Preparation and characterization of Co(II) ion-imprinted composite membrane based on a novel functional monomer

In this paper, a novel functional monomer, N-(pyrrolidin-2-ylmethyl) methacrylamide (PMMA), was synthesized. Using Co(II) ions as the template, 2,2-azoisobutyronitrile (AIBN) as the initiator, ethylene glycol dimethacrylate (EDGMA) as the crosslinking agent, twenty-seven Co(II) ion-imprinted composite membranes (Co(II)-PMMAIICM1~27) and their corresponding non-imprinted composite membranes (PMMA-NICM1~27) were prepared.

Available online 24 January 2022

1387-1811/© 2022 The Authors Published by Elsevier Inc This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Preparation and characterization of Co(II) ion-imprinted composite

membrane based on a novel functional monomer

Li Zhaob,1, Deqiong Hua,c,1, Huiling Chenga,*

aFaculty of Science, Kunming University of Science and Technology, Kunming, Yunnan, 650500, China

bFaculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650051, China

cYunnan Chihong Resources Comprehensive Utilization Co Ltd, Qujing, Yunnan, 655000, China

A R T I C L E I N F O

Keywords:

Novel functional monomer

Ion-imprinted composite membrane

Membrane selectivity permeation

Cobalt

A B S T R A C T

In this paper, a novel functional monomer, N-(pyrrolidin-2-ylmethyl) methacrylamide (PMMA), was synthesized Using Co(II) ions as the template, 2,2-azoisobutyronitrile (AIBN) as the initiator, ethylene glycol dimethacrylate (EDGMA) as the crosslinking agent, twenty-seven Co(II) ion-imprinted composite membranes (Co(II)-PMMA- IICM1~27) and their corresponding non-imprinted composite membranes (PMMA-NICM1~27) were prepared Additionally, the related parameters of the imprinting system were systematically optimized Co(II) ion- imprinted composite membranes (Co(II)-PMMA-IICM16) were prepared using Nylon-6 as the supporting mem-brane, which was soaked in a pre-polymerized solution of N,N-dimethylformamide and water (DMF: H2O (v/v) =

1: 1) for 180 s, using a molar ratio of template, monomer, and crosslinker of 1: 4: 50 The obtained material had a

higher adsorption capacity (Q e =428.24 mg g− 1) and imprinting factor (IF = 2.36) The surface and internal

porosity of Co(II)-PMMA-IICM16 were characterized by scanning electron microscopy and a nitrogen adsorption apparatus In addition, it was found that the adsorption process of Co(II)-PMMA-IICM16 prepared under optimal conditions was better described by a Langmuir isotherm adsorption model, which verified that the adsorption involved monolayer adsorption The kinetics data was more closely fit by a pseudo-second-order kinetics model, indicating that this adsorption process proceeded via chemisorption The permeation experiments indicated that

a “delayed” permeation mass transfer mechanism also occurred (β(Co(II)/Cd(II)) =2.11 and β(Co(II)/Cu(II)) =1.55) The Co(II) ion-imprinted composite membrane prepared in this paper demonstrated a relatively better imprinting effect, a specific recognition ability for template ions, and good selective permeability These results validated that the design of this novel functional monomer was reasonable, and that it has potential applications in various fields where adsorption is necessary

1 Introduction

Cobalt (Co) is a naturally occurring heavy metal that exists in three

oxidation states (0, +2, and +3), the most of which is +2 Co(II) is highly

susceptible to corrosion by alkali, water, and air, and as a result, it has

been widely used in various applications including electroplating,

mineral processing, and forging [1,2] Along with economic

develop-ment and social progress, significant amounts of Co(II) have been

released into the environment At low levels, Co(II) ion acts as a nutrient

and has benefits for both humans and plants [3] In contrast, beyond

permissible levels, it may lead to various acute or chronic reactions such

as those affecting the gastrointestinal tract, asthma, pneumonia and so

on [4–6] Therefore, the effective treatment and removal of Co(II) ions from wastewater is still highly important

Ion imprinting technology is derived from molecular imprinting technology, which is also a biomimetic recognition technology that can introduce ion recognition sites into some polymeric materials [7–10] Ion-imprinted polymers prepared by ion imprinting have high affinities and selectivity for template ions [11] Membrane separation technology

is one of the most promising separation techniques due to its many advantages such as continuous operation, easy amplification, and low energy consumption However, current commercially-available ultra-filtration, microultra-filtration, and reverse osmosis membranes lack pre-determined selectivity, and it is difficult to achieve the selective

* Corresponding author

E-mail address: ynchenghl@163.com (H Cheng)

