adsorption of resorcinol and catechol from aqueous solution by aminated hypercrosslinked polymers

REACTIVE

Reactive & Functional Polymers 64 (2005) 63–73

&FUNCTIONALPOLYMERS

www.elsevier.com/locate/react

Adsorption of resorcinol and catechol from aqueoussolution by aminated hypercrosslinked polymers

Yue Sun *, Jinlong Chen, Aimin Li, Fuqiang Liu, Quanxing Zhang

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University,

Nanjing 210093, PR China

Received 20 October 2004; received in revised form 7 March 2005; accepted 17 March 2005

Available online 22 June 2005

Abstract

A series of aminated hypercrosslinked polymers (AH-1, AH-2 and AH-3) were prepared on the basis of the conven-

tional chloromethylated low-crosslinked macroporous styrene–divinylbenzene copolymers by controlling the post-

crosslinking reaction and surface modification with dimethylamine. Adsorption behaviour of resorcinol and catechol

from aqueous solution onto the above aminated hypercrosslinked polymers and the hypercrosslinked polymeric adsor-

bent NDA-100 without amino groups was compared. It was found that the aminated hypercrosslinked polymers had

higher adsorption capacities than NDA-100 due to the Lewis acid–base interaction between the phenolic compounds

and the tertiary amino groups on the polymer matrix. Specific surface area and micropore structure of the adsorbent, in

company with tertiary amino groups on the polymer matrix mutually affect the adsorption performance towards the

both phenolic compounds. Additionally, more catechol is adsorbed than resorcinol due to the affinity towards water

and position of the hydroxyl group on the benzene ring of the compound. Besides, thermodynamic study was carried

out to interpret the adsorption mechanism. Kinetic study testified that the tertiary amino groups on the polymer matrix

could decrease the adsorption rate and increase the adsorption apparent activation energy.

� 2005 Elsevier B.V. All rights reserved.

Keywords: Hypercrosslinked polymer; Resorcinol; Catechol; Adsorption; Thermodynamics; Kinetics

1. Introduction

Phenol and its derivatives are versatile raw

materials in chemical industry. As a result, they

1381-5148/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.reactfunctpolym.2005.03.004

* Corresponding author.

are widely found in the effluence of their manu-

facture and subsequent usage. Because of their

high toxicity, high oxygen demand and low bio-

degradability, phenolic compounds are consid-

ered as primary pollutants in wastewater [1,2].Consequently, the removal or destruction of phe-

nols from such streams has become significant

ed.

64 Y. Sun et al. / Reactive & Functional Polymers 64 (2005) 63–73

environmental concern. During the past decades,

resin adsorption technology as a method of

recovery of resource has been widely adopted

for the treatment of effluents containing phenolic

compounds [3–5]. In comparison with such classi-cal adsorbents as silica gels, aluminas and acti-

vated carbons, polymeric adsorbents are of

particular interest for their high chemical stabil-

ity, easy regeneration, excellent selectivity and

longevity. Among them, the commercial resin

Amberlite XAD-4 has ever been considered the

best one for the removal of phenolic compounds

from wastewater [3,6]. Subsequently, the hyper-crosslinked polymers developed by Davankov

were discovered better contact with aqueous

phase and larger adsorption capacities towards

aromatic organics, especially the phenols with

limited solubility such as 2-naphthol [7–9]. How-

ever, they appear to possess much lower adsorp-

tion capabilities towards organic compounds

that exhibit a strong affinity towards water [10].Consequently, there is a clear need for new poly-

functional adsorption materials to efficiently re-

move these strongly hydrophilic compounds

from aqueous media. Thus, some scientists have

made more efforts on chemical modification of

polymeric adsorbents to improve their adsorption

properties owing to the presence of some special

interactions between adsorbates and adsorbents[11–14]. In the present study, three aminated

hypercrosslinked polymeric adsorbents (AH-1,

AH-2 and AH-3) were prepared from the conven-

tional chloromethylated low-crosslinked macro-

porous styrene–divinylbenzene copolymers by

controlling the post-crosslinking reaction and sur-

face modification with dimethylamine expectantly

to remove hydrophilic phenolic compounds. Twoderivatives of phenol, namely resorcinol and cat-

echol, are chosen as the target solutes since both

of them have high toxicity, high solubility and

further, widely exist in the effluents of many

industries such as textile, paper, pulp, steel, petro-

chemical, petroleum refinery, rubber, dyes, plas-

tics, pharmaceutical and so on [15]. The focus

of this work is to compare the adsorption ofthe two phenolic compounds from aqueous solu-

tion onto the aminated hypercrosslinked resins

(AH-1, AH-2, AH-3) and the hypercrosslinked

resin (NDA-100). The adsorption thermodynam-

ics and kinetics were further discussed.

