Enhanced adsorption of p-arsanilic acid from water by amine modified

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Enhanced adsorption of p-arsanilic acid from water by amine modified UiO-67 as examined using EXAFS, XPS and DFT calculations Chen Tian, Jian Zhao, Xinwen Ou, Jieting Wan, Yuepeng Cai, Zhang Lin, Zhi Dang, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05761 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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Environmental Science & Technology

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Enhanced adsorption of p-arsanilic acid from water by amine

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modified UiO-67 as examined using EXAFS, XPS and DFT

3

calculations

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Chen Tian, †,§ Jian Zhao, ‡,§ Xinwen Ou, † Jieting Wan, † Yuepeng Cai, ‖ Zhang Lin, *,†

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Zhi Dang, † and Baoshan Xing

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†

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Restoration in Industry Clusters (Ministry of Education), Guangdong Engineering and Technology

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Research Center for Environmental Nanomaterials, South China University of Technology,

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Guangzhou 510006, China

⊥

School of Environment and Energy, The Key Laboratory of Pollution Control and Ecosystem

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‡

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Science and Ecology (Ministry of Education), Ocean University of China, Qingdao 266100, China

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‖

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College of Environmental Science and Engineering, Key Laboratory of Marine Environmental

School of Chemistry and Environment, South China normal University, Guangzhou 510006, China

⊥

Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

§Equal

contribution

15 16 17

* Email: [email protected] (Dr. Zhang Lin); phone: 86-20-39380503; fax: 86-20-

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39380508

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1

ACS Paragon Plus Environment


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TOC

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2


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ABSTRACT

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p-Arsanilic acid (p-ASA) is an emerging organoarsenic pollutant comprising both

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inorganic and organic moieties. For the efficient removal of p-ASA, adsorbents with

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high adsorption affinity are urgently needed. Herein, amine modified UiO-67 (UiO-67-

27

NH2) metal-organic frameworks (MOFs) were synthesized, and their adsorption

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affinities towards p-ASA were 2 times higher than that of the pristine UiO-67. Extended

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X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and

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density functional theory (DFT) calculation results revealed the adsorption through a

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combination of As-O-Zr coordination, hydrogen bonding, and π-π stacking, among

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which As-O-Zr coordination was the dominant force. Amine groups played a

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significant role in enhancing the adsorption affinity through strengthening the As-O-Zr

34

coordination and π-π stacking, as well as forming new adsorption sites via hydrogen

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bonding. UiO-67-NH2s could remove p-ASA at low concentrations (< 5 mg L-1) in

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simulated natural and waste waters to an arsenic level lower than that in the drinking

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water standard of World Health Organization (WHO) and the surface water standard of

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China, respectively. This work provided an emerging and promising method to increase

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the adsorption affinity of MOFs towards pollutants containing both organic and

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inorganic moieties, via modifying functional groups based on the pollutant structure to

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achieve synergistic adsorption effect.

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3



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INTRODUCTION

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As a typical phenylarsonic acid compound, p-arsanilic acid (p-ASA) is an emerging

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micropollutant, since its concentrations in water and soil were reported to be ranged

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from 0.5 to 5000 μg L-1 and 0.2 to 1000 μg kg-1, respectively.1-3 p-ASA is widely used

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as animal feed additives, due to its broad antimicrobial and nontoxic properties.4,5

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However, after excreted in manure, the water-soluble p-ASA would be degraded into

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high-toxic inorganic arsenic species (such as arsenate) in 30 days via the biotic and

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abiotic routes, thus leading to severe arsenic pollution in soil and groundwater.6,7 For

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example, in the Pearl River Delta of southern China, the concentration of p-ASA was

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detected up to 771 μg kg-1 in the surface soils surrounding swine farms, and the arsenic

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content was found to be far beyond the local background level.2 Therefore, the removal

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of p-ASA is crucial for maintaining the quality and safety of water and soil.

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To date, studies on p-ASA removal mainly focused on two types of techniques

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including degradation and adsorption.8,9 Comparing with the former technique,

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adsorption exhibited the advantages of higher efficiency, simpler operation, and lower

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risk of releasing inorganic arsenic into the environment. Several types of adsorbents

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have been studied for p-ASA adsorption, including ferric and manganese binary

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oxide,10 iron humate,11 and metal-organic frameworks (MOFs).12 Among these

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adsorbents, MOFs are attracting more attentions because of their high porosity,

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controllable structure, and high adsorption capacity to p-ASA (up to 791 mg g-1).13

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However, the adsorption affinity of MOFs to p-ASA is relatively weak, since the

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adsorption is mainly depended on the electrostatic attraction and pore filling. Therefore, 4


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to achieve efficient and quick removal of p-ASA, especially at environmentally relevant

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concentrations (e.g., below 5 mg L-1), specific adsorption sites in MOF structures

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towards p-ASA are required.

