments (Popli and Patel, 2015), and adsorption (Tian et al., 2015; Damjanovic et al., 2010; Panic et al., 2013).

Among these decontamination methods, adsorption is considered by many authors as a superior technique due to its high efficiency, low cost of implementation, ample availability and simplicity in terms of design (Tian et al., 2015; Panic et al., 2013; Tanhaei et al., 2015). The most commonly used adsorbents for the removal of dyes from wastewater are as follows: aluminophosphate (Kannan et al., 2013), calcium carbonate (Chong et al., 2014), perlite (Vijayakumar et al., 2012), modified silica (Donia et al., 2009) but also by-products or waste from various industries and agriculture (Sharma et al., 2011). For the phenol removal from industrial wastewater, the most used adsorbents are as follows: granular activated carbon (Dabrowski et al., 2005), synthetic resins (Abburi, 2003), natural zeolites (Damjanovic et al., 2010) and poly(methyl methacrylate) (Al-Muhtaseb et al., 2011).

However classical adsorbents, have some shortcomings due to difficult separation. For this reason, magnetic nanoparticles have received special attention because they have a much higher stability in suspension medium, a very high adsorption capacity and high efficiency in terms of separation (Ali, 2012; Tang and Lo, 2013). Among the nanomaterials with magnetic properties, those based on iron oxides have reached special attention because of their ability to separate easily from solutions using a magnet and of their biocompatibility (Kyzas and Matisa, 2015; Li et al., 2015); they can also be functionalized with polymeric molecules or inorganic materials surface to improve the surface reactivity (Yang et al., 2015; Ge et al., 2012). Considering the excellent adsorption capacity of activated carbon, many studies have been focused on the development of novel adsorbents that combine the adsorption capacity of carbon materials with the magnetic properties of iron oxides (Qiu et al., 2014, 2015; Zhu et al., 2013).

Up to date, many studies were focused on single component adsorption of dyes and phenolic compounds, respectively onto various adsorbents (Fan et al., 2012; Fu et al., 2015; Kannan et al., 2013; Chong et al., 2014; Vijayakumar et al., 2012), but limited data are available for multi-component adsorption. In the recent years, several simultaneous adsorption of different dyes (Ghaedi et al., 2015; Turabik, 2008), dyes and heavy metals (Tovar-Gómez et al., 2015; Deng et al., 2013), phenol and heavy metals (Yang et al., 2015; Han et al., 2015), phenol and resorcinol (Kumar et al., 2011) have been reported but there are no reports regarding the simultaneous adsorption of dyes and phenolic compounds. Considering the real possibility that different dyes and phenolic compounds to be present together in many industrial effluents, the study of their simultaneous adsorption is of great interest.

The current study introduces a novel, one-step and cost-effective solvothermal method to synthesize magnetic iron oxide/carbon nanocomposites. The feasibility of this method was examined by using the prepared composites as adsorbents for the removal of some hazardous pollutants from single and bicomponent aqueous solution. Methyl orange and phenol were used as pollutant models, considering the lack of reports regarding the simultaneous magnetic removal of these pollutants.

2

2 Materials and methods

2.1

2.1 Materials

The starting reagents for the adsorbents preparation were as follows: iron chloride hexahydrate extra pure (FeCl3·6H2O) (Ph Eur, from Scharlau), 1,2 propanediol (C3H8O2) (12PG, for synthesis, purity >99%, from Merck), potassium hydroxide (Sigma–Aldrich, ⩾85%), granular activated carbon, analytical reagent grade, (purchased from Utchim, Romania), ethanol (C2H6O) (analytical reagent grade, from Merck). Anionic dye, methyl orange (C14H14N3NaO3S) (Sigma–Aldrich) and phenol (C6H6O) (Sigma–Aldrich) were used as adsorbates. All the chemicals were used as received, without further purification.

2.2

2.2 Preparation of magnetic adsorbents

In a typical synthesis, the necessary amount of FeCl3 was dissolved in 30 mL of 1,2-propanediol, followed (in the case of nanocomposites) by the addition of the corresponding amount of activated carbon; the suspension was magnetically stirred about 30 min, in order to facilitate the adsorption of Fe3+ cations on activated carbon. The necessary amount of KOH (considering 50% excess and the minimum purity of 85%) was separately dissolved in 30 mL of 1,2-propanediol (at 50 °C). The obtained solution was then added on the suspension Fe3+/activated C/1,2-propanediol with formation of brown-black slurry, left under magnetically stirring for other 30 min. The resulted slurry was transferred into a Teflon-lined stainless steel autoclave of 75 mL capacity. The autoclave was placed into an oven and kept at 195 °C for 12 h, and then allowed to cool to room temperature.

The precipitate was separated from the liquid phase by filtration. The product was washed with ethanol and distilled water–ethanol mixture (1:1) several times to remove the residual organics and Cl− ions (detected by reaction with Ag+ ions) from the final product. The washed precipitate was left to dry at room temperature for one day and then was grinded and the obtained powder was further characterized without thermal treatment. Table 1 presents the characteristics of the synthesized samples.

Table 1 Characteristics of the synthesized samples.
Sample’s symbol Sample V12PG (mL) m (FeCl3·6H2O) (g) m (KOH) (g) mC (g) Time (h) Temp. (°C)
MNP1 FexOy 60 8.1096 8.70 – 12 195
MNP2 FexOy/C 60 8.1096 8.70 0.48 12 195
MNP3 FexOy/2C 60 8.1096 8.70 0.96 12 195

2.3

2.3 Characterization methods

Thermal behavior of the powders synthesized at 195 °C was studied using a Netzsch STA 449C instrument, in air atmosphere at a flow rate of 20 mL min−1. The TG/DSC curves were recorded in the range of 25–1000 °C with a heating rate of 10 K min−1, using alumina crucibles. The phase composition of the samples was determined by XRD, using a Rigaku Ultima IV diffractometer (CuKα radiation). FTIR spectra were carried out using a Shimadzu Prestige-21 spectrometer in the range 400–4000 cm−1, using KBr pellets and resolution of 4 cm−1. Specific surface area of the powders, SBET, was measured by BET (Brunauer, Emmett, and Teller) nitrogen gas adsorption technique using a Micromeritics ASAP 2020 instrument. The pore distribution was computed using the Barret–Joyner–Hallenda (BJH) method from the desorption curves. The morphology of the nanopowders was investigated by scanning electron microscopy (SEM), using a FEI Quanta FEG 250 microscope and by transmission electron microscopy (TEM) using a FEI TECNAI TWIN XT TEM (200 kW, G2) microscope. The behavior in external magnetic field of the obtained nanocomposites was studied under AC (50 Hz) applied magnetic fields of amplitudes up to 160 kA m−1 with an installation described elsewhere (Mihalca and Ercuta, 2003).

