Development of Novel Diol-Functionalized Silica Particles toward Fast

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Development of novel diol-functionalized silica particles towards fast and efficient boron removal Yupan Tang, Tai-Shung Chung, Martin Weber, and Christian Maletzko Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03115 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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Industrial & Engineering Chemistry Research

Development of novel diol-functionalized silica particles towards fast and efficient boron removal

Yu Pan Tanga, Tai Shung Chunga,*, Martin Weberb, Christian Maletzkoc

a

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 b

Advanced Materials & Systems Research, BASF SE, RAP/OUB - B001, 67056 Ludwigshafen, Germany

c

Performance Materials, BASF SE, G-PM/PU, 67056 Ludwigshafen, Germany

*

Corresponding author

Email: [email protected] Fax: (65)67791936

1 ACS Paragon Plus Environment


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Abstract

2

Ion-exchange adsorption may be a promising way to tackle the boron contamination in

3

various waterbodies on condition that an effective boron-specific adsorbent with fast sorption

4

kinetics, high efficiency and capacity, easy regeneration and low cost is accessible. In this

5

work, a group of novel silica-based adsorbents were synthesized for boron removal, with the

6

objectives of assessing their adsorption behaviors and improving their boron separation

7

performance. The adsorption efficiency was systematically evaluated and optimized under

8

various synthesis and operating conditions, i.e., reactant ratio, chelating temperature, particle

9

loading, contact time and ion strength. In addition, the adsorption kinetics and isotherm were

10

adequately demonstrated. The adsorption kinetics followed the pseudo-second order kinetic

11

model while the adsorption isotherm was described by Langmuir, Freundlich and Sips

12

models. The silica adsorbent exhibited a high adsorption rate; equilibrium was reached in few

13

minutes, due to its high hydrophilicity and non-tortuous structure. A high adsorption capacity

14

was predicted and a heterogeneous sorption behavior was validated by the isotherm models.

15

Finally, regeneration performance of the adsorbent in both batch experiments and liquid

16

chromatographic (LC) column-based experiments demonstrated that the adsorption capacity

17

was marginally sacrificed (less than 10%) after three cycles of measurements, illustrating

18

promising reusability. These findings may open up new ways to design high-performance

19

boron-specific adsorbents.

20 21

Key words: boron removal; diol-functionalized silica; adsorption kinetics; adsorption

22

isotherm; regeneration


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1.

2

Boron is not only an essential micronutrient for living beings, but also an important raw

3

material for numerous industries, e.g. the production of fiberglass, detergents, fuel cell, etc.1,2

4

However, it becomes toxic to plants and animals once exceeding a certain critical dose.3,4 The

5

boron concentration in drinking water has been regulated in most countries and regions, for

6

example, 2.4 mg L-1 recommended by WHO.5 In certain industries such as the semiconductor

7

manufacturing sector, the control of boron in ultra-pure water is much more stringent because

8

boron is considered as a p-type impurity which may not only invert the n-type silicon but also

9

cause an adverse effect on the concentration of carriers.6 Therefore, it is of paramount

10

importance to remove boron from waterbodies in order to produce eligible water for various

11

uses.

Introduction

12 13

A number of technologies such as reverse osmosis (RO),7-10 forward osmosis (FO),11-14 ion

14

exchange with boron-specific resins (BSRs),15-18 hybrid adsorption-membrane filtration

15

(AMF),19-21 and membrane distillation (MD)22-24 have been proposed to remove the boron

16

from water. We have summarized the state-of-the-art deboronation technologies in a recent

17

review article.25 We found that no simple and effective solution has been developed yet. RO

18

seems to have great potential owing to its superior capability in desalination. However, it is

19

largely incapacitated due to the facts that: (1) the boric acid molecule has no charge at the

20

neutral condition and (2) its size is similar to the water molecule.26 In other words, boron can

21

hardly be effectively separated by either size exclusion or Donnan exclusion. As a

22

consequence, a low rejection of 40-80% to the non-dissociated boric acid was attained in RO

23

processes under normal operation conditions.27,28 On the other hand, multi-pass RO systems

24

are proposed to reduce boron concentration effectively,7 which, nevertheless, involves

25

additional energy and costs. The AMF process is attractive because it combines the sorption 3 ACS Paragon Plus Environment


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process with membrane separation. The technology has not been fully developed yet and the

2

effective methods to regenerate adsorbents remain to be demonstrated.29,30 MD have been

3

revealed to have very high rejections to non-volatile solutes. However, it is still considered to

4

be an energy-intensive process unless powered by waste heat or solar energy.31

5 6

Among various approaches, ion-exchange adsorption may be the most efficient way for

7

boron removal.18,32 In order to form a stable complex, the adsorbent should bear diol or

8

polyol groups which must orient properly to match the structural parameters of the

9

tetrahedrally coordinated boron.3,33 For example, a typical commercially available BSR

10

consists of a macro-porous polystyrene backbone and a boron-specific functional group based

11

on N-methyl-D-glucamine (NMDG).34 However, such kind of resin suffers from a number of

12

disadvantages. Firstly, the hydrophobic polystyrene polymer hinders the diffusion of

13

hydrophilic solute molecules into the resin and leads to slow adsorption.18 Secondly, the

14

frequent swelling and shrinking of the resin during the adsorption-desorption cycles could

15

shorten its lifetime and lower its packing density.35 Dead volumes are also often found within

16

the packed column.36 Thirdly, the pressure drop across the packed ion exchange column can

17

be significant which leads to high energy consumption.37,38

18 19

Exploration of new high-performance adsorbents is one of the most important imperatives for

20

the large-scale implementation of this treatment. As backbone materials, inorganic silica

21

particles may be more advantageous than the reported polymeric resins. First of all, due to

22

their hydrophilic feature, silica particles may have high wettability in boron solutions such

23

that the contact of functional groups with boron may be sufficient and fast. Secondly, a high

24

density of diol functionalization may be achievable due to their versatility in chemical

25

reactions. Thirdly, other than the polymeric BSRs, the non-tortuous structure of the silica


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particles may facilitate the boron diffusion and hence, result in a fast adsorption process.