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Our previous work revealed that roxarsone (ROX) could be efficiently adsorbed by

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Fe3O4-graphene nanocomposites through coordination, hydrogen bonding, and π-π

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stacking.14 Considering the similar structure of p-ASA with ROX, new strategies are

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expected to increase the adsorption affinity through the mentioned interactions. Such

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strategies should point to utilizing MOFs which contain metal clusters for the As-O-M

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(M stands for the metal clusters in MOFs) coordination, organic linkers with benzene

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rings for π-π stacking, and functional groups for forming hydrogen bonds with p-ASA.

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UiO-67 is a Zr-based MOF which comprises Zr6O4(OH)4 clusters as 12-connected

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nodes and linear 4,4’-biphenyldicarboxylic acid (BPDC) ligands.15 UiO-67 showed

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high application potential in practical water treatments due to its simple synthesis

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method and superior chemical stability.16 It consists of substantial octahedral (16 Å)

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and tetrahedral (12 Å) cages,17,18 and consequently can expose abundant active sites for

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p-ASA adsorption. Considering the -NH2 and -OH groups in p-ASA molecules, the

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incorporation of -NH2 groups to the BPDC ligands of UiO-67 (denote as UiO-67-NH2)

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is expected to enhance the adsorption affinity through hydrogen bonding.19-21 Moreover,

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other interactions are also expected between UiO-67-NH2 and p-ASA. It was reported

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that the missing-linkers in UiO-66 could induce defects in the Zr nodes, which could

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provide more Zr-OH groups and greatly enhance the capture of ROX via the stronger

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As-O-Zr coordination.22,23 Similarly, this effect is also predicted between the Zr nodes 5



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with defects in UiO-67-NH2 and p-ASA. In addition, the BPDC ligands in UiO-67-NH2

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are also expected to bind p-ASA through π-π stacking.24,25

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Therefore, we expect the defective UiO-67-NH2 to be a powerful and advanced

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material for the efficient adsorption of p-ASA from aqueous media. Herein, we aimed

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to: (1) determine the adsorption capacity, affinity and rate of defective UiO-67-NH2

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towards p-ASA in comparison with the pristine UiO-67, to confirm the enhanced

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adsorption of UiO-67-NH2; (2) analyze the contribution of As-O-Zr coordination, π-π

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stacking and hydrogen bonding to the adsorption via extended X-ray absorption fine

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structure (EXAFS), x-ray photoelectron spectroscopy (XPS), and density functional

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theory (DFT) calculations; and (3) investigate the mechanism of amine modification on

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the enhancement of adsorption affinity. This study provides useful information on the

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design of novel and efficient materials for p-ASA removing via understanding the

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adsorption mechanism, and offers a new alternative to increase the adsorption affinity

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via the synergistic effect between the introduced functional groups.

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EXPERIMENTAL SECTION

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Synthesis and characterization of UiO-67 and UiO-67-NH2s

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The defective UiO-67 and its -NH2 modified derivatives were prepared using

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biphenyl-4,4’-dicarboxylic acid (BPDC) as ligand by following previously reported

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methods.22,26 The missing-linker-induced defects in the MOFs were produced by adding

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concentrated HCl during preparation. Two types of UiO-67-NH2s with different amine

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contents were obtained. The as-prepared UiO-67-NH2 that theoretically had one amine 6


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group per BPDC ligand was abbreviated as UiO-67-NH2(1), while the other one that

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had two -NH2 groups per BPDC ligand as UiO-67-NH2(2).

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The crystal structure, surface morphology, specific surface area and surface charge

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of UiO-67 and UiO-67-NH2s were analyzed using X-ray diffraction (XRD), scanning

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electron microscope (SEM), Brunauer-Emmett-Teller (BET) N2 adsorption/desorption

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method and zeta potential analyzer, respectively. Zeta potentials of the materials at pHs

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3-10 were determined using a NanoBrook Omni zeta potential analyzer (Brookhaven,

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USA). The concentration of the tested MOFs was 1 g L-1, and the pH was adjusted by

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HNO3 or KOH solution. Details of the characterization are provided in Section S1.

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Adsorption experiments

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For adsorption kinetic experiments, 0.045 g adsorbents were added to 300 mL p-ASA

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(Aladdin Bio-Chem Technology, China) solution at a concentration of 50 mg L-1, and

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shaken at pH 4.0 ± 0.1 and 25 C with a speed of 200 rpm for 1 min to 24 h. The

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adsorption kinetics curves were fitted with the pseudo-second-order non-linear kinetic

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model (SI Section S1). Adsorption isotherm studies were conducted with the p-ASA

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concentrations from 1 to 100 mg L-1, while the adsorbent concentration was kept at 0.15

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g L-1 in 30 mL solution (pH 4.0 ± 0.1). The mixtures were shaken under 25 C with a

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speed of 200 rpm for 24 h. After centrifugation separation, the residual p-ASA

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concentrations in the supernatants were measured by a UV-vis spectrometer (UV-2600,

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Shimadzu) at 252 nm. The adsorption isotherms were fitted with Langmuir and

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Freundlich models (SI Section S1). The effect of pH on adsorption was investigated by

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conducting adsorption experiments within the pH range of 3.0-10.0 at the p-ASA 7



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concentration of 50 mg L-1 and the adsorbent dosages of 0.15 g L-1.

