Removal Mechanisms of Phosphate by Lanthanum Hydroxide

Oct 16, 2017 - Department of Civil and Environmental Engineering, Louisiana State ...... Mayer , B. K.; Baker , L. A.; Boyer , T. H.; Drechsel , P.; G...
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Article Cite This: Environ. Sci. Technol. 2017, 51, 12377-12384

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Removal Mechanisms of Phosphate by Lanthanum Hydroxide Nanorods: Investigations using EXAFS, ATR-FTIR, DFT, and Surface Complexation Modeling Approaches Liping Fang,† Qiantao Shi,‡ Jessica Nguyen,§ Baile Wu,† Zimeng Wang,§ and Irene M. C. Lo*,† †

Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Center for Environmental Systems, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States § Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States Downloaded via IOWA STATE UNIV on January 26, 2019 at 13:49:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Lanthanum-based materials are effective for sequestering phosphate in water, however, their removal mechanisms remain unclear, and the effects of environmentally relevant factors have not yet been studied. Hereby, this study explored the mechanisms of phosphate removal using La(OH)3 by employing extended X-ray absorption spectroscopy (EXAFS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), density functional theory (DFT) and chemical equilibrium modeling. The results showed that surface complexation was the primary mechanism for phosphate removal and in binary phosphate configurations, namely diprotonated bidentate mononuclear (BM-H2) and bidentate binuclear (BB-H2), coexisting on La(OH)3 in acidic conditions. By increasing the pH to 7, BM-H1 and BB-H2 were the two major configurations governing phosphate adsorption on La(OH)3, whereas BB-H1 was the dominant configuration of phosphate adsorption at pH 9. With increasing phosphate loading, the phosphate configuration of on La(OH)3 transforms from binary BM-H1 and BB-H2 to BB-H1. Amorphous Ca3(PO4)2 forms in the presence of Ca, leading to enhanced phosphate removal at alkaline conditions. The contributions of different mechanisms to the overall phosphate removal were successfully simulated by a chemical equilibrium model that was consistent with the spectroscopic results. This study provides new insights into the molecular-level mechanism of phosphate removal by La(OH)3.

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INTRODUCTION Conventional biological processes have poor ability to remove phosphate from sewage, causing excessive discharge of phosphorus (P) into natural water bodies. As a consequence, this can lead to eutrophication with algal blooms and red tides, damaging the water quality and eventually threatening wildlife and human health.1,2 Adsorption is an economical and efficient solution for removing phosphate from water,3 and metal (hydr)oxide-based adsorbents are deemed promising for phosphate removal because of their strong affinity for phosphate.4−6 In the past decades, extensive studies have shown that the mechanism of phosphate adsorption on typical minerals like Fe/Al (hydr)oxides is likely through the formation of innersphere surface complexes;5,7 phosphate can substitute for either one or two hydroxyl groups on the surface of metal (hydr)oxides to form monodentate (MD), bidentate (BD) or coexisting MD/ BD binding surface complexes depending on the pH and surface loading.7−9 Structural information (e.g., surface-specific bond distance) obtained from interpreting the surface configuration of phosphate on Fe/Al (hydr)oxides at the molecular-scale, can be useful in predicting the phosphate adsorption with metal (hydro)oxides under different environmental conditions.8 © 2017 American Chemical Society

As one of the most promising adsorbents, besides commercialized lanthanum modified bentonite clay (i.e., Phoslock), lanthanum hydroxide (i.e., La(OH)3) is receiving increasing attention for its high potential in effectively removing phosphate even at trace levels.10,11 To date, a number of lanthanum-based adsorbents with high phosphate adsorption capacities have been developed to tackle the problem of phosphate-contaminated water.10,12,13 Although tremendous efforts have been made to develop lanthanum-based adsorbents for phosphate removal, there has been less attention paid to developing a full understanding of the mechanisms of phosphate adsorption on La(OH)3 at the molecular level. Using Fourier transform infrared spectroscopy (FTIR) and macroscopic batch methods, Xie and colleagues reported that phosphate ions form deprotonated MD complexes on La(OH)3 at pH > 8.2.13 Using X-ray diffraction (XRD) and 31P solid-state nuclear magnetic resonance (31P NMR), Zhang et al.10 and Dithmer et al.14 observed from their Received: Revised: Accepted: Published: 12377

July 25, 2017 October 10, 2017 October 16, 2017 October 16, 2017 DOI: 10.1021/acs.est.7b03803 Environ. Sci. Technol. 2017, 51, 12377−12384