2

Therefore, silica particles are promising adsorbents in virtue of their high hydrophilicity, high

3

density of functional groups, fast sorption kinetics and low cost. So far there are only limited

4

studies to explore silica as an adsorbent for boron removal.35,39-41 However, all these studies

5

are restricted to the widely known NMDG functionalization. Thus, breakthroughs on silica

6

functionalization are urgently needed.

7 8

In this work, a group of novel diol-functionalized silica particles were synthesized for boron

9

separation from water. The effects of synthesis and adsorption parameters were

10

systematically studied and compared with regard to the adsorption performance. The

11

adsorption behaviors were correlated with the pseudo-second order kinetic model and three

12

isotherm models in order to assess the optimal contact time and adsorption capacity.

13

Regeneration of the functionalized silica was also investigated. Finally, a LC column-based

14

system was demonstrated for continuous operations of boron removal from boron-containing

15

water. Results indicated that the silica particles exhibited an extremely high adsorption rate,

16

outstanding adsorption capacity and excellent reusability, which may be promisingly applied

17

for commercial use.

18

2.

19

2.1. Materials

20

The 3-aminopropyl-functionalized silica gel with a functional group loading of 1 mmol g-1

21

and an average particle size of 50 µm was purchased from Sigma-Aldrich. Glycidol (96%)

22

from Sigma-Aldrich, sodium chloride from Merck and sulfuric acid (96.3%) from J.T. Baker

23

were used as received without further purification. Boric acid (99.9%) provided by Sigma-

24

Aldrich was used to prepared the boron solution.

Experimental



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2.2. Synthesis of diol-functionalized silica particles

2

The diol-functionalized silica particles were synthesized by grafting glycidol on the 3-

3

aminopropyl-functionalized silica (Silica-NH2) via the epoxy-amine nucleophilic addition

4

reaction.42,43 Figure 1 illustrates the synthesis routes. Briefly, the Silica-NH2 particle was first

5

dispersed in DI water to a solid content of 10 wt% via vigorous ultra-sonication for 2 h.

6

Argon-purged glycidol was subsequently introduced dropwise into the silica suspension at

7

ambient temperature (23 ºC). The reactant ratio was varied based on their epoxy-to-amine

8

molar ratio, which was pre-set at 1:1, 2:1, 3:1, 5:1 and 10:1. The mixture was kept at 60 ºC in

9

a water bath with stirring to proceed the reaction for 3.5 h. Afterwards, the products were

10

extracted via centrifugation, washed with DI water to remove the excess glycidol and finally

11

dried at 100 ℃. These diol-functionalized particles were referred as Silica-N1, Silica-N2,

12

Silica-N3, Silica-N4, Silica-N5, respectively, according to the reactant ratios.

13

Figure 1

14

2.3. Batch experiments for boron removal

15

Effects of various parameters in synthesis and chelation processes were studied in batch

16

experiments. These variables of interest include reactant ratio, chelating temperature, particle

17

loading, contact time and ion strength, etc. In one experiment, a pre-determined amount of

18

silica particles was introduced in a 10 mL boron solution. The solution pH value was kept at

19

neutral (about 6.8) and the boron concentration was fixed at about 11.5 ppm unless otherwise

20

stated. The adsorbent loading, denoted as the molar ratio of the tertiary amines on silica

21

particles to the boron molecules (Equation 1), was varied in the range of 0-100. =

22 23


(1)

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where nA and nB are the amount of substance of the tertiary amines on the silica particles and

2

that of boron molecules in the solution, respectively.

3 4

The ion strength was adjusted by adding NaCl to the boron solution. The mixture was

5

vigorously stirred for a pre-set time duration. Afterwards, the silica particles were isolated via

6

centrifugation and the supernatant was collected for boron analyses by an inductively coupled

7

plasma optical emission spectrometer (ICP-OES). The boron rejection, R, was described by

8

the following equation: (%) = 1 −

9 10

× 100%

(2)

where Cp and Cf are the boron concentrations of supernatant and feed, respectively. The boron uptake (Q, mg g−1) was calculated based on the following equation: =

! " (#) ! " ()

(3)

11

The adsorption isotherm of Silica-N4 was carried out under the following conditions: a

12

contact time of 10 min, the chelating temperature at 23 ºC and a particle loading of 50 mol

13

mol−1 (= molar ratio of the tertiary amines on silica particles to the boron molecules). The

14

initial boron concentration was varied from 1 to 1000 ppm.

15

2.4. Multi-cycle studies

16

Reusability is one of the critical indexes of an eligible adsorbent. Regeneration of the BSRs

17

were generally accomplished via acidity adjustment.3 The recycling capability of the silica

18

adsorbents was evaluated as follows. Sulfuric acid was employed as the regenerant and its

19

concentration was varied from 0.125 M to 2.0 M. In general, 10 mL of acid was introduced to

20

the silica after it being saturated and isolated. The mixture was centrifuged after stirring on a

21

Stuart roller mixer for 2 h. The supernatant was analyzed by ICP-OES and the sedimentary

22

particles were washed thoroughly with DI water to remove the acid and bring pH back to 7 ACS Paragon Plus Environment


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neutral. A fresh boron feed solution of 10 mL was then introduced for the 2nd and 3rd

2

sorption-desorption cycles in the same manner.

3

2.5. LC column-based continuous adsorption

4

Figure 2 shows the LC column-based bench-scale system customized for continuous

5

adsorption measurements. The column was packed with 4.0 g of diol-functionalized silica

6

particles. Its internal diameter is 1.0 cm and effective length is 12.0 cm. The experiments

7

were performed at a constant flow rate of 0.5 ml min−1 and an ambient temperature of 23 °C.

8

The boron feed concentration and pH were kept at 50 ppm and neutral, respectively. A

9

circulation pump was employed to regulate the feed flow rate. Boron concentrations in the

10

feed and effluent were analyzed using ICP-OES. Regeneration of the column was carried out

11

using 0.125 M sulfuric acid.

