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Fe3O4 nanocomposites
Water Research 126 (2017) 179e188
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Highly efficient and selective phosphate removal from wastewater by magnetically recoverable La(OH)3/Fe3O4 nanocomposites Baile Wu a, Liping Fang a, John D. Fortner b, Xiaohong Guan c, Irene M.C. Lo a, * a
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, United States c State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2017 Received in revised form 18 September 2017 Accepted 19 September 2017 Available online 20 September 2017
The use of lanthanum (La)-based materials for phosphate removal from water and wastewater has received increasing attention. However, challenges remain to enhance phosphate sorption capacities and recover La-based sorbents. In this study, magnetic La(OH)3/Fe3O4 nanocomposites with varied La-to-Fe mass ratios were synthesized through a precipitation and hydrothermal method. Based upon preliminary screening of synthesized La(OH)3/Fe3O4 nanocomposites in terms of phosphate sorption capacity and La content, La(OH)3/Fe3O4 nanocomposite with a La-to-Fe mass ratio of 4:1 was chosen for further characterization and evaluation. Specifically, for these materials, magnetic separation efficiency, phosphate sorption kinetics and isotherm behavior, and solution matrix effects (e.g., coexisting ions, solution pH, and ionic strength) are reported. The developed La(OH)3/Fe3O4 (4:1) nanocomposite has an excellent magnetic separation efficiency of >98%, fast sorption kinetics of 30 min, high sorption capacity of 83.5 mg P/g, and strong selectivity for phosphate in presence of competing ions. Phosphate uptake by La(OH)3/Fe3O4 (4:1) was pH-dependent with the highest sorption capacities observed over a pH range of 4e6. The ionic strength of the solution had little interference with phosphate sorption. Sorptiondesorption cyclic experiments demonstrated the good reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite. In a real treated wastewater effluent with phosphate concentration of 1.1 mg P/L, 0.1 g/L of La(OH)3/Fe3O4 (4:1) efficiently reduced the phosphate concentration to below 0.05 mg P/L. Electrostatic attraction and inner-sphere complexation between La(OH)3 and P via ligand exchange were identified as the sorption mechanisms of phosphate by La(OH)3/Fe3O4 (4:1). © 2017 Elsevier Ltd. All rights reserved.
Keywords: Eutrophication Lanthanum Magnetic nanoparticles Phosphate Sorption
1. Introduction Excess phosphorus (P) released into water bodies can trigger eutrophication, affecting water quality and aquatic ecosystem health (Correll, 1998). Studies show that a P concentration above 0.02 mg P/L generally accelerates eutrophication in lakes (Heathwaite and Sharpley, 1999), leading many places to make recommendations on concentration limits for P in water bodies. For example, the United States Environmental Protection Agency (US EPA) recommends that any streams entering a lake or reservoir should have total P concentrations not exceeding 0.05 mg P/L (Loganathan et al., 2014), and the European Union (EU) defines the
* Corresponding author. E-mail address: [email protected] (I.M.C. Lo). https://doi.org/10.1016/j.watres.2017.09.034 0043-1354/© 2017 Elsevier Ltd. All rights reserved.
cut-off total P concentrations in lakes as non-risk and risk conditions of eutrophication to be < 0.01 mg P/L and > 0.1 mg P/L, respectively (Loganathan et al., 2014). For discharges from wastewater treatment plants, the permissible concentration of P will be lowered from 1e2 mg P/L to 0.1 mg P/L in the EU under the Water Framework Directive (Shepherd et al., 2016). However, these are stringent P discharge standards; traditional techniques such as chemical precipitation and biological treatment are unable to reduce phosphate concentrations to below 0.1 mg P/L (Sengupta and Pandit, 2011). Hence, other techniques are needed to meet stringent discharge requirements. Sorption is a preferable approach for phosphate removal due to its simplicity of design, effectiveness even at low P concentrations (Loganathan et al., 2014), and potential for recovery. Many sorbents such as activated carbon, resins, industrial byproducts, and waste biomass have been explored for removing phosphate from water
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(Rittmann et al., 2011), yet low sorption capacity and poor selectivity are inherent limitations for most of these materials. Due to lanthanum's strong affinity with phosphate, a number of studies have developed La-modified sorbents such as La-modified bentonite clay (Phoslock®) (Haghseresht et al., 2009), La-doped silica spheres (Huang et al., 2014b), and La-treated lignocellulosic sorbents (Shin et al., 2005) for the removal of phosphate. These sorbents have exhibited marginally enhanced phosphate sorption capacities e for example the maximum sorption capacity of Phoslock® is only 9.5e10.5 mg P/g (Haghseresht et al., 2009). Further, these sorbents are difficult to separate from wastewater after treatment. Traditional methods to recover sorbents include centrifugation and filtration; however, centrifugation is energy-intensive and filtration is prone to filter blockages (Chen et al., 2009). Magnetic separation is faster and more effective in separating nanoparticles from wastewater compared to centrifugation and filtration (Tang and Lo, 2013). In addition, the incorporation of magnetic nanoparticles (e.g., Fe3O4) with La facilitates the separation and recovery of sorbents. The La acts as active sites for the removal of phosphate in water; while Fe3O4 allows for the magnetic separation. To date, only a few studies have reported on the use of magnetic La-based sorbents for phosphate removal (Lai et al., 2016; Rashidi Nodeh et al., 2017), and these studies have typically been restricted by complicated synthesis procedures, the use of graphene as substrate, which is relatively expensive, or low sorption capacities (i.e., 27.8 mg P/g for Fe3O4@SiO2@La2O3). Additionally, the sorption mechanisms of phosphate by La-based materials have not been characterized in detail. Therefore, the objectives of this work are to: (1) develop magnetic La-based nanocomposite (i.e., La(OH)3/Fe3O4) through a facile precipitation and hydrothermal method; (2) demonstrate selective removal of phosphate by La(OH)3/Fe3O4 with a higher sorption capacity than other sorbents; and (3) elucidate sorption mechanisms through batch experiments (e.g., effects of pH and ionic strength), zeta potential, and spectroscopic investigations (Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS)). 2. Materials and methods 2.1. Materials and chemicals All chemicals used in this study were analytical reagents of high purity. The FeCl3$6H2O (98%), FeCl2$4H2O (99%), trisodium citrate dihydrate, lanthanum (III) nitrate hydrate (La(NO3)3$xH2O, x ¼ 3e5, 99.9%), and KH2PO4 (99%) were purchased from Sigma-Aldrich in the US. The deionized (DI) water with a specific conductivity of 18 MU cm was used to prepare all solutions unless otherwise specified. A 1000 mg P/L stock solution of phosphate was prepared by dissolving 2.2171 g of KH2PO4 in 500 mL DI water. All phosphate in this study refers to orthophosphate unless otherwise specified. 2.2. Synthesis of Fe3O4 nanoparticles and La(OH)3/Fe3O4 nanocomposites 2.2.1. Synthesis of Fe3O4 nanoparticles Fe3O4 nanoparticles were prepared through a modified coprecipitation method (Hu et al., 2005). First, 100 mL of DI water was bubbled by nitrogen gas for 30 min. Then 2.164 g of FeCl3$6H2O and 0.796 g of FeCl2$4H2O were dissolved in the above 100 mL deoxygenated DI water with nitrogen gas purging. NH3$H2O (28% in H2O) was added dropwise into the above mixture until the pH reached 11. The solution was mixed for 10 min, and then a 2-mL solution that contained 0.6 g of trisodium citrate dihydrate was injected rapidly into the mixture. The mixture was then stirred for another 1 h with nitrogen gas purging. Finally, the black products were
separated from the mixture by a magnet, and rinsed with deoxygenated DI water three times. 2.2.2. Synthesis of La(OH)3/Fe3O4 nanocomposites La(OH)3/Fe3O4 nanocomposites were prepared by a precipitation and hydrothermal method. The precipitation method was adopted because of its simplicity, high efficiency, and low cost. The subsequent hydrothermal treatment could benefit the crystallinity of the products (Xiao et al., 2014). The as-prepared Fe3O4 nanoparticles (0.45 g) were dispersed in 60 mL of DI water under ultrasonication for 10 min. Then 0.45 g of La(NO3)3$xH2O in 10 mL DI water was added into the Fe3O4 suspension. The resulting mixture was stirred for 10 min before adding 0.45 g of NaOH dissolved in a 10 mL DI water. After being stirred for another 5 h, the mixture was transferred to a Teflon-lined stainless-steel autoclave (100 mL capacity) and heated at 180 C for 10 h. Finally, the autoclave was cooled down to room temperature and the products were washed with water three times. To synthesize La(OH)3/Fe3O4 nanocomposites with different La contents, the mass ratios of La(NO3)3$xH2O and Fe3O4 (La-to-Fe) were varied from 1:1, 2:1, 4:1, to 5:1 by adjusting the amount of La(NO3)3$xH2O in the initial synthetic solution. The same amount of La(NO3)3,xH2O and NaOH was used in each synthesis. La(OH)3/Fe3O4 nanocomposites with four La-to-Fe mass ratios of 1:1, 2:1, 4:1, and 5:1 were obtained using the above method. 2.3. Characterization of magnetic sorbents The crystal structure of the magnetic sorbents was analyzed using an X-ray diffraction spectrometer (PW-1830, Philips, France) with Cu Ka radiation (l ¼ 1.5406 Å) over the 2q range of 5-90 . The morphology of the sorbents was studied using a transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) at an accelerating voltage of 20 kV. The magnetic properties of the materials were analyzed by a vibrating sample magnetometer (VSM, Lake Shore 7037, USA) at room temperature. Nitrogen adsorptiondesorption isotherms were measured at 77 K using a surface area analyzer (NOVA-3200e, Quantachrome, USA). Prior to the surface area analysis, the samples were degassed at 80 C for 24 h. The specific surface area was determined from the adsorption branch of isotherm using the BrunauerEmmettTeller (BET) method in a relative pressure range of 0.05e0.30. The total pore volume was calculated from the adsorbed nitrogen amount at a relative pressure of 0.98. The La content in each La(OH)3/Fe3O4 nanocomposite was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, Perkin-Elmer, USA). The surface charge of each sorbent was analyzed on a zeta potential analyzer (Zetaplus, LaborScience S.A., Greece) using 1 mM KCl as background electrolyte. The zeta potential values obtained were averaged over ten measurements. The functional groups of the materials were determined on an FTIR spectrometer (Vertex 70, Bruker, USA). FTIR spectra were recorded from 400 to 4000 cm1 at a resolution of 4 cm1 and were averaged over 128 scans. KBr powder was used as a background material. The chemical compositions of the materials were analyzed using an XPS (PHI 5600, Physical Electronics Inc., USA) with Al Ka radiation (1486.6 eV). All binding energies were referenced to the C1s peak at 285.0 eV. 2.4. Batch experiments To determine the time needed to reach sorption equilibrium, the sorption kinetics of phosphate on the as-prepared La(OH)3/Fe3O4 (4:1) nanocomposite were conducted by mixing 0.025 g of sorbents with 50 mL of 5.0 mg P/L phosphate solutions at 23 C. Samples were drawn periodically for phosphate concentration analysis. Sorption
