Getting insight into the influence of coexisting airborne nanoparticles on gas adsorption performance over porous materials

Journal of Hazardous Materials 386 (2020) 121928

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Getting insight into the influence of coexisting airborne nanoparticles on gas adsorption performance over porous materials

T

Yi Xinga, Yongkang Cuia, Ziyi Lia,*, Yingshu Liua,*, Danqi Baoa, Wei Sua, Chuen-Jinn Tsaib, Chao-Heng Tsengc, Angus Shiuec, David Y.H. Puid,e, Ralph T. Yangf a

School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China Institute of Environmental Engineering, National Chiao Tung University, University Road, Hsinchu 30010, Taiwan c Institute of Environment Engineering and Management, National Taipei University of Technology, Taipei 10608, Taiwan d Particle Technology Laboratory, Mechanical Engineering, University of Minnesota, 111 Church St., S.E., Minneapolis 55455, USA e School of Science and Engineering, Chinese University of Hong Kong, Shenzhen, Guangdong 518172, China f Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: L. Eder

Adsorption as one of the most important air cleaning methods has been extensively applied during which the coexisting airborne nanoparticles (NPs) with sizes close to adsorbent pore sizes could inevitably influence gas adsorption processes. In this work, the influence of sub-20 nm NPs on toluene adsorption on ZSM-5 zeolites exchanged with different cations (Li+, Na+ and K+) were studied based on gas-and-particle coexisting adsorption/filtration tests. Affinities for both toluene and NPs on adsorbents follow Li-ZSM-5 > Na-ZSM-5 > KZSM-5 regarding the orders of charge density, pore size, and internal and external specific surface areas. The toluene adsorption was shown to be impaired by coexisting NPs from perspectives of thermodynamics and kinetics. For Li-ZSM-5, Na-ZSM-5 and K-ZSM-5, significant relative reductions of 10.4 %, 10.5 % and 16.0 % in toluene adsorption capacity at the lower feed concentration, and of 20.3 %, 15.2 % and 2.3 % in mass transfer coefficient at the higher feed concentration were observed, respectively. The influential mechanisms regarding competitiveness between toluene and NPs in interaction with cationic and porous surfaces were accordingly proposed, which are of practical significance for selecting robust adsorbents under realistic harsh air conditions.

Keywords: Adsorption Airborne nanoparticles Zeolite Toluene Gas purification

⁎

Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (Y. Liu).

https://doi.org/10.1016/j.jhazmat.2019.121928 Received 28 September 2019; Received in revised form 17 December 2019; Accepted 17 December 2019 Available online 19 December 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 386 (2020) 121928

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1. Introduction

soft X-ray assisted gas-to-particle conversion. Besides, the use of activated carbons as disordered microporous adsorbents without periodic cationic sites was hard to manifest the effect of NPs within a short duration. Kuo et al. (Kuo et al., 2013) studied the effects of micron- and submicron-sized oil aerosols on the cyclohexane adsorption capacities on activated charcoals, and found that liquid aerosols with sizes of 0.2 μm were more likely to form a film covering the carbon surface leading to pore blockage while solid aerosols with sizes of 0.15 and 10 μm shortened the penetration time. However, in their work, the aerosols were preloaded onto the adsorbents prior to gas adsorption instead of in-situ mixture of particles and gases resembling the realistic particle-gas coexisting conditions, and thus the demonstration of the microscopic interactions of NPs and gas with adsorbents were to some extent incomplete. Therefore, to gain a systematic and deeper insight into the effect of NPs on gas adsorption, a sophisticated selection of adsorbent coupling with favorable experimental methodology is highly required. In this work, the comprehensive effect of sub-20 nm NPs on adsorption of toluene as a typical organic pollutant gas on ZSM-5 as a commonly used zeolite was studied. The methodology of an in-situ gasand-particle coexisting adsorption breakthrough test was adopted. The interaction between NPs and ZSM-5 with different cations and corresponding influential rules on toluene adsorption thermodynamics and kinetics were explored in detail.

