Tuning Pb(II) Adsorption from Aqueous Solutions on Ultrathin Iron

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Tuning Pb(II) Adsorption from Aqueous Solutions on Ultrathin Iron Oxychloride (FeOCl) Nanosheets Jinming Luo, Meng Sun, Cody Ritt, Xia Liu, Yong Pei, John C. Crittenden, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07027 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Environmental Science & Technology

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Tuning Pb(II) Adsorption from Aqueous Solutions

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on Ultrathin Iron Oxychloride (FeOCl) Nanosheets

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Revised: January 27, 2019

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Jinming Luo,†,⊥ Meng Sun,*,‡,⊥ Cody L. Ritt,‡ Xia Liu,ǁ Yong Pei,ǁ John C. Crittenden,†

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and Menachem Elimelech*,‡

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† Brook Byers Institute for Sustainable Systems and School of Civil and Environmental

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Engineering, Georgia Institute of Technology, 828 West Peachtree Street, Atlanta, GA

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30332, United States

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‡ Department of Chemical and Environmental Engineering, Yale University, New

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Haven, Connecticut 06520-8286, United States

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ǁ Department of Chemistry, Key Laboratory for Green Organic Synthesis and

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Application of Hunan Province, Key Laboratory of Environmentally Friendly Chemistry

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and Applications of Ministry of Education, Xiangtan University, Xiangtan, Hunan

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Province 411105, China

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* Corresponding authors:

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Email: [email protected]

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Email: [email protected]; Tel. +1 (203) 432-2789

ACS Paragon Plus Environment


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ABSTRACT

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Structural tunability and surface functionality of layered two-dimensional (2-D) iron

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oxychloride (FeOCl) nanosheets are critical for attaining exceptional adsorption properties. In

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this study, we combine computational and experimental tools to elucidate the distinct

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adsorption nature of Pb(II) on 2-D FeOCl nanosheets. After finding promising Pb(II)

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adsorption characteristics by bulk FeOCl sheets (B-FeOCl), we applied computational

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quantum mechanical modeling to mechanistically explore Pb(II) adsorption on representative

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FeOCl facets. Results indicate that increasing the exposure of FeOCl oxygen and chlorine sites

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significantly enhances Pb(II) adsorption. The (110) and (010) facets of FeOCl possess distinct

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orientations of oxygen and chlorine, resulting in different Pb(II) adsorption energies.

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Consequently, the (110) facet was found to be more selective toward Pb(II) adsorption than the

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(010) facet. To exploit this insight, we exfoliated B-FeOCl to obtain ultrathin FeOCl

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nanosheets (U-FeOCl) possessing unique chlorine- and oxygen-enriched surfaces. As we

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surmised, U-FeOCl nanosheets achieved excellent Pb(II) adsorption capacity (709 mg g-1 or

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3.24 mmol g-1). Moreover, U-FeOCl demonstrated rapid adsorption kinetics, shortening

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adsorption equilibration time to one-third of the time for B-FeOCl. Extensive characterization

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of FeOCl-Pb adsorption complexes corroborated the simulation results, illustrating that

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increasing the number of Pb-O and Pb-Cl interaction sites led to the improved Pb(II) adsorption

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capacity of U-FeOCl.

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KEYWORDS: adsorption, iron oxychloride, heavy metal, coordination, EXAFS

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1


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TOC Art

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2



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INTRODUCTION

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Extensive use of lead (Pb)-containing products such as power cells, mineral resources, and

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cosmetics1, 2 have accelerated the global occurrence of Pb in the atmosphere, sediments, and

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waters,3, 4 thereby escalating environmental risks.5-10 Recent increases in the bioaccumulation

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of Pb in the food chain have given rise to the possibility of increased Pb ingestion by humans.

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Inherently toxic to brain tissue, the nervous system, and the reproductive system,11 Pb poses

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significant risks for human health. Therefore, mitigation of Pb contamination has become a

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vital issue in separation science and environmental remediation.