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isotherm experiments at temperatures of 23 C and 37 C were performed to find out the maximum sorption capacity. La(OH)3/ Fe3O4 (4:1) nanocomposites (0.004 g) were dispersed into 40 mL of phosphate solution with initial concentration ranging from 0.5 to 15 mg P/L. pH values of the solutions were maintained at 7.0 throughout the kinetic and isotherm studies by using a 10 mM N-2hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) buffer solution (Biswas et al., 2007). HEPES is widely used for its minimal influence on metal complexation (Pan et al., 2017). The vials were shaken in a mechanical shaker for 2 h at 200 rpm. The effects of competing ions on phosphate sorption were examined by adding 2 2þ 2þ common coexisting ions (i.e., Cl, NO 3 , SO4 , HCO3 , Ca , and Mg ) to the phosphate solutions in separate vials at 23 C. To mimic the typical phosphate concentration of real wastewater, an initial phosphate concentration of 2 mg P/L was selected (He et al., 2015; Li et al., 2009). Various concentrations of ions (10, 50, 100 mg/L) were used. The sorbent dosage was 0.1 g/L and the pH was maintained at 7.0 by using the 10 mM HEPES buffer solution. The vials were shaken in the mechanical shaker for 2 h at 200 rpm. For the solution pH effect studies, 0.004 g of the sorbents were suspended in 40 mL of phosphate solution with initial concentration of 5 mg P/L at 23 C. The pH, ranging from 3.0 to 11.0, was adjusted with a 0.1 M HCl or 0.1 M NaOH solution. The pH was measured using a pH meter (Inolab WTW series pH720) when the solution has been stable for 2 min. NaCl with concentrations of 0.001 M, 0.05 M, and 0.1 M were used to control the ionic strength. The initial phosphate concentration was 5 mg P/L and the sorbent dosage was 0.1 g/L. Five sorptiondesorption cycles were performed in phosphate-spiked DI water (phosphate only) to evaluate the reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite. The magnetic sorbent (0.025 g) was suspended in 50 mL of phosphate solution with initial concentration of 2 mg P/L. The pH was maintained at 7.0 by using the 10 mM HEPES buffer solution. After being mixed for 1 h at 23 C, the phosphateadsorbed nanocomposites were magnetically separated and then suspended in 50 mL of 1 M NaOH solution for 2 h at 23 C. The regenerated La(OH)3/Fe3O4 (4:1) was washed by DI water to remove the remaining NaOH solution until pH 7 and then reused in the succeeding cycle. A fresh 1 M NaOH solution was used for each desorption process. The phosphate desorption efficiencies were calculated as a ratio of the desorbed to the initially adsorbed phosphate. To demonstrate the applicability of La(OH)3/Fe3O4 (4:1) nanocomposite, phosphate removal from real treated wastewater effluent was also studied. The treated wastewater effluent was collected after dichlorination at the Stonecutters Island Sewage Treatment Works in Hong Kong. Prior to being characterized, the treated wastewater effluent was filtered using a 0.45 mm membrane filter and stored at 4 C. All experiments were performed in triplicate.
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increased as the La-to-Fe mass ratio increased from 1:1 to 4:1, then leveling off at 5:1 (Fig. 1). This is likely due to the higher La loadings in resulting nanocomposites. The increase in La content in the La(OH)3/Fe3O4 nanocomposites was further confirmed by ICP-OES measurements: the wt% of the La in La(OH)3/Fe3O4 nanocomposites with La-to-Fe mass ratios of 1:1, 2:1, and 4:1 were 12.3%, 15.1%, and 27.8%, respectively. Comparable results of phosphate sorption capacity and La content were obtained for La-to-Fe mass ratios of 4:1 and 5:1. To minimize the use of La chemicals, La(OH)3/Fe3O4 (4:1) nanocomposite was selected for further studies, including detailed material characterization and phosphate sorption performance.
3.2. Characterization of La(OH)3/Fe3O4 (4:1) nanocomposite A suite of techniques including XRD, TEM, BET, and VSM were used to characterize the La(OH)3/Fe3O4 (4:1) nanocomposite. Figure S1 shows the XRD patterns of Fe3O4, La(OH)3, and La(OH)3/ Fe3O4 (4:1). The observed diffraction patterns of pure Fe3O4 are in good agreement with cubic iron oxide phase (JCPDS card 19e0629) (Fang et al., 2017). All diffraction peaks of the pure La(OH)3 product are well indexed as the hexagonal phase (JCPDS 36e1481) of La(OH)3 (Hu et al., 2007). Both diffractions peaks of cubic iron oxide phase and hexagonal La(OH)3 phase were observed in the La(OH)3/ Fe3O4 (4:1). Fig. 2 shows the TEM images of Fe3O4 nanoparticles and the La(OH)3/Fe3O4 (4:1) nanocomposite. Fe3O4 nanoparticles have an average diameter of 10 nm (Fig. 2a). Further loading of La(OH)3 yielded a hybridized La(OH)3/Fe3O4 structures with a rough surface (Fig. 2bec). During the precipitation of La(OH)3, the Fe3O4 nanoparticles were almost entrapped within the whole material matrix. Thus, the formation of the nanocomposite resulted in the increase of size, and the particle size of the La(OH)3/Fe3O4 (4:1) was estimated to be 150e250 nm (Fig. 2b). Concurrent elemental mapping confirms the formation of La(OH)3/Fe3O4 nanocomposite, as La, Fe, and O elements exist throughout the material matrix (Fig. 2def). The BET specific surface area of Fe3O4 and La(OH)3/Fe3O4 (4:1) was 109.1 and 89.6 m2/g, respectively. The decrease of the specific surface area after La(OH)3 modification may be due to the formation of aggregated nanocomposites and the increase in the particle
2.5. Analytical methods Phosphate concentration was determined using the ammonium molybdate method and a UV/vis spectrometer (Lambda 25, PerkinElmer, USA). The concentrations of anions in the treated wastewater effluent were determined using ion chromatography (HIC20 A super, Shimadzu, Japan). Total organic carbon (TOC) was analyzed using a TOC analyzer (TOC, Shimadzu, Japan). ICP-OES was used to detect metal ions. 3. Results and discussion 3.1. Phosphate sorption by La(OH)3/Fe3O4 nanocomposites as a function of the La-to-Fe mass ratio Sorption of phosphate by La(OH)3/Fe3O4 nanocomposites
Fig. 1. Phosphate sorption capacity and La content of the La(OH)3/Fe3O4 nanocomposite with different mass ratios. Experimental conditions for sorption experiments: dosage of sorbent: 0.1 g/L; initial phosphate concentration: 10 mg P/L; pH: 7.0; temperature: 23 C.
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Fig. 2. TEM image of Fe3O4 nanoparticles (a) and La(OH)3/Fe3O4 (4:1) nanocomposites (bec); La. Fe, and O elemental mappings (def) of the selected La(OH)3/Fe3O4 (4:1) nanocomposite (c).
size of the nanocomposites (Fig. 2aec). The total pore volume of the Fe3O4 and La(OH)3/Fe3O4 (4:1) was 0.21 cm3/g and 0.33 cm3/g, respectively. The saturation magnetization value of the La(OH)3/Fe3O4 (4:1) nanocomposite was about 13.3 emu/g (Fig. 3a), which was lower than that of Fe3O4 nanoparticles (60.6 emu/g), but still considered superparamagnetic (Chen et al., 2010). The resulting saturation magnetization of 13 emu/g is sufficient to separate the La(OH)3/ Fe3O4 (4:1) nanocomposites from water. The magnetic separability of the Fe3O4 and La(OH)3/Fe3O4 (4:1) was tested in a magnetic separation unit with magnetic field strength of 0.2 T (Tang et al., 2014). The magnetic separation efficiency was calculated based on the suspension turbidity before and after separation. A linear relationship between magnetic nanoparticle concentration and turbidity was found (Figure S2). High separation efficiencies of 98% for La(OH)3/Fe3O4 (4:1) and 99% for Fe3O4 were achieved within 5 min (Fig. 3b), which was faster than conventional sedimentation or filtration (Drenkova-Tuhtan et al., 2017). Such ease of magnetic separation for La(OH)3/Fe3O4 (4:1) nanocomposites not only enables the recovery of the sorbent but also subsequent phosphate recovery through the sorbent regeneration. 3.3. Phosphate sorption by La(OH)3/Fe3O4 (4:1) nanocomposite 3.3.1. Sorption kinetics The kinetics of phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite is shown in Fig. 4a. The phosphate uptake was very rapid in the first 5 min and sorption equilibrium was achieved within 30 min. To further understand the phosphate sorption process, the kinetic data were fitted using following pseudosecond-order model, as follows (Ho, 2006):
qt ¼
q2e k2 t 1 þ qe k2 t
(1)
where qt and qe are the sorption capacities (mg/g) at time t and at equilibrium, respectively. k2 is the rate constants of pseudo-secondorder sorption (mg/g/min). t is the contact time (min). Results show that the experimental kinetic data can be well described by this model (R2 ¼ 0.911). Similar results were observed for phosphate sorption by other lanthanum-based sorbents, such as La(OH)3-modified exfoliated vermiculites (Huang et al., 2014a), lanthanum-treated lignocellulosic sorbents (Shin et al., 2005), lanthanum hydroxide (Xie et al., 2014), lanthanum-modified SBA15 (Yang et al., 2011), and La(OH)3-modified polyacrylonitrile nanofibers (He et al., 2015). The sorption rate constant k2 was determined to be 0.2198 g/(mg min), which was significantly higher than those of other lanthanum-based sorbents (e.g., 0.00175 g/(mg min)).