Article

Environmental Science & Technology experiments that a new crystal phase of LaPO4·xH2O was found after phosphate adsorption. However, the exact structural information on phosphate coordination with La(OH)3 remains unclear. In addition, current knowledge about the mechanisms of phosphate adsorption on La(OH)3 has been obtained under different experimental conditions. As pH and surface loading may critically influence the local atomic environment of phosphate,9,15 a systematic investigation across a wide range of water chemistry parameters is warranted. As an element-specific technique, extended X-ray absorption fine structure (EXAFS) spectroscopy has shown advantages in directly probing the surface configuration of oxyanions (e.g., phosphate, arsenate, and sulfate) on mineral surfaces.9,16−19 Although two research groups both characterized the structure of phosphate complexes on La-bearing adsorbents by applying La K-edge EXAFS,12,14 it is more straightforward to investigate the mechanisms of phosphate removal by a pure phase of La(OH)3 using P K-edge EXAFS, as this can could give more accurate information on the surface configuration of P with La. Moreover, EXAFS is not able to reflect the protonation states of phosphate on the surface of La(OH)3 under different pHs, without the complementation of other techniques such as attenuated total reflectance (ATR)-FTIR spectroscopy with density functional theory (DFT).20 To address the aforementioned research gaps, the objectives of this study were to determine the molecular configurations and protonation states of phosphate in association with La(OH)3 as a function of pH, P loading and coexisting ion concentrations by combining batch sorption experiments, EXAFS, ATR-FTIR, DFT calculation, and chemical equilibrium modeling techniques. La(OH)3 were used as the model in this study. Batch studies were first undertaken to investigate how background ions (e.g., Cl, NO3−, HCO3−, SO42−, Ca and Mg), natural organic matter (NOM) and pH influence phosphate adsorption on La(OH)3. ATR-FTIR spectroscopy was used to identify the phosphate-La(OH)3 complexes that formed under different pH values and phosphate loadings, and DFT calculations were used to predict the IR spectra of phosphate after adsorption on La(OH)3 and to compare the predictions compared with experimental IR data. Further, the local structural environment of the phosphate was directly probed under different environmental conditions by means of P K-edge EXAFS. Finally, a generic double layer model incorporating the surface configurations of phosphate and possible precipitation of calcium phosphate based on the multiple techniques was developed, successfully predicting the phosphate removal by La(OH)3 under different environmental conditions.

Batch Experiments. The synthesized La(OH)3 nanorods have an excellent adsorption capacity and a high La use efficiency (SI Figure S1 and Table S1). Batch studies were conducted to examine the effects of background ions (e.g., Cl−, HCO3−, SO42−, NOM, Ca2+, and Mg2+) and pH on the adsorption of phosphate on the as-synthesized La(OH)3. An appropriate amount of phosphate stock solution was spiked into 0.025g/L of the La(OH)3 suspension, to achieve an initial phosphate concentration of 0.16 mM. The concentration of background inorganic anions ranged from 0 to 16.7 mM, and the concentration of NOM ranged from 0 to 2.5 mM as C. The pH of the solution was kept at 7.0 ± 0.1, with either NaOH or HCl. The effects of cations (i.e., Ca and Mg) on the adsorption of phosphate were investigated at pH values of 7.0 and 9.0. The effects of pH were investigated with and without the addition of Ca in a pH range of 3.0−10.0. The ionic strength of the bath studies was maintained using 0.01 M NaCl. The concentration of phosphate was determined using the ammonium molybdate method with a UV/vis spectrometer (Lambda 25, PerkinElmer).3 EXAFS Analysis. To probe the local coordination environment of phosphorus on the surface of La(OH)3, the extended X-ray fine structure (EXAFS) spectra were obtained in the fluorescence mode at beamline 4B7A of the Beijing Synchrotron Radiation Facility (BSRF; Beijing, China). A Si (111) crystal was used as the monochromator during measurement, and the energy of the electrons was 2.5 GeV in the storage ring. The scanned energy ranged between −200 and 1000 eV from the P K-edge, with minimum energy steps of 0.2 eV and a dwell time of 1 s per point. The EXAFS samples were prepared using a similar protocol to the batch experiments with an initial P concentration of 0.04 mM (0.08 mM for the samples with a higher P loading or in the presence of Ca) in a La(OH)3 dosage of 0.025 g/L at the desired pH values. The obtained samples were dried and directly pressed into tablets and pasted onto a sample holder made from stainless steel. The EXAFS data analysis was performed using Athena and Artemis.17 Details of the analysis procedures are given in the SI Text S2. ATR-FTIR Experiments. In order to obtain information on the protonation state of the loaded phosphate on La(OH)3, the ATR-FTIR spectra of the phosphate-loaded La(OH)3 were recorded on a Bruker Vertex 70 Hyperion 1000 spectrometer (Bruker, Germany) equipped with a platinum diamond ATR crystal accessory and MCT-A detector cooled by liquid N2. The samples of the phosphate-loaded La(OH)3 under different experimental conditions (i.e., pH and surface loading) were prepared using the same protocol as the batch experiments, and their ATR-FTIR spectra were immediately collected at 256 scans per spectrum with a 4 cm−1 resolution. The ATR-FTIR spectra of all samples were subtracted from their corresponding IR spectra of filtrate solution. For the ATR-FTIR spectra of the aqueous phosphate at pH values ranging from 3.0 to 10.0, a drop (16.1 mM) of phosphate solution at given pH conditions was added to the ATR crystal and the IR spectra were collected as the average of 256 scans at a 4 cm−1 resolution. The water spectrum was used as the background and subtracted from each individual spectrum. The second-derivative was used to locate the IR peak position of the phosphate during curve fitting. DFT Calculation. To verify the experimental IR spectra of the phosphate with La(OH)3, theoretical calculations of the IR frequency of the surface complexes of the phosphate on the La(OH)3 proceeded using the generalized gradient approximation (GGA) with the function parametrized by Perdew, Burke and Enzerhof (PBE) in the DMol3 of Material Studio