2. Experimental

2.1. Materials

Resorcinol and catechol were purchased from

Shanghai Chemical Reagent Plant (Shanghai

City, China) and dissolved in deionized water in

adsorption tests without pH adjustment. The

chloromethylated low-crosslinked macroporousstyrene–divinylbenzene copolymers (the DVB

content 6% and the chlorine content 19.5%),

1,2-dichloroethane, zinc chloride, ethanol and

aqueous dimethylamine (40 wt%) were all sup-

plied by Langfang Electrical Resin Co. Ltd. (He-

bei Province, China).

2.2. Synthesis of adsorbents

2.2.1. Post-crosslinking

In a 100 mL three-necked round-bottomed flask

equipped with a mechanical stirrer, a thermometer

and a reflux condenser, 10 g of chloromethylated

low-crosslinked macroporous styrene–divinylben-

zene copolymer beads were swollen in 60 mL 1,2-

dichloroethane. Under mechanical stirring,weighted zinc chloride was added at 298 K. The

mixture was further stirred at 388 K for 2–12 h

to get post-crosslinked polymer beads with differ-

ent residual chloride content. Then the polymer

beads were filtered and extracted with ethanol

for 8 h in a Soxhlet apparatus and dried under vac-

uum at 333 K.

2.2.2. Chemical modification of the post-crosslinked

polymers

In a 100 mL three-necked round-bottomed flask

equipped with a mechanical stirrer, a thermometer

and a reflux condenser, 10 g of the post-cross-

linked polymer beads of a known residual chloride

content were swollen in 60 g of aqueous dimethyl-

amine (40 wt%). Under mechanical stirring, theamination reaction continued for 12 h at 318 K.

Finally, the reaction mixture was filtered and the

modified adsorbents AH-1, AH-2 and AH-3 were

Y. Sun et al. / Reactive & Functional Polymers 64 (2005) 63–73 65

extracted with ethanol for 8 h in a Soxhlet appara-

tus and then dried under vacuum at 333 K.

2.3. Characterization of adsorbents

Chlorine content of the polymers was mea-

sured using the Volhard method [16]. Tertiary

amino group content of the aminated polymers

was measured according to the literature [3].

Specific surface area and pore structure of the

resins were measured with an ASAP-2010C

Micromeritics Instrument (Micromeritics Instru-

ment Corp., Norcross, GA, USA) with nitrogenas the adsorbate following the BET method.

Infrared spectra of the polymeric adsorbents

were obtained using a Nicolet 170 SX IR spec-

trometer (Nicolet Instrument Corp., Madison,

WI, USA) employing a pellet of powdered

potassium bromide and resin.

2.4. Static adsorption

Static adsorption of resorcinol and catechol on

the adsorbents at three temperatures (283, 298 and

313 K) was conducted as follows: 0.100 g of dry re-

sin was introduced into a flask and 100 mL of

aqueous solution of resorcinol or catechol was

added into each flask. The initial concentrations

(C0) of the solutions ranged from 100 to1000 mg/L. The flasks were completely sealed

and placed in a constant temperature shaker (Taic-

ang Guangming Experimental Instrument Co.

Ltd., Jiangsu Province, China) at a pre-settled

temperature and shaken at 200 rpm for 24 h to en-

sure the adsorption process reaching equilibrium.

The equilibrium concentration (Ce) of resorcinol

and catechol was determined using a Helious Betra

CH CH2 CH CH2

CH2 CH2Cl+ NH CH3

CH3

Fig. 1. Scheme of am

UV–VIS spectrometer (Unicam, Cambridge, UK).

The corresponding equilibrium adsorption capac-

ity Qe (mmol/g) was calculated via the following

equation:

Qe ¼ V ðC0 � CeÞ=WM ; ð1Þwhere V is the volume of solution (L), W is the

mass of dry resin (g) and M is the molecular

weight of resorcinol or catechol.

