Adsorption Performance of Glyphosate on Modified Shell Powder/Ce-N-Tio 2

. Glyphosate is a typical dissolved organic phosphorus that can cause adverse effects on plants, animals, and humans. Modified shell powder/Ce-N-TiO 2 was synthesized by high-temperature doping, and the prepared modified shell powder/Ce-N-TiO 2 was characterized and used response surface methodology to find out the optimum condition for adsorption removal rate. The zeta characterization indicates that the doping of Ce shifts the isotropic potential of the material to the right. The FT-IR and XPS characterization indicated that glyphosate was successfully adsorbed, and the two-two interactions of pH, contact time and dosing amount had a more obvious effect on adsorption by response surface analysis. The process of glyphosate adsorption by modified shell powder/Ce-N-TiO 2 was consistent with the quasi-secondary kinetic model and Langmuir model, and the maximum adsorption amount was 66.98 mg/g. The above results may be useful for the study of other systems of glyphosate wastewater treatment.


Introduction
Glyphosate is a broad spectrum, post emergency and non-selective herbicide, widely used for post-emergence weed control [1][2][3].Considering that come into contact with glyphosate is toxicologically harmful and can cause to serious health problems, such as cancer, liver and tissue rupture, endoderm destruction, lymphoma, etc [4][5].So it is essential to choose a reliable application for removing glyphosate.
Many of the available treatment technologies share several drawbacks in respect of their applicability, mainly in view of the expansion of the scale [6][7][8][9][10].So on, adsorption exist as an operative correction strategy to remove glyphosate pollutants from the aqueous phase, in existence some advantages, such as low cost and simple operation [11][12][13].
In this study, TiO2 was modified by heat treatment and doping, and glyphosate was used as the target pollutant.The surface structure, functional groups and bonding energy changes were investigated before and after adsorption by means of scanning electron microscopy, infrared spectroscopy and XPS characterization, and its adsorption properties on glyphosate were explored by adsorption kinetics and thermodynamics.

Experimental method
Preparation of modified shell powder/Ce-N-TiO2 Taken a 100ml beaker and added 60ml isopropanol into it, then added 0.173g prepared shell powder (calcination temperature 900 °C, sieving particle size 120 mesh, starch dosage 15%, calcination time 2.5h and alkali treatment) was optimized by single factor orthogonal experiment.Shaken with ultrasonic for 10min, then dropped 15ml tetrabutyl titanate under magnetic stirring, and stirred uniformly for 20min to formed solution A. Measured 20ml isopropanol and 2.4ml deionized water, then mixed with 1.313g cerium nitrate hexahydrate and 0.655g urea, and the pH was adjusted to 3 with nitric acid to formed solution B. Slowly dropped solution B into solution a, stirred at a uniform speed for 2h to form sol. Aged at room temperature for 12h, dried and ground in a 65 °C blast drying oven, and calcine in a 450 °C muffle furnace for 4h to obtain modified shell powder/Ce-N-TiO2.
Adsorption kinetics experiment.The glyphosate solution with 100ml initial concentration of 1000mg/L was poured into the 250ml cone bottle, adjusted the initial pH to 4.0±0.2.Added 0.5g modified shell powder/Ce-N-TiO2 mixed, shaken for 7h at 25°C and 160r/min.Sampled every 0.5h and measured the absorbance, calculated the adsorption amount.Each experiment was conducted in parallel three times.In addition, the quasi first-order kinetic model is shown in equation ( 1) and the quasi second-order kinetic model is shown in equation ( 2) to fit the data.
in which qe and qt are the equilibrium adsorption capacity of glyphosate and the adsorption at time t (mg/g), k1 is the equilibrium constant of the quasi-firstorder kinetic model (min -1 ), k2 is the equilibrium constant of the quasi-second-order kinetic model in which b is Langmuir adsorption equilibrium constant (L/mg), qmax is the maximum adsorption amount (mg/g), Ce is the adsorption equilibrium concentration (mg/L), K is Freundlich adsorption equilibrium constant.

Determination method of glyphosate concentration and calculation method of adsorption capacity
The concentration of glyphosate was determined by nitrite ultraviolet spectrophotometry.
Formula ( 5) is used as the evaluation index of glyphosate adsorption effect of modified shell powder/Ce-N-TiO2.

Surface morphology analysis
The surface morphology of modified shell powder/Ce-N-TiO2 before and after adsorption was analyzed by scanning electron microscope.The results are shown in Figure 1 and Figure 2.  It can be seen from SEM Figure 1 and Figure 2 before and after adsorption.Before adsorption, the surface of the material is blocky, and after adsorption, there is obvious aggregation on the surface, and the surface is dense honeycomb.

Zeta potential analysis
The zeta potential change curves before and after glyphosate adsorption by modified shell powder/Ce-N-TiO2 are shown in Figure 3.As shown in Figure 3, the isoelectric point of modified shell powder/N-TiO2 is 5.7, and that of modified shell powder/Ce-N-TiO2 is 7.The isoelectric point of modified shell powder/Ce-N-TiO2 shifts left after glyphosate adsorption.The isoelectric point of metal oxides is determined by the deprotonation and protonation of hydroxyl groups on their surfaces.The leftward shift of modified shell powder/Ce-N-TiO2 isoelectric point after glyphosate adsorption indicates that glyphosate forms negatively charged inner layer complexes with the surface of modified shell powder/Ce-N-TiO2.

Surface functional group analysis
The functional group changes of the modified shell/Ce-N-TiO2 before and after adsorption were analyzed by infrared spectroscopy.The results show that in Figure 4.As shown in Figure 4, a symmetric stretching vibration of the carboxylate was observed at 1401.30 cm - 1 after the adsorption of glyphosate, indicating the adsorption of glyphosate by it [14].