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MATERIALS AND METHODS Material Synthesis and Characterization. La(OH)3 nanorods were synthesized by precipitating an appropriate amount of analytical-grade La(NO3)3 in NaOH, and hydrothermally treating the suspension at 180 °C for 12 h.21 The white product was washed and dried prior to further use. A detailed description of the synthesis of La(OH)3 nanorods is found in the Supporting Information (SI) Text S1. All chemicals used in this study were of analytical grade. The morphology and particle size of the as-synthesized La(OH)3 were determined by transmission electron microscopy (TEM; JEOL JEM-2010, Japan) at an accelerating voltage of 200 kV, and by scanning electron microscopy (SEM; JEOL JSM-6700F, Japan). The structure of La(OH)3 was confirmed by X-ray diffraction (XRD; PANalytical X′pert Pro, Netherlands). 12378

DOI: 10.1021/acs.est.7b03803 Environ. Sci. Technol. 2017, 51, 12377−12384

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Environmental Science & Technology

used to establish the equilibrium between phosphate species in the aqueous phases and on the surface of the La(OH)3. Based on the findings of EXAFS, ATR-FTIR, and the DFT calculations, bidentate surface complexes with two different protonation levels (H2 and H1) were employed in the model.25 However, the model does not differentiate mononuclear and binuclear coordination to avoid over fitting the data with excessive fitting parameters. Possible precipitation of calcium and magnesium phosphate and the aqueous speciation reactions were also calculated in the model. The logKs values for the two surface complexation reactions were optimized by minimizing the sum of the residual squares between the experimental and simulated data for both 0.06 and 0.16 mM total P adsorption experiments. The log Ks values for the two precipitation reactions were subsequently optimized by fitting the experimental results with the addition of Ca and Mg, and the obtained constants were close to the values in the literature. The model was implemented by MINEQL+ 5.0,26 with the optimization executed by MINFIT.27

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.

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RESULTS AND DISCUSSION Characterization of the Synthesized La(OH)3 Nanorods. Figure 1 shows the XRD pattern of the synthesized material, where a highly crystalline structure was observed in line with that for a typical La(OH)3 (JCPDS 83−2034), and is significantly distinct from that of LaPO4. The XRD reflections can be indexed to a hexagonal phase [P63/m(no.176)] of La(OH)3.28 Electron microscopy study confirms the nanorodlike structure of the synthesized La(OH)3. A detailed discussion is given in the SI Figure S3. Batch Studies. The competitive adsorption experiment is an intuitive approach to examine the selectivity of phosphate adsorption on La(OH)3, which can indirectly reflect whether inner-sphere complexation occurs when phosphate is adsorbed on La(OH)3, since the adsorption through the outer-sphere complexation is generally affected by background ions like sulfate and bicarbonate in water.29 As shown in SI Figure S4, the presence of different anions such as SO42−, Cl−, HCO3− or natural organic matter (NOM) exhibits a negligible effect on the adsorption of phosphate on La(OH)3 at pH 7.0 ± 0.1, even when the molar ratio of the respective background anion to phosphate is up to 35 for Cl−, 40 for SO42−, 100 for HCO3− and 2.5 for NOM (as C), respectively. It should be noted that the given initial phosphate concentration (i.e., 0.16 mM) exceeds

