Surface-Tailored Nanocellulose Aerogels with Thiol-Functional

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Surface-tailored nanocellulose aerogels with thiol-functional moieties for highly efficient and selective removal of Hg (II) ions from water Biyao Geng, Haiying Wang, Shuai Wu, Jing Ru, Congcong Tong, Yufei Chen, Hongzhi Liu, Shengchun Wu, and XuYing Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03188 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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ACS Sustainable Chemistry & Engineering

1

Surface-tailored

nanocellulose

aerogels

with

2

thiol-functional moieties for highly efficient and

3

selective removal of Hg(II) ions from water

4

Biyao Geng†a,b, Haiying Wang†c, Shuai Wu†c, Jing Rua,b, Congcong Tonga,b, Yufei

5

Chena,b, Hongzhi Liu*a,b, Shengchun Wu*c, Xuying Liud

6

a

7

High-efficiency Utilization, No. 666 Wusu Street, Lin’an District, Hangzhou 311300,

8

China

9

b

Zhejiang Provincial Collaborative Innovation Center for Bamboo Resources and

School of Engineering, Zhejiang Agriculture & Forestry University, No. 666 Wusu

10

Street, Lin’an District, Hangzhou 311300, China

11

c

12

University, No. 666 Wusu Street, Lin’an District, Hangzhou 311300, China

13

d

14

Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

School of Environmental and Resource Sciences, Zhejiang Agriculture & Forestry

International Center for Young Scientists (ICYS), National Institute for Materials

15 16 17

*Corresponding authors

18

To whom the correspondence should be addressed, Tel: +86-0571-63746552, Fax:

19

+86-0571-63730919

20

E-mail: [email protected] (Prof. Hongzhi Liu) & [email protected] (Prof.

21

Shengchun Wu)

22 23

†The co-first authors. B. Geng, H. Wang, and S. Wu contributed equally to this work.

24 25 26

1

ACS Paragon Plus Environment


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Abstract: Developing an easily recyclable and reusable biosorbent for highly

2

efficient removal of very toxic Hg(II) ions from waterbody is of special significance.

3

Herein, a thiol-functionalized nanocellulose aerogel-type adsorbent for the highly

4

efficient capture of Hg(II) ions, was fabricated through a facile freeze-drying of

5

bamboo-derived TEMPO-oxidized nanofibrillated cellulose (TO-NFC) suspension in

6

the presence of hydrolyzed 3-mercaptopropyl-trimethoxysilane (MPTs) sols. Notably,

7

the modified aerogel was able to effectively and selectively remove more than 92%

8

Hg(II) ions even in a wide range of Hg(II) concentration (0.01~85 mg/L) or the

9

coexistence with other heavy metals. Besides, the adsorption capacity of the aerogel

10

was little compromised by the variation in pH values of Hg(II) solutions over a wide

11

pH range. The fitting results of adsorption models suggested the monolayer

12

adsorption and chemisorptive characteristic with the maximal uptake capacity as high

13

as 718.5 mg/g. The adsorption mechanism of the MPTs-modified TO-NFC aerogel

14

toward Hg(II) was studied in detail. For the simulated chloralkali wastewater

15

containing Hg(II) ions, the novel aerogel-type adsorbent exhibited a removal

16

efficiency of 97.8%. Furthermore, its adsorption capacity for Hg(II) was not

17

apparently deteriorated after four adsorption/desorption cycles while almost

18

maintaining the original structural integrity.

19 20

Keywords: aerogel, thiol, bamboo, nanofibrillated cellulose (NFC), adsorption,

21

Hg(II)

22

2


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1

Introduction

2

Mercury (Hg) is a ubiquitous metal contaminant that is very poisonous to

3

living organisms even in the trace concentrations. With the rapid development

4

of industrial processes in recent decades, the amounts of industrial effluents

5

containing Hg(II) ions continue to increase, which poses a serious threat to

6

ecological systems and even human health.1 The US Environmental Protection

7

Agency sets a limit of Hg(II) ions concentration of 10 µg/L for wastewater

8

discharge and 2 µg/L for drinking water.2 Therefore, there is an urgent demand

9

to develop cost-effective methods for highly effective removal of very toxic

10

Hg(II) pollutants from water body.

11

Among various treatment techniques for water pollution, adsorption is

12

considered to be one of the most effective and economic approaches with some

13

distinguished advantages, such as good efficiency, low operation cost, and

14

simplicity of design. To date, a variety of inorganic,3-4 organic,5 and

15

organic/inorganic hybrid adsorbents,6-8 have been developed to remove Hg(II)

16

from water. However, many conventional adsorbents (e.g. activated carbon and

17

clays) exist in the form of powder or particulates, and display inconvenient

18

recyclability or expensive regeneration cost, which would increase the expense

19

for water treatment. Although some magnetic particles were attempted to load

20

onto sorbents to improve their recyclability,9-14 either a high preparation cost,

21

poor feasibility in the practical recovery, or unsatisfactory Hg(II) sorption

22

performance, largely restricted their practical applications. On the other hand, 3



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1

Hg(II) ions often coexist with other heavy metal ions15 in the wastewater. The

2

presence of other metal ion species would reduce removal efficiency of Hg(II)

3

ions due to the interference effect on the adsorbents. Therefore, in addition to

4

easy recyclability and excellent adsorption efficiency, a high adsorption

5

selectivity is also preferred for an ideal adsorbent for Hg(II).