The Zeta potential was measured by electrophoresis using a Zetameter (Zetameter System 3.0+ model, made by Zetameter Company, Inc., USA) for a suspension of 1 g L−1 powder in 10 mg L−1 NaCl.

The leached Fe concentration in the supernatant was measured by atomic absorption spectroscopy using a Varian SpectrAA 110 instrument.

2.4

2.4 Adsorption experiments

The adsorption experiments were performed at 25 °C, in a thermostated shaker with an operating speed of 200 rpm. The initial concentrations of each pollutant, MO and phenol, were varied between 20 and 100 mg L−1. The adsorbent mass ranged from 0.5 to 4 g L−1 and the pH value from 4 to 10.

The adsorbent was separated from the aqueous solution by the aid of a magnet. The concentration was monitored using a UV–Vis spectrophotometer model UVmini-1240 SHIMADZU. The absorbance values were measured at the wavelength of maximum absorbance, namely 646 nm for MO and 270 nm for phenol.

The amount of pollutant adsorbed, qt (mg g−1), was calculated according to Eq. (1) and the percentage of pollutant removed, R (%), was calculated by Eq. (2):

(1)
q t = ( C 0 - C t ) · V m
(2)
R = C 0 - C e C 0 · 100 where C0, Ct and Ce (mg L−1) are the concentrations of pollutant, initially, at any time t, and at equilibrium respectively, V the volume of solution (L) and m the mass of adsorbent (g).

2.5

2.5 Kinetic studies

The kinetic experiments were performed in single and binary system, using solutions of MO, phenol and their mixture with initial concentration of 50 mg L−1 for each pollutant. The adsorption kinetic of MO and phenol onto MNP2 nanocomposite was investigated only in single system while the adsorption of MO and phenol in case of MNP3, was conducted both in single and in binary systems.

In this study, the kinetic data were fitted with the linearized form of the Ho and McKay pseudo-second-order equation (Ho and McKay, 1999) expressed as follows:

(3)
t q t = 1 k 2 q e 2 + t q e where qt and qe are the amounts of solute adsorbed at time t and at equilibrium per unit mass of adsorbent (mg g−1); k2 is the adsorption rate constant (g mg−1 min−1).

2.6

2.6 Adsorption isotherms

Important information on the adsorption mechanism, the surface properties of the sorbent and the affinities between the sorbent and sorbate is provided by the parameters obtained from different isotherm models. In this study, four of the most used isotherms namely Langmuir, Freundlich, Redlich–Peterson and Sips were used to fit the equilibrium experimental data of MO and phenol adsorption onto MNP2 and MNP3 nanocomposites.

Langmuir theory assumes finite number of identical sites homogeneously distributed over the adsorbent surface, monolayer adsorption and no interactions between the adsorbed molecules. The Langmuir isotherm is represented by the equation (Hamdaoui and Naffrechoux, 2007a):

(4)
q e = q m K L C e 1 + K L C e where qe is the amount of solute adsorbed per unit mass of adsorbent at equilibrium (mg g−1), qm is the maximum monolayer adsorption capacity (mg g−1), Ce is the equilibrium concentration of the solute in the bulk solution (mg L−1) and KL is the Langmuir sorption constant (L mg−1).

The Freundlich isotherm describes the adsorption on heterogeneous surfaces, with interaction between adsorbed molecules; it is an empirical equation expressed as (Hamdaoui and Naffrechoux, 2007a):

(5)
q e = K F C e 1 / n where KF is the Freundlich constant (mg1−1/n L1/n g−1), indicating the adsorption capacity of the adsorbent and n (dimensionless) is a constants related to the intensity of adsorption.

Redlich–Peterson isotherm is a combination of Langmuir and Freundlich isotherms. This is an empirical with three parameters equation expressed as (Hamdaoui and Naffrechoux, 2007b):

(6)
q e = K RP C e 1 + α RP C e β where KRP is the Redlich–Peterson constant (L g−1), αR is also a constant ((L mg−1)β) and β is an exponent varying between 0 and 1. At high concentrations of the adsorbate, Redlich–Peterson equation reduces to the Freundlich Eq. (5). For β = 1, Eq. (6) reduces to the Langmuir equation (4).

Sips isotherm is an empirical equation with three parameters, a combination of the Langmuir and Freundlich models, expressed by the equation (Hamdaoui and Naffrechoux, 2007b):

(7)
q e = q m K S C e n 1 + K S C e n where KS is the Sips constant ((L mg−1)n) and n is the Sips model exponent. For n = 1, Eq. (7) reduces to Langmuir isotherm (Eq. (4)) and for low equilibrium concentration, close to 0, the Sips isotherm reduces to the Freundlich isotherm (Eq. (5)).

The isothermal studies were performed in single and binary systems. In the single system adsorption studies, the initial concentrations of MO and phenol varied from 20 to 100 mg L−1. In the simultaneous adsorption studies, the initial concentration of one component was fixed (phenol – 50 mg L−1 or MO – 20 mg L−1) while the concentration of the other component (MO or phenol) varied from 20 to 100 mg L−1. The adsorption of MO and phenol onto MNP2 nanocomposite was conducted only in single system while in case of MNP3, the adsorption of MO and phenol was conducted both in single and in binary systems.

The parameters of the four above mentioned isotherms were determined by nonlinear regression analysis, using ORIGIN 8 software. The coefficient of determination (R2) (Eq. (8)) (Ho, 2006) and the Chi-square (χ2) test (Eq. (9)) (Foo and Hameed, 2010) were employed to find the best fitted isotherm model for the experimental equilibrium data.

(8)
R 2 = ∑ q e , m - q ¯ e , exp 2 ∑ q e , m - q ¯ e , exp 2 + ∑ q e , m - q e , exp 2
(9)
χ 2 = ∑ q e , exp - q e , m 2 q e , m where q e , m and q e , exp are the equilibrium amounts adsorbed per unit mass of adsorbent (mg g−1) obtained from isotherm model and experiment, respectively; q ¯ e , exp is the average of q e , exp (mg g−1).

