Adsorption of Low-Molecular-Weight Amines in Aqueous Solutions to

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Adsorption of Low-molecular-weight Amines in Aqueous Solutions to Zeolites —An Approach to Impeding Lowmolecular-weight Amines to Regenerate N-nitrosamines Xiaoyan Guo, Hailan Yun, Man Zhang, Qilin Li, Qi-Xing Zhou, huaiqi shao, wanli hu, Chunyu Li, and Shougang Fan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01948 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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Industrial & Engineering Chemistry Research

Adsorption of Low-molecular-weight Amines in Aqueous Solutions to Zeolites —An Approach to Impeding Low-molecular-weight Amines to Regenerate N-nitrosamines Xiaoyan Guoa*, Hailan Yuna, Man Zhanga, Qilin Lib, Qixing Zhoua, Huaiqi Shaoc, Wanli Hua, Chunyu Lia, Shougang Fana

a: Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria/ Tianjin Key Laboratory of Environmental Remediation and Pollution Control /College

of

Environmental

Science

and

Engineering,

Nankai

University,

Tongyan Road 38#, Haihe Education Park, Jinnan District, Tianjin 300350, China; b: Department of Civil & Environmental Engineering, George R. Brown School of Engineering, Rice University, 6100 Main Street, Houston, TX 77005 c: College of Material Science and Chemical Engineering, Tianjin University of Science & Technology, Thirteenth Street 29, TEDA, Tianjin 300457, China

1

ACS Paragon Plus Environment


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ABSTRACT: The combined effects of the porous structure, acidity and ion exchange properties of zeolites and the structure of the degraded products of nitrosamines in aqueous solution on adsorption were evaluated. The degraded products of nitrosamines including aliphatic low-molecular-weight amines such as methylamine, dimethylamine, ethylamine, diethylamine and aromatic amine such as aniline all exhibited great adsorptive affinity to HZSM-5 zeolites, and aliphatic low-molecular-weight amines exhibited much greater adsorptive affinity to HY zeolite than to NaY and HZSM-5 zeolites, but aromatic amines exhibited little adsorptive affinity to HY and NaY zeolites. Results of the pH-dependency experiments further indicated that the acidity and polarity of adsorbates and the acidity of adsorbents played a combinational role in determining

the

significance

of

the

adsorptive

interactions

between

low-molecular-weight amines in aqueous solution and zeolites. We propose that the acid-base interaction and electrostatic interaction and shape-selective adsorption property of HZSM-5 zeolites are all responsible for the adsorption of aliphatic and aromatic low-molecular-weight amines to HZSM-5 zeolites in aqueous solution, and the acid-base interaction and electrostatic interaction and ion exchangeable property of Y-type zeolites all contribute to the adsorption of aliphatic amines to Y-type zeolites in aqueous solution. The findings of the present study might have significant implications for the removal of the low-molecular-weight amines in aqueous solution and thoroughly controlling N-nitrosamines in water with zeolites.

KEY WORDS 2


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low-molecular-weight amine; adsorption; zeolite; aqueous solution; N-nitrosamine

INTRODUCTION 3



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N-nitrosamines are considered as a group of emerging disinfection byproducts with high carcinogenic potency and have been tried to remove from water by the methods such as UV photolysis1, photocatalytic degradation2-4, etc.. However, these N-nitrosamines-degradation methods raised a serious concern on their degraded products such as methylamine (MA), dimethylamine (DMA), ethylamine (EA), diethylamine (DEA), and the like low-molecular-weight amine, which, in particular, secondary amines including DMA are important precursors for corresponding nitrosamines and are easy to reform N-nitrosamines5, resulting in inefficient removal of N-nitrosamines from water. Therefore, it is significant for thoroughly controlling N-nitrosamines in water to investigate the removal of low-molecular-weight amines. Zeolite has been proved to be a promising adsorbent for removal of contaminant in aquatic environment, such as phenol6, methylene blue7, naphthalene8, based on its low cost and valuable properties such as ion-exchange capability, compared to activated carbon. However, adsorption of low-molecular-weight amines in aquatic environment by using zeolite has not been conducted. It is noted that Parrillo et al. 9 has verified there were higher adsorption capabilities of MA and EA in gas phase on HZSM-5 zeolite based on the interactions of each base（amines）with the Br¢nsted acid sites on HZSM-5 zeolite. Compared with the amines in gas phase, the low-molecular-weight amines in aqueous solution can also exist in the forms of molecules when the solution pH is above pKa of amines. Meanwhile, zeolites present a certain acidity based on the Si/Al ratio. So it is possible that the amines in aqueous solution can be adsorbed to zeolite based on the acid-base interaction. In our previous study, coal activated carbon and HY zeolite were respectively used to treat 90 mg/L DMA aqueous solution and 45% and 94% of adsorption efficiencies were correspondingly obtained under the similar conditions of 0.5g/100mL solution. It is obvious that zeolite presents a stronger adsorptive affinity to DMA than activated carbon. Thus far, the adsorptive efficacies of low-molecular-weight amines in aqueous solution to zeolites have not been evaluated systematically. Many of the low-molecular-weight amines, such as MA, DMA, EA, DEA and aniline (AN), are from the degradation of N-nitrosamines prevalently present in water, just like 4