3.3.2. Sorption isotherms The sorption isotherm of the La(OH)3/Fe3O4 (4:1) nanocomposite was measured to assess its phosphate uptake capacity (Fig. 4b) as a function of phosphate concentration. Two different models (i.e., Langmuir and Freundlich) were employed to fit the isotherm data. To minimize the respective error functions, a nonlinear optimization technique was used to calculate the isotherm parameters (Tran et al., 2017). The equations of isotherm models and methods of statistical analysis are provided in Text S1. Isotherm data are well described by the Langmuir model, suggesting a monolayer phosphate sorption onto the homogeneous sites of the La(OH)3/Fe3O4 (4:1) nanocomposite (Table S1). The
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provide more adsorption sites (i.e, surface hydroxyl groups) than lanthanum oxide (Zhang et al., 2012). Therefore, our La(OH)3/Fe3O4 (4:1) sorbents possess higher phosphate sorption capacity (83.5 mg P/g) than lanthanum oxide functionalized Aerosil (La150A; 71.8 mg P/g) with comparable La contents (27.8% vs. 32.1%) and BET surface area (89.6 m2/g vs. 87 m2/g). Moreover, the positive surface charge of the La(OH)3/Fe3O4 (4:1) at pH < 7.9 facilities the interaction with negatively charged phosphate species (i.e., H2PO 4 and HPO2 4 ) through electrostatic attraction (Fig. 6b), thus enhancing the phosphate sorption capacity. In addition, the isotherm of phosphate sorption by Fe3O4 nanoparticles is shown in Fig. 4b, and the maximum sorption capacity was estimated to be 4.5 mg P/g at 23 C. Comparing the maximum sorption capacity of the La(OH)3/Fe3O4 (4:1) nanocomposite with that of Fe3O4, the contribution of Fe3O4 to the total phosphate removal appears to be minor.
Fig. 3. VSM analysis of Fe3O4 nanoparticles and the La(OH)3/Fe3O4 (4:1) nanocomposite (a) and magnetic separation of the Fe3O4 and La(OH)3/Fe3O4 (4:1) nanocomposite through a magnetic separation unit (b).
maximum phosphate sorption capacity was estimated to be 83.5 mg P/g and 122.2 mg P/g at 23 C and 37 C, respectively, indicating that sorption process was endothermic. Compared to other La-based sorbents reported for phosphate (Table 1), La(OH)3/ Fe3O4 (4:1) nanocomposite clearly exhibits superior capacity. For example, the maximum sorption capacities of magnetic Fe3O4@SiO2@La2O3 and commercial Phoslock® were only 27.8 mg P/g and 10.6 mg P/g, respectively, which were much lower than that of the developed La(OH)3/Fe3O4 (4:1) nanocomposite. The higher phosphate sorption capacity of La(OH)3/Fe3O4 (4:1) to other La-based sorbents could be understood from the differences in the contents of active sites (i.e., La) that are responsible for phosphate uptake, the density of surface hydroxyl groups on the active sites, and the surface charge on the sorbent surface. The La(OH)3/Fe3O4 (4:1) has a La content of 27.8%, which is much higher than that of Fe3O4@SiO2@La2O3 (11.2%) and is comparable to La(OH)3-modified exfoliated vermiculites (31.2%). Higher La content will lead to higher phosphate sorption capacity, as La mainly serves as active sites for phosphate sorption. In addition, phosphate uptake by Labased sorbents has been shown to occur through ligand exchange at surface hydroxyl sites and electrostatic attraction (Xu et al., 2017). Previous study suggests lanthanum hydroxides may
3.3.3. Effects of coexisting ions In addition to rapid sorption kinetics and high sorption capacity, an effective sorbent should have good selectivity toward phosphate as wastewater is often a mixture of different constituents, including 2 potentially competing anions such as Cl, NO 3 , SO4 and HCO3 and cations like Ca2þ and Mg2þ. To evaluate the selectivity of La(OH)3/ Fe3O4 (4:1) nanocomposite for phosphate removal, the sorption studies of phosphate in the presence of these coexisting ions were conducted and the results are presented in Fig. 4c. For this analysis, the initial concentration of phosphate was held constant at 2 mg P/ L, whereas the concentrations of other ions varied from 10 to 100 mg/L, which are typical concentration ranges of anions and cations in wastewater (Warwick et al., 2013). There was negligible 2 2þ interference from common ions like Cl, NO 3 , SO4 , HCO3 , Ca , 2þ and Mg with concentrations up to 100 mg/L, indicating the strong selectivity of the La(OH)3/Fe3O4 (4:1) material. This is in line with the previous findings on phosphate removal by La-based materials such as lanthanum hydroxides (Xie et al., 2014). Such strong selectivity could facilitate the recovery of phosphate with high purity through the regeneration of the exhausted La(OH)3/Fe3O4 (4:1) nanocomposite, further supporting the practical utilization of this magnetic sorbent for wastewater treatment. 3.3.4. Effects of solution pH and ionic strength The effect of pH, ranging from 3 to 11, on the La(OH)3/Fe3O4 (4:1) phosphate sorption were investigated and the results are presented in Fig. 4d. Phosphate sorption by the La(OH)3/Fe3O4 nanocomposite (4:1) was strongly pH-dependent. The uptake of phosphate increased as the pH increased from 3.0 to 4.3, and this high level was maintained between pH 4.3 and 6. With a further increase of pH from 6 to 11, phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite decreased dramatically. Such behavior has also been observed during phosphate sorption onto other La-based sorbents (He et al., 2015; Zhang et al., 2011). The main species of phosphate was monovalent H2PO 4 in the pH range of 3e6, and the surface of La(OH)3/Fe3O4 was protonated (pHPZC ¼ 7.9, Fig. 6b). Consequently, the positive surface charge of the La(OH)3/Fe3O4 likely facilitates the interaction with H2PO 4 through electrostatic attraction, thus enhancing the phosphate sorption capacity. In the pH range of 6e8, a decrease of phosphate sorption was observed, which could be due to the lower affinity of the surface for divalent HPO2 compared to monovalent H2PO 4 4 . Previous studies also suggested that lanthanum ions have greater affinity for monovalent H2PO 4 due to the hydroxylation of lanthanum ions (Haghseresht et al., 2009; Zhang et al., 2011). When pH was increased to 7.9, the surface of La(OH)3/Fe3O4 became negatively charged, which was not favorable for electrostatic attraction. However, there was still phosphate sorption for solutions over pH 8, which is due to the
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Fig. 4. Phosphate sorption kinetics (a), isotherms (b), effects of coexisting ions (c), and effects of pH and ionic strength (d) on phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite. Experimental conditions for (a) 0.5 g/L; 5 mg P/L; pH 7.0; 23 C; (b) 0.1 g/L; 0.5e15 mg P/L; pH 7.0; 23 and 37 C; (c) 0.1 g/L; 2 mg P/L; pH 7.0; 23 C; and (d) 0.1 g/L; 5 mg P/L; 23 C.
Table 1 Comparison of the sorption capacities of different La-based sorbents for phosphate removal. Sorbent
La (wt BET surface area %) (m2/g)
Experimental conditions (pH; temperature; dosage of sorbent; initial P concentration range)
Sorption capacity (mg P/g)
Reference
La(OH)3/Fe3O4 (4:1) Fe3O4@SiO2@La2O3 La5EV La150-A
27.8
7.0; 23 C; 0.1 g/L; 0.5e15 mg P/L
83.5
This study
11.2 31.5 32.1
47.33 39.1 87
6.59; 25 C; 1 g/L; 0e200 mg P/L 5.0; 25 C; 1 g/L; 1e100 mg P/L n.a.; 25 C; 1 g/L; 10e120 mg P/L
27.8 79.6 71.8
La200MOSF La-NN-M41 HMS-1/5 La100SBA-15 FMS-0.2La Phoslock®
36.6 8.6 22.4 23.3 30.02 4.9
172 134 420.38 227 67.4 39.3
n.a.; 25 C; 1 g/L; 1e100 mg P/L 7.0; 25 C; 1 g/L; n.a. 5.0; 25 C; 0.5 g/L; n.a. n.a.; 25 C; 1 g/L; 10e80 mg P/L n.a.; 25 C; 0.5 g/L; n.a. n.a.; 23 C; 0.05e5 g/L; 10 mg P/L
70.4 54.3 47.9 45.6 44.82 10.6
Lai et al., 2016 Huang et al., 2014a Emmanuelawati et al., 2013 Yang et al., 2012 Zhang et al., 2011 Huang et al., 2014b Yang et al., 2011 Huang et al., 2015 Haghseresht et al., 2009
89.6
Note: n.a.: not available.
involvement of other interactions between the phosphate and the sorbent that are discussed in Section 3.6, below. Lanthanum (La) and iron (Fe) release from the La(OH)3/Fe3O4 (4:1) during phosphate sorption at different pH conditions were measured (Fig. S3). Within pH 3e11, no Fe release was detected. The extent of La release depends on solution pH. When pH was higher than 5.6, La release was negligible, whereas significant La release
was found under acidic conditions with up to 23.6% of La released from the sorbents at pH 3.6. The negligible La release at pH > 5.6 is suitable for the use of La(OH)3/Fe3O4 for phosphate removal from treated effluent, as the pH range of treated effluent usually varies from 6.5 to 8.5 (Metcalf and Eddy, 2003). The tendency of La to release from the La(OH)3/Fe3O4 (4:1) at low pH conditions is similar to the behaviors of other La-based sorbents. Shin et al. observed
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that around 45.6% of La was released from La-treated juniper bark (La/JB02) at pH 4.1 (Shin et al., 2005). Qiu et al. found that 3.9% of La was desorbed from nano-La(III) (hydr)oxides modified-wheat straw at pH 3.0, and it further reached to 54% when solution pH decreased to 1.9 (Qiu et al., 2017). Fig. 4d shows the effects of ionic strength on phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite. Over the pH range of 3e11, the phosphate sorption data under three ionic strengths nearly overlapped, indicating the negligible effect of ionic strength on phosphate sorption. Such sorption behavior, independent of ionic strength, is typical for an inner-sphere complexation. This is because inner-sphere complexation is not affected or increased marginally by increasing ionic strength, whereas outer-sphere complexation is suppressed by increasing ionic strength due to the competitive sorption from background ions and the electric double-layer contraction (Gu et al., 2016; Su et al., 2013).