6

As a class of highly interconnected porous and ultra-light solid materials,

7

aerogels display many unique characteristics as an ideal adsorbent, e.g. large

8

specific surface area, high porosity, and ease of separation from water after

9

adsorption.16-17 With growing concerns about the sustainability of adsorbent

10

materials, considerable efforts have recently been directed to developing

11

aerogel-type

12

Nanocelluloses (NCs) refer to a family of novel cellulosic materials with the

13

lateral dimension in the order of nano-sized range.21 In addition to the “green”

14

advantages and ease to surface modification associated with natural cellulose,

15

NCs possess a larger specific surface, a higher aspect ratio, and impressive

16

mechanical properties. Among one sub-category of NCs, nanofibrillated

17

cellulose (NFC), also known as cellulose nanofiber, is obtained from cellulose

18

fibers by mechanical disintegration22 or its combination with various

19

pre-treatments, such as enzyme,23-24 TEMPO-mediated oxidation,25 and

20

carboxymethylation.26 Unlike rodlike and rigid nanocrystalline cellulose (NCC)

21

isolated by acid hydrolysis, NFC is characterized by a long, flexible, and

22

entangled network of cellulose nanofibers (i.e., 2∼60 nm in diameter and

bio-sorbents

derived

from

renewable

4


resources.18-20

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1

several micrometers in length).27 Moreover, the cost of NFC is more

2

competitive in comparison to NCC.28 These advantages would enable NFC to

3

serve as a promising nano-sized building block for the preparation of biobased

4

aerogels.18, 21, 29

5

To date, the reports regarding nanocellulose aerogel-based sorbents mainly

6

dealt with the clean-up of oily liquids from water21, 30-32 and dye27, 33-34, and

7

only relatively limited efforts were devoted to the removal of heavy metal

8

ions35-38, especially Hg(II). Furthermore, these prior NFC aerogel-type sorbents

9

suffered from either inferior adsorption capacity of Hg(II) (157.5 mg/g),35

10

which was possibly caused by insufficient Hg(II)-binding sites or abilities on

11

the surfaces.

12

It have been recognized that thiol groups displayed strong affinities toward

13

Hg(II) ions. Although various categories of sorbents bearing thiol groups (-SH)

14

have been developed to tackle Hg(II) pollutions so far, these adsorbents still

15

suffer from some drawbacks to be overcome, such as complicated preparation

16

routes, inferior adsorption capacity due to a relatively low grafted ratio of

17

thiols39-40 and poor recyclability after the use39, 41-43. To date, there are yet no

18

attempts, in which -SH groups are introduced onto the surfaces of nanocellulose

19

aerogels for the removal of Hg(II). In view of abundant hydroxyl groups

20

available to chemical modification as well as nano-sized cellulose fibril units,

21

NFC would function as an ideal precursor to immobilize a high concentration

22

of thiols on the surfaces. 5



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1

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Herein, it was for the first time demonstrated that directly freeze-drying

2

mercaptosilylated

TEMPO-oxidized

nanofibrillated

cellulose

(TO-NFC)

3

suspension, yielded a flexible aerogel-type biosorbent bearing a high content of

4

-SH groups (3.33 mmol/g) on the surfaces. The MTPs-hydrolyzed crosslinking

5

strengthened three-dimensional scaffold of TO-NFC aerogel and improved its

6

structural durability, while the presence of abundant -SH groups significantly

7

increased its adsorption capacity toward Hg(II) ions. Moreover, its adsorption

8

capacity was much less markedly deteriorated even in both low and high pH

9

ranges, and the aerogel-type adsorbents still displayed a highly selective

10

removal efficiency on Hg(II) ions even in the co-existence with multiple kinds

11

of heavy metal ions and the Hg(II)-containing simulated chloralkali wastewater

12

with complicated compositions. Its underlying adsorption mechanism of Hg(II)

13

was studied as well. After multiple adsorption-desorption cycles, the removal

14

efficiency of Hg(II) still remained at a level of more than 93%, demonstrating

15

good reusability.

16 17

Results and discussion

18

Structural characterization

19

TO-NFC with carboxylate groups was isolated from bamboo pulp by

20

TEMPO-oxidized pre-treatment followed by mechanical disintegration,44,

21

while the non-charged NFC was prepared by high-density ultrasonic

22

disintegration.46 The surface charge content of TO-NFC was determined via 6


45

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1

conductometric titration to be 1.0±0.07 mmol/g corresponding to the degree of

2

substitution of carboxylate groups (DO) being approximately 0.17, while the

3

charge content was found to be negligible for NFC. Accordingly, the zeta

4

potential value of the former one was much more negative than that of the latter

5

(i.e. -49.5±0.9 vs. -16.8±0.8 mV).

6

As shown in Figure S1, TEM images of both NFC suspensions revealed

7

the network structure consisting of many entangled nanofibrils. However, the

8

extents of nanofibrillation differed largely between them. The average diameter

9

of the nanofibers was ∼20.5 nm for NFC and ∼9.4 nm for TO-NFC,

10

respectively.

11

The mercaptosilylated aerogels were prepared by directly freeze-drying

12

NFC or TO-NFC suspension in the presence of acid-hydrolyzed MPTs sols.

13

Figure 1a shows characteristic FT-IR spectra of the aerogels before and after

14

mercaptosilylation. After the modification with MPTs, the characteristic

15

absorption peaks associated with MPTs were identified for both of NFC-Si-SH

16

and TO-NFC-Si-SH aerogels. The absorption peak at 2856 cm-1 and 1255 cm-1

17

were attributed to C-H stretching and in-plane bending vibrations in the

18

mercaptopropyl moieties,47 respectively, whereas the minor absorption at

19

2543 cm-1 was related to the stretching vibration of thiol groups. And a new

20

band at ca. 795 cm-1 originated from the stretching vibrations of Si-C and/or

21

Si-O bonds appeared.48

7



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1 2

Figure 1. (a) FT-IR spectra of native and MTPs-modified nanocellulose

3

aerogels; (b) the high-resolution core-level Si 2p spectra of the TO-NFC-Si-SH

4

aerogel from XPS analysis. EDX spectrum and element mapping of the

5

mercaptosilylated TO-NFC aerogel: (c) EDX spectrum and element analysis results;

6

(d) EDX mapping of Si and S elements; (e) EDX mapping of Si element; (f) EDX

7

mapping of S element.

8

For the NFC aerogel and its mercapotsilyated one (Figure 1a), the O-H

9

bending vibration of absorbed water appeared at 1640 cm-1.49 In the case of the

10

TO-NFC aerogel, a sharper peak at ca. 1605 cm-1 was visible, which was 8


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1

attributed to the stretching vibration of carboxylate groups (–COO-)49

2

overlapping with O-H bending vibration of absorbed water.50 After the

3

mercaptosilylation of TO-NFC, the –COO- stretching vibration band at ca.