2017 (BIOVIA Inc.). The double numerical plus polarization (DNP) function basis set (basis file: 3.5) was employed with the effective core potentials used as lanthanum valence electron wave functions. The use of the DNP function basis set has been suggested to be more accurate than that of the Gaussian 6-31+G (d,p) basis set.22,23 A conductor-like screening model (COSMO) was applied to simulate the water solvent environment. A simple cluster model of a lanthanum hydroxide unit cell containing an edge-sharing seven-coordinated hexagonal cluster for calculating possible configurations with phosphate was built (SI Figure S2).17 The size of the cluster in the model has been proven to sufficiently satisfy the basic demands for the calculations of the interfacial configurations and IR frequency of small oxyanions like phosphate complexation on minerals.24 This approach is also suggested to be more convincing than that using periodic slab models that cannot calculate the vibrational spectra of surface complexes on minerals or metal oxides very well.8,24 Chemical Equilibrium Modeling. A chemical equilibrium model including adsorption and precipitation reactions was developed to simulate different phosphate removal mechanisms. A generic double layer surface complexation model was

Figure 2. 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. 12379

DOI: 10.1021/acs.est.7b03803 Environ. Sci. Technol. 2017, 51, 12377−12384

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Despite the prediction showing that the SI values of several calcium-based minerals, including hydroxyapatite, amorphous Ca3(PO4)2 and β- Ca3(PO4)2, exceed 0 at pH ≥ 7, amorphous Ca3(PO4)2 appears to have a high possibility of being present in our system (SI Figure S5). This is also confirmed by XRD analysis, showing that no new crystalline minerals such as hydroxyapatite or brushite formed at pH 9 (SI Figure S6).31 In addition, we also found about 4.6% of total Ca lost from the aqueous solution after the reaction of phosphate with La(OH)3 at pH 9 (Figure 2B). XPS analysis indicates that the Ca 2p spectrum of the solid sample obtained after sorption experiments at pH 9 has a comparable binding energy of 347.6 eV, which is in line with that for amorphous Ca3(PO4)2 (SI Figure S7).32 By combining this evidence, we conclude that the enhanced sorption of phosphate in the presence of Ca in alkaline conditions is likely due to the coprecipitation of amorphous Ca3(PO4)2. Phosphorus K-Edge EXAFS Analysis. EXAFS was employed to directly probe the local coordination environment of P at the surface of La(OH)3 nanorods under different environmental conditions, such as pH and surface loadings of phosphate. The Fourier transform (FT) EXAFS spectra of P K-edge for the phosphate-loaded La(OH)3 are given in Figure 4 and the corresponding fitted parameters are listed in Table 1. The EXAFS fitting results show a significant oscillation at a distance of 1.52 to 1.57 Å for all samples, and this can be assigned to the first shell of P coordination with 4.0 oxygen atoms. This P−O bonding distance is closely in line with previous findings by P K-edge EXAFS fitting (i.e., 1.51−1.52 Å),9 and DFT calculation (i.e., 1.56−1.57 Å).8,33 Two P−La distances were obtained from the EXAFS analysis for the La(OH)3 sample after phosphate adsorption, that is, a distance of 2.73− 2.89 Å, and 3.17−3.21 Å, respectively (Figure 3 and Table 1). Interestingly, the coordination of P−La with two different distances only appears at pH values ranging from 3 to 7 when the surface loading of P is 1.6 mmol/g, however, at pH 9 only the coordination of P−La with a distance of 3.19 Å from P is present (Figure 3 and Table 1). Besides, the value of the coordination number (CN) can well reflect the surface configuration of P with La(OH)3. As shown in Table 1, both bidentate binuclear and bidentate mononuclear inner-sphere complexes of P on La (OH)3 exist at pH values from 3 to 7, but only the bidentate binuclear (BB) surface complex forms at pH 9. The shortest P−La distances of 2.73−2.89 Å are probably due to the bidentate mononuclear (BM; 1V/2E) configuration that forms at pH between 3 and 7 with a P loading of 1.6 mmol/g, since these P−La distances are too short for the monodentate mononuclear configuration. The BM configuration of P on La(OH)3 is expected to be reasonable as similar surface complexation has been frequently observed for P and As(V) (analogue of P) on metal (hydr)oxides.9,34,35 Our results are partially in agreement with Xu and colleagues12 regarding the La L-edge EXAFS finding of formation of BB surface complexes of phosphate with lanthanum/alumina. Further, our EXAFS data also do not support the formation of lanthanum phosphate since the reported value of 4.2 Å for the La−P distance of LaPO4 is too large compared with that in our study (Table 1). In addition, the results also suggest that the increase of P loading on La(OH)3 from 2.5 to 5 mM leads to a BB configuration of P with La at pH 7 (Figure 3 and Table 1), showing a similar pattern for the P configuration with iron hydroxide reported by Abdala and colleagues.9 Moreover, the EXAFS spectra show that the distance of La−P decreases from 3.19 to 2.76 Å after the addition of Ca at pH 9, and the CN value changes from 1.9 to 1.3,

