Surface-Tailored Nanocellulose Aerogels with ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

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∥ †

Zhejiang Provincial Collaborative Innovation Center for Bamboo Resources and High-efficiency Utilization, No. 666 Wusu Street, Lin’an District, Hangzhou 311300, China ‡ School of Engineering and §School of Environmental and Resource Sciences, Zhejiang Agriculture & Forestry University, No. 666 Wusu Street, Lin’an District, Hangzhou 311300, China ∥ International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

ABSTRACT: Developing an easily recyclable and reusable biosorbent for highly efficient removal of very toxic Hg(II) ions from bodies of water is of special significance. Herein, a thiolfunctionalized nanocellulose aerogel-type adsorbent for the highly efficient capture of Hg(II) ions was fabricated through a facile freeze-drying of bamboo-derived 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO) oxidized nanofibrillated cellulose (TO-NFC) suspension in the presence of hydrolyzed 3-mercaptopropyltrimethoxysilane (MPTs) sols. Notably, the modified aerogel was able to effectively and selectively remove more than 92% Hg(II) ions even in a wide range of Hg(II) concentrations (0.01−85 mg/ L) or coexistence with other heavy metals. Besides, the adsorption capacity of the aerogel was not compromised much by the variation in pH values of Hg(II) solutions over a wide pH range. The fitting results of adsorption models suggested the monolayer adsorption and chemisorptive characteristics with the maximal uptake capacity as high as 718.5 mg/g. The adsorption mechanism of the MPTs-modified TO-NFC aerogel toward Hg(II) was studied in detail. For the simulated chloralkali wastewater containing Hg(II) ions, the novel aerogel-type adsorbent exhibited a removal efficiency of 97.8%. Furthermore, its adsorption capacity for Hg(II) was not apparently deteriorated after four adsorption/desorption cycles while almost maintaining the original structural integrity. KEYWORDS: Aerogel, Thiol, Bamboo, Nanofibrillated cellulose (NFC), Adsorption, Hg(II)

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inorganic hybrid adsorbents6−8 have been developed to remove Hg(II) from water. However, many conventional adsorbents (e.g., activated carbon and clays) exist in the form of powder or particulates and display inconvenient recyclability or high regeneration costs, which would increase the expense for water treatment. Although some magnetic particles were attempted to load onto sorbents to improve their recyclability,9−14 either a high preparation cost, poor feasibility in the practical recovery, or unsatisfactory Hg(II) sorption performance largely restricted their practical applications. On the other hand, Hg(II) ions often coexist with other heavy metal ions15 in wastewater. The presence of other metal ion species would reduce the removal efficiency of Hg(II) ions due to the interference effect on the

INTRODUCTION

Mercury (Hg) is a ubiquitous metal contaminant that is very poisonous to living organisms even in the trace concentrations. With the rapid development of industrial processes in recent decades, the amounts of industrial effluents containing Hg(II) ions continue to increase, which poses a serious threat to the ecological systems and even human health.1 The US Environmental Protection Agency sets a limit of Hg(II) ion concentration of 10 μg/L for wastewater discharge and 2 μg/ L for drinking water.2 Therefore, there is an urgent demand to develop cost-effective methods for highly effective removal of very toxic Hg(II) pollutants from bodies of water. Among the various treatment techniques for water pollution, adsorption is considered to be one of the most effective and economic approaches with some distinguished advantages, such as good efficiency, low operation cost, and simplicity of design. To date, a variety of inorganic,3,4 organic,5 and organic/ © 2017 American Chemical Society

Received: September 9, 2017 Revised: October 8, 2017 Published: October 25, 2017 11715

DOI: 10.1021/acssuschemeng.7b03188 ACS Sustainable Chem. Eng. 2017, 5, 11715−11726

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) FT-IR spectra of native and MTPs-modified nanocellulose aerogels. (b) High-resolution core-level Si 2p spectra of the TO-NFC-Si-SH aerogel from XPS analysis. EDX spectrum and element mapping of the mercaptosilylated TO-NFC aerogel: (c) EDX spectrum and element analysis results; (d) EDX mapping of Si and S elements; (e) EDX mapping of Si element; (f) EDX mapping of S element.

adsorbents. Therefore, in addition to easy recyclability and excellent adsorption efficiency, a high adsorption selectivity is also preferred for an ideal adsorbent for Hg(II). As a class of highly interconnected porous and ultralight solid materials, aerogels display many unique characteristics as ideal adsorbents, e.g. large specific surface area, high porosity, and ease of separation from water after adsorption.16,17 With growing concerns about the sustainability of adsorbent materials, considerable efforts have recently been directed to developing aerogel-type biosorbents derived from renewable resources.18−20 Nanocelluloses (NCs) refer to a family of novel cellulosic materials with lateral dimensions in the nanosized range.21 In addition to the “green” advantages and ease to surface modification associated with natural cellulose, NCs possess a larger specific surface, a higher aspect ratio, and impressive mechanical properties. Among one subcategory of NCs, nanofibrillated cellulose (NFC), also known as cellulose nanofiber, is obtained from cellulose fibers by mechanical disintegration22 or its combination with various pretreatments, such as enzyme,23,24 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation,25 and carboxymethylation.26 Unlike rodlike and rigid nanocrystalline cellulose (NCC) isolated by acid hydrolysis, NFC is characterized by a long, flexible, and entangled network of cellulose nanofibers (i.e., 2− 60 nm in diameter and several micrometers in length).27

