One-Step Carbothermal Synthesis of Super Nanoadsorbents for Rapid and Recyclable Wastewater Treatment

Abstract

As a potential magnetic super adsorbent in wastewater treatment, Fe₃O₄ has been researched intensively up to date. However, its key problem of poor comprehensive magnetic properties is still challenging. In this work, an effective solution to this problem has been developed by a one-step carbothermal synthesis of Fe₃O₄ crystals, which are merited with pure-stoichiometry (FeO-phase free), high crystallinity, small-size (~10 nm), strong magnetism and sensitive magnetic response. The unveiled saturation magnetization of Fe₃O₄ nanoparticles reaches as high as 90.32 emu·g⁻¹, and the fastest magnetic response time is as short as only 5 s. Such magnetic Fe₃O₄ super adsorbents exhibit outstanding performance when applied as an adsorbent for wastewater treatment. They can quickly and effectively adsorb methylene blue with an adsorption capacity of 62.5 mg·g⁻¹, which is much higher than that of Fe₃O₄ adsorbents prepared by other methods reported in the literature. Importantly, this capacity is refreshable after removing the adsorbed methylene blue just by ultrasonic cleaning. With such combined outstanding magnetic properties and recyclable adsorption capacity, the problems associated with the conventional adsorbent solid–liquid separation could be resolved, thus making a forward development towards industrial wastewater treatment.

1. Introduction

As an important functional magnetic material, Fe₃O₄ has an inverse cubic spinel structure (two Fe³⁺ with one Fe²⁺) in which oxygen atoms form an fcc close-packed structure [1,2,3,4]. In recent years, Fe₃O₄ nanoparticles have been used as adsorbents for wastewater purification and have gradually attracted great attention from scholars at home and abroad, due to their stable physical and chemical properties, strong magnetism, large specific surface area, and ease of dispersion [5,6,7,8,9,10,11,12].

With the development of industrial technology, water pollution is currently one of the most serious environmental problems, causing potential harm to the health of humans and other organisms [13,14,15]. Especially, dye waste treatment in industrial wastewater has become one of the biggest challenges due to its high organic content, complex composition, deep color and contribution to large changes in water quality [16,17,18]. With the wastewater treatment techniques constantly innovating, various methods such as electrochemical and catalytic degradation [19], membrane filtration, adsorption, and biological treatment have come into play [20,21,22,23,24]. According to reports, adsorption is recognized as the most effective method for the decolorization of printing and dyeing wastewater. Among many adsorbents that have been used in wastewater decolorization (such as activated carbon, kaolin, bagasse charcoal, and biochar), Fe₃O₄ has attracted widespread attention, due to its low price and ease of separation of solid and liquid dye components. Therefore, it has an important environmental significance that calls for the development of a cost-effective magnetic Fe₃O₄ adsorbent for removing methylene blue from waste liquid.

In this work, aiming at reusable magnetic adsorbents for efficient wastewater treatment, a solution to the key problem of poor magnetic responsiveness of Fe₃O₄ is developed by synthesizing purely-stoichiometric, monodisperse, and small-sized crystals using a one-step carbothermal strategy. Based on the original one-step reduction method [25,26], in this experiment, the carbon source is provided by adding an appropriate amount of citric acid during the sol-gel process. Some parameters, such as the amount of ferric nitrate and citric acid in the sol-gel process, reduction temperature, holding time as well as the ratio of dry gel and picric acid, have been explored and optimized to accurately control the preparation of Fe₃O₄ nanoparticles. This work uses methylene blue (MB) solution to simulate wastewater, and conducts a series of adsorption experiments using Fe₃O₄ nanoparticles as the MB adsorbent. The amount of adsorbent, reaction time, and initial concentration of MB on the adsorption performance of the material were explored. The purpose of this study is to prepare Fe₃O₄ nanoparticles by a one-step carbothermal reduction method with high magnetic properties and excellent magnetic response, that can effectively remove MB from aqueous solution with excellent recyclability. This study paves an avenue to manufacture Fe₃O₄ nanoparticles towards industrial dye wastewater treatment.