Table 1. Structure Parameters Derived from P K-Edge EXAFS Analysis samples

path

CNa

R (Å)b

f

σ2 (Å2)c ΔE0 (eV)d R-factore

(a) pH 3

P−O 4.0 P−Lag 1.4 (7) P−Lah 2.0 (1)

1.52 (3) 2.73 (13) 3.21 (18)

0.005 (1) 0.014 (8) 0.025 (9)

(b) pH 5

P−O 4.0f P−Lag 1.0 (4) P−Lah 2.0 (4)

1.52 (2) 2.89 (13) 3.18 (16)

0.003 (2) 0.012 (7) 0.013 (6)

(c) pH 7_1i P−O 4.0f P−Lag 1.2 (4) P−Lah 2.0 (1)

1.57 (2) 2.85 (14) 3.21 (5)

0.004 (3) 0.006 (3) 0.006 (5)

(d) pH 7_2i P−O 4.0f P−Lag P−Lah 1.9 (3)

1.52 (1)

0.002 (1)

3.17 (4)

0.004 (3)

1.53 (2)

0.003 (1)

3.19 (5)

0.013 (5)

1.53 (2) 2.76 (9)

0.007 (1) 0.014 (3)

(e) pH 9.0

P−O 4.0f P−Lag P−Lah 1.9 (4)

(f) pH 9+Ca P−O 4.0f P−Lag 1.3 (4) P−Lah

2.5

0.018

0.7

0.027

13.3

0.016

2.4

0.023

9.1

0.030

3.6

0.029

a

Coordination number. bInteratomic distance. cDebye−Waller factor. Threshold 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 mononuclear configuration (BM). hP−La distance represents a bidentate binuclear configuration (BB). ipH 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. d

the estimated adsorption capacity of La(OH)3 nanorods (SI Figure S1). The highly selective adsorption of La(OH)3 for phosphate indicates that dominance of inner-sphere complexes of phosphate with La(OH)3 is likely.13 Possible effects of cations (e.g., Ca2+ and Mg2+) on phosphate adsorption were also investigated in batch studies. Figure 2A shows that the presence of Ca can significantly enhance the adsorption of phosphate at pH 9.0 ± 0.1, while a negligible effect was observed for the case of Mg. In addition, the effect of Ca was further investigated together with the pH condition, and the findings are given in Figure 2B. The results show that phosphate adsorption gradually decreases from 6.45 to 1.5 mmol/g with the pH increasing from 4 to 10 when more phosphate than can be removed by La(OH)3 nanorods is present in the water (e.g., 0.16 mM). The presence of Ca (2.5 mM) shows insignificant effects in acidic to neutral conditions, while a dramatic increase of phosphate adsorption was found at pH > 7.5 (Figure 2B). This is in line with the findings reported by Lin and colleagues, where notably enhanced adsorption of phosphate on zirconium oxide was also observed.30 They suggested that this was due to the occurrence of coprecipitation of CaHPO4. It is well-known that Ca can precipitate phosphate from neutral to alkaline conditions when the saturation index (SI) of calcium and phosphate-based minerals exceeds 0. Hence, a simple calculation was carried out to predict the possible precipitation under our experimental conditions using MINEQL+ 5.0.26 12380

DOI: 10.1021/acs.est.7b03803 Environ. Sci. Technol. 2017, 51, 12377−12384

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Environmental Science & Technology

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.

