Removal of phenolic pollutants from wastewater streams using ionic liquids

Journal Pre-proofs Removal of phenolic pollutants from wastewater streams using ionic liquids Olalla G. Sas, Pablo B. Sánchez, Begoña González, Ángeles Domínguez PII: DOI: Reference:

S1383-5866(19)32452-9 https://doi.org/10.1016/j.seppur.2019.116310 SEPPUR 116310

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

10 June 2019 31 October 2019 8 November 2019

Please cite this article as: O.G. Sas, P.B. Sánchez, B. González, A. Domínguez, Removal of phenolic pollutants from wastewater streams using ionic liquids, Separation and Purification Technology (2019), doi: https://doi.org/ 10.1016/j.seppur.2019.116310

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Removal of phenolic pollutants from wastewater streams using ionic liquids

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Olalla G. Sasa , Pablo B. Sánchezb , Begoña Gonzáleza , Ángeles Domíngueza a Advanced

3 4

5

Separation Processes Group, Department of Chemical Engineering, Vigo, Spain b Departamento de Física Aplicada, Universidad de Vigo, Spain

Abstract

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The removal of toxic phenolic compounds from wastewater streams with ionic liquids as extracting agents

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have been addressed with experiments and simulations. Three phenolic compounds, phenol, chlorophenol

8

and o-cresol, were extracted from aqueous streams at different concentrations using five ionic liquids allowing

9

to study the effect of both ionic structures, the anion and the cation, on the extraction efficiency. Besides, the

10

interactions between phenols and ions were analysed by means of atomistic molecular dynamic simulations

11

and their solvation environment described. Finally, the accomplishment of the regulations regarding phenol

12

concentrations in wastewater streams was discussed.

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Keywords: phenolic pollutants, ionic liquids, extraction, molecular dynamics, green chemistry

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1. Introduction

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Phenols and their derivative compounds are used in different industries such as the manufacture of

16

pesticides, explosives or dyes[1]. As pollutants they are present in wastewater of oil refineries, coke plants,

17

plastics, leather, paint and pharmaceutical industries within a concentration range that can vary between

18

a 0.1 and 3900 mg·L−1 [2]. Thus, phenolic compounds as phenol, o-cresol or chlorinated derivatives are

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categorized as priority pollutants by World Health Organization (WHO)[3] due to their high toxicity for

20

aquatic life and human health. According to the actual environmental regulations, these compounds must

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be removed from wastewater prior to discharging. Different world organisations, such as Environmental

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Protection Agency (EPA) and the European Union, have fixed the threshold quantity for phenolic compounds

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in wastewaters in 1 mg·L−1 [4, 5].

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Several techniques can be used to treat these industrial wastewaters, nonetheless they all present severe

25

drawbacks. Carbon adsorption needs high amounts of adsorbent, the thermal decomposition and the cat-

26

alytic oxidation require elevated amount of energy and the biological treatments have limitations to treat

27

high concentrations of phenolic compounds due to the survival of the microorganisms[6]. Extraction with

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polar organic solvents, such as alcohols, acetates, toluene, benzene, chloroform, ethers and ketones, can be Email address: [email protected] (Pablo B. Sánchez) Preprint submitted to Elsevier

October 31, 2019

1

used for the separation of these components[1, 6]. Organic solvents used in this process are generally toxic,

2

flammable and volatile, causing harmful environmental effects. It is therefore necessary to find new solvents

3

capable of replacing traditional organic solvents, reducing the environmental impact, the energy consump-

4

tion and the installation and operational costs of the extraction process[5, 6]. Among the alternatives some

5

studies[1, 7, 8] have pointed out the extraction with ionic liquids (ILs) as a reliable alternative.

6

ILs are a family of chemicals that have attracted attention in the last decades due to their very particular

7

properties[9]. Their chemical structure, consisting of large ions with highly delocalized charges, lead to

8

very low volatility (vapour pressure is close to zero)[10], good thermal[11] and chemical stability[12] and

9

most important in this case, the ability to dissolve polar and apolar compounds depending on the selected

10

ionic structures[13]. All these properties make them an alternative to conventional solvents in liquid-liquid

11

extraction processes.