Environmental and human health threats caused by particulate and gas pollutions are one of the major issues of global concern. Many purification technologies have been developed, such as three-way catalytic converters of automobile exhausts, indoor air purification, and adsorption of industrial flue gas (Wang et al., 2014a; Li et al., 2018; Wang et al., 2011). The coexistence of solid particles and harmful gases in the environment is a common phenomenon. On one hand, in the atmosphere, nucleation mode particles (< 20 nm) are formed through the gas interaction and condensation, and subsequent gas-particle reactions as well as particle agglomeration lead to the secondary formation of particles in diverse conditions (Hossain et al., 2012; Kim and Pui, 2015; Djikaev and Ruckenstein, 2019). Besides, during the longdistance transmission of polluted gases, low-volatile compounds such as polycyclic aromatic hydrocarbons, dioxins, and metal oxides, are likely to spontaneously transform into particulate state through various transformation routes (Johansson et al., 2017). On the other hand, bag filters and electrostatic precipitators as primary industrialized particle collectors show insufficient removal efficiency for ultrafine particles accounting for more than 90 % in number concentration, respectively associated with the thermal rebound (Kleinhans et al., 2018) of particles from filter surfaces due to increased Brownian diffusion and collision speed with decreasing particle size, and the partial charging (Jaworek et al., 2017) resulting in insufficient migration for small particles (especially < 30 nm) in the electric field (Li et al., 2015; Jaworek et al., 2018). Adsorption as one of the most significant gas purification methods has been extensively applied, for which the adsorbent materials with the pore size range from micropore (< 2 nm) to mesopore (2–50 nm) are normally used. During practical gas adsorption processes, coexisting airborne nanoparticles (NPs) with very small sizes (e.g. < 20 nm) could inevitably diffuse into the pores of adsorbents or deposit onto the interior surfaces (Yin et al., 2019; Xing et al., 2018), thereby leading to pore blockage, specific surface area reduction, cationic adsorption site occupation, and other potential influences on physical and chemical properties of adsorbents. Kim et al. (2016a) examined the mechanisms of sub-3 nm particles filtration on granular activated carbons, and proposed that the particle removal efficiency increases due to the internal deposition on pore surfaces corresponding to the slight change in the parameter of the single sphere efficiency equation. Khachikian and Harmon (Khachikian and Harmon, 2000) showed that particle deposition and diffusion reduce the specific surface area of various adsorbents by up to 50 %, the pore volume at pore size range of 2–35 nm, and the adsorption energy. Ding et al. (2008) speculated that the degree of the influence of NPs on the adsorbents is closely related to the pore size distribution, which could be significant when the particle size is close to the pore size. Einvall et al. (2007) showed catalysts deactivation after exposure to K2SO4 particles, and attributed it to the coverage of active sites and the decrease in surface area. Xing et al. (2018) demonstrated the reductions of 5.3 %, 15.0 %, and 36.4 % in pore volumes of SBA-15 adsorbents with primary pore sizes of 9.6, 10.8, and 14.6 nm by coexisting NPs, respectively. On the basis of above evidences of interactions between NPs and porous materials, it should be noted that the service life of an adsorbent could be shortened due to NPs loading. The further influence of NPs on gas adsorption performance is therefore of great significance in practical use regarding appropriate selection of robust adsorbents. However, little literature has clearly addressed this long-sought-after phenomenal issue, which might be due to inappropriate choice of porous materials and experimental difficulties in obtaining highlyconcentrated nanoparticles and in-situ particle-and-gas coexisting conditions. Kim et al. (2017) indicated that NPs collected on activated carbons did not significantly impact toluene adsorption, associated with insufficient pore blockage by NPs with low total number concentrations (∼105 #/cm3) which were generated from toluene molecules through

2. Experimental section 2.1. Material preparation and characterizations Three ZSM-5 zeolites with different cations (Li+, Na+, and K+), LiZSM-5, Na-ZSM-5, and K-ZSM-5, were used as adsorbents in this work. Na-ZSM-5 was purchased from Luoyang Jalon Micro-nano New Materials Co., Ltd. Li-ZSM-5 and K-ZSM-5 were prepared based on NaZSM-5 with ion exchange methods as follows. Crude Na-ZSM-5 (5 g) was added into 250 ml of 2 mol L−1 alkali metal cation solution (LiCl, NaCl or KCl), and was ion-exchanged for 5 days at 70 ℃ under 200 rpm. The solution was replaced by a fresh one each day and the powder samples were collected with suction filtration. The final samples were washed at least 5 times with 500 ml deionized water to remove excess metal cations, dried at 120 ℃, calcined at 500 ℃ for 4 h in tubular furnace, and finally stored in a series of 5–10 ml glass vials for future use. For each adsorbent, the structures and crystallinities were characterized using X-ray diffraction (XRD) in the range of 5–40° at a scanning speed of 0.1°⋅min−1, recorded by an X-ray diffractometer (D8 Advace, Bruker, Germany). The ion exchange rate was tested using an inductively coupled optical atomic emission spectrometry (ICP 2070, Baird, Bedford, MA, USA). The morphology of the adsorbent loaded with NPs was observed using a transmission electron microscope (Tecnai G2 F20 s-twin, FEI, USA).The porous properties were characterized with Ar (87 K) adsorption and desorption experiments using a physisorption apparatus (Autosorb-1, Quantachrome, USA), where the pore size distribution was calculated according to the NLDFT model, the total pore volume was calculated based on Ar uptake at P/P0 of 0.99, and the BET specific (SBET) and the external surface areas (Sexternal) were calculated based on BET and t-plot methods at P/P0 of 0.05–0.25, respectively. The adsorption isotherm of toluene on each adsorbent was obtained using a vapor adsorption apparatus (3H-2000PW, Beishide Instrument Technology, Beijing Co., Ltd). 2.2. Gas–particle adsorption/filtration test The gas-particle adsorption/filtration experimental system for simultaneous measurement of the particle removal efficiency curve and the gas breakthrough curve is shown in Fig. 1. For NPs filtration, ambient air was pumped into a buffer tank, dried with a silica gel column 2

