Effective adsorption of phosphate from aqueous solution by La-based metal–organic frameworks

Abstract

In this study, lanthanide-based metal–organic frameworks (La-MOFs) were prepared by a hydrothermal method and employed for the adsorption of phosphate from water. The structure and properties of La-MOFs have been verified by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and zeta potential measurements. Afterward, its performance as an adsorbent for phosphate removal was investigated. Experiments were performed to study the effects of various conditions on the phosphate adsorption, including adsorbent dosages, contact time and the initial pH. The results indicated that the phosphate adsorption on La-MOFs was pH-dependent. The experimental data were interpreted elaborately by different models of adsorption kinetics and isotherms. The results showed that the kinetic data fitted well to the pseudo-second-order models, indicating that the adsorption behaviors were mainly ascribed to both physisorption and chemisorption. The equilibrium data were best described by the Langmuir isotherm model with a maximum adsorption capacity of about 142.04 mg g⁻¹. Moreover, the adsorbed phosphate could be almost desorbed completely with NaOH solution for reusability. To summarize, La-MOFs are a promising adsorbent for the phosphate adsorption from water.

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1 Introduction

Phosphorus is treated as the basic element and major nutrient for the development of biosomes and the normal functioning of ecosystems. However, the excessive application of phosphorus as fertilizer and detergent unavoidably causes a great number of pollutants, environment problems and severe eutrophication which contribute to aquatic species death, algal bloom and parasite infection.^1–3 Phosphorus usually exists in the form of phosphate in the aquatic environment at low concentrations, including inorganic phosphate, organic phosphate and polyphosphate.⁴ On the basis of the Environmental Protection Agency (EPA), the maximum allowable level and strict discharge limit of phosphate ions are 0.1 mg L⁻¹ and less than 0.05 mg L⁻¹, respectively.^5,6 As a result, it is necessary to develop optimal methods for removal of phosphorus from wastewater.

In recent years, a large number of methods have been discovered to decrease and control the concentration of phosphate in wastewater, such as chemical treatment (precipitation with iron salts, alum, or lime) and biological process. Compared to these methods, adsorption is the most effective approach to decontaminate water due to its high efficiency, low cost, and ease of operation.^7–9 Even more important, adsorbed phosphorus could be recycled if the adsorption amount is high and an appropriate desorption approach can be found. Hence, the applications of proper adsorption techniques and adsorbents are vitally important to ensure the efficiency of wastewater treatment.

Moreover, a number of adsorption materials have been developed for removal of phosphate, as summarized recently. Among them, metal–organic frameworks (MOFs), a class of hybrid porous materials composed of metal-oxo clusters and organic building blocks, have been developed as efficient sorbents attributed to their fascinating structures and unusual properties, such as ultrahigh surface areas, uniform but tunable cavities, and tailorable chemistry.¹⁰ Zubair et al. used MOFs to adsorb hazardous organics from water.¹¹ And the adsorption of phthalic acid and diethyl phthalate from water with MOFs was studied by Nazmul et al.¹² Here, phosphate was used as a model pollutant to study the adsorption performance of MOFs.

Lanthanum is a rare earth element that is considered to be environmentally friendly and is relatively abundant in the earth's crust.^13–15 Lanthanum is a trivalent metal with strong Lewis acidic. Because of the hard Lewis basic property of phosphate, lanthanum demonstrates strong ligand adsorption for phosphate to form the lanthanum–phosphate complex. A lot of researches in the past have demonstrated that lanthanum-containing materials for the phosphate removal from water were promising.^16–24

Therefore, the aim of this study was to prepare La-MOFs and employ for the adsorption of pollutants from water. The prepared La-MOFs exhibited excellent performance and adsorption capability for the phosphorus removal from water. The physicochemical properties of La-MOFs were measured by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscope, and zeta potential measurements. Meanwhile, the effects of different factors on the phosphate removal efficiency and adsorption capacity, such as adsorbent dosages, contact time, initial concentration and pH, were also studied. The adsorption kinetics and isotherms models were used for evaluating the experimental data.

2 Materials and methods

2.1 Chemicals

The chemicals used in the study were of analytical grade and without any further purification, lanthanum nitrate dodecahydrate (La(NO₃)₃·12H₂O), sodium hydroxide, ammonium molybdate, antimony potassium tartrate and potassium dihydrogen phosphate, ascorbic acid, terephthalic acid (H₂BDC) were received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Hydrochloric acid, sulfuric acid, N,N-dimethyl formamide (DMF) and acetone were received from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China).

