Removal Mechanisms of Phosphate by Lanthanum Hydroxide

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Removal Mechanisms of Phosphate by Lanthanum Hydroxide Nanorods: Investigations using EXAFS, ATRFTIR, DFT and Surface Complexation Modeling Approaches Liping Fang, Qiantao Shi, Jessica Nguyen, Baile WU, Zimeng Wang, and Irene M. C. Lo Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03803 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Environmental Science & Technology

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Removal Mechanisms of Phosphate by Lanthanum Hydroxide

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Nanorods: Investigations using EXAFS, ATR-FTIR, DFT and Surface

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Complexation Modeling Approaches

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Liping Fang,† Qiantao Shi,‡ Jessica Nguyen,§ Baile Wu,† Zimeng Wang,§ Irene M.C.

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Lo*,†

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†

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Science and Technology, Hong Kong, China

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‡

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Jersey 07030, USA.

Department of Civil and Environmental Engineering, The Hong Kong University of

Center for Environmental Systems, Stevens Institute of Technology, Hoboken, New

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§

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Rouge, LA 70803, USA.

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*Corresponding Author:

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Fax: (852) 23581534; email: [email protected].

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Notes

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The authors declare no competing financial interest.

Department of Civil and Environmental Engineering, Louisiana State University, Baton

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ABSTRACT

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Lanthanum-based materials are effective for sequestering phosphate in water, however

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their removal mechanisms remain unclear, and the effects of environmentally relevant

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factors have not yet been studied. Hereby, this study explored the mechanisms of

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phosphate removal using La(OH)3 by employing extended X-ray absorption spectroscopy

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(EXAFS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-

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FTIR), density functional theory (DFT) and chemical equilibrium modeling. The results

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showed that surface complexation was the primary mechanism for phosphate removal

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and in binary phosphate configurations, namely diprotonated bidentate mononuclear

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(BM-H2) and bidentate binuclear (BB-H2), coexisting on La(OH)3 in acidic conditions.

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By increasing the pH to 7, BM-H1 and BB-H2 were the two major configurations

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governing phosphate adsorption on La(OH)3, while BB-H1 was the dominant

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configuration of phosphate adsorption at pH 9. With increasing phosphate loading, the

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phosphate configuration of on La(OH)3 transforms from binary BM-H1 and BB-H2 to

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BB-H1. Amorphous Ca3(PO4)2 forms in the presence of Ca, leading to enhanced

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phosphate removal at alkaline conditions. The contributions of different mechanisms to

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the overall phosphate removal were successfully simulated by a chemical equilibrium

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model that was consistent with the spectroscopic results. This study provides new

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insights into the molecular-level mechanism of phosphate removal by La(OH)3.


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INTRODUCTION

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Conventional biological processes have poor ability to remove phosphate from sewage,

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causing excessive discharge of phosphorus (P) into natural water bodies. As a

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consequence, this can lead to eutrophication with algal blooms and red tides, damaging

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the water quality and eventally threatening wildlife and human health.1, 2 Adsorption is an

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economical and efficient solution for removing phosphate from water,3 and metal

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(hydr)oxide-based adsorbents are deemed promising for phosphate removal because of

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their strong affinity for phosphate.4-6 In the past decades, extensive studies have shown

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that the mechanism of phosphate adsorption on typical minerals like Fe/Al (hydr)oxides

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is likely through the formation of inner-sphere surface complexes;5,

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substitute for either one or two hydroxyl groups on the surface of metal (hydr)oxides to

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form monodentate (MD), bidentate (BD) or coexisting MD/BD binding surface

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complexes depending on the pH and surface loading.7-9 Structural information (e.g.

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surface-specific bond distance) obtained from interpreting the surface configuration of

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phosphate on Fe/Al (hydr)oxides at the molecular-scale, can be useful in predicting the

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phosphate adsorption with metal (hydro)oxides under different environmental

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conditions.8

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phosphate can

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As one of most promising adsorbents, besides commercialized lanthanum modified

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bentonite clay (i.e. Phoslock®), lanthanum hydroxide (i.e. La(OH)3) is receiving

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increasing attention for its high potential in effectively removing phosphate even at trace

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levels.10,

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adsorption capacities have been developed to tackle the problem of phosphate-

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contaminated water.10,

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To date, a number of lanthanum-based adsorbents with high phosphate

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Although tremendous efforts have been made to develop 3 ACS Paragon Plus Environment


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lanthanum-based adsorbents for phosphate removal, there has been less attention paid on

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developing a full understanding of the mechanisms of phosphate adsorption on La(OH)3

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at the molecular level. Using Fourier transform infrared spectroscopy (FTIR) and

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macroscopic batch methods, Xie and colleagues reported that phosphate ions form

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deprotonated MD complexes on La(OH)3 at pH > 8.2.13 Using X-ray diffraction (XRD)

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and 31P solid-state nuclear magnetic resonance (31P NMR), Zhang et al.10 and Dithmer et

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al.14 observed from their experiments that a new crystal phase of LaPO4·xH2O was found

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after phosphate adsorption. However, the exact structural information of phosphate

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coordination with La(OH)3 remains unclear. In addition, current knowledge about the

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mechanisms of phosphate adsorption on La(OH)3 has been obtained under different

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experimental conditions. As pH and surface loading may critically influence the local

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atomic environment of phosphate,9, 15 a systematic investigation across a wide range of

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water chemistry parameters is warranted.

