Knitted Microporous Polymers as Efficient Adsorbents for Wastewater Treatment： Effects of Skeleton Structure and Pore Distribution

ZOU Zhijuan()， SONG Kunpeng()， 2*， FU Yufang()

1 College of Chemistry and Chemical Engineering， China West Normal University， Nanchong 637002， China 2 Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province， Nanchong 637002， China

Abstract： Hyper-cross-linked microporous organic polymers (MOP) with controlled skeleton structure and pore distribution were prepared by Friedel-Crafts alkylation reaction. The hyper-cross-linked polymers(HCPs) produced by knitting aromatic functional groups posses the typical micro- and meso-porous composite structure and specific surface areas of up to 957 m2·g-1. The obtained materials were evaluated as adsorbents for methylene blue (MB) and subjected to several batch adsorption tests to investigate the effects of adsorbent dosage， concentration of MB， temperature， and pH on MB removal. The maximum adsorbed capacity (qm) of KAPs-Ph(381 mg·g-1， knitted using benzene) exceeded those of less mesoporous KAPs-PhPh3(310 mg·g-1 knitted using 1，3，5-triphenylbenzene) and chloromethyl polystyrene resin(58 mg·g-1). Moreover， KAPs-Ph could be regenerated by Soxhlet extraction with ethanol and reused for up to 15 times with minimal loss of adsorption capacity. The results illustrate that adsorption performance can be improved by controlling the pore structure of the adsorbing materials， and KAPs-Ph has a potential application values for the industrial removal of organic dyes from wastewater.

Key words： microporous organic polymer (MOP)； controllable synthesis； wastewater treatment； catalyst regeneration

Introduction

Organic dyes， widely used in the textile industry for fiber coloration[1]， are prevalent water contaminants which posing a significant environmental threat due to being hardly degradable under natural conditions[2]. To mitigate these problems， a variety of dye removal technologies have been established，e.g.， adsorption[3]， photocatalytic degradation[4]， chemical oxidation[5]， membrane filtration[6]， and electro-oxidation[7]. Among these methods， adsorption is one of the most effective methods， exhibiting the advantages of low cost and easy applicability[8]. Porous carbon is a popular high-surface-area adsorbent for dye removal， which featuring a highly disorganized structure of aromatic sheets/strips and offering a range of active sites due to the presence of several functional groups within its pores[9-10]. Moreover， the cost-effectiveness， ready availability， and appropriate thermal and mechanical properties make carbon-based materials even more attractive for adsorption-based dye removal in water treatment[11]. However， the above materials are hard to regenerate， with the increased processing cost significantly narrowing their application scope[12-13]. Despite the development of nano-sized adsorbents such as graphene[14-15]， carbon nanotubes[16]， and metal-organic frameworks[17]， the removal of these nano-sized adsorbents incurs high costs and thus remains an issue to be solved.

In the past decades， microporous organic polymers(MOPs) have attracted increased attention due to their well-defined porosity， high surface area， low density， and easy of application. Although the above properties make MOPs efficient absorbents for toxic chemicals such as metal ions[18]， dyes[19]， and organic solvents[20-21]， most of these materials are prepared by expensive materials or catalysts， which limit their industrial-scale synthesis and application range.

The above scale-up issues can be solved by utilizing hyper-cross-linked polymers(HCPs) to prepare a new class of porous structures comprising rigid rod-like organic linkers. Importantly， this technique allows one-pot preparation of MOPs with tunable properties， organic functionality， high thermal and mechanical stability， large pore size， and high surface area. For example， Friedel-Crafts alkylation catalyzed by inexpensive FeCl3can be used to synthesize HCPs and be readily scaled up in a relatively cost-effective way[22-23]. To date， HCPs have been demonstrated to be efficient organic polymer-based adsorbents for dyes. For example， Lietal. have reported a novel MOP featuring a combination of sodium acrylate-functionalized HCP groups with magnetic Fe3O4nanoparticles for adsorption of Rhodamine B. The above composite exhibits an adsorption capacity as high as 216 mg·g-1and is reused for up to five times[24]. Although various synthetic approaches to diverse classes of porous materials are currently being developed[25-26]， only few reports describe the preparation of HCPs with tunable pore distributions and skeleton structures for the removal of organic pollutants， and the data on such materials comprise variable building blocks being particularly scarce.

