Adsorption Characteristics of Glyphosate on Cross-Linked Amino-Starch

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Adsorption Characteristics of Glyphosate on Cross-Linked AminoStarch Lei Guo,*,† Yuanchao Cao,† Kaijin Jin,† Ling Han,‡ Guiying Li,† Junshen Liu,† and Songmei Ma† †

School of Chemistry and Materials Science, Ludong University, Yantai 264025, People’s Republic of China Wanhua Energysav Science & Technology Group CO., Ltd., Yantai 264025, People’s Republic of China

‡

ABSTRACT: Cross-linked amino-starch (CAS) was prepared via a dry process and used to adsorb glyphosate from aqueous solution. Effects of adsorption time, glyphosate concentration, temperature, pH, and ionic strength on the adsorption of glyphosate by CAS were thoroughly studied. It shows that CAS can effectively remove glyphosate from the solution. The adsorption equilibrium is attained in about 60 min, and the adsorption process follows the pseudo second-order kinetic model. Sips isotherm model is more suitable than Langmuir and Freundlich isotherm models for the adsorption process, and the maximum adsorption capacity is 3.61 mmol·g−1 from Sips isotherm fitting. The result of thermodynamics study indicates that the adsorption is a spontaneous and exothermic process. The results of isotherm and thermodynamics study show that the adsorption of glyphosate by CAS is attributed to a combination of chemical and physical processes. The adsorption process is severely affected by pH and ionic strength of the solution and the optimal adsorption performance is achieved at pH = 6.0.

1. INTRODUCTION Glyphosate (N-(phosphonomethyl) glycine) is a broadspectrum, nonselective and efficient herbicide. It has a broad range of applications, such as no till farming in rice and wheat in agriculture, exploitation and utilization of grassland and pasture in animal husbandry, and mass killing of grass in the fields of construction and traffic. It accounts for 60% of the herbicide sales at home and abroad.1 Significant use of glyphosate leads to an increase in concerns about its impacts on the environment in which the glyphosate entered from various sources such as industrial effluents, agricultural runoff, and chemical spills.2,3 Different symptoms caused by poisonous glyphosate for human such as cardiac and respiratory problems, irritation, and anaphylaxis.4 There is still no standard value considering the presence of glyphosate in drinking water, but the limitation for any herbicide in EU is no more than 0.1 μg·L−1. It is urgent to find an effective and economical approach to reduce the glyphosate content in effluent and realize harmless emission. Many methods, such as electrochemical oxidation,5 photocatalytic degradation,6 and adsorption,7−9 have been performed to solve glyphosate contamination. Recently, adsorption is considered as the promising process because of the cost-effectiveness without secondary-pollution and a variety of adsorbents. A range of materials, for instance, nanosized copper hydroxide modified resin,7 industrial residuals,10 magnetic MnFe2O4−graphene hybrid composite,11 and resin,12 were used to be adsorbents of glyphosate. Jia et al. prepared nanosized copper hydroxide modified resin and used it as an adsorbent of glyphosate with a maximum adsorption capacity of 113.7 mg·g−1 in the presence 18% NaCl.7 Hamoudi et al. prepared graphene oxide/MnFe2O4 © XXXX American Chemical Society

composites and used them to adsorb glyphosate. It was found that the adsorption of glyphosate reached equilibrium within 8 h at 25 °C with a maximum adsorption capacity of 39 mg·g−1 at 5 °C.11 Xiao et al. compared the adsorption performance of glyphosate on three resins and found that the maximum adsorption capacity of glyphosate was observed at pH of 2.51, and glyphosate was adsorbed in the form of HOPO 2 − CH 2 NH 2 + CH 2 COOH and HOPO 2 − CH 2 NH 2 + CH2COO−.12 Starch is considered to be an abundant and renewable natural polysaccharide, extracted from corn, sweet potato, and wild acorns. Starch derivatives, such as starch phosphate,13 dithiocarbamate-modified starch,14 water-insoluble starch sulfate,15 and starch 5-choloromethyl-8-hydroxyquinoline polymer,16 were used as adsorbent materials. Modified starch via dry or semidry process shows good surface activity because the reaction often takes place on the surface of starch granules.13 In the present study, cross-linked amino-starch (CAS) was prepared via a dry process as a promising procedure and used to adsorb glyphosate from aqueous solution. Effects of adsorption time, glyphosate concentration, temperature, pH and ionic strength on the adsorption of glyphosate by CAS were thoroughly studied. With the aim to explore the adsorption mechanism of glyphosate on CAS, adsorption kinetics, isotherm and thermodynamics were also investigated. Received: September 20, 2017 Accepted: January 10, 2018