3

3 Results and discussion

The purpose of our study was the synthesis of magnetic iron oxide/carbon nanocomposites with high specific surface, in order to be used as adsorbents for hazardous pollutants. The only magnetic iron oxides are magnetite (Fe3O4) and maghemite (γ-Fe2O3) both having spinel structure. Magnetite is oxidized slowly, at room temperature, at maghemite which, in turn is transformed, at higher temperatures, in hematite (Tang et al., 2003). It is really difficult to synthesize pure magnetite and to stabilize it, in order to be used in air, without any special conditions. According to the literature, when Fe3O4 is heated in an oxidizing atmosphere at low temperature, γ-Fe2O3 is obtained, while, at higher temperature α-Fe2O3 is formed (Correa et al., 2006). It has been reported that the transition of Fe3O4 to γ-Fe2O3 takes place at around 250 °C (Haneda and Morrish, 1977) while γ-Fe2O3 is completely transformed in α-Fe2O3 around 600 °C (Nakatani and Matsuoka, 1983). Our previous studies, regarding the synthesis of composites based on magnetite by a similar solvothermal method, that used amine as precipitant agent (Stoia et al., 2015) showed that, even if magnetite has resulted from the synthesis, in time (about a month) it was quantitatively oxidized at maghemite.

Our original solvothermal method to obtain magnetic iron oxide/carbon nanocomposites, uses 1,2-propanediol as solvent, KOH as precipitant and activated carbon, with a specific surface area of 890 m2 g−1. The powders obtained at 195 °C were all magnetic, with no phase separation under the action of a magnet, suggesting the formation of a homogeneous composite.

3.1

3.1 Characterization of adsorbents

FT-IR spectroscopy represents, beside X-ray diffractometry, an excellent tool for the identification of the different iron oxides, due to the fact that each one presents different characteristic bands, while the diffraction patterns of magnetite and maghemite are almost identical, characteristic to spinel phases. According to the literature data, the characteristic absorption bands of the Fe–O bond in bulk Fe3O4 are located at 570 cm−1 and 375 cm−1 (Namduri and Nasrazadani, 2008; Ma et al., 2003). In case of maghemite there are several characteristic bands located at 556 cm−1, 638 cm−1 and 696 cm−1 (Ercuta and Chirita, 2013) while hematite has two strong absorption bands around 590 cm−1 and 470 cm−1 (Basavaraja et al., 2011). Due to these significant differences, the FTIR technique was confirmed in the literature to be a valuable tool to distinguish the two magnetic iron oxides Fe3O4 and γ-Fe2O3 that can be also used for their quantification (Namduri and Nasrazadani, 2008).

In order to evidence the nature of the obtained iron oxide phases and to study the changes that occur during the thermal treatment, the initial powders were annealed in air at 300 °C, 500 °C and 700 °C and were characterized by FTIR spectroscopy and X-ray diffractometry.

Fig. 1 presents the FTIR spectra of the powders MNP1 (a), MNP2 (b) and MNP3(c), annealed at different temperatures. The evolution of FTIR spectra of all powders is similar. Thus, in case of the initial, as synthesized powders, the FTIR spectra present two bands, located at 580 cm−1 and 440 cm−1 characteristic to magnetite (Fe3O4) (Ercuta and Chirita, 2013). The weak shoulder around 630 cm−1 may be due to the presence of maghemite (γ-Fe2O3) in a small extent (Li et al., 2012). This hypothesis was confirmed by the appearance of two supplementary bands, located around 630 cm−1 and 690 cm−1 in case of the powders annealed at 300 °C, when magnetite is oxidized to maghemite, according to the literature (Haneda and Morrish, 1977). In case of the powders annealed at 500 °C, some differences appear. Thus, MNP1 and MNP2 spectra exhibit two clear bands around 560 cm−1 and 450 cm−1, characteristic to α-Fe2O3 (hematite), with a shoulder around 634 cm−1, due to the presence of some residual maghemite, probably. The spectrum of MNP3 powder annealed at 500 °C, is almost identical with the one of the powder annealed at 300 °C, evidencing that in this case the transition maghemite to hematite did not occur yet, probably due to the presence of the carbon which stabilizes the maghemite phase (Lu et al., 2007; Xie et al., 2004; Xu et al., 2012).

FT-IR spectra of the magnetic nanopowders: (a) MNP1, (b) MNP2 and (c) MNP3.
Figure 1
FT-IR spectra of the magnetic nanopowders: (a) MNP1, (b) MNP2 and (c) MNP3.

Fig. 2 presents the XRD patterns of the powders MNP1 (a), MNP2 (b) and MNP3 (c) annealed at different temperatures. The evolution of the crystalline phases with the annealing temperature confirms the conclusions of FTIR study. Thus, in case of powders MNP1 (Fig. 2(a)) and MNP2 (Fig. 2(b)), both diffraction patterns of the initial powder and the one annealed at 300 °C evidence the presence of a spinel phase, that can be magnetite, Fe3O4 (PDF file no. 01-088-0866) or maghemite γ-Fe2O3 (PDF file no. 00-039-1346) (The Powder Diffraction File, 2012). It is difficult to make a distinction between magnetite and maghemite phase, due to the fact that both phases have characteristic diffraction peaks at very close value of 2θ. According to the literature, magnetite has its diffraction peaks at lower 2 theta values, compared to maghemite; this difference is the base of a new method for the quantification of magnetite–maghemite mixture using conventional X-ray diffraction technique (Kim et al., 2012). The interplanar distance corresponding to 2θ = 43.284° is also helpful to differentiate between the XRD pattern of the maghemite (d(4 0 0) = 2.0886 Å, 2θ = 43.28°) and magnetite (d(400) = 2.0993 Å, 2θ = 43.052°) phases due to the shift at 2θ value (Guivar et al., 2014). In our case, the interplanar distance d400 for the initial powders, synthesized at 195 °C, was 2.0995 Å for MNP1, 2.096 Å for MNP2 and 2.096 Å for MNP3, close to the value characteristic to magnetite. With the increase in annealing temperature at 300 °C, the values of d400 significantly decreased, becoming: 2.092 Å for MNP1, 2.0875 Å for MNP2 and 2.0883 Å for MNP3, confirming the transition of magnetite into maghemite. Only a slight shift of the diffraction peak maxima to higher 2 theta values was observed by annealing of all powders at 300 °C. It is possible thus that our initial powder to be a mixture of magnetite and maghemite, as evidenced by FTIR analysis.

XRD patterns of magnetic nanopowders: (a) MNP1, (b) MNP2 and (c) MNP3.
Figure 2
XRD patterns of magnetic nanopowders: (a) MNP1, (b) MNP2 and (c) MNP3.