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As an element-specific technique, extended X-ray absorption fine structure (EXAFS)

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spectroscopy has shown advantages in directly probing the surface configuration of

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oxyanions (e.g. phosphate, arsenate and sulfate) on mineral surfaces.9, 16-19 Although two

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research groups both characterized the structure of phosphate complexes on La-bearing

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adsorbents by applying La K-edge EXAFS,12, 14 it is more straightforward to investigate

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the mechanisms of phosphate removal by a pure phase of La(OH)3 using P K-edge

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EXAFS, as this can could give more accurate information on the surface configuration of

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P with La. Moreover, EXAFS is not able to reflect the protonation states of phosphate on

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the surface of La(OH)3 under different pHs, without the complementation of other


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techniques such as attenuated total reflectance (ATR)-FTIR spectroscopy with density

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functional theory (DFT).20

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To address the aforementioned research gaps, the objectives of this study were to

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determine the molecular configurations and protonation states of phosphate in association

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with La(OH)3 as a function of pH, P loading and co-existing ion concentrations by

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combining batch sorption experiments, EXAFS, ATR-FTIR, DFT calculation and

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chemical equilibrium modeling techniques. La(OH)3 were used as the model in this study.

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Batch studies were first undertaken to investigate how background ions (e.g. Cl, NO3-,

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HCO3-, SO42-, Ca and Mg), natural organic matter (NOM) and pH influence phosphate

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adsorption on La(OH)3. ATR-FTIR spectroscopy was used to identify the phosphate-

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La(OH)3 complexes that formed under different pH values and phosphate loadings, and

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DFT calculations were used to predict the IR spectra of phosphate after adsorption on

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La(OH)3 and to compare the predictions compared with experimental IR data. Further,

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the local structural environment of the phosphate was directly probed under different

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environmental conditions by means of P K-edge EXAFS. Finally, a generic double layer

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model incorporating the surface configurations of phosphate and possible precipitation of

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calcium phosphate based on the multiple techniques was developed, successfully

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predicting the phosphate removal by La(OH)3 under different environmental conditions.

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MATERIALS AND METHODS

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Material Synthesis and Characterization. La(OH)3 nanorods were synthesized by

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precipitating an appropriate amount of analytical-grade La(NO3)3 in NaOH, and



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hydrothermally treating the suspension at 180 °C for 12 h.21 The white product was

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washed and dried prior to further use. A detailed description of the synthesis of La(OH)3

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nanorods is found in the Supporting Information (Text S1). All chemicals used in this

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study were of analytical grade.

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The morphology and particle size of the as-synthesized La(OH)3 were determined by

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transmission electron microscopy (TEM; JEOL JEM-2010, Japan) at an accelerating

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voltage of 200 kV, and by scanning electron microscopy (SEM; JEOL JSM-6700F,

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Japan). The structure of La(OH)3 was confirmed by X-ray diffraction (XRD; PANalytical

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X'pert Pro, Netherlands).

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Batch Experiments. The synthesized La(OH)3 nanorods have an excellent adsorption

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capacity and a high La use efficiency (Figure S1 and Table S1). Batch studies were

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conducted to examine the effects of background ions (e.g. Cl-, HCO3-, SO42-, NOM, Ca2+

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and Mg2+) and pH on the adsorption of phosphate on the as-synthesized La(OH)3. An

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appropriate amount of phosphate stock solution was spiked into 0.025g/L of the La(OH)3

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suspension, to achieve an initial phosphate concentration of 0.16 mM. The concentration

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of background inorganic anions ranged from 0 to 16.7 mM, and the concentration of

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NOM ranged from 0 to 2.5 mM as C. The pH of the solution was kept at 7.0 ± 0.1, with

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either NaOH or HCl. The effects of cations (i.e. Ca and Mg) on the adsorption of

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phosphate were investigated at pH values of 7.0 and 9.0. The effects of pH were

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investigated with and without the addition of Ca in a pH range of 3.0 to 10.0. The ionic

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strength of the bath studies was maintained using 0.01 M NaCl. The concentration of


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phosphate was determined using the ammonium molybdate method with a UV/vis

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spectrometer (Lambda 25, Perkin-Elmer, USA).3

