Phosphate removal using zinc ferrite synthesized through a facile solvothermal technique

Powder Technology 301 (2016) 723–729

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Phosphate removal using zinc ferrite synthesized through a facile solvothermal technique Wei Gu, Qiang Xie, Chongyang Qi, Lianqin Zhao, Deyi Wu ⁎ School of Environmental Science and Engineering, Shanghai Jiao Tong University, No. 800, Dongchuan Rd., Shanghai 200240, China

a r t i c l e

i n f o

Article history: Received 18 March 2016 Received in revised form 3 July 2016 Accepted 6 July 2016 Available online 6 July 2016 Keywords: Zinc ferrite Solvothermal synthesis Phosphate Adsorption Magnetic separation

a b s t r a c t To develop a phosphate adsorbent in powder form that is easily separated from water, we prepared magnetic spinel zinc ferrite using a facile solvothermal technique. Characterization of zinc ferrite was done by VSM, XRD, TEM, and FTIR measurements. We found that zinc ferrite crystallized as a cubic ZnFe2O4 phase (JCPDS card no. 891010). It had a saturation magnetization of 34.95 emu/g, which allowed easy separation using a magnet. Phosphate adsorption under different initial phosphate concentrations, solution pH values, ionic strengths, temperatures, contact times, as well as in the presence of competitive ions, was investigated. Data from kinetic experiments fit well the pseudo-second-order model. The maximum adsorption capacity obtained by fitting adsorption isotherm data to the Langmuir model ranged within 5.23–6.28 mg/g at different temperatures. Thermodynamic parameters indicate that phosphate adsorption by zinc ferrite is an endothermic and spontaneous process. The amount of phosphate adsorbed increased with decreasing pH and increasing ionic strength. Zinc ferrite showed good selectivity for phosphate. Results suggest that phosphate adsorbed onto the zinc ferrite surface via formation of an inner-sphere complex. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Phosphorus is an essential element for all living things, including aquatic organisms. However, accumulation of excess phosphorus in closed water bodies such as lakes can lead to eutrophication and thus deterioration of water quality [1,2]. Removal of phosphate from water/ wastewater before its release into water bodies is thus essential to their protection from eutrophication. Adsorption is a promising technology for the removal of phosphate from water/wastewater and is superior to chemical precipitation treatment and biological treatment technologies when the phosphate concentration is low [3–5]. In fact, advanced further treatments for phosphate removal from wastewater, such as adsorption technology, are often needed after chemical and biological treatments to meet increasingly stringent requirements [3–5]. Therefore, recent investigations on the development of adsorbents with high performance in phosphate adsorption from water/wastewater have been undertaken [6–17]. An adsorbent is most efficient in its powder form because of the high specific surface area and number of surface reaction sites, as well as the short intraparticle diffusion distance. Unfortunately, finely powdered adsorbents cannot be easily recovered for reuse once dispersed in water because of the difficulty in separating them from the liquid. Although granulation can overcome the separation problem, the resulting ⁎ Corresponding author. E-mail address: [email protected] (D. Wu).

http://dx.doi.org/10.1016/j.powtec.2016.07.015 0032-5910/© 2016 Elsevier B.V. All rights reserved.

