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Abstract

Synthetic resin adsorbents comprise agglomerated gel-type microparticles. Although the excellent adsorption capacities of resins are useful in adsorbing various organic substances, the adsorbed molecules cannot be completely desorbed. A majority of the irreversible adsorbed molecules can be attributed to slow mass transfer within microparticles of resins and it may lower the recovery efficiency of resins. This study focuses on reducing the irreversible adsorption that occurs within a resin (FPX-66) to improve the overall recovery efficiency of the adsorbate. In order to reduce the irreversible adsorption capacity of the resin, styrene monomer was polymerized in microparticles of the resin. Effects of polymerized styrene concentrations and multi-cycle polymerization on irreversible adsorption capacity of the resin were investigated. The experimental results indicated that using one cycle of polymerization with 30–40 vol% of styrene solution is the most effective modification process. The irreversible adsorption capacity could be lowered by half with the above process.

Keywords

Adsorption desorption irreversible adsorption recovery efficiency synthetic resin

Introduction

Synthetic resin adsorbents consisting of styrene–divinylbenzene copolymers are widely used to remove or purify organic substances such as pesticides (Naushad et al., 2014; Otero et al., 2014; Schummer et al., 2013; Zhang et al., 2013), antibiotics (Chao et al., 2014; Jia et al., 2014; Zou et al., 2015), polyphenols, polypeptides, and carotenoids (Huang et al., 2008; Kühn et al., 2014; Monsanto et al., 2015; Sevillano et al., 2014; Weisz et al., 2010). In general, resin macroparticles comprise agglomerated microparticles and can be classified into three types of pores: (1) macropores, which are external pores among resin microparticles; (2) micropores, which are the internal pores of resin microparticles; and (3) gel pores, which exist only in the swollen state of resin microparticles (Sederel and Dejong, 1973). In general, (2) and (3) are not distinct and both pore volumes considered to be micropore volumes. Synthetic resins have good adsorption capacities; however, a part of adsorbed molecules may not be completely desorbed (Buran et al., 2014; Hui et al., 2010; Kim et al., 2014; Liu et al., 2010). For example, the adsorbed molecules in micropores and gel pores would require longer desorption time than those in macropores of resins. Two different adsorption capacities, the reversible adsorption capacity (mainly macropore) and irreversible adsorption capacity (mainly micropores and/or gel pores), exist within resins. The sum of reversible adsorption capacity and irreversible adsorption capacity is total adsorption capacity. As a result, in a fixed-bed operation, tailing behaviour is commonly observed, limiting the efficiency of the desorption process. Consequently, the adsorption/desorption performance of the fixed bed is hindered and the recovery efficiency lowering of desorption process is a serious problem.

In order to increase the efficiency of separation process, there are many reports on the modification of synthetic resins to increase their total adsorption capacities. For example, Sukhorukov et al. (1998) coated resin particles with polyelectrolyte multilayers through consecutive adsorptions, and Wang et al. (2015) introduced acetamide to a hyper-cross-linked resin and evaluated the adsorption behaviour. In addition, Chao et al. (2015) impregnated polystyrene-type resins with metal ions to increase the adsorption of antibiotics, and Negrea et al. (2011) loaded Fe (III) in a styrene-type resin to remove As (V). However, these studies did not investigate the reduction of irreversible adsorption capacity to increase the recovery efficiency. This study focuses on using a styrene monomer polymerization technique in resin particles to reduce the irreversible adsorption capacity and increase the recovery efficiency of resin adsorbents.

Materials and methods

In this study, two modification techniques (one-time polymerization and repeat polymerizations) were employed by polymerizing a styrene monomer within resin microparticles. The macro- and micropore volumes of the resins were then determined by the carbon tetrachloride vaporization technique (CTVT) (Kinoshita et al., 2014). The irreversible amount adsorbed (IRAA) by resins was determined using the static adsorption/desorption technique (ADT) (Kinoshita et al., 2014).

Adsorbent

The adsorbent used in this study was FPX-66 (Dow Chemical Company, USA), which was formed with styrene–divinylbenzene copolymers and had a macro-reticular structure. The resin particles were soaked and washed with isopropanol (IPA), methanol, distilled water, 0.1 M NaOH aqueous solution, and distilled water to remove residual monomers and oligomers. The resins were preserved with IPA in a glass bottle. The characteristics of FPX-66 are listed in Table 1.

Table 1.

Characteristics of FPX-66.

Particle size (mm)	Surface area (m²/g)	Pore volume (cm³/g)	Mean pore diameter (Å)
0.60–0.75^a	700^a	1.7^a	240^b
0.60–0.75^a	910^b	1.9^b	240^b

Provided by Dow Chemical, USA.

Provided by Organo Corporation, Japan.

Chemicals

During the modification process, styrene monomer, IPA and 2-2′-azobis(isobutyronitrile) (AIBN) were employed as the adsorbate, solvent and initiator of polymerization, respectively. The styrene monomer was washed with 10 wt% NaOH aqueous solution and distilled water to remove polymerization inhibitors before use. Carbon tetrachloride (CCl₄) was provided by Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Phenol and sodium hydroxide were provided by Junsei Chemical Co., Ltd. (Tokyo, Japan). All chemicals except styrene were used without further purification.