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Fig. 1. Gas–NPs adsorption/filtration experimental system.

and cleaned with a high-efficiency particulate air (HEPA) filter. Based on the sublimation of tungsten oxide excited by a certain voltage, NPs were generated under the condition of clean dry air as carried gas with flow velocity controlled by a mass flow controller (MFC1) in a nanoparticle generator (Yin et al., 2019; Xing et al., 2018). The real-time number concentration and the size distribution of generated and filtered NPs were measured using a scanning mobility particle sizer (SMPS; model 3938, TSI, Inc., USA). A pair of adsorption tube and dummy tube with the inner diameter of 7 mm, was packed with adsorbents-loaded clay pellets. The following expression was used to evaluate the particle removal efficiency (Kim and Pui, 2015):

Ei =

Cpi, in − Cpi, out Cpi, in

× 100%

existence, effects of NPs on toluene adsorption could be analyzed. 2.3. Adsorption thermodynamics model The adsorption thermodynamics for toluene on ZSM-5 can be obtained from adsorption isotherm data fitted with Freundlich or Langmuir isotherm models as:

qF = KF C1/ n qL =

Q C0 t f − mZSM − 5

(

∫0

tf

Ct dt

)

(4)

where KF (L/g) and KL (L/mmol) are Freundlich and Langmuir equilibrium constants, respectively (Ng et al., 2003); C is the adsorbate concentration at the equilibrium, mmol/L; n is Freundlich exponent (dimensionless); qm is the maximum adsorption capacity, mmol/g. Other significant thermodynamic parameters such as changes in Gibb's free energy (ΔG), changes of enthalpy (ΔH), and changes in the entropy (ΔS) can be obtained to quantify adsorption energy and judge the direction in the adsorption process using the van't Hoff equation as:

(1)

where Cpi, in and Cpi,out are the upstream and downstream number concentrations of NPs, respectively. For the gas adsorption experiment, toluene gas was supplied from the cylinder (purity of 99.99 %) purchased from Beijing Huayuan Gas Industry Co., Ltd., and mixed with clean dry carrier gas. The toluene feed concentration was controlled with MFC2 based on a fixed carrier gas. A mixture of 75 ± 1 mg (250–425 μm) of the adsorbent and 700–800 mg of same-sized inert were placed in the same adsorption tube. The toluene concentration in the outlet gas stream as a function of adsorption time was plotted as a breakthrough curve, recorded by a VOC detector (ppbRAE 3000; PGM-7340, USA), assuming 10 % of the feed concentration as the breakthrough point. The adsorption capacity, q0 (mmol/g), of the adsorbent was calculated using a numerical integration method with the formula as follows:

q0 =

qm KL C 1 + KL C

(3)

lnK e = −

ΔH ΔS + RT R

(5)

where T is the absolute temperature, K; R is the universal gas constant, 8.314 J/(mol K); Ke is the dimensionless equilibrium constant which can be obtained by converting the units of KF (the best isotherm model fitted), that are given initially in L/g into L/mol with the multiplication by the molecular weight of the adsorbate (92.14 g/mol for toluene), and subsequently making the multiplication by the unitary standard concentration of the adsorbate (1 mol/L) and the division by the activity coefficient (dimensionless). The conversion step is given in detail in previous literatures (Lima et al., 2019a, b); ΔH and ΔS are respectively determined from the slope and the intercept by plotting lnKe versus the 1/T at 1/288, 1/298 and 1/308 /K employed in this work. ΔG can be calculated as:

(2)

where Q is the total gas flow rate, m /min; mzsm-5 is the adsorbent packing mass, g; C0 is the feed concentration of toluene, mmol/L; tf is the adsorption time, min; and q0 is the adsorption capacity, mmol/g. For the gas/particle simultaneous removal test, the in-situ mixture of toluene and NPs were created in the chamber of the particle generator, and the flow of coexisting gas-particle was carried to the adsorption column. The existence of toluene was proved to have negligible effects on NPs generation. The feed concentrations of gas (C0) and particle (Cpi,in) were controlled with respective MFCs and separately measured by passing the mixture flow through the dummy column. After the flow was stabilized, the adsorption was started under appropriate face velocity controlled with MFC3 to obtain breakthrough curves. Comparing toluene breakthrough curves with and without NPs 3

ΔG= ΔH- TΔS

(6)

2.4. Adsorption kinetics model To investigate the adsorption kinetics of toluene on adsorbents, the constant pattern wave propagation model (Lee et al., 2008; Li et al., 2017; Liu et al., 2016) was applied to fit with the breakthrough curve based on the assumptions of no chemical reaction, negligible bed 3

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representative value with corresponding error bar obtained using the standard deviation equation. The analysis of statistical differences for both q0 and Kp were performed using the pairwise t-test method (De Winter, 2013), to assess whether the difference between the means of two samples were statistically significant. The two paired samples herein refer to the toluene adsorption test results (q0 or Kp) with and without coexisting NPs, on the same adsorbent at the same C0. In using t-test method, the value of t can be calculated based on the average and the standard deviation value of the difference between paired samples, and the p-value can then be obtained by checking the critical value from t-Table (Box, 1987). The result of “p-value < 0.05” is considered to have statistical differences (Pivnenko et al., 2016) indicative of the importance of the NPs effect. The t-test results for both q0 and Kp including mean values and standard deviations of two paired samples (Toluene and Toluene/NPs), and p-value at each condition are summarized in Table S2. 3. Results

Fig. 2. XRD patterns of Li-ZSM-5, Na-ZSM-5 and K-ZSM-5.