Previously， we proposed a low-cost method of synthesizing large-surface-area microporous via one-step knitting of rigid aromatic blocks with an external cross-linker[18， 27]. Herein， we described the design of novel “dual-porous” materials(KAPs-Ph， KAPs-PhPh3， and chloromethyl polystyrene resin(CPR)) based on constituent block control(benzene and 1，3，5-triphenylbenzene). The pore distribution and skeleton strength of KAPs were also controlled， and the above materials were evaluated for methylene blue (MB) adsorbents. Finally， we addressed the pore volume， pore size/distribution， and adsorption parameter optimization to produce a highly efficient and MB-selective adsorbent for water treatment， focused on increasing the specific surface area， mesoporosity， recyclability， and network rigidity of KAPs-Ph.

1 Experimental

1.1 Materials

Benzene，ferric chloride anhydrous， methanol， MB， and 1，2-dichloroethane(analytical reagent) were obtained from Nation Medicines Corporation Ltd., China and used as received CPR(100-200 mesh). Formaldehyde dimethyl acetal(Adamas-beta?， China)， and 1，3，5-triphenylbenzene(Adamas-beta?， China) were also used as received.

1.2 Synthesis of organic polymer networks

Different types of KAPs(KAPs-Ph and KAPs-PhPh3) were prepared from different monomers using external cross-linked reactions described elsewhere[23]. Specifically， KAPs-Ph was synthesized from benzene(1.56 g， 0.02 mol)， formaldehyde dimethyl acetal(4.56 mg， 0.06 mol)， ferric chloride anhydrous(9.75 g， 0.06 mol) and 1，2-dichloroethane(20 mL)， whereas 1，3，5-triphenylbenzene(3.06 g， 0.01 mol)， formaldehyde dimethyl acetal(4.56 mg， 0.06 mol)， ferric chloride anhydrous(9.75 g， 0.06 mol) and 1，2-dichloroethane(20 mL) were used to prepare KAPs-PhPh3.

1.3 Characterization

Transmission electron microscope(TEM) imaging was performed using an FEI Tecnai G220 electron microscope(operating voltage is 200 kV， FEI Company， Holland)， N2adsorption isotherms(77.3 K), and pore size distributions were determined using a micromeritics ASAP 2020 M surface area and porosity analyzer(Micromeritics， American). Prior to analysis， samples were degassed at 180 ℃ for 8 h under vacuum(1 Pa). Fourier transform infrared(FT-IR) spectra were recorded under ambient conditions at 4 000- 400 cm-1using a VERTEX 70 FT-IR spectrometer(Bruker， Germany). Thermogravimetric analysis (TGA) was performed at a heating rate of 10 ℃·min-1under nitrogen(Pyris 1， Perkin Elmer Instruments， Germany)， and polymer morphologies were investigated by field emission scanning electron microscope(FE-SEM， FEI Sirion 200).

1.4 Adsorption experiments

Working solutions of MB were prepared by dilution of its stock solution(1 000 mg·L-1) in distilled water. Bath adsorption experiments were performed in a 100 mL eggplant bottle that containing 20 mg of KAPs(or CPR) and 30 mL of MB solutions with initial concentrations of 100-300 mg·L-1. The above mixtures were shaken in a mechanical shaker at 30 ℃ and 300 r/min for predetermined time of 5-300 min， after centrifuged， the MB concentration in the supernatant was determined from its UV-VIS absorption at 664 nm. The pH of the MB solution(200 mg·L-1) was adjust in the range of 3-10 from HCl and NaOH solution, and the effect on MB adsorption by KAPs-Ph was also investigated. The maximum adsorbed capacityqm(mg·g-1)， the capacity adsorbed at equilibriumqe(mg·g-1)， and the capacity of MB adsorbed at timet，qt(mg· g-1) were calculated as

(1)

whereC0is the concentrations at time=0(mg·g-1)，Ceis the equilibrium absorbate concentrations (mg·g-1)，Vis the solution volume(L) andWis the mass of the dry adsorbent(g).

Two adsorption models(Langmuir and Freundlich) and three kinetic models(pseudo-first-order， pseudo-second-order， and intraparticle diffusion) were investigated to describe the adsorbate-adsorbent interaction， evaluate adsorbent performance， and understand the adsorption dynamics in the MB-KAPs system.

2 Results and Discussion

The properties of the investigated KAPs were unaffected by changes of humidity or pH. Figure 1 shows that these materials exhibit high thermal stability(decomposition temperature(Tdec)>400 ℃) in air， theTdecof KAPs-PhPh3exceeding that of KAPs-Ph indicats that the former polymer possesses a stronger skeleton.

