Porous carbon materials containing magnetite are used due to their high sorption characteristics [1] and high electrical capacity of the electric double layer [2]. The use of magnetic sorbents is important for the purification of natural and technological matrices from oil products and heavy metals [3], as well as for the microconcentration of impurities [4–6] in qualitative and quantitative analysis.

Methods for producing magnetic carbon sorbents, as a rule, are based on the deposition of magnetite on a carbon matrix [4–6] or its organic precursor [2]. Iron(II) compounds act as precursors. The presence of ferrous iron compounds in the system, which are easily oxidized in an aqueous medium, complicates the synthesis procedure: it is required to maintain pH and temperature in narrow ranges, and the reaction is carried out in an inert atmosphere [5, 6]. An alternative method consists in semicoking coals impregnated with iron salts [1] in an evacuated furnace. Small changes in conditions can bring about the formation of antiferromagnetic wustite, nonmagnetic hematite, or weakly magnetic maghemite instead of magnetite, which has a pronounced magnetization. The noted difficulties of depositing magnetite on a carbon matrix, which can be overcome in laboratory conditions, complicate the scaling of the process for industrial use; therefore, the development of new methods for obtaining magnetic carbon sorbents, devoid of these drawbacks, has both scientific and practical significance.

Magnetite is a decomposition product of some labile iron compounds. In [7], the thermal decomposition of iron(II) oxalate in the atmosphere of the reaction products was investigated. It is shown that the active decomposition of iron(II) oxalate starts at 360°C, the main products at 400°C are magnetite (80%) and cementite (20%). When the temperature rises to 500°C, the transition of cementite to α-iron occurs, and at temperatures above 550°C magnetite turns into wustite. It is noted that in the region of 400°C a significant portion of magnetite is in the superparamagnetic nanoscale state [7]. In the same work, it is shown that the use of the decomposition reaction of labile iron compounds for the production of magnetite is promising; however, the applicability of this approach for the deposition of magnetite on carbon matrices in order to yield magnetic carbon materials has not yet been studied.

This work is aimed at researching the possibility of yielding magnetic porous carbon materials based on the reaction of thermal decomposition of labile iron compounds deposited on a carbon matrix, using the example of iron(II) oxalate.

EXPERIMENTAL

The starting reagents used were FeSO4·7H2O (chemically pure, Kupavnareaktiv JSC) and K2C2O4·H2O (chemically pure, Cherkassk State Plant of Chemical Reagents). BAU-A activated carbon (GOST (State Standard) 6217–74, Vekton LLC) served as a model carbon matrix.

In the first series of experiments, the technique of thermolysis of iron(II) oxalate in the absence of a carbon matrix was worked out. Iron(II) oxalate was prepared by mixing 1 M solutions of iron sulfate and potassium oxalate with a volume ratio of 2 : 3. The yielded yellow precipitate was washed with distilled water by decantation, filtered through a “white ribbon” filter, and dried on a filter. The dried substance was transferred into a porcelain crucible and calcined in a muffle furnace at 400°C for 2 h.

In order to produce a magnetic carbon sorbent, 2 g of BAU-A activated carbon (air-dry state) was placed in an open-type reactor, 20 mL of 1 M FeSO4 solution was added, then 30 mL of 1 M K2C2O4 solution was added in a jet with stirring. The mixture was washed with distilled water by decantation, filtered through a white ribbon filter. The filter with the intermediate product was dried in an oven at a temperature of 120–130°С, the dried substance was calcined in a muffle furnace for 2 h at a temperature of 400°С. The calcination product, after cooling, was washed with distilled water and ethanol, ensuring the visual purity of the wash water.

X-ray fluorescence analysis (XRF) of the composition of the mineral component was carried out on a Spectroscan MAKS-G instrument according to the method providing for the determination of copper, iron, manganese, and zinc in the range of 0.10–18, 0.40–63, 0.03–3.5, and 0.010–4.5%, respectively.Footnote 1 Phase composition of the objects were examined on a DRON-7 X-ray powder diffractometer (λ = 1.5406 Å, accumulation time 1.5 s, scanning step 0.02°) according to the procedure [8] using the COD database (Rev 89244 2013.10.11).

