Preliminary study of phosphate adsorption onto cerium oxide nanoparticles for use in water purification. Nanoparticles synthesis and characterization

In this study, the synthesis and characterization of cerium oxide nanoparticles (CeO 2 -NPs) and their adsorption potential for removing phosphate from water was evaluated using a multi-factor experimental design to explore the effect of various factors on adsorption. The objective function selected was the percentage of phosphate removed from water, in which the phosphate concentration and the CeO 2 -NP concentration are quantitative variables (factors in the experimental design). A lineal polynomial fitted the experimental results well (R 2 =0.9803). The nanostructure was studied by transmission electron microscopy and high-resolution transmission electron microscopy techniques before and after the adsorption process. During the adsorption and desorption processes several changes in the morphology and surface chemistry of the CeO 2 -NPs were observed.


INTRODUCTION
Cerium nanoparticles (CeO 2 -NPs) have been the subject of recent studies due to their applications in catalysis, fuel cells, optical films, and in other fields (Campbell and Peden, 2005;Yuan et al., 2009).In recent years, promising results have been obtained in water treatment for chromium and arsenate removal by using CeO 2 nanoparticles or CeO 2 nanocomposites (Peng et al., 2005;Xiao et al., 2009;Recillas et al., 2010).Understanding the environmental process on a molecular level is key to several environmental processes (Al-Abadleh and Grassian, 2003).It is also important to evaluate the environmental impact after the complete adsorption-desorption process that occurs on the NPs.The NP synthesis conditions determine the specific adsorption behavior of the resultant NPs, because the redox properties can be modified, and their dual reversible reaction can modify the overall adsorption process.
Phosphate is a key contaminant in the eutrophication process (Fernández et al., 2003;Nowack and Stone, 2006;Laney et al., 2007).Therefore, the removal of phosphate from wastewater by chemical and biological treatments has been widely investigated; this usually results in complex biological operations or the production of chemical sludge (Barnard, 1983;Scheer and Seyfried, 1997).The adsorption and precipitation of phosphate by using fly ash and modified fly ash have been investigated in the search for economical adsorbents for phosphate (Xu et al., 2010).Nevertheless, the inconvenience of the precipitation process is that it requires a large amount of chemicals and produces a great deal of wastewater sludge (Ozacar, 2003).Even though the adsorption technique is useful and economical, only a few studies consider the recovery of the adsorbent used and the pollutant, which are crucial issues for the use of NPs in environmental processes where economics play a decisive role.

P r e -p r i n t
In the present study, a Box-Hunter spherical experimental design (Box et al., 1978) and surface characterization techniques were used to study phosphate adsorption behavior on CeO 2 -NPs.This experimental design is based on determining the coefficients that fit a polynomial function to describe the system under study and the influence of the proposed factors on the target function (Sánchez et al., 2000).In this case, the phosphate adsorption percentage was selected as the objective function.As a secondary objective, the chemical surface and nanostructure changes during the adsorption-desorption process were studied by infrared attenuated total reflectance (ATR-IR), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM).

CeO 2 nanoparticle preparation
CeO 2 nanoparticles were synthesized in aqueous solution, using milli-Q grade water.All reagents were purchased from Sigma-Aldrich and used as received.Briefly, the CeO 2 -NP synthesis was based on the methodology proposed by Zhang et al. (2004), from Ce(NO 3 ) 3 salt oxidized under basic pH conditions to Ce 4+ using 0.5 M hexamethylenetetramine (HMT) .

Characterization and stability of nanoparticles
For full characterization of the NPs, the obtained nanoparticle suspension was analyzed using dynamic light scattering (DLS) to determine the nanoparticle size distribution in a Nanoparticles Analysis System (Malvern, UK).Zeta potential (ZP) measurements were also performed to study the surface properties and any changes after the experiments.X-Ray diffraction spectra (using a PANalytical X´Pert diffractometer with a Cu Kα radiation source) were obtained to determine the crystalline phase of the samples.TEM images of the samples P r e -p r i n t were also taken using a JEOL 1010 operating at an accelerating voltage of 80 kV after the nanoparticle synthesis to characterize the NPs before and after the phosphate adsorptiondesorption process.The samples were ultrasonically suspended in ethanol and then dropped onto amorphous carbon specimen grids.HRTEM was used to analyze the CeO 2 -NPs after the phosphate adsorption process.After the desorption treatment process with NaOH, the morphology, electron diffraction pattern, and the elemental analysis of the NPs were determined; the latter being performed by using energy dispersive X-ray spectroscopy (EDS).
Inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent Equipment (Model 7500ce) was used to analyze the initial and final phosphate concentration in solutions.
Table 1 and Fig. 1 show some of the main characteristics of the used nanoparticles as they were synthesized.

