Activation of Lattice Oxygen in LaFe (Oxy)hydroxides for Efficient

Jun 26, 2019 - In the magnified FTIR spectra (Figure 3a), a new peak at 1057 cm–1 and a shoulder peak at 1015 cm–1 appeared after phosphate adsorp...
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Activation of Lattice Oxygen in LaFe (Oxy)hydroxides for Efficient Phosphorus Removal Jie Yu,†,‡,§ Chao Xiang,†,‡,§ Gong Zhang,§ Hongjie Wang,*,†,‡ Qinghua Ji,*,§ and Jiuhui Qu§ †

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College of Environmental Science and Engineering, Beijing Key Lab for Source Control Technology of Water Pollution, Beijing Forestry University, Beijing 100083, China ‡ Xiong’an Institute of Eco-Environment, Hebei University, Baoding 071002, China § Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Lanthanum (La)-based materials have been recognized as promising adsorbents for aqueous phosphate removal. The incorporation of base metals into La (oxy)hydroxides represents an effective strategy to improve adsorption performance. Understanding how base metals affect phosphate adsorption is challenging but essential for the development of effective materials for phosphorus control. Herein, we demonstrated a high-performance LaFe (oxy)hydroxide and studied its mechanisms on phosphate adsorption. The P K edge X-ray absorption near edge structure (XANES) analysis showed that PO43− was preferentially bonded with La, and the lattice oxygen in LaFe (oxy)hydroxide was demonstrated to be the active site. The O K edge XANES suggested that Fe optimized the electron structure of La, and Fe/La metal orbital hybridization resulted in the shift of oxygen p character to unoccupied states, facilitating phosphate adsorption. Furthermore, surface analysis showed that the pore size and volume were increased due to the introduction of Fe, which enabled efficient utilization of the active sites and fast adsorption kinetics. The dual effects of Fe in LaFe (oxy)hydroxide greatly enhance the effectiveness of La and represent a new strategy for the development of future phosphorus-control materials.

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INTRODUCTION Phosphorus (P) is one of the essential elements and an important nutrient for the growth of organisms and the normal functioning of ecosystems.1 However, excessive amounts of phosphate may cause eutrophication and subsequent deterioration of water quality.2 In the last few decades, adsorption has been regarded as an economical and efficient method for the removal of phosphate. Consequently, diverse bimetallic hydroxides materials have been developed as adsorbents for phosphorus removal. For instance, for separation convenience and highly efficient reusability, magnetic Fe-Zr binary oxide was synthesized and used as adsorbent for removing phosphate.3 A Fe-Mn binary oxide adsorbent with a Fe/Mn molar ratio of 6:1, synthesized by a simultaneous oxidation and coprecipitation process, yielded reasonably high phosphate affinity.4 Furthermore, Ce-Zr binary oxide nanoparticles, synthesized by a simple and low-cost solvothermal process, increased phosphate adsorption capacity up to 112.23 mg P/ g.5 Bimetallic hydroxides have been demonstrated to have good phosphate removal performances. Lanthanum (La) is a rare earth element which is considered to be environmentally friendly.6−9 Previous studies suggest that La-based materials show excellent performance for phosphate adsorption.10,11 However, La is one of the most expensive elements on earth; this hinders its large-scale practical © XXXX American Chemical Society

application. One feasible solution is to develop La-incorporated composites, which can improve the economical efficiency and practicability of the application. A key problem to be solved is how to improve the utilization of La in adsorbents. To address this problem, several La-incorporated materials, such as La metal organic frameworks, La3+/La(OH)3 loaded magnetic cationic hydrogel composites, La-modified zeolites and La-porous carbon composites, have been synthesized and studied for phosphate adsorption.12−15 In these materials, La showed a strong affinity for phosphate and offered rapid accessibility of active La for phosphate coordination. However, the adsorption mechanism remains unclear, which limits the design and development of high-performance La-incorporated adsorbents. The electron structure of La can be tuned by changing the support material to enable excellent interaction with phosphate, which is vital for the improvement of adsorption performance and effective use of La.16,17 For example, the accessibility of La to phosphate adsorption can be enhanced and the crystallization of La phosphate can be promoted by Received: Revised: Accepted: Published: A