Figure 2 shows FE-SEM and TEM images of KAPs-Ph and KAPs-PhPh3， the images revealed that KAPs exhibited a porous surface， and KAPs-Ph featured larger pores than KAPs-PhPh3. The FT-IR spectra of KAPs-Ph and KAPs-PhPh3(Fig. 3) displayed a series of bands around 2 930 cm-1that were assigned to the —CH2— stretch， the result confirmed the successful preparation of these polymers. Moreover， peaks at 1 250-950 cm-1and 900-650 cm-1were attributed to benzene skeleton stretching and benzene C—H out-of-plane/in-plane bending vibrations， respectively.

Fig. 1 TGA for KAPs-Ph and KAPs-PhPh3

(a)(b)

(c)(d)

Fig. 2 FE-SEM images of KAPs-Ph:(a) TEM images of KAPs-Ph; (b) FE-SEM images of KAPs-Ph; (c) TEM images of KAPs-Ph; (d) TEM images of KAP-Ph

Fig. 3 FT-IR spectra of KAPs-Ph and KAPs-PhPh3

Microporous polymers prepared from different monomers generally feature remarkably different skeletal structures and pore distributions，e.g. the network obtained through 1，3，5-triphenylbenzene was much more rigid than benzene. As shown in Table 1， KAPs-Ph exhibited a specific surface area of 908 m2·g-1and a micropore fraction of 15.7 %， whereas the respective values for KAPs-PhPh3equaled 957 m2·g-1and 71.0%. Conversely， CPR had almost no micropores and exhibited a specific surface area of only 24 m2·g-1. In table 1,SBETis calculated from N2adsorption isotherms at 77.3 K using the Brunauer-Emmett-Teller (BET) equation，SLis calculated from N2adsorption isotherms at 77.3 K using the Langmuir equation，DPis average pore size calculated from 4 V·A-1， pore volume is calculated from N2isotherm at 77.3 K andP/P0=0.995， micropore fraction is calculated from total pore volume and micropore volume.

Surface areas and pore size distribution of the above three materials were tested by N2sorption analysis(Fig. 4(a))， the steep N2gas uptake at low relative pressures(P/P0<0.001) indicate abundant microporosity， the result is in accordance with the data of Table 1.

Table 1 Surface area and porosity of utilized adsorbents

At low relative pressures， the flat part of the isotherm middle section corresponded to a type I isotherm according to the International Union of Pure and Applied Chemistry(IUPAC) classification. At higher relative pressures， however， the isotherms of the three catalysts were markedly different. The isotherm of KAPs-PhPh3resembled the catalyst that we previous described and classified as type I， while KAPs-Ph showed significant pore filling at high relative pressures and classified as type II character[28]. Therefore， the isotherm of KAPs-Ph was classified as type II with some type IV character， the insignificant hysteresis observed upon desorption indicates the presence of micro- and meso-prous. Based on the obtained pore size distributions(Fig. 4(b))， we concluded that MB adsorption was favored by mesoporosity and rigid skeleton structure and KAPs-PhPh3exhibited a certain(although small) extent of mesoporosity， and the surface area of CPR was very small. The average pore sizes of KAPs-Ph， KAPs-PhPh3， and CPR equaled 5.2， 1.3， and 35.0 nm respectively， and the corresponding pore size distributions were calculated using density functional theory.

(a)

(b)

Fig. 4 Specific surface areas and pore size distribution: (a) nitrogen adsorption and desorption isotherms at 77.3 K and(b) pore size distributions of the investigated catalysts

2.1 Adsorption of MB onto KAPs-Ph

Adsorption isotherms are usually determined under equilibrium conditions. First， we examined the adsorption performance of KAPs-Ph， KAPs-PhPh3， and CPR. As shown in Table 1， under identical conditions， KAPs-Ph and KAPs-PhPh3showed a much better performance than CPR in controlled experiments due to their larger specific surface areas. Although the surface area of KAPs-Ph was smaller than that of KAPs-PhPh3， the KAPs-Ph featured increased micro- and meso-prous and thus showed the highest MB adsorption capacity. Therefore， we selected KAPs-Ph as adsorbent model to further investigate the influence of temperature， pH and concentration on adsorption performance.

2.2 Effect of agitation time and MB concentration

A series of experiments were carried out at different initial MB concentrations(100-300 mg·L-1) at a temperature of 30 ℃， and the results showed that a contact time of 300 min was required to reach adsorption equilibrium(Fig. 5).

(a)

(b)

Fig. 5 Adsorption capacity of KAPs-Ph for various initial MB concentrations: (a) remove rate and (b) adsorbed amount

As can be seen from Fig. 5， the capacity of MB adsorbed on KAPs-Ph increased with time and at some point reached a constant value. The time required to attain this equilibrium was denoted as the equilibration time， the capacity of dye adsorbed at this time reflect the maximum adsorption capacity under these conditions.

