Selective Adsorption of p-Xylene from Pure Terephthalic Acid

Article pubs.acs.org/jced

Selective Adsorption of p‑Xylene from Pure Terephthalic Acid Wastewater on Modified and Formed Zeolites Zhaoyang Qi, Miaomiao Zhou, Ling Li, Ting Qiu, and Changshen Ye* School of Chemical Engineering, Fuzhou University, Fuzhou 350116, China ABSTRACT: Different zeolites were screened to achieve the aim of effective selective removal of p-xylene (PX) from the pure terephthalic acid (PTA) wastewater, and the ZSM-5 zeolite modified by NH4Cl was chosen as a suitable adsorbent because of the maximum equilibrium adsorption capacity and the largest selectivity factor. The selectivity factors of PX to Co2+ and PX to Mn2+ can attain 1778.069 and 1875.650, respectively, and the saturation adsorption capacity of PX even increases to 126.10 mg·g−1. First, the adsorption experiments were conducted at 298.15, 308.15, 318.15, and 328.15 K with the initial concentration of PX ranging from 50 to 180 mg·L−1 in a batch adsorber. The Langmuir model agreed better with the experimental results than did the Freundlich model. The adsorption parameters of thermodynamics are ΔS = −17.83 J·mol−1·K−1, ΔH = −12.33 kJ mol−1, and ΔG < 0, respectively. It is demonstrated that the adsorption of PX on the H-ZSM-5 zeolite is a spontaneous and exothermic physisorption. Second, the kinetics of PX adsorbed on the H-ZSM-5 zeolite was studied by batch adsorption. The results showed that the pseudo-second-order rate model can be used to better describe the adsorption behavior of PX on the H-ZSM-5 zeolite. The H-ZSM-5 zeolite is successfully regenerated using the high-temperature roasting method. On the basis of this, the molding process of the solid H-ZSM-5 zeolite was further studied. Finally, fixed bed adsorption was conducted in a tubular glassware instrument to obtain the breakthrough curves. The influences of operation parameters on the adsorption process in the fixed bed were discussed. The results showed that H+exchanged ZSM-5 zeolite was a highly efficient material for selectively adsorbing PX from PTA wastewater.

1. INTRODUCTION At present, the domestic quantity of wastewater from the purified terephthalic acid (PTA) industry has exceeded 180 million tons per year.1 It is estimated that the production of 1 ton of PTA will generate approximately 3 tons of PTA wastewater, which consists of p-toluylic acid, PX, metal ions, and other pollutants. PX is the raw material used to produce PTA, and Co2+ and Mn2+ ions can be used as catalysts in the PTA industry. All of them are costly and harmful to the body when ingested. To lower the cost and protect the environment, Co2+ and Mn2+ ions and all kinds of organic matter should be recycled from PTA wastewater. Currently, aerobic bioremediation and anaerobic biotreatment are the main industrial methods used to treat PTA wastewater. However, the biological degradation of these impurities is difficult, and the organic matter in PTA wastewater cannot be recovered and reused with these methods. Accordingly, Qiu and co-workers2,3 reported a process to deal with PTA wastewater. This technological process includes liquid−liquid extraction, ultrafiltration, and reverse osmosis, which could effectively recycle metal ions and organics as shown in Figure 1. However, it was found that the working life of a reverse osmosis membrane is short because its base material is polyamide, which could be decomposed by residual extractant p-xylene. The wastewater after extraction technology still contains PX (40−50 ppm) and Co2+ and Mn2+ ions (30 ppm © 2017 American Chemical Society

Figure 1. Technological process: (1) PTA wastewater, (2) PX, (3) extraction phase, (4) separator, (5) extraction column, (6) transfer pump, (7) cooling unit, (8) separator, (9) transfer pump, (10) pxylene transfer pump, (11) recycling of organic matter, (12 ultrafiltration concentrated water, (13) ultrafiltration−reverse osmosis system, (14) pure water, (15) reverse osmosis concentrated water, and (16) metal ion adsorption apparatus.

