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Strong adsorption of phosphate by amorphous zirconium oxide nanoparticles
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 0 1 8 e5 0 2 6
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Strong adsorption of phosphate by amorphous zirconium oxide nanoparticles Yu Su a, Hang Cui a, Qi Li a,*, Shian Gao a, Jian Ku Shang a,b a
Environment Functional Materials Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China b Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
article info
abstract
Article history:
Phosphate removal is important in the control of eutrophication of water bodies.
Received 11 March 2013
Adsorption is one of the promising approaches for the removal of phosphate, which could
Received in revised form
serve as a supplement for the biological phosphate removal process commonly used in the
20 May 2013
wastewater treatment industry to meet the discharge requirement when the biological
Accepted 23 May 2013
performance is deteriorated from changes of operation conditions. Amorphous zirconium
Available online 5 June 2013
oxide nanoparticles were synthesized by a simple and low-cost hydrothermal process, and their phosphate removal performance was explored in aqueous environment under
Keywords:
various conditions. A fast adsorption of phosphate was observed in the kinetics study, and
Eutrophication
their adsorption capacity was determined at about 99.01 mg/g at pH 6.2 in the equilibrium
Phosphate removal
adsorption isotherm study. Commonly coexisting anions showed no or minimum effect on
Amorphous ZrO2 nanoparticles
their phosphate adsorption performance. The phosphate adsorption showed little pH
Adsorption
dependence in the range from pH 2 to 6, while it decreased sharply with the pH increase
Inner-sphere
above pH 7. After adsorption, phosphate on these am-ZrO2 nanoparticles could be easily
complexing mechanism
desorbed by NaOH solution washing. Both the macroscopic and microscopic techniques demonstrated that the phosphate adsorption mechanism of am-ZrO2 nanoparticles followed the inner-sphere complexing mechanism, and the surface hydroxyl groups played a key role in the phosphate adsorption. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
With the rapid development of modern agriculture and industry, more and more wastewater is being produced every day. If wastewater is discharged into rivers, lakes and oceans without proper treatment, it could do harm to the environment and subsequently on the human health (Shannon et al., 2008). Many nutrients, such as nitrogen and phosphorus, exist in wastewater, which are needed for aquatic organisms to grow. The excessive presence of nitrogen and phosphorus in
water bodies could cause eutrophication and subsequently stimulate blue-green algae blooms when proper temperature conditions are present (Boujelben et al., 2008). It had been found that blue-green algae have the ability of biological fixation of nitrogen, and planktonic N2-fixing cyanobacteria bloom in freshwaters when phosphorus is abundant while nitrogen availability is low (Schindler et al., 2008; Conley et al., 2009). Consequently, the removal of phosphorus is essential to preventing eutrophication of natural water bodies, especially for freshwaters. The World Health Organization (WHO)
* Corresponding author. 72 Wenhua Road, Shenyang, Liaoning Province, 110016, PR China. Tel.: þ86 24 83978028; fax: þ86 24 23971215. E-mail addresses: [email protected], [email protected] (J.K. Shang). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.05.044
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 0 1 8 e5 0 2 6
suggested a maximum discharge limit of phosphorus of 0.5e1.0 mg/L (Galalgorchev, 1992), which is now being adopted in many countries and regions. For example, the emission standard for the newly-built urban sewage treatment plant in China will be upgraded to GB 1A (Total P at 0.5 mg/L) from GB 1B (Total P at 1.0 mg/L) (Ren et al., 2012). In aqueous environment, phosphorus exists in the pentavalent form as orthophosphate, pyrophosphate, longer-chain polyphosphates, organic phosphate esters and phosphodiesters, and organic phosphonates, and these various phosphorus compounds could be hydrolyzed to orthophosphate, the only form of phosphorus that could be utilized by bacteria, algae, and plants (Correll, 1998). Various techniques, such as chemical precipitation, adsorption, biological removal, reverse osmosis, membrane, ion exchange, and constructed wetlands, have been studied for the removal of phosphate from wastewater (Choi et al., 2012; de-Bashan and Bashan, 2004; Morse et al., 1998; van Voorthuizen et al., 2005; Zhang et al., 2012a, b). Among these approaches, the biological process is considered a costeffective and environmentally sound alternative to the chemical precipitation treatment, and is now widely used for the removal of phosphate at industrial level (Gebremariam et al., 2011). However, its effectiveness could be affected by many factors, such as volatile fatty acids, cations, temperature, sludge quality and settlement, and pH (Mulkerrins et al., 2004). So the biological process is usually supplemented by an additional treatment to meet the discharge requirement, when its performance is deteriorated from the fluctuations in operating conditions. The adsorption process requires simple operating conditions, possesses stable phosphate removal effect, and produces little sludge compared with other methods. Thus, it is promising as a supplement to the biological process for the phosphate removal, and it also could be easily applied for phosphate removal on small-scale treatment facilities or wastewater with relatively low phosphate concentrations. For this purpose, various adsorbents have been examined, including zero-valent iron (Almeelbi and Bezbaruah, 2012), activated carbon (Wang et al., 2012), aluminum oxide/hydroxide (Li and Guan, 2011), goethite (Gu et al., 2011), calcite (Liu et al., 2012), zeolite (Schick et al., 2012), red mud (Castaldi et al., 2012), and fly ash (Lu et al., 2009). Because of their large surface areas and preferred surface properties, synthesized metal oxides at nano-size have demonstrated effective phosphate adsorption (Lu et al., 2013; Yang et al., 2007; Yu et al., 2012). Zirconium based oxides could be an attractive adsorbent choice for water treatment because of their non-toxicity, good resistance to oxidant agents and acids/bases, high thermal stability, and very low solubility in water (Bergamaschi et al., 2005; Blaney et al., 2007; Suzuki et al., 2000). The use of zirconium based oxides for phosphate adsorption had been reported in literature (Awual et al., 2011). Most of the studies were focused either on fibers (Awual et al., 2011) or activated carbon (Okumura et al., 1999, 1998) loaded with zirconium hydroxide, or binary oxides of zirconium with other metals (for example, FeeZr binary oxides) (Ren et al., 2012). No report is available on the study of the phosphate adsorption by more stable amorphous ZrO2 nanoparticles, while amorphous adsorbents may have high adsorption capacity because of their
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porous and highly hydrated structure. In our recent study, amorphous zirconium oxide nanoparticles were synthesized by a simple and low-cost hydrothermal process and were found to have very strong adsorption on arsenic species from water (Cui et al., 2012). Phosphorus and arsenic belong to the same element group (main group V) and have similar electronic structures. They form similar components in water (phosphate and arsenate), and our recent study indicated that a strong adsorption competition existed between arsenate and phosphate (Cui et al., 2012). Thus, amorphous zirconium oxide nanoparticles may be a good choice for the removal of phosphate from water. In this study, the phosphate removal performance of amorphous zirconium oxide nanoparticles was examined in details by both the kinetics study and equilibrium adsorption isotherm study. The influences of various experimental parameters, such as solution pH, ionic strength, and competitive anions on phosphate removal were investigated. The phosphate desorption from these am-ZrO2 nanoparticles and their reuse for phosphate adsorption were explored. Moreover, the phosphate adsorption mechanism of am-ZrO2 nanoparticles was investigated by both the macroscopic and microscopic techniques.
2.
Experimental section
2.1.
Chemicals and material
Zirconium oxychloride octahydrate (ZrOCl28H2O, 99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P.R. China) was used as the raw material, deionized water was used as the solvent, and aqueous ammonia (25 wt%, Tianjin Kermel Chemical Reagents Development Center, Tianjin, P.R. China) was used as the solvent, and aqueous ammonia (25 wt%, Tianjin Kermel Chemical Reagents Development Center, Tianjin, P.R. China) was used as the precipitation agent in the hydrothermal process. Sodium dihydrogen phosphate anhydrous (NaH2PO4, AR, Sinopharm Chemical Reagent Co. Ltd, Shanghai, P.R. China) was used to prepare phosphate stock solution. Concentrated hydrochloride acid (HCl, 32e38%, Tianda Chemical Reagents Factory, Tianjin, P.R. China) and sodium hydroxide (NaOH, 98%, Tianda Chemical Reagents Factory, Tianjin, P.R. China) were used to adjust solution pH. Sodium chloride (NaCl, 99.5%, Shenyang Chemical Reagents Factory, Shenyang, P.R. China), sodium sulfate anhydrous (Na2SO4, 99%, Tianjin Da Mao Chemical Reagents Factory, Tianjin, P.R. China), and sodium bicarbonate (NaHCO3, AR, Sinopharm Chemical Reagent Co. Ltd, Shanghai, P.R. China) were used in the competing ion effect experiment. Sodium chloride was also used in the ionic strength effect experiment.