2.5. Kinetic adsorption

Kinetic adsorption of resorcinol and catechol

onto the resins NDA-100, AH-1, AH-2 and AH-

3 was carried out in the way similar to the static

adsorption tests, except for that the initial concen-

tration of each adsorbate was settled at 1000 mg/L

in all cases at 298 and 313 K. The instantaneous

adsorbate uptakes on the resins were calculated

by measuring the concentration of adsorbate insolution at different contact time.

3. Results and discussion

3.1. Characteristics of the polymeric adsorbents

Three post-crosslinked polymers were preparedfrom the chloromethylated low-crosslinked macro-

porous polymer beads by controlling the residual

chloride content, and then the hypercrosslinked

resins AH-1, AH-2 and AH-3 containing tertiary

amino groups were obtained by the amination of

these post-crosslinked polymers with dimethyl-

amine. The amination reaction scheme is shown

in Fig. 1. The hypercrosslinked resin NDA-100without amino groups was prepared in the way

CH CH2 CH CH2

CH2 CH2N(CH3)2

ination process.

Table 1

Characteristics of the four adsorbents

Property NDA-100 AH-1 AH-2 AH-3

Structure St-DVBa St-DVB St-DVB St-DVB

Hypercrosslinked Amine-modified Amine-modified Amine-modified

Hypercrosslinked Hypercrosslinked Hypercrosslinked

Polarity Weak polar Moderate polar Moderate polar Moderate polar

BET surface area (m2/g) 934 819 726 483

Micropore area (r 6 1 nm, m2/g) 561 463 394 261

Micropore volume (r 6 1 nm, cm3/g) 0.25 0.21 0.18 0.12

Tertiary amino group (mmol/g) 0 1.51 2.10 2.75

Average pore diameter (nm) 2.4 2.4 2.6 3.0

Average particle size (mm) 0.4–0.6 0.4–0.6 0.4–0.6 0.4–0.6

Chlorine content before amination (%) 6.02 6.02 8.10 10.50

Chlorine content after amination (%) 0.85 0.76 0.84

Colour Brown Brown Yellow Yellow

a Abbreviated form of styrene–divinylbenzene.


similar to AH-1, AH-2 and AH-3, but without

chemical modification process.

The characteristics of the tested adsorbents are

presented in Table 1. It can be seen from Table 1

that the post-crosslinked polymers can be ami-

nated by dimethylamine effectively, and the resid-

ual chlorine content is very low after amination.

The hypercrosslinked resin NDA-100 has the larg-est surface area, micropore area and micropore

volume among the four resins but without tertiary

amino groups on the matrix. The content of ter-

tiary amino groups of AH-1, AH-2 and AH-3 in-

creases in turn but all the specific surface area,

Fig. 2. IR spectr

micropore area and micropore volume accordingly

decrease. The presence of tertiary amino groups on

the aminated hypercrosslinked polymers was fur-

ther supported by the absorbance bands at 2772

and 1347 cm�1 in IR spectra from Fig. 2.

3.2. Static equilibrium adsorption

Fig. 3 shows the equilibrium adsorption iso-

therms of resorcinol and catechol on the adsor-

bents at 283 K. The data were analyzed using the

empirical Freundlich equation [17]:

logQe ¼ logKF þ 1=n logCe; ð2Þ

a of AH-1.

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 (a)

Qe (

mm

ol/g

)

Ce (mmol/L)

0 2 4 6 80.00.20.40.60.81.01.21.41.61.82.02.2 (b)

Qe (

mm

ol/g

)

Ce (mmol/L)

Fig. 3. Adsorption isotherms of: (a) resorcinol and (b) catechol onto (e) NDA-100, (h) AH-1, (n) AH-2 and (·) AH-3 at 283 K.


where Qe is the equilibrium adsorption capacity of

adsorbent (mmol/g); Ce is the equilibrium concen-

tration of adsorbate (mmol/L), KF and n are the

characteristic constants. All the isotherm parame-

ters could be determined from the experimental

data by plotting log Qe against log Ce on the basisof the above equation. The Freundlich isotherms

parameters KF, n, and the square of the correlative

coefficients R2 are listed in Table 2.

All the equations are reliable because of the

square of all the correlative coefficients R2 > 0.98.