Received: October 4, 2016 Accepted: January 20, 2017 Published: February 3, 2017 1047

DOI: 10.1021/acs.jced.6b00861 J. Chem. Eng. Data 2017, 62, 1047−1057

Journal of Chemical & Engineering Data

Article

molding of adsorbents, and the fixed bed adsorption process were also investigated.

each), so it is necessary to remove PX completely before ultrafiltration. Selective adsorption PX technology is considered to be one of the most promising approaches to achieving the above aim between liquid−liquid extraction and ultrafiltration mode as shown in Figure 2. Zeolite is a natural aluminosilicate

2. EXPERIMENTAL SECTION 2.1. Materials. All zeolites (5A, 13X, NaY, and ZSM-5) were purchased from Yuan Li Co Ltd., Tian Jin, China. Co(CH3COO)2·4H2O and Mn(CH3COO)2·4H2O were used to prepare the solution containing Co2+ and Mn2+ ions. Analytical reagents were used in this work without further purification. All chemicals were provided by Aldrich Chemical Co. Ltd. or Sinopharm Chemical Reagent Co. Ltd., China. The initial pH of the solutions was regulated with 0.1 mol·L−1 sodium hydroxide and hydrochloric acid. 2.2. Adsorption Parameters. The adsorption percentage, η (%), and the adsorption capacity, Qe (mg·g−1), can be expressed as follows, respectively, η=

C0 − Ce × 100% C0

Qe = Figure 2. New technological process: (1) PTA wastewater, (2) PX, (3) extraction phase, (4) separator, (5) extraction column, (6) transfer pump, (7) cooling unit, (8) separator, (9) transfer pump, (10) pxylene transfer pump, (11) recycling of organic matter, (12) ultrafiltration concentrated water, (13) p-xylene adsorber, (14) ultrafiltration−reverse osmosis system, (15) pure water, (16) reverse osmosis concentrated water, and (17) metal ion adsorption apparatus.

(1)

(C0 − Ce) m −1

(2) −1

where C 0 (mg·L ) and C e (mg·L ) are the initial concentration of adsorbate in liquid and the equilibrium concentration of adsorbate in liquid, respectively. The quantity of adsorbent in a liter of aqueous solution is represented by m (g·L−1). All experiments were conducted three times under the same conditions, and the relative errors were not more than 5%. The selectivity coefficient of PX for a given metal ion was defined by the distribution coefficient. The distribution coefficients (Kd) of PX, Co2+, and Mn2+ were calculated with eq 3:15

mineral. According to previous reports,4−6 zeolites and ionexchanged zeolites are considered to be promising and less expensive adsorbents for PX. Guo7 presented a process to separate PX from mixtures of ethylbenzene (EB), o-xylene (OX), and PX using the H/ZSM-5 zeolite. The selectivity factors of PX to EB and OX reach 7.95 and 104, respectively.7 Smolin8 employed Na-X and Li-X zeolites to adsorb PX from C8 aromatics. The results showed that the selectivity factors of PX to MX, PX to OX, and PX to EB were between 1.68 and 5.60.8 Hsiao et al.,9 Moya-Korchi,10 and Santacesaria et al.11,12 carried out a detailed experimental study of the equilibrium behaviors of KY, BaX, and K-BaY zeolites, respectively. The adsorption equilibrium has been measured at temperatures of 293 to 353 K, and all kinds of zeolites have similar adsorption capacities of PX (1−1.75 mol·kg−1) in these works. Erdem et al.13 have studied the adsorption of heavy metal cations on natural zeolites. The findings indicated that the adsorption depends on the charge density and hydrated ion radius, and the adsorption order of various ions is H+ > Co2+ > Cu2+ > Zn2+ > Mn2+. Motsi et al.14 have pointed out that the adsorption selectivity sequence on natural zeolites is shown as Fe3+ > Zn2+ > Cu2+ > Mn2+, according to the equilibrium studies. Zeolites mainly depend on ion-exchange action to adsorb metal ions, so adjusting the SiO2/Al2O3 ratio and using the method of cations exchange could achieve the point of selective removal of PX from industrial wastewater. It is clear that both organic matter and ions can be adsorbed by zeolites. Therefore, in this work the major interest lies in ion exchange with zeolites to selectively adsorb PX from PTA effluents. Influences of various parameters such as the dosage of zeolites and the initial pH value were discussed by batch adsorption. The adsorption equilibria and kinetics of PX on ion-exchanged zeolites, the

Kd = Q e/Ce

(3)

Then the selectivity factor (K) of zeolite for PX with respect to one of the two metal ions can be obtained with eq 4: K = Kd(PX)/Kd (metal ion)

(4)

