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Cite This: J. Chem. Eng. Data 2019, 64, 3535−3546

Tyrosine-Immobilized Montmorillonite: An Efficient Adsorbent for Removal of Pb2+ and Cu2+ from Aqueous Solution Yuting Chu, Sidi Zhu, Fengyun Wang,* Wu Lei, Mingzhu Xia,* and Chuan Liao

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School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China ABSTRACT: Herein, a new effective material, tyrosine-functionalized montmorillonite (Tyr-Mt), was prepared and used in the adsorption of Pb2+ and Cu2+ ions. Tyr-Mt is synthesized by modifying sodium montmorillonite (Na-Mt) with L-tyrosine. The X-ray diffraction, ζ-potential, Fourier transform infrared spectroscopy, Brunauer−Emmett−Teller, and thermal analysis results showed that the modifier was successfully inserted into the interlayer of montmorillonite. In single-ion systems, various parameters were thoroughly evaluated, such as the amount of the modifier, pH value, the initial concentration of M2+, and contact time, affecting the adsorption processes in sodium montmorillonite (Na-Mt) and Tyr-Mt composites. The adsorption experiments of Pb2+ and Cu2+ showed that the adsorption effect of Tyr-Mt was better than that of Na-Mt. The balanced adsorption of Pb2+ by Tyr-Mt was 106.21 (higher than that by Na-Mt, 89.08 mg/ g) and of Cu2+ was 28.31 mg/g (higher than that by Na-Mt, 23.93 mg/g). The adsorption model conforms to the Langmuir model, indicating that the active sites on the adsorbent surface are the same. The kinetics conforms to the pseudo-second-order kinetics, and the adsorption process is a spontaneous endothermic process. In the binary co-adsorption system, the adsorption capacity for heavy metal ions was lower than that in a single adsorption system. Tyr-Mt is used in the adsorption of heavy ions due to its high efficiency and low cost. This study will provide a simple and convenient method for the application of other modified clays to achieve high adsorption capacity for toxic cations in soil and aqueous solutions.

1. INTRODUCTION Due to the development of the modern industrial economy, the discharge of heavy metal sewage is becoming more and more serious, which has a great threat to biological health and the environment.1−3 Lead (Pb) is a very toxic heavy metal. The increase of lead load in the human body causes certain damage to the neural behavioral function. Lead poisoning can lead to slow reactions and impaired vision in children.4,5 Copper (Cu) is an essential trace element for the human body, but when the concentration of copper exceeds 5 mg/L, it also causes poisoning, leading to neurological disorders, memory loss, liver damage, and even death.6 Heavy metal ions (M2+) tend to accumulate in organisms because they are not biodegradable. Removal of M2+ (in this paper, we studied Pb2+ and Cu2+) from water is a problem that must be considered by human beings. Many methods are used for removing heavy metal ions, such as membrane separation,7,8 electrocoagulation,9,10 ultrafiltration,11 and others.12,13 The adsorption method has become the preferred method to remove heavy metal ions due to its simple operation and high efficiency. Many materials are used as adsorbents at present, e.g., metal organic framework materials,14,15 marine algae,16 nanocomposite hydrogels,17,18 nanofibers,19 and clay minerals.20 Among them, clay minerals are widely used because of their low cost and good adsorption effect. Montmorillonite is used to adsorb heavy metals due to its good cation exchange capacity and large specific surface area. © 2019 American Chemical Society

The modification of montmorillonite can improve the layer spacing, change the surface hydrophilicity into hydrophobicity, and improve its ability to absorb heavy metal ions and organic pollutants.21,22 Nowadays, many researchers are studying how to modify montmorillonite. Hu et al.23 used chitosan-saturated montmorillonite to adsorb Cd(II), Pb(II), and Cu(II). They found that the adsorption effect of modified montmorillonite on heavy metal ions was improved. Similar to common methods such as chemical precipitation and ion exchange, conventional adsorption also has secondary pollution problems (such as the dissolution of heavy metal hydroxides in acid rain, the re-release of heavy metal ions after the degradation of the organic adsorbent/complexing agent by environmental microorganisms, etc.). We selected a modifier with chelating ability to modify montmorillonite, and the adsorbed heavy metal can be locked in the interlayer of montmorillonite. In this study, we used a green and environmentally friendly modifier, tyrosine, to modify montmorillonite. The as-prepared material was used for adsorbing the heavy metal ions (Pb2+ and Cu2+; referred to as M2+ in this study). X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, thermal analysis (TG/DTG), Brunauer−Emmett−Teller (BET) analysis, and ζ-potential measurements were performed to study the structure of materials. In a single adsorption Received: April 8, 2019 Accepted: June 19, 2019 Published: July 2, 2019 3535

DOI: 10.1021/acs.jced.9b00304 J. Chem. Eng. Data 2019, 64, 3535−3546

Journal of Chemical & Engineering Data

Article

Table 1. CAS Registry Number, Supplier, Grade, and Purity of the Chemicals component

CAS

suppliers (China)

grade

purity (%)

sodium montmorillonite L-tyrosine (C9H11NO3) lead nitrate [Pb(NO3)2] copper nitrate trihydrate [Cu(NO3)2·3H2O] hydrochloric acid (HCl) sodium hydroxide (NaOH) sodium pyrophosphate (Na4P2O7·10H2O) ammonium chloride (NH4Cl) ethanol (C2H5OH) calcium chloride (CaCl2)

85049-30-5 60-18-4 10099-74-8 10031-43-3 7647-01-0 1310-73-2 13472-36-1 12125-02-9 64-17-5 10043-52-4

Wancheng Co. Ltd. (Heishan, Liaoning) Shanghai Sinopharm Group Shanghai Sinopharm Group Shanghai Sinopharm Group Luoyang Chemical Reagent Factory Shanghai Shiyi Chemicals Co. Ltd. Cdkelong Chemicals Co. Ltd. Cdkelong Chemicals Co. Ltd. Cdkelong Chemicals Co. Ltd. Cdkelong Chemicals Co. Ltd.