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Adsorption, considered a state of the art technology for heavy metal removal,12, 13 is already

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well-developed and appears to be more economical and effective than other methods.14-18 To

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date, a number of adsorbents have been developed for aqueous Pb(II) adsorption. Biochar,

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produced from dehydrated banana peels via hydrothermal carbonization, exhibited an excellent

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Pb(II) adsorption capacity of 359 mg g-1. The presence of Pb-O and Pb-O-C interactions

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illustrated an ion exchange mechanism dominating the adsorption of Pb(II) on biochar

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adsorbents.19 Double network hydrogels, produced from waste cotton fabrics, exhibited a Pb(II)

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adsorption capacity of 382.8 mg g-1, owing to an abundance of oxygen-containing groups (i.e.,

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C-O-H, C-O-C, and C=O) across the framework.20 In another work, a carbomethoxy-

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functionalized metal-organic framework (MOF) exhibited a modest Pb(II) adsorption capacity

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with relatively high adsorption kinetics (0.162 g mg-1 min-1), while also validating the vital role

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of O atoms of the carbomethoxy group in capturing Pb(II).21 Additionally, observations of

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newly developed transition metal (e.g., Mn and Fe) oxide adsorbents indicate that the

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constituent O atoms could also interact with Pb(II).22, 23

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Previous studies have presented data exemplifying the effect of Pb(II) interactions with

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chemical groups or constituent anions (e.g., oxygen in WO3).24 However, fundamental

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understanding of adsorption mechanisms, which would provide insight into enhancing

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adsorptive properties, is still lacking. Furthermore, adsorbents enabling multicomponent Pb(II)

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interactions are rarely reported. In particular, the interaction of constituent anions (e.g., O, Cl) 3


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during the adsorption process is still unclear; therefore, investigation of the impact of

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multicomponent interactions on adsorption performance (i.e., adsorption capacity and kinetics)

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is of great value.

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Iron oxychloride (FeOCl), possessing a two-dimensional (2-D) nanosheet structure, has

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drawn increasing attention in recent years due to its unique structure-dependent catalytic

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activity.25,

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unique atomic coordination27 are features which may offer exciting opportunities for FeOCl in

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adsorption processes. The co-existence of Fe(II)/Fe(III) in the architecture of FeOCl is an

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impetus for research efforts to optimize the bicomponent functionality, possibly leading to the

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discovery of novel material properties.28, 29 Moreover, chlorine and oxygen atoms diversify the

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internal coordination environment of FeOCl while also varying coordination modes in

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accordance with external environmental changes.30 These interesting properties have allowed

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researchers to introduce various metal ions into the FeOCl nanosheet framework through sheet

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intercalation and delamination.31,

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investigation of metal ion interaction with FeOCl may provide insight into novel adsorption

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mechanisms.

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Attributes such as scalability, compatibility with multiple iron valences, and

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Inspired by these pioneering works, we believe the

In this work, we first present the Pb(II) adsorption efficiency of bulk FeOCl sheets (B-FeOCl)

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followed by the exploration of Pb(II) adsorption mechanisms on different FeOCl facets using

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density functional theory (DFT). Insights gained from DFT informed the exfoliation of B-

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FeOCl sheets to produce ultrathin FeOCl nanosheets (U-FeOCl). Adsorption performance of

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both U-FeOCl nanosheets and B-FeOCl sheets was investigated and compared to demonstrate

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the enhanced Pb(II)-binding interactions offered by U-FeOCl, as was predicted by DFT. This

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work highlights the novel application of FeOCl for heavy metal adsorption and provides the

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scientific base for enhancing adsorptive properties of 2-D metal oxychlorides.

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EXPERIMENTAL SECTION

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Chemicals and materials. Ferric chloride hexahydrate (FeCl3·6H2O, >98.0%), iron oxide

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(Fe2O3, ≥99%), magnetic iron oxide (Fe3O4, 97%), lead nitrate (Pb(NO3)2, ≥99.0%), nitric acid 4



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(HNO3, 70%), hydrochloric acid (HCl, 37%), sodium chloride (NaCl, ≥99.5%), potassium

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chloride (KCl, ≥99.0%), calcium chloride (CaCl2, ≥97.0%), magnesium chloride (MgCl2,

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≥98.0%), sodium hydroxide (NaOH, ≥98.0%), were purchased from Sigma-Aldrich.

