Article pubs.acs.org/est
Gold Nanoparticle−Aluminum Oxide Adsorbent for Efficient Removal of Mercury Species from Natural Waters Sut-I Lo,† Po-Cheng Chen,† Chih-Ching Huang,*,‡,§ and Huan-Tsung Chang*,† †
Department of Chemistry, National Taiwan University, Taipei, Taiwan Institute of Bioscience and Biotechnology and §Center for Marine Bioenvironment and Biotechnology (CMBB), National Taiwan Ocean University, Keelung, Taiwan
‡
S Supporting Information *
ABSTRACT: We report a new adsorbent for removal of mercury species. By mixing Au nanoparticles (NPs) 13 nm in diameter with aluminum oxide (Al2O3) particles 50−200 μm in diameter, Au NP− Al2O3 adsorbents are easily prepared. Three adsorbents, Al2O3, Au NPs, and Au NP−Al2O3, were tested for removal of mercury species [Hg2+, methylmercury (MeHg+), ethylmercury (EtHg+), and phenylmercury (PhHg+)]. The Au NP adsorbent has a higher binding affinity (dissociation constant; Kd = 0.3 nM) for Hg2+ ions than the Al2O3 adsorbent (Kd = 52.9 nM). The Au NP−Al2O3 adsorbent has a higher affinity for mercury species and other tested metal ions than the Al2O3 and Au NP adsorbents. The Au NP−Al2O3 adsorbent provides a synergic effect and, thus, is effective for removal of most tested metal ions and organic mercury species. After preconcentration of mercury ions by an Au NP−Al2O3 adsorbent, analysis of mercury ions down to the subppq level in aqueous solution was performed by inductively coupled plasma mass spectrometry (ICP-MS). The Au NP−Al2O3 adsorbent allows effective removal of mercury species spiked in lake water, groundwater, and seawater with efficiencies greater than 97%. We also used Al2O3 and Au NP−Al2O3 adsorbents sequentially for selectively removing Hg2+ and MeHg+ ions from water. The low-cost, effective, and stable Au NP−Al2O3 adsorbent shows great potential for economical removal of various mercury species.
1. INTRODUCTION Heavy-metal pollution is an important environmental concern because it can cause serious human health problems.1−7 For example, various mercury species can cause diseases such as Alzheimer’s-like dementia, deafness, loss of muscle coordination, and memory loss.8−11 Organic mercury species, including methylmercury (MeHg+), dimethylmercury (Me2Hg+), ethylmercury (EtHg+), and phenylmercury (PhHg+), are more toxic to living systems than inorganic species.12−14 The US Environmental Protection Agency has set the maximum level of mercury contamination in drinking water at 2 ppb (10 nM).15 Owing to its high toxicity and bioaccumulation factor of up to 106 in the food chain, the monitoring and removal of mercury contamination in natural water or wastewater generated by various industries, such as metal mining and metal finishing, are very important.16−23 Pristine natural waters (e.g., ocean water, lake water, freshwater, and river water) usually contain Hg in the low ppt range, whereas contaminated natural waters reportedly contain as much as several ppb.24−26 Traditional techniques for removal of mercury species from water include ion exchange, amalgamation, electrodeposition, chemical precipitation, reverse osmosis, photochemical methods, flotation, mechanical filtration, membrane separation, and selective liquid−liquid extraction.27−39 However, most of these methods are ineffective in removing certain mercury species and/or are not costeffective. Thus, in the last two decades, strategies that use © 2012 American Chemical Society
solid-phase adsorbents having greater affinity and capacity for mercury species have been investigated. Adsorbents such as 2aminothiazole, dimethyl sulfoxide, dithizone, 2-mercapto benzimidazole, 6-mercaptopurine, and thiosemicarbazide are commonly used for removing mercury ions from natural waters via strong Hg−S bonding.40−44 However, tedious, complicated processes are required to prepare these adsorbents, and thus, they are typically costly. For the past several years, attempts have been made to remove mercury species using nanomaterials such as silver, gold, alumina, selenium, ferrous oxide, manganese dioxide, and polymers.45−56 These nanomaterials have a high surface-area-to-volume ratio and are easily anchored onto solid supports and conjugated with recognition elements for high affinity toward mercury species. Among them, gold nanoparticles (Au NPs) are advantageous because Au has a high affinity for mercury and forms Au−Hg amalgams.53−56 Although unmodified gold-substrate-based collectors have been used to enrich elemental mercury (Hg0) and mercury ions (Hg2+ and MeHg+), their selectivity for different mercury species remains a challenge.55 In this paper, we report a new adsorbent Au NP−Al2O3 that was prepared by mixing Au NPs 13 nm in diameter and Received: Revised: Accepted: Published: 2724
October 17, 2011 December 25, 2011 February 6, 2012 February 6, 2012 dx.doi.org/10.1021/es203678v | Environ. Sci. Technol. 2012, 46, 2724−2730
Environmental Science & Technology
Article
aluminum oxide particles 50−200 μm in diameter. Selective removal of organic and inorganic mercury species was achieved by controlling the amount of Au NPs in the Au NP−Al2O3 adsorbent. An inductively coupled plasma mass spectroscopy (ICP-MS) technique using this adsorbent was applied to analysis of various organic mercury species in water samples; the technique offers the advantages of sensitivity, selectivity, and simplicity.