12

When thinking of extracting phenolic compounds from wastewater streams, the interactions of extracting

13

agents with the pollutants (phenolic compounds) and the solvent (water) are a key issue to deal with. Aiming

14

to obtain two phases (aqueous and organic), ILs leading to hydrophobic interactions and consequently to

15

immiscibility with aqueous systems are required[14]. This issue has been addressed from experimental[15, 16]

16

and computational[17] perspectives. Freire and coworkers[15] found the subsequent trends: C(CN)3– <

17

PF6– < NTf2– when studying the hydrophobicity of anions and CnC1Im+ < CnC1Py+ < CnC1Pyr+ <

18

CnC1Pip+ for cationic structures. Authors also reported an increase in the solubility of water in IL when

19

shortening the length of the cationic alkyl chains. Both patterns are in good agreement with COSMO-

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RS predictions[17]. While hydrophobicity is a necessary condition for IL to extract phenols from aqueous

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streams, a suitable design of the extraction process requires also the understanding at molecular level of the

22

interactions taking place between phenols and ILs.

23

ILs were proposed as alternatives to volatile organics solvents in extractions process more than two

24

decades ago[18]. Since then several authors studied the phenol extraction using ILs[19–21], in most cases

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the selected anions were BF4–, PF6– or NTf2–. Besides, the effect of adding ionic salts over the extraction

26

process has been considered with interesting results[1]. In our group we have also reported the performance

27

of extraction of phenols from streams with several ILs[5].

28

In this work the role of five ionic liquids, [C4Py][NTf2], [C4C1Py][NTf2], [C4C1Pip][NTf2], [C4C1Pyr][NTf2]

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and [C4C1Pyr][Nf2] as extracting agent for phenolic compounds was studied. Calculations of the extraction

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efficiency with these five ILs were done according to the procedure that we had previously optimised in our

31

group. Besides, we have combined for first time the experimental work with atomistic molecular dynamics

32

(AMD). By means of the computing experiments based AMD) simulations we have obtained insights on the

33

solvation environment of phenols in ILs and the interaction energies between phenols and ions.

2

(a) Phenol

(b) o-Chlorophenol

(c) o-cresol

Figure 1: Chemical structures of the extracted phenolic compounds

1

2. Materials and methods

2

2.1. Chemicals

3

In the table 1 the general information about the ILs used to extract the phenolic compounds is shown.

4

Further details can be consulted together with an extensive literature review of their physico chemical

5

properties in NIST database - CAS numbers are given in table 1 -. The NTf2– based ionic liquids were

6

supplied by Iolitec (Germany) and [C4C1Pyr][Nf2] by Solvionic (France). Prior to use, all ionic liquid were

7

dried under vacuum (P = 0.2 Pa) using moderate temperature (T = 343.15 K) and for at least 48 h until

8

constant density. To prevent moisture absorption, storage under argon atmosphere in sealed flasks was done. Full name

Abbreviature

CAS number

1-butylpyridinium bis(trifluoromethylsulfonyl)imide 4-butyl-1-methylpyridinium bis(trifluoromethylsulfonyl)imide 4-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide 4-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide 4-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide

[C4Py][NTf2] [C4C1Py][NTf2] [C4C1Pip][NTf2] [C4C1Pyr][NTf2] [C4C1Pyr][Nf2]

187863-42-9 344790-86-9 623580-02-9 223437-11-4 1057745-51-3

Table 1: General information of the ILs used to extract phenolic compounds

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The phenolic compounds o-cresol (99.0%) and chlorophenol (99.0%) were provided from Merck Schuclardt

10

(Germany), and phenol (99.0%) from Sigma-Aldrich (Germany) - see chemical structures in figure 1 -.

11

The reactants used in the 4-amineantipyrine analytical method, potassium hexacyanoferrate (III) (98.0%),

12

potassium sodium L(+)-tartrate tetrahydrate, ammonium chloride (99.0%), 4- amineantipyrine (98.0%) and

13

ammonium hydroxide (solution 25%) were supplied by VWR Prolabo Chemicals (Belgium). The components

14

used to determine the water content, Coulomat CG and Coulomat AG were purchased from Sigma-Aldrich.

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2.2. Apparatus

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The phenolic solutions were carried out using a Mettler Toledo Excellence plus XP205 analytical balance

17

with an uncertainty of ± 3·10−4 g. To perform the extraction process a Polyscience thermostatic bath (±

18

0.01K) coupled to a Phoenix Instrument RSM-03-10K magnetic stirrer was used. A digital thermometer

19

ASL model F200 with an uncertainty of ± 0.01 K controlled the temperature of the process. 3

1

The determination of the concentrations of phenolic compounds in the aqueous phase were carried out

2

using a Jasco V-630 UV/Vis spectrophotometer with an uncertainty of ± 0.002 Abs (from 0 to 0.5 Abs) and

3

± 0.003 (from 0.5 to 1 Abs), respectively. The pH values of the phenolic solutions were determined using a

4

pHmeter Crison Basic 20 with an uncertainty of ±0.01 pH.