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To detect the reusability of UiO-67 and UiO-67-NH2s, each adsorbent (5 mg) was

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firstly added to 30 mL p-ASA solution (pH 4.0 ± 0.1). After shaking for 24 h (25 C,

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200 rpm), the obtained adsorbents after adsorption were moved to 30 mL desorption

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solution containing 0.5 mol L-1 NaOH/ethanol, and shaking with a speed of 200 rpm at

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25 C for 1 h. The regenerated adsorbents were dried in a vacuum oven at 110 °C for

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12 h before the next adsorption cycle. The removal of p-ASA by UiO-67-NH2s in

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simulated natural and waste waters was studied in the presence of dissolved organic

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matter (DOM, 0-16 mg C L-1) and in swine manure lixivium (added p-ASA of 1-5 mg

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L-1). Experimental details were displayed in Section S1.

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EXAFS and XPS analyses

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EXAFS and XPS were used to investigate the interaction between p-ASA and UiO-

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67s.27 The samples on p-ASA (50 mg L-1) adsorbed UiO-67 and UiO-67-NH2s (0.15 g

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L-1) were prepared at pH 4.0, in a manner similar to the adsorption experiments. The K-

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edge EXAFS spectra of As in p-ASA-loaded samples were acquired at the beamline

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4W1B of the Beijing Synchrotron Radiation Facility (BSRF). All EXAFS data were

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collected in the fluorescence mode at room temperature, using a Lytle detector equipped

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with Soller slits and Ge filter for screening scattering and fluorescence background. The

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maximum absorption edge As was set to 11875 eV. The EXAFS data analysis was

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performed using the ATHENA and ARTEMIS interfaces to the IFEFFIT version 1.2.11

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program package.28 Details of the EXAFS data collection and analysis are provided in

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Section S1. 8


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XPS spectra were analyzed using an Axis Ultra DLD instrument (Kratos Analytical,

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U.K.) with an A1 Kα X-ray source. Spectra were recorded at a pass energy of 160 eV

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for survey scans and 40 eV for high-resolution scans. High-resolution scans were

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carried out in an energy range of 295-280 eV for C 1s, and 405-395 eV for N 1s XPS

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spectra.

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Computational calculation

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DFT calculations were performed using the Vienna ab initio simulation package

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(VASP). The Perdew-Burke-Ernzerhof (PBE) functional, and the potential projector

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augmented wave (PAW) pseudo-potentials were used for all the calculations. The plane-

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wave cutoff energy was set to 500 eV, which has been previously tested and shown to

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be appropriate for UIO-67.29 Scalar relativistic effects were incorporated into the

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effective core potentials via explicit mass-velocity and Darwin corrections. Detailed

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calculation methods are provided in Section S1.

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RESULTS AND DISCUSSION

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Characterization of the as-prepared MOFs

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The as-synthesized UiO-67 and UiO-67-NH2s exhibited typical characteristic peaks

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of Fm-3m symmetric space groups in the XRD patterns (Figure S1), consistent with the

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reported UiO-67 topology.30 The identical XRD peaks of UiO-67-NH2s with the

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pristine UiO-67 implied that introducing -NH2 groups did not change the framework

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structure. FTIR spectra of UiO-67-NH2s showed C-N and N-H stretching vibrations at

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1257 and 3300-3500 cm-1, respectively (Figure S2). The mole ratios of -NH2:Zr 9



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calculated from XPS data were 0.88 and 1.80 for UiO-67-NH2(1) and UiO-67-NH2(2),

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respectively (Table S1), which were similar to their theoretical values (1.0 for UiO-67-

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NH2(1) and 2.0 for UiO-67-NH2(2)). These results confirmed the successful amine

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modification of UiO-67. The structural illustrations of UiO-67, UiO-67-NH2(1) and

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UiO-67-NH2(2) are shown in Figure 1. The BET surface areas of UiO-67, UiO-67-

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NH2(1) and UiO-67-NH2(2) were 1871, 750, and 465 m2 g-1, respectively (Figure S3,

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and Table S1). The lower surface areas of UiO-67-NH2s compared with the pristine

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UiO-67 were probably due to the partial blocking of the pores by -NH2 groups on the

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surface (Figure 1a-1c).