analytical analytical analytical analytical analytical analytical analytical analytical analytical analytical

>95.0 ≥99.0 ≥99.0 ≥99.0 36.0−38.0 ≥96.0 99 ≥99.5 ≥99.7 96.0

powder was seal-preserved. According to the amount of tyrosine added in the preparation process, the samples were labeled as Tyr-Mt-0.1, Tyr-Mt-0.2, Tyr-Mt-0.6, Tyr-Mt-1.0, and Tyr-Mt-1.2. 2.3. Preparation of Pb 2+ Solutions and Cu 2+ Solutions. A total of 1598.5 mg of Pb(NO3)2 was weighed and predissolved with 2% dilute nitric acid, and the constant volume was adjusted to 1 L. A standard reserve solution of 1000 mg/L Pb2+ was prepared. Moreover, 3801.73 mg of Cu(NO3)2·3H2O was weighed and predissolved with 2% dilute nitric acid, and the volume was adjusted to 1 L. The Cu2+ standard storage solution of 1000 mg/L was prepared. We prepared 50−600 mg/L Pb2+ solution according to different dilution ratios; the 25−180 mg/L Cu2+ solutions were prepared by the same method. 2.4. Characterization. The diffraction patterns of the prepared materials were recorded with a Bruker D8 X-ray diffractometer (λCuKα = 1.5406 Å, 40 kV, 40 mA, 2θ = 4−40°, scanning speed = 2°/min). Fourier transform infrared spectroscopy (FT-IR) was performed using a Nicolet IS-10 FT-IR spectrometer. Thermogravimetric analysis was carried out using an STA 449 F3 Jupiter thermogravimetric analyzer. The temperature was in the range of 313−1173 K under a flowing nitrogen atmosphere. The ζ-potential-measuring instrument involved in this project was nano-zs90 model equipment produced by the British company Malvern. Lead and copper concentration decays were determined by a WA 2081 flame atomic absorption spectrophotometer (Hlntrely Instrument, Beijing). 2.5. Batch Adsorption Experiments. The adsorption capacity [qe (mg/g)] of Tyr-Mt was compared with that of NaMt by intermittent adsorption experiments of M2+. The effects of dosage of the modifier, initial M2+ concentration, pH value, temperature, and contact time on the modification effect were studied. By comparing the 15 °C under the condition of adsorption ability, pH = 5, can get the best dosage of modifier. Add 0.1 M NaOH or HCl solution, change the pH value from 1 to 5, and determine the pH parameter. The pH of the solution was determined using a pH meter (WIGGENS PHT 810). Under the condition of 15 °C and pH = 5, the concentration of Pb 2+ changes from 100 to 500 mg/L and Cu 2+ concentration changes from 25 to 125 mg/L, determine the initial metal ion concentration of effective dose. Also, 25−55 °C temperature is implemented. Add 0.1 g of adsorbent to 100 mL of 300 mg/L Pb 2+ or 75 mg/L Cu 2+ solution, determination of the effect of contact time under 15 °C, time changes ranging from 2 to 120 min to get the best reaction time. In the binary adsorption system, each sample

system, the parameters of the amount of the modifier added, the initial concentration of M2+, pH value, temperature, and contact time were tested. The binary co-adsorption systems were also studied for Pb2+ and Cu2+ under different concentrations, and the adsorption capacities were compared with the values in a single-component system. In addition, the mechanism of the adsorption was studied.

2. MATERIALS AND EXPERIMENTAL METHODS 2.1. Materials. The CAS Registry Numbers, suppliers, grades, and purities of the chemicals in this study are listed in Table 1. The cation exchange capacity (CEC) of the clay was 89.93 mmol/100 g and was determined by the ammonium chloride−ethanol method.24 2.2. Preparation of Tyrosine-Immobilized Montmorillonite. The procedure for the preparation of Na-Mt is as follows: 20 g of the purchased sodium montmorillonite was added to 1000 mL of deionized water, stirred at 15 °C for 1 h, and left standing for 10 min, and then the upper float was discarded and the remaining was washed with deionized water. The above steps were repeated three times, and the residual suspension liquid was filtered under vacuum. The sample was dried at 100 °C for 24 h. The product was ground in an agate mortar and passed through 200-mesh screens before being used. The pretreated sodium montmorillonite is called Na-Mt. The procedure for the preparation of Tyr-Mts is as follows: Different quantities of L-tyrosine (Figure 1) were weighed,

Figure 1. Structural diagram of tyrosine.

added into 200 mL of deionized water, and stirred until the modifier was fully dissolved. The solution was prepared for further use. In this part of the experiment, only the amount of L-tyrosine was changed, and the weights were 32.61 mg (0.1 CEC), 65.23 mg (0.2 CEC), 195.68 mg (0.6 CEC), 326.14 mg (1.0 CEC), and 391.37 mg (1.2 CEC). The prepared Na-Mt (2 g) was poured into the prepared tyrosine solution and stirred and mixed at 15 °C, a few drops of 0.1 M HCl solution were added to adjust the pH to 5, and the contents were sealed and stirred for 24 h. After the reaction was completed, the suspension was vacuumed and filtered using a Buchner funnel. The filter cake was washed with deionized water several times to ensure no residual modifier was left. Then, the cake was dried at 85 °C for 24 h in the oven. The dried solid content was ground and passed through a 200-mesh sieve. The sieved 3536



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Figure 2. XRD patterns of (a) Na-Mt and Tyr-Mt with different modifier dosages and (b) tyrosine at 4−40°.

contained a mixture of Pb2+ and Cu2+ with different concentrations. After adsorption, the residual M2+ concentration of the supernatant was ensured by centrifugation with an atomic absorption spectrophotometer (AAS). Adsorption capacity can be calculated by the following formula qe = (C0 − Ce)

V m

(1)

where Ce (mg/L) refers to the equilibrium M2+ concentration and C0 (mg/L) refers to the initial M2+ concentration. m (g) is the quantity of the adsorbent used (Na-Mt or Tyr-Mt), and V (L) represents the volume of the solution. To reduce systematic errors, all experiments in this study were repeated three times, and the standard deviation was controlled within ±3%.