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DOI: 10.1021/acs.jced.7b00842 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

2. EXPERIMENTAL SECTION 2.1. Materials. Corn starch (GB/T 12309-1990, Chinese Standards), purchased from Zhucheng Xingmao Corn Developing Co., Ltd. (Weifang, China), was dried at 105 °C before it was used. Glyphosate was purchased in the form of glyphosate isopropylamine salt (40 wt % in H2O, Sigma−Aldrich). The active ingredient of the Monsanto herbicides known as Roundup is mainly the isopropylamine salt of glyphosate. Epichlorohydrin (ECH), NH4OH, and all other commercial chemicals were analytical grade and used without further purification. The water used in all of the experiments was doubly distilled. 2.2. Preparation of Cross-Linked Amino-Starch. CAS was prepared in two steps via a dry process. Corn starch was activated with NaOH, and then cross-linked cationic starch was prepared by reacting active starch with ECH and NH4OH. The cross-linking of starch with ECH in the presence of NH4OH via wet process has been described in detail elsewhere.17 An increase in the amount of ECH added can improve the mechanical stability of the modified starch. A typical crosslinking reaction was carried out as follows: Corn starch (16.2 g, dried) was mixed with NaOH (4 g, 50% W/W) to give a homogeneous paste via a high-speed mixer (HR2874, Philips, China). The mixture was heated for 2 h at 50 °C in an oven. After cooling to room temperature, the active starch was mixed with ECH and NH4OH to give a homogeneous paste via a high-speed mixer (HR2874, Philips, China). The mole ratio of starch, ECH, and NH4OH was 1:1:0.7. The mixture was heated for 4 h at 50 °C in an oven. The product was washed three times with deionized water and one time with ethanol. The product was then dried at 50 °C in vacuum. The nitrogen content of the amino-starch was 1.81 mmol· g−1, measured by the Kjeldahl method (GB/T 22427.10-2008, Chinese Standards). The structures of the amino-starch were analyzed by using a FTIR spectrometer (Tensor 37, Bruker, German). The surface morphology of starch, CAS and CAS adsorbed glyphosate were characterized by a scanning electron microscopy (JSM-5610LV, Jeol, Japan). 2.3. Batch Adsorption Experiments. Batch methods were adopted in the adsorption experiments. Fifty milligrams of CAS was added to 50 mL of glyphosate aqueous solution in a series of 100 mL glass-stoppered Erlenmeyer flasks. The suspension was stirred at a uniform speed of 120 rpm on a water bath oscillator with constant temperature (SHA-C, China). After a certain adsorption time, the suspension was filtered through a 0.2 μm nylon membrane by using a syringe filter, and the concentration of glyphosate in the aqueous phase was determined by using a UV−vis spectrophotometer (UV2550, Shimadzu, Japan) at the maximum wavelength of 242 nm according to the Chinese Standard Method (GB/T 126862004, Chinese Standards). The initial pH value of the glyphosate solution was adjusted by adding either 0.1 M HCl or 0.1 M NaOH solution. NaCl and Na2SO4 were used to examine the impact of ionic strength on the adsorption and repeated three times in each case. The adsorption capacity was calculated according to eq 1 Q=

(C i + C t )V m

respectively. V (mL) is the volume of the adsorption solution, and m (mg) is the mass of the adsorbent.

