Facile Synthesis, Characterization, and Adsorption Insights of Lanthanum Oxide Nanorods

This study synthesized lanthanum oxide (La2O3) nanorods to develop a practical approach for the removal of arsenic from groundwater. La2O3 nanorods were synthesized by a simple hydrothermal process followed by calcination at 500 ◦C and were characterized by spectroscopic and microscopic techniques. To evaluate the adsorption mechanism of La2O3 nanorods, adsorption parameters including solution pH, temperature, equilibrium isotherms, and kinetics for arsenic were studied. The results suggested that the arsenic uptake was a rate-limiting, monolayer adsorption interaction on the La2O3 nanorods homogeneous surface. In addition, it was found that the adsorptive removal behavior of La2O3 for As(V) was sensitive to the initial pH and temperature, and the maximum uptake amount of as prepared La2O3 was found to be 260.56 mg/g of As(V) at pH 6.0 and 25 ◦C. Furthermore, the uptake capacity of La2O3 nanorods for As(V) increased with temperature. The resultant thermodynamic parameters (∆G0, ∆H0, and ∆S0) suggested an endothermic adsorption of As(V) on La2O3. The adsorption capacity of La2O3 was higher than that of several reported nanocomposites, suggesting its practical applicability and novelty for As-contaminated wastewater treatment.


Introduction
Lanthanides are widely used in hydrogen storage, electrodes, gate insulators, adsorbents, and superconductors, owing to their versatility and multifunctionality [1][2][3][4]. In lanthanide series, lanthanum (La) is one of the lightest elements that has been widely used in various applications, including optoelectronic devices [5], phosphors [6], solid electrolytes [7], catalysts [8,9], sorbents [10], and gas sensors [11] in the form of oxide, carbonate, and phosphate. Currently, lanthanum oxide (La 2 O 3 ) is used in various applications, including water treatment [12,13], catalytic exhaust gas converters [14], and high-k gate dielectric materials [15]. Various methods have been adopted for the preparation of La 2 O 3 nanostructures, including precipitation, thermal decomposition, hydrothermal, sol gel, and pyrolysis methods [13,[15][16][17][18][19][20][21][22][23][24]. The adsorption properties of La 2 O 3 mainly depend on its texture and surface morphology of materials. With this, there is still a research gap in developing a novel surface texture and morphology for La 2 O 3 in water treatment. Thus, this article focuses on the synthesis of nanostructured La 2 O 3 through simple precipitation followed by calcination at 500 • C and its adsorptive removal ability for As(V) in groundwater.
Arsenic is a highly toxic pollutant in the environment, occurring through natural and anthropogenic sources. Among existing arsenic forms in the environment, inorganic arsenic is more prevalent in +3 or

Synthesis of La 2 O 3 Nanorods
All chemicals were of analytical grade and used without further purification. La 2 O 3 was synthesized by simple hydrothermal precipitation followed by calcination. La(NO 3 ) 3 ·6H 2 O powder was dissolved in water and stirred vigorously to obtain a clear solution. The solution was adjusted to pH 10 using 5 M NaOH solution and stirred for 30 min and filtered. The obtained precipitate was then collected. The precipitate was evaporated to dryness at 100 • C and calcined at 500 • C for 4 h. The calcined products were stored and used for further studies. The calcination process was carried out at different time (1-6 h) intervals and different calcination temperatures (300, 400, 500, and 600 • C), but the morphology and surface area of the material remained the same at the above calcination at 500 • C for 4 h. Hence, the calcination condition 500 • C for 4 h was chosen as the optimum condition in the preparation of La 2 O 3 in this study.