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. XRD Analysis. Figure 2a shows the XRD patterns of Na-Mt and Tyr-Mt with different dosages of the modifier. The diffraction peak position at 2θ = 7.16° and the layer spacing d = 1.23 nm are obvious for the Na-Mt layer spacing distribution range.25 It can be observed that the diffraction peak began to move to the left and the layer spacing increased after the Na-Mt was modified by tyrosine, which proves the feasibility of tyrosine as a modifier. However, new diffraction peaks appeared in Tyr-Mt-1.0 and Tyr-Mt-1.2, with 2θ = 18.1 and 20.4°, respectively (Figure 2b) because some of the L-tyrosine did not dissolve in the process of the preparation of Tyr-Mt-1.0 and Tyr-Mt-1.2. The number of cations that could be exchanged between layers decreased when the amount of tyrosine was further increased; the change of the layer spacing of Tyr-Mt began to level off, and finally, the layer spacing increased to only 1.53 nm (Tyr-Mt-1.2). 3.1.2. FT-IR Analysis. FT-IR spectra of Na-Mt and Tyr-Mt0.6 were recorded between 4000 and 550 cm−1. As seen in Figure 3, the peak at 3607 cm−1 is attributed to the stretching vibration of the structural hydroxyl group of Al−OH and the broadband characterization at 3404 cm−1 is attributed to the vibration of hydrogen bonds of water molecules adsorbed within the interlayer space of Na-Mt. The peak at 1611 cm−1 is due to the H−O−H bending vibration of water molecules.26 The peak appearing at 1111 cm−1 is ascribed to the stretching vibration of Si−O outside of the plane and the peak at 984 cm−1 is due to stretching vibration of Si−O in the plane, which are the typical bands of the silicate structure.27 The peak at 3199 cm−1 is due to the C−H stretching vibration peaks of the benzene ring. Multiple peaks appear at 600−1600 cm−1 due to

Figure 3. FT-IR spectra of Na-Mt, tyrosine, and Tyr-Mt-0.6 recorded between 4000 and 550 cm−1.

bending vibrations of −CC−H of the aromatic ring.28 The characteristic peaks of Tyr-Mt-0.6 not only contained the characteristic peaks of tyrosine but also showed the characteristic adsorption bands of Na-Mt. This indicated that tyrosine was successfully modified on Na-Mt. 3.1.3. TG/DTG Analysis. The TG/DTG curves of Na-Mt and Tyr-Mt-0.6 are shown in Figure 4. There was a significant weight loss due to the evaporation of water molecules between the montmorillonite layers within the range of 346−445 K, and the DTG curve of Na-Mt showed a weight loss of 6.1% at 445 K. Tyr-Mt-0.6 started to show obvious weight loss before 528 K, where tyrosine began to decompose itself, and the weight

Figure 4. TG/DTG curves of Na-Mt and Tyr-Mt-0.6 at 313−1173 K. 3537



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the modified montmorillonite, resulting in a decrease in the average pore diameter. This series of numerical changes indicated that the modification of tyrosine successfully affected the micropore structure of montmorillonite, and it occupied some of the pores of the montmorillonite. This is similar to other material studies.36,37 3.1.5. ζ-Potential Analysis. ζ-Potential variation trends of Na-Mt, tyrosine, and Tyr-Mt-0.6 were tested under different pH conditions. As shown in Figure 6, there were a large

loss rate of this segment was 7.6%. The mass fraction of the modifier dosage is about 0.98% (195.68 mg/2 g), and the weight loss of Tyr-Mt-0.6 is 1.5% more than that of Na-Mt, mainly including the amount of the modifier and excess water molecules.29,30 In the range of 589−922 K, a large segment of weight loss (3.8%) occurred in Na-Mt, which was caused by the removal of the hydroxyl group formed by the basic structure of montmorillonite. At this time, Na-Mt would gradually lose its adsorption capacity. Overall, the weight loss of Tyr-Mt-0.6 was higher due to the presence of the tyrosine modifier. Moreover, the interactions between the modifier and Na-Mt could be revealed by TG/DTG curves: the weight losses between 373 and 473 K are caused by the physically adsorbed modifiers, whereas those between 473 and 673 K are related to the electrostatically intercalated modifiers.21,31 3.1.4. N2 Adsorption−Desorption Isotherm Analysis. Figure 5 shows the N2 adsorption−desorption isotherms of

Figure 6. ζ-Potential values of Na-Mt, tyrosine, and Tyr-Mt-0.6 at different pH values.

number of −OH on the surface of montmorillonite in the pH range of 2−10, which made ζ-potentials of both Na-Mt and Tyr-Mt-0.6 negative. The ζ-potential became positive for tyrosine when the pH < 4. Comparing ζ-potential trends before and after the modification of montmorillonite, the value of ζ-potential of Tyr-Mt-0.6 changes was between that of Tyr and Na-Mt, this is because the charge on the Tyr surface neutralizes the negative charge on the Na-Mt surface. 3.2. Adsorption Studies. 3.2.1. Effect of Histidine Dose on Tyrosine Efficiency. Figure 7 shows the adsorption curves

Figure 5. N2 adsorption−desorption isotherms of Na-Mt and Tyr-Mt0.6.