The equilibrium adsorption capacity increased from 100 to 381 mg·g-1as the initial dye concentration increased from 100 to 300 mg·L-1， the result revealed that KAPs-Ph efficiently adsorbed MB from aqueous solution due to its porous structure and large internal pore surface area. Commonly， the adsorption of solutes by porous adsorbents features three consecutive mass transport steps[29]. The first is solute transport from the particle surface into its interior(pore diffusion)， the second is adsorbate migration through the solution and finally adsorption at interior active sites. So it required a relatively long contact time and explained the large equilibration time of 300 min observed for KAPs-Ph.

2.3 Effect of pH

Figure 6 showed that KAPs-Ph exhibited a high MB adsorption capacity which was practically independent of pH due to the buffering effect of the adsorbent[30].

Fig. 6 Effect of solution pH on MB adsorption by KAPs-Ph

2.4 Adsorption isotherms

The adsorption isotherm reflected the distribution of adsorbate molecules between the liquid phase and the solid phase at equilibrium， and fitted by different models which were important for successful adsorbent design and optimization[31]. Herein， the adsorption isotherms were fitted using the well-known Langmuir and Freundlich models. The Langmuir model assumed monolayer adsorption onto a surface containing a finite number of uniform adsorption sites with no adsorbate transmigration in the surface plane[32]. And the Freundlich model assumed heterogeneous surface energies， with the energy term in the Langmuir equation being a function of surface coverage[32]. The quality of the obtained fits illustrated the applicability of the above models， and were determined from the corresponding correlation coefficientsR2.

2.4.1Langmuirmodel

The Langmuir model can be described by the following linear equation：

(2)

The fact that experimental data could be well fitted by the Langmuir model, indicated that the surface of KAPs-Ph was homogeneous，i.e.， the adsorption of each MB molecule had the same activation energy， and the results implied that the outer surface of KAPs-Ph was covered by a monolayer of dye molecules.

Fig. 7 Isotherms for MB adsorption on KAPs-Ph at 30 ℃：(a) Langmuir isotherm and(b) Freundlich isotherm

The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless equilibrium parameter(RL)[33]defined by

(3)

wherebis the Langmuir constant andC0is the maximal dye concentration(mg·L-1). Thus， depending on the above parameters， adsorption was classified as either unfavorable(RL>1)， linear(RL=1)， favorable(1>RL>0)， or irreversible(RL=0). Herein，RLis determined as 0.008， confirming that the adsorption of MB on KAPs-Ph is favorable under the conditions used in this study.

2.4.2Freundlichmodel

The well-known logarithmic form of the Freundlich model is given by

(4)

whereqeis the capacity of adsorbate adsorbed at equilibrium(mg·g-1)，Ceis the equilibrium adsorbate

concentration， andKF(mg·g-1) is the adsorption capacity of the adsorbent defined as the adsorption or distribution coefficient which represents the quantity of MB adsorbed onto KAPs-Ph at a unit equilibrium concentration. The slope 1/n(ranging between 0 and 1) is a measure of surface heterogeneity， with values close to zero reflecting increased heterogeneity[34]. Generally， slope values below one indicate normal Langmuir adsorption， whereas slope values above one indicate cooperative adsorption[35]. In this study， the plot of lnqevslnceyielded a straight line(Fig. 7(b))， revealing that the adsorption of MB can also be described by the Freundlich model. The corresponding Freundlich constants(KFandn) are listed in Table 2， which compares the parameters obtained for the two models. The related correlation coefficients revealed that the Langmuir model yield a somewhat better fit than the Freundlich one at different temperatures. Moreover， the value of 1/n=0.14 at 30 ℃ indicated favorable adsorption[36].

Table 2 Langmuir and Freundlich isotherm parameters for MB adsorption on KAPs-Ph.

2.4.3Adsorptionkinetics

The adsorption rate constant can be determined using the pseudo-first-order equation of Langergren and Svenska[37]：

ln(qe-qt)=lnqe-k1t，

(5)

whereqeandqtare the capacity of MB adsorbed(mg·g-1) at equilibrium and at timet(min)， respectively， andk1is the corresponding rate constant(min-1). The values ofk1were calculated from the plots of ln(qe-qt)vs.t(Fig. 8(a)) for different MB concentrations. Although the correlation coefficients obtained at high concentrations exceeded 0.85， the experimentalqevalues did not agree with the ones calculated based on these linear plots(Table 3)， implying that the adsorption of MB onto KAPs-Ph could not be described by a pseudo-first-order model.