2. Materials and Methods

2.1. Synthesis of Fe₃O₄ Nanoparticles

The dry gel is prepared by the citric acid sol-gel method. Herein, the organic fuel citric acid C₆H₈O₇·H₂O is used as the carbon source, and the soluble metal salt Fe(NO₃)·9H₂O is used as the iron source, which were dissolved in deionized water according to a certain molar ratio. After stirring, the solvent was evaporated and the pH adjusted to form a red-brown gel. The resulting viscous solution was poured into a glass, and dried at 100 °C in an oven, to achieve complete dehydration, which was later ground into a powder. The C₆H₈O₇·H₂O-Fe (NO₃)·9H₂O xerogel and picric acid were put into an autoclave at a ratio of 12:1 with sealing under an argon atmosphere, and then a carbothermal reduction reaction was conducted. In this process, picric acid explosion not only results in dispersed particles with reduced particle size, but also the CO produced could be used as a reducing agent. Through changing the three process parameters of reduction temperature, holding time, and the ratio of dry gel and picric acid in the carbothermic reduction process, the controllable preparation of single-phase nano Fe₃O₄ particles at 450–650 °C was finally realized.

2.2. Adsorption Experiment

An aqueous stock solution of MB (1000 mg·L⁻¹) was prepared by dissolving MB in deionized water. Then MB aqueous solutions of different concentrations (40, 60, 80, and 100 mg·L⁻¹) were prepared by successive dilution of the stock solution with deionized water. 50 mL each of MB dye solutions of different concentrations were taken and 40 mg Fe₃O₄ adsorbent was added to it. Then the solutions were shaken sufficiently in a neutral environment with a temperature of 20 °C and a pH of 7. At different time intervals, 5 mL of suspension samples were taken for external magnetic separation, and the supernatant was collected to measure its absorbance at the maximum absorption wavelength of MB at 664 nm by UV-Vis spectrophotometry, following which the adsorption capacity was calculated using the formulae [7,10]

A = KCL,

(1)

Q = [(C₀−C_e) V/W] × 100%,

(2)

E = [(C₀−C_e)/C₀] × 100%,

(3)

where A is the absorbance, K is the absorption coefficient of the sample, C is the concentration of the sample to be tested, L is the length of the sample in the optical path, Q is the absorption capacity of the material for MB, E is the removal rate, and C₀ is the initial dye concentration (mg/L), C_e is the equilibrium concentration of the dye in the reaction equilibrium, V is the volume of the reaction solution, and W is the weight of the adsorbent (g).

2.3. Samples Characterization

The phase structure of the samples 1–6 was studied using XRD patterns. Jade software was used to fit the strongest FeO and Fe₃O₄ diffraction peaks for semi-quantitative analysis to obtain the mass fraction of each component. FESEM (field-emission scanning electron microscope) was used to observe the surface morphology and particle size of the samples 1–5 to study the influence of different content of FeO on the shape and size of the product. Transmission electron microscopy (TEM) was used to characterize the morphology of the Fe₃O₄ (S6) nanoparticles. The room temperature magnetic hysteresis (M-H) loops of samples 1–6 were measured using a vibrating sample magnetometer (VSM) system with a maximum magnetic field of 20,000 Oe. The pure phase Fe₃O₄ (S6) was used for the adsorption experiment, and the absorbance of the solution to be tested was measured with an ultraviolet-visible spectrophotometer.

3. Results and Discussion

3.1. Characterizations of Fe₃O₄/FeO and Fe₃O₄ Nanoparticles

For convenience, the amount of xerogel and picric acid were fixed, and the ratio of ferric nitrate to citric acid was varied in the following discussion. In Figure 1a, when the molar ratio of ferric nitrate to citric acid is 1:0.5 mol, an obvious peak of FeO in samples S1–S3 is present, and that of Fe₃O₄ becomes weak, indicating that the content of Fe₃O₄ in the product diminishes.