Na-Mt and Tyr-Mt-0.6. According to IUPAC classification criteria, the adsorption−desorption isotherms of Na-Mt and Tyr-Mt-0.6 are all summarized in type IV isotherms. Meanwhile, the isothermal curves all showed an H3-type hysteresis ring. According to the definition, it was proved that the material had a mesoporous structure. The specific surface area (SBET), total pore volume (Vt), and average pore radius (Da) of the samples were obtained through the calculation of the BET curve, and the detailed data are shown in Table 2. Table 2. Specific Surface Area (SBET), Total Pore Volume (Vt), and Average Pore Radius (Da) of Na-Mt and Tyr-Mt0.6 sample

SBET (m2/g)

Vt (cm3/g)

Da (nm)

Na-Mt Tyr-Mt-0.6

130.88 133.82

0.17 0.16

4.96 2.38

Figure 7. Adsorption curves of Tyr-Mt with different tyrosine contents.

Tyr-Mt-0.6 has specific surface area and total pore volume similar to those of Na-Mt. In most cases, the surface area of organo-Mts is decreased compared with the raw Na-Mt due to the coverage of surface active sites by the modifier.32−34 After modification, the specific surface area and pore volume of TyrMt-0.6 increase, which is mainly caused by the increase of intercalation of tyrosine.35 However, Tyr-Mt-0.6 has a smaller average micropore radius. This phenomenon may be due to a large number of micropores with small diameter produced by

of Tyr-Mt with different tyrosine contents on Pb2+ and Cu2+ (C0 = 100 mg/L). The adsorption capacity of Tyr-Mt to different ions was similar, that is, the adsorption capacity of Tyr-Mt to Pb2+ increased from 58.0 mg/g (Tyr-Mt-0.2) to 64.77 mg/g (Tyr-Mt-0.6) when the tyrosine content was low. The influence of the addition of the modifier on its adsorption property began to weaken when the concentration of tyrosine 3538



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Figure 8. Effect of solution pH and C0 on (a) Pb2+ and (b) Cu2+ adsorption.

Table 3. Comparison of the Maximum Adsorption Capacities of Different Adsorbents for the Removal of Pb2+ and Cu2+ Pb2+

Cu2+

adsorbent

qm (mg/g)

references

adsorbent

qm (mg/g)

references

Mt-TOA Ca-montmorillonite paper sludge redox polymer sepiolite Arg-Mt Na-Mt Tyr-Mt-0.6

33.10 13.65 103.50 21.99 30.50 144.93 101.11 115.65

5 38 40 41 43 45 this work this work

Mt-TOA pristine natural zeolite polydopamine-treated zeolite base-treated black tea leaf power tree fern Arg-Mt Na-Mt Tyr-Mt-0.6

303.951 14.93 28.58 43.18 11.70 31.43 28.04 30.50

6 39 39 42 44 45 this work this work

Figure 9. Effect of reaction time on the adsorption capacity for (a) Pb2+ and (b) Cu2+.

metal ions. In addition, the hydrated surface of Tyr-Mt-0.6 is protonated and excess hydrogen ions and heavy metal ions competitively adsorb to occupy the active site when the pH value is low. The adsorption capacity of Tyr-Mt-0.6 also began to increase with the increase of C0 of Pb2+ and Cu2+ as can be seen from the general trend of each curve. The qe (mg/g) greatly improved (Figure 8a) until C0 of Pb2+ reached 300 mg/ L and then increased slowly. Also, the qe (mg/g) greatly improved (Figure 8b) until C0 of Cu2+ reached 75 mg/L and then increased slowly. The changing trend is mainly due to the following reasons: First, there were a large number of active sites at the beginning of the adsorption reaction, and adsorbents occupied the adsorption sites rapidly. With the extension of time, adsorbents gradually occupied the adsorption active sites. M2+ needs to overcome a large mass transfer resistance for further adsorption, resulting in a slow adsorption rate. Finally, because the active site of the adsorbent is basically occupied and the adsorption has been saturated,

in the solution reached a certain level (0.8−1.2 CEC). The adsorption of Cu2+ showed the same trend. This is because when the amount of the modifier increases, it is not completely dissolved in water. The amount of the modifier does not significantly contribute to the increase of the layer spacing but can occupy the active site, leading to steric hindrance, thus affecting the adsorption of heavy metal ions. To sum up, the optimal preparation conditions for tyrosine-modified montmorillonite were as follows: the optimum amount of tyrosine was 195.68 mg (0.6 CEC), and the synthetic material was called Tyr-Mt-0.6. 3.2.2. Effects of pH Value and Initial M2+ Concentration. Figure 8 shows the adsorption capacity curve of Tyr-Mt-0.6 under different adsorption conditions. The qe (mg/g) increases with the increase of pH value from 1 to 5 at the same initial M2+ concentration. Combined with ζ-potential analysis, it can be seen that with the increase of pH value, the more negative ζ-potential facilitates the electrostatic adsorption of heavy 3539



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Table 4. Related Parameters of Different Isotherms for M2+ Adsorbed on Na-Mt and Tyr-Mt-0.6 Pb2+

Cu2+

isotherm

parameters

Na-Mt

Tyr-Mt-0.6

Na-Mt

Tyr-Mt-0.6

Langmuir

qmax (mg/g) KL (L/mg) RL R2 KF (L/mg) n R2 qm (mg/g) β E R2 a b R2

101.63 0.024 0.066−0.457 0.996a 18.60 3.62 0.879 88.92 236.92 0.046 0.944 16.86 −2.03 0.768

125.47 0.029 0.054−0.407 0.999a 28.37 4.10 0.944 101.11 15.48 0.18 0.750 26.26 −5.49 0.799

27.68 0.064 0.111−0.384 0.975a 5.63 3.07 0.977 23.09 16.18 0.18 0.771 −2.07 0.63 0.716

41.22 0.030 0.213−0.575 0.995a 3.24 1.97 0.949 28.09 20.15 0.16 0.840 5.29 −2.21 0.641

Freundlich

D−R

BET

a

Bold entries indicate better correlation coefficient.

