Adsorption performance of TF-Zr on Pb2+ and F–
The effect of pH valueGenerally, the pH of the solution is one of the most crucial factors affecting adsorption. The effect of pH on the adsorption of the prepared TF-Zr on single-component Pb2+ at a concentration of 10 mg/L and two-component Pb2+ and F- at a concentration of 10 mg/L, respectively, was examined at the pH of 2.5–7.0, The results are shown in Fig. 1a. Pb2+ adsorption experiments were not performed at pH over 7 because Pb2+ would form metal precipitates under alkaline conditions, leading to an overestimation of the adsorption capacity of Pb2+. Accordingly, the adsorbed Pb2+ increased significantly as the pH increased from 2.5 to 7.0 and remained almost constant thereafter, which was due to the reduced competition between Pb2+ and H+ at the same adsorption sites of the adsorbent, and this intense competition occurs at lower pH values.Fig. 1The effect of pH (a) and TF-Zr dosage (b) (the volume of the sample is 50 mL) on adsorption of single-component Pb2+ (10 mg/L) and two-component Pb2+ (10 mg/L) and F- (10 mg/L) (pH = 4.5 for 90 min).The existence of F– could greatly influence the adsorption of Pb2+ by TF-Zr material. It was found that for the adsorption of single component Pb2+ by TF-Zr, the adsorption capacity was 2.08 mg/g and the adsorption equilibrium was reached at pH = 4.5. However, when 10 mg/L F– was added in the solution with 10 mg/L Pb+, the adsorption capacity and the pH at adsorption equilibrium was significantly increased to 4.71 mg/g and 5.5 respectively, which may be attributed to the strong affinity of Zr (IV) on TF-Zr for F–34, resulting in the enhanced electrostatic attraction between TF-Zr and Pb2+. In addition, the effect of pH on the adsorption of F- by TF-Zr is tiny35.The effect of TF-Zr dosageIn Fig. 1b, it was found that with the increase of TF-Zr dosage, the adsorption capacity of TF-Zr on Pb2+ increased from 1.91 to 2.65 mg/g in the single-component system with only Pb2+ and from 3.50 to 4.51 mg/g in the co-existence of 10 mg/L F– and 10 mg/L Pb2+, both of which reached the adsorption equilibrium at the dosage of 0.1 g TF-Zr. The higher TF-Zr dosage provided a large number of adsorption active sites for the whole reaction and increased the surface contact area of the solid–liquid phase in the system. At the same time, the presence of F- had a facilitating effect on the adsorption of Pb2+ by TF-Zr, which was attributed to enhanced electrostatic attraction between TF-Zr and Pb2+, similar as the effect of pH.Pb2+ initial concentration and adsorption kineticsCommonly, the initial concentration of Pb2+ in the solution is an important factor affecting the mass transfer resistance between the aqueous solution and the adsorbent36. According to Fig. 2a, in the initial adsorption stage, the adsorption capacity increased with the increase of initial concentration of Pb2+. Besides, the adsorption reaction of TF-Zr on Pb2+ mainly occurred within 20 min, after which the adsorption efficiency gradually declined and slowly tended to equilibrium. At lower initial concentration of Pb2+ (3–8 mg/L), there were sufficient number of active sites on the surface of 0.1 g TF-Zr for Pb2+ adsorption, thus TF-Zr had higher adsorption efficiency for Pb2+. While, at higher initial concentration of Pb2+ (12 mg/L), the adsorption efficiency was declined by nearly 4.35%. It was possible that the number of metal ions was more than that of active adsorption sites, leading to the lower adsorption of Pb2+ by TF-Zr. As a whole, the adsorption of Pb2+ by TF-Zr was limited by the adsorption time and the initial concentration of Pb2+.Fig. 2(a) The effect of initial Pb2+ concentration on the adsorption of Pb2+ by 0.1 g TF-Zr; The curves of (b) proposed primary dynamic and (c) proposed secondary dynamic; (d) Adsorption capacity of TF-Zr on Pb2+ and F– under different concentration ratios of F– and Pb2+.The proposed the pseudo-first-order and pseudo-second-order kinetic models were employed to investigate the kinetics of TF-Zr adsorption and analyze the removal mechanism of TF-Zr during the adsorption process. The proposed kinetic models were shown in the following mathematical expressions (1) and (2), respectively:$$ {\text{q}}_{{\text{t}}} = {\text{ q}}_{{\text{e}}} \left( {{1} – {\text{e}}^{{ – {\text{k}}}} 1^{t} } \right), $$
