AbstractHighly efficient removal of phosphate from water bodies is of great importance to controlling eutrophication. Herein, the lanthanum-based perovskites (i.e., LaMnO3, LaFeO3) were synthesized and employed for phosphorus adsorption. Adsorption experiments showed that the phosphate removal by LaMnO3 was well fitted with the pseudo second-order kinetic model, with a multi-stage adsorption process. The LaMnO3 exhibited a maximum adsorption capacity of 51.3 mg/g for phosphate calculated using the Langmuir model, which was 2.1 times higher than that of LaFeO3 under neutral conditions. Characterization and DFT calculation results further revealed that the La element on the surface of perovskite was the main adsorption site via inner-sphere complex and/or electrostatic interactions, whereas the B-site metal (i.e. Mn element) could trigger the surface electronic structure modulation such as the reduction of surface bonding barrier, for further improving the phosphate adsorption. Additionally, the LaMnO3 adsorbent exhibited commendable performance in treating natural water with low phosphorus concentration and good regenerative capability. This work provides insight into the development of novel perovskite-type adsorbents for efficiently removing phosphorus in environmental remediation.
Graphical Abstract![]() IntroductionNowadays, phosphorus has been recognized as a determinant of eutrophication in lakes [1–3]. The releasing of excess phosphorus into rivers and lakes can lead to severe damage to aquatic ecosystems [4], which has aroused extensive concern worldwide. To date, the removal of phosphorus especially with low concentrations is still a research emphasis and hot spot in water treatment. Phosphorus removal technologies are largely composed of physicochemical, biological, and adsorption techniques [5–10]. Although physicochemical techniques such as chemical precipitation and biological methods have been reported to efficiently remove phosphorus, their expensive and intricate operations severely restricted the industrial application. In contrast, the adsorption method exhibits a promising prospect in treating phosphate-containing wastewater, owing to its simplicity, low cost, high treatment efficiency and environment friendly nature.
In recent decades, a wide range of adsorbents, from natural and modified clay minerals, agricultural waste to metal oxides/hydroxides and biopolymer have been developed for lead, cadmium, arsenic and phosphate elimination from aqueous solutions [11,12]. Various adsorbents materials with different elements often exhibit varied properties and mechanisms toward contaminants removal, thus making it imperative to thoroughly investigate their underlying adsorption performance and mechanism. For instance, manganese-lanthanum (Mn-La) binary oxide have been proposed to combine the characteristic lattice oxygen of Mn oxides and La (hydro) oxides with Lewis acidity sites, which could be a promising adsorbent for removal of cationic heavy metals [12]. Currently, La-based adsorbent materials have aroused widespread attention in the field of phosphate pollution improvement, since the La has a strong binding ability toward phosphate ions (Ksp = 3.7 × 10−23) [13]. Up to now, a great deal of La-modified materials involving natural clay, carbon-based materials and especially metal oxide materials have been employed to remove phosphorus in water [14,15]. For instance, Yang et al., loaded lanthanum on Fe3O4 by chemical co-sedimentation to obtain the La-Fe3O4 adsorbent, which showed 3~1000 times higher uptake of phosphate in water than that of unloaded lanthanum, and achieved 97.8% of lanthanum removal in 7h [16]. Zhang et al., immobilised hydrated La(III) oxide (HLO) nanoclusters within the network pores of polystyrene anion exchanger D-201, with working capacity 2–4 times higher than a commercial Fe(III) oxide- based nanocomposite in batch runs [17]. Therefore, La-based metal oxide adsorbents are promising materials for treatment of phosphorus-containing water, and core of this method is the development of excellent adsorption oxide materials.