April 1, 2019 June 18, 2019 June 26, 2019 June 26, 2019 DOI: 10.1021/acs.est.9b01939 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

prepared by dilution of the stock solution and used for the adsorption experiments. Adsorption kinetics of phosphate on the as-prepared materials were examined by mixing 100 mg of adsorbent with 500 mL of 25.0 mg L−1 P solution. The solutions were maintained at pH 7.0 ± 0.1 and shaken at 300 rpm for adsorption kinetic experiments. Samples were collected for phosphate concentration analysis at specific time intervals from 3 to 30 min. Adsorption isotherm experiments were conducted with initial P concentrations ranging from 10 to 200 mg L−1. Desorption and reusability were conducted using 1 M NaOH. The LaFe (oxy)hydroxide was first saturated with phosphate for 24 h and separated from the solution. Then it was immersed in NaOH solutions for another 24 h. The initial phosphate concentration and adsorbent dosage were 50 mg L−1 and 200 mg L−1, respectively. All experiments were performed in triplicate at 25 °C. Characterization. Scanning electron microscopy (SEM, Tescan Inc., U.S.A.) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) were used to characterize the morphology of the as-prepared LaFe (oxy)hydroxide nanocomposite. Brunauer−Emmett−Teller (BET) surface areas and pore size distributions were examined by a Micromeritics ASAP 2020 static volumetric analyzer. To determine the content of Fe and La in LaFe (oxy)hydroxide, samples were digested in 6 mol L−1 HCl and the concentrations of Fe and La were determined by using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8300, PerkinElmer, U.S.A.). FTIR spectra were recorded on a FTIR spectrometer (Vertex 70, Bruker, U.S.A.) from 400 to 4000 cm−1 at a resolution of 4 cm−1 and averaged over 128 scans, with KBr pellets as background. XPS measurements were performed using an X-ray photoelectron spectrometer (PHI 5600, Physical Electronics Inc., U.S.A.) with Al Kα radiation (1486.6 eV), and all binding energies were referenced to the C 1s peak at 285.0 eV. The O and P K edge XANES measurements were taken at beamline 4B7A at the Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). The O K edge XANES of the LaFe (oxy)hydroxide before phosphate adsorption and the P K edge XANES after phosphate adsorption were analyzed in total-electron-yield (TEY) mode. The scanned energy range was between 510 and 570 eV for O K edge XANES, with a minimum energy step of 0.1 eV, and between 2129 and 2200 eV for P K edge XANES, with a minimum energy step of 0.2 eV and dwell time of 1 s per point. The samples were directly pressed into tablets and pasted onto sample holders made from copper and stainless steel for O and P K edge XANES analyses, respectively.

using support materials with macropores (average pore diameters >50 nm).18 Xu et al. prepared a novel lanthanum/ aluminum-hydroxide composite (LAH) and reported that phosphate was bonded on La sites by forming inner sphere bidentate−binuclear complexes and oxygen defects exhibited on LAH surfaces, which could be active adsorption sites for phosphate.19 The findings demonstrate that lattice oxygen in La-based materials plays a vital role during phosphate adsorption. The covalency of metal-oxygen bonds is crucial to trigger lattice oxygen oxidation and enable nonconcerted proton−electron transfers, which may participate in interfacial reactions related to oxyanions in water.20 Lattice oxygen is the critical structure in bimetallic hydroxides that bridges different metal ions. Therefore, the electron structure of La−O−M in La-incorporated bimetallic hydroxides may be changed, which can further influence its binding affinity toward phosphate. Iron (Fe) is a typical face-centered cubic structure metal and its hydroxide has been widely explored for the adsorption of group V elements (e.g., phosphate, arsenate, antimonate).21−23 Due to its large surface area and high charge density, Fe hydroxide exhibits great potential for the adsorption of oxyanions and might be an ideal candidate for the development of high-performance La-incorporated bimetallic hydroxides. In this paper, LaFe (oxy)hydroxide nanocomposites were prepared and the mechanism for phosphate adsorption was extensively investigated. The O K edge X-ray absorption near edge structure (XANES) and X-ray photoelectron spectra (XPS) analyses were applied for confirming the change in the state of O on the surface of LaFe (oxy)hydroxide. The P K edge XANES was used to study the binding sites of phosphate on LaFe (oxy)hydroxide. Moreover, the pore and surface structure of LaFe (oxy)hydroxide, which was demonstrated to be a critical factor for phosphate adsorption, was also investigated.