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chemicals are analytical grade and used as received. Water was treated by a Milli-Q ultrapure

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water purification system (Millipore, Billerica, MA) to produce deionized (DI) water. Stock

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solutions of Pb(II) were prepared by dissolving Pb(NO3)2 in DI water.

All

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Synthesis of FeOCl Sheets. B-FeOCl was prepared by a facile annealing method,

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where a precursor, FeCl3‧6H2O, was placed in a sealed crucible and heated in a microwave

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oven for 5 min. After heating, the prepared B-FeOCl sheets were allowed to cool to room

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temperature. U-FeOCl nanosheets were prepared via sonication of B-FeOCl sheets dispersed

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in DI water for 30 minutes. Dispersed nanosheets were run through several centrifugation-

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rinsing cycles with DI water. The supernatant was then decanted and the remaining

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concentrated U-FeOCl solution was vacuum-filtered and dried to remove remnant moisture.

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Material Characterization. Crystalline phases of the samples were investigated via

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Rigaku SmartLab X-ray diffractometer (XRD) using Cu Kα radiation (λ = 1.5418 Å). XRD

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was operated at 45 kV and 200 mA with a scanning speed of 2o min-1 and 2θ range of 5o to 80o.

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PDXL 2 Rigaku data analysis software was used to obtain lattice constants and crystallite sizes.

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The sample morphologies were examined using a Hitachi SU8230 UHR Cold Field Emission

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Scanning Electron S3 Microscope (FESEM) and a FEI Tecnai Osiris 200kV transmission

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electron microscopy (TEM). High spatial resolution X-ray energy dispersive spectroscopy

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(EDS) was performed with a Bruker XFlash 5060FQ Annular EDS detector. X-ray

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photoelectron spectroscopy (XPS) spectra were obtained using monochromatic 1486.7 eV Al

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Kα X-ray source on a PHI VersaProbe II X-ray photoelectron spectrometer with a 0.47 eV

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system resolution. The energy scale was calibrated using the Cu 2p3/2 (932.67 eV) peak of a

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clean copper plate and the Au 4f7/2 (84.00 eV) peak of a clean gold foil. For XPS, a 23.5 eV

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pass energy, 20 ms integration time, and 100 meV step size were chosen; additionally, the C

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1s peak was calibrated to 284.8 eV for all spectra obtained. XPS data analysis was performed

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using the Multipak software provided by Physical Electronics, INC. Fe K-edge X-ray 5


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absorption fine structure spectroscopy (XAFS) spectra were collected on beamline 1W1B at

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Beijing Synchrotron Radiation Facility. The ring storage energy of the synchrotron radiation

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accelerator during data collection was 2.5 GeV with a current intensity of 50 mA. Fe K-edge

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spectrum of the sample was collected using transmission mode. Fe foil, Fe3O4, and Fe2O3

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powder were used as standard compounds. The tested FeOCl-based adsorbents were

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homogenized and compressed into 2-3 mm slices, bearing a diameter of 1 cm. Athena software

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was used to process the normalization and liner combination fitting (LCF) of the X-ray

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absorption near edge structure (XANES) spectra. Inductively coupled plasma mass

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spectrometry (ICP-MS, ELAN DRC-e, Perkin Elmer, Waltham, MA) was used to quantify the

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Pb(II) ion concentration. Nitric acid solutions (1% HNO3) consisting of Pb(II) concentrations

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ranging from 0 to 200 μg L-1 were used to calibrate the system. Indium nitrate (40 μg L-1) was

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used as an internal standard for ICP-MS.

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Adsorption Performance Experiments. Pb(II) solution (40 mL) with initial

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concentrations ranging from 10 to 800 mg L-1, was prepared by dilution of concentrated stock

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solution with DI water. Adsorption isotherms of Pb(II) at 15, 25, and 35 °C were determined

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by batch experiments, with an adsorbent dosage of 0.5 g L-1 for all experiments. The adsorbed

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amount (qe) for Pb(II) was calculated using

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qe =

V ( C0 − Ce ) m

(1)

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where V is the volume of the Pb(II) solution, m is the mass of the FeOCl adsorbent, and C0 and

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Ce are the initial and equilibrium solution concentrations of Pb(II), respectively.

