Study on influencing factors and mechanism of removal of Cr(VI) from soil suspended liquid by bentonite-supported nanoscale zero-valent iron

In order to clarify the mechanism and effect of bentonite-supported nanoscale zero-valent iron (nZVI@Bent) on Cr(VI) removal in soil suspended liquid, nZVI@Bent was prepared by liquid-phase reduction method in this research. A number of factors, including the mass ratio of Fe2+ to bentonite during preparation of nZVI@Bent, nZVI@Bent dosage, soil suspended liquid pH value and reaction temperature were assessed to determine their impact on the reduction of Cr(VI) in soil suspended liquid. The nZVI@Bent was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to analyze the mechanism of removal of Cr(VI) from the soil. The results showed that the temperature of soil suspended liquid had a significant effect on the removal efficiency. Calculated by the Arrhenius formula, nZVI@Bent removes Cr(VI) from the soil suspended liquid as an endothermic reaction with a reaction activation energy of 47.02 kJ/mol, showed that the reaction occurred easily. The removal of mechanism Cr(VI) from the soil by nZVI@Bent included adsorption and reduction, moreover, the reduction process can be divided into direct reduction and indirect reduction. According to XPS spectrogram analysis, the content of Cr(III) in the reaction product was 2.1 times of Cr(VI), indicated that the reduction effect was greater than the adsorption effect in the process of Cr(VI) removal. The experiment proved that nZVI@Bent can effectively remove Cr(VI) from soil suspension, and can provide technical support for repairing Cr(VI)-polluted paddy fields.


Results and discussion
Effect of Fe 2+ to Bentonite Mass Ratio on Reduction Efficiency of Cr(VI). As shown in Fig. 1, nZVI@Bent particles had great effects on Cr(VI) removal in contaminated soil suspended liquid. When the reaction time was 240 minutes, the removal efficiency of Cr(VI) from soil suspended liquid by nZVI@Bent particles with the mass ratio of Fe 2+ to bentonite of 1:3 and 1:2 was only 87.58% and 89.26%, respectively, which efficiency was not satisfactory. However, when the mass ratio of Fe 2+ to bentonite increased to 1:1 and 1:0.5, the removal efficiency of Cr(VI) from soil suspended liquid could reach 95.33% and 99.46% respectively. The results showed that the efficiency of nZVI@Bent particles remove Cr(VI) from soil suspended liquid increased with the increase of the mass ratio of Fe 2+ to bentonite. The experimental result of this paper was similar to the finding of a previous  26 , in which the removal efficiency of phosphorus in the water rose with the increase of the nZVI in graphene supported nanoscale zero-valent iron (G-nZVI) materials. This may be because that when the quality of Fe 2+ was lower in the preparation process, nZVI particles in nZVI@Bent were less and too dispersed on bentonite, even some of nZVI had been coated by bentonite. These reasons lead to the reduction of the number of molecules with reactive activity in nZVI@Bent. Cr(VI) cannot combine with reactive molecules, resulting in a decrease in the ability of nZVI@Bent to remove Cr(VI) from soil suspended liquid.