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N-nitrosodimethylamine

(NDMA),

N-nitrosodiethylamine

(NDEA)

and

p-nitrosodiphenylamine (NDPA). Additionally, the porous structure, acidity and ion exchange properties of zeolites might have considerable effects on the adsorption of matters. The overall objective of the present study is to investigate the adsorptive affinity of different low-molecular-weight amines in aqueous solution to zeolites with different porous structure, acidity and ion exchange properties, in particular, to investigate how the structure of low-molecular-weight amines and the porous structure, acidity and ion exchange properties of zeolites might affect adsorption. The chemicals selected were MA, DMA, EA, DEA and AN. The adsorbents included two kinds of zeolites with different porous structures (HZSM-5 and Y-type zeolites). Specifically, HZSM-5 zeolites contained HZSM-5(25) (25 refers to Si/Al ratio) and HZSM-5(360) (360 refers to Si/Al ratio) with different acidity; Y-type zeolites involved HY and NaY with different acidity and ion exchange properties. Adsorption isotherms were obtained to compare adsorption affinity of different adsorbate-adsorbent combinations. The effect of pH on adsorption was also evaluated to understand further the mechanisms on controlling adsorptive interactions because the pH can significantly affect the existing states of amines in aqueous solution. EXPERIMENTAL SECTION Materials. Commercially available rods of H-ZSM-5(25), HZSM-5(360), HY and NaY were purchased from Nankai Catalyst Co. (Tianjin, China). Surface area, pore size and pore volume were determined by the adsorption of nitrogen on a iQ autosorb sorptometer (Quantachrome). The acidic properties of zeolites, i.e. the acid amount were characterized by temperature-programmed desorption (TPD) of ammonia (NH3). MA (40% water solution), DMA (40% water solution), DEA (99% water solution) were purchased from Tianjin Kermel Chemical Reagent Co. (Tianjin, China), EA (67% water solution) and AN (99.5% water solution) were respectively procured from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China) and Tianjin Damao Chemical Reagent Factory, Tianjin, China). NDMA (99.5% purity) was purchased from Wako Pure Chemical, Japan. All adsorbates were of analystical grade and were used without further purification. Selected physical-chemical properties of the adsorbates 5