agglomeration of adsorbent particles reduces the adsorptive capacity of the adsorbent. In recent years, magnetic nanoparticles have emerged as a novel material for phosphate adsorbents [18–25]. These nanoparticles have the unique advantage of being easily separated from aqueous media for reuse. The direct use of magnetite nanoparticles as adsorbents was studied by Daou et al. [18], de Vicente et al. [19], and Yoon et al. [20]. However, Fe2+ in the nanoparticles is unstable, readily undergoing oxidation to maghemite, thus necessitating their preparation under an inert atmosphere to prevent the formation of iron oxides and iron hydroxides such as goethite. An approach recently attempted to protect the magnetite nanoparticles from oxidation while retaining their magnetic property is coating them with a silica layer. This is followed by surface functionalization of the silica-coated magnetite nanoparticles with other materials for their use in adsorption. A number of magnetic nanoparticle materials with this core/shell structure have been developed into adsorbents for the removal of pollutants from water. Zirconia [21, 22], layered double hydroxide [23,24], and hydrous lanthanum oxide [25] have been successfully used to functionalize silica-coated magnetite nanoparticles for phosphate removal. More recently, we prepared hydrous zirconia-coated magnetite nanoparticles using a one-step coprecipitation method and found that the material shows high performance in phosphate adsorption and is easily separable [26]. Fe2+ in magnetite may be replaced with other divalent metal cations M while retaining the spinel structure to generate different magnetic nanoparticle materials with the formula MFe2O4, where M represents a divalent metal cation [27]. To address the stability problem, we have

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Fig. 1. The TEM image (left) and the XRD pattern (right) of zinc ferrite material.

considered using spinel ferrites with divalent cations that are relatively resistant to oxidation–reduction in magnetic nanoparticle materials for phosphate removal. However, previous studies have focused on unmodified or modified magnetite nanoparticles; few investigations on spinel ferrites for phosphate removal have been done. In the present study, spinel zinc ferrite (ZnFe2O4) was synthesized by a solvothermal method, and its use in phosphate removal was investigated for the first time. Unlike Fe(II), zinc is stable in air. Zinc ferrite was characterized through various techniques, and phosphate adsorption under different conditions in solution was explored. 2. Materials and methods 2.1. Synthesis of zinc ferrite All chemicals used were of analytical grade. Potassium dihydrogen phosphate (KH2PO4), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), and sodium acetate trihydrate (NaCH3COO·3H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ethylene glycol was obtained from Yonghua Chemical Co. (Jiangsu, China). The ZnFe2O4 nanoparticles were prepared through a solvothermal method according to the procedure [28] with minor modifications. Zn(CH3COO)2·4H2O (15 mmol), Fe(NO3)3·9H2O (30 mmol), and NaCH3COO·3H2O (60 mmol) were dissolved in 70 mL of ethylene glycol. After ultrasonic treatment of the mixture for 1 h, a homogeneous phase was formed. It was then transferred to a 100 mL Teflon-lined stainless-steel autoclave and sealed. The autoclave was heated at 180 °C for 14 h in an oven and then allowed to cool to room

temperature. The resulting precipitate was sequentially rinsed with deionized water and 95% ethyl alcohol thrice and dried in an oven at 45 °C for 24 h. The solid product was ground, passed through a 180 μm sieve, and stored for later experiments. 2.2. Characterization of zinc ferrite The magnetic hysteresis loop of zinc ferrite was obtained on a vibrating sample magnetometer (Lakeshore). Its BET specific surface area after pretreatment at 180 °C for 2 h was determined through the N2 adsorption method using an ASAP 2010 M + C analyzer (Micromeritics Inc.). Its mineralogical composition was characterized by X-ray diffraction (XRD) performed on a D8 Advance diffractometer (Bruker-AXS) using copper Kα radiation (30 kV, 15 mA) at a scanning rate of 10°/min. Its morphology was observed with a Tecnai G2 F20 transmission electron microscope (FEI; 200 kV, 0.24 nm point-to-point resolution) after it was dispersed in absolute ethanol. Its zeta potential was determined on a DelsaNano C zeta potential analyzer (Beckman Coulter, USA). Fourier transform infrared spectra (FTIR) were recorded with an FTIR spectrophotometer (Shimadzu IR Prestige-21) through the KBr method. 2.3. Adsorption kinetics and isotherms For the kinetic studies, a 5 mg/L phosphate solution was used. Adsorbent (0.1 g) was added to a 100 mL phosphate solution in a 250 mL glass vessel. The resulting adsorbent–adsorbate suspension was transferred to a shaker and then agitated at a speed of 180 rpm. The experiments were performed at three temperatures (298 ± 0.1, 308 ± 0.1, 318 ±

Fig. 2. The magnetic hysteresis curve (left) and the photograph of separation result (right) of zinc ferrite material.