The reaction rate is another key indicator in evaluating the ability of nZVI@Bent remove Cr(VI) from soil suspended liquid. Apparent rate constant k obs indicate the reaction rate of nZVI@Bent to remove Cr(VI) from soil suspended liquid. The larger the apparent rate constant, the faster the reaction rate is 28 . Figure 1 showed that the removal rate of Cr(VI) changed slightly after 60 min, so the data of the first 60 min in this experiment was selected to use ln(c t /c 0 ) for linear fitting of t to obtain the apparent rate constant k obs of the reaction. The fitting results were shown in Table 1, when the mass ratio of Fe 2+ to bentonite was 4 different treatments, ln(c t /c 0 ) had a very significant linear relationship with t. The apparent rate constant k obs increased with the increasing proportion of Fe 2+ in the preparation process. This illustrated that the removal rate of Cr(VI) from the soil suspended liquid by nZVI@Bent was promoted with the increase of the proportion of Fe 2+ mass in the preparation process. This was probably attributed to the increase of nZVI content in nZVI@Bent caused a large amount of Cr(VI) to combine with nZVI and react at the initial stage of the reaction, thus greatly increased the reaction rate. Figure 2 showed the effect of nZVI@Bent dosage on removal efficiency. When the dosage of nZVI@Bent was 2.00, 3.00, 4.00, 5.00 and 6.00 g/L, the removal efficiency of Cr(VI) in soil suspended liquid was 72.08%, 91.74%, 95.33%, 99.10% and 99.75%, respectively. Through comparison, the removal efficiency of Cr(VI) was increased by 27.67% when the dosage increased from 2.00 g/L to 6.00 g /L. The dosage of nZVI@Bent had a significant effect on the removal efficiency of Cr(VI) in soil suspended liquid. The efficiency of nZVI@Bent to remove Cr(VI) in soil suspended liquid increased when the dosage increased. The trend of removal efficiency of Cr(VI) in soil suspended liquid was the same as that of nanoscale valent iron-copper bimetal (nZVI/Cu) with different dosage reported by previous study 28 . This was because the number of nZVI with reactivity and the reaction area will increase as the content of nZVI@Bent in soil suspended liquid increase. More Cr(VI) will be combined with nZVI@Bent and removed by the reaction, and the removal efficiency of Cr(VI) in soil suspended liquid will increase.

Effect of nZVI@Bent dosage on the reduction efficiency of Cr(VI).
The experimental data of the first 60 minutes were selected to make a linear fitting of t with ln(c t /c 0 ) ( Table 2). The standard deviation showed that the ln(c t /c 0 ) of Cr(VI) reaction in soil suspended liquid with different dosage had an extremely significant linear correlation with t. Moreover, the apparent rate constant of Cr(VI) k obs in soil suspended liquid was increased with the increase of nZVI@Bent. Previous researcher 29 found that the apparent www.nature.com/scientificreports www.nature.com/scientificreports/ rate constant k obs of removal of Cr(VI) from wastewater by nZVI increased with the increase of the amount of nZVI added to the wastewater containing Cr(VI), which was a similar result with this study. Similar results further proved that the removal rate of Cr(VI) in soil suspended liquid by nZVI@Bent was promoted with the nZVI@Bent dosage increased. It can be explained that with the increased of nZVI@Bent particles, reactive molecules also increased. At the beginning of the reaction, a large amount of Cr(VI) will be attached to nZVI@Bent particles and removed, resulted in an increase in the reaction rate. Figure 3 showed that the removal efficiency of Cr(VI) in soil suspended liquid was 99.87%, 99.59%, 95.10%, 93.75% and 87.29%, when the reaction at pH 3, 5, 7, 9 and 11, respectively. With the increase of pH value, the removal efficiency of Cr(VI) in the soil suspended liquid significantly decreased, which indicated that nZVI@Bent was beneficial to the removal of Cr(VI) in the soil suspended liquid under acidic environment. The removal efficiency of nZVI@Bent decreased when the soil suspended liquid changed from acidic to alkaline. Zhu et al. 30 also found that the removal efficiency of Cr(VI) by nanoscale valent iron/Ni bimetal materials (nZVI/Ni) decreased from 99.84% to 52.63% when the pH of soil suspended liquid increased from 5 to 7 and other conditions remained unchanged. This was because nZVI@Bent consumes H + in the process of removing Cr(VI), while in an acidic environment, there was a large amount of free H + in the soil suspended liquid. A large amount of H + can promote the reaction direction of Cr(VI) removal from nZVI@Bent. Moreover, under the acidic environment, the Fe 2 O 3 and FeOOH oxide layer attached to the surface of nZVI@Bent will be corroded to expose more reaction sites. More Cr(VI) contacted reaction sites and then were removed, so as to improve the efficiency of removing Cr(VI) from the soil. However, when the soil suspended liquid was alkaline, Fe 2+ and Fe 3+ will form a large amount of Fe(OH) X (X = 2,3) accumulated with excessive OH − in the suspended liquid. The Fe(OH) X (X = 2,3) formed a layer of iron hydroxide film covered the surface of nZVI@Bent prevented the contact of nZVI@Bent with Cr(VI), resulted in a decline of the reduction rate.