The values of KF, a relative indicator of adsorption

capacity from Freundlich theory [17], indicate that

the aminated hypercrosslinked polymers AH-1,AH-2 and AH-3 have more effective adsorption

sites than NDA-100. The exponent n is larger than

one in all cases, an indication of favorable pro-

cesses. The adsorption capacities (Qe) on the

adsorbents calculated with Freundlich equations

corresponding to the same residual concentration

Table 2

Freundlich isotherms parameters for adsorption of resorcinol and ca

Adsorbate Adsorbent KF

Resorcinol NDA-100 0.4441

Resorcinol AH-1 0.5937



Catechol NDA-100 0.5109

Catechol AH-1 1.015

Catechol AH-2 1.145

Catechol AH-3 1.024

a Calculated by isotherm equations when Ce = 4.0 mmol/L.

(Ce) of 4.0 mmol/L are also listed in Table 2. At

283 K, when Ce = 4.0 mmol/L, the adsorption

capacities of AH-1, AH-2 and AH-3 are 25.6%,

47.7%, 42.2% towards resorcinol and 54.7%,

70.9%, 60.6% for catechol higher than those of

NDA-100, respectively.Generally speaking, surface area, micropore

structure, as well chemical surface heterogeneity

of a hypercrosslinked polymer usually put much

influence on its adsorption capacity [13,14,18].

All the specific surface area, micropore area and

micropore volume of NDA-100 are the largest

among the tested resins, but its capacity is the low-

est, thus the significant difference in adsorptioncapacity is not only related to the specific surface

area or micropore structure. It is reasonable that

the higher adsorption capacities of the aminated

hypercrosslinked polymers are attributed to the

presence of tertiary amino group on their net-

works. According to Lewis acid–base theory, the

techol on the adsorbents

n R2 Qe(mmol/g)a

2.286 0.9846 0.8144

2.547 0.9866 1.023

2.499 0.9926 1.203

2.503 0.9909 1.158

2.028 0.9955 1.012

3.245 0.9987 1.556

3.357 0.9970 1.730

3.001 0.9988 1.625


benzene ring of aromatic-based resin and the ter-

tiary amino groups on it can be viewed as Lewis

bases, while phenolic compounds can be viewed

as Lewis acids [12]. Therefore, the Lewis acid–base

interaction may occur between phenolic com-pounds and the benzene ring as well as the tertiary

amino group of the resins, which leads to the for-

mation of a hydrogen bonded complex. The inter-

action can be interpreted qualitatively in terms of

the Lewis acidity and basicity [19,20]. The solvato-

chromic parameters a and b for resorcinol and cat-

echol are given in Table 3. For the values of

benzene ring and tertiary amino group on resinsare not available, they are approximated by the

values of the close homologues, namely, benzene

and triethyl amine, respectively. A larger a corre-

sponds to a greater ability of a molecule, or actu-

ally of the functional group to accept electrons or

serve as a Lewis acid in an association complex.

Conversely, a larger b corresponds to a greater

ability of a molecule or group, to donate electronsor serve as a Lewis base. From Table 3, it will be

found that both resorcinol and catechol can serve

as a Lewis acid and also a Lewis base. The fact

that triethyl amine has a larger value of b than

benzene, indicates that tertiary amino group on

the resin network acts as a stronger Lewis base

than benzene ring of the resin, which leads to a

stronger Lewis acid–base interaction between theboth phenolic compounds and tertiary amino

group than that between them and benzene ring.

This is probably the primary reason for the in-

creased adsorption capacity of the aminated

hypercrosslinked polymers over that of NDA-

100. In addition, the adsorption capacity towards

resorcinol and catechol on AH-2 with moderate

content of tertiary amino groups is highest amongthe three aminated hypercrosslinked polymers but

not AH-3, which has the most tertiary amino

groups. It is because that all the specific surface

Table 3

Acidity and basicity parametersa

Solute Resorcinol Catechol Benzene Triethyl amine

a 1.10 0.85 0 0

b 0.58 0.52 0.14 0.71

a Taken from [19,20].

area, micropore area and micropore volume of

the polymers decrease with increasing content of

tertiary amino groups. In a word, specific surface

area, micropore structure and content of tertiary

amino groups together dominate the adsorptionproperty of the aminated hypercrosslinked

polymers.

Resorcinol has a larger value of a than catechol

indicating a stronger Lewis acid–base interaction

between it and the tertiary amino group on the re-

sin, but as is evident from Table 2, the adsorption

capacity of catechol is larger than that of resor-

cinol onto the aminated resins. The difference inadsorbability can be explained in terms of the sol-

ubility and polarity of adsorbate and the position

of hydroxyl group on the benzene ring. The solu-

bility of resorcinol in water is much higher than

that of catechol (229 and 46 wt%, respectively

[21]), thus it shows stronger affinity towards water.