for P adsorption on La(OH)3, so direct assignment of these vibrational modes for P−La(OH)3 is difficult. Hence, DFT calculation was further applied to predict the IR frequencies of P−La(OH)3 with different configurations. On the basis of the EXAFS findings (Figure 3 and Table 1), six surface configurations [i.e., deprotonated, monoprotonated and diprotonated bidentate mononuclear (BM-H0, BM-H1, and BM-H2), deprotonated, monoprotonated and diprotonated bidentate binuclear (BB-H0, BB-H1, and BB-H2)] are considered in the DFT calculation (SI Figure S10). As shown in SI Figure S11, DFT calculation suggests that none of the individual surface complex configurations can reproduce all the observed IR peaks for the P−La(OH)3 at pH 3 to 7 (R2 ranges from 0.958 to 0.994). The results indicate that a combination of BM-H2 with BB-H2 satisfactorily correlates with the observed IR frequencies for the P−La(OH)3 at pH 3, and the combination of BM-H1 with BB-H2 complexes matches with that at pH 5 and 7. The formation of the BB-H2 configuration in the pH range of 3−7 is reasonable, and is in line with a previous study by Shi et al.17 who reported a similar finding for As(V) (analogue of phosphate) adsorption on La(OH)3. All of this evidence indicates that pH is a decisive factor on the protonation status of interfacial P configurations. In addition, the presence of two P−La(OH)3 surface complexes at pH from 3 to 7 is also in line with our EXAFS findings (Figure 3 and Table 1), which is not unusual and has been extensively reported in the literature for the P configuration on iron oxides.8,24,37 Nevertheless, at pH 9, the calculated IR frequencies of the BM-H1 or BB-H1 configuration is found highly correlated with the fitted IR frequencies derived from the experimental IR spectra of the samples (R2 = 0.941, SI Figure S11). In combination with our EXAFS finding, our data suggest that BB-H1 likely governs the surface configuration of P on La(OH)3 at pH 9 (Figure 3 and Table 1). The effect of P loading on the P configuration was also investigated by ATR-FTIR, and the corresponding results are depicted in Figure 4 and SI Figure S12. The results show that the peak intensity increases with increasing P loading, which agrees with the finding of the batch studies (SI Figure S1). Further, the fitted IR spectrum of the lowest P loading

which indicates the possible transformation of the surface complexation of P with La(OH)3 from a BB to a BM configuration (Figure 3). This is likely due to the coprecipitation of amorphous Ca3(PO4)2 by Ca2+, leading to a lower P loading on the La(OH)3 (Figure 2B). Oscillation accounting for Ca−P was not observed in the sample, and is probably due to the weaker backscattering of Ca than La. ATR-FTIR Analysis and DFT Calculation. The protonation status of the P complexation on La(OH)3 is further crucial information for predicting P adsorption behavior.17,36 However, EXAFS is unable to identify these data, while ATRFTIR analysis is a powerful tool to complement such limitation. The ATR-FTIR spectra of the dissolved phosphate and the phosphate-loaded La(OH)3 under different pH conditions are depicted in Figure S8 and Figure 4A, where typical P−O fingerprint features are observed in the frequency range of 900−1200 cm−1. Frequencies below 900 cm−1 are not considered due to overlap with the IR background of La(OH)3. The IR spectra of the P−La(OH)3 are markedly different from those of the dissolved P species (i.e., H3PO4, H2PO4− and HPO42−) (Figure 4A, Figures S8 and S9), indicating the formation of inner-sphere complexes for P−La(OH)3.24 With the increase of pH from 3 to 9, a significant peak shift to a lower frequency is observed, due to the change of each characteristic band as a result of protonation/deprotonation change of the loaded P on La(OH)3.17 Each band in the P−O fingerprint range of 900−1200 cm−1 can be resolved through curve fitting and then compared with the calculated IR frequencies. As shown in Figure 5A the fitted ATR-FTIR spectrum exhibits vibrational modes at 1102, 1073, 1046, 1018, 991, 956, and 918 cm−1, for the P−La(OH)3 at a pH of 3. A further increase of pH to 5 results in one distinct group of vibrational modes at 1078, 1045, 1008, 962, and 917 cm−1, while similar vibrational modes appear at pH 7. In comparison to that at pH 5, the relative peak intensity at 1006 to 1046 cm−1 is significantly enhanced at pH 7 (Figure 4A). At pH 9, the fitted IR spectrum exhibits distinct vibrational modes at 1075, 1048, 1017, 988 cm−1 and very weak bands at 955 and 918 cm−1 (Figure 4A). In the meantime, there is no data on the characteristic peak positions 12381

DOI: 10.1021/acs.est.7b03803 Environ. Sci. Technol. 2017, 51, 12377−12384

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Environmental Science & Technology

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.