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Langmuir, Freundlich, and Dubinin–Radushkevich (D-R) isotherm models were used for

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fitting the temperature-dependent adsorption data. Thermodynamic parameters, such as

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standard Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were calculated

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according to the van’t Hoff equation (Section 1, Supporting Information). Pb(II) adsorption

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kinetics were examined with an initial Pb(II) concentration of 25 mg L-1 and placed under

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constant stirring to reduce mass transfer resistance. To analyze the effect of solution pH on

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Pb(II) adsorption, isotherms were determined for pH values varying from 1.0 to 5.0. Desired 6



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pH was obtained via dropwise addition of 0.1 M HCl or 0.1 M NaOH to the solution. Batch

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adsorption experiments were run in triplicates under stirring at 240 rpm for 24 h. After

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equilibration, all samples were filtered through 0.45 µm cellulose acetate membranes to

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separate the FeOCl nanosheets for analysis of Pb(II) in solution. The impact of competing

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cations on the adsorption of Pb(II) was evaluated by adding NaCl, KCl, CaCl2, and MgCl2 at

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concentrations of 0.01 and 0.1 M. Residual concentrations of Pb(II) in solution were

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determined with the use of ICP-MS.

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Density Functional Theory Calculations. Structure optimization calculations were

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performed within the framework of density functional theory (DFT) as implemented in the

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Vienna Ab initio Simulation Package (VASP).33, 34 Exchange-correlation interactions were

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treated by generalized gradient approximation (GGA), parameterized by Perdew, Burke, and

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Ernzerhof (PBE).35 We described the interaction between ions and electrons using the projected

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augmented wave (PAW).36 The periodic unit was optimized by the conjugate gradient

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algorithm with an energy cutoff of 500 eV until the force on each atom was less than 0.01 eV

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Å-1. A vacuum space exceeding 20 Å was employed to minimize interactions between the two

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periodic units. The Brillouin-zone was meshed by the γ-centered Monkhorst-Pack method with

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5×5×1 k-points for geometry optimizations. As a typical layered 2-D nanomaterial, FeOCl is

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constructed by stacking individual layers through the van der Waals force.37 The (010) facet of

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FeOCl was positioned perpendicular to the Y-axis and terminated via weak interactions with

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adjacent layers of Cl atoms (Figure S1). The (010) facet of FeOCl was represented as a

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monolayer slab containing four atomic layers to simplify structural optimization and calculate

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adsorption energy at the facet.38 In contrast, the in-plane (110) facet possessed strong

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intermolecular chemical bonds between adjacent atomic layers. Therefore, minimizing the

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influence of adjacent atoms required an eight-layer slab model to accurately represent the (110)

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facet structure located at the edge of FeOCl. The three bottom slab layers in the (110) facet

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were restricted during calculations. Adsorption energy (Ead) on the slab surfaces is defined as

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Ead = Etot − Esurf − Eatom, where Etot is the total energy of the adsorption model, Esurf is the

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optimized energy on the slab surface in the absence of Pb atom, and Eatom is the energy of a Pb 7


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atom under vacuum. Therefore, a more negative Ead is indicative of better Pb(II) adsorption

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and can provide insight into the mechanisms of adsorption at the different FeOCl facets.39

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Furthermore, the differential charge density (Δρ), used to calculate the charge distribution

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across the adsorption complexes, was defined as Δρ = ρ[FeOCl-Pb] −ρ[FeOCl] −ρ(Pb).