Effect of pH of soil suspended liquid on reduction efficiency of Cr(VI).
The nZVI@Bent particles were separated from the soil suspended liquid and scanned by SEM after reaction under different pH conditions. It was found that as the pH dropped from 11 to 3, more flaky structures appeared on the surface of the nZVI@Bent particles, and the particle size of the material became larger. A small amount of nZVI less than 100 nm will be found on the surface of nZVI@Bent particles under alkaline conditions. Most nZVI particles showed a flake structure under acidic conditions. Under alkaline conditions, nZVI was corroded by OH − in the environment to generate Fe(OH) 3 Table 2. Apparent rate constants k obs of reactions with different nZVI@Bent dosages. Note: ** indicates an extremely significant correlation (p ≤ 0.001). www.nature.com/scientificreports www.nature.com/scientificreports/ reacted with Cr(VI) in soil suspended liquid, causing nZVI to be oxidized into a flake structure exceeding 100 nm, thus increasing nZVI's particle size.
When the pH of soil suspended liquid was 3, 5, 7, 9 and 11, the experimental data of the first 60 minutes were selected to fit t linearly with ln(c t /c 0 ) ( Table 3). The linear relationship between ln(c t /c 0 ) and t was very significant (0.9683 ** , 0.9880 ** , 0.9787 ** , 0.9553 ** , and 0.9453 ** ). As the pH of the soil suspended liquid increased, the apparent rate constant k obs of nZVI@Bent removing Cr(VI) became smaller. This was because there was a layer of hydroxide of Fe on the surface of nZVI@Bent particle under alkaline conditions, which hindered Cr(VI) from reacting with nZVI, thus reduced the reaction rate. Fig. 4, at 15, 25 and 35 °C, Cr(VI) removal efficiency in the soil suspended liquid after reaction equilibrium were 68.88%, 95.33% and 99.95%, respectively. The removal efficiency of Cr(VI) from soil suspended liquid increased with the increase of temperature.

Effect of reaction temperature on reduction efficiency of Cr(VI). As shown in
The nZVI@Bent particles reacted at different temperatures were separated from the suspension for SEM scanning. With the increase of temperature, the surface of nZVI@Bent particles had more flaky structures, and the material size became larger. Under the condition of low temperature, there were fewer active particles on the nZVI@Bent surface. nZVI@Bent cannot react with Cr(VI) in soil suspended liquid, thus retaining its own structure. However, with the increase of temperature, more particles gain energy and become active particles to react with Cr(VI) in suspended liquid, and more nZVI particles react to form large flake structures, thus increasing their particle sizes. Table 4 showed the fitting of the reaction of nZVI@Bent for removing Cr(VI) from soil suspended liquid at different temperatures with a pseudo-first-order model reaction. The results showed that the reaction conformed to the pseudo-first-order kinetics model when the reaction ambient temperature was 15, 25 and 35 °C respectively. The apparent rate constant k obs of nZVI@Bent for removing Cr(VI) from soil suspended liquid increased with the increase of reaction temperature. Due to the reason that the energy in reaction system increased with the temperature rose, there were more reactant molecules and higher reaction rates; the rate of remove Cr(VI) from soil solution by nZVI@Bent was boosted by the rising of reaction environment temperature.
