Facile preparation of biosurfactant-functionalized Ti2CTX MXene nanosheets with an enhanced adsorption performance for Pb(II) ions

Journal Pre-proof Facile preparation of biosurfactant-functionalized Ti2CTX MXene nanosheets with an enhanced adsorption performance for Pb(II) ions Shuhao Wang, Yilin Liu, Qiu-Feng Lü, Huiping Zhuang PII:

S0167-7322(19)33996-0

DOI:

https://doi.org/10.1016/j.molliq.2019.111810

Reference:

MOLLIQ 111810

To appear in:

Journal of Molecular Liquids

Received Date: 18 July 2019 Revised Date:

9 September 2019

Accepted Date: 24 September 2019

Please cite this article as: S. Wang, Y. Liu, Q.-F. Lü, H. Zhuang, Facile preparation of biosurfactantfunctionalized Ti2CTX MXene nanosheets with an enhanced adsorption performance for Pb(II) ions, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.111810. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1

Facile preparation of biosurfactant-functionalized Ti2CTX MXene

2

nanosheets with an enhanced adsorption performance for Pb(II) ions

3

Shuhao Wang, Yilin Liu, Qiu-Feng Lü*, Huiping Zhuang

4

Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), College of Materials

5

Science and Engineering, Fuzhou University, 2 Xueyuan Road, Fuzhou 350116, China

＞ 7

Abstract

8

The aim of this study is to improve the adsorption performance for Pb(II) ions of a novel

9

two-dimensional Ti2CTX MXene. Three biosurfactants, chitosan (CS, a cationic surfactant),

10

lignosulfonate (LS, an anionic surfactant) and enzymatic hydrolysis lignin (EHL, a non-ionic

11

surfactant), were used to functionalize the Ti2CTX nanosheets. The samples were characterized by

12

using Field-emission scanning electron microscopy, Fourier infrared spectroscopy, X-ray diffraction

13

and Nitrogen adsorption-desorption isotherms, their adsorption performances for Pb(II) ions were

14

investigated by bath adsorption experiments under various adsorption conditions including different

15

types of biosurfactants, pH value, initial Pb(II) ions concentration, adsorbent concentration, contact

1＞

time and adsorption temperature. Results revealed that the non-ionic surfactant EHL prevents

17

Ti2CTX nanosheets from restacking and introduces active functional groups so that the adsorption

18

performance of the EHL-functionalized Ti2CTX nanosheets are promoted. The EHL-functionalized

19

Ti2CTX displayed a maximum adsorption capacity of 232.9 mg g-1 for Pb(II) ions. Moreover,

* Corresponding author. Tel.: +86 591 22 866 540; fax: +86 591 22 866 539. E-mail address: [email protected]; [email protected] (Q.-F. Lü). 1

1

reaction kinetics, isotherms and thermodynamics studies were analyzed to describe the adsorption

2

behaviors, and the experimental data were fitted well with pseudo-second order model and

3

Frendlich isotherm. The results showed that the biosurfactants-functionalized Ti2CTX shows an

4

improved adsorption performance in removal of Pb(II) ions and has great potential as an

5

environmental adsorption material.

＞

Keywords: Biosurfactants, Ti2CTX MXene, Adsorption, Pb(II) ions

7

1. Introduction

8

Water pollution caused by heavy-metals has directly threatened human beings and

9

environmental safety, which attracts worldwide concern over the last few decades[1-3]. Lead (Pb),

10

one of toxic heavy metals, must be removed from drinking water and wastewater, as its toxicity and

11

non-biodegradable could causes food poisoning, kidney organism and nerve system illnesses

12

easily[4]. Thus, studies on the recovery of lead ions from aqueous solution are necessary to reduce

13

the probability and danger degree that lead ions produce. Several techniques have been used for the

14

removal of Pb(II) ions and other heavy metals from water and wastewater such as membrane

15

separation, adsorption and electrochemical deposition[5]. Among those techniques, adsorption is

1＞

particularly attractive as a method to remove heavy metal ions due to its lower cost and easy-handle

17

design[6]. The adsorbent materials play an important role when adsorption technique is used to deal

18

with contaminants like heavy metals discharged from industries. Researchers have made continuous

19

efforts to develop better and more easily available adsorbent materials for Pb(II) ions (e.g., plant

20

sorbents[7], carbonaceous adsorbents[5, 8], bacteria[9]). 2

1

In recent years, two-dimensional (2D) materials have been one of the most widely studied

2

classes of materials[10]. Many 2D materials possess properties of large specific surface area and

3

abundant surface functional groups, which demonstrate their great potential for water treatment and

4

adsorption of heavy metal ions[11]. 2D transition metal carbides (MXenes) as a new family of

5

two-dimensional materials have garnered great attention owing to their unique physical and

＞

chemical properties[12]. Several MXenes (such as Ti3C2TX and Ti2CTX) have been prepared by

7

etching of certain elements from their layered hexagonal MAX precursors (Ti3AlC2 and Ti2AlC)[12,

8

13]. Reports also showed that the easy availability and multiple active sites on MXenes’ surfaces

9

make them easier to remove heavy metal ions, organic dyes and other pollutants from aqueous

10

solution[1, 4, 14, 15]. Peng et al. reported a chemical exfoliation and alkalization intercalation

11

alk-MXene material, which exhibits an efficient Pb(II) ions uptake performance of 140 mg g-1[4].

12

Recently, Gu et al. also reported an improved adsorption performance of Hg(II) ions via etched

13

Ti3C2TX shell aerogel spheres[14].

14

However, MXenes can be easily oxidized in water but contact with water is inevitable during

15

the adsorption process, which decreased their adsorption active sites and limited their adsorption

1＞

performances[16]. Besides, restacking is an intractable problem in the preparation of 2D materials

17

such as graphene and MXenes[17, 18]. Steps taken to solve this problem is necessary since the

18

restacking of MXenes cause a waste of massive active sites.