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EXPERIMENTAL SECTION Synthesis of LaFe (Oxy)hydroxide Nanocomposite. Analytical grade Fe(NO3)3·9H2O, La(NO3)3·6H2O, C6H8O7· H2O, and ethanol were used to prepare the LaFe (oxy)hydroxide nanocomposite. In a typical synthesis, 21.0 mg of citric acid monohydrate was dissolved in 10 mL distilled water, and to this solution, ammonium hydroxide was slowly added to adjust the pH to 10. Fe(NO3)3·9H2O and La(NO3)3·6H2O were dissolved in 25 mL of ethanol under magnetic stirring to obtain a nitrate−ethanol solution. Then, the stabilized nitrate− ethanol solution was mixed with 10 mL of the abovementioned citric acid monohydrate solution under vigorous stirring. After 60 min, the formed transparent solution was poured into a 50 mL poly(tetrafluoroethylene) (PTFE) tube and transferred to a stainless-steel autoclave for solvothermal treatment at 150 °C. The resultant solid materials were then washed repeatedly with ethanol and deionized water until the solution was at a neutral pH. The obtained precipitate was dried at 60 °C for 24 h and then ground into a fine powder. Samples were named according to the analytical results for the contents of Fe and La in the sample synthesis process, i.e., FOH, FL5:1, FL4:1, FL2:1, FL3:2, and FL1:1. Pure LOH adsorbents were also prepared. (Details are provided in the Supporting Information, Section 1.) Adsorption Experiments. A stock P solution (1000 mg L−1) was prepared by dissolving KH2PO4 in deionized water. Phosphate solutions with different concentrations were

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RESULTS AND DISCUSSION Characterization of Materials. The physiochemical properties of the synthesized LaFe (oxy)hydroxide nanocomposite materials are summarized in Table 1. The BET surface area of FOH was 55.98 m2g−1, and the BET surface area of LaFe (oxy)hydroxide decreased from about 101.48 m2 g−1 for FL5:1 to 49.38 m2 g−1 for FL1:1 with increasing La content. The decrease of the BET surface area may be due to the formation of aggregated nanocomposites and the increase in the particle size of the nanocomposites. Iron acted as a dispersant, increasing the specific surface area of the nanocomposites when compared with that of pure FOH. The FL2:1 had the maximum total pore volume of 0.45 cm3 g−1. The SEM images showed that the as-obtained LaFe (oxy)hydroxide B

DOI: 10.1021/acs.est.9b01939 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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around 532 eV is attributed to bands of primarily La 5d character. Two different regions can be identified in the spectrum of LaFe (oxy)hydroxide. The first two peaks below 540 eV in each spectrum are the oxygen 2p states hybridized with the transition metal 3d ones, which are both split into t2g and eg orbitals in the octahedral symmetry. The structures at the edge below 540 eV arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of Fe/La character.26,27 The structure above 540 eV is attributed to bands of higher-energy metal states, like Fe 4sp and La 6sp.28 The peak intensity of t2g and eg nonbonding orbitals for LaFe (oxy)hydroxide decreased compared with those for FOH, suggesting the formation of bridging O2− through the interaction of Fe 5d-O 2p orbitals with La 5d-O 2p orbitals. The formation of bimetallic LaFe (oxy)hydroxide affects O states, leading to a split of the two groups of bands (t2g and eg) into four: spin-up t2g, spin-up eg, spin-down t2g, and spin-down eg. A greater number of unoccupied molecular orbitals can provide a greater number of active sites for binding PO43−. Phosphate Adsorption Performance of LaFe (Oxy)hydroxide Nanocomposites. Adsorption experiments were performed using LaFe (oxy)hydroxide nanocomposites for the removal of aqueous phosphate. The adsorption isotherms of LaFe (oxy)hydroxide for phosphate are shown in Figure 2. Regarding the adsorption capacity, the Langmuir model (Table S1) fits well for phosphate adsorption on LaFe (oxy)hydroxide. The phosphate adsorption isotherms of LF2:1 obtained at different pH values are presented in Figure 2a. The LaFe (oxy)hydroxide prepared at pH 10 demonstrated better adsorption performance. Figure 2b shows the phosphate adsorption isotherms of LF2:1 obtained at different temperatures, which indicated that the LaFe (oxy)hydroxide prepared at 150 °C had better adsorption capacity. The phosphate adsorption isotherms of LaFe (oxy)hydroxide (with different Fe/La molar ratios) prepared with optimum pH and temperature are presented in Figure 2c. The corresponding

