One-stage method for removing dyes under the action of underwater plasma and ferrites of cobalt, nickel, and titanium

Pulsed underwater direct current discharge is considered as a tool for a one-step process for ferrite synthesis and organic dye removal. The formation of cobalt, nickel and titanium ferrites during the discharge �ring process was con�rmed by methods of light scattering dynamics and X-ray phase analysis. The transformation of dye molecules (�uorescein, methylene green) during the combined action of plasma and ferrites was detected by UV absorption spectroscopy. The contributions of the separate action of plasma and ferrites to the process of dye removal from the solution were investigated. It was found that the synthesized structures have a high sorption capacity. It was found that �uorescein can be used as an indicator for the presence of nickel ferrites.


Introduction
Various combinations of gas discharge with aqueous solutions are being studied more intensively.At the same time, there are more and more publications dealing with applied research on such systems [1][2][3][4].Among the various areas that continue to be of importance are plasma medicine, the removal of organic and inorganic contaminants, the effects on the seeds of various crops in agriculture and the synthesis of nanostructures.A trend towards more complex structures has emerged in the synthesis of nanostructures.The rst studies in this direction showed the possibility of obtaining nanostructures of noble metals (Ag, Au, Pd, Pt) from the corresponding salts or acids [5][6][7][8].It was also shown that it is possible to obtain bimetallic structures when plasma is used in contact with aqueous solutions [9].It was found that the synthesized nanoparticles have biocidal properties [10].The next step was to obtain oxide structures of D elements.Aqueous solutions of metal salts and electrode materials in buffer solutions (to generate conductivity) were used as starting materials [11][12][13][14][15][16][17][18][19].The results obtained showed that the method is quite promising, but to obtain pure structures it is necessary to wash or calcine the synthesis products.Purer structures can be obtained if the plasma is ignited in the presence of water (above the surface or inside the volume).In this case, the source of the metal particles is the electrode material [20,21].
Recent publications show that it is possible to obtain more complex structures (layered double hydroxides and spinel-like structures) by solution plasma.In the work of [22], the authors obtained the structures of nickel ferrites under the conditions of contact glow discharge in alkaline solutions in the presence of ethanol.Sintered iron and nickel powder was used as the anode.The results of the analysis showed that during the discharge the anode material is sprayed and a powder containing nickel ferrite, nickel and iron oxides is formed in the solution.Smirnova et al. showed the possibility of obtaining cobalt ferrite from nitrate solutions by a direct current glow discharge [23].The production method proposed by the authors is a two-stage process.In the rst stage, the ultra ne particles are obtained from oxynitrites and iron and cobalt oxides in the solution.In the second stage of thermal annealing, cubic spinel-like structures are formed.In [24], a one-step process for the production of nickel ferrite using a pulsed underwater direct current plasma was proposed.Iron and nickel electrodes were used as metal sources and the plasma was excited in distilled water.The resulting nickel ferrite structures had an impurity in the form of iron oxide (ε-Fe 2 O 3 ), which improved the magnetic properties.
In addition to the magnetic and catalytic properties, various ferrites are considered to be effective magnetic sorbents for the removal of organic and inorganic (heavy metal ions) contaminants [25][26][27].In [25], rapid adsorption of uorescein on cobalt ferrite was observed (95% in 4 min).The authors explained this effect with heterogeneous physical absorption.Composites based on chitosan/Fe 2 O 3 /NiFe 2 O 4 showed effective sorption of the dye methylene green (~ 80% in 30 min).NiFe 2 O 4 structures are also considered as sorbents for lead ions [26] or pharmaceuticals [27].
To increase the catalytic and sorption activity, nickel and cobalt ferrites are often doped or composites are prepared on their basis [28][29][30][31].It has been found that the introduction of ferrites into the structure of titanium dioxide or the production of composites based on TiO 2 /CoFe 2 O 4 (or TiO 2 /NiFe 2 O 4 ) leads to improved catalytic properties or imparts magnetic properties.Regarding titanium ferrite, its magnetic and biocidal properties are mainly investigated [32][33][34].The authors of [35] have successfully used a composite material based on TiFe 2 O 4 as a catalyst for the production of biofuel.
In this work, we present an environmentally friendly method for the synthesis of cobalt, nickel and titanium ferrites by pulsed underwater plasma without chemical reagents.Previously, the synergistic effect of the action of the plasma, the chemical particles and the sorption properties of the structures synthesized during the plasma action was shown [36].We present the results of studies on the combined effect of plasma and the structures formed during its combustion on dye solutions.The dyes uorescein (a class of xanthene dyes) and methylene green (a thiazine dye) were chosen as objects of research as model compounds of the dye classes most frequently used in the textile industry.The novelty of the work is a new reagent-free synthesis of ferrites (especially titanium ferrite) under plasma conditions in contact with liquid and the demonstration of the possibility of a combined effect of plasma and synthesized particles on dye solutions for the purpose of their removal.