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The as-prepared MOFs are thermally stable up to 320 °C as detected by the TG-MS

185

(Figure S4) and temperature-dependent XRD (Figure S5). Therefore, based on the TGA

186

data on the weight loss at 320-500 °C (Table S2, Figure S6), the numbers of the missing-

187

linker induced defects for each unit of UiO-67, UiO-67-NH2(1) and UiO-67-NH2(2)

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were calculated as 4, 3.2 and 3.2, respectively. These defects would provide more

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binding centers towards p-ASA through the external Zr-OH groups,31 thus facilitating

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the p-ASA adsorption.

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Adsorption kinetics

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Adsorption kinetics of p-ASA by UiO-67 and UiO-67-NH2s was investigated and the

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results are shown in Figure 2a. All the kinetic curves were fitted using the pseudo-

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second order model, which was commonly used to describe adsorption kinetics in

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which chemisorption controlled the adsorption rate and the number of active sites

196

determined the adsorption capacity.32 The obtained kinetic parameters are shown in 10


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Table S3. Good fits of the tested MOFs with the pseudo-second order model (R2 > 0.99)

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implied the adsorption of p-ASA on the tested MOFs was controlled by chemical

199

interactions. The increased adsorption rates (k2) of UiO-67-NH2s relative to the pristine

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UiO-67 implied the enhancement of chemisorption between p-ASA and the amine

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modified MOFs. Moreover, in comparison with other porous MOFs (such as meso-ZIF-

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8 in ref.13), substantially enhanced adsorption rates were observed for UiO-67 and UiO-

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67-NH2s, which was possibly due to the stronger chemisorption between UiO-67s and

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p-ASA molecules.

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Adsorption isotherms

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Adsorption isotherms of UiO-67, UiO-67-NH2(1) and UiO-67-NH2(2) towards p-

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ASA are shown in Figure 2b. All isotherms were fitted well with Langmuir and

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Freundlich33,34 models (Table S4), but the R2 values of Langmuir model were higher.

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Therefore, the following discussion was mainly based on Langmuir-fitting results of all

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the MOFs. Maximum adsorption capacities (Qm) calculated from Langmuir model

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followed an order of UiO-67 > UiO-67-NH2(1) > UiO-67-NH2(2) (Figure 2b, Table S4).

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However, this order changed to an opposite sequence after normalizing by the surface

213

area (Qm/Asurf in Figure 2c), indicating higher adsorption capacities of UiO-67-NH2s

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than the pristine UiO-67 deducting the effect of surface area. This observation could be

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explained by the steric hindrance of the introduced -NH2 groups situating in the aperture

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of the nanopores (Figure 1 and Table S1). The entry of p-ASA (with a molecule

217

diameter of 6.625 Å13) to the internal part of UiO-67-NH2s would be obstructed because

218

of their narrower apertures than the pristine UiO-67. 11



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It was reported that materials with a small Qm and large affinity coefficient (KL) could

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have a larger value of qe when compared with materials with a large Qm and small KL,

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at low pollutant concentrations.35 Therefore, considering p-ASA as a micropollutant,

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the KL value was more significant than Qm. As shown in Table S4, KL values of UiO-

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67-NH2s were much higher than that of the pristine UiO-67, and increased with

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increasing -NH2 content. In order to further evaluate the adsorption affinity at different

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p-ASA concentrations, single point sorption coefficients (K)36 based on the Langmuir

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fitting results were calculated when Ce = 0.03 (K0.03) and 0.3 (K0.3) mg L-1. As shown

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in Table S4, both of K0.03 and K0.3 followed the order of UiO-67-NH2(2) > UiO-67-

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NH2(1) > UiO-67, indicating that UiO-67-NH2s had higher affinities towards p-ASA.

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The higher affinities made UiO-67-NH2s having higher qe compared with the pristine

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UiO-67 at low concentrations of p-ASA, as shown in the insert of Figure 2b. This is

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very important for the practical application of UiO-67-NH2s, considering that the

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concentrations of p-ASA in environment are usually very low.

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The comparison of adsorption capacity and affinity of the present UiO-67s with other

234

adsorbents in the published literature is listed in Table S5. The Qm value of UiO-67

235

(454.54 mg g-1) was higher than most of the reported absorbents. Despite that

236

mesoporous ZIF-8 had the highest Qm (791 mg g-1) and Qm/Asurf (0.70 mg m-2), UiO-67-

237

NH2(2) was more superior for p-ASA capture in practical applications due to its much

238

higher adsorption capacity and affinity toward p-ASA (qe0.03 = 4.50 mg g-1 and K0.03 =

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150 L g-1) than ZIF-8 (qe0.03 = 0.76 mg g-1 and K0.03 = 25.3 L g-1) at low concentrations.