(1)
$$ {\text{q}}_{{\text{t}}} = {\text{ k}}_{{2}} {\text{q}}_{{\text{e}}}^{{2}} {\text{t}}/ \, \left( {{1} + {\text{k}}_{{2}} {\text{q}}_{{\text{e}}} {\text{t}}} \right). $$
(2)
where qe and qt are the amount of Pb2+ adsorbed at equilibrium and at any time t, k1 and k2 are the equilibrium rate constants, and t is the reaction time (min).The linear fitting of TF-Zr for single-component Pb2+ and two-components F– and Pb2+ adsorption are shown in Fig. 2b and c. Accordingly, the proposed secondary kinetic results showed a qe value of 2.364 mg/g for Pb2+ adsorption in the absence of F–. While the qe value of TF-Zr for Pb2+ was 4.608 mg/g in the existence of F–, which was elevated by 2.224 mg/g in comparison with that without F- addition. The fitted parameters are shown in Table 1. It was found that the adsorption of Pb2+ on TF-Zr fitted the second-order kinetic model well (R2 = 0.998) in the absence of F–, demonstrating the adsorption of Pb2+ on TF-Zr was mainly dominated by chemisorption. Similar results were obtained even if F– existed. In the meantime, the adsorption of F– on TF-Zr was consistent with the proposed the pseudo-second-order kinetic model (R2 = 0.997) and was predominant by chemisorption as well. The positive effect of F- on the adsorption of Pb2+ on TF-Zr was also quantified by fitting the qe values obtained from the the pseudo-first-order and pseudo-second-order kinetic models, where the qe values in the system containing F- were higher than the qe values in the system without F–.Table 1 Relevant parameters of the proposed primary and secondary dynamics.The effect of different F– and Pb2+ concentration ratiosAs the results mentioned above, the existence of F– in the adsorption system could significantly improve the adsorption performance of Pb2+ by the adsorbent TF-Zr. If the adsorbent would apply in actual soils for simultaneous adsorbing F– and Pb2+, the concentration ratios of F– and Pb2+ could be an important factor affecting the adsorption performance of TF-Zr. It has been identified that lead and fluorine are present in various forms in actual soli and the concentration of both is a factor that impacts their forms37. Therefore, the effect of the concentration ratio of F– and Pb2+ in aqueous solution on the adsorption characteristics of the adsorbent TF-Zr was investigated. At ambient temperature, F– and Pb2+ can easily react and form PbF2 sediment. The solubility product (ksp) of PbF2 refers to 3.3 × 10–8, and ion product (Qc) is calculated by the following Eq. (3):$$ {\text{Q}}_{{\text{c}}} = \left( {{\text{F}}^{ – } } \right)^{{2}} \cdot\left( {{\text{Pb}}^{{{2} + }} } \right). $$
(3)
In this study, it was calculated that Qc < ksp existed in both F– and Pb2+ coexisting solutions, which indicated that there was no precipitation of PbF2. Meanwhile, the concentrations of Pb2+ and F– in the solutions were examined by ICP-OES and fluorine ion-selective electrode techniques, respectively. As shown in Fig. S1. both F– and Pb2+ existed in the ionic form.The influence of TF-Zr on Pb2+ adsorption under different F– concentrations (different ratio of F– and Pb2+) was shown in Fig. 2d. The adsorption efficiency of TF-Zr on Pb2+ increased with the increase of F- concentration, and the adsorption of Pb2+ by the adsorbent tended to equilibrium when the concentration ratio of F–:Pb2+ was 30:10, at which time the removal efficiency of lead was 99.99%. Under this condition, the corresponding adsorption capacity was 5.02 mg/g, which was 2.37 mg/g higher than that in the absence of F-. it was mainly due to that on the one hand, Pb2+ could coordinate with the neighboring phenolic hydroxyl groups on TF-Zr, on the other hand, the electrostatic adsorption of Pb2+ could possibly happen on the TF-Zr surface after F– adsorption38. It was also found that the adsorption efficiency of TF-Zr on F– increased and then decreased with the increase of F– concentration, which was attributed to the facts that when the amounts