According to the Arrhenius formula, the equation of ln k obs = −5655.3 T 1 + 14.569 was obtained by drawing curves of apparent rate constants with temperature. The apparent activation energy of removing Cr(VI) from soil suspended liquid by nZVI@Bent is 47.02 kJ/mol, and the preexponential factor k 0 was 2.124 × 10 6 min −1 . It indicated that the Cr(VI) reaction in soil suspended liquid removed by nZVI@Bent was an endothermic reaction. When the temperature of the reaction system increased, the reaction would be moved forward, resulted in more Cr(VI) reacted with nZVI@Bent and be removed.  Table 3. Apparent rate constants k obs of pH of different soil suspended liquid. Note: ** indicates an extremely significant correlation (p ≤ 0.001). www.nature.com/scientificreports www.nature.com/scientificreports/ Optimal conditions for removal of Cr(VI) by nZVI@Bent. At the ratio 1:0.5 of Fe to bentonite in nZVI@Bent particles, nZVI@Bent had the highest removal rate of Cr(VI) in soil suspended liquid. At the dosage 5 g/L and 6 g/L of nZVI@Bent, the removal rate of Cr(VI) reached more than 99%. Considering the economic factor of practical application, the better dosage of nZVI@Bent was 5 g/L. Under the pH 3 and 5 of the reaction environment, the removal rate of Cr(VI) reached more than 99%. Considering the higher acidity in the soil will limit the growth of crops, the pH 5 of the reaction environment was suitable condition. At the temperature 35 °C, the removal rate of Cr(VI) was the highest. Consequently, the optimum condition for nZVI@Bent to remove Cr(VI) from soil suspended liquid was the ratio 1:0.5 of Fe to bentonite in nZVI@Bent particles, the dosage 5 g/L, the reaction environment pH 5 and the reaction environment of 35 °C.
Shi 31 , Wang 32 conducted a study of the removal effect of nZVI@Bent on Cr(VI) in water, which is relatively easy compared with the removal of paddy Cr(VI) in this paper. However, the optimal conditions for the removal of Cr(VI) by nZVI@Bent prepared by different scholars can be compared. Table 5 showed that different scholars used the same supported material, the optimum removal conditions of different removal materials were different because of the different Fe to bentonite mass ratio controlled by each scholar during the preparation process. Although the nZVI@Bent material prepared by Shi et al. 31 had the advantages of low dosage and low iron content, it required low pH in the environment and high reaction temperature, which brought similar problems of high cost in practical application. Although the nZVI@Bent material prepared by Wang et al. 32 had the advantage of low dosage, the high iron content of the material will make the nZVI@Bent had a large amount of iron left in the water after removing pollutants, resulting in secondary pollution. Moreover, nZVI@Bent prepared by Wang et al. 32 required a high acidity environment (pH 2), which has strict requirements on the pH of the wastewater. The nZVI@Bent material prepared in this study had the disadvantage of large dosage, but it had the advantages of appropriate iron content, close to neutral reaction environment and relatively low reaction environment temperature. The advantages of nZVI@Bent in this study will avoid secondary iron pollution after the reaction, acidification of paddy fields and high removal cost. So this research is meaningful and can bring technical support for actual production and life.
Previous studies had also used different supported materials to prepare composite nZVI, but they had some drawbacks in their application to the removal of Cr(VI). For example, MCM-41-supported nanoscale zero-valent iron prepared by Petala et al. 33 needed to be performed in a low pH environment when removing Cr(VI), which will cause the harm of water acidification. The attapulgite-supported nanoscale zero-valent iron prepared by Zhang et al. 34 required a long contact time to remove Cr(VI), which greatly affected the removal efficiency of Cr(VI)-containing wastewater. Therefore, nZVI@Bent has certain advantages in removing Cr(VI).

Removal of Cr(III) by nZVI@Bent.