This may be one of the possible reasons for its

lower adsorption capacity. In addition, the match-ing of polarity between adsorbent and adsorbate is

also an important factor affecting adsorption of

phenolic compounds. The tertiary amino nitrogen

on the resins has a large dipole moment, and the

dipole moment of catechol is larger than that of

resorcinol (2.620 and 2.071 D, respectively [22]),

therefore, the interaction between the resins and

catechol is expected to be stronger than that be-tween the resins and resorcinol. Furthermore,

some researches revealed that the same functional

group but at the ortho position greatly enhances

the adsorption energy of such compound as cate-

chol used in the present study [15]. Thus cumula-

tive effects of solubility and position of the

hydroxyl group at the ortho position may probably

account for higher adsorbability of catechol thanresorcinol onto not only aminated hypercross-

linked polymers but also NDA-100.

3.3. Thermodynamics of resorcinol and catechol

adsorbed onto NDA-100, AH-1, AH-2 and AH-3

As discussed above, the aminated hypercross-

linked polymers demonstrated excellent adsorp-tion capabilities towards resorcinol and catechol.

Comparing with NDA-100, the thermodynamic

study of AH-1, AH-2 and AH-3 was thus carried


out to explore the adsorption mechanism. The

isosteric enthalpy change of adsorption herein

was calculated by Van�t Hoff equation [23].

logð1=CeÞ ¼ logKe þ ð�DH=2.303RT Þ; ð3Þ

where Ce is the equilibrium concentration of solute

in mol/L at the absolute temperature T; DH is the

isosteric enthalpy change in adsorption and R is

the gas constant. Ce was obtained from the well-

suited isotherms corresponding to a definite Qe va-

lue at different temperatures (283, 298 and 313 K).

DH was then calculated from the slope of the lineplotted by log (1/Ce) versus 1/T, and some of the

plotted lines are shown in Fig. 4.

The adsorption free energy change was calcu-

lated from the Freundlich isotherm by employing

the Gibbs equation [1,23]:

DG ¼ �nRT ; ð4Þ

0.0031 0.0032 0.0033 0.0034 0.0035 0.00362.2

2.4

2.6

2.8

3.0

3.2

3.4(a)

log

(1/C

e(mol

/L))

1/T (K)

Fig. 4. Determination of isosteric enthalpy changes of adsorption towa

1, (m) AH-2 and (d) AH-3 at adsorption capacity Qe = 0.5 mmol/g.

Table 4

Calculated thermodynamic parameters for adsorption of resorcino

(Qe = 0.5 mmol/g)

Adsorbate Adsorbent DH (kJ/mol) DG (kJ/mol)

283 K 2

Resorcinol NDA-100 �24.93 �5.38 �Resorcinol AH-1 �31.37 �5.99 �Resorcinol AH-2 �31.17 �5.88 �Resorcinol AH-3 �30.27 �5.89 �Catechol NDA-100 �25.31 �4.77 �Catechol AH-1 �33.76 �7.64 �Catechol AH-2 �33.26 �7.90 �Catechol AH-3 �36.72 �7.06 �

where n is the parameter of the Freundlich equa-

tion and DG is the free energy change in an adsorp-

tion process.

The adsorption entropy change then could be

calculated via the Gibbs–Helmholtz relationship[23]:

DS ¼ ðDH � DGÞ=T . ð5ÞWhen the equilibrium adsorption capacity is

determined as 0.5 mmol/g adsorbent, the values

of the isosteric enthalpy change (DH, kJ/mol), free

energy change (DG, kJ/mol) and entropy change(DS, J/mol K) calculated from the data obtained

in the present study were thoroughly presented in

Table 4.

