ARTICLE pubs.acs.org/IECR
As(III) and As(V) Adsorption by Hydrous Zirconium Oxide Nanoparticles Synthesized by a Hydrothermal Process Followed with Heat Treatment Cui Hang,† Qi Li,*,† Shian Gao,† and Jian Ku Shang†,‡ †
Materials Center for Water Purification, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, People’s Republic of China ‡ Department of Materials Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States
bS Supporting Information ABSTRACT: Hydrous zirconium oxide (ZrO2 3 xH2O) were synthesized by a low-cost hydrothermal process followed with heat treatment. ZrO2 3 xH2O nanoparticles ranged from 6 nm to 10 nm and formed highly porous aggregates, resulting in a large surface area of 161.8 m2 g1. The batch tests on the laboratory water samples demonstrated a very high degree of As(III) and As(V) removal by ZrO2 3 xH2O nanoparticles. The adsorption mechanism study demonstrated that both arsenic species form inner-sphere surface complexes on the surface of ZrO2 3 xH2O nanoparticles. Higher arsenic removal effect of these ZrO2 3 xH2O nanoparticles were demonstrated, compared with commercially available Al2O3 and TiO2 nanoparticles. Ionic strength and competing ion effects on the arsenic adsorption of these ZrO2 3 xH2O nanoparticles were also studied. Testing with natural lake water confirmed the effectiveness of ZrO2 3 xH2O nanoparticles in removing arsenic species from natural water, and the immobilization of ZrO2•xH2O nanoparticles on glass fiber cloth minimized the dispersion of nanoparticles into the treated body of water. The high adsorption capacity of ZrO2 3 xH2O nanoparticles is shown to result from the strong inner-sphere surface complexing promoted by the high surface area, large pore volume, and surface hydroxyl groups of zirconium oxide nanoparticles.
1. INTRODUCTION Arsenic contamination in natural water is a widespread problem in many regions of the world.1 Chronic exposure to arsenic could lead to liver, lung, kidney, bladder, and skin cancers,2 cause cardio vascular system problems,3 and affect the mental development of children.4 Thus, the removal of arsenic contamination from water supply is critical to the health and everyday life quality of people living near bodies of water with arsenic contamination. In order to reduce the health risk to human beings, the U.S. Environmental Protection Agency (USEPA) revised the Maximum Contaminant Level (MCL) for arsenic in drinking water from 50 μg L1 to 10 μg L1 in 2001.5 In a natural water environment, most arsenic pollution exists as two major inorganic species, namely, As(III) (arsenite) and As(V) (arsenate). Among the various established techniques for arsenic removal from contaminated water sources,6 the adsorption is believed to be simple and cost-effective, especially when the arsenic concentration is low as that in the natural water environment.7 As(III) exists mainly as nonionic H3AsO3 in natural water, so it usually does not have a high affinity to adsorbent surfaces, compared with charged As(V).8 Thus, it could not be effectively removed until a pretreatment was adopted by oxidizing it to As(V) and/or adjusting the pH value of water.6,911 However, for the treatment of large natural bodies of water contaminated with arsenic, it is difficult or even impossible to conduct such a pretreatment and after-treatment pH adjustment. Novel adsorbents should be developed, which could remove both As(III) and As(V) without the requirement of pretreatment under near neutral pH conditions to largely simplify the treatment process and reduce the treatment cost. r 2011 American Chemical Society
Many natural materials have been studied extensively for the removal of arsenic, because of their low costs.12,13 However, their adsorption kinetics is usually slow and their capability is low. There might also be impurity and stability issues with these natural materials, which would cause secondary pollution to the treated water.8,14 Recently, synthesized metal oxides at the nanosize level—especially aluminum, iron, titanium and cupricbased oxides—have demonstrated superior performance for arsenic adsorption, because of their large surface areas and preferred surface properties.7,8,1519 Zirconium-based oxides are stable, nontoxic, and not dissolvable in water, and they could be an attractive choice for drinking water purification. Reports of using zirconium-based oxides