La-based perovskite oxides (LaBO3) with B cations being mostly transition metals always possess an excellent thermal stability, high ionic conductivity, good electron mobility and redox capacity compared to single metal oxides or common composite metal oxides, making them have extensive application prospects in the field of environmental pollution control [18–22]. It is generally believed that about 90% of the metal elements in the periodic table can be used to synthesize perovskites, which opens up a wide range of possibilities for modulating perovskite properties and leads to its wider application in industrial multiphase catalysis [23]. However, research on the adsorption of phosphate by La-based perovskite is limited [24]. Importantly, most previous studies about the La-based oxides adsorbents just focused on their physical properties like the structure, surface area and particle size, but the surface chemical properties and interface energy of oxides is also a critical issue in promoting the phosphate adsorption, which deserved to be deeply studied [25].
Inspired by this point, La-based manganese and iron perovskites absorbents were synthesized by sol-gel method and utilized as absorbents to remove phosphate from water [26]. The B-site metals of La-based perovskites (LaBO3, B = Mn, Fe) are assumed to be able to modulate the surface chemical properties while maintaining a stable and similar structure. The phase composition, morphology and physicochemical properties of LaMnO3 and LaFeO3 were systematically characterized using various techniques, such as X-ray powder diffraction (XRD), scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET) method. The effects of operating parameters (dosage, initial pH, temperature, coexisting ions) on the removal of phosphate, as well as adsorption kinetics and adsorption isotherms of phosphate over LaMnO3 were explored. The phosphate removal mechanism over perovskite oxide was further revealed via X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR), Density functional theory (DFT) calculations and so on. Finally, the adsorbent regeneration and phosphorus removal in actual surface water were carried out to investigate the general applicability of perovskite absorbents.
Materials and Methods2.1. ChemicalsSulfuric acid (H2SO4, GR) and nitric acid (HNO3, GR) were purchased from Xilong Scientific Co., Ltd. (China). Potassium dihydrogen phosphate (KH2PO4, GR), sodium hydroxide (NaOH, AR), lanthanum nitrate (La(NO3)3, AR), iron nitrate (Fe(NO3)3, AR), manganese nitrate (Mn(NO3)2, AR), citric acid (C6H8O7, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Ascorbic acid (C6H8O6, AR), ammonium molybdate ((NH4)2MoO4, AR), potassium antimony tartrate (C8H4K2OSb2, AR). All chemicals were used as received without further purification. The deionized water with resistivity of 18.8 MΩ·cm−1 (SY-BJ30L, China) was employed for all solutions preparation.
2.2. Synthesis of Perovskite OxidesThe perovskite used in the experiment was prepared by sol-gel method. 2 mmol citric acid was dissolved into 30ml DI water, Fe(NO3)3 and Mn(NO3)2 were dissolved into 20ml DI water according to the stoichiometric ratio of LaFeO3 and LaMnO3. The beaker containing citric acid was placed in a magnetic heating stirrer and heated to 40°C. Fe(NO3)3 and Mn(NO3)2 were dripped into the citric acid with a rubber-tipped burette respectively, to prepare LaFeO3 and LaMnO3 perovskites precursor, at the end of the process, warmed up to 80°C and timed. The mixture was stirred for 4 hours, and then transferred to a 100°C oven for 24h. The formed solid samples were then crushed to a crucible and placed in a muffle furnace. At first, the dry gel was heated to 280°C at a rate of 3.85°C/min and then cultured for 1 h, followed by calcination at 700°C at a rate of 2.17°C/min and held for 8 h. Then, the particles containing Fe and Mn elements were obtained by cooling down to room temperature and were named as LFO (LaFeO3) and LMO (LaMnO3).