Table 1. General Characteristics of LaFe (oxy)hydroxide nanocomposites Adsorbent

Atomic ratio (Fe/La)

Surface area (m2 g−1)

FOH FL5:1 FL4:1 FL2:1 FL3:2 FL1:1

4.8 4.0 1.9 1.5 1.1

55.98 101.48 93.69 73.58 50.80 49.38

nanocomposites were almost-spherical nanoagglomerates, which exhibited a porous and loose morphology (Figure S1). By comparison, pure lanthanum hydroxide showed a rod-like morphology according to the SEM images of LOH (Figure S2a). This morphology was significantly changed to almostspherical nanoagglomerates due to the addition of Fe, which increased the surface area and pore volume of the hydroxides. HRTEM (Figure S2b) image showed that the FOH nanoparticles had an average diameter of about 5 nm. Further loading of La yielded a hybridized LaFe (oxy)hydroxide (Figure 1a,b), which demonstrated an increase of the particle size (∼10 nm). Concurrent elemental mapping (Figure 1c) confirmed that the La and Fe elements were uniformly distributed throughout the hydroxide nanocomposites; blue corresponds to Fe and purple corresponds to La. As shown in Figure S3, X-ray diffraction patterns for LaFe (oxy)hydroxides indicate that LaFe (oxy)hydroxides are amorphous and have a disordered structure. XPS spectra (Figure 1d,e) showed the peak position of Fe at 710.9 eV for Fe 2p1/2 and 724.4 eV for Fe 2p2/3.24 The typical peaks located at 854.7 and 851.1 eV and those at 837.8 and 834.2 eV can be attributed to La 3d3/2 and La 3d5/2, respectively.25 These results confirmed the successful preparation of LaFe (oxy)hydroxide. The O K edge XANES spectra of pure FOH, LOH, and LaFe (oxy)hydroxide nanocomposites are shown in Figure 1f. The broad peak of pure LOH centered

Figure 1. Characterization of LaFe (oxy)hydroxides. HR-TEM images at low magnification (a) and high magnification of FL2:1 (b); Fe and La elemental mappings (c); Fe 2p (d) and La 3d (e) XPS spectra of FL2:1; O K edge XANES spectra of pure FOH, FL2:1, and LOH (f). C

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Figure 2. Phosphate adsorption performance of LaFe (oxy)hydroxides. (a) Adsorption isotherms of LaFe (oxy)hydroxide obtained at different pH. (b) Adsorption isotherms of LaFe (oxy)hydroxide obtained at different temperature (and pH = 10). (c) Adsorption isotherms of LaFe (oxy)hydroxide obtained at different Fe/La molar ratios (initial P concentration = 10 to 200 mg L−1, sorbent dosage = 200 mg L−1, temperature = 25 °C, pH = 7.0 ± 0.2, equilibrium time = 24 h). (d) Adsorption kinetic of LaFe (oxy)hydroxide using the mass transfer model (initial P concentration = 25 mg L−1, sorbent dosage = 200 mg L−1, temperature = 25 °C, pH = 7.0 ± 0.2 (dashed lines are model fitting results)).