2.2. am-ZrO2 nanoparticle synthesis and characterization am-ZrO2 nanoparticles were prepared by a hydrothermal process as detailed in our previous report (Cui et al., 2012). The Fourier transform infrared (FTIR) spectra of am-ZrO2 nanoparticles before and after phosphate adsorption were measured on a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, U.S.A.), and their zeta
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potential values were analyzed by an electrophoretic spectroscopy (JS84H, Shanghai Zhongchen Digital Instrument Co., Ltd, Shanghai, P.R. China). The semi-quantitative chemical composition and surface chemical states of the samples were examined by X-ray photoelectron Spectrometer (XPS) using an ESCALAB250 X-ray Photoelectron Spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, U.S.A.) with an Al K anode (1486.6 eV photon energy, 0.05 eV photon energy resolution, 300 W).
2.3.
Phosphate adsorption/desorption experiments
All the phosphate adsorption experiments were carried out at about 25 C in 250 mL glass beakers. Phosphate solution of 200 mL was used for each experiment with a fixed dose of amZrO2 at 0.1 g/L. The mixture was magnetically stirred to ensure good contact between am-ZrO2 and phosphate in water. The pH of phosphate solution was adjusted with dilute HCl and NaOH. After the adsorption experiment, the supernatant solution was collected for the remaining phosphate concentration analysis by centrifugation at 12,000 rpm to separate the adsorbent. Phosphate concentration in the supernatant solution was analyzed on an ion chromatograph (Dionex ICS 1100 Ion Chromatography, Thermal SCIENTIFIC, Sunnyvale, CA, U.S.A.). For the kinetic study of phosphate adsorption on amZrO2 nanoparticles, two initial phosphate concentrations of 5 and 10 mg/L were used, respectively, the solution pH was about 6, and the contact time was from 0 h to 8 h. For the equilibrium adsorption isotherm study, the initial phosphate concentration ranged from 5 to 50 mg/L, the solution pH was about 6, and the contact time was 8 h. For the solution pH and ionic strength effect studies, the initial phosphate concentration was 5 mg/L, the solution pH was adjusted from 2 to 10, the concentrations of NaNO3 were 0.1, 0.01, 0.001 M, respectively, and the contact time was 8 h. For the coexisting anion competing effect study, the initial phosphate concentrations was 5 mg/L, the concentrations of Cl-, SO4 2 and HCO3 were 0, 5 and 10 mM, the solution pH was about 6, and the contact time was 8 h. For the phosphate desorption study, NaOH aqueous solution was chosen as the desorption solution, and different NaOH concentrations of 0, 0.001, 0.01, 0.1 and 0.5 M were used to examine its effect on the desorption effect. After the phosphate adsorption, 0.1 g am-ZrO2 nanoparticles were dispersed into 50 mL NaOH solutions and the desorption time was 12 h. For the reuse experiments, the initial phosphate concentration was 5 mg/L and the contact time was 12 h. After being separated by centrifugation, am-ZrO2 nanoparticles were desorbed with 0.1 M NaOH solution. The desorbed nanoparticles were washed to neutral pH and dried at 80 C for 12 h before they were used for phosphate adsorption again. Two times of desorption and reuse experiments were conducted.
3.
5 nm. Their BET specific surface area was about 327 m2/g, and most pores were mesoporous. The mass density of am-ZrO2 nanoparticles was determined at 5.85 g/cm3. Thus, their theoretical specific surface area was calculated to be 205 m2/g, which was smaller than the BET specific surface area (327 m2/ g). This difference also indicated that these am-ZrO2 nanoparticles were highly porous. The isoelectric point of am-ZrO2 nanoparticles was determined at pH 5.9. Detailed characterization results could be found in our previous report (Cui et al., 2012). The kinetics of the phosphate adsorption from aqueous solutions onto am-ZrO2 nanoparticles are shown in Fig. 1a. It is apparent that the adsorption process could be divided into two stages, a rapid stage at the very beginning followed by a gradually slower stage until the adsorption equilibrium was achieved with the increase of the adsorption time. For example, when the initial phosphate concentration was 5 mg/ L, the adsorption rate at the very beginning was fast. Within just 0.5 h, about 96.8% phosphate was adsorbed onto am-ZrO2 nanoparticles, and the remaining phosphate concentration dropped to about 0.16 mg/L, which already satisfied the discharge requirement of 0.5e1.0 mg/L (Galalgorchev, 1992;
Results and discussion
3.1. Kinetics studies on phosphate adsorption by amZrO2 nanoparticles am-ZrO2 nanoparticles had a very poor crystallization and nonuniform shapes, with an average particle size of about
Fig. 1 e (a) The phosphate uptake of am-ZrO2 nanoparticles with the change of contact time at different initial phosphate concentrations: 5 mg/L (;), and 10 mg/L (:); (b) The fitting curves by the pseudo-second-order rate model of the kinetic study (adsorbent dose at 0.1 g/L, pH 6.2, 25 C).
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details in the adsorption mechanism study in the following sections.
Ren et al., 2012). This fast adsorption rate may be attributed to the fine size and subsequent large surface area of am-ZrO2 nanoparticles, which could enhance their contact efficiency with phosphate in water and be favorable for the diffusion of phosphate ions from bulk solution onto active sites of amZrO2 nanoparticles. With the further increase of adsorption time, the phosphate removal percentage gradually increased to over 99% and the adsorption equilibrium was established at 8 h. The phosphate adsorption process with the initial concentration of 10 mg/L was similar to that of 5 mg/L. With the increase of the initial phosphate concentration, a higher adsorption amount was observed, and the time to reach equilibrium increased. From the kinetic study, it could be seen that 8 h was adequate for the adsorption equilibrium to occur. So we conducted the following adsorption experiments with the contact time of 8 h. The adsorption kinetic data could be fitted into various rate models to provide the understanding of the adsorption mechanism (Chen et al., 2007). The linear pseudo-first-order rate equation could be given as Eq. (1): log qe qt
¼ logqe
k1 t 2:303
3.2. Equilibrium adsorption isotherm study on phosphate adsorption by am-ZrO2 nanoparticles The adsorption capacity of am-ZrO2 nanoparticles on phosphate was investigated by the equilibrium adsorption isotherm study, and the results are shown in Fig. 2. The adsorption data was fitted with both Freundlich and Langmuir as given in Eq. (3) (Deliyanni et al., 2007) and Eq. (4) (Reed and Matsumoto, 1993), respectively: 1
rate
equation could
t 1 t ¼ þ qt k2 q2e qe
(3)
qe ¼ qmax KL Ce =ð1 þ KL Ce Þ
(4)
where qe is the amount (mg/g) of phosphate adsorbed at equilibrium, ce is the equilibrium phosphate concentration (mg/L) in the solution, KF and n are the Freundlich constants, KL is the Langmuir constant, and qmax is the Langmuir monolayer adsorption capacity. The parameters obtained in fitting the experimental data are summarized in Table 2. It is clear that the phosphate adsorption data could be best fitted with the Langmuir isotherm because the R2 fitted with this model is closer to1 (R2 ¼ 0.998), compared with that produced by the Freundlich isotherm (R2 ¼ 0.974). The Langmuir adsorption capacity (qmax) was determined at about 99.01 mg/g, which was among the highest reported values in the literature. Table 3 summarized the reported adsorption capacities of various adsorbents on phosphate. The phosphate adsorption capacity of am-ZrO2 nanoparticles was very close to the highest phosphate adsorption capacity of 102.3 mg/g recently reported for FeeZr binary oxide (Ren et al., 2012), while pure am-ZrO2 nanoparticles should have a better stability than FeeZr binary oxide in aqueous environment. From Fig. 2, it could also be observed that the phosphate amount that am-ZrO2 nanoparticles adsorbed at low equilibrium concentration was quite high. For example, when the equilibrium concentration was just 0.0089 mg/L, the phosphate amount that am-ZrO2 nanoparticles adsorbed was over 44.4 mg/g, even higher than the maximum adsorption capacities of some reported adsorbents in literature with much higher equilibrium phosphate concentrations (usually over 1000 times) (Ogata et al., 2012; Wang et al., 2012; Zamparas et al., 2012; Zhang et al., 2009; Zhang et al., 2012a, b). Such a performance is beneficial to its potential application as the supplement to the biological process for the phosphate removal because the phosphate concentration in streams after biological process is usually not very high.