1 These authors contributed to the work equally and should be regarded as co-first authors

https://doi.org/10.1016/j.micromeso.2022.111707

Received 18 December 2021; Received in revised form 11 January 2022; Accepted 14 January 2022

separation of individual substances [12,13] Membranes prepared via a

combination of ion imprinting techniques allow for the simple, fast, and

effective separation of specific ions

Ion-imprinted composite membranes (IICMs) are comprised of an

ion-imprinted polymer layer on either the surface or pores of an existing

membrane, which combines the advantages of an ion-imprinted

poly-mer with the stability of a membrane The recognition ability of IICMs is

closely related to the properties of the complexes formed between

template ions and functional monomers Therefore, selecting functional

monomers that match the functional groups and structures of template

ions is the key to preparing IICMs with good performance [14–17]

Suitable functional monomers can be selected according to the structure

of the template ions Acidic template ions can be selected for basic

functional monomers, and mixed functional monomers can also be

considered for some complex template ions Acrylic acid, acrylamide,

and vinyl pyridine are the mostly common functional monomers used to

prepare IICMs [18–23] However, these functional monomers and

template ions only weakly bind to each other, so it is difficult to use them

for ion imprinting Therefore, much research has focused on designing

new functional monomers which possess strong ionic bonding abilities

with the template ions for ion imprinting In recent years, some studies

have reported the development of novel functional monomers For

example, Betra et al [24] synthesized a novel polymerizable functional

monomer containing three functional groups, which was then

poly-merized with styrene to prepare a molecular-imprinted polymer

mem-brane which selectively recognized imprinted molecules Fang et al

[25] designed and synthesized a heterocyclic compound containing a

positively-charged imidazole moiety, tetra-bromine-bi-4,5-2 (methylene

bi-imidazole) acridine This compound was used as a functional

mono-mer, antimony potassium tartrate as a template, and bulk

polymeriza-tion was used to successfully prepare a novel Sb(III)-ion-imprinted

polymer (CFM-IIP) Xu et al [26] synthesized thymine-isocyanate

trie-thoxysilane (T-IPTS) as a functional monomer to successfully prepare

mercury-imprinted polymers capable of specifically recognizing

mer-cury ions Some studies have made progress by synthesizing novel

functional monomers with different target ions, as well as analyzing the

feasibility, recognition mechanism, and new characterization methods

Additionally, identifying and pioneering a new research field using

novel functional monomers for ion imprinting has promising prospects

In this paper, an absorbent material, Co(II) ion-imprinted composite

membranes (Co(II)-PMMA-IICMs) was prepared by Co(II) ion as the

template, N-(pyrrolidin-2-ylmethyl) methacrylamide (PMMA) as the

novel function monomer, a commercial membrane as the support

membrane Additionally, the related parameters of the influence factors

were systematically optimized The surface morphology, internal

structure, adsorption characteristics, and mass transfer mechanism of

the optimal imprinted composite membrane were studied The results

showed that the membrane had good specific adsorption recognition

and permeation selectivity for Co(II) template ions, and it is expected

that the membrane can be used for the removal of Co(II) from aqueous

solutions

2 Experimental

2.1 Materials and instruments

Analytically pure reagents used in this experiment, including cobalt

chloride hexahydrate (CoCl2⋅6H2O), were purchased from Fangchuan

Chemical Reagent Technology (Tianjin, China) Ethylene glycol

dime-thacrylate (EDGMA), 2,2-azoisobutyronitrile (AIBN), NaOH, HCl, and

CH3CH2COOH were purchased from Aladdin Industrial Corporation

(Shanghai, China) and used as received Polyvinylidene fluoride (PVDF),

Nylon-6, and polytetrafluoroethylene membrane (PTFE), with pore sizes

of 0.45 μm and thicknesses of 125 μm, were obtained from Shanghai

Yadong Heji Rosin Co., Ltd, (Shanghai, China) and used to produce

adsorbent The synthetic procedure of N-(pyrrolidin-2-ylmethyl)

methacrylamide (PMMA) is detailed in the Supplementary Materials Document

Ultraviolet–visible spectrophotometry (UV–Vis) (UV-2500, Shi-madzu, Japan) was used to obtain the UV–Vis spectra of Co(II)-IICMs and NICMs An N2 adsorption apparatus (MFA-140, Builder) and scan-ning electron microscopy (SEM, Nova Nano SEM 450FEI-IMC, USA) were employed to characterize the surface area and porosity of Co(II)- IICMs and NICMs, respectively Inductively coupled plasma optical emission spectroscopy (ICP-OES) (Avio 500, PerkinElmer, America) was used to analyze the concentration of metal ions