3.4. Reusability of La(OH)3/Fe3O4 (4:1) nanocomposite and phosphate removal from treated wastewater effluent Five consecutive phosphate sorption/desorption cycles were
Fig. 5. Reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite under 5 consecutive sorption-desorption cycles. Initial phosphate concentration: 2 mg P/L, dosage of sorbent: 0.5 g/L; desorption solution: 1 M NaOH, sorption-to-desorption volume ratio: 1:1 (a), and phosphate removal from real treated sewage effluent by using the La(OH)3/ Fe3O4 (4:1) nanocomposite at different dosages (insert is the characteristics of the treated sewage effluent) (b).
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performed to evaluate the reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite. Fig. 5a shows that the regenerated La(OH)3/Fe3O4 (4:1) nanocomposite exhibited almost no decrease in sorption performance compared to the fresh magnetic sorbent, and the phosphate desorption efficiencies remained higher than 70% for all 5 cycles, suggesting high potential for La(OH)3/Fe3O4 (4:1) nanocomposite as a recovery/recycle platform. To reduce P-loading and prevent eutrophication in natural water bodies, different countries and regions have applied strict P effluent limits (Sengupta and Pandit, 2011). However, selective removal of phosphate from wastewater is still a challenge. Based on enhanced phosphate removal performance of the La(OH)3/Fe3O4 (4:1) nanocomposite, the material's applicability for treatment of real wastewater effluent was also tested. As shown in Fig. 5b, the concentration of phosphate was reduced from 1.1 to 0.05 mg P/L within 1 h at a dosage of 0.1 g/L, even in the presence of excess competitive ions. The release/leaching of La during the sorption process was also monitored and no La ions were detected by ICP-OES (pH 7.0). Moreover, ca. 65% of the adsorbed phosphate could be recovered within 1 h by simply using 1 M NaOH as desorption solution. These results further support the potential of using the La(OH)3/Fe3O4 (4:1) nanocomposite as a magnetic recoverable sorbent for selective phosphate removal from wastewater effluent.
Fig. 6. FTIR spectra of the La(OH)3/Fe3O4 (4:1) nanocomposite before and after phosphate sorption at pH 6.5 (a); and zeta potential of the La(OH)3/Fe3O4 (4:1) nanocomposite before and after phosphate sorption at pH 6.5.
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3.5. Toxicity analysis The ecotoxicological effects of lanthanum to aquatic organisms are of paramount importance when applying La-based sorbents for wastewater treatment. The acute toxicity of lanthanum oxide (La2O3) nanoparticles on two biomonitoring aquatic species, freshwater microalgae Chlorella sp. and the crustacean Daphnia magna, has been studied. No toxic effects of La2O3 were observed on Chlorella sp. even at higher La2O3 concentration (1000 mg/L) after 72 h exposure. For Daphnia magna, no significant toxic effects were detected at La2O3 concentration of 250 mg/L, whereas considerable toxic effects were noted in higher La2O3 concentrations. The median effect concentrations (EC50) of La2O3 against Daphnia magna was 500 mg/L (Balusamy et al., 2015). The toxicity of La2O3 towards microbial organisms, such as Escherichia coli, Staphylococcus carnosus, Penicillium roqueforti, and Chlorella vulgaris, also depends on the presence of phosphate (Gerber et al., 2012). As La2O3 converts to LaPO4, toxicity toward microorganisms decreases. In other words, the toxicity of La2O3 can be controlled by the addition of phosphate. In this study, a low dosage of La(OH)3/Fe3O4 (4:1) (e.g., 0.1 g/L) can be adopted to treat the real wastewater effluent efficiently. Moreover, the release of La from the La(OH)3/ Fe3O4 (4:1) at pH > 5.6 is undetectable. These findings suggest that it is possible to use La(OH)3/Fe3O4 (4:1) for phosphate removal from wastewater without causing substantial adverse effects on aquatic microorganisms. 3.6. Removal mechanisms of phosphate by La(OH)3/Fe3O4 (4:1) nanocomposite To gain insight into the sorption mechanisms of phosphate by the La(OH)3/Fe3O4 (4:1) nanocomposite, a matrix of complimentary
analyses was performed. First, FTIR analysis confirmed that phosphate was indeed adsorbed to the La(OH)3/Fe3O4 (4:1) nanocomposite. As shown in Fig. 6a, a new peak at 1053 cm1 corresponding to the v3 band vibration of HPO2 4 or H2PO4 was observed for the La(OH)3/Fe3O4 upon phosphate sorption (Zhang et al., 2016). This observation is in line with expected phosphate species at pH 6.5. The zeta potential measurements of the La(OH)3/Fe3O4 nanocomposite, before phosphate sorption, showed that the surface of the La(OH)3/Fe3O4 was positively charged over a wide pH range (Fig. 6b), with a point of zero charge (pHPZC) of ca. 7.9. The positive surface charge of La(OH)3/Fe3O4 decreased upon phosphate sorption (0.1 g/L, 5 mg P/L), with a corresponding shift in pHPZC to ca. 5.8. Such a shift of pHPZC to lower pH values due to the accumulation of negative charge within the shear plane, is typically regarded as inner-sphere complexation phenomena via ligand exchange (Xu et al., 2017). Such a ligand exchange can be described in the following reactions. La OH þ H2 PO 4 4La H2 PO4 þ OH
(2)
La ¼ OH þ HPO2 4 4La ¼ HPO4 þ 2OH
(3)
La≡OH þ PO3 4 4La≡PO4 þ 3OH
(4)
The involvement of ligand exchange was further evidenced by the observed change of solution pH during the sorption process. The initial pH of 2 mg P/L phosphate solution was 5.82 and after adding the La(OH)3/Fe3O4 (4:1) nanocomposite, the pH value gradually increased to 7.58 after 2 h. The increase in pH value during sorption is attributed to the release of OH upon ligand
Fig. 7. Wide scan (a), P 2p (b), La 3 d (c), and Fe 2p (d) XPS spectra of the La(OH)3/Fe3O4 (4:1) nanocomposite before and after phosphate sorption at pH 6.5.
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exchange as described above. To further probe the interactions between the phosphate ions and the La(OH)3/Fe3O4 (4:1) nanocomposite, the XPS spectra of the particles before and after phosphate sorption were evaluated. Wide scan XPS spectra of the fresh La(OH)3/Fe3O4 indicated the presence of Fe, O, and La elements (Fig. 7a). The appearance of the P 2p spectra at a binding energy of ~133.9 eV after phosphate sorption demonstrated the successful loading of phosphate on La(OH)3/ Fe3O4 (Fig. 7aeb). The high-resolution XPS spectra of La 3 d before and after phosphate sorption are reported in Fig. 7c. The representative peaks of La 3d5/2 are centered at 833.2 eV and 837.1 eV, and peaks of La 3d3/2 were located at 850.1 eV and 853.8 eV. After phosphate sorption, the binding energies of the La 3d5/2 and La 3d3/ 2 shifted to higher values (ca. 0.8e1.1 eV), indicating a possible electron transfer in the valence band of La 3 d and the formation of La-O-P inner-sphere complexation. These observations are in agreement with a previous study of phosphate removal by nanoLa(III) (hydr)oxides modified wheat straw (Qiu et al., 2017). No obvious changes of Fe 2 P spectra were observed after phosphate sorption (Fig. 7d). 4. Conclusions In this work, magnetic La(OH)3/Fe3O4 nanocomposites of various La-to-Fe mass ratios were developed through a facile precipitation and hydrothermal process for selective phosphate removal from wastewater. La(OH)3/Fe3O4 (4:1) nanocomposites exhibit a high magnetic separation efficiency of 98%, fast equilibrium time of 30 min, high sorption capacity of 83.5 mg P/g, excellent selectivity for phosphate in the presence of competing ions, good reusability, and economic merits (Text S2). Taken together, these properties underpin the potential applicability of using the La(OH)3/Fe3O4 (4:1) nanocomposite materials to remove phosphate from treated wastewater effluent. As demonstrated here for a real wastewater, residual phosphate concentration of 0.05 mg P/L after treatment is well below stringent discharge standards of P. Mechanistically, all experimental evidence indicates that phosphate sorption can be attributed to electrostatic attraction exerted by the positively charged surface of La(OH)3/Fe3O4 (4:1) nanocomposite and the inner-sphere complexation between La(OH)3 and P, via ligand exchange. Overall, our findings advance the use of La-based sorbents, and highlight the described La(OH)3/Fe3O4 (4:1) nanocomposite as a promising platform sorbent for selective phosphate removal and recovery from wastewater. Acknowledgement This work is financially supported by the Research Grants Council of Hong Kong (GRF16207916; T21-711/16-R-1). Technical supports from Materials Characterization and Preparation Facility (MCPF) and Advanced Engineering Material Facility (AEMF) of the Hong Kong University of Science and Technology are appreciated. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.watres.2017.09.034. References Balusamy, B., Tastan, B.E., Ergen, S.F., Uyar, T., Tekinay, T., 2015. Toxicity of lanthanum oxide (La2O3) nanoparticles in aquatic environments. Environ. Sci. Process. Impacts 17 (7), 1265e1270. Biswas, B.K., Inoue, K., Ghimire, K.N., Ohta, S., Harada, H., Ohto, K., Kawakita, H., 2007. The adsorption of phosphate from an aquatic environment using metalloaded orange waste. J. Colloid Interface Sci. 312 (2), 214e223.