12

Figure 2

13

2.6. Characterizations

14

The successful surface grafting of diols was evidenced by X-ray Photoelectron Spectroscopy

15

(XPS, Kratos AXIS UltraDLD, Kratos Analytical Ltd., England). The characterization

16

procedures were introduced elsewhere.44,45 Generally, wide scans in the binding energy range

17

of 0-1100 eV and narrow scans of core-level C1s were performed on the pristine and diol-

18

functionalized silica particles. All binding energies (BEs) were referenced to that of the

19

neutral C 1s hydrocarbon peak at 284.7 eV. The core-level C1s spectra were deconvoluted

20

with the aid of a XPSSPEAK41 software by applying the respective binding energy of each

21

element-related structure involved in different chemistry environments. Surface elemental

22

stoichiometries were determined from peak-area ratios, after correcting with the

23

experimentally determined sensitivity factors. The morphology of the silica particles was

24

observed by a field emission scanning electron microscope (FESEM JEOL JSM-6700F). To


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prevent charging effects, the samples were coated with platinum using a JEOL JFC-1300

2

Platinum coater.

3

3.

4

3.1. Characterizations of the diol-functionalized silica particles

5

Figure 3 summarizes the XPS wide scanned spectra of the pristine and diol-functionalized

6

silica particles. The peaks of Si 2p, O 1s, C 1s and N 1s can be identified from all the spectra.

7

Core-level spectra of C1s and their de-convolution results were plotted in Figure 4. The C1s

8

spectrum of the pristine silica was deconvoluted into three peaks at the binding energies of

9

approximately 284.0, 284.7 and 286.0 eV, corresponding to the C-Si, C–C, C-N bonds,

10

respectively.46 Upon reaction with glycidol, one more peak appears at 287.0 eV, attributed to

11

the C-O groups.20

Results and discussion

12

Figure 3

13

Figure 4

14

Figure 5 presents the oxygen-to-silicon atomic ratios of the silica particles derived from XPS

15

analyses. The three dotted lines indicate the ratios for three ideal scenarios: the pristine silica,

16

the functionalized silica with all primary amine groups completely and solely converted to

17

secondary amine groups (Figure 1 (b)), and the one with those completely converted to

18

tertiary amine groups (Figure 1 (a)). It is interesting to observe that the experimental values

19

are all higher than the ideal values. In addition to the instrumental error, the physical

20

adsorption of excess glycidol on the particles via hydrogen bonding may be another reason.

21

Figure 5

22

Figure 6 shows the FESEM morphology. The particle size of the pristine silica is in the range

23

of 40-80 µm which is quite consistent with the reported value from the supplier. Figure 6 (b)

24

presents the image of the pristine particles after 2 h ultra-sonication. It appears that the

25

particle size does not change much in comparison to that of the pristine silica. However, the



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1

particle size significantly decreases after diol functionalization possibly due to the trituration

2

by the vigorous magnetic stirring during the reaction with glycidol.

3

Figure 6

4

3.2. Parameter studies on the adsorption behaviors

5

3.2.1.

6

Figure 7 (a) shows the boron adsorption performance of the five diol-functionalized silica and

7

their counterpart Silica-NH2 at adsorbent loadings of 10 and 50. The dosage of the reactant

8

glycidol defines the grafting density of the diol groups on the silica particles. The silica-NH2

9

has very low boron rejections at both adsorbent loadings. The boron rejection gradually

10

increases to a plateau as a function of epoxy-to-amine ratio due to the increasing diol content.

11

Figure 7 (b) shows good linear correlations between the boron rejection and the oxygen-to-

12

silicon ratio. A high grafting density is always preferred for a high adsorption capacity.

13

Silica-N4 was chosen for further studies in what follows since the rejections reached plateaus

14

at a reactant ratio of 5 for both loadings.

15

Effects of the reactant ratios

Figure 7

16

3.2.2. Effects of the adsorbent loadings and chelating temperatures

17

Adsorbent loading is another important factor in determining the effectiveness of boron

18

removal. Due to the kinetic and thermodynamic limitations, adsorption efficiency can also be

19

improved by providing more active sites.20 However, a high adsorbent loading would cause a

20

high volume of regenerant and high cost. To search for the optimal adsorbent loading, the

21

chelating temperature was kept at 60 °C for Silica-N5 while two temperatures, 23 and 60 °C,

22

were employed for Silica-N4 for comparison. Figure 8 shows the performance as a function

23

of adsorbent loading. The boron rejection dramatically increases before reaching a plateau at

24

a loading of 50. Since the chelation is a reversible process, chemical equilibrium at the solid-

25

liquid interface is responsible for such behavior. 10 ACS Paragon Plus Environment

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1

Figure 8

2

Another interesting observation is that the adsorption capacity is not affected by the chelation

3

temperature as shown in Figure 8 (b). This is somewhat at odds with our previous work

4

This is probably due to the inherently high chelation affinity between the boron molecules

5

and the diol groups as well as the low barrier for the boron diffusion.

6

3.2.3. Adsorption kinetics

7

Adsorption of boron was measured as a function of contact time to evaluate the adsorption

8

kinetics. The Silica-N4 was used for the measurements at two loadings. The boron rejection

9

in Figure 9 (a) exhibits a substantial increase at first and then turns to a plateau. Both loadings

10

display a similar trend and have the inflection points at 5 min, suggesting fast adsorption

11

processes for both cases.

20

.