(1)
and the pseudo-second-order expressed as Eq. (2):
qe ¼ KF Cen
be
(2)
where qe and qt are the amounts (mg/L) of phosphate adsorbed at equilibrium and at time t, respectively, and k1 (h1) and k2 (g mg1 h1) are the rate constants of pseudo-first-order and pseudo-second-order adsorption processes, respectively. The fitting curves by the pseudo-first-order rate model were demonstrated in Figure S1 in the supplementary data, and the fitting curves by the pseudo-second-order rate model were demonstrated in Fig. 1b. The kinetics parameters obtained in fitting the experimental data are summarized in Table 1. It was found that the kinetics data could be best fitted into the pseudo-second-order rate model with R2 higher than 0.99 for both 5 mg/L and 10 mg/L initial phosphate concentrations, while the pseudo-first-order rate model could not fit the data well. Similar kinetic study results had been reported by Rodrigues et al. (Rodrigues and da Silva, 2010) and Tang et al. (Tang et al., 2012) on various phosphate adsorbents. The pseudo-second-order rate model indicates a chemisorptions (Azizian, 2004) occurred between phosphate and am-ZrO2 nanoparticles involving valency forces through sharing or exchange of electrons between sorbent and sorbate (the replacement of eOH by phosphate), which will be discussed in
Table 1 e The kinetic model parameters for the adsorption of phosphate on am-ZrO2 nanoparticles at pH 6.2 and T about 25 C. Initial concentration of phosphate (mg/L) 5 10
Pseudo-first-order model qt ¼ qe(1-e-k1t)
Pseudo-second-order model qt ¼ k2q2et/(1 þ k2qet)
qe(mg g1)
k1(h1)
R2
qe(mg g1)
k2(g mg-1h1)
R2
48.44 67.76
5.323 0.250
0.937 0.521
48.45 67.29
1.262 0.079
1 0.999
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charged am-ZrO2 nanoparticle surface and the negatively charged phosphate species could decrease the phosphate adsorption on am-ZrO2 nanoparticles, and the negative effect increased with the pH increase. So, less adsorption could occur with the pH increase as demonstrated in Fig. 3.
3.4. Ionic strength effect on phosphate adsorption by am-ZrO2 nanoparticles
Fig. 2 e The adsorption equilibrium isotherm of phosphate on am-ZrO2 nanoparticles at pH 6.2 and 25 C: Freundlich isotherm fitting curve (dd), and isotherm fitting curve ($$$$$$).
3.3. Solution pH effect on phosphate adsorption by amZrO2 nanoparticles The solution pH effect on phosphate adsorption by am-ZrO2 nanoparticles was demonstrated in Fig. 3. The phosphate adsorption on am-ZrO2 nanoparticles was evidently dependent on pH. Under the acidic pH conditions, the adsorption performance just decreased very slightly from pH 2 to pH 6, while it dropped much faster with the pH increase after pH 7. Similar solution pH effect had also been observed in previous reports for various adsorbents (Lu et al., 2013). Under the solution pH range investigated in this study (pH 2e10), the dominant species of phosphate in the solution are H2 PO4 and HPO4 2 . Low solution pH in the acidic region is beneficial for the protonation of adsorbent surface, which could enhance the electrostatic attraction between the adsorbent surface and the phosphate anions to facilitate the phosphate adsorption. Yet the phosphate adsorption on am-ZrO2 nanoparticles is a chemisorption dominated process, and the adsorption from the electrostatic attraction is not a major contribution in this process. So the phosphate adsorption performance of amZrO2 nanoparticles just increased very slightly from pH 6 to pH 2. When the solution pH increased from the neutral to basic region, am-ZrO2 nanoparticles had negatively charged surface and the charge intensity increased with the pH increase. Thus, the coulomb repulsive interaction between the negatively
The phosphate adsorption mechanism on am-ZrO2 nanoparticles was first investigated by the macroscopic technique of evaluating the ionic strength effect on the adsorption behavior. The solution ionic strength was adjusted by the addition of NaNO3 with different concentrations into the phosphate solutions. Fig. 3 demonstrated that the phosphate adsorption increased with the increase of the solution ionic strength. Similar observations had been reported in literature for the phosphate adsorption on different adsorbents (Tanada et al., 2003). As suggested by McBride (1997), the adsorption of phosphate would decrease with the increase of ionic strength if phosphate formed outer-sphere surface complexes, while the adsorption of phosphate would either not change or increase with the increase of ionic strength if phosphate formed inner-sphere complexes. Thus, the results in this study demonstrated that the adsorption of phosphate on am-ZrO2 nanoparticles follows the inner-sphere complex mechanism.
3.5.
The phosphate adsorption mechanism onto am-ZrO2 nanoprticles was also investigated by another macroscopic technique of electrophoretic mobility measurement. Fig. 4 showed the zeta potentials of pure am-ZrO2 nanoparticles and amZrO2 nanoparticles after the phosphate adsorption (5 mg/L phosphate solution). Without the phosphate adsorption, the isoelectric point (IEP) of am-ZrO2 nanoparticles was about pH 5.0, while the IEP of am-ZrO2 nanoparticles decreased to about pH 3.9 after phosphate adsorption. The IEP of metal oxides is determined by protonation and deprotonation of surface hydroxyl groups. The formation of outer-sphere surface complexes cannot shift the IEP because there is no chemical reaction between the adsorbate and adsorbent surface that could change the surface charge (Ren et al., 2012; Yang et al., 2007; Zhang et al., 2009). Thus, it is clear that a specific adsorption rather than a purely electrostatic interaction existed between the phosphate and am-ZrO2 nanoparticles to form inner-sphere phosphate anionic charged surface complexes on am-ZrO2 nanoparticles.
3.6.
Table 2 e Langmuir and Freundlich isotherm constants for phosphate adsorption on am-ZrO2 nanoparticles at pH 6.2 and T about 25 C. Langmuir isotherm qm (mg/g) 99.01
Freundlich isotherm -1
2
KL (L mg )
R
KF
0.31
0.998
50.48
n 0.164
R2 0.974
Zeta potential study
FTIR study
The phosphate adsorption mechanism was further investigated by the microscopic technique of FTIR spectroscopy. Fig. 5 demonstrated the FTIR spectra of am-ZrO2 nanoparticles before and after phosphate adsorption, respectively. Without the phosphate adsorption, the FTIR spectrum of am-ZrO2 nanoparticles had strong hydroxyl stretching (3363 cm-1) and bending (1638 cm-1) vibrations of physically adsorbed H2O, and the deformation vibration (1338 cm-1) of ZreOH (Cui et al., 2012), indicating that am-ZrO2 nanoparticles had high
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Table 3 e Phosphate adsorption capacities of various adsorbents. Adsorbent Fee Al e Mn trimetal oxide Fee Mn binary oxide FeeZr binary oxide Mesoporous ZrO2 Innovative modified bentonites Tantalum Hydroxide Iron-doped activated carbon Granular boehmite Carbonated ferric green rust Lanthanum(III)-coordinated diamino-functionalized 3D hybrid mesoporous silicates Lanthanum-doped activated carbon fiber Sepiolite am-ZrO2
pH
Adsorption capacity (mg/g)
6.8 5.6 5.5 About 6.8 7 2 3.78 about 6. 84 3. 5, 7 6
48.3 33.2 102.3 91.05 11.15 78.5 14.12 8.38 51.02 54.3
No data 4.8 6.2
adsorption capacities to H2O and hydroxyl groups existed on their surface, which was in accordance with the results of zeta potential test. After the phosphate adsorption, the FTIR spectrum of am-ZrO2 nanoparticles demonstrated obvious changes. The ZreOH deformation peak disappeared, while the peaks of physically adsorbed H2O greatly weakened. These observations indicated that the replacement of eOH and H2O occurred during the phosphate adsorption. Additionally, a new peak appeared at 1037 cm-1, which was broad and intensive. This new peak could be assigned to the asymmetry vibration of PeO bond, indicating that the surface hydroxyl groups were replaced by the adsorbed phosphate (Zhang et al., 2009). The FTIR study results demonstrated that the substitution of ZreOH groups by phosphate species played a key role in their adsorption on am-ZrO2 nanoparticles.