4

1605 cm−1 for the TO-NFC aerogel was shifted to 1630 cm−1. And a minor

5

shoulder at 1731 cm-1 for the TO-NFC-Si-SH one was due to the vibration of

6

–COOH groups that was possibly converted from some portions of COO-

7

anions during the hydrolysis.51

8

We further studied the Si linkage structure of MPTs grafting onto the

9

TO-NFC aerogel in terms of XPS, the high-resolution core-level Si 2p spectra

10

of the TO-NFC-Si-SH aerogel is presented in Figure 1b. The binding energy

11

peak of Si 2p in the Si-O-Si bonds of the MPTs appears at about 101.3 eV.52

12

However, two peaks of Si 2p appeared at 100.6 and 102.2 eV, both of which

13

can be assigned to Si-C bonds of the mercaptopropyl and Si-O-C ones formed

14

due to the self-polycondensation of MPTs, respectively.52 The above results

15

evidenced that the thiols were successfully attached onto the backbone of

16

TO-NFC or NFC aerogels through silylation.

17

Table 1.Mass and molar percentages of both carbon and sulfur elements for

18

various aerogels together with their substitution degree of thiol groups (-SH)

Element percentages (wt%)

Molar ratio

DSSH

Samples C

S

nS / nc

NFC

40.79

0.00

0.00

-

NFC-Si-SH

37.12

9.19

0.09

0.77

TO-NFC

37.66

0.00

0.00

-

TO-NFC-Si-SH

31.89

10.66

0.13

1.21

9



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1

The content of thiol groups on the surfaces of aerogels had a profound

2

effect on the ultimate adsorption effect of Hg(II). The carbon and sulfur element

3

contents of both NFC-Si-SH and TO-NFC-Si-SH aerogels were measured via

4

elemental analysis to determine degree of substitution by thiol groups per one

5

anhydroglucose unit (DSSH), and the results are listed in Table 1. The calculated

6

DSSH of TO-NFC-Si-SH aerogel was 1.21, which was higher than that 0.77 of

7

NFC-Si-SH one. This result was probably attributed to the fact that TO-NFC

8

had a smaller nanofibril diameter than NFC and thus a higher specific surface

9

area. Consequently, the former was more susceptible to the modification by

10

MPTs. Notably, the sulfur content of the mercaptosilylated aerogel in our case

11

was remarkably higher than that of the previously reported thiol-containing

12

inorganic adsorbents for Hg(II) (less than 5 wt%32,

13

advantageous in achieving a high adsorption capacity of Hg(II).

35

). This was evidently

14

The content and distribution of both Si and S elements within the

15

TO-NFC-Si-SH aerogel was further evaluated by wavelength-dispersive X-ray

16

spectroscopy (EDX). Based on the element composition results in Figure 1c,

17

the molar amounts of S and Si were almost equivalent. The mapping pictures

18

revealed that the distribution of both elements appeared quite homogeneous,53

19

as manifested by Figure 1d-f. It suggested that the coverage by the

20

poly(3-mercaptopropylsiloxane) was rather uniform.

21

The apparent and actual densities, porosity as well as specific surface area

22

data of unmodified and mercaptosilylated aerogels, are listed in Table 2. 10


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1

Regardless of the modification or not, all these nanocellulose-based aerogels

2

exhibited ultra-light and highly porous (∼99%) characteristics. A slight decrease

3

in porosity was noted after the modification. This may be due to the thickening

4

of cellulosic scaffold after the modification, reducing the void volume fraction

5

within the aerogel. Nevertheless, the BET surface area of the TO-NFC aerogel

6

almost remained unchanged before and after the modification. Owing to the

7

somewhat thicker diameter of cellulose nanofibrils (See Figure S1), the

8

relatively lower surface area was noted for the NFC and its MPTs-modified

9

aerogels, as compared to TO-NFC counterpart ones. Unlike the TO-NFC

10

aerogel, NFC aerogels exhibited the decreased specific surface area after the

11

modification by MPTs.

12

Table 2. Structural characteristics of native and mercaptosilylated aerogels Density Samples

Porosity

BET surface area

ρa (kg/m3)a

ρs (kg/m3)b

（%））

(m2/g)

NFC

7.21

1500

99.51

29.99

NFC-Si-SH

12.11

1280

99.05

18.47

TO-NFC

6.94

1500

99.53

43.51

TO-NFC-Si-SH

11.37

1269

99.10

43.57

13

Note: a) ρa is the apparent density of the aerogels; b) ρs is the density of the

14

solid scaffold.

15

The microstructure of TO-NFC and TO-NFC-Si-SH aerogels was further

16

examined by SEM and the pictures are shown in Figure 2. Both unmodified and

17

mercaptosilylated TO-NFC aerogels displayed an interconnected porous

18

morphology consisting of many thin sheets (Figure 2a & 2c). These sheets were 11



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1

considered to be formed by self-aggregation of cellulose nanofibrils during the

2

freezing-drying step, at which the ice crystals were formed.54-56 But at the

3

magnified images in Figure 2b & 2d, the surface texture of the TO-NFC-Si-SH

4

aerogel appeared somewhat coarser than that of the TO-NFC one presumably

5

due to the coverage by polysiloxane layers

6 7

Figure 2. SEM micrographs of cryo-fractured cross-section surfaces of aerogel

8

absorbent (a) TO-NFC aerogel, and (c) TO-NFC-Si-SH aerogel. Inserts: the

9

magnified images of (b) & (d).

