Abstract
Vernadite has excellent oxidation and adsorption performance, suggesting that it has good application prospects for the removal of phenolic substances and heavy metals from wastewater. In this study, after vernadite was synthesized by two different methods, the removal performance difference between the samples synthesized by the new and traditional methods (Ver-H and Ver-OH, respectively) was explored by sample characterization, phenol degradation, and Pb2+ adsorption experiments. The results show that, compared with Ver-OH, Ver-H has a larger particle size, specific surface areas, and total organic carbon (TOC) degradation capacity; the equilibrium degradation capacity of TOC of Ver-H was increased by 31.3%. The difference in the amount of TOC degradation may be attributed to more Mn(IV) oxygen vacancies in Ver-H, which facilitate the removal of intermediate products formed during phenol degradation. In addition, the larger specific surface areas provide the mineral surface with a larger number of active sites; Ver-H can therefore adsorb more intermediate products to promote their mineralization into CO2. The adsorptions of Pb2+ by Ver-H and Ver-OH are both consistent with Langmuir isothermal adsorption, and the maximum adsorption capacities are 569.79 g/kg and 623.10 g/kg, respectively. The lack of significant difference indicates that both vernadites have strong adsorption capacities for Pb2+.
1. Introduction
Manganese oxides are widely distributed in the environment as important components of soil and sediments, with additional prevalence in oceans, rivers, and lakes. Owing to their high activity, catalytic efficiency, low toxicity, and abundance, they are widely used in the fields of adsorption and oxidative degradation, as well as in capacitors [1–6]. Vernadite is a layered manganese oxide commonly found in nature; it is formed by the close hexagonal accumulation of oxygen ions and water molecules [7]. It has a high specific surface area, structural defects, and a mixed valence state of Mn [8, 9] and plays a pivotal role in the biogeochemical cycle of metals and carbon. It is often used as a heavy metal adsorbent and oxidant for various organic and inorganic pollutants. For example, Zhang et al. [10] studied the adsorption and oxidation differences of vernadite with different Mn average oxidation states (MnAOS) on As and Cr, demonstrating that the MnAOS determines the oxidation capacity of vernadite on As to a certain extent. Higher MnAOS corresponds to greater As oxidation capacity; however, its influence on Cr is small. Dong et al. [11] studied the removal effect and mechanism of vernadite toward phenol under different experimental conditions. The results showed that vernadite has a strong phenol removal effect, and the removal mechanism is adsorption and oxidation, with pH being the most influential factor. When the pH was lower than 4, oxidation dominated; at pH exceeding 4, the removal of phenol was mainly due to vernadite adsorption.
Vernadite has a poorly crystalline lamellar structure that is unstable in the environment and easily aged into other Mn ores. Therefore, few reports exist regarding the application and synthesis of vernadites. At present, the main synthetic method for vernadite is the redox method [12–14]. Mn2+ is oxidized by MnO4− at room temperature under weakly alkaline conditions to eventually form vernadite.
In consideration of the complexity and high cost of this traditional synthesis method, a simpler and more economical method is adopted in this study: KMnO4 is used to oxidize H2O2 under acidic conditions to synthesize vernadite. After the physical properties of vernadite synthesized by these two methods were compared, the removal performance of vernadite for phenol, TOC, and Pb2+ was investigated.
2. Materials and Methods
2.1. Reagents and Water
All chemical reagents required for the experiment were of analytical grade, and ultrapure water was used in the experiment.
2.2. Vernadite Preparation
2.2.1. Preparation of Vernadite by the New Method
First, 10.0000 g KMnO4 was weighed in a 500 mL conical flask. Next, 300 mL ultrapure water and 1.75 mL of 98% concentrated H2SO4 were added with a magnetic stirrer for dissolution in acidic conditions; then, 100 mL of H2O2 (1.17 mol/L) was added dropwise to the KMnO4 solution at a speed of 5 mL/min. After standing for 30 min, the supernatant was poured out, and the collected solid was washed with deionized water until the conductivity of the filtrate was lower than 20 μs/cm. The solid was freeze-dried, ground, and sieved to 100 mesh for later use. The obtained sample was named Ver-H.