Moreover, the cost of NFC is more competitive in comparison to that of NCC.28 These advantages would enable NFC to serve as a promising nanosized building block for the preparation of biobased aerogels.18,21,29 To date, the reports regarding nanocellulose aerogel-based sorbents mainly dealt with the cleanup of oily liquids from water21,30−32 and dye,27,33,34 and only relatively limited efforts were devoted to the removal of heavy metal ions,35−38 especially Hg(II). Furthermore, these prior NFC aerogel-type sorbents suffered from inferior adsorption capacity of Hg(II) (157.5 mg/g),35 which was possibly caused by insufficient Hg(II)-binding sites or abilities on the surfaces. It has been recognized that thiol groups displayed strong affinities toward Hg(II) ions. Although various categories of sorbents bearing thiol groups (−SH) have been developed to tackle Hg(II) pollutions so far, these adsorbents still suffer from some drawbacks to be overcome, such as complicated preparation routes, inferior adsorption capacity due to a relatively low grafted ratio of thiols,39,40 and poor recyclability after use.39,41−43 To date, there are yet no attempts, in which −SH groups are introduced onto the surfaces of nanocellulose aerogels for the removal of Hg(II). In view of abundant hydroxyl groups available to chemical modification as well as nanosized cellulose fibril units, NFC would function as an ideal 11716


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ACS Sustainable Chemistry & Engineering precursor to immobilize a high concentration of thiols on the surfaces. Herein, it was for the first time demonstrated that directly freeze-drying mercaptosilylated TEMPO-oxidized nanofibrillated cellulose (TO-NFC) suspension yielded a flexible aerogeltype biosorbent bearing a high content of −SH groups (3.33 mmol/g) on surfaces. The MTPs-hydrolyzed cross-linking strengthened the three-dimensional scaffold of the TO-NFC aerogel and improved its structural durability, while the presence of abundant −SH groups significantly increased its adsorption capacity toward Hg(II) ions. Moreover, its adsorption capacity was much less markedly deteriorated even in both low and high pH ranges, and the aerogel-type adsorbents still displayed a highly selective removal efficiency on Hg(II) ions even in the coexistence with multiple kinds of heavy metal ions and the Hg(II)-containing simulated chloralkali wastewater with complicated compositions. The underlying adsorption mechanism of Hg(II) was studied as well. After multiple adsorption−desorption cycles, the removal efficiency of Hg(II) still remained at a level of more than 93%, demonstrating good reusability.

1731 cm−1 for TO-NFC-Si-SH was due to the vibration of −COOH groups that was possibly converted from some portions of COO− anions during the hydrolysis.51 We further studied the Si linkage structure of MPTs grafting onto the TO-NFC aerogel in terms of XPS, the high-resolution core-level Si 2p spectra of the TO-NFC-Si-SH aerogel is presented in Figure 1b. The binding energy peak of Si 2p in the Si−O−Si bonds of the MPTs appears at about 101.3 eV.52 However, two peaks of Si 2p appeared at 100.6 and 102.2 eV, both of which can be assigned to Si−C bonds of the mercaptopropyl and Si−O−C ones formed due to the selfpolycondensation of MPTs, respectively.52 The above results evidenced that the thiols were successfully attached onto the backbone of TO-NFC or NFC aerogels through silylation. The content of thiol groups on the surfaces of aerogels had a profound effect on the ultimate adsorption effect of Hg(II). The carbon and sulfur element contents of both NFC-Si-SH and TO-NFC-Si-SH aerogels were measured via elemental analysis to determine degree of substitution by thiol groups per one anhydroglucose unit (DSSH), and the results are listed in Table 1. The calculated DSSH of the TO-NFC-Si-SH aerogel

RESULTS AND DISCUSSION Structural Characterization. TO-NFC with carboxylate groups was isolated from bamboo pulp by TEMPO-oxidized pretreatment followed by mechanical disintegration,44,45 while the noncharged NFC was prepared by high-density ultrasonic disintegration.46 The surface charge content of TO-NFC was determined via conductometric titration to be 1.0 ± 0.07 mmol/g corresponding to the degree of substitution of carboxylate groups (DO) being approximately 0.17, while the charge content was found to be negligible for NFC. Accordingly, the zeta potential value of the former one was much more negative than that of the latter (i.e., −49.5 ± 0.9 vs −16.8 ± 0.8 mV). As shown in Figure S1, TEM images of both NFC suspensions revealed the network structure consisting of many entangled nanofibrils. However, the extents of nanofibrillation differed largely between them. The average diameter of the nanofibers was ∼20.5 nm for NFC and ∼9.4 nm for TONFC, respectively. The mercaptosilylated aerogels were prepared by directly freeze-drying NFC or TO-NFC suspension in the presence of acid-hydrolyzed MPTs sols. Figure 1a shows characteristic FTIR spectra of the aerogels before and after mercaptosilylation. After the modification with MPTs, the characteristic absorption peaks associated with MPTs were identified for both of NFCSi-SH and TO-NFC-Si-SH aerogels. The absorption peak at 2856 and 1255 cm−1 were attributed to C−H stretching and inplane bending vibrations in the mercaptopropyl moieties,47 respectively, whereas the minor absorption at 2543 cm−1 was related to the stretching vibration of thiol groups. And a new band at ca. 795 cm−1 originated from the stretching vibrations of Si−C and/or Si−O bonds appeared.48 For the NFC aerogel and its mercapotsilyated version (Figure 1a), the O−H bending vibration of absorbed water appeared at 1640 cm−1.49 In the case of the TO-NFC aerogel, a sharper peak at ca. 1605 cm−1 was visible, which was attributed to the stretching vibration of carboxylate groups (−COO−)49 overlapping with the O−H bending vibration of absorbed water.50 After the mercaptosilylation of TO-NFC, the −COO− stretching vibration band at ca. 1605 cm−1 for the TO-NFC aerogel was shifted to 1630 cm−1. And a minor shoulder at