XRD patterns of the powders annealed at 500 °C show some differences between MNP1, MNP2 and MNP3 powders, as shown by the FTIR study also. In case of MNP1 and MNP2, two crystalline phases are evidenced in their XRD patterns: a spinel phase, most probably maghemite (PDF file no. 00-039-1346) and a rhombohedral phase, identified as α-Fe2O3 (hematite) (PDF file no. 01-089-0598). In case of MNP3 powder, the spinel phase, identified as maghemite was the only crystalline phase in the system, confirming the conclusions of the FTIR study. All powders annealed at 700 °C contain α-Fe2O3 as single crystalline phase according to the PDF file no.01-089-0598 (The Powder Diffraction File, 2012).

The synthesized powders have been characterized by thermal analysis, in order to evidence the presence of carbon in the synthesized composites. Fig. 3 presents the TG and DSC curves of the powders MNP1, MNP2 and MNP3.

DSC (a) and TG (b) curves of the as synthesized powders: MNP1, MNP2 and MNP3.
Figure 3
DSC (a) and TG (b) curves of the as synthesized powders: MNP1, MNP2 and MNP3.

The shape of the thermal curves is similar for the MNP2 and MNP3 nanocomposites, while the one of MNP1 (without carbon) is different. Thus, the powder without carbon exhibits only two decomposition steps: one up to 120 °C, due to dehydration, associated on DSC to an endothermic effect. The second step, in the range 200–400 °C may be due to the dehydroxylation of the residual –OH groups with some structural changes (crystallization), taking into account the small exothermic effect registered on DSC around 330 °C. Despite the fact that no mass change has been registered up to 1000 °C, a clear exothermic effect is evidenced on DSC curve around 520 °C, due to the transition of γ-Fe2O3 to α-Fe2O3 (Sidhu, 1988), as evidenced by XRD analysis.

The thermal curves of composites MNP2 and MNP3 (with carbon) show a distinct thermal behavior. Their thermal decomposition takes place in three steps. The first one, in the range 70–120 °C, associated with an endothermic effect on DSC curve (Fig. 3(a)), corresponds to the elimination of water and ethanol adsorbed on particles’ surface. The second step, in the range 250–400 °C, with similar mass losses in all samples, associated with a weak exothermic effect on both DSC curves, can be assigned to dehydroxylation of residual –OH groups (Lepp, 1957). The third step in the range 400–650 °C, with large but different mass losses accompanied by a strong exothermic effect on DSC curve, in both MNP2 and MNP3, is due to the burning of the carbon present in the samples. This large exothermic effect present on the DSC curves overlaps with the exothermic effect of the transition γ-Fe2O3 → α-Fe2O3, visible in case of MNP2 (with lower carbon content). The mass loss of powder MNP2 in this step is 17% (calculated 17.142%), as compared with 29% (calculated 29.268%) for the powder MNP3, in agreement with the active carbon mass introduced in the synthesis. In order to confirm the assignment of the third thermal decomposition step to the burning of carbon, we have also characterized by thermal analysis the powder MNP3 annealed at 300 °C and 500 °C for 2 h. The obtained thermal curves are presented in Fig. 4. The powder annealed at 300 °C undergoes an initial endothermic dehydration step, and then the mass remains constant up to 450 °C, when the combustion of the carbon from nanocomposite takes place. The mass loss corresponding to this step is of 28%, very close to the one of the initial sample, and thus the carbon is not affected by the thermal treatment at 300 °C. In case of the powder annealed at 500 °C the second, exothermic, step is absent, so the entire carbon has burned during the heating at this temperature.

DSC and TG curves of MNP3 powder, annealed at 300 and 500 °C.
Figure 4
DSC and TG curves of MNP3 powder, annealed at 300 and 500 °C.

Based on the FTIR, XRD and thermoanalytical studies, it can be concluded that the initial powders contain a mixture of magnetite and maghemite as magnetic oxides. To have a stable adsorbent with constant properties, we decided to thermally treat all powders, to promote the formation of maghemite magnetic phase (γ-Fe2O3). In order to not affect the carbon content in the composites and the specific surface area, we have treated the powders at 100 °C for 6 h.

The SEM images of the obtained powders (Fig. 5(a)–(c)) evidence the fine nature of the formed maghemite nanoparticles. All powders exhibit quasi-spherical nanoparticles, as evidenced in the TEM images (Fig. 5(d)–(f)), with diameters between 5 and 10 nanometers. The EDX spectra highlight the increase in the carbon content from MNP2 to MNP3 and also the presence of residual potassium, resulted from precipitation, in each sample.

SEM and TEM images of nanopowders MNP1 (a, d); MNP2 (b, e) and MNP3 (c, f).
Figure 5
SEM and TEM images of nanopowders MNP1 (a, d); MNP2 (b, e) and MNP3 (c, f).

Fig. 6 presents the nitrogen adsorption–desorption isotherms (a) and pore size distribution (b) for the powders thermally treated at 100 °C for 6 h. All powders present type IV adsorption–desorption isotherms, characteristic to mesoporous adsorbents (with pores in the range 2–50 nm). Fig. 6(b) shows that MNP2 and MNP3 nanocomposites have mainly two types of pores with diameter around 13 nm and 9 nm, respectively. The pores with diameter around 13 nm are related to the presence of carbon in the materials, as the powder without carbon has a much smaller fraction of this kind of pores.

N2 adsorption–desorption isotherms (a) and pore size distribution (b) for powders MNP1, MNP2 and MNP3.
Figure 6
N2 adsorption–desorption isotherms (a) and pore size distribution (b) for powders MNP1, MNP2 and MNP3.

The textural characteristics of the powders MNP1, MNP2 and MNP3 are shown in Table 2. The specific surface area of the synthesized powders evidenced the positive effect of activated carbon on this parameter; thus the increase in the activated carbon amount increases the specific surface area of the composite.

Table 2 Textural characteristics of the powders.
Sample S (BET) (m2 g−1) Vpores (BJH – adsorption cumul. volume of pores) (cm3 g−1) Spores (BJH – adsorption cumul. surface area of pores (m2 g−1) Dpores (average) (nm)
MNP1 184.9 0.237 197.6 4.794
MNP2 189.4 0.255 187.7 5.442
MNP3 394.1 0.251 202.4 4.965
Carbon 889.9 0.199 179.0 4.442

Fig. 7 shows the magnetization cycle of the powder MNP3, with the highest carbon content, hence with the lowest saturation magnetization, due to the presence of non-magnetic carbon. The shape of the magnetization curve evidences a superparamagnetic behavior, characteristic to monodomenial magnetic nanoparticles (smaller than critical size). The value of maximum magnetization for this powder is 30 emu g−1, and thus all powders can be magnetically separated from aqueous solutions, with a magnet.

