Sorption of Non-ionic Aromatic Organics to Mineral Micropores

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Environmental Processes

Sorption of nonionic aromatic organics to mineral micropores: Interactive effect of cation hydration and mineral charge density Erdan Hu, Xinglei Zhao, Shangyue Pan, Ziwei Ye, and Feng He Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00145 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Environmental Science & Technology

Sorption of nonionic aromatic organics to mineral micropores:

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Interactive effect of cation hydration and mineral charge

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density

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Erdan Hu, Xinglei Zhao, Shangyue Pan, Ziwei Ye, Feng He*

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College of Environment, Zhejiang University of Technology, Hangzhou 310014, China

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Submission Date: 02/19/2019

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*Corresponding author: Feng He; Phone: 86-571-88871509; Fax: 86-571-88871509; E-mail:

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[email protected]

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(Emails: Erdan Hu, [email protected]; Xinglei Zhao, [email protected]; Shangyue Pan, [email protected]; Ziwei Ye, [email protected]; Feng He, [email protected])

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Abstract

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The influence of K+ and Ca2+ on the sorption of nonionic aromatic contaminants

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(1,4-dinitrobenzene and p-xylene) on a series of microporous zeolite minerals (HZSM-5) with

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various surface charge densities were investigated. For zeolites with high or low charge density

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(> 1.78 sites/nm2 or < 0.16 sites/nm2), K+ and Ca2+ had negligible influence on the sorption of

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organics, which mainly occurred at the hydrophobic nanosites. For zeolites with charge density

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in the moderate range (0.16~1.78 sites/nm2), the sorption of organics was strongly dependent

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on cation hydration effect. K+ with lower hydration free energy greatly favored sorption of

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organics to the micropores compared to Ca2+. Differential scanning calorimetry and X-ray

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photoelectron spectroscopy results indicated that K+ can reduce the water affinity and promote

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specific sorption of organics in the zeolites with moderate charge density. The above

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mechanisms were successfully applied to explain the retention of 1,4-dinitrobenzene and

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p-xylene on four natural minerals (smectite, illite, kaolinite and mordenite). This study shed

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new insights on how cation hydration influences sorption interactions of nonionic aromatic

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contaminants at mineral-water interfaces as a function of mineral charge density.

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TOC Graphic

45 0.60

Mex+

K+

Mex+

sorption Mex+ Ca2+

Surface charge density

Mex+

SiO2/Al2O3=5.4 Blank + K 2+ 0.30 Ca

0.45

0.15

Qe (mmol/kg)

Mex+

Mex+

750 SiO2/Al2O3=46 600 450 300 150 800

SiO2/Al2O3=81

Mex+

700 600 500 0.00

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0.09 0.18 Ce (mM)

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0.27


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Introduction

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Molecular-scale chemical and biochemical processes at mineral-water interfaces (MWI) play

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important roles in the fate and transport of contaminants in soils and sediments. For soils and

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sediments with low organic carbon (< 0.1%), sorption of contaminants into the mineral phase is

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commonly assumed to be the major mechanism for contaminant retention.1-3 In particular, clay

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minerals such as smectites, palygorskite, and zeolites are critical players in transport and fate of

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contaminants and have drawn significant attention as suitable materials for environmental

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remediation.4-12

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Micropores (< 2 nm) are ubiquitous in clay minerals and have been shown to determine

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contaminant sorption and transport through both chemical and physical interactions in limited

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flow regimes. Sorption capacities of microporous minerals are generally high due to their large

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specific surface area and strong sorption potential.13,14 The desorption rate, however, is

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typically slow due to hindered diffusion. Previous studies indicated that both biotic and abiotic

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transformations of organic contaminants can be inhibited in micropores.3 As a result, sorption

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in mineral micropores can increase the persistence of organic contaminants in the subsurface.3

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Therefore, the understanding of sorption interactions of organic contaminants with mineral

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micropores is of significant importance in assessing the long-term ecotoxicological risk of

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organic contaminants in soils and sediments and in designing microporous mineral materials

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for contaminant elimination. 4 ACS Paragon Plus Environment

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Possible interactions for organic contaminants with mineral micropores include: (1) ion

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exchange of organic cations; 15-18 (2) site-specific interactions between exchangeable cations in

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micropores and organic substituents carrying a partial negatively charged function groups; 15,

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19-24

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cations or the terminal hydroxyls with organic contaminants;

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donor-acceptor (EDA) interactions between aromatic rings of contaminants and oxygens of

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siloxane surfaces; 19, 26-28 (5) nonspecific van der Waals interactions between the neutral portion

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of the organic guest species and the clay siloxane surface.20, 21 Hence, sorption of organic

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contaminants into mineral micropores is dependent on their molecular structure, the

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characteristics of mineral micropores, and the specific ionic composition and pH of the

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aqueous phase.

(3) hydrogen bond interactions of the water molecules surrounding the exchangeable 16, 21, 22, 25

(4) electron

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Metal ions commonly found in natural environment (Na+, K+, Ca2+, Mg2+) can often replace

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the original cations in minerals via ion exchange, and dramatically influence the sorption of

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organic contaminants at mineral-water interfaces. This is because they could control clay

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interlayer environments and interact with sorbed organic contaminants.18,

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Previous

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studies have focused on the hydration effects of metal ions on the sorption behaviors of

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organics such as nitroaromatic compounds (NACs),22, 26, 30, 32, 33 pesticides,20, 23 organic cations

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(CXHYN amines)34,

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minerals that were exchanged with weakly hydrated cations (e.g., K+, Cs+, NH4+) was

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in clay-water systems. It has revealed that sorption of organics to



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significant. However, the presence of strongly hydrated cations (e.g., Ca2+, Mg2+) led to lower

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sorption.19-23, 26, 30-32, 34, 35 This is due to that uncharged siloxane surfaces between charged sites

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on mineral surface are relatively hydrophobic and can interact with nonpolar moieties of

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organic contaminants that access these mineral surfaces. When minerals are saturated with

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strongly hydrated divalent cations, the water cluster surrounding exchangeable cations

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prohibits organic molecules from approaching the hydrophobic domains, thus weakening their

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interactions. Besides, strongly bounded water or strongly hydrated adsorbed cations, can block

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the electron donor sites and thus reduce the strength of interactions between exchangeable ions

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and polar functional groups in organic contaminants.19, 20, 22, 24, 28

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Up to now, although the hydration effect of metal ions on the sorption behaviors of organics

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to mineral micropores has been extensively studied,20, 22, 23, 26, 30, 32-35 the interactive role of

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surface chemistry, especially the surface charge density of mineral micropores on these

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processes was not fully understood. Boyd and co-workers have done a lot of work on the

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sorption of organics on minerals especially smectite and provided significant mechanistic

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understanding on the effect of cation hydration and surface charge density on the sorption

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behaviors. 20-24 For example, they found that the sorption of NACs to a variety of smectite

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surfaces was controlled largely by the hydration characteristics of the exchangeable cations,

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which regulates cation-nitroaromatic complexation.21, 24. But the surface charge densities of the

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clay minerals they used were usually limited in intermediate range (e.g., CEC = 80-132

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cmol/kg).