W. Gu et al. / Powder Technology 301 (2016) 723–729

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Fig. 4. Adsorption isotherms of phosphate on zinc ferrite. Adsorbent dose: 1 g/L, agitation speed: 180 rpm, contact time: 24 h.

Fig. 3. Plot of amount adsorbed versus contact time. Adsorbent dose: 1 g/L, initial P concentration: 5 mg/L, agitation speed: 180 rpm, reaction temperature: 298 ± 0.1 K; 308 ± 0.1 K; 318 ± 0.1 K.

equilibrium, the solid was separated by using a magnet, and then the final phosphate concentrations in the solutions were determined.

0.1 K). After a specified reaction time, solid/liquid separation was quickly done by using a permanent neodymium–iron–boron magnet, and phosphate concentrations of the supernatants were determined. The amount of adsorbed phosphate was calculated from the difference between the initial and final phosphate concentrations. Mean data for all experiments performed in duplicate are reported. Adsorption isotherm experiments were conducted in the same fashion as the measurements for adsorption kinetics, except that working solutions with phosphate concentrations ranging from 0 to 80 mg/L were used and the reaction time was fixed at 24 h.

3. Results and discussion 3.1. Characterization of zinc ferrite The morphology of zinc ferrite was observed by TEM. As shown in Fig. 1, zinc ferrite nanoparticles aggregated to form clusters with diameters mostly ranging from 100 to 200 nm. This aggregation may be explained by surface tension and magnetic attraction. The XRD pattern of zinc ferrite (Fig. 1) shows that zinc ferrite was well crystallized. According to the 2θ values and the radiation intensities of the peaks, it may be indexed to the cubic zinc diiron(III) oxide (ZnFe2O4) phase with cell parameters of a = b = c = 0.8429 nm, for which α = β = γ = 90° (JCPDS card no. 89-1010). The size of single particles is difficult to observe from the TEM image because of nanoparticle agglomeration. However, the average crystallite size could be calculated (15.2 nm, based on half-width of the most intense peak (311) and the Debye– Scherrer equation [29]) after subtraction of the standard instrumental broadening. The BET specific surface area of zinc ferrite was found to be 34.94 m2/g. The absence of a magnetization hysteresis loop in the magnetization curve of zinc ferrite nanoparticles (Fig. 2) suggests that they are superparamagnetic [30]. The saturation magnetization of zinc ferrite nanoparticles was 34.95 emu/g, which was high enough for them to be readily separated from suspension by a magnet (Fig. 2).

2.4. Effect of pH and ionic strength on adsorption The experiments were carried out under the following conditions: 1 g/L adsorbent dose; 5 mg/L initial phosphate concentration; 298 ± 0.1 K reaction temperature; 24 h reaction time. To investigate the influence of pH, the pH of the suspension was adjusted to values ranging from 2.5 to 10.5 by addition of HCl or NaOH solution. To determine the effect of ionic strength, the initial phosphate solution was mixed with NaCl solution at a concentration of 0.001 to 1.0 mol/L to obtain different ionic strengths.

2.5. Adsorption selectivity To determine the adsorption selectivity of zinc ferrite toward phosphate and to examine how other anions affect phosphate adsorption, ions that commonly coexist with phosphate, namely, nitrate, sulfate, and bicarbonate, were added to a 5 mg/L phosphate solution. The concentration of phosphate was ~0.16 mmol/L, while that of coexisting anions was ~ 16 mmol/L. Phosphate solutions with and without competitive ions were allowed to react with zinc ferrite for 24 h to attain