of TF-Zr adsorbent were constant, the active sites were limited. At this time, too much F– cannot interact with the limited adsorption sites (mainly Zr ions). In conclusion, the presence of F- favored the adsorption of Pb2+ by TF-Zr, which was consistent with the above obtained results.The exploration of adsorption mechanismSEM–EDSThe microscopic morphology of TF-Zr was obtained by SEM scanning, finding that the surface of TF-Zr was a non-uniform porous structure (Fig. 3a). It was observed that there was nearly no changes in the microscopic morphology of TF-Zr before and after adsorption (Figs. 3b and 4c). The results of EDS demonstrated that Zr was successfully loaded onto the TF surface (Fig. S2a). The signal peaks of Pb and F were observed in Fig. S2b and c, respectively, indicating that F and Pb were successfully adsorbed by TF-Zr. Besides, the amount of Pb increased from 1.68 to 1.78 with the addition of F-, reflecting that the existence of F- had a facilitating effect on the adsorption of Pb2+ by TF-Zr. The Mapping element analysis of TF-Zr was shown in Fig. 3d–f, revealing that Zr (IV) was uniformly dispersed on the TF surface. In addition, the presence of lead and fluorine elements on the surface of TF-Zr also proved that Pb2+ and F– reacted with the function groups on the surface of TF-Zr.Fig. 3SEM of TF-Zr before and after adsorption (a) TF-Zr, (b) TF-Zr adsorbed lead, (c) TF-Zr adsorbed both fluorine and lead and Mapping of TF-Zr adsorbed both fluorine and lead, (d) Zr, (e) Pd (f) F.Fig. 4(a) FT-IR; (b) XPS spectra of TF and TF-Zr before and after adsorption; (c) XPS O 1 s of TF and TF-Zr before and after TF-Zr adsorption; (d) XPS Zr 3d before and after TF-Zr adsorption.FT-IRAs shown in Fig. 4a, the peak at 3400–3500 cm–1 is caused by OH stretching vibration, and the peak at 1398 cm–1 is caused by OH bending vibration39, indicating that there are a large number of hydroxyl groups on TF surface40. The peaks near 1620 cm–1 and 1454 cm–1 are attributed to the characteristic peaks of the carbon skeleton structure of the benzene ring, and the peaks near 1040 cm–1 are caused by the C–O stretching vibration on the benzene ring. When Zr is loaded onto TF, the peak at 624 cm–1 is caused by the tensile vibration of Zr–O–C indicating that Zr is successfully loaded onto TF surface41. After the adsorption of Pb2+, the peak of OH bending vibration at 1398 cm–1 shifted to 1381 cm–1. Simultaneously, the peaks at 3461 cm-1 belonging to OH stretching vibration shifted to 3429 cm–1. These results indicated that the material TF-Zr was able to adsorb Pb2+, which was mainly because Pb2+ could react with the hydroxyl groups on the surface of tannins to form stable chelates. Besides, when F- was added, the shift of OH peak at 1398 cm–1 was stronger to 1377 cm–1 in comparison with that without F–, which was possible that the absorption of Pb2+ was intensified by the addition of F–.XPSXPS was utilized to determine the surface elemental composition and atomic binding energy of TF and TF-Zr before and after adsorption, through which could reveal the elemental changes and chemical reactions on the surface of the materials TF and TF-Zr42. As shown in Fig. 4b, for TF-Zr, two peaks of Zr 3d3/2 and 3d5/2 appeared at 185.54 eV and 183.19 eV, respectively, indicating that Zr (IV) was successfully loaded on the TF surface. The peak of Pb 4f appeared after the adsorption of Pb2+ by TF-Zr (Fig. 4b) and the Pb 4f5/2 and Pb 4f7/2 peaks with 144.1 eV and 139.2 eV were found in the high-resolution spectrum of Pb (Fig. S3), which confirmed the successful adsorption of Pb2+ onto the TF-Zr surface. In addition, the TF-Zr with simultaneous adsorption of Pb2+ and F– also showed the Pb 4f. peak and F 1 s peak (Fig. S3), confirming that Pb2+ and F– could be adsorbed onto the TF-Zr surface. All of these were in accordance with the results of EDS.To reveal the mechanism of Pb2+ adsorbing on TF-Zr, the high-resolution spectrum of O element was analyzed (Fig. 4c). It was found that three different types of O-linked bonds were existed on TF-Zr at 533.13 eV, 532.42 eV and 530.85 eV, corresponding to C–O, C=O and Zr–O, respectively