Magnetization curve of the powder MNP3.
Figure 7
Magnetization curve of the powder MNP3.

3.2

3.2 Adsorption studies

3.2.1

3.2.1 Effect of pH

Fig. 8 shows the effect of pH value on MO and phenol (Ph) adsorption onto MNP1, MNP2 and MNP3 magnetic nanopowders, studied in the range of 3.6–10.5 where the adsorbents have demonstrated good stability, as it can be seen in Table 3.

Effect of pH on MO and Ph adsorption onto MNP1, MNP2 and MNP3 nanopowders: initial concentration 50 mg L−1, adsorbent dose 1 g L−1, equilibrium time 8 h.
Figure 8
Effect of pH on MO and Ph adsorption onto MNP1, MNP2 and MNP3 nanopowders: initial concentration 50 mg L−1, adsorbent dose 1 g L−1, equilibrium time 8 h.
Table 3 Concentration of leached iron as a function of pH.
pH Concentration of leached iron ions (mg L−1)
MNP1 MNP2 MNP3
2 19.1 16.5 8.76
4 2.23 0.57 0.48
6 0.98 0.76 0.59
8 1.07 0.81 0.49
10 1.41 0.96 0.69
12 20.2 17.1 5.18

It can be noticed that in the range of pH values between 4 and 10, the leaching of Fe is practically negligible. Also it can be observed the decrease in the leaching of Fe with the increase in carbon content. This behavior confirms the literature reports (Lu et al., 2007; Xie et al., 2004) regarding the possibility to increase the stability against oxidation and acid leaching by preparing nanocomposites of magnetic nanoparticles and carbon.

While the MNP1 magnetic nanopowder (without carbon) doesn’t show adsorbent capacity neither for MO nor for phenol, both nanosorbents, MNP2 and MNP3, demonstrate better removal efficiency for MO compared with phenol. The adsorbent MNP3, with high carbon content, shows much higher removal efficiency, for both MO and phenol, compared to MNP2 on the entire range of solution pH. As can be observed, in case of both adsorbents MNP2 and MNP3, the removal efficiency of MO is a little higher at low pH value than those at high pH value, indicating that the adsorption mechanism for MO is influenced by the solution pH. It is known that MO, an anionic dye, is negatively charged at all over the pH value. At low pH value, the adsorption mechanism of MO is controlled by electrostatic attractions between the positively charged surface of adsorbent, as a result of the protonation process, and the negatively charged MO molecule (Asuha et al., 2011; Chen et al., 2011). It is important to note the high removal efficiency of MO by the two adsorbents also at neutral pH which suggests another mechanism involving important non-electrostatic interactions between the delocalized π-electrons on the surface of adsorbent and the free electrons of the dye molecule present in aromatic ring (Belhachemi and Addoun, 2011). Similar results regarding the pH influence on the MO adsorption were reported by Chen et al. (2011) and by Asuha et al. (2011). As results from Fig. 8, the pH value has no influence on the removal efficiency of phenol indicating that in this case the mechanism is controlled only by non-electrostatic interactions (Moreno-Castilla, 2004).

Zeta potential values for the three nanopowders, using solution of 10 mg L−1 NaCl were as follows: −95 mV for MNP1, −72 mV for MNP2 and −56 mV for MNP3. These negative values show that the working conditions are far from the pHpzc and that particles surface is negatively charged. The surface charge, however, decreases with the carbon content, due to the partial cover of maghemite particles surface by carbon. It is clear, and in accordance with the literature data, (Bavio and Lista, 2013; Zhang et al., 2012) that we worked above pHpzc. This explains the insignificant adsorption capacity for the naked γ-Fe2O3 powder, as a result of electrostatic repulsion between MO and the negatively charged surface of the adsorbent particles. It results that carbon is the main responsible for the adsorption capacity of our materials (γ-Fe2O3/C nanocomposites).

According to these results, the adsorption studies of MO and phenol were further conducted at the natural pH of their solutions namely pH – 4.8 for MO and pH – 4.5 for phenol respectively.

3.2.2

3.2.2 Effect of adsorbent dose

The effect of adsorbent dose, ranging from 0.5 g L−1 to 4.0 g L−1, on the removal efficiency of MO and phenol respectively is shown in Fig. 9.

Effect of adsorbent dose on MO and Ph adsorption: initial concentration 100 mg L−1 for both MO and Ph.
Figure 9
Effect of adsorbent dose on MO and Ph adsorption: initial concentration 100 mg L−1 for both MO and Ph.

It can be observed that MNP1 doesn’t show adsorption capacity neither for MO nor for phenol, regardless of the dose of adsorbent used; this may be explained by the lack of active sites on the γ-Fe2O3 surface. Unlike MNP1, the removal efficiency of MNP2 and MNP3 increases with adsorbent dose, both for MO and for phenol which may be explained by the increase in the available active sites on the sorbents surface. MNP3 shows a sharp increase in MO removal efficiency from 43.84% to 99.63% as the adsorbent dose increased from 0.5 to 2 g L−1; the increase in MNP3 dose from 2 to 4 g L−1 has practically an insignificant effect, increasing the MO removal efficiency only from 99.63% to 99.90%. MNP2 adsorbent shows almost the same removal efficiency (97%) as MNP3 only for 4 g L−1 dose, which confirm its lower adsorption capacity compared to MNP3.

In the case of phenol, both adsorbents, MNP3 and MNP2 show a moderate growth of removal efficiency with adsorbent dose but in this case too, the adsorbent capacity of MNP3 is higher compared to MNP2.

3.2.3

3.2.3 Effect of initial pollutant concentration

Fig. 10 shows the effect of different initial concentrations of MO and phenol, respectively on the amount adsorbed at equilibrium onto MNP1, MNP2 and MNP3 nanopowders.

Effect of initial concentration on (a) MO and (b) phenol adsorption onto MNP1, MNP2 and MNP3 nanopowders: adsorbent dose 1 g L−1.
Figure 10
Effect of initial concentration on (a) MO and (b) phenol adsorption onto MNP1, MNP2 and MNP3 nanopowders: adsorbent dose 1 g L−1.

The results shown in Fig. 10 confirm that MNP1 has no adsorption capacity neither for MO nor for phenol and it was not further used in adsorption studies. Regarding MNP2 and MNP3 nanopowders, the results are in accordance with the literature data showing the increase in the amount adsorbed at equilibrium with increasing the initial concentration of MO and phenol respectively.