3. RESULTS AND DISCUSSION 3.1. IR and SEM Characteristics of the Adsorbents. The IR spectra of the native corn starch, CAS, and CAS adsorbed glyphosate are shown in Figure 1. The nitrogen content of CAS

Figure 1. IR spectra of starch, CAS, and CAS adsorbed glyphosate.

was 1.81 mmol·g−1. In the IR spectra of CAS, the band at 2849 cm−1 appears corresponding to C−H (in CH2N groups) vibrations and the band at about 3400 cm−1 is attributed to O− H (in starch and ECH) vibrations. The IR spectra of CAS adsorbed glyphosate shows typical absorption bands of amino acids at 1627 and 1387 cm−1 corresponding to antisymmetrical and symmetrical stretching bands of ionized carboxylic groups. Bands resulting from P−O (in PO2(OH)− group) stretching at 1157 cm−1and 1100 cm−1 and P−O (in PO32− group) stretching at 1020 cm−1 can be observed in the spectrum of CAS adsorbed glyphosate.18 These bands can prove that glyphosate is adsorbed by CAS. The scanning electron micrographs of starch, CAS, and CAS adsorbed glyphosate (CAS-G) are shown in Figure 2. As can be seen, the starch granules are almost spherically shaped and smooth with a diameter of about 10 μm. After reacting with ECH and NH4OH, the surface of the CAS granules are rough and have multiple collapses, leading to an increase in its specific surface area. Solid-phase reactions usually take place on the surface of starch granules and cross-linked starch phosphate carbamates using a dry process show the similar morphology with CAS.13 When comparing CAS and CAS-G, the surface morphology is similar, but the surface of the CAS-G particles are smoother due to the adsorption of glyphosate. 3.2. Adsorption Kinetics of Glyphosate on CAS. The effect of time on the adsorption of glyphosate by CAS was investigated at three different temperatures, as shown in Figure 3. The adsorption capacities increase rapidly during the first 20 min, and the maximum adsorption capacity is reached in about 60 min. Other materials used to adsorb glyphosate had a longer equilibrium time, so rapid removal of glyphosate by CAS is feasible.11,19,20 Glyphosate is almost negatively charged at pH 5.64,21 as shown in Figure 4, while CAS covers with positive charges at the same pH.22 The adsorption of glyphosate on

(1)

where Q is the adsorption capacity of glyphosate (mmol·g−1), and C i and C t (mmol·L −1 ) are the initial and final concentrations of the glyphosate in the adsorption solution, B



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Figure 2. SEM images of starch, CAS, and CAS-G.

Figure 4. Species of glyphosate depending on the pH and protonation constants from Motekaitis and Martell.21

surface are empty and the concentration of glyphosate is high. With the extending of contact time, the adsorption sites decrease and ultimately attain saturation, so the rate slowly reduces to zero, and the dynamic equilibrium is reached. Besides, it is clear that the adsorption capacity decreases with the rising of temperature. Kinetics analysis is required to comprehend the adsorption rate-limiting step of the transport of solute between liquid and solid and design of the process. Up to now, the pseudo firstorder23 and second-order equations24 are the two most used kinetic models to examine the controlling mechanism of adsorption process between liquid and solid. Pseudo first-order

Figure 3. Effect of the adsorption time on the adsorption capacities of glyphosate on CAS (Ci, 1 mmol·L−1; pH, 5.64): □, 293 K; ○, 313 K; Δ, 333 K; ------, pseudo-first-order; , pseudo-second-order.