2.3. Experimental ProceduresFor exploring the phosphate adsorption behavior of the prepared perovskites, typically, 0.05 g of obtained sample were dispersed into 250 mL phosphate solution of 2.5 mg/L. Potassium dihydrogen phosphate (KH2PO4) of analytical grade was used to prepare the phosphate solution. The initial pH was adjusted with 0.1M H2SO4 and 0.1M NaOH, all the adsorption experiments were carried out at room temperature (30°C) excepting the temperature effect experiments and constant stirring rate of 200 rpm. The concentrations of chloride ions, nitrate ions and sulfate ions in solution were monitored by ion chromatography (ICS-600, American), respectively. At given time intervals, solution samples were filtered through 0.45μm membrane syringe filters prior to measurement. The phosphate concentration was determined by an ammonium molybdate spectrophotometry [27] with a UV-Vis spectrophotometer (UV-1800, China). To assess the reusability of LaMnO3 absorbent, the used adsorbent after adsorption equilibrium was treated with 2 M NaOH (the resolution rate reaches 100% at 12 h) solution for phosphate desorption [28]. Samples were withdrawn and subsequently filtered to the next adsorption cycle and this process was repeated for ten times. To quantify the adsorption selectivity of adsorbents, the distribution ratio (KD, mL g−1), selectivity coefficients of phosphate (α) with respect to other anions were determined calculated by Eq. (1) and (2) respectively [29, 30]. In addition, the selectivity (%) to phosphate of adsorbent was defined as the mole ratio of the removed phosphate upon anions interference to the phosphate concentration in solution at equilibrium, as shown in the following Eq. (3).
where qe is the amount of adsorbed phosphate at adsorption equilibrium (mg/g), qei is the amount of adsorbed phosphate upon anion interference (mg/g), Ce is the initial concentration of phosphate (mg/L), KD1 and KD2 represent the distribution coefficients of phosphate and D2 (D2 is Cl−, NO3 −, or SO4 2−), respectively.
Adsorption isotherm experiment: in a typical run, 0.05 g of adsorbents was dispersed in a conical flask with 250 mL solution containing KH2PO4 at different concentrations (5, 7.5, 10, 20, 30, mg /L, pH = 5). The solution was shaken in a shaking incubator for 72 h, and the phosphate concentration was measured immediately. The adsorption isotherm model was fitted by Langmuir model (Eq. 4) and Freundlich model (Eq. 5).
where qm is the maximum adsorption capacity of langmuir adsorption (mg/g), KL is the langmuir adsorption constant (L/mg), and KF is the Freundlich isotherm equation constant.
In order to investigate the adsorption kinetic process of LaMnO3, 0.1 g of adsorbent was dispersed into the solutions with pre-set phosphate concentrations of 2.5 and 5 mg/L (initial pH = 5), respectively. Solution samples were then taken periodically for phosphate concentration analysis. The adsorption kinetics was described with pseudo-first-order kinetic model (Eq. 6), pseudo- second-order kinetic model (Eq. 7) and intra-particle diffusion model (Eq. 8).
where qt is the amount of phosphate adsorbed at time t (mg/g), k1 is the first-order rate constant (min), and k2 is the second-order rate constant (g/mg/min), kd is the intraparticle diffusion rate constant (mg/g/min), C is the intercept (mg/g), and t is the time (min).
2.4. Characterization Methods and DFT CalculationThe products obtained during different stages were characterized by X-ray diffraction (XRD) using a D8 ADVANCE and DAVINCI DESIGN (Brook, American) with Cu Kα radiation in the range of 2θ = 10~80°, and at a scanning rate of 0.02° s−1. The morphology and surface element compositions of as-prepared samples were observed by scanning electron microscope (SEM, Zeiss Sigma 300, Germany) with operation voltage of 3 keV and Energy Dispersion Spectroscopy (EDS, Oxford X-MAX, Britain). The composition of sorbents before and after P uptake were inspected by using energy dispersive X-ray spectroscopy (XPS, ESCAL AB 250X1, Thermo, American), all spectra were calibrated using the C1s signal located at 284.8eV. BET surface areas and BJH pore distributions of as-synthesized products were determined with a surface area analyzer (3Flex 5.02, American) basing on N2 adsorption-desorption isotherms. Before measurement, solid samples need to be pretreated at 150°C under vacuum for 6 hours to remove surface impurities.