When the molar ratio of ferric nitrate to citric acid is 1:0.8 mol, it can be observed that the main phase of samples S4 and S5 is Fe₃O₄, with a trace amount of FeO. On this basis, without changing the ratio of raw materials, the FeO content is reduced by adjusting the temperature. When the reaction temperature reaches 650 °C, there is only a single phase of Fe₃O₄ in the product without other peaks (Figure 1c). The strong and sharp diffraction peaks indicate that the sample has good crystallinity. The XRD pattern can be matched to the series of Bragg reflections corresponding to the standard phase of the spinel structure of Fe₃O₄. Six peaks at 30.16°, 35.49°, 43.01°, 53.78°, 57.21°, and 62.73° can be indexed as (220), (311), (400), (422), (511), and (440) of the cubic structure of Fe₃O₄ nanoparticles.

According to the related reports of cited literature and experimental results, the precursor formed by ferric nitrate and citric acid sol-gel is first reduced to Fe₂O₃ during the carbothermal reduction stage. The remaining free -C- in the precursor further reduces Fe₂O₃ to form Fe₃O₄. When the free -C- in the system is in excess, FeO will be gradually formed. However, it can be seen from Figure 1 that when the molar amount of citric acid is reduced from 0.8 mol to 0.5 mol, the product contains a large amount of FeO. This may be due to the fact that in the process of preparing the precursor, a longer drying time is required for the formation of 1.05 mol dry gel. A part of Fe₂O₃ gets reduced to Fe₃O₄ after drying in an oven at 100 °C for 24 h. In the carbothermal reduction stage, the Fe₃O₄ contained in the original xerogel is reduced to FeO.

The (311) diffraction peaks corresponding to the FeO/Fe₃O₄ mixture S1, S2, and S3 shifted to a higher 2θ (Figure 1b), indicating that at the mixing ratio of 1:0.5 ferric nitrate and citric acid, the doping of FeO content increases. Additionally, when the FeO content is large, it can be seen from the interlayer spacing of Fe₃O₄ that the Fe₃O₄ diffraction peak intensity is not sharp enough.

Morphological studies. Figure 2(S1–S5) shows the observed SEM images of five Fe₃O₄/FeO mixed samples. With the decrease of FeO content, the particles are obviously smaller and the size is uniform. The small spherical particles can be clearly seen from the images. The morphology of Fe₃O₄ NPs and the corresponding SAED images were obtained by high resolution transmission electron microscopy (HRTEM). Fe₃O₄ NPs (Figure 2(S6)) are spherical and uniformly sized, with a diameter of 10 nm. The HRTEM clear parts of multiple particles were selected separately, and the interplanar spacing was measured, which corresponds to the Fe₃O₄ standard card 89-4391.

It can be seen in Figure 2a that the lattice stripes of Fe₃O₄ are clear and continuous, indicating that Fe₃O₄ nanoparticles have a good crystal structure. The lattice fringe spacing of ~10 nm for Fe₃O₄ NPs is ~0.25 nm and 0.20 nm, corresponding to the (311) plane and (400) plane of the NP, respectively. The FFT of the corresponding area (Figure 2a) is displayed as a polycrystalline FFT pattern.

The difference in the crystallization process will affect the distribution of Fe²⁺ and Fe³⁺ in the octahedral sites, thereby affecting the super-exchange between Fe ions. It can be seen from XRD and TEM that the nano Fe₃O₄ particles synthesized by one-step carbothermal synthesis have high crystallinity, more than 82%, indicating the presence of fewer lattice defects. Therefore, the super-exchange between Fe-O-Fe is enhanced, and hence the magnetic properties are improved [27].

