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Publicly Available Published by De Gruyter January 25, 2019

New polymeric adsorbent materials used for removal of phenolic derivatives from wastewaters

  • Corneliu-Mircea Davidescu , Radu Ardelean EMAIL logo and Adriana Popa EMAIL logo

Abstract

Phenolic compounds are produced in thermal cracking processes, drugs and herbicides synthesis and other industrial processes. Such compounds exhibit high toxicity for aquatic environment and for aquatic life. So, due to their high toxicity is important to treat waters with phenols content. For the treatment of waste waters containing phenols or phenolic compounds several unconventional methods are applied, such as: inverse osmosis, coagulation, solvent extraction, flotation–coagulation combined processes, adsorption, and anaerobic processes. From all used remediation processes adsorption has a higher applicability degree due to its main advantages: simplicity, ease of use and operation and high efficiency. Through time activated carbon and ashes were used as adsorbent materials for phenols remediation, but such materials present the main disadvantage of low regeneration degree. Thus, it is important to develop and use new adsorbents with higher regeneration degree and longer life time. Polymeric materials have been used for removal of organic compounds and/or metal ions from contaminated water due to their versatility in functionality, morphology and texture properties. Chemical modification of polymeric matrices with pendant functional groups is a valuable method used to improve the surface and interface chemistry of polymeric adsorbents, to achieve better adsorption performance and to design tailor-made adsorbents with respect to specific pollutants. In present study new adsorbent materials were obtained starting from chloromethylated styrene-divinylbenzene copolymers with different degrees of crosslinking (6.7%, 12% and respectively 15% DVB), functionalized by reaction with 3-hydroxibenzaldehyde. The polymeric intermediates were further modified by polymer-analogous reaction with iso-propylamine and diethylphosphite with the aim to improve their adsorptive properties. The obtained polymeric adsorbents were tested for remediation of waters containing phenol (P), 2,3-dimethylphenol (2,3-DMP) and 2,4,6-trimethyl-phenol (2,4,6-TMP). Based on obtained experimental data the adsorption mechanism, process kinetics and thermodynamics were studied.

Introduction

Phenols represent a class of pollutants with high persistence and possible accumulation in the environment, causing important damage to the environment health [1], [2], [3]. It is well known that phenol and phenolic compounds have a high toxicity. Due to large spread of the different industrial technologies, phenols and phenolic derivatives generated are present in industrial effluents such as: effluents from oil refining, olive oil extraction, paper milling, wood processing, coal gasification, textile and resin manufacturing. Discharging of such effluents without any treatment lead to concentrations of phenols and phenolic derivatives higher then maximum admissible values [4]. For example, the fertilizer industry is responsible for the generation of waste containing different phenolic compounds (such as 2-methylphenol, 4-methylphenol and 2,4,6-trimethylphenol) [5]. Similarly, the paper industry is generating wastewaters containing different quantities of phenols [6]. Environmental protection is a matter of great actual interest, in the attempt to maintain ecological balance, to improve the quality of the environment and ensure living and working conditions suitable for current and future generations [2], [3], [7].

Polymers modification by the “one-pot” Kabachnik–Fields synthesis represent one possible method used to produce polymeric resins containing grafted α-amino-phosphonate groups. Such pendant groups grafted on the polymeric chain modify adsorbent surface chemistry allowing specific adsorbent-adsorbate interactions and being responsible for the improvement of adsorption capacities towards organic compounds.

Polymeric adsorbents with porous structure show enhanced efficiency for removal, by adsorption processes, of alcohols, aldehydes, ketones, phenols, furans and acids from aqueous solutions through Van der Waals intermolecular interactions [8]. To improve the adsorption capacity and selectivity towards phenolic derivatives, polymeric adsorbents are often modified by introducing various nonpolar and/or polar functional groups in the polymeric matrix. Changes in the porous structure and texture of the polymeric adsorbents (specific surface, pore volume and dimensions) can improve their adsorption capacity [8]. Hydrogen bonds are commonly involved in adsorption processes, due to their low energy and high selectivity. Phenolic derivatives are potential hydrogen bonding donors [9]. The adsorption capacity of different macromolecular adsorbents can be increased by inserting pendant functional groups able to form hydrogen bonds. This can be realized by grafting nonpolar and/or polar functional groups such as phenolic hydroxyl [9], carbonyl [10], acetyl [11], [12], amine [13], [14] or amino-phosphonate groups [8], [14] into the polymeric matrix.

Experimental data presented in literature confirm that adsorption of phenolic compounds on modified polymeric adsorbents is usually driven by hydrogen bonding, hydrophobic interactions and ππ stacking [15].

In order to describe the efficiency of the new adsorbent materials it is important to identify and better understand the nature of the adsorbate-adsorbent surface interactions [16].