10

Compressive properties

11

Excellent shape recovery or mechanical durability is of particular

12

significance for recycling and reusing of aerogel-type adsorbents. For this

13

purpose, shape-recovery properties of the unmodified and MTPs-modified

14

aerogels were evaluated by compression tests (Figure S2a). The native aerogels

15

(i.e. both TO-NFC and NFC) were very fragile, while the modified ones

16

behaved much more flexible, and could be manipulated without breaking upon

17

multiple cyclic compressions. This high flexibility was rarely observed in

18

traditional inorganic silica aerogels. A similar flexible behavior has been 12


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1

observed

for

methyltrimethoxysilane

(MTMs)-modified

nanocellulose

2

sponges.31, 56 The enhanced flexibility after the silylation was attributed to a

3

decrease in cross-linking density within NFCs as well as the repulsive

4

interactions existing between alkyl groups of polysiloxanes.57 The stress-strain

5

curves of the aerogels that were subjected to the compression and unloading,

6

are recorded in Figure 3a. The optimum overall performance was found for the

7

TO-NFC-Si-SH aerogel, which exhibited the linear stress−strain behavior

8

below 5% strain, which was associated with elastic deformation of cellulosic

9

scaffolds at low strains. In the strain range of 5∼50%, the gradual transition

10

from linear to nonlinear behavior occurred due to the progressive collapse of

11

the scaffold.54, 58-59

12 13

Figure 3. (a) Compressive stress−strain curves of different aerogels; (b)

14

thickness recovery of different aerogels upon the unloading from a compressed

15

state (ε= 50%). Note: the relative thickness of the aerogels after unloading was

16

illustrated by the columns.

17 18

Elastic modulus (E) and stress at 50% compression strain (σ=50%) were significantly

increased

after

MTPs

modifications.

13


The

modulus

of


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1

TO-NFC-Si-SH aerogel (E = 94.5 kPa) was higher than that of the other

2

reported silylated NFC one with a higher density (i.e.,ρs =1680kg/m3,E = 27

3

kPa)57 and NFC foams (i.e., ρs =1680kg/m3, E =52kPa)57. It was probably caused

4

by rigid polysiloxane layers, electrostatic repulsion from negatively charged

5

carboxylate groups, and the formed hydrogen bonds between hydroxyls of

6

NFCs and oxygen atoms of polysiloxanes, all of which made the modified

7

aerogel stronger upon compression.

8

Besides, the shape recovery property was evaluated by comparing the

9

residual strain (εfinal) of unloading compressed aerogel specimens. The

10

thickness recovery, expressed as the ratio of the original thickness, was then

11

plotted for various aerogels (Figure 3b). The thickness recovery ratio of both

12

NFC and TO-NFC ones was increased after the modification. Although the

13

highest recovery ratio was achieved for the NFC-Si-SH aerogel, i.e. 80% of its

14

original thickness, the thickness of TO-NFC-Si-SH aerogel was also recovered

15

up to 76%. This result was presumably because a higher DS may lead to a

16

higher crosslinking extent of polymercaptosiloxanes, thereby yielding less

17

elasticity of the aerogel. A similar mechanism was also reported in the flexible

18

silica aerogels.60, 61 Although the recovery ratio of the TO-NFC-Si-SH aerogel

19

under the aforesaid compressive test did not achieve 100%, the aerogel

20

exhibited outstanding flexibility under the hand force with a recovery ratio even

21

up to 100 % (Figure S2b).

22

Adsorption properties of Hg(II) ions 14


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1

Effects of adsorption conditions

2

Considering the complexity of the adsorption process onto the aerogels,

3

the nature of all the used components as well as possible operating variables in

4

a real system, it was imperative to clarify effects of the working parameters on

5

the adsorption efficiency. Thus, we investigated effects of solution pH, sorbent

6

dosage, and initial Hg(II) concentrations on removal efficiency of Hg(II) ions in

7

terms of the TO-NFC-Si-SH aerogel, since it had a higher DS of thiol groups in

8

addition to the presence of negatively-charged carboxylate ones.

9 10

Figure 4. (a) Adsorption of Hg(II) and (b) Zeta potentials of aquesous milled

11

TO-NFC-Si-SH aerogel suspension at different pH values. Note: Hg(II)

12

concentration=30 mg/L, m(aerogels)/V(solution) = 0.2 g/L, temperature = 25o C,

13

adsorption time = 6h.

14

It is known that the solution pH value is among the important factors

15

during the adsorption process of metal ions since it would influence the

16

ionization level of the sorbents and species forms of the adsorbates.56 Effects

17

of pH values on removal efficiency of Hg(II) were examined (Figure 4a), and

18

the zeta potential values at different pH values (1~11) was also determined for 15



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Page 16 of 38

1

the TO-NFC-Si-SH aerogel (Figure 4b). In the pH values ranging from 5 to 9,

2

the highest removal efficiency (∼97%) was achieved. Within this pH range,

3

the surface zeta potential of the aerogel was found to be the most negative

4

(less than -33mV), which would yield the strongest electrostatic attraction to

5

Hg(II)

6

Therefore, the TO-NFC-Si-SH aerogel exhibited the optimum removal

7

capacity in this case. With further decreasing pH values, the absolute value of

8

zeta potential was markedly reduced so that the surface activity of the aerogel

9

toward Hg(II) was weakened. Meanwhile, the intense competition between H+

10

and Hg2+ ions for active binding sites on the aerogel could occur. As a result, a

11

lower removal efficiency of Hg(II) was achieved. But it needed to note that

12

the efficiency of the aerogel was still close to 80% at pH=1.

ions for the capture by active adsorption groups on the surfaces.