As can be seen from the above, the optimum condition for nZVI@Bent to remove Cr(VI) from soil suspended liquid was the ratio 1:0.5 of Fe to bentonite in nZVI@Bent particles, the dosage 5 g/L, the reaction environment pH 5 and the reaction environment of 35 °C. Under these conditions, the removal rate of Cr(III) by nZVI@Bent was 92.36%. Most of Cr(III) was removed and further immobilized free Cr(III), which made Cr more stable and reduced the harm brought by Cr. Figure 5(a) showed the structure and morphology of nZVI@Bent's. The size of nanoscale zero-valent iron particles was 40-80 nm and had been loaded on bentonite and only a small amount of agglomeration occurred. The nanoscale zero-valent iron particles can be dispersed more on bentonite and effectively prevent agglomeration of zero-valent iron nanoparticles. Figure 5   www.nature.com/scientificreports www.nature.com/scientificreports/ surface which reacted with Cr(VI) in the soil suspended liquid, and then a large number of Fig. 5(b) flake structures appeared. XRD characterization. X-ray diffraction was used to explore the nZVI@Bent particle characterizations conducted before and after the reaction with Cr(VI). Figure 6(a) showed that there were obvious Fe 0 peaks and weak Fe 2 O 3 peaks in freshly prepared nZVI@Bent particles. The obvious Fe 0 peaks indicated that nZVI had been loaded on bentonite surface. The weak Fe 2 O 3 peaks indicated that a small amount of Fe 2 O 3 was attached to the surface of nZVI@Bent. The XRD pattern of biochar supported nanoscale zero-valve iron prepared by Li et al. 35 showed obvious peaks of Fe 2 O 3 and FeOOH, nZVI supported with biochar will not effectively reduce the oxidation. However, there were no obvious peaks of Fe 2 O 3 and FeOOH in Fig. 6(a), which indicated that nZVI supported with bentonite can effectively prevent the oxidation of nZVI. Figure 6(b), showed that nZVI@Bent particles had no obvious Fe 0 peak after reacting with soil suspended liquid containing Cr(VI) pollution, but had obvious Fe 2 O 3 and FeOOH peaks, it suggested that nZVI had been nearly completely reacted with soil suspended liquid containing Cr(VI) pollution, and most of them had been oxidized to form Fe 2 O 3 and FeOOH. The reaction equation is as follows:     3 2 Mechanism of removing Cr from soil suspended liquid by nZVI@Bent. Removal of Cr(VI) from soil suspended liquid by nZVI@Bent can be divided into two processes: reduction and adsorption 36 . Cr(VI) in nZVI@Bent reduced soil suspended liquid can be roughly divided into two processes, direct reduction and indirect reduction 10,37,38 . In the direct reduction process, Cr(VI) will be adsorbed on the surface of nZVI@Bent to directly react with the surface containing nZVI to form Cr(III)(Eqs. f,g). nZVI will be oxidized to Fe 2+ for indirect reduction with Cr(VI): Fe 2+ reduced Cr(VI) in soil suspended liquid to Cr(III) (Eq. h), and Cr(III) can combine with OH − in soil suspended liquid to form Cr(OH) 3 (Eq. i), Fe 2+ was oxidized to Fe 3+ , and Fe 3+ of the reaction system combines with Cr(III) and free OH − in the soil suspended liquid to form Fe(III)-Cr(III) complex (Eqs. j,k) 39,40 . The peak area of Fe 3+ increases obviously ( Fig. 7(b)), and conversely, the peak area of Fe 2+ decreased ( Fig. 7(a)). The proportion of Fe 2+ in nZVI@Bent particles decreased from 64.50% before the reaction to 45.57% after the reaction. The proportion of Fe 3+ increased from 35.50% before reaction to 54.43% after reaction. The change of the peak area of Fe 2+ and Fe 3+ further confirmed the removal process of Fe 2+ and Cr(VI) reaction being oxidized to Fe 3+ and then forming Fe(III)-Cr(III) complex. With the formation of these complexes and their attachment to the surface of nZVI@Bent, the contact of Cr(VI) with nZVI@Bent will be hindered, resulted in the reduction ability of nZVI@Bent. This hypothesis was also consistent with the fact that the nZVI@Bent reaction rate in batch experiments was very high at the initial stage of the reaction and became slower with the reaction proceeds. XPS analysis confirmed that there was no material containing Cr in the freshly prepared nZVI@Bent particle. After the reaction with soil suspended liquid containing Cr(VI), the materials containing Cr appeared on the surface of the nZVI@Bent particle (Fig. 7(c)). There was a larger peak area for Cr(III) than that for Cr(VI), and the ratios of Cr(III) and Cr(VI) on the surface of nZVI@Bent particles after the reaction were 67.35% and 32.65%, respectively. It indicated that the reduction of Cr(VI) in the system of nZVI@Bent particles and the soil suspended liquid containing Cr(VI) was the main process compared with the adsorption. The reduction reaction was also more intense and most of the Cr(VI) was removed by the reduction reaction, and the amount of Cr(VI) left in the soil greatly decreased.