M2+ is no longer adsorbed, and at this time, the adsorption equilibrium is reached. The saturated extent of adsorption of Tyr-Mt-0.6 for Pb2+ reached nearly 115.65 mg/g (Figure 8a, C0 = 500 mg/L) and for Cu2+ reached nearly 30.50 mg/g (Figure 8b, C0 = 150 mg/L). Table 3 reports the maximum uptake of different adsorbents compared with Tyr-Mt-0.6 for the separation of M2+. Tyr-Mt-0.6 had a better adsorption effect on Pb2+ and Cu2+. 3.2.3. Effect of Contact Time on Adsorption Properties. As can be seen from the variation trends of the two curves in Figure 9a, the adsorption rates of Na-Mt and Tyr-Mt-0.6 were relatively high at the beginning of the Pb2+ adsorption process (t = 2−12 min) but significantly decreased at the middle stage of the adsorption process (t = 12−20 min), and finally, adsorption equilibrium appeared almost at about t = 30 min. In Figure 9b, the trend of the curve also shows that the adsorption increased significantly in the initial 10 min, but at the middle stage (t = 10−15 min), the adsorption rate began to decrease, and finally, the adsorption equilibrium appeared at about t = 20 min. In this study, four points in Figure 9 were selected as the adsorption equilibrium points of Na-Mt and Tyr-Mt-0.6 at this concentration. Pb2+ adsorption equilibrium points were t = 30 min, qe = 106.21 mg/g (Tyr-Mt-0.6) and t = 30 min, qe = 89.08 mg/g (Na-Mt), and Cu2+ adsorption equilibrium points were t = 20 min, qe = 28.31 mg/g (Tyr-Mt0.6) and t = 20 min, qe = 23.93 mg/g (Na-Mt). By comparing the adsorption capacities of Na-Mt and Tyr-Mt-0.6 at the adsorption equilibrium, it can be known that the latter shows better adsorption performance, which is mainly because the modification process improves the montmorillonite interval. In addition, the adsorption contact time was set at 60 min to ensure the equilibrium of the adsorption process in subsequent experiments. 3.2.4. Studies on Adsorption Isotherms. The linearization expressions of the adsorption models of Langmuir, Freundlich, D−R, and BET isotherm equations (eqs 2−5, respectively) are as follows46−50 qe = qmax KLCe/(1 + KLCe)

(2)

ln qe = ln Ce/n + ln KF

(3)

ln qe = ln qm − βε

2

Ce b − 1Ce 1 = + qe(C0 − Ce) ab C0 ab RL =

1 1 + KLC0

1 zyz ji ε = RT lnjjj1 + z j Ce zz{ k

E=

1 2β

(5)

(6)

(7)

(8)

where qe (mg/g) represents the equilibrium adsorption quantity of Na-Mt or Tyr-Mt-0.6, Ce (mg/L) refers to the equilibrium M2+ concentration in solution, KL (L/mg) is the Langmuir constant, and qmax (mg/g) is the maximum adsorption capacity of the adsorbent. The related adsorption process is favorable where the RL value is between 0 and 1. KF (L/g) corresponds to the Freundlich constant. n (dimensionless) in the Freundlich equation refers to the degree of nonlinearity. In the D−R equation, (mol2 kJ−2) is a constant (its value is related to the average adsorption free energy) and the value ε is the Bollani potential, which can be calculated by eq 7. R (J mol−1 K−1) represents the molar constant of the gas, and T (K) is the absolute temperature. The E value of adsorption, that is, the free energy, can be calculated by eq 8. In the BET equation, a and b are BET constants. The related parameters are listed in Table 4. The adsorption data fitted better with the Langmuir isotherm than other isotherms based on coefficient R2. This means that the adsorption sites are identical in essence. The adsorption process was favorable because the values of RL (Table 4) were between 0 and 1, within the scope of our study. 3.2.5. Thermodynamic Parameters of Adsorption. At the 15−55 °C range, determination of M2+ adsorption on Na-Mt and Tyr-Mt-0.6 on M2+ was carried out. Thermodynamic parameters of enthalpy (ΔH°), Gibbs free energy (ΔG°), and entropy (ΔS°) were calculated by eqs 9−11, respectively ΔG° = −RT ln K

(4)

ΔG° = ΔH ° − T ΔS 3540

(9) (10) DOI: 10.1021/acs.jced.9b00304 J. Chem. Eng. Data 2019, 64, 3535−3546


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Figure 10. Plots of ln K vs 1/T for the estimation of thermodynamic parameters for the adsorption of (a) Pb2+ and (b) Cu2+ on Na-Mt and TyrMt-0.6.

Table 5. Thermodynamic Parameters for the Adsorption of M2+ on Na-Mt and Tyr-Mt-0.6 ΔG° (kJ/mol) adsorbate 2+

Pb

Cu2+

adsorbent

ΔH° (kJ/mol)

Na-Mt Tyr-Mt-0.6 Na-Mt Tyr-Mt-0.6

4.19 4.23 1.91 1.95

−1

ΔS° (J mol

−1

K )

15.07 15.21 7.48 7.95

288 K

308 K

328 K

R2

−0.18 −3.55 −0.24 −0.31

−0.44 −3.80 −0.40 −0.34

−0.77 −4.05 −0.54 −0.36

0.9897 0.9916 0.9955 0.9936

Table 6. Linear Fitting Parameters of the Pseudo-First-Order Model and Pseudo-Second-Order Model of Pb2+ and Cu2+ Adsorption Processes by Na-Mt and Tyr-Mt-0.6 adsorbate 2+

Pb

adsorbent

qe‑exp (mg/g)

Na-Mt Tyr-Mt-0.6

Cu2+

89.08 106.21

Na-Mt

23.93

Tyr-Mt-0.6

29.27

qecal (mg/g)

kinetics model pseudo-first-order model pseudo-second-order model pseudo-first-order model pseudo-second-order model pseudo-first-order model pseudo-second-order model pseudo-first-order model pseudo-second-order model