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are listed in Table 1. Adsorption experiments. Adsorption experiments were conducted using a batch adsorption approach. Adsorption experiments were carried out in 100-mL flasks equipped with glass stoppers. Duplicate samples were done for each adsorption isotherm data point, and triplicate samples were done for each pH effect data point. For adsorption isotherm experiments, the pH for MA, DMA, EA and DEA on HZSM-5(25) and HZSM-5(360) was between 8.5 and 9.3 (measured at the end of batch sorption ) and the pH for MA, DMA, EA and DEA on HY and NaY was 9.6-10.5 and 10.6-11.1, respectively, except that the pH for AN on HZSM-5(25) and HZSM-5(360) was 7.8, and 8.5 on HY and NaY. For the pH effect experiments, the equilibrium pH was set over a range of 4-12 (pH adjusted with NaOH and HCl). For the selective adsorption of photocatalytic degradated products of NDMA, a mixed solution was prepared for 100mL, containing NMDA(0.01mM) ， DMA(0.8mM) , and MA(0.2mM). Conrespondingly, pure NDMA(0.01mM), DMA(0.8mM) , MA(0.2mM) solution will be prepared for control. Prior to initiating an adsorption experiment, a certain amount of zeolites (precalculated to ensure data compatibility for different adsorbate-adsorbent combinations while maintaining analytical accuracy) was activated at 550℃ for 2 h and was transferred to a 100-mL flask. Afterwards, a stock solution of an adsorbate was added to the flask using a microsyringe. The flasks were then filled with 100 mL of MilliQ high purity water and were mixed at 150 r/min with a rotary shaker at 25℃ for more than 10 days. The time required to reach adsorption equilibrium was predetermined. Then, the flasks were removed from the shaker and were left undisturbed on a flat surface to allow complete settling of the zeolites. Aliquots of the aqueous solution were then withdrawn from the flasks and were filtered through 0.45µm filter for measurement. The filtered solution was analyzed by ion chromatograph (IC,Dionex DX-120) equipped with a IonPac CS12A for cations and a conductivity detector for MA, DMA, EA and DEA, and by high performance liquid chromatograph (HPLC Waters 1525) equipped with a Waters Xterra RP18 column and UV detector at 230nm for AN and at 6


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228 nm for NDMA, respectively. The adsorbed mass at each equilibrium concentration was calculated as the difference between the total and solution-phase mass. Table 1. Summary of adsorbate properties [molecular size, acid dissociation constant (pKa), n-Octanol-water partition coefficient (KOW), and water solubility (Csat)] and Freundlich model coefficients (KF and n) obtained from adsorption results.

molecular adsorbate

logKOW

size(nm)

0.18×0.30a

MA

0.18×0.42a

DMA

0.18×0.42a

EA

0.18×0.68a

DEA

0.43×0.59a

AN

a

KF

Csat pKa

adsorbent (ug/L)

10.62b

10.64b

10.63b

10.98b

4.62b

-0.57c

-0.38c

-0.13c

0.58c

0.96d

1-n n

n

R2

(mmol L /kg) HZSM-5(25)

625±1

0.387±0.02

0.969

HZSM-5(360)

453±1

0.495±0.02

0.986

HY

1445±1

0.179±0.004

0.972

NaY

606±1

0.314±0.02

0.974

HZSM-5(25)

601±1

0.234±0.02

0.947

HZSM-5(360)

430±1

0.354±0.009

0.993

HY

1343±1

0.486±0.03

0.971

NaY

411±1

0.496±0.03

0.957

HZSM-5(25)

623±1

0.343±0.02

0.965

HZSM-5(360)

428±1

0.487±0.03

0.951

HY

1066±1

0.393±0.02

0.972

NaY

296±1

0.569±0.03

0.967

HZSM-5(25)

428±1

0.454±0.03

0.941

HZSM-5(360)

399±1

0.224±0.02

0.953

HY

1060±1

0.235±0.01

0.979

NaY

191±1

0.423±0.02

0.971

HZSM-5(25)

529±1

0.574±0.02

0.991

HZSM-5(360)

392±1

0.521±0.04

0.946

HY

-

-

-

NaY

-

-

-

soluble

soluble

soluble

miscible

3.4×107d

calculated by the method of B3LYP/6-31G in the Gaussian 03 software. bfrom Smith10. cfrom Sangster11 .dfrom 7



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Mohapatra12. The errors of KF and n are all less than 5%, which are satisfactory for fitting by Freundlich model.

RESULTS AND DISCUSSION Characterization of Zeolites. The pore size, BET surface area, pore volume and total acidic amount of the four types of zeolites are given in Table 2. Results of the pore size and pore volume analysis show that HZSM-5 and Y-type zeolites have different pore structures. Data of the total acidic amounts confirm that the acidity of HZSM-5(25) is considerably higher than that of HZSM-5(360), and the acidity of NaY is weaker than that of HY. Table 2. Pore size, BET surface area, pore volume and total acid amount for HZSM-5(25), HZSM-5(360), HY and NaY BET Pore Zeolite

SiO2/Al2O3

Pore

Total

volume(cm3/g)

amount(µmol/g)

346

0.2380

1297.54

283

0.2337

221.25

surface size(nm)