Analytical Methods
A D/Max-2500 X-ray diffractometer (Rigaku, Tokyo, Japan) was used to evaluate the crystallinity and textural properties of the prepared adsorbents. The elemental composition was analyzed using a PHI Quantera-II XPS (Ulvac-PHI, Kanagawa, Japan). A scanning electron microscope (S-4300 and EDX-350, Hitachi, Tokyo, Japan) was used to investigate the surface morphology of the adsorbents. HR-TEM (JEM-4010, JEOL, Peabody, MA, USA) was used to measure the shape and particle size of the adsorbents. The samples for TEM were prepared by dispersing the final samples in distilled water, and this dispersant was then dropped on carbon-copper grids covered by an amorphous carbon film. To prevent agglomeration of the adsorbent, the copper grid was placed on a filter paper at the bottom of a Petri dish. N 2 adsorption-desorption isotherms of the prepared materials were constructed using an Autosorb-1 (Quantachrome Instruments, Boynton Beach, FL, USA) instrument that was also used to measure the surface area, pore volume, and pore diameter. FT-Raman spectroscopy was carried out with BRUKER OPTICKGMBH and ESCALAB-210 (Barcelona, Spain) instruments. The pH at the potential of zero-point charge (pHzpc) of the sample was measured using the pH drift method. The pH of a solution of 0.01 M NaCl was adjusted between 2 and 12 by adding either HCl or NaOH. Absorbent (0.1 g) was added to 50 mL of the solution. After the pH had stabilized (after 24 h), the final pH was recorded. The graph of the final pH vs. initial pH was used to determine the point at which the initial pH and final pH values were equal. This was taken as the pHzpc of the sample.

Adsorption Process
As(V) stock solutions were freshly prepared by dissolving the desired amount of Na 2 HAsO 4 ·7H 2 O (99% purity, Junsi, Japan). The required arsenic experimental solutions were prepared by diluting the stock solution. The batch studies were carried out using 0.1 g/L La 2 O 3 in a 50 mL Falcon tube contain metal ion solution (10 and 100 mg/L) at 25 • C and pH 6.0. The adsorption isotherms of As(V) on the adsorbent were studied at different temperatures (25,35, and 50 • C) at pH 6.0 and a predetermined equilibrium time of 60 min. The solution pH influence on arsenic adsorption was estimated under pH ranges from 2.0 to 12.0 that were adapted using 0.1 M of HCl or NaOH solution under constant conditions. To evaluate the influence of associate ions on the adsorption efficiency of La 2 O 3 for As(V), NaCl, KCl, CaCl 2, MgCl 2 , Na 2 SO 4 , Na 3 PO 4 , NaNO 3 , and NaHCO 3 salts were used to adapt the ionic concentration (0.05-0.5 mg/L), respectively. The adsorbed amount (q e ) of As(V) was calculated using where the initial and equilibrium concentrations of As(V) (mg/L) are indicated by C 0 and C e , respectively, V is the aqueous phase volume (L), and m is the adsorbent weight (g). To increase the precision and accuracy of the data, duplicates, blanks, and reference standards were used in this study. Figure 1a indicates the FT-IR spectra of the prepared nanomaterial. Figure 1a clearly shows a sharp band at 3576 cm −1 , suggesting the O-H bond vibrates from the water molecules on the external surface of the material. The sharp bands at 1462 and 1318 cm −1 were assigned to the asymmetric stretching mode of the CO 3 2− group. It is highly probable that all CO 3 2− groups occupy identical sites in the crystal lattice, so the lanthanum oxide precursor might be lanthanum carbonate due to the basic properties of La 2 O 3 which can further react with atmospheric CO 2 and water [33,34]. The sharp bands at 498, 672, 846, and 1050 cm −1 were assigned to the stretching vibration of the La-O bond in La 2 O 3 [12,35]. The XRD pattern in Figure 1b [36]. The remaining diffraction peaks showed a small amount of La(OH) 3 in the La 2 O 3 product [23,36]. The crystallite size was calculated by Debye-Scherer's formula, D = kλ/β cosθ, where D is the average crystalline size of the nanorod, λ is the wave length of radiation, β is the full width at a half maximum of the peak at 30.02 • , and θ is Bragg's angle. The resultant mean crystalline size of the nanorods is 19.3 nm. The full-range XPS spectra of La 2 O 3 are displayed in Figure 1c, exhibiting the characteristic La and O1s peaks. The distinguished sharp peaks centered at 838.75 and 853.06 eV ascribed to La 3d 5/2 and 3d 3/2 , respectively, suggest La(III) [37]. Further, the high-resolution XPS of La 3d and O 1s (Figure 1d,e) confirmed the existence of La 2 O 3 . The La3d high-resolution XPS clearly showed that the bonding energy peaks at 832.68 and 849.84 eV are attributed to the corresponding two spin−orbits of 3d 5/2 and 3d 3/2 respectively, and the same is also authenticated from the reported literature [38,39]. Further, the peaks at 836.32 and 853.57 eV can be attributed to the satellite peaks of the 3d 5/2 and 3d 3/2 spin−orbits, respectively [38], while the O 1s spectrum demonstrated a fine deconvolution with the two peaks. The peaks at 528.11 and 531.50 eV are designated to the oxygen anions from metallic oxygen (La−O) [37,38]. Further the SEM-EDX ( Figure S1a) found La and oxygen along with traces of carbon from carbonate around 0.5 k eV due to exposure to the natural atmosphere. These results confirmed the formation of La 2 O 3 with traces of carbonate at the surface, which is authenticated by the reported results [33,34]. Further, the SEM-EDX ( Figure S1b) of La 2 O 3 , which was prepared under N 2 atmosphere, is not shown with authentic carbon peaks, which are responsible for carbonate functions.