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Recently, Droge et al., reported that surface charge densities of the 6 ACS Paragon Plus Environment

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minerals (with a relatively wider range) and the presence of inorganic cations Na+ and Ca2+ in

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solution significantly influenced the ion exchange sorption of a wide variety of organic cations

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(CXHYN amines) to phyllosilicate clay minerals such as illite, kaolinite, and bentonite.16, 35 But

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for nonionic organics, the main sorption mechanism is no longer ion exchange but one or more

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interactions include specific interactions with the exchangeable cations or water molecules

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surrounding the cations, EDA interactions, and nonspecific van der Waals interactions with the

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neutral siloxane surface.19, 22-24, 36, 37 How metal ions affect sorption of nonionic organics to

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mineral micropores especially related to surface charge density at sufficiently low or high

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charge density needs further investigation.

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In this study, we investigated the impact of two typical metal ions K+ and Ca2+ on the

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sorption behavior of nonionic aromatic organics onto microporous clay minerals with various

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surface charge densities. Two neutral molecules at typical environmental conditions but with

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different polarity, 1,4-dinitrobenzene and p-xylene, were selected as the representative nonionic

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aromatic organics. A series of HZSM-5 zeolites with various surface charge densities were

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used as the model microporous clay minerals. HZSM-5 zeolites are composed of [SiO4]4- and

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[AlO4]5- tetrahedra arranged in three dimensional frameworks by sharing oxygen atoms, which

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creates interconnecting cages and channels (Figure S1A). 38, 39 Although natural minerals such

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as smectite have sheet structures, they are also composed of tetrahedra [SiO4]4- and octahedra

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[AlO3(OH)3]6-, and the negative permanent charges created by isomorphic substitution are

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balanced by those of interlayer cations (Figure S1C, D). Therefore, the pore surface chemical

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properties of both natural minerals and zeolites are similar despite their physical difference in 7 ACS Paragon Plus Environment


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pore structure. We chose zeolites as model minerals because of their high microporosity,

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well-defined pore structure, and particularly a wide range of surface charge density that can be

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adjusted by dealumination. To reveal the reaction mechanisms, differential scanning

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calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) analyses were conducted to

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probe coordination environments of nonionic organics and water on mineral micropores in the

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absence and presence of K+ and Ca2+. To demonstrate that the sorption mechanisms proposed

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from the zeolite studies are applicable to natural minerals, sorption of 1,4-dinitrobenzene and

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p-xylene onto four natural aluminosilicate minerals (smectite, illite, kaolinite and mordenite)

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with different micropores structures (Figure S1) were also performed. The results from this

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study may provide a better understanding of the fate of nonionic organic contaminants in soils

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and sediments, and will facilitate successful environmental remediation procedures.

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Experimental Section

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Mineral Sorbents. A series of HZSM-5 zeolites (SiO2/Al2O3 = 5.4, 27, 36, 46, 81, 470) that

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have a well defined micropore structure and varying surface cation densities were supplied by

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Nankai University Catalyst Co., Ltd. (Tianjin, China). Natural minerals smectite, illite, and

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kaolinite were supplied by Aladdin Reagent (Shanghai, China) while mordenite was obtained

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from Longyue Filter Material Co., Ltd. (Gongyi, China). Table 1 summarizes the key

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properties of the sorbents used in this study. To remove the non-structural water, mineral

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sorbents were calcined at 380 oC for 12 h before use. 8 ACS Paragon Plus Environment

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Chemicals. Standards of 1,4-dinitrobenzene (1.4-DNB, 99.9%) and P-xylene (PX, 99.5%)

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were purchased from Aladdin Reagent (Shanghai, China). Table 2 summarizes the selected

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properties of these two compounds. Stock solutions of K+ and Ca2+ were prepared from KCl

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(≥99.5%) and CaCl2 (≥96.0 %), respectively, both of which were obtained from Ling Feng

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Chemical Reagent (Shanghai, China). HPLC grade methanol was supplied by CNW

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Technologies (Dusseldorf, Germany). Laboratory distilled, deionized water (resistivity of 18.2

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MΩ·cm) was used in preparation of all aqueous solutions. All samples were stored at 4 oC in

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the dark, and analyzed within 2 days.

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Batch Sorption Experiments. Sorption of 1,4-DNB and PX on a series of HZSM-5 zeolites

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with SiO2/Al2O3 ratios of 5.4, 27, 36, 47, 81 and 470, and four natural minerals (smectite, illite,

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kaolinite and mordenite) was conducted with batch experiments at 25 °C. The solid-to-solution

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ratios were adjusted to ensure 20−80% uptake of initially added sorbates. For the sorption of

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1,4-DNB, 10-30,000 mg of HZSM-5 zeolites or natural minerals were transferred to 250 mL

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brown glass bottles containing 150 mL solutions of KCl (0-100 mM) or CaCl2 (0-50 mM) and

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1,4-DNB (0.05-0.30 mM). For the sorption of PX, 6.4-12,800 mg of sorbents were added to 64

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mL serum bottles. To avoid solute evaporation loss, the serum bottles were filled up with PX

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solutions (0.1-0.5 mM) in the absence or presence of KCl/CaCl2 until less than 1 mL headspace

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was left. The calculation of Setchenow constant and molar activity corresponding to the

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investigated ionic strength is provided in SI (Figure S2). Control bottles without sorbents were 9 ACS Paragon Plus Environment


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also carried out in parallel. The bottles were immediately sealed and agitated in the dark at 120

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rpm for 12 h, preliminary tests indicated that apparent equilibrium can be achieved in less than

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2 h (Figure S3). The supernatant was then withdrawn from each bottle and filtered with 0.22

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μm PTFE membrane filter. After filtration, 1,4-DNB or PX in the solution was determined

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using high-performance liquid chromatography (HPLC, Agilent 1260) equipped with a 4.6 mm

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×100 mm reverse-phase Eclipse Plus-C18 column and an UV detector. The HPLC mobile

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phase was methanol/water (50:50, v/v) for 1,4-DNB and acetonitrile/methanol/water (60:20:20,

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v/v) for PX with flow rate of 1.0 mL/min. The UV wavelength used for 1,4-DNB and PX was

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262 nm and 210 nm, respectively. Preliminary experiments have suggested that adding K+ or

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Ca2+ to exchange with HZSM-5 first and then adding organics to achieve sorption equilibrium

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did not resulted in any sorption difference.