A high selectivity coefficient indicates that the adsorption separation of two substances will be easy. 2.3. Zeolite Screening. The adsorption experiments of PX and heavy ions on different kinds of zeolites were performed in a batch adsorber. A 100 mL solution containing 50 mg·L−1 PX and 30 mg·L−1 Co2+ and Mn2+ was set up in a conical flask. Then the adsorbent, 0.1 g of zeolites, was placed into the conical flask, which was shaken at 200 rpm for 2 h at a temperature of 298 K. Subsequently, the concentrations of residual metal ions in the liquid phase were determined by inductively coupled plasma (ICPE-9000, Shimadzu Corporation), and the PX concentrations were measured by capillary gas chromatography with headspace (GC-2014C Shimadzu). 2.4. Batch Adsorption. The batch adsorption experiments were carried out in accordance with the methods described in section 2.3. The differences are that the concentrations, adsorption temperature, and initial pH will be set according to the specific requirements of the experiments. 2.5. Adsorbent Molding. The particle adsorbent was made with H-ZSM-5 zeolite, pseudoboehmite, sesbania powder, and 5 wt % HNO3. H-ZSM-5 zeolite powder, binder pseudoboehmite, and pore-forming material sesbania powder were mixed adequately, and then 5 wt % HNO3 and deionized water were 1048



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electrostatic force on PX. After ion exchange, H+ occupies the exchangeable/adsorbable sites that Co2+ and Mn2+ cannot exchange with them. Before and after H+ ion exchange, there was no significant difference in the saturated adsorption capacity. 3.1.3. Characterization. The reference adsorbent is an ionexchanged ZSM-5 zeolite. The average pellet diameter calculated from the particle size distribution analysis was 0.62 mm. The structure and texture of the adsorbent were characterized by SEM (S-4800, Hitachi) analysis, X-ray microanalysis (Miniflex II, Rigaku), and N2 adsorption (Autochem 2920, Micrometrics). SEM analysis (Figure 3) showed that the crystal size is 0.5 to 1 μm. The X-ray diffractograms (Figure 4) indicated that the adsorbent is an MFI-type zeolite in accordance with the diffraction peak doublets. In addition, the ion-exchange process has no obvious effect on the crystallinity of ZSM-5 zeolites. The N2 adsorption was measured using a Micrometrics ASAP2020 at 77 K. The samples (ca. 0.15 g) were degassed in situ under vacuum at 573 K for 4 h prior to analysis. The pore diameter distribution is obtained by the Horvath−Kawazoe method. The results of N2 adsorption analysis are summarized in Table 3, which also explains why the saturation adsorption capacity of H-ZSM-5 zeolites slightly increases compared to that of ZSM-5 zeolites, probably because the kinetic diameter of H+(0.1 nm) is smaller and it could enlarge the pore canal after ion exchange with Na+ (0.196 nm) to make the average pore diameter of H-ZSM-5 zeolites larger than that of ZSM-5 zeolites, which can also be proven by the N2 adsorption experiment. In view of the adsorption capacity and selectivity factor, H-ZSM-5 zeolites are selected as a suitable adsorbent for the adsorption process. 3.2. Ascertaining the Dosage of Adsorbent. The influences of adsorbent dosage on absorbing Co2+, Mn2+, and p-xylene on the H-ZSM-5 zeolites were discussed at 298.15 K as shown in Figure 5. The results illustrated that the adsorption percentage increased with the increase in adsorbent dosage. The reason for this is that the zeolites provide more adsorption surface and active sites. As the dosage of adsorbent reached 1 g· L−1, the adsorption percentages of Co2+, Mn2+, and PX were 0.97, 0.92, and 96.57% on H-ZSM-5 zeolites, respectively. 1 g· L−1 zeolites was adopted for the following studies on account of these results. 3.3. Influence of the Initial pH. More metal ions had been removed by H-ZSM-5 zeolites with the increase in pH in Figure 6. Compared to the heavy metal ions, zeolites adsorb H+ ions preferentially from solution.16,17 Accordingly, the more acidic the solution, the more H+ ions that will be adsorbed on adsorbents. The metal ions were removed quickly in the form of a metal hydroxide precipitate when the pH is in excess of neutrality. When the pH value is greater than 7, the precipitation reaction is the main reason for removing metal ions from solution. However, the adsorption percentage of PX is invariant over the whole range of pH, which indicates that

placed into the powdery mixture. The total mixture was kneaded for 30 min to obtain molding precursor. The conventional extrusion molding method was used, and the size of the molding catalyst was φ = 1.5 mm × 7 mm. The molding adsorbents were dried in natural air for 24 h. Thereafter, the molding adsorbents were placed in the vacuum drying oven, in which the drying temperature was 110 °C, the degree of vacuum was 0.1 MPa, and the drying time was 2 h. The dried catalysts were transferred to muffle furnace, the roasting temperature was 750 °C, and the roasting time was 2 h. Through the above steps, H-ZSM-5 zeolites were molded.