Table 1. Mass and molar percentages of both carbon and sulfur elements for various aerogels together with their substitution degree of thiol groups (-SH)

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element percentages (wt %)

molar ratio

samples

C

S

nS/nc

NFC NFC-Si-SH TO-NFC TO-NFC-Si-SH

40.79 37.12 37.66 31.89

0.00 9.19 0.00 10.66

0.00 0.09 0.00 0.13

DSSH 0.77 1.21

was 1.21, which was higher than that (0.77) of NFC-Si-SH. This result was probably attributed to the fact that TO-NFC had a smaller nanofibril diameter than NFC and thus a higher specific surface area. Consequently, the former was more susceptible to the modification by MPTs. Notably, the sulfur content of the mercaptosilylated aerogel in our case was remarkably higher than that of the previously reported thiolcontaining inorganic adsorbents for Hg(II) (less than 5 wt %32,35). This was evidently advantageous in achieving a high adsorption capacity of Hg(II). The content and distribution of both Si and S elements within the TO-NFC-Si-SH aerogel was further evaluated by wavelength-dispersive X-ray spectroscopy (EDX). Based on the element composition results in Figure 1c, the molar amounts of S and Si were almost equivalent. The mapping pictures revealed that the distribution of both elements appeared quite homogeneous,53 as manifested by Figure 1d−f. It suggested that the coverage by the poly(3-mercaptopropylsiloxane) was rather uniform. The apparent and actual densities, porosity, and specific surface area data of unmodified and mercaptosilylated aerogels are listed in Table 2. Regardless of the modification or not, all these nanocellulose-based aerogels exhibited ultralight and highly porous (∼99%) characteristics. A slight decrease in porosity was noted after the modification. This may be due to the thickening of cellulosic scaffold after the modification, reducing the void volume fraction within the aerogel. Nevertheless, the BET surface area of the TO-NFC aerogel almost remained unchanged before and after the modification. 11717


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the compression and unloading are recorded in Figure 3a. The optimum overall performance was found for the TO-NFC-SiSH aerogel, which exhibited the linear stress−strain behavior below 5% strain, which was associated with elastic deformation of cellulosic scaffolds at low strains. In the strain range of 5− 50%, the gradual transition from linear to nonlinear behavior occurred due to the progressive collapse of the scaffold.54,58,59 Elastic modulus (E) and stress at 50% compression strain (σ = 50%) were significantly increased after MTPs modifications. The modulus of TO-NFC-Si-SH aerogel (E = 94.5 kPa) was higher than that of the other reported silylated NFC one with a higher density (i.e., ρs = 1680 kg/m3, E = 27 kPa)57 and NFC foams (i.e., ρs = 1680 kg/m3, E = 52 kPa).57 It was probably caused by rigid polysiloxane layers, electrostatic repulsion from negatively charged carboxylate groups, and the formed hydrogen bonds between hydroxyls of NFCs and oxygen atoms of polysiloxanes, all of which made the modified aerogel stronger upon compression. Besides, the shape recovery property was evaluated by comparing the residual strain (εfinal) of unloading compressed aerogel specimens. The thickness recovery, expressed as the ratio of the original thickness, was then plotted for various aerogels (Figure 3b). The thickness recovery ratio of both NFC and TO-NFC ones was increased after the modification. Although the highest recovery ratio was achieved for the NFCSi-SH aerogel, i.e., 80% of its original thickness, the thickness of TO-NFC-Si-SH aerogel was also recovered up to 76%. This result was presumably because a higher DS may lead to a higher cross-linking extent of polymercaptosiloxanes, thereby yielding less elasticity of the aerogel. A similar mechanism was also reported in the flexible silica aerogels.60,61 Although the recovery ratio of the TO-NFC-Si-SH aerogel under the aforesaid compressive test did not achieve 100%, the aerogel exhibited outstanding flexibility under the hand force with a recovery ratio even up to 100% (Figure S2b). Adsorption Properties of Hg(II) Ions. Effects of Adsorption Conditions. Considering the complexity of the adsorption process onto the aerogels, the nature of all the used components as well as possible operating variables in a real system, it was imperative to clarify effects of the working parameters on the adsorption efficiency. Thus, we investigated effects of solution pH, sorbent dosage, and initial Hg(II) concentrations on removal efficiency of Hg(II) ions in terms of the TO-NFC-Si-SH aerogel, since it had a higher DS of thiol groups in addition to the presence of negatively charged carboxylate ones.