First-principal calculation in this work was performed using the Vienna AB-initio Simulation Package (VASP) [31]. The generalized gradient approximation by Perdew-Burke-Ernzerhof (PBE) was employed to describe the exchange-correlation functional [32]. Use the projector augmented wave (PAW) method to describe the interaction between the valence electrons and the atomic [33]. The energy cutoff for the plane wave expansion was set to 450 eV. A2×2×1 Monkhorst Pack k-point setup was used for geometry optimization. The adsorption energies (Eads, kcal/mol) of phosphate on the surface of perovskites were calculated by Eq. 9:
where EAB is the total energy of perovskite adsorbed to phosphate, and EA and EB represent the energies of adsorbate phosphate and adsorbent of perovskite, respectively.
Results and discussion3.1. CharacterizationsThe XRD patterns of the obtaining samples are shown in Fig. 1a and Fig. S1a. As can be seen, the diffraction peaks of two samples can be well matched with the standard diffraction pattern of LaMnO3 (PDF#01-082-8384) and LaFeO3 (PDF#01-070-7777) [34,35], indicating the successful synthesis of perovskites. Using the Scherrer equation, the average grain size of LaMnO3 is calculated to be 19.2 nm at a peak of 32.2°, which is smaller compared with that of LaFeO3 (45.9 nm). Smaller grains may result in larger grain boundary areas, providing more reaction sites and improving the reactivity of material.
The surface morphologies of two perovskites were also observed by SEM. As can be seen from Fig. S1c, LaFeO3 consists of closely connected spherical particles with a grain size of 40–200nm, with a relatively compact skeleton structure. No obvious pore structure and relatively smooth outer surface was observed with a low specific surface area of 3.35 m2/g (Table S1). In contrast, the LaMnO3 with a mesoporous structure are connected with each other to form a rough, small and porous sponge structure by the accumulation of particles forming clefts (Fig. 1c and Fig. S1d), resulting in an increased specific surface area of 12.67 m2/g (Fig. 1b and Table S1). Fig. 1b illustrates the N2 adsorption-desorption isothermal curve of LaMnO3, which exhibited an isotherm with a H3-type hysteresis loop type IV isotherm [24], also verifying the porous property of LaMnO3 material. The porous structure of LaMnO3 may provide a greater number of adsorption sites for the removal of phosphate. The EDS images (Fig. 1d) further confirmed that the LaMnO3 contained lanthanum, manganese, and oxygen elements with an even distribution on its exterior.
3.2. Phosphate Adsorption Behavior3.2.1. Effect of adsorbent dosageThe dosage of absorbent directly relates to the content of surface binding sites, thus affecting the performance of phosphate removal. The effect of absorbents dosage (0.1 g/L, 0.2 g/L, 0.3 g/L) on the phosphate removal are shown in Fig. 2a. As can be seen, the removal efficiency increased with increasing absorbents dosage. Besides, LaMnO3 exhibited higher removal efficiency (48.5%~ 73.9%) and adsorption capacity (6.16~12.13 mg/g) for phosphate than that of LaFeO3 (20.2%~45.5%; 3.79~5.05 mg/g), possibly owing to a larger BET area (Table S1) and plentiful binding sites of LaMnO3 absorbent. However, the excessive adsorbent in the solution tends to be agglomerated that concealed part of adsorption sites on the adsorbent surface, in turn restraining the chemical bonding with phosphate and reducing the phosphate adsorption capacities. Thus, a further increase in absorbents just caused a slight enhancement in the removal efficiency at the LaMnO3 dosage of 0.3 g/L. Considering the adsorption performance and acceptable cost of adsorbent, the adsorbent dosage of 0.2 g/L is selected to apply in the subsequent experiments.
3.2.2. Effect of solution pHPhosphate adsorption is influenced by both the surface charge of adsorbent and the protonation state of phosphate in solution [36]. Consequently, the initial pH (pH0) of the solution plays a crucial role in the phosphate elimination. Fig. 2b presented the effect of solution pH (3, 5, 7, 9, and 11) on phosphate adsorption. It can be seen that the phosphate removal and adsorption capacity of perovskite absorbents increased dramatically as the pH0 of solution raised from 3 to 5, and then decreased gradually at pH0 > 5. The LaMnO3 and LaFeO3 exhibited the maximum capacities of 12.30 and 4.48 mg/g at pH 5.