still remains after loading different amounts of P on the surface La(OH)3 and the crystal structure of P−La(OH)3 is markedly different from that of LaPO4 (Figure 1).38 With the increase of Ca concentration from 0 to 5 mM, the ATR-FTIR spectra also show significantly enhanced peak intensities within the range of 900−1200 cm−1 (SI Figure S15), due to the enhanced P removal.39 The IR spectra of the P−La(OH)3 samples became broader after the introduction of Ca, which is likely due to the presence of an amorphous Ca3(PO4)2 phase.40 This has also been demonstrated by our theoretical calculation and XPS study (SI Figures S5 and S7). Chemical Equilibrium Modeling. The surface complexation model successfully predicts the variation of surface complex speciation of phosphate on La(OH)3 as a function of pH, P loading and the absence and presence of Ca (Figure 2B and SI Table S2). The predicted protonation levels of the surface complexes from acid, neutral to alkaline pH, as well as at excessively high phosphate concentration, followed the spectroscopically and computationally determined results (SI Figure S16). The model predicted the formation of amorphous Ca3(PO4)2 when the combination of pH and the concentration of Ca enable its saturation (Figure 2, SI Figures S5 and S16). Precipitation enhances the overall removal of the dissolved phosphate ions and may also outcompete the adsorption contribution, leading to the transformation of a BB to BM configuration as shown in our EXAFS data (Table 1).41 The model also shows a fine prediction of phosphate removal by La(OH)3 in the presence of Ca and Mg at pH 9.0 ± 0.1 (Figure 2A); magnesium phosphate has higher solubility than calcium phosphate, and the model only predicted minor precipitation for the highest (i.e., 8 mM) Mg concentration. Overall, the proposed mechanisms of adsorption and precipitation for phosphate removal are quantitatively verified by the surface complexation model. Our results indicate that the model has the potential to accurately extrapolate the removal of phosphate by La(OH)3 under different environmental conditions, such as pH, P loading and the presence of Ca and Mg (SI Figure S16). Environmental Significance. Given the important role of La-based sorbents for managing phosphate in eutrophic lake and sewage effluents,12,14 this work provides new insights into the molecular-level removal mechanism of phosphate by means

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, 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.

(i.e., 0.4 mmol/g) exhibits more distinct vibrational modes at 1020 and 988 cm−1 than those with higher P loadings (i.e., 1007−1009 cm−1, Figure 4B). The unique IR vibrational modes at 1020 and 988 cm−1 likely correspond to the BB-H1 configuration because of their high correlation with the calculated IR frequencies of BB-H1 (SI Figure S13). With the increase of P loading, the vibrational modes at 1020 and 988 cm−1 shift to a single vibrational mode at about 1008 cm−1 that may be due to the formation of a BB-H2 or BM-H1 configuration (Figure 4B). Interestingly, Abdala et al.9 reported a similar finding, where P formed a BB configuration on goethite at low P loading and then transformed to a BM and subsequently, a BB configuration with increase of P loading. These findings are also in accordance with the above EXAFS results, showing that the BB configuration of phosphate formation occurred with an increase of P loading (Figure 3 and Table 1). In order to examine the possible transformation of the crystal structure reported by other groups,10,14 XRD analysis was carried out to confirm the crystal structure of La(OH)3 after P adsorption at pH 9. As shown in SI Figure S14, the nanorod-like structure of La(OH)3 12382

DOI: 10.1021/acs.est.7b03803 Environ. Sci. Technol. 2017, 51, 12377−12384

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Environmental Science & Technology

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

The authors are thankful for the technical support from the Advanced Engineering Material Facility of the Hong Kong University of Science and Technology (AEMF-HKUST). EXAFS beam time was granted by the 4B7A endstation of the Beijing Synchrotron Radiation Facility at the Institute of High Energy Physics, Chinese Academy of Sciences. Dr. Lei Zheng from 4B7A beamline is gratefully acknowledged for his support in the measurements and data reduction. Jessica Nguyen was supported by a Huel Perkins Diversity Fellowship.

of batch studies, EXAFS, ATR-FTIR, and DFT calculations (Figure 6). The combination of multiple spectroscopic techniques with theoretical calculations (e.g., DFT) allows us to build the entire picture of surface complexation of phosphate with La(OH)3, and provides a reliable approach in future efforts to probe the interfacial chemistry of other oxyanions.36 Our findings on the P configurations on La(OH)3 and possible precipitation of phosphate in the presence of Ca or Mg give concrete evidence for developing surface complexation models, which can be used to predict phosphate removal using La(OH)3 during lake restoration or sewage treatment. The prediction of phosphate removal should consider specific environmental variables such pH values (e.g., pH from 5 to 9 for natural lakes, and about 7.5 for sewage),3,42 surface P loading, and the presence of Ca and Mg at high pH values (e.g., pH 9). In spite of the high efficiency of La in phosphate removal and the relatively low acute toxicity,12 the ecological safety in regard to the extensive application of La-based sorbents requires caution, in particular, the long-term effects on aquatic organisms should be carefully evaluated in future work.