2.2 Synthesis of N-(pyrrolidin-2-ylmethyl) methacrylamide

The synthetic method of N-(pyrrolidin-2-ylmethyl) methacrylamide

is shown in Fig 1, whose steps are detailed below

A mixture of N-Boc prolinol 1 (10.05 g, 50 mmol) and anhydrous potassium carbonate (6.9 g, 50 mmol) was formed in a round bottom flask, and anhydrous dichloromethane (150 mL) was used to dissolve the mixture P-toluene sulfonyl chloride (11.5 g, 60 mmol) was added to the mixed solution in batches, and the reaction was carried out for 24 h Once the TLC indicated the disappearance of the reagents, the reaction solution was poured into cold water (200 mL) and stirred for 30 min, and the methylene chloride layer was washed three times with water (50 mL

×3) After drying with anhydrous sodium sulfate, the organic phase was

concentrated in vacuo, and compound 2 was obtained without additional

purification

Compound 2 and sodium azide (6.5 g, 100 mmol) were dissolved in dry DMF in an oil bath at 92 ◦C, and the reaction was allowed to proceed for 24 h After the reaction was completed, the mixture was cooled to room temperature and dissolved in ethyl acetate (200 mL) Then, after washing the organic layer 3 times with water and drying and concen-trating with anhydrous sodium sulfate, compound 3 was obtained Compound 3 and palladium on carbon (0.1 g) were dissolved in methanol (100 mL) and allowed to react at room temperature under a hydrogen atmosphere for 2 days After the TLC indicated the disap-pearance of the reactants, the solution was filtered, and compound 4 was obtained

Next, anhydrous dichloromethane (100 mL) was used to dissolve a mixture of anhydrous potassium carbonate (6.9 g, 50 mmol) and methacryloyl chloride (5 mL) with magnetic stirring in an ice bath The dichloromethane solution containing compound 4 was added into the mixture, and the reaction was allowed to proceed overnight Once the TLC indicated the disappearance of the raw materials, the reaction mixture was poured into cold water (200 mL) and stirred for 1 h Then, the methylene chloride layer was washed three times with water (50 mL

×3) After drying with anhydrous sodium sulfate, the organic phase was

concentrated in vacuo, and compound 5 was obtained

Finally, anhydrous dichloromethane (30 mL) was used to dissolve a mixture of compound 5 and trifluoroacetate (20 mL), and the reaction proceeded for 24 h After the TLC indicated the consumption of reagents, saturated sodium carbonate solution was slowly added dropwise to the mixture until no more gas evolution was observed Then, the solution was stirred for 1 h, and dichloromethane (50 mL × 3) was used to extract the solution After drying with anhydrous sodium sulfate, the organic

phase was concentrated in vacuo Light-green oily compounds (3.19 g)

were obtained by column chromatography using dichloromethane as the eluent The total yield of the five-step sequence was 38%

The intermediates involved in the synthesis process were reported according to Refs [27,28], and final product N-(pyrrolidin-2-ylmethyl) methacrylamide was characterized as follows:

1H NMR (400 MHz, CDCl3, ppm) δ: 5.21 (s, 1H), 5.10 (s, 1H), 4.96

(brs, 1H), 4.17 (d, J = 7.3 Hz, 1H), 3.55–3.63 (m, 3H), 3.35–3.41 (m, 1H), 2.07 (d, J = 5.2 Hz, 1H), 1.90 (s, 3H), 1.85 (d, J = 6.0 Hz, 2H),

1.70–1.74 (m, 1H), 1.51–1.60 (m, 1H); 13C NMR (100 MHz, CDCl3, ppm)

δ: 173.09, 141.33, 116.67, 66.95, 60.99, 50.20, 28.42, 24.78, 19.77

2.3 Preparation of Co(II) ion-imprinted composite membrane

Co(II)-PMMA-IICM were prepared according to the procedure shown

in Fig 2 The detailed procedure is as follows:

First, 0.0238 g CoCl2⋅6H2O and different amounts of functional

monomer N-(pyrrolidin-2-ylmethyl) methacrylamide (PMMA) were

mixed and dissolved in solvents, and then shocked for 3 h at 25 ◦C Then,

the crosslinker, ethylene glycol dimethacrylate (EGDMA), and 10.00 mg

initiator, 2,2-azoisobutyronitrile (AIBN), were added into this mixture

After being ultrasonicated for 5 min, a pre-polymerization complex was

formed Next, different support membranes were soaked in a pre-

polymerization solution for a certain period of time at room

tempera-ture The polymerization was carried out for about 24 h at 60 ◦C Finally,

the obtained Co(II)-PMMA-IICM were washed by a mixture of methanol

and acetic acid in a volume ratio of 9: 1 The as-prepared membranes

were dried for 24 h after the template ions were completely removed

using Soxhlet extraction

Non-imprinted composite membranes (PMMA-NICM) were prepared

according to the abovementioned method, but the template (CoCl2

6H2O) was absent

2.4 Batch adsorption experiments

First, 20.0 mg Co(II)-PMMA-IICM16 and 10.00 mL Co(II) solution

with a fixed initial concentration were added into a stoppered 25.00 mL

Erlenmeyer flask Then, the flask containing Co(II) ions was shocked at

room temperature After 12 h, the solution was analyzed by UV–Vis to determine the concentration of Co(II) ions