5

2.3. Extraction procedure

6

Ternary solutions of phenolic compound + water + IL were weighted and introduced in glass and sealed

7

flasks. Phase volume ratio between ionic phase and aqueous phase (VIL :VW ) was 1:2. This ratio were

8

previously optimised[5] and no significant improvements were found to justify higher proportions of ILs. Then

9

systems were placed in thermostatic bath at constant temperature (T = 298.15 K) under vigorous stirring to

10

ensure intimate contact. After 120 min, stirring was removed. In order to let complete separation between

11

phases ternary systems were left in thermostatic bath at 298.15 K for 120 min. No modifications in the pH

12

were done, resulting in all cases in values around 6. Following, the samples of the aqueous phase were collected

13

with a syringe and the phenolic composition was measured by means of the 4-amineantipyrine analytical

14

method was applied[22, 23]. The performance of the extraction process (%E) is calculated according the

15

equation 1.

E% = 16

C0 − C 100 C0

(1)

being C0 and C denote the initial and equilibrium concentrations of the phenolic compounds in the

17

aqueous phase expressed in mg L−1 .

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2.4. Atomistic MD

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All chemicals were represented according to Optimised Potential for Liquid Simulations All Atom (OPLS-

20

AA) [24] whose functional form for the potential energy can be found in SI. OPLS-AA parameters were used

21

to describe interactions of phenols while for ILs we have taken parametrisation from Padua and Canongia-

22

Lopes[25, 26]. In all cases fix-charged models were used to describe coulombic interactions. Molecular

23

dynamics (MD) simulations were performed using the LAMMPS[27, 28] open source package. A cutoff of

24

12 Å was applied to LJ interactions and electrostatics were handled using the particle-particle particle-mesh

25

[29] method with a precision of 10−4 in electrostatic energy. The timestep was set to 1 fs.

26

Simulation boxes were created from Packmol[30] and fftool[31] utilities. The resulting initial config-

27

urations were initially equilibrated to constant volume in N pT mode. With this purpose Nosé-Hoover

28

thermostat and barostat at 333 K and 1 bar was applied. Then density was averaged and particles relocated

29

according to the rescaling of the simulation box. Finally production trajectories were produced in NVT

30

mode for 20 ns.

31

Productions trajectories were carried out in periodic cubic boxes containing 200 ion pairs and 20 phenol

32

molecules, resulting in systems with xPh ≈ 0.10, suitable for obtaining accurate statistics of the solvation 4

1

environment of phenols. Density of pure IL was compute to validate the model. Deviation with experimental

2

values are under 3%. Results are given in supplementary information (see table S1).

3

3. Results

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The extraction efficiency of five ILs - [C4Py][NTf2] , [C4C1Py][NTf2], [C4C1Pip][NTf2], [C4C1Pyr][NTf2]

5

and [C4C1Pyr][Nf2] - was determined. In figure 2 we show the results for these phenolic structures when

6

the concentration of phenol in the aqueous stream is 15000 mg L−1 , the highest value studied in this work

7

- lower concentrations are given in tables S2 to S6. Results in figure 2 indicate the good performance of

8

the extractions with efficiencies higher than 90% in all cases. The hydrophobic character of the extracting

9

agents and their affinity for the phenolic compounds may well explain this behaviour. A detailed analysis

10

of these features are presented in the next sections. Phenol o−cresol

extraction efficiency %

100

Chlorophenol

98

96

94

92

90

[C4Py][NTf2]

[C4C1Py][NTf2]

[C4C1Pip][NTf2]

[C4C1Pyr][NTf2]

[C4C1Pyr][Nf2]

Figure 2: Extraction efficiency (see eq. 1) to remove phenol, o-cresol and 2-chlorophenol from water (C0 = 15000 mg L−1 ) using five ILs.

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The extraction efficiency (E%) is weakly affected by initial concentration in the wastewater streams.

12

Efficiency values are slightly lower when concentrations are under 10 mg L−1 but they remain constant up

13

to the highest charges, 15000 mg L−1 , indicating that no saturation in the ionic phase occurs - see table S2

14

to S6.