Characterization
Metals 2020, 10, x FOR PEER REVIEW 4 of 15 eV can be attributed to the satellite peaks of the 3d5/2 and 3d3/2 spin−orbits, respectively [38], while the O 1s spectrum demonstrated a fine deconvolution with the two peaks. The peaks at 528.11 and 531.50 eV are designated to the oxygen anions from metallic oxygen (La−O) [37,38]. Further the SEM-EDX ( Figure S1a) found La and oxygen along with traces of carbon from carbonate around 0.5 k eV due to exposure to the natural atmosphere. These results confirmed the formation of La2O3 with traces of carbonate at the surface, which is authenticated by the reported results [33,34]. Further, the SEM-EDX ( Figure S1b) of La2O3, which was prepared under N2 atmosphere, is not shown with authentic carbon peaks, which are responsible for carbonate functions.  The morphologies of the La 2 O 3 samples were evaluated using SEM and TEM images. The resultant width of the prepared nanorod was close in agreement with Scherer's crystalline size. A Brunauer-Emmett-Teller (BET) measurement was conducted to find the pore volume, pore diameter, and surface area of the prepared La 2 O 3 nanorods. From the BET results, we found that the surface area of La 2 O 3 nanorods was 18.76 m 2 /g with a pore volume of 0.378 cm 3 /g and diameter of 1.785 nm. The overall microscopic results suggest that the prepared La 2 O 3 is classified as a nano-sized rod-shaped material.
Metals 2020, 10, x FOR PEER REVIEW 5 of 15 The morphologies of the La2O3 samples were evaluated using SEM and TEM images. Figure 2 shows a typical SEM and TEM image of La2O3 nanorods. The SEM images of the prepared compound confirmed the formation of nanoparticles. The nanorod texture from the TEM (Figure 2a) observations revealed rod-like nanostructures. The nanostructures of the synthesized La2O3 exhibit uniform morphology of the rods with a width of 18 nm and length of 160-200 nm. The resultant width of the prepared nanorod was close in agreement with Scherer's crystalline size. A Brunauer-Emmett-Teller (BET) measurement was conducted to find the pore volume, pore diameter, and surface area of the prepared La2O3 nanorods. From the BET results, we found that the surface area of La2O3 nanorods was 18.76 m 2 /g with a pore volume of 0.378 cm 3 /g and diameter of 1.785 nm. The overall microscopic results suggest that the prepared La2O3 is classified as a nano-sized rod-shaped material.   Figure 3a indicates the influence of the solution initial pH on the adsorptive removal of As(V) by the La 2 O 3 nanorods. The uptake of As(V) was rapidly increased from pH 2.0-3.0, reaching a plateau around 99.8% removal at pH 3.0-7.0. As pH is further increased (7.0-11.0), As(V) adsorptive removal slowly decreased. The slow increase in As(V) removal at pH 2.0-3.0 may be attributed to the predominance of As(V) in the molecular state of H 3 AsO 4 below pH 2.3 [40]. The maximum removal efficiency remained constant when As(v) pH increased to 3.0-7.0, possibly due to the more negatively charged As(V) species, leading to high adsorption on the positive surface of the La 2 O 3 nanorods (pH < pHPZC (6.0) ( Figure S2). A rapid decline emerged in the alkali pH 8.0-12.0 due to the presence of As(V) as AsO 4 3− in aqueous media, leading to electrostatic repulsion with the negative surface charge of the La 2 O 3 nanorods (pH > pHPZC (6.0)). However, there is significant adsorption removal under alkali conditions. Hence, the influence of the solution pH on As(V) adsorptive removal may be explained by two mechanisms: (1) electrostatic interaction and (2) surface complexation through ligand exchange [41].  [42]. At pH 8.0-11.0, the significant adsorption of As(V) may be due to the surface complexation of arsenic species through surface functional groups. However, the adsorption removal above pH > 10 was not significant. As shown in Figure 3b, the adsorptive removal of As(V) was efficient (99.30%) even at a low La 2 O 3 dosage (0.1 g/L). However, as the La 2 O 3 concentration increased from 0.1 to 1.0 g/L, the uptake capacity remained constant with an average removal efficiency of 99.59 ± 0.12%.