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Reusability 12


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The regeneration efficiency of UiO-67s is shown in Figure S7. Despite the adsorption

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capacities slightly decreased after the first recycle, UiO-67 and UiO-67-NH2(2) still

243

exhibited considerable adsorption capacities after four cycles regeneration (over 85%

244

of the original capacity). FTIR analysis showed the -NH and C-N vibration bands still

245

present in UiO-67-NH2(2) after the first adsorption cycle (Figure S8), suggesting that

246

the -NH2 moieties remained stable during the repeated adsorption process. Meanwhile,

247

XRD results showed that the crystal structure of UiO-67 and UiO-67-NH2(2) did not

248

change after regeneration (Figure S9). These results revealed that the UiO-67 and UiO-

249

67-NH2(2) could be regenerated and reused for multiple times, thus confirming the

250

practical value and application of UiO-67s as the adsorbents of p-ASA.

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Mechanism of p-ASA adsorption on UiO-67-NH2s

252

To get insight into the adsorption mechanism of UiO-67-NH2s towards p-ASA,

253

electrostatic interaction, As-O-Zr coordination, hydrogen bonding, and π-π interaction

254

were taken into consideration.37 The effects of pH on the adsorption of p-ASA were

255

studied to investigate the contribution of electrostatic interaction. Figure S10a showed

256

that UiO-67 and UiO-67-NH2s exhibited the highest adsorption capacity at pH 4.0, and

257

then sharply decreased with the increasing pHs. The isoelectric points of the three UiO-

258

67s were determined to be between pH 4 and 5, and thus their zeta potentials were quite

259

low at pH 4.0 (Figure S10b). Moreover, the aqueous dissociation constants (pKa) of p-

260

ASA molecules were 1.91, 4.13 and 9.19, respectively (Figure S11). Therefore, the

261

arsenic group in p-ASA is electroneutral at pH 4.0.38 These results indicated that the

262

electrostatic attraction was not the dominant force for the p-ASA adsorption at pH 4.0. 13



263

It has been well-documented that Zr-based materials could efficiently capture

264

arsenate groups due to the bidentate binuclear and monodentate mononuclear

265

complexes forming between the Zr-OH groups and As.22,39 For the as-prepared UiO-67

266

and UiO-67-NH2s, these Zr-OH groups mainly originated from the defects present in

267

Zr-O clusters.23,40 To further reveal the role of As-O-Zr coordination in p-ASA

268

adsorption, x-ray adsorption spectroscopy of As was determined. The As-O and As-Zr

269

shells could be obviously isolated in the k3-weighted As K-edge EXAFS spectra and

270

the corresponding Fourier-transforms (Figure 3), indicating the formation of As-O-Zr

271

inner-sphere complexes after adsorption. In all the samples, the As-O first-neighbor

272

contributions were fit with 3.6-3.9 oxygen atoms at 1.69 Å (Table S6), which agreed

273

with the DFT calculation in the previous study.39

274

The second-neighbor contributions were fitted with the As-Zr atom distances of 3.44,

275

3.41, and 3.40 Å for UiO-67, UiO-67-NH2(1) and UiO-67-NH2(2), respectively (Table

276

S6). The As-Zr distances were consistent with that in the bidentate binuclear complex

277

obtained from the DFT calculation (see details in the DFT Calculation section), which

278

was also observed in other Zr-based MOFs.41 Moreover, the As-Zr distances in UiO-

279

67-NH2(1) and UiO-67-NH2(2) were shorter than that in UiO-67, indicating that the

280

amine modification of UiO-67 shortened the As-Zr atom distance, and thus formed

281

stronger inner-sphere complexes. The enhanced As-O-Zr coordination was one of the

282

explanations for the higher adsorption affinity of UiO-67-NH2s towards p-ASA than

283

the pristine UiO-67.

284

The higher coordination number (CN) of the As-Zr contribution in UiO-67-NH2s 14


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than the pristine UiO-67 (Table S6) also indicated stronger As-O-Zr coordination. Such

286

enhancement could be ascribed to the synergistic effect of other interactions, such as π-

287

π stacking and hydrogen bonding. Moreover, according to the Langmuir fitting results,

288

the maximum molecule uptake of p-ASA by UiO-67 was found to be 5.3:1 (p-ASA:Zr6

289

in Figure 4a), which exceeded the stoichiometric p-ASA:Zr6 value calculated by As-O-

290

Zr coordination (4:1 of p-ASA:Zr6 based on the bidentate binuclear complex forming

291

between p-ASA and the defective UiO-67 unit). Considering the absence of -NH2

292

groups in UiO-67 ligands, the hydrogen bonds between the pristine UiO-67 and p-ASA

293

were perceived to be relatively weak. Thus, the higher p-ASA uptake than the

294

stoichiometric value was possibly due to the π-π stacking. Further indication was

295

conducted with the high resolution C 1s XPS spectra (Figure 4b and 4c). For UiO-67s

296

before p-ASA adsorption, three peaks were observed at 284.6 eV, 285.9 eV and 288.5

297

eV, which belonged to the sp2 C=C, C-N, and O-C=O groups in the BPDC linkers.42,43

298

The characteristic peak of π-π* component appeared at 291.2 eV44 after adsorption,

299

suggesting the contribution of π-π stacking for p-ASA adsorption. Meanwhile, the π-π*

300

proportions of UiO-67-NH2(1) and UiO-67-NH2(2) were 2.11% and 7.78%,

301

respectively, which were both higher than that of the pristine UiO-67, suggesting the

302

enhancement of π-π interaction after the -NH2 modification.