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RESULTS AND DISCUSSION

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Adsorption of Lead on Bulk FeOCl Slice (B-FeOCl). We applied B-FeOCl, for the first

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time, toward the application of Pb(II) adsorption. As shown in Figure S2a, adsorption of Pb(II)

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increases concurrently with solution temperature. The Langmuir isotherm model was a better

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fit for the experimental data than the Freundlich model (Table S1), suggesting that adsorption

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of Pb(II) on B-FeOCl occurs in a monolayer of equally available adsorption sites.17 Predictions

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based on the Langmuir model indicate that the maximum Pb(II) adsorption capacity for B-

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FeOCl is 409.71 mg g-1 at 35 oC (Table S1). The Dubinin-Radushkevich (D-R) model (Figure

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S2b) predicts a free energy (E) of 9.92 J mol-1 (Table S2), indicating a chemisorption process.40

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Thermodynamic parameters, including standard Gibbs free energy (ΔG°), enthalpy (ΔH°),

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and entropy (ΔS°) of the adsorption process were calculated according to the van't Hoff

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equation (Figure S2c). For experiments conducted at temperatures of 15, 25, and 35 °C, ΔG°

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values were calculated as -3.38, -4.82, and -5.71 kJ mol-1, respectively (Table S3).

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Theoretically, negative ΔG° values indicate that the adsorption of Pb(II) onto B-FeOCl is

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thermodynamically favorable. ΔH° and ΔS° values were calculated as 30.29 kJ mol-1 and 0.12

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kJ mol-1 K-1, respectively. The coupling of negative ΔG° and positive ΔH° values suggests that

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Pb(II) adsorption on B-FeOCl occurs as a spontaneous, endothermic reaction.41 Furthermore,

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positive ΔS° values reveal a thermodynamically favorable increase in system randomness

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occurring at the interface between the adsorbent (B-FeOCl) and adsorbate (Pb(II)) as

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temperature increases.42 These thermodynamic insights predict an essential improvement in

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Pb(II) adsorption at elevated temperatures, corroborating the temperature dependency of Pb(II)

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adsorption on B-FeOCl observed initially.

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TABLE 1 8



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Insight into Pb(II)-FeOCl Interactions. Our reported Pb(II) adsorption capacities for B-

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FeOCl, although higher than materials from many other studies,16, 23, 43-52 were still not the

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highest (Tables 1 and S4). Therefore, further investigation of FeOCl and its adsorption

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mechanisms was essential for optimizing the adsorption performance. To gain further insight,

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a DFT study was conducted to evaluate the surface geometries and atomic configurations of

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the FeOCl-Pb adsorption complexes. Due to the predominance of (010) and (110) facets in

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FeOCl, these facets were systematically investigated to assess their influence on Pb(II)

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adsorption. In brief, one equivalent and two non-equivalent53 adsorption complexes are formed

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at the (010) and (110) facets, respectively. Pb(II) adsorption at the (010) facet is equivalently

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bonded with the nearest four Cl atoms (bond length of 2.76 Å) and O atom (bond length of

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2.88 Å) (Figure 1A), whereas non-equivalent adsorption complexes, found on the (110) facet,

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result in two forms of Pb(II) complexation. For non-equivalent complexation, Pb(II) may bind

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with two O atoms and one Cl atom (Figure 1B) or two Cl atoms and one O atom (Figure 1C),

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denoted as (110)-a and (110)-b, respectively. In (110)-a bonding, Pb(II) is strongly bound to O

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(bond length of 2.30 Å), but weakly bound to Cl (bond length of 2.86 Å), whereas the

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interactions of Pb(II) with O (bond length of 2.20 Å) and Cl (bond length of 2.76 Å ) are both

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enhanced in (110)-b bonding due to a reduction in their respective atomic distances. This

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enhancement predicts that (110)-b adsorption should be more favorable than both (110)-a and

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(010) adsorption. Adsorption energies (Ead) between Pb(II) and (010), (110)-a, and (110)-b

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facets were calculated as -3.69, -3.68, and -3.96 eV, respectively. The more negative adsorption

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energy between Pb(II) and (110)-b indicates a more stable atomic configuration, thus

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corroborating our previous prediction that (110)-b is the most favorable site for Pb(II)

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complexation.

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To further explore the interactions between Pb(II) and constituent non-metal atoms found in

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FeOCl, we evaluated the binding energy of Pb-O and Pb-Cl at the (010) and (110)-b facets.

