Highly Efficient Reduction of Vanadium (V) with Histidine

: In this paper, histidine was applied to treat vanadium-containing wastewater. Several independent experimental parameters, including H 2 SO 4 concentration, dosage of histidine, reaction time and reaction temperature, were investigated and optimized through response surface methodology. The influence on the reduction process decreased in the following order: dosage of histidine > reaction temperature > reaction time. The reduction efficiency could be achieved at 95.77% under the following reaction conditions: H 2 SO 4 concentration of 0.2 mol/L, reaction temperature of 90 ◦ C, dosage of histidine at n(His)/n(V) = 3.6, reaction time of 60 min and stirring rate at 500 rpm. The reduction kinetics was followed successfully with the pseudo-first-order kinetics model and the Ea for reduction of vanadium was calculated to be 25.31 kJ/mol. The reduction kinetics was affected by these factors and the kinetics model could be described by an Equation. This paper provides a versatile strategy for treatment of wastewater containing V(V) and shows a bright tomorrow for wastewater treatment.


Introduction
Vanadium is a critical rare metal playing a pivotal role in various sectors [1][2][3][4][5].However, excessive exposure to vanadium is harmful to life and leads to issues like pulmonary tumors, kidney lesions, and nervous system damage at higher concentrations [6][7][8][9].A large amount of vanadium-containing wastewateris produced during the vanadium-producing process, some of which might be distributed to the soil or groundwater and threaten the environment [10][11][12][13][14]. Vanadium is an environmental hazardous element according to the United Nations Environment Programme, and the minimum concentration is limited to 0.2 µg/L [15], while the concentrations of vanadium in wastewater collected from Chengde, China, ranged from 0.05 to 0.2 g/L, and 0.076 to 0.208 mg/L in Panzhihua, China [16][17][18][19][20]. Hence, efforts should be made to prevent it.
In this paper, histidine is applied to treat vanadium-containing wastewater, during which histidine is actually used to reduce vanadium rather than just precipitate vanadium.The experimental parameters (concentration of H 2 SO 4 , dosage of histidine, reaction temperature and reaction time) of the reduction process are investigated.Response surface methodology (RSM) was used to optimize the reaction conditions and order the influences of the parameters on the reduction process.Reduction kinetics behavior is also analyzed with the pseudo-first-order kinetics model.

Materials
The main components, included Na 3 VO 4 and C 6 H 9 N 3 O 2 (collected from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), were used directly.

Experimental Procedure
For the reduction experiment, a series of experiments was conducted.Firstly, the vanadium solution was prepared by dissolving a certain amount of Na 3 VO 4 in deionized water.Then, 100 mL 0.05 M Na 3 VO 4 solution was added to a 250 mL beaker placed in a thermostatic water bath with a temperature precision of ±0.1 • C.After the reaction temperature was heated to a predetermined temperature, amounts of C 6 H 9 N 3 O 2 (n(His)/n(V) = 0.6-3.6)and H 2 SO 4 (0.1-0.4 mol/L) were added to the above vanadium solution and stirred well.A total of 2 mL of the sample were collected every 10 min and the concentration of vanadium (V) in the sample was measured and determined by ICP-OES [43][44][45].Each experiment was conducted three times to ensure the accuracy of the data.The reduction efficiency (η) was calculated via Equation (1): where C 0 and C t , are the concentrations of vanadium (V) at initial and time t, respectively, in mg/L.

Response Surface Optimization
The interactions are ignored in the above experiments, and optimal reaction conditions cannot be determined.Thus, RSM is applied to solve the problem [45][46][47][48].In this paper, response surface methodology (RSM)conducted by Design Expert 8.0 software was applied to optimize the experimental process and order the significance of the experimental parameters.The experimental parameters affecting the reduction process were selected as A (reaction temperature), B (reaction time) and C (n(His)/n(V)), and the reduction efficiency of vanadium was selected as the response.The independent variables and factor levels are detailed in Table 1.