2

acid

area(m /g) 0.54 × HZSM-5

25 0.56 0.54 ×

HZSM-5

360 0.56

HY

5.6

0.74

564

0.3987

1037.57

NaY

5.3

0.74

552

0.3491

814.59

Adsorption isotherms. The adsorption results of the five chemicals on four zeolites are shown in Table 1 and Figures 1 and 2. The adsorption data were fitted with the Freundlich isotherm: q=KFCWn, where q (mmol/kg) and CW (mmol/L) are equilibrium concentrations of an adsorbate on the adsorbent and in the aqueous solution, respectively; KF (mmol1-nLn/kg) is the Freundlich affinity coefficient; and n (unitless) is the Freundlich linearity index. In general, adsorption was highly nonlinear and the Freundlich model provided reasonably good fits to the data. Figure 1 compares the adsorption affinities of different chemicals to a given adsorbent. Adsorption affinities to HZSM-5(25) increased as AN/DEA ﹤ 8


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EA/DMA/MA. Similar results to HZSM-5(360) were obtained as AN﹤DEA﹤ EA/DMA﹤MA. These trends correlated with the molecular size of adsorbates shown in Table 1. Because the molecular sizes of MA, DMA and EA are less than the pore sizes of HZSM-5(25) and HZSM-5(360), MA, DMA and EA can be free to diffuse into the cavities of zeolites, resulting in higher adsorptive affinity. An interesting observation was that, even though the molecular sizes of DEA and AN are slightly larger than the pore sizes of HZSM-5(25) and HZSM-5(360), HZSM-5(25) and HZSM-5(360) still represented certain adsorptive affinity to DEA and AN, only lower than that of EA/DMA/MA. Thus, the results indicate that HZSM-5 zeolites have good shape-selective adsorption property, and their frameworks are so flexible that they can adsorb the molecules which are slightly larger than the zeolites’ pore sizes, just with significantly lower adsorption capacity and rate than smaller molecules. Moreover, the adsorptive affinities of five amines to HZSM-5(25) and HZSM-5(360) were related to the amines’ hydrophobicity, shown by the water solubility(Csat) and n-octanol-water partition coefficient(KOW) values in Table 1. Additionally, it can be seen from Figure 1 that there is no adsorption for the aromatic amine AN in HY and NaY zeolites, but different adsorptive trends of the other four aliphatic amines were observed on HY and NaY zeolites: there were no significant differences for the adsorptive affinities of the four amines to HY; adsorption affinities of the four amines to NaY remarkably followed the order of DEA﹤EA﹤DMA﹤MA. It should be noted that the pore sizes of HY and NaY zeolites was considerably larger than the molecular sizes of five amines, the results, especially AN, indicated that HY and NaY zeolites had no shape-selective adsorption property for amines. However, the pKa value of AN is remarkably lower than that of other four aliphatic amines, HY and NaY also possessed a certain acidity determined by NH3-TPD presented in Table 2, hence acidity-base adsorption mechanism may be responsible for the four aliphatic amines to HY and NaY. The pKa values of four aliphatic amines were so close that there were no obvious differences for the adsorptive affinities of four aliphatic amines to HY. The distinct adsorptive affinity trends of four aliphatic amines to NaY correlated to amines’ hydrophobicity, shown by n-octanol-water partition coefficient(KOW) values in Table 1. 9



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Figure 1. comparison of the adsorption isotherms of five adsorbates on a given adsorbent. MA, DMA, EA, DEA, and AN are acronyms for methylamine, dimethylamine, ethylamine, diethylamine, and aniline, respectively. Error bars, in most cases smaller than the symbols, represent standard deviations of duplicate samples.

Figure 2 compares the adsorption of a given chemical to the four different adsorbents. The adsorbed concentrations were normalized for the BET surface area. The figure shows that, for MA, DMA, EA and DEA, the effect of the adsorbent properties on the adsorption affinity was similar: the adsorption was most strongest on HY; the adsorption was very close on HZSM-5(25) and HZSM-5(360); the adsorption was weakest on NaY. For AN, there was no adsorption on HY and NaY, the adsorption was also very similar on HZSM-5(25) and HZSM-5(360). These results seem to indicate that the low-molecular-weight amines presented different adsorptive properties on HZSM-5 and Y-type zeolites: For HZSM-5 zeolites, the acidity (different Si/Al ratio) played an insignificant role in adsorption; for Y-type zeolites, the acidity (different cation, H+ and Na+) had important effects on adsorption. 10


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Figure 2. comparison of the adsorption isotherms of a given adsorbate on four different adsorbents. Solid-phase concentrations are normalized with the BET surface area. Error bars, in most cases smaller than the symbols, represent standard deviations of duplicate samples.