303

Considering the higher adsorption affinity of UiO-67-NH2s than the pristine UiO-67,

304

additional mechanism other than As-O-Zr coordination and π-π interaction should also

305

contribute to the adsorption by UiO-67-NH2s. As mentioned above, the good fitting of

306

the adsorption kinetic to the pseudo-second order model suggested that the adsorption 15



307

capacities of UiO-67s were determined by the number of active sites on the

308

adsorbents.32 Since the surface area of UiO-67-NH2(2) was only 60% of UiO-67-NH2(1)

309

(Table S1), the active sites in UiO-67-NH2(2) could also conclude to be ~60% of those

310

in UiO-67-NH2(1). Therefore, the adsorption capacity of UiO-67-NH2(2) should be

311

about 60% of UiO-67-NH2(1) theoretically. However, the Langmuir-fitted adsorption

312

capacity and the molecular uptake of p-ASA:Zr6 by UiO-67-NH2(2) were calculated to

313

be 178 mg g-1 and 2.3:1, respectively, which were much higher than 60% of the p-ASA

314

amount adsorbed by UiO-67-NH2(1) (Table S4 and Figure 4a). This result suggested

315

that the additional -NH2 groups provided more active sites, thus enhancing the

316

adsorption capacity and affinity of UiO-67-NH2(2). Considering the previous

317

discussion, hydrogen bonds between UiO-67-NH2s and p-ASA were expected to be the

318

key role for the enhancement. XPS N 1s results provided strong evidence for the

319

hydrogen bond formation (Figure 4d and 4e). Before adsorption, the symmetrical peaks

320

locating at 399.4 eV were attributed to the free -NH245 in the BPDC linkers of UiO-67-

321

NH2s (Figure 4d). No signals of H-bonded -NH2 were observed in the spectra. After

322

adsorption, although no -NH2 groups existing on the pristine UiO-67 structure, the peak

323

for free -NH2 was still observed (Figure S12), which belonged to the adsorbed p-ASA.

324

Moreover, a new peak assigned to H-bonded -NH2 at 400.2 eV45 was observed in the N

325

1s spectra of UiO-67-NH2s after adsorption (Figurer 4e), and the peak ratio increased

326

with the amine content in UiO-67-NH2s. This result indicated that these hydrogen bonds

327

increased the sites for p-ASA adsorption on UiO-67-NH2s.

328 16


Page 16 of 36

Page 17 of 36


329

DFT calculations

330

To further elucidate the roles of -NH2 groups in UiO-67-NH2s for p-ASA adsorption,

331

the possible interaction scenarios of UiO-67 and UiO-67-NH2(2) with p-ASA were

332

further investigated at the molecular level through DFT calculation. For UiO-67, it was

333

obvious that the As-O-Zr coordination had the highest interaction energy (-181.5 kJ

334

mol-1 in Figure S13a) and was considered to be the most favorable adsorption force.

335

The optimal configurations of the Zr nodes and p-ASA molecules suggested the

336

formation of bidentate binuclear As-O-Zr complexes after adsorption (Figure S13a),

337

which was consistent with the aforementioned EXAFS analysis. In addition, the

338

relatively high adsorption energy (-40.2 kJ mol-1) of the π-π interaction between p-ASA

339

and UiO-67 (face-centered distance, 3.8 Å, Figure S13b) indicated that the π-π stacking

340

also played an important role in the adsorption.

341

After amine modification, the optimal configurations between UiO-67-NH2(2) and

342

p-ASA are shown in Figure 5. The As-O-Zr bidentate binuclear complex, π-π stacking

343

and hydrogen bonds were all present in the UiO-67-NH2(2) samples after adsorption.