In present study styrene-co-divinylbenzene chelating resins containing amino-phosphonate groups were obtained, characterized and used as adsorbents of phenol derivatives from water. Such adsorbents were obtained by functionalization of macroporous resins cross-linked with 6.7%, 12% and respectively 15% divinylbenzene.

Experimental

Reagents

The following reagents were used as received from the suppliers without further purification: iso-propylamine (Fluka, 99%), tetrahydrofurane (Sigma–Aldrich, 99%), diethyl phosphite (Fluka), acetone (Chimopar, p.a., 99.9%), ethanol (Chimopar, p.a.), diethylether (Chimopar, purum, 97.0%).

Three sorts of chloromethylated styrene-divinylbenzene copolymers with different reticulation degree were used as starting materials in the polymer-analogous functionalization reactions: S-6.7%DVB (Purolite Victoria Romania, 14.22% (w) chlorine, 4.01 mmol Cl/g of copolymer) and two sorts of copolymers obtained by courtesy of Dr. Stela Drăgan (Institute of Macromolecular Chemistry “Petru Poni” Iași, Romania): S-12% DVB 10.32 % ((w chlorine, 2.91 mmol Cl/g of ))copolymer and S-15%DVB (10.21 % (w) chlorine, 2.88 mmol Cl/g of copolymer).

Synthesis of functionalized copolymers by polymer-analogous modification reactions

Reaction of chloromethylated copolymers with 3-hidroxy-benzaldehyde

The chloromethylated copolymers were functionalized by reaction with 3-hidroxy-benzaldehyde using the synthesis protocol previously reported [14] and presented in Scheme 1.

Scheme 1: 
              Functionalization of source copolymers by reaction with 3-hidroxy-benzaldehyde3-hidroxy-benzaldehyde.
Scheme 1:

Functionalization of source copolymers by reaction with 3-hidroxy-benzaldehyde3-hidroxy-benzaldehyde.

Functionalization with aminophosphonate pendant groups by “one-pot” Kabachnik–Fields reaction

Five gram of styrene-divinylbenzene copolymer (with 6.7%, 12% and respectively 15%DVB) functionalized with 3-benzaldehyde groups were added in 50 mL tetrahydrofuran. The mixture was then maintained for 2 h at room temperature for swelling the copolymer beads. Then, iso-propylamine and diethyl phosphite were added to achieve a molar ratio aldehyde groups:iso-propylamine:diethylphosphite=1:1.5:1. The mixture was kept under stirring for 24 h at a reaction temperature of 60°C. After cooling, the polymer beads separated by filtration were washed with acetone (3×20 mL), ethanol (3×20 mL) and diethyl ether (3×20 mL), and finally dried at 50°C for 24 h.

Characterization of the polymeric adsorbents

The chemical transformations by polymer-analogous reactions were validated by using Fourier-transform Infrared Spectroscopy. The FTIR spectra (KBr pellets) were recorded on a Shimadzu Prestige-21 FTIR spectrophotometer in the range 4000–400 cm−1.

Chlorine and phosphorus content in the polymeric adsorbents were determined by standard analytical methods and enabled the calculation of functionalization degrees and reaction yields.

The fraction of aromatic rings bearing pendant functional groups was evaluated by assuming the statistical structure of the repetitive unit of the copolymers as presented in Fig. 1a for the source chloromethylated copolymer, in Fig. 1b for the intermediate copolymer functionalized with aromatic aldehyde groups and in Fig. 1c for the adsorbents functionalized with amino-phosphonate active groups.

Fig. 1: 
            Statistical structure modeling of the repetitive unit of the copolymers: (a) source; (b) intermediate bearing aromatic aldehyde groups; (c) polymeric adsorbents with amino-phosphonate active centers.
Fig. 1:

Statistical structure modeling of the repetitive unit of the copolymers: (a) source; (b) intermediate bearing aromatic aldehyde groups; (c) polymeric adsorbents with amino-phosphonate active centers.

Morphological and textural characterization of functionalized styrene-divinylbenzene copolymers were performed by determination of the specific surface area by Brunauer, Emmett and Teller-BET method (Quantachrome NOVA 1200 E). The pore volume and average pore radius were calculated by the Haul–Dümbgen method [9], [15], [17] assuming cylindrical pore shape and using eq. 1:

(1) r p = ( 2 V p / S sp ) × 103  (nm)

where

V p = ( 1 / ρ ap ) ( 1 / ρ r )  (mL/g)

The apparent density (ρap) was determined using a mercury pycnometer at 1.333·10−2 Pa and the specific density (ρr) was measured in n-heptane.

Further information on morphology and texture was obtained by imaging using Scanning Electron Microscopy (SEM FEI Quanta FEG 250).