The adsorption capacity Q (mg.g− 1) of Co(II)-PMMA-IICM16 and PMMA-NICM16 and the imprinting factor IF at equilibrium were calcu-lated by the following equations:

IF = Q Co(II)− PMMA− IICM

/

where C 0 and C (mg.mL− 1) are the initial and equilibrium

concentra-tions of the Co(II) ions, respectively, V (mL) is the volume of the solu-tion, and m is the mass of the Co(II)-PMMA-IICM16

2.4.1 Adsorption isotherms

The adsorption behavior of Co(II)-PMMA-IICM16 was described by the Langmuir and Freundlich models:

Langmuir ⋅

Freundlich ⋅

where Q (mg⋅g− 1) is the amount of Co(II) ions on Co(II)-PMMA-IICM16

at equilibrium, C (mg⋅mL− 1) is the concentration of the Co(II) ions at

equilibrium, qm (mg⋅g− 1) is the maximum adsorption capacity, KL

(mL⋅mg− 1) and KF (mg⋅g− 1) represent the equilibrium constants of the

Fig 1 Synthetic method of N-(pyrrolidin-2-ylmethyl) methacrylamide

Fig 2 Schematic of the preparation process of Co(II)-PMMA-IICM

Langmuir and the Freundlich models, respectively

2.4.2 Adsorption kinetics

Pseudo-first-order and pseudo-second-order kinetic models were

used to study the adsorption kinetics of Co(II) on Co(II)-PMMA-IICM16,

as shown in equations (5) and (6):

Pseudo − first − order ⋅ kinetic ⋅

Pseudo − second − order ⋅ kinetic⋅

model:⋅t/Q t=t/Q + Q2/

where Qe (mg⋅g− 1) is obtained from the abovementioned contents, Qt

(mg⋅g− 1) is the amount of Co(II) adsorbed at time t, K1 (min− 1) and K2

(g⋅mg− 1⋅min− 1) are the rate constants of the pseudo-first-order, and

pseudo-second-order kinetic models

2.4.3 Adsorption thermodynamics

The thermodynamic parameters, including the enthalpy change

(ΔH θ ), Gibbs free energy change (ΔG θ ), and entropy change (ΔS θ), were

calculated by using the following Van’t Hoff expressions:

lnK = − ΔH θ/

RT + ΔS θ/

ln(Q / C) = ΔS θ/

R − ΔH θ/

where R (8.314 J mol− 1 K− 1) is the universal gas constant, T (K) is the

absolute temperature, and Kd is the thermodynamic equilibrium

constant

2.5 Permeation experiments

Competitive penetration tests were used to test the selective

permeation performance of Co(II)-PMMA-IICM16 First, Co(II)-PMMA-

IICM16 was fixed on an H-shaped permeation device (Fig 3), with a total

volume of 200 mL and an area of 1.5 cm2 at the joint of the two cells

Then, 100.00 mL of an aqueous solution containing Co(II), Cd(II), and

Cu(II) ions with identical concentrations of 25 mg mL− 1 was added into

the left cell, and an identical volume of deionized water was placed into the right cell Then, competitive penetration experiments were carried out using a mechanical stirrer with a fixed speed at 25 ◦C The amount of metal ions in the receiving solution was determined via ICP-OES

The permeation flux J (mg⋅cm− 2 s− 1), permeability coefficient P

(cm2⋅s− 1), and permeation selectivity factor β were obtained from the

following equations:

J i=ΔC i V/ΔtA⋅i = Co(II), ⋅Cu(II), ⋅or⋅Cd(II) (11)

β Co2+/j=P Co2+

/

where A (cm2), d (cm), and V (mL) represent the effective membrane

area, membrane thickness, and the volume of the feeding and receiving

solutions, respectively ∇C i /∇t represents the changes in the

concen-trations in the receiving solution, and CFi and CRi are the corresponding

ion concentrations in the feeding and receiving pools, respectively

3 Results and discussion

3.1 Effects of various parameters during the preparation of Co(II)- PMMA-IICM

3.1.1 Effects of support membrane species

The properties of the supporting membrane are one of the important factors affecting the selectivity and permeability of ion-imprinted composite membranes To obtain a better Co(II)-PMMA-IICM, a series

of (Co(II)-PMMA-IICM1~3) and corresponding non-imprinted mem-branes (PMMA-NICM1~3) were prepared using polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and Nylon-6 microfiltration membranes The properties of these composites were tested using Co(II) solutions