MNP2 shows different behavior in case of MO adsorption compared with phenol. The amount adsorbed at equilibrium increases continuously for initial concentrations of MO in the range 0–40 mg L−1 and, then remains practically constant over 40 mg L−1; this behavior can be related to the surface saturation of MNP2 adsorbent. In the case of phenol, MNP2 nanopowder shows a moderate, continuous, increase in amount adsorbed at equilibrium, on the entire range of initial concentrations.

It can be noted that MNP3 nanopowder, unlike MNP2, shows a significant increase in adsorbed amount at equilibrium on the entire range of initial concentrations, for both MO and for phenol, in accordance with its better adsorption capacity, in comparison with MNP2.

3.2.4

3.2.4 Effect of contact time

The contact time between adsorbate and adsorbent is an important parameter in assessing the performance of adsorbents. The shorter the contact time in adsorption process, the lower are the operational costs that recommend the adsorbent for large-scale industrial application. Both nanocomposites, MNP2 and MNP3 were studied regarding the effect of time contact on MO and on phenol adsorption. Nanopowder MNP3 that showed the best adsorption capacity was investigated as adsorbent for the removal of MO and phenol using single and binary-component solutions, respectively.

The effect of contact time on MO and phenol adsorption onto MNP2 using single-component solutions is shown in Fig. 11(a). Fig. 11(b) shows the effect of contact time on MO and phenol adsorption onto MNP3 using single and binary-component solutions.

Effect of contact time on the sorption of (a) MO and Ph onto MNP2 using single-component solutions and (b) MO and Ph onto MNP3 using single and binary-component solutions: adsorbent dose 1 g L−1.
Figure 11
Effect of contact time on the sorption of (a) MO and Ph onto MNP2 using single-component solutions and (b) MO and Ph onto MNP3 using single and binary-component solutions: adsorbent dose 1 g L−1.

It is evident the sharp increase in the adsorbed amount of MO and phenol in the first 60 min of the process, onto both MNP2 and MNP3 nanopowders. This behavior can be attributed to the large number of vacant surface sites available for adsorption in the initial stage of the process; then, near equilibrium, the available sites for adsorption become fewer and are difficult to be occupied because of the repulsive forces between the solute molecules on the solid and the solution (Al-Muhtaseb et al., 2011).

It can be noticed that phenol has reached equilibrium faster (about 60–100 min) compared with MO (about 100–200 min) in case of both adsorbents, MNP2 and MNP3. Also, the equilibrium time was the same for the adsorption of MO and phenol in single and binary-component solutions.

3.2.5

3.2.5 Kinetic studies

Fig. 12 shows the plot of the linearized form of the pseudo-second-order equation (Eq. (3)) for the adsorption of MO and Ph onto MNP2 using single-component solutions (Fig. 12a) and for the adsorption of MO and Ph onto MNP3 using single and binary-component solutions (Fig. 12b).

Plots of t/q versus t for the adsorption of (a) MO and Ph onto MNP2 using single-component solutions and (b) MO and Ph onto MNP3 using single and binary-component solutions: adsorbent dose 1 g L−1.
Figure 12
Plots of t/q versus t for the adsorption of (a) MO and Ph onto MNP2 using single-component solutions and (b) MO and Ph onto MNP3 using single and binary-component solutions: adsorbent dose 1 g L−1.

The adsorption kinetic parameters are reported in Table 4. The linear plots of t/q versus t (Fig.12), the high values of the determination coefficients (R2) and the experimental values of qe very close to the calculated ones (Table 4) indicate that the adsorption process of MO and phenol, in the both single and binary systems, is properly described by the pseudo-second-order model.

Table 4 Kinetic parameters of the pseudo-second-order model for the adsorption of MO and phenol onto MNP2 and MNP3 nanopowders.
Adsorbent System Pollutant Initial concentration (mg L−1) k2 (g mg−1min−1) qe (mg g−1) R2
Calculated Experimental
MNP2 Single MO 50 0.0019 ± 0.0005 27.3 ± 0.7 26.1 ± 1.0 0.99654
Phenol 50 0.0054 ± 0.0007 6.28 ± 0.12 5.90 ± 0.35 0.99825
MNP3 Single MO 50 0.0024 ± 0.0004 46.2 ± 0.4 45.2 ± 2.2 0.99966
Phenol 50 0.0061 ± 0.0012 20.2 ± 0.2 19.8 ± 0.9 0.99958
Binary MO–Phenol 50–50 0.0031 ± 0.0003 46.4 ± 0.2 45.8 ± 2.3 0.99991
Phenol–MO 50–50 0.170 ± 0.041 7.9 ± 1.1 7.87 ± 0.51 0.99883

It can be noted that the rate of phenol adsorption is higher than that of MO in case of both adsorbents, MNP2 and MNP3, using single component solutions, in agreement with the shorter time required for reaching the equilibrium. This can be attributed to the higher molecular weight of MO than that of phenol, which affects the mass transfer mechanisms involved in adsorption process (Tovar-Gómez et al., 2012). The rate constants of MO and phenol in binary-component system are higher than those for single-component system, using MNP3 as adsorbent. This is likely due to the synergistic effect between MO and phenol in the adsorption process.

3.2.6

3.2.6 Adsorption isotherms

3.2.6.1
3.2.6.1 Single component adsorption isotherms

The fit of the equilibrium experimental data with the four isotherm models for the single adsorption of MO and phenol onto MNP2 and MNP3 is shown in Figs. 13 and 14, respectively.

Isotherm plots for the adsorption of MO (a) and phenol (b) onto MNP2.
Figure 13
Isotherm plots for the adsorption of MO (a) and phenol (b) onto MNP2.
Isotherm plots for the adsorption of (a) MO and (b) phenol onto MNP3 in single system.
Figure 14
Isotherm plots for the adsorption of (a) MO and (b) phenol onto MNP3 in single system.

The parameters of the four investigated isotherms with the corresponding coefficient of determination (R2) and Chi-square value (χ2) are listed in Tables 5 and 6 in the case of MNP2 and MNP3 adsorbents. Comparing the R2 and χ2 values shown in Table 5 for each model, it can be noticed that the Sips isotherm best fitted the equilibrium data of MO and phenol adsorption onto MNP2 adsorbent. The Sips isotherm being a combination of the Langmuir and Freundlich models, this result suggests that at low equilibrium concentration, the adsorption process of MO and phenol is carried out according to the Freundlich isotherm and proceeds according to the Langmuir isotherm at high concentrations.