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EXAFS Analysis. To probe the local coordination environment of phosphorus on the

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surface of La(OH)3, the extended X-ray fine structure (EXAFS) spectra were obtained in

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the fluorescence mode at beamline 4B7A of the Beijing Synchrotron Radiation Facility

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(BSRF; Beijing, China). A Si (111) crystal was used as the monochromator during

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measurement, and the energy of the electrons was 2.5 GeV in the storage ring. The

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scanned energy ranged between -200 and 1000 eV from the P K-edge, with minimum

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energy steps of 0.2 eV and a dwell time of 1 s per point. The EXAFS samples were

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prepared using a similar protocol to the batch experiments with an initial P concentration

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of 0.04 mM (0.08 mM for the samples with a higher P loading or in the presence of Ca)

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in a La(OH)3 dosage of 0.025 g/L at the desired pH values. The obtained samples were

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dried and directly pressed into tablets and pasted onto a sample holder made from

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stainless steel. The EXAFS data analysis was performed using Athena and Artemis.17

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Details of the analysis procedures are given in the Supporting Information (Text S2).

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ATR-FTIR Experiments. In order to obtain information on the protonation state of

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the loaded phosphate on La(OH)3, the ATR-FTIR spectra of the phosphate-loaded

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La(OH)3 were recorded on a Bruker Vertex 70 Hyperion 1000 spectrometer (Bruker,

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Germany) equipped with a platinum diamond ATR crystal accessory and MCT-A

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detector cooled by liquid N2. The samples of the phosphate-loaded La(OH)3 under

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different experimental conditions (i.e. pH and surface loading) were prepared using the

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same protocol as the batch experiments, and their ATR-FTIR spectra were immediately 7 ACS Paragon Plus Environment


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collected at 256 scans per spectrum with a 4 cm-1 resolution. The ATR-FTIR spectra of

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all samples were subtracted from their corresponding IR spectra of filtrate solution. For

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the ATR-FTIR spectra of the aqueous phosphate at pH values ranging from 3.0 to 10.0, a

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drop (16.1 mM) of phosphate solution at given pH conditions was added to the ATR

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crystal and the IR spectra were collected as the average of 256 scans at a 4 cm-1 resolution.

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The water spectrum was used as the background and subtracted from each individual

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spectrum. The second-derivative was used to locate the IR peak position of the phosphate

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during curve fitting.

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DFT Calculation. To verify the experimental IR spectra of the phosphate with

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La(OH)3, theoretical calculations of the IR frequency of the surface complexes of the

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phosphate on the La(OH)3 proceeded using the generalized gradient approximation (GGA)

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with the function parameterized by Perdew, Burke and Enzerhof (PBE) in the DMol3 of

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Material Studio 2017 (BIOVIA Inc.). The double numerical plus polarization (DNP)

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function basis set (basis file: 3.5) was employed with the effective core potentials used as

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lanthanum valence electron wave functions. The use of the DNP function basis set has

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been suggested to be more accurate than that of the Gaussian 6-31+G (d,p) basis set.22, 23

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A conductor-like screening model (COSMO) was applied to simulate the water solvent

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environment. A simple cluster model of a lanthanum hydroxide unit cell containing an

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edge-sharing seven-coordinated hexagonal cluster for calculating possible configurations

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with phosphate was built (Figure S2).17 The size of the cluster in the model has been

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proven to sufficiently satisfy the basic demands for the calculations of the interfacial

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configurations and IR frequency of small oxyanions like phosphate complexation on 8 ACS Paragon Plus Environment

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minerals.

This approach is also suggested to be more convincing than that using

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periodic slab models that cannot calculate the vibrational spectra of surface complexes on

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minerals or metal oxides very well.8, 24

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Chemical Equilibrium Modeling. A chemical equilibrium model including

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adsorption and precipitation reactions was developed to simulate different phosphate

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removal mechanisms. A generic double layer surface complexation model was used to

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establish the equilibrium between phosphate species in the aqueous phases and on the

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surface of the La(OH)3. Based on the findings of EXAFS, ATR-FTIR and the DFT

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calculations, bidentate surface complexes with two different protonation levels (H2 and

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H1) were employed in the model.25 However, the model doesn’t differentiate

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mononuclear and binuclear coordination to avoid over fitting the data with excessive

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fitting parameters. Possible precipitation of calcium and magnesium phosphate and the

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aqueous speciation reactions were also calculated in the model. The logKs values for the

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two surface complexation reactions were optimized by minimizing the sum of the

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residual squares between the experimental and simulated data for both 0.06 and 0.16 mM

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total P adsorption experiments. The log Ks values for the two precipitation reactions were

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subsequently optimized by fitting the experimental results with the addition of Ca and

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Mg, and the obtained constants were close to the values in the literature. The model was

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implemented by MINEQL+ 5.0 26, with the optimization executed by MINFIT. 27

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RESULTS AND DISCUSSION

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Characterization of the Synthesized La(OH)3 Nanorods. Figure 1 shows the XRD

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pattern of the synthesized material, where a highly crystalline structure was observed in



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line with that for a typical La(OH)3 (JCPDS 83-2034), and is significantly distinct from

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that of LaPO4. The XRD reflections can be indexed to a hexagonal phase [P63/m(no.176)]

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of La(OH)3.28 Electron microscopy study confirms the nanorod-like structure of the

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synthesized La(OH)3. Detailed discussion is given in the Supporting Information (Figure

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S3).