Adsorption-desorption study
A Box-Hunter spherical experimental design was used to study the influence on the adsorption behavior of the initial concentration of phosphate and CeO 2 -NPs.In order to limit the range of the initial concentration of phosphate, the adsorption capacity of three initial phosphate concentrations (100, 50, 10 mg L -1 ) were tested, while the concentration of CeO 2 -NPs was maintained constant (320 mg L -1 ) and the contact time was fixed at 24 hours to ensure that equilibrium conditions were reached.Previous experiments (data not shown) performed at 100 mg L -1 of phosphate and 320 mg L -1 of CeO 2 -NPs showed that at 3 hours of contact time nearly 100% of the phosphate was adsorbed, and from this time phosphate was not detectable in solution.Once the range of the studied variables was limited, the ranges of the variables used in the Box-Hunter spherical method were: phosphate: 0-170 mg L -1 , CeO 2 -NPs suspension 0-320 mg L -1 .These ranges cover the typical values found for phosphate concentration in most waters and wastewaters.The exact values used in the experiment are P r e -p r i n t presented in Table 2.The objective function of the experimental design was the removal percentage of phosphate from the solution.The experimental design was statistically validated by using the Sigmaplot 11.0 software package (Systat Software Inc, San Jose, USA), from four replicated experiments (Table 2).
The adsorption processes were performed as follows: Equal volumes of potassium phosphate solutions and CeO 2 -NPs suspensions were poured into a vessel and stirred at 150 rpm at room temperature and pH 7 for 24 hours; the samples were separated by centrifugation (10000 g, 10 min) and the liquid-phase phosphate concentration was analyzed by using ICP-MS.The desorption study was performed using the same procedure used for adsorption, although only the concentrations of phosphate of 170 mg L -1 and CeO 2 -NPs suspension of 320 mg L -1 were tested.The solid phases obtained after adsorption and centrifugation were dried at room temperature for 24 hours.In the same vessel the following solvents for desorption were used: deionized water, 0.1 M NaOH, and 0.5 M NaOH.The suspensions were stirred for 24 hours, separated by centrifugation (10000 g, 10 min), and the concentration of phosphate in the liquid phase determined.The initial and final concentrations of phosphate were again measured using ICP-MS.All desorption experiments were carried out in triplicate and the average values are presented.The standard deviation was very low for all experiments (< 5%) and is not presented.

Synthesis of CeO 2 nanoparticles
Synthesized CeO 2 nanoparticles crystallize in a cubic fluorite structure (Fig. 1a) with the predominant crystallographic planes exposed at the (111) surface (Fig. 1b), which are responsible for the catalytic behavior (Zhang et al., 2004).The average diameter obtained was P r e -p r i n t 11.7 ± 1.6 nm (Fig. 1c).The size distribution was obtained after image analysis of different TEM images, by counting at least 500 NPs.These nanocrystals have more cerium atoms than oxygen atoms per unit surface; unlike (100)-terminated CeO 2 -NPs, which are predominantly oxygen terminated (Trovarelli, 2002).This fact is related to the storage and release of oxygen and the promotion of noble-metal activity and dispersion (Stanek et al., 2008).Both phenomena are controlled by the type, size, and distribution of oxygen vacancies as these are the most relevant surface defects (Carrettin et al., 2004).

Adsorption-desorption study
The The use of this experimental method permits to study the removal capacity of CeO 2 -NPs using a relatively small number of experiments and provides a reliable tool to predict adsorption at any concentrations, being some values close to 100% removal (Table 2).This is an important advance since most of the studies published test randomly the effect of different parameters on the adsorption process and it has been validated in studies in different research areas (San Sebastián et al., 2003).
In relation to the adsorption capacities observed, they are within the range of 0.3-0.4mg of phosphate per gram of CeO 2 -NPs.In general, these values are higher than those found by other authors.Thus, Huang and Chiswell (2000) found a capacity of 0.30-0.33mg phosphate per gram of air-dried spent alum, whereas other authors report the maintenance of P r e -p r i n t concentrations of total phosphorous using activated alumina columns (Donnert and Manfred, 1999).
Phosphate speciation can have an important role on the adsorption process.In this ).According to Lin et al. (2011), the physical adsorption of phosphate onto active carbon is not pH-dependent.However, adsorption onto other surfaces, such as particles of Al/SiO 2 /Fe 3 O 4 , was found to be clearly pH-dependent, being 4.5 the optimum pH for phosphate removal.In this case, the increase in the adsorption capacity was attributed to changes on the surface of Al/SiO 2 /Fe 3 O 4 particles and not due to changes in phosphate speciation.At acid pH the surface charge of cerium oxide nanoparticles is positive (Di et al., 2006), thus the adsorption capacity of phosphate at acid pH could be higher, although in this study we preferred to work under neutral pH, as it is more common in real waters.
Adsorption experiments were performed with the aim to recover both nanoparticles and the adsorbed material.Deionized water and NaOH (0.5 M and 0.1 M) were selected as solvents for desorption.The phosphate desorption percentages obtained were 27% and 8% for NaOH 0.5 M and 0.1 M, respectively, whereas deionized water did not produce any desorption at the evaluated concentrations.This opens the research on using effective solutions for desorbing valuable materials such as phosphate, which is a fundamental point for an economical and environmentally friendly overall process.In this sense, some authors have proved the recovery of high quality phosphate after the desorption process from wastewater (Ebie et al., 2008;Midorikawa et al., 2008), although, to our knowledge, this process has not been tested using nanoparticles.
P r e -p r i n t