2.2.2. Preparation of Vernadite by Traditional Methods
According to the methods described by Catts and Langmuir [15] and Luo et al. [16], vernadite was synthesized by oxidizing Mn2+ with KMnO4 under alkaline conditions. (1) Firstly, three solutions of A, B, and C were configured: 75.14 g MnCl2·4H2O was weighed and dissolved in 1280 mL ultrapure water, which was set as A; 40.00 g KMnO4 was treated as the above methods named solution B; 28 g of NaOH was weighed and dissolved by addition of 1440 mL of ultrapure water as solution C. (2) Then, solution B was slowly added into solution C within 5 minutes with stirring. After that, solution A was added slowly to the above mixed solution in 35 minutes with stirring, which showed a black color. (3) The supernatant of the suspension was poured after standing for 4 hours, and the left suspension was centrifuged and the residue was collected. (4) The residue was washed with 1 mol/L NaCl solution for 5 times and shook overnight at the last time. (5) Finally, the solid was washed until the supernatant conductance was less than 20 μs/cm and the pH was about 6.2. The sample was freeze-dried, ground, and sieved to 100 mesh for later use. The sample is named Ver-OH.
2.3. Sample Characterization
The X-ray powder diffraction (XRD) patterns of the prepared samples were determined using a D8 ADVANCE polycrystalline X-ray diffractometer equipped with Cu Kα radiation at a voltage of 35 kV and current of 38 mA. The scan range was from 5° to 85°. The morphology of the samples was determined by field-emission scanning electron microscopy (SEM) using a Czech Fei Nova NanoSEM450 microscope. The working voltage was 10 kV. Fourier-transform infrared spectroscopy (FTIR) was performed using an INVENIO S spectrometer. The test range was 4000–400 cm−1. Thermogravimetric (TG) analysis was performed using a Germany Netzsch STA409PC synchronous thermal analyzer in the temperature range 30–930°C at a heating rate of 10°C/min. The specific surface areas (SSAs) of the samples were measured using an automatic specific surface and pore size distribution analyzer (Micromeritics ASAP 2020) and calculated using the Brunauer–Emmett–Teller (BET) theory. The MnAOS was determined using the oxalic acid method [5]. The chemical composition of the samples was determined according to Catts and Langmuir [15].
2.4. Phenol Degradation
Vernadite powder (0.0625 g) was placed in a 250 mL triangular flask, to which 100 mL of ultrapure water was added before the flask was placed in a magnetic mixer for suspension stirring. The pH of the mineral suspension was adjusted to 3.00 with 0.1 mol/L HCl and 0.1 mol/L NaOH until the pH value fluctuated by less than 0.05 within 24 h. After pH stabilization, the sample suspension was transferred to a 250 mL double-layer beaker and 25 mL of a 500 mg/L phenol solution was added. After the above treatment, the vernadite concentration was 0.5 g/L and the initial concentration of phenol was 100 mg/L. The phenol degradation reaction was performed at 25°C with agitation. According to the set time gradient, 15 mL of the reaction solution was sampled each time and filtered through a 0.22 μm microporous membrane to obtain the test solution, from which the concentrations of phenol and TOC were determined in the later period. Two parallel samples were used for each group, and the results were averaged.
The phenol concentration was determined by high-performance liquid chromatography (HPLC, Agilent 1260). The basic test parameters were as follows: fluorescence detector, excitation wavelength 270 nm, emission wavelength 300 nm; C18 column, column temperature 25°C; mobile phase volume (acetonitrile): pure water ; flow rate 0.3 mL/min; injection volume 5 μL. The TOC concentration was measured using a TOC analyzer (multi N/C 3100 TOC in Jena, Germany).