3.1. Characterization results

temperature variation with time, and the plug flow with negligible axial diffusion. The final form of the breakthrough curve equations based on Freundlich and Langmuir adsorption isotherms are given as follows (Li et al., 2017):

t = t1 +

ρq0 1 1 − x n−1 ln ln2x − K G αC0 n−1 1 − 21 −n

(7)

t = t1 +

ρq0 ⎛ 1 x ⎞ ln2x + ln K G αC0 ⎝ KL C0 1 − x ⎠

(8)

2

2

⎜

XRD patterns results of adsorbents as shown in Fig. 2. Three alkali metal-modified ZSM-5 have typical ZSM-5 crystalline structure as shown in Figs. S4 and S5, with the intensity of diffraction peak following the order of Na-ZSM-5 > Li-ZSM-5 > K-ZSM-5. Alkali metals with negative charge in the zeolite framework may to some extend affect the diffraction of framework atoms, which is more significant with higher ion exchange degree (Otomo et al., 2016), consistent with current results of Na+ (90 %), Li+ (91 %), and K+ (93 %). Fig. 3 shows pore size distributions for three adsorbents with corresponding textural properties listed in Table 1 including total pore volumes, BET and external specific surface areas, and average pore sizes (Dp). It can be clearly seen that the incorporation of greater cations renders smaller micropore volume, specific areas, and pore size (Ji et al., 2019). Based on the ionic radius of Li+, Na+ and K+ of 0.76, 1.02 and 1.83 Å (Frising and Leflaive, 2008), respectively, this result is attributed to the shielding effects of cations on 10-membered rings in ZSM-5, resulting larger space occupation of the cations in the pore windows. The characterization results for the generated tungsten oxide NPs based on 10 measurement tests are given in Fig. 4. It can be seen that the generation of sub-20 nm NPs with a good monodispersity at a geometric mean size of 5.04 nm and high total number concentration (∼4.7 × 107 counts/cm3) was successfully achieved at the specific condition (flow rate of 1.5 L/min, voltage of 1.78 V, and current of 12 A). In addition, the inserted TEM picture shows that the generated NPs have highly spherical shape which is critical for eliminating undesirable gas adsorption on particles or special interaction of NPs on surfaces. In

⎟

where t1/2 is the adsorption time when x reaches 0.5; x is defined as C/ C0; ρ is the packing density of the fixed bed, kg/m3; α is the mass transfer area per unit volume of the bed, m2/m3; KGα is the overall mass transfer coefficient that is estimated by the slope of breakthrough curve when x is 0.5 as:

dx K αC | x = 0.5 = G 0 (x − x n ) dt ρq0

(9)

The external diffusion coefficient, Kf can be given by:

Kf =

1.09 1/3 1/3 Sc Re Dm (0.0001 < Re < 72) εds

(10)

where Dm can be calculated as:

Dm =

1 MN

+

1 Mt

T 3/2 × 5.36 × 10−5

P (VN1/3 + Vt1/3)2

(11)

where Sc=μ/(ρNDm) and Re=(ρNu0ds)/μ; ρN is the density of the carrier nitrogen gas, kg/m3; μ is the dynamic viscosity of air, Pa⋅s; Dm is the molecule diffusion coefficient of toluene in carrier gas; MN and Mt are the molecular weights of N2 and toluene, g/mol; VN and Vt are the molar volumes of N2 and toluene, cm3/mol, respectively. The internal diffusion coefficient, KP, can be obtained following the relation as:

1 1 C0 = + KG α Kf α KP ρq0

(12)

All of the parameters related to the adsorption kinetics modelling are listed in the Table S1 of the Supplementary Material. 2.5. Statistical analysis The sets of toluene adsorption tests with and without NPs generation to investigate the effect of NPs on toluene adsorption were repeated at least 10 times at each condition. The average value over 10 test results for each adsorption parameter (q0 or Kp) were taken as a

Fig. 3. Ar adsorption-desorption isotherms (87 K) and pore size distributions of Li-ZSM-5, Na-ZSM-5 and K-ZSM-5. 4

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(8–20 nm) with increasing particle size. The mechanisms of capturing NPs particularly those less than 100 nm are mainly Brownian diffusion (Kim and Pui, 2015) and electrostatic interaction (Givehchi et al., 2015). The decrease in filtration efficiency with decreasing particle size for 8–20 nm could be triggered by the thermal rebound when a bounce of particles on the surface of filter medium due to a kinetic energy higher than the adhesion one (Gensch and Weber, 2017), and it might happen at a relative larger particle size in this work due to the low packing density (3.2 mg/cm3) for the filter bed. As the particle size further reduces, the adsorptive interaction between NPs and zeolite surfaces could play a significant role in particle capture. The order of NPs removal efficiency with the minimum value follows Li-ZSM-5 (96 %) > Na-ZSM-5 (93 %) > K-ZSM-5 (87 %). The cations with smaller ionic radius on the zeolite presents greater charge density and electrostatic interaction with adsorbates, and correspondingly the higher external specific surface could also be a reason, since by analogy with gas adsorption, particle with size of nanometers can be retained by the porous surface. Three adsorbents show almost the same particle sizes for minimum removal efficiencies, which could be associated with their similar pore sizes (Xing et al., 2018).