for arsenic removal are available in the literature.2027 However, only As(V) removal was examined in most of them. Until now, no report had been made on the removal of more mobilized/toxic As(III) from water by pure nanostructured ZrO2, and the arsenic adsorption mechanism by ZrO2 is absent. In this study, we synthesized ZrO2 3 xH2O nanoparticles by a hydrothermal process followed with heat treatment. We observed that these nanoparticles had strong arsenic adsorption effects for both As(III) and As(V) in lab-prepared and natural water samples, and determined that the arsenic adsorption followed the inner-sphere complexing mechanism. These ZrO2 3 xH2O Received: October 2, 2011 Accepted: December 4, 2011 Revised: December 3, 2011 Published: December 05, 2011 353
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1 h at 450 °C. ZrO2 3 xH2O nanoparticles were also immobilized onto glass fiber cloth by dispersing 2 g of ZrO2 3 xH2O nanoparticles into 20 mL of deionized water with stirring and immersing a glass fiber cloth (10 cm 10 cm, 5.56 g) in the suspension for 2 h. The weight percentage of ZrO2 3 xH2O nanoparticles on the treated glass fiber cloth was ∼29%, which was chosen because it provided a relatively high material loading while the loaded nanoparticles had strong adhesion to the glass fiber cloth for a stable adsorbent. 2.3. ZrO2 3 xH2O Nanoparticle Characterization. X-ray diffraction (XRD) analysis was conducted in a D/MAX-2004-X-ray powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu (0.15418 nm) radiation at 56 kV and 182 mA. BrunauerEmmettTeller (BET) measurement was conducted by N2 adsorptiondesorption isotherm with an Autosorb-1 Series Surface Area and Pore Size Analyzers (Quantachrome Instruments, Boynton Beach, FL). Both transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM) were used to examine the powder morphology. TEM analysis was conducted on a JEOL Model 2010 TEM microscope (JEOL, Ltd., Tokyo, Japan) operated at 200 kV, with point-to-point resolution of 0.28 nm, and TEM samples were made by dispersing a thin film of ZrO2 3 xH2O powder on a Cu grid precoated with a thin and flat carbon film. A SUPRA35 FESEM microscope (ZEISS, Germany) was used in the SEM imaging. The SEM sample was made by dispersing ZrO2 3 xH2O nanoparticles in ethyl alcohol, applying a drop of the dispersion on a conductive carbon tape, and drying in air. Prior to imaging, the sample was sputtered with gold for 20 s (Emitech K575 Sputter Coater, Emitech Ltd., Ashford Kent, UK). The zetapotential curve of ZrO2 3 xH2O nanoparticles dispersed in water was measured at various pH values with the electrophoretic spectroscopy (Model JS84H, Shanghai Zhongchen Digital Instrument Co., Ltd., Shanghai, PRC). To determine the isoelectric point (IEP), all electrophoretic mobility (EM) experiments were conducted at room temperature and under N2 atmosphere to eliminate CO2 from the system. The background electrolyte was NaCl (0.1 M). Fourier transform infrared spectroscopy (FTIR) (Bruker TENSOR 27, MCT detector, the resolution of 4 cm1) was used to measure the infrared spectra of these powders. To ensure quantitative ZrOH analysis, the samples were mixed with KBr at a fixed ratio (1%). A SETSYS Evolution18 thermal analyzer (SETARAM, France) was used to conduct the thermogravimetric analysis and differential scanning calorimeter analysis (TG-DSC). The heating-up speed was 5 °C/min, and the temperature range was from room temperature to 800 °C. 2.4. Arsenic Removal from Water Samples. The arsenic adsorption experiments were carried out at ∼25 °C. All suspensions were sealed and magnetically stirred during the adsorption process. Since ZrO2 3 xH2O is not known to be a strong oxidant, no obvious oxidation of As(III) to As(V) was observed under our experimental conditions (see Figure S1 in the Supporting Information). After recovering the adsorbent via centrifugation at 12 000 rpm, one drop of concentrated HCl was added into the clear solution to preserve its arsenic species. The concentrations of arsenic species in water samples were analyzed using an atomic fluorescence spectrophotometer (AFS-9800, Beijing KeChuangHaiGuang Instrument Inc., Beijing, PRC). Each arsenic concentration measurement result was the average of the data from three experiments. Detailed information on experiment conditions could be found in the Supporting Information.