Single Factor Experiments
The existing forms of vanadium in wastewater were closely related to the pH and concentration of vanadium in wastewater, and their existing forms greatly influenced the reduction process of vanadium.The effect of the H 2 SO 4 concentrationon the vanadium reduction process was studied experimentally.During the reduction process, the concentrations used were 0.1, 0.2, 0.3 and 0.4 mol/L.Figure 1 shows that the change in the concentration of H 2 SO 4 had a great influence.When the concentration of H 2 SO 4 was 0.1 mol/L, the reduction efficiency of vanadium increased along with the dosage of histidine, and the reduction efficiency increased from 40.81% to 74.63%.Although the reduction of vanadium could be enhanced by increasing H 2 SO 4 concentration, the reduction efficiency of vanadium decreased gradually with increasing H 2 SO 4 concentrations.When the dosage of histidine was set to n(His)/n(V) = 3.6 and the concentration of sulfuric acid was 0.2 mol/L, the greatest reduction efficiency reached 95.77%.When the concentration of H 2 SO 4 was low, the pH of the solution was high, and vanadium mainly existed in the anionic form of metavanadate or pyro-vanadate.At this time, the redox ability of vanadium was weak, and it was not easy to reduce to low valence.With increasing H 2 SO 4 concentration, the oxidation ability of vanadium gradually increased, and vanadium was easily reduced to a low valence by a reducing agent.With the continuous increase in the concentration of H 2 SO 4 , pentavalent vanadium easily hydrolyzed at high temperature, which was not conducive to the reduction of vanadium.Therefore, 0.2 mol/L was confirmed as the best concentration for further experiments.
The existing forms of vanadium in wastewater were closely related to the pH and concentration of vanadium in wastewater, and their existing forms greatly influenced the reduction process of vanadium.The effect of the H2SO4concentrationon the vanadium reduction process was studied experimentally.During the reduction process, the concentrations used were 0.1, 0.2, 0.3 and 0.4 mol/L.
Figure 1 shows that the change in the concentration of H2SO4 had a great influence.When the concentration of H2SO4 was 0.1 mol/L, the reduction efficiency of vanadium increased along with the dosage of histidine, and the reduction efficiency increased from 40.81% to 74.63%.Although the reduction of vanadium could be enhanced by increasing H2SO4 concentration, the reduction efficiency of vanadium decreased gradually with increasing H2SO4 concentrations.When the dosage of histidine was set to n(His)/n(V)= 3.6 and the concentration of sulfuric acid was 0.2 mol/L, the greatest reduction efficiency reached 95.77%.When the concentration of H2SO4 was low, the pH of the solution was high, and vanadium mainly existed in the anionic form of metavanadate or pyro-vanadate.At this time, the redox ability of vanadium was weak, and it was not easy to reduce to low valence.With increasing H2SO4 concentration, the oxidation ability of vanadium gradually increased, and vanadium was easily reduced to a low valence by a reducing agent.With the continuous increase in the concentration of H2SO4, pentavalent vanadium easily hydrolyzed at high temperature, which was not conducive to the reduction of vanadium.Therefore, 0.2 mol/L was confirmed as the best concentration for further experiments.From the results shown above, it could be seen that the dosage of histidine also had a significant impact on the reduction efficiency.The influence of the dosage of histidine on the reduction efficiency of vanadium was studied.During the reduction process, the dosage of histidine (n(His)/n(V)) was set to 0.6, 1.2, 1.8, 2.4, 3.0 and 3.6.
Figure 2 shows that, with increasing histidine dosage, the reduction efficiency also increased significantly.The reduction efficiency increased with histidine dosage in both low-temperature and high-temperature media.When the reaction temperature was 30 °C and the dosage of histidine was n(His)/n(V) = 0.6, the reduction efficiency increased from 11.22% to 20.73% as the reaction time increased from 10 min to 60 min, while the increase From the results shown above, it could be seen that the dosage of histidine also had a significant impact on the reduction efficiency.The influence of the dosage of histidine on the reduction efficiency of vanadium was studied.During the reduction process, the dosage of histidine (n(His)/n(V)) was set to 0.6, 1.2, 1.8, 2.4, 3.0 and 3.6.
Figure 2 shows that, with increasing histidine dosage, the reduction efficiency also increased significantly.The reduction efficiency increased with histidine dosage in both low-temperature and high-temperature media.When the reaction temperature was 30 • C and the dosage of histidine was n(His)/n(V) = 0.6, the reduction efficiency increased from 11.22% to 20.73% as the reaction time increased from 10 min to 60 min, while the increase was slow and insignificant.When the dosage of histidine increased to n(His)/n(V) = 3.6, the reduction efficiency increased from 34.15% to 54.59% as the reaction time increased from 10 min to 60 min, and the total reduction rate increased by 22.93 and 33.86%, respectively.The reduction efficiency increased more obviously at high temperatures.When the reaction temperature was 90 • C and the dosage of histidine was n(His)/n(V) = 0.6, the reduction efficiency increased from 34.47% to 53.49% as the reaction time increased from 10 min to 60 min.When the dosage of histidine was n(His)/n(V) = 3.6, the reduction efficiency increased from 52.32% to 95.77% as the reaction time increased from 10 min to 60 min, and the total reduction efficiency increased by 17.65 and 42.28%, respectively.With the increase in the dosage of histidine and the concentration of reactants, the number of reactive compounds in the system increased gradually, which promoted the reaction and strengthened the reduction of vanadium.Therefore, n(His)/n(V) = 3.6 was selected.
When the reaction temperature was 90 °C and the dosage of histidine was n(His)/n(V) = 0.6, the reduction efficiency increased from 34.47% to 53.49% as the reaction time increased from 10 min to 60 min.When the dosage of histidine was n(His)/n(V) = 3.6, the reduction efficiency increased from 52.32% to 95.77% as the reaction time increased from 10 min to 60 min, and the total reduction efficiency increased by 17.65 and 42.28%, respectively.With the increase in the dosage of histidine and the concentration of reactants, the number of reactive compounds in the system increased gradually, which promoted the reaction and strengthened the reduction of vanadium.Therefore, n(His)/n(V) = 3.6 was selected.As an important factor in the chemical reaction process, reaction temperature affected the viscosity and activity of reactants and had a very important influence on the chemical reaction process.The effect of the reaction temperature on the vanadium reduction process was studied.During the reduction process, the reaction temperatures were set to 30 °C, 45 °C, 60 °C, 75 °C and 90 °C.
Figure 2 indicated that the reduction efficiency of vanadium significantly improved with increasein reaction temperature, regardless of whether the amount of histidine was small or large.When the dosage of histidine was n(His)/n(V) = 0.6, the reduction efficiency increased from 20.73% to 53.49% as the reaction temperature increased from 30 °C to 90 °C, which was an increase of 32.76%.With increasing histidine dosage, the influence of the reaction temperature was further strengthened.When the dosage of histidine was n(His)/n(V) = 3.6, the reduction efficiency of vanadium increased from 54.59% to 95.73% as the reaction temperature increased from 30 °C to 90 °C, which was an improvement of 41.14%.With increasing temperature, the movement rate of vanadium ions and histidine in the reaction system was accelerated, the reactivity increased and the viscosity of the Figure 2 indicated that the reduction efficiency of vanadium significantly improved with increasein reaction temperature, regardless of whether the amount of histidine was small or large.When the dosage of histidine was n(His)/n(V) = 0.6, the reduction efficiency increased from 20.73% to 53.49% as the reaction temperature increased from 30 • C to 90 • C, which was an increase of 32.76%.With increasing histidine dosage, the influence of the reaction temperature was further strengthened.When the dosage of histidine was n(His)/n(V) = 3.6, the reduction efficiency of vanadium increased from 54.59% to 95.73% as the reaction temperature increased from 30 • C to 90 • C, which was an improvement of 41.14%.With increasing temperature, the movement rate of vanadium ions and histidine in the reaction system was accelerated, the reactivity increased and the viscosity of the solution decreased, hence increasing the reaction rate and the contact between vanadium ions and histidine, which contributed to accelerating the reduction process and improving the reduction efficiency.
Above all, it was concluded that increasing the dosage of histidine, raising the reaction temperature and prolonging the reaction time could enhance the reduction process and improve the reduction efficiency.When the reaction temperature was 90 • C, H 2 SO 4 concentration was 0.2 mol/L, the dosage of histidine was n(His)/n(V) = 3.6, the reaction time was 60 min, and the reduction efficiency of vanadium reached 95.77%.Otherwise, the reaction conditions investigated above are suitable for simulated vanadium-containing wastewater and, for real wastewater or industrial application, the reaction conditions should be optimized.

Response Surface Methodology
To better describe the simulated results, the natural log model (shown in Equation (S6)) was applied: According to the results shown in Equation ( 2), the coefficients for each factor were 0.35 (reaction temperature), 0.25 (reaction temperature) and 0.41 (dosage of histidine) and the positive coefficients corresponded to a positive effect.The detailed interaction of parameters was discussed in the Supporting Information, and the results indicated that RSM (shown in Figure 3) was an effective statistical tool for optimizing the reaction conditions.The influence on the reduction process decreased in the following order: dosage of histidine > reaction temperature > reaction time.
concentration was 0.2 mol/L, the dosage of histidine was n(His)/n(V) = 3.6, the reaction time was 60 min, and the reduction efficiency of vanadium reached 95.77%.Otherwise, the reaction conditions investigated above are suitable for simulated vanadium-containing wastewater and, for real wastewater or industrial application, the reaction conditions should be optimized.

Response Surface Methodology
To better describe the simulated results, the natural log model (shown in Equation (S6))was applied: According to the results shown in Equation ( 2), the coefficients for each factor were 0.35 (reaction temperature), 0.25 (reaction temperature) and 0.41 (dosage of histidine) and the positive coefficients corresponded to a positive effect.The detailed interaction of parameters was discussed in the Supporting Information, and the results indicated that RSM (shown in Figure 3) was an effective statistical tool for optimizing the reaction conditions.The influence on the reduction process decreased in the following order: dosage of histidine > reaction temperature > reaction time.The reduction process of vanadium using histidine through various variables could be investigated through these model equations.Different parameters, R 2 , p values, F values and adjusted R 2 values, were measured as standards that were helpful for determining the accuracy of every coefficient to evaluate the significance of the predicted model.The ANOVA results (shown in Table 2) confirmed that the Model F value of 118.63 implies that the model is significant.There is only a 0.01% chance that a large "Model F Value" could occur due to noise.Values of "Prob > F" less than 0.0500 indicated The reduction process of vanadium using histidine through various variables could be investigated through these model equations.Different parameters, R 2 , p values, F values and adjusted R 2 values, were measured as standards that were helpful for determining the accuracy of every coefficient to evaluate the significance of the predicted model.The ANOVA results (shown in Table 2) confirmed that the Model F value of 118.63 implies that the model is significant.There is only a 0.01% chance that a large "Model F Value" could occur due to noise.Values of "Prob > F" less than 0.0500 indicated that the model terms are significant.In this case, A, B, C, AC, and C 2 are significant model terms.The R 2 value reflected how much variability in the observed response values could be expressed by the experimental factors, as well as their interactions, by establishing a relationship between the predicted and experimental results.An R 2 close to one revealed good fitting of the experimental data to the predicted model equation.The regression model produced a higher R 2 of up to 0.9935, signifying excellent fit between the model and the experimental data.The Predicated-R 2 of up to 0.8958 was in reasonable agreement with the adjusted R 2 of 0.9851.Adequate precision was helpful for evaluating the signal-to-noise ratio.A ratio greater than 4 was desirable.Here, a higher adequate precision of 40.991 revealed an adequate signal.This regression model could be applied to navigate the design space.

Reduction Kinetics Analysis
The reduction behavior kinetics was analyzed by the pseudo-first-order model (shown in Equation ( 3)) [49][50][51][52] Firstly, the reducing data were fitted with Equation ( 4) and the related results were summarized in Table 3.The R 2 values were calculated as 0.9694, 0.9832, 0.9818, 0.9869 and 0.9673, respectively, which were all close to 1, indicating that the pseudo-first-order model was reasonable for simulating the reduction behavior kinetics.Meanwhile, the Ea of the reduction process was 25.31 kJ/mol based on the Arrhenius equation as in Equation ( 5) (shown in Figure 4a).This was far less than the reduction with oxalic acid [53], which indicated that the reduction of vanadium was easier.The reduction process was significantly affected by the experimental parameters according to the results analyzed above.To further understand the reduction kinetics behavior, the pseudo-first-order model, Arrhenius equation and the reaction factors (H 2 SO 4 concentration [H 2 SO 4 ], dosages of histidine [n(His)/n(V)] and reaction temperatures [T]) were combined in analysis (shown in Equation ( 6)).
After transformation, Equation ( 7) was obtained: ductant for treatment of simulated vanadium-containing wastewater, but there is a long way to go before its industrial application.This paper focused only on the reduction behavior of vanadium (V), and some other issues were ignored, including the treatment of products of the oxidation of histidine and reduction products of vanadium (V), high reaction temperature and high concentration of H2SO4 used, etc.In the future, the issues mentioned above will be our focus.All the experimental data were fitted and the results are shown in Table 4 and Figure 4b-d.The exponent orders in Equation (8) were calculated as 0.01152, 0.02151 and 0.09919, respectively.Therefore, the reduction kinetics equation could be described as follows:

Figure 1 .
Figure 1.Effect of the H2SO4 concentration on the reduction efficiency of vanadium.

Figure 1 .
Figure 1.Effect of the H 2 SO 4 concentration on the reduction efficiency of vanadium.

Figure 2 .
Figure 2. Effect of the dosage of histidine on the reduction efficiency of vanadium.

Figure 2 .
Figure 2. Effect of the dosage of histidine on the reduction efficiency of vanadium.As an important factor in the chemical reaction process, reaction temperature affected the viscosity and activity of reactants and had a very important influence on the chemical reaction process.The effect of the reaction temperature on the vanadium reduction process was studied.During the reduction process, the reaction temperatures were set to 30 • C, 45 • C, 60 • C, 75 • C and 90 • C.Figure2indicated that the reduction efficiency of vanadium significantly improved with increasein reaction temperature, regardless of whether the amount of histidine was small or large.When the dosage of histidine was n(His)/n(V) = 0.6, the reduction efficiency increased from 20.73% to 53.49% as the reaction temperature increased from 30 • C to 90 • C, which was an increase of 32.76%.With increasing histidine dosage, the influence of the reaction temperature was further strengthened.When the dosage of histidine was n(His)/n(V) = 3.6, the reduction efficiency of vanadium increased from 54.59% to 95.73% as the reaction temperature increased from 30 • C to 90 • C, which was an improvement of 41.14%.With increasing temperature, the movement rate of vanadium ions and histidine in the reaction system was accelerated, the reactivity increased and the viscosity of the solution decreased, hence increasing the reaction rate and the contact between vanadium ions and histidine, which contributed to accelerating the reduction process and improving the reduction efficiency.Above all, it was concluded that increasing the dosage of histidine, raising the reaction temperature and prolonging the reaction time could enhance the reduction process and improve the reduction efficiency.When the reaction temperature was 90 • C, H 2 SO 4 concentration was 0.2 mol/L, the dosage of histidine was n(His)/n(V) = 3.6, the reaction time was 60 min, and the reduction efficiency of vanadium reached 95.77%.Otherwise, the reaction conditions investigated above are suitable for simulated vanadium-containing wastewater and, for real wastewater or industrial application, the reaction conditions should be optimized.

Figure 3 .
Figure 3. Perturbation plot for the reduction efficiency of vanadium (A, reaction temperature; B, reaction time; C, n(His)/n(V)).

Figure 3 .
Figure 3. Perturbation plot for the reduction efficiency of vanadium (A, reaction temperature; B, reaction time; C, n(His)/n(V)).

Table 1 .
Independent variables and factor levels.

Table 2 .
Analysis of variance (ANOVA) for the response.

Table 3 .
Apparent rate constants K correlation coefficients.

Table 4 .
Apparent rate constants and correlation coefficients for the experimental parameters.