They were then centrifuged at a relative centrifugal force (RCF) of 25,000g (Au NPs adsorbents) or 500g (Al2O3 or Au NP−Al2O3 adsorbents) for 10 min. The supernatants were then collected and acidified with 2% HNO3 before the metal ion concentrations were determined by ICP-MS. These processes were also applied for removing the metal ions and organic mercury species in solution. For selective removal of Hg2+ and MeHg+, the Al2O3 adsorbent (0.1 g) was first separately added to mixtures (1 mL) of Hg2+ and MeHg+ having various concentrations ([Hg2+]/ [MeHg+] = 2 μM/1 μM, 1 μM/1 μM, and 1 μM/2 μM) to adsorb inorganic mercury. After centrifugation (RCF 500g; 10 min), the Au NP−Al2O3 adsorbent (0.1 g) was added to the supernatant to adsorb the remaining MeHg+. The mixture was then subjected to centrifugation (RCF 500g; 10 min). Both Al2O3 and Au NP−Al2O3 pellets were then treated separately with HCl solution (5%, 1 mL) to desorb mercury ions from their surfaces. Then, the solutions were analyzed separately by ICP-MS. 2.5. Concentration of Mercury Ions by Au NP−Al2O3 Adsorbents. Hg2+, MeHg+, EtHg+, or PhHg+ (1.0 pM−5 nM) was adsorbed by Au NP−Al2O3 (0.01 g) in 50 mL of Trisborate (pH 7.0) solution for 30 min. They were then centrifuged at RCF of 500g for 10 min. The supernatants (49.5 mL) were then removed, and the resulted pellet solutions were treated separately with HCl solution (10%, 0.5 mL) to desorb mercury ions from their surfaces. The concentration of desorbed mercury ions were determined by ICP-MS. 2.6. Column Adsorption of Au NP−Al2O3 Adsorbents. In the column adsorption mode, the Au NP−Al2O3 adsorbent (0.5 g) that had been dispersed in ultrapure water was packed in a column (0.5 cm in diameter). Aliquots (50 mL) of solutions containing mercury species (0.1 μM each) were separately passed through the column at a flow rate of 2.5 mL/min. The unadsorbed mercury species were collected and acidified with 2% HNO3 before ICP-MS analysis. Aliquots of HCl solution (5%, 10 mL) were used to elute the adsorbed mercury species at a flow rate of 1.0 mL/min. The eluates were collected and acidified with HNO3 (final concentration 2%) before ICP-MS analysis. Between each run, the column was equilibrated with 50 mL of ultrapure water. 2.7. Analyses of Mercury Ions in Natural Waters. For simplicity, we used batch adsorption to validate the practicality of this approach for analyses of natural waters. Inorganic and organic mercury species (0.5 mL, 2.0 μM) were spiked separately to 0.5 mL in lake water, groundwater, and seawater. Aliquots of the Au NP−Al2O3 adsorbent (0.01 g) were added separately to the spiked samples (1.0 mL). The mixtures were then equilibrated and analyzed according to the processes described above. After centrifugation (RCF 500g; 10 min), HCl solutions (1 mL, 5%) were added separately to the pellets. All solutions were acidified again with 2% HNO3 before ICP-MS analysis.