3.2. Adsorption kinetics Fig. 3 shows the amount of phosphate adsorbed per unit mass of adsorbent after various contact times and at different ambient temperatures. The results indicate that the amount adsorbed increased with increasing temperature at a given reaction time. At a given temperature,

Table 1 Parameters of kinetic models for phosphate adsorption onto zinc ferrite. Temperature (K)

Pseudo-first-order model

(min 298 308 318

qe

k1 −1

)

0.0038 0.0020 0.0037

Pseudo-second-order model r2

(mg/g) 2.50 1.48 2.54

0.7792 0.9354 0.7982

k2

qe

(g / (mg · min))

(mg/g)

0.0073 0.0077 0.0078

3.13 3.36 3.52

r2

0.9990 0.9996 0.9997

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Table 2 Results of fitting of the two adsorption isotherm models. Temperature (K)

Freundlich model KF ((mg/g)/(mg/L)1/n)

n

r2

KL (L/mg)

Langmuir model qm (mg/g)

r2

298 308 318

2.1249 2.2502 3.3086

3.8898 3.7184 6.4960

0.8843 0.7911 0.9895

0.5304 0.6352 0.6233

5.2329 5.5726 6.2842

0.9895 0.9918 0.9930

the adsorption process could be divided roughly into two stages. The initial rapid stage occurs within a reaction time of about 240 min. This stage involves the diffusion of phosphate ions from the bulk solution to the nanocluster surface [31]. The second, slower stage is a rate-controlled step. Adsorption in this stage is mainly controlled by intraparticle diffusion [31]. This slow reaction step may involve the diffusion of phosphate into clusters containing closely packed zinc ferrite nanoparticles. After 960 min, the adsorption capacity was N90% of its maximum. Maximum values of adsorption capacity attained were 2.85 mg/g (298 K), 3.28 mg/g (308 K), and 3.42 mg/g (318 K). Adsorption reached equilibrium within 1440 min, after which the adsorption capacity did not increase. This phenomenon could be explained by the fact that active adsorption sites on the surface were finite. Two adsorption kinetic models, the pseudo-first-order and the pseudo-second-order models, were used to describe our experimental data. The pseudo-first-order model could be described by Eq. (1): dqt ¼ k1 ðqe −qt Þ dt

ð1Þ

where qe and qt are the amounts of phosphate adsorbed per unit mass of adsorbent (mg/g) at equilibrium and at reaction time t (min), respectively; k1 is the rate constant (min−1). This equation may be linearized to Eq. (2): ln ðqe −qt Þ ¼ ln qe −k1 t

ð2Þ

ð3Þ

where k2 is the rate constant (g / (mg ∙ min)). After integration and rearrangement, it may be linearized to Eq. (4): t 1 t ¼ þ qt k2 qe 2 qe

ln qe ¼ ln K F þ

1 ln C e n

Ce 1 Ce ¼ þ qe K L qm qm

ð5Þ

ð6Þ

where Ce (mg/L) and qe (mg/g) are the phosphate equilibrium concentration and the amount of adsorbed phosphate at equilibrium, respectively; qm is the maximum adsorption capacity (mg/g); KF and KL are constants for the Freundlich and Langmuir models, respectively; and n is related to the adsorption intensity. The fitting results and the relevant parameters are listed in Table 2. The r2 value for the Langmuir isotherm model is higher than that for the Freundlich model, suggesting that adsorption occurs at specific homogeneous sites in a monolayer process [14,35]. The maximum adsorption capacity obtained by fitting adsorption isotherm data to the Langmuir model ranged within 5.23–6.28 mg/g at different temperatures. Previous studies on the direct use of magnetite in phosphate removal obtained maximum adsorption capacities of 5.2 mg/g [18] and 5.03 mg/g [20]. Therefore, the adsorption capacity of zinc ferrite is comparable to that of magnetite while having better stability. 3.4. Thermodynamic study of the adsorption process