Determination of adsorption capacity

The adsorption capacity of the polymeric adsorbents was evaluated in batch experiments: 0.200 g of polymeric adsorbent were introduced into 25 mL of a phenolic compound solution with the concentration of 3.0 mmol/L. The content of the Erlenmeyer flasks was maintained under continuous mixing for 24 h at 200 rpm, in a Julabo SW22 shaker, at four different temperatures (293, 298, 303 and 308 K).

The solution was tested hourly in the first 8 h. Every hour a sample of 1 mL of solution was analyzed to determine the phenolic compound uptake. After measurements the sample was returned to the solution. The adsorption equilibrium was reached after 24 h.

At the end of the adsorption process the solution was analyzed by UV spectrophotometry to determine the residual concentrations of phenolic compounds using a Shimadzu UV mini 1240 UV-VIS spectrophotometer. The measurements were conducted at the wavelength of 270 nm for phenol solution, at 269 nm for 2,3-dimethylphenol solution and at 267 nm for 2,4,6-trimethylphenol solution.

Influence of the contact time, temperature, nature of adsorbate and adsorbate, initial concentrations of phenol derivatives on the adsorption process were studied.

In the kinetic studies, the initial concentration of phenolic compounds solutions was in all cases 3 mmol/L and in thermodynamic studies we used four different initial concentrations of adsorbate: 3 mmol/L, 2.5 mmol/L, 2 mmol/L and 1 mmol/L.

Kinetic studies

The performance of the adsorption process on the polymeric adsorbents was evaluated by kinetic studies.

Lagergren [18] pseudo-first-order rate equation and Ho and Mckay [19] pseudo second order rate equation were used to evaluate obtained experimental adsorption data.

Lagergren pseudo-first order rate equation was used in the linear form as expressed in eq. 2:

(2) l n ( q e q t ) = ln q e k 1 t

where: qt – adsorption capacity at time t, mmol·g−1;

q e – equilibrium adsorption capacity, mmol·g−1;

k 1 – pseudo-first order rate constant, h−1;

t – time in hours, h.

Ho and McKay pseudo-second order equation was used in the linear form as expressed in eq. 3:

(3) t q t = 1 k 2 q e 2 + t q e

where: qt – adsorption capacity at time t, mmol·g−1;

q e – equilibrium adsorption capacity, mmol·g−1;

k 2 – pseudo-second-order rate constant, L·mmol·h−1;

t – time in hours, h.

Adsorption isotherms

Adsorption isotherms are useful tools for the characterization of type and possible mechanism of adsorption. Theoretical maximum adsorption capacities of studied adsorbent materials and the equilibrium thermodynamic constants were evaluated using Langmuir [20], Freundlich [21] and Redlich–Peterson [22] isotherms.

The Langmuir isotherm eq. 4 was used in the linear form eq. 5:

(4) q e = q max K L C e 1 + K L C e

(5) C e q e = 1 K L q max + C e q max

where: qe – equilibrium adsorption coefficient, mmol·g−1;

q max – calculated saturation adsorption coefficient, mmol·g−1;

C e – equilibrium adsorbate concentration, mmol·L−1;

K L – Langmuir thermodynamic equilibrium constant.

The Freundlich isotherm eq. 6 was used in the linear form eq. 7:

(6) a = α C e 1 n

(7) ln a = ln α + 1 n ln C e

where: a – equilibrium adsorption coefficient, mmol·g−1;

C e – equilibrium adsorbate concentration, mmol·L−1;

α – measure of the adsorption capacity of the adsorbent;

1 n – constant depending on the nature of adsorption forces.

The Redlich–Peterson adsorption isotherm eq. 8 was used in the linear form eq. 9:

(8) q e = K RP C e 1 + α R C e β

(9) ln ( K RP C e q e 1 ) = β ln C e + ln α R

where: qe – equilibrium adsorption coefficient, mmol·g−1;

C e – equilibrium adsorbate concentration, mmol·L−1;

K RP – Redlich–Peterson thermodynamic equilibrium constant;

α R – constant;

β – exponent with values between 0 and 1.

For β=0 the Redlich–Peterson isotherm reduces to Henry’s law, for β=1 reduces to Langmuir isotherm and for high values of Ce the Redlich–Peterson reduces to Freundlich isotherm.

Thermodynamic activation parameters

The activation enthalpy (ΔH*) and entropy (ΔS*) change associated to the adsorption equilibrium could be evaluated from eq. 10:

(10) ln K L = Δ S * R Δ H * R T

thus one could also evaluate the Gibbs free energy change for the adsorption-desorption process.