12

Figure 9

13

A pseudo-second order kinetic model as expressed in Equation (3) was employed to fit the

14

experimental data.47

$

=

1

& %& '(

+

1

'(

(4)

15

where k2 is the rate constant (g mg−1 min−1) and Qeq is the amount of boron adsorbed at

16

equilibrium (mg g−1). The pseudo-first order model was not suitable for this work as it is for

17

high concentration range.40,48 Since the boron concentration is only 11.5 ppm (Section 2.3),

18

the pseudo-second order model was chosen. Figure 9 (b) presents excellent linear fitting for

19

both cases with the R2 > 0.9997. Based on the second order model, the half-adsorption time

20

(t1/2) was estimated by the following equation: *⁄& =

%&

1

(5) '(

21

The half-adsorption time is often used as a measure of the adsorption rate. It is defined as the

22

time consumed for the adsorbent to take up one-half of its equilibrium value. 11 ACS Paragon Plus Environment


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1 2

Table 1 shows the values of k2, Qeq and t1/2. It appears that a higher adsorption rate but a

3

lower equilibrium uptake are achieved for the case with a higher adsorbent loading, which

4

agrees well with those reported in the literature.39 The half-adsorption time attained in this

5

work is much shorter than those reported in the literature,40 indicating fast adsorption

6

kinetics. This may be due to the high hydrophilicity and the non-porous structure of the silica

7

particles which induce rapid wetting and provide excellent contact with boron molecules. In

8

general, the adsorption time is the sum of boron diffusion time and adsorption time, while the

9

diffusion consists of external diffusion in the bulk solution and internal diffusion in the

10

adsorbent matrix.40 In addition, diffusion is the dominant step in the adsorption process.34 The

11

slow adsorption observed for the BSRs may be caused by the slow internal diffusion rate of

12

boron molecules due to their tortuous hydrophobic matrix. Different from BSRs, the silica

13

adsorbents have a non-tortuous structure. The absence of internal diffusion results in their

14

extremely fast adsorption of boron molecules.

15

Table 1

16

3.2.4. Effects of ion strength

17

In seawater and produced wastewater, large amounts of cations and anions coexist with boron.

18

Therefore, the effects of ion strength are worthy of study. As shown in Figure 10 (a), the

19

boron rejections have very similar values with and without the presence of 0.6 M NaCl at

20

different adsorbent loadings. Figure 10 (b) displays that the NaCl concentration has

21

negligibly negative effects on boron rejection.

22

Figure 10


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1

3.3. Adsorption isotherm

2

The boron adsorption isotherm of Silica-N4 was studied at 23 °C and a adsorbent loading of

3

50. Figure 11 (a) shows the boron rejection as a function of boron concentration in the feed

4

while Figure 11 (b) shows the isotherm.

5

Figure 11

6

The adsorption isotherms are generally described by Langmuir and Freundlich isotherm

7

models

8

homogeneous adsorbent surface. The binding sites are also assumed to be energetically

9

equivalent and distant from each other. The Langmuir model can be represented by the

10

35,49

. The Langmuir model assumes a monolayer coverage of adsorbate over a

following equation: 1

'(

11

=

1

,,*

+

1 ,,* %. /'(

(6)

12

where Qm,1 is the adsorption capacity when the adsorbent surface is completely covered with

13

adsorbate. ceq is the boron concentration at equilibrium in the aqueous phase (mg L−1) and kL

14

is the Langmuir adsorption constant (L mmol−1). Figure 11 (c) shows the plot of 1/Qeq versus

15

1/ceq and Table 2 summarizes the values of Qm,1 and kL. Although it seems a good fitting with

16

R2 > 0.9999, the data points at the high concentration range diverge from the fitting line as

17

zoomed in the insert of Figure 11 (c). It should be noted that the speciation of boron highly

18

relies on its concentration. The mononuclear B(OH)3/B(OH)4- dominates at low boron

19

concentrations while polyborate species are dominant at concentrations higher than 220

20

ppm.1,3,4 The divergence is probably attributed to the formation of polyborates. The

21

adsorption at high boron concentrations may not be in a monolayer manner as proposed in

22

Figure 12. Thus, the Langmuir model may not be applicable to describe the whole

23

concentration range.

24

Figure 12



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Page 14 of 31

1

On the other hand, the Freundlich adsorption isotherm model describes the reversible

2

adsorption and is not restricted to the formation of the monolayer. A linear Freundlich

3

equation was therefore applied to the data:50 log

4

'(

= log %3 + log /'(

(7)

5

where kF is the Freundlich constant and n is the index of heterogeneity, which varies from 0

6

to 1; n=1 for homogenous materials. Figure 11(d) shows the plot of log Qeq versus log ceq and

7

Table 2 summarizes the values of kF and n values. It is interesting to observe that n is equal to

8

0.8864 at the low boron concentration range of 1-50 ppm and 0.3798 at the high range of 50-

9

1000 ppm. This phenomenon implies that the adsorption gradually becomes heterogeneous at

10

elevated concentrations. It is in consistency with the observation from Langmuir fitting. In

11

order to attain a close fitting, the Sips isotherm model, which combines the Langmuir and

12

Freundlich sorption, was employed to describe the adsorption behaviors:51,52

13 1

'(

14

=

1

,,&

+

1 5 ,,& (%4 /'( )

(8)

15

where Qm,2 is the adsorption capacity (mg g−1) and kS is the affinity constant for adsorption (L

16

mg−1). The equation becomes the Langmuir adsorption equation when n = 1, which means

17

that the sorption is homogeneous. Figure 11 (b) shows the fitting curve and Table 2

18

summarizes the values of Qm,2, kS and n. The Qm,2 calculated from the Sips model is much

19

higher than that predicted by the Langmuir model. This is probably attributed to the

20

formation of polyborate species at high boron concentrations as aforementioned, which

21

significantly increases the adsorption capacity. In addition, n is equal to 0.5407, indicating

22

that the adsorption is heterogeneous.

23

Table 2

24


Page 15 of 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

3.4. Multi-cycle studies

2

The ability to regenerate adsorbent materials is an important measure of their reusability. An

3

ideal adsorbent material must be easily regenerated in multiple cycles without sacrificing its

4

adsorption efficiency. High stability of the adsorbent material under acidic conditions as well

5

as minimum weight loss during regeneration are key factors for its large scale applications in

6

industries. To evaluate the reusability of the diol-functionalized silica, multi-cycle tests were

7

carried out. Figure 13 shows the results at different acid concentrations. The boron uptake at

8

equilibrium was normalized based on that of the first cycle. The boron uptake decreases to 95%

9

in the second cycle and shows almost no further decreasing in the third cycle. The decrease is

10

probably due to two reasons: the loss of silica particles during the centrifugation and the

11

incomplete decomposition of the diol-boron complex. Furthermore, the effectiveness of

12

regeneration seems not to be affected by the acid concentration in the range of 0.125-2.0 M.