Table 5 Isotherm parameter values for the adsorption of MO and phenol onto MNP2 in single solutions.
Isotherm model Parameter Pollutant
MO Phenol
Langmuir qm (mg g−1) 32.6301 19.3563
KL (L mg−1) 0.57847 0.00916
R2 0.98143 0.99413
χ2 3.01919 0.06514
Freundlich KF (((mg1−(1/n)L1/n)g−1)) 16.55866 0.38826
n 5.81112 1.43704
R2 0.99745 0.98742
χ2 0.41434 0.13955
Redlich–Peterson KRP (L mg−1) 68.00942 0.14947
αR ((L mg−1)β) 3.55101 2.91E−04
β 0.86355 1.67943
R2 0.99835 0.99437
χ2 0.26822 0.06249
Sips qm (mg g−1) 54.6915 12.6863
KS (L mg−1) 0.39563 5.11E−03
n 0.33227 1.34021
R2 0.99868 0.99542
χ2 0.21444 0.05079

The values in bold represent the best results obtained.

Table 6 Isotherm parameter values for the adsorption of MO and phenol onto MNP3 in single and binary solutions.
Isotherm model Parameter Pollutant
Single system Binary system
MO Ph MO-50 mg L−1 Ph Ph-20 mg L−1 MO
Langmuir qm (mg g−1) 72.6774 42.3423 71.0174 19.0257
KL (L mg−1) 0.42064 4.59E−02 0.42379 0.05963
R2 0.98799 0.99599 0.95574 0.9986
χ2 8.5388 0.59539 30.1084 0.05057
Freundlich KF (((mg1−(1/n)L1/n)g−1)) 27.82727 5.3126 28.21102 3.81457
n 3.6275 2.30321 3.87281 3.04473
R2 0.93911 0.98496 0.89968 0.9844
χ2 43.29047 2.23311 68.23998 0.56409
Redlich–Peterson KRP (L mg−1) 30.12775 1.88255 25.77693 0.94041
αR ((L mg−1)β) 0.40602 0.03956 0.2829 0.02857
β 1.0063 1.02574 1.07431 1.11775
R2 0.98401 0.99469 0.94448 0.9994
χ2 11.36684 0.78828 37.76983 0.02167
Sips qm (mg g−1) 70.38593 42.67517 64.48114 17.42514
KS (L mg−1) 0.41774 0.04668 0.35261 0.03538
n 1.10963 0.98926 1.59379 1.25029
R2 0.98474 0.99466 0.95074 0.99955
χ2 10.85025 0.79313 33.50947 0.01628

The values in bold represent the best results obtained.

According to the same criteria, the equilibrium data of MO and phenol adsorption onto MNP3 were best fitted by the Langmuir isotherm model indicating the homogeneous nature of the adsorbent surface (Makrigianni et al., 2015; Tanhaei et al., 2015; Chen et al., 2011).

The adsorption selectivity of MO and phenol (SMO/Ph) on MNP2 and MNP3 adsorbents was calculated with the following equation (Tovar-Gómez et al., 2015; Ortega et al., 2013):

(10)
S MO / Ph = q MO q Ph

Using the qm values from Langmuir model (Tables 5 and 6), the calculated values of SMO/Ph are 1.68 and 1.72 for the adsorption of MO and phenol onto MNP2 and MNP3, respectively. These very close values show that, both adsorbents MNP2 and MNP3 have practically the same adsorption capacity toward MO which is approximately 1.7 times greater than that of phenol.

The practically identical values of SMO/Ph demonstrate that the selectivity is not influenced by the different textural characteristics of the two adsorbents MNP2 and MNP3 (i.e. specific surface area). The higher affinity of both adsorbents, MNP2 and MNP3 toward MO can be explained by the lower solubility of MO (5 g L−1) compared with phenol (83 g L−1).

Comparing the maximum adsorption capacity (qm) of the two adsorbents, resulted from the Langmuir isotherm, (Tables 5 and 6), one can notice the higher adsorption capacity of MNP3 than that of MNP2, for both MO and phenol. These results can be related to the larger carbon content and respectively larger specific surface area of MNP3 (SBET = 394.06 m2 g−1) adsorbent compared with MNP2 (SBET = 189.42 m2 g−1).

3.2.6.2
3.2.6.2 Binary adsorption isotherms

The fit of the equilibrium experimental data with the four isotherms is shown in Fig. 15(a) and (b) for the simultaneous adsorption of MO in the presence of fixed concentration of phenol (50 mg L−1) and of phenol in the presence of fixed concentration of MO (20 mg L−1), respectively onto MNP3 adsorbent.

Isotherm plots for the adsorption of MO and phenol onto MNP3 in binary system. The fixed concentration of phenol and MO was 50 mg L−1 and 20 mg L−1, respectively.
Figure 15
Isotherm plots for the adsorption of MO and phenol onto MNP3 in binary system. The fixed concentration of phenol and MO was 50 mg L−1 and 20 mg L−1, respectively.

The corresponding parameters of the four investigated isotherms in binary system and the R2 and χ2 values are listed in Table 6.

Based on the highest value of R2 and of the lowest value of χ2, it can be concluded that the Langmuir isotherm is the best fit model for the binary adsorption of MO–Ph while Sips isotherm is the best fit model for the binary adsorption of Ph–MO.

The effect of the simultaneous presence of the two pollutants, MO and Ph, on the bicomponent removal performance of MNP3 has been investigated by calculating the ratio of adsorption capacity (Rq), defined as (Agarwal et al., 2013):

(11)
R q , i = q m , i q 0 , i where q m , i and q 0 , i are the adsorption capacity of pollutant i in the bicomponent solution and in the single component solution, respectively, in the same operating condition. According to the literature data (Agarwal et al., 2013) (a) if R q , i > 1 , the presence of other pollutants in multi-component systems improves the adsorption of pollutant i (i.e., synergistic adsorption), (b) if R q , i = 1 , there is no effect of the presence of other pollutants in multi-component systems on the adsorption of pollutant i and (c) if R q , i < 1 , the adsorption of pollutant i is reduced by the presence of other pollutants in multi-component systems (i.e., antagonistic adsorption).

The adsorption of phenol is significantly affected by the presence of MO in the binary system. The maximum adsorption capacity of phenol (qm value from Langmuir model, Table 6) is 42.34 mg g−1 in single component solution and 19.03 mg g−1 in bicomponent solution, respectively. The value Rq,Ph = 0.45 is far less than 1, thereby confirming the antagonistic adsorption of phenol in the presence of MO in the bicomponent solutions. On the other hand, the MO adsorption is practically not affected by the presence of phenol in the binary system. The maximum adsorption capacity of MO is 72.68 mg g−1 in single component solution and 71.02 mg g−1 in bicomponent solution, resulting the value Rq,MO = 0.98, which is almost equal to 1.