TEM and HRTEM
TEM images of CeO 2 -NPsPO 4 and CeO 2 -NPsPO 4 -NaOH are shown in Fig. 2. The NPs obtained after the phosphate adsorption process show particles with a homogeneous size and similar irregular shape (Fig. 2a), whereas at higher amplification (Fig. 2b) the shape of the particles is roughly spherical.The estimated diameter of the NPs is around 12 nm (Fig. 2b).
Changes in the morphology and size of the nanostructure were found after the desorption treatment with NaOH solutions.At low amplification a homogeneous material was observed (Fig. 2c), whereas at higher amplification fused elongated NPs were observed (Fig. 2d).The diameter of the fused particles is around 24 nm (Fig. 2d).
Fig. 3a shows the HRTEM images of NPs after adsorption treatment (CeO 2 -NPsPO 4 ); these are mainly the NPs bounded by ( 111) planes.This result is expected because this plane is the most stable (Gross et al., 1997).The lattice spacing as measured from the TEM image is approximately 0.32 nm for (111), in accordance with the expected fluorite structure, and can be described as a face-centered cubic packing of cations, with anions in all of the tetrahedral holes (the Ce 4+ cation is surrounded by 8 O 2 -ions with each O 2 -coordinated to 4 Ce 4+ ).Fig. 3b shows the electron diffraction pattern of the CeO 2 -NPsPO 4 .The ring patterns confirm the nanocrystalline structure and are consistent with the indexed cubic cerium oxide with a fluorite structure.The four rings (from inner to outer) correspond to the (111), ( 200), (220), and (300) reflections.The most intense line arises from (111), which corresponds to a 0.54 nm lattice constant (calculated from the radii of Debye-Scherrer rings); this is expected for the fluorite structure of CeO 2 (Strom and Jun, 1980).After the desorption process (CeO 2 -NPsPO 4 -NaOH) the lattice spacing and the nanoparticle size remained unchanged (Fig. 3c).
The CeO 2 -NPsPO 4 elemental analysis results showed the presence of Ce, O, P, Na, and K after the desorption process (Fig. 3d) and were compared with those of CeO 2 -NPsPO 4 -P r e -p r i n t NaOH.As expected, a decrease in the phosphate concentration and an increase in sodium concentration were observed.

CONCLUSIONS
The high capacity for phosphate adsorption at low and high concentrations found for CeO 2 -NPs is a promising result for water and wastewater treatment processes.The use of a Box-Hunter spherical experimental design to study the adsorption process resulted in a better interpretation of the optimum conditions of adsorption than has been previously obtained, and with a relatively small number of experiments.During the phosphate adsorption and desorption processes several changes in the surface chemistry and morphology of NPs were observed.Estimated surface area (m 2 g -1 ) 121

Tables
*HMT: Hexamethylenetetramine P r e -p r i n t P r e -p r i n t P r e -p r i n t P r e -p r i n t P r e -p r i n t response of the objective function (percentage of phosphate removal) as a result of the initial phosphate and CeO 2 -NPs concentrations was tested in the experimental design.The surface obtained fitted well with lineal equation such Eq. 1, where x and y are the normalized values of the factors considered: F = y o + ax + by (Eq. 1) being R 2 = 0.9803, y o = 50.05,a = 0.227 and b = -0.313.

Table 1 .
Main characteristics of the CeO 2 nanoparticles used.Values presented were obtained as nanoparticles were synthesized.

Table 2 :
Tested conditions according to the Box-Hunter experimental design matrix.Results of objective function (percentage removal of phosphate after adsorption) are also presented, being x and y are the normalized factors considered in the experimental design.