Results and discussions

The chemical transformations of the functional groups of the copolymers (Schemes 1 and 2) are validated by data obtained by FT-IR Spectroscopy. Presence of amino-phosphonate functional groups on styrene-divinylbenzene copolymer surface was evidenced by recording the FT-IR spectra for the prepared polymeric adsorbents.

Scheme 2: 
          Kabachnik–Fields “one-pot” reaction.
Scheme 2:

Kabachnik–Fields “one-pot” reaction.

A typical spectra is presented in Fig. 2, for the best performing polymeric adsorbent S-15%DVD-iPrAFO:

Fig. 2: 
          FT-IR spectra (KBr) of S-15%DVB-iPrAFO.
Fig. 2:

FT-IR spectra (KBr) of S-15%DVB-iPrAFO.

FT-IR spectra remarks:

  1. the decrease in the intensity of the absorption bands in the region 1700–1600 cm−1 assigned to C=O aromatic stretching indicate that the reaction took place at the level of the –CHO groups;

  2. the single sharp band at 3441 cm−1 indicate a secondary amine;

  3. the increase in the intensity of adsorption band at around 1644 cm−1 is indicative for N–H valence vibrations; the N–H wagging band for substituted amino groups band was observed at 762 cm−1;

  4. the bands located at 1260 and 1028 cm−1 can be associated with the stretching vibrations of P=O and P–O–R bonds [17].

The main characteristics of the copolymers functionalized with amino-phosphonate pendant groups by “one-pot” polymer-analogous reactions are given in Table 1.

Table 1:

Characteristics of polymeric adsorbents functionalized with amino-phosphonates pendant groups.

Polymer adsorbent x y z P (wt.%) Fd (mmol/g)
S-6.7%DVB-i-PrAFO 0.53 0.49 0.37 4.90 1.59
S-12%DVB-i-PrAFO 0.42 0.39 0.36 4.84 1.55
S-15%DVB-i-PrAFO 0.40 0.38 0.35 4.76 1.54

Based on statistical modeling of the repetitive unit of the copolymers (Fig. 1) one can conclude that the functionalization took place on more than one third of the aromatic rings in the structure of the polymeric support and that the functionalization degrees (Fd) are over 1.5 mmol active groups per gram of copolymer. The obtained copolymers are ensuring therefore a sufficient concentration of active centers per unit mass of copolymer being suitable for use as adsorbents.

Morphological characterization

The morphology, texture and porosity properties of the adsorbents are key factors determining the adsorption rate and adsorption mechanism. A critical parameter is represented by the porous structure of the adsorbent materials. The data obtained for specific surface (Ssp), pore volume (Vp) and average pore radius (rp) are summarized in Table 2.

Table 2:

Texture properties of the polymeric materials.

Copolymer Ssp (m2/g) Vp (mL/g) rp (nm)
S-6.7%DVB–CH2Cl 33.24 0.67 40.33
S-12%DVB–CH2Cl 44.00 1.32 59.80
S-15%DVB–CH2Cl 57.00 1.38 58.32
S-6.7%DVB–CHO 31.62 0.51 87.58
S-12%DVB–CHO 25.59 0.77 60.09
S-15%DVB–CHO 33.44 0.54 51.77
S-6.7%DVB–iso–PrAFO 31.34 0.49 70.05
S-12%DVB–iso–PrAFO 27.23 0.79 57.94
S-15%DVB–iso–PrAFO 35.12 0.59 51.60

Analysing data presented in Table 2 we can observe that the specific surface of adsorbent materials is decreasing as consequence of the functionalization process. Decreasing of the material pore volume is a direct consequence of material functionalization, when the functional groups grafted on polymer structure are located not only at the surface but also inside the pores, the polymer analogous reactions being conducted in solvents with good solvation capacity. According to IUPAC classification, all the polymeric adsorbents are to be considered as macroporous, based on their average pore radius.

Adsorption of the phenolic derivatives onto the polymeric supports

The experimental data indicated that all the chemically modified copolymers are efficient in the adsorption of phenol and alkylated phenols: 2,4-dimethylphenol and respectively 2,4,6-trimethylphenol.

The adsorption process is rapid, over 80% of the maximum adsorption capacity being achieved in the first 4 h. After 8 h, the equilibrium adsorption capacity is practically attained.

Typical adsorption profiles are presented in Figs. 3 and 4 on the best performing polymeric adsorbent (S-15%DVB-iso-PrAFO). Such adsorption profiles are observed for all adsorbate-adsorbent pairs and in the temperature range studied.

Fig. 3: 
            Adsorption capacity vs. time on S-15%DVB-iso-PrAFO, at 298 K and initial concentration of phenolic derivative, Co=3 mmol/L.
Fig. 3:

Adsorption capacity vs. time on S-15%DVB-iso-PrAFO, at 298 K and initial concentration of phenolic derivative, Co=3 mmol/L.