2.5. Pb2+ Isothermal Adsorption
In the adsorption experiment, 0.7500 g of vernadite and 1.75 g of NaNO3 were weighed in a 250 mL triangular flask. After adding 150 mL of ultrapure water, the mixture was suspended and stirred on a magnetic stirrer to prepare a sample suspension of 5 g/L. The suspension pH was adjusted to 4.50 with 0.1 mo1/L HNO3 and 0.1 mo1/L NaOH until the pH value did not fluctuate by more than 0.05 within 24 h. A series of Pb(NO3)2 solutions (0, 1, 2, 3, 4, 5, 6, 7, and 8 mL) were added into the centrifugal tubes, the volume was corrected to 10 mL with 0.15 mol/L NaNO3, and then, 5 mL vernadite suspension was added. The centrifuge tube was kept in an oscillator at at a speed of 250 rpm for 24 h. The pH of the system was adjusted to twice with 0.1 mol/L HNO3 and NaOH during the reaction process, and the amount of HNO3 and NaOH solution added was recorded. After reacting for 24 h, the supernatant was filtered through a 0.22 μm filter membrane. The equilibrium concentration of Pb2+ in the supernatant was analyzed using an atomic absorption spectrometer (Agilent AAS240FS), and the adsorption capacity of Pb2+ was calculated. The above experiment was repeated three times, and the results were averaged.
2.6. Data Analysis
2.6.1. Langmuir Isothermal Adsorption Equation
To understand the adsorption behavior of vernadite toward heavy metal ions, the Langmuir isothermal adsorption equation was used to fit the adsorption process of Pb2+ by vernadite. The equation is as follows: where is the adsorption capacity of vernadite per unit weight to Pb2+ (g/kg), is the maximum adsorption capacity of Pb2+ (g/kg), is a constant related to the adsorption binding energy, and is the concentration of Pb2+ in the equilibrium solution (mg/L).
2.6.2. Dynamic Equation
To understand the chemical behavior of vernadite in degrading phenol, a pseudo-second-order kinetic equation was used in this study to linearly fit the kinetic curve of vernadite toward phenol degradation in 12 h. The pseudo-second-order kinetic equation is as follows: where and are the removal amounts (mg/g) at equilibrium and at time , respectively, and is the rate constant (mg/(g·h)) of the pseudo-second-order equation. The rate constant and correlation coefficient were obtained by fitting.
3. Results and Discussions
3.1. XRD Analysis
The XRD patterns of the synthesized samples are shown in Figure 1. The two spectral lines are similar with diffraction peaks at 37° and 66°, representing the (11, 20) and (31, 02) crystal plane diffraction peaks of vernadite, respectively, which is consistent with the relevant literature [17, 18]. As shown in the figure, the diffraction peaks of the synthesized minerals are low in intensity with broad half-peak widths, indicating that the minerals synthesized by the new method are structurally consistent with those synthesized by the traditional method, both of which are pure and slightly crystallized vernadite.
3.2. SEM Analysis
SEM images of the vernadite synthesized by the two methods are shown in Figure 2. It can be seen clearly from the figure that the structures of the two minerals are well distributed, while the morphologies differ. The morphology of Ver-H is nanosheet-like, whereas Ver-OH is granular. The particle size of Ver-H is somewhat larger than that of Ver-OH. Both crystals show obvious aggregation, with greater agglomeration in the particles of Ver-OH. The above results show that the vernadite synthesized by the new method has increased particle size and morphological changes.
(a)
(b)
3.3. TG Analysis
No significant difference appears between the TG curves of Ver-H and Ver-OH (Figure 3). The total weight losses of Ver-H and Ver-OH are 24.19% and 23.69%, respectively, and both have three obvious weight loss peaks. The rapid and large weight loss of the two samples before 300°C mainly originates from surface-adsorbed water and water molecules located between the octahedral layers of vernadite [19], indicating that both samples have high hygroscopicity. The weight loss in the range of 470–640°C may arise from the partial conversion of MnO2 to Mn2O3 in vernadite [20, 21]. The final weight loss peaks of Ver-H and Ver-OH differ significantly: the weight loss temperature of Ver-OH (870–940°C) is significantly higher than that of Ver-H (680–740°C). The weight losses of the two minerals at this temperature may be caused by the transformation of Mn2O3 to Mn3O4 [22, 23]. The results of the TG analysis showed that both Ver-H and Ver-OH had good thermal stability.