the cycle number. After the fifth regeneration cycle, the removal efficiency of phosphate by LaFe (oxy)hydroxide (FL2:1) was maintained at more than 70% and the phosphate removal efficiency of the FOH adsorbent decreased to be 60%, suggesting that the LaFe (oxy)hydroxide adsorbent can be used repeatedly. Mechanism Study. The FTIR spectra of LaFe (oxy)hydroxide nanocomposites before and after phosphate adsorption are shown in Figure S7. Broad peaks at 3433 and 1621 cm−1 were observed for all LaFe (oxy)hydroxide nanocomposites, which can be assigned to the adsorbed water and the bending mode of the H−O−H bond on the metal hydroxides.30 Peaks of pristine LaFe (oxy)hydroxide nanocomposites at 581 and 464 cm−1 were due to the stretching of Fe−O bonds in LaFe (oxy)hydroxide. In the magnified FTIR spectra (Figure 3a), a new peak at 1057 cm−1 and a shoulder peak at 1015 cm−1 appeared after phosphate adsorption, which belonged to the ν3 vibration of phosphate groups. Additional weaker peaks at 615 and 541 cm−1 with a shoulder peak at 572 cm−1 were attributed to the ν4 vibration from phosphate groups, which proved that phosphate was successfully adsorbed on LaFe (oxy)hydroxide. High-resolution XPS analysis was performed to gain insight into the chemical structure of the LaFe (oxy)hydroxide. Wide scan XPS spectra of the pristine LaFe (oxy)hydroxide nanocomposite indicated the presence of Fe, La, and O elements (Figure S8). The appearance of the P 2p spectra at a binding energy of ∼133.7 eV after phosphate sorption demonstrated the successful loading of phosphate on LaFe (oxy)hydroxide. Figure 3b−d presents the XPS spectra of O 1s, Fe 2p, and La 3d before and after PO43− adsorption, respectively. The broad and asymmetric O 1s XPS spectra correspond to two chemical states of O, including crystal lattice oxygen (OL) and hydroxyl

parameters (Table S1) show that the experimental equilibrium adsorption data are well-described by the Langmuir model, representing a homogeneous monolayer chemisorption process. The maximum adsorption capacities (Qmax) of phosphate onto LaFe (oxy)hydroxide (FL2:1) were calculated to be 123.46 mg P g−1. The Qmax for pure LOH was 75.75 mg P g−1, which is much lower than that of FL2:1(Figure S4). Adsorption kinetic curves are shown using the mass transfer model for phosphate adsorption (Figure 2d, Table S2). Mass transfer coefficients (Kf) were calculated as 9.42 × 10−6, 2.14 × 10−6, 1.39 × 10−6, 4.11 × 10−6, 3.83 × 10−6, and 2.41 × 10−6m s−1 for FOH, FL5:1, FL4:1, FL2:1, FL3:2, and FL1:1. FL2:1 possessed the fastest phosphate adsorption kinetics compared with FL5:1, FL4:1, FL3:2, and FL1:1. FOH has a larger mass transfer coefficient compared with that of the LaFe (oxy)hydroxide nanocomposites, probably due to its less effective active sites, and can reach the absorption equilibrium in short time. The fast kinetics are likely due to the rapid reaction of PO43− on active sites and the easily accessible pathway for ion diffusion.29 Furthermore, adsorption kinetics of a low concentration phosphate on the as-prepared materials were examined by mixing 50 mg of adsorbent (FL2:1) with 250 mL of 1.0 mg L−1 P solution (Figure S5). The results showed that the phosphate concentration dropped to 0.19 mg L−1 after 3 min, and it dropped further to 0.08 mg L−1 after 5 min. Finally, the phosphate concentration stayed at 0.03 mg L−1 from 10 to 30 min. The results revealed that LaFe (oxy)hydroxides showed good adsorption capacity at lower PO43− concentration. The adsorption−regeneration cycles were carried out five times (Figure S6). The value of cycle 1 was assigned to the adsorption capacity of the original LaFe (oxy)hydroxide. In general, the adsorption capacity decreased with the increase of D