Effects of pH on adsorption. Figure 3 shows that the pH effect on adsorption varied for different chemicals. For AN, the pH effect on adsorption to two HZSM-5 zeolites was minimal. Changing the pH over the range of 4-12 should have significantly affected the protonation-deprotonation transition of HZSM-5 and Y-type zeolites surface groups such as hydroxyl. However, it appeared that such a transition had little effect on the adsorptive affinity of nonpolar compounds. The effect of pH on the adsorption of DMA was insignificant when the pH was below the compound’s pKa(10.64), but the adsorption was considerably hindered when the pH was above the 11



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pKa. Moreover, this pH effect was more significant for the adsorption to HZSM-5 zeolites than to Y-type zeolites. A similar trend of the pH effect was observed on DEA. Surprisingly, when the pH was above the pKa (10.63), the adsorption of EA to HZSM-5 and Y-type zeolites was considerably stronger than that at the lower pH values (pH 10.98 or below). Moreover, this pH effect was more significant for the adsorption to HY zeolite than to HZSM-5 zeolites. Another interesting observation shown in Figure 3 was that the adsorption of MA to HZSM-5 zeolites significantly increased when the pH exceeded its pKa, but the adsorption to Y-type zeolites was first markedly decreased and then increased. In fact, the predominant fraction of low-molecular-weight amine was the positive ionized form when the solution pH was below its pKa, conversely, it existed in the form of nonionized form, which were shown in Fig.S1. Additionally, the zeta potentials of zeolites (HZSM-5(25), HZSM-5(360), HY, NaY) surface in aqueous solution also depended on the solution pH, as presented in Fig.S2. The pH dependence results suggest that the electrostatic interaction of the positive ion of the amine adsorbate and the surface hydroxyl of zeolite adsorbent plays a role in the adsorption of amine in aqueous solution to zeolites.

12


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Figure 3. Changes of distribution coefficients(Kd, which is defined as the equilibrium adsorption capacity of absorbent at unit equilibrium concentration of absorbate.) with the pH at single-point initial concentrations for adsorption of all adsorbates to HZSM-5(25), HZSM-5(360), HY and NaY zeolites. The initial concentrations for methylamine on HZSM-5(25), HZSM-5(360), HY and NaY zeolites are 2.903mmol/L, those for dimethylamine and ethylamine are 3.333mmol/L, those for diethylamine are 2.466mmol/L, those for aniline are 0.968mmol/L. Error bars, in most cases smaller than the symbols, represent standard deviations of triplicate samples. Vertical dashed lines represent pKa values of the respective adsorbates.

Mechanistic aspects. As discussed earlier, HZSM-5 zeolites exhibited strong adsorptive affinity to four aliphatic amines MA, DMA, EA, DEA, and aromatic amine AN in aqueous solution. Likewise, Parrillo et al.9 also reported that there were stronger 13