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3.1. Effect of the anion

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Anions play a major role in most of the ILs properties[32] as it happens with hydrophobic interactions

17

that are crucial in the extraction efficiency. Hereby two anions, NTf2– and Nf2–, sharing a common cation,

18

C4C1Pyr+, have been used to extract phenolic compounds. Results at different initial concentrations are 5

1

plotted in figure 3. In all cases, the concentration of phenols in the aqueous stream after extraction is

2

larger when [C4C1Py][NTf2] is used as expected based on the extracting efficiency results shown in figure

3

2. This trend is accomplished regardless the phenolic structure. Since extraction efficiency for o-cresol and

4

chlorophenol are larger than obtained for phenol, concentration in aqueous phase after extraction is lower. 1000 a

b

c

C /mg L−1

800 600 400 200 0 1

10

100 1000 10000 C0 /mg L−1

1

10

100 1000 10000 C0 /mg L−1

1

10

100 1000 10000 100000 C0 /mg L−1

Figure 3: Phenolic concentration in the aqueous phase for a) phenol b) o-cresol c) chlorophenol after extraction using [C4C1Pyr][Nf2] (red line) and [C4C1Pyr][NTf2] (blue line) as extracting agents

5

Nf2– based ILs lead to lower viscosities and higher conductivities than those composed by NTf2–[33],

6

however it would not explain their extracting abilities that should be sought in the interactions between

7

solvents - water and IL - and phenols. Both anions lead to highly hydrophobic ILs, but larger size of

8

NTf2– would sharpen this property[34]. Interactions between the anions and phenol have been addressed

9

through AMD. In figure 4, the radial distribution functions (RDFs) between phenol and both anions are

10

plotted. Solvation shells shown in RDFs are similar when oxygen (O) - see figure 4a - is the interaction site

11

- peak is slightly higher for NTf2–. When representing fluorine - see figure 4b -, we observe that solvation of

12

phenol is constrained by the presence of trifluoromethyl groups that are only present in NTf2– that would

13

be responsible for the lower extraction efficiency of NTf2– based IL.

14

3.2. Effect of the cation structure

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Alike previously done for the anion, we have addressed the effect of the cationic structure over the

16

performance of the extraction. Results are plotted in figure 5. In spite of the differences in the final

17

concentration of phenols in the aqueous phase, they do not show an unambiguous trend for all phenols. We

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observe how C4C1Py+ leads to better extraction efficiency - mainly in figures 5a and c - followed by C4Py+.

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As it happened in previous section differences are more noticeable in figure 5a. More substituted phenols -

20

see figures 5b and c - lead to similar extraction efficiencies in which no significant differences can be reported

21

for the studied cations.

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The high solubility of chlorophenols in NTf2– ILs with different cations[35] and the partition coefficients

23

between other hydrophobic ILs and water[36] are consistent with the efficiencies obtained in figure 5b in 6

3

(ONf2)(OPh) (ONTf2)(OPh)

(FNf2)(OPh) (FNTf2)(OPh)

2 g(r)

O..anion ... Ph

F..anion ... Ph

1

0

0

5

10

15

20

0

5

10

r/Å

15

20

25

r/Å

Figure 4: RDFs between phenol and the ILs: [C4C1Pyr][Nf2] and [C4C1Pyr][NTf2]. Chosen sites are oxygen (O) and fluorine (F) for the anion and oxygen for phenol. Nf2– lines are drawn in blue and NTf2– are drawn in red 1000 a

b

[C4Py][NTf2] [C4C1Py][NTf2] [C4C1Pip][NTf2] [C4C1Pyr][NTf2]

c

C /mg L−1

800 600 400 200 0 1

10

100 1000 10000 C0 /mg L−1

1

10

100 1000 10000 C0 /mg L−1

1

10

100 1000 10000 100000 C0 /mg L−1

Figure 5: Phenolic concentration in aqueous phase after extraction for a) phenol b) o-cresol c) chlorophenol using [C4Py][NTf2], [C4C1Py][NTf2], [C4C1Pip][NTf2], [C4C1Pyr][NTf2] as extracting agents

1

which negligible variations between ILs are found. RDFs in figure 6 show weaker interactions between

2

aromatic cations - C4Py+ and C4C1Py+ - and phenol. According to figure 5a, aromatics lead to slightly

3

higher extraction efficiencies than aliphatic cations which indicates that reasons should be sought behind

4

interactions between cation and phenol. Besides, increasing the substitution in the aromatic ring does not

5

affect to the solvation shell indicated by RDFs.

6

3.3. Effect of the phenols: structure and concentration

7

Results in figures 3 and 5 show how concentration of phenols in the aqueous media after extraction

8

follows the same trend, from higher to lower: phenol > o-cresol > chlorophenol. It indicates that the

9

highest extraction efficiency is obtained for chlorophenol followed by o-cresol and phenol (see figure 2).