3.4. FTIR Analysis
The samples synthesized by the two methods were characterized by FTIR, as shown in Figure 4. The results show that the absorption bands of Ver-OH and Ver-H are similar, located at 3378, 1630, 920, 738, 585, and 550 cm−1. The band at 3378 cm−1 is due to the stretching vibration of free hydroxyl groups on the mineral surface, and that at 1630 cm−1 is caused by the H–O–H bending vibrations of water molecules, indicating that the synthesized minerals contain crystalline water [24, 25]. The peak at 920 cm–1 can be attributed to the bending vibration of hydroxyl groups around the vacancy in the [MnO6] layer [26]. The strength of this peak is slightly lower for Ver-H than for Ver-OH, indicating that the number of hydroxyl groups around the vacancy in Ver-H is lower than that in Ver-OH. Generally, the number of oxygen vacancies decreases with a decrease in MnAOS, which in turn affects the number of surrounding hydroxyl groups. Therefore, the MnAOS of Ver-H may be lower than that of Ver-OH. The absorption bands located at 738, 585, and 550 cm−1 may correspond to Mn–O–Mn stretching vibrations in the mineral [27].
3.5. MnAOS, Chemical Composition, and SSAs
Data for the MnAOS, SSAs, and elemental contents of the synthetic vernadite samples are shown in Table 1. The chemical compositions of Ver-H and Ver-OH are similar, while the MnAOS of Ver-H is lower than that of Ver-OH; this is consistent with the relevant conclusions from the FTIR spectra. The resulting difference in MnAOS in Ver-OH and Ver-H may be related to the K content in the mineral [28]. The SSAs of Ver-H is increased compared to that of Ver-OH, possibly because of the agglomeration of smaller-sized particles in Ver-OH. The vernadite synthesized using the new method has a large SSAs.
3.6. Performance Comparisons of the Two Vernadites in Phenol Degradation
The corresponding phenol and TOC degradation rates, as well as the pseudo-secondary kinetic curves, were obtained from the kinetic experiments of phenol degradation by the two vernadites (Figure 5). As shown in Figures 5(a) and 5(e), the trends in the phenol degradation rates of Ver-H and Ver-OH are consistent. The degradation rate of phenol by both minerals reaches 45% after 3 minutes of the reaction, indicating that phenol is quickly removed after contact with vernadite. This may be due to the strong oxidation capacity and the large number of adsorption sites in vernadite, which can quickly oxidize and adsorb phenol. The phenol degradation rate is increased rapidly in the first 30 minutes of the reaction with increasing reaction time, and then, the increase rate decreases. This may be due to the occupation of almost all of the active sites on the surfaces of the two vernadites by phenol in the initial stages of the reactions. When Mn(IV) is reduced to Mn(III) and Mn(II), some of the low-valence Mn ions are adsorbed on the mineral surface. These ions compete with phenol and its intermediates, which inhibits further degradation of phenol. After 6 h of reaction, the degradation rate is constant, and the reaction reaches equilibrium. No significant difference in the phenol degradation rate is observed between Ver-H and Ver-OH during the reaction process, indicating that the vernadite synthesized by both methods has a strong degradation effect on phenol. The dynamic change pattern of TOC is similar to that of phenol, with rapid degradation in the early stage and a slow increase in the later stage.