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Batch Studies. The competitive adsorption experiment is an intuitive approach to

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examine the selectivity of phosphate adsorption on La(OH)3, which can indirectly reflect

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whether inner-sphere complexation occurs when phosphate is adsorbed on La(OH)3,

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since the adsorption through the outer-sphere complexation is generally affected by

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background ions like sulfate and bicarbonate in water.

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presence of different anions such as SO42-, Cl-, HCO3- or natural organic matter (NOM)

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exhibits a negligible effect on the adsorption of phosphate on La(OH)3 at pH 7.0 ± 0.1,

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even when the molar ratio of the respective background anion to phosphate is up to 35 for

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Cl-, 40 for SO42-, 100 for HCO3- and 2.5 for NOM (as C), respectively. It should be noted

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that the given initial phosphate concentration (i.e. 0.16 mM) exceeds the estimated

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adsorption capacity of La(OH)3 nanorods (Figure S1). The highly selective adsorption of

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La(OH)3 for phosphate indicates that dominance of inner-sphere complexes of phosphate

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with La(OH)3 is likely. 13

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As shown in Figure S4, the

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Possible effects of cations (e.g. Ca2+ and Mg2+) on phosphate adsorption were also

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investigated in batch studies. Figure 2A shows that the presence of Ca can significantly

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enhance the adsorption of phosphate at pH 9.0 ± 0.1, while a negligible effect was

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observed for the case of Mg. In addition, the effect of Ca was further investigated

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together with the pH condition, and the findings are given in Figure 2B. The results show 10 ACS Paragon Plus Environment

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that phosphate adsorption gradually decreases from 6.45 to 1.5 mmol/g with the pH

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increasing from 4 to 10 when more phosphate than can be removed by La(OH)3 nanorods

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is present in the water (e.g. 0.16 mM). The presence of Ca (2.5 mM) shows insignificant

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effects in acidic to neutral conditions, while a dramatic increase of phosphate adsorption

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was found at pH > 7.5 (Figure 2B). This is in line with the findings reported by Lin and

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colleagues, where notably enhanced adsorption of phosphate on zirconium oxide was also

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observed.30 They suggested that this was due to the occurrence of co-precipitation of

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CaHPO4.

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It is well known that Ca can precipitate phosphate from neutral to alkaline conditions

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when the saturation index (SI) of calcium and phosphate-based minerals exceeds 0.

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Hence, a simple calculation was carried out to predict the possible precipitation under our

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experimental conditions using MINEQL+ 5.0.

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SI values of several calcium-based minerals, including hydroxyapatite, amorphous

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Ca3(PO4)2 and β- Ca3(PO4)2, exceed 0 at pH ≥ 7, amorphous Ca3(PO4)2 appears to have a

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high possibility of being present in our system (Figure S5). This is also confirmed by

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XRD analysis, showing that no new crystalline minerals such as hydroxyapatite or

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brushite formed at pH 9 (Figure S6).31 In addition, we also found about 4.6% of total Ca

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lost from aqueous solution after the reaction of phosphate with La(OH)3 at pH 9 (Figure

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2B). XPS analysis indicates that the Ca 2p spectrum of the solid sample obtained after

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sorption experiments at pH 9 has a comparably binding energy of 347.6 eV, which is in

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line with that for amorphous Ca3(PO4)2 (Figure S7).32 By combining this evidence, we

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conclude that the enhanced sorption of phosphate in the presence of Ca in alkaline

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conditions is likely due to the co-precipitation of amorphous Ca3(PO4)2.

26

Despite the prediction showing that the



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Phosphorus K-edge EXAFS Analysis. EXAFS was employed to directly probe the

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local coordination environment of P at the surface of La(OH)3 nanorods under different

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environmental conditions, such as pH and surface loadings of phosphate. The Fourier

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transform (FT) EXAFS spectra of P K-edge for the phosphate-loaded La(OH)3 are given

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in Figure 4 and the corresponding fitted parameters are listed in Table 1.