Adsorption Studies
Metals 2020, 10, x FOR PEER REVIEW 6 of 15 Figure 3a indicates the influence of the solution initial pH on the adsorptive removal of As(V) by the La2O3 nanorods. The uptake of As(V) was rapidly increased from pH 2.0-3.0, reaching a plateau around 99.8% removal at pH 3.0-7.0. As pH is further increased (7.0-11.0), As(V) adsorptive removal slowly decreased. The slow increase in As(V) removal at pH 2.0-3.0 may be attributed to the predominance of As(V) in the molecular state of H3AsO4 below pH 2.3 [40]. The maximum removal efficiency remained constant when As(v) pH increased to 3.0-7.0, possibly due to the more negatively charged As(V) species, leading to high adsorption on the positive surface of the La2O3 nanorods (pH < pHPZC (6.0) ( Figure S2). A rapid decline emerged in the alkali pH 8.0-12.0 due to the presence of As(V) as AsO4 3− in aqueous media, leading to electrostatic repulsion with the negative surface charge of the La2O3 nanorods (pH > pHPZC (6.0)). However, there is significant adsorption removal under alkali conditions. Hence, the influence of the solution pH on As(V) adsorptive removal may be explained by two mechanisms: (1) electrostatic interaction and (2) surface complexation through ligand exchange [41]. As confirmed by FT-IR and XPS, there are active surface functional groups, lanthanum oxide, hydroxides, and traces of nitrate (Figure 1a) on the La2O3 surface. At low pH solution (3.0-7.0), La2O3 nanorods protonated the surface functions and formed positively charged surface functions that could adsorb negatively charged arsenic species (H2AsO4 − and HAsO4 2− ) through electrostatic attraction and surface complexation [42]. At pH 8.0-11.0, the significant adsorption of As(V) may be due to the surface complexation of arsenic species through surface functional groups. However, the adsorption removal above pH>10 was not significant. As shown in Figure 3b, the adsorptive removal of As(V) was efficient (99.30%) even at a low La2O3 dosage (0.1 g/L). However, as the La2O3 concentration increased from 0.1 to 1.0 g/L, the uptake capacity remained constant with an average removal efficiency of 99.59 ± 0.12%.  Adsorption kinetic studies were carried out to determine the removal of As(V) (10-100 mg/L) using the as-prepared La 2 O 3 nanorods (Figure 3b). The adsorptive removal of As(V) on La 2 O 3 increased rapidly as the contact time increased from 0-60 min. After 60 min, increasing the contact time caused the adsorption removal to remain constant. Pseudo first-order (PFO) (Equation (2)) and pseudo second-order (PSO) (Equation (3)) kinetics are used to investigate the adsorption kinetics of As(V) onto La 2 O 3 : PFO kinetic equation : q = q e 1 − 10 −k 1 ×t/2.303 , where the PFO and PSO rate constants are indicated by k 1 and k 2 , respectively. The resultant kinetic model fitting linear plots are shown in Figure S3. The calculated parameters for the adsorption kinetics of As(V) onto La 2 O 3 nanorods at various metal concentrations (10-100 mg/L) are tabulated in Table 1. From the results, the PSO equation was found to be the best-fitting model considering their correlation coefficients R 2 (PSO correlation coefficient) approximately equal to one. This was further confirmed by the close agreement of the theoretical q e (mg/g) and predicted q e (mg/g) of the PSO equation. Thus, the adsorption of As(V) on La 2 O 3 was a rate-limiting PSO reaction. Equilibrium adsorption isotherms are an important parameter for understanding adsorption mechanisms and describing the interaction between the adsorbate and adsorbent. An As(V) adsorption isotherm was obtained by varying the initial As(V) concentration (10-100 mg/L) (Figure 3d) and fitting the experimental data using Langmuir (Equation (4)), Freundlich (Equation (5)), and Temkin (Equation (6)) isotherm models to evaluate the adsorption process mechanism: C e /q e = 1/q m K L + C e /q m , log q e = log K F + n −1 log C e , where the constants q m , K L and n, and K F are predicted from the intercept and slope of the plots of C e /q e vs. C e (Equation (3)), logC e vs. logqe (Equation (4)), and q e vs. logC e (Equation (5)) ( Figure S4), respectively. These adsorption models represent the adsorption equilibrium at interfaces of the adsorbent with metal ions in a solution.
To determine the isotherm model most suitable in describing the As(V) adsorption process, data analysis was conducted using linear fitting isotherm models ( Figure S3) and was considered based on the value of the correlation coefficients (R 2 ) tabulated in Table 2. Langmuir isotherms produced higher and more precise R 2 values (close to 1) than the other tested isotherms. This was further confirmed from Pearson's chi-square (χ 2 ) analysis, which compared all isotherms on the same abscissa and ordinate to evaluate the best fitted model for the equilibrium adsorption isotherm data. The χ 2 is calculated using Equation (7) where q model and q exp (mg/g) are the adsorption capacities determined from the model fitting and the experimental data, respectively. The χ 2 values for each model are shown in Table 2. From Table 2, it can be found that the resulting Pearson's chi-square (χ 2 ) errors have are lower for the Langmuir model fitting than others at three temperatures suggesting the Langmuir isotherm well explains the adsorption equilibrium data. Thus, the results of the isotherm suggest that monolayer adsorption occurred on La 2 O 3 's energetically homogeneous surface. The maximum uptake capacity (q e ) was found to be 260.56 mg/g at pH 6.0 and 25 • C. Further, the adsorption capacity of La 2 O 3 , prepared at N 2 atmosphere, has been found to be 272.12 mg/g. This higher adsorption of La 2 O 3 (at N 2 atmosphere) may be caused by the absence of carbonate on the La 2 O 3 surface ( Figure S1b). Further, the adsorption capacity of La 2 O 3 nanorods for As(V) was comparable or more than the reported hybrid composites (Table 3), though there are similar reports offering comparable or lower As(V) removal efficiencies [25,40,[42][43][44][45] as compared with the present material. Thus, these results prompted the potential of as prepared La 2 O 3 nanorods for arsenic-contaminated wastewater treatment.