13

Figure 13

14 15

3.5. Applications in LC column experiments

16

Figure 14 shows the breakthrough curve for three adsorption-desorption cycles in the LC

17

column-based system as shown in Figure 2. The adsorption capacity is 19.03 mg g−1

18

calculated from the first cycle, which is lower than the theoretical value derived from the Sips

19

isotherm model. This is because of the low boron concentration in the feed such that the

20

formation of polyborate species is limited. In other words, the adsorption behaviors may

21

follow the Langmuir calculation. In addition, the adsorption capacity is only sacrificed less

22

than 10% of its original value after three cycles of measurements, implying a promising

23

reusability of the diol-functionalized silica particles.

24

Figure 14

25 26

4.

Conclusions



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

In this work, a group of novel boron-specific silica particles were developed as adsorbents for

2

boron separation from water. The variables of interest, i.e. reactant ratio, chelating

3

temperature, particle loading, contact time and ion strength, were carefully tuned and

4

optimized in batch experiments. Adsorption behaviors were correlated with the pseudo-

5

second order kinetic model and three isotherm models. Regeneration of the adsorbent was

6

also investigated in both batch experiments and LC column-based experiments. The

7

following conclusions can be drawn from this work:

8

1. The diol-functionalized silica particles showed promising boron adsorption performance

9

under ambient conditions and neutral pH.

10

2. The half-adsorption time of Silica-N4 is much shorter than the commercially available

11

BSRs and those adsorbents reported in literatures. The extremely fast adsorption kinetics is

12

probably due to the high hydrophilicity and non-tortuous structure of the silica particles.

13

3. Adsorption isotherm was described and compared by three sorption models. The sorption

14

behavior follows the Sips model due to the heterogeneity of the adsorbent. A high

15

adsorption capacity was predicted by the Sips model.

16

4. The adsorbent was easily regenerated by using 0.125 M sulfuric acid. Both the batch

17

experiments and the continuous column adsorption experiments demonstrated that the

18

adsorption capacity is marginally sacrificed (less than 10%) after three cycles of

19

measurements, illustrating a promising reusability of the diol-functionalized silica.

20

Acknowledgements

21

Dr. Tang and Prof Chung would like to thank BASF SE, Germany for funding this work with

22

a grant number of R-279-000-411-597. Thanks are also given to Dr. S. Japip, Dr. N. Widjojo

23

and Dr. M. Jung for their kind help and suggestions.

24

Figures and tables

25

Figure 1. Synthesis routes of the diol-functionalized silica particle. 16 ACS Paragon Plus Environment

Page 16 of 31

Page 17 of 31

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1

Figure 2. Schematic illustration of the liquid chromatographic column-based testing system.

2

Figure 3. XPS wide scanned spectra of the pristine and diol-functionalized silica particles.

3

Figure 4. C 1s core-level spectra of the pristine and diol-functionalized silica particles.

4

Figure 5. Oxygen-to-Silicon atomic ratios of the pristine and diol-functionalized silica

5

particles.

6

Figure 6. FESEM morphology of (a) silica-NH2, (b) silica-NH2 after 2 h ultra-sonication and

7

(c)-(g) Silica-N1, Silica-N2, Silica-N3, Silica-N4 and Silica-N5.

8

Figure 7. (a) Boron rejection performance of silica particles under different reaction

9

conditions (chelating temperature at 60 °C) and (b) Linear correlation between the boron

10

rejection and the O/Si atomic ratio.

11

Figure 8. Effects of adsorbent loading on boron rejections of (a) Silica-N5 at 60 °C and (b)

12

Slica-N4 at 60 and 23 °C.

13

Figure 9. (a) Boron rejection as a function of contact time (Silica-N4, chelating temperature

14

at 23 °C), and (b) linear fitting by the pseudo-second order kinetic model.

15

Figure 10. Effects of ion strength on boron rejection of Silica-N4 (chelating temperature at

16

23 °C and adsorbent loading at 50).

17

Figure 11. (a) Boron rejection of Silica-N4 as a function of the initial feed concentration, (b)

18

Boron adsorption isotherm and the fitting curve by the Sips model, (c) Langmuir isotherm

19

plot and (d) Freundlich isotherm plot.

20

Figure 12. Chelation mechanisms of boric acid with diols at (a) a low boron concentration

21

(220 ppm) where B4O4(OH)4 takes place.

22

Figure 13. Multi-cycle experiments for Silica-N4.

23

Figure 14. Breakthrough curve for three cycles.

24

Table 1. Fitting parameters from the pseudo-second order kinetic models.

25

Table 2. Fitting parameters from the three isotherm models. 17 ACS Paragon Plus Environment


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

References (1) Parks, J. L.; Edwards, M.: Boron in the environment. Crit. Rev. Env. Sci. Technol. 2005, 35, 81-114. (2)

Kot, F. S.: Boron in the environment. In Boron Separation Processes; Kabay, N., Bryjak, M., Hilal, N., Eds.; Elsevier: Amsterdam, 2015.

(3)

Hilal, N.; Kim, G. J.; Somerfield, C.: Boron removal from saline water: A comprehensive review. Desalination 2011, 273, 23-35.

(4)

Tagliabue, M.; Reverberi, A. P.; Bagatin, R.: Boron removal from water: needs, challenges and perspectives. J. Clean. Prod. 2014, 77, 56-64.

(5)

WHO: Boron in drinking-water, Background document for development of WHO guidelines for drinking-water quality, WHO/HSE/WSH/09.01/2; World Health Organization press: Switzerland, 2009; Vol. WHO/HSE/WSH/09.01/2.