19

Biosurfactants, widely known as non-toxic and eco-friendly materials, have been extensively

20

used in absorbents because of their high biocompatibility, low toxicity and good activity[17, 19, 20].

21

Biosurfactants can be classified as cationic, anionic and non-ionic surfactants like their synthetic

22

counterparts[19, 20]. Lately mixtures of biosurfactants and nanomaterials have become promising 3

1

adsorbents for the remediation of heavy metal ions removal[17, 21]. Our previous study also

2

demonstrated that biosurfactants (chitosan and lignosulfonate) can effectively impede the

3

aggregation of polypyrrole and increase the activity points of polypyrrole-based composites[22].

4

In this work, novel biosurfactant-functionalized Ti2CTX MXene nanosheets were prepared, and

5

the biosurfactants are introduced in the preparation of MXene adsorbents for the first time.

＞

Considering the cost performance, the charge of the active groups and easy-accessibility, three types

7

of biosurfactants, chitosan (CS, cationic surfactants), lignosulfonate (LS, anionic surfactants) and

8

enzymatic hydrolysis lignin (EHL, non-ionic surfactants) were chosen. The biosurfactants may not

9

only prevent Ti2CTX nanosheets from oxidation and restacking but also increase the active sites and

10

hydrophilic of Ti2CTX adsorbent. The adsorption performances for Pb(II) ions of the different

11

biosurfactants-functionalized

12

EHL-functionalized Ti2CTX nanosheets (Ti2CTX-EHL) produced in this work exhibited better

13

morphology and enhanced adsorption performances than those of CS-functionalized Ti2CTX

14

nanosheets (Ti2CTX-CS), LS-functionalized Ti2CTX nanosheets (Ti2CTX-LS) and pristine Ti2CTX.

15

Further, reaction kinetics, isotherms and thermodynamics studies were investigated to better

1＞

understand the characteristics of Pb(II) ions removal processes and then a possible adsorption

17

mechanism is proposed.

18

2. Material and methods

19

2.1. Materials

20

Ti2CTX

nanosheets

were

tested,

compared

and

analyzed.

Ti2AlC (500 meshes) was purchased from Laizhou Kaikai Ceramic Materials Co., Ltd. Sodium 4

1

lignosulfonate (LS) and enzymatic hydrolysis lignin (EHL) are purchased from Shandong Shuntong

2

group and Shandong Longlive Bio-technology Co., Ltd. Chitosan (CS), Pb(NO3)2 and other

3

reagents and chemicals are obtained from Chinese Medicine Group Chemical Reagent Co., Ltd.

4

2.2. Preparation of biosurfactant-functionalized Ti2CTX

5

Ti2CTX was successfully synthesized by etching Al from Ti2AlC powder (500 meshes). 1g

＞

Ti2AlC powder was added to a premixed solution of 1g LiF and 10 mL 12 mol L-1 HCl (in 5 min),

7

after which the solution was stirred at 40 °C for 48 h to ensure complete etching of Al[16]. After

8

etching, the solution was centrifuged for 5 min at 3500 rpm. The centrifuge cycle was repeated until

9

a black supernatant was obtained. After that, etched sample was bath sonicated in 100 mL DI water

10

for 2h under the protection of argon atmosphere. Then, the solution was centrifuged at 3500 rpm for

11

1 h and the supernatant, which consists of monolayered Ti2CTX, was collected for further

12

experiments. 0.1 g CS, LS and EHL was dissolved in a 5 mL solution of 1.0 wt% icy acetic acid, DI

13

water and 1.0 wt% ammonia, respectively, to prepare biosurfactant solutions. The biosurfactant

14

solution was added into 50 mL Ti2CTX supernatant, and then the mixture was continuously stirred

15

for 4 h for the solid contact between Ti2CTX nanosheets and biosurfactant. Finally, the mixed

1＞

solutions were dried using a freeze-dryer for 24 h to obtain biosurfactant-functionalized Ti2CTX

17

power. The resulting samples functionalized with CS, LS, and EHL biosurfactants were labeled as

18

Ti2CTX-CS, Ti2CTX-LS, and Ti2CTX-EHL. The preparation process of biosurfactant-functionalized

19

Ti2CTX was showed in Scheme 1.

5

1

2

Scheme 1. The preparation process of biosurfactant-functionalized Ti2CTX.

2.3. Characterizations of biosurfactant-functionalized Ti2CTX

3

Field-emission scanning electron microscopy (FESEM, Carl Zeiss ULTRA 55) were used to

4

view the morphology of biosurfactant-functionalized Ti2CTX. Fourier infrared (FT-IR) spectroscopy

5

was measured using a Nicolet 5700 spectrophotometer to detect the functional groups of the

＞

samples. Wide-angle X-ray diffraction (XRD) were performed using an Ultima III Xray model

7

diffractometer (Rigaku, Japan) at a scanning rate of 5° min-1 in a reflection mode over a 2θ range

8

from 5° to 80° with Cu Kα (k=0.15406 nm) radiation resource. X-ray photoelectron spectroscopy

9

(XPS) spectra was obtained using an X-ray photoelectron spectrometer (ESCALAB 250,

10

Thermo-Fisher Scientific, USA). Zeta-potentials of various biosurfactant-functionalized Ti2CTX at

11

different pH values were recorded by a Zetasizer (NanoZS90 Malvern). Brunauere-Emmette-Teller

12

(BET) specific surface areas and porous size distributions were performed using a Micromeritics

13

3Flex analyzer.

14

2.4. Adsorption experiments

15

Adsorption experiments in this study were carried out by a batch method. Typically, 40 mg ＞

1

power sample (Ti2CTX-CS, Ti2CTX-LS, Ti2CTX-EHL or pristine Ti2CTX) was added in 25 mL 200

2

mg L-1 Pb(II) ions aqueous solution (prepared from Pb(NO3)2) for comparison. The adsorption

3

solution was ultrasonic dispersion for 5 min, and then the adsorption process proceeded at 30 °C for

4

24 h. After reaching equilibrium, the adsorption solution was passed through a glass filter lined with

5

filter paper, and the concentration of Pb(II) after adsorption was analyzed by molar titration[23, 24].