Modification of FPX-66

One-time polymerization: FPX-66 (1 g), IPA, and AIBN (5 mmol/L) were added in a 30-mL shaded glass bottle that contained 20 mL of known concentrations of styrene at 20℃. At the end of a seven-day adsorption process, molecules might be adsorbed in deep area of microparticles, and hence could not be desorbed quickly in this condition (Kinoshita et al., 2014). The adsorbent was removed and washed with 5 mL of distilled water to remove excess adsorbate attached on the external surfaces of the resin particles. The resulting adsorbents were heated and polymerized in an oven (DSR-111, Isuzu Manufacturing Ltd., Japan) at 80℃ for three days. The resulting resin was referred to as FPX-66-M1.

Repeat polymerizations: The effect of repeated modifications (repeat polymerizations) with 33 vol% of styrene solution was also investigated. FPX-66 (64 g), IPA including AIBN (5 mmol/L, 200 mL), and 100 mL of styrene were added in a 400-mL shaded glass bottle at 20℃. After seven days of adsorption, the adsorbents were removed and washed with a few hundred millilitres of water to remove excess adsorbate attached on the external surfaces of the macro- and microparticles. The resulting adsorbents were heated and polymerized in the oven at 80℃ for three days. Each modified resins were tested as follows (2.4, 2.5). This polymerization procedure was repeated three times. The resulting resins were referred to as FPX-66-M2.

Determination of IRAA

The IRAA was determined by ADT. For the adsorption test, a known amount of resin (0.1 g) and phenol aqueous solution (0.1 M, 20 mL) were added into a glass bottle, and adsorption was allowed to occur for seven days at 20℃ to diffuse molecules deeply in microparticles and could not be desorbed easily in this condition (Kinoshita et al., 2014). Subsequently, adsorbents were washed with 1 mL of distilled water and vacuumed to remove excess adsorbate molecules on external resin surfaces. The washed resins and NaOH aqueous solution (0.1 M, 200 mL) were added into an Erlenmeyer flask to determine the amount desorbed from the resin particles. These solution concentrations were measured by UV-visible spectrophotometry (UV-1700, SHIMADZU, Japan) and calculated by the following equations:

q_{e} = \frac{(C_{0} - C_{e}) V}{m}

(1)

q_{d} = \frac{C_{d} V}{m}

(2)

q_{i} = q_{e} - q_{d}

(3) where q_e (mg/g) denotes the total amount adsorbed at equilibrium, C₀ (mg/L) is the initial concentration of the solution, C_e (mg/L) is the concentration of the solution at equilibrium, V (L) is the solution volume, m (g) is the mass of the dry resin, q_d (mg/g) is the desorbed amount (=reversible adsorption capacity), C_d (mg/L) is the desorbed solution concentration, q_i (mg/g) is the irreversible amount (=irreversible adsorption capacity).

Determination of macro- and micropore volumes

Mercury porosimetry and nitrogen adsorption techniques are generally used to determine macro- and micropore volumes, respectively. However, gel porosity, only existing in wet state, cannot be measured by these techniques because both measure resin particles in the dry state. In addition, the high pressure of mercury porosimetry and the low temperature of nitrogen adsorption might change or deform the structure of the resin. Therefore, CTVT (Kinoshita et al., 2014) was applied in this study.

The technique process was carried out as follows. A known amount of resin (0.1 g) was added into a glass bottle containing 10 mL of CCl₄, and the resulting mixture was mixed at 20℃ for seven days. At the end of the adsorption process, excess CCl₄ was removed and resin weight was measured at predetermined times using an electronic balance until a stable weight was reached. The evaporation process was conducted at room temperature (about 20℃) and normal atmospheric pressure (about 1 atm) without any ventilation in the electronic balance. Resin weight was plotted against time; the obtained plots were used to determine the macro- and micropore volumes of the resins, as shown in Figure 1.

Figure 1.

Vaporization process of CCl₄ molecules on FPX-66 by the carbon tetrachloride vaporization technique.

When adsorbed CCl₄ molecules are released from adsorbents, vaporization occurs in the following order: (i) vaporization of CCl₄ molecules on the external surfaces of resins; (ii) vaporization of CCl₄ molecules in the macropores of resins (macropore volume); and (iii) vaporization of CCl₄ molecules in the micropores of resins (micropore volume). Consequently, the macropore volume (V_M) and micropore volume (including microgel volume) (V_m) could be obtained using equations (4) and (5), respectively:

V_{M} = \frac{Δ W_{1} - Δ W_{2}}{d \times m}

(4)

V_{m} = \frac{Δ W_{2}}{d \times m}

(5) where ΔW₁ and ΔW₂ (g) are the weights of CCl₄ existing within the resin macro- and micropores which can be determined by the intersections of each approximation straight lines, respectively (Figure 1), d (1.594 g/cm³ at 20℃) is the density of CCl₄, and m (g) is the mass of the dry resin, respectively.