nanoparticles successfully removed most of the arsenic contamination from natural water samples of Lake Yangzonghai to meet the USEPA standard for arsenic in drinking water. To facilitate the removal of adsorbent from the aqueous environment and avoid the dispersion of nanoparticles into the environment, we also immobilized ZrO2 3 xH2O nanoparticles onto a glass fiber cloth, which demonstrated an even better arsenic removal performance than dispersed ZrO2 3 xH2O nanoparticles in water.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Material. ZrO2 3 xH2O nanoparticles were prepared by a hydrothermal process. Zirconium oxychloride octahydrate (ZrOCl2 3 8H2O, 99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, PRC) was used in the hydrothermal process as the raw material. Deionized water (DI) was used as the solvent in the hydrothermal process. Aqueous ammonia (25 wt %, Tianjin Kermel Chemical Reagents Development Center, Tianjin, PRC) was used as the precipitation agent in the hydrothermal process. Sodium metaarsenite (NaAsO2, 95%, Shanghai Tian Ji Chemical Institute, Shanghai, PRC) and sodium hydrogen arsenate heptahydrate (Na2HAsO4 3 7H2O, 98%, Alfa Aesar, Shore Road Heysham, Loncs, England) was used to prepare As(III) and As(V) stock solutions. Concentrated hydrochloric acid (HCl, 32%38%, Tianda Chemical Reagents Factory, Tianjin, PRC) and sodium hydroxide (NaOH, 98%, Tianda Chemical Reagents Factory, Tianjin, PRC) was used to adjust solution to neutral environment in the isotherm study. Commercially available titanium dioxide (TiO2, 99%, particle size of 50100 nm, surface area of 14 m2 g1, Shenyang Shen Yi Fine Chemicals Co., Ltd., Shenyang, PRC) and aluminum oxide (Al2O3, particle size of 3050 nm, surface area of 148 m2 g1, Shanghai Wu Si Farm Chemical Agents Co., Shanghai, PRC) powders were used to compare with our ZrO2 3 xH2O nanoparticles on the As(III) removal effect. Sodium chloride (NaCl, 99.5%, Shenyang Chemical Reagents Factory, Shenyang, PRC), sodium nitrate (NaNO3, 99%, Tianjin Chemical Industry Co. Ltd., Tianjin, PRC), anhydrous sodium sulfate (Na2SO4, 99%, Tianjin Da Mao Chemical Reagents Factory, Tianjin, PRC), and sodium fluoride (NaF, 98%, Shenyang Xin Xi Reagents Factory, Shenyang, PRC) were used in the experiments on the competing ion effect. Sodium chloride was also used in the experiments on the ionic strength effect. 2.2. ZrO2 3 xH2O Nanoparticle Synthesis. ZrO2 3 xH2O nanoparticles were prepared by a simple and robust hydrothermal process followed with heat treatment. The ZrO2 3 xH2O precursor solution was prepared at room temperature by the following hydrothermal process. First, ZrOCl2 3 8H2O (0.035 mol) was dissolved in 20 mL of deionized water. The solution was stirred magnetically for 5 min, and then aqueous ammonia (25 wt %) was slowly added with rigorous stirring until the pH of the mixture was 11. The above mixture was stirred for another 5 min, and put into a polytetrafluoroethylene (PTFE) container. The container was then sealed in a stainless-steel pressure vessel, and the pressure vessel was placed in a thermostatic oven maintained at 150 °C for 3 h. The pressure in the pressure vessel was ∼1.5 MPa. The mixture was then cooled to room temperature, and the resultant precipitate was filtered and washed repeatedly with deionized water until neutral pH and no Cl ion could be detected by AgNO3 precipitation. The precipitate was dried at 110 °C for 8 h, crushed into fine powders, and calcinated in air for 354
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nanoparticles obtained by the hydrothermal process followed with the heat treatment at 450 °C for 1 h is demonstrated in Figure 1a. Most diffraction peaks belong to the tetragonal phase, and only a few diffraction peaks come from the monoclinic phase. This observation indicates that the hydrothermal process used is beneficial to the crystal phase transition of ZrO2 3 xH2O, compared with the solid-state reaction to produce ZrO2 3 xH2O, which usually requires an elevated heat treatment temperature at ∼1200 °C for the phase transition from monoclinic phase to tetragonal phase. This effect is due to the role of alkalinity in the hydrothermal process.28 The crystallite size of the tetragonal phase is ∼7.7 nm, obtained from the strongest XRD peak (at 2θ ≈ 30° for tetragonal phase) by the Scherrer’s formula:29 D ¼ 0:9λ=βcos θ
ð1Þ
where λ is the average wavelength of the X-ray radiation, β is the line-width at half-maximum peak position, and θ is the diffracting angle. Figure 1b and 1c show the HRTEM images of ZrO 3 xH2O nanoparticles. They are nanosized particles with nonuniform shapes, and the average particle size is ∼610 nm (Figure 1b). The high-magnification image of one ZrO2 3 xH2O nanoparticle verifies its crystalline structure (Figure 1c). One set of lattice planes could be clearly observed with a d-spacing at ∼0.294 nm, corresponding to the (111) plane of the crystalline tetragonal phase. Figure 1d shows the FESEM image of ZrO2 3 xH2O nanoparticles. Nanoparticles are aggregated into a highly porous structure, which is beneficial for achieving relatively high adsorption efficiency, because it provides access to the entire adsorbent contact area for As(III) and As(V) in water, and it is also beneficial to their removal from the aqueous environment after the treatment. 