before adsorption of Pb2+. However, after adsorption of Pb2+, two new peaks appeared at 532.86 eV and 531.82 eV, corresponding to the C–O–Pb and C=O-Pb bond, respectively, while the Zr–O peak was unvaried. Moreover, the increase of the C–O–Pb peak was larger than that of C=O–Pb, indicating that Pb2+ was mainly coordinated to the hydroxyl group on the TF-Zr surface during the adsorption. Similar results were observed after the simultaneous adsorption of Pb2+ and F–, which indicated that the addition of F would not change the adsorption mechanism of Pb2+ by TF-Zr, but enhance the adsorption capacity of Pb2+ by intensifying the electrostatic adsorption between TF-Zr and Pb2+.It was found from the high-resolution spectrum of Zr element (Fig. 4d) that the binding energy of Zr had no change before and after adsorption of Pb2+, while the binding energies corresponding to the Zr 3d3/2 and Zr 3d5/2 peaks increased from 185.54 eV and 183.19 eV to 185.64 eV and 183.28 eV, respectively with F addition. These results indicated the adsorption process of Pb and F by TF-Zr was independent. Besides, the adsorption of F by TF-Zr was mainly due to the chemical reaction of Zr and F, which decreased the electron cloud density of Zr, resulting in an increase in the binding energy. Similar results were also reported by Wolter and Dou43,44. Meanwhile, the binding energy of the F1s peak was 685.2 eV after the adsorption of F and Pb by TF-Zr, which was closed to that of ZrF4 with 685.1 eV, also indicating the reaction of F- with Zr (IV).The application of TF-Zr in tea garden soilChanges of soil pHIt is reported that soil pH may affect the forms of Pb and F in soil, which can further affect the uptake of F and Pb by tea plants. Thus, in this study, the prepared material TF-Zr with different dosages were added in the tea garden soil to investigate its effect on the soil pH. Importantly, a wet experimental environment was simulated as that of tea garden soil in Ya’an city. The experimental results were showed in Fig. 5. The addition of TF-Zr could significantly reduce the soil pH at the initial period (20 days). When different amount of TF-Zr (0, 0.8, 1.4, 2.0 g) were added after 20 days, the soil pH was CK (control) > 0.8 > 1.4 > 2.0 g. Specifically, the soil pH with 2.0 g of TF-Zr addition was significantly reduced by 0.57 compared with the control group (P < 0.05), which was due to the release of H+ caused by the weak acidity of bayberry tannins. Subsequently, the soil pH gradually increased in the treatment groups with different amount of TF-Zr addition after 40 and 60 days, while the soil pH in the control group showed insignificant changes. It was proved that the increase of soil pH could be the result of the catalytic effect of hydrogen ions in the esterification reaction45. However, in our experiment, it may be due to the reaction between hydrogen ions and other substances in the soil that causes an increase in soil pH. Meanwhile, with the prolongation of the application time of TF-Zr, the pH value of the soil increased, which shows that the soil itself has a strong buffering effect. Although the pH value of the soil decreased at the beginning, through our tests, we found that the content of the two elements in the soil was decreasing in the mobile component, and the content of the residual state increased, indicating that at this time, pH is not the key to affect the morphology of F and Pb, but rather the TF-Zr material itself reacts with the fluorine and lead in the soil.Fig. 5The soil pH with different TF-Zr dosages at different experimental days.Changes of F forms in soilIt has been identified that different forms of F exist in tea garden soil, including water-soluble state(water-F), exchangeable state(Ex-F), ferromanganese oxidation state(Fe/Mn-F), organic bound state(Or-F) and residue state(Res-F), and their mobility and biological effectiveness in soil vary greatly46. It was commonly believed that the main forms of F that could migrate from soil to tea trees were the water-soluble and exchangeable states instead of total F47. In this work, the effect of TF-Zr applying in tea garden soil on different forms of