＞

Then, according to the characteristic and adsorption performance, Ti2CTX-EHL was chosen to

7

further investigate the influences of adsorbent concentration, adsorbent temperature, contact time,

8

initial pH value, and initial Pb(II) concentration. The desired pH value was adjusted using 1 mol L-1

9

HNO3 or 1 mol L-1 NaOH aqueous solution. Further, the adsorption kinetics, isotherms and

10

thermodynamics were analyzed based on the data of adsorption experiments. The adsorption

11

capacity (Q) and removal rate (q) were calculated according equations (1) and (2), respectively.

12

Q = (C0-C)×V/m q = (C0-C)/C0×100%

13

(1) (2)

14

Where, Q (mg g-1) is adsorption capacity, C0 (mg L-1) is the initial concentration of Pb(II) ions

15

solution, C (mg L-1) is the concentration of Pb(II) solution after adsorption, V (L) is the volume of

1＞

Pb(II) solution, m (g) is the mass of adsorbent and q (%) is removal rate. Mathematical models and

17

correlative formulas are showed in Supplemental file.

18

3. Results and discussion

19

3.1. Characterization

20

3.1.1. Morphology 7

1

The general morphologies and structures of Ti2CTX and the biosurfactant-functionalized

2

Ti2CTX were characterized by FE-SEM. In Fig. 1, it is very clear that the as-synthesized Ti2CTX and

3

biosurfactant-functionalized Ti2CTX samples possess a typical 2D structure with a few layers like

4

graphene[25]. The dispersion and ductility of Ti2CTX nanosheets was improved when the

5

biosurfactant (CS, LS, or EHL) was introduced into the Ti2CTX nanosheets, especially with the

＞

functionalization of EHL. At a low magnification of the Ti2CTX-EHL images (Fig. S1), Ti2CTX with

7

a great quantity and quality of monolayers can be clearly observed. Besides, the corresponding

8

elemental mapping of Ti2CTX-EHL (Fig. S2) showed that the carbon, titanium, oxygen, fluorine

9

and chlorine elements uniformly distribute on the Ti2CTX-EHL nanosheets. The reason is that

10

nanosheets of 2D nanomaterials can be peeled easier by nonionic surfactants at the same

11

concentration, and the dispersibility of the system can also be enhanced[17]. Although ionic

12

surfactants greatly increase the conductivity of system and interaction between biosurfactants and

13

Ti2CTX, non-ionic biosurfactants (EHL in this case) are easier to prepare Ti2CTX with monolayered

14

structure[4, 17, 26]. 2D nanomaterials adsorbents with monolayered structure can availably improve

15

their adsorption capacities to some extent[27].

8

1

Fig. 1. FE-SEM images of Ti2CTX (a1, a2, a3), Ti2CTX-CS (b1, b2, b3),

2

Ti2CTX-LS (c1, c2, c3) and Ti2CTX-EHL (d1, d2, d3).

3

3.1.2. FT-IR

4

FT-IR spectra of Ti2CTX, Ti2CTX-CS, Ti2CTX-LS and Ti2CTX-EHL were carried out and

5

presented in Fig. 2a. The peaks at 3400 and 1629 cm-1 could be ascribed to the hydrogen-bonded

＞

hydroxyl (—OH O) in Ti2CTX[14]. The vibration of Ti—O may account for the peak at 565 cm-1,

7

which further demonstrate the presence of —OH[28]. Upon functionalization with biosurfactants,

8

the spectra of Ti2CTX-CS and Ti2CTX-LS didn't display obvious changes. Particularly, in the

9

spectrum of Ti2CTX-EHL, peaks at 3124 and 1390 cm-1 corresponded to the stretching of C—H and 9

1

C—O bands of substantially aromatic groups in EHL[29, 30]. The result showed that functional

2

groups of EHL have been introduced into the Ti2CTX system successfully, which may offer more

3

adsorption action sites with high activity[21, 31].

(a)

(b)

Ti2CTX-CS Ti2CTX-LS Ti2CTX-EHL

3200 2400 1600 -1 800 Wavenumber (cm ) 4

5

Ti2CTX

Intensity (a.u.)

Transmittance (a.u.)

Ti2CTX

Ti2CTX-CS Ti2CTX-LS Ti2CTX-EHL

20

40 60 2 Theta (degree)

80

Fig. 2. FT-IR spectra (a) and XRD curves (b) of Ti2CTX, Ti2CTX-CS, Ti2CTX-LS and Ti2CTX-EHL.

3.1.3. XRD

＞

The crystal phase of Ti2CTX, Ti2CTX-CS, Ti2CTX-LS and Ti2CTX-EHL were investigated by

7

XRD (Fig. 2b). The peaks of Ti2CTX appearing at about 7.5° can be indexed to the (002) plane of

8

MXenes, which corresponds to an interlayer spacing of about 1.18 nm[32]. After the addition of

9

biosurfactants, the (002) peaks of the functionalized samples downshifted to lower angles, which

10

demonstrates the increment of interlayer spacing due to the intercalation of small-sized

11

biosurfactants molecules[13, 33]. The interlayer spacing of Ti2CTX-CS, Ti2CTX-LS and

12

Ti2CTX-EHL are of 1.43, 1.55 and 1.47 nm, respectively. The larger interlayer spacing of these

13

samples after functionalization indicated that the biosurfactants have successfully prevented Ti2CTX

14

nanosheets from restacking. 2D materials with a large interlayer distance are normally considered to 10

1

possess excellent performances for adsorption[34].