2. EXPERIMENTAL METHODS 2.1. Materials. Tris(hydroxymethyl)aminomethane (Tris), boric acid, trisodium citrate, and all metal salts used in this study were obtained from Aldrich (Milwaukee, WI, USA). Hydrogen tetrachloroaurate(III) trihydrate and neutral activated Al2O3 (50−200 μm; 135−165 m2 g−1) were purchased from Acros (Geel, Belgium). Milli-Q ultrapure water was used for all experiments. Tris-borate (50 mM) buffer prepared from Tris (50 mM) was adjusted to pH 7.0 with boric acid. Standard metal ion solutions (100 mM) except for solutions of organic mercury ions were prepared in HNO3 (100 mM). Phenylmercury (PhHg+; 100 mM) was prepared in dimethyl sulfoxide. Methylmercury (MeHg+; 100 mM) and ethylmercury (EtHg+; 100 mM) ions were prepared in ethanol. All standard solutions were subjected to serial dilution with ultrapure water to obtain standard solutions with the required concentrations. 2.2. Preparation and Characterization of Au NPs. Spherical Au NPs (13.3 nm) were prepared by a 4.0 mMcitrate-mediated reduction of 1.0 mM HAuCl4.57−59 Their sizes were verified by an H7100 transmission electron microscope (TEM; Hitachi High-Technologies Corporation, Tokyo, Japan); Au NPs appeared to be nearly monodisperse with an average size of 13.3 ± 1.2 nm. A Cintra 10e double-beam UV− vis spectrophotometer (GBC, Victoria, Australia) was used to measure the extinctions of Au NP solutions. The concentration of the 15 nM Au NPs was determined by Beer’s law using an extinction coefficient of 2.06 × 108 M−1 cm−1 at 520 nm for the 13 nm Au NPs. The zeta potentials (ξ) of Au NPs were measured using a Zetasizer 3000HS analyzer (Malvern Instruments, Malvern, UK). 2.3. Preparation and Characterization of an Au NP− Al2O3 Adsorbent. Au NP−Al2O3 adsorbents were prepared by soaking 5 g of neutral activated Al2O3 in 50 mL of Au NPs (15 nM) at room temperature for 2 h. After the supernatant was decanted, the Au NP−Al2O3 adsorbents were washed twice with ultrapure water. After most of the water was removed, the adsorbents were dried in an oven at 110 °C. The dry adsorbents (0.01 g) were then redispersed in Tris-borate buffer (1 mL, 5 mM, pH 7.0). The solution was then subjected to UV−vis extinction measurement. Solutions containing Au NP− Al2O3 adsorbents and one of the mercury species (0−50 μM) were also subjected to UV−vis extinction measurements. To determine the amount of Au NPs in Al2O3 by ICP-MS, they were dissolved in 2% HNO3. The signals (m/z 197) of Au ions against the concentration of (spiked) standard Au ions were used to establish linear plots. 2.4. Batch Adsorption of Au NP−Al2O3 Adsorbents. Batch adsorption was conducted by adding aliquots of Au NPs, Al2O3, and Au NP−Al2O3 adsorbents separately to solutions of Tris-borate buffer (1 mL, 5 mM, pH 7.0) containing one of the tested metal ions (Cd2+, Co2+, Cr3+, Ni2+, and Pb2+) or mercury species (MeHg+, EtHg+, PhHg+, and Hg2+) at various concentrations (0−2000 μM). These solutions were subjected to gentle shaking for 30 min at 50 rpm at room temperature.