344

Moreover, it was obvious that the binding energies of As-O-Zr coordination and π-π

345

interaction in UiO-67-NH2(2) were both higher than those in the pristine UiO-67, while

346

the As-Zr atom distance and the benzene rings face-centered distance were shorter

347

(Figure 5, S13). These results indicated that As-O-Zr coordination and π-π stacking

348

were enhanced after -NH2 modification. Moreover, -NH2 groups in UiO-67-NH2(2) also

349

provided extra adsorption sites for p-ASA through hydrogen bonds with an optimal

350

configuration of NH…O and a bonding energy of -52.4 kJ mol-1 (Figure 5c), thus 17



351

further increasing the adsorption affinity. These results were in agreement with the

352

EXAFS and XPS analyses. The interaction energies followed the order: UiO-67-NH2(2)

353

and p-ASA > water molecule and p-ASA > p-ASA and p-ASA according to DFT

354

calculation (Figure 5, S13), suggesting that p-ASA molecules were monolayerly

355

adsorbed on UiO-67-NH2(2) because free p-ASA molecules cannot be further adsorbed

356

by the bound p-ASA on UiO-67-NH2(2). Overall, p-ASA would form a monolayer

357

adsorption on the surface of UiO-67-NH2(2), under the combined interactions of As-O-

358

Zr coordination, π-π stacking and hydrogen bonding (Figure 6). It should be noted that

359

amine groups on UiO-67-NH2(2) played a significant role in enhancing the adsorption

360

affinity through forming new adsorption sites via hydrogen bonding, as well as

361

strengthening the As-O-Zr coordination and π-π stacking.

362 363

Adsorption behavior of amine modified UiO-67 in simulated natural and waste waters

364

To test if UiO-67-NH2s could efficiently remove p-ASA for practical application,

365

DOM containing water and swine manure lixivium were used to simulated natural and

366

waste water, respectively. Adsorption results indicated that at a p-ASA concentration of

367

0.5 mg L-1, the removal rate of UiO-67-NH2(2) was slightly decreased from 99.6% to

368

98.7% when the DOM concentration increased from 0 to 16 mg C L-1, which were

369

higher than those of UiO-67 (Figure 7a). When the DOM concentration was 16 mg C

370

L-1, the residual arsenic concentrations after adsorbed by UiO-67-NH2(1) and UiO-67-

371

NH2(2) were 4.15 and 2.26 μg L-1, respectively (Figure 7b), which were lower than the

372

arsenic standard in drinking water of WHO (10 μg L-1). The adsorption results of UiO18


Page 18 of 36

Page 19 of 36


373

67-NH2s in swine manure lixivium at different p-ASA concentrations are shown in

374

Figure 7c. It can be seen that when the initial concentration of p-ASA was 5 mg L-1, a

375

dosage of 0.15 g L-1 UiO-67-NH2(2) could decrease the arsenic concentration to 28.1

376

μg L-1, which satisfied the arsenic standard in surface water of China (50 μg L-1,

377

GB3838-2002). To reduce the arsenic concentration to the same level (below 50 μg L-

378

1

379

respectively, which were 26.7 and 44.8 times higher than that of UiO-67-NH2(2) (Table

380

S7). Moreover, since UiO-67 has already been studied as a water-treatment adsorbent

381

for a period of time,16 its modification is supposed to be more simple and commercially

382

available, compared with synthesizing a new MOF. These results suggested that the

383

amine modified UiO-67 was a promising candidate for removing p-ASA from natural

384

and waste waters.

), the needed dosages of graphene and activated carbon were 4 g L-1 and 6.7 g L-1,

385 386

ENVIRONMENTAL IMPLICATIONS

387

The defective UiO-67-NH2 designed based on the structural features of p-ASA

388

showed high adsorption affinity towards p-ASA. The study of adsorption mechanism

389

indicated that the contribution of different adsorption forces followed an order of As-

390

O-Zr coordination > π-π stacking > H-bonding. The -NH2 groups in UiO-67-NH2

391

played an important role for the efficient adsorption, including enhancing the As-O-Zr

392

coordination and π-π stacking, as well as providing new adsorption sites for p-ASA

393

adsorption through H-bonding. These synergistic interactions resulted in strong affinity

394

of UiO-67-NH2 towards p-ASA, which were beneficial for the removal of p-ASA in 19



395

simulated natural and waste waters. This study revealed great potential of -NH2

396

modified UiO-67s for the removal of p-ASA in practical applications. Moreover, this

397

work also provided a new route for increasing the adsorption affinity of adsorbents

398

towards targeted pollutants through engineering different functional groups to achieve

399

synergistic effects.

400 401

ASSOCIATED CONTENT

402

Supporting Information

403

Thirteen figures, seven tables and experimental details are presented in Supporting

404

Information section. The material is available free of charge via the Internet at

405

http://pubs.acs.org.

406

AUTHOR INFORMATION

407

Corresponding Authors

408

* Email: [email protected] (Dr. Zhang Lin); phone: 86-20-39380503; fax: 86-20-

409

39380508

410

Notes

411

The authors declare no competing financial interest.

412 413

ACKNOWLEDGMENTS

414

This work was supported by the National Natural Science Foundation of China (grant

415

no. 21477129), the Guangdong Innovative and Entrepreneurial Research Team

416

Program (No. 2016ZT06N569), the China Postdoctoral Science Foundation (no. 20


Page 20 of 36

Page 21 of 36


417

2016M600654), and the Fundamental Research Funds for the Central Universities (no.