2

3.1.4. Nitrogen adsorption-desorption isotherms

3

The nitrogen adsorption-desorption isotherms and pore diameter distributions of Ti2CTX and

4

Ti2CTX-EHL were showed in Fig. S3. The BET-specific surface area of Ti2CTX-EHL (22.50 m2 g-1)

5

was larger than that of pristine Ti2CTX (13.3 m2 g-1), indicating more monolayers existed in

＞

Ti2CTX-EHL due to the spacer effect of EHL. Besides, Ti2CTX-EHL displayed larger pore volume

7

and the pore size of Ti2CTX-EHL centered at approximately 4 nm, which demonstrate the

8

introduction of biosurfactants promote the formation of mesoporous structure. The larger surface

9

area and minor-sized mesoporous structure could provide more adsorption active sites, and further

10

improve their adsorption performances[16, 35].

11

3.2. Adsorption performances

12

3.2.1. Effect of different biosurfactant-functionalized Ti2CTX

13

To investigate the adsorption capacities and adsorptivity of Pb(II) ions onto Ti2CTX,

14

Ti2CTX-CS, Ti2CTX-LS and Ti2CTX-EHL, batch experiments were studied and the results of

15

adsorption capacities were showed in Fig. 3. The pH values of Ti2CTX, Ti2CTX-CS, Ti2CTX-LS and

1＞

Ti2CTX-EHL after adsorption declined to 2.74, 2.77, 2.69 and 3.81, respectively, which can be

17

resulted from the released H+ ions during the adsorption process[13]. Clearly, Ti2CTX-EHL possess

18

the largest adsorption capacity (Q) and removal rate (q) than those of other samples, which is up to

19

122.4 mg g-1 and 97.9%, respectively, whereas the adsorption capacities of Ti2CTX, Ti2CTX-CS and

20

Ti2CTX-LS are 81.5, 93.5 and 104.38 mg g-1, respectively. The improved adsorption performances

21

for Pb(II) ions after the functionalization of biosurfactants are attributed to the abundant functional 11

1

groups and active sites of the biosurfactants[31, 36]. CS (a cation surfactant, Table 1) has a

2

tendency to bond with anion Ti2CTX (with groups like —OH) via strong electrostatic attractions,

3

but is easy to become competitors to Pb(II) during the adsorption possess[9]. Comparatively, LS (an

4

anion surfactant, Table 1) is hard to interact on the surface of Ti2CTX[26]. Especially, non-ionic

5

EHL can effectively peel and interact with Ti2CTX, which could maximize the efficiency of

＞

biosurfactant. Thus, Ti2CTX-EHL can be considered as an excellent adsorbent and is used for further

7

adsorption experiments.

100

Q q

90

80

q (%)

Q (mg g-1)

120

80 40 70 0

Ti2CTX Ti2CTX-CS Ti2CTX-LS Ti2CTX-EHL

8

Fig. 3. Adsorption performances of Ti2CTX, Ti2CTX-CS, Ti2CTX-LS and Ti2CTX-EHL for Pb(II)

9

ions.

10

Table 1. Structures and functional groups of CS, LS and EHL. Biosurfactant

Molecular formula

Chitosan (CS)

12

Functional groups

Classification

—NH3+

Cationic

—OH

surfactant

Lignosulfonate (LS)

—SO3-

Anionic

—OH

surfactant

Non-ionic Enzymatic hydrolysis lignin (EHL)

1

—OH

surfactant

3.2.2. Effect of initial pH value of adsorption solution

2

The effect of initial pH value of solution on Pb(II) adsorption by Ti2CTX-EHL was measured in

3

the range of 1-6 because the Pb(II) ions has a tendency to precipitate when pH 6, which results an

4

inaccurate interpretation of adsorption behavior[37, 38]. As shown in Fig. 4a, the adsorption

5

capacities increased with incremental solution pH and reached a maximum value of 122.4 mg g-1 at

＞

the initial pH value of 5. This can be explained by ionization state of the functional groups in the

7

Ti2CTX-EHL sample. At lower pH value, the deprotonation of functional groups on the surface of

8

Ti2CTX-EHL is limited, therefore the ions exchange between the H+ from hydroxyl (such as

9

[Ti-O]-H+) and Pb(II) can be largely reduced[14, 22], which can be further explained by zeta

10

potential analysis. The zeta potentials of Ti2CTX and Ti2CTX-EHL (Fig. 4b) decreased with the

11

increment of pH values and achieved pHZPC at 6.9 and 3.2, respectively, indicating more negative

12

charges were accumulated on the surface of the adsorbent, and which is favorable for adsorption

13

process[39]. Besides, the result also showed that the introduction of biosurfactant EHL decreases

14

the surface potential of Ti2CTX, and a lower potential is more favorable to adsorb cation like Pb(II) 13

ions.

120

Q

100

q

105

Q (mg g )

-1

(b)

90 80

90

70 75 60 1

2

3 4 pH value

5

6

Zeta potential (mV)

(a)

q (%)

1

Ti2CTX Ti2CTX-EHL

40 20

zpc6.9

0 -20

zpc3.2

-40 2

4 6 pH value

8

10

2

Fig. 4. (a) Effect of initial pH value of solution on Pb(II) ions adsorption onto Ti2CTX-EHL,

3

and (b) Zeta potentials of Ti2CTX and Ti2CTX-EHL at different pH values.

4

3.2.3. Effect of contact time

5

Adsorption time allowed in the adsorption system is an important factor that influences the

＞

binding of metal ions to the adsorbent. The adsorption capacity and removal rate of Pb(II) from

7

aqueous solution versus contact time were carried out using Ti2CTX-EHL adsorbent and showed in

8

Fig. 5a. The adsorption capacity and removal rate of Ti2CTX-EHL for Pb(II) improved rapidly

9

within the first 10 min, and about 90% removal rate was achieved. The reason for such an

10

adsorption behavior may be that there are substantial vacant active sites on the surface of

11

adsorbents[40]. After that, the adsorption rate decreases with the contact time prolonged. Finally,

12

the adsorption reached equilibrium in 24 h, and the adsorption capacity and removal rate were up to

13

122.4 mg g-1 and 97.9%, respectively.