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Low-cost and large-scale synthesis of functional porous materials for phosphate removal with high performance. Nanoscale 5 (13), 6173e6180. Fang, L., Wu, B., Lo, I.M.C., 2017. Fabrication of silica-free superparamagnetic ZrO2@ Fe3O4 with enhanced phosphate recovery from sewage: performance and adsorption mechanism. Chem. Eng. J. 319, 258e267. Gerber, L.C., Moser, N., Luechinger, N.A., Stark, W.J., Grass, R.N., 2012. Phosphate starvation as an antimicrobial strategy: the controllable toxicity of lanthanum oxide nanoparticles. Chem. Commun. 48 (32), 3869e3871. Gu, C., Wang, Z., Kubicki, J.D., Wang, X., Zhu, M., 2016. X-ray absorption spectroscopic quantification and speciation modeling of sulfate adsorption on ferrihydrite surfaces. Environ. Sci. Technol. 50 (15), 8067e8076. Haghseresht, F., Wang, S., Do, D.D., 2009. A novel lanthanum-modified bentonite, Phoslock, for phosphate removal from wastewaters. Appl. Clay Sci. 46 (4), 369e375. 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Mater. 166 (2), 891e896. Loganathan, P., Vigneswaran, S., Kandasamy, J., Bolan, N.S., 2014. Removal and recovery of phosphate from water using sorption. Crit. Rev. Environ. Sci. Technol. 44 (8), 847e907. Metcalf, E., Eddy, E., 2003. Wastewater Engineering: Treatment and Reuse. McGrawHill. Inc., New York. Pan, C., Troyer, L.D., Liao, P., Catalano, J.G., Li, W., Giammar, D.E., 2017. Effect of humic acid on the removal of chromium(VI) and the production of solids in iron electrocoagulation. Environ. Sci. Technol. 51 (11), 6308e6318. Qiu, H., Liang, C., Yu, J., Zhang, Q., Song, M., Chen, F., 2017. Preferable phosphate sequestration by nano-La(III) (hydr)oxides modified wheat straw with excellent properties in regeneration. Chem. Eng. J. 315, 345e354. Rashidi Nodeh, H., Sereshti, H., Zamiri Afsharian, E., Nouri, N., 2017. Enhanced removal of phosphate and nitrate ions from aqueous media using nanosized lanthanum hydrous doped on magnetic graphene nanocomposite. J. Environ. 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Highly efficient and selective phosphate removal from wastewater by magnetically recoverable La(OH)3/Fe3O4 nanocomposites Baile Wu a, Liping Fang a, John D. Fortner b, Xiaohong Guan c, Irene M.C. Lo a, * a
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, United States c State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2017 Received in revised form 18 September 2017 Accepted 19 September 2017 Available online 20 September 2017
The use of lanthanum (La)-based materials for phosphate removal from water and wastewater has received increasing attention. However, challenges remain to enhance phosphate sorption capacities and recover La-based sorbents. In this study, magnetic La(OH)3/Fe3O4 nanocomposites with varied La-to-Fe mass ratios were synthesized through a precipitation and hydrothermal method. Based upon preliminary screening of synthesized La(OH)3/Fe3O4 nanocomposites in terms of phosphate sorption capacity and La content, La(OH)3/Fe3O4 nanocomposite with a La-to-Fe mass ratio of 4:1 was chosen for further characterization and evaluation. Specifically, for these materials, magnetic separation efficiency, phosphate sorption kinetics and isotherm behavior, and solution matrix effects (e.g., coexisting ions, solution pH, and ionic strength) are reported. The developed La(OH)3/Fe3O4 (4:1) nanocomposite has an excellent magnetic separation efficiency of >98%, fast sorption kinetics of 30 min, high sorption capacity of 83.5 mg P/g, and strong selectivity for phosphate in presence of competing ions. Phosphate uptake by La(OH)3/Fe3O4 (4:1) was pH-dependent with the highest sorption capacities observed over a pH range of 4e6. The ionic strength of the solution had little interference with phosphate sorption. Sorptiondesorption cyclic experiments demonstrated the good reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite. In a real treated wastewater effluent with phosphate concentration of 1.1 mg P/L, 0.1 g/L of La(OH)3/Fe3O4 (4:1) efficiently reduced the phosphate concentration to below 0.05 mg P/L. Electrostatic attraction and inner-sphere complexation between La(OH)3 and P via ligand exchange were identified as the sorption mechanisms of phosphate by La(OH)3/Fe3O4 (4:1). © 2017 Elsevier Ltd. All rights reserved.
Keywords: Eutrophication Lanthanum Magnetic nanoparticles Phosphate Sorption
1. Introduction Excess phosphorus (P) released into water bodies can trigger eutrophication, affecting water quality and aquatic ecosystem health (Correll, 1998). Studies show that a P concentration above 0.02 mg P/L generally accelerates eutrophication in lakes (Heathwaite and Sharpley, 1999), leading many places to make recommendations on concentration limits for P in water bodies. For example, the United States Environmental Protection Agency (US EPA) recommends that any streams entering a lake or reservoir should have total P concentrations not exceeding 0.05 mg P/L (Loganathan et al., 2014), and the European Union (EU) defines the
* Corresponding author. E-mail address: [email protected] (I.M.C. Lo). https://doi.org/10.1016/j.watres.2017.09.034 0043-1354/© 2017 Elsevier Ltd. All rights reserved.
cut-off total P concentrations in lakes as non-risk and risk conditions of eutrophication to be < 0.01 mg P/L and > 0.1 mg P/L, respectively (Loganathan et al., 2014). For discharges from wastewater treatment plants, the permissible concentration of P will be lowered from 1e2 mg P/L to 0.1 mg P/L in the EU under the Water Framework Directive (Shepherd et al., 2016). However, these are stringent P discharge standards; traditional techniques such as chemical precipitation and biological treatment are unable to reduce phosphate concentrations to below 0.1 mg P/L (Sengupta and Pandit, 2011). Hence, other techniques are needed to meet stringent discharge requirements. Sorption is a preferable approach for phosphate removal due to its simplicity of design, effectiveness even at low P concentrations (Loganathan et al., 2014), and potential for recovery. Many sorbents such as activated carbon, resins, industrial byproducts, and waste biomass have been explored for removing phosphate from water
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(Rittmann et al., 2011), yet low sorption capacity and poor selectivity are inherent limitations for most of these materials. Due to lanthanum's strong affinity with phosphate, a number of studies have developed La-modified sorbents such as La-modified bentonite clay (Phoslock®) (Haghseresht et al., 2009), La-doped silica spheres (Huang et al., 2014b), and La-treated lignocellulosic sorbents (Shin et al., 2005) for the removal of phosphate. These sorbents have exhibited marginally enhanced phosphate sorption capacities e for example the maximum sorption capacity of Phoslock® is only 9.5e10.5 mg P/g (Haghseresht et al., 2009). Further, these sorbents are difficult to separate from wastewater after treatment. Traditional methods to recover sorbents include centrifugation and filtration; however, centrifugation is energy-intensive and filtration is prone to filter blockages (Chen et al., 2009). Magnetic separation is faster and more effective in separating nanoparticles from wastewater compared to centrifugation and filtration (Tang and Lo, 2013). In addition, the incorporation of magnetic nanoparticles (e.g., Fe3O4) with La facilitates the separation and recovery of sorbents. The La acts as active sites for the removal of phosphate in water; while Fe3O4 allows for the magnetic separation. To date, only a few studies have reported on the use of magnetic La-based sorbents for phosphate removal (Lai et al., 2016; Rashidi Nodeh et al., 2017), and these studies have typically been restricted by complicated synthesis procedures, the use of graphene as substrate, which is relatively expensive, or low sorption capacities (i.e., 27.8 mg P/g for Fe3O4@SiO2@La2O3). Additionally, the sorption mechanisms of phosphate by La-based materials have not been characterized in detail. Therefore, the objectives of this work are to: (1) develop magnetic La-based nanocomposite (i.e., La(OH)3/Fe3O4) through a facile precipitation and hydrothermal method; (2) demonstrate selective removal of phosphate by La(OH)3/Fe3O4 with a higher sorption capacity than other sorbents; and (3) elucidate sorption mechanisms through batch experiments (e.g., effects of pH and ionic strength), zeta potential, and spectroscopic investigations (Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS)). 2. Materials and methods 2.1. Materials and chemicals All chemicals used in this study were analytical reagents of high purity. The FeCl3$6H2O (98%), FeCl2$4H2O (99%), trisodium citrate dihydrate, lanthanum (III) nitrate hydrate (La(NO3)3$xH2O, x ¼ 3e5, 99.9%), and KH2PO4 (99%) were purchased from Sigma-Aldrich in the US. The deionized (DI) water with a specific conductivity of 18 MU cm was used to prepare all solutions unless otherwise specified. A 1000 mg P/L stock solution of phosphate was prepared by dissolving 2.2171 g of KH2PO4 in 500 mL DI water. All phosphate in this study refers to orthophosphate unless otherwise specified. 2.2. Synthesis of Fe3O4 nanoparticles and La(OH)3/Fe3O4 nanocomposites 2.2.1. Synthesis of Fe3O4 nanoparticles Fe3O4 nanoparticles were prepared through a modified coprecipitation method (Hu et al., 2005). First, 100 mL of DI water was bubbled by nitrogen gas for 30 min. Then 2.164 g of FeCl3$6H2O and 0.796 g of FeCl2$4H2O were dissolved in the above 100 mL deoxygenated DI water with nitrogen gas purging. NH3$H2O (28% in H2O) was added dropwise into the above mixture until the pH reached 11. The solution was mixed for 10 min, and then a 2-mL solution that contained 0.6 g of trisodium citrate dihydrate was injected rapidly into the mixture. The mixture was then stirred for another 1 h with nitrogen gas purging. Finally, the black products were