2.2 Preparation of La-MOFs

The procedure to obtain La-MOFs was performed as described by Serra-Crespo.²⁵ In this case, 0.670 g of La (NO₃)₃·12H₂O and 0.514 g of H₂BDC were dissolved in 30 mL of DMF. After 30 min stirring, the solution was transferred to a Teflon-lined autoclave and heated at 130 °C for 3 days. Then the product was washed three times with acetone. The solid was collected by centrifugation and dried at 60 °C.

2.3 Adsorption experiment

Kinetics studies were carried out using 0.015 g La-MOFs in 30 mL solution containing 50, 80, and 100 mg L⁻¹ phosphate, respectively. The experiment was performed in 50 mL beakers at 120 rpm on a magnetic stirrer at room temperature (25 °C). Samples were drawn at 0–80 min, and the last sampling time was sufficient to attain adsorption equilibrium.

In the effect of dosage experiment, La-MOFs with different dosages (0.005–0.030 g/30 mL) were added to 100 mg L⁻¹ phosphate solution. Experiment was carried out in 50 mL beakers at 120 rpm on a magnetic stirrer at room temperature, and the solution pH value was about 6.32 without any artificially adjusting.

In the pH effect experiment, the original pH of the phosphate solution (100 mg L⁻¹) were adjusted to the value of 2–10 by acid or base solutions, and then 0.015 g La-MOFs were added to 30 mL phosphate solutions. Experiment was carried out at room temperature in 50 mL beakers at 120 rpm on a magnetic stirrer. According to the ammonium molybdate spectrophotometric method, the concentration of each solution was measured by a UV-visible spectrophotometer (UV-5100B, Metash Instruments (Shanghai) Co., Ltd, China) at the wavelength of UV-maximum (λ_max) 880 nm.

The amounts of phosphate removed by La-MOFs were calculated by the following equation described as:

where q_e is the amount of phosphate adsorbed on La-MOFs (mg g⁻¹); C₀ and C_e are the initial and equilibrium concentrations of phosphate in solution (mg L⁻¹); V is the volume of solution (L); m is the dosage of La-MOFs.

2.4 Characterization methods

The crystalline structure and composition of the La-MOFs were identified by a X-ray diffraction (XRD, Bruker Corporation, Germany) with the range 2θ of 5–70° at a scan speed of 0.03° s⁻¹ and Cu Kα radiation (40 mA, 45 kV). Scanning electron microscopy (SEM, S4800, Hitachi Corporation, Japan) was used to observe the morphologies of La-MOFs. Fourier transform infrared spectroscopy (FTIR) was taken with a Vertex 70 FTIR spectrophotometer (Bruker Corporation, Germany) at room temperature. The zeta potentials of adsorbent before and after adsorption of phosphate were determined by a nano ZS90 zetasizer analyzer (Malvern Instruments Ltd, UK). A precision pH meter (Starter 3100, Ohaus Instruments (Shanghai) Co., Ltd, China) was used to measure the pH values of solutions.

3 Results and discussion

3.1 Characterization of La-MOFs

Fig. 1a showed the XRD pattern of La-MOFs. It matched well with the diffraction data of La-based MOFs previously reported and demonstrated that pure phases were obtained. As is well known, peak signals' change of MOFs depends on the guest molecules inside the pores because of the breathing effect. The first peaks at 9.5° suggest that the pattern corresponded to the structure of MIL-53 material, but in comparison to MIL-53, the position of the peak signal is slightly changed. This may due to the presence of residual unreacted H₂BDC in the reaction or washing process was trapped in.²⁶ Moreover, the diffraction angles 2θ = 16.61°, 18.54° and 26.47° corresponded to the crystal surface (020), (012) and (133). In the synthesis of other La-MOFs, the similar findings were also mentioned.²⁷

Fig. 1 Characterization of La-MOFs: (a) XRD pattern; (b) FTIR spectra; (c) SEM image; (d) zeta potential.

Further confirmation of La-MOFs was provided by the FTIR spectra. As shown in Fig. 1b, the asymmetric and symmetric stretching vibrations of the carboxylate groups have bands at 1562 and 1387 cm⁻¹. The bands at 3050, 831, 675, and 753 cm⁻¹ are attributed to the aromatic skeleton vibration of the benzene ring, ν_=C–H of benzene, and δ_=C–H out of the face of benzene. The bands at 1655 and 2937 cm⁻¹ are due to ν_C=O and the asymmetric stretching vibration of the methyl group of the DMF molecules. The broad band at 3480 cm⁻¹ belongs to the typical band of the water molecules.²⁷

In addition, SEM image of La-MOFs were presented in Fig. 1c. As seen the image, the La-MOFs were mainly a prism-like morphology, a uniform ordered structure with size about 5.55 ± 1.75 μm. Besides, the La-MOFs particles were more aggregated with a smooth surface. And the sizes of particle were obtained from numerous particles by measuring from SEM image.