From Table 4, exothermic adsorption processes

are testified by the negative values of all the isos-

teric enthalpy changes. The larger absolute values

of adsorption isosteric enthalpy changes of AH-1,

0.0031 0.0032 0.0033 0.0034 0.0035 0.00362.4

2.8

3.2

3.6

4.0

4.4(b)

log

(1/C

e(m

ol/L

))

1/T (K)

rds: (a) resorcinol and (b) catechol onto (.) NDA-100, (n) AH-

l and catechol within the temperature range of 283–313 K

DS (J/mol K)

98 K 313 K 283 K 298 K 313 K

4.97 �4.66 �69.08 �66.98 �64.76

5.74 �5.99 �89.68 �86.01 �81.09

5.75 �5.25 �89.36 �85.30 �82.81

5.17 �5.20 �86.15 �84.23 �80.10

4.54 �4.12 �72.58 �69.70 �67.70

7.71 �6.96 �92.30 �87.42 �85.62

7.93 �7.60 �89.61 �85.00 �81.98

6.88 �6.56 �104.81 �100.13 �96.36


AH-2 and AH-3 mean the interactions between the

two phenolic compounds and the aminated hyper-

crosslinked resins are stronger than that between

them and NDA-100 due to the Lewis acid–base

interaction. The adsorption free energy changesare always negative proving that the adsorption

processes of resorcinol and catechol on adsorbent

surface are all spontaneous. In addition, the abso-

lute values of adsorption free energy changes of

the aminated hypercrosslinked resins are larger

than that of NDA-100, suggesting that the adsorp-

tion of resorcinol and catechol is easier onto the

former resins. The entropy changes during theadsorption of resorcinol and catechol on all

the employed adsorbents are negative, indicating

that the activity of adsorbate molecules on the

adsorbents is weaker than those in the aqueous

solution. In comparison with NDA-100, the abso-

lute values of the adsorption entropy changes of

AH-1, AH-2 and AH-3 are large, which indicates

that the adsorbate molecules are more tight and

0 50 100 150 200 2500.0

0.2

0.4

0.6

0.8

1.0

1.2 (a)

Q (

mm

ol/g

)

t (min)

0 50 100 150 200 2500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6(c)

Q (

mm

ol/g

)

t (min)

Fig. 5. Effect of contact time on the uptake of: (n) resorcinol at 313 K

298 K onto: (a) NDA-100; (b) AH-1; (c) AH-2; (d) AH-3 (C0 = 1000

ordered on the aminated hypercrosslinked resins.

Furthermore, the fact that the absolute values of

the isosteric enthalpy change, free energy change

and entropy change of adsorption on the aminated

hypercrosslinked resins towards catechol are alllarger than those for resorcinol, indicates the bet-

ter adsorbed property of the former. This is con-

sistent with the conclusion made in the static

equilibrium adsorption study.

3.4. Kinetics of resorcinol and catechol adsorbed

onto NDA-100, AH-1, AH-2 and AH-3

Kinetic sorption of resorcinol and catechol

onto NDA-100, AH-1, AH-2 and AH-3 was car-

ried out to explore the feasibility of this type of

resins in the removal of the two phenolic com-

pounds from water stream. The influence of con-

tact time on the uptake of resorcinol and catechol

onto NDA-100, AH-1, AH-2 and AH-3 was

shown in Fig. 5.

0 50 100 150 200 2500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 (b)

Q (

mm

ol/g

)

t (min)

0 50 100 150 200 2500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6(d)

Q (

mm

ol/g

)

t (min)

; (·) resorcinol at 298 K; (n) catechol at 313 K; (s) catechol at

mg/L).

Table 5

Kinetic parameters for adsorption of resorcinol and catechol onto NDA-100, AH-1, AH-2 and AH-3

Adsorbate Adsorbent Temperature (K) ln (1 � Qt/Qe) = �k t Ea (kJ/mol)

k (min�1) R2

Resorcinol NDA-100 298 0.0343 0.9848 22.50

Resorcinol NDA-100 313 0.0530 0.9895

Resorcinol AH-1 298 0.0303 0.9905 24.21

Resorcinol AH-1 313 0.0484 0.9952

Resorcinol AH-2 298 0.0224 0.9934 32.62

Resorcinol AH-2 313 0.0421 0.9908

Resorcinol AH-3 298 0.0180 0.9806 34.67

Resorcinol AH-3 313 0.0352 0.9910

Catechol NDA-100 298 0.0258 0.9806 24.82

Catechol NDA-100 313 0.0417 0.9851

Catechol AH-1 298 0.0238 0.9839 26.19

Catechol AH-1 313 0.0395 0.9881

Catechol AH-2 298 0.0187 0.9899 32.70

Catechol AH-2 313 0.0352 0.9939

Catechol AH-3 298 0.0148 0.9874 35.13

Catechol AH-3 313 0.0292 0.9833


The adsorption rate constants for the removal

of resorcinol and catechol by the resins were deter-

mined using the pseudo first-order kinetic expres-

sion [24]:

lnð1� Qt=QeÞ ¼ �kt; ð6Þwhere Qt is the uptake (mmol/g) of adsorbate attime t; Qe is the equilibrium uptake (mmol/g); k

and t are the rate constant and time, respectively.