Plasma setup
The DC power supply BP-0.5-2 (LLC "TD ARS THERM", Russia) with an output voltage of up to 10 kV and a ballast resistor of 0.5 kΩ ignitions the discharge.The voltage and current waveforms were recorded using the ADS-2072 dual-channel digital oscilloscope (LLC "Aktakom", Russia).An industrial voltage divider with a division ratio of 1:1000 was used to measure the voltage drop.The experiments were performed in a cell with a xed solution volume of 200 mL and a at optical quartz window.The optical emission spectra during the burning discharge were registered through the quartz window with the ber optic spectrometer AvaSpec ULS3648 (Avantes BV, Netherlands) in the range of 200-950 nm.Iron, nickel, cobalt and titanium wires (Tangda Co., China, purity 99.99%) with a diameter of 1.0 mm were used as electrodes for the experiments.The electrodes were used without any pretreatment.The electrodes were housed in a refractory ceramic mullite tube, which allowed a constant distance of 3 mm between the electrodes to be achieved.The average discharge current was 0.25 A for all tests.Three series of tests were carried out with three different pairs of electrodes: Fe-Co, Fe-Ni, Fe-Ti.In each series of experiments, in the rst case the anode was iron and the cathode was another metal; in the second case wires of cobalt, nickel or titanium were used as the anode and the cathode was iron.The electrodes were weighed both before and after ignition of the discharge in order to calculate the speed (w av ) at which the nanoparticles were formed.The AND HR-150AZ analytical balance (AND, Tokyo, Japan), which has a 5% margin of error, was used for these measurements.
Underwater plasma was ignited in solutions of methylene green (MG) (Sigma-Aldrich) and uorescein (FS) (Sigma Aldrich) dyes with an initial concentration of 100 mg/L.The degree of dye removal was assessed spectrophotometrically by changes in the A values at the dye absorption maximum.
Absorption spectra were recorded using a spectrophotometer (SF 56, Russia) in the wavelength range of 300-800 nm.
The samples were designated according to the materials of the anode-cathode electrodes.

Characterization
Powder X-ray diffraction (XRD) was performed on D2 PHASER diffractometer (Brucker, Germany) in the range of 2θ = 5-70° equipped with a Cu-Kα radiation source (λ = 1.54 Å, scan step size: 0.02°, generator voltage: 30 kV, tube current: 10 mA).The surface morphology of samples was investigated using scanning electron microscopy (SEM) (TESCAN VEGA 3 SBH, Czech Republic).The textural characteristics of the powders were calculated from nitrogen adsorption-desorption isotherms at 77 K (NOVAtouch NT LX, Quantachrome apparatus, USA).Before measurements, the powders were degassed in vacuum for 2 hours at 150ºC.Fourier Transform Infrared (FTIR) spectra of samples were registered by VERTEX 80v spectrometer (Brucker Optics, Germany) in the range of 400-4000 cm-1 with resolution of 0.2 cm-1.The average size of particles and zeta potential was determined by Dynamic light scattering (DLS) in Zetasizer Nano (Malvern Instruments Ltd., UK) in the aqueous dispersion immediately after plasma treatment.

Plasma characteristics
A study of the electrical properties of the underwater plasma showed that its parameters (average current, voltage, duration of the discharge pulse, frequency) do not depend on the material of the electrode pairs and the polarity of the electrode material (Fig. 1).The value of the average pulse frequency was 5-9 Hz and the pulse duration was 1.2-1.5 ms.The average value of the energy of a single discharge was 9-11 J, and the average value of the input power was 62-72 W.
During the discharge process, electrodes are sputtered.Table 1 shows the changes in the masses of the electrodes after the experiment.It was noted that the cathode sputters less.The reason for this may be near-electrode processes, described in detail in the work [37].The process of electrode sputtering during plasma treatment is also con rmed by the emission spectra data (Fig. 2).The analysis of the obtained results shows that the change in polarity of the electrode materials affects only the intensity of the bands of the metal atoms.The differences lie in a different set of lines associated with iron.When using Fe and Ni electrodes, more Fe lines are registered in the emission spectrum in the 330-350 nm range.And there are no bands associated with FeO (transition to ground state 5Δi) (Fig. 2b).It has already been established that the appearance of the FeO band is due to the occurrence of chemiluminescence processes [38].The probable reason for the absence of this band could be processes involving Ni atoms that suppress the reaction between iron and oxygen atoms.This assumption is supported by a different set of Fe emission lines and a higher intensity of the O emission lines.

Characterization of obtained structures
The phase composition of the resulting structures was investigated using X-ray phase analysis (Fig. 3).
The result of the plasma chemical synthesis is a mixture of iron oxide and spinel-like structures with cubic structure and characteristic peaks at 36.15º, 51.63º and 64.12º, corresponding to the planes (311), ( 422) and (440), respectively.The X-ray diffraction patterns contain re ections related to ε-Fe 2 O 3 [39].
The presence of such an impurity improves the magnetic properties of the resulting materials [24,40].
The structural features of the synthesized ferrites are listed in Table S1 in the supplementary information (SI) le.
The morphology of the resulting powders was investigated by scanning electron microscopy.The data are shown in Fig. 4. The analysis of the images showed that the anode material (Fe:Me ratio) determines the surface morphology.At a higher iron content (Fe anode), the particles can take the form of spherical plates (Co cathode), acicular structures (ε-Fe 2 O 3 ) with spheres on the surface (NiFe 2 O 4 ) or at cubic plates (Ti cathode).At an Fe:Me = 1:1 content, the surface morphology changes.This con rms the statement that the preparation method has a great in uence on the morphology of the synthesized structures.The large sizes of the structures obtained are explained by the high magnetism of the ferrites formed.