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Due to the large bond length between Pb and O at the (010) facet, the binding energy of Pb-O

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was negligible; therefore, the binding energy of Pb-Cl at the (010) facet was calculated as -0.92

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eV by averaging the overall binding energy of the complex. Due to the identical bond lengths 9


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of Pb-Cl on the (010) and (110)-b facets, the binding energy of Pb-O on the (110)-b facet, with

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a total binding energy of -3.96 eV, can be estimated as -2.11 eV. For the (110)-b facet, the total

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Pb-O binding energy was slightly more negative than the total Pb-Cl binding, indicating that O

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sites play a slightly greater role in Pb(II) adsorption at this facet.

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Adsorption charge densities of complexes on different facets were investigated to elucidate

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interatomic interactions (Figure 1D-F). It was found that exposed Cl atoms are able to donate

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electrons as Pb(II) approaches the (010) facet, thus changing the electron cloud density around

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the Pb(II) atom (Figure 1D). Alternatively, the distance between Pb(II) and O at the (010) facet

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(2.88 Å) is too substantial to allow for redistribution of the electron cloud density around O.

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As a result, the FeOCl-Pb adsorption complex on the (010) facet eventually configures into a

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typical μ4-coordination. In contrast, both O and Cl atoms at the (110)-a and (110)-b facets

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contribute to the redistribution of the electron cloud around Pb(II) (Figure 1E-F), hence

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forming a μ3-coordination. Although the two adsorption complexes at the (110) facet show the

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same coordination configuration, the binding energies for these two complexes are expected to

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be quite different. We attribute this phenomenon to the self-tuning of atomic distances among

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O, Cl, and Pb atoms. For example, the atomic distances between Pb(II) and anionic constituents

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at the (110)-b facet are shorter than the atomic distances at the (110)-a facet, causing more

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electron accumulation around the Pb(II) atom.

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DFT calculations demonstrated that the adsorption of Pb(II) on FeOCl is predominantly due

263

to the interactions between Pb and Cl atoms; however, increasing the exposure of O atoms in

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FeOCl will also largely improve adsorption performance as the most favorable facet for Pb(II)

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adsorption, (110)-b, is significantly influenced by O. Therefore, increasing both O and Cl sites

266

is requisite for optimizing the Pb(II) adsorption performance of FeOCl.

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FIGURE 1

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Preparation and Characterization of FeOCl Nanosheets. Inspired by insights

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gained from the DFT study, we attempted exposing more Cl- and O-rich surface sites located

270

throughout FeOCl to improve Pb(II) adsorption. B-FeOCl can be regarded as a stacked 10



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composite of lamellar FeOCl nanosheets, where the nanosheets are tightly bound through van

272

der Waals forces and the exterior of each nanosheet is decorated by Cl atoms (Figure S3).54 To

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expose more binding sites, an ultrasonic probe sonicator was employed to exfoliate B-FeOCl

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sheets into ultrathin FeOCl (U-FeOCl) nanosheets, thus unveiling numerous exterior Cl sites

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(Figure 2A). Without exfoliation, B-FeOCl were platelet-shaped sheets (Figure 2B, SEM top-

276

view image) possessing a distinct hierarchical structure less than 100 nm in thickness (Figure

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2B inset, SEM cross-section image). The crystalline structure of B-FeOCl was evidenced by

278

HRTEM images, indicating lattice fringes of 3.44, 2.54, and 1.90 Å, which correspond to the

279

(110), (021), and (200) facets, respectively (Figure 2C). The selected area electron diffraction

280

(SAED) pattern of B-FeOCl presents a highly ordered diffraction spot matrix (Figure 2C, inset).

281

Diffraction spots in a typical parallelogram can be assigned to the orthogonal planes of {200}

282

and {002} as well as their vector sum, the {021} plane, along the {0-12} zone in orthorhombic

283

FeOCl,29 therefore verifying our observations from TEM imaging. In contrast, the morphology

284

and crystal structure of U-FeOCl differed from B-FeOCl, as U-FeOCl consisted of sheets

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smaller in size (

Tuning Pb(II) Adsorption from Aqueous Solutions on Ultrathin Iron

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