DOI: 10.1021/acs.est.9b01939 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 3. Mechanism study on phosphate adsorption using LaFe (oxy)hydroxides. (a) Magnified FTIR spectra of the as-prepared LaFe (oxy)hydroxide before and after P adsorption (adsorbent dosage = 0.5 g L−1, initial P concentration = 50.0 mg L−1, pH = 7.0, equilibrium time = 24 h), (b) O 1s, (c) La 3d, and (d) Fe 2p of FL2:1 before and after phosphate adsorption. (e) P K edge XANES spectra of FOH, LOH, and LaFe (oxy)hydroxide after phosphate adsorption. (f) Linear combination fit of FL2:1 after phosphate adsorption (adsorbent dosage = 0.5 g L−1, initial P concentration = 50.0 mg L−1, pH = 7.0, equilibrium time = 24 h).

Figure 4. Fitting peak intensity of phosphate-adsorbed FOH and LaFe (oxy)hydroxides. (a) FOH, (b) FL5:1, (c) FL4:1, (d) FL2:1, (e) FL3:2, (f) FL1:1. Experimental conditions: adsorbent dosage = 0.5 g L−1, initial P concentration = 50.0 mg L−1, pH = 7.0, equilibrium time = 24 h.

E

DOI: 10.1021/acs.est.9b01939 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology oxygen (OH).31 The XPS signal of OL is attributed to the La− O and Fe−O bonds in the LaFe (oxy)hydroxide crystal lattice (peak position: ∼529.7 eV). The OH is related to La−OH and Fe−OH groups, mainly resulting from chemisorbed water (peak position: ∼531.4 eV). The peak intensity of OL decreased and shifted after PO43− adsorption, indicating that the adsorption of phosphate might result in a change in the state of O. This result suggested that the lattice oxygen is probably the active site for phosphate adsorption. The binding energies of La 3d3/2 and La 3d5/2 had a noticeable shift after PO43− adsorption. In contrast, the peak positions of iron at 710.9 and 724.4 eV did not change. It can be deduced from the above results that the major contribution for phosphate adsorption on LaFe (oxy)hydroxide was the lattice oxygen of La−O. To further confirm this hypothesis, the chemical states of P bonded on LaFe (oxy)hydroxide were investigated using P K edge XANES analysis (Figure 3e). A strong white line at ∼2153 eV was characterized after PO43− adsorption on all samples and was assigned to a transition of the P 1s electron in the unoccupied states. The pre-edge feature and the shoulder peak indicated ligand−metal covalent bonds by the orbital hybridization between metal and PO43−. Figure 4 shows the fitted peak intensity of the white line, pre-edge features, and shoulder peaks for phosphate-adsorbed FOH and LaFe (oxy)hydroxide. The fitting results are summarized in Table 2. The peak intensities of the white line of pure FOH and

LaFe (oxy)hydroxide (Figures 3f and S9 and Table 3). Results showed that La sites accounted for 87.3%, 89.9%, 89.8%, Table 3. Linear Combination Fitting Results of P K edge XANES Spectra for the Phosphate Adsorbed LaFe (Oxy)hydroxides Percent composition

a

Height Pre-edge feature

White Line

Shoulder Peak

Fit parameters (Rfactor)

FOH LF5:1-P LF4:1-P LF2:1-P LF3:2-P LF1:1-P LOH

0.446 1.600 1.448 1.674 1.947 0.789 1.508

7.201 0.809 0.959 3.502 0.677 2.048 5.448

5.608 5.835 4.271 5.431 5.110 1.177

0.001388 0.001106 0.000855 0.000916 0.000954 0.001154 0.000660

Samples

La−P

Fe−P

R-factor

χ2

FL5:1-P FL4:1-P FL2:1-P FL3:2-P FL1:1-P

0.873 0.899 0.898 0.915 0.864

0.127 0.101 0.102 0.085 0.136

0.033973 0.037221 0.036969 0.038539 0.076657

0.01244 0.01365 0.01416 0.01364 0.02717

R-factor; χ2 = goodness of fit.