Fig. 4: 
            Solution residual concentration vs. time for adsorption at 298 K on the adsorbent S-15%DVB-iso-PrAFO at initial concentration of phenolic derivative, Co=3 mmol/L.
Fig. 4:

Solution residual concentration vs. time for adsorption at 298 K on the adsorbent S-15%DVB-iso-PrAFO at initial concentration of phenolic derivative, Co=3 mmol/L.

The equilibrium adsorption capacities for the adsorbate-adsorbent pairs studied are presented in Fig. 5.

Fig. 5: 
            Equilibrium experimental adsorption capacities for an initial concentration of phenolic derivative solution, Co=3 mmol/L.
Fig. 5:

Equilibrium experimental adsorption capacities for an initial concentration of phenolic derivative solution, Co=3 mmol/L.

As can be seen, all the obtained polymeric absorbent are efficient in the adsorption of phenolic derivatives from aqueous solutions. The adsorption capacities of the functionalized polymer matrices are significantly influenced by the nature and structure of the adsorbate, very probable due to solubility and/or polarity differences which modify the transport properties of the adsorbate from the bulk solution to the active adsorption centers located at the surface or inside the pores of the polymeric support. The intensity of interaction forces between the adsorbate and the adsorbent are also, probable, affected. The best results were obtained in the case of 2,4,6-trimethylphenol which is less soluble and considerably more hydrophobic in comparison with phenol or 2,3-dimethylphenol.

Important differences in adsorption performances are observed in relation with the texture and porosity properties of the macromolecular support. The performance of the adsorbents can be directly correlated with the resin reticulation degree. The best results were observed (Fig. 5) for the adsorbent with the highest divinylbenzene content (15 %DVB), probably ensuring the most stable porous structure and texture of the adsorbent.

Important differences in adsorption performances are observed in relation with the texture and porosity properties of the macromolecular support. The performance of the adsorbents can be directly correlated with the resin reticulation degree. The best results were observed (Fig. 5) for the adsorbent with the highest divinylbenzene content (15 %DVB), probably ensuring the most stable porous structure and texture of the adsorbent.

The percent recovery, R% was calculated using eq. 11 for an initial concentration of adsorbate solution Co=3 mmol/L:

(11) R % = C o C e C o 100

Data concerning percent recovery are presented in Table 3. The best recovery values were obtained when using copolymer supports with higher crosslinking degree (15 %DVB) and using 2,4,6-trimethylphenol as adsorbate and are significantly better than previous reported results [14].

Table 3:

Percent recovery for the adsorbate-adsorbent pairs studied.

Adsorbate Percent recovery, R% on adsorbent
S-6.7DVB-isoPrAFO S-12DVB-isoPrAFO S-15DVB-isoPrAFO
Phenol 27.50 37.63 44.17
2,3-DMP 53.57 58.53 62.23
2,4,6-TMP 69.50 73.53 76.73

The superficial charge of the adsorbent (parameter which can affect the adsorbent performance) can be changed by modifying the pH of the solution. Experimental studies showed that for such polymeric adsorbents, the optimum pH for the adsorption process is represented by the pH obtained by dissolution of the phenolic compounds [8], [14], [23] in water, no prior pH adjustment being required.

Kinetic studies

The obtained experimental data were mathematically modeled using pseudo-first-order and pseudo-second-order kinetic models. Kinetic studies were performed for all obtained adsorbent materials. Typical plots are presented in Fig. 6a and b for the best performing adsorbate-adsorbent pair: adsorption of 2,4,6-trimethylphenol on S-15% DVB-isoPrAFO polymer adsorbent. Calculated kinetic parameters are summarized in Table 4.

Fig. 6: 
            The kinetic model plot for 2,4,6-TMP adsorption onto S-15%DVB-iso-PrAFO adsorbent (a) pseudo-first order kinetic model; (b) pseudo-second order kinetic model.
Fig. 6:

The kinetic model plot for 2,4,6-TMP adsorption onto S-15%DVB-iso-PrAFO adsorbent (a) pseudo-first order kinetic model; (b) pseudo-second order kinetic model.

Table 4:

Kinetic parameters for adsorption of phenol, 2,3-dimethylphenol and 2,4,6-trimethylphenol onto synthesized adsorbents.