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DSC Analysis. DSC analysis was carried out using a TA Q2000 DSC (New Castle DE,

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America) to study the thermal dehydration behavior of the wet zeolites before and after being

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exchanged with K+ and Ca2+. K+ was exchanged into the framework cages and channels of

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HZSM-5 (5.4), HZSM-5 (46), and HZSM-5 (470) by equilibrating 5 g of dry zeolite in 100 mL

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of 1.0 mol/L KCl solution for 48 h under constant stirring. The treated zeolite was separated

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from the solution by filtration followed by rinsing with deionized water to remove the excess

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K+, and calcined at 120 oC for 12 h and then 380 oC for another 12 h. Potassium-exchanged

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ZSM-5 zeolites were finally obtained by repeating this process twice. Calcium-exchanged

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zeolites were prepared similarly except 1.0 mol/L CaCl2 solution was used. Wet zeolites were 10 ACS Paragon Plus Environment

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prepared by equilibrating these zeolites with saturated water vapor for longer than 3 months,

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thus their micropores were saturated with water.10 Powders of wet zeolites were loaded into

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alumina pans and heated under a flow of 60 mL/min dry nitrogen to 200 oC at a rate of

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5 °C/min. The heat flow between the pan loaded with zeolite sample and an empty alumina

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sample pan, which was used as reference, was recorded.

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XPS Analysis. XPS analysis was performed by an ESCALAB 250 Xi XPS system (Thermo

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Fisher Scientic, England) and excited by monochromatic Al Ka radiation (1486.6 eV). All

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binding energies were calibrated using the C(1s) carbon peak (284.6 eV). Binding energies

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were measured with a precision of ±0.05 eV.

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Results and Discussion

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1. Impact of K+ and Ca2+ on 1,4-DNB and PX sorption to mineral micropores

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The sorption of both 1,4-DNB and PX on all six zeolites (SiO2/Al2O3 = 5.4, 27, 36, 46, 81,

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and 470) in the absence and presence of K+ and Ca2+ (0.01 M) exhibited Freundlich-type

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isotherms (Figure 1). The values of Freundlich coefficients (kf) and Freundlich exponents (nf)

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are summarized in Table S1 and S2. The nonlinear isotherms suggested that sorption sites in

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zeolites are heterogeneous. An increase of kf, indication of a higher sorption capacity of

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zeolites, was observed with increasing SiO2/Al2O3 ratio. To quantitatively compare the sorption 11 ACS Paragon Plus Environment


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capacity, the sorption distribution coefficients (Kd=Qe/Ce) at a fixed initial concentration (0.08

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mM) within linear portion of all isotherms were calculated and listed in Table S1 and S2.

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When SiO2/Al2O3 ratio of HZSM-5 zeolites increased from 5.4 to 470, Kd value increased from

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1.82 to 6192.38 L/kg for 1,4-DNB and 2.27 to 34428.57 L/kg for PX, respectively, in the

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absence of K+ or Ca2+. Since the BET surface area of these zeolites are comparable, this

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increase of sorption can be explained by that dealumination lowers the surface charge density

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and weakens the electrostatic field in the zeolites micropores. In general, zeolites with higher

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SiO2/Al2O3 ratios have more hydrophobic micropores, and thus possess greater sorption

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capacity towards organic sorbates in the presence of water.10, 11 Furthermore, it appears that the

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increase of sorption capacity with SiO2/Al2O3 ratio for 1,4-DNB was less significant than that

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for PX (Figure S4A). Particularly, for 1,4-DNB, the Kd value was almost a constant when

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SiO2/Al2O3 ratios were at the range of 81 to 470. But for PX, the Kd value increased more than

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three times with the increase of SiO2/Al2O3 ratio from 81 to 470. With an octanol-water

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partition coefficient (log Kow) of 1.49,43 1,4-DNB is less hydrophobic than PX (log Kow =

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3.27).43 Meanwhile, 1,4-DNB has two highly polar nitro groups so that it is able to enter the

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hydration shell and interact with cations and water molecules in the micropores.36 Besides

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hydrophobic interaction, 1,4-DNB can also sorb to minerals via electron donor-acceptor (EDA)

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interactions between aromatic rings and lone electron pairs of siloxane oxygens, and through

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formation of inner- or outer-sphere complexes between the –NO2 groups and weakly hydrated

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exchangeable cations (eg., K+, Cs+) in mineral micropores, as illustrated in Figure 2A.19, 22, 26

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In contrast, PX is a relatively nonpolar aromatic molecule with two prominent hydrophobic 12 ACS Paragon Plus Environment

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alkyl groups. It is much less likely to move into the hydrophilic sorption sites. In this case,

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nonspecific hydrophobic interaction is the dominant mechanism, and specific cation-π EDA

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interactions may be also involved in the sorption processes (Figure 2B).19, 28 This explains the

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weaker correlation of 1,4-DNB sorption affinity with surface hydrophobicity of mineral

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micropores than PX.

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It is interesting to find that 1,4-DNB sorption behaviors on HZSM-5 zeolite with SiO2/Al2O3

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ratio of 5.4 in the absence and presence of K+ and Ca2+ were almost identical. While for

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zeolites with SiO2/Al2O3 ratio of 27, 36, and 46, sorption of 1,4-DNB was significantly

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enhanced in the presence of K+ while the presence of Ca2+ had little effect. When SiO2/Al2O3

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ratio was further increased to 81 and 470, sorption of 1,4-DNB was not influenced by both K+

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and Ca2+, same as the case with SiO2/Al2O3 ratio of 5.4. For PX, similar trend was observed. It

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was observed that the metal loading on zeolites increased with the increase of surface charge

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density, and the uptakes of K+ and Ca2+ on zeolites with same surface charge density are

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comparable (Table S3). It is also found that ionic strength (consequently molar activity) in the

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range of natural environmental from 0.01 to 0.15 M had negligible effect on sorption of

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1,4-DNB or PX on zeolites (Figure S5). Therefore, the effects of K+ and Ca2+ on sorption of

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1,4-DNB and PX to the zeolites are likely caused by their different cation hydration effect.