3.7.
XPS study
The surface O 1s spectra of am-ZrO2 nanoparticles before and after phosphate adsorption were analyzed by high-resolution XPS scan, and were demonstrated in Fig. 6a and b,
Fig. 3 e Effect of pH and ionic strength on phosphate adsorption by am-ZrO2 nanoparticles (initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, 25 C).
22.86 98.06 99.01
Ref. (Lu et al., 2013) (Zhang et al., 2009) (Ren et al., 2012) (Liu et al., 2008) (Zamparas et al., 2012) (Yu et al., 2012) (Wang et al., 2012) (Ogata et al., 2012) (Barthelemy et al., 2012) (Zhang et al., 2011) (ZhangLiu et al., 2012a, b) (Yin et al., 2011) Present study
respectively. The O 1s peak could be best fitted with three overlapped O 1s peaks of oxide oxygen (O2), hydroxyl group (-OH), and adsorbed water (H2O). The data were fitted using Lorentzian peak shape, and the fitting parameters could be found in Table S1 in the supplementary data. Fig. 6a demonstrated that the eOH percentage was about 42.8% for these am-ZrO2 nanoparticles before the phosphate adsorption, while Fig. 6b demonstrated that the eOH percentage largely dropped to about 20.8% after the phosphate adsorption. The XPS analysis results suggested that hydroxyl groups existed on the surface of am-ZrO2 nanoparticles, and the eOH percentage in the total surface oxygen dropped sharply by the adsorption of phosphate. Thus, the decrease of eOH groups could be attributed to the replacement of eOH by phosphate during the adsorption process. So the XPS analysis also suggested that the surface hydroxyl groups played the key role in the phosphate adsorption, which is in accordance with results of ionic strength, zeta potential and FTIR studies.
3.8. Coexisting anion effect on phosphate adsorption by am-ZrO2 nanoparticles Due to the complexity of natural water contents, there might be competition from other species which may largely affect the phosphate removal performance on am-ZrO2 nanoparticles. Coexisting anions, such as Cl, SO4 2 and HCO3 , are generally present in wastewater and might interfere with the phosphate adsorption on am-ZrO2 nanoparticles through competitive adsorptions. Herein, the effects of these coexisting anions at two concentrations (5 and 10 mM) on the phosphate adsorption were examined, and the results were demonstrated in Fig. 7. The initial concentration of phosphate was about 5 mg/L, i.e. about 0.05 mM, and the am-ZrO2 loading was 0.1 g/L. The presence of Cl, SO4 2 and HCO3 , even at very high concentrations compared with that of phosphate (100 times as 5 mM and 200 times as 10 mM), had no or just a slight effect on the phosphate adsorption. Thus, am-ZrO2 nanoparticles could remove phosphate effectively even when exceptional high concentrations of these competing anions existed, which is beneficial to their potential industrial applications.
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Fig. 4 e Zeta potential of am-ZrO2 nanoparticles before (-) and after phosphate adsorption (C) with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
3.9. Desorption of phosphate from am-ZrO2 nanoparticles and their reuse The phosphate adsorbed on am-ZrO2 nanoparticles could be desorbed with NaOH solution, and the desorption results with NaOH solutions of different concentrations were demonstrated in Fig. 8. It is clear that the amount of desorbed phosphate increased with the increase of the alkalinity of NaOH solution. Without NaOH addition (pH 7), a desorption effect of just 5.33% of adsorbed phosphate was obtained when the extracting solution was only 0.01 M NaNO3 solution. When the NaOH concentration increased to 0.1 M (pH 13), however, the desorption effect increased to 91.67%. The further increase of NaOH concentration to 0.5 M (pH 13.7) did not significantly enhance the desorption of phosphate. The results demonstrated that the phosphate adsorbed on am-ZrO2 nanoparticles could be easily desorbed using a 0.1 M NaOH
Fig. 5 e FTIR spectrum of am-ZrO2 nanoparticles before (-) and after phosphate adsorption (C) with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
Fig. 6 e The surface O 1s spectra of am-ZrO2 nanoparticles (a) before and (b) after phosphate adsorption with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
solution. The desorbed am-ZrO2 nanoparticles were reused for phosphate removal, which demonstrated that they could keep about 80e90% of its original phosphate removal capability after being desorbed for two times. Thus, these
Fig. 7 e Effect of coexisting anions on the phosphate removal performance with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 0 1 8 e5 0 2 6
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Innovation Promotion Association, Chinese Academy of Sciences (Grant No. Y2N5711171).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.05.044.
references
Fig. 8 e Desorption of phosphate from phosphate-loaded am-ZrO2 by NaOH solutions with different pH values.
recovered am-ZrO2 nanoparticles could be reutilized for phosphate removal from water, which could significantly reduce the operation cost.
4.
Conclusions
In summary, am-ZrO2 nanoparticles synthesized from a simple and low-cost hydrothermal process demonstrated an effective phosphate removal performance in water in terms of both the adsorption speed and adsorption capacity. Their adsorption capacity on phosphate was determined at about 99.01 mg/g at pH 6.2, which was among the highest reported values in the literature. The adsorption mechanism of phosphate onto am-ZrO2 nanoparticles was determined to follow the inner-sphere complexing mechanism, and the surface eOH groups played a major role in the phosphate removal. am-ZrO2 nanoparticles could remove phosphate effectively even when competing anions with exceptionally high concentrations were present. Thus, it may have the potential to be a promising adsorbent as the supplement to the biological process for the phosphate removal, and it also could be applied for phosphate removal on small-scale treatment facilities or wastewater with relatively low phosphate concentrations. Further work is now underway to immobilize them onto various highly porous substrates and synthesize highly porous, nanostructured ZrO2 spheres in millimeter size based on these ZrO2 nanoparticles to avoid the difficulty in the separation of nanoadsorbents, to test their continuous treatment performance in flow-through, fixed bed reactors, and to use the pore surface diffusion modeling to predict their performance in full-scale fixed bed systems.
Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant No. 51102246), the Knowledge Innovation Program of Institute of Metal Research, Chinese Academy of Sciences (Grant No. Y0N5A111A1), and the Youth
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Galalgorchev, H., 1992. Who guidelines for drinking-water quality. In: Iwsa Specialized Conference on Quality Aspects of Water Supply, 11, pp. 1e16. Gebremariam, S.Y., Beutel, M.W., Christian, D., Hess, T.F., 2011. Research advances and challenges in the microbiology of enhanced biological phosphorus removal e a critical review. Water Environmental Research 83, 195e219. Gu, Z.M., Buyuksonmez, F., Gajaraj, S., Beighley, R.E., 2011. Adsorption of phosphate by goethite and zeolite: effects of humic substances from green waste compost. Compost Science Utility 19, 197e204. Li, C., Guan, X.H., 2011. Competitive adsorption of three different phosphate species on aluminum hydroxide. Fresenius Environmental Bulletin 20, 1936e1941. Liu, H.L., Sun, X.F., Yin, C.G., Hu, C., 2008. Removal of phosphate by mesoporous ZrO2. Journal of Hazardous Materials 151, 616e622. Liu, Y., Sheng, X., Dong, Y.H., Ma, Y.J., 2012. Removal of highconcentration phosphate by calcite: effect of sulfate and pH. Desalination 289, 66e71. Lu, J.B., Liu, H.J., Liu, R.P., Zhao, X., Sun, L.P., Qu, J.H., 2013. Adsorptive removal of phosphate by a nanostructured Fe-AlMn trimetal oxide adsorbent. Powder Technology 233, 146e154. Lu, S.G., Bai, S.Q., Zhu, L., Shan, H.D., 2009. Removal mechanism of phosphate from aqueous solution by fly ash. Journal of Hazardous Materials 161, 95e101. McBride, M.B., 1997. A critique of diffuse double layer models applied to colloid and surface chemistry. Clays and Clay Minerals 45, 598e608. Morse, G.K., Brett, S.W., Guy, J.A., Lester, J.N., 1998. Review: phosphorus removal and recovery technologies. Science of the Total Environmental 212, 69e81. Mulkerrins, D., Dobson, A.D.W., Colleran, E., 2004. Parameters affecting biological phosphate removal from wastewaters. Environmental International 30, 249e259. Ogata, F., Tominaga, H., Kangawa, M., Inoue, K., Kawasaki, N., 2012. Characteristics of granular boehmite and its ability to adsorb phosphate from aqueous solution. Chemical and Pharmaceutical Bulletin (Tokyo) 60, 985e988. Okumura, M., Fujinaga, K., Seike, Y., 1999. Adsorption of phosphate with activated carbon loaded with zirconium and its application to a simple in situ preconcentration method for phosphate in environmental water samples. Bunseki Kagaku 48, 215e224. Okumura, M., Fujinaga, K., Seike, Y., Hayashi, K., 1998. A simple in situ preconcentration method for phosphate phosphorus in environmental waters by column solid phase extraction using activated carbon loaded with zirconium. Analytical Science 14, 417e419. Reed, B.E., Matsumoto, M.R., 1993. Modeling cadmium adsorption by activated carbon using the Langmuir and Freundlich isotherm expressions. Separation Science and Technology 28, 2179e2195. Ren, Z.M., Shao, L.N., Zhang, G.S., 2012. Adsorption of phosphate from aqueous solution using an iron-zirconium binary oxide sorbent. Water Air Soil Pollution 223, 4221e4231. Rodrigues, L.A., da Silva, M., 2010. Adsorption kinetic, thermodynamic and desorption studies of phosphate onto hydrous niobium oxide prepared by reverse microemulsion method. Adsorption 16, 173e181. Schick, J., Caullet, P., Paillaud, J.L., Patarin, J., Freitag, S., MangoldCallarec, C., 2012. Phosphate uptake from water on a
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Strong adsorption of phosphate by amorphous zirconium oxide nanoparticles Yu Su a, Hang Cui a, Qi Li a,*, Shian Gao a, Jian Ku Shang a,b a
Environment Functional Materials Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China b Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
article info
abstract
Article history:
Phosphate removal is important in the control of eutrophication of water bodies.
Received 11 March 2013
Adsorption is one of the promising approaches for the removal of phosphate, which could
Received in revised form
serve as a supplement for the biological phosphate removal process commonly used in the
20 May 2013
wastewater treatment industry to meet the discharge requirement when the biological
Accepted 23 May 2013
performance is deteriorated from changes of operation conditions. Amorphous zirconium
Available online 5 June 2013
oxide nanoparticles were synthesized by a simple and low-cost hydrothermal process, and their phosphate removal performance was explored in aqueous environment under
Keywords:
various conditions. A fast adsorption of phosphate was observed in the kinetics study, and
Eutrophication
their adsorption capacity was determined at about 99.01 mg/g at pH 6.2 in the equilibrium
Phosphate removal
adsorption isotherm study. Commonly coexisting anions showed no or minimum effect on
Amorphous ZrO2 nanoparticles
their phosphate adsorption performance. The phosphate adsorption showed little pH
Adsorption
dependence in the range from pH 2 to 6, while it decreased sharply with the pH increase
Inner-sphere
above pH 7. After adsorption, phosphate on these am-ZrO2 nanoparticles could be easily
complexing mechanism
desorbed by NaOH solution washing. Both the macroscopic and microscopic techniques demonstrated that the phosphate adsorption mechanism of am-ZrO2 nanoparticles followed the inner-sphere complexing mechanism, and the surface hydroxyl groups played a key role in the phosphate adsorption. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
With the rapid development of modern agriculture and industry, more and more wastewater is being produced every day. If wastewater is discharged into rivers, lakes and oceans without proper treatment, it could do harm to the environment and subsequently on the human health (Shannon et al., 2008). Many nutrients, such as nitrogen and phosphorus, exist in wastewater, which are needed for aquatic organisms to grow. The excessive presence of nitrogen and phosphorus in
water bodies could cause eutrophication and subsequently stimulate blue-green algae blooms when proper temperature conditions are present (Boujelben et al., 2008). It had been found that blue-green algae have the ability of biological fixation of nitrogen, and planktonic N2-fixing cyanobacteria bloom in freshwaters when phosphorus is abundant while nitrogen availability is low (Schindler et al., 2008; Conley et al., 2009). Consequently, the removal of phosphorus is essential to preventing eutrophication of natural water bodies, especially for freshwaters. The World Health Organization (WHO)
* Corresponding author. 72 Wenhua Road, Shenyang, Liaoning Province, 110016, PR China. Tel.: þ86 24 83978028; fax: þ86 24 23971215. E-mail addresses: [email protected], [email protected] (J.K. Shang). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.05.044
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 0 1 8 e5 0 2 6
suggested a maximum discharge limit of phosphorus of 0.5e1.0 mg/L (Galalgorchev, 1992), which is now being adopted in many countries and regions. For example, the emission standard for the newly-built urban sewage treatment plant in China will be upgraded to GB 1A (Total P at 0.5 mg/L) from GB 1B (Total P at 1.0 mg/L) (Ren et al., 2012). In aqueous environment, phosphorus exists in the pentavalent form as orthophosphate, pyrophosphate, longer-chain polyphosphates, organic phosphate esters and phosphodiesters, and organic phosphonates, and these various phosphorus compounds could be hydrolyzed to orthophosphate, the only form of phosphorus that could be utilized by bacteria, algae, and plants (Correll, 1998). Various techniques, such as chemical precipitation, adsorption, biological removal, reverse osmosis, membrane, ion exchange, and constructed wetlands, have been studied for the removal of phosphate from wastewater (Choi et al., 2012; de-Bashan and Bashan, 2004; Morse et al., 1998; van Voorthuizen et al., 2005; Zhang et al., 2012a, b). Among these approaches, the biological process is considered a costeffective and environmentally sound alternative to the chemical precipitation treatment, and is now widely used for the removal of phosphate at industrial level (Gebremariam et al., 2011). However, its effectiveness could be affected by many factors, such as volatile fatty acids, cations, temperature, sludge quality and settlement, and pH (Mulkerrins et al., 2004). So the biological process is usually supplemented by an additional treatment to meet the discharge requirement, when its performance is deteriorated from the fluctuations in operating conditions. The adsorption process requires simple operating conditions, possesses stable phosphate removal effect, and produces little sludge compared with other methods. Thus, it is promising as a supplement to the biological process for the phosphate removal, and it also could be easily applied for phosphate removal on small-scale treatment facilities or wastewater with relatively low phosphate concentrations. For this purpose, various adsorbents have been examined, including zero-valent iron (Almeelbi and Bezbaruah, 2012), activated carbon (Wang et al., 2012), aluminum oxide/hydroxide (Li and Guan, 2011), goethite (Gu et al., 2011), calcite (Liu et al., 2012), zeolite (Schick et al., 2012), red mud (Castaldi et al., 2012), and fly ash (Lu et al., 2009). Because of their large surface areas and preferred surface properties, synthesized metal oxides at nano-size have demonstrated effective phosphate adsorption (Lu et al., 2013; Yang et al., 2007; Yu et al., 2012). Zirconium based oxides could be an attractive adsorbent choice for water treatment because of their non-toxicity, good resistance to oxidant agents and acids/bases, high thermal stability, and very low solubility in water (Bergamaschi et al., 2005; Blaney et al., 2007; Suzuki et al., 2000). The use of zirconium based oxides for phosphate adsorption had been reported in literature (Awual et al., 2011). Most of the studies were focused either on fibers (Awual et al., 2011) or activated carbon (Okumura et al., 1999, 1998) loaded with zirconium hydroxide, or binary oxides of zirconium with other metals (for example, FeeZr binary oxides) (Ren et al., 2012). No report is available on the study of the phosphate adsorption by more stable amorphous ZrO2 nanoparticles, while amorphous adsorbents may have high adsorption capacity because of their
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porous and highly hydrated structure. In our recent study, amorphous zirconium oxide nanoparticles were synthesized by a simple and low-cost hydrothermal process and were found to have very strong adsorption on arsenic species from water (Cui et al., 2012). Phosphorus and arsenic belong to the same element group (main group V) and have similar electronic structures. They form similar components in water (phosphate and arsenate), and our recent study indicated that a strong adsorption competition existed between arsenate and phosphate (Cui et al., 2012). Thus, amorphous zirconium oxide nanoparticles may be a good choice for the removal of phosphate from water. In this study, the phosphate removal performance of amorphous zirconium oxide nanoparticles was examined in details by both the kinetics study and equilibrium adsorption isotherm study. The influences of various experimental parameters, such as solution pH, ionic strength, and competitive anions on phosphate removal were investigated. The phosphate desorption from these am-ZrO2 nanoparticles and their reuse for phosphate adsorption were explored. Moreover, the phosphate adsorption mechanism of am-ZrO2 nanoparticles was investigated by both the macroscopic and microscopic techniques.