FT-IR spectra analysis. Figure 2b shows the FT-IR spectra of the obtained samples. With the decrease of FeO content in the samples, the characteristic peak of Fe₃O₄ becomes more obvious. The sample has vibration peaks at around 1620 cm⁻¹ and 3400 cm⁻¹, which are attributed to the H₂O and O-H stretching vibration absorption peaks contained in the ferrite. It can be seen from the figure that the absorption vibration peak of the self-burning powder at about 570 cm⁻¹ is the vibration peak of Fe₃O₄. From the enlarged FTIR image, it is observed that the characteristic peaks of the samples with more FeO content around 570 cm⁻¹ have shifted to the left relative to the Fe₃O₄ samples, and the infrared absorption band gradually assumes a broad and flat form. However, the S1 and S2 samples have no characteristic peak near 550 cm⁻¹, indicating that some Fe₃O₄ is reduced to FeO excessively. This result is due to a difference in the temperatures of the carbothermal reduction reaction, which deforms the inside of the particles after heating, thereby changing the bond length, and increasing the bond frequency [27,28].

VSM analysis. The magnetic properties are summarized in

Table 1. Magnetic Properties of Fe₃O₄/FeO Samples.

Sample	Preparation Conditions			Fe3O4 (wt%)	FeO (wt%)	MS (emu/g)	Mr (emu/g)	HC (Oe)
	Molar Ratio	T (°C)	T (min)					+	−
1	1:0.5	450	50	32.3	67.7	26.72	8.26	489.7	552.2
2	1:0.5	600	20	56.7	43.3	32.69	9.73	581.4	487.7
3	1:0.5	600	30	66.5	33.5	38.08	14.26	633.8	714.1
4	1:0.8	550	20	75.5	24.5	70.57	19.52	431.8	311.4
5	1:0.8	600	30	87.7	12.3	88.57	35.77	392.6	327.1
6	1:0.8	650	15	100	0	90.32	36.56	319.2	320.4

. The saturation magnetization (Ms) values of the S1, S2, S3, S4, S5, and S6 (Fe₃O₄) samples are determined as 26.72, 32.69, 38.08, 70.57, 88.57, and 90.32 emu/g respectively (Figure 3a–f). It can be noticed that as the FeO content decreases, the Ms value tends to increase. The Ms value of Fe₃O₄ nanoparticles is significantly higher than that of other samples, and even higher than the Ms (86.2 emu/g) we previously reported. This result can be owed to the different crystal sizes. From Table 1, we know that the coercivity of Fe₃O₄ nanoparticles is large, which is due to the one-step carbon thermal synthesis process. Spin barrier, spin tilt and spin glass effect due to in-situ chemical disorder on the surface of nanoparticles [25]. The following figures provide some useful information about the magnetic response of various samples. It is these magnetic properties that render potential applications in magnetic adsorbents [29,30].

3.2. Magnetic Response

The samples are prone to agglomeration due to their large specific surface area and magnetic dipole interaction, which makes them difficult to be used directly. In order to study the magnetic response behavior of nanoparticles with different contents, local high temperature, high pressure, or strong shock wave generated by ultrasonic cavitation is used to greatly weaken the interaction between nanoparticles. The magnetic response tests are performed by using samples of the same quality and dissolving and dispersing them in ethanol solution under ultrasonication for 15 min. The time for which the test sample was adsorbed on the bottle wall in the magnet environment is recorded, and the same sized bottle and magnet were used to ensure comparability [31].