Adsorbent Adsorbate 293 K
298 K
303 K
308 K
k 1 [h−1] R 2 k 1 [h−1] R 2 k 1 [h−1] R 2 k 1 [h−1] R 2
Pseudo-first order kinetic model
S-6.7%DVB-iso-PrAFO P 0.389 0.9642 0.474 0.9695 0.486 0.9926 0.555 0.9917
2,3-DMP 0.551 0.9781 0.568 0.9924 0.661 0.9904 0.610 0.9852
2,4,6-TMP 0.446 0.9986 0.484 0.9897 0.529 0.9955 0.586 0.9944
S-12%DVB-iso-PrAFO P 0.465 0.9244 0.509 0.9795 0.599 0.9912 0.558 0.9852
2,3-DMP 0.555 0.9797 0.572 0.9933 0.658 0.9940 0.620 0.9839
2,4,6-TMP 0.532 0.9863 0.522 0.9964 0.593 0.9950 0.661 0.9912
S-15%DVB-iso-PrAFO P 0.501 0.9617 0.524 0.9837 0.541 0.9937 0.516 0.9790
2,3-DMP 0.558 0.9803 0.585 0.9935 0.657 0.9932 0.533 0.9711
2,4,6-TMP 0.541 0.9871 0.560 0.9963 0.603 0.9973 0.695 0.9941
293 K
298 K
303 K
308 K
k 2 h−1·L·mmol−1 R 2 k 2 h−1·L·mmol−1 R 2 k 2 h−1·L·mmol−1 R 2 k 2 h−1·L·mmol−1 R 2
Pseudo-second order kinetic model
S-6.7%DVB-

iso-PrAFO
P 3.407 0.9998 6.148 0.9994 9.804 0.9997 16.33 0.9996
2,3-DMP 4.624 0.9996 6.260 0.9997 9.199 0.9986 12.946 0.9995
2,4,6-TMP 3.313 0.9994 4.277 0.9995 5.533 0.9998 7.550 0.9990
S-12%DVB-iso-PrAFO P 4.451 0.9995 6.826 0.9992 12.051 0.9996 13.996 0.9993
2,3-DMP 4.648 0.9997 6.260 0.9998 9.025 0.9989 12.437 0.9996
2,4,6-TMP 3.746 0.9999 4.774 0.9998 6.030 0.9991 7.722 0.9991
S-15%DVB-iso-PrAFO P 5.017 0.9991 6.467 0.9998 9.619 0.9988 11.541 0.9992
2,3-DMP 4.658 0.9998 6.245 0.9999 8.785 0.9991 10.739 0.9997
2,4,6-TMP 3.820 0.9994 4.839 0.9998 6.052 0.9989 7.720 0.9990

Is well known that the kinetic models are often used to establish the nature of the adsorption mechanism and to establish the stages which are determining the process rate limiting step, including chemical reaction and/or mass transport [24].

From the obtained data we can conclude that the adsorption process of studied phenolic compounds is better described by the pseudo-second-order kinetic model, due to the values of correlation coefficients much closer to unity. This observation can suggest that the phenolic derivatives adsorption onto the functionalized S-DVB copolymers seems to be chemisorption. As expected, the temperature increase lead to an increase of the adsorption speed, evidenced by the increase of the adsorption rate constants.

Experimental data confirm that the phenolic compounds adsorption is a rather slow process, due to the week driving forces [25]. Slow adsorption rates are often associated to the steric hindrance effects, limiting the access of bulky adsorbents at the active centers located in the internal pores of the adsorbent [26].

Adsorption isotherms

Liquid–liquid adsorption process can be evaluated by performing dynamic studies and by performing equilibrium tests [27]. Isotherms used to obtain information on adsorption mechanism were: Langmuir isotherm (which provide information about adsorption process nature), Freundlich isotherm (which can be an indication of an heterogeneous adsorption process [28]) and the three parameter Redlich–Peterson isotherm (often providing information on the heterogeneity of the used adsorbents [29]).

In Fig. 7 are presented the linear forms of Langmuir, Freundlich and Redlich–Peterson isotherms obtained for the adsorption 2,4,6-TMP onto S-15%DVB-iso-Pr-AFO adsorbent.

Fig. 7: 
            The Linear form of adsorption isotherms (2,4,6-TMP onto S-15%DVB-iso-Pr-AFO). (a) Langmuir isotherm; (b) Freundlich isotherm; (c) Redlich–Peterson isotherm.
Fig. 7:

The Linear form of adsorption isotherms (2,4,6-TMP onto S-15%DVB-iso-Pr-AFO). (a) Langmuir isotherm; (b) Freundlich isotherm; (c) Redlich–Peterson isotherm.

Similar plots were obtained for all the pairs adsorbate-adsorbent studied.

Based on calculated kinetic parameters is possible to determine which model best describes the adsorption of phenolic compounds onto the functionalized SDVB copolymers. The evaluated the kinetic parameters associated to each model were presented in Table 5.

Table 5:

Parameters of Langmuir, Freundlich and Redlich–Peterson isotherms for adsorption of phenol derivatives on the synthesized adsorbents.