It can be seen from Fig 4 that the Co(II)-PMMA-IICM3 had a better adsorption capacity (692.89 mg g− 1) than PMMA-NICM3, and its

imprinting factor (IF) reached 1.24 This may be due to the fact that the

Nylon-6 membranes are more hydrophilic than the PTFE and PVDF membranes That is, the Nylon-6 membrane pores were more easily filled with solvent and solute molecules because they were more easily adsorbed onto the membranes, which increased the adsorption capacity

of the Nylon-6 membrane [29] Therefore, the following experiments used Nylon-6 as the support membrane

3.1.2 Effects of functional monomer dosages

To investigate the effect of functional monomer dose during poly-merization on Co(II)-PMMA-IICMs, several Co(II)-PMMA-IICM3~6 were fabricated using template-to-monomer ratios of 1:2, 1:4, 1:6, and 1:8 to identify the appropriate ratio As shown in Fig 5, when too little or too much was used, the adsorption of the Co(II)-PMMA-IICMs depended on the amount of functional monomer due to the specific recognition ability

of the imprinted membrane majority When an insufficient amount of functional monomer was used, it is difficult to form stable complexes with imprinted ions In contrast, excessive functional monomer can produce many non-specific binding sites in the membrane, which can reduce the recognition ability and selectivity of the membrane [30,31] According to the imprinting factor and adsorption capacity of the imprinted composite membranes, the optimal amount of functional monomer was 4 mmol Therefore, the molar ratio of the template ion to functional monomer was 1:4

3.1.3 Effects of the amount of cross-linker

Effects of the amount of cross-linker (EGDMA) on the adsorption capacity was also investigated The crosslinking agent is the main component in ion-imprinted composite membranes and can fix template-monomer complexes in the polymer matrix The amount of

Fig 3 Schematic of the H-Shape permeation device

crosslinking agent can also affect the structural integrity, accessibility,

and quantity of recognition sites, thereby changing the mesh structure

and adsorption properties of composite membranes [32] Fig 6 shows

the adsorption capacity of Co(II)-IICM3,7~12 and NICM3,7~12 prepared

from different molar ratios of template ions and crosslinking agents

(1:10, 1:20, 1:30, 1:40, 1:50, 1:60, and 1:70, respectively)

Fig 6 shows that the maximum amount of adsorbed Co(II) ion

(625.84 mg g− 1) was obtained when using Co(II)-PMMA-IICM12

pre-pared with a 1:50 M ratio of Co(II) ion to EGDMA This material had a

better imprinting factor (1.40) If too little crosslinking agent was used,

the template-monomer chelate structure did not form during pre- polymerization, Moreover, the lack of a crosslinking agent was not helpful for forming imprinted sites [33], which gave the imprinted composite membrane a poor selectivity However, the use of excessive crosslinking agent increased the rigidity of the membrane, and made the substrate close to the imprinting site As a result, a template: functional monomer: crosslinker ratio of 1:4:50 was selected to prepare the Co (II)-PMMA-IICM

Fig 4 The adsorption capacity of support membrane species

Fig 5 The adsorption capacity of functional monomer dosages

3.1.4 Effects of solvent ratio

Co(II) ions are water-soluble, while cross-linking agents and

func-tional monomers are hydrophobic To maintain the binding spaces of Co

(II) ions during polymerization, the imprinting process requires a

ho-mogenous mixture of Co(II) ions, functional monomers, and cross-

linking agents Therefore, the pore-forming solvent must be mixed

with water and an organic solvent [34] In this work, organic solvents

(CH3OH, CH3CH2OH, (CH3)2CHOH, and CH3CH2CN) and H2O at a

volume ratio of 1:1 (v/v) were used Additionally, different volume

ra-tios of N, N-dimethylformamide (DMF) and H2O (1:1, 1:3, 2:3, 3:1, 3:2,

3:7, 7:3) were used during preparation

The results in Table 1 show that the optimal solvent composition was

1:1 (v/v) DMF:H2O, and the experiment achieved a better imprinting

factor of 2.36 using Co(II)-PMMA-IICM16 (428.24 mg g− 1), compared

with PMMA-NICM16 (181.68 mg g− 1) The influence of solvent

composition on the imprinting factor was greater than that on the

adsorption amount The imprinting factor is used as the main means to

evaluate adsorption Therefore, Co(II)-PMMA-IICMs using an optimum

volume ratio (v/v) of DMF:H2O 1:1 was used in the following

experiments

3.1.5 Effects of soaking time on membrane

In the pre-polymerization system, a short soaking time was not favorable for the deposition of the imprinted polymer, making it difficult for the membrane to be covered However, excessive soaking times resulted in the formation of thick imprinted polymer deposits on the composite membrane Under these conditions, it is difficult for the substrate to access the recognition sites in the imprinted membrane interior, which is not conducive to the adsorption and recognition of the substrate by the membrane [35] Therefore, soaking times of 30 s, 60 s,