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The EXAFS fitting results show a significant oscillation at a distance of 1.52 to 1.57 Å

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for all samples, and this can be assigned to the first shell of P coordination with 4.0

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oxygen atoms. This P-O bonding distance is closely in line with previous findings by P

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K-edge EXAFS fitting (i.e. 1.51 – 1.52 Å),9 and DFT calculation (i.e. 1.56 – 1.57 Å). 8, 33

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Two P-La distances were obtained from the EXAFS analysis for the La(OH)3 sample

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after phosphate adsorption, i.e. a distance of 2.73 to 2.89 Å, and 3.17 – 3.21 Å,

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respectively (Figure 3 and Table 1). Interestingly, the coordination of P-La with two

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different distances only appears at pH values ranging from 3 to 7 when the surface

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loading of P is 1.6 mmol/g, however, at pH 9 only the coordination of P-La with a

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distance of 3.19 Å from P is present (Figure 3 and Table 1). Besides, the value of the

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coordination number (CN) can well reflect the surface configuration of P with La(OH)3.

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As shown in Table 1, both bidentate binuclear and bidentate mononuclear inner-sphere

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complexes of P on La (OH)3 exist at pH values from 3 to 7, but only the bidentate

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binuclear (BB) surface complex forms at pH 9. The shortest P-La distances of 2.73 to

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2.89 Å are probably due to the bidentate mononuclear (BM; 1V/2E) configuration that

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forms at pH between 3 and 7 with a P loading of 1.6 mmol/g, since these P-La distances

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are too short for the monodentate mononuclear configuration. The BM configuration of P

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on La(OH)3 is expected to be reasonable as similar surface complexation has been 12 ACS Paragon Plus Environment

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frequently observed for P and As(V) (analogue of P) on metal (hydr)oxides.9, 34, 35 Our

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results are partially in agreement with Xu and colleagues

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EXAFS finding of formation of BB surface complexes of phosphate with

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lanthanum/alumina. Further, our EXAFS data also do not support the formation of

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lanthanum phosphate since the reported value of 4.2 Å for the La-P distance of LaPO4 is

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too large compared with that in our study (Table 1). In addition, the results also suggest

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that the increase of P loading on La(OH)3 from 2.5 to 5 mM leads to a BB configuration

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of P with La at pH 7 (Figure 3 and Table 1), showing a similar pattern for the P

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configuration with iron hydroxide reported by Abdala and colleagues.9 Moreover, the

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EXAFS spectra show that the distance of La-P decreases from 3.19 to 2.76 Å after the

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addition of Ca at pH 9, and the CN value changes from 1.9 to 1.3, which indicates the

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possible transformation of the surface complexation of P with La(OH)3 from a BB to a

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BM configuration (Figure 3). This is likely due to the co-precipitation of amorphous

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Ca3(PO4)2 by Ca2+, leading to a lower P loading on the La(OH)3 (Figure 2B). Oscillation

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accounting for Ca-P was not observed in the sample, and is probably due to the weaker

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backscattering of Ca than La.

12

regarding the La L-edge

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ATR-FTIR Analysis and DFT Calculation. The protonation status of the P

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complexation on La(OH)3 is further crucial information for predicting P adsorption

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behavior.

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analysis is a powerful tool to complement such limitation. The ATR-FTIR spectra of the

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dissolved phosphate and the phosphate-loaded La(OH)3 under different pH conditions are

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depicted in Figure S8 and Figure 4A, where typical P-O fingerprint features are observed

17, 36

However, EXAFS is unable to identify these data, while ATR-FTIR



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in the frequency range of 900 – 1200 cm-1. Frequencies below 900 cm-1 are not

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considered due to overlap with the IR background of La(OH)3.

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The IR spectra of the P-La(OH)3 are markedly different from those of the dissolved P

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species (i.e. H3PO4, H2PO4- and HPO42-) (Figure 4A, Figures S8 and S9), indicating the

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formation of inner-sphere complexes for P-La(OH)3.24 With the increase of pH from 3 to

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9, a significant peak shift to a lower frequency is observed, due to the change of each

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characteristic band as a result of protonation/deprotonation change of the loaded P on

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La(OH)3.17 Each band in the P-O fingerprint range of 900 – 1200 cm-1 can be resolved

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through curve fitting and then compared with the calculated IR frequencies. As shown in

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Figure 5A the fitted ATR-FTIR spectrum exhibits vibrational modes at 1102, 1073, 1046,

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1018, 991, 956 and 918 cm-1, for the P-La(OH)3 at a pH of 3. A further increase of pH to

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5 results in one distinct group of vibrational modes at 1078, 1045, 1008, 962 and 917 cm-

297

1

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relative peak intensity at 1006 to 1046 cm-1 is significantly enhanced at pH 7 (Figure 4A).