(6)

Wen, R.; Deng, S.; Zhang, Y.: The removal of silicon and boron from ultra-pure water by electrodeionization. Desalination 2005, 181, 153-159.

(7)

Xu, J.; Gao, X.; Chen, G.; Zou, L.; Gao, C.: High performance boron removal from seawater by two-pass SWRO system with different membranes. Wa. Sci. Technol. 2010, 10, 327-336.

(8)

Dydo, P.; Nemś, I.; Turek, M.: Boron removal and its concentration by reverse osmosis in the presence of polyol compounds. Sep. Purif. Technol. 2012, 89, 171-180.

(9)

Rahmawati, K.; Ghaffour, N.; Aubry, C.; Amy, G. L.: Boron removal efficiency from Red Sea water using different SWRO/BWRO membranes. J. Membr. Sci. 2012, 423424, 522-529.

(10) Hu, J.; Pu, Y.; Ueda, M.; Zhang, X.; Wang, L.: Charge-aggregate induced (CAI) reverse osmosis membrane for seawater desalination and boron removal. J. Membr. Sci. 2016, 520, 1-7. 18 ACS Paragon Plus Environment

Page 18 of 31

Page 19 of 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


(11) Jin, X.; Tang, C. Y.; Gu, Y.; She, Q.; Qi, S.: Boric Acid Permeation in Forward Osmosis

Membrane

Processes:

Modeling,

Experiments,

and

Implications.

Environmental science & technology 2011, 45, 2323-2330. (12) Kim, C.; Lee, S.; Shon, H. K.; Elimelech, M.; Hong, S.: Boron transport in forward osmosis: Measurements, mechanisms, and comparison with reverse osmosis. J. Membr. Sci. 2012, 419–420, 42-48. (13) Valladares Linares, R.; Li, Z. Y.; Sarp, S.; Park, Y. G.; Amy, G.; Vrouwenvelder, J. S.: Higher boron rejection with a new TFC forward osmosis membrane. Desalin. Water Treat. 2015, 55, 2734-2740. (14) Luo, L.; Zhou, Z.; Chung, T.-S.; Weber, M.; Staudt, C.; Maletzko, C.: Experiments and Modeling of Boric Acid Permeation through Double-Skinned Forward Osmosis Membranes. Environmental science & technology 2016, 50, 7696-7705. (15) Kabay, N.; Yilmaz, Đ.; Bryjak, M.; Yüksel, M.: Removal of boron from aqueous solutions by a hybrid ion exchange–membrane process. Desalination 2006, 198, 158165. (16) Kabay, N.; Sarp, S.; Yuksel, M.; Kitis, M.; Koseoğlu, H.; Arar, Ö.; Bryjak, M.; Semiat, R.: Removal of boron from SWRO permeate by boron selective ion exchange resins containing N-methyl glucamine groups. Desalination 2008, 223, 49-56. (17) Yan, C.; Yi, W.; Ma, P.; Deng, X.; Li, F.: Removal of boron from refined brine by using selective ion exchange resins. J. Hazard. Mater. 2008, 154, 564-71. (18) Nasef, M. M.; Nallappan, M.; Ujang, Z.: Polymer-based chelating adsorbents for the selective removal of boron from water and wastewater: A review. React. Funct. Polym. 2014, 85, 54-68.



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

(19) Geffen, N.; Semiat, R.; Eisen, M. S.; Balazs, Y.; Katz, I.; Dosoretz, C. G.: Boron removal from water by complexation to polyol compounds. J. Membr. Sci. 2006, 286, 45-51. (20) Tang, Y. P.; Yuwen, S.; Chung, T. S.; Weber, M.; Staudt, C.; Maletzko, C.: Synthesis of hyperbranched polymers towards efficient boron reclamation via a hybrid ultrafiltration process. J. Membr. Sci. 2016, 510, 112-121. (21) Wei, Y.-T.; Zheng, Y.-M.; Chen, J. P.: Design and fabrication of an innovative and environmental friendly adsorbent for boron removal. Water Res. 2011, 45, 2297-2305. (22) Boubakri, A.; Bouguecha, S. A.-T.; Dhaouadi, I.; Hafiane, A.: Effect of operating parameters on boron removal from seawater using membrane distillation process. Desalination 2015, 373, 86-93. (23) Francis, L.; Ghaffour, N.; Alsaadi, A. S.; Nunes, S. P.; Amy, G. L.: Performance evaluation of the DCMD desalination process under bench scale and large scale module operating conditions. J. Membr. Sci. 2014, 455, 103-112. (24) Hou, D.; Wang, J.; Sun, X.; Luan, Z.; Zhao, C.; Ren, X.: Boron removal from aqueous solution by direct contact membrane distillation. J. Hazard. Mater. 2010, 177, 613-9. (25) Tang, Y. P.; Luo, L.; Thong, Z.; Chung, T. S.: Recent advances in membrane materials and technologies for boron removal. Journal of Membrane Science 2017, 541, 434-446. (26) Kochkodan, V.; Darwish, N. B.; Hilal, N.: The chemistry of boron in water. In Boron Separation Processes; Kabay, N., Bryjak, M., Hilal, N., Eds.; Elsevier: Amsterdam, 2015. (27) Oo, M. H.; Song, L.: Effect of pH and ionic strength on boron removal by RO membranes. Desalination 2009, 246, 605-612. (28) Yavuz, E.; Arar, Ö.; Yüksel, M.; Yüksel, Ü.; Kabay, N.: Removal of boron from geothermal water by RO system-II-effect of pH. Desalination 2013, 310, 135-139.