F was investigate to evaluate the migration ability of fluorine from soil to tea trees. As shown in Fig. 6a, the content of different forms of F in all treatment groups changed greatly after 60 day’ simulating experiment. It was found that the content of water-F and Ex-F gradually and significantly decreased and the content of Fe/Mn-F and Or-F decreased slightly, while the content of Res-F gradually increased. It was obviously found that after 60 days, the water-F and Ex-F could gradually and significantly transform to Res-F in all treatment groups.Fig. 6(a) The changes of different forms of F in soil with different amount of TF-Zr addition. (b) The changes of different forms of Pb in soil with different amount TF-Zr addition.Most importantly, the changes of different forms of F with the addition of TF-Zr were quite different from those of the control group. With different amount of TF-Zr addition (0.8–2.0 g), the content of various forms of soil F in all the experimental periods changed significantly in comparison with those of the control group, especially the water-F, Ex-F. With the increase of TF-Zr addition from 0.8 to 2.0 g, the contents of Water-F and Ex-F in soil decreased gradually and significantly. Especially when 2.0 g TF-Zr was added, the water-F and Ex-F deceased by 86.07% and 71.65% after 60 day’ simulating experiment, which was 12.54 and 3.80 times higher than those of the control group. Besides, the contents of Res-F increased to 99.03% after 60 day’ simulating experiment, which was higher than that of the control group (98.37%). These results showed that adding TF-Zr could promote the transformation of Water-F and Ex-F in soil to Res-F, and the more TF-Zr was added, the more effective Water-F and Ex-F were immobilized. Thus, these findings confirmed that the addition of TF-Zr could effectively prevent fluoride migrating from tea garden soil to tea plants. It was identified that TF-Zr was a natural polymer composite and an organic material. When applied in adsorption of F in soil, it could facilitate the formation of soil agglomerates and provided more adsorption sites on the surface of the soil, resulting in more fluorine to be adsorbed onto the surface of the material and further be immobilized48. The ability of TF-Zr to reduce the bio-availability of soil fluorine was based on the fact that the Zr(IV) pair on the TF-Zr surface had a strong affinity for F, which could make the Zr-F structure easily formed.Changes of Pb forms in soilThe forms of heavy metals in soil can greatly affect the soil environmental quality and uptake of heavy metals by plants. It has been identified that the exchangeable state is considered to be the most readily available for plants uptake in soil, while the residual state is extremely stable, which can be tightly bound to the mineral lattice and within the crystalline oxide and is difficult to release into the soil environment49. Therefore, enhancing the transformation of exchangeable state of heavy metals to residual state has been regarded as a feasible and effective way to reduce the pollution and threat of heavy metals in soil. In this work, the effect of TF-Zr applying in tea garden soil on different forms of Pb was investigated to evaluate the immobilization ability and remediation performance of soil Pb. In order to better observe the influence of TF-Zr on different forms of Pb in soil, a certain amount of exogenous lead (about 1250 mg/kg dry soil) was added to the original soil in this study. Therefore, the content of exchangeable state (Ex-Pb) in the soil was relatively high at the initial experimental stage, accounting for 77.36% of the total Pb content. As shown in Fig. 6b, the content of different forms of Pb in all treatment groups changed greatly after 60 days’ simulating experiment, especially Ex-Pb and residue state (Res-Pb). It was found that the content of Ex-Pb gradually and significantly decreased, while the content of Res-F gradually and significantly increased, indicating the gradual and significant transformation of Ex-Pb to Res-Pb in all treatment groups with 60 days’ experiment.Besides, it was obviously found that the