separated from the mixture by a magnet, and rinsed with deoxygenated DI water three times. 2.2.2. Synthesis of La(OH)3/Fe3O4 nanocomposites La(OH)3/Fe3O4 nanocomposites were prepared by a precipitation and hydrothermal method. The precipitation method was adopted because of its simplicity, high efficiency, and low cost. The subsequent hydrothermal treatment could benefit the crystallinity of the products (Xiao et al., 2014). The as-prepared Fe3O4 nanoparticles (0.45 g) were dispersed in 60 mL of DI water under ultrasonication for 10 min. Then 0.45 g of La(NO3)3$xH2O in 10 mL DI water was added into the Fe3O4 suspension. The resulting mixture was stirred for 10 min before adding 0.45 g of NaOH dissolved in a 10 mL DI water. After being stirred for another 5 h, the mixture was transferred to a Teflon-lined stainless-steel autoclave (100 mL capacity) and heated at 180 C for 10 h. Finally, the autoclave was cooled down to room temperature and the products were washed with water three times. To synthesize La(OH)3/Fe3O4 nanocomposites with different La contents, the mass ratios of La(NO3)3$xH2O and Fe3O4 (La-to-Fe) were varied from 1:1, 2:1, 4:1, to 5:1 by adjusting the amount of La(NO3)3$xH2O in the initial synthetic solution. The same amount of La(NO3)3,xH2O and NaOH was used in each synthesis. La(OH)3/Fe3O4 nanocomposites with four La-to-Fe mass ratios of 1:1, 2:1, 4:1, and 5:1 were obtained using the above method. 2.3. Characterization of magnetic sorbents The crystal structure of the magnetic sorbents was analyzed using an X-ray diffraction spectrometer (PW-1830, Philips, France) with Cu Ka radiation (l ¼ 1.5406 Å) over the 2q range of 5-90 . The morphology of the sorbents was studied using a transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) at an accelerating voltage of 20 kV. The magnetic properties of the materials were analyzed by a vibrating sample magnetometer (VSM, Lake Shore 7037, USA) at room temperature. Nitrogen adsorptiondesorption isotherms were measured at 77 K using a surface area analyzer (NOVA-3200e, Quantachrome, USA). Prior to the surface area analysis, the samples were degassed at 80 C for 24 h. The specific surface area was determined from the adsorption branch of isotherm using the BrunauerEmmettTeller (BET) method in a relative pressure range of 0.05e0.30. The total pore volume was calculated from the adsorbed nitrogen amount at a relative pressure of 0.98. The La content in each La(OH)3/Fe3O4 nanocomposite was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, Perkin-Elmer, USA). The surface charge of each sorbent was analyzed on a zeta potential analyzer (Zetaplus, LaborScience S.A., Greece) using 1 mM KCl as background electrolyte. The zeta potential values obtained were averaged over ten measurements. The functional groups of the materials were determined on an FTIR spectrometer (Vertex 70, Bruker, USA). FTIR spectra were recorded from 400 to 4000 cm1 at a resolution of 4 cm1 and were averaged over 128 scans. KBr powder was used as a background material. The chemical compositions of the materials were analyzed using an XPS (PHI 5600, Physical Electronics Inc., USA) with Al Ka radiation (1486.6 eV). All binding energies were referenced to the C1s peak at 285.0 eV. 2.4. Batch experiments To determine the time needed to reach sorption equilibrium, the sorption kinetics of phosphate on the as-prepared La(OH)3/Fe3O4 (4:1) nanocomposite were conducted by mixing 0.025 g of sorbents with 50 mL of 5.0 mg P/L phosphate solutions at 23 C. Samples were drawn periodically for phosphate concentration analysis. Sorption
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isotherm experiments at temperatures of 23 C and 37 C were performed to find out the maximum sorption capacity. La(OH)3/ Fe3O4 (4:1) nanocomposites (0.004 g) were dispersed into 40 mL of phosphate solution with initial concentration ranging from 0.5 to 15 mg P/L. pH values of the solutions were maintained at 7.0 throughout the kinetic and isotherm studies by using a 10 mM N-2hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) buffer solution (Biswas et al., 2007). HEPES is widely used for its minimal influence on metal complexation (Pan et al., 2017). The vials were shaken in a mechanical shaker for 2 h at 200 rpm. The effects of competing ions on phosphate sorption were examined by adding 2 2þ 2þ common coexisting ions (i.e., Cl, NO 3 , SO4 , HCO3 , Ca , and Mg ) to the phosphate solutions in separate vials at 23 C. To mimic the typical phosphate concentration of real wastewater, an initial phosphate concentration of 2 mg P/L was selected (He et al., 2015; Li et al., 2009). Various concentrations of ions (10, 50, 100 mg/L) were used. The sorbent dosage was 0.1 g/L and the pH was maintained at 7.0 by using the 10 mM HEPES buffer solution. The vials were shaken in the mechanical shaker for 2 h at 200 rpm. For the solution pH effect studies, 0.004 g of the sorbents were suspended in 40 mL of phosphate solution with initial concentration of 5 mg P/L at 23 C. The pH, ranging from 3.0 to 11.0, was adjusted with a 0.1 M HCl or 0.1 M NaOH solution. The pH was measured using a pH meter (Inolab WTW series pH720) when the solution has been stable for 2 min. NaCl with concentrations of 0.001 M, 0.05 M, and 0.1 M were used to control the ionic strength. The initial phosphate concentration was 5 mg P/L and the sorbent dosage was 0.1 g/L. Five sorptiondesorption cycles were performed in phosphate-spiked DI water (phosphate only) to evaluate the reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite. The magnetic sorbent (0.025 g) was suspended in 50 mL of phosphate solution with initial concentration of 2 mg P/L. The pH was maintained at 7.0 by using the 10 mM HEPES buffer solution. After being mixed for 1 h at 23 C, the phosphateadsorbed nanocomposites were magnetically separated and then suspended in 50 mL of 1 M NaOH solution for 2 h at 23 C. The regenerated La(OH)3/Fe3O4 (4:1) was washed by DI water to remove the remaining NaOH solution until pH 7 and then reused in the succeeding cycle. A fresh 1 M NaOH solution was used for each desorption process. The phosphate desorption efficiencies were calculated as a ratio of the desorbed to the initially adsorbed phosphate. To demonstrate the applicability of La(OH)3/Fe3O4 (4:1) nanocomposite, phosphate removal from real treated wastewater effluent was also studied. The treated wastewater effluent was collected after dichlorination at the Stonecutters Island Sewage Treatment Works in Hong Kong. Prior to being characterized, the treated wastewater effluent was filtered using a 0.45 mm membrane filter and stored at 4 C. All experiments were performed in triplicate.
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increased as the La-to-Fe mass ratio increased from 1:1 to 4:1, then leveling off at 5:1 (Fig. 1). This is likely due to the higher La loadings in resulting nanocomposites. The increase in La content in the La(OH)3/Fe3O4 nanocomposites was further confirmed by ICP-OES measurements: the wt% of the La in La(OH)3/Fe3O4 nanocomposites with La-to-Fe mass ratios of 1:1, 2:1, and 4:1 were 12.3%, 15.1%, and 27.8%, respectively. Comparable results of phosphate sorption capacity and La content were obtained for La-to-Fe mass ratios of 4:1 and 5:1. To minimize the use of La chemicals, La(OH)3/Fe3O4 (4:1) nanocomposite was selected for further studies, including detailed material characterization and phosphate sorption performance.
3.2. Characterization of La(OH)3/Fe3O4 (4:1) nanocomposite A suite of techniques including XRD, TEM, BET, and VSM were used to characterize the La(OH)3/Fe3O4 (4:1) nanocomposite. Figure S1 shows the XRD patterns of Fe3O4, La(OH)3, and La(OH)3/ Fe3O4 (4:1). The observed diffraction patterns of pure Fe3O4 are in good agreement with cubic iron oxide phase (JCPDS card 19e0629) (Fang et al., 2017). All diffraction peaks of the pure La(OH)3 product are well indexed as the hexagonal phase (JCPDS 36e1481) of La(OH)3 (Hu et al., 2007). Both diffractions peaks of cubic iron oxide phase and hexagonal La(OH)3 phase were observed in the La(OH)3/ Fe3O4 (4:1). Fig. 2 shows the TEM images of Fe3O4 nanoparticles and the La(OH)3/Fe3O4 (4:1) nanocomposite. Fe3O4 nanoparticles have an average diameter of 10 nm (Fig. 2a). Further loading of La(OH)3 yielded a hybridized La(OH)3/Fe3O4 structures with a rough surface (Fig. 2bec). During the precipitation of La(OH)3, the Fe3O4 nanoparticles were almost entrapped within the whole material matrix. Thus, the formation of the nanocomposite resulted in the increase of size, and the particle size of the La(OH)3/Fe3O4 (4:1) was estimated to be 150e250 nm (Fig. 2b). Concurrent elemental mapping confirms the formation of La(OH)3/Fe3O4 nanocomposite, as La, Fe, and O elements exist throughout the material matrix (Fig. 2def). The BET specific surface area of Fe3O4 and La(OH)3/Fe3O4 (4:1) was 109.1 and 89.6 m2/g, respectively. The decrease of the specific surface area after La(OH)3 modification may be due to the formation of aggregated nanocomposites and the increase in the particle
2.5. Analytical methods Phosphate concentration was determined using the ammonium molybdate method and a UV/vis spectrometer (Lambda 25, PerkinElmer, USA). The concentrations of anions in the treated wastewater effluent were determined using ion chromatography (HIC20 A super, Shimadzu, Japan). Total organic carbon (TOC) was analyzed using a TOC analyzer (TOC, Shimadzu, Japan). ICP-OES was used to detect metal ions. 3. Results and discussion 3.1. Phosphate sorption by La(OH)3/Fe3O4 nanocomposites as a function of the La-to-Fe mass ratio Sorption of phosphate by La(OH)3/Fe3O4 nanocomposites
Fig. 1. Phosphate sorption capacity and La content of the La(OH)3/Fe3O4 nanocomposite with different mass ratios. Experimental conditions for sorption experiments: dosage of sorbent: 0.1 g/L; initial phosphate concentration: 10 mg P/L; pH: 7.0; temperature: 23 C.
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Fig. 2. TEM image of Fe3O4 nanoparticles (a) and La(OH)3/Fe3O4 (4:1) nanocomposites (bec); La. Fe, and O elemental mappings (def) of the selected La(OH)3/Fe3O4 (4:1) nanocomposite (c).
size of the nanocomposites (Fig. 2aec). The total pore volume of the Fe3O4 and La(OH)3/Fe3O4 (4:1) was 0.21 cm3/g and 0.33 cm3/g, respectively. The saturation magnetization value of the La(OH)3/Fe3O4 (4:1) nanocomposite was about 13.3 emu/g (Fig. 3a), which was lower than that of Fe3O4 nanoparticles (60.6 emu/g), but still considered superparamagnetic (Chen et al., 2010). The resulting saturation magnetization of 13 emu/g is sufficient to separate the La(OH)3/ Fe3O4 (4:1) nanocomposites from water. The magnetic separability of the Fe3O4 and La(OH)3/Fe3O4 (4:1) was tested in a magnetic separation unit with magnetic field strength of 0.2 T (Tang et al., 2014). The magnetic separation efficiency was calculated based on the suspension turbidity before and after separation. A linear relationship between magnetic nanoparticle concentration and turbidity was found (Figure S2). High separation efficiencies of 98% for La(OH)3/Fe3O4 (4:1) and 99% for Fe3O4 were achieved within 5 min (Fig. 3b), which was faster than conventional sedimentation or filtration (Drenkova-Tuhtan et al., 2017). Such ease of magnetic separation for La(OH)3/Fe3O4 (4:1) nanocomposites not only enables the recovery of the sorbent but also subsequent phosphate recovery through the sorbent regeneration. 3.3. Phosphate sorption by La(OH)3/Fe3O4 (4:1) nanocomposite 3.3.1. Sorption kinetics The kinetics of phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite is shown in Fig. 4a. The phosphate uptake was very rapid in the first 5 min and sorption equilibrium was achieved within 30 min. To further understand the phosphate sorption process, the kinetic data were fitted using following pseudosecond-order model, as follows (Ho, 2006):
qt ¼
q2e k2 t 1 þ qe k2 t
(1)
where qt and qe are the sorption capacities (mg/g) at time t and at equilibrium, respectively. k2 is the rate constants of pseudo-secondorder sorption (mg/g/min). t is the contact time (min). Results show that the experimental kinetic data can be well described by this model (R2 ¼ 0.911). Similar results were observed for phosphate sorption by other lanthanum-based sorbents, such as La(OH)3-modified exfoliated vermiculites (Huang et al., 2014a), lanthanum-treated lignocellulosic sorbents (Shin et al., 2005), lanthanum hydroxide (Xie et al., 2014), lanthanum-modified SBA15 (Yang et al., 2011), and La(OH)3-modified polyacrylonitrile nanofibers (He et al., 2015). The sorption rate constant k2 was determined to be 0.2198 g/(mg min), which was significantly higher than those of other lanthanum-based sorbents (e.g., 0.00175 g/(mg min)).