The pseudo-second-order model may be represented by Eq. (3): dqt ¼ k2 ðqe −qt Þ2 dt

homogeneously distributed on the surface [33]. Adsorption occurs in a monolayer where adsorbate molecules do not interact [33,34]. The two models may be expressed as Eqs. (5) and (6), respectively:

To understand better the effect of temperature on adsorption, we determined the change in thermodynamic parameters Gibb's free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) on the basis of Eqs. (7)–(9), using data from adsorption kinetic studies [36]: ΔG ¼ −RT ln K e ; K e ¼

ð4Þ

qe m Ce

ΔG ¼ ΔH  −TΔS

ð8Þ

Table 1 displays the results of fitting the kinetic data to the two kinetic models. From the comparison of the correlation coefficients (r2) of the two kinetic models, the pseudo-second-order model evidently fits better the experimental data, thus implying that the adsorption is a chemisorption process [14,32]. Results in Table 1 also indicate that qe increases with increasing temperature, implying that adsorption on the ferrite surface is thermodynamically favorable. Meanwhile, k2 also increases as the temperature increases from 298 to 318 K, which means that elevating temperature not only increases the adsorption capacity, but also the reaction rate. 3.3. Adsorption isotherms Fig. 4 shows the isotherms for phosphate adsorption on zinc ferrite at different temperatures. Again, these results evidence that the amount of phosphate adsorbed increases with increasing temperature. Freundlich and Langmuir isotherm models are widely applied to adsorption processes. In the former, the adsorption surface is assumed to be heterogeneous and having sites with different adsorption energies [33]. The latter model supposes a finite number of binding sites

ð7Þ

Fig. 5. Plot of ln Ke versus 1/T.

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Table 3 Thermodynamic parameters for phosphate adsorption onto zinc ferrite obtained in the study. T (K)

ΔG° (kJ/mol)

ΔH° (kJ/mol)

ΔS° (J/K mol)

298 308 318

−1.19 −1.68 −2.17

13.36 13.36 13.36

48.81 48.81 48.81

ln K e ¼ −

ΔH  ΔS þ RT R

ð9Þ

where m is the adsorbent dose (g/L), Ce is the equilibrium concentration (5 mg/L), qe is the amount of phosphate adsorbed at equilibrium (mg/g) after an equilibration time of 24 h, and T is the reaction temperature (K). A plot of ln Ke versus 1/T (Fig. 5) was constructed from the experimental data. The negative values of ΔG° (−1.19 to −2.17 kJ/mol) in Table 3 indicate that phosphate adsorption onto zinc ferrite is a spontaneous process. The positive values of ΔH° and ΔS° suggest that the adsorption is endothermic and that the randomness at the solid–liquid interface increases after adsorption [37]. 3.5. Effect of pH and ionic strength on phosphate adsorption pH is usually an important parameter during adsorption; it influences not only the surface properties of the adsorbent material, but also the species distribution on the material. As shown in Fig. 6, acidic conditions enhanced phosphate adsorption by zinc ferrite; the amount of phosphate adsorbed decreased with increasing pH. This behavior could be explained by the stronger negative charge of the adsorbent surface at higher pH (Fig. 6). Additionally, phosphate becomes more negatively charged with increasing pH (i.e., H3PO4 → H2PO− 4 → HPO24 − → PO34 −), leading to charge repulsion between phosphate and the surface. Another contribution to poorer adsorption at higher pH is competition of phosphate with hydroxyl. Phosphate solutions (5 mg/L) with different concentrations of NaCl were prepared to investigate the influence of ionic strength on phosphate adsorption by zinc ferrite. As shown in Fig. 7, the amount of phosphate adsorbed increased with increasing ionic strength (i.e., NaCl concentration). This effect was especially significant at lower NaCl concentrations. Phosphate adsorption is known to involve the formation of outer- and inner-sphere complexes. Formation of the former may be retarded at high ionic strength, as other anions can compete for the positively charged adsorption sites. In contrast, an increase in ionic strength has no effect or may even enhance the formation of an inner-sphere complex [26,38]. Therefore, phosphate adsorption by zinc ferrite may be considered to involve the formation of an inner-sphere complex.