13

When the pH value was increased from 9 to 11, the removal efficiency of

14

Hg(II) somewhat declined and but still remained at a level of ∼90%. Since the

15

zeta potential value only became slightly less negative in this case (-33.0 vs

16

-31.8 mV), the reduced surface activity on the adsorbent was hard to account

17

for a decrease in the removal efficiency. The decrease was likely because the

18

more preferable species of Hg(II) at higher pH values were Hg(OH)3 ,

19

Hg(OH)2, and Hg(OH)+ compound forms, which had smaller effective size

20

and higher mobility than Hg(II).62-66 Some studies have revealed that Hg(OH)2

21

was able to dissolve in case the initial Hg(II) concentration was less than

22

120 mg/L in the solution,65, 66 which has been confirmed by our experimental

-

16


Page 17 of 38

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1

observation. Because the Hg(II) concentration adopted in this work was

2

30 mg/L, the hydrolysis of Hg(II) ions would not disturb their adsorption onto

3

the aeorgel. These above results manifested that the adsorption capacity of the

4

TO-NFC-Si-SH aerogel was less sensitive to the variation of pH values. This

5

was quite different from the widely investigated chitosan-based adsorbents,

6

whose adsorption capacity of Hg(II) was drastically decreased at low pH

7

values due to the protonation of free amino groups although the adsorption

8

capacity was very high at the pH value close to 7.14,

9

advantage of the TO-NFC-Si-SH aerogel was undoubtedly preferred in the

10

practical treatment of Hg(II)-containing wastewater that could be acidic in

11

nature. Herein, pH=7.0 was chosen for the subsequent adsorption studies due

12

to the optimal removal effect.

67-69

Therefore, this

13

Sorbent dosage is another important parameter for the cost-effective

14

application of adsorbents. Figure S3a presents the effects of the TO-NFC-Si-SH

15

aerogel dosage on removal efficiency. With increasing the dosage from 0.05 to

16

0.2 g/L, the removal efficiency was increased from 66 to 97%, followed by a

17

level-off. The minimal dosage of the TO-NFC-Si-SH aerogel to attain the

18

adsorption equilibrium was much lower than that of previously reported

19

sorbents.7, 11 This

20

consideration of wastewater treatment.42, 70-71 The dosage value of 0.2 g/L was

21

chosen from the consideration of the cost-effective adsorption in the following

22

work.

advantage

was thus

favorable

17


from

the

economic


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Effects of initial concentrations of Hg(II) ions on the removal efficiency of

2

the TO-NFC-Si-SH aerogel, are shown in Figure S3b. The initial concentration

3

Hg(II) was examined in a wide concentration range, i.e. 0.01, 0.3, 1, 15, 30, 50,

4

85, and 97 mg/L. The highest removal efficiency up to 99.5% was achieved at

5

the initial Hg(II) concentration of 1 mg/L. When the initial concentration of

6

Hg(II) was reduced to 0.01 mg/L, the efficiency was slightly decreased to

7

94.4%. With further increasing Hg(II) concentration up to 97 mg/L, the

8

efficiency still maintained above 85%. To gain an in-depth insight into adsorption

9

and applicative characteristics of the TO-NFC-Si-SH aerogel for the advanced

10

treatment of Hg(II)-containing wastewaters, the adsorption capacity at the initial

11

Hg(II) concentration of 1 mg/L was examined. Both residual concentration and

12

removal efficiency of Hg(II) as a function of time for the TO-NFC-Si-SH aerogel, are

13

shown in Figure 5a. The adsorption equilibrium of the TO-NFC-Si-SH aerogel was

14

rapidly reached less than 1h. And the removal efficiency and residual concentration of

15

Hg(II) ion in the solution were determined to be ∼99.5% and ∼4.5 µg/L, respectively.

16

This residual concentration has been below the limit for the wastewater discharge

17

(10 µg/L) and was even close to the one (2 µg/L) for drinking water set by US

18

Environmental Protection Agency. The superior adsorption performance at the low

19

concentration of Hg(II) demonstrated that the bio-derived aerogel could be used as a

20

highly sensitive sorbent.

18


Page 18 of 38

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1 2

Figure 5. (a) The residual concentration (left Y axis) and removal efficiency (right

3

Y axis) of 1mg/L Hg(II) as a function of time for the TO-NFC-Si-SH aerogel; (b)

4

Comparison of removal efficiency as a function of contact time for different

5

samples. Note: m(aerogels)/V(solution) = 0.2 g/L, pH=7, temperature = 25o C.

6

Since activated carbon is well known to be one of the most widely used

7

adsorbents in the practical treatment of wastewater, we further compared the

8

adsorption efficiency of TO-NFC-Si-SH aerogel with that of commercial active

9

carbon with a high specific surface up to 1482 m2/g (Figure S4a). Noteworthy, at the

10

same initial concentration (30 ppm) of Hg(II) and adsorbent dosage, the removal

11

efficiency and time to attain ultimate adsorption equilibrium of the TO-NFC-Si-SH

12

aerogel were markedly superior to that of the activated carbon, as illustrated in Figure

13

5b. It needed to be mentioned that the TO-NFC-Si-SH aerogel used in this work had a

14

specific surface area of only 43 m2/g (Figure S4b). Therefore, we can conclude that

15

the impressive Hg(II) removal capability of the TO-NFC-Si-SH aerogel should be

16

attributed to a high density of active adsorption groups on the surfaces rather than

17

physical adsorption dominated by its specific surface area.

18

Adsorption kinetics, equilibrium and thermodynamics 19



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1 2

Figure 6. (a) Adsorption kinetics of the TO-NFC-Si-SH aerogel in an aqueous Hg(II)

3

solution; (b) Adsorption isotherms of Hg(II) on the TO-NFC-Si-SH aerogel at the

4

different initial Hg(II) concentrations ranging from 1 to 410 mg/L. Note:

5

m(aerogels)/V(solution) = 0.2 g/L, pH=7, temperature = 25o C.