14

(b)

100

80

-1

90 Q 80

q

100

0

0 200 400 600 800 1000 -1 Initial Pb (II) ions concentration (mg L )

25

(c)

100 300

150 120 100

200

100

Q q

80

q (%)

-1

-1

90

(d) Q (mg g )

5 10 15 20 Contact time (h)

60 40

70 0

Q q

70

0 1 2 3 -1 Absorbent dosage concentration (g L )

90

Q q

q (%)

100 90

Q (mg g )

100 200

Q (mg g )

110

q (%)

-1

Q (mg g )

120

q (%)

(a)

80

60 30 40 50 Adsorption temperature (°C)

1

Fig. 5. Effect of contact time (a), initial Pb(II) ions concentration (b), adsorbent dosage

2

concentration (c) and adsorption temperature (d) on Pb(II) ions adsorption onto Ti2CTX-EHL.

3

3.2.4. Effect of initial Pb(II) ions concentration

4

The removal of Pb(II) from aqueous solutions was carried out in the initial Pb(II) ions

5

concentrations range from 50 to 1000 mg L-1. It was clearly observed in Fig. 5b that the adsorption

＞

capacity increases with the increment of initial Pb(II) ions concentration. However, the removal rate

7

increases with the initial Pb(II) ions concentration below 200 mg g-1 and then decreases within the

8

concentration from 200 to 1000 mg g-1. This is because with a limited active sites on adsorbents,

9

superfluous Pb(II) ions cannot be absorbed, which could lead to an abatement of Pb(II) ions

10

removal rate[3]. Thus, other experiments in this study were carried out at the initial Pb(II) ions 15

1

concentration of 200 mg L-1 to achieve higher removal rate of Pb(II) ions. The adsorption attained

2

saturation with a maximum adsorption capacity of 232.9 mg g-1 when the initial Pb(II) ions

3

concentration is 1000 mg L-1.

4

3.2.5. Effect of adsorbent dosage

5

The effect of adsorbent dosage on adsorption of Pb(II) ions was investigated by varying the

＞

Ti2CTX-EHL concentration from 0.4 to 3.2 g L-1 (Fig. 5c). The removal rate increased from 66.6%

7

to 98.9%, whereas the adsorption capacity decreased from 306.8 to 61.8 mg g-1. Besides, the

8

removal rate showed a rapid increment at the beginning (from 0.4 to 1.6 g L-1), which may be due to

9

the increased surface area and active sites because of the augment of adsorbent [41]. However,

10

excess adsorbent (more than 1.6 g L-1) can decrease the effective surface area, which cause a waste

11

of active sites and in turn depress the adsorption capacity[13, 42]. Thus, the optimum adsorbent

12

concentration of 1.6 g L-1 was picked up for other experiments.

13

3.2.6. Effect of adsorption temperature

14

The effect of temperature on Pb(II) adsorption by Ti2CTX-EHL was tested at 303, 313 and 323

15

K. Fig. 5d showed that the adsorption capacity and removal rate decrease as the adsorption

1＞

temperature increase, demonstrating relatively low temperatures benefit the removal of Pb(II).

17

Although higher temperatures may promote dehydration process of Pb(II) ion sheaths and improve

18

mass transfer rate, the interaction between the adsorbate and the adsorption sites will be weakened,

19

which results in a decrease of adsorption performances[43, 44]. Besides, MXene is a kind of

20

two-dimensional materials with a strong tendency to oxidize, especially when it was exposed to

21

high temperature aqueous solution, leading to a further reduction of functional group adsorption

22

sites on the surface of Ti2CTX-EHL (such as —OH)[45]. 1＞

1

3.3. Mathematical models

2

3.3.1. Adsorption kinetics

3

To further investigate the process of Pb(II) ions adsorption on Ti2CTX-EHL, kinetic data was

4

analyzed by pseudo-first order and pseudo-second kinetic models[16, 46]. Based on the results of

5

effect of contact time, the values of correlation coefficients are listed in Table 2 and lines obtained

＞

from the plot with linear regression are presented in Fig. S4. It can be seen that better coefficient R2

7

value (0.9999) is obtained by the pseudo-second order model. Besides, the adsorptive capacity (Qe,

8

122.7 mg g-1) calculated by the pseudo-second order model is close to experimental value (122.4

9

mg g-1). Thus, it can be inferred that the removal of Pb(II) ions by the Ti2CTX-EHL is mainly

10

controlled by the chemical adsorption including ions exchange reaction and chelation [14, 21].

11

Table 2. Kinetics parameters for Pb(II) ions adsorption onto Ti2CTX-EHL. Parameter

Value

Parameter

Pseudo-first order model

12

3.3.2

Value

Pseudo-second order model

Qe [mg g-1]

8.75

Qe [mg g-1]

122.72

k1 [h-1]

0.26

k2 [g mg-1 h-1]

0.11

R2

0.7176

R2

0.9999

Adsorption isotherm

13

In addition, the Langmuir, Freundlich and Temkin models were used to investigate the

14

adsorption mechanism of Pb(II) ions adsorption onto Ti2CTX-EHL (Table 3, Fig. S5). The

15

adsorption data based on the initial Pb(II) ions concentration can be better fitted by the Langmuir

1＞

isotherm (coefficient of 0.9997) than Freundlich and Temkin isotherm (coefficient of 0.7418 and

17

1

0.9011). The Langmuir isotherm model is normally used for the adsorption process controlled by

2

monolayers adsorption on the adsorbent surface[42]. Additionally, all adsorption active sites are

3

equivalent and evenly distributed on the surface of adsorbents in Langmuir model[3]. Therefore, it

4

can be concluded that Pb(II) ions are uniformly adsorbed on the surface of Ti2CTX-EHL and exist as

5

a form of monolayer.