418

2017PY009 and 2017BQ054). The authors thank the beamline 4W1B (Beijing

419

Synchrotron Radiation Facility) for providing the beam time.

420 421

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422

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Organic Frameworks with Inherent Recognition Sites for Selective Phosphate Sensing

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Enhanced Removal of Organophosphorus Pesticides from Aqueous Solution by Zr-

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Based MOFs of UiO-67. Acs Appl. Mater. Inter. 2015, 7, (1), 223-231.

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Hupp, J. T.; Farha, O. K. High Efficiency Adsorption and Removal of Selenate and

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Selenite from Water Using Metal-Organic Frameworks. J. Am. Chem. Soc. 2015, 23,

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(123), 7488-7494.

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Coupled with Uio-66 (NH2) via a Facile Condensation Process for Enhanced Visible-

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Light-Driven Photocatalysis. J. Alloy. Compd. 2017, 690, 123-130.

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as Efficient and Stable Visible-Light-Driven Photocatalyst for H2 Evolution and CO2

561

Reduction. Applied Catalysis B: Environmental 2017, 200, 448-457.

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565

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566

and their Immobilization Capacities. Langmuir 2004, 20, (6), 2309-2314.

567

27



568 569

FIGURES

570

571 572

Figure 1. Perspective views of (a) UiO-67, (b) UiO-67-NH2(1), and (c) UiO-67-NH2(2);

573

(d) Structure of the defective Zr6 node; (e) The BPDC linker for UiO-67 and its -NH2

574

modified forms for UiO-67-NH2(1) and UiO-67-NH2(2). In (a), (b), and (c), Zr6 nodes,

575

framework carbon and oxygen atoms are shown in cyan, white, and gray, respectively.

576

Modified -NH2 moieties are shown in red.

577

28


Page 28 of 36

Page 29 of 36


578

579

580 581

Figure 2. (a) Adsorption kinetics, (b) adsorption isotherms, and (c) surface area-

582

normalized isotherms of p-ASA adsorbed by UiO-67, UiO-67-NH2(1) and UiO-67-

583

NH2(2).

29



584

585

586

587 588

Figure 3. Arsenic K-edge data for p-ASA adsorbed UiO-67 and UiO-67-NH2 samples:

589

(a) filtered k3-weighted EXAFS data, (b) magnitude part of Fourier transformed R-

590

space, and (c) real part of Fourier transformed R-space. Experimental and calculated

591

curves are displayed as black solid lines and red open circles, respectively. Results of

592

the k3-weighted fitting are listed in Table S6. 30


Page 30 of 36

Page 31 of 36


593

594

595

31



596

597 598

Figure 4. The uptake of p-ASA by UiO-67, UiO-67-NH2(1) and UiO-67-NH2(2), and

599

related XPS analysis after p-ASA adsorption. (a) The uptake of p-ASA (adsorbed p-

600

ASA / Zr6 node) by UiO-67, UiO-67-NH2(1) and UiO-67-NH2(2); XPS C 1s and N 1s

601

spectra of UiO-67, UiO-67-NH2(1) and UiO-67-NH2(2) before (b, d) and after (c, e) p-

602

ASA adsorption.

603

32


Page 32 of 36

Page 33 of 36


604

605

606 607

Figure 5. The optimal configurations obtained via DFT calculations of (a) As-O-Zr

608

coordination, (b) π-π stacking, (c) and H-bonding between UiO-67-NH2(2) and p- ASA

609

(Magenta for As, cyan for Zr, red for O, blue for N, gray for C, and white for H). The

610

interaction energy between UiO-67-NH2(2) and p-ASA was calculated to be -297.4 kJ

611

mol-1 (As-O-Zr coordination π-π stacking, and H-bonding).

612 33



613 614 615 616

Figure 6. The adsorption mechanism of UiO-67-NH2 towards p-ASA

34


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Page 35 of 36


617

618

619 620

Figure 7. p-ASA adsorption by UiO-67, UiO-67-NH2(1) and UiO-67-NH2(2) in the

621

presence of DOM or in swine manure lixiviums. (a) Effect of DOM on the removal

622

rates of p-ASA; (b) the residual arsenic concentrations (C0 = 0.5 mg L-1) in the presence

623

of DOM; (c) the residual arsenic concentrations after adsorption in swine manure 35



624

lixiviums with different initial concentrations of p-ASA. The adsorbent concentrations

625

were kept at 0.15 g L-1 in 30 mL solution. The mixtures were shaken at pH 4.0 ± 0.1

626

and 25 C with a speed of 200 rpm for 12 h.

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Enhanced adsorption of p-arsanilic acid from water by amine modified

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