91.5%, and 86.4% of phosphate hybridization sites in LF5:1, FL4:1, FL2:1, FL3:2, and FL1:1, respectively. P K edge XANES spectra also proved that La greatly contributed to the adsorption of phosphate. However, the maximum adsorption capacities were not positively correlated with the content of La in LaFe (oxy)hydroxide nanocomposites. This result suggested that La was not the only factor in improving the phosphate adsorption performance. Surface and pore structure were important factors in adsorption. Therefore, surface analysis of the adsorbents was performed and the results are presented in Figure 5. The pore size distribution revealed that all adsorbents had pore sizes above 2 nm (Figure 5a). The average pore size of LaFe (oxy)hydroxide was larger than that of FOH and tended to increase with increasing La content. As previously mentioned, the LaFe (oxy)hydroxide (FL2:1), which exhibited the largest pore size and pore volume, achieved the highest phosphate adsorption capacity. This pore structure can provide more active sites and fast ion diffusion. However, with the increase of the La content of materials from FL5:1 to FL1:1, the specific surface areas decreased from 101.48 to 49.38 m2 g−1. The above result indicated that the pore size and volume played more important roles than the surface area in phosphate adsorption. In general, the electron structure of La in LaFe (oxy)hydroxide was changed upon the incorporation of Fe, which activated the lattice oxygen and promoted its binding affinity toward phosphate. In comparison with those of the pure FOH and LOH samples, the charge transfers between Fe and La and the hybridization orbitals between Fe/La and O atoms were observed in the LaFe (oxy)hydroxide; once Fe was incorporated into the La, the energy of the O 2p hybridization orbitals rapidly decreased compared with those in FOH or LOH alone which indicates that the electrostatic interaction between them influences the bond lengths of La−O/Fe−O, resulting in the performance optimization of phosphate adsorption. Furthermore, the pore structure of the LaFe (oxy)hydroxides was optimized due to the introduction of Fe, which increased the pore size and volume. This improvement enabled a full utilization of the active sites and resulted in the high adsorption capacity of LaFe (oxy)hydroxide (Scheme 1).

Table 2. Fitting Results of P K edge XANES Spectra for the Phosphate Adsorbed LaFe (Oxy)hydroxides

Samples

Fit parametersa

LOH were much higher than that of LaFe (oxy)hydroxide, indicating a strong covalent interaction between LaFe (oxy)hydroxide and PO43−. The XANES spectrum for PO43− adsorbed on FOH showed a pre-edge feature on the low energy side of the white line. This pre-edge feature was due to the electronic transition of a phosphorus 1s electron into an Fe(3d)-O(2p)-P(3p) antibonding molecular orbital.32 Therefore, this feature indicated that PO43− was coordinated with Fe atoms in the second shell, and it provided direct evidence for inner-sphere surface complexation of phosphate on FOH. However, this feature was not observed for PO43− adsorbed on LaFe (oxy)hydroxide, indicating that the binding site of PO43− was different from that of FOH.33 Simultaneously, all the preedge peaks at ∼2151.9 eV and the shoulder peak at ∼2155 eV for P-adsorbed LaFe (oxy)hydroxides indicated the formation of inner-sphere surface complexation Fe/La−PO4.34 All Lacontaining samples exhibited similarly shaped resonances, emphasizing that La played a major role in phosphate adsorption. Using Fe−P and La−P as references, XANES linear combination fitting (LCF) was performed to quantify the contribution of Fe and La in the hybridization of P on

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ENVIRONMENTAL IMPLICATIONS In this study, a LaFe (oxy)hydroxide nanocomposite with rationally designed chemical composition and pore structure was developed. The LaFe (oxy)hydroxide nanocomposite F

DOI: 10.1021/acs.est.9b01939 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 5. Surface and pore structure analysis of LaFe (oxy)hydroxides. (a) BJH pore size distributions. (b) Calculated pore volume.

combination fit of P K edge XANES spectra for phosphate adsorbed FL5:1, FL4:1, FL3:2, and FL1:1 samples (PDF)

Scheme 1. Proposed Mechanisms of Phosphate Removal by LaFe (Oxy)hydroxide

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. Tel. (8610)6279-0565. ORCID

Gong Zhang: 0000-0001-6117-095X Qinghua Ji: 0000-0001-5444-895X Jiuhui Qu: 0000-0001-9177-093X Notes