Effects of Coexisting Ions
Real environmental water contains several ions that may influence the adsorptive removal of As(V). To assess the influence of competitive ions on As(V) adsorptive removal, 0.05-0.5 mg/L salt solutions of K + , Na + , Mg 2+ , Ca 2+ , CO 3 2− , SO 4 2− , PO 4 3− , and NO 3 − ions, respectively, were used and the results are shown in Figure 4. It was clearly observed from Figure 4, as salt concentration increased from 0.05 to 0.5 mg/L, that the adsorptive removal of As(V) decreased slightly from 99% to 90% and 83% with the respective cations, Mg 2+ and Ca 2+ . These results suggested that the tested cations were not significantly influenced by That PO 4 3− decreased the adsorptive removal of As(V) significantly might be because As(V) and Metals 2020, 10, 1001 9 of 14 PO 4 3− are in the same symmetry (tetrahedral) and compete with each other for the surface sites at the nanocomposite surface [47,48]. Further, the results of the adsorption capacity of La 2 O 3 , prepared at N 2 atmosphere, confirmed that the carbonate species predominately influence the adsorptive removal of As(V) ( Figure S1b).
not significantly influenced by the adsorptive removal of As(V), where 7.7%, 92.7%, 39.5%, and 96.7% of the adsorptive removal of As(V) are shown in the presence of CO3 2− , SO4 2− ,PO4 3− , and NO3 − anions, respectively. The adsorptive removal of As(V) on the adsorbent occurs between As(V) ions and La2O3 ions at the adsorbent interface. K + and Na + exhibit a positive influence on the adsorption affinity of La2O3 for As(V) than the other investigated cations and anions. However, CO3 2− and PO4 3− are significantly influenced by the adsorption affinity of La2O3. The significant effects of CO3 2− on the adsorptive removal of arsenic might be due to an inner-sphere surface complexation [46]. That PO4 3− decreased the adsorptive removal of As(V) significantly might be because As(V) and PO4 3− are in the same symmetry (tetrahedral) and compete with each other for the surface sites at the nanocomposite surface [47,48]. Further, the results of the adsorption capacity of La2O3, prepared at N2 atmosphere, confirmed that the carbonate species predominately influence the adsorptive removal of As(V) ( Figure S1b).