Furthermore， pseudo-second-order equilibrium adsorption[38]can be expressed as

(6)

(a)

(b)

Fig. 8 Models of MB adsorption on KAPs-Ph：(a) pseudo-first-order model and(b) pseudo-second-order model

The mechanism of diffusion occurring during adsorption was identified by using the intraparticle diffusion model[1]：

qe=kpt0.5+B，

(7)

wherekp(mg·g-1·min-1/2) is the intraparticle diffusion constant， andBis a constant describing boundary layer effects. WhenB=0， the adsorption kinetics is controlled only by intraparticle diffusion， the adsorption kinetics being quite complex ifB≠0. The values ofkpandBobtained from plots ofqevs.t0.5(Fig. 9) suggested that intraparticle diffusion played an important role in the adsorption of MB on KAPs-Ph， and a non-zeroBvalue implying that intraparticle diffusion was not the exclusive rate-controlling step[39].

Table 3 Kinetics parameters of pseudo-first-order and pseudo-second-order models of MB adsorption on KAPs-Ph

Fig. 9 Intraparticle diffusion kinetics for MB adsorption on KAPs-Ph

2.4.4Adsorptionthermodynamics

To evaluate the effect of temperature and understand the nature and feasibility of MB adsorption， we performed experiments at 0， 15， and 30 ℃ with an initial MB concentration of 300 mg·L-1， and used the following equations to calculate thermodynamic parameters[40].

(8)

ΔG=RTlnKD，

(9)

(10)

whereKDis the equilibrium partition constant， ΔG(kJ·mol-1) is the Gibbs free energy change， ΔH(kJ·mol-1) is the enthalpy change， ΔS(kJ·mol-1) is the entropy change，R(8.314 J·mol-1·K-1) is the universal gas constant， andT(K) is the temperature.

The values of ΔGare calculated from theKDvalues for each temperature， and the values of ΔSand ΔHare calculated from the slope and intercept of the plot of lnKDvs. 1/T， respectively(Table 4). The positive value of ΔH(21.03 kJ·mol-1) confirmed that the adsorption process was endothermic， in agreement with the results of Refs.[41-42]， and the negative value of ΔGshowed that MB adsorption was spontaneous[43]. Moreover， ΔGbecame more negative with increasing temperature， indicating that the adsorption of MB on KAPs-Ph was more favorable at higher temperatures.

2.5 Regenerability

Multi-cycle regenerability is a key property of an effective absorbent， helping reduce the adsorption process cost，and is very important for industrial applications. In this study， the used adsorbent was regenerated by Soxhlet extraction with ethanol， with the effect of regeneration time shown in Fig. 10. Compared to those of fresh adsorbents， the adsorption capacities of KAPs-Ph and KAPs-PhPh3after the second recycling decreased by about5% and 50%， respectively. The results indicate the good regenerability of the former adsorbent and the adsorbent can retain a high adsorption capacity up to the 15th cycle. Conversely， KAPs-PhPh3and CPR featured good cycling performance but low adsorption capacity， especially in the case of CPR， whose adsorption capacity decreased to almost zero after eight cycles. The above difference is ascribed to the optimized skeleton structure of KAPs and their mesoporosity. These confirm that the efficient， durable KAPs-Ph can be easily regenerated and recycled after adsorption， and have a great potential for MB removel in water.

Table 4 Thermodynamic parameters of MB adsorption on KAPs-Ph at different temperatures

Fig. 10 Recycle runs of KAPs-Ph， KAPs-PhPh3， and CPR

3 Conclusions

Herein， we have successfully prepared a dual-porous organic polymer(KAPs-Ph) with pronounced microporosity， high BET surface area， and rigid skeleton by “knitting” rigid aromatic blocks with an external cross-linker， achieving excellent adsorption capacity for MB.

The adsorption capacity of KAPs-Ph for MB was not influenced by pH. Moreover， the adsorption of MB at a concentration of 300 mg·L-1was described by a pseudo-first-order model， and a pseudo-second-order model which are applied at higher concentrations. The results suggesting that the chemisorption of MB on KAPs-Ph was essentially irreversible. The above adsorption was best fitted by the Langmuir model， with theqmaxequaling 380 mg·g-1. Thermodynamic studies revealed that adsorption was spontaneous and endothermic， and the rigid structure of the polymer remarkably improved its regenerability. Thus， the obtained results may constitute a new paradigm for the development of porous polymers for efficient water treatment.