Table 2. Structural Characteristics of Native and Mercaptosilylated Aerogels density samples NFC NFC-Si-SH TO-NFC TO-NFC-SiSH

ρa (kg m−3)a ρs (kg m−3)b 7.21 12.11 6.94 11.37

1500 1280 1500 1269

porosity (%)

BET surface area (m2/g)

99.51 99.05 99.53 99.10

29.99 18.47 43.51 43.57

a ρa is the apparent density of the aerogels. bρs is the density of the solid scaffold.

Owing to the somewhat thicker diameter of cellulose nanofibrils (See Figure S1), the relatively lower surface area was noted for the NFC and its MPTs-modified aerogels, as compared to TO-NFC counterpart ones. Unlike the TO-NFC aerogel, NFC aerogels exhibited the decreased specific surface area after the modification by MPTs. The microstructure of TO-NFC and TO-NFC-Si-SH aerogels was further examined by SEM, and the pictures are shown in Figure 2. Both unmodified and mercaptosilylated TONFC aerogels displayed an interconnected porous morphology consisting of many thin sheets (Figure 2a and c). These sheets were considered to be formed by self-aggregation of cellulose nanofibrils during the freezing-drying step, at which the ice crystals were formed.54−56 But in the magnified images in Figure 2b and d, the surface texture of the TO-NFC-Si-SH aerogel appeared somewhat coarser than that of TO-NFC presumably due to the coverage by polysiloxane layers Compressive Properties. Excellent shape recovery or mechanical durability is of particular significance for recycling and reusing aerogel-type adsorbents. For this purpose, the shape-recovery properties of the unmodified and MTPsmodified aerogels were evaluated by compression tests (Figure S2a). The native aerogels (i.e., both TO-NFC and NFC) were very fragile, while the modified ones behaved much more flexibly and could be manipulated without breaking upon multiple cyclic compressions. This high flexibility was rarely observed in traditional inorganic silica aerogels. A similar flexible behavior has been observed for methyltrimethoxysilane (MTMs) modified nanocellulose sponges.31,56 The enhanced flexibility after the silylation was attributed to a decrease in cross-linking density within NFCs as well as the repulsive interactions existing between alkyl groups of polysiloxanes.57 The stress−strain curves of the aerogels that were subjected to

Figure 2. SEM micrographs of cryo-fractured cross-section surfaces of aerogel absorbent (a) TO-NFC aerogel and (c) TO-NFC-Si-SH aerogel. (insets) Magnified images of b and d. 11718


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Figure 3. (a) Compressive stress−strain curves of different aerogels. (b) Thickness recovery of different aerogels upon the unloading from a compressed state (ε = 50%). Note: the relative thickness of the aerogels after unloading is illustrated by the columns.

Figure 4. (a) Adsorption of Hg(II) and (b) zeta potentials of aquesous milled TO-NFC-Si-SH aerogel suspension at different pH values. Note: Hg(II) concentration = 30 mg/L, m(aerogels)/V(solution) = 0.2 g/L, temperature = 25 °C, adsorption time = 6 h.

values were Hg(OH)3−, Hg(OH)2, and Hg(OH)+ compound forms, which had smaller effective size and higher mobility than Hg(II).62−66 Some studies have revealed that Hg(OH)2 was able to dissolve in case the initial Hg(II) concentration was less than 120 mg/L in the solution,65,66 which has been confirmed by our experimental observation. Because the Hg(II) concentration adopted in this work was 30 mg/L, the hydrolysis of Hg(II) ions would not disturb their adsorption onto the aeorgel. These above results manifested that the adsorption capacity of the TO-NFC-Si-SH aerogel was less sensitive to the variation of pH values. This was quite different from the widely investigated chitosan-based adsorbents, whose adsorption capacity of Hg(II) was drastically decreased at low pH values due to the protonation of free amino groups although the adsorption capacity was very high at the pH value close to 7.14,67−69 Therefore, this advantage of the TO-NFC-SiSH aerogel was undoubtedly preferred in the practical treatment of Hg(II)-containing wastewater that could be acidic in nature. Herein, pH = 7.0 was chosen for the subsequent adsorption studies due to the optimal removal effect. Sorbent dosage is another important parameter for the costeffective application of adsorbents. Figure S3a presents the effects of the TO-NFC-Si-SH aerogel dosage on removal efficiency. With increasing the dosage from 0.05 to 0.2 g/L, the removal efficiency was increased from 66 to 97%, followed by a level-off. The minimal dosage of the TO-NFC-Si-SH aerogel to attain the adsorption equilibrium was much lower than that of previously reported sorbents.7,11 This advantage was thus favorable from the economic consideration of wastewater

It is known that the solution pH value is among the important factors during the adsorption process of metal ions since it would influence the ionization level of the sorbents and species forms of the adsorbates.56 Effects of pH values on removal efficiency of Hg(II) were examined (Figure 4a), and the zeta potential values at different pH values (1−11) was also determined for the TO-NFC-Si-SH aerogel (Figure 4b). In the pH values ranging from 5 to 9, the highest removal efficiency (∼97%) was achieved. Within this pH range, the surface zeta potential of the aerogel was found to be the most negative (less than −33 mV), which would yield the strongest electrostatic attraction to Hg(II) ions for the capture by active adsorption groups on the surfaces. Therefore, the TO-NFC-Si-SH aerogel exhibited the optimum removal capacity in this case. With further decreasing pH values, the absolute value of zeta potential was markedly reduced so that the surface activity of the aerogel toward Hg(II) was weakened. Meanwhile, the intense competition between H+ and Hg2+ ions for active binding sites on the aerogel could occur. As a result, a lower removal efficiency of Hg(II) was achieved. But it needed to note that the efficiency of the aerogel was still close to 80% at pH = 1. When the pH value was increased from 9 to 11, the removal efficiency of Hg(II) somewhat declined and but still remained at a level of ∼90%. Since the zeta potential value only became slightly less negative in this case (−33.0 vs −31.8 mV), the reduced surface activity on the adsorbent was hard to account for a decrease in the removal efficiency. The decrease was likely because the more preferable species of Hg(II) at higher pH 11719