CAS depends on the electrostatic interaction between CAS and glyphosate. At the beginning of the adsorption, the adsorption rate is high because the adsorption sites on the adsorbent C



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quantity. n and s are heterogeneity factor of Freundlich and Sips, respectively. KL, KF, and KS are the isotherm constant of Langmuir, Freundlich and Sips, respectively. The three models were used by nonlinear regression analysis in the study. The adsorption equilibrium isotherms are shown in Figure 5, and the model constants are tabulated in Table 2.

and second-order equations are generally expressed as eqs 2 and 3 Q = Q e(1 − e K1t ) Q=

(2)

K 2Q e 2t 1 + K 2Q et

(3) −1

where Q and Qe (mmol·g ) are the adsorption capacity at time t (min) and equilibrium, respectively; K1 (min−1) and K2 [g· (mmol·min) −1] refer to the pseudo first-order and secondorder kinetic rate constants, respectively. The kinetics fitting plots for the adsorption of glyphosate on CAS are described in Figure 3 by using nonlinear regression analysis, and the parameters of the two kinetic models are summarized in Table 1. The values of R2 demonstrated that both the kinetic models Table 1. Pseudo First and Pseudo-second Order Kinetic Parameters for the Adsorption of Glyphosate on CAS (Ci, 1 mmol L−1; pH, 5.64) temperature kinetic models pseudo-first-order

pseudo-secondorder

experiments

293 K

313 K

333 K

Qe/(mmol·g−1) K1/(min−1) R2 Qe/(mmol·g−1)

parameters

0.98 0.15 0.9911 1.06

0.94 0.13 0.9950 1.03

0.86 0.11 0.9958 0.95

K2/[g.(mmol·min) −1 ] R2 Qe‑exp/(mmol·g−1)

0.23

0.19

0.17

0.9991 1.00

0.9965 0.96

0.9961 0.86

Figure 5. Effect of the equilibrium concentration on the adsorption capacities of glyphosate on CAS and adsorption equilibrium isotherms (adsorption time, 60 min; pH, 5.64): □, 293 K; ○, 313 K; Δ, 333 K; ------, Langmuir;......, Freundlich; , Sips.

We can conclude that Sips model is more suitable for the adsorption process than the other two models by comparing the values of R2. Sips isotherm is the combination of both Langmuir and Freundlich isotherms. At low adsorbate concentrations, it actually reduces to a Freundlich isotherm, while at high adsorbate concentrations it shows the characteristic of the Langmuir isotherm with a saturated adsorption capacity. The maximum adsorption capacities of CAS from Sips isotherm fitting is 3.61 mmol·g−1 at 293 K. The Sips model exponent s indicates surface heterogeneity. The deviation of s value from unity will be higher for a highly heterogeneous system. The value of s (1.52, 0.65 and 0.30) indicates that the CAS surface is heterogeneous in nature and this surface heterogeneity is favorable for the adsorption of glyphosate on CAS. The adsorption of glyphosate on other amino functional composites was also observed to obey Sips equation well.30 3.4. Adsorption Thermodynamics of Glyphosate on CAS. The effect of temperature on the adsorption of glyphosate by CAS was investigated, and the results are shown in Figure 6A. It is clear that the adsorption capacity decreased from 0.99 to 0.87 mmol·g−1 with the rising temperature from 293 to 333 K, suggesting the adsorption process was exothermic. Thermodynamic parameters such as the change in Gibbs free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) were determined according to eqs 7 and 8

have performed well and the pseudo second-order kinetic model is more suitable for the adsorption process than the pseudo first-order kinetic model. Chen et al. studied the adsorption kinetics of glyphosate on resin D301 and got similar results.25 It indicates that this adsorption is mainly controlled by the surface control, rather than the adsorbate diffusion, and the chemical reaction is the primary rate-limiting step in the whole adsorption process.26 3.3. Adsorption Isotherms of Glyphosate on CAS. Equilibrium adsorption isotherms involve the interaction between the adsorbent and the solute at different concentrations. Langmuir27 and Freundlich,28 as two-parameter adsorption isotherms, were widely applied to describe the adsorption equilibrium between the liquid and solid phases. Sips is a three-parameter adsorption isotherm model, which may provide more information about the adsorption process than two-parameter isotherm models.29 Their nonlinear equations are expressed as eqs 4−6:

