3.1 Characterization of magnetic nanoparticles
3.1.1. TEM analysis
Figure 2(a) is the TEM image of Cu-Fe3O4 magnetic nanoparticles. It can be seen from the TEM that the grains of Cu-Fe3O4 magnetic nanoparticles are mostly spherical or spheric-like, and the grain size is relatively uniform(Meng et al. 2005, Vijayakumar et al. 2000). Spherical or spheric-like nanoparticles have a large specific surface area, which is conducive to improving the surface activity of the Cu-Fe3O4 nanocatalyst. It can be seen from the EDS test results: copper was successfully loaded on the magnetic nanoparticles. The loading capacity was slightly higher than the theoretical loading capacity, which may be due to the residual Cu2+ ions in the solution on the surface of nanoparticles.
3.1.2 XRD test
To explore the loading components of the loaded nanoparticles, XRD characterization was carried out. According to the XRD peaks of the samples synthesized by the anti-coprecipitate method, as shown in Fig. 2(b), corresponding to PDF card 19-0629, the synthesized samples were confirmed to be magnetic nanoparticles Fe3O4. The half-height and width of the strongest peak of the sample were calculated by the Debye-Scherrer formula (1), so the grain size of Fe3O4 prepared by the reverse coprecipitation method was 13.5nm.
D = 0.89λ / β cosθ (2)
In formula (2), λ is the X-ray wavelength and D is the grain diameter.
The diffraction peaks of loaded and unloaded nanoparticles at 2θ = 30.1°, 35.5°, 43.1°, 57.0° and 62.7° correspond to planes 220, 311, 222, 422 and 511 of the face-centered cubic structure Fe3O4(Nikazar et al. 2014), which are the characteristic peaks of Fe3O4. According to the XRD patterns of the loaded components of magnetic Fe3O4, the loaded catalysts were not damaged by the loaded transition metal. However, Cu has no obvious corresponding diffraction peak of metal oxide, which indicates that Cu is difficult to be detected in uniform and less dispersed microcrystals.
The intensity of the main diffraction peak of magnetic Fe3O4 decreases with the introduction of Cu, which may be because the metal compound has a higher absorption coefficient of X-ray so that the X-ray absorbed by Cu and the intensity of the corresponding main diffraction peak decreases. At the same time, the diffraction peak of magnetic Fe3O4 becomes wider after loading copper, indicating that the grain size becomes larger after loading copper, and vice versa. This result can be interpreted that the dispersion of magnetic Fe3O4 is improved after loading copper.
3.1.3 BET analysis and potential test
After the composition was determined, the pore size, particle size and potential of Cu-Fe3O4 nanoparticles before and after loading were tested. The specific surface area and pore volume of magnetic nanoFe3O4 loaded with Cu were tested by BET, and the Zata potential on its surface was measured to analyze the dispersion among nanoparticles. The results are shown in Table 1.
Pore size, particle size and zeta of Fe3O4 loaded with copper before and after loading
Average pore diameter
The results show that the preparation of nanoparticles is negatively charged surface, the surface of the nanoparticles after load average potential absolute value is higher than before the load, and the highest potential absolute value of a load of copper, for 28 mv, the potential absolute value is close to 30 mv, Zeta potential increased to reduce the phenomenon of reunion, increased the system stability, better dispersion(Crundwell 2016), It is consistent with XRD results. However, the Zeta potential of the studied system is also affected by other factors, such as the change of the pH value of the dispersion system, the change of the conductivity of the solution, and the concentration of surfactant, among which the most important factor is the pH value(Hong et al. 2019). In the experimental process, the Zeta potential of the actual reaction system was not involved too much due to the test conditions.
Compared with N2-BET and particle size analysis, the total pore volume of the magnetic nanoparticles loaded with copper is 0.3165 cc/g, and the surface area is 167.683 m2/g, indicating that the magnetic nanoparticles loaded with Copper have a larger specific surface area, while the average pore size decreases, which is more favorable for the adsorption reaction of gas on the particle surface.