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Figure 5. (a) Residual concentration (left Y axis) and removal efficiency (right Y axis) of 1 mg/L Hg(II) as a function of time for the TO-NFC-Si-SH aerogel. (b) Comparison of removal efficiency as a function of contact time for different samples. Note: m(aerogels)/V(solution) = 0.2 g/L, pH = 7, temperature = 25 °C.

Figure 6. (a) Adsorption kinetics of the TO-NFC-Si-SH aerogel in an aqueous Hg(II) solution. (b) Adsorption isotherms of Hg(II) on the TONFC-Si-SH aerogel at the different initial Hg(II) concentrations ranging from 1 to 410 mg/L. Note: m(aerogels)/V(solution) = 0.2 g/L, pH = 7, temperature = 25 °C.

treatment.42,70,71 The dosage value of 0.2 g/L was chosen from the consideration of the cost-effective adsorption in the following work. The effects of initial concentrations of Hg(II) ions on the removal efficiency of the TO-NFC-Si-SH aerogel are shown in Figure S3b. The initial concentration Hg(II) was examined in a wide concentration range, i.e. 0.01, 0.3, 1, 15, 30, 50, 85, and 97 mg/L. The highest removal efficiency up to 99.5% was achieved at the initial Hg(II) concentration of 1 mg/L. When the initial concentration of Hg(II) was reduced to 0.01 mg/L, the efficiency was slightly decreased to 94.4%. With further increasing Hg(II) concentration up to 97 mg/L, the efficiency still maintained above 85%. To gain an in-depth insight into adsorption and applicative characteristics of the TO-NFC-SiSH aerogel for the advanced treatment of Hg(II)-containing wastewaters, the adsorption capacity at the initial Hg(II) concentration of 1 mg/L was examined. Both residual concentration and removal efficiency of Hg(II) as a function of time for the TO-NFC-Si-SH aerogel are shown in Figure 5a. The adsorption equilibrium of the TO-NFC-Si-SH aerogel was rapidly reached in less than 1 h. And the removal efficiency and residual concentration of Hg(II) ion in the solution were determined to be ∼99.5% and ∼4.5 μg/L, respectively. This residual concentration has been below the limit for the wastewater discharge (10 μg/L) and was even close to the one (2 μg/L) for drinking water set by US Environmental Protection Agency. The superior adsorption performance at the

low concentration of Hg(II) demonstrated that the bioderived aerogel could be used as a highly sensitive sorbent. Since activated carbon is well-known to be one of the most widely used adsorbents in the practical treatment of wastewater, we further compared the adsorption efficiency of TO-NFC-SiSH aerogel with that of commercial active carbon with a high specific surface up to 1482 m2/g (Figure S4a). Noteworthy, at the same initial concentration (30 ppm) of Hg(II) and adsorbent dosage, the removal efficiency and time to attain the ultimate adsorption equilibrium of the TO-NFC-Si-SH aerogel were markedly superior to that of the activated carbon, as illustrated in Figure 5b. It needed to be mentioned that the TO-NFC-Si-SH aerogel used in this work had a specific surface area of only 43 m2/g (Figure S4b). Therefore, we can conclude that the impressive Hg(II) removal capability of the TO-NFCSi-SH aerogel should be attributed to a high density of active adsorption groups on the surfaces rather than physical adsorption dominated by its specific surface area. Adsorption Kinetics, Equilibrium, and Thermodynamics. The investigation of adsorption kinetics represents one of the important approaches in evaluating the performance of a given sorbent and an irreplaceable means in gaining useful information regarding rates and mechanism of sorption process.72 For this purpose, the kinetics of the TO-NFC-SiSH aerogel during the adsorption process was analyzed by fitting experimental data using the pseudo-first-order and pseudo-second-order models (the data illustrated in Figure 11720