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(1) Mayer, B. K.; Baker, L. A.; Boyer, T. H.; Drechsel, P.; Gifford, M.; Hanjra, M. A.; Parameswaran, P.; Stoltzfus, J.; Westerhoff, P.; Rittmann, B. E. Total Value of Phosphorus Recovery. Environ. Sci. Technol. 2016, 50 (13), 6606−6620. (2) Seviour, R. J.; Mino, T.; Onuki, M. The microbiology of biological phosphorus removal in activated sludge systems. Fems Microbiol Rev. 2003, 27 (1), 99−127. (3) Fang, L.; Wu, B.; Lo, I. M. C. Fabrication of silica-free superparamagnetic ZrO2@Fe3O4 with enhanced phosphate recovery from sewage: Performance and adsorption mechanism. Chem. Eng. J. 2017, 319, 258−267. (4) Fang, L.; Huang, L.; Holm, P. E.; Yang, X.; Hansen, H. C. B.; Wang, D. Facile upscaled synthesis of layered iron oxide nanosheets and their application in phosphate removal. J. Mater. Chem. A 2015, 3 (14), 7505−7512. (5) Liu, Y.-T.; Hesterberg, D. Phosphate Bonding on Noncrystalline Al/Fe-Hydroxide Coprecipitates. Environ. Sci. Technol. 2011, 45 (15), 6283−6289. (6) de Vicente, I.; Huang, P.; Andersen, F. Ø.; Jensen, H. S. Phosphate Adsorption by Fresh and Aged Aluminum Hydroxide. Consequences for Lake Restoration. Environ. Sci. Technol. 2008, 42 (17), 6650−6655. (7) Li, W.; Feng, X.; Yan, Y.; Sparks, D. L.; Phillips, B. L. Solid-State NMR Spectroscopic Study of Phosphate Sorption Mechanisms on Aluminum (Hydr)oxides. Environ. Sci. Technol. 2013, 47 (15), 8308− 8315. (8) Kubicki, J. D.; Paul, K. W.; Kabalan, L.; Zhu, Q.; Mrozik, M. K.; Aryanpour, M.; Pierre-Louis, A.-M.; Strongin, D. R. ATR−FTIR and Density Functional Theory Study of the Structures, Energetics, and Vibrational Spectra of Phosphate Adsorbed onto Goethite. Langmuir 2012, 28 (41), 14573−14587. (9) Abdala, D. B.; Northrup, P. A.; Arai, Y.; Sparks, D. L. Surface loading effects on orthophosphate surface complexation at the goethite/water interface as examined by extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. J. Colloid Interface Sci. 2015, 437, 297−303. (10) Zhang, Y.; Pan, B.; Shan, C.; Gao, X. Enhanced Phosphate Removal by Nanosized Hydrated La(III) Oxide Confined in Crosslinked Polystyrene Networks. Environ. Sci. Technol. 2016, 50 (3), 1447−1454. (11) Wasay, S. A.; Haron, J.; Tokunaga, S. Adsorption of fluoride, phosphate, and arsenate ions on lanthanum impregnated silica gel. Water Environ. Res. 1996, 68 (3), 295−300. (12) Xu, R.; Zhang, M.; Mortimer, R. J. G.; Pan, G. Enhanced Phosphorus Locking by Novel Lanthanum/Aluminum-Hydroxide

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b03803. Detailed synthesis of La(OH)3 nanorods, EXAFS data collection and analysis, adsorption of phosphate on La(OH)3, comparison of La use efficiencies of La-based sorbents, cluster structure of La(OH)3, electron microscopic images of La(OH)3, competitive adsorption of background anions with phosphate, calculated saturation indexes of calcium phosphate species, X-ray diffractograms of La(OH)3, Ca 2p XPS spectra, ATR-FTIR spectra of aqueous phosphate and phosphate loaded La(OH)3, phosphate species distribution, configurations of P on La(OH)3, correlations of the calculated and experimental IR frequencies of phosphate, reaction equations for surface complexation modeling, comparison of experimental results with surface complexation modeling results, are provided (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Fax: (852) 23581534; e-mail: [email protected]. ORCID

Irene M. C. Lo: 0000-0002-7001-6900 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the Research Grants Council of Hong Kong (GRF16207916; T21-711/16-R-1). 12383

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