To characterize the sorption properties of the product, we employed the methods of adsorption of saturated vapors of benzene and methylene blue dye from an aqueous solution. The concentration of methylene blue was registered by a photometric method using a KFK-3-ZOMZ photometer.Footnote 2

RESULTS AND DISCUSSION

Upon yielding a magnetic carbon sorbent with calcination under conditions of air feeding, a black magnetic powder with a reddish tint is formed, which indicates the formation of iron oxide in the form of maghemite. The calcination temperature (400°C) is insufficient to oxidize the carbon matrix with the formation of carbon monoxide for the subsequent reduction of iron, for example, to wustite. In the absence of air feeding, the product was completely black and magnetic (Fig. 1): decomposition of oxalate is not accompanied by partial oxidation of iron, and magnetite is formed.

Fig. 1.
figure 1

A test tube with the produced carbon material (a); carbon material that is attracted to a permanent magnet in air (b); view of the test tube after adding a solution of methylene blue to the material (c); material collected by a magnet on the wall of a test tube after adsorption of methylene blue (d).

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The main elements of the mineral part in the initial activated carbon BAU-A are copper, iron, manganese, and zinc (Fig. 2), the content of which is <0.1 (less than the detection limit by the method used), 0.09 ± 0.04, 0.08 ± 0.03, and 0.04 ± 0.02 %, respectively. After thermolysis of the deposited salt, a sharp increase in the iron content in the sample to 6.0 ± 1.3% is observed, which indicates the deposition of magnetite on the carbon matrix; the weight fraction of magnetite is 8.3%. The content of copper and manganese in the sample keeps at the same level (<0.1 and 0.09 ± 0.03%, respectively), while the proportion of zinc falls below the detection limit. The decrease in the amount of zinc is associated with its binding by oxalate ions, which were taken in excess. The stability constants of metal oxalate complexes increase in the series manganese(II) < iron(II) << zinc(II) <copper(II) [9]; therefore, zinc leaching with an excess of oxalate is quite probable.

Fig. 2.
figure 2

X-ray fluorescence spectrum of the original activated carbon (BAU-A) and the produced magnetic carbon material (BAU-A-M).

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The X-ray diffraction patterns of both samples show broad peaks in the angular range of 15–35° and 40–45°, which are characteristic of carbon turbostratic structures (Fig. 3). The main component of the mineral component is silicon dioxide. On the X-ray diffraction pattern of the yielded magnetic sorbent, reflexes of the phases of mixed iron oxide, magnetite (Fe3O4), are recorded. The small width of the magnetite reflexes indicates a rather large volume of particles.

Fig. 3.
figure 3

Powder diffraction patterns of the original activated carbon (BAU-A) and the produced magnetic carbon material (BAU-A-M).

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Sorption characteristics of the initial BAU-A are consistent with those reported in the literature (Table 1). After the deposition of magnetite, a slight decrease in the sorption characteristics of activated carbon is observed, which can be explained by a decrease in the specific pore volume, as well as a change in the granulometric composition of the sorbent during washing. Partial coating of the amorphous carbon matrix with iron oxide results in a decrease in the background signal level in the X-ray diffraction patterns.

Table 1. Sorption characteristics of the original activated carbon (BAU-A) and the produced magnetic carbon material (BAU-A-M)
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The magnetite precursor, iron(II) oxalate, was deposited on a carbon matrix in a mode close to mass crystallization, which promotes rapid deposition and the production of small crystals. In this case, filling the pores with magnetite is unlikely. Magnetite produced by thermal decomposition of iron(II) oxalate has low specific surface area, the specific adsorption of saturated benzene vapors is 20 mg g–1. From the weight fraction of magnetite determined by X-ray fluorescence analysis, its volume (per 1 g of the composite) can be estimated as 0.016 cm3 g–1. This value is much less than the characteristic pore volume of activated carbon BAU-A (0.33 cm3 g–1 [10]). A slight decrease in the specific adsorption values of benzene and methylene blue may evidence in favor of partial clogging of pores with magnetite particles deposited on their outer surface.

CONCLUSIONS

Thermal decomposition of labile iron salts deposited on porous carbon matrices can be used to produce magnetic sorbents. The proposed method of deposition, tested on iron oxalate and activated carbon BAU-A, is simple to implement, does not require the use of an inert atmosphere, and contributes to the preservation of the sorption characteristics of the initial porous carbon material.