Manganese Doping of Magnetic Iron Oxide Nanoparticles - American

Feb 13, 2012 - (Sigma-Aldrich, St. Louis, MO), MnO2 resin, 75−150 μm (Eichrom. Technologies, Lisle ... 0. 27 ± 8. 1 mmol doped Fe3O4. 47. 4.5. 31 ...
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Manganese Doping of Magnetic Iron Oxide Nanoparticles: Tailoring Surface Reactivity for a Regenerable Heavy Metal Sorbent Cynthia L. Warner, Wilaiwan Chouyyok, Katherine E. Mackie, Doinita Neiner, Laxmikant V. Saraf, Timothy C. Droubay, Marvin G. Warner, and R. Shane Addleman* Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: A method for tuning the analyte affinity of magnetic, inorganic nanostructured sorbents for heavy metal contaminants is described. The manganese-doped iron oxide nanoparticle sorbents have a remarkably high affinity compared to the precursor material. Sorbent affinity can be tuned toward an analyte of interest simply by adjustment of the dopant quantity. The results show that following the Mn doping process there is a large increase in affinity and capacity for heavy metals (i.e., Co, Ni, Zn, As, Ag, Cd, Hg, and Tl). Capacity measurements were carried out for the removal of cadmium from river water and showed significantly higher loading than the relevant commercial sorbents tested for comparison. The reduction in Cd concentration from 100 ppb spiked river water to 1 ppb (less than the EPA drinking water limit of 5 ppb for Cd) was achieved following treatment with the Mn-doped iron oxide nanoparticles. The Mn-doped iron oxide nanoparticles were able to load ∼1 ppm of Cd followed by complete stripping and recovery of the Cd with a mild acid wash. The Cd loading and stripping is shown to be consistent through multiple cycles with no loss of sorbent performance.

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INTRODUCTION The environmental and health issues associated with heavy metal (i.e., Hg, Pb, Cd, etc.) contaminated waters have become more significant as population density and industrialization have increased globally. Concurrently, the remediation of heavy metal contaminated waters has become more challenging as more information about their deleterious health effects pushes drinking water and effluent regulatory limits down to the low ppb level.1,2 An abundance of materials have been reported in the literature for the collection of heavy metals. While most of the reported materials have some advantageous properties, few sorbents can meet the feasibility, cost-effectiveness, recoverability, and toxicity requirements to be a viable option for treatment of wastewater systems.3,4 Nanomaterials have seen increased attention for heavy metal remediation applications since the nanostructured surfaces offer large surface areas that provide enhanced capacity as well as the ability to enhance contaminant affinity with chemical modification of the surface.5−8 Sorbents comprised of bare magnetite and maghemite9−11 as well as examples of each of these iron oxide phases coated with affinity ligands or polymers12−26 have been used increasingly for the targeting of contaminants from wastewater. They are attractive candidates due to the oxides’ inherent affinity for many toxic chemicals but have the added advantage that they are also easily removed through magnetic manipulation.8 While the addition of reactive surface chemistries to these magnetic materials can greatly enhance their affinity toward some contaminants, the organic ligand modified and polymer coated nanoparticles do not always make good candidates when reuse of the sorbent © 2012 American Chemical Society

material is desired. Repeated exposure to environmental matrices and the chemical regeneration solutions (i.e., acidic solutions) can damage or remove the organic surface coating. Furthermore, organic modification of the surface of the sorbent material adds complexity to the production process and can significantly reduce synthetic yield, increasing sorbent material cost. Reusability of the nanostructured sorbent provides significant cost reduction, and the ability to remove the adsorbed analyte from the sorbent is beneficial when recovery of the metal is desired for applications such as recycling and resource recovery. These desired features suggest that tuning the analyte affinity through direct modifications to the oxide itself would yield a better material, particularly for applications in harsher matrices such as highly saline and biological fluids or processes where repetitive utilization is desired. Various metal oxides (e.g., MnO2, TiO2, Al2O3, SiO2, Fe2O3) have been shown to have a high affinity for many of the more common contaminants (e.g., Hg, Pb, etc.) encountered in wastewater treatment. Further, it has been shown that the addition of a metal dopant to many of these oxides can increase the affinity toward a particular analyte when compared to the native oxide material.27−29 For example, Bartos and co-workers showed that the introduction of Co and Fe dopants into MnO2 increased the oxide affinity toward Ra.27 Others have also shown the increased affinity of titanate and Received: October 27, 2011 Revised: January 12, 2012 Published: February 13, 2012 3931