3.2. Surface Properties of ZrO2 3 xH2O Nanoparticles. Figure 2a shows the BET measurement curve of these ZrO2 3 xH2O nanoparticles. The BET surface specific area of these ZrO2 3 xH2O nanoparticles was found to be 161.8 m2/g, corresponding to an average particle diameter of ∼7 nm, close to the calculated tetragonal phase crystallite size and the TEM observation. The pore size distribution of these nanoparticles is demonstrated in Figure 2b, which suggests that most pores are mesopores. The average pore diameter was determined to be 10.65 nm, which should reflect the interparticle porosity in the ZrO2 3 xH2O nanoparticle aggregates. The specific pore volume was measured to be 0.43 cm3 g1. Thus, these ZrO2 3 xH2O nanoparticles created by the hydrothermal process have a relatively large surface area and pore volume, which is beneficial for its adsorption capability. In the recent adsorption mechanism study of arsenic on TiO2 by Pena et al.,17 they suggested that the presence of high-affinity surface hydroxyl groups makes it an effective adsorbent. Figure 2c demonstrates that the isoelectric point (IEP) of these ZrO2 3 xH2O nanoparticles is located at pH ∼3. In the near neutral or weak alkali environment, they are negatively charged, indicating the existence of surface hydroxyl groups. The IEP value of these ZrO2 3 xH2O nanoparticles is lower than most reported ZrO2 IEP values (pH 411) in the literature,30,31 suggesting that these ZrO2 3 xH2O nanoparticles have specific surface electric properties. Thus, the large surface area and the existence of surface hydroxyl groups on these ZrO2 3 xH2O nanoparticles (see more evidence from FTIR studies below) indicate that they would be effective in removing As(III) in water by adsorption. Figure 2d shows the TG-DSC curves of ZrO2 3 xH2O nanoparticles, which
Figure 1. (a) X-ray diffraction (XRD) pattern, (b and c) high-resolution transmission electron microscopy (HRTEM) images, and (d) fieldemission scanning electron microscopy (FESEM) image of ZrO2 3 xH2O nanoparticles.
3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology of ZrO2•xH2O Nanoparticles. The X-ray diffraction pattern of ZrO2 3 xH2O 355
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Figure 2. (a) BET curve, (b) mesopore size distribution, (c) zetapotential measurement, and (d) TG-DSC curves of ZrO2 3 xH2O nanoparticles.
Figure 3. Adsorption kinetics of arsenic on ZrO2 3 xH2O nanoparticles: (a) initial As(III) concentration ≈ 0.105 mg L1, (b) initial As(V) concentration ≈ 0.081 mg L1, (c) initial As(III) concentration ≈ 1 mg L1, (d) initial As(V) concentration ≈ 0.95 mg L1.
experienced an obvious weight loss (∼3.7%) and could be roughly divided into three stages. During the first stage (from the room temperature to 150 °C), a weight loss of 2.8% was observed, which could be attributed to the elimination of physically absorbed water.32,33 The endothermic peak of physically absorbed water is at the 107 °C. During the second stage (from 150 °C to 450 °C), a weight loss of 0.9% was observed, which could be contributed to the loss of surface hydroxyl groups.32,33 From the first-stage weight loss and the second-stage weight loss, the average value of x for H2O in these ZrO2 3 xH2O nanoparticles could be determined
to be x ≈ 0.27. The third stage is from 450 °C to 800 °C, over which the weight remains constant. In this temperature range, there is no exothermic peak, corresponding to the structural conversion. 3.3. Kinetic Studies on Arsenic Adsorption by ZrO2 3 xH2O Nanoparticles. Figure 3 demonstrates the decrease of both As(III) and As(V) concentration in laboratory-prepared water samples with the increase of the treatment time. From Figure 3, it 356
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is clear that, with increased adsorbent loading, the arsenic removal speed increases and the final arsenic concentration in the treated water samples decreases for both As(III) and As(V). Figure 3a summarizes the As(III) concentration change with the time increase when the initial As(III) concentration is ∼0.105 mg L1. It demonstrates that As(III) could be adsorbed onto ZrO2 3 xH2O nanoparticles in a short time. For example, ∼66% of the As(III) in the solution was adsorbed in just 10 min when the ZrO2 3 xH2O loading concentration was just 0.15 g L1. The equilibrium As(III) concentration was 6.5 μg L1 after the treatment, which meets the USEPA standard for arsenic in drinking water. Figure 3b summarizes the change in As(V) concentration with increasing time when the initial As(V) concentration is 0.0813 mg L1. Most of the As(V) could also be adsorbed onto ZrO2 3 xH2O nanoparticles with a relatively low loading concentration. For example, ∼88.2% of the As(V) in the solution was adsorbed when the loading concentration was just 0.02 g L1, and the equilibrium As(V) concentration was 9.6 μg L1 after the treatment. For a relatively high arsenic concentration (∼1.0 mg L1), ZrO2 3 xH2O nanoparticles also were also effective. When the ZrO2 3 xH2O loading concentration was just 0.5 g L1, the equilibrium As(III) concentration could be reduced from ∼1.0 mg L1 initially to 99% (see Figure 3c). When the ZrO2 3 xH2O loading concentration was just 0.3 g L1, the equilibrium As(V) concentration could also be reduced from ∼0.95 mg L1 to