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At pH 9, the fitted IR spectrum exhibits distinct vibrational modes at 1075, 1048, 1017,

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988 cm-1 and very weak bands at 955 and 918 cm-1 (Figure 4A). In the meantime, there is

301

no data on the characteristic peak positions for P adsorption on La(OH)3, so direct

302

assignment of these vibrational modes for P-La(OH)3 is difficult. Hence, DFT calculation

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was further applied to predict the IR frequencies of P-La(OH)3 with different

304

configurations. On the basis of the EXAFS findings (Figure 3 and Table 1), six surface

305

configurations

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mononuclear (BM-H0, BM-H1 and BM-H2), deprotonated, monoprotonated and

307

diprotonated bidentate binuclear (BB-H0, BB-H1 and BB-H2)] are considered in the DFT

, while similar vibrational modes appear at pH 7. In comparison to that at pH 5, the

[i.e.

deprotonated,

monoprotonated

and


diprotonated

bidentate

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calculation (Figure S10). As shown in Figure S11, DFT calculation suggests that none of

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the individual surface complex configurations can reproduce all the observed IR peaks

310

for the P-La(OH)3 at pH 3 to 7 (R2 ranges from 0.958 to 0.994). The results indicate that

311

a combination of BM-H2 with BB-H2 satisfactorily correlates with the observed IR

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frequencies for the P-La(OH)3 at pH 3, and the combination of BM-H1 with BB-H2

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complexes matches with that at pH 5 and 7. The formation of the BB-H2 configuration in

314

the pH range of 3 to 7 is reasonable, and is in line with a previous study by Shi et al.17

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who reported a similar finding for As(V) (analogue of phosphate) adsorption on La(OH)3.

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All of this evidence indicates that pH is a decisive factor on the protonation status of

317

interfacial P configurations. In addition, the presence of two P-La(OH)3 surface

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complexes at pH from 3 to 7 is also in line with our EXAFS findings (Figure 3 and Table

319

1), which is not unusual and has been extensively reported in the literature for the P

320

configuration on iron oxides.8, 24, 37 Nevertheless, at pH 9, the calculated IR frequencies

321

of the BM-H1 or BB-H1 configuration is found highly correlated with the fitted IR

322

frequencies derived from the experimental IR spectra of the samples (R2=0.941, Figure

323

S11). In combination with our EXAFS finding, our data suggest that BB-H1 likely

324

governs the surface configuration of P on La(OH)3 at pH 9 (Figure 3 and Table 1).

325

The effect of P loading on the P configuration was also investigated by ATR-FTIR,

326

and the corresponding results are depicted in Figures 4 and S12. The results show that the

327

peak intensity increases with increasing P loading, which agrees with the finding of the

328

batch studies (Figure S1). Further, the fitted IR spectrum of the lowest P loading (i.e. 0.4

329

mmol/g) exhibits more distinct vibrational modes at 1020 and 988 cm-1 than those with

330

higher P loadings (i.e. 1007 – 1009 cm-1, Figure 4B). The unique IR vibrational modes at 15 ACS Paragon Plus Environment


331

1020 and 988 cm-1 likely correspond to the BB-H1 configuration because of their high

332

correlation with the calculated IR frequencies of BB-H1 (Figure S13). With the increase

333

of P loading, the vibrational modes at 1020 and 988 cm-1 shift to a single vibrational

334

mode at about 1008 cm-1 that may be due to the formation of a BB-H2 or BM-H1

335

configuration (Figure 4B). Interestingly, Abdala et al. 9 reported a similar finding, where

336

P formed a BB configuration on goethite at low P loading and then transformed to a BM

337

and subsequently, a BB configuration with increase of P loading. These findings are also

338

in accordance with the above EXAFS results, showing that the BB configuration of

339

phosphate formation occurred with an increase of P loading (Figure 3 and Table 1). In

340

order to examine the possible transformation of the crystal structure reported by other

341

groups,10, 14 XRD analysis was carried out to confirm the crystal structure of La(OH)3

342

after P adsorption at pH 9. As shown in Figure S14, the nanorod-like structure of

343

La(OH)3 still remains after loading different amounts of P on the surface La(OH)3 and the

344

crystal structure of P-La(OH)3 is markedly different from that of LaPO4 (Figure 1).38

345

With the increase of Ca concentration from 0 to 5 mM, the ATR-FTIR spectra also show

346

significantly enhanced peak intensities within the range of 900 – 1200 cm-1 (Figure S15),

347

due to the enhanced P removal.39 The IR spectra of the P-La(OH)3 samples became

348

broader after the introduction of Ca, which is likely due to the presence of an amorphous

349

Ca3(PO4)2 phase.40 This has also been demonstrated by our theoretical calculation and

350

XPS study (Figures S5 and S7).