180 s, 360 s, 1800 s, and 3600 s were selected for the study, as shown in

Fig 7

It is evident from Fig 7 that the adsorption ability of Co(II) ions increased with soaking times less than 180 s The better adsorption ca-pacity of Co(II)-PMMA-IICM16 and PMMA-NICM16 were 428.24 and 181.68 mg g− 1 separately, and the imprinting factor is calculated to be 2.36 from these two values After 180 s, the adsorption of Co(II) ions decreased to 324.42 mg g− 1 The ion-imprinted composite membrane showed the best affinity and specific recognition to Co(II) ions when the base membrane was soaked for 180 s

The optimized Co(II)-PMMA-IICM16 had a relatively good adsorption capacity and high imprinting factor when the following preparation conditions were used: a Nylon-6 microporous membrane as the

sup-porting membrane, a solvent volume ratio of 1:1 (v/v) of DMF: H2O, a

molar ratio of template ions, functional monomer, and cross-linker of 1:4:50, and a membrane soaking time of 180 s

3.2 Characterization 3.2.1 SEM

The surface topographies of Nylon-6, Co(II)-PMMA-IICM16, and PMMA-NICM16 were obtained using SEM (Fig 8), and indicate that the surface morphology and pore structures of Co(II)-PMMA-IICM16 and PMMA-NICM16 were different than the Nylon-6 membrane The surface

of the Nylon-6 base membrane (Fig 8a) exhibited a symmetrical and flat network structure compared with Co(II)-PMMA-IICM16 (Fig 8b) The corresponding surface of PMMA-NICM16 (Fig 8c) appears rough, indi-cating that the surfaces of the base membranes of Co(II)-PMMA-IICM16 and PMMA-NICM16 were coated with a thin polymer layer

Fig 6 The adsorption capacity of amount of cross-linker

Table 1

The adsorption capacity of various solvent ratios (mCo(II)-PMMA-IICM =20 mg; CCo

(II) =25 mg mL− 1; t = 12 h; pH = 7; T = 25 ◦C)

Membrane Solvent ratio (v:v) Q (Co(II)-PMMA-IICM)