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adsorptive affinity for MA, EA and other amines on HZSM-5 zeolites. However, it should be noted that the maximum adsorption amounts of amines investigated by Parrillo et al. in gas phase( for example, the maximum adsorption amounts of MA and EA was 2613 mmol and 2422 mmol on per kilogram on per gram HZSM-5 zeolite (Si/Al=35), respectively ) were markedly higher than those obtained in our present research in aqueous phase(for instance, the maximum adsorption amounts of MA and EA was 762 mmol and 568 mmol on per kilogram HZSM-5 zeolite (Si/Al=25), respectively, presented in Fig.1 ). Parrillo et al.13 confirmed that the adsorption of amines on HZSM-5 zeolite in gas phase must be associated with the interactions of each base（amines）with the Br¢nsted acid sites on HZSM-5 zeolite, and the gas-phase proton affinities of HZSM-5 zeolite were significantly greater than the heats of protonation in aqueous phase HZSM-5, which were due to the solvent effects in Br¢nsted acid solution. Therefore, the solvent effects of water resulted in less adsorption amounts of amines on HZSM-5 zeolite in aqueous solution than that in gas phase. Further specifically, water has an influence on the existed states of amines and HZSM-5 zeolites in aqueous solution. From the Fig.S1, we can see that amines molecule dissociated and existed in the two possible forms in aqueous solution: neutral and ionized amines, which varied with the solution pH14. When the solution pH value was lower than pKa, the ionized DMA was the main form, and vice verse. Besides, the zeta potentials of HZSM-5 zeolites surface in aqueous solution also depended on the solution pH, as presented in Fig.S2. The negative charges at the HZSM-5 zeolites surface increased with the solution pH rising, resulting from the hydroxyl at the HZSM-5 zeolite surface was ionized to strip proton. The pH of zero point of charge (pHZPC) of the zeolite HZSM-5(25) and HZSM-5(360) was 5.0, it can be inferred the positively charged low-molecular-weight amines can interact with negatively charged sorbents HZSM-5(25) and HZSM-5(360) zeolites via electrostatic interaction when the pH was between the pHZPC of HZSM-5 zeolites and the pKa of amines. Correspondingly, we also noticed that the adsorptive affinity of amines to HZSM-5 zeolites varied with the pH value in aqueous solution. Among those, the primary amines MA and EA presented highest adsorptive affinity to HZSM-5 zeolites at pH 14


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value of 12 (pH﹥pKa) due to the stronger acid-base interaction between the primary amines with strong basicity and HZSM-5 zeolites with Br¢nsted acid sites; whereas the adsorptive affinity of the secondary amines DMA and DEA were highest when the pH value was equal to the pKa, which resulted from the electrostatic interaction between the ionized secondary amines and negatively charged HZSM-5 zeolites; additionally, the poor polarity and weak basicity of AN were responsible for the unchangeable adsorptive affinity with pH rising. All the results indicated that the basicity and polarity of amines may play important roles in adsorption to HZSM-5 zeolites in aqueous solution. Besides, the good shape-selective adsorption property of HZSM-5 zeolites can be a key influence factor. Generally, occurrence of shape selectivity is related to the presence of micropores15. HZSM-5 has a two-dimensional micropore system consisting of two types of intersecting channels that have 10-membered ring openings. The sinusoidal channels have near circular openings with a size of 0.51×0.55 nm and the linear channels have elliptical openings with a size of 0.54×0.56 nm. The sinusoidal and linear channels intersect to form a larger cavity to make the HZSM-5 have stronger absorptive ability15. The molecular sizes of MA, DMA, EA, which are 0.18×0.30 nm, 0.18×0.24 nm and 0.18×0.24 nm, respectively, are all smaller than the pore sizes of HZSM-5 zeolites. As a result, the stronger absorption was obtained for MA, DMA, EA on HZSM-5 zeolites. As for DEA with the molecular size of 0.18×0.68 nm and AN with that of 0.43×0.59 nm, the reduced adsorbed amounts were observed for the sake of a little bigger molecular sizes than the pore sizes of HZSM-5 zeolites. It should be noted that the sizes of ionized amines were smaller than the ones of neutral amines, the good shape-selective adsorption property of HZSM-5 zeolites was also suitable for the ionized amines. All the above discussion indicated that the acid-base interaction and shape-selective adsorption property of HZSM-5 zeolites were mainly responsible for the adsorption of amines to HZSM-5 zeolites in aqueous solution when the pH was above the pKa of amines, and the electrostatic interaction and shape-selective adsorption property of HZSM-5 zeolites played a leading role in adsorbing of amines to HZSM-5 zeolites in aqueous solution when the pH was below the pKa of amines. In comparison with HZSM-5 zeolites, Y-type zeolites presented strong adsorptive 15