As reported, the tertiary dissociation constants of phosphate are 2.1, 7.2, and 12.3 respectively, that is, the phosphate fractions in water were dominated in the form of monovalent H2PO4 − in the pH range of 2.1~7.2, while HPO4 2− is the major species within the pH range of 7.2~12.3 [37]. It is assumed that the adsorption energy of H2PO4 − is lower than that of HPO4 2−, thus H2PO4 − is more easily adsorbed on the surface of oxides compared to HPO4 2− [38]. That was why the adsorption of phosphate decreased with increasing pH. Moreover, under alkaline conditions, hydroxide ions will occupy the adsorption site and compete with phosphate on the surface, also diminishing the reduction of adsorption efficiency. However, in the strong acidic condition (pH = 3), the decreased adsorption efficiency might be attributed to the metal elements dissolution as well as the destroy of adsorbent and its pore structure [39].
3.2.3. Effect of temperatureTo investigate the effect of temperature on the phosphate removal by perovskite absorbents, various temperatures ranging from 30°C to 50°C were chosen for the experiment. As shown in Fig. 2c, when the temperature was raised from 30°C to 50°C, the adsorption capacity over LaFeO3 rose slightly from 6.14 mg/g to 8.57 mg/g, indicating that the adsorption process most likely underwent chemical interaction process. High temperature was in favor of phosphate removal, probably owing to an endothermic property of the phosphate adsorption [40]. Increasing the temperature was also beneficial for mass transport process by enhancing driving force of phosphate ions onto the LaFeO3 and reducing the energy barrier of adsorption reaction. In contrast, the phosphate removal remained relatively stable (>95.0%) for LaMnO3 at 30°C ~ 50°C. It might be ascribed to the following aspect. At a lower temperature (30°C), the surface adsorption sites of the LaMnO3 adsorbent have already completely been occupied, and in this case, the increased temperature on adsorption efficiency for LaMnO3 is not obvious.
3.2.4. Effect of coexisting anionsUnderstanding the adsorption preference of adsorbent for coexisting anions is crucial, since some natural anions including NO3 −, SO4 2− and Cl−, always coexist in wastewater or natural waters. Therefore, the impact of these coexisting anions at different concentration levels (50 mg/L and 100 mg/L) on phosphate removal by LaMnO3 was investigated. As shown in Fig. 2d, little influence was observed with the presence of SO4 2− and Cl−, while the NO3 − has a slight inhibitory effect for phosphate adsorption. At the anion concentration of 50mg/L, the selectivity coefficients (α) of phosphate in the presence of NO3 −, SO4 2− and Cl− were determined as 7.2, 31.2, and 17.2 (all the α > 1.0), respectively, confirming the good selectivity performance of LaMnO3 for phosphate removal [29]. In addition, compared with the phosphate adsorbents in Table S4, the selectivity (%) to phosphate of LaMnO3 is calculated as 82%~99%, which is also superior to most of lanthanide-based adsorbents reported in previous literatures, further verifying its preferential performance of LaMnO3 for phosphate adsorption.
3.2.5. Adsorption isotherms of LaMnO3The LaMnO3 was selected as a representative perovskite to investigate the adsorption properties of phosphate in the subsequent experiments, in terms of its better adsorption capacity of phosphate from aqueous solution. The Langmuir and Freundlich adsorption isotherm models were employed to fit the experimental data (Table S2) obtained from the adsorption of different initial concentrations of phosphate by LaMnO3 at pH 5, respectively. As illustrated in Fig. 3a and Table S3 the Langmuir model can better fit the phosphate adsorption process with a fitting coefficient (R2) of 0.993, compared with that of Freundlich model (R2 = 0.925), indicating that the adsorbed molecules mainly existed as monolayer coverings on the absorbent surface [41].