48.15 89.36 54.44 109.17 9.80 23.48 9.75 28.20

R2

k 1.97 8.61 2.23 8.17 2.06 1.10 2.07 5.63

× × × × × × × ×

−1

10 10−3 10−1 10−3 10−1 10−1 10−1 10−2

0.9925 0.9997a 0.9775 0.9945a 0.9806 0.9992a 0.9693 0.9970a

a

Bold entries indicate better correlation coefficient.

ln K =

−ΔG° −ΔH ° ΔS° = + RT RT R

ji 1 zy t 1 = + jjjj zzzzt 2 jq z qt k 2q2 k 2{

(11)

The qe and Ce values were calculated by the Langmuir equation, and K was obtained by qe/Ce. K referred to the partition coefficient, and ΔG° could be calculated by eq 9 at different temperatures. A straight line in Figure 10 gives ln K versus 1/T for the adsorption, which provided intercepts ΔS°/ R and slope −ΔH°/R. Table 5 shows the negative ΔG° values of Tyr-Mt-0.6 and Na-Mt, illustrating that the adsorption processes were spontaneous. The positive value of enthalpy change (ΔH°) corresponds to the endothermic adsorption. The entropy change (ΔS°) provides no obvious change, which indicates that the disorder degree of the adsorption system has a little change and the layered structure of montmorillonite is basically intact. 3.2.6. Adsorption Kinetics. Adsorption kinetics has two classical kinetic models: pseudo-first-order model and pseudosecond-order model; the formulas are as follows51,52 ln(q1 − qt ) = ln q1 − k1t

(13)

where q1 (mg/g) is the adsorbed amount of M2+ at equilibrium for the pseudo-first-order model and q2 (mg/g) is that for the pseudo-second-order model. qt (mg/g) is the adsorbed amount at time “t” in each model. k1 (min−1) represents the rate constant of the pseudo-first-order model, and k2 (g mg−1 min−1) is for the pseudo-second-order model. Table 6 lists the linear fitting parameters of the pseudo-first-order model and pseudo-second-order model of Pb2+ and Cu2+ adsorption processes. The adsorption model conforms to the pseudosecond-order kinetic model due to the greatest values of the correlation coefficient (R2), which indicates that the whole adsorption process is controlled by chemical adsorption. This is consistent with other studies.53 In addition, the smaller the k2 value, the stronger the affinity of the adsorbent site to M2+. The adsorption process is faster, and the adsorption process is more favorable. The k2 of Tyr-Mt-0.6 after adsorbing Pb2+ is 8.17 × 10−3, which is lower than that of Na-Mt (8.61 × 10−3). In the same case, the k2 of Tyr-Mt-0.6 after adsorbing Cu2+ is 5.63 × 10−2, which is lower than that of Na-Mt (1.10 × 10−1).

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Figure 11. Adsorption capacities of Pb2+ and Cu2+ by Tyr-Mt-0.6 in binary-system (a) solutions containing 300 mg/L Pb2+ and 25−125 mg/L Cu2+ and (b) solutions containing 75 mg/L Cu2+ and 100−500 mg/L Pb2+.

Figure 12. Desorption rates of (a) Pb2+ and (b) Cu2+ adsorption on Na-Mt and Tyr-Mt-0.6.

These results suggest that Pb2+ and Cu2+ preferred to adsorb on Tyr-Mt-0.6 relative to Na-Mt. 3.3. Co-adsorption of Pb2+ and Cu2+. We prepared two groups for experiments by mixing Pb2+ with Cu2+ for the coadsorption study. One group contained 300 mg/L Pb2+ in each solution, and C0 values of Cu2+ were 25, 50, 75, 100, and 125 mg/L. Another group contained 75 mg/L Cu2+ in each solution, and C0 values of Pb2+ were 100, 200, 300, 400, and 500 mg/L. The obtained adsorption experimental data were used to draw a three-dimensional surface in Figure 11. It can be found from the surface variation trend in Figure 11a that when the content of Cu2+ in the solution increases, the adsorption capacity for Pb2+ begins to decrease, whereas with the increase of Pb2+ in the solution, the adsorption capacity for Pb2+ begins to rise, and the qe variation trend for Cu2+ in Figure 11b is similar. It is clear that Pb2+ and Cu2+ also have competitive adsorption in the binary system. In Figure 11a, the qe (Pb2+) of the solution is 111.3 mg/g when the concentration of Pb2+ is 400 mg/L, and then when the solution becomes a co-adsorption system (400 mg/L Pb2+, 25 mg/L Cu2+), its qe decreases to 78.5 mg/g, with a decrease in the rate of 29.5%. In Figure 11b, the qe for Cu2+ in solution is 28.3 mg/ g when the concentration is 125 mg/L, and then when the solution becomes a binary system (125 mg/L Cu2+, 100 mg/L Pb2+), its qe decreases to 16.5 mg/g, with a decrease in the rate of 41.7%. In the binary system, the Pb2+ and Cu2+ would compete to occupy the adsorption active sites. As a result, the adsorption capacity in the binary system is obviously lower than that in the single system.

3.4. Desorption. The adsorption stability of adsorbent materials is an important parameter, which can provide important information for the study of the adsorption mechanism. In this study, hydrochloric acid (HCl) solution was used as an eluent, and to prepare hydrochloric acid solution with pH = 3−6, Na-Mt (0.1 g) and Tyr-Mt-0.6 were put into 100 mL of 300 mg/L Pb2+ solution or 100 mL of 75 mg/L Cu2+ solution and then stirred for 2 h at pH = 5 and 15 °C. All of the experiments were carried out four times. After the reaction, the samples were filtered through the filter membrane, and the filter cakes were put into 100 mL of solution with the pH value of 3−6 and stirred for 3 days. The solution was then centrifuged, and the concentration of the supernatant M2+ was determined by AAS. The desorption rate was calculated using the following equation desorption ratio = VDC D/mqe × 100%