Table 1 Texture properties for modified ZSM-5. Materials

VP (cm3/g)

SBET (m2/g)

Sexternal (m2/g)

DP (nm)

Ion exchange rate (%)

Li-ZSM-5 Na-ZSM-5 K-ZSM-5

0.443 0.408 0.239

443 416 338

70 43 33

0.54 0.52 0.49

91 90 93

3.3. Adsorption of toluene on ZSM-5 For adsorption thermodynamics of toluene, the fitting results of toluene adsorption isotherms (See Fig. S1) on three ZSM-5 are listed in Table 2. The Freundlich model with higher R2 is more suitable which is consistent with the results in literatures (Zhang et al., 2019a; Hor et al., 2016). The value of n as the indicator of heterogeneity of active sites and adsorption intensity follows the orders of Li-ZSM-5 > Na-ZSM5 > K-ZSM-5 (dos Santos et al., 2014). The dynamic adsorption capacities (qF) obtained from the breakthrough curves shown in Fig. 6 for each sample follows the same orders at both C0 of 0.27 × 10−3 and 1.69 × 10-3 mol/m3. Barrel (Barrer and Gibbons, 1963) and Kiselev (Kiselev and Du, 1981) simulated the electric field in NaX by attributing partial charge to sodium ions and oxygen atoms in the cavity, and proposed that the contribution of ion–quadrupole interaction to gas–solid interaction energies of CO2 was estimated to be 41 %. As a polar gas molecule with higher polarizability (Li et al., 2009) as compared to CO2, toluene would have greater affinity with the porous surface contributed from gas–cation interactions. The increase in adsorption intensity with increasing charge/diameter of cations indicative of charge density (Rasouli et al., 2012) renders Li-ZSM-5 the best toluene adsorption thermodynamics (Fan et al., 2015). In addition, the larger VP (0.216 cm3/g) and SBET (335 m2/g) of Li-ZSM-5 allow greater adsorption capacity as compared to Na-ZSM-5 and K-ZSM-5 in the micropore filling stage as indicated by the isotherms (Fig. S1) at lower pressure range (Zhang et al., 2019b). For adsorption kinetics of toluene, fitting parameters obtained from the model fitting wiexperimental breakthrough curves (Fig. 6) are listed in Table 3. It can be seen that the model exhibits good fitting results for both cases with and without NPs. Much smaller value of KP than Kfα indicates much larger internal diffusion resistance than external diffusion resistance. Considering the kinetic diameter of toluene (0.53 nm) (J.R. L et al., 2009) comparable to the pore size of ZSM-5 (Table 1), internal diffusion should be the rate-controlling step in adsorption. The

Fig. 4. Particle size distributions and morphologies of generated NPs.

comparisons to conventional particle generation methods including the atomization (Wang et al., 2006; Kim et al., 2016b), the evaporation and condensation with a tube furnace (∼1000 ℃) (Kim and Pui, 2015) and the soft X-ray-radiolysis (Kim et al., 2017) respectively causing unstable gas moisture control, high temperature or low number concentration in the NPs flow, in this work, a precise and flexible generation of the wellmonodispersed sub-20 nm high-concentration NPs flow without moisture at room temperature ought to be more favorable for investigating effects of NPs on toluene adsorption on zeolites in the subsequent in-situ mixed gas-NPs adsorption tests. 3.2. Filtration of NPs Fig. 5 shows the filtration efficiency curve of pure NPs on pure clay pellets and those loaded with Li-ZSM-5, Na-ZSM-5 and K-ZSM-5. For each ZSM-5, the removal efficiency curve of NPs shows a sharp decrease (< 8 nm), then a minimum value at ∼8 nm and a gradual rise

Table 2 Fitting results of Langmuir and Freundlich isotherm models. Models Langmuir