3. RESULTS AND DISCUSSION 3.1. Adsorbent Preparation. 3.1.1. Zeolite Screening. The screening results of the zeolite adsorption are shown in Table 1. There is a change in the adsorption selectivity factors Table 1. Selectivity with Different Types of Zeolitesa

a

zeolites

5A

13X

NaY

ZSM-5

selectivity factor of PX to Co2+ selectivity factor of PX to Mn2+ SiO2/Al2O3

0.075 0.093 2

0.029 0.044 3

0.053 0.072 4

219.122 245.887 80

The relative error is less than 5%.

of PX to Co2+ and Mn2+ on different zeolites, and the adsorption selectivity factor increases as SiO2/Al2O3 increases. Through comparative analysis, it is found that the ZSM-5 zeolite is suitable for adsorbing PX. To get a larger adsorption selectivity factor, the method of ion exchange was used to modify ZSM-5 zeolites. 3.1.2. Adsorption Modification. Zeolites contain many cations that can exchange reversibly with cations in the liquid without changing the framework. The adsorbent capacity, selectivity, and kinetics of cations decide the adsorption order. Through the ion exchange, the pore size and other parameters of zeolites can be controlled to reach the aim of selective adsorption. Therefore, zeolites can avoid adsorbing undesired components through exchange with the metal ion. In this study, synthesized zeolites were ion-exchanged with iron, cobalt, or hydrogen. Typically, cetyltrimethylammonium bromide (CTMAB) 5%, Co(CH3COOH)2 5%, and NH4Cl 10% are dissolved in demineralized water, and 2−4 g of zeolites was added to 200 mL of the solution for ion exchange. Ion exchange occurred in the temperature range of (363 to 368) K and for 2 to 3 h with stirring at 500 rpm. The samples were washed and filtered several times and dried overnight at 343 K. Finally, the obtained Co-ZSM-5 and H-ZSM-5 zeolites were roasted for 2 h at 973 K. Table 2 shows that the adsorption selectivity of PX to Co2+ and Mn2+ on H+ ion-exchanged ZSM-5 zeolites is more appropriate. The reason probably is that the ZSM-5 zeolites with surface hydrophobicity contain a few of the exchangeable/ adsorbable sites for the metal ion but impose a large Table 2. Selectivity with Different Ion-Exchanged Zeolitesa

a

zeolites

ZSM-5

Co-ZSM-5

H-ZSM-5

CTMA-ZSM-5

selectivity factor of PX to Co2+ selectivity factor of PX to Mn2+ adsorption capacity of PX (mg·g−1)

219.122 245.889 120.152

765.829 724.783 112.675

1778.069 1875.650 126.100

153.836 170.754 76.325

The relative error is less than 5%. 1049



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Figure 3. SEM photographs of the H-ZSM-5 adsorbent.

Figure 5. Influence of adsorbent dosage on the adsorption of metal ions and PX on H-ZSM-5 zeolites (CCo2+ = 30 mg·L−1, CMn2+ = 30 mg· L−1, CPX = 50 mg·L−1, t = 60 min, s = 200 rpm, T = 298.15 K, pH 7, and V = 100 mL).

Figure 4. XRD patterns for zeolites (A) ZSM-5 and (B) H-ZSM-5.

Table 3. Properties of the Different Adsorbents items

ZSM-5

H-ZSM-5

BET surface area (m2·g−1) average pore diameter (nm) micropore volume (m3·g−1)

412.169 0.562 0.102

438.497 0.582 0.104

adsorption mechanisms of PX do not contain exchanged ions and there is no competitive adsorption between metal ions and PX. 3.4. Ascertain the Adsorption Time. The adsorption of PX on H-ZSM-5 zeolites was studied at 298.15 K and an initial concentration of 50 mg·L−1, when the adsorption time was between 0.5 and 50 min. As can be seen in Figure 7, PX was rapidly adsorbed on zeolites and then reached equilibrium in 20 min. Thus, 50 min of adsorption time was enough to achieve equilibrium. Similarly, the adsorption of Co2+ and Mn2+ is also a rapid process, as shown in Figure 8. 3.5. Adsorption Thermodynamics. The adsorption equilibria of PX on H-ZSM-5 zeolites were measured at 298, 308, 318, and 328 K. The experimental data are presented in Figure 9 and correlated to different adsorption isotherms, namely, the Langmuir isotherms and the Freundlich isotherms. In general, the Langmuir isotherm18−20 can be written as Ce C 1 = + e Qe Q mKL Qm

Figure 6. Influence of pH on the adsorption of metal ions and pxylene on H-ZSM-5 zeolites (CCo2+ = 30 mg·L−1, CMn2+ = 30 mg·L−1, CPX = 50 mg·L−1, m = 1 g·L−1, s = 200 rpm, T = 298.15 K, t = 60 min, and V = 100 mL).