351

Chemical Equilibrium Modeling. The surface complexation model successfully

352

predicts the variation of surface complex speciation of phosphate on La(OH)3 as a

353

function of pH, P loading and the absence and presence of Ca (Figure 2B and Table S2). 16 ACS Paragon Plus Environment

Page 16 of 34

Page 17 of 34


354

The predicted protonation levels of the surface complexes from acid, neutral to alkaline

355

pH, as well as at excessively high phosphate concentration, followed the

356

spectroscopically and computationally determined results (Figure S16). The model

357

predicted the formation of amorphous Ca3(PO4)2 when the combination of pH and the

358

concentration of Ca enable its saturation (Figure 2, Figures S5 and S16). Precipitation

359

enhances the overall removal of the dissolved phosphate ions and may also outcompete

360

the adsorption contribution, leading to the transformation of a BB to BM configuration as

361

shown in our EXAFS data (Table 1).41 The model also shows a fine prediction of

362

phosphate removal by La(OH)3 in the presence of Ca and Mg at pH 9.0 ± 0.1 (Figure 2A);

363

magnesium phosphate has higher solubility than calcium phosphate, and the model only

364

predicted minor precipitation for the highest (i.e. 8 mM) Mg concentration. Overall, the

365

proposed mechanisms of adsorption and precipitation for phosphate removal are

366

quantitatively verified by the surface complexation model. Our results indicate that the

367

model has the potential to accurately extrapolate the removal of phosphate by La(OH)3

368

under different environmental conditions, such as pH, P loading and the presence of Ca

369

and Mg (Figure S16).

370

371

Environmental Significance. Given the important role of La-based sorbents for

372

managing phosphate in eutrophic lake and sewage effluents ,12, 14 this work provides new

373

insights into the molecular-level removal mechanism of phosphate by means of batch

374

studies, EXAFS, ATR-FTIR and DFT calculations (Figure 6). The combination of

375

multiple spectroscopic techniques with theoretical calculations (e.g. DFT) allows us to

376

build the entire picture of surface complexation of phosphate with La(OH)3, and provides 17 ACS Paragon Plus Environment


377

a reliable approach in future efforts to probe the interfacial chemistry of other

378

oxyanions.36 Our findings on the P configurations on La(OH)3 and possible precipitation

379

of phosphate in the presence of Ca or Mg give concrete evidence for developing surface

380

complexation models, which can be used to predict phosphate removal using La(OH)3

381

during lake restoration or sewage treatment. The prediction of phosphate removal should

382

consider specific environmental variables such pH values (e.g. pH from 5 to 9 for natural

383

lakes, and about 7.5 for sewage),3, 42 surface P loading, and the presence of Ca and Mg at

384

high pH values (e.g. pH 9). In spite of the high efficiency of La in phosphate removal and

385

the relatively low acute toxicity,12 the ecological safety in regard to the extensive

386

application of La-based sorbents requires caution, in particular, the long-term effects on

387

aquatic organisms should be carefully evaluated in future work.

388

389

Supporting Information

390

Detailed synthesis of La(OH)3 nanorods, EXAFS data collection and analysis,

391

adsorption of phosphate on La(OH)3, comparison of La use efficiencies of La-based

392

sorbents, cluster structure of La(OH)3, electron microscopic images of La(OH)3,

393

competitive adsorption of background anions with phosphate, calculated saturation

394

indexes of calcium phosphate species, X-ray diffractograms of La(OH)3, Ca 2p XPS

395

spectra, ATR-FTIR spectra of aqueous phosphate and phosphate loaded La(OH)3,

396

phosphate species distribution, configurations of P on La(OH)3, correlations of the

397

calculated and experimental IR frequencies of phosphate, reaction equations for surface


Page 18 of 34

Page 19 of 34


398

complexation modelling, comparison of experimental results with surface complexation

399

modelling results, are provided in the Supporting Information.

400

401

ACKNOWLEDGEMENTS

402

This work was supported financially by the Research Grants Council of Hong Kong

403

(GRF16207916; T21-711/16-R-1). The authors are thankful for the technical support

404

from the Advanced Engineering Material Facility of the Hong Kong University of

405

Science and Technology (AEMF-HKUST). EXAFS beam time was granted by the 4B7A

406

endstation of the Beijing Synchrotron Radiation Facility at the Institute of High Energy

407

Physics, Chinese Academy of Sciences. Dr. Lei Zheng from 4B7A beamline is gratefully

408

acknowledged for his support in the measurements and data reduction. Jessica Nguyen

409

was supported by a Huel Perkins Diversity Fellowship.



410

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411 412

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for Phosphorus Speciation in Soils and Other Environmental Systems All rights reserved.