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analysis. The concentrations in the control (no nanoparticles) and the test solutions (after being contacted with nanoparticle materials) were analyzed using ICP-MS. All batch experiments were performed in triplicate, and the averaged values were reported. Cd Adsorption Capacity Measurements. The sorption capacities of Cd for the Mn-doped nanoparticles was measured along with the undoped nanoparticles, Darco KB-B activated carbon (Sigma-Aldrich, St. Louis, MO), MnO2 resin, 75−150 μm (Eichrom Technologies, Lisle, IL), and Chelex 100 (Bio-Rad, Hercules, CA) for comparison to commercially available Cd sorbents. These measurements were carried out in the same fashion as the batch contact measurements, except that the initial Cd concentration was varied until the maximum sorption capacity was obtained. This was accomplished by using a large molar excess of Cd to the binding sites on the nanoparticle/sorbent materials (e.g., 0.02 to ∼4.8 mg L−1 of Cd at L/S ratio of 25 000 mL g−1). Sorbent Recycling Experiments. The recyclability of the Mndoped nanoparticle sorbents was investigated using 1 ppm of Cd spiked filtered river water. The sorbent was initially contacted with the Cd solution for 2 h with shaking. The nanoparticles were separated from the Cd solution using a 1.2 T NdFeB magnet, and then the isolated nanoparticles were washed with DI water to remove the unbound Cd. For desorption of Cd, 5 mL of 0.01 M HCl was added into a bottle after the DI water was removed, and the eluent solution was separated from the nanoparticles after 1 h of continuous shaking. After another DI rinse, the material was again contacted with the Cd solution. This contact and stripping process was repeated multiple times, and the supernatant was collected for ICP-MS analysis before and after stripping to determine complete removal of the Cd from the sorbent as well as sorbent efficiency after multiple regeneration cycles. The experiments were performed in triplicate, and the averaged values were reported.

ferrite sorbents toward arsenic by introduction of a Ce dopant into the oxide structures.28,29 The known heavy metal sorbent properties of MnO2 and iron oxides (i.e., Fe3O4 and Fe2O3) suggested a hybrid nanomaterial of Mn doped iron oxide with the appropriate dopant concentrations should yield a magnetically active material that performs as a sorbent on par with or better than each of the native oxides. Precise control over the Mn dopant included in the iron oxide lattice should permit the tailoring of the sorbent reactivity toward the analyte of interest. The doping method we describe in this paper yields nanoparticles with increased chemical reactivity, while still maintaining the magnetic character of the iron oxide nanoparticles for retrieval and manipulation. Although we use a commercial magnetite nanopowder in this study to demonstrate synthetic ease and potential low cost, the method is applicable to iron oxides of any size and morphology. We show that the level of Mn dopant in the ferrite has a significant effect on the reactivity of the materials toward selected heavy metal analytes (e.g., Co, Ni, Cu, Zn, As, Ag, Cd, Hg, and Tl). The chemical affinity and total capacity of the Mn-doped nanoparticles are shown to be superior to a number of other sorbents for these select heavy metals in Columbia River water. The doped nanoparticles show the ability to reduce Cd content in spiked river water (100 ppb) to ∼1 ppb, below the EPA drinking water limit of 5 ppb.30 Additionally, we show the reuse of the Mn-doped iron oxide nanoparticles through multiple collection and stripping cycles for the uptake of Cd from spiked river water.

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RESULTS AND DISCUSSION Synthesis and Physical Properties of the Materials. To explore the conditions needed to Mn dope the iron oxide, samples were treated with varied molar ratios of MnSO4 and the KMnO4 oxidant. EDS analysis was then used to determine the Fe to Mn ratios following treatment. Initial experiments varied the amount of MnSO4 relative to the amount of KMnO4 to determine the relation of each to the doping process (detailed data from EDS analysis can be found in Table S1). It was ultimately found that an equimolar amount of MnSO4 and KMnO4 was required to dope the material at each particular Mn concentration. The series of Mn-doped iron oxides used in this study contained 4.5%, 9%, and 13.7% Mn (synthetic details in Table S1). STEM and BET were used to determine changes to the material surface morphology following the doping process; the results are summarized in Table 1. STEM showed an average