The maximum adsorption capacity (qm) obtained from the Langmuir model is calculated to be 51.3 mg/g, which is close to the experimentally measured qm value of 50.8 mg/g. The constant separation factors (RL) ranged from 0.1220 to 0.023, significantly lower than 1, suggesting a favorable adsorption of phosphate by LaMnO3 adsorbent [41]. Table S4 displays the comparative qm obtained from Langmuir fitting for different adsorbents reported in literature. It can be seen that the adsorption capacity of LaMnO3 is comparable or surpasses that of conventional modified bentonite, clay, nanocomposite and other non-metallic minerals adsorbents, demonstrating a commendable performance of LaMnO3 [26].
3.2.6. Adsorption kinetics of LaMnO3The effects of reaction time on the adsorption capacity of phosphate by LaMnO3 were evaluated with the initial phosphate concentrations of 2.5 mg/L and 5.0 mg/L (Table S5). As shown in Fig. 3b, the fitting result was more consistent with the pseudo-second- order equation than that of pseudo-first-order process, thus implying that the phosphate adsorption by LaMnO3 is mainly controlled by chemical processes [42]. Most of phosphate was adsorbed by LaMnO3 in 60 min, and then the adsorption slowly approached equilibrium after 240 min. With the initial phosphate concentration raising from 2.5 mg/L to 5 mg/L, the equilibrium adsorption capacity of LaMnO3 were found to be 12.21 mg/g and 22.29 mg/g, which were close to the measured maximum adsorption capacities of 12.63 mg/g and 21.62 mg/g, respectively.
The solid/liquid adsorption process typically progresses through three stages including external diffusion, internal diffusion, and adsorption. To gain better understanding of the phosphate diffusion and adsorption processes, the internal diffusion model was applied. As illustrated in Fig. 3c, the fitted plot can be divided into 3 straight lines with different slopes, indicating that the phosphate adsorption by LaMnO3 is a multi-stage adsorption process. As shown in Table S6, the constant C1 < C2 and diffusion rate constant Kd1 > Kd2 demonstrated that the first stage facilitated the mass diffusion and phosphate adsorption. In this stage, a huge hydraulic mass transfer force would form at the solid-liquid interface with phosphate rapidly occupying the surface-active sites [43]. In the second stage, the Kd2 decreased accompanied by a sharp increase in the constant C2, which can be ascribed to the gradually saturated surface of adsorbent and an electrostatic repulsive force generated by the adsorbed phosphate leading to a reduction of mass transfer force.
In the third stage, the adsorption sites were nearly depleted, and the phosphate adsorption moved towards equilibrium, significantly slowing down the adsorption rate. Of note, a higher initial phosphate concentration (5.0 mg/L) would lead to an extended period of internal diffusion and adsorption of particles with larger K values at various stages. This could be attributed to the enhanced driving force exerted by the gradient difference at an elevated phosphate concentration, thereby expediting the adsorption process.
3.3. Adsorption MechanismTo clarify the mechanism of phosphate removal from solution, comprehensive characterizations of LaMnO3 adsorbent were carried out by various means. SEM and surface elemental mapping of LaMnO3 after phosphate adsorption is shown in Fig. 4. It is observed that the surface of used LaMnO3 exhibited some interconnected needle-like nanostructures with many flocculent deposits and a linear morphology, implying the formation of a new phase attached to the adsorbent surface. EDS elemental mapping also revealed that the phosphorus was successfully adsorbed by LaMnO3 perovskite.
Based on the XPS analysis of LaMnO3 adsorbent before and after phosphate uptake (Fig. 5a,b), a new P 2p spectra located at 133.1 eV appeared only after phosphate adsorption. Noted that this P 2p peak does not correspond to the standard P 2p characteristic peak of pure KH2PO4 (134.0 eV), suggesting that an inner spherical complex via La-O-P chemotactic bond might be formed between phosphate and the LaMnO3 oxides [44]. From the La 3d high-resolution spectra (Fig. 5c), two double-split peaks for fresh LaMnO3 adsorbent located at 833.7 and 850.5 eV belonged to the characteristic peaks of La 3d5/2 and La 3d3/2, with an energy difference of 16.8 eV [45]. After phosphate uptake, the La 3d peaks shifted to higher binding energies (~1.2 eV), proving the formation of stronger ionic bonding between La3+ and PO4 2− ions, and the La 3d valence band underwent electron transfer to generate La-O-P inner complex.