Freundlich

Fig. 5. Particle removal efficiencies of NPs on Li-ZSM-5, Na-ZSM-5 and K-ZSM5. 5

Equation

qL =

qm KL C 1 + KL C

qF = KF C1/ n

Parameters

Li-ZSM-5

Na-ZSM-5

K-ZSM-5

qm (mmol/g) KL (L/mmol) R2 KF (L/g) n R2

1.281 255.1 0.9854 19.9 1.60 0.9908

1.260 248.8 0.9877 19.7 1.58 0.9987

1.176 246.2 0.9824 18.218 1.57 0.9968

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et al., 2018; Lin et al., 2004) tends to result in higher frequency of collision between the adsorbate and the pore wall, leading to the accumulation of adsorbates around active sites at the pore aperture (Lin et al., 2004; Yang et al., 2018). Therefore, the pore size becomes the main factor for steric hindrance limiting the internal diffusion. K-ZSM-5 with DP of 0.49 nm shows the largest diffusion resistance while Li-ZSM5 and Na-ZSM-5 with similar DP shows slight difference in KP. At lower C0 with smaller driving force, the electrostatic interaction from cation might exert restraining force on adsorbates (Xu et al., 2016; Newsome and Coppens, 2015), becoming the controlling resistance limiting the internal diffusion particularly on Li-ZSM-5 with stronger electric field. Koriabkina et al. (Koriabkina et al., 2002) found that the electrostatic interaction between n-ethane and cations in ZSM-5 framework increases the internal diffusion resistance and the restraining force gradually weakens or disappears with increasing C0 because of saturated adsorption sites. 3.4. Effects of NPs on toluene adsorption Fig. 7 depicts the q0 and KP averaged over 10 test results for toluene on ZSM-5 with and without coexisting NPs at each condition. The negative effects of coexisting NPs on toluene adsorption can be observed from perspectives of thermodynamics and kinetics as evidenced by the reductions of q0 and KP as shown in Table S3, respectively. The t-test results for the statistical difference between two paired samples (toluene and toluene/NPs) at each condition are shown in Table S2. It can be observed that statistical differences (p-value < 0.05) indicative of the importance of NPs effect on toluene adsorption exist in all cases except for KP on Li-ZSM-5 at 0.27 × 10−3 mol/m3 (p-value = 0.6751). Therefore, the influence of coexisting NPs on gas adsorption is statistically convincible. For Li-ZSM-5, Na-ZSM-5 and K-ZSM-5, q0 were decreased by 10.4 %, 10.5 %, and 16.0 % at C0 of 0.27 × 10−3 mol/m3 and by 1.7 %, 2.2 %, and 5.6 % at C0 of 1.69 × 10−3 mol/m3, respectively. The order of LiZSM-5 < Na-ZSM-5 < K-ZSM-5 is consistent with that of saturated adsorption capacity (qF) shown in Table S3. Other key thermodynamic parameters such as ΔH, ΔS and ΔG, for toluene adsorption with and without NPs obtained by fitting static toluene isotherms (Fig. S2) using van’t Hoff (Fig. S3) and Gibbs free energy equations are also included in Table 4. The absolute values of ΔH and ΔS were decreased by 2.21 %, 2.79 %, and 6.67 %, and 3.11 %, 3.68 % and 4.67 % on Li-ZSM-5, NaZSM-5 and K-ZSM-5, respectively. The absolute value of ΔG reduces with NPs coexistence indicative of the reduced spontaneity of toluene adsorption, which is more significant on K-ZSM-5. To sum up, the effects of NPs on toluene adsorption thermodynamics follow the order of Li-ZSM-5 < Na-ZSM-5 < K-ZSM-5 and are more significant at lower C0. For the effect of NPs on toluene adsorption kinetics as shown in Fig. 7(b), KP were decreased by 0.3 %, 3.9 %, and 4.9 % at C0 of 0.27 × 10−3 mol/m3, and more significantly by 20.3 %, 15.2 %, and 2.3 % at C0 of 1.69 × 10−3 mol/m3 on Li-ZSM-5, Na-ZSM-5 and K-ZSM5, respectively. Note that little statistical difference (p-value = 0.6751) indicates insignificant NPs effects for the case of KP on Li-ZSM-5 at 0.27 × 10−3 mol/m3. The effects on adsorption kinetics are more significant at higher C0, which is inconsistent with the more significant effects on adsorption thermodynamics at lower C0. The reductions in KP on three adsorbents follow opposite orders at lower C0 (Li-ZSM-5 < NaZSM-5 < K-ZSM-5) and higher C0 (Li-ZSM-5 > Na-ZSM-5 > K-ZSM-5). This fact can be highly associated with toluene adsorption kinetics itself and interactions of NPs with adsorbents as will be discussed in Section 4.

Fig. 6. Experimental and model-fitting results of breakthrough curves at different C0 on (a) Li-ZSM-5; (b) Na-ZSM-5; (c) K-ZSM-5.

increased KP with increasing C0 representative of larger driving force can be observed. However, the order of the internal diffusion resistance inversely proportional to KP on three adsorbents varies with C0: Li-ZSM5 < Na-ZSM-5 < K-ZSM-5 at 1.69 × 10−3 mol/m3, and Li-ZSM5 > Na-ZSM-5 > K-ZSM-5 at 0.27 × 10−3 mol/m3. This could be explained by two factors that dominate internal diffusion resistance, the steric hindrance due to confined porous structures and the restraining force due to gas-surface electrostatic interactions (Tsai et al., 2008; Koriabkina et al., 2002). Higher C0 with greater driving force (Zhang