＞

Table 3. Isotherm parameters for Pb(II) ions adsorption onto Ti2CTX-EHL. Parameter

Value

Parameter

Langmuir isotherm model

7

Value

Parameter

Freundlich isotherm model

Value

Temkin isotherm model

Qe [mg g-1]

250

KF [(mg g-1) (L mg-1)1/n]

43.15

KT (L g-1)

2.97

KL [L mg-1]

0.037

1/n

0.289

B

33.758

R2

0.9997

R2

0.7418

R2

0.9011

3.3.3

Adsorption thermodynamic

8

The thermodynamic parameters and fitted line for the adsorption were showed in Table 4 and

9

Fig. S6. The calculated ∆H° and ∆S° were -74.14 kJ mol-1 and -216.48 J mol-1 K-1. The negative

10

value of ∆H° indicated that the adsorption process was exothermic, high enthalpy changes ( 40 kJ

11

mol-1) also confirmed that the processes of Pb(II) onto Ti2CTX-EHL is chemisorption[22]. The

12

Gibb's free energies (∆G) at all temperatures were negative and increased along with the

13

temperature increasing, demonstrating the reaction is spontaneous and favorable for relatively low

14

temperature[35].

15

Table 4. Thermodynamic parameters for Pb(II) ions adsorption onto Ti2CTX-EHL. T (K)

Thermodynamic parameters ∆G (kJ mol-1)

303

∆Нº (kJ mol-1)

-8.55 18

∆Sº (J mol-1 K-1)

1

3.4

313

-6.38

323

-4.21

-74.14

-216.48

Adsorption mechanism

2

The Ti2CTX-EHL shows an outstanding adsorption performance for Pb(II) ions among the

3

reported MXene-based adsorbents and some previous Pb(II) adsorbents (Table S1, S2) and based on

4

the above analysis, a possible adsorption mechanism of Ti2CTX-EHL can be summarized as follows

5

and was shown in Scheme 2. Firstly, Ti2CTX nanosheets with tons of adsorption active sites are

＞

expected to have strong affinity with Pb(II) ions. Then Pb(II) ions are adsorbed onto the

7

Ti2CTX-EHL by chelation and ions exchange with functional groups and polycyclic structures on

8

the Ti2CTX nanosheets and EHL. Also, from XPS spectrum (Fig. S6) it can be observed that

9

Ti2CTX-EHL show obvious Pb 4f peaks (Pb 4f 7/2 at 136.1 eV and Pb 4f 5/2 at 140.7 eV). Specially,

10

Pb 4f 7/2 with a lower energy level compared to Pb(NO3)2 (139.7 eV) indicated the presence of

11

strong affinity of newly formed Pb—O groups, further demonstrating the interaction between

12

functional groups and Pb(II) ions[47]. Thus, Pb(II) ions cover the surface of Ti2CTX-EHL and reach

13

adsorption equilibrium after fill the active sites. Chemisorption plays a leading role during the

14

adsorption process.

19

1

2

Scheme 2. The Adsorption mechanism for Pb(II) ions on Ti2CTX-EHL.

4 Conclusions

3

In summary, a new type of Ti2CTX-based adsorbent was synthesized by the functionalization of

4

inexpensive and renewable biosurfactants. Among various biosurfactant-functionalized Ti2CTX, the

5

Ti2CTX-EHL adsorbent displays a better structure and extraordinary adsorption performance for

＞

Pb(II) ions (as high as 232.9 mg g-1). Because non-ionic surfactant EHL improves the preparation of

7

Ti2CTX nanosheets, which substantially increase adsorption active sites and ions exchange

8

efficiency. Besides, EHL also enhances synergistic effect of the functional groups and chelation to

9

Pb(II) ions so that the chemisorption has also been enhanced. This work paves a facile way to

10

prepare efficient and environmental Ti2CTX-based adsorption materials for the removal of Pb(II)

11

ions from waste water.

20

References

1

[1] A. Shahzad, K. Rasool, W. Miran, M. Nawaz, J. Jang, K.A. Mahmoud, D.S. Lee,

2

Two-dimensional Ti3C2Tx MXene nanosheets for efficient copper removal from water, ACS

3

Sustainable Chem. Eng. 5 (2017) 11481-11488.

4

[2] H. Yousefzadeh, A. Salarian, H.S. Kalal, Study of Pb (II) adsorption from aqueous solutions by

5

TiO2 functionalized with hydroxide ethyl aniline (PHEA/n-TiO2), J. Mol. Liq. 263 (2018) 294-302.

＞

[3] J. Yang, J.-X. Wu, Q.-F. Lü, T.-T. Lin, Facile preparation of lignosulfonate–graphene

7

oxide–polyaniline ternary nanocomposite as an effective adsorbent for Pb (II) ions, ACS

8

Sustainable Chem. Eng. 2 (2014) 1203-1211.

9

[4] Q. Peng, J. Guo, Q. Zhang, J. Xiang, B. Liu, A. Zhou, R. Liu, Y. Tian, Unique lead adsorption

10

behavior of activated hydroxyl group in two-dimensional titanium carbide, J. Am. Chem. Soc. 136

11

(2014) 4113-4116.

12

[5] S. Mishra, N. Verma, Surface ion imprinting-mediated carbon nanofiber-grafted highly porous

13

polymeric beads: Synthesis and application towards selective removal of aqueous Pb(II), Chem.

14

Eng. J. 313 (2017) 1142-1151.

15

[6] L. Sun, H. Yu, B. Fugetsu, Graphene oxide adsorption enhanced by in situ reduction with

1＞

sodium hydrosulfite to remove acridine orange from aqueous solution, J. Hazard. Mater. 203-204

17

(2012) 101-110.

18

[7] S.A. Koksharov, S.V. Aleeva, O.V. Lepilova, Description of adsorption interactions of lead ions

19

with functional groups of pectin-containing substances, J. Mol. Liq. 283 (2019) 606-616.

20

[8] X. Huang, M. Pan, The highly efficient adsorption of Pb (II) on graphene oxides: A process 21

1

combined by batch experiments and modeling techniques, J. Mol. Liq. 215 (2016) 410-416.

2

[9] W. Huang, Z.-m. Liu, Biosorption of Cd (II)/Pb (II) from aqueous solution by

3

biosurfactant-producing bacteria: isotherm kinetic characteristic and mechanism studies, Colloids

4

Surf., B 105 (2013) 113-119.