(14)

where CD (mg/L) is the M2+ concentration in the desorption solution and VD (L) is the volume of the desorption solution. m (g) is the quantity of the adsorbent and, in this case, is 0.1 g. qe (mg/g) is the amount of M2+ adsorbed onto the adsorbent in the adsorption. Figure 12 shows the performance of Tyr-Mt-0.6 and Na-Mt after desorption. The desorption rate of Tyr-Mt-0.6 was lower than that of Na-Mt, whereas the locking ability of Tyr-Mt-0.6 on M2+ was significantly better than that of Na-Mt, indicating that Tyr-Mt-0.6 is more difficult to be eluted by acid rain in nature (pH = 4−5.5).54 Under weak acidic conditions (pH > 3542



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Figure 13. XRD patterns of (a) Na-Mt and Na-Mt after adsorbing Pb2+ and Cu2+ and (b) Tyr-Mt-0.6 and Tyr-Mt-0.6 after adsorbing Pb2+ and Cu2+

4), Tyr-Mt-0.6 can fix metal ions and form complexes with amino groups in tyrosine, locking them in the montmorillonite layer.

4. MECHANISM 4.1. XRD Analysis of Heavy Metal Ions Adsorbed by Tyr-Mt-0.6. The XRD patterns of Na-Mt and Tyr-Mt-0.6 with the samples after adsorbing Pb2+ and Cu2+ are shown in Figure 13. Carbonate is an impurity of montmorillonite, and ion exchange occurred between Na-Mt and M2+. The peak of PbCO3 occurred (Figure 13a), but copper carbonate is unstable and instead Cu(OH)2 occurred. The interlayer space of Na-Mt is wider after the adsorption of Pb2+, and no obvious change is seen after adsorbing Cu2+. This is because the ionic radius of Pb2+ (1.19 A) is larger than that of the Na+ (0.95 A) between the layers; both the ionic radii are also larger than the ionic radius of Cu2+ (0.72 A). However, no peaks of PbCO3 and Cu(OH)2 were found in Figure 13b, indicating that the adsorption of Tyr-Mt-0.6 on Pb2+ and Cu2+ was not an ion-exchange process. The interlayer space of Tyr-Mt-0.6 becomes smaller after the adsorption of M2+. The results showed that M2+ may adsorb on the surface of Tyr-Mt-0.6, and it is also possible that amino acids and M2+ form complexes and enter the interlayer of Tyr-Mt-0.6. However, the arrangement of substances between the layers may change, for example, from inclined to flat, resulting in slightly reduced interlayer spacing. 4.2. FT-IR Analysis of Tyr-Mt-0.6 after Adsorption of Heavy Metal Ions. Figure 14 shows the FT-IR spectra of TyrMt-0.6 and Tyr-Mt-0.6 after adsorbing Pb2+ and Cu2+. It can be found that after the adsorption of Pb2+ and Cu2+ the characteristic peak is only offset, which indicates that the adsorption of heavy metal ions does not damage the structure of montmorillonite. The characteristic peaks of the benzene ring (600−1600 cm−1) begin to show a gentle trend, which is because the amount of the modifier is not very large. After adsorption of heavy metal ions, tyrosine undergoes a complexation reaction with heavy metal ions, leading to a less obvious peak. 4.3. Scanning Electron Microscopy (SEM) Analysis of Heavy Metal Ions Adsorbed by Tyr-Mt-0.6. Figure 15 shows the SEM image of Na-Mt and Tyr-Mt-0.6 after adsorption of heavy metal ions. It can be seen in the figure that when Na-Mt adsorbs heavy metal ions the layered structure still exists, only with the occurrence of marginal scaling, indicating that heavy metal ions exchange with the

Figure 14. FT-IR spectra of Tyr-Mt-0.6 and Tyr-Mt-0.6 after adsorbing Pb2+ and Cu2+.

interlayer sodium ions of sodium montmorillonite and increase the interlayer spacing. There is also a layered structure when Tyr-Mt-0.6 adsorbs M2+ because M2+ accesses the interlayer and interacts with the modifier. This does not break the interlayer structure of the material. 4.4. Possible Mechanisms. The possible mechanisms for the adsorption of M2+ on Na-Mt and Tyr-Mt-0.6 are shown in Figure 16. According to the above analysis, we believe that the main mechanism of Na-Mt adsorption of heavy metal ions is ion exchange, and the mechanism of Tyr-Mt-0.6 adsorption of heavy metal ions is that tyrosine and heavy metal ions first form complexes, which are then locked in the montmorillonite layer. After absorbing M2+, the layered structures of the two materials were not destroyed.

5. CONCLUSIONS Herein, Tyr-Mt was successfully prepared by modifying Na-Mt with L-tyrosine and applied to adsorb Pb2+ and Cu2+ in aqueous solutions. XRD, FT-IR, TG/DTG, and ζ-potential analyses confirmed that tyrosine has successfully modified NaMt. The results of batch adsorption experiments showed that Tyr-Mt had better adsorption capacity for M2+ than Na-Mt under the same conditions. The optimal addition amount was 3543



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Figure 15. SEM images of Na-Mt, Tyr-Mt-0.6, and the materials after adsorbing Pb2+ and Cu2+: (a) Na-Mt, (b) Tyr-Mt-0.6, (c) Na-Mt after adsorbing Pb2+, (d) Tyr-Mt-0.6 after adsorbing Pb2+, (e) Na-Mt after adsorbing Cu2+, and (f) Tyr-Mt-0.6 after adsorbing Cu2+.

0.6 CEC. The higher the pH value, the better the adsorption effect. The adsorption capacity increased with the increasing initial M2+ concentration and then reached the equilibrium. The adsorption isotherm simulation was studied, and both NaMt and Tyr-Mt-0.6 in M2+ adsorption processes were consistent with the Langmuir model. The simulation of the kinetic equation shows that the adsorption process conforms to the pseudo-second-order kinetic characteristics. The adsorption effects of two metal ion mixtures were studied, and it was found that the adsorption effect decreased when

another metal ion was present. In this paper, an environmentally friendly modifier, tyrosine, was used to modify montmorillonite. The adsorption capacity for heavy metal ions has been found to increase after modification. However, tyrosine is a small molecule and there is no obvious increase in the layer spacing of montmorillonite after the modification. In addition, the modifier also occupies certain active sites and influences the adsorption capacity. In the future study, we should explore more modifiers with chelating abilities, which can obviously increase the interlamellar spacing. 3544



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(8) Efome, J. E.; Rana, D.; Matsuura, T.; Lan, C. Q. Experiment and modeling for flux and permeate concentration of heavy metal ion in adsorptive membrane filtration using a metal-organic framework incorporated nanofibrous membrane. Chem. Eng. J. 2018, 352, 737− 744. (9) Lu, J.; Li, Y.; Yin, M.; Ma, X.; Lin, S. Removing heavy metal ions with continuous aluminum electrocoagulation: A study on back mixing and utilization rate of electro-generated Al ions. Chem. Eng. J. 2015, 267, 86−92. (10) Al-Shannag, M.; Al-Qodah, Z.; Bani-Melhem, K.; Qtaishat, M. R.; Alkasrawi, M. Heavy metal ions removal from metal plating wastewater using electrocoagulation: Kinetic study and process performance. Chem. Eng. J. 2015, 260, 749−756. (11) Huang, J.; Yuan, F.; Zeng, G.; Li, X.; Gu, Y.; Shi, L.; Liu, W.; Shi, Y. Influence of pH on heavy metal speciation and removal from wastewater using micellar-enhanced ultrafiltration. Chemosphere 2017, 173, 199−206. (12) Li, Z.; Chen, J.; Guo, H.; Fan, X.; Wen, Z.; Yeh, M. H.; Yu, C.; Cao, X.; Wang, Z. L. Triboelectrification-Enabled Self-Powered Detection and Removal of Heavy Metal Ions in Wastewater. Adv. Mater. 2016, 28, 2983−2991. (13) Hao, J.; Ji, L.; Li, C.; Hu, C.; Wu, K. Rapid, efficient and economic removal of organic dyes and heavy metals from wastewater by zinc-induced in-situ reduction and precipitation of graphene oxide. J. Taiwan Inst. Chem. Eng. 2018, 88, 137−145. (14) Peng, Y.; Huang, H.; Zhang, Y.; Kang, C.; Chen, S.; Song, L.; Liu, D.; Zhong, C. A versatile MOF-based trap for heavy metal ion capture and dispersion. Nat. Commun. 2018, 9, No. 187. (15) Li, J.; Wang, X.; Zhao, G.; Chen, C.; Chai, Z.; Alsaedi, A.; Hayat, T.; Wang, X. Metal−organic framework-based materials: superior adsorbents for the capture of toxic and radioactive metal ions. Chem. Soc. Rev. 2018, 47, 2322−2356. (16) Foroutan, R.; Esmaeili, H.; Abbasi, M.; Rezakazemi, M.; Mesbah, M. Adsorption behavior of Cu(II) and Co(II) using chemically modified marine algae. Environ. Technol. 2018, 39, 2792−2800. (17) Ma, J.; Liu, Y.; Ali, O.; Wei, Y.; Zhang, S.; Zhang, Y.; Cai, T.; Liu, C.; Luo, S. Fast adsorption of heavy metal ions by waste cotton fabrics based double network hydrogel and influencing factors insight. J. Hazard. Mater. 2018, 344, 1034−1042. (18) Dil, N. N.; Sadeghi, M. Free radical synthesis of nanosilver/ gelatin-poly (acrylic acid) nanocomposite hydrogels employed for antibacterial activity and removal of Cu(II) metal ions. J. Hazard. Mater. 2018, 351, 38−53. (19) Morillo Martín, D.; Faccini, M.; García, M. A.; Amantia, D. Highly efficient removal of heavy metal ions from polluted water using ion-selective polyacrylonitrile nanofibers. J. Environ. Chem. Eng. 2018, 6, 236−245. (20) Sellaoui, L.; Soetaredjo, F. E.; Ismadji, S.; Bonilla-Petriciolet, A.; Belver, C.; Bedia, J.; Ben Lamine, A.; Erto, A. Insights on the statistical physics modeling of the adsorption of Cd2+ and Pb2+ ions on bentonite-chitosan composite in single and binary systems. Chem. Eng. J. 2018, 354, 569−576. (21) Zhu, R.; Chen, Q.; Zhou, Q.; Xi, Y.; Zhu, J.; He, H. Adsorbents based on montmorillonite for contaminant removal from water: A review. Appl. Clay Sci. 2016, 123, 239−258. (22) Ma, L.; Chen, Q.; Zhu, J.; Xi, Y.; He, H.; Zhu, R.; Tao, Q.; Ayoko, G. A. Adsorption of phenol and Cu(II) onto cationic and zwitterionic surfactant modified montmorillonite in single and binary systems. Chem. Eng. J. 2016, 283, 880−888. (23) Hu, C.; Zhu, P.; Cai, M.; Hu, H.; Fu, Q. Comparative adsorption of Pb(II), Cu(II) and Cd(II) on chitosan saturated montmorillonite: Kinetic, thermodynamic and equilibrium studies. Appl. Clay Sci. 2017, 143, 320−326. (24) Doğan, M.; Alkan, M.; Onganer, Y. Adsorption of methylene blue from aqueous solution. Water, Air, Soil Pollut. 2000, 120, 229− 248. (25) Kul, A. R.; Koyuncu, H. Adsorption of Pb(II) ions from aqueous solution by native and activated bentonite: kinetic,

Figure 16. Possible mechanisms for the adsorption of M2+ on Na-Mt and Tyr-Mt-0.6.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.W.). *E-mail: [email protected] (M.X.). ORCID

Fengyun Wang: 0000-0002-2359-9875 Funding

This work was supported by the National Natural Science Foundation of China (Nos. 51572130, 51672134, and 51572121). Notes

The authors declare no competing financial interest.

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