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To further quantify this effect of cation hydration on 1,4-DNB and PX sorption, the Kd

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values in the presence of K+ (Kd(K)) and Ca2+ (Kd(Ca)) as a function of SiO2/Al2O3 are 13 ACS Paragon Plus Environment


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compared in Figure S4B. For 1,4-DNB, the Kd(K)/Kd(Ca) values increased from 0.9 to 2.7 as

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SiO2/Al2O3 ratios increased from 5.4 to 46, then diminished to 1.0 as SiO2/Al2O3 ratio further

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increased to 470. For PX, the sorption enhancement by K+ compared to Ca2+ was much weaker

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than 1,4-DNB. The Kd(K)/Kd(Ca) values increased by only 60% as SiO2/Al2O3 ratios increased

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from 5.4 to 46, then decreased.

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Exchangeable cations associated with minerals influence organic contaminants sorption by

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changing interface environments. Previous studies reported that minerals loaded with weakly

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hydrated cations (e.g., K+, Cs+) manifested a higher affinity for organics compared with

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minerals loaded with more strongly hydrated cations (e.g., Ca2+) because of the lower blocking

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effect of water.19, 20, 26, 31 1,4-DNB and PX showed high affinity for zeolites with moderate

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charge densities (SiO2/Al2O3 ratios of 27, 36, and 46）in the presence of K+, which imply the

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development of specific or nonspecific interactions between sorbate molecules and sorbent.

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The comparatively low hydration enthalpy of K+ (-78.6 kcal/mol)44 results in a smaller

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hydration sphere surrounding it. This facilitated the specific interactions of polar functional

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groups (-NO2) of the 1,4-DNB with K+ and/or polarized cation-bridging water molecules, as

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well as the cation-π interaction between K+ and the planar surface of PX which has an aromatic

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π-donor system. It also resulted in less obscuration of zeolites surfaces between exchangeable

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cations, hence make uncharged regions of the basal siloxane surfaces (hydrophobic nanosites)

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accessible for organics.20, 21, 30 While for zeolite with too high charge density (SiO2/Al2O3 ratio

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of 5.4), the distance between the sorbed cations was very close, water molecules in the 14 ACS Paragon Plus Environment

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micropores were strongly bounded due to the overlap of cationic electric effects. Organics

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sorption was strongly inhibited by both K+ and Ca2+, as the large hydration shell of metal

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cations may intrude or shield the sorption sites for organics in pore spaces, only a very weak

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sorption at the uncharged regions can be allowed. When the charge density of the zeolite was

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relatively low (SiO2/Al2O3 ratio of 81 and 470), the sorbed cations were farther apart on the

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basal internal surface, water molecules in the micropores were loosely bounded and can be

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easily out-competed by organics, even in the presence of strongly hydrated adsorbed Ca2+.

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Sorption at the hydrophobic nanosites was the dominant mechanism due to the very low charge

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density. As a result, metal ions with different hydration enthalpy caused little difference on the

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sorption behaviors of organics in these two cases. These results imply that during the

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assessment of the impact of metal ions on the sorption interactions of nonionic organics at

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mineral-water interfaces, charge density of minerals should be taken into account.

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2. DSC analysis of wet zeolites and implications for organics sorption

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To help understand the sorption behaviors of organics, interactions between the sorbed water

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and micropore surfaces were investigated by DSC analysis. Water molecules are present in the

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zeolite micropores in several types: coordinated water which is bonded to surface cations,

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“zeolitic water” that is hydrogen bonded with coordinated water molecules or silanol groups,

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and “loosely bound” water filling the hydrophobic pore spaces via capillary condensation.13,

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45-47

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heating, whereas the coordinated water needs more drastic conditions (several hundred oC).48, 49

“Loosely bound” water and “zeolitic water” are relatively easy to be removed upon



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It was reported that wet silicoaluminate zeolites had major, rapid mass losses between 30 and

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200 oC, with slower and less significant mass losses at higher temperatures.49, 50 All samples

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exhibited large endothermic peaks in the range of 30-200 oC (Figure 3), which are attributed to

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the removal of loosely bound and zeolitic water. These water molecules have weak interactions

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with micropore surfaces and can be easily out-competed by organic compounds.

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It is noteworthy that the major endotherm temperatures and dehydration enthalpies obtained

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from DSC curves decreased as the SiO2/Al2O3 ratio of zeolite increased (Table S4). Besides,

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the endotherm peaks observed for the zeolites with SiO2/Al2O3 ratio of 5.4 were larger and

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broader than zeolites with SiO2/Al2O3 ratios of 46 and 470. This suggests that the overall water

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affinity for zeolites of low SiO2/Al2O3 ratios was much higher than those of high SiO2/Al2O3

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ratios due to the high charge density that resulted in strong electrostatic interactions among

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cations, water molecules, and frameworks in zeolite micropores.51 Thus, sorption of organics

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through competing with water molecules was lowered in the zeolites of low SiO2/Al2O3 ratios

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(Figure 1).

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The water affinity of zeolite micropores also depends on the types of surface cations. For

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zeolites with SiO2/Al2O3 ratio of 5.4, it appears that Ca/ZSM-5 (5.4) peaked at higher

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temperature (98 oC) than HZSM-5 (5.4) (86 oC) and K/ZSM-5 (5.4) (83 oC) (Figure 3A). With

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smaller ionic radius and higher ion electric charge, Ca2+ has higher hydration free energy

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(-369.9 kcal/mol ) than H+ (-252.6 to -262.5 kcal/mol) and K+ (-78.6 kcal/mol),44, 52 leading to 16 ACS Paragon Plus Environment

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stronger interaction with water molecules in Ca/ZSM-5 (5.4) than in HZSM-5 (5.4) and

336

K/ZSM-5 (5.4).51 The dehydration behaviors of zeolites with higher SiO2/Al2O3 ratio of 46 (i.e.,

337

HZSM-5 (46), K/ZSM-5 (46), and Ca/ZSM-5 (46)) were quite different, and more positively

338

correlated to the hydration free energies of the surface cations (Figure 3B). It was found that

339

K/ZSM-5 (46) displayed a single endotherm peak at 42 oC, HZSM-5 (46) exhibited a major

340

endotherm peak at 45 oC with a small shoulder at around 78 oC, while for Ca/ZSM-5 (46), two

341

overlapping endotherm peaks at 45 oC and 78 oC were observed. The DSC results clearly

342

indicate that water binding in K/ZSM-5 (46) was relatively weaker and can be more easily

343

removed compared to Ca/ZSM-5 (46), which held water molecules more tightly and retained to

344

higher temperatures. However, when SiO2/Al2O3 ratio further increased to 470, negligible

345

difference in the endotherm temperatures was observed with zeolites in the forms of H+, K+,

346

and Ca2+. All zeolites (HZSM-5 (470), K/ZSM-5 (470), and Ca/ZSM-5 (470)) exhibited a low

347

endothermic peak at approximately 41 oC. The low temperature endothermic peak indicated the

348

relatively loosely bounded water in these zeolites, which resulted in greater sorption of

349

organics through displacing water. Overall, the DSC results were consistent with the sorption

350

behaviors of organics.