3.3.2. Sorption isotherms The sorption isotherm of the La(OH)3/Fe3O4 (4:1) nanocomposite was measured to assess its phosphate uptake capacity (Fig. 4b) as a function of phosphate concentration. Two different models (i.e., Langmuir and Freundlich) were employed to fit the isotherm data. To minimize the respective error functions, a nonlinear optimization technique was used to calculate the isotherm parameters (Tran et al., 2017). The equations of isotherm models and methods of statistical analysis are provided in Text S1. Isotherm data are well described by the Langmuir model, suggesting a monolayer phosphate sorption onto the homogeneous sites of the La(OH)3/Fe3O4 (4:1) nanocomposite (Table S1). The
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provide more adsorption sites (i.e, surface hydroxyl groups) than lanthanum oxide (Zhang et al., 2012). Therefore, our La(OH)3/Fe3O4 (4:1) sorbents possess higher phosphate sorption capacity (83.5 mg P/g) than lanthanum oxide functionalized Aerosil (La150A; 71.8 mg P/g) with comparable La contents (27.8% vs. 32.1%) and BET surface area (89.6 m2/g vs. 87 m2/g). Moreover, the positive surface charge of the La(OH)3/Fe3O4 (4:1) at pH < 7.9 facilities the interaction with negatively charged phosphate species (i.e., H2PO 4 and HPO2 4 ) through electrostatic attraction (Fig. 6b), thus enhancing the phosphate sorption capacity. In addition, the isotherm of phosphate sorption by Fe3O4 nanoparticles is shown in Fig. 4b, and the maximum sorption capacity was estimated to be 4.5 mg P/g at 23 C. Comparing the maximum sorption capacity of the La(OH)3/Fe3O4 (4:1) nanocomposite with that of Fe3O4, the contribution of Fe3O4 to the total phosphate removal appears to be minor.
Fig. 3. VSM analysis of Fe3O4 nanoparticles and the La(OH)3/Fe3O4 (4:1) nanocomposite (a) and magnetic separation of the Fe3O4 and La(OH)3/Fe3O4 (4:1) nanocomposite through a magnetic separation unit (b).
maximum phosphate sorption capacity was estimated to be 83.5 mg P/g and 122.2 mg P/g at 23 C and 37 C, respectively, indicating that sorption process was endothermic. Compared to other La-based sorbents reported for phosphate (Table 1), La(OH)3/ Fe3O4 (4:1) nanocomposite clearly exhibits superior capacity. For example, the maximum sorption capacities of magnetic Fe3O4@SiO2@La2O3 and commercial Phoslock® were only 27.8 mg P/g and 10.6 mg P/g, respectively, which were much lower than that of the developed La(OH)3/Fe3O4 (4:1) nanocomposite. The higher phosphate sorption capacity of La(OH)3/Fe3O4 (4:1) to other La-based sorbents could be understood from the differences in the contents of active sites (i.e., La) that are responsible for phosphate uptake, the density of surface hydroxyl groups on the active sites, and the surface charge on the sorbent surface. The La(OH)3/Fe3O4 (4:1) has a La content of 27.8%, which is much higher than that of Fe3O4@SiO2@La2O3 (11.2%) and is comparable to La(OH)3-modified exfoliated vermiculites (31.2%). Higher La content will lead to higher phosphate sorption capacity, as La mainly serves as active sites for phosphate sorption. In addition, phosphate uptake by Labased sorbents has been shown to occur through ligand exchange at surface hydroxyl sites and electrostatic attraction (Xu et al., 2017). Previous study suggests lanthanum hydroxides may
3.3.3. Effects of coexisting ions In addition to rapid sorption kinetics and high sorption capacity, an effective sorbent should have good selectivity toward phosphate as wastewater is often a mixture of different constituents, including 2 potentially competing anions such as Cl, NO 3 , SO4 and HCO3 and cations like Ca2þ and Mg2þ. To evaluate the selectivity of La(OH)3/ Fe3O4 (4:1) nanocomposite for phosphate removal, the sorption studies of phosphate in the presence of these coexisting ions were conducted and the results are presented in Fig. 4c. For this analysis, the initial concentration of phosphate was held constant at 2 mg P/ L, whereas the concentrations of other ions varied from 10 to 100 mg/L, which are typical concentration ranges of anions and cations in wastewater (Warwick et al., 2013). There was negligible 2 2þ interference from common ions like Cl, NO 3 , SO4 , HCO3 , Ca , 2þ and Mg with concentrations up to 100 mg/L, indicating the strong selectivity of the La(OH)3/Fe3O4 (4:1) material. This is in line with the previous findings on phosphate removal by La-based materials such as lanthanum hydroxides (Xie et al., 2014). Such strong selectivity could facilitate the recovery of phosphate with high purity through the regeneration of the exhausted La(OH)3/Fe3O4 (4:1) nanocomposite, further supporting the practical utilization of this magnetic sorbent for wastewater treatment. 3.3.4. Effects of solution pH and ionic strength The effect of pH, ranging from 3 to 11, on the La(OH)3/Fe3O4 (4:1) phosphate sorption were investigated and the results are presented in Fig. 4d. Phosphate sorption by the La(OH)3/Fe3O4 nanocomposite (4:1) was strongly pH-dependent. The uptake of phosphate increased as the pH increased from 3.0 to 4.3, and this high level was maintained between pH 4.3 and 6. With a further increase of pH from 6 to 11, phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite decreased dramatically. Such behavior has also been observed during phosphate sorption onto other La-based sorbents (He et al., 2015; Zhang et al., 2011). The main species of phosphate was monovalent H2PO 4 in the pH range of 3e6, and the surface of La(OH)3/Fe3O4 was protonated (pHPZC ¼ 7.9, Fig. 6b). Consequently, the positive surface charge of the La(OH)3/Fe3O4 likely facilitates the interaction with H2PO 4 through electrostatic attraction, thus enhancing the phosphate sorption capacity. In the pH range of 6e8, a decrease of phosphate sorption was observed, which could be due to the lower affinity of the surface for divalent HPO2 compared to monovalent H2PO 4 4 . Previous studies also suggested that lanthanum ions have greater affinity for monovalent H2PO 4 due to the hydroxylation of lanthanum ions (Haghseresht et al., 2009; Zhang et al., 2011). When pH was increased to 7.9, the surface of La(OH)3/Fe3O4 became negatively charged, which was not favorable for electrostatic attraction. However, there was still phosphate sorption for solutions over pH 8, which is due to the
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Fig. 4. Phosphate sorption kinetics (a), isotherms (b), effects of coexisting ions (c), and effects of pH and ionic strength (d) on phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite. Experimental conditions for (a) 0.5 g/L; 5 mg P/L; pH 7.0; 23 C; (b) 0.1 g/L; 0.5e15 mg P/L; pH 7.0; 23 and 37 C; (c) 0.1 g/L; 2 mg P/L; pH 7.0; 23 C; and (d) 0.1 g/L; 5 mg P/L; 23 C.
Table 1 Comparison of the sorption capacities of different La-based sorbents for phosphate removal. Sorbent
La (wt BET surface area %) (m2/g)
Experimental conditions (pH; temperature; dosage of sorbent; initial P concentration range)
Sorption capacity (mg P/g)
Reference
La(OH)3/Fe3O4 (4:1) Fe3O4@SiO2@La2O3 La5EV La150-A
27.8
7.0; 23 C; 0.1 g/L; 0.5e15 mg P/L
83.5
This study
11.2 31.5 32.1
47.33 39.1 87
6.59; 25 C; 1 g/L; 0e200 mg P/L 5.0; 25 C; 1 g/L; 1e100 mg P/L n.a.; 25 C; 1 g/L; 10e120 mg P/L
27.8 79.6 71.8
La200MOSF La-NN-M41 HMS-1/5 La100SBA-15 FMS-0.2La Phoslock®
36.6 8.6 22.4 23.3 30.02 4.9
172 134 420.38 227 67.4 39.3
n.a.; 25 C; 1 g/L; 1e100 mg P/L 7.0; 25 C; 1 g/L; n.a. 5.0; 25 C; 0.5 g/L; n.a. n.a.; 25 C; 1 g/L; 10e80 mg P/L n.a.; 25 C; 0.5 g/L; n.a. n.a.; 23 C; 0.05e5 g/L; 10 mg P/L
70.4 54.3 47.9 45.6 44.82 10.6
Lai et al., 2016 Huang et al., 2014a Emmanuelawati et al., 2013 Yang et al., 2012 Zhang et al., 2011 Huang et al., 2014b Yang et al., 2011 Huang et al., 2015 Haghseresht et al., 2009
89.6
Note: n.a.: not available.
involvement of other interactions between the phosphate and the sorbent that are discussed in Section 3.6, below. Lanthanum (La) and iron (Fe) release from the La(OH)3/Fe3O4 (4:1) during phosphate sorption at different pH conditions were measured (Fig. S3). Within pH 3e11, no Fe release was detected. The extent of La release depends on solution pH. When pH was higher than 5.6, La release was negligible, whereas significant La release
was found under acidic conditions with up to 23.6% of La released from the sorbents at pH 3.6. The negligible La release at pH > 5.6 is suitable for the use of La(OH)3/Fe3O4 for phosphate removal from treated effluent, as the pH range of treated effluent usually varies from 6.5 to 8.5 (Metcalf and Eddy, 2003). The tendency of La to release from the La(OH)3/Fe3O4 (4:1) at low pH conditions is similar to the behaviors of other La-based sorbents. Shin et al. observed
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that around 45.6% of La was released from La-treated juniper bark (La/JB02) at pH 4.1 (Shin et al., 2005). Qiu et al. found that 3.9% of La was desorbed from nano-La(III) (hydr)oxides modified-wheat straw at pH 3.0, and it further reached to 54% when solution pH decreased to 1.9 (Qiu et al., 2017). Fig. 4d shows the effects of ionic strength on phosphate sorption by the La(OH)3/Fe3O4 (4:1) nanocomposite. Over the pH range of 3e11, the phosphate sorption data under three ionic strengths nearly overlapped, indicating the negligible effect of ionic strength on phosphate sorption. Such sorption behavior, independent of ionic strength, is typical for an inner-sphere complexation. This is because inner-sphere complexation is not affected or increased marginally by increasing ionic strength, whereas outer-sphere complexation is suppressed by increasing ionic strength due to the competitive sorption from background ions and the electric double-layer contraction (Gu et al., 2016; Su et al., 2013).