Qe =

Q mCe 1 + KLCe

(4)

Q e = KFCe1/ n

(5)

ln K a 0 = −

1/ s

Qe =

Q m(KSCe)

1/ s

1 + (KSCe) −1

(6)

ΔH ° ΔS ° + RT R

ΔG° = ΔH ° − T ΔS °

−1

(7) (8)

Ka0

where Ce (mmol·L ) and Qe (mmol·g ) are the equilibrium glyphosate concentration and equilibrium adsorption capacity, respectively. Qm (mmol·g−1) is the maximal adsorption

where is the thermodynamic equilibrium constant; R, the gas constant, is equal to 8.314 J·(mol·K) −1, and T is the absolute temperature in Kelvin. Ka0 can be obtained from KL D



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Table 2. Adsorption Isotherm Model Constants and Correlation Coefficients for the Adsorption of Glyphosate on CAS (Adsorption Time, 60 min; pH, 5.64) temperature isotherms Langmuir

Freundlich

Sips

experiments

parameters

293 K

313 K

333 K

Qm/(mmol·g−1) KL/(L·mmol−1) R2 KF/[(mmol·g)·(mmol·L)−1/n] n R2 Qm/(mmol·g−1) KS/(L·mmol−1) s R2 Qm‑exp/(mmol·g−1)

3.27 25.54 0.9696 2.81 4.37 0.9139 3.61 17.61 1.52 0.9852 3.35

3.57 6.13 0.9655 2.47 3.14 0.8317 3.27 8.61 0.65 0.9831 3.27

3.85 2.43 0.8890 2.18 2.49 0.7776 3.14 4.99 0.30 0.9816 3.23

Ka 0 =

KL × 106 mmol/L 169

(9)

The linear fitting plots of ln Ka0 versus 1/T for the adsorption of glyphosate on CAS are shown in Figure 6B, and the calculated values of the thermodynamic parameters are given in Table 3. The negative values of ΔH0 suggest that the adsorption process is exothermic and the increase in ΔG0 with the increase in temperature indicates that the adsorption process was spontaneous and more favorable at low temperature.31 The adsorption process of glyphosate on MnFe2O4graphene hybrid composite11 and modified polystyrene resin12 were also found to be exothermic. Generally, the change of free energy for physical adsorption is smaller than that for chemisorption. The former is in the range of −20 kJ·mol−1 to 0, and the latter is in the range of −80 to −400 kJ·mol−1.32 The range of ΔG for the adsorption of glyphosate on CAS is −30.0 to −20.0 kJ·mol−1. The adsorption of glyphosate on CAS could be considered as a physical adsorption enhanced by the chemical effect. 3.5. Effect of pH on the Adsorption of Glyphosate on CAS. The surface properties and protonation of functional groups on adsorbents are often deeply affected by the pH of the aqueous solution. The influence of the initial pH on the adsorption of glyphosate by CAS is shown in Figure 7. The adsorption capacity increases with the increase of pH first and then decreased. The maximal adsorption capacity (1.00 mmol· g−1) is reached at about pH 6.0. Glyphosate is an amphiprotic compound, containing phosphoric groups, carboxyl groups, and amino groups. According to Figure 4, GpsH2− is the main species at about pH 6.0.21 GpsH2− has the deprotonated phosphonate and carboxylic groups, whereas the nitrogen bears a positive charge. CAS mainly exists in the form of quaternary ammonium salt at low pH (pH < 6).22 At about pH 6, the maximum adsorption capacity is reached due to better electrostatic forces between cationic CAS and anionic glyphosate. The glyphosate is acid form in GpsH3 at lower pH value (pH < 6), so the weak attraction between adsorbent and adsorbate making it difficult to attract. At higher pH value (pH > 6), CAS is tertiary amine forms, and the attraction between adsorbent and adsorbate is also weak.33 3.6. Effect of Ionic Strength on the Adsorption of Glyphosate on CAS. The impact of ionic strength on the adsorption of glyphosate by CAS is shown in Figure 8. The concentration increase of salt, either NaCl or Na2SO4, leads to the decrease of adsorption capacity. The adsorption capacity