Adsorbent S-6.7%DVB-iso-PrAFO
S-12%DVB-iso-PrAFO
S-15%DVB-iso-PrAFO
Temperature, K 293 K 298 K 303 K 308 K 293 K 298 K 303 K 308 K 293 K 298 K 303 K 308 K
Isotherm
P
 Langmuir
KL 0.414 0.378 0.349 0.316 0.444 0.401 0.362 0.322 0.478 0.431 0.379 0.328
qmax 0.222 0.219 0.214 0.208 0.346 0.344 0.339 0.332 0.382 0.375 0.362 0.355
R2 0.987 0.989 0.991 0.987 0.995 0.988 0.969 0.986 0.990 0.987 0.988 0.985
 Freundlich
A 0.065 0.062 0.058 0.055 0.097 0.092 0.087 0.083 0.120 0.117 0.111 0.107
 1/n 0.672 0.688 0.703 0.726 0.718 0.728 0.756 0.788 0.711 0.713 0.748 0.755
R2 0.9991 0.9994 0.9993 0.9986 0.9966 0.9992 0.9991 0.9993 0.9989 0.9992 0.9977 0.9999
 Redlich–Peterson
α 0.63 0.602 0.586 0.551 0.523 0.483 0.434 0.406 0.586 0.577 0.525 0.508
β 0.791 0.760 0.739 0.688 0.831 0.810 0.769 0.717 0.851 0.823 0.782 0.756
KRP 0.108 0.100 0.093 0.086 0.150 0.139 0.126 0.118 0.195 0.188 0.172 0.163
R2 0.9945 0.9957 0.9995 0.9983 0.9969 0.9838 0.9671 0.9879 0.998 0.9968 0.9958 0.9988
2,3-DMP
 Langmuir
KL 0.731 0.694 0.646 0.607 1.096 0.951 0.834 0.704 1.654 1.416 1.214 1.042
qmax 0.436 0.423 0.417 0.399 0.386 0.407 0.418 0.443 0.360 0.373 0.389 0.407
R2 0.991 0.989 0.989 0.994 0.986 0.990 0.989 0.991 0.990 0.989 0.992 0.989
 Freundlich
A 0.169 0.163 0.159 0.153 0.201 0.194 0.188 0.181 0.226 0.22 0.214 0.207
 1/n 0.679 0.685 0.689 0.692 0.606 0.628 0.65 0.683 0.543 0.568 0.601 0.626
R2 0.9992 0.9988 0.999 0.9993 0.9996 0.9993 0.9993 0.9991 0.9994 0.9995 0.9996 0.9997
 Redlich–Peterson
α 0.711 0.675 0.649 0.615 1.477 1.319 1.179 1.014 2.242 1.951 1.733 1.531
β 0.815 0.801 0.789 0.775 0.829 0.803 0.782 0.762 0.850 0.823 0.796 0.769
KRP 0.29 0.280 0.270 0.261 0.497 0.451 0.412 0.367 0.724 0.648 0.582 0.524
R2 0.9994 0.9994 0.9987 0.9992 0.9989 0.9994 0.9997 0.9999 0.9981 0.9988 0.9993 0.9997
2,4,6-TMP
 Langmuir
KL 2.031 1.600 1.278 1.049 2.910 2.205 1.753 1.476 3.885 2.821 2.148 1.750
qmax 0.406 0.432 0.463 0.493 0.398 0.426 0.456 0.481 0.395 0.425 0.460 0.492
R2 0.987 0.989 0.991 0.992 0.984 0.987 0.988 0.990 0.980 0.984 0.987 0.989
 Freundlich
α 0.288 0.278 0.269 0.259 0.325 0.318 0.311 0.305 0.359 0.353 0.349 0.344
 1/n 0.553 0.593 0.633 0.659 0.513 0.561 0.598 0.627 0.488 0.541 0.587 0.621
R2 0.9987 0.9991 0.9996 0.9995 0.9992 0.9994 0.9991 0.9992 0.9997 0.9993 0.9993 0.9995
 Redlich–Peterson
A 2.583 2.025 1.617 1.339 3.706 2.808 2.250 1.939 5.062 3.616 2.760 2.287
β 0.874 0.854 0.833 0.815 0.896 0.860 0.828 0.795 0.905 0.867 0.824 0.784
KRP 0.987 0.807 0.682 0.592 1.395 1.200 0.954 0.854 1.904 1.465 1.204 1.058
R2 0.9975 0.9976 0.9985 0.999 0.9952 0.997 0.9979 0.9988 0.9936 0.9958 0.9976 0.9989

The data presented in Table 5 clearly confirm that the adsorption-desorption equilibrium does not follow with sufficient accuracy the Langmuir model adsorption isotherm which presume identical active centers, with uniform distribution and monomolecular layer adsorption. Analysing the obtained data one can observe that the temperature increase leads at decrease of the equilibrium constants, which means that the temperature increase is not favorable for the phenolic compounds’ adsorption.