(mg.g − 1 ) Q (mg.g(PMMA-NICM) − 1 ) IF

12 CH 3 OH: H 2 O (1: 1) 625.84 447.03 1.40

13 CH 3 CH 2 OH: H 2 O (1:

14 (CH 3 ) 2 CHOH: H 2 O

15 CH 3 CH 2 CN: H 2 O (1:

16 DMF: H 2 O (1: 1) 428.24 181.68 2.36

17 DMF: H 2 O (1: 3) 639.48 586.94 1.09

18 DMF: H 2 O (2: 3) 344.46 268.24 1.28

19 DMF: H 2 O (3: 1) 243.75 233.98 1.04

20 DMF: H 2 O (3: 2) 301.17 229.26 1.31

21 DMF: H 2 O (3: 7) 379.30 292.92 1.29

22 DMF: H 2 O (7: 3) 251.89 179.46 1.40

It was also clearly shown that Co(II)-PMMA-IICM16 (Fig 8b) had a

greater pore diameter than PMMA-NICM16 (Fig 8c), which can be

attributed to the influence in the membrane surface area and pore

structure during imprinting Ion-imprinted cavities were located at the

surface and inside Co(II)-PMMA-IICM16, and these cavities can match

the size and function of imprinted molecules Furthermore, these results

are consistent with those obtained in adsorption experiments

3.2.2 BET

To determine the effect of the template (Co(II) ions) during

prepa-ration and Co(II) adsorption ability, Co(II)-PMMA-IICM16 and PMMA-

NICM16 were subjected to BET analysis, which is depicted in Fig 9a and

b, respectively Both the pore diameter distributions and surface areas

were tested The results shown in Fig 9, according to the IUPAC

clas-sification method, show that Co(II)-PMMA-IICM16 and PMMA-NICM16

were a type IV isotherms and typical mesoporous materials In addition,

the slopes of the adsorption-desorption isotherms of Co(II)-PMMA-

IICM16 and PMMA-NICM16 were significantly different, indicating that

the membrane pore structure of Co(II)-PMMA-IICM16 and PMMA-

NICM16 were obviously different

As summarized in Table 2, all related parameters of Co(II)-PMMA- IICM16 and PMMA-NICM16 were displayed, including the specific sur-face area and average pore diameter which values are distributed at 9.019, 9.660 m2 g− 1 and 11.768, 10.518 nm, respectively It can be inferred that both Co(II)-PMMA-IICM16 and PMMA-NICM16 belong to mesoporous material according to pore diameter, the Co(II)-PMMA- IICM16 is higher than PMMA-NICM16 between two kinds of value Large surface area can provide more recognition sites to facilitate cobalt adsorption, and the pore sizes of the materials further demonstrate that there is a “template effect” in Co(II)-PMMA-IICM16

3.3 Adsorption isotherms

The Co(II) adsorption capacity using Co(II)-PMMA-IICM16 and PMMA-NICM16 with an initial concentration range of 5–30 mg mL− 1 was studied at a pH of 7 The results are shown in Fig 10 The adsorption capacities of Co(II)-PMMA-IICM16 and PMMA-NICM16 gradually increased as the initial concentration of Co(II) ions increased As the

Fig 7 The adsorption capacity as a function of membrane soaking time

Fig 8 SEM images of the surfaces of Nylon-6(a), Co(II)-PMMA-IICM16 (b), and PMMA-NICM16 (c)

initial concentration of Co(II) increased, the mass transfer driving force

between the solution and the membrane increased, which increased the

adsorption capacity

Additionally, the adsorption ability of Co(II)-PMMA-IICM16 was

relatively higher than PMMA-NICM16 because PMMA-NICM16 may not have had any cavities which matched the Co(II) ions The PMMA- NICM16 could adsorb Co(II) ions to some extent, but not selectively Co (II)-PMMA-IICM16 contains many cavities which match the shape and size of Co(II) ions, and these cavities could specifically recognize and memorize Co(II) ions Thus, the adsorption capacity of PMMA-NICM16 was lower than Co(II)-PMMA-IICM16

The data was fitted as 1/Ce versus 1/Qe and lnCe versus lnQe to study the Langmuir and Freundlich models, respectively, the results are shown

in Fig 11a and b KL, qm, n, and KF were calculated using the slope and

intercept of the correlation lines, and the results are summarized in

Fig 11 The linear correlation coefficient (R2) of the plot of the Freundlich model was relatively better than the Langmuir model, which indicates that the adsorption process was closer to a Freundlich model

Fig 9 The N2 adsorption-desorption isotherms of Co(II)-PMMA-IICM16 (a) and PMMA-NICM16 (b) (insets are the pore diameter distributions)

Table 2

The corresponding parameters of Co(II)-PMMA-IICM16 and PMMA-NICM16

Material Surface area

(m 2 g − 1 ) Pore volume (cm 3 g − 1 ) Average pore diameter (nm) Co(II)-PMMA-

IICM 16

PMMA-PMMA-

NICM 16

Fig 10 Effect of initial Co(II) concentration on the adsorption capacity of Co(II)-PMMA-IICM16

This suggests that adsorption may have occurred through a multilayer

adsorption process

3.4 Adsorption kinetics

The residence times of the adsorbate at the solid-solution interface

and solute adsorption rate were determined using kinetics models using

data obtained from experiments performed between 5 and 180 min

Fig 12 shows that the adsorption rate of Co(II)-PMMA-IICM16 and the

corresponding PMMA-NICM16 rapidly increased within 30 min, and the

adsorption amount of Co(II)-PMMA-IICM16 increased much more

rapidly than PMMA-NICM16 Finally, the adsorption amount tended to

reach equilibrium at 45 min using Co(II)-PMMA-IICM16 and PMMA-

NICM16 This may be due to the fact that the initial adsorption included adsorption by surface imprinted pores As the adsorption time increased, the surface imprinted pores became saturated When the imprinted ions were transferred into the membrane interior, the resistance increased, which decreased the adsorption rate Finally, the adsorption amount slowly increased and finally reached equilibrium, but there were no pores which corresponded to the template ion structure in PMMA- NICM16 The adsorption was mainly affected by the non-specific adsorption of the surface non-imprinted composite membrane, and so adsorption equilibrium was quickly reached

Plots of t versus log (Qe-Qt) for the Co(II) ions adsorption kinetics

behavior are shown in Fig 13a, and the constants of Co(II)-PMMA- IICM16 and PMMA-NICM16 were calculated from the correlation line

Fig 11 The Langmuir model (a) and Freundlich model (b) for the adsorption of the Co(II) ions on the Co(II)-PMMA-IICM16