The kinetic data obtained within 80 min were plot-

ted relating ln (1 � Qt/Qe) with the contact time t

at 298 and 313 K, respectively, and k was therefore

obtained from the slope of the linear relationship.

The apparent activation energy was calculated via

the Arrhenius equation:

k ¼ k0 expð�Ea=RT Þ; ð7Þ

where k is the rate constant (1/min); k0 is a param-

eter related to temperature (1/min); Ea is the

apparent activation energy (J/mol); R is the gas

constant (8.314 J/mol K); and T is the absolute

temperature (K). The values of the rate constantsand the square of the correlation coefficients along

with the apparent activation energy are listed in

Table 5.

Fig. 5 shows the sorption equilibrium com-

pleted within about 3 h of contact time with the

initial resorcinol or catechol concentration C0 of

1000 mg/L and the higher temperature is favor-

able to attaining equilibrium but leads to lower

sorption uptake. From Table 5, it will be found

that the adsorption of the two phenolic com-

pounds onto the resins conformed to be the

first-order process because the square of all the

correlation coefficients (R2) are higher than 0.98.

Under the same temperature, for both adsor-bates, the values of rate constant k decrease in

the following order: AH-3 < AH-2 < AH-

1 < NDA-100. Some researchers have studied

the effect of chemical surface heterogeneity on

the adsorption of dissolved aromatic compounds

on activated carbon and found that water adsorp-

tion happened on the hydrophilic, polar oxygen

groups and then water clusters were built up onthese groups by hydrogen-bonding interaction

[25]. The water clusters can effectively reduce

the width of the pores and then the diffusion rate

will slow down. Similarly it may occur on AH-1,

AH-2 and AH-3 for the presence of tertiary ami-

no group on the resin matrix and then water

adsorption can happen to slow down the adsorp-

tion rate [26]. The order of apparent activationenergy required for the adsorption of both resor-

cinol and catechol onto the resins is: NDA-

100 < AH-1 < AH-2 < AH-3. This may also be

attributed to the Lewis acid–base interaction.


4. Conclusions

Post-crosslinking reaction and surface modifi-

cation of the conventional chloromethylated

low-crosslinked macroporous polymers werestudied and three aminated hypercrosslinked res-

ins AH-1, AH-2 and AH-3 were obtained. The

adsorption capacities of both resorcinol and cate-

chol on the aminated hypercrosslinked resins

AH-1, AH-2 and AH-3 are higher than on the

hypercrosslinked resin NDA-100 without amino

groups. Specific surface area, micropore structure

and content of tertiary amino groups play acombined role during the adsorption of the both

compounds onto the aminated hypercrosslinked

polymers.

Negative values of the isosteric enthalpy

changes indicate that the adsorption towards res-

orcinol and catechol on the tested resins is exother-

mic. The negative values of adsorption free energy

changes demonstrate the spontaneous processes.The entropy changes are negative, indicating

the weaker activity of adsorbate molecules on the

adsorbents than in the aqueous solution. The

absolute values of the above three thermodynamic

parameters of AH-1, AH-2 and AH-3 are all larger

than those of NDA-100 indicating that the ami-

nated hypercrosslinked resins exhibited better

adsorption abilities towards resorcinol andcatechol.

Kinetic sorption study of resorcinol and cate-

chol onto NDA-100, AH-1, AH-2 and AH-3 indi-

cates a first-order kinetic behaviour. The aminated

hypercrosslinked resins show a lower adsorption

rate than NDA-100 for the presence of water clus-

ters. Furthermore, the values of apparent activa-

tion energy required for the adsorption increasewith the rising content of tertiary amino groups

on the resin matrix for Lewis acid–base

interaction.

Acknowledgements

The authors thank the National Natural Sci-ence Foundation of China (No. 20274017). We

are grateful to the Analytical Center at Jiangsu

Polytechnic University for the measurement of

the specific surface area and pore structure, and

the State Key Laboratory for Coordination

Chemistry at Nanjing University for the IR

data.

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adsorption of resorcinol and catechol from aqueous solution by aminated hypercrosslinked polymers

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