351 352

3. N1s XPS spectra

353 354

N1s XPS spectra of 1,4-DNB sorbed on HZSM-5 zeolites with different SiO2/Al2O3 ratios in

355

the absence and presence of K+ and Ca2+ were obtained. As shown in Figure 4, the N1s spectra 17 ACS Paragon Plus Environment


356

of 1,4-DNB can be fitted into three peaks at 400, 402, and 406 eV. The peak at 400 eV was

357

assigned to the C-N structures.53, 54 The most intense feature at 402 eV was attributed to the

358

nitro groups of the 1,4-DNB molecules when they were weakly adsorbed on hydrophobic

359

siloxane surfaces via electron donor-acceptor (EDA) interactions between aromatic rings and

360

lone electron pairs of siloxane oxygens. The same peak was observed with nitro-compounds

361

sorbed on clean silicon surface in previous studies.55, 56 The signal at 406 eV can be attributed

362

to the -NO2 groups which did not interact with zeolite surfaces.53, 54, 57, 58

363 364

For zeolite with high charge density (SiO2/Al2O3 ratio of 5.4), the N1s XPS spectrum shows

365

only the presence of 400 and 402 eV peaks, suggesting 1,4-DNB can only sorb on the

366

hydrophobic siloxane surface. While for zeolite with moderate (SiO2/Al2O3 ratio of 46) or low

367

charge density (SiO2/Al2O3 ratio of 470), all three peaks can be observed. This indicates that

368

besides hydrophobic sorption (402 eV), specific interactions between 1,4-DNB and surface

369

cations were also involved. 1,4-DNB may bound to surface cations or cation-bridging water

370

molecules by one of its -NO2 groups, making the other -NO2 group unreacted to either

371

hydrophobic siloxane sites or surface cations, thus produce the peak at 406 eV for these two

372

zeolites. Furthermore, for HZSM-5 (5.4) and HZSM-5 (470), the ratio of the N1s (400 eV) and

373

N1s (402 eV) peak areas N1s (400 eV):N1s (402 eV), was comparable in the absence and

374

presence of K+ and Ca2+ (i.e., 0.2-0.3 for HZSM-5 (5.4) and 0.4-0.6 for HZSM-5 (470),

375

respectively). But for HZSM-5(46), N1s (400 eV):N1s (402 eV) ratio was much higher (1.0),

376

and significantly increased to 2.4 in the presence of K+, while Ca2+ had little effect (Table S5). 18 ACS Paragon Plus Environment

Page 18 of 32

Page 19 of 32


377

We can infer that the specific interactions between -NO2 groups and surface cations were most

378

favored, and closely related to hydration ability of the sorbed cations in the zeolites with

379

moderate charge density. Previous studies have reported that the cation-NO2 interaction can

380

cause the increase of bond strength of C-N (400 eV) for nitroaromatic molecules compared to

381

uncomplexed counterparts.21

382 383

4. Impact of K+ and Ca2+ on 1,4-DNB and PX sorption to natural minerals

384 385

Sorption of 1,4-DNB and PX on four natural minerals, smectite, illite, kaolinite and

386

mordenite was also performed (Figure 5). It is noteworthy that the four minerals have more

387

significant difference in surface area than in surface charge density (Table 1), which is contrast

388

to the characteristics of the HZSM-5 zeolites. Therefore, although smectite has the highest

389

charge density (1.20 sites/nm2), it still had the highest sorption capacity for 1,4-DNB and PX

390

with the BET surface area of 240 m2/g, followed by illite (0.96 sites/nm2), mordenite (0.82

391

sites/nm2) and kaolinite (negligible), with BET surface areas of 69, 24 and 14 m2/g,

392

respectively, in the absence of K+ or Ca2+. Nonetheless, the effects of K+ and Ca2+ on 1,4-DNB

393

and PX sorption on these minerals exhibited the same trend as on HZSM-5 zeolites. With the

394

decrease of the surface charge density of the minerals, the Kd(K)/Kd(Ca) value for 1,4-DNB

395

adsorption increased first from 1.4 (smectite) to 4.9 (illite) and 29.7 (mordenite) and then

396

decreased to 1.1 (kaolinite). While for PX, the impact of K+ and Ca2+ was less significant, the

397

Kd(K)/Kd(Ca) value increased from 1.4 (smectite and illite) to 2.1 (mordenite) then decreased 19 ACS Paragon Plus Environment


398

to 1.2 (kaolinite) as the surface charge density of the minerals decreased (Table S6 and S7).

399

These results further confirm that even for minerals with very different structures and sorption

400

capacities, the impact of K+ and Ca2+ on the sorption of nonionic organics on mineral

401

micropores is dependent on the surface charge density of the minerals. When the surface

402

charge density of minerals is too high or too low, K+ and Ca2+ caused little difference in the

403

sorption behaviors of nonionic organics

404 405

Environmental Implications

406 407

Exploring the sorption mechanism of organic contaminants at mineral-water interfaces is of

408

great importance in resolving their fate, transport, and elimination in the environment. In

409

addition to the sorbent and sorbate properties, metal ions commonly found in nature (Na+, K+,

410

Ca2+, Mg2+) also influence the interaction processes as they can replace the original cations in

411

minerals via ion exchange and change interface environments. The results from this study

412

demonstrate that the influence of metal ions (i.e. K+ and Ca2+) on the sorption of nonionic

413

organics (i.e. 1,4-DNB and PX) to mineral micropores is dependent on the charge density of

414

the minerals. When minerals with relatively high or low charge density, hydrophobic

415

interaction is the dominant mechanism and the specific interactions that caused by different

416

cation hydration can be neglected. As a result, metal ions with different hydration enthalpy

417

cause little difference on the sorption behaviors of organics in these two cases. When the

418

charge density of minerals is in the moderate range, hydrophobic partitioning and specific 20 ACS Paragon Plus Environment

Page 20 of 32

Page 21 of 32


419

interactions are comparable. The presence of weakly hydrated cation (K+) manifest a much

420

higher affinity for organics compared with more strongly hydrated cation (Ca2+). The results

421

obtained from this work will shed new light on the impact of metal ions on the retention of

422

nonionic organics in mineral micropores, and build a more complete, and descriptive

423

understanding of the long-term transport behavior of these species in the environment.