6

The investigation of adsorption kinetics represents one of the important

7

approaches in evaluating the performance of a given sorbent and an irreplaceable

8

means in gaining useful information regarding rates and mechanism of sorption

9

process.72 For this purpose, the kinetics of the TO-NFC-Si-SH aerogel during the

10

adsorption process was analyzed by fitting experimental data using the

11

pseudo-first-order and pseudo-second-order models (the data illustrated in Figure 6a),

12

respectively. The linear fitting results are summarized in Table S1. The theoretical qe

13

value matched the experimental data more closely (i.e. 139.52 vs 140.25 mg/g), and a

14

much higher correlation coefficient (R2) was found for the pseudo-second-order

15

equation. This result suggested that the pseudo-second-order kinetics was much better

16

than the pseudo-first-order in an attempt to describe the kinetics for Hg(II) adsorption

17

onto the TO-NFC-Si-SH aerogel. And the adsorption process of Hg (II) onto the

18

aerogel was dominated by chemisorption, which was in coincidence with the 20


Page 20 of 38

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1

conclusion drawn in other thiol-functional sorbents.39-43

2

To examine interactive behaviors between the adsorbent and adsorbate at the

3

equilibrium and to estimate the maximum Hg(II) adsorption capacity of the

4

TO-NFC-Si-SH aerogel, effects of initial Hg(II) concentrations on the adsorption was

5

also presented. The isotherm of Hg(II) adsorption on the TO-NFC-Si-SH aerogel as

6

illustrated in Figure 6b, was fitted with the widely used Langmuir and Freundlich

7

isotherm models, respectively. And the corresponding fitting parameters are also

8

outlined in Table S2. Compared with the Freundlich isotherm, the Langmuir one

9

appeared more suitable in describing adsorption behaviors of Hg(II) onto the

10

TO-NFC-Si-SH aerogel due to a higher correlation coefficient (R2=0.998 vs 0.835) of

11

the latter, suggesting the monolayer adsorption.73 The value of the separation factor

12

constant (RL) lied between 0 and 1, indicative of a favorable adsorption process.6 The

13

theoretical maximal adsorption capacity for Hg(II) was estimated to be 729.9 mg/g,

14

which was quite close to the experimental value of 718.5 mg/g corresponding to ca.

15

3.58 mmol/g.

16

The maximum adsorption capacity (qm) of the TO-NFC-Si-SH aerogel was

17

compared to that of various previously reported biosorbents in term of the removal

18

effect of Hg(II) (Table S5). It was found that the qm value of the TO-NFC-Si-SH

19

aerogel was superior to that of these reported biosorbents except commercial chitosan

20

(CS) powder74 with a high surface area. However, these (CS)-based materials tend to

21

suffer from the drawbacks, such as inconvenience to be recycled for the reuse, much

22

inferior adsorption capacity at low pH values (a reduction of even more than 50%), 21



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1

and poor durability due to the intrinsic brittleness.14,67-69 Although it has been

2

reported that the coating of CS materials with magnetic particles was an alternative to

3

markedly improve the recyclable performance,9-11, 14 the adsorption capacity of these

4

magnetic sorbents, e.g. magnetic CS-phenylthiourea (CSTU) resin (135±3 mg/g),11

5

magnetic CS-glutaraldehyde (MCS-GA) (96 mg/g),14 still remained relatively lower

6

in comparison to TO-NFC-Si-SH aerogel in our work. Moreover, our aerogel-type

7

adsorbent not only displayed the super-sorption capacity for Hg(II), but also was

8

allowed to be readily recycled or collected due to its high floatability and good

9

mechanical flexibility. It could be more practically feasible in the treatment of

10

Hg(II)-containing wastewater.

11 12

Figure 7. Plot of ln Kd versus 1/T for Hg(II) adsorption on the TO-NFC-Si-SH aerogel

13

at different temperatures.

14

Thermodynamic studies can provide the detailed information regarding inherent

15

energetic changes during the process of adsorption. In this work, effects of

16

temperature on Hg(II) ion adsorption onto the TO-NFC-Si-SH aerogel were illustrated

17

by drawing a linear plot of ln Kd versus 1/T in Figure 7, and the estimated 22


Page 22 of 38

Page 23 of 38

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1

thermodynamic parameters and correlation coefficients are summarized in Table S3.

2

Under the steady-state reaction conditions, the Gibbs free energy (∆Go) ranged from

3

-13.37 to -20.72 kJ/mol, and ∆Ηo and ∆So were equal to 48.16 kJ/mol and

4

0.21 kJ/mol·K, respectively. The positive values of ∆Ηo revealed that the adsorption

5

of Hg(II) ions on the TO-NFC-Si-SH aerogel was endothermic.39 The negative value

6

of ∆Go indicated that the adsorption of Hg(II) was spontaneous and the ∆Go values

7

decreased with elevating temperature. This implies that a higher temperature favored

8

the spontaneous adsorption of Hg(II) ions by the TO-NFC-Si-SH aerogel.

9

Adsorption mechanism

10

Since both thiol and carboxyl groups on the TO-NFC-Si-SH aerogel had binding

11

affinities toward Hg(II), there existed possible competitions between both kinds of

12

active groups during the adsorption. To elaborate underlying adsorption mechanism of

13

the TO-NFC-Si-SH aerogel, we first estimated the numbers of thiol and carboxyl

14

groups anchored onto the TO-NFC-Si-SH aerogel (see experimental section in SI).

15

Based on the sulfur content (Table 1) of TO-NFC-Si-SH and DO value of TO-NFC,

16

the amount of thiol and carboxyl groups on the TO-NFC-Si-SH aerogel were

17

determined to be 3.33 mmol/g and 0.47 mmol/g, respectively. Assuming that each two

18

negatively charged carboxylates bound one Hg(II) ion, the adsorption capacity of

19

Hg(II) by thiols was about 3.34 mmol/g after the adsorption one by carboxyls (i.e.

20

0.24 mmol/g) was deducted from the experimental adsorption amount of Hg(II) (i.e.

21

3.58 mmol/g). It is supposed that each thiol on the TO-NFC-Si-SH aerogel was likely

22

to complex with average one Hg(II) ion on maximum, which was supported by the

23



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1

equivalent molar amount between S and Hg elements in the EDX analysis result

2

of TO-NFC-Si-SH aerogel after the adsorption of Hg(II) (Figure 8d). Since the

3

theoretical adsorption contribution from carboxyl ones only occupied about 6.7% in

4

the total amount of adsorbed Hg(II), the super-sorption capacity of Hg(II) for the

5

TO-NFC-Si-SH aerogel predominantly arose from the contribution of thiol groups

6

having much greater quantities other than carboxyl ones. According to the hard-soft

7

acid base (HSAB) theory, Hg(II) ions are classified as a Lewis soft acid, while thiol

8

and carboxylate groups belong to Lewis soft and hard bases, respectively.75 Based on

9

the rule that soft base-soft acid gives priority to the formation of a stable complex,75

10

the thiols on the TO-NFC-Si-SH aerogel tended to preferentially complex with Hg(II)

11

ions in comparison to the carboxyls.