The zeta potential of La-MOFs before and after adsorption was demonstrated in Fig. 1d. It can be seen that the point of zero charge (pH_pzc) for La-MOFs before adsorption was 10.56, the higher pH_pzc indicates the La-MOFs is more pH-independence. Moreover, it revealed that the surface charge of La-MOFs was positive when the pH was lower than pH_pzc and the adsorbent was much easier to absorb negative ions because of electrostatic attraction. Therefore, La-MOFs were beneficial for the adsorption of phosphate ions. Whereas, the pH_pzc for La-MOFs after adsorption was lower than the one before adsorption due to the absorbed phosphate ions on the surface of La-MOFs.

3.2 Effect of different conditions on the adsorption of phosphate

On the adsorption of phosphate, the effects of adsorbent dosage, contact time and original solution pH were conducted. The removal efficiencies and adsorption capacities of phosphate by adsorbent at different dosages (0.005–0.030 g/30 mL) were shown in Fig. 2a. It can be concluded that the removal efficiency of phosphate increased with the amounts from 0.005 g to 0.020 g and reached a stage. After that, as more adsorbent was added to the solution, the removal efficiency remained equilibrium. This was because the active adsorption sites were limited at less amounts of La-MOFs, leading to the lower removal efficiency of phosphate. However, when the amounts increased to above 0.020 g, the active adsorption sites were abundant, and removal efficiency was closed to 100%. It indicated that almost all phosphate ions were absorbed in solution. However, the adsorption capacity at the adsorbent dosage of 0.020 g was lower than 0.015 g because of the excess of adsorbent. Therefore, based on economical and practical considerations, the adsorbent dosage of 0.015 g was selected as the optimum dosage.

Fig. 2 Effect of different conditions on the adsorption of phosphate: (a) adsorbent dosage, (b) contact time and (c) initial solution pH (phosphate concentration: 100 mg L⁻¹, adsorbent dosage: 0.5 g L⁻¹ and temperature: 25 °C).

Fig. 2b depicted the effect of contact time on phosphate adsorption over La-MOFs. More than 70% of phosphate was removed within 20 min which showed that La-MOFs exhibited excellent adsorption performance, and then the removal efficiency was stable at about 90%. The fast adsorption rate at the incipient stage could be attributed to the increase of driving force provided by the concentration gradient of phosphate in aqueous solution and the existence of the great number of available active sites on the surface of La-MOFs.

The pH of solution is the most important factor to influence the surface acid–base properties of the adsorbent. In this study, the initial solution pH ranging from 2.0 to 10.0 was conducted. As Fig. 2c showed, the adsorption for phosphate on La-MOFs presented the lower capacity in the beginning, and then, with the increase of initial pH value, the capacity tended to increase, because the La-MOFs surface possessed more positive charge favoring electrostatic attraction. However, as the pH increased to 10.0, the capacity decreased dramatically, because of the abundance of OH⁻ and the less attractive or more repulsive electrostatic interaction at higher solution pH values.

3.3 Adsorption kinetic

In order to research the adsorption mechanisms and potential rate controlling step of phosphate removal, the pseudo-first-order and pseudo-second-order model were used for describing the process of phosphate adsorption on La-MOFs. Table 1 exhibited some correlative parameters of the kinetic models, such as the linear forms, the way of plots and the correlation coefficients (R²). Fig. 3 showed the linear forms of two adsorption kinetic models by fitting with the experimental data, respectively.^28–30

Table 1 Adsorption kinetic models, the corresponding linear forms and parameters of La-MOFs obtained at four phosphate initial concentrations

C0 (mg L^−1)	Pseudo-first-order model			Pseudo-second-order model
	log(qe − qt) = log qe − k1t/2.303
	qe (mg g^−1)	k1 (min^−1)	R^2	qe (mg g^−1)	k2 (g (mg^−1 min^−1))	R^2
50	141.28	0.026	0.9786	101.10	0.00528	0.9938
80	207.42	0.02564	0.9090	130.34	0.00398	0.9925
100	161.44	0.02515	0.9411	91.48	0.00518	0.9884

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Fig. 3 Adsorption kinetics of phosphate on La-MOFs at three different initial phosphate concentrations using (a) pseudo-first-order model and (b) pseudo-second-order model (phosphate concentration: 50, 80 and 100 mg L⁻¹, adsorbent dosage: 0.5 g L⁻¹, temperature: 25 °C, and pH: 6.32).

So it was clear that the experimental data were fitted better by the pseudo-second order model with R² of 0.9884–0.9938 than the pseudo-first-order model with R² of 0.9090–0.9786. The high R² values of the pseudo-second-order kinetic equations implied the good feasibility for the phosphate adsorption on La-MOFs. In addition, the equilibrium adsorption capacities calculated were more approximate to the experimental values, further verifying the applicability of the pseudo-second-order model. These results suggested that the phosphate adsorption process was chemical adsorption achieved by sharing electrons between phosphate ions and adsorbent.