SEM characterization.
The main principle of adsorption concerned with both the large specific surface area of nZVI@Bent, and was more adsorption sites on its surface. Not only Cr(VI) was directly adsorbed by nZVI@Bent in the soil suspended liquid, but also Cr(OH) 3 and Fe(III)-Cr(III) complexes produced during the reduction process were adsorbed by nZVI@Bent, further reduced soil Cr pollution.
Based on the above removal reaction process, the reduction process (Fig. 8) and the reduction process equation of Cr(VI) can be roughly inferred as follows 31,41-43 : www.nature.com/scientificreports www.nature.com/scientificreports/ Cr(VI) is reduced to Cr(III) equation: 3 3 Fe(III)-Cr(III) complex formation process:

Conclusions
(1) Nanoscale zero-valent iron was loaded on bentonite, and the agglomeration of nanoscale zero-valent iron after loading significantly reduced. (2) The ability of nZVI@Bent particles to remove Cr(VI) from soil suspended liquid increased with the increase of the mass ratio of Fe 2+ to bentonite, nZVI@Bent particles dosage and the temperature in the reaction environment. The ability of nZVI@Bent particles to remove Cr(VI) from soil suspended liquid decreased with the increase of pH in the reaction environment. Moreover, the reaction temperature had the greatest influence on the ability of nZVI@Bent particles to remove Cr(VI) in soil extract. When the temperature increased from 15 °C to 35 °C, other conditions remained unchanged, the removal rate of Cr(VI) in soil extract increased from 68.88% to 99.95%, and the removal rate increased by 31.07%. (3) When the ratio of Fe 2+ to bentonite in nZVI@Bent particles was 1:0.5, the dosage was 5 g/L, the reaction environment pH was 5 and the temperature was 35 °C, the removal rate of Cr(VI) in soil suspension by nZVI@Bent was the optimum reaction conditions for the removal of Cr(VI) (4) Removal of Cr(VI) from soil extract contaminated by Cr(VI) by nZVI@Bent can be divided into two processes: reduction and adsorption, and the reduction effect mainly proceeds. The final product of reduction was the complex of Cr(OH) 3 and Fe(III)-Cr(III).   NaBH 4 droplets, continue stirred the mixture in the flask for 40 minutes (all these processes had been protected by nitrogen). The above mixture was filtered with a vacuum pump to obtain nZVI@Bent particles, which were quickly rinsed with anhydrous ethyl alcohol for 3 times, then dried in a vacuum drying oven at 75 °C for 12 h, and put into a dryer filled with nitrogen for later use.

Materials and methods
Collection of contaminated soil and preparation of soil suspended liquid. The contaminated soil used in this experiment was collected from the vicinity of a chromium slag storage plant in Jilin province, China. After the soil dried, passed 100 mesh sieve for later use. The properties of the soil were as following: pH 5.25, total organic matter content 42.5 g/kg, alkali-hydrolyzed nitrogen content 153.5 mg/kg, available phosphorus content 63.2 mg/kg, available potassium content 101.5 mg/kg, and clay proportion was 19.6%, respectively. Weigh 1.00 g of contaminated soil into a 50 ml polyethylene centrifuge tube, adding 25.00 ml distilled water, then put centrifuge tube into a water bath oscillator at the temperature of 25 °C and oscillated for 2 hours, and then, taking out for standby, and the pH of soil suspended liquid was adjusted to be 7, and more mud was prepared.