4. Discussion In general, it may be natural to speculate that during the interaction with porous materials NPs could enter into pore channels and impede the adsorption process (Kim et al., 2017). However, in current case, it 6

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Table 3 Fitting parameters of breakthrough curves using the constant-pattern wave model. Adsorbents

Adsorbates

C0 (10−3 mol/m3)

q0 (mmol/g)

KP (10−2s-1)

Kfα (104s−1)

KGα (103s−1)

R2

Li-ZSM-5

toluene toluene/NPs toluene toluene/NPs

1.69 1.69 0.27 0.27

0.233 0.229 0.106 0.095

8.11 6.46 3.51 3.50

3.97 3.97 3.97 3.97

4.08 3.27 4.91 4.46

0.99 0.99 0.99 0.99

Na-ZSM-5

toluene toluene/NPs toluene toluene/NPs

1.69 1.69 0.27 0.27

0.225 0.220 0.095 0.085

8.08 6.85 4.11 3.95

3.97 3.97 3.97 3.97

3.95 3.34 5.15 4.50

0.98 0.98 0.98 0.96

K-ZSM-5

toluene toluene/NPs toluene toluene/NPs

1.69 1.69 0.27 0.27

0.213 0.201 0.081 0.068

7.53 7.36 4.89 4.65

3.97 3.97 3.97 3.97

3.53 3.28 5.20 4.43

0.97 0.91 0.91 0.94

Fig. 7. Effects of NPs on (a) q0 and (b) KP for toluene adsorption on three adsorbents.

hindrance originated from comparable gas kinetic diameter and effective pore size (Tsai et al., 2008) and the restraining force originated from cationic attraction (Koriabkina et al., 2002; Newsome and Coppens, 2015), acting as controlling resistances at higher (Li-ZSM5 < Na-ZSM-5 < K-ZSM-5) and lower (Li-ZSM-5 > Na-ZSM-5 > KZSM-5) C0, respectively. For capturing pure NPs on ZSM-5, electrostatic interactions and external porous surface play significant roles as shown in Fig. 8a2, giving the same thermodynamic order as that of toluene on three materials. Considering similar interactions with adsorbents, coexisting gases and NPs on ZSM-5 might form competitiveness which affects gas adsorption thermodynamics and kinetics based on proposed mechanisms as follows. As evidenced by Wang et al. who addressed the deterioration of CO2 separation performance on flat-sheet polysulfone membranes, the occupation of the effective membrane area and the increase of the mass transfer resistance can be caused by the fine particles deposited on the membrane (Wang et al., 2014b). In terms of the effect of NPs on adsorption thermodynamics, the reductions in q0, qF, ΔH and ΔS caused by the occupation of original affinities by NPs could stem from two aspects. On one hand, the deposition and dynamic diffusion of NPs on the external surface (Kim et al., 2017; Kuo et al., 2013) may cause pore blockage (Pelekani and Snoeyink, 1999) (see Fig. S5) that prevents gas molecules from entering pores (Fig. 8a2) and overall, reduces the effective pore volume (Xing et al., 2018) and the specific surface area (Wang et al., 2014b) of the adsorbent. On the other hand, the electrostatic interaction between cations and NPs hamper the binding of gases with original adsorption sites particularly distributed around external surfaces (Fig. 8a3) regarding the inaccessibility of NPs into pores. In the water treatment field, it had been found that the natural organic matter may compete with aromatic molecules via pore blockage and direct site competition (Wang et al., 2009).With the strongest charge density and the largest

Table 4 Changes in thermodynamic properties for toluene and toluene/NPs on each adsorbent. Materials

Adsorbates

ΔH (kJ/mol)

ΔS (J/mol/K)

ΔG (kJ/mol) 288K

298K

308K

Li-ZSM-5

toluene toluene/NPs

−20.30 −19.85

−48.05 −46.55

−6.46 −6.45

−7.12 −7.03

−7.51 −6.81

Na-ZSM-5

toluene toluene/NPs

−19.08 −18.55

−41.52 −39.99

−5.98 −5.98

−6.71 −6.63

−7.16 −6.48

K-ZSM-5

toluene toluene/NPs

−17.32 −16.16

−34.07 −32.48

−5.50 −5.51

−6.29 −6.23

−6.82 −6.16

seems to contradict with the fact that the pore sizes of ZSM-5 (∼0.5 nm) is much smaller than the diameter of NPs (2–20 nm). There ought to be existing other unrevealed effects regarding gas-particlesurface synergetic interactions. We attempt to explain the influential mechanisms of NPs on gas adsorption as depicted in Fig. 8, based on above series of results of single and binary toluene/NPs adsorption properties and adsorbent characterizations. The adsorption affinity of toluene on ZSM-5 is mainly composed of the micropore filling contributed by specific microporosity (Zhang et al., 2019b) and the electrostatic interaction between zeolitic cations and methyl or π bond of benzene ring (Kustov et al., 2018) of toluene molecules as shown in Fig. 8a1. The adsorption thermodynamics follows the order of Li-ZSM-5 > Na-ZSM-5 > K-ZSM-5, consistent with the orders of charge density (charge/diameter of cation), specific surface area and pore volume. In kinetics, these two types of affinities would in turn hinder the internal diffusion of adsorbates due to the steric 7