Fig 12 Kinetic adsorption curves of Co(II)-PMMA-IICM16 and PMMA-NICM16

The linear correlation coefficients (R2) of these plots were relatively

worse

Plots of t versus t/Qt for the Co(II) ions adsorption kinetic behavior

are shown in Fig 13b Compared with the plots in Fig 13a, the linear

correlation (R2) demonstrated that a pseudo-first-order kinetics model

was not as good as the pseudo-second-order kinetics model The rate

constant K2 for Co(II)-PMMA-IICM16 and PMMA-NICM16 were 0.0024

and 0.0027 g mg⋅min− 1, respectively, which may be due to diffusion

resistance of the Co(II) ions to functional monomers [36]

The above data indicate that the entire adsorption process of Co(II)

ions on Co(II)-PMMA-IICM16 and PMMA-NICM16 was mainly controlled

by chemical adsorption In other words, the adsorption capacity of

ad-sorbents was proportional to the number of active sites on their surface,

and chemisorption may be the rate-limiting step in the overall

adsorp-tion process

3.5 Adsorption thermodynamics

To study the effects of temperature on the adsorption amounts of the

Co(II)-PMMA-IICM16, experiments were carried out between 15 and

55 ◦C The adsorption thermodynamics were studied by calculating

several parameters From Table 3, it can be seen that the value of ΔG θ

was negative, indicating that the adsorption of Co(II) on the Co(II)-

PMMA-IICM16 was a spontaneous process The values of ΔH θ and ΔS θ

were − 5288 J mol− 1 and 3.574 J mol− 1 K− 1, respectively The negative

value of ΔH θ indicates that the adsorption reaction was exothermic

Therefore, a higher temperature hindered the adsorption of Co(II) ions

on Co(II)-PMMA-IICM16 The positive values of ΔSθ indicate that the

randomness of the solid/dissolved interface increased during adsorption

[37]

3.6 Effects of pH during Co(II) adsorption

The adsorption process was significantly affected by the pH of aqueous solution, which can affect the protonation of the amino group and the degree of ionization of the adsorbate The effects of the pH value

of Co(II) ion solution on Co(II)-PMMA-IICM16 adsorption were studied and discussed in this section, and the results are shown in Fig 14 When the pH increased from 3 to 7, the amount of adsorbed Co(II) increased from 312.56 mg g− 1 to 359.47 mg g− 1, and reached a maximum value (359.47 mg g− 1) when the pH was 7 Further increasing the pH caused the adsorption amount to decrease to 348.49 mg g− 1 The reason for the initial increase followed by the decrease may be that the amino group was protonated [38], which affected the surface morphology of Co(II)-PMMA-IICM16

3.7 Permeation selectivity of membranes

Permeation selectivity experiments provided a key understanding of the relationship between the template imprinting sites and arrangement

of functional groups According to the various ways of transferring imprinting ions in the membrane, the mass transfer mechanisms of the imprinted membrane can be classified as either “promoting” or

“delaying” permeation [39] Due to the presence of a concentration gradient, imprinted ions and other ions spread in the same direction during the promoting permeation process In this process, the imprinted ions are preferentially adsorbed onto the holes with specific recognition abilities, which can slow down other ion penetration rates and promote the mass transfer of target ions Moreover, during delayed permeation, the specific recognition holes interact with target ions to reach satura-tion, but other ions which do not interact with the recognition sites can diffuse to the other side of the membrane

The permeation selectivity of the Co(II)-PMMA-IICM16, which pos-sesses the optimal adsorption capacity and imprinting factor, was studied and discussed in this part Co(II), Cd(II), and Cu(II) ions were add to the feeding solution at identical concentrations, and the results are shown in Table 4 The permeation flux J of Co(II), Cd(II) and Cu(II)

ions on Co(II)-PMMA-IICM16 reached 4.09 × 10− 5, 8.39 × 10− 5 and 66.25 × 10− 5 mg cm− 2 s− 1, respectively Additionally, the permeability

coefficient P of Co(II), Cd(II), and Cu(II) ions reached 7.57 × 10− 10, 1.60

×10− 9 and 1.17 × 10− 9 cm2 s− 1, respectively The transmittance of Co (II) ions was significantly lower than Cu(II) and Cd(II) ions, possibly because the holes on the Co(II)-PMMA-IICM16 interacted with the Co(II)

Fig 13 The pseudo-first-order (a) and pseudo-second-order (b) kinetic models of Co(II)-PMMA-IICM16 and PMMA-NICM16

Table 3

The thermodynamics model parameters

T (K) ΔG θ (J⋅mol − 1 ) ΔH θ (J⋅mol − 1 ) ΔS θ (J⋅mol − 1 ⋅K − 1 )

273 − 6264

283 − 6299

293 − 6335

303 − 6371