Thermodynamic Parameters
The thermodynamic parameters, enthalpy (∆H 0 ), entropy (∆S 0 ), and Gibbs free energy (∆G 0 ) of As(V) adsorption using La 2 O 3 can be calculated from RT logKc vs. T ( Figure 5). The values of the free energy change (∆G 0 ) are calculated using where the ideal gas constant is indicated by R in 8.314 kJ/mol, and temperature is indicated by T in " • C". Thus, the ∆H 0 and ∆S 0 can be measured from the slope and intercept of the RT logKc vs. T plot ( Figure 5).
where the ideal gas constant is indicated by R in 8.314 kJ/mol, and temperature is indicated by T in "°C". Thus, the ΔH 0 and ΔS 0 can be measured from the slope and intercept of the RT LogKc vs. T plot ( Figure 5). Based on Table 4, the negative ΔG 0 values suggest that the process of As(V) adsorptive removal by La2O3 is spontaneous. The high ΔG 0 at 50 °C confers a favorable sorption at higher temperatures. The positive ΔH 0 suggests that the process of As(V) adsorptive removal by La2O3 is endothermic. This is attributed to the assumption that the exothermic energy of the ions attached to the solid surface is lower than the endothermic energy of dehydration [49]. The positive value of ΔS 0 suggest increased randomness at the solid-liquid interface during the process of As(V) adsorptive removal by La2O3. Therefore, from the overall thermodynamic results, it is suggested that the uptake of As(V) on La2O3 is an endothermic and spontaneous process.

As(V) Adsorption Mechanism Using La2O3
Based on batch adsorption studies, including pH, kinetics, and equilibrium isotherms results, the adsorptive removal of As(V) with La2O3 involves electrostatic interaction and surface complexation. From the pH experiment, high adsorption was obtained between pH 3.0-7.0, suggesting an electrostatic attraction between anionic arsenic species and the positive surface charge Based on Table 4, the negative ∆G 0 values suggest that the process of As(V) adsorptive removal by La 2 O 3 is spontaneous. The high ∆G 0 at 50 • C confers a favorable sorption at higher temperatures. The positive ∆H 0 suggests that the process of As(V) adsorptive removal by La 2 O 3 is endothermic. This is attributed to the assumption that the exothermic energy of the ions attached to the solid surface is lower than the endothermic energy of dehydration [49]. The positive value of ∆S 0 suggest increased randomness at the solid-liquid interface during the process of As(V) adsorptive removal by La 2 O 3 . Therefore, from the overall thermodynamic results, it is suggested that the uptake of As(V) on La 2 O 3 is an endothermic and spontaneous process.