On the basis of this equation, the Langmuir isotherm model parameters and the statistical fits are tabulated in Table 4. The Freundlich isotherm is an empirical equation used to describe

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the nonideal process. The Freundlich isotherm is indicated as follows:21 1 ln Ce + ln KF n

ln Q e =

(6)

The Freundlich isotherm model parameters and the statistical fits are given in Table 4. From the experimental results, the PX isotherms in different adsorbents are very steep and have similar tendencies. Therefore, it is important to obtain a good description of the experimental data. In this study, the enthalpy change (ΔH) and entropy change (ΔS) of PX adsorption at given loadings was approximately calculated by the following equations: ln KD = Figure 7. Influence of adsorption time on the adsorption of PX on HZSM-5 zeolites (CPX = 50 mg·L−1, m = 1 g·L−1, s = 200 rpm, T = 298.15 K, pH 7, and V = 100 mL).

KD =

ΔS ΔH − R RT

(7)

Qe Ce

(8)

The values of ΔH and ΔS were obtained from the slope and intercept of ln KD versus 1/T plots. The Gibbs’ free energy change (ΔG) was computed with the following formula: ΔG = ΔH − T ΔS

(9)

The results are exhibited in Table 5. According to experimental data, the isosteric heat of adsorption of PX is low, and the process of adsorption of PX on the H-ZSM-5 zeolite can release heat. Therefore, the adsorption process is a spontaneously exothermic physisorption process, and low temperature will contribute to the adsorption of PX on zeolites. 3.6. Adsorption Kinetics. The dosage of the H-ZSM-5 zeolite was 1 g·L−1, and the concentration of solutions ranged from 50 to 180 mg·L−1. The examples were collected and determined in the following intervals: 0.5, 1, 2, 5, 10, 20, 30, and 50 min. The relative errors were approximately ±2.5%. The results of the adsorption kinetics of PX on the modified zeolites are shown in Figure 10. The results indicated that the adsorption rate is rapid during the initial period. Within 10 min, PX combines easily with the active sites, and the driving force of mass transfer for adsorption is very high, which leads to a rather high adsorption rate. After 10 min, the adsorption rates become slower because of slow diffusion of PX in the adsorbed particles. The effect of the solution concentrations on the uptake of PX is also shown in Figure 10. The increase in the initial concentration will lead to a larger mass-transfer force. Therefore, it can be found that the adsorption capacity of PX increased as a result of the increase in concentration. The pseudo-first-order and pseudo-second-order rate equations were adopted in the adsorption kinetics of PX. The pseudo-first-order rate can be written as the following linear form:10

Figure 8. Influence of adsorption time on the adsorption of heavy metal ions on H-ZSM-5 zeolites (CCo2+ = 30 mg·L−1, CMn2+ = 30 mg· L−1, m = 1 g·L−1, s = 200 rpm, T = 298.15 K, pH 7, and V = 100 mL).

ln(Q e − Q t ) = ln Q e − k1t

(10)

The pseudo-second-order rate equation can be expressed as11 t 1 t = + 2 Qt Q k 2Q e e

Figure 9. Experimental isotherms on H-ZSM-5 zeolites.

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(11) DOI: 10.1021/acs.jced.6b00861 J. Chem. Eng. Data 2017, 62, 1047−1057


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Table 4. Parameters of the Adsorption Isotherm of PX on the H-ZSM-5 Zeolite Langmuir

Freundlich

adsorbate

T(K)

Qm(mg·g−1)

KL(L·g−1)

R2

KF(mg·g−1)

n

R2

H-ZSM-5

298.15 308.15 318.15 328.15

125.16 119.33 115.34 114.15

0.419 0.484 0.859 0.615

0.996 0.997 0.998 0.997

52.93 49.90 47.94 46.06

4.29 4.39 4.30 4.23

0.540 0.564 0.577 0.512

Table 5. Thermodynamic Parameters for PX Adsorbed on the H-ZSM-5 Zeolite ΔG (kJ·mol−1) −1

−1

−1

adsorbate

ΔH (kJ·mol )

ΔS (J·mol ·K )

298.15K

308.15K

318.15K

328.15K

H-ZSM-5

−12.33

−17.83

−7.02

−6.84

−6.66

−6.48

temperature. In Figure 11, with increasing regeneration times from 0 to 12, a decrease in the adsorption capacity is observed