3.2 Cu-Fe3O4 magnetic particles removal of HCN
3.2.1 Cu-Fe3O4 catalyzed degradation of HCN in pseudo-homogeneous liquid phase
Cu-Fe3O4 liquid phase catalytic degradation of HCN without applying voltage: A certain amount of Cu-Fe3O4 magnetic nanoparticles were measured and dispersed in the reactor. HCN gas with a certain O2 was injected at the bottom of the reactor and the purified gas was discharged from the top of the reactor. The gas products were collected during the reaction. After the reaction, magnetic solid-liquid separation was performed to characterize the gas, liquid and solid samples. Reaction conditions: initial pH 6.00±0.01 and T=25°C, initial concentration of HCN were 200ppm, the gas flow was 300ml/min, a small amount of catalyst Cu-Fe3O4.
HCN degradation results are shown in Fig. 3(a). The purification effect of Cu-loaded Fe3O4 magnetic nanoparticles is significant. Within 100min, HCN decreases rapidly and then gradually decreases. The purification efficiency is stable above 70%, which is better than that of the control group.
The gas after the reaction was analyzed by gas chromatography for decomposition products, and the results were shown in Fig. 3(b): There is a small spike near the O2+N2 peak, which is calibrated as CO2. Since a mixture of N2-HCN and a certain concentration of O2 is passed into the reaction system, the reaction system is closed, there is no source of CO2, and the only C-containing substance is HCN, so it can be judged that CO2 is the product of HCN oxidation. The proportion of O2+N2 in the mixed gas is very large, and its chromatographic peak and CO2 peak cross, so the CO2 quantitative calibration result is too small.
The inlet gases were N2, HCN and O2 before the reaction, and there was no source of CO2 in the inlet. However, more NH3 and CO2 were detected in the products after the reaction, and the proportion of N2 increased. It can be seen that the products of HCN purified by loaded Cu nanoparticles include CO2, N2 and NH3.
The reaction takes place in solution, and part of HCN is directly oxidized to CO2 and N2 under the action of Cu-Fe3O4 magnetic nanoparticles(Liu et al. 2021, Yin et al. 2021). The reaction formula is as follows:
The Gibbs free energy of the related reaction equation is calculated, and the order of Gibbs free energy is: (3)>(4)>(5). It can be seen that reaction (3) and (4) are the main reactions and contribute the most to the products, while the contribution of reaction formula (5) is small, so the content of CO in the reaction products is low and cannot be detected.
To further explore the changes of Cu-Fe3O4 magnetic nanoparticles during the reaction, the Raman test was carried out on the magnetic nanoparticles after the reaction. According to The Raman test in Fig. 3(c), it can be seen that the characteristic peak value of Fe3O4 decreases at 278cm−1, 389cm−1 and 680cm−1, which is because the signal of the iron element is relatively reduced after the loading of copper. In the process of static purification of HCN, magnetic nanoparticles dissolve some metal elements into the liquid phase. In the liquid phase dissolved state participate in the reaction, but due to low sensitivity, so not detected. ICP inductively coupled plasma analysis showed that 2.183µg/mL copper and 6.477µg/mL iron were found in the supernatant after the static reaction of Cu-loaded magnetic nanoparticles with HCN. Specific reactions are as follows:
HCN(g) →HCN(l) →CN−+H+ (6)
Due to the low content of iron in the solution, Fe(CN)2, Fe(CN)3, [Cu(CN)4]3− are formed only in the form of macromolecular colloidal particles, which are not easy to flocculate and precipitate.
To explore the changes of elemental groups in the liquid, the magnetic particles before and after loading Cu were used to purify HCN liquid for FT-IR analysis. The FT-IR analysis results are shown in Fig. 3(d). At 2100~2200cm−1, an obvious CN− absorption peak appears, indicating that part of CN− appears in the liquid after the reaction of HCN with Cu-loaded magnetic particles. However, due to the low copper content in the solution, The characteristic peak of 918-923 cm−1 attributed to Cu2+ and the peak of copper oxide were not detected in the infrared spectrum(Siddiqui et al. 2016). The infrared spectrum at 3200-3800cm−1 is to characterize the stretching vibration of -OH, and the obvious wide peak at 3452cm−1 is mainly due to the vibration of -OH generated by water or hydroxide in the reactant.