3.4. Reusability of La(OH)3/Fe3O4 (4:1) nanocomposite and phosphate removal from treated wastewater effluent Five consecutive phosphate sorption/desorption cycles were
Fig. 5. Reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite under 5 consecutive sorption-desorption cycles. Initial phosphate concentration: 2 mg P/L, dosage of sorbent: 0.5 g/L; desorption solution: 1 M NaOH, sorption-to-desorption volume ratio: 1:1 (a), and phosphate removal from real treated sewage effluent by using the La(OH)3/ Fe3O4 (4:1) nanocomposite at different dosages (insert is the characteristics of the treated sewage effluent) (b).
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performed to evaluate the reusability of the La(OH)3/Fe3O4 (4:1) nanocomposite. Fig. 5a shows that the regenerated La(OH)3/Fe3O4 (4:1) nanocomposite exhibited almost no decrease in sorption performance compared to the fresh magnetic sorbent, and the phosphate desorption efficiencies remained higher than 70% for all 5 cycles, suggesting high potential for La(OH)3/Fe3O4 (4:1) nanocomposite as a recovery/recycle platform. To reduce P-loading and prevent eutrophication in natural water bodies, different countries and regions have applied strict P effluent limits (Sengupta and Pandit, 2011). However, selective removal of phosphate from wastewater is still a challenge. Based on enhanced phosphate removal performance of the La(OH)3/Fe3O4 (4:1) nanocomposite, the material's applicability for treatment of real wastewater effluent was also tested. As shown in Fig. 5b, the concentration of phosphate was reduced from 1.1 to 0.05 mg P/L within 1 h at a dosage of 0.1 g/L, even in the presence of excess competitive ions. The release/leaching of La during the sorption process was also monitored and no La ions were detected by ICP-OES (pH 7.0). Moreover, ca. 65% of the adsorbed phosphate could be recovered within 1 h by simply using 1 M NaOH as desorption solution. These results further support the potential of using the La(OH)3/Fe3O4 (4:1) nanocomposite as a magnetic recoverable sorbent for selective phosphate removal from wastewater effluent.
Fig. 6. FTIR spectra of the La(OH)3/Fe3O4 (4:1) nanocomposite before and after phosphate sorption at pH 6.5 (a); and zeta potential of the La(OH)3/Fe3O4 (4:1) nanocomposite before and after phosphate sorption at pH 6.5.
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3.5. Toxicity analysis The ecotoxicological effects of lanthanum to aquatic organisms are of paramount importance when applying La-based sorbents for wastewater treatment. The acute toxicity of lanthanum oxide (La2O3) nanoparticles on two biomonitoring aquatic species, freshwater microalgae Chlorella sp. and the crustacean Daphnia magna, has been studied. No toxic effects of La2O3 were observed on Chlorella sp. even at higher La2O3 concentration (1000 mg/L) after 72 h exposure. For Daphnia magna, no significant toxic effects were detected at La2O3 concentration of 250 mg/L, whereas considerable toxic effects were noted in higher La2O3 concentrations. The median effect concentrations (EC50) of La2O3 against Daphnia magna was 500 mg/L (Balusamy et al., 2015). The toxicity of La2O3 towards microbial organisms, such as Escherichia coli, Staphylococcus carnosus, Penicillium roqueforti, and Chlorella vulgaris, also depends on the presence of phosphate (Gerber et al., 2012). As La2O3 converts to LaPO4, toxicity toward microorganisms decreases. In other words, the toxicity of La2O3 can be controlled by the addition of phosphate. In this study, a low dosage of La(OH)3/Fe3O4 (4:1) (e.g., 0.1 g/L) can be adopted to treat the real wastewater effluent efficiently. Moreover, the release of La from the La(OH)3/ Fe3O4 (4:1) at pH > 5.6 is undetectable. These findings suggest that it is possible to use La(OH)3/Fe3O4 (4:1) for phosphate removal from wastewater without causing substantial adverse effects on aquatic microorganisms. 3.6. Removal mechanisms of phosphate by La(OH)3/Fe3O4 (4:1) nanocomposite To gain insight into the sorption mechanisms of phosphate by the La(OH)3/Fe3O4 (4:1) nanocomposite, a matrix of complimentary
analyses was performed. First, FTIR analysis confirmed that phosphate was indeed adsorbed to the La(OH)3/Fe3O4 (4:1) nanocomposite. As shown in Fig. 6a, a new peak at 1053 cm1 corresponding to the v3 band vibration of HPO2 4 or H2PO4 was observed for the La(OH)3/Fe3O4 upon phosphate sorption (Zhang et al., 2016). This observation is in line with expected phosphate species at pH 6.5. The zeta potential measurements of the La(OH)3/Fe3O4 nanocomposite, before phosphate sorption, showed that the surface of the La(OH)3/Fe3O4 was positively charged over a wide pH range (Fig. 6b), with a point of zero charge (pHPZC) of ca. 7.9. The positive surface charge of La(OH)3/Fe3O4 decreased upon phosphate sorption (0.1 g/L, 5 mg P/L), with a corresponding shift in pHPZC to ca. 5.8. Such a shift of pHPZC to lower pH values due to the accumulation of negative charge within the shear plane, is typically regarded as inner-sphere complexation phenomena via ligand exchange (Xu et al., 2017). Such a ligand exchange can be described in the following reactions. La OH þ H2 PO 4 4La H2 PO4 þ OH
(2)
La ¼ OH þ HPO2 4 4La ¼ HPO4 þ 2OH
(3)
La≡OH þ PO3 4 4La≡PO4 þ 3OH
(4)
The involvement of ligand exchange was further evidenced by the observed change of solution pH during the sorption process. The initial pH of 2 mg P/L phosphate solution was 5.82 and after adding the La(OH)3/Fe3O4 (4:1) nanocomposite, the pH value gradually increased to 7.58 after 2 h. The increase in pH value during sorption is attributed to the release of OH upon ligand
Fig. 7. Wide scan (a), P 2p (b), La 3 d (c), and Fe 2p (d) XPS spectra of the La(OH)3/Fe3O4 (4:1) nanocomposite before and after phosphate sorption at pH 6.5.
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exchange as described above. To further probe the interactions between the phosphate ions and the La(OH)3/Fe3O4 (4:1) nanocomposite, the XPS spectra of the particles before and after phosphate sorption were evaluated. Wide scan XPS spectra of the fresh La(OH)3/Fe3O4 indicated the presence of Fe, O, and La elements (Fig. 7a). The appearance of the P 2p spectra at a binding energy of ~133.9 eV after phosphate sorption demonstrated the successful loading of phosphate on La(OH)3/ Fe3O4 (Fig. 7aeb). The high-resolution XPS spectra of La 3 d before and after phosphate sorption are reported in Fig. 7c. The representative peaks of La 3d5/2 are centered at 833.2 eV and 837.1 eV, and peaks of La 3d3/2 were located at 850.1 eV and 853.8 eV. After phosphate sorption, the binding energies of the La 3d5/2 and La 3d3/ 2 shifted to higher values (ca. 0.8e1.1 eV), indicating a possible electron transfer in the valence band of La 3 d and the formation of La-O-P inner-sphere complexation. These observations are in agreement with a previous study of phosphate removal by nanoLa(III) (hydr)oxides modified wheat straw (Qiu et al., 2017). No obvious changes of Fe 2 P spectra were observed after phosphate sorption (Fig. 7d). 4. Conclusions In this work, magnetic La(OH)3/Fe3O4 nanocomposites of various La-to-Fe mass ratios were developed through a facile precipitation and hydrothermal process for selective phosphate removal from wastewater. La(OH)3/Fe3O4 (4:1) nanocomposites exhibit a high magnetic separation efficiency of 98%, fast equilibrium time of 30 min, high sorption capacity of 83.5 mg P/g, excellent selectivity for phosphate in the presence of competing ions, good reusability, and economic merits (Text S2). Taken together, these properties underpin the potential applicability of using the La(OH)3/Fe3O4 (4:1) nanocomposite materials to remove phosphate from treated wastewater effluent. As demonstrated here for a real wastewater, residual phosphate concentration of 0.05 mg P/L after treatment is well below stringent discharge standards of P. Mechanistically, all experimental evidence indicates that phosphate sorption can be attributed to electrostatic attraction exerted by the positively charged surface of La(OH)3/Fe3O4 (4:1) nanocomposite and the inner-sphere complexation between La(OH)3 and P, via ligand exchange. Overall, our findings advance the use of La-based sorbents, and highlight the described La(OH)3/Fe3O4 (4:1) nanocomposite as a promising platform sorbent for selective phosphate removal and recovery from wastewater. Acknowledgement This work is financially supported by the Research Grants Council of Hong Kong (GRF16207916; T21-711/16-R-1). Technical supports from Materials Characterization and Preparation Facility (MCPF) and Advanced Engineering Material Facility (AEMF) of the Hong Kong University of Science and Technology are appreciated. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.watres.2017.09.034. References Balusamy, B., Tastan, B.E., Ergen, S.F., Uyar, T., Tekinay, T., 2015. Toxicity of lanthanum oxide (La2O3) nanoparticles in aquatic environments. Environ. Sci. Process. Impacts 17 (7), 1265e1270. Biswas, B.K., Inoue, K., Ghimire, K.N., Ohta, S., Harada, H., Ohto, K., Kawakita, H., 2007. The adsorption of phosphate from an aquatic environment using metalloaded orange waste. J. Colloid Interface Sci. 312 (2), 214e223.
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