Based on the values of the correlation coefficients we have concluded that the adsorption process of the studied phenolic compounds is better described by Freundlich adsorption isotherm, indicating a significant degree of heterogeneity of the polymeric adsorbent surface. Surface heterogeneity is increasing when the value of the 1/n is decreasing. The values n>1 are confirming a favorable adsorption-desorption equilibrium.

The Redlich–Peterson adsorption model is also respected with very good accuracy. The values of the β constant are pointing out that Langmuir ideal adsorption process assumptions are not respected and supporting the conclusion that the adsorption mechanism is, most probably, hybrid.

Analysing obtained data can observe that the temperature increase leads at decrease of the equilibrium constants, which means that the temperature increase is not favorable for the phenolic compounds adsorption. This observation is in correlation with the value of the adsorption enthalpy.

In all tested isotherms, the values proportional with the adsorption capacity (KL, KF and respectively β) decrease with increasing temperature, consistent with a thermodynamic exothermic adsorption process.

Based on performed experimental studies, the phenol derivatives adsorption is considered a complex equilibrium process involving possible interactions such as: ππ dispersion interaction, electron-donor-acceptor complex formation and/or hydrogen bond formation [25], [30], [31], [32], [33], [34], [35], [36].

Thermodynamic activation parameters

To better understand the adsorption process is important to evaluate the values of the standard activation entropy and enthalpy change. A typical plot is depicted in Fig. 8.

Fig. 8: 
          Plot of ln KL vs. f(1/T) for the adsorption of 2,4,6-TMP on polymeric adsorbents.
Fig. 8:

Plot of ln KL vs. f(1/T) for the adsorption of 2,4,6-TMP on polymeric adsorbents.

Thermodynamic data obtained based on such plots for the adsorbate-adsorbent pairs studied are presented in Table 6.

Data presented in Table 6 confirm that the adsorption process of phenolic compounds onto functionalized S-DVB copolymers is an exothermic process, not favored by increasing temperature. In all cases Gibbs free energy has a small negative value indicating that the adsorption process is spontaneous.

Table 6:

Thermodynamic activation parameters.

Adsorbate Thermodynamic property S-6,7%DVB-iso-PrAFO S-12%DVB-iso-PrAFO S-15%DVB-iso-PrAFO
Phenol ΔH* (kJ/mol) −13.351 −15.988 −18.859
ΔS* (J/mol·K) −52.884 −61.277 −70.398
2,3-DMP ΔH* (kJ/mol) −18.437 −21.875 −23.107
ΔS* (J/mol·K) −71.775 −73.844 −74.667
2,4,6-TMP ΔH* (kJ/mol) −33.132 −34.052 −40.048
ΔS* (J/mol·K) −107.236 −108.521 −125.591

Conclusions

Polymer analogous reaction synthesis routes for obtaining styrene-divinylbenzene copolymers functionalized with pendant aminophosphonate groups are simple and efficient.

The obtained functionalized macromolecular adsorbents are efficient for phenol and phenol derivatives removal from aqueous solutions.

Based on experimental data it was observed that the adsorption capacity increases with the increase of the cross-linking degree of the copolymer matrix. The maximum adsorption capacity for all studied phenolic compounds was exhibited by S-15%DVB functionalized copolymer. The adsorption capacity depends on the nature of phenolic derivative used and on the polymeric support properties. The best results were obtained at 293 K, in the case of highly alkylated 2,4,6-trimethylphenol substrate with a maximum adsorption capacity of 0.292 mmol/g of copolymer, equivalent of a recovery of 76.73 %.

Adsorption process mechanism is best described by Freundlich and Redlich–Peterson adsorption isotherms.

The adsorption process is exothermic, thermodynamically spontaneous and obey a pseudo-second-order reaction kinetics.

The data obtained suggested that the adsorption of phenol and phenol derivatives on functionalized polymer adsorbents is a complex process involving physical and/or chemical adsorption.


Article note

A collection of papers presented at the 17th Polymers and Organic Chemistry (POC-17) conference held 4–7 June 2018 in Le Corum, Montpelier, France.


Acknowledgments

The authors are grateful to Dr. Ecaterina Stela Dragan, (“Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi) for providing chloromethylated copolymers (S-12%DVB and S-15%DVB) and for the morphological characterization of the initial chloromethylated copolymers, intermediate products bearing aldehyde pendant groups and aminophosphonate adsorbent copolymers.

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Published Online: 2019-01-25
Published in Print: 2019-03-26

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