Figure 6. (A) Effect of the temperature on the equilibrium adsorption capacity (Ci, 1 mmol·L−1; adsorption time, 60 min; pH, 5.64) and (B) the plot of ln Ka0 versus 1/T for the adsorption of glyphosate on CAS.

using the activity other than concentration. KL (L·mmol−1) in Langmuir isotherm represents the adsorption affinity. The concentration is infinitely close to the activity, and the density of the solution is extremely close to the density of the water (1 g·mL−1) at infinite dilution. To eliminate the unit of the equilibrium constant, the standard thermodynamics equilibrium constant (Ka0) of the adsorption process is given by eq 9 E



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Table 3. Thermodynamics Parameters for the Adsorption of Glyphosate on CAS (Ci, 1 mmol/L; Adsorption Time, 60 min; pH, 5.64) T/K 293 303 313 323 333

KL/(L·mmol−1)

Ka0

25.54 6.13 2.43 1.08 0.39

× × × × ×

1.51 3.63 1.44 6.39 2.31

ΔG/(kJ·mol−1)

ΔH/(kJ·mol−1)

−28.78 −26.95 −25.12 −23.28 −21.45

−82.47

5

10 104 104 103 103

ΔS/[J·(mol·K)

−1

]

−183.24

electrostatic attraction with CAS due to its double charge at the same salt concentration. de Jonge considered that the change of ion strength in solution alters the net charge of glyphosate and the solution pH.33

4. CONCLUSIONS Adsorption characteristics of glyphosate on CAS were thoroughly studied, and CAS exhibits the potential as a costeffective and efficient adsorbent. According to kinetic data, the adsorption equilibrium is attained in about 60 min, and the pseudo second-order kinetics model is well fitted for the adsorption process. Sips isotherm is more suitable than Langmuir and Freundlich isotherms for the adsorption of glyphosate on CAS, and the maximum adsorption capacity is 3.61 mmol·g−1 computed from Sips isotherm fitting at 293 K. The thermodynamics study indicates that the adsorption is a spontaneous and exothermic process. The adsorption mechanism of glyphosate by CAS is a chemical adsorption process combined with physical adsorption. The results also demonstrate that pH and the ion strength have a significant influence on the adsorption capacity. The adsorption capacity reaches a maximum value at pH 6.0. The adsorption capacity decreases with the rising of the ion strength. CAS is a feasible and promising adsorbent to remove glyphosate from aqueous solution.

Figure 7. Effect of pH on the adsorption capacities of glyphosate on CAS (Ci, 1 mmol·L−1; adsorption time, 60 min; temperature, 293 K).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-535-6672176. Fax: +86-535-6696162. E-mail: [email protected]. ORCID

Lei Guo: 0000-0003-4061-2629 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (Nos. 21206066, 21171085, 21204035, and 21304043).

Figure 8. Effect of salt concentration on the adsorption capacities of glyphosate on CAS (Ci, 1 mmol·L−1; adsorption time, 60 min; temperature, 293 K; pH, 5.64): □, NaCl; ○, Na2SO4.

■

more significantly decreases by Na2SO4 salinity than NaCl salinity at the same salt concentration below 1 mol·L−1. At a salt concentration above 1 mol·L−1, the adsorption capacity of glyphosate on CAS is close to zero. The phenomenon may be explained by the newly emerged anions, which interfere with the adsorption process. The adsorption mechanism between glyphosate and CAS is the electrostatic interaction. In the background electrolyte, Cl− or SO42− competes with glyphosate to be adsorbed on the active sites of CAS. With the increase in the salt concentration, Cl− or SO42− takes more and more advantages, leading to the decrease of the glyphosate adsorption. In comparison with Cl−, SO42− shows higher

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Adsorption Characteristics of Glyphosate on Cross-Linked Amino-Starch

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