3. RESULTS AND DISCUSSION 3.1. Characterization of Au NP−Al2O3 Adsorbents. The citrate-stabilized Au NPs (13 nm) were adsorbed on Al2O3 (50−200 μm) mainly through electrostatic interactions. Al2O3 particles were chosen as supports for Au NPs because they are inexpensive and have been used to concentrate trace metal ions.60−63 The unadsorbed Au NPs were easily separated from the as-prepared Au NP−Al2O 3 adsorbent in Tris-borate buffer (1 mL, 5 mM, pH 7.0) through gravitation; the former were well dispersed, whereas the latter were precipitated. 2725
dx.doi.org/10.1021/es203678v | Environ. Sci. Technol. 2012, 46, 2724−2730
Environmental Science & Technology
Article
The photograph in Figure 1A clearly shows the adsorption of Au NPs (rose red) on Al2O3 particles (white powder) in the Au
Figure 1. (A) Photographs of (a) Al2O3 (0.2 g) and (b) Au NP−Al2O3 (0.2 g), (B) SEM image of the Au NP−Al2O3 adsorbent and (C) UV− vis extinction spectra of (a) citrate-capped Au NPs (1.5 nM) and (b) Au NP−Al2O3 (0.01 g/mL) in 5 mM Tris-borate (pH 7.0). The two solutions have different Au NP concentrations; thus, the adsorbance values also differ. The extinction in (C) is plotted in arbitrary units (a.u.).
NP−Al2O3 adsorbent (pink). A scanning electron microscope (SEM) image of Au NP−Al2O3 (Figure 1B) confirms that Au NPs adsorbed on Al2O3 surfaces did not form aggregates. The Au NP and Au NP−Al2O3 adsorbents in solution both exhibited unique surface plasmon resonance (SPR) absorption bands at 520 nm (Figure 1C), further confirming that the Au NPs were well dispersed both in solution and on the Al2O3 surfaces. From the ICP-MS data, we estimated that there were 670 μg of Au NPs per 1 g of Al2O3 particles. 3.2. Affinity of Au NP and Al2O3 Adsorbents for Various Metal Species. We first used Au NPs as an adsorbent for extracting various metal ions and mercury species. The SPR absorption band of the Au NPs differs slightly in the absence and presence of various mercury species (1.0 μM). TEM images (see Figure S1 of the Supporting Information) reveal that, in the absence and presence of mercury species (1.0 μM), Au NPs have almost the same size. Note that, in the presence of mercury species such as Hg2+ ions, a submonolayer of Hg reportedly forms on the surface of the Au NPs.64,65 When the mercury species concentrations were increased from 0 to 50 μM, the SPR absorption bands underwent various shifts (Figure 2A). At low concentrations (10 μM), the SPR absorption bands of the Au NPs remained almost the same for the solutions containing Hg2+ and MeHg+ ions. In contrast, EtHg+ and PhHg+ induced greater aggregation of Au NPs at concentrations higher than 50 and 5 μM, respectively. The values of Ex650/520 were plotted against the mercury species concentrations to express the molar ratio of aggregated to dispersed Au NPs (Figure 2B). The extinction coefficients at 520 (Ex520)
Figure 2. (A) UV−vis extinction spectra of Au NPs (1.5 nM) in the absence and presence of 0.25, 0.50, 1.0, 5.0, 10, 25, and 50 μM (a) Hg2+ ions, (b) MeHg+ ions, (c) EtHg+ ions, and (d) PhHg+ ions, respectively. (B) Ex650/520 values of Au NPs (1.5 nM) upon addition of 0−50 μM Hg2+, MeHg+, EtHg+, or PhHg+ ions. Other conditions are the same as those described in Figure 1.
and 650 nm (Ex650) are related to the quantities of dispersed and aggregated Au NPs, respectively.67−70 With increasing hydrophobicity of the mercury species, the extent of Au NP aggregation increased in the order of PhHg+ ≫ EtHg+ > MeHg+ > Hg2+ (see Figure S2 of the Supporting Information). As a control, other metal ions, including Cd2+, Co2+, Cr3+, Ni2+, and Pb2+ (10 μM each), were examined and did not induce any shift in the SPR absorption band (not shown). When Al2O3 particles (0.1 g/mL) were used as the adsorbent, most of the metal ions (1.0 μM) and Hg2+ (1.0 μM) were quantitatively removed (>99%) from the aqueous solutions. However, the organic mercury species MeHg+, EtHg+, and PhHg+ were not removed effectively (