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ACS Sustainable Chemistry & Engineering 6a), respectively. The linear fitting results are summarized in Table S1. The theoretical qe value matched the experimental data more closely (i.e., 139.52 vs 140.25 mg/g), and a much higher correlation coefficient (R2) was found for the pseudosecond-order equation. This result suggested that the pseudosecond-order kinetics was much better than those of pseudofirst-order in an attempt to describe the kinetics for Hg(II) adsorption onto the TO-NFC-Si-SH aerogel. And the adsorption process of Hg (II) onto the aerogel was dominated by chemisorption, which was in coincidence with the conclusion drawn in other thiol-functional sorbents.39−43 To examine interactive behaviors between the adsorbent and adsorbate at the equilibrium and to estimate the maximum Hg(II) adsorption capacity of the TO-NFC-Si-SH aerogel, effects of initial Hg(II) concentrations on the adsorption was also presented. The isotherm of Hg(II) adsorption on the TONFC-Si-SH aerogel as illustrated in Figure 6b, was fitted with the widely used Langmuir and Freundlich isotherm models, respectively. And the corresponding fitting parameters are also outlined in Table S2. Compared with the Freundlich isotherm, the Langmuir one appeared more suitable in describing adsorption behaviors of Hg(II) onto the TO-NFC-Si-SH aerogel due to a higher correlation coefficient (R2 = 0.998 vs 0.835) of the latter, suggesting the monolayer adsorption.73 The value of the separation factor constant (RL) was between 0 and 1, indicative of a favorable adsorption process.6 The theoretical maximal adsorption capacity for Hg(II) was estimated to be 729.9 mg/g, which was quite close to the experimental value of 718.5 mg/g corresponding to ca. 3.58 mmol/g. The maximum adsorption capacity (qm) of the TO-NFC-SiSH aerogel was compared to that of various previously reported biosorbents in term of the removal effect of Hg(II) (Table S5). It was found that the qm value of the TO-NFC-Si-SH aerogel was superior to that of these reported biosorbents except commercial chitosan (CS) powder74 with a high surface area. However, these (CS)-based materials tend to suffer from the drawbacks, such as inconvenience to be recycled for the reuse, much inferior adsorption capacity at low pH values (a reduction of even more than 50%), and poor durability due to the intrinsic brittleness.14,67−69 Although it has been reported that the coating of CS materials with magnetic particles was an alternative to markedly improve the recyclable performance,9−11,14 the adsorption capacity of these magnetic sorbents, e.g. magnetic CS-phenylthiourea (CSTU) resin (135 ± 3 mg/g),11 magnetic CS-glutaraldehyde (MCS-GA) (96 mg/ g),14 still remained relatively lower in comparison to TO-NFCSi-SH aerogel in our work. Moreover, our aerogel-type adsorbent not only displayed the supersorption capacity for Hg(II) but also was allowed to be readily recycled or collected due to its high floatability and good mechanical flexibility. It could be more practically feasible in the treatment of Hg(II)containing wastewater. Thermodynamic studies can provide the detailed information regarding inherent energetic changes during the process of adsorption. In this work, effects of temperature on Hg(II) ion adsorption onto the TO-NFC-Si-SH aerogel were illustrated by drawing a linear plot of ln Kd versus 1/T in Figure 7, and the estimated thermodynamic parameters and correlation coefficients are summarized in Table S3. Under steady-state reaction conditions, the Gibbs free energy (ΔG°) ranged from −13.37 to −20.72 kJ/mol, and ΔH° and ΔS° were equal to 48.16 and 0.21 kJ/mol·K, respectively. The positive values of

Figure 7. Plot of ln Kd versus 1/T for Hg(II) adsorption on the TONFC-Si-SH aerogel at different temperatures.

ΔH° revealed that the adsorption of Hg(II) ions on the TONFC-Si-SH aerogel was endothermic.39 The negative value of ΔG° indicated that the adsorption of Hg(II) was spontaneous, and the ΔG° values decreased with elevating temperature. This implies that a higher temperature favored the spontaneous adsorption of Hg(II) ions by the TO-NFC-Si-SH aerogel. Adsorption Mechanism. Since both thiol and carboxyl groups on the TO-NFC-Si-SH aerogel had binding affinities toward Hg(II), there existed possible competitions between both kinds of active groups during the adsorption. To elaborate underlying adsorption mechanism of the TO-NFC-Si-SH aerogel, we first estimated the numbers of thiol and carboxyl groups anchored onto the TO-NFC-Si-SH aerogel (see the experimental section in the SI). Based on the sulfur content (Table 1) of TO-NFC-Si-SH and DO value of TO-NFC, the amount of thiol and carboxyl groups on the TO-NFC-Si-SH aerogel were determined to be 3.33 and 0.47 mmol/g, respectively. Assuming that each two negatively charged carboxylates bound one Hg(II) ion, the adsorption capacity of Hg(II) by thiols was about 3.34 mmol/g after the adsorption one by carboxyls (i.e., 0.24 mmol/g) was deducted from the experimental adsorption amount of Hg(II) (i.e., 3.58 mmol/g). It is supposed that each thiol on the TO-NFC-Si-SH aerogel was likely to complex with average one Hg(II) ion on maximum, which was supported by the equivalent molar amount between S and Hg elements in the EDX analysis result of TO-NFC-Si-SH aerogel after the adsorption of Hg(II) (Figure 8d). Since the theoretical adsorption contribution from carboxyl ones only occupied about 6.7% in the total amount of adsorbed Hg(II), the supersorption capacity of Hg(II) for the TO-NFC-Si-SH aerogel predominantly arose from the contribution of thiol groups having much greater quantities other than carboxyl ones. According to the hard−soft acid base (HSAB) theory, Hg(II) ions are classified as a Lewis soft acid, while thiol and carboxylate groups belong to Lewis soft and hard bases, respectively.75 Based on the rule that soft base-soft acid gives priority to the formation of a stable complex,75 the thiols on the TO-NFC-Si-SH aerogel tended to preferentially complex with Hg(II) ions in comparison to the carboxyls. Also, effects of mercapsilylation on adsorption properties of NFC without carboxyl groups and TO-NFC aerogels were further compared. Figure 5b shows the dependence of removal efficiency as a function of contact time for the native and MPTs-modified aerogels. Compared to the NFC aerogel that almost did not adsorb Hg(II) ions, the TO-NFC one rapidly 11721