3.4 Adsorption isotherm

Adsorption isotherm models were employed for describing equilibrium studies. In this study, equilibrium data were used for fitting with the Langmuir and Freundlich models.

The Langmuir model is based on the following assumptions: (1) monolayer adsorption; (2) the adsorbed anions are dependent; and (3) all sites are equivalent. The Langmuir model is expressed as:^28,30,31

where q_m is the maximum adsorption capacity (mg g⁻¹) and K_L is the Langmuir constant (mg⁻¹).

The Freundlich model is applicable for the heterogeneous surface of adsorbent with multi-layers adsorption. In this model, the maximum adsorption capacity is uncertain and adsorption sites are not equal with different energy. The Freundlich model is expressed as:^28,30,31

where K_F is the Freundlich constant [(mg g⁻¹) (mg L⁻¹)⁻¹], and n indicates sorption favorability.

As shown in Fig. 4, the Langmuir isotherm exhibited a better fit with the higher correlation coefficient (R² = 0.9392). Furthermore, the q_m calculated by this function was quite close to the values actually determined. The results revealed the homogeneous monolayer adsorption nature of phosphate onto La-MOFs.

Fig. 4 The Langmuir isotherm model (phosphate concentration: 10–200 mg L⁻¹, adsorbent dosage: 0.5 g L⁻¹, temperature: 25 °C, and pH: 6.32).

3.5 Adsorption mechanisms

It is known that phosphate acid can dissociate to form different ionic species of H₂PO₄⁻, HPO₄²⁻ and PO₄³⁻, depending on the pH of solution, which can be presented as:
where pK₁ = 2.15, pK₂ = 7.20, and pK₃ = 12.33.

When the pH value was lower than 2.15, the main existence form of the phosphate in solution was H₃PO₄, and the binding capacity between H₃PO₄ and adsorbent was weak. In the pH range of 2.15 and 7.20, H₂PO₄⁻ was the main species in solution. Due to the electrostatics attraction between La³⁺ and H₂PO₄⁻, La³⁺ in MOFs was easy to form a complex with H₂PO₄⁻, the high phosphate adsorption capacity was formed. As the pH increased, the increasing concentration of OH⁻ made the surface of adsorbent negatively charged. The adsorption of phosphate could be unfavorable because of electrostatic repulsion between the adsorption surface and the anionic ions (HPO₄²⁻). On the other hand, based on the surface complexation, a certain amount of coordinated water was chemically adsorbed on the surface of La in the solution. After further dissociation, a large number of OH⁻ was formed on the surface of the adsorbent. The OH⁻ exchanged with the phosphate ions in the water, and the phosphate ions were adsorbed on the surface of the adsorbent. However, the replacement of the OH⁻ would further increase the solution pH, which would have an adverse effect on the adsorption of phosphate. Fig. 5 showed the proposed mechanisms for phosphate removal on La-MOFs.

Fig. 5 The proposed mechanisms for the removal of phosphate by La-MOFs.

3.6 Desorption

Recovery and repeated usability of the adsorbent is important for the practical applications of real effluent. In this work, La-MOFs was regenerated by using 1 mol L⁻¹ NaOH solution.

As shown in Fig. 6, after three adsorption–desorption cycles, the phosphate sorption capacities are observed with only slight losses in their initial adsorption capacities. This indicates that La-MOFs can be repeatedly used for the removal of phosphate from aqueous solution.

Fig. 6 Adsorption–desorption cycles using 1.0 mol L⁻¹ NaOH solution.

4 Conclusions

In this study, La-MOFs were synthesized for the adsorption of phosphate. The effects of different conditions for the phosphate adsorption were studied particularly. The adsorption kinetics followed the pseudo-second-order model, and the isotherm data were well described by the Langmuir isotherm model with maximum adsorption capacity of 142.04 mg g⁻¹. The studies of XRD, SEM, FTIR and zeta potential indicated that the adsorption mechanisms were not only electrostatic attraction but also ligand exchange reactions between phosphate ions and La-MOFs. Furthermore, NaOH solution was selected as the elution agent for the regeneration of sorbents. After three adsorption–desorption cycles, the capacities for the regenerated sorbents were yet higher than 90%. Therefore, La-MOFs are promising materials for adsorption of phosphate from aqueous solution.

Acknowledgements

This work was financially supported by the National Natural Science Foundation, China (Grant No. 51578264), the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2013EEM004), and the Shandong Provincial Science and Technology Development Program, China (Grant No. 2014GSF117008).

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