As(V) Adsorption Mechanism Using La 2 O 3
Based on batch adsorption studies, including pH, kinetics, and equilibrium isotherms results, the adsorptive removal of As(V) with La 2 O 3 involves electrostatic interaction and surface complexation. From the pH experiment, high adsorption was obtained between pH 3.0-7.0, suggesting an electrostatic attraction between anionic arsenic species and the positive surface charge of the nanorods ( Figure 6). However, arsenic removal decreased when pH was increased to 7.0-12.0, implying an electrostatic repulsion between the negative surface charge of the adsorbent and anionic species of As(V). Regardless, there is still significant adsorptive removal, revealing that the adsorption process involved chemical surface complexation predominantly at higher pH values (3.0-7.0). Thus, both electrostatic interaction and chemical surface complexation led to the adsorption process of As(V) at pH 3.0-7.0, which is further confirmed by FTIR results (Figure 7). Figure 7 clearly showed -OH (~3576 cm −1 ) and La-O (~498 cm −1 ) peaks altered by the adsorption of As(V), suggesting an adsorption process through surface complexation along with electrostatic interactions. The overall results of the adsorption process suggest that the electrostatic attraction of arsenic species through the protonated surface of La 2 O 3 and surface complexation are predominant in acidic conditions, while electrostatic repulsion and surface complexation of arsenic species with the La 2 O 3 surface are predominant in basic conditions. These representations are clearly shown in Figure 6 for better understanding.
showed -OH (~3576 cm ) and La-O (~498 cm ) peaks altered by the adsorption of As(V), suggesting an adsorption process through surface complexation along with electrostatic interactions. The overall results of the adsorption process suggest that the electrostatic attraction of arsenic species through the protonated surface of La2O3 and surface complexation are predominant in acidic conditions, while electrostatic repulsion and surface complexation of arsenic species with the La2O3 surface are predominant in basic conditions. These representations are clearly shown in Figure 6 for better understanding.

Conclusions
La2O3 was successfully synthesized to remove As(V) from contaminated water. The FT-IR image showed nanoparticles with an elemental composition of La and O as La2O3. Using TEM, the La2O3 nanorod dimensions were measured, obtaining a width of 18 nm and length of 160-200 nm. The batch studies conferred that the adsorptive removal efficiency of La2O3 nanorods for As(V) was sensitive to pH, temperature, and initial metal concentration. As(V) was most effectively removed at pH 3.0-7.0. The kinetic data were well explained by the PSO kinetics and the equilibrium isotherm data agreed with the Langmuir isotherm model fitting. These results allow one to conclude that the uptake efficiency of La2O3 for As(V) was rate-limiting kinetics with monolayer sorption on the homogeneous energetic surface of the nanorods. The maximum adsorptive removal capacity of La2O3 has been found to be 260.56 mg/g at pH 6.0 and 25 °C, which is comparable with or more than the reported

Conclusions
La 2 O 3 was successfully synthesized to remove As(V) from contaminated water. The FT-IR image showed nanoparticles with an elemental composition of La and O as La 2 O 3 . Using TEM, the La 2 O 3 nanorod dimensions were measured, obtaining a width of 18 nm and length of 160-200 nm. The batch studies conferred that the adsorptive removal efficiency of La 2 O 3 nanorods for As(V) was sensitive to pH, temperature, and initial metal concentration. As(V) was most effectively removed at pH 3.0-7.0. The kinetic data were well explained by the PSO kinetics and the equilibrium isotherm data agreed with the Langmuir isotherm model fitting. These results allow one to conclude that the uptake efficiency of La 2 O 3 for As(V) was rate-limiting kinetics with monolayer sorption on the homogeneous energetic surface of the nanorods. The maximum adsorptive removal capacity of La 2 O 3 has been found to be 260.56 mg/g at pH 6.0 and 25 • C, which is comparable with or more than the reported amounts of some hybrid materials. Therefore, La 2 O 3 is a potential material for As-contaminated wastewater treatment.