The authors declare no competing financial interest.

showed excellent performance in phosphate removal. The results of XANES analyses indicated that lattice oxygen is the active site for phosphate adsorption. Moreover, PO43− was preferentially bonded with La to form inner-sphere surface complexes of La−PO4. The incorporation of Fe changes the electron structure of La and increases the pore size and volume of the LaFe (oxy)hydroxide nanocomposite, which allows the maximum utilization of La. These findings open new possibilities for the development of highly efficient adsorbents with activated lattice oxygen and present a new strategy to promote the effective utilization of rare earth elements.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51578067, 51608516 and 51778054).

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REFERENCES

(1) Cordell, D.; Drangert, J. O.; White, S. The story of phosphorus: Global food security and food for thought. Change-Human Policy Dimens. 2009, 19 (2), 292−305. (2) Conley, D. J.; Paerl, H. W.; Howarth, R. W.; Boesch, D. F.; Seitzinger, S. P.; Havens, K. E.; Lancelot, C.; Likens, G. E. Ecology. Controlling eutrophication: nitrogen and phosphorus. Science 2009, 323 (5917), 1014. (3) Long, F.; Gong, J. L.; Zeng, G. M.; Chen, L.; Wang, X. Y.; Deng, J. H.; Niu, Q. Y.; Zhang, H. Y.; Zhang, X. R. Removal of phosphate from aqueous solution by magnetic Fe-Zr binary oxide. Chem. Eng. J. 2011, 171 (2), 448−455. (4) Zhang, G.; Liu, H.; Liu, R.; Qu, J. Removal of phosphate from water by a Fe-Mn binary oxide adsorbent. J. Colloid Interface Sci. 2009, 335 (2), 168−74. (5) Su, Y.; Yang, W.; Sun, W.; Li, Q.; Shang, J. K. Synthesis of mesoporous cerium-zirconium binary oxide nanoadsorbents by a solvothermal process and their effective adsorption of phosphate from water. Chem. Eng. J. 2015, 268, 270−279. (6) Fang, L.; Wu, B.; Chan, J. K.M.; Lo, I. M.C. Lanthanum oxide nanorods for enhanced phosphate removal from sewage: A response surface methodology study. Chemosphere 2018, 192, 209−216. (7) Fang, L.; Liu, R.; Li, J.; Xu, C.; Huang, L. Z.; Wang, D. Magnetite/Lanthanum hydroxide for phosphate sequestration and recovery from lake and the attenuation effects of sediment particles. Water Res. 2018, 130, 243. (8) Fang, L. P.; Shi, Q. T.; Nguyen, J.; Wu, B. L.; Wang, Z. M.; Lo, I. M. C. Removal mechanisms of phosphate by lanthanum hydroxide nanorods: investigations using EXAFS, ATR-FTIR, DFT, and surface

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b01939. Fitting results and calculated parameters of the Langmuir adsorption isotherm model for phosphate adsorption on the LaFe (oxy)hydroxides adsorbents (25 °C); parameters of kinetics models for phosphate adsorption on the LaFe (oxy)hydroxides adsorbents; SEM image of LaFe (oxy)hydroxides sample; SEM image of LOH (a), HR-TEM images of FOH (b), FL5:1 (c), FL4:1 (d), FL3:2 (e), and FL1:1 (f); X-ray diffraction patterns for LaFe (oxy)hydroxides sample; phosphate adsorption performance of as-prepared LOH adsorbents; adsorption kinetic of LaFe (oxy)hydroxide using the mass transfer model, initial P concentration = 1 mg L−1, sorbent dosage = 200 mg L−1, temperature = 25 °C, pH = 7.0 ± 0.2; reusability of LaFe (oxy)hydroxides regenerated by 1 M NaOH; FTIR spectra of the as-prepared FOH, LF5:1, LF4:1, and LF2:1 adsorbents before and after P adsorption; XPS spectra of FL2:1 before and after phosphate adsorption; linear G

DOI: 10.1021/acs.est.9b01939 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.est.9b01939 Environ. Sci. Technol. XXXX, XXX, XXX−XXX