The supernatant filtered through 0.45μm membrane by centrifugation in centrifugal containing mud tube. The concentration of Cr total in the filtrate was analyzed using a flame atomic absorption spectrophotometer (17.21 mg/L), Cr(VI) in the filtrate was measured using the diphenylcarbazide spectrophotometric method (15.68 mg/L) as the initial concentration of Cr(VI) in soil suspended liquid. The initial concentration of Cr(III) (1.53 mg/L) is equal to the initial concentration of Cr total minus the initial concentration of Cr(VI).
Batch experiments. The conduct batch experiments were conducted by using soil suspended liquid prepared in the above step. The dilute NaOH and H 2 SO 4 solutions were used to adjust pH values. The effect of mass ratio of Fe 2+ to bentonite during nZVI@Bent preparation (1:0.5, 1:1, 1:2 and 1:3), nZVI@Bent dosage (2.00, 3.00, 4.00, 5.00 and 6.00 g/L), pH of soil suspended liquid (3, 5, 7, 9 and 11) and reaction temperature (15, 25 and 35 °C) were studied. The reaction solution was separately centrifuged at a reaction time of 5, 10, 20, 20, 30, 40, 50, 60, 80, 100, 120, 180 and 240 min, respectively, and the supernatant was passed through a 0.45μm filter, and the residual concentration of Cr(VI) in the filtrate was measured using the diphenylcarbazide spectrophotometric method to calculate the removal efficiency of Cr(VI).
Structural characterization. Scanning electron microscope (SEM, SU8010, Hitachi) was used to morphology the surface characteristics of nZVI@Bent particles before and after the reaction. Before the reaction, the test material was freshly prepared nZVI@Bent particles. After the reaction, the test material was freshly prepared nZVI@Bent particles which reacted with the soil suspended liquid containing Cr(VI) at pH = 7 and the reaction temperature was 25 °C.
X-ray diffraction (XRD, D/MAX 2250 V, Rigaku) (Cu, Ka) was used to measure the X-ray diffraction patterns of nZVI@Bent particles before and after the reaction (the same test material as that of SEM). The X-ray diffraction spectra were acquired from 5 to 80 degrees, and the scanning speed was 2°/min. X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fischer) was employed to analyze the nZVI@Bent particle surface elemental valence before and after the reaction (the same test material as that of SEM). The X-ray source was Al-kα ray (hv = 1486.6ev) with a working voltage of 12.5 kV.
Data processing. Removal efficiency of Cr(VI) and Cr(III) from soil suspended liquid by nZVI@Bent: t 0 0 where: ω(%) is the removal efficiency of Cr(VI) or Cr(III) in the soil suspended liquid by nZVI@Bent, c 0 is the initial concentration of Cr(VI) or Cr(III) in the soil suspended liquid (mg/L), and c t is the concentration of Cr(VI) or Cr(III) in soil suspended liquid at time t (mg/L).
Kinetic model. Determining the reduction kinetics of Cr(VI) by nZVI@Bent with pseudo-first-order model 33,44 : t SA s m 0 where: k SA is the specific surface area reaction rate coefficient of the material [L/(min•m 2 )], a s is the specific surface area of the material (m 2 /g), ρ m is the concentration of the material (g/L), k SA , a s , and ρ m are the constants in one reaction and k SA , a s , and ρ m can be expressed by a pseudo-first-order model apparent rate constant k obs (min −1 ), integral formula (2) to obtain: t o bs 0 apparent activation energy calculation The apparent activation energy of nZVI@Bent particles for removing Cr(VI) from soil suspended liquid solution contaminated by Cr(VI) was calculated by Arrhenius formula 45 .
obs a 0 where: R is the molar gas constant (J/mol•K); T is the thermodynamic temperature (K); E a is the apparent activation energy (kJ/mol); k 0 is the preexponential factor (min −1 ).