2.
Experimental section
2.1.
Chemicals and material
Zirconium oxychloride octahydrate (ZrOCl28H2O, 99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, P.R. China) was used as the raw material, deionized water was used as the solvent, and aqueous ammonia (25 wt%, Tianjin Kermel Chemical Reagents Development Center, Tianjin, P.R. China) was used as the solvent, and aqueous ammonia (25 wt%, Tianjin Kermel Chemical Reagents Development Center, Tianjin, P.R. China) was used as the precipitation agent in the hydrothermal process. Sodium dihydrogen phosphate anhydrous (NaH2PO4, AR, Sinopharm Chemical Reagent Co. Ltd, Shanghai, P.R. China) was used to prepare phosphate stock solution. Concentrated hydrochloride acid (HCl, 32e38%, Tianda Chemical Reagents Factory, Tianjin, P.R. China) and sodium hydroxide (NaOH, 98%, Tianda Chemical Reagents Factory, Tianjin, P.R. China) were used to adjust solution pH. Sodium chloride (NaCl, 99.5%, Shenyang Chemical Reagents Factory, Shenyang, P.R. China), sodium sulfate anhydrous (Na2SO4, 99%, Tianjin Da Mao Chemical Reagents Factory, Tianjin, P.R. China), and sodium bicarbonate (NaHCO3, AR, Sinopharm Chemical Reagent Co. Ltd, Shanghai, P.R. China) were used in the competing ion effect experiment. Sodium chloride was also used in the ionic strength effect experiment.
2.2. am-ZrO2 nanoparticle synthesis and characterization am-ZrO2 nanoparticles were prepared by a hydrothermal process as detailed in our previous report (Cui et al., 2012). The Fourier transform infrared (FTIR) spectra of am-ZrO2 nanoparticles before and after phosphate adsorption were measured on a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, U.S.A.), and their zeta
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potential values were analyzed by an electrophoretic spectroscopy (JS84H, Shanghai Zhongchen Digital Instrument Co., Ltd, Shanghai, P.R. China). The semi-quantitative chemical composition and surface chemical states of the samples were examined by X-ray photoelectron Spectrometer (XPS) using an ESCALAB250 X-ray Photoelectron Spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, U.S.A.) with an Al K anode (1486.6 eV photon energy, 0.05 eV photon energy resolution, 300 W).
2.3.
Phosphate adsorption/desorption experiments
All the phosphate adsorption experiments were carried out at about 25 C in 250 mL glass beakers. Phosphate solution of 200 mL was used for each experiment with a fixed dose of amZrO2 at 0.1 g/L. The mixture was magnetically stirred to ensure good contact between am-ZrO2 and phosphate in water. The pH of phosphate solution was adjusted with dilute HCl and NaOH. After the adsorption experiment, the supernatant solution was collected for the remaining phosphate concentration analysis by centrifugation at 12,000 rpm to separate the adsorbent. Phosphate concentration in the supernatant solution was analyzed on an ion chromatograph (Dionex ICS 1100 Ion Chromatography, Thermal SCIENTIFIC, Sunnyvale, CA, U.S.A.). For the kinetic study of phosphate adsorption on amZrO2 nanoparticles, two initial phosphate concentrations of 5 and 10 mg/L were used, respectively, the solution pH was about 6, and the contact time was from 0 h to 8 h. For the equilibrium adsorption isotherm study, the initial phosphate concentration ranged from 5 to 50 mg/L, the solution pH was about 6, and the contact time was 8 h. For the solution pH and ionic strength effect studies, the initial phosphate concentration was 5 mg/L, the solution pH was adjusted from 2 to 10, the concentrations of NaNO3 were 0.1, 0.01, 0.001 M, respectively, and the contact time was 8 h. For the coexisting anion competing effect study, the initial phosphate concentrations was 5 mg/L, the concentrations of Cl-, SO4 2 and HCO3 were 0, 5 and 10 mM, the solution pH was about 6, and the contact time was 8 h. For the phosphate desorption study, NaOH aqueous solution was chosen as the desorption solution, and different NaOH concentrations of 0, 0.001, 0.01, 0.1 and 0.5 M were used to examine its effect on the desorption effect. After the phosphate adsorption, 0.1 g am-ZrO2 nanoparticles were dispersed into 50 mL NaOH solutions and the desorption time was 12 h. For the reuse experiments, the initial phosphate concentration was 5 mg/L and the contact time was 12 h. After being separated by centrifugation, am-ZrO2 nanoparticles were desorbed with 0.1 M NaOH solution. The desorbed nanoparticles were washed to neutral pH and dried at 80 C for 12 h before they were used for phosphate adsorption again. Two times of desorption and reuse experiments were conducted.
3.
5 nm. Their BET specific surface area was about 327 m2/g, and most pores were mesoporous. The mass density of am-ZrO2 nanoparticles was determined at 5.85 g/cm3. Thus, their theoretical specific surface area was calculated to be 205 m2/g, which was smaller than the BET specific surface area (327 m2/ g). This difference also indicated that these am-ZrO2 nanoparticles were highly porous. The isoelectric point of am-ZrO2 nanoparticles was determined at pH 5.9. Detailed characterization results could be found in our previous report (Cui et al., 2012). The kinetics of the phosphate adsorption from aqueous solutions onto am-ZrO2 nanoparticles are shown in Fig. 1a. It is apparent that the adsorption process could be divided into two stages, a rapid stage at the very beginning followed by a gradually slower stage until the adsorption equilibrium was achieved with the increase of the adsorption time. For example, when the initial phosphate concentration was 5 mg/ L, the adsorption rate at the very beginning was fast. Within just 0.5 h, about 96.8% phosphate was adsorbed onto am-ZrO2 nanoparticles, and the remaining phosphate concentration dropped to about 0.16 mg/L, which already satisfied the discharge requirement of 0.5e1.0 mg/L (Galalgorchev, 1992;
Results and discussion
3.1. Kinetics studies on phosphate adsorption by amZrO2 nanoparticles am-ZrO2 nanoparticles had a very poor crystallization and nonuniform shapes, with an average particle size of about
Fig. 1 e (a) The phosphate uptake of am-ZrO2 nanoparticles with the change of contact time at different initial phosphate concentrations: 5 mg/L (;), and 10 mg/L (:); (b) The fitting curves by the pseudo-second-order rate model of the kinetic study (adsorbent dose at 0.1 g/L, pH 6.2, 25 C).
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details in the adsorption mechanism study in the following sections.