The Mn 2p spectra (Fig. 5d) maintained unchanged after phosphate adsorption indicated that the Mn element may be less directly involved in the phosphate adsorption. Fig. 5(e) shows the O 1s peak spectra which is divided into three overlapping peaks located at 529.2, 531.1, and 532.9 eV, corresponding to the M-O (lattice oxygen), -OH/P-O (surface hydroxyl oxygen), and H2O (adsorbed water), respectively [46]. It can be observed that the relative content of -OH increased from 43.35% to 55.99% after adsorption of phosphate, while the content of lattice oxygen decreased dramatically from 46.12% to 28.04%. This implied that the lattice oxygen plays a crucial role in the adsorption process as a main adsorption site. It is assumed that P reacts with lattice oxygen on the absorbent surface to form new hydroxyl oxides, thus increasing the content of –OH group. FTIR analysis (Fig. 5f) revealed a new absorption peak at ~1050 cm−1 after phosphate uptake, which was attributed to the typical characteristic of asymmetric stretch vibration of O-P-O [47]. Slightly weakening of the peak intensity at 610 cm−1 (M-O bond) after phosphate adsorption also proved the participation of lattice oxygen in the adsorption process [48].
3.3.1. DFT calculationThe adsorption configurations and interaction energies of phosphate molecules adsorbed on perovskite interface were estimated by DFT, aiming to elucidate the binding mode between the adsorbent with phosphate and also insight into the adsorption mechanism of phosphate by perovskites. Compared with that of LaFeO3, the La−O bond length of LaMnO3 decreased from 2.6872, 2.5633 Å to 2.5082, 2.5366 Å respectively (Fig. 6 and Table S7). In addition, the phosphate adsorption energies (Eads) of −10.61, −12.66 kJ/mol were obtained for LaFeO3 and LaMnO3 respectively, indicating the LaMnO3 perovskite has a great affinity towards phosphate ions. The charge distribution of LaFeO3 and LaMnO3 was further analyzed via the charge density difference, respectively. Observing the differential charge density (Fig. 6c,d), the charge accumulation (yellow region) is close to La-O bonds, and the charge depletion (blue region) is close to P-O bonds, demonstrated that the stable La-O-P bonds are formed at interface between phosphate and perovskite [49,50]. In addition, more charge accumulation was observed at interface of LaMnO3 compared with that of LaFeO3, which might be ascribed to the electronegativity of Mn being less than that of Fe, triggering the LaMnO3 easily donating electrons to form positive ions on the surface that attracted phosphate molecules.
3.4. Practical Exploration of AbsorbentThe regeneration of adsorbent is crucial for the technology scale-up in practical application. As indicated in Fig. 7a, during the recycling removal of phosphorus, the removal efficiency was still maintained at >70% after 10 regenerations of LaMnO3. Just a slight decrease of phosphorus removal in the cycle adsorption might be due to the high strength in binding force of La and P [16]. In brief, the LaMnO3 has sufficient chemical stability over several adsorption-desorption repetitions, and it can be regenerated via dilute alkaline treatment, demonstrating a good reusability of the catalyst with superior potential in environmental remediation. Furthermore, the phosphorus removal from actual surface water was also examined by using LaMnO3 adsorbent. As shown in Fig. 7b, the LaMnO3 exhibited a commendable performance in removing phosphorus in surface water (Xunsi River in Wuhan city, China). Despite the presence of ions and organic matter in surface water (Table S8), the LaMnO3 adsorbent still exhibits excellent selectivity and removal capacity for phosphorus. During the process of treating surface water under natural conditions, the concentrations of competing anions (i.e., Cl−, SO4 2− and NO3 −) exhibited slight variation, which were determined as 78.7~81.9 mg/L, 42.5~39.3 mg/L, 52.7~47.3 mg/L, respectively, indicating these anions might have non-significant impacts on the phosphorus adsorption process, even in natural water. The phosphorus concentration decreased from 0.32 mg/L to < 0.01 mg/L in 90 min at pH 5.0 and 7.9, fulfilling the rigorous requirements for averting eutrophication (P < 0.02 mg/L) [46]. Overall, the LaMnO3 exhibits immense potential as an efficient adsorbent for the elimination and immobilization of phosphorus concentrations in eutrophic waters.