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Figure 8. (a) Wide-scan XPS spectrum of TO-NFC-Si-SH aerogel before and after adsorption. High-resolution core-level spectra of S 2p before (b) and after (c) adsorption. (d) EDX element analysis spectrum of the TO-NFC-Si-SH aerogel after the adsorption of Hg(II) ions. (e) EDX mapping image of Si, S, and Hg elements. (f) Magnified image of EDX element mapping. (g) Hg element mapping. (h) S element mapping. (i) Schematic illustration of proposed scheme adsorption mechanism of Hg(II) ions by the TO-NFC-Si-SH aerogel.

achieved the adsorption equilibrium with a removal efficiency of only 23%. The low adsorption capacity of the TO-NFC aerogel should arise from the contribution of its carboxylate groups that has had a lower affinity to Hg(II) than thiols, as mentioned earlier.75 But regardless of the presence of carboxyl groups, both of MPTs-modified aerogels exhibited the almost same equilibrium efficiency that was higher than 90%. Again, it was clearly demonstrated that thiol groups played a predominant role in the removal of Hg(II) in our case. But the equilibrium one was attained for the TO-NFC-Si-SH aerogel within 5 h, which was shorter than that of the NFC-SiSH one merely containing thiol groups (i.e., 10 h). This may be

attributed to the coexistence of both thiol and carboxyl groups on the surfaces of TO-NFC-Si-SH aerogel, yielding more accessible active sites for the rapid uptake of Hg(II) ions. In order to identify the binding of thiol groups with Hg(II), the variation of XPS spectra of the TO-NFC-Si-SH aerogel before and after the adsorption is also given (Figure 8a). It was noted that new peaks for Hg 4f5, Hg 4d3, and Hg 4d5 were visible after the adsorption. Figure 8b and c further presents high-resolution S 2p core-level spectra of the TO-NFC-Si-SH aerogel. Before the adsorption, two major peaks at 163.7 and 162.5 eV were attributed to the C−S and S−H bonds of MPTs. After the adsorption, the S 2p binding energies of C−S and S− 11722


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ACS Sustainable Chemistry & Engineering H bonds were slightly decreased, which may be due to the electron-donating effect from S atoms of C−S and S−H bonds to Hg(II). Besides, a new peak assigned to S−Hg bond was visible at 161.3 eV, confirming that the complexation between Hg(II) and S species indeed occurred. The conclusion was also supported by visualizing the EDX mapping pictures (Figure 8e−h), in which the yellow points denoting Hg elements appeared to closely attach with red points representing S elements, as shown by the inset of Figure 8f. The aforementioned adsorption contribution arising from carboxyl groups was also supported by the variation of both CO groups in the FT-IR adsorption of the TO-NFC-Si-SH aerogel and its XPS O 1s core-level spectrum before and after the Hg(II) adsorption (Figure S5). On the basis of the aforementioned results, the dominant adsorption mechanism of Hg(II) onto the TO-NFC-Si-SH aerogel was proposed in Figure 8i. The negatively charged surface characteristics of the aerogel yielded electrostatic interaction to free Hg(II) ions, which then caused these ions to rapidly approach the surfaces for the adsorption predominantly by more active and abundant thiol groups anchored onto the aerogel. In this case, each Hg(II) ion was bound by approximately one thiol through chemical complexation. Besides, a small amount of Hg(II) ions was captured through electrostatic interaction by carboxyl groups on the surfaces. Selective Removal of Hg(II) Ions and Application. Considering that Hg(II) often coexists with other heavy metals in real wastewater, we further investigated selective adsorption properties of TO-NFC-Si-SH aerogel for Hg(II) ions in the presence of other common heavy metal ions. In this work, Cu(II), Cd(II), Pb(II), and Zn(II) were chosen as the coexisted metal species because they have been reported to have interfering effects on the adsorption of Hg(II) ions onto adsorbents.11,15,76 And the initial concentration of all heavy metal ion species was fixed at 30 ppm and the competitive adsorption efficiencies of these metal ions are illustrated in Figure 9. In the coexistence with the other heavy metal ions, the