Ren et al., 2012). This fast adsorption rate may be attributed to the fine size and subsequent large surface area of am-ZrO2 nanoparticles, which could enhance their contact efficiency with phosphate in water and be favorable for the diffusion of phosphate ions from bulk solution onto active sites of amZrO2 nanoparticles. With the further increase of adsorption time, the phosphate removal percentage gradually increased to over 99% and the adsorption equilibrium was established at 8 h. The phosphate adsorption process with the initial concentration of 10 mg/L was similar to that of 5 mg/L. With the increase of the initial phosphate concentration, a higher adsorption amount was observed, and the time to reach equilibrium increased. From the kinetic study, it could be seen that 8 h was adequate for the adsorption equilibrium to occur. So we conducted the following adsorption experiments with the contact time of 8 h. The adsorption kinetic data could be fitted into various rate models to provide the understanding of the adsorption mechanism (Chen et al., 2007). The linear pseudo-first-order rate equation could be given as Eq. (1): log qe qt
¼ logqe
k1 t 2:303
3.2. Equilibrium adsorption isotherm study on phosphate adsorption by am-ZrO2 nanoparticles The adsorption capacity of am-ZrO2 nanoparticles on phosphate was investigated by the equilibrium adsorption isotherm study, and the results are shown in Fig. 2. The adsorption data was fitted with both Freundlich and Langmuir as given in Eq. (3) (Deliyanni et al., 2007) and Eq. (4) (Reed and Matsumoto, 1993), respectively: 1
rate
equation could
t 1 t ¼ þ qt k2 q2e qe
(3)
qe ¼ qmax KL Ce =ð1 þ KL Ce Þ
(4)
where qe is the amount (mg/g) of phosphate adsorbed at equilibrium, ce is the equilibrium phosphate concentration (mg/L) in the solution, KF and n are the Freundlich constants, KL is the Langmuir constant, and qmax is the Langmuir monolayer adsorption capacity. The parameters obtained in fitting the experimental data are summarized in Table 2. It is clear that the phosphate adsorption data could be best fitted with the Langmuir isotherm because the R2 fitted with this model is closer to1 (R2 ¼ 0.998), compared with that produced by the Freundlich isotherm (R2 ¼ 0.974). The Langmuir adsorption capacity (qmax) was determined at about 99.01 mg/g, which was among the highest reported values in the literature. Table 3 summarized the reported adsorption capacities of various adsorbents on phosphate. The phosphate adsorption capacity of am-ZrO2 nanoparticles was very close to the highest phosphate adsorption capacity of 102.3 mg/g recently reported for FeeZr binary oxide (Ren et al., 2012), while pure am-ZrO2 nanoparticles should have a better stability than FeeZr binary oxide in aqueous environment. From Fig. 2, it could also be observed that the phosphate amount that am-ZrO2 nanoparticles adsorbed at low equilibrium concentration was quite high. For example, when the equilibrium concentration was just 0.0089 mg/L, the phosphate amount that am-ZrO2 nanoparticles adsorbed was over 44.4 mg/g, even higher than the maximum adsorption capacities of some reported adsorbents in literature with much higher equilibrium phosphate concentrations (usually over 1000 times) (Ogata et al., 2012; Wang et al., 2012; Zamparas et al., 2012; Zhang et al., 2009; Zhang et al., 2012a, b). Such a performance is beneficial to its potential application as the supplement to the biological process for the phosphate removal because the phosphate concentration in streams after biological process is usually not very high.
(1)
and the pseudo-second-order expressed as Eq. (2):
qe ¼ KF Cen
be
(2)
where qe and qt are the amounts (mg/L) of phosphate adsorbed at equilibrium and at time t, respectively, and k1 (h1) and k2 (g mg1 h1) are the rate constants of pseudo-first-order and pseudo-second-order adsorption processes, respectively. The fitting curves by the pseudo-first-order rate model were demonstrated in Figure S1 in the supplementary data, and the fitting curves by the pseudo-second-order rate model were demonstrated in Fig. 1b. The kinetics parameters obtained in fitting the experimental data are summarized in Table 1. It was found that the kinetics data could be best fitted into the pseudo-second-order rate model with R2 higher than 0.99 for both 5 mg/L and 10 mg/L initial phosphate concentrations, while the pseudo-first-order rate model could not fit the data well. Similar kinetic study results had been reported by Rodrigues et al. (Rodrigues and da Silva, 2010) and Tang et al. (Tang et al., 2012) on various phosphate adsorbents. The pseudo-second-order rate model indicates a chemisorptions (Azizian, 2004) occurred between phosphate and am-ZrO2 nanoparticles involving valency forces through sharing or exchange of electrons between sorbent and sorbate (the replacement of eOH by phosphate), which will be discussed in
Table 1 e The kinetic model parameters for the adsorption of phosphate on am-ZrO2 nanoparticles at pH 6.2 and T about 25 C. Initial concentration of phosphate (mg/L) 5 10
Pseudo-first-order model qt ¼ qe(1-e-k1t)
Pseudo-second-order model qt ¼ k2q2et/(1 þ k2qet)
qe(mg g1)
k1(h1)
R2
qe(mg g1)
k2(g mg-1h1)
R2
48.44 67.76
5.323 0.250
0.937 0.521
48.45 67.29
1.262 0.079
1 0.999
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charged am-ZrO2 nanoparticle surface and the negatively charged phosphate species could decrease the phosphate adsorption on am-ZrO2 nanoparticles, and the negative effect increased with the pH increase. So, less adsorption could occur with the pH increase as demonstrated in Fig. 3.
3.4. Ionic strength effect on phosphate adsorption by am-ZrO2 nanoparticles
Fig. 2 e The adsorption equilibrium isotherm of phosphate on am-ZrO2 nanoparticles at pH 6.2 and 25 C: Freundlich isotherm fitting curve (dd), and isotherm fitting curve ($$$$$$).
3.3. Solution pH effect on phosphate adsorption by amZrO2 nanoparticles The solution pH effect on phosphate adsorption by am-ZrO2 nanoparticles was demonstrated in Fig. 3. The phosphate adsorption on am-ZrO2 nanoparticles was evidently dependent on pH. Under the acidic pH conditions, the adsorption performance just decreased very slightly from pH 2 to pH 6, while it dropped much faster with the pH increase after pH 7. Similar solution pH effect had also been observed in previous reports for various adsorbents (Lu et al., 2013). Under the solution pH range investigated in this study (pH 2e10), the dominant species of phosphate in the solution are H2 PO4 and HPO4 2 . Low solution pH in the acidic region is beneficial for the protonation of adsorbent surface, which could enhance the electrostatic attraction between the adsorbent surface and the phosphate anions to facilitate the phosphate adsorption. Yet the phosphate adsorption on am-ZrO2 nanoparticles is a chemisorption dominated process, and the adsorption from the electrostatic attraction is not a major contribution in this process. So the phosphate adsorption performance of amZrO2 nanoparticles just increased very slightly from pH 6 to pH 2. When the solution pH increased from the neutral to basic region, am-ZrO2 nanoparticles had negatively charged surface and the charge intensity increased with the pH increase. Thus, the coulomb repulsive interaction between the negatively
The phosphate adsorption mechanism on am-ZrO2 nanoparticles was first investigated by the macroscopic technique of evaluating the ionic strength effect on the adsorption behavior. The solution ionic strength was adjusted by the addition of NaNO3 with different concentrations into the phosphate solutions. Fig. 3 demonstrated that the phosphate adsorption increased with the increase of the solution ionic strength. Similar observations had been reported in literature for the phosphate adsorption on different adsorbents (Tanada et al., 2003). As suggested by McBride (1997), the adsorption of phosphate would decrease with the increase of ionic strength if phosphate formed outer-sphere surface complexes, while the adsorption of phosphate would either not change or increase with the increase of ionic strength if phosphate formed inner-sphere complexes. Thus, the results in this study demonstrated that the adsorption of phosphate on am-ZrO2 nanoparticles follows the inner-sphere complex mechanism.
3.5.
The phosphate adsorption mechanism onto am-ZrO2 nanoprticles was also investigated by another macroscopic technique of electrophoretic mobility measurement. Fig. 4 showed the zeta potentials of pure am-ZrO2 nanoparticles and amZrO2 nanoparticles after the phosphate adsorption (5 mg/L phosphate solution). Without the phosphate adsorption, the isoelectric point (IEP) of am-ZrO2 nanoparticles was about pH 5.0, while the IEP of am-ZrO2 nanoparticles decreased to about pH 3.9 after phosphate adsorption. The IEP of metal oxides is determined by protonation and deprotonation of surface hydroxyl groups. The formation of outer-sphere surface complexes cannot shift the IEP because there is no chemical reaction between the adsorbate and adsorbent surface that could change the surface charge (Ren et al., 2012; Yang et al., 2007; Zhang et al., 2009). Thus, it is clear that a specific adsorption rather than a purely electrostatic interaction existed between the phosphate and am-ZrO2 nanoparticles to form inner-sphere phosphate anionic charged surface complexes on am-ZrO2 nanoparticles.
3.6.