12

Also, effects of mercapsilylation on adsorption properties of NFC without

13

carboxyl groups and TO-NFC aerogels were further compared. Figure 5b shows the

14

dependence of removal efficiency as a function of contact time for the native and

15

MPTs-modified aerogels. Compared to the NFC aerogel that almost did not adsorb

16

Hg(II) ions, the TO-NFC one rapidly achieved the adsorption equilibrium with a

17

removal efficiency of only 23%. The low adsorption capacity of the TO-NFC aerogel

18

should arise from the contribution of its carboxylate groups that has had a lower

19

affinity to Hg(II) than thiols, as mentioned earlier.75 But regardless of the presence of

20

carboxyl groups, both of MPTs-modified aerogels exhibited the almost same

21

equilibrium efficiency that was higher than 90%. Again, it was clearly demonstrated

22

that thiol groups played a predominant role in the removal of Hg(II) in our case. But 24


Page 24 of 38

Page 25 of 38

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1

the equilibrium one was attained for the TO-NFC-Si-SH aerogel within 5h, which was

2

shorter than that of the NFC-Si-SH one merely containing thiol groups (i.e. 10 h).

3

This may be attributed to the coexistence of both thiol and carboxyl groups on the

4

surfaces of TO-NFC-Si-SH aerogel, yielding more accessible active sites for the rapid

5

uptake of Hg(II) ions.

6 25



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1

Figure 8. (a) Wide-scan XPS spectrum of TO-NFC-Si-SH aerogel before and after

2

adsorption; the high-resolution core-level spectra of S 2p before (b) and after (c)

3

adsorption; (d) EDX element analysis spectrum of the TO-NFC-Si-SH aerogel after

4

the adsorption of Hg(II) ions; (e) EDX mapping image of Si, S, and Hg elements; (f)

5

the insert: the magnified image of EDX element mapping; (g) Hg element

6

mapping; (h) S element mapping; (i) Schematic illustration of proposed scheme

7

adsorption mechanism of Hg(II) ions by the TO-NFC-Si-SH aerogel.

8

In order to identify the binding of thiol groups with Hg(II), the variation of XPS

9

spectra of the TO-NFC-Si-SH aerogel before and after the adsorption is also given

10

(Figure 8a). It was noted that new peaks for Hg 4f5, Hg 4d3, and Hg 4d5 were visible

11

after the adsorption. Figure 8b-c further present high-resolution S 2p core-level

12

spectra of the TO-NFC-Si-SH aerogel. Before the adsorption, two major peaks at

13

163.7 and 162.5 eV were attributed to the C-S and S-H bonds of MPTs. After the

14

adsorption, the S 2p binding energies of C-S and S-H bonds were slightly decreased,

15

which may be due to the electron-donating effect from S atoms of C-S and S-H bonds

16

to Hg(II). Besides, a new peak assigned to S-Hg bond was visible at 161.3 eV,

17

confirming that the complexation between Hg(II) and S species indeed occurred. The

18

conclusion was also supported by visualizing the EDX mapping pictures (Figure

19

8e-h), in which the yellow points denoting Hg elements appeared to closely attach

20

with red points representing S elements, as shown by the insert of Figure 8f.

21

The aforementioned adsorption contribution arising from carboxyl groups, was

22

also supported by the variation of both C=O groups in the FT-IR adsorption of 26


Page 26 of 38

Page 27 of 38

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1

the TO-NFC-Si-SH aerogel and its XPS O1s core-level spectrum before and

2

after the Hg(II) adsorption (Figure S5).

3

On the basis of the aforesaid results, the dominant adsorption mechanism of

4

Hg(II) onto the TO-NFC-Si-SH aerogel was proposed in Figure 8i. The

5

negatively-charged surface characteristics of the aerogel yielded electrostatic

6

interaction to free Hg(II) ions, which then caused these ions to rapidly approach the

7

surfaces for the adsorption predominantly by more active and abundant thiol groups

8

anchored onto the aerogel. In this case, each Hg(II) ion was bound by approximately

9

one thiol through chemical complexation. Besides, a small amount of Hg(II) ions was

10

captured through electrostatic interaction by carboxyl groups on the surfaces.

11

Selective removal of Hg(II) ions and application

12

Considering that Hg(II) often coexist with other heavy metals in the practical

13

wastewater, we further investigated selective adsorption properties of TO-NFC-Si-SH

14

aerogel for Hg(II) ions in presence of other common heavy metal ions. In this work,

15

Cu(II), Cd(II), Pb(II), and Zn(II) were chosen as the co-existed metal species because

16

they have been reported to have interfering effects on the adsorption of Hg(II) ions

17

onto adsorbents.11, 15, 76 And the initial concentration of all heavy metal ion species

18

was fixed at 30 ppm and the competitive adsorption efficiencies of these metal ions

19

are illustrated in Figure 9. In the co-existence with the other heavy metal ions, the

20

removal efficiency of Hg(II) was still the highest and reached 93%, which was only

21

slightly lower than that in the presence of Hg(II) alone (97%). For the other four kinds

22

of metal ion species, the efficiency in the co-existed state was decreased to varying 27



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1

extent in comparison to their states alone. Comparatively, the best adsorption

2

selectivity of the aerogel was noted for Hg(II), since the adsorption capacity of Hg(II)

3

ions was much less affected by the presence of other heavy metal ion species. Based

4

on the HSBA theory, Hg(II) and Cd(II) ions were soft acids, while Cu(II), Pb(II), and

5

Zn(II) ones were borderline ones. Thus, thiol groups belonging to the soft base would

6

give priority to the complexation with the soft acid. Besides, because chemical

7

hardness of Hg(II) was lower than that of Cd(II), the thiol groups would bind Hg(II)

8

more stably and preferentially.75

9 10

Figure 9. Effect of co-existing heavy metal ions on Hg(II) adsorption capacity onto

11

the TO-NFC-Si-SH aerogel.