5

[10] S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutierrez, T.F. Heinz, S.S. Hong, J.

＞

Huang, A.F. Ismach, E. Johnston-Halperin, M. Kuno, V.V. Plashnitsa, R.D. Robinson, R.S. Ruoff, S.

7

Salahuddin, J. Shan, L. Shi, M.G. Spencer, M. Terrones, W. Windl, J.E. Goldberger, Progress,

8

challenges, and opportunities in two-dimensional materials beyond graphene, ACS Nano 7 (2013)

9

2898-926.

10

[11] J. Zhu, E. Ha, G. Zhao, Y. Zhou, D. Huang, G. Yue, L. Hu, N. Sun, Y. Wang, L.Y.S. Lee, Recent

11

advance in MXenes: A promising 2D material for catalysis, sensor and chemical adsorption, Coord.

12

Chem. Rev. 352 (2017) 306-327.

13

[12] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M.W.

14

Barsoum, Twodimensional nanocrystals produced by exfoliation of Ti3AlC2, Adv. Mater. 23 (2011)

15

4248-4253.

1＞

[13] S. Li, L. Wang, J. Peng, M. Zhai, W. Shi, Efficient thorium (IV) removal by two-dimensional

17

Ti2CTx MXene from aqueous solution, Chem. Eng. J. 366 (2019) 192–199.

18

[14] A. Shahzad, M. Nawaz, M. Moztahida, J. Jang, K. Tahir, J. Kim, Y. Lim, V.S. Vassiliadis, S.H.

19

Woo, D.S. Lee, Ti3C2Tx MXene core-shell spheres for ultrahigh removal of mercuric ions, Chem.

20

Eng. J. 368 (2019) 400-408.

21

[15] C. Peng, P. Wei, X. Chen, Y. Zhang, F. Zhu, Y. Cao, H. Wang, H. Yu, F. Peng, A hydrothermal

22

etching route to synthesis of 2D MXene (Ti3C2, Nb2C): Enhanced exfoliation and improved 22

1

adsorption performance, Ceram. Int. 44 (2018) 18886-18893.

2

[16] L. Wang, H. Song, L.-Y. Yuan, Z. Li, P. Zhang, J.K. Gibson, L. Zheng, H. Wang, Z. Chai, W.-Q.

3

Shi, Effective removal of anionic Re (VII) by surface-modified Ti2CTx MXene nanocomposites:

4

Implications for Tc (VII) sequestration, Environ. Sci. Technol. 53 (2019) 3739−3747.

5

[17] F. Zhang, S. Li, Q. Zhang, J. Liu, S. Zeng, M. Liu, D. Sun, Adsorption of different types of

＞

surfactants on graphene oxide, J. Mol. Liq. 276 (2019) 338-346.

7

[18] M. Zhu, Y. Huang, Q. Deng, J. Zhou, Z. Pei, Q. Xue, Y. Huang, Z. Wang, H. Li, Q. Huang,

8

Highly flexible, freestanding supercapacitor electrode with enhanced performance obtained by

9

hybridizing polypyrrole chains with MXene, Adv. Energy Mater. 6 (2016) 1600969.

10

[19] W. Peng, L. Chang, P. Li, G. Han, Y. Huang, Y. Cao, An overview on the surfactants used in ion

11

flotation, J. Mol. Liq. 286 (2019) 110955.

12

[20] H. Saleem, P. Pal, M.A. Haija, F. Banat, Regeneration and reuse of bio-surfactant to produce

13

colloidal gas aphrons for heavy metal ions removal using single and multistage cascade flotation, J.

14

Cleaner Prod. 217 (2019) 493-502.

15

[21] Z. Liu, Z. Li, H. Zhong, G. Zeng, Y. Liang, M. Chen, Z. Wu, Y. Zhou, M. Yu, B. Shao, Recent

1＞

advances in the environmental applications of biosurfactant saponins: A review, J. Environ. Chem.

17

Eng. 5 (2017) 6030-6038.

18

[22] J. Zhou, Q.-F. Lü, J.-J. Luo, Efficient removal of organic dyes from aqueous solution by rapid

19

adsorption onto polypyrrole–based composites, J. Cleaner Prod. 167 (2017) 739-748.

20

[23] Q.F. Lü, M.R. Huang, X.G. Li, Synthesis and heavymetalion sorption of pure

21

sulfophenylenediamine copolymer nanoparticles with intrinsic conductivity and stability, Chem. -

22

Eur. J. 13 (2007) 6009-6018. 23

1

[24] T. Suzuki, D. Tiwari, A. Hioki, Precise chelatometric titrations of zinc, cadmium, and lead with

2

molecular spectroscopy, Anal. Sci. 23 (2007) 1215-1220.

3

[25] Y. Yoon, M. Lee, S.K. Kim, G. Bae, W. Song, S. Myung, J. Lim, S.S. Lee, T. Zyung, K.S. An, A

4

strategy for synthesis of carbon nitride induced chemically doped 2D MXene for highperformance

5

supercapacitor electrodes, Adv. Energy Mater. 8 (2018) 1703173.

＞

[26] C.A. Voigt, M. Ghidiu, V. Natu, M.W. Barsoum, Anion adsorption, Ti3C2Tz MXene multilayers,

7

and their effect on claylike swelling, J. Phys. Chem. C 122 (2018) 23172-23179.

8

[27] S. Chowdhury, R. Balasubramanian, Recent advances in the use of graphene-family

9

nanoadsorbents for removal of toxic pollutants from wastewater, Adv. Colloid Interface Sci. 204

10

(2014) 35-56.

11

[28] Q. Xue, H. Zhang, M. Zhu, Z. Pei, H. Li, Z. Wang, Y. Huang, Y. Huang, Q. Deng, J. Zhou,

12

Photoluminescent Ti3C2 MXene quantum dots for multicolor cellular imaging, Adv. Mater. 29 (2017)

13

1604847.