10

Partition coefficients of phenol and o-cresol isomers[37] in phosphonium IL and water are consistent

11

with these results. Authors pointed out the increase of hydrophobicity caused by the substituents of the 7

3

(NC4Py+)(OPh) (NC4C1Py+)(OPh) (NC4C1Pyr+)(OPh)

2 g(r) 1

0

0

5

10

15

20

r/Å Figure 6: RDFs of phenol and the cation in [C4Py][NTf2], [C4C1Py][NTf2] and [C4C1Pyr][NTf2]. Chosen sites are oxygen (O) in phenol and nitrogen (N) for the anions. Lines in green for [C4Py][NTf2], blue for [C4C1Py][NTf2] and red for [C4C1Pyr][NTf2]

1

phenolic ring may well explain this trend. It would also explain that in spite of the higher interaction

2

energies - see figure 8 - between phenol and IL the extraction efficiency is better for o-cresol. This analysis

3

becomes more complex when phenol is chloro-substituted. While competition between oxygen and chloride

4

in chlorophenol for hydrogen bond donnors may well be behind the lower peak found in figure 7a, a decrease

5

in the solubility of chlorophenols when aromatic rings are highly substituted reported by Sulaiman and

6

coworkers[35]. However, the complexity of interactions in ternary systems and the small variations in the

7

extraction efficiencies imposes to be cautions when analysing the thermodynamics of the studied systems.

3

(O)(O1) (O)(O2) (O)(O3)

a

(N)(O1) (N)(O2) (N)(O3)

b

2 g(r) 1

0

0

5

10

15

20

0

r/Å

5

10

15

20

25

r/Å

Figure 7: RDFs between phenols and the ions in [C4C1Pyr][Nf2]. Chosen sites are oxygen (O) in (a) Nf2– and nitrogen (N) in (b) C4C1Pyr+. (O1), (O2) and (O3) are oxygens in phenol, chlorophenol and o-cresol respectively

8

Interaction energies in figure 8 in good agreement with solvation shells shown by RDFs indicate how 8

1

stronger interactions between phenols and IL are not conclusive for the extraction efficiency. Hydrophobic

2

interactions of ILs and phenols which will both favour desirable an efficient extraction also play a very

3

relevant role. This hydrophobicity also reduces the risks of pollution of the wastewater streams with ILs.

4

5

1

H NMR analysis were performed to make sure aqueous phase is not containing significant amount of ILs.

Results can be consulted in SI (figure S3).

Interaction energy / kJmol

−1

0

−30

−60

−90

phenol

o−cresol

chlorophenol Ph−Anion Ph−Cation

−120

Figure 8: Interaction energies between the phenolic compounds and ions in [C4C1Pyr][Nf2]: cation = C4C1Pyr+, anion = Nf2–.

6

Finally, it is important to remark that according to legal regulations concentrations of phenolic com-

7

pounds, all wastewater effluents in EU must be under 1 mg L−1 . These values are accomplished with a single

8

extraction step for initial concentrations up to 50 mg L−1 . Higher charges will require further extraction

9

step to obtain pourable water.

10

4. Conclusions

11

A summary of the main conclusions is listed below:

12

• Extraction efficiency (E%) is in all cases higher than 90%. Efficiency of the extraction decreases when

13

concentration of phenols in aqueous phase is small and no saturation takes place in the ionic phase

14

when initial concentration of phenols in aqueous phase reaches 15000 mg L−1 .

15

• Anion play the most important role when looking at the ionic structure of the extracting agent. In our

16

case Nf2– leads to higher extraction efficiencies than NTf2–. On the cation side, C4C1Py+ is the most

17

convenient structure followed by C4Py+ suggesting better performance when substituted aromatic

18

structures are used.

19

• Substituted phenols - chlorophenol and o-cresol - are better extracted by ILs that phenol. Reasons

20

may rely on the higher hydrophobicity of the chemicals since interactions between phenols and ILs do

21

not account for this behaviour. 9

1

• Solvation shells indicate that sharper peaks between oxygen of anion and phenols lead to lower ex-

2

traction efficiencies. It would indicate that hydrogen bonds are not the driving force of the extraction

3

process. Hydrophobic interactions are expected to play a relevant role for phenols to move from

4

aqueous to ionic phase.

5

5. Acknowledgements

6

This work was financed by Ministerio de Economía y Competitividad under the project CTM2013-46093-

7

P. OGS and PBS acknowledge Ministerio de Economía y Competitividad and Xunta de Galicia for their

8

predoctoral and postdoctoral fellowships.

9

6. References

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11

Highlights

• Role of hydrophobic ionic liquids for removal of pollutant phenols from aqueous currents. • Calculation of extraction efficiency for three phenolic structures with five extracting agents • Effect of the anion and the cation on the efficiency of the whole process • Analysis of the solvation shells and the interaction energies between the phenols and ions

1

DISCHARGEABLE WATER

+ IL WASTEWATER

PHENOLIC COMPOUNDS