12

In order to preliminarily survey the feasibility of the as-prepared TO-NFC-Si-SH

13

aerogel for the removal of Hg(II) from wastewater samples containing complicated

14

compositions, a batch adsorption experiment was conducted on the Hg(II)-containing

15

simulated chloralkali wastewater that was prepared according to the formulation

16

described in the previous literature.77 And its specific characteristics and composition

17

are summarized in Table S4. Since the simulated chloralkali wastewater contained 28


Page 28 of 38

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1

high concentrations of many other ion species, the adsorption of Hg(II) onto the

2

adsorbents was expected to be interfered. Noteworthy, after the adsorption by the

3

TO-NFC-Si-SH aerogel, it was revealed that 97.8% Hg(II) ions can be removed from

4

the simulated wastewater. This result evidenced that the aerogel-type adsorbent would

5

have a promising potential in removing Hg(II) ions from wastewater under practical

6

conditions. In the next-step study, we will evaluate the performance of this adsorbent

7

in the real wastewater containing Hg(II) from a variety of sources, such as Hg mining

8

industry and fluorescence lamp factories, etc.

9

Reuse of adsorbents

10

For an adsorbent material, the reusability is also of very important concerns in

11

the practical applications because desorption results may facilitate us to get a better

12

understanding of the feasibility to recycle the adsorbent and to recover Hg(II) from

13

aqueous solutions. In this work, the reusability of the TO-NFC-Si-SH aerogel was

14

investigated by using 1M hydrochloric acid solution with 5 wt% thiourea as an eluent

15

to regenerate TO-NFC-Si-SH aerogel.

16 17

Figure 10. Adsorption–Desorption cycles of the TO-NFC-Si-SH aerogel for Hg(II) 29



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1

Page 30 of 38

solution.

2

The reusability of the TO-NFC-Si-SH aerogel is presented in Figure10. The

3

removal percentage of Hg(II) was slightly decreased from 97.5 to 93.2% after four

4

adsorption/desorption cycles. In each cycle, the desorption percentage was more than

5

90%. Surprisingly, the adsorption efficiency was not declined significantly and the

6

adsorbent maintained overall structural integrity after four cycles. These results

7

showed that TO-NFC-Si-SH aerogel would be used for the multiple treatments of

8

Hg(II)-containing wastewater, in which the regeneration of adsorbents was preferred

9

to reduce operation costs.

10 11 12

Conclusions In this work, a novel nanocellulose aerogel containing both thiol and

13

carboxyl

groups

(TO-NFC-Si-SH),

was

fabricated

14

freeze-drying of TEMPO oxidation NFC suspension in the presence of

15

mercaptopropylsiloxane (MPTs) sols. The aerogel-type adsorbent exhibited

16

excellent shape recovery properties upon the release of compression. Due to the

17

abundant thiol groups anchored onto the surfaces, it could highly effective

18

remove off Hg(II) ions up to 92% in a wide initial concentration range from

19

0.01 to 85 mg/L, and the adsorption capacity was less compromised by the

20

variation in pH values of Hg(II) solutions over a wide pH range (including very

21

acidic conditions). The adsorption of Hg(II) onto the aerogel well fitted

22

Langmuir isotherm and pseudo-second order kinetics with the maximum 30


through

a

facile

Page 31 of 38

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1

adsorption capacity as high as 718.5 mg/g that surpassed the almost majority of

2

reported biosorbents but possessed the optimum flexibility and ease of

3

recyclability. And thermodynamic calculations suggested that the adsorption

4

process of Hg(II) onto the modified aerogel was an endothermic process. The

5

detailed adsorption mechanism of Hg(II) onto the TO-NFC-Si-SH aerogel was

6

investigated. Furthermore, the adsorbent still exhibited excellent selective

7

removal effect on Hg(II) ions from the aqueous solution containing five kinds

8

of heavy metal ion species. And for simulated chloralkali wastewater

9

containing Hg(II) ions, the novel TO-NFC-Si-SH aerogel also displayed a high

10

removal

efficiency

up to

97.8%.

Noteworthy,

the

easily

recyclable

11

super-sorbents could be reused without the pronounced loss in the removal

12

efficiency of Hg(II) after multiple consecutive adsorption/desorption cycles.

13

Superior adsorption performance for Hg(II) ions in combination with excellent

14

selectivity and reusability, would make this thiol-functionalized aerogel derived

15

from renewable resources a very promising biosorbent for potential

16

applications in the practical treatment of wastewater.

17 18

Associated content

19

Supporting information

20

Experimental section; Figure S1: TEM images of NFC (a) and TO-NFC (b);

21

Figure S2: Compression test pictures of the TO-NFC-Si-SH aerogel; Figure S3:

22

Removal efficiency curves of Hg(II) against adsorbent dosage (a) and the initial

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Hg (II) concentrations (b); Figure S4: N2 adsorption-desorption isotherms; Figure S5:

2

FT-IR spectra (a) and XPS O1s core-level spectra (b) before and after the adsorption

3

of Hg(II) onto the TO-NFC-Si-SH aerogel; Table S1: Kinetic models parameters;

4

Table S2: Parameters of the isotherms models; Table S3: Thermodynamic parameters;

5

Table S4: Composition of stimulated chloralkali wastewater used in this work; Table

6

S5: Comparison of the previously reported bio-sorbents with our aerogel adsorbent.

7 8

Acknowledgements

9

The authors are grateful for the financial supports from Public Welfare Projects of

10

Zhejiang Province (No. 2016C33029 & 2017C33113 & 2015C33050), National

11

Natural Science Foundation of China (No. 21677131), and Scientific Research

12

Foundation of Zhejiang Agriculture & Forestry University (No. 2013FR088).

13

14

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For Table of Contents Use Only

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A reusable thiol-functionalized nancellulose aerogel-type bio-sorbent has been prepared for highly efficient and selective removal of Hg(II) ions from water.

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Surface-Tailored Nanocellulose Aerogels with Thiol-Functional

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