The fact that the maximum adsorption amount for phenol decreases significantly in the presence of MO (Rq,Ph = 0.45) while for MO is kept almost constant (Rq,MO = 0.98) in the presence of phenol is due to much stronger interactions between MO and the surface of MNP3 adsorbent as compared with those involved in the phenol adsorption. These results are in perfect agreement with the literature reports (Tovar-Gómez et al., 2015) regarding the complex adsorption mechanism of dyes which involves (as we discussed at the pH influence) both electrostatic and non-electrostatic interactions, unlike the adsorption mechanism of phenol which involves only non-electrostatic interactions. The efficiency of MNP3 nanopowder was compared with other reported results regarding the adsorption of MO and phenol onto different adsorbents (Tables 7 and 8).

Table 7 Comparison of the removal efficiency of MO by different adsorbents.
Adsorbent qm (mg g−1) Adsorbent dose (g L−1) Equilibrium time (min) Adsorption condition References
Surfactant modified silkworm exuviate 87.3 2 60 pH = 7.0, T = 30.2 °C Chen et al. (2011)
Rectorite/iron oxide nanocomposites 0.36 2 1 T = 25 °C Wu et al. (2011)
SiO2-coated Fe3O4 magnetic nanoparticles (SMNPs) 53.19 1 30 pH = 2.66, T = 25 °C Shariati-Rad et al. (2014)
γ-Fe2O3/SiO2/chitosan composite 34.29 1 300 T = 37 °C Zhu et al. (2011)
γ-Fe2O3 crosslinked chitosan composite 29.46 1 100 pH = 6.6, T = 27 °C Zhu et al. (2010)
Magnetic cellulose beads 1.47 250 180 pH-7.0 Luo and Zhang (2009)
Magnetic halloysite nanotubes/iron oxide composites 0.65 2 480 – Xie et al. (2011)
γ-Fe2O3/2C nanocomposite 72.68 1 100 pH = 4.8, T = 25 °C This study
Table 8 Comparison of the removal efficiency of phenol by different adsorbents.
Adsorbent qm (mg g−1) Adsorbent dose (g L−1) Equilibrium time (min) Adsorption condition References
Activated carbon (ACK1) 17.83 2 120 pH = 7.0, T = 20 °C Kilic et al. (2011)
Poly(methyl methacrylate) (PMMA) 33.1 2 30 T = 25 °C Al-Muhtaseb et al. (2011)
Magnetic hydroxyapatite 18.0 – 80 pH = 7.0, T = 25 °C Wang (2011)
Modified natural red clay (HDTMA-clay) 1.13 4 360 pH = 5.8, T = 20 °C Plaska et al. (2012)
Soil (Adhanur-India) 34.27 14 360 pH-6.0 T = 30 °C Subramanyam and Das (2009)
Magnetic nanopowder 13.5 2 22 h pH = 6.5, T = 25 °C Mihoc et al. (2014)
Porous hydroxyapatite PHAp 9.2 2 150 T = 22–25 °C Bahdod et al. (2009)
γ-Fe2O3/2C nanocomposite 42.34 1 60 pH = 4.5, T = 25 °C This study

3.2.7

3.2.7 Adsorbent reusability

In order to evaluate the possibility of regeneration and reuse of MNP3 adsorbent, four cycles of adsorption–desorption have been performed using single and binary-component solutions of MO and Ph. Desorption behavior was studied using aqueous solution of ethanol (anhydrous ethanol:water = 2:1). The removal efficiency is shown in Fig. 16.

Removal efficiency of MO and phenol (Ph) in the following: (a) single-component solution and (b) binary-component solution, using MNP3 adsorbent. Initial concentration of MO, Ph and MO–Ph mixture: 50, 50, 50–50 mg L−1; adsorbent dose 1 g L−1.
Figure 16
Removal efficiency of MO and phenol (Ph) in the following: (a) single-component solution and (b) binary-component solution, using MNP3 adsorbent. Initial concentration of MO, Ph and MO–Ph mixture: 50, 50, 50–50 mg L−1; adsorbent dose 1 g L−1.

Results showed (Fig. 16(a)) that in four cycles, the removal efficiency gradually decreased from 91% to 60% in case of MO and from 41% to 34% for phenol in single-component solution. In binary-component solution (Fig. 16(b)), the removal efficiency decreased from 90% to 56% in case of MO and from 16% to 6% for phenol.

These results indicate that MNP3 adsorbent shows a good potential for reusability.

4

4 Conclusions

New magnetic adsorbents with varied carbon content were successfully synthesized by a facile one-step solvothermal method, using non-toxic and cost-effective precursors. The removal efficiency of both, MO and phenol can be significantly improved by increasing the carbon content of the magnetic nanopowder. These results can be related to the larger specific surface area of MNP3 (SBET = 394.06 m2 g−1) adsorbent compared with MNP2 (SBET = 189.42 m2 g−1). The adsorbed amount of MO and phenol increased sharply in the first 60 min of the adsorption process. The investigated adsorbents showed a higher adsorption capacity toward MO than that of phenol. The sorption kinetics of MO and phenol were well described by the pseudo second-order model in both single and binary-component solutions. The single-pollutant adsorption showed that the capacity of the two investigated adsorbents, MNP2 and MNP3, for adsorbing MO was approximately 1.7 times greater than that of phenol, which can be explained by the lower solubility of MO (5 g L−1) compared with phenol (83 g L−1). From the four investigated isotherm models, Langmuir, Freundlich, Redlich–Peterson and Sips, in single component adsorption, the equilibrium data were better described by Langmuir isotherm for both, MO and phenol adsorption onto MNP2 and MNP3 adsorbents, respectively. The maximum monolayer adsorption capacity was 72.68 and 42.34 mg g−1, for the adsorption of MO and phenol respectively, onto MNP3 adsorbent. In the bicomponent adsorption of the two pollutants, the MO adsorption was practically not affected by the presence of phenol while the adsorption of phenol was significantly reduced in the presence of MO. This behavior was explained by the much stronger interactions between MO and adsorbent, involving both electrostatic and non-electrostatic interactions, unlike the adsorption mechanism of phenol which involves only non-electrostatic interactions.

The desorption studies performed using single and binary-component solution of MO and phenol demonstrated the good potential of regeneration and reuse of MNP3 adsorbent.

Considering the facile and inexpensive method, which allows the synthesis of nanocomposites with tailored adsorption capacity and magnetic separation, effective in single and bicomponent adsorption of MO and phenol, the adsorbents used in this study offer many promising benefits for commercial purpose in the future.

Acknowledgments

This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI, project number PN-II-RU-TE-2014-4-0514.

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