14

[29] Z. Ma, S. Li, G. Fang, N. Patil, N. Yan, Modification of chemical reactivity of enzymatic

15

hydrolysis lignin by ultrasound treatment in dilute alkaline solutions, Int. J. Biol. Macromol. 93

1＞

(2016) 1279-1284.

17

[30] W. Qiao, S. Li, G. Guo, S. Han, S. Ren, Y. Ma, Synthesis and characterization of

18

phenol-formaldehyde resin using enzymatic hydrolysis lignin, J. Ind. Eng. Chem. 21 (2015)

19

1417-1422.

20

[31] M. Nadeem, A. Mahmood, S. Shahid, S. Shah, A. Khalid, G. McKay, Sorption of lead from

21

aqueous solution by chemically modified carbon adsorbents, J. Hazard. Mater. 138 (2006) 604-613.

22

[32] L. Wang, H. Song, L. Yuan, Z. Li, Y. Zhang, J.K. Gibson, L. Zheng, Z. Chai, W. Shi, Efficient 24

1

U (VI) reduction and sequestration by Ti2CTx MXene, Environ. Sci. Technol. 52 (2018)

2

10748-10756.

3

[33] A. VahidMohammadi, J. Moncada, H. Chen, E. Kayali, J. Orangi, C.A. Carrero, M. Beidaghi,

4

Thick and freestanding MXene/PANI pseudocapacitive electrodes with ultrahigh specific

5

capacitance, J. Mater. Chem. A 6 (2018) 22123-22133.

＞

[34] J. Luo, W. Zhang, H. Yuan, C. Jin, L. Zhang, H. Huang, C. Liang, Y. Xia, J. Zhang, Y. Gan, X.

7

Tao, Pillared structure design of MXene with ultralarge interlayer spacing for high-performance

8

lithium-Ion capacitors, ACS Nano 11 (2017) 2459-2469.

9

[35] W. Mu, S. Du, X. Li, Q. Yu, H. Wei, Y. Yang, S. Peng, Removal of radioactive palladium based

10

on novel 2D titanium carbides, Chem. Eng. J. 358 (2019) 283-290.

11

[36] N. Guo, M. Li, X. Sun, F. Wang, R. Yang, Enzymatic hydrolysis lignin derived hierarchical

12

porous carbon for supercapacitors in ionic liquids with high power and energy densities, Green

13

Chem. 19 (2017) 2595-2602.

14

[37] R. Herrera-Urbina, D. Fuerstenau, The effect of Pb (II) species, pH and dissolved carbonate on

15

the zeta potential at the quartz/aqueous solution interface, Colloids Surf., A 98 (1995) 25-33.

1＞

[38] G. Issabayeva, M.K. Aroua, N.M.N. Sulaiman, Removal of lead from aqueous solutions on

17

palm shell activated carbon, Bioresour. Technol. 97 (2006) 2350-2355.

18

[39] T. Xu, R. Fu, L. Yan, A new insight into the adsorption of bovine serum albumin onto porous

19

polyethylene membrane by zeta potential measurements, FTIR analyses, and AFM observations, J.

20

Colloid Interface Sci. 262 (2003) 342-350.

21

[40] Q.-F. Lü, Z.-K. Huang, B. Liu, X. Cheng, Preparation and heavy metal ions biosorption of graft

22

copolymers from enzymatic hydrolysis lignin and amino acids, Bioresour. Technol. 104 (2012) 25

1

111-118.

2

[41] Y. Li, J. Sun, Q. Du, L. Zhang, X. Yang, S. Wu, Y. Xia, Z. Wang, L. Xia, A. Cao, Mechanical

3

and dye adsorption properties of graphene oxide/chitosan composite fibers prepared by wet

4

spinning, Carbohydr. Polym. 102 (2014) 755-761.

5

[42] Y. Jin, C. Zeng, Q.-F. Lü, Y. Yu, Efficient adsorption of methylene blue and lead ions in

＞

aqueous solutions by 5-sulfosalicylic acid modified lignin, Int. J. Biol. Macromol. 123 (2019)

7

50-58.

8

[43] D. Zhao, X. Yang, H. Zhang, C. Chen, X. Wang, Effect of environmental conditions on Pb(II)

9

adsorption on β-MnO2, Chem. Eng. J. 164 (2010) 49-55.

10

[44] X. Zhang, Q. Lin, S. Luo, K. Ruan, K. Peng, Preparation of novel oxidized mesoporous carbon

11

with excellent adsorption performance for removal of malachite green and lead ion, Appl. Surf. Sci.

12

442 (2018) 322-331.

13

[45] C.J. Zhang, S. Pinilla, N. McEvoy, C.P. Cullen, B. Anasori, E. Long, S.-H. Park, A.

14

Seral-Ascaso, A. Shmeliov, D. Krishnan, C. Morant, X. Liu, G.S. Duesberg, Y. Gogotsi, V. Nicolosi,

15

Oxidation Stability of Colloidal Two-Dimensional Titanium Carbides (MXenes), Chem. Mater. 29

1＞

(2017) 4848-4856.

17

[46] K. Yang, Z. Lou, R. Fu, J. Zhou, J. Xu, S.A. Baig, X. Xu, Multiwalled carbon nanotubes

18

incorporated with or without amino groups for aqueous Pb (II) removal: comparison and

19

mechanism study, J. Mol. Liq. 260 (2018) 149-158.

20

[47] Y. Xia, T. Yang, N. Zhu, D. Li, Z. Chen, Q. Lang, Z. Liu, W. Jiao, Enhanced adsorption of Pb(II)

21

onto modified hydrochar: Modeling and mechanism analysis, Bioresour. Technol. 288 (2019).

2＞

• A novel 2D adsorbent Ti2CTX was synthesized and functionalized by biosurfactants. • EHL in Ti2CTX-EHL promoted to form Ti2CTX nanosheets. • EHL provided a large number of adsorption active sites. • EHL as a non-ionic biosurfactant can increase the adsorption capacity of Ti2CTX.