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affinity to aliphatic amines except for the little adsorptive affinity to aromatic amines. In the mean time, the adsorption properties of the primary amines and secondary amines on Y-type zeolites are similar to those on HZSM-5 zeolites. These results indicated that the acid-base interaction and electrostatic interaction may also play key roles in the adsorption of aliphatic amines to Y-type zeolites. It is noticed that HY zeolites show highest adsorptive affinity among these kinds of zeolites, which inferred that the acid-base interaction between HY zeolite with highest acid amounts and aliphatic amines was predominant, thus further confirming little adsorptive affinity of AN with weak basicity. In the mean time, the pHZPC of HY and NaY zeolite was 3.0 and 1.2, respectively, which were lower than the ones of HZSM-5 zeolites, presented in Fig.S2. It may be inferred that the electrostatic interaction played the more essential role in adsorbing of amines to HY and NaY zeolites than to HZSM-5 zeolites in aqueous solution when the pH was below the pKa of amines. Additionally, it should be noted that NaY zeolite should be easily ion exchanged. For example, Ikeda et al.16 reported that the methylammonium ion is adsorped on the site in the cage of zeolite 4A by an ion-exchanged mechanism with Na+ consisting of three elementary processs, that is, ( ) the adsorption-desorption process of Na+; (ⅱ) the diffusion process of methylammonium ion on the surface; ( ⅲ ) the adsorption process of the methylammonium ion on the site in the cage of 4A zeolites. For further confirming the influences of ion exchange properties of NaY zeolite on adsorption of amines in aqueous solution, take the DMA for example, Fig.S3 compares the Na+ amount released from NaY with the DMA+ amount decreased from the aqueous solution, with HY as comparison. From the results, we can see that Na+ amounts released from NaY and HY in DMA solution are more than those in aqueous solution and at the mean time dimethylammonium ion amounts are decreased, which may be explained by dimethylammonium ion exchange for Na+. Moreover, it is noted that the amounts of Na+ released from NaY and HY are greater than that of dimethylammonium ion adsorbed, which may be due to hydrolysis of the dimethylammonium ion in solution, followed by hydronium ion exchange for Na+ 16. However, at the present pH (the pH value of the HY and NaY suspension solution with DMA is 8.6 and 9.2, respectively, 16


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when the adsorption equilibrium is reached.), the hydronium ion seems not to be responsible for Na+ release. On the other hand, the fact that the amounts of dimethylammonium ion adsorbed to HY are greater than that to NaY may be owing to HY’bigger pore volume, which can allow more dimethylammonium ion to enter into the cage of HY, compared to NaY. These results indicated that the stronger ion exchangeable property may lead to higher adsorption amounts of dimethylamine to HY than NaY. All the above discussion indicated that the acid-base interaction and electrostatic interaction and ion exchangeable property of Y-type zeolites all contribute to the adsorption of amines to Y- type zeolites in aqueous solution. Finally, HY zeolite was used to treat the photocatalytic degraded products of NDMA3, a kind of typical N-nitrosamines, containing DMA, MA and few of NDMA. A striking observation presented in Fig.S4 is that NDMA has little effect on the adsorption of DMA and MA, and there are almost same dynamic adsorptive trends for DMA and MA, whether another low-molecular-weight amine is coexisted or not, compared with the pure controlled samples. These results indicated that it is feasible and of potential for selective removal of degraded products of N-nitrosamines through adsorbing to zeolites. The mechanism of selective adsorption of degraded products of N-nitrosamines will be further systematically investigated. CONCLUSIONS A

promising

environmental

application

of

zeolites

is

adsorbents

for

low-molecular-weight amines in aqueous solution. Findings in this study indicate that the porous structure, acidity and ion exchange properties of zeolites and the acidity and size of amines can play important roles in adsorption. Additionally, the pH has a significant effect on the adsorption of polar amines and, particularly, the enhanced adsorption at pH of higher than pKa for primary amine and at pH of equal to pKa for secondary amine has not been reported previously. Thus, it is possible that the removal of the degraded products of N-nitrosamines can be achieved by controlling the solution chemistry and selecting specific properties for zeolites to enhance the adsorptive affinity with target low-molecular-weight amines. ACKNOWLEDGMENTS 17



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This work was supported by the National Natural Science Foundation of China (Grant No. 50808102) and the Natural Science Foundation of Tianjin City (14JCYBJC23100 and 15JCYBJC48100). We gratefully acknowledge the valuable suggestion from Prof. Chen Wei of Nankai University. SUPPORTING INFORMATION Distributions of neutral and ionized amines, and zeta potentials of zeolites(HZSM-5(25), HZSM-5(360), HY, NaY) surface in aqueous solution with the solution pH, and released Na+ and deceased ionized DMA in the kinetic process of DMA adsorption on HY and NaY in aqueous solution, and time profiles of the adsorbed NDMA, MA and DMA amounts to HY zeolite in a typical case for NDMA photocatalytic degradation products. CORRESPONDING AUTHOR *E-mail: [email protected]