was much less affected by the presence of other heavy metal ion species. Based on the HSBA theory, Hg(II) and Cd(II) ions were soft acids, while Cu(II), Pb(II), and Zn(II) ones were borderline ones. Thus, thiol groups belonging to the soft base would give priority to the complexation with the soft acid. Besides, because chemical hardness of Hg(II) was lower than that of Cd(II), the thiol groups would bind Hg(II) more stably and preferentially.75 In order to preliminarily survey the feasibility of the asprepared TO-NFC-Si-SH aerogel for the removal of Hg(II) from wastewater samples containing complicated compositions, a batch adsorption experiment was conducted on the Hg(II)containing simulated chloralkali wastewater that was prepared according to the formulation described in the previous literature.77 And its specific characteristics and composition are summarized in Table S4. Since the simulated chloralkali wastewater contained high concentrations of many other ion species, the adsorption of Hg(II) onto the adsorbents was expected to be interfered. Noteworthy, after the adsorption by the TO-NFC-Si-SH aerogel, it was revealed that 97.8% Hg(II) ions can be removed from the simulated wastewater. This result evidenced that the aerogel-type adsorbent would have a promising potential in removing Hg(II) ions from wastewater under practical conditions. In the next-step study, we will evaluate the performance of this adsorbent in the real wastewater containing Hg(II) from a variety of sources, such as Hg mining industry and fluorescence lamp factories, etc. Reuse of Adsorbents. For an adsorbent material, the reusability is also of very important concerns in the practical applications because desorption results may facilitate us to get a better understanding of the feasibility to recycle the adsorbent and to recover Hg(II) from aqueous solutions. In this work, the reusability of the TO-NFC-Si-SH aerogel was investigated by using 1 M hydrochloric acid solution with 5 wt % thiourea as an eluent to regenerate TO-NFC-Si-SH aerogel. The reusability of the TO-NFC-Si-SH aerogel is presented in Figure 10. The removal percentage of Hg(II) was slightly

Figure 9. Effect of coexisting heavy metal ions on Hg(II) adsorption capacity onto the TO-NFC-Si-SH aerogel.

Figure 10. Adsorption−desorption cycles of the TO-NFC-Si-SH aerogel for Hg(II) solution.

removal efficiency of Hg(II) was still the highest and reached 93%, which was only slightly lower than that in the presence of Hg(II) alone (97%). For the other four kinds of metal ion species, the efficiency in the coexisted state was decreased to varying extent in comparison to their states alone. Comparatively, the best adsorption selectivity of the aerogel was noted for Hg(II), since the adsorption capacity of Hg(II) ions

decreased from 97.5 to 93.2% after four adsorption/desorption cycles. In each cycle, the desorption percentage was more than 90%. Surprisingly, the adsorption efficiency was not declined significantly and the adsorbent maintained overall structural integrity after four cycles. These results showed that TO-NFCSi-SH aerogel would be used for the multiple treatments of Hg(II)-containing wastewater, in which the regeneration of adsorbents was preferred to reduce operation costs. 11723


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ORCID

CONCLUSIONS In this work, a novel nanocellulose aerogel containing both thiol and carboxyl groups (TO-NFC-Si-SH) was fabricated through a facile freeze-drying of TEMPO oxidation NFC suspension in the presence of mercaptopropylsiloxane (MPTs) sols. The aerogel-type adsorbent exhibited excellent shape recovery properties upon the release of compression. Due to the abundant thiol groups anchored onto the surfaces, it could highly effective remove off Hg(II) ions up to 92% in a wide initial concentration range from 0.01 to 85 mg/L, and the adsorption capacity was less compromised by the variation in pH values of Hg(II) solutions over a wide pH range (including very acidic conditions). The adsorption of Hg(II) onto the aerogel well fitted Langmuir isotherm and pseudo-second order kinetics with the maximum adsorption capacity as high as 718.5 mg/g that surpassed the most of the majority of reported biosorbents but possessed the optimum flexibility and ease of recyclability. And, thermodynamic calculations suggested that the adsorption process of Hg(II) onto the modified aerogel was an endothermic process. The detailed adsorption mechanism of Hg(II) onto the TO-NFC-Si-SH aerogel was investigated. Furthermore, the adsorbent still exhibited excellent selective removal effect on Hg(II) ions from the aqueous solution containing five kinds of heavy metal ion species. And for simulated chloralkali wastewater containing Hg(II) ions, the novel TO-NFC-Si-SH aerogel also displayed a high removal efficiency up to 97.8%. Noteworthy, the easily recyclable supersorbents could be reused without the pronounced loss in the removal efficiency of Hg(II) after multiple consecutive adsorption/desorption cycles. Superior adsorption performance for Hg(II) ions in combination with excellent selectivity and reusability makes this thiol-functionalized aerogel derived from renewable resources a very promising biosorbent for potential applications in the practical treatment of wastewater.

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Hongzhi Liu: 0000-0002-1725-0976 Notes

The authors declare no competing financial interest. ⊥ The cofirst authors, B.G., H.W., and S.W., contributed equally to this work.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from Public Welfare Projects of Zhejiang Province (nos. 2016C33029, 2017C33113, and 2015C33050), National Natural Science Foundation of China (no. 21677131), and Scientific Research Foundation of Zhejiang Agriculture & Forestry University (no. 2013FR088).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03188. Experimental section. Figure S1: TEM images of NFC (a) and TO-NFC (b). Figure S2: Compression test pictures of the TO-NFC-Si-SH aerogel. Figure S3: Removal efficiency curves of Hg(II) against adsorbent dosage (a) and the initial Hg (II) concentrations (b). Figure S4: N2 adsorption−desorption isotherms. Figure S5: FT-IR spectra (a) and XPS O 1s core-level spectra (b) before and after the adsorption of Hg(II) onto the TO-NFC-Si-SH aerogel. Table S1: Kinetic models parameters. Table S2: Parameters of the isotherm models. Table S3: Thermodynamic parameters. Table S4: Composition of simulated chloralkali wastewater used in this work. Table S5: Comparison of the previously reported biosorbents with our aerogel adsorbent (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-0571-63746552. Fax: +86-0571-63730919. E-mail: [email protected] (H.L.) *E-mail: [email protected] (S.W.). 11724


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