3.2.3 Electrochemical catalytic degradation of HCN with Cu-Fe3O4 in pseudo-homogeneous liquid phase
Considering the high toxicity of HCN and to facilitate the detection and analysis of the products, deionized water and a certain amount of Cu-Fe3O4 magnetic nanoparticles were added to the reactor as the reaction medium. After victimization, HCN was passed through, and HCN gas was first absorbed in the liquid phase. Then, the influence of voltage and reaction time on the removal effect under different conditions such as electrolysis, electrolysis + aeration, electrolysis + micro-electrolysis, electrolysis + micro-electrolysis + aeration is discussed. The removal rate is converted according to the difference of component concentration before and after the reaction. Under the above reaction conditions, the reaction was conducted for 2h under different voltages (0.8, 1.0, 1.5, 1.9 and 2.5V), respectively. The removal effects of electrolysis, electrolysis + aeration, electrolysis + micro-electrolysis, electrolysis + micro-electrolysis + aeration and other conditions are shown in Fig. 4(a).
It can be seen from Fig. 4(a) that in the same reaction time, the increase of voltage contributes to the degradation of HCN. Under different voltages, the order of purification effect is as follows: electrolysis + micro-electrolysis + aeration > electrolysis + micro-electrolysis > electrolysis + aeration > electrolysis + micro-electrolysis + aeration. The degradation efficiency of HCN in the electrolysis + micro-electrolysis + aeration group is significant, but there is little difference between electrolysis and electrolysis aeration. When the voltage increases from 1.0V to 1.5V, the purification effect of electrolysis + micro-electrolysis increases significantly compared with electrolysis and electrolysis aeration. The removal rate at 1.5V is more than 85%, and the gap between the purification effect of electrolysis + micro-electrolysis + aeration decreases gradually. The purification effect of electrolysis + micro-electrolysis and electrolysis + micro-electrolysis + aeration (especially electrolysis + micro-electrolysis + aeration) increases gradually when the voltage is greater than 1.5V. Thus the addition of iron-carbon particles on the electrochemical degradation of HCN has an obvious effect, the reason may be that the addition of iron-carbon particles uniform discharge reaction system, forming a micro-electric field in the surface of the iron-carbon particles, each of the iron-carbon particles surfaces micro reaction area, at the same time, iron-carbon particles and Cu-Fe3O4 magnetic nanoparticles mixing in the system, contact closely, Reduce the distance between the reacting substances. Then, with the increase of voltage, the system without iron and carbon particles can also form a uniform and stable electric field under the electric field stimulation, which increases the degradation rate of HCN.
Because after the voltage of 1.5 V, the degradation rate was significantly increased in each group, increasing the voltage degradation efficiency after little change, at the same time, from the angle of the combination of reaction conditions, when other things being equal, pure electrolytic purification effect of HCN is limited, if you want to achieve a better removal effect electrolysis voltage may be increased, but this will no doubt increase the power consumption, both purification efficiency and power consumption of two ways, 1.5V was selected for subsequent experiments.
Figure 4(b) shows the removal effect of HCN at 1.5V voltage for different reaction periods. As can be seen from the figure, with the extension of reaction time, the purification effect of A, B, C and D is getting better and better, and the differences among the four gradually narrow. When the reaction time was 5h, the removal rate was more than 95%. When the reaction time is less than 4h, the purification effect decreases in the order of D, C, B and A at the same time. There is little difference between the purification effect of D and C, and the purification effect of B and A, but the effect of the former two is significantly better than the latter two, especially when the reaction 2h, the difference is the largest (C is more than 85%, while B is less than 60%). These results indicate that Cu-Fe3O4 magnetic nanoparticles have a significant degradation efficiency of HCN under the synergistic effect of iron and carbon particles micro-electrolysis. When the reaction time increased from 2h to 5h, the purification efficiency of D did not increase significantly. In the same reaction time, the removal effect changes in the order of electrolysis + micro-electrolysis + aeration > electrolysis + micro-electrolysis > electrolysis + aeration > electrolysis. Especially for 1h reaction, the removal rate of HCN by electrolysis + micro-electrolysis + aeration is close to 60%, which is much higher than 12% by pure electrolysis, indicating that certain purification effects can be achieved in a short time under combined conditions. To a certain extent, this indicates that in the actual treatment of HCN gas, due to the continuous flow of gas into the reaction system, a certain removal rate can be achieved under the condition of limited residence time.