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Fig. 8. Schematic diagram of influential mechanisms of NPs on gas adsorption.

exchanged with larger cations (e.g. K-ZSM-5) might be a good choice in terms of a high resistance of adsorption kinetics to NPs. To gain more reliable references for wider applications, broader knowledge about effects of other types of NPs on adsorption in other different adsorbentadsorbate pairs are necessary, which merits further investigation.

effective pore size, Li-ZSM-5 renders the greatest resistance to NPs influence on toluene adsorption thermodynamics, followed by Na-ZSM-5 and then K-ZSM-5. Although Li-ZSM-5 has the strongest capture of NPs, the differences from those on Na-ZSM-5 and K-ZSM-5 (Fig. 5) especially close to the pore size could be negligible as compared to the equilibrium adsorbed amount of gases located by cationic sites inside the pores. However, the gap between the total number concentration of NPs (> 107 counts/cm3) and the gas feed concentration makes differences, as evidenced by the significantly enhanced effects of NPs on q0 as C0 reduced from 1.69 × 10−3 to 0.27 × 10−3 mol/m3. In terms of the effect of NPs on adsorption kinetics, the influential mechanism varying with C0 is mainly related to different controlling resistance at different C0. For the higher C0 with higher driving force as shown in Fig. 8a3, the dominant steric hindrance is highly associated with the accessible interspace (Yang et al., 2018; Wang et al., 2009) that could be reduced by NPs at any sizes captured on the external surface. For Li-ZSM-5 having the strongest capture of NPs with largest average pore size and external specific surface area, the enhancement of steric hindrance could be the most prominent leading to the greatest reduction in adsorption kinetics, followed by Na-ZSM-5 and then KZSM-5. For the lower C0 with lower driving force as shown in Fig. 8a4, the negative effect of increased steric hindrance still exist because of the same deposition of NPs. However, the restraining force from cations around the surface as the controlling resistance could to some extent be released by NPs regarding overlapping preference for adsorption sites (Shu et al., 2013). This can be explained by the fact that different-sized deposited NPs strongly interacting with cations might interfere with the original electric field restraining the gases and obstruct the gas adsorption by increasing gas-cation distance. Li-ZSM-5 with the largest charge density and NPs deposition amount might exhibit highest release of the restraining force, which could to a large extent offset the negative effect of steric hindrance. As a result, the case of KP on Li-ZSM5 at 0.27 × 10−3 mol/m3 exhibiting negligible effects of NPs with little statistical difference (p-value = 0.6751) could be explained. The above discussion reveals the effect of NPs on gas adsorption from the synergetic interaction of cations and porosities on ZSM-5 which behaves differently in adsorption thermodynamics and kinetics in the gas-particle-adsorbent system. It is also of practical significance for selecting robust adsorbents under coexisting toluene/particle conditions according to the feeds in real applications and the needs of adsorption capacity or rate. For instance, if C0 is low, an adsorbent exchanged with smaller cations (e.g. Li-ZSM-5) would be a good choice for minimizing the influences from NPs; if C0 is high and the adsorption rate is required to be fast based on a short cycling time, an adsorbent

5. Conclusions The effects of coexisting NPs on toluene adsorption on ZSM-5 with different cations were systematically studied. Affinities for pure NPs and toluene on adsorbents originating from porosities and cations follow the order of Li-ZSM-5 > Na-ZSM-5 > K-ZSM-5, which results in restraining force and steric hindrance as controlling resistances for toluene adsorption kinetics at lower and higher C0, respectively. The toluene adsorption thermodynamics and kinetics are shown to be impaired by coexisting NPs. Relative reductions of q0 by NPs are significant at lower C0 following Li-ZSM-5 (10.4 %) < Na-ZSM-5 (10.5 %) < K-ZSM-5 (16.0 %). Relative reductions of KP are significant at higher C0 following Li-ZSM-5 (20.3 %) > Na-ZSM-5 (15.2 %) > K-ZSM5 (2.3 %). The influential mechanisms regarding competitiveness between toluene and NPs in interaction with cationic and porous surfaces were proposed, for thermodynamics that NPs occupies adsorption sites by pore blockage and interference with cationic electrostatic interactions, and for kinetics that NPs increase the steric hindrance and might partially offset this negative effect by releasing the electrostatic restraining force. It is of practical significance for selecting robust adsorbents under realistic harsh air conditions. Author contribution section Yi Xing: Writing-Original Draft, Investigation. Yongkang Cui: Data Curation, Investigation. Ziyi Li: Conceptualization, Methodology, Writing - Review & Editing. Yingshu Liu: Funding acquisition, Supervision. Danqi Bao: Investigation. Wei Su: Visualization. ChuenJinn Tsai: Writing - Review & Editing. Chao-Heng Tseng: Funding acquisition. Angus Shiue: Formal analysis. David Y.H. Pui: Resources. Ralph T. Yang: Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 8

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Acknowledgments

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