424

Moreover, these new findings may have broad chemical and environmental implications for

425

future studies on remediation of nonionic organic pollutants by clay minerals, especially

426

metal-modified clay minerals.

427 428

Acknowledgement

429 430

This work was supported in parts by the National Natural Science Foundation of China

431

(21607131, 41877357) and the Natural Science Foundation of Zhejiang Province

432

(LR16E080003).

433 434

Supporting Information Available

435 436

Additional information on the Freundlich isotherm fits of 1,4-DNB and PX sorption on

437

ZSM-5 zeolites and natural minerals, sorption distribution coefficients (Kd) of 1,4-DNB and

438

PX onto HZSM-5 zeolites and comparison of the effects of K+ and Ca2+ (Kd(K)/Kd(Ca)) as a

439

function of SiO2/Al2O3 ratio, the uptakes of K+ and Ca2+ on zeolite surface during the ion 21 ACS Paragon Plus Environment


440

exchange, DSC data of the wet ZSM-5 zeolites, deconvolution results of XPS N1s spectra,

441

formation of micropores in the zeolites and sheet silicate minerals, sorption kinetics of

442

1,4-DNB and PX on HZSM-5 zeolites, and impact of ionic strength on sorption of 1,4-DNB

443

and PX on HZSM-5 zeolites.

444 445

Literature Cited

446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476

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NH4+, K+, Mg2+, and Ca2+ on the cesium adsorption/desorption in binding sites of vermiculitized biotite. Environ. Sci. Technol. 2017, 51, (23), 13886-13894. (30) Chatterjee, R.; Laird, D. A.; Thompson, M. L. Interactions among K+-Ca2+ exchange, sorption of m-dinitrobenzene, and smectite quasicrystal dynamics. Environ. Sci. Technol. 2008, 42, (24), 9099-9103. (31) Liu, C.; Gu, C.; Yu, K.; Li, H.; Teppen, B. J.; Johnston, C. T.; Boyd, S. A.; Zhou, D. Integrating structural and thermodynamic mechanisms for sorption of PCBs by montmorillonite. Environ. Sci. Technol. 2015, 49, (5), 2796-2805. (32) Jaynes, W. F.; Boyd, S. A. Hydrophobicity of siloxane surfaces in smectites as revealed by aromatic hydrocarbon adsorption from water. Clays and Clay Minerals. 1991, 39, (4), 428-436. (33) Charles, S.; Teppen, B. J.; Li, H.; Laird, D. A.; Boyd, S. A. Exchangeable cation hydration properties strongly influence soil sorption of nitroaromatic compounds. Soil Sci. Soc. Am. J. 70, 2006, 1480-1479. (34) Jolin, W. C.; Goyetche, R.; Carter, K.; Medina, J.; Vasudevan, D.; MacKay, A. A. Predicting organic cation sorption coefficients: Accounting for competition from sorbed inorganic cations using a simple probe molecule. Environ. Sci. Technol. 2017, 51, (11), 6193-6201. (35) Droge, S. T. J.; Goss, K.-U. Development and evaluation of a new sorption model for organic cations in soil: Contributions from organic matter and clay minerals. Environ. Sci. Technol. 2013, 47, (24), 14233-14241. (36) Qu, X.; Liu, P.; Zhu, D. Enhanced sorption of polycyclic aromatic hydrocarbons to tetra-alkyl ammonium modified smectites via cation-pi interactions. Environ. Sci. Technol. 2008, 42, (4), 1109-1116. (37) Ringwald, S. C.; Pemberton, J. E. Adsorption interactions of aromatics and heteroaromatics with hydrated and dehydrated silica surfaces by Raman and FTIR spectroscopies. Environ. Sci. Technol. 2000, 34, (2), 259-265. (38) Lone, K. G.; Stepanov, V. G.; Echevskii, G. V.; Paukshtis, E. A. Study of nature of the factors determining activity, stability and selectivity of zeolite catalysts. Zeolites 1984, 4 (2), 114-119. (39) Lukyanov, D. B. Effect of SiO2/Al2O3 ratio on the activity of HZSM-5 zeolites in the different steps of methanol conversion to hydrocarbons. Zeolites 1992, 12 (3), 287-291. (40) http://www.iza-structure.org/databases. (41) Gast, R. G. Surface and Colloid Chemistry. In J. B. Dixon and S. B. Weed (ed.) Minerals in soil environments. SSSA, Madison, WI. 1977, pp: 27-73. (42) Lee, J. F.; Mortland, M. M.; Chiou, C. T.; Kile, D. E.; Boyd, S. A. Adsorption of benzene, toluene, and xylene by two tetramethylammonium-smectites having different charge densities. Clays and Clay Minerals. 1990, 38, (2), 113-120. (43) Lee, C. M.; Rene P. Schwarzenback.; Philip M. Gschwend.; Dieter M. Imboden. Environmental organic chemistry, second edition. J. Chem. Educ. 2003, 80, 1199-1208. (44) Yu, H.; Whitfield, T. W.; Harder, E.; Lamoureux, G.; Vorobyov, I.; Anisimov, V. M.; MacKerell, A. D., Jr.; Roux, B. Simulating monovalent and divalent ions in aqueous solution using a drude polarizable force field. J. Chem. Theory Comput. 2010, 6, (3), 774-786. 24 ACS Paragon Plus Environment