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No part of this periodical may be reproduced or transmitted in any form or by any means,

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TOC/graphic abstract

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545


Table 1. Structure parameters derived from P K-edge EXAFS analysis. Samples Path CN a R (Å) b σ2 (Å2) c ∆E0 (eV) d

(a) pH 3

(b) pH 5

(c) pH 7_1i

(d) pH 7_2i

(e) pH 9.0

(f) pH 9+Ca

546 547 548 549

a

P-O

4.0f

1.52 (3)

0.005 (1)

P-Lag

1.4 (7)

2.73 (13)

0.014 (8)

P-Lah

2.0 (1)

3.21 (18)

0.025 (9)

P-O

4.0 f

1.52 (2)

0.003 (2)

P-Lag

1.0 (4)

2.89 (13)

0.012 (7)

P-Lah

2.0 (4)

3.18 (16)

0.013 (6)

P-O

4.0 f

1.57 (2)

0.004 (3)

1.2 (4)

2.85 (14)

0.006 (3)

P-Lah

2.0 (1)

3.21 (5)

0.006 (5)

P-O

4.0 f

1.52 (1)

0.002 (1)

P-Lag

-

-

-

P-Lah

1.9 (3)

3.17 (4)

0.004 (3)

P-O

4.0 f

1.53 (2)

0.003 (1)

P-Lag

-

-

-

P-Lah

1.9 (4)

3.19 (5)

0.013 (5)

P-O

4.0 f

1.53 (2)

0.007 (1)

P-Lag

1.3 (4)

2.76 (9)

0.014 (3)

P-Lah

-

-

-

P-Lag

b

c

R-factor e

2.5

0.018

0.7

0.027

13.3

0.016

2.4

0.023

9.1

0.030

3.6

0.029

coordination number. interatomic distance. Debye-Waller factor. dthreshold energy shift. egoodness-of-fit parameter: R-factor = Σ(χdata – χfit)2/Σ(χdata)2. fthe parameters were fixed during fitting. Parentheses: the estimated parameter uncertainties are listed in parentheses, representing the errors in the last digit. gP-La distance represents a bidentate 27 ACS Paragon Plus Environment


550 551 552

mononuclear configuration (BM). hP-La distance represents a bidentate binuclear configuration (BB). i pH 7_1 represents the sample with lower P loading of 1.6 mmol/g, while pH 7_2 represents the sample with a higher P loading of 3.2 mmol/g.

553


Page 28 of 34

Page 29 of 34


554 555 556

Figure 1. X-ray diffraction (XRD) pattern of the as-synthesized La(OH)3 before and after loading different amounts of phosphate at pH 7.0.

557



558

559 560 561 562 563 564 565 566

Figure 2. The effect of Mg and Ca on the removal of phosphate by La(OH)3 at pH 9.0 ± 0.1 (A). The initial concentration of phosphate was 0.16 mM, and the dosage of La(OH)3 was 0.025 g/L. The band area represents the simulated phosphate removal by La(OH)3 at pH 9.0 ± 0.1 in the presence of different concentrations of Ca or Mg; the experimental results and surface complexation modeling results for the effects of pH on the phosphate removal by La(OH)3 in the presence and absence of Ca (B). The initial phosphate concentration was 0.08 and 0.16 mM, and the dosage of La(OH)3 was 0.025 g/L.

567


Page 30 of 34

Page 31 of 34


568

569 570 571 572 573 574 575

Figure 3. Normalized k2-weighted experimental (dots) and their fitted (lines) Phosphorus (P) K-edge EXAFS spectra of phosphate removal by La(OH)3 (A), the corresponding Fourier transformed magnitude (B), and real parts of the Fourier transform. Data sets represent the samples of P-La(OH)3 with P loading of 1.6 mmol/g at pH 3 (a), pH 5 (b), pH 7 (c), pH 7 with a higher P loading of 3.2 mmol/g (d), pH 9 (e), and in the presence of Ca (5 mM) at pH 9.0 (f). The corresponding parameters are listed in Table 1.

576



577

578 579 580

Figure 4. ATR-FTIR spectra of the phosphate loaded La(OH)3 at pH 3, pH 5, pH 7 and pH 9 (A), and the phosphate loaded La(OH)3 with a phosphate loading of 0.4, 1.6, 3.2 32 ACS Paragon Plus Environment

Page 32 of 34

Page 33 of 34

581 582 583


and 5.5 mmol/g at pH 7 (B). Slurries were obtained by performing phosphate adsorption experiments at a La(OH)3 dosage of 0.025 g/L and an initial phosphate concentration of 0.16 mM.

584

`

585 586 587 588 589

Figure 5. ATR-FTIR spectra of the phosphate loaded La(OH)3 at pH 9.0 in the presence of 5 mM and 2.5 mM (A-B), and the absence (C) of Ca. Slurries were obtained by performing phosphate adsorption experiments at a La(OH)3 dosage of 0.025 g/L and an initial phosphate concentration of 0.16 mM at pH 9.0.

590



591

592 593 594

Figure 6. A schematic diagram of a potential surface complexation of phosphate on La(OH)3 under different pH conditions.

595 596


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Removal Mechanisms of Phosphate by Lanthanum Hydroxide

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