REFERENCES (1) Mitch,W.A.; Sharp, J.O.; Trussell, R.R.; Valentine, R.L.; Alvarez-Cohen, L.; Sedlak,D.L. N-Nitrosodimethylamine (NDMA) as a Drinking Water Contaminant: A Review. Environ. Eng. Sci. 2003,20, 389. (2)Lee, J.; Choi,W.; Yoon, J. Photocatalytic degradation of N-nitrosodimethylamine: mechanism, product Distribution, and TiO2 surface modification. Environ. Sci. Technol. 2005,39, 6800. (3) Guo,X.; Li,Q.; Zhang, M.; Long,M.; Kong,L.; Zhou,Q.; Shao, H.; Hu,W.; Wei,T. Enhanced photocatalytic performance of N-nitrosodimethylamine on TiO2 nanotube based on the role of singlet oxygen. Chemosphere 2015,120,521. (4) Guo,X.; Shao, H.; Kong, L.; Long, M.; Zhang, M.; Zhou, Q.; Hu,W. Probing the effect of nanotubes on N-nitrosodimethylamine photocatalytic degradation efficiency and reaction pathway. Chem. Eng. Sci. 2016,144,1. (5) Liang, S.; Min, J. H.; Davis, M. K.; Green, J. F.; Remer, D. S. Use of pulsed-UV processes to destroy NDMA. J. AWWA. 2003, September, 121. (6) Yousef,R.; El-Eswed,B.; Al-Muhtaseb, A. Adsorption characteristics of natural zeolites as solid adsorbents for phenol removal from aqueous solutions: Kinetics, mechanism, and thermodynamics studies. Chem. Eng. J. 2011,171,1143. 18


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(7) Rida,K.; Bouraoui,S.; Hadnine, S. Adsorption of methylene blue from aqueous solution by kaolin and zeolite. Appl. Clay Sci. 2013,83-84,99. (8) Chang, C.; Chang,C.; Chen, K.; Tsai,W.; Shie, J.; Chen.,Y. Adsorption of naphthalene on zeolite from aqueous solution. J. Colloid Interf. Sci. 2004,277,29. (9) Parrillo,D.J.; Adamo,A.T.; Kokotailo,G.T.; Gorte, R.J. Amine adsorption in H-ZSM-5. Appl. Catal. 1990,67,107. (10) Smith., D.A. Metabolism, Pharmacokinetics and Toxicity of Functional Groups: Impact of the Building Blocks of Medicinal Chemistry on ADMET; RSC Drug Discovery, Royal Society of Chemistry, 2010. (11) Sangster., J. Octanol-water partition coefficients: fundamentals and physical chemistry. Vol. 2 of Wiley series in solution chemistry, John Wiley and Sons Ltd, Chichester, 1997. (12) Mohapatra., P.K. Textbook of Environmental Biotechnology, I. K. International Publishing House, 2007. (13) Parrillo,D.J.; Gorte,R.J.; Farneth,W.E. A Calorimetric Study of Simple Bases in H-ZSM-5: A Comparison with Gas-Phase and Solution-Phase Acidities. J. Am. Chem. Soc. 1993,115,12441. (14) Schwarzenbach, R.P.; Gschwend, P.M.; Imboden, D.M. Environmental Organic Chemistry, Wiley, Hoboken, 2003. (15) Devriesea,L.I.; Martens, J.A.; Thybaut,J.W.; Marin,G.B.; Baron,G.V.; Denayer, J. F.M. A new methodology to probe Shape Selectivity in Porous Adsorbents. Microporous Mesoporous Mater. 2008,116,607. (16) Ikesa, T.; Sasaka,M.; Mlkami, N.; Yasunaga,T. Kinetic Studies of Ion Exchange of the Methylammonium Ion for Na+ in Zeolite 4A Using the Pressure-Jump Method. J. Phys. Chem. 1981, 85, 3896.

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Graphical Abstract

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Adsorption of Low-Molecular-Weight Amines in Aqueous Solutions to

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