Fig. 7. Influence of ionic strength on phosphate removal by zinc ferrite.

3.6. Adsorption selectivity Results in Section 3.5 may also indicate that Cl− does not compete effectively with phosphate for adsorption sites even at a very high concentration. Competing anions other than chloride, such as nitrate, sulfate, and bicarbonate, are commonly present in wastewater. Fig. 8 2− shows that presence of the oxyanions NO− 3 and SO4 slightly enhanced phosphate removal. This unexpected effect suggests that zinc ferrite has high selectivity for phosphate. Equilibrium pH values for phosphate solutions, as well as for phosphate/nitrate, phosphate/sulfate, and phosphate/bicarbonate mixtures, are 6.34, 6.09, 6.07, and 8.64, respectively. Thus, the slight enhancement of phosphate adsorption in the presence of nitrate and sulfate may be due to the increase in ionic strength and the reduction of pH. However, phosphate adsorption was greatly suppressed by HCO2− 3 . This may be attributed to competition and the resulting substantial rise in pH. of phosphate with HCO2− 3

3.7. FTIR spectroscopy FTIR spectra of zinc ferrite before and after phosphate adsorption (Fig. 9) have peaks at low wavenumbers, which are characteristic of spinel ferrites [39]. Peaks at 422 and 576 cm−1 may be assigned to Fe–O and Zn–O stretching vibrations, respectively, in the normal spinel configuration. The band at 3460 cm−1 may be ascribed to adsorbed water and to the stretching vibration of surface hydroxyls. The peak at 1654 cm− 1 is attributed to the bending vibration of hydroxyls of water molecules adsorbed on the ferrite surface [40,41]. The two bands at 1320 and 1424 cm−1, which weakened greatly after phosphate

Fig. 6. Influence of solution pH on adsorption of phosphate (left) and zeta potential of zinc ferrite.

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Appendix A

Fig. 8. Influence of competitive anions on phosphate adsorption by zinc ferrite.

adsorption, may be due to OH groups on zinc ferrite. The change in these two peaks after adsorption may arise from the displacement of surface hydroxyls by phosphate. On the other hand, a new peak at 1055 cm−1 emerged with phosphate adsorption; this peak is due to asymmetric P–O stretching vibration [31,40,41].

4. Conclusions A new adsorbent for phosphate removal from water, zinc ferrite nanoparticles, was obtained through a facile solvothermal method. The material has a saturation magnetization of 34.95 emu/g and is thus easily separable with the use of magnet. The material shows a maximum of 5.23–6.28 mg/g for the adsorption capacity for phosphate, which is comparable to that of magnetite. However, zinc ferrite is superior to magnetite because of its stability in air. Phosphate adsorption by zinc ferrite was enhanced at lower pH and with increasing ionic strength. Furthermore, zinc ferrite shows good selectivity for phosphate. Zinc ferrite nanoparticles thus have potential use as magnetic adsorbent for phosphate in water.

Acknowledgements The authors express thanks for financial support from the National Natural Science Foundation Project (21507084).

Fig. 9. FTIR spectra of zinc ferrite with and without adsorbed phosphate.

BET FTIR PZC TEM VSM XRD Ce C0 qe qm qt KF KL ΔG° ΔH° ΔS°

Brunauer-Emmett-Teller Fourier transform infrared spectra point of zero charge transmission electron microscope vibrating sample magnetometer X-ray diffraction equilibrium concentration of phosphate initial concentration of phosphate adsorption capacity of phosphate at equilibrium the maximum adsorption capacity of phosphate adsorption capacity of phosphate at the reaction time t (min) a constant for Freundlich isotherm model a constant for Langmuir isotherm model standard free energy change standard enthalpy change standard entropy change

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