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(45) Brauner, K.; Preisinger, A. Struktur und entstehung des sepioliths. Tschermaks Mineral. Petrogr. Mitt. 1956, 6(1-2), 120-140. (46) van Reeuwijk, L. P. The thermal dehydration of natural zeolites. Landbouwhogeschool. 1974. (47) Leherte, L.; Andre, J. M.; Derouane, E. G.; Vercauteren, D. P. Self-diffusion of water into a ferrierite-type zeolite by molecular-dynamics simulations. J. Chem. Soc., Faraday Trans. 1991, 87, (13), 1959-1970. (48) Ruiz-Hitzky, E. Molecular access to intracrystalline tunnels of sepiolite. J. Mater. Chem. 2001, 11, (1), 86-91. (49) Frost, R. L.; Ding, Z. Controlled rate thermal analysis and differential scanning calorimetry of sepiolites and palygorskites. Thermochimica Acta. 2003, 397, (1-2), 119-128. (50) Alver, B. E.; Sakizci, M.; Yorukogullari, E. Investigation of clinoptilolite rich natural zeolites from Turkey: a combined XRF, TG/DTG, DTA and DSC study. J. Therm. Anal. Calorim. 2010, 100, (1), 19-26. (51) Sun, P.; Navrotsky, A. Enthalpy of formation and dehydration of alkaline earth cation exchanged zeolite beta. Microporous Mesoporous Mater. 2008, 109, (1-3), 147-155. (52) Tawa, G. J.; Topol, I. A.; Burt, S. K.; Caldwell, R. A.; Rashin, A. A. Calculation of the aqueous solvation free energy of the proton. J. Chem. Phys. 1998, 109, (12), 4852-4863. (53) Nowak, A. M.; McCreery, R. L. Characterization of carbon/nitroazobenzene/titanium molecular electronic junctions with photoelectron and Raman spectroscopy. Anal. Chem. 2004, 76, (4), 1089-1097. (54) Guo, P.; Tang, L.; Tang, J.; Zeng, G.; Huang, B.; Dong, H.; Zhang, Y.; Zhou, Y.; Deng, Y.; Ma, L.; Tan, S. Catalytic reduction-adsorption for removal of p-nitrophenol and its conversion p-aminophenol from water by gold nanoparticles supported on oxidized mesoporous carbon. J. Colloid Interface Sci. 2016, 469, 78-85. (55) Tian, F.; Cui, Y.; Teplyakov, A. V. Nitroxidation of H-Terminated Si(111) surfaces with nitrobenzene and nitrosobenzene. J. Phys. Chem. C 2014, 118, (1), 502-512. (56) Perrine, K. A.; Leftwich, T. R.; Weiland, C. R.; Madachik, M. R.; Opila, R. L.; Teplyakov, A. V. Reactions of aromatic bifunctional molecules on silicon surfaces: Nitrosobenzene and nitrobenzene. J. Phys. Chem. C 2009, 113, (16), 6643-6653. (57) Ortiz, B.; Saby, C.; Champagne, G. Y.; Belanger, D. Electrochemical modification of a carbon electrode using aromatic diazonium salts. 2. Electrochemistry of 4-nitrophenyl modified glassy carbon electrodes in aqueous media. J. Electroanal. Chem. 1998, 455, (1-2), 75-81. (58) Toupin, M.; Belanger, D. Spontaneous functionalization of carbon black by reaction with 4-nitrophenyldiazonium cations. Langmuir 2008, 24, (5), 1910-1917.



600 601

Page 26 of 32

Table 1. Selected properties of the HZSM-5 zeolites and natural minerals used in this study

Sorbent

Formula

HZSM-5 (5.4)

H26 [Al26Si70O192]·16(H2O)b

SiO2/Al2O3 molar ratioa 5.4

Surface charge density (sites/nm2) 1.78c

BET surface Particle sizea areaf (m2/g) (μm)

Pore size a (nm)

700

1.00

0.56

27

0.45

c

365

1.00

0.56

H5.1 [Al5.1Si90.9O192]·16(H2O)b

36

0.35c

350

1.00

0.56

H4[Al4Si92O192]·16(H2O)b

46

0.27c

350

1.00

0.56

c

HZSM-5 (27)

b

H6.6 [Al6.6Si89.4O192]·16(H2O)

HZSM-5 (36) HZSM-5 (46)

b

HZSM-5 (81)

H2.3 [Al2.3Si93.7O192]·16(H2O)

81

0.16

360

1.00

0.56

HZSM-5 (470)

H0.4 [Al0.4Si95.6O192]·16(H2O)b

470

0.03c

365

1.00

0.56

Mordenite

Na8[Al8Si40O96]·24(H2O)a

10

0.82c

24

1.38

1.20

Smectite

(Na0.41K0.27Ca0.22)(Si7.51Al0.49)(Al2.58Fe0.54Mg0.52)O20(OH)4

a

―

1.20

d

240

1.20

1.10

Illite

(Na0.08K0.75Ca0.03)(Si7.30Al0.70)(Al3.71Fe0.05Mg0.19)O20(OH)4a

―

0.96d

69

2.50

1.09

Kaolinite

Al2(Si2O5)(OH)4a

―

―e

14

2.62

1.01

602 603

Notes:

604 605 606 607

a

Value reported by supplier. b Estimated from the SiO2/Al2O3 mole ratios and the general molecular formula of ZSM-5 [Nan [AlnSi96-nO192]·16H2O. c Estimated from the formula and cell parameters of zeolites reported in Database of Zeolite Structures. 40 d Estimated from the formula as described by Gast.41 e The exchangeable cation in kaolinite by isomorphous substitutions is negligible.42 f Determined in this study using 5-point BET surface area measurement


Page 27 of 32

608 609


Table 2. Selected physicochemical properties of organic compounds investigated.

Compound

Structure

Molecular Weight (g/mol)

Water Solubility Sw (mg/L)

Log Kow

pKa

NO2

1,4-Dinitrobenzene (1,4-DNB)

168.1

69.0

1.49

2.7

106.2

180.4

3.27

―

O2N CH3

p-Xylene (PX)

H3C

610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 27 ACS Paragon Plus Environment


Page 28 of 32

642

Qe (mmol/kg)

400

0.30

0.08

Ce (mM)

0.16

350

0.07

0.14

0.24

600

0.09

0.27

450 0.00

300

0.12

0.24

0.10

0.20

0.30

600 0.00

0.15

0.20

Ce (mM)

2500 HZSM-5 (81) + HZSM-5 (81) w/ K 2+ HZSM-5 (81) w/ Ca

2000

HZSM-5 (470) + HZSM-5 (470) w/ K 2+ HZSM-5 (470) w/ Ca

1500

1000

800

Ce (mM)

0.30

200

0 0.00

0.36

1000

160

0.20


Qe (mmol/kg)

320

0.24

100

1400

1200

0.16


Ce (mM)


0.10

0.08 Ce (mM)

Qe (mmol/kg)

Qe (mmol/kg)

0.18

180

60 0.00

0.30

640

0 0.00


750

Ce (mM)