Table 2 e Langmuir and Freundlich isotherm constants for phosphate adsorption on am-ZrO2 nanoparticles at pH 6.2 and T about 25 C. Langmuir isotherm qm (mg/g) 99.01
Freundlich isotherm -1
2
KL (L mg )
R
KF
0.31
0.998
50.48
n 0.164
R2 0.974
Zeta potential study
FTIR study
The phosphate adsorption mechanism was further investigated by the microscopic technique of FTIR spectroscopy. Fig. 5 demonstrated the FTIR spectra of am-ZrO2 nanoparticles before and after phosphate adsorption, respectively. Without the phosphate adsorption, the FTIR spectrum of am-ZrO2 nanoparticles had strong hydroxyl stretching (3363 cm-1) and bending (1638 cm-1) vibrations of physically adsorbed H2O, and the deformation vibration (1338 cm-1) of ZreOH (Cui et al., 2012), indicating that am-ZrO2 nanoparticles had high
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Table 3 e Phosphate adsorption capacities of various adsorbents. Adsorbent Fee Al e Mn trimetal oxide Fee Mn binary oxide FeeZr binary oxide Mesoporous ZrO2 Innovative modified bentonites Tantalum Hydroxide Iron-doped activated carbon Granular boehmite Carbonated ferric green rust Lanthanum(III)-coordinated diamino-functionalized 3D hybrid mesoporous silicates Lanthanum-doped activated carbon fiber Sepiolite am-ZrO2
pH
Adsorption capacity (mg/g)
6.8 5.6 5.5 About 6.8 7 2 3.78 about 6. 84 3. 5, 7 6
48.3 33.2 102.3 91.05 11.15 78.5 14.12 8.38 51.02 54.3
No data 4.8 6.2
adsorption capacities to H2O and hydroxyl groups existed on their surface, which was in accordance with the results of zeta potential test. After the phosphate adsorption, the FTIR spectrum of am-ZrO2 nanoparticles demonstrated obvious changes. The ZreOH deformation peak disappeared, while the peaks of physically adsorbed H2O greatly weakened. These observations indicated that the replacement of eOH and H2O occurred during the phosphate adsorption. Additionally, a new peak appeared at 1037 cm-1, which was broad and intensive. This new peak could be assigned to the asymmetry vibration of PeO bond, indicating that the surface hydroxyl groups were replaced by the adsorbed phosphate (Zhang et al., 2009). The FTIR study results demonstrated that the substitution of ZreOH groups by phosphate species played a key role in their adsorption on am-ZrO2 nanoparticles.
3.7.
XPS study
The surface O 1s spectra of am-ZrO2 nanoparticles before and after phosphate adsorption were analyzed by high-resolution XPS scan, and were demonstrated in Fig. 6a and b,
Fig. 3 e Effect of pH and ionic strength on phosphate adsorption by am-ZrO2 nanoparticles (initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, 25 C).
22.86 98.06 99.01
Ref. (Lu et al., 2013) (Zhang et al., 2009) (Ren et al., 2012) (Liu et al., 2008) (Zamparas et al., 2012) (Yu et al., 2012) (Wang et al., 2012) (Ogata et al., 2012) (Barthelemy et al., 2012) (Zhang et al., 2011) (ZhangLiu et al., 2012a, b) (Yin et al., 2011) Present study
respectively. The O 1s peak could be best fitted with three overlapped O 1s peaks of oxide oxygen (O2), hydroxyl group (-OH), and adsorbed water (H2O). The data were fitted using Lorentzian peak shape, and the fitting parameters could be found in Table S1 in the supplementary data. Fig. 6a demonstrated that the eOH percentage was about 42.8% for these am-ZrO2 nanoparticles before the phosphate adsorption, while Fig. 6b demonstrated that the eOH percentage largely dropped to about 20.8% after the phosphate adsorption. The XPS analysis results suggested that hydroxyl groups existed on the surface of am-ZrO2 nanoparticles, and the eOH percentage in the total surface oxygen dropped sharply by the adsorption of phosphate. Thus, the decrease of eOH groups could be attributed to the replacement of eOH by phosphate during the adsorption process. So the XPS analysis also suggested that the surface hydroxyl groups played the key role in the phosphate adsorption, which is in accordance with results of ionic strength, zeta potential and FTIR studies.
3.8. Coexisting anion effect on phosphate adsorption by am-ZrO2 nanoparticles Due to the complexity of natural water contents, there might be competition from other species which may largely affect the phosphate removal performance on am-ZrO2 nanoparticles. Coexisting anions, such as Cl, SO4 2 and HCO3 , are generally present in wastewater and might interfere with the phosphate adsorption on am-ZrO2 nanoparticles through competitive adsorptions. Herein, the effects of these coexisting anions at two concentrations (5 and 10 mM) on the phosphate adsorption were examined, and the results were demonstrated in Fig. 7. The initial concentration of phosphate was about 5 mg/L, i.e. about 0.05 mM, and the am-ZrO2 loading was 0.1 g/L. The presence of Cl, SO4 2 and HCO3 , even at very high concentrations compared with that of phosphate (100 times as 5 mM and 200 times as 10 mM), had no or just a slight effect on the phosphate adsorption. Thus, am-ZrO2 nanoparticles could remove phosphate effectively even when exceptional high concentrations of these competing anions existed, which is beneficial to their potential industrial applications.
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Fig. 4 e Zeta potential of am-ZrO2 nanoparticles before (-) and after phosphate adsorption (C) with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
3.9. Desorption of phosphate from am-ZrO2 nanoparticles and their reuse The phosphate adsorbed on am-ZrO2 nanoparticles could be desorbed with NaOH solution, and the desorption results with NaOH solutions of different concentrations were demonstrated in Fig. 8. It is clear that the amount of desorbed phosphate increased with the increase of the alkalinity of NaOH solution. Without NaOH addition (pH 7), a desorption effect of just 5.33% of adsorbed phosphate was obtained when the extracting solution was only 0.01 M NaNO3 solution. When the NaOH concentration increased to 0.1 M (pH 13), however, the desorption effect increased to 91.67%. The further increase of NaOH concentration to 0.5 M (pH 13.7) did not significantly enhance the desorption of phosphate. The results demonstrated that the phosphate adsorbed on am-ZrO2 nanoparticles could be easily desorbed using a 0.1 M NaOH
Fig. 5 e FTIR spectrum of am-ZrO2 nanoparticles before (-) and after phosphate adsorption (C) with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
Fig. 6 e The surface O 1s spectra of am-ZrO2 nanoparticles (a) before and (b) after phosphate adsorption with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
solution. The desorbed am-ZrO2 nanoparticles were reused for phosphate removal, which demonstrated that they could keep about 80e90% of its original phosphate removal capability after being desorbed for two times. Thus, these
Fig. 7 e Effect of coexisting anions on the phosphate removal performance with initial phosphate concentration at 5 mg/L, adsorbent dose at 0.1 g/L, and pH 6.2.
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 0 1 8 e5 0 2 6
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Innovation Promotion Association, Chinese Academy of Sciences (Grant No. Y2N5711171).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.05.044.
references
Fig. 8 e Desorption of phosphate from phosphate-loaded am-ZrO2 by NaOH solutions with different pH values.
recovered am-ZrO2 nanoparticles could be reutilized for phosphate removal from water, which could significantly reduce the operation cost.
4.
Conclusions
In summary, am-ZrO2 nanoparticles synthesized from a simple and low-cost hydrothermal process demonstrated an effective phosphate removal performance in water in terms of both the adsorption speed and adsorption capacity. Their adsorption capacity on phosphate was determined at about 99.01 mg/g at pH 6.2, which was among the highest reported values in the literature. The adsorption mechanism of phosphate onto am-ZrO2 nanoparticles was determined to follow the inner-sphere complexing mechanism, and the surface eOH groups played a major role in the phosphate removal. am-ZrO2 nanoparticles could remove phosphate effectively even when competing anions with exceptionally high concentrations were present. Thus, it may have the potential to be a promising adsorbent as the supplement to the biological process for the phosphate removal, and it also could be applied for phosphate removal on small-scale treatment facilities or wastewater with relatively low phosphate concentrations. Further work is now underway to immobilize them onto various highly porous substrates and synthesize highly porous, nanostructured ZrO2 spheres in millimeter size based on these ZrO2 nanoparticles to avoid the difficulty in the separation of nanoadsorbents, to test their continuous treatment performance in flow-through, fixed bed reactors, and to use the pore surface diffusion modeling to predict their performance in full-scale fixed bed systems.
Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant No. 51102246), the Knowledge Innovation Program of Institute of Metal Research, Chinese Academy of Sciences (Grant No. Y0N5A111A1), and the Youth
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