Page 20 of 31

Page 21 of 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


(29) Abdulgader, H. A.; Kochkodan, V.; Hilal, N.: Hybrid ion exchange – Pressure driven membrane processes in water treatment: A review. Sep. Purif. Technol. 2013, 116, 253264. (30) Koltuniewicz, A. B.; Witek, A.; Bezak, K.: Efficiency of membrane-sorption integrated processes. J. Membr. Sci. 2004, 239, 129-141. (31) Wang, P.; Chung, T.-S.: Recent advances in membrane distillation processes: Membrane development, configuration design and application exploring. J. Membr. Sci. 2015, 474, 39-56. (32) Guan, Z.; Lv, J.; Bai, P.; Guo, X.: Boron removal from aqueous solutions by adsorption – A review. Desalination 2016, 383, 29-37. (33) Wang, B.; Guo, X.; Bai, P.: Removal technology of boron dissolved in aqueous solutions – A review. Colloids. Surf. A Physicochem. Eng. Asp. 2014, 444, 338-344. (34) Lou, J.; Foutch, G. L.: Ion exchange borate kinetics. In Boron Separation Processes; Kabay, N., Bryjak, M., Hilal, N., Eds.; Elsevier: Amsterdam, 2015. (35) Li, X.; Liu, R.; Wu, S.; Liu, J.; Cai, S.; Chen, D.: Efficient removal of boron acid by Nmethyl-d-glucamine functionalized silica–polyallylamine composites and its adsorption mechanism. J. Colloid Interface Sci. 2011, 361, 232-237. (36) Wei, Y.-T.; Zheng, Y.-M.; Chen, J. P.: Functionalization of regenerated cellulose membrane via surface initiated atom transfer radical polymerization for boron removal from aqueous solution. Langmuir 2011, 27, 6018-6025. (37) Hwang, S.-J.; Lu, W.-J.: Ion exchange in a semifluidized bed. Ind. Eng. Chem. Res. 1995, 34, 1434-1439. (38) Güler, E.; Kaya, C.; Kabay, N.; Arda, M.: Boron removal from seawater: State-of-theart review. Desalination 2015, 356, 85-93.



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

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(39) Sanfeliu, C.; Martínez-Máñez, R.; Sancenón, F.; Soto, J.; Puchol, V.; Amorós, P.; Marcos, M. D.: Low-cost materials for boron adsorption from water. J. Mater. Chem. 2012, 22, 25362-25372. (40) Liu, H.; Ye, X.; Li, Q.; Kim, T.; Qing, B.; Guo, M.; Ge, F.; Wu, Z.; Lee, K.: Boron adsorption using a new boron-selective hybrid gel and the commercial resin D564. Colloids. Surf. A Physicochem. Eng. Asp. 2009, 341, 118-126. (41) Ben Amor, T.; Dhaouadi, I.; Lebeau, B.; Tlili, M.; Ben Amor, M.: Synthesis, characterization and application of glucamine-modified mesoporous silica type SBA-15 for the removal of boron from natural water. Desalination 2014, 351, 82-87. (42) Labouriau, A.; Smith, B. F.; Khalsa, G. R. K.; Robison, T. W.: Boric acid binding studies with diol containing polyethylenimines as determined by

11

B NMR

spectroscopy. J. Appl. Polym. Sci. 2006, 102, 4411-4418. (43) Shechter, L.; Wynstra, J.; Kurkjy, R. P.: Glycidyl ether reactions with amines. Industrial & Engineering Chemistry 1956, 48, 94-97. (44) Tang, Y. P.; Wang, H.; Chung, T. S.: Towards high water permeability in triazineframework-based microporous membranes for dehydration of ethanol. Chemsuschem 2015, 8, 138-147. (45) Tang, Y. P.; Cai, T.; Loh, D.; O'Brien, G. S.; Chung, T. S.: Construction of antifouling lumen surface on a poly(vinylidene fluoride) hollow fiber membrane via a zwitterionic graft copolymerization strategy. Sep. Purif. Technol. 2017, 176, 294-305. (46) Wang, H.; Cheng, F.; Shen, W.; Cheng, G.; Zhao, J.; Peng, W.; Qu, J.: Amino acidbased anti-fouling functionalization of silica nanoparticles using divinyl sulfone. Acta Biomater. 2016, 40, 273-281. (47) Ho, Y. S.: Review of second-order models for adsorption systems. J. Hazard. Mater. 2006, 136, 681-689.


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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


(48) Azizian, S.: Kinetic models of sorption: a theoretical analysis. J. Colloid Interface Sci. 2004, 276, 47-52. (49) Morisada, S.; Rin, T.; Ogata, T.; Kim, Y. H.; Nakano, Y.: Adsorption removal of boron in aqueous solutions by amine-modified tannin gel. Water Res. 2011, 45, 4028-34. (50) Foo, K. Y.; Hameed, B. H.: Insights into the modeling of adsorption isotherm systems. Chem. Eng. J. 2010, 156, 2-10. (51) Jeppu, G. P.; Clement, T. P.: A modified Langmuir-Freundlich isotherm model for simulating pH-dependent adsorption effects. J. Contam. Hydrol. 2012, 129-130, 46-53. (52) Japip, S.; Wang, H.; Xiao, Y.; Chung, T. S.: Highly permeable zeolitic imidazolate framework (ZIF)-71 nano-particles enhanced polyimide membranes for gas separation. J. Membr. Sci. 2014, 467, 162-174.



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Page 24 of 31

Figures and Tables

(a)

HO

O

OH

N

SiO2

OH

OH SiO2

NH2

HO

3-Aminopropyl-functionalized silica gel

(b)

SiO2

N H

OH OH

Figure 1. Synthesis routes of the diol-functionalized silica particle.

Figure 2. Schematic illustration of the liquid chromatographic column-based testing system.