The pseudo-second-order kinetic fitting parameters for the degradation of phenol and TOC by Ver-H and Ver-OH are shown in Table 2. The results show that the phenol degradation amount and reaction rate constants of Ver-H and Ver-OH were not significantly different. However, the TOC degradation amount of Ver-H was significantly larger than that of Ver-OH. The equilibrium degradation amounts of TOC are 58.2 mg/g and 40.0 mg/g for Ver-H and Ver-OH, respectively, indicating a 31.3% increase for Ver-H compared to that for Ver-OH. This indicates that both synthesized vernadites have strong phenol removal ability and that the vernadite synthesized by the new method has better TOC degradation ability. Generally, higher MnAOS values correspond to stronger mineral oxidation capacities [29]. However, the Ver-H MnAOS was smaller than that of Ver-OH, and there was no significant difference in the phenol degradation amount in this study. This may be because, while Ver-OH has more Mn than Ver-H and thus a higher MnAOS, the Mn in it is mostly lower-valence Mn(III). Relatively more Mn(IV) is present in Ver-H; therefore, no significant difference appeared in the phenol removal capacity between Ver-H and Ver-OH. In contrast, the higher TOC degradation amount in Ver-H may be due to its higher content of Mn(IV) oxygen vacancies [30], thus facilitating the removal of intermediate products formed during phenol degradation. In addition, Ver-H has larger SSAs, which increases the number of surface active sites that can absorb more intermediate products, thereby facilitating their mineralization into CO2. In summary, both Ver-H and Ver-OH show strong phenol degradation performances; however, Ver-H has a better degradation effect on TOC and stronger mineralization ability.
3.7. Lead Adsorption Performance
The isothermal adsorption curves of Pb2+ on the two vernadite samples are shown in Figure 6; both curves are L-shaped. For a low equilibrium concentration of Pb2+, the adsorption amount increases rapidly with increasing Pb2+ concentration and tends to stabilize as the concentration approaches a certain value. The correlation parameters obtained using the Langmuir equation are shown in Table 3, and the correlation coefficients are 0.9011 and 0.9900, respectively, indicating that the adsorption kinetics of Pb2+ by the two vernadites is consistent with the Langmuir isothermal adsorption model. The maximum adsorption capacities of Ver-H and Ver-OH are 569.79 g/kg and 623.10 g/kg, respectively, showing that the adsorption capacity of Ver-OH for Pb2+ is slightly larger than that of Ver-H. In general, the adsorption performance of Mn oxide minerals for heavy metal ions is positively correlated with the SSAs; however, related studies have shown that MnAOS also affects the heavy metal adsorption performance and that higher MnAOS values correspond to stronger adsorption capacities [31–33]. This indicates that the MnAOS of vernadites is decisive in determining the Pb2+ adsorption in this study; thus, the Pb2+ adsorption capacity of Ver-OH is slightly higher than that of Ver-H.
4. Conclusion
In this study, phase-pure vernadite was successfully synthesized by oxidizing H2O2 with KMnO4 under dilute acid conditions. Compared to the traditional method, the new method consumes a small amount of concentrated sulfuric acid instead of a large amount of sodium hydroxide; this method is simpler, economical, and environmentally friendly. Moreover, the vernadite synthesized using the new method has a larger particle size and SSAs. The vernadite types synthesized by the two methods both have strong phenol degradation abilities with the degradation amounts of 71.6 mg/g and 70.2 mg/g, respectively. The kinetic fitting results show that the TOC degradation amount of Ver-H (58.2 mg/g) is significantly larger than that of Ver-OH (40.0 mg/g), an increase of 31.3%. Therefore, the vernadite synthesized by the new method has a stronger ability to mineralize organic matter. The adsorptions of Pb2+ by Ver-H and Ver-OH are both consistent with Langmuir isothermal adsorption, and the maximum adsorption capacities are 569.79 g/kg and 623.10 g/kg, respectively. The lack of significant difference indicates that both vernadites have strong adsorption capacities for Pb2+.
Data Availability
The [DATA TYPE] data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Acknowledgments
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: this work is financially supported by the National Natural Science Foundation of China (41867004) and Jiangxi Province Graduate Student Innovation Special Fund Project (YC2020-S256), which are gratefully acknowledged