It can be seen from the test results (Figure 4) that samples other than S1 could be dispersed in ethanol to form a uniform suspension when there is no external magnetic field, which can be quickly assembled and separated when an external magnetic field is applied. As the FeO content in the S1 particles is higher, the particle size is larger, and FeO is insoluble in ethanol, resulting in a weaker dispersion in the ethanol solution compared to the other four samples. Samples S4 and S5 showed a relatively faster adsorption and the solutions were clear and free of impurities. The other solutions were all turbid. As the size of the nanoparticles decreases, the turbidity of the solution increases, and the rate of adsorption by the magnet is faster. This shows that samples with higher FeO content have weaker magnetic properties and are not easily soluble in ethanol. The S6 sample is more soluble in the ethanol and is adsorbed by the magnet within 5 s. The adsorbed solution is clearer than the other five samples, indicating that the prepared Fe₃O₄ nanoparticles have high magnetic response sensitivity. Based on the rapid separation technology, the synthesized nano Fe₃O₄ particles can be used as a magnetic adsorbent to treat other pollutants such as heavy metal ions and colored dyes in wastewater.

3.3. Adsorption Performance

Methylene blue dye is easily soluble in water and difficult to eliminate. Due to the small size of Fe₃O₄ particles, large specific surface area and high surface activity, it increases the chance of contact with the coloring agent. It can reach the adsorption equilibrium in a short time, and quickly reduce the chroma of wastewater.

It can be seen from Figure 5a that in the initial stage of adsorption, the adsorption capacity increased rapidly with the increase of time. When the adsorption reached equilibrium, the adsorption capacity tended to be constant. This is because in the initial stage of adsorption, there are many vacancies on the surface of the adsorbent, and adsorption occurs easily. As the adsorption continues, the vacancies on the surface of the adsorbent lessen, and the adsorption rate significantly declines until an equilibrium is reached. The illustration in Figure 5a shows the adsorption process of MB on Fe₃O₄ at different durations. The color of the MB solution was dark blue in the beginning, and it became light blue after 2 min of adsorption with the adsorbent. After 160 min, the solution is nearly colorless, the solid–liquid separation effect of Fe₃O₄ is obvious, and the solution is very clear. Figure 5b is the relationship between absorbance and methylene blue concentration curve, fitting a linear standard curve and standard curve equation, bringing the absorbance of the sample to be measured into the equation, so as to calculate the concentration to be measured.

Figure 5b shows that, as the mass of adsorbent increases, the removal effect of MB gradually increases. When the dosage of adsorbent is 200 mg, the removal effect of MB is the best. Figure 5d is the adsorption isotherm drawn based on the Langmuir model [5,7]. R² reaches 0.82, indicating that the adsorption of MB on Fe₃O₄ conforms to the law of Langmuir isotherm adsorption equation. Calculated from the equation, the maximum adsorption capacity of the sample is 62.5 mg/g. The Langmuir isotherm equation can be expressed as:

$$\frac{1}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{q}}_{\max }}+\frac{1}{\begin{array}{ccc}\mathrm{b}& {\mathrm{q}}_{\max }& {\mathrm{c}}_{\mathrm{e}}\end{array}}$$

wherein, C_e is the equilibrium concentration of the solution of MB dye, q_e is the adsorption amount of MB, q_max is theoretical MB maximum amount of adsorption, b is the adsorption constant.

The adsorption capacity of the Fe₃O₄ nano particles used in this experiment is greater than that of other adsorbents reported in the literature, such as Modified Bagasse Charcoal (q_max = 3.52 mg·g⁻¹) [32], modified biochar (q_max = 28.21 mg·g⁻¹) [33] and steel slag-montmorillonite composite adsorbent (q_max = 12.45 mg·g⁻¹) [34]. Most importantly, the magnetic adsorbent in this study can easily and quickly separate MB from the solution under the action of an external magnetic field, due to the high saturation magnetization of Fe₃O₄ nanoparticles, which simplifies the adsorption process and improves the adsorption efficiency. It is worth noting that, compared with Fe₃O₄ and based composite adsorbents prepared by other methods in previously reported works as summarized in

Table 2. Adsorption capacity of Fe₃O₄ adsorbents prepared by different methods.

Adsorbents	Synthesis Methods	Size (nm)	Ms (emu·g−1)	Adsorption Equilibrium Time	Adsorption Capacity (mg·g−1)	Ref.
Fe3O4	one-step reduction	~10	90.32	20 min	62.5	This work
	hydrothermal	13~23	67.77	20 min	40.1	[35]
	co-precipitation	6~8	74.3	-	34.9	[36]
	polyol	18~35	78.2	-	4.98
	co-precipitation	15~30	65.33	40 min	53.1	[37]
GNS/Fe3O4 composite	-	200	42.9	30 min	43.82	[38]
ZrO2/Fe3O4	one-step co-precipitation	1.18~4.56	12.7	72 h	29.5	[39]

, Fe₃O₄ prepared by one-step carbothermal reduction developed in this work has exhibited good magnetic properties, magnetic response, recyclability and better performance in wastewater treatment. In addition, the materials after the wastewater treatment experiment can be recycled and reused just by facile ultrasonic cleaning to be discussed as following, which greatly reduces the total processing cost.

3.4. Recycling of Fe₃O₄ Nanoparticles

The recyclability of Fe₃O₄ as an adsorbent is a common issue that deserves investigation in the pursuit of practical applications. In the recycling experiments, the Fe₃O₄ which adsorbed MB is separated from the solution with an external magnet, and then is re-dispersed into acidic methanol solution. After proper ultrasonic cleaning, Fe₃O₄ will be re-collected and dried for reuse to adsorb MB again. This process is repeated four times. As shown in Figure 6a, Fe₃O₄ magnetic powder after repeated use still retains its magnetic properties in aqueous solution. After detecting the absorbance of the supernatant after adsorption, it is concluded that Fe₃O₄ can adsorb MB many times, and the adsorption capacity is basically unchanged. It shows that the structure and magnetic properties of Fe₃O₄ are stable and that it could be reused.

In order to further study the adsorption mechanism of Fe₃O₄ NPs, XRD detection was performed on the Fe₃O₄ nanoparticles before and after adsorption. As shown in Figure 6b, seven identical characteristic peaks of Fe₃O₄ appeared before and after adsorption, and the interlayer spacing corresponding to the characteristic peaks of Fe₃O₄ before and after adsorption was almost unchanged, which indicates the adsorbate molecules did not enter or change the interior structures of the magnetic powder. Therefore, the adsorption of MB by magnetic Fe₃O₄ is the physical adsorption onto the outer surface by direct contact. Such physical adsorption is featured with low adsorption resistance between the adsorbate and the adsorbent, enabling Fe₃O₄ NPs to quickly adsorb MB molecules on its surface, when MB is in contact with Fe₃O₄ NPs. In addition, the physical adsorption benefits the separation between the adsorbate and the adsorbent during the reuse step. Therefore, such Fe₃O₄ NPs are endowed with both wastewater treatability and recyclability.

4. Conclusions

In summary, the purely-stoichiometric, monodisperse and small-sized (~10 nm) Fe₃O₄ nanoparticles were successfully synthesized by a one-step carbothermal method, in which the crystallinity of Fe₃O₄ nanoparticles was over 82%. Therefore, it has a high sensitivity magnetic response, a fast separation within 5 s under the action of an external magnetic field, and a high saturation magnetization of 90.32 emu/g. The performance of such Fe₃O₄ nanoparticles in the removal of methylene blue from aqueous solutions was studied in detail. The results show that they have a significant quick decolorization effect on methylene blue with a high adsorption capacity of 62.5 mg·g⁻¹. The adsorption process conforms to the Langmuir adsorption isotherm which indicates that a single molecule adsorption is the most likely to occur. Importantly, Fe₃O₄ nanoparticles’ quick dye adsorption capability is accompanied with their recyclability by separating the adsorbed molecules from them by just using ultrasonic cleaning. Considering the simplicity of the whole manufacturing and wide application of magnetic nanomaterials as adsorbents, this method is promising to effectively solve the traditional problem of poor magnetic response of Fe₃O₄ nanomaterials used in wastewater treatment.