Page 25 of 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


O1s

N1s

Silica-N5

C1s

Si 2p

Silica-N4 Silica-N3 Silica-N2 Silica-N1 Silica-NH2 1100

1000

900

800

700

600 500 400 Binding energy (eV)

300

200

100

0

Figure 3. XPS wide scanned spectra of the pristine and diol-functionalized silica particles. Silica-NH2

Silica-N1 C-C

C-C C-N

C-Si C-N

290

288

C-O

286

284

282

280

Silica-N2

290

288

286

284

282

280

288

286

284

282

280

288

286

284

282

280

Silica-N3

288

286

284

282

280

Silica-N4

290

290

C-Si

290

Silica-N5

288

286

284

282

280

290

Figure 4. C 1s core-level spectra of the pristine and diol-functionalized silica particles. 25 ACS Paragon Plus Environment


2.4

2.3 O/Si atomic ratio

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

2.2

2.1

2

1.9 SILICA-NH2 SILICA-N1

SILICA-N2

SILICA-N3

SILICA-N4

SILICA-N5

ce (ppm)

Figure 5. Oxygen-to-Silicon atomic ratios of the pristine and diol-functionalized silica particles.

Figure 6. FESEM morphology of (a) silica-NH2, (b) silica-NH2 after 2 h ultra-sonication and (c)-(g) Silica-N1, Silica-N2, Silica-N3, Silica-N4 and Silica-N5.


Page 26 of 31

Page 27 of 31

100

(b)

90

y = 221.32x - 430.57 R² = 0.9408

90

80

80

70

Silica loading 50

60

Silica loading 10

Boron rejection (%)

Boron rejection (%)

(a)

70 60 50 40 30 20

Silica loading=50

10

Silica loading=10

50 40 30

y = 160.36x - 322.25 R² = 0.9947

20 10 0

0

2 0

2

4 6 Epoxy/amine ratio

8

2.05

2.1

10

2.15 2.2 2.25 O/Si atomic ratio

2.3

2.35

Figure 7. (a) Boron rejection performance of silica particles under different reaction conditions (chelating temperature at 60 °C) and (b) Linear correlation between the boron rejection and the O/Si atomic ratio. (a)

(b)

100

100

Boron rejection (%)

Boron rejection (%)

80 60 40 20

80 60 60 °C 40

23 °C

20 0

0 0

20

40 60 80 Silica loading (mol/mol)

0

100

20


100

Figure 8. Effects of adsorbent loading on boron rejections of (a) Silica-N5 at 60 °C and (b) Slica-N4 at 60 and 23 °C. (b)

100

200

80

y = 0.9921x + 0.6017 R² = 0.9999

150

60

t/Qt

(a) Boron rejection (%)

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


100

40 Silica-N4, Loading 100

20

y = 0.8203x + 0.9448 R² = 0.9997

50

Silica-N4, Loading 50 0

0 0

50

100 Time (min)

150

0

50

100 Time (min)

150

Figure 9. (a) Boron rejection as a function of contact time (Silica-N4, chelating temperature at 23 °C), and (b) linear fitting by the pseudo-second order kinetic model.



(a)

80

Boron rejection (%)

Boron rejection (%)

90

(b)

100

60 Without NaCl

40

0.6M NaCl

80

70

60

20 50

0 0

20


0

100

0.05 0.2 0.4 NaCl concentration (M)

0.6

Figure 10. Effects of ion strength on boron rejection of Silica-N4 (chelating temperature at 23 °C and adsorbent loading at 50).

(a)

(b)

90

25 20

70

Qeq (mg/g)

Boron rejection (%)

80

60 50 40 30

15 10 5

20 10

0 0

100 200 300 400 500 600 700 800 900 1000 Feed B concentration (ppm)

10

(c)

0

100 200 300 400 500 600 700 800 900 ceq (ppm)

2

(d) y = 1.4681x + 0.0751 R² = 0.9999

8

y = 0.3798x + 0.2771 R2 = 0.9984

1.5

6

log Qeq

1

1/Qeq (g/mg)

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

Page 28 of 31

0.15 0.1

4

0.05 2

0.5 0

y = 0.5919x - 0.201 R² = 0.9578

-0.5

0

y = 0.8864x – 0.2239 R2 = 0.9964

-1 0

0.005

0.01

6

7

0

-1.5 0

1

2

3 4 1/ceq (ppm-1)

5

-1

-0.5

0

0.5

1 1.5 log ceq

2

2.5

3

Figure 11. (a) Boron rejection of Silica-N4 as a function of the initial feed concentration, (b) Boron adsorption isotherm and the fitting curve by the Sips model, (c) Langmuir isotherm plot and (d) Freundlich isotherm plot.


Page 29 of 31

(a)

C C

C

OH

C

OH

HO

O B

OH

O

OH B O

OH (b)

C C

O

O B

O

O

H

B O

B O

B O

H

Figure 12. Chelation mechanisms of boric acid with diols at (a) a low boron concentration (220 ppm) where B4O4(OH)4 takes place.

1.2

Cycle 1

Cycle 2

Cycle 3

1

Normalized Q

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


0.8 0.6 0.4 0.2 0 0.125

0.500 1.000 Acid concentration (M)

Figure 13. Multi-cycle experiments for Silica-N4.


2.000


120

100

80 Rejection (%)

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

Page 30 of 31

60

40

20

0 0

200

400 600 Volume (mL)

800

1000

Figure 14. Breakthrough curve for three cycles.

Table 1. Fitting parameters from the pseudo-second order kinetic models. Case

Conditions

K2 (g mg-1 min-1)

Qeq (mg g-1)

t1/2 (min)

1

Silica-N4, loading 100

1.949

1.007

0.509

2

Silica-N4, loading 50

0.713

1.219

1.151

Table 2. Fitting parameters from the three isotherm models. Langmuir isotherm model

Sips isotherm model

Freundilich isotherm model

Qm,2 (mg g−1)

kS (L mg−1)

n

Qm,1 (mg g−1)

kL (L mg−1)

kF

57.09

0.0006

0.5407

13.26

0.0514

0.6295


n 0.5919

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Highlights: • A group of novel diol-functionalized silica adsorbents were synthesized for boron removal. • The silica particles showed fast adsorption kinetics, high capacity and excellent reusability. • Adsorption behaviors were correlated well with the kinetic model and three isotherm models. • Continuous boron removal by a LC column-based system was demonstrated under ambient conditions.

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Development of Novel Diol-Functionalized Silica Particles toward Fast

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