Action of plasma and ferrites on the dye's solutions
In a series of experiments on the formation of nanostructures during the combustion of underwater discharge plasma in dye solutions (combined effect of plasma and nanostructures), the effect of the electrode material and its polarity on the kinetics of the changes in the A values at the absorption maximum was determined.In a series of experiments with FS, an increase in color intensity (increase in optical density at the absorption maximum) was observed independent of the electrode materials and their polarity (Fig. 5a).This is typical for the occurrence of a dimeric structure of dyes in solutions.In the case of the MG dye, the effect of the plasma and the resulting structures leads to a decrease in the dye content (Fig. 5b).The recorded effects can be the result of at least two factors: the action of active particles formed in the solution under the in uence of the plasma (products of water plasmolysis: atoms, radicals) and the heterogeneous interaction with the resulting structures.These factors can act both together and separately.
To test this hypothesis, a series of experiments were performed on the effects of plasma on dye solutions using inert graphite electrodes with low sputtering speed [41].The results show that the effect of the plasma leads to the destruction of the dyes (55% FS and 88% MG for 30 minutes) (Fig. 6).
In parallel, a series of experiments were carried out with separately synthesized structures.In the case of the MG dye, structures based on NiFe 2 O 4 and structures obtained under the conditions of Ti, Co anode and Fe cathode act as sorbents (Fig. 7a).The sorption process removes 95-98% of the dye in 60 minutes.Comparison with the data shown in Fig. 5 shows that the effect of the plasma accelerates the process.In the presence of structures with an iron anode and a Ti, Co cathode in solution, an increase in optical density is observed at the absorption maximum of this dye (Fig. 7b).This may indicate the process of dye molecules with the formation of dimeric structures [42].In this case, the structures synthesized in the plasma act as a bridge for the dimerization of dye molecules [43].According to the Xray phase analysis data, this bridge could be ε-Fe 2 O 3 .Thus, it can be argued that the effect of plasma prevents the agglomeration of methylene green dye molecules due to the interaction with chemically active particles.
Separate experiments with uorescein and synthesized ferrites showed that in the presence of CoFe2O4 there was a rapid decrease in the FS dye content (Fig. 8a).The sorption rate was found to be lower than reported in [25] (95% in 4 minutes).The process of dye sorption on TiFe2O4 structures is slower at short times, but a larger amount of dye is removed.In both cases, the synthesized ferrites act as sorbents, which is due to their surface properties (Table S2 in SI).The calculated sorption capacity values showed a high qe over FS and MG of the synthesized structures (Table S3 in SI).
In the case of NiFe 2 O 4 , an increase in absorption intensity was observed at the maximum (Fig. 7b).
Previously, a similar effect (increased absorption intensity of these dyes) was observed in the presence of colloidal silver and gold nanoparticles [44].The authors explained this effect by the interaction of dye molecules with metal molecules due to surface plasmons.In our case, large particles (120-2500 nm) are formed by underwater plasma (Table S4 in SI).NiFe 2 O 4 particles aggregate in solution due to their magnetic properties and have a positive charge in solution according to DLS analysis (Table S4 in SI).
Fluorescein is an anionic dye and the sorption process occurs due to electrostatic interaction.During this process, conditions are created under which the dye molecules can approach each other and form dimers on the surface of the nickel ferrite.This hypothesis is con rmed by IR spectroscopy data (Fig. S1 in SI).The effect of the formation of dimer structures on the surfaces of iron oxides (goethite, magnetite) was also recorded in [45].
The effect of underwater plasma on the methylene green dye is due to the interaction of chemically reactive particles with dye molecules.This leads to the breaking of nitrogen double bonds, which are responsible for the intensity of the color.In addition, the action of the active particles leads to the destruction of the sulfur bridge, which in turn leads to the destruction of the dye molecule (Scheme 1).The presence of the ferrite structures can act as a sorbent or as a means of transport for dye molecules into the plasma zone.
The effect of chemically active particles on solutions of xanthene dyes leads to the destruction of the oxygen bridge responsible for the coloration [46].No further destruction process takes place, as the resulting biproduct binds to the surface of the resulting ferrite/iron oxide particles via an -O-bond (Scheme 2) [45].Since in the process of plasma synthesis at the initial stage the ferrite particles are small, when several dye molecules bind to the surface, distance conditions are created for the dimerization process.It expressed as an increase in the intensity of uorescein absorption by the plasma action in the presence of ferrites.On the other hand, the results obtained both with and without the plasma, showed that uorescein is an indicator of the presence of nickel ferrite.

Conclusion
The work presents a new approach to the use of solution plasma for the one-step creation of magnetic structures and the removal of organic contaminants from liquid media using the example of methylene green and uorescein dyes.

Supplementary Files
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Table 1
Electrode's mass change during plasma action