Figure 10. Kinetics of PX adsorption on the H-ZSM-5 zeolite (CPX = 50, 80, 100, 120, 150 mg·L−1; m = 1 g·L−1; s = 200 rpm; T = 298.15 K; pH 7; and V = 100 mL). Figure 11. Relationship between the adsorption capacity and regeneration time (CPX = 180 mg·L−1, m = 1 g·L−1, s = 200 rpm, T = 298.15 K, pH 7, and V = 100 mL).

where Qt (mg·g−1) is the adsorbance of PX on the adsorbent at time t (min) and k1 (min−1) and k2 (g·mg−1·min−1) are the constants for two rate models. The kinetic parameters of two rate equations are presented in Table 6. The above two rate equations can well describe the adsorption behavior of PX on the H-ZSM-5 zeolite because the correlation coefficients all approached 1. Besides, the adsorption capacities in equilibrium calculated by two rate equations were in good agreement with those determined by experiments. 3.7. Desorption. Desorption is an important means of adsorbent regeneration. The H-ZSM-5 zeolite can retain its nature and structural stability after high-temperature baking; however, PX will be converted to CO2 and H2O. Therefore, the regeneration of zeolites can be achieved by roasting at high

for PX from 121.8 to 83.2 mg·g−1. The adsorption capacity gradually tends to stabilize after eight uses. In view of the regeneration times and the slight decrease in the adsorption capacity the last few times, the high-temperature roasting method is an effective way to reuse the H-ZSM-5 zeolite. 3.8. Production of the Granular Adsorbent. The factors, water/powder ratio and consumption of different powders, that would influence the compressive strength, the adsorption behavior of PX, Co2+, and Mn2+ on the granular adsorbent, and the adsorption capacity of the shaped H-ZSM-5 zeolite are discussed. In this section, the experimental conditions are as follows: CPX = 50 mg·L−1, CCo2+ = 30 mg·L−1, CMn2+ = 30 mg·

Table 6. Parameters of Adsorption Kinetics of PX on the H-ZSM-5 Zeolite pseudo-first-order model −1

−1

−1

−1

pseudo-second-order model 2

initial concentration (mg·L )

Qe,exp (mg·g )

Q1e (mg·g )

k1 (min )

R

50 80 100 120 150

46.90 76.71 94.28 111.17 118.17

46.92 76.73 94.30 111.23 118.21

0.121 0.139 0.116 0.120 0.129

0.966 0.922 0.976 0.989 0.978

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−1

Q2e (mg·g )

k2 (g·mg−1·min−1)

R2

47.08 74.89 94.79 111.86 119.33

0.062 0.085 0.024 0.019 0.015

0.999 0.999 0.999 0.999 0.999



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L−1, m = 2 g·L−1, t = 60 min, s = 200 rpm, T = 298.15 K, pH 7, and V = 100 mL. 3.8.1. Ascertaining the Water/Powder Ratio. In the process of molding zeolite, the water/powder ratio is one of the key influencing factors. The impact of the water/powder ratio on the compressive strength and adsorption capacity of PX, Co2+, and Mn2+ of the shaped H-ZSM-5 zeolite is presented in Figures 12−14 under the same extrusion conditions. The result

Figure 14. Influence of the water/powder ratio on the adsorption of PX on the shaped H-ZSM-5 zeolite.

dosage from 0 to 40% is presented in Figures 15−17. The result illustrates that the compressive strength increased with

Figure 12. Influence of the water/powder ratio on the compressive strength of the shaped H-ZSM-5 zeolite.

Figure 15. Influence of the pseudoboehmite dosage on the compressive strength of the shaped H-ZSM-5 zeolite.

the increase in the pseudoboehmite dosage. This is because the pseudoboehmite’s own bond action can increase the binding power of the powder zeolite and then increase the compressive strength. On the basis of these results, the adsorption percentage of PX decreased and the adsorption percentage of Co 2+ and Mn 2+ increased with the increase in the pseudoboehmite dosage. A 20% pseudoboehmite dosage was used for subsequent studies. 3.8.3. Ascertaining the Dosage of Sesbania Powder. The role of sesbania powder is to make a large hole, and the sesbania powder dosage effects the strength and adsorption capacity of PX, Co2+, and Mn2+ of the shaped H-ZSM-5 zeolite when the dosage is varied from 0 to 40%. The results are presented in Figures 18−20. The results illustrate that the compressive strength decreased quickly with the increase in the pseudoboehmite dosage. This is because the number of large holes in the pore volume increased with the increase in starch content and then decreased the compressive strength. The result of the adsorption percentages of PX, Co2+, and Mn2+ of

Figure 13. Influence of the water/powder ratio on the adsorption of PX on the shaped H-ZSM-5 zeolite.

illustrated that the compressive strength increased with the increase in the water/powder ratio and then decreased with the increase in the water/powder ratio. This is because the water/ powder ratio can influence the extruded pressure and surface roughness of the shaped H-ZSM-5 zeolite. In view of the very small effect of the water/powder ratio on the adsorption of PX on the shaped H-ZSM-5 zeolite, a 0.45 mL·g−1 water/powder ratio was used for subsequent studies. 3.8.2. Ascertaining the Dosage of Pseudoboehmite. The effect of the pseudoboehmite dosage on the compressive strength and adsorption capacity of PX, Co2+, and Mn2+ of the shaped H-ZSM-5 zeolite by varying the pseudoboehmite 1053



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Figure 16. Influence of the pseudoboehmite dosage on the adsorption of PX on the shaped H-ZSM-5 zeolite.

Figure 19. Influence of the sesbania powder dosage on the adsorption of PX on the shaped H-ZSM-5 zeolite.

Figure 17. Influence of the pseudoboehmite dosage on the adsorption of PX on the shaped H-ZSM-5 zeolite.

Figure 20. Influence of the sesbania powder dosage on the adsorption of PX on the shaped H-ZSM-5 zeolite.

dosage. This is because increases in both the number of large holes and the pore volume can reduce the internal diffusion effect and provide a greater effective adsorption surface area and more adsorption sites. In light of these results, 30% of the sesbania powder dosage was appropriate. When the water/powder ratio is 0.45 mL·g −1 , the consumption of H-ZSM-5 zeolite, pseudoboehmite, and sesbania powder is 56, 14, and 30 wt %, respectively. The shaped H-ZSM-5 zeolite with a columnar construction of 1.5 × 7 mm2 has better adsorption. The particle strength can attain 85.38 N·cm−1, and the maximum adsorption capacity of the granular absorbent reaches 62.4 mg·g−1 for PX. 3.9. Dynamic Adsorption. The dynamic adsorption experiments were performed in a tubular glassware instrument22 as shown in Figure 21 to achieve the breakthrough curves. Breakthrough curves are always valuable for industrial applications of fixed adsorption beds, especially when operating conditions need to be optimized or a fixed bed has to be designed and amplified. The influences of operating conditions, such as the inlet flow rate, concentration of PX, and length− diameter ratio of the adsorption column, were discussed.

Figure 18. Influence of the sesbania powder dosage on the compressive strength of the shaped H-ZSM-5 zeolite.

the shaped H-ZSM-5 zeolite illustrated that the adsorption percentage increased with the increase in the sesbania powder 1054



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temperature. The inlet flow rate was varied from 5 to 10 mL· min−1, and the initial concentration of PX was 50 mg·L−1. The influence of the inlet flow rate is shown in Figure 23 and Table

Figure 21. Flowchart of the dynamic adsorption experimental apparatus. (1) PTA simulated wastewater reservoir, (2) pump, (3) fixed bed, (4) wastewater reservoir after disposal, and (5) outlet of sampling.

Figure 23. Breakthrough curves of PX for different inlet flow rates.

3.9.1. Dynamic Adsorption of Co2+ and Mn2+. The preliminary studies were carried out with identical columns made of a perspex tube of 1.6 cm internal diameter and 24 cm height using Co2+ and Mn2+ solutions of 30 mg·L−1 in water with an inlet flow rate of 5 mL·min−1, and columnar H-ZSM-5 zeolites filled in the fixed adsorption bed. The breakthrough curves of Co2+ and Mn2+ at room temperature are presented in Figure 22. According to the breakthrough curves, the

Table 8. Adsorption Capacity of the H-ZSM-5 Zeolite Sorbent under Different Inlet Flow Ratesa inlet flow rate (mL·min−1) −1

Qe (mg·g ) a

adsorption capacities of Co2+ and Mn2+ are 0.819 and 0.414 mg·g−1 as shown in Table 7, similar to the static adsorption of powder. The low adsorption capacity was a precondition of the selective removal of PX. 3.9.2. Influence of the Inlet Flow Rate. The adsorption of PX on the columnar H-ZSM-5 zeolite was studied at a perspex tube of 1.6 cm internal diameter and 24 cm height at room Table 7. Adsorption Capacity of Co2+ and Mn2+

Qe (mg·g )

Co2+

Mn2+

0.819

0.414

8

10

44.01

41.56

38.53

The relative deviations are less than 5%.

8. The adsorption capacity decreased from 44.01 to 38.53 mg· g−1 as the inlet flow rate increased from 5 to 10 mL·min−1. The adsorption process was mainly controlled by internal diffusion for the granular adsorbent. Also, the retention time of solution became longer when the inlet flow rate was low. Therefore, PX can contact the adsorbent in the column adequately at low flow velocity, which leads to a longer breakthrough time. 3.9.3. Influence of the Concentration of PX. The concentration of the solution of PX is the other factor influencing dynamic adsorption. Dynamic adsorption in a fixed bed with a bed height of 24 cm was conducted at room temperature. The inlet flow rate was 5 mL·min−1. From Figure 24 and Table 9, the time of breakthrough with initial concentrations of 100 and 80 mg·L−1 was obviously shorter than that with an initial concentration of 50 mg·L−1, and the adsorption capacity of PX also increased. The larger driving force at a higher PX concentration made the adsorption capacity of PX increase. However, the increase in the concentration of PX was not proportional to the increase in the adsorption capacity. The higher the concentration, the longer the breakthrough time. 3.9.4. Influence of Bed Height. The adsorption of PX on the columnar H-ZSM-5 zeolite was studied at a perspex tube of internal diameter 1.6 cm at room temperature. The bed height was varied from 16 to 32 cm, the initial concentration of PX was 50 mg·L−1, and the inlet flow rate was 5 mL·min−1. The breakthrough curves are shown in Figure 25. The corresponding adsorption capacities are shown in Table 10. With the increase in bed height, breakthrough points moved to the right. The reason was that the increase in the bed height provided a

Figure 22. Breakthrough curves of Co2+ and Mn2+.

−1

5

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and that the ion-exchange modified ZSM-5 zeolites should be an appropriate adsorbent to selectively remove PX from PTA wastewater. On the basis of experimental investigations and material characterization tests, H+ can exchange Na+ from ZSM-5 zeolite efficiently and then improve the selectivity and the saturation capacity. The selectivity factors of PX to Co2+ and PX to Mn2+ can approach 1778.069 and 1875.650, respectively, and the saturation capacity even increase to 126.10 mg·g−1. Therefore, H+ is a relatively suitable ion for exchanging the Na+ from the ZSM-5 zeolite. Experimental data in the batch adsorption at temperatures of between 298 and 328 K are consistent with the Langmuir isotherm model. The adsorption heat of PX is −12.33 kJ·mol−1, and the adsorption of PX on the zeolites is a spontaneously exothermic physisorption process. Either the pseudo-secondorder rate model or the pseudo-first-order rate model can well describe the adsorption behavior of PX on the H-ZSM-5 zeolite. When the water/powder ratio is 0.45 mL·g −1 , the consumption of H-ZSM-5 zeolite, pseudoboehmite, and sesbania powder is 56, 14, and 30 wt %, respectively, and the shaped H-ZSM-5 zeolite with a columnar construction of 1.5 mm × 7 mm has better adsorptivity. The particle strength can attain 85.38 N·cm−1, and the maximum adsorption capacity of PX is 62.4 mg·g−1. The high-temperature roasting method is an effective regenerating method for the H-ZSM-5 zeolite. The results of fixed bed adsorption indicate that H+exchanged ZSM-5 zeolite will become potential adsorption material for industrial applications in removing PX from PTA wastewater selectively.

Figure 24. Breakthrough curves of PX for different initial concentrations.

Table 9. Adsorption Capacity of the H-ZSM-5 Zeolite Sorbent in Solutions with Different Concentrations of PXa PX concentration (mg·L−1) −1

Qe (mg·g ) a

50

80

100

44.01

50.02

53.10


■

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 18050282203. E-mail: [email protected]. ORCID

Changshen Ye: 0000-0003-1673-6218 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support for this work from the Fujian Province Department of Science & Technology (2014Y0066).

■

Figure 25. Breakthrough curves of PX for different bed heights.

Table 10. Adsorption Capacity of the H-ZSM-5 Zeolite Sorbent at Different Bed Heightsa bed height (cm) −1

Qe (mg·g ) a

16

24

32

45.02

44.01

42.70

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4. CONCLUSIONS The findings of this investigation demonstrated that the adsorption selectivity factor increases as SiO2/Al2O3 increases 1056



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Selective Adsorption of p-Xylene from Pure Terephthalic Acid

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