changes of different forms of Pb with the addition of TF-Zr were quite different from those of the control group. After 20 days’ experiment, the Ex-Pb content decreased by 51.88%, 56.19% and 63.95% with the addition of 0.8, 1.4 and 2.0 g TF-Zr respectively, which was 2.08, 2.26 and 2.57 times higher than that of the control group (without TF-Zr addition). Meanwhile, the Res-Pb content in the total Pb in soil with the addition of 0.8, 1.4 and 2.0 g TF-Zr accounted for 22.01%, 23.30% and 33.11% after 20 days respectively, which increased by 18.39%,19.68% and 29.49% compared with the control group. These results indicated that the addition of TF-Zr could significantly accelerate the transformation of Ex-Pb to Res-Pb in soil and the more TF-Zr was added, the faster Ex-Pb was transformed to Res-Pb. After 60 days, the decrease of Ex-Pb with 2.0 g TF-Zr addition was as high as 751.25 mg/g and the increase of Res-Pb reached to 471.90 mg/g. Moreover, the changes of Fe/Mn-Pb and organic bound state(Or-Pb) contents were similar with that of Ex-Pb, but it was not as obvious as the changes of Ex-Pb content, which was possibly due to the relative lower content of Fe/Mn-Pb and Or-Pb in comparison with Ex-Pb. In conclusion, the application of TF-Zr in tea garden soil could promote and accelerate the transformation of Ex-Pb to Res-Pb, and also promote the transformation of other forms of Pb to Res-Pb, which could effectively prevent the pollution of Pb in soil and control the migration of Pb to tea trees. The ability of TF-Zr to reduce the pollution and threat of Pb in soil was based on the complexation of hydroxyl group on the surface of TF-Zr with the Ex-Pb, in which the highly reactive Ex-Pb could be converted to the most stable form (Res-Pb).The analysis of Pearson correlationThe immobilization of Pb and F in soil by application of the material TF-Zr could be affected by many factors, such as soil pH, TF-Zr dosage and different forms of Pb and F. The Pearson correlation analysis were carried out to reveal the potential important factors, as shown in Tables 2 and 3. During the whole 60 days’ experiment, the soil pH value was significantly correlated with TF-Zr dosage (positive) and different forms of Pb and F (positive or negative) (P < 0.01) only with 20 days (Tables S2-1, S2-2 and S3-1, S3-2). It was mainly because the excess addition of TF-Zr could release H+ in soil at the initial periods, which would cause the decrease of soil pH and affect the existing forms of Pb and F. However, the influence of TF-Zr addition was insignificant on soil pH thereafter.Table 2 Pearson correlation analysis of TF-Zr dosage, pH, different fractions of fluorine at different test days.Table 3 Pearson correlation analysis of TF-Zr dosage, pH, different fractions of lead at different test days.Besides, the TF-Zr dosage showed a significantly negative correlation (P < 0.01) with Water-F, Ex-F, Fe/Mn-F, Ex-Pb and Or-Pb and a significantly positive correlation (P < 0.01) with Res-F and Res-Pb, which indicated the addition of TF-Zr could reduce the active forms of F and Pb and increase the stable form of F and Pb (Tables 2 and 3). These findings were in line with the changes of different forms of soil F and Pb above mentioned. From the correlation analysis of different forms of F, it was obvious that Water-F, Ex-F and Fe/Mn-F were significantly positive correlation (P < 0.01) with each other, while they were negatively correlated with Res-F suggesting that Water-F, Ex-F and Fe/Mn-F could transform to Res-Pb. The correlation analysis results of different forms of Pb were similar with those of F. The Ex-Pb, Car-Pb and Or-Pb were significantly positive correlation (P < 0.01) with each other, while they were negatively correlated with Res-Pb suggesting that Ex-Pb, Car-Pb and Or-Pb could transform to Res-Pb. However, the Fe/Mn-Pb had little correlation with other forms of Pb. This was probably due to insignificant changes of Fe/Mn-Pb after adding of different of TF-Zr during the whole experimental periods.
Efficient and simultaneous immobilization of fluoride and lead in water and tea garden soil by bayberry tannin foam loaded zirconium