Results and discussion

Modification of resins by one-time polymerization (FPX-66-M1)

The relationships between the IRAA of phenol onto the modified PX-66-M1 and micropore volumes are shown in Figure 2. Both the IRAA and the micropore volumes of FPX-66-M1 were similarly affected by the styrene polymerization within resin particles. This result reveals two important things: (1) the IRAA is related to the micropore volume of resin; and (2) the trend of micropore volume changes can be easily estimated by CTVT. Therefore, in order to reduce the IRAA, the reduction of micropore volume is required and the volume can be easily estimated by CTVT. According to Figure 2, styrene polymer chain can block the micropore volume of the resin and half micropore volume can be reduced by thus-modification using the 30–40 vol% of styrene solution.

Figure 2.

Effect of styrene concentration on the irreversible adsorption and micropore volume (FPX-66-M1).

The styrene concentration dependency on reversible amount adsorbed (RAA) and macropore volume are also illustrated in Figure 3. It reveals from the graph that the RAA is affected by the macropore volume. Although macropore volumes are almost constant in the low concentration range of styrene solution (0–40 vol%), those volumes are decreasing with increasing styrene solution concentration (50–85 vol%).

Figure 3.

Effect of styrene concentration on the reversible amount adsorbed and macropore volume (FPX-66-M1).

Two pore volumes, macro- and micropores, are compared in Figure 4. The gray and dark areas denote the macro- and micropore volumes of virgin FPX-66, respectively. The micropore volumes of FPX-66-M1 are smaller than that of FPX-66 and the macropores are not changed at low styrene concentrations (0–40 vol%). This result might be attributed to the polymerized styrene molecules only in the resin microparticles. As a result, the micropore volumes decrease with increasing styrene concentration. At higher concentrations of styrene solution, excess styrene is present in both macro- and micropores (or gel pores) of the resin. Some of the styrene polymerized on microparticle surfaces of resin; hence, the macropore volumes were slightly decreased in high concentration of styrene solutions (50–85 vol%). Although styrene molecules are almost straight-chain (Outer et al., 1950), small amounts of monomers tend to form branched chains (Bamford and Dewar, 1948; Khuong et al., 2005). Furthermore, the agglomeration of microparticles of the resin might suppress the mobility of styrene molecules (Sobani et al., 2014). Consequently, styrene would form branched or entanglement chains within the microparticles of the resins; the resulting chains might form new networks with the pores. These pores could capture adsorbate molecules and hence the curve of the micropore volume against styrene concentration might be a quadratic function-like shape.

Figure 4.

Dependency of styrene concentration on macro- and micropore volumes (FPX-66-M1).

Modification of resins by repeat polymerizations (FPX-66-M2)

The pore volumes obtained by repeat polymerizations using 33 vol% of styrene solution are shown in Figure 5. The gray and dark areas denote the macro- and micropore volumes of virgin FPX-66, respectively. Both pore volumes, macropore volume (V_M) and micropore volume (V_m), decreased with increasing number of modifications. For example, the micropore volume was reduced by about 50% when the number of modifications was increased from one to three. However, the significant reduction of macropore volumes was observed when the number of modifications was increased. The majority of styrene molecule polymerization in the microparticles might occur after the first modification. During the second modification, the styrene molecules might polymerize within micro/macro particle as get caught in the residual styrene chains. Therefore, the entanglement chains of styrene polymers might be expanded. This process might decrease the macro- and micropore volume of the resins.

Figure 5.

Dependency of number of modifications on macro- and micropore volume (FPX-66-M2).

Comparison between FPX-66-M1 and FPX-66-M2

Once again, the object of this study is reducing the IRAA without lowering the RAA in order to increase the recovery efficiency of resin. Two kinds of modified resin, FPX-66-M1 and FPX-66-M2, are compared in Figure 6. FPX-66-M1 using 30–40 vol% of styrene solution could reduce the IRAA by half without significantly reducing of the RAA. On the other hand, also FPX-66-M2 could reduce the IRAA by half when the number of modification repeats three times, while significant reduction of the RAA was observed. The entanglement styrene polymer chain by multi-cycle polymerization might cause the reduction of macropore volumes. Therefore it is concluded that one-time polymerization using 30–40 vol% of styrene solution is the most effective modification method for FPX-66.

Figure 6.

Correlations between irreversible amount adsorbed and RAA for FPX-66-M1 and FPX-66-M2.

Conclusion

In this study, in order to increase recovery efficiency of resin, two modification techniques were developed. Both techniques, one-time polymerization and repeat polymerizations, aimed at reducing the IRAA of resins by decreasing the micropore volume. Experimental results indicated that styrene polymer chain could block micropores (or gel pores) and disturb the adsorption within the microparticles. However, dense styrene solutions were not effective because excess polymer chain decreased RAA. The one-time polymerization technique, employing 30–40 vol% of styrene solution, could decrease IRAA by 50% without significantly changing RAA. On the other hand, the repeat polymerizations technique also could reduce IRAA but resulted in significantly hindering the RAA. Therefore, the one-time polymerization technique seems to be a more effective technique for resin modification. This technique can improve the recovery efficiency of the resin significantly.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Optimization of a modification technique for reducing irreversible adsorption within synthetic resins