XPS analysis
The modified shell powder/Ce-N-TiO2 before and after adsorption were analyzed by XPS.The results show that in Figure 5.As shown in Figure 5 after the adsorption of glyphosate, element P appeared, indicating that glyphosate was successfully adsorbed by modified shell powder/Ce-N-TiO2.

Response surface analysis
In this experiment, three factors were selected for investigation: pH, contact time and dosing amount, and response value: removal rate of glyphosate.The response surface analysis of three factors and three levels was carried out by using BOX-Behnken design principle, and 17 experimental groups were obtained to analyze the interaction between the influencing factors and obtain the optimization equation in the reaction process.The experimental design factor levels are shown in the Table 1 , and the results of experimental runs are shown in the Table 2.The data in Table 2  The actual and predicted values of this polynomial model are compared in Figure 6 and Figure 7.As can be seen from Table 3, the P-value of this model is less than 0.0500, which is a good fit and significant.The fitted correlation coefficient R 2 =0.9466 and adjusted coefficient of determination Radj 2 =0.8780, this regression equation has credibility and can respond to the true value.Figure 8 shows that the surfaces are relatively flat, indicating that the interaction between these two influencing factors does not have a significant effect on the removal rate.Figure 9 shows the effect of the interaction between pH and dosing amount, and it can be seen that the interaction between the two is significant.Figure 10 shows the effect of the interaction between contact time and dosing amount, and it can be seen that the interaction is significant.According to the response surface analysis above, the effect of three factors on the removal rate of modified shell powder/Ce-N-TiO2 on glyphosate containing wastewater, and the interaction between two factors were the same as the regression data analysis.This response surface model is reliable for optimization analysis and prediction.

Adsorption kinetics
Under the conditions of initial concentration of 1000 mg/L, pH = 4.0±0.2,temperature of 25℃ and rotating speed of 160 r/min, the effects of different contact time on the removal of glyphosate from modified shell powder/Ce-N-TiO2 were investigated.The results are found in the Figure 11.As seen in Figure 11 that the adsorption capacity gradually increased in the first 4.5h and reached the highest value of 66.15 mg/g at 5h.The quasi-first-order and quasi-second-order kinetic models are used to fit the data in the Figure 12 and Figure 13.It can be seen from the Figure 10 , Figure 13 and Table 4 that the adsorption process of modified shell powder/Ce-N-TiO2 is better matched with the quasisecond-order kinetic model, and the adsorption process is mainly chemical adsorption.

Adsorption thermodynamics
Under the conditions of pH = 4.0±0.2,rotating speed of 160r/min and contact time of 6h, the adsorption isotherms of glyphosate at 15, 25 and 35°C were carried out respectively.Results are presented in the Figure 14.As can be seen in Figure 14 with the increase of temperature, the adsorption counts of glyphosate by modified shell powder/Ce-N-TiO2 decreases slightly, indicating that low temperature is conducive to adsorption, which is an exothermic reaction process.The maximum adsorption capacity was 66.98 mg/g at 15℃.Langmuir and Freundlich adsorption isotherms were used for fitting, and the results are shown in the Figure 15 and Figure 16.It can be seen from the Figure 15 , Figure 16 and Table 5 that the Langmuir model has a higher degree of fitting and is more suitable to describe the adsorption of glyphosate on modified shell powder/Ce-N-TiO2.It shows that the adsorption process can be explained by single-layer coverage and uniform distribution.

Conclusion
(1)The successful adsorption of glyphosate by modified shell powder/Ce-N-TiO2 and the formation of negatively charged inner layer complexes on the surface of modified shell powder/Ce-N-TiO2 were demonstrated by SEM, zeta potential, FT-IR and XPS characterization.
(2)The effects of pH, contact time and dosing amount on the removal rate were analyzed by response surface analysis, which indicated that the interaction of two-two factors had significant effects on the adsorption and removal of glyphosate.
(3)Modified shell powder/Ce-N-TiO2 adsorption of glyphosate was in accordance with the quasi-secondary kinetic model and Langmuir model, and the maximum adsorption amount was 66.98 mg/g.
[g/(mg•min)].Adsorption thermodynamics experiment.Added separately 100ml glyphosate solution with initial concentration of 1000, 900, 800, 700, 600 and 500mg/L into several 250ml conical flasks.Adjusted the initial pH to 4.0±0.2.Added 0.5g modified shell powder/Ce-N-TiO2 into it, oscillated for 6h at 15, 25 and 35°C and 160r/min.Measured the change of glyphosate concentration in the solution and calculated the adsorption capacity.Each temperature gradient was parallel three times.Langmuir equation (3) and Freundlich equation (4) are used for linear fitting.

Fig. 6 .
Fig. 6.Distribution of true and predicted values of glyphosate removal rate.

Fig. 7 .
Fig. 7. Distribution of true and predicted values of glyphosate removal rate.

Fig. 8 .
Fig. 8. Interaction of pH and contact time.

Fig. 10 .
Fig. 10.Interaction of contact time and dosing amount.

Fig. 11 .
Fig. 11.Effect of adsorption time on the removal of glyphosate by modified shell powder/Ce-N-TiO2.

Table 1 .
Experimental design factor level table.

Table 2 .
Experimental design and results.

Table 3 .
Variance analysis results of response surface regression model.

Table 4 .
Kinetic fitting parameters for the adsorption of glyphosate by modified shell powder/Ce-N-TiO2.

Table 5 .
Adsorption isotherm fitting parameters for glyphosate adsorption by modified shell powder/Ce-N-TiO2.