ConclusionIn this work, two typical perovskites, LaMnO3 and LaFeO3, were applied to remove phosphate from aqueous solution. The performance of phosphate adsorption by perovskites was investigated in various reaction conditions. Experiments results demonstrated that the LaMnO3 was a superior adsorbent for phosphate removal compared with LaFeO3. 98.4% of phosphorus could be removed by LaMnO3 in 1.5h under the optimal conditions (P0 = 2.5mg/L, dosage = 0.2g/L, pH = 5). Meanwhile, the LaMnO3 possessed a good recycle stability for efficient phosphate removal and also showed the practicality of water treatment in natural water, Adsorption kinetics indicated that the adsorption of phosphorus by LaMnO3 was consistent with Langmuir isotherm and pursues the pseudo- second-order model with an adsorption capacity of 51.3 mg P/g. Mechanism investigation revealed that the efficient phosphate removal by LaMnO3 could be owing to electrostatic interactions and stable inner-sphere complexation between La-O bonds and phosphate. Noted that the Mn element at B site improves the porosity and interface electronic structure of perovskite in comparison with that of Fe element, finally promoting the adsorption performance of phosphorus. In summary, the LaMnO3 perovskite possesses the potential to become a high-profile absorbent for phosphate removal in wastewater treatment and eutrophication control.
AcknowledgementsThis study was supported by the National Natural Science Foundation of China (Grant No. 22006034), the Hubei Provincial Natural Science Foundation of China (Grant No. 2020CFB485 and 2022CFB986), and the Hubei Province Yangtze River Water Ecological Environmental Protection Research Project (Grant No. 2022-LHYJ-02-0506-04).
NotesAuthor Contribution L. W. (PhD student) conducted all the experiments and wrote the manuscript. H. L. (Professor) directed the research. H. D. (PhD student) and Y. C. (PhD student) helped in developing the methodology and material characterization. H. W. (Postdoc) and Y. X (Associate Professor), who had supervised the work and made all the possible corrections to the manuscript. References1. Schindler DW, Carpenter SR, Chapra SC, Hecky RE, Orihel DM. Reducing Phosphorus to Curb Lake Eutrophication is a Success. Environ. Sci. Technol. 2016;50(17)8923–8929.
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![]() Fig. 1(a) XRD patterns; (b) N2 adsorption desorption isotherms; (c) SEM images and (d) EDS mapping of LaMnO3. ![]() Fig. 2Effect of (a) absorbent dosage, (b) solution pH and (c) temperature on the phosphate removal (C0 = 2.5 mg P/L, adsorbent dosage = 0.2 g/L; without pH adjustment (initial pH 5.6); room temperature; time = 1.5h); (d) Effect of coexisting anions on the adsorption of phosphate by LaMnO3 (C0 = 10 mg P/L, adsorbent dosage = 0.2 g/L; pH = 5; room temperature; time = 24h). ![]() Fig. 3(a) Isotherm model for phosphate adsorption; (b) Pseudo-first order kinetic model and Pseudo-second-order kinetic model; (c) Intra-particle diffusion model of phosphate adsorption by LaMnO3 (adsorbent dosage = 0.2 g/L; pH = 5; room temperature). ![]() Fig. 5XPS spectra of (a) wide scan, (b) P 2p, (c) La 3d, (d) Mn 2p, (e) O 1s; and (f) FTIR spectra of LaMnO3 before and after phosphate adsorption. ![]() |
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