The gas products of HCN were degraded by electrolysis + Microelectrolysis + aeration for chromatographic analysis, and the results were shown in Fig. 4(c). The carrier gas used is N2, and the gas composition mainly consists of H2, O2 and a small amount of CO2. The wide peak corresponding to the retention time of 14.767min is presumed to be a water leak, which has not been calibrated due to limited conditions. During the reaction process, H2 and O2 are precipitated in cathodic and anodic electrolysis respectively. The theoretical ratio of H2 and O2 is 2∶1, but the actual ratio is close to 1, which may be caused by the mixing of O2 in the mixture entering the system. The reaction system is closed, and there is no CO2 in the system. The only source of CO2 may be generated after the HCN reaction. The reaction takes water as the medium, and the CO2 produced spills out of the system with airflow. No peaks of other gas components were detected, so it can be inferred that HCN mixture gas is oxidized to CO2 and N2 through a series of complex reactions under the joint action of O2 in the mixed flow and reactive oxygen species generated by electrolysis after entering the liquid phase.
3.3 reaction mechanism
Electrochemistry works with Cu-Fe3O4 magnetic nanoparticles to purify HCN gas. Under the catalysis of Cu-Fe3O4 magnetic nanoparticles, the addition of electrochemistry greatly reduces the activation energy of the reaction and improves the reaction efficiency. Meanwhile, the addition of iron-carbon microelectrode forms numerous micro-discharge regions, and the electric field distribution in the reactor is more uniform. It can achieve a higher purification effect under smaller voltage and reduce energy consumption. A complete active region was formed between each pair of iron and carbon particles to reduce the reaction distance of HCN and enhance the transfer rate of HCN in the reactor. A single pair of microelectronic fields composed of iron-carbon particles was taken as a unit element to form a complex reaction chain in the whole reaction system. The pseudo-homogeneous reaction mechanism of the electrochemical coordination of Cu-Fe3O4 magnetic nanoparticles was analyzed by taking a single unit element of the microelectronic field as an example.
Its reaction mechanism is shown in Fig. 5:
Under the combined action of oxygen and electric field, the HCN degradation reaction is strengthened. In addition to direct oxidation (reaction 3-5), CNO intermediates(Li &Sarathy 2020, Öğütveren et al. 1999) are also generated during the HCN degradation process, which is further oxidized into CO2, N2 and H2O under the action of the catalyst. reactions are as follows:
In the anode zone:
In addition, CN− loses electrons at the anode and is oxidized to acid radical, which is further oxidized to CO2, H2O(Abdel-Rahman et al. 2019, Liu et al. 2021, Zhu et al. 2019), etc:
In the cathode zone, With the participation of oxygen, H2O2 will be generated in the cathode region as follows:
H2O2 produced in the system can also react with Cu+ and Fe2+ in the solution (under alkaline conditions) to generate hydroxyl radicals:
2Cu++H2O2 → 2Cu2++•OH+OH− (22)
Because ·OH and H2O2 both have strong oxidation capacity, part of HCN is removed by oxidation reaction under the action of ·OH. At the same time, after oxidation, the obtained Cu2+ and Fe3+ ions are reduced to Cu+ and Fe2+ after electron gain at the cathode, which makes the Fenton reaction continue and produce a large amount of ·OH.
In addition, after the reaction, the color of the solution changed from colorless to light brown. After the detection of Fe3+, the potential difference between iron and carbon in the iron-carbon particles formed countless tiny galvanic cells, in which the iron was corroded into divalent iron ions into the solution. The results indicate that the electrode directly and indirectly participates in the reaction under-voltage and micro-electrolysis:
Fe-2e−→Fe2+ E(Fe/ Fe2+)=0.44V (23)
It has been found that in the presence of copper cyanide complex, H2O2 can oxidize Cu+ in [Cu(CN)3]2− and [Cu(CN)2]− to Cu2+ and generate CNO− at the same time(Nguyen et al. 2013). The reaction process is as follows:
Similarly, the complex formed by Fe2+ and CN− also has a similar reaction: