The Inﬂuence of Pluronic F-127 Modiﬁcation on Nano Zero-Valent Iron (NZVI): Sedimentation and Reactivity with 2,4-Dichlorophenol in Water Using Response Surface Methodology

: Nano zero-valent iron (NZVI) is widely used for reducing chlorinated organic pollutants in water. However, the stability of the particles will a ﬀ ect the removal rate of the contaminant. In order to enhance the stability of nano zero-valent iron (NZVI), the particles were modiﬁed with F-127 as an environmentally friendly organic stabilizer. The study investigated the e ﬀ ect of the F-127 mass ratio on the colloidal stability of NZVI. Results show that the sedimentation behavior of F-NZVI varied at di ﬀ erent mass ratios. A biphasic model was used to describe the two time-dependent settling processes (rapid sedimentation followed by slower settling), and the settling rates were calculated. The surface morphology of the synthesized F-NZVI was observed with a scanning electron microscope (SEM), and the functional groups of the samples were analyzed with Fourier Transform Infrared Spectroscopy (FTIR). Results show that the F-127 was successfully coated on the surface of the NZVI, and that signiﬁcantly improved the stability of NZVI. Finally, in order to optimize the removal rate of 2,4-dichlorophenol (2,4-DCP) by F-NZVI, three variables were tested: the initial concentration 2,4-DCP, the pH, and the F-NZVI dosage. These were evaluated with a Box-Behnken Design (BBD) of response surface methodology (RSM). The experiments were designed by Design Expert software, and the regression model of ﬁtting quadratic model was established. The following optimum removal conditions were determined: pH = 5, 3.5 g · L − 1 F-NZVI for 22.5 mg · L − 1 of 2,4-DCP.


Introduction
Chlorophenols (CPs) are widely used in wood preservatives, pesticides, lubricants, dyestuffs, and the synthesis of other chemicals [1][2][3]. At present, CPs have raised great concern in the protection of human health, due to their high toxicity and long persistence in the environment [4,5]. 2,4-dichlorophenol (2,, as a long-lived pollutant, is one of the representative phenolic compounds and a key intermediate in the synthesis of chloride-based herbicides. It has been listed as a priority pollutant by the U.S. Environmental Protection Agency (EPA) (2003) [6,7]. Removal methods of 2,4-DCP from water have become important in the environment field.
Over the past years, nano zero-valent iron (NZVI) has been reported to be a promising nanomaterial for the treatment of chlorinated phenols in wastewater, drinking water, and groundwater because of its large specific surface area, excellent adsorption, low redox potentials, small particle size, and high reaction activity [8][9][10][11]. In fact, the NZVI is applied for the removal of various pollutants in water,

Results and Discussion
2.1. Structure and Performance of F-NZVI (F-127/Fe, m/m)

Morphology of NZVI and F-NZVI
The SEM images of F-NZVI are depicted in Figure 1. It could be observed that the F-NZVI particles were spherical and organized in a chain-like structure. The spherical form of F-NZVI suggests that numerous reactive sites could be provided with a chain-like structure, due to the magnetic interactions between the F-NZVI particles, which could be maintained in a stable state [30]. The diameters of F-NZVI (m/m, 2:1) range from 70 to 80 nm. The shape and diameter of the bare NZVI was irregular in previous research [31]. Compared with bare NZVI, F-NZVI shows a significantly increased dispersity. Obviously, F-127 as a modifier can increase the dispersity of NZVI.  Figure 2 refers to the FTIR spectra of NZVI, F-127, and F-NZVI, respectively. The presence of O-H at 3430 and 1633 cm −1 indicates that some hydroxyl groups existed on the surface of F-127, NZVI, and F-NZVI samples [32]. The FTIR bands in the F-127 spectra have peaks at 2899 cm −1 , which could be attributed to the stretching -CH from the alkyl groups, and a strong band ranging from 1100 to 1150 cm −1 refers to C-O [33]. The stretching bands at 1360 and 1472 cm −1 confirms asymmetric and symmetric stretching of CH3 groups, respectively [34]. The results correspond to the molecular formula of F-127. In the FTIR spectra of NZVI, the peak at 669 cm −1 relates to the partial oxidation loaded on the NZVI and confirms that NZVI was partly oxidized [35]. Changes between the adsorption peaks of F-NZVI and the FTIR spectra of NZVI were observed. For example, the peaks at 1132 and 1440 cm −1 were significantly increased and the Fe-O vibrations were significantly reduced. The results demonstrate that F-127 was combined firmly with the NZVI.  Figure 2 refers to the FTIR spectra of NZVI, F-127, and F-NZVI, respectively. The presence of O-H at 3430 and 1633 cm −1 indicates that some hydroxyl groups existed on the surface of F-127, NZVI, and F-NZVI samples [32]. The FTIR bands in the F-127 spectra have peaks at 2899 cm −1 , which could be attributed to the stretching -CH from the alkyl groups, and a strong band ranging from 1100 to 1150 cm −1 refers to C-O [33]. The stretching bands at 1360 and 1472 cm −1 confirms asymmetric and symmetric stretching of CH 3 groups, respectively [34]. The results correspond to the molecular formula of F-127. In the FTIR spectra of NZVI, the peak at 669 cm −1 relates to the partial oxidation loaded on the NZVI and confirms that NZVI was partly oxidized [35]. Changes between the adsorption peaks of F-NZVI and the FTIR spectra of NZVI were observed. For example, the peaks at 1132 and 1440 cm −1 were significantly increased and the Fe-O vibrations were significantly reduced. The results demonstrate that F-127 was combined firmly with the NZVI.  Figure 2 refers to the FTIR spectra of NZVI, F-127, and F-NZVI, respectively. The presence of O-H at 3430 and 1633 cm −1 indicates that some hydroxyl groups existed on the surface of F-127, NZVI, and F-NZVI samples [32]. The FTIR bands in the F-127 spectra have peaks at 2899 cm −1 , which could be attributed to the stretching -CH from the alkyl groups, and a strong band ranging from 1100 to 1150 cm −1 refers to C-O [33]. The stretching bands at 1360 and 1472 cm −1 confirms asymmetric and symmetric stretching of CH3 groups, respectively [34]. The results correspond to the molecular formula of F-127. In the FTIR spectra of NZVI, the peak at 669 cm −1 relates to the partial oxidation loaded on the NZVI and confirms that NZVI was partly oxidized [35]. Changes between the adsorption peaks of F-NZVI and the FTIR spectra of NZVI were observed. For example, the peaks at 1132 and 1440 cm −1 were significantly increased and the Fe-O vibrations were significantly reduced. The results demonstrate that F-127 was combined firmly with the NZVI.

Sedimentation test of F-NZVI Particles with Different Mass Ratios
The colloidal stabilities of F-NZVI suspensions with different mass ratios were investigated by measuring the sedimentation kinetics of F-NZVI dispersions ( Figure 3). As shown in Figure 3, the downward trend of the relative concentration of F-NZVI can be divided into a two-stage process: at the beginning, an obviously rapid settling of F-NZVI particles; then, a much slower settling. The biphasic model was used to describe the two-stage process and expressed as Equation (1) [36]: where C is the concentration of F-NZVI at time t and C 0 is the initial concentration of F-NZVI at t = 0; P rapid and P slow are the settling compartments; kr and ks are the settling rate constants for the compartments (s −1 ); and t is time (s).
Catalysts 2020, 10, x FOR PEER REVIEW 5 of 12   Table 3 lists a total of 17 experimental runs and the corresponding results. The experimental runs, with a different combination of three factors, aim to investigate the effects of these factors on the 2,4-DCP removal rate (V). According to the RSM analysis, a quadratic polynomial equation, which expressed the relationship between the removal rate of 2,4-DCP and the independent variables, was established to obtain the predicted values of response. The model, in accordance with actual factors, is shown as Equation (2). The fitting curves of F-NZVI with different mass ratios are presented in Figure 3. They indicate that the experimental data were well-fitted to the biphasic model, with the square of the correlation coefficient of the linear regression (R 2 ) > 0.9. The calculated results, according to the biphasic rate model, are shown in Table 1. The results indicate that the colloidal stability of NZVI can be significantly improved by F-127 at a suitable mass ratio. Significant and rapid sedimentation of the NZVI and F-NZVI (m/m, 1:1) are observed at the outset of the experiment. From 0 to 2000 s, the settling rate of F-NZVI (m/m, 1:1) is faster than NZVI. F-127 can self-assemble to micelle in water and form a core-shell structure, and this can affect the core-shell structure of NZVI. When the mass ratio of F-NZVI is 1:1 (m/m), the particle has a thin shell. Van der Waals forces inside the nuclear shell affect the repulsion force coming from the outside, making the particles less stable in water. The downturn of the settlement curve of F-NZVI (m/m, 2:1) is gentle. After 1 h, the relative concentration of F-NZVI (m/m, 2:1) is 0.39, and the concentration of F-NZVI (m/m, 2:1) is 0.39 g·L −1 . The colloidal stability of NZVI (m/m, 2:1) is good. The downturn of the settlement curve of F-NZVI (m/m, 4:1) is the same as F-NZVI (m/m, 2:1). The relative concentration of F-NZVI (m/m, 4:1) is stable at around 0.46, and the concentration of F-NZVI (m/m, 4:1) is 0.46 g·L −1 . The colloidal stability is best when the core-shell structure of F-NZVI has mass ratios of 2:1 and 4:1, lipophilic groups inside the core shell structure, and hydrophilic groups with negative charge outside the core shell structure.The repulsive forces between F-NZVI particles, caused by hydrophilic groups and the steric barrier effect, strengthens the stability of NZVI. Table 1. Settling rate constants (k) and settled fractions (F) in the rapid and slow compartments of particle sedimentation, calculated from the biphasic rate model (R 2 > 0.9). Zeta potential is a key index of the stability of NZVI and a higher absolute value indicates more stability [37]. Zeta potentials of F-NZVI with different mass ratios are listed in Table 2. The results indicate that F-127 can change the surface charge of NZVI from positive to negative, suggesting interactions between the positively charged NZVI and the negatively charged F-127. The distinctions of zeta potentials between NZVI and F-NZVI indicate that F-127 was successfully coated on the surface of NZVI. The dispersion of the F-NZVI particles could be attributed to the adsorption of F-127 on the surface of the NZVI particles, which increases the electrostatic repulsion effect. Additionally, it was interesting to observe that the negative charge of F-NZVI changed when the mass ratios of F-NZVI increased from 0 to 3:1, while it decreased to some degree. As the mass ratio increased to 4:1, the zeta potential decreased. The nanoparticles were more stable, since steric effect impeded the release of a negative charge. This indicates that a steric effect also played an important role in stabilizing the F-NZVI particles. F-127 plays a significant effect on the aggregation and colloidal stability of F-NZVI. The mass ratio of 2:1 is better, and the mass ratio of 4:1 is the best. Combined with the zeta potentials of F-NZVI, we adopted the F-NZVI (m/m, 2:1) for the following test.  Table 3 lists a total of 17 experimental runs and the corresponding results. The experimental runs, with a different combination of three factors, aim to investigate the effects of these factors on the 2,4-DCP removal rate (V). According to the RSM analysis, a quadratic polynomial equation, which expressed the relationship between the removal rate of 2,4-DCP and the independent variables, was established to obtain the predicted values of response. The model, in accordance with actual factors, is shown as Equation (2). Table 4 presents the results of analysis of variance (ANOVA) for the response surface quadratic model. F-value evaluates the statistical significance of modelling, and P-value determines the significance of factors for the response. Generally, a relatively large F-value and small P-value are significant. Therefore, the model is significant, due to the F-value (3482.27) and P-value (less than 0.0500). The Lack of Fit F-value of 3.93 implies the error is not significant, relative to the pure error. The R 2 value of 0.9998 and the adjusted R 2 value of 0.9995 in the quadratic model imply the veracity and suitability of the model. The predicted R 2 value of 0.9972 is in reasonable agreement with the adjusted R 2 value of 0.9995. Furthermore, the relatively low value of coefficient of variation (CV% = 0.41%) suggests that this model improved the precision and reliability of the tests. The P-value of AB, AC, and BC are 0.0003, <0.0001, and 0.0026, respectively, which means that these model terms are significant.

Mass
As a result, the established model is proved to be highly accurate in the prediction of values in the 2,4-DCP removal test.

Quantitative Effects of the Factors
A three-dimensional contour map presents the relationships among the two independent variables and the removal rate of 2,4-DCP. Figure 4 shows the change regulation of the V value under the effects of two independent variables.   As demonstrated in Figure 4a,b, the value of V gradually increases to the peak value as the initial concentration of 2,4-DCP increases from 5 to 22.5 mg/L, and, then, descends as A continuously increases. Meanwhile, the V value reaches the peak value with pH (B) of 5 and also displays a decrease as B continuously increases. The V value reaches the maximum value when A = 22.5 mg/L and B = 5. A suitable concentration of 2,4-DCP or pH facilitates the removal rate of 2,4-DCP. High or low A or B values may decrease the removal rate of 2,4-DCP. Figure 4c,d shows the interaction between the initial concentration of 2,4-DCP and F-NZVI dosages on the removal rate of 2,4-DCP. The V value gradually reaches the peak value with a moderate F-NZVI dosage, and decreases as C increases. Obviously, when increasing the initial concentration of 2,4-DCP within an appropriate range, the removal rate improves. V reaches the maximum value when A = 22.5 mg/L and C = 3.5 g/L. However, excess F-NZVI dosages would lead to a decline of the efficiency of the 2,4-DCP removal rate because excess F-NZVI dosages may affect dispersion and the molecules of 2,4-DCP cannot migrate to the iron surface.
Figure 4e,f shows the interaction between pH and F-NZVI dosages on the efficiency of the 2,4-DCP removal rate. It is clear that as the pH and F-NZVI dosages increase, the efficiency of the 2,4-DCP removal rate increases and, then, starts to slightly decrease. The V value reaches the maximum value with a moderate B value of 5 and a C value of 3.5 g/L. The passivation layer formed on the surface of F-NZVI, with a high pH, hinders electron transfer from highly active zero-valent iron cores. Hence, the removal rate of 2,4-DCP is reduced.
In short, A, B, and C are key points of variables that affect the removal rate of 2,4-DCP on account of the P-value of <0.05. The results also show that A, B, and C significantly affect the removal rate of 2,4-DCP as a single independent factor, which is consistent with the results of the ANOVA.

Preparation and Characterization of Bare NZVI and F-NZVI Particles
F-NZVI particles with different mass ratios of F-127/NZVI were prepared by a liquid phase reducing method with ferrous iron and sodium borohydride in an N 2 atmosphere. The chemistry reaction experiments were put into a 1000 mL three-necked round-bottomed flask, in which the middle neck was connected to the agitator, the second neck was connected to N 2 gas, and the third neck was connected to NaBH 4 solution. Take the F-NZVI samples with 2:1(m/m) F-127/NZVI as an example. Firstly, 1 g F-127 was placed in 250 mL ultrapure water in a 1000 mL three-necked flask at room temperature. It was stirred mechanically (250 r/min) for 25 min, before 2.484 g ferrous sulfate was added to the F-127 solution. The mixture solution was continuously stirred (250 r/min) for 15 min. Subsequently, a freshly prepared NaBH 4 solution (0.678 g NaBH 4 in 250 mL ultrapure water) was added drop-wise (5 mL/min) into the mixture solution of F-127 and Ferrous sulfate, and stirred constantly at 250 r/min under a N 2 atmosphere. The solution was stirred (250 r/min) for another 30 min until the NaBH 4 solution was fully incorporated into the mixture solution. The black particles were produced, and the solution turned black. Afterwards, the F-NZVI particles were collected by vacuum filtration and washed three times with ultrapure water and anhydrous ethanol. Finally, the fresh F-NZVI was dried by Freeze Drier (FD-1 A-50) and kept in a sealed bottle, prior to use. (Equation (3)) [31]: The pristine NZVI was prepared, following the method as described above, except no F-127 was added in this experiment.

Stability Study of F-NZVI Particles with Different Mass Ratios
The colloidal stabilities of F-NZVI particles were evaluated by measuring the sedimentation kinetics of F-NZVI particles and the zeta potential at different mass ratios (F-127/NZVI:0, 1:1, 2:1, 3:1, 4:1). In the sedimentation experiments, the concentration of F-NZVI particles was 1 g L −1 . F-NZVI suspensions with different mass ratios were sonicated for 10 min, and a 1 ml F-NZVI suspension was transferred to a glass cuvette for spectrophotometric analysis. The sedimentation of F-NZVI was measured by monitoring the optical absorbance at 508 nm by UV-vis spectrophotometry in intervals for 100 min. The measurements were carried out at room temperature (27 • C), and all the experiments were run in duplicates or triplicates. Surface charges of F-NZVI with different mass ratios were measured with a zeta potential analyzer (90 Plus Zeta, Brookhaven Instruments, Holtsville, NY, America).

RSM Experimental Design
The RSM is extensively used for evaluating the relationship between a combination of experimental factors and response values, using multiple regression equations. It aims to find the optimum removal conditions. The removal experiments were designed by the Design Expert software (8.0.6, Stat-Ease, Inc., Minneapolis, MN, America) to formulate optimization of the experimental factors. The effect of three independent experimental variables on the removal rate of V (response value) was developed by BBD at three levels: the initial concentration of 2,4-DCP (mg L −1 , (A)), pH (B), and the F-NZVI dosage (g L −1 , (C)). Table 3 presents the level of experimental variables and experimental orders. The sum of 17 operational variables from the study was applied toward the construction of a quadratic model. After experiments were completed, a second order polynomial model has been expressed as Equation (4): where V refers to the removal rate of 2,4-DCP; A, B, and C are independent variables (the initial concentration of 2,4-DCP, the pH, and the F-NZVI dosage, respectively); α 0 is the constant coefficient; α 1 , α 2 , and α 3 are linear coefficients; α 11 , α 22 , and α 33 are quadratic coefficients; finally, α 12 , α 13 and α 23 are interaction coefficients. Table 5 displays the parameters, and we assess the fit quality of the polynomial model equation by the coefficient of determination R 2 and adjusted R 2 . Model terms are chosen or rejected depending on the probability of P value with a 95% confidence level. 3D surface plots not only visualize the relationship between the removal rate and experimental factors utilized in the design but also determine the optimum condition, based on the main parameters.

Conclusions
In this work, F-127 was developed to modify the NZVI, and it was found that F-127 significantly increased the stability of NZVI. SEM analysis indicated that F-127, as a modifier, can enhance the dispersity of NZVI. Compared with the traditional NZVI, F-NZVI had better stability. Zeta potential and FTIR analysis indicated that F-127 played a significant effect on the aggregation and sedimentation behavior of F-NZVI. The sedimentation tests indicated that there is a two-stage process (rapid sedimentation and slow suspension) of F-NZVI in water. F-127 had an important effect on electrostatic repulsion and the steric effect that makes F-NZVI stable. The experimental conditions were optimized through RSM. The maximal removal rate of 72.29% was obtained under optimal operational conditions, with a pH of 5, F-NZVI dosage of 3.5 g/L, and an initial 2,4-DCP concentration of 22.5 mg/L.