480

900

120

0.3

0.24

1050

Qe (mmol/kg)

Qe (mmol/kg)

Qe (mmol/kg)

240

0.16 Ce (mM)

300 HZSM-5 (5.4) + HZSM-5 (5.4) w/ K 2+ HZSM-5 (5.4) w/ Ca

0.18

0.08

Ce (mM)

0.6

0.0 0.12

150 0.00

0.21

700

500 0.00

0.21

1.2

0.9

0.14


Ce (mM)

(B)

Ce (mM)

600

0.07

450

Qe (mmol/kg)

Qe (mmol/kg)

Qe (mmol/kg)

800


300


525

175 0.00

300

100 0.00

0.24

875

700

600

200

0.15

0.00 0.00

HZSM-5 (27) + HZSM-5 (27) w/ K 2+ HZSM-5 (27) w/ Ca Qe (mmol/kg)

HZSM-5 (5.4) + HZSM-5 (5.4) w/ K 2+ HZSM-5 (5.4) w/ Ca

Qe (mmol/kg)

0.45

750

500

0.60

(A)

0.09

0.18

0.27

Ce (mM)

500 0.05

0.10 Ce (mM)

Figure 1. Effects of K+ and Ca2+ (I = 0.01 M with KCl or CaCl2) on sorption of DNB (A) and PX (B) onto HZSM-5 zeolites with different SiO2/Al2O3 ratios (i.e., HZSM-5 (x), x: 5.4, 27, 36, 46, 81, and 470) at 25 oC. Qe is the amount of organics sorbed and Ce is the organics concentration in the water phase at equilibrium. The solid lines represent fitted Freundlich-type isotherms.

643 644 645 646 28 ACS Paragon Plus Environment

Page 29 of 32


647 (A)

NO2

NO2

H

H O2N

O2N

O

Si

(B)

Si

Si

O

Si

O

O

Si

Si

Al

Si

O

O

Si

H3C

O

Si

Si O

O Si

Si O

Si

Al

Si

O

H

H H3C

Si

H H O

CH3

CH3

O

O

O

O

hydrophobic interaction

O

H

H H H cation-polar x+ H O OMe O O

n-π EDA

O

O

O

H O

H

H

H H H cation-π O OMex+ O O Si

Al

Si

O

O

Si

Si

O

O H H O

Si

Al O

Figure 2. Illustration of the sorption mechanism of 1,4-DNB (A) and PX (B) sorption onto zeolite micropores.

648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 29 ACS Paragon Plus Environment


Page 30 of 32

670 30

Heat flow (mW)

(A)

25

Ca/ZSM-5 (5.4)

20

K/ZSM-5 (5.4)

HZSM-5 (5.4)

15 0

90

T (℃ )

180

270

22

Heat flow (mW)

(B)

20 K/ZSM-5 (46) Ca/ZSM-5 (46)

18

HZSM-5 (46)

16 0

90

T (℃ )

180

270

22

Heat flow (mW)

(C)

20

K/ZSM-5 (470)

Ca/ZSM-5 (470)

18

HZSM-5 (470)

16 0

90

T (℃ )

180

270

Figure 3. DSC curves of the wet ZSM-5 (5.4) (A), ZSM-5 (46) (B) and ZSM-5 (470) (C) zeolites in the temperature range of 30 to 200 °C (at a heating rate of 5 °C/min).

671


Page 31 of 32


(A)

HZSM-5 (5.4)

Intensity (a.u.)

+

K -HZSM-5 (5.4)

2+

Ca -HZSM-5 (5.4)

410

(B)

405

400

395

390

405

400

395

390

405

400

395

390

Binding Energy (eV)

HZSM-5 (46)

Intensity (a.u.)

+

K -HZSM-5 (46)

2+

Ca -HZSM-5 (46)

410

(C)

Binding Energy (eV)

HZSM-5 (470)

Intensity (a.u.)

+

K -HZSM-5 (470)

2+

Ca -HZSM-5 (470)

410

Binding Energy (eV)

Figure 4. XPS N1s core level spectra of 1,4-DNB sorbed on HZSM-5 with SiO2/Al2O3 ratios of 5.4 (A), 46 (B), and 470 (C) in the absence and presence of K+ and Ca2+. HZSM-5, K+-HZSM-5, Ca2+-HZSM-5 represent the zeolites that have absorbed 1,4-DNB in the absence and presence of K+ and Ca2+.



0.52

80

Qe (mmol/kg)

60

Smectite + Smectite w/ K 2+ Smectite w/ Ca

0.39

0.26

40

0.13

20

0 0.00

Mordenite + Mordenite w/ K 2+ Mordenite w/ Ca

Qe (mmol/kg)

(A)

0.00 0.06

0.12

0.00

0.18

0.03

10.0

0.08

0.16

0.24

0.000 0.00

0.04

0.12

Qe (mmol/kg)

Qe (mmol/kg)

0.09

Mordenite + Mordenite w/ K 2+ Mordenite w/ Ca

0.06

0.03

0.12

0.24

0.00 0.00

0.36

0.04

Ce (mM)

Ce (mM)

0.028

2.4 Illite + Illite w/ K 2+ Illite w/ Ca

0.021

Kaolinite + Kaolinite w/ K 2+ Kaolinite w/ Ca

Qe (mmol/kg)

Qe (mmol/kg)

0.08

0.12 Smectite + Smectite w/ K 2+ Smectite w/ Ca

1.0

0.014

1.2

0.007

0.6

0.0 0.00

0.12

Ce (mM)

2.0

1.8

0.08

0.005

4.0

0.0 0.00

0.12

0.010

Ce (mM)

3.0

0.08

Kaolinite + Kaolinite w/ K 2+ Kaolinite w/ Ca

Qe (mmol/kg)

Qe (mmol/kg)

0.015

2.5

(B)

0.09

0.020 Illite + Illite w/ K 2+ Illite w/ Ca

5.0

0.0 0.00

0.06 Ce (mM)

Ce (mM)

7.5

Page 32 of 32

0.15

0.30

0.45

0.000 0.00

0.04 Ce (mM)

Ce (mM))

Figure 5. Effects of K+ and Ca2+ (I = 0.01 M with KCl or CaCl2) on sorption of DNB (A) and PX (B) onto smectite, illite, kaolinite and mordenite at 25 oC. Qe is the amount of organics sorbed and Ce is the organics concentration in the water phase at equilibrium. The solid lines represent fitted Freundlich-type isotherms.

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Sorption of Non-ionic Aromatic Organics to Mineral Micropores

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