EXPERIMENTAL SECTION

Materials and Methods. Iron(II,III) oxide nanopowder, potassium permanganate, and manganese sulfate were purchased from Sigma-Aldrich (St. Louis, MO) and used as received. All water used in the synthesis of the materials was 18.2 MΩ·cm. River water for uptake experiments was collected from the Columbia River and filtered through a 0.22 μm cellulose filter prior to use. Synthesis of the Mn-Doped Iron Oxide. In a typical synthesis, 1 g of iron oxide nanopowder was suspended in 50 mL of nanopure water and sonicated for 30 min. To this mixture, 10 mL of MnSO4 in water at a chosen concentration was added and stirred for 50 min, followed by the addition of 10 mL of KMnO4 in water at the same concentration as the MnSO4. This was then stirred for another 1 h at room temperature. The reaction mixture was washed with nanopure water until the supernatant was free of any traces of potassium permanganate. The washed material was then dried in air at 120 °C for 1 h. For experiments involving different dopant amounts, the reactions were carried out in an identical fashion, but the molarities of the MnSO4 and KMnO4 were adjusted. Characterization of Mn-Doped Iron Oxide. The nanoparticles were analyzed before and after the doping procedure using X-ray powder diffraction (XRD), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS), and vibrating sample magnetometry (VSM). Inductively coupled plasma−mass spectrometry (ICP-MS) was used for determining heavy metal uptake. Detailed descriptions of the characterization conditions can be found in the Supporting Information. Batch Contact Conditions. Filtered river water was spiked with metal ions of Co, Ni, Cu, Zn, As, Ag, Cd, Hg, and Tl at a concentration of 50 ppb of each metal. 4.9 mL of the metal solution (pH ∼ 7.0) was placed in a polypropylene tube and spiked with 0.1 mL of nanoparticles suspended in deionized (DI) water to obtain a liquid-to-solid ratio of 104 (L/S in mL g−1). The tubes were shaken for 2 h at 200 rpm on an orbital shaker. The nanoparticles were then separated from the solution using a 1.2 T NdFeB magnet, and the supernatant was removed and stored in 2% HNO3 prior to metal

Table 1. Surface Properties of Mn-Doped Fe3O4 sample

specific surface area (m2 g−1)

atomic % Mn

Fe3O4 1 mmol doped Fe3O4 3 mmol doped Fe3O4 5 mmol doped Fe3O4

42 47 69 59

0 4.5 9 13.7

particle size by TEM (nm) 27 31 34 26

± ± ± ±

8 9 10 9

particle size of between 25 and 35 nm before and after treatment so no significant change in core size occurs following the doping process (Figure 1 and Table 1). BET analysis of the material before and after doping shows minimal changes to the nanoparticles’ surface area. The Fe3O4 precursor nanoparticle has a specific surface area (SSA) of 42 m2 g−1. With the 3932

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Figure 2. Powder XRD of (a) the undoped Fe3O4 nanoparticles, (b) 4.5% Mn-doped Fe3O4, (c) 9% Mn-doped Fe3O4, and (d) 13.7% Mndoped Fe3O4.

Figure 1. STEM of (a) iron oxide core, (b) 4.5% Mn-doped, (c) 9% Mn-doped, and (d) 13.7% Mn-doped iron oxide nanoparticle. Scale bar = 20 nm.

addition of Mn dopant the surface area is found to be 47 m2 g−1for 4.5% Mn doped, 69 m2 g−1 for the 9% Mn doped, and 59 m2 g−1 for the 13.7% Mn doped. Given the broad size distribution for the precursor and doped materials, these values are well within the expected range of surface area if no change to the surface occurs during the doping process. XRD analysis of the nanoparticles before and after doping shows Bragg reflections indexed to the inverse cubic spinel phase of iron oxide (PDF # 01-88-0315, overlaid XRD patterns shown in Figure 2). Each sample contains identical peaks regardless of Mn content, and there is no evidence of peaks corresponding to MnO2 or Mn3O4, confirming that the Mn is incorporated into the ferrite structure rather than precipitating as a manganese oxide on the surface of the iron oxide. The unit cell constants are a = 8.3713, 8.3659, 8.3685, and 8.3694 Å for the undoped Fe3O4, 4.5% Mn-doped, 9% Mn-doped, and 13.7% Mn-doped Fe3O4, respectively (Table S2). The oxidative conditions for the doping result in the conversion of magnetite to maghemite, which have theoretical unit cell constants of 8.39 and 8.34 Å, respectively;31 thus, the initial change in the unit cell constant is consistent with this oxidative conversion. Following the initial lattice contraction from the oxidation, the unit cell then increases with Mn addition; this suggests lattice expansion is occurring when the Fe ions in the lattice are being replaced by the larger radius Mn2+ ions. These results are consistent with those described by others for evidence of lattice expansion due to Mn dopant inclusion.32,33 The particle size obtained from X-ray diffraction on these nanoparticles is also consistent with the nanoparticle sizes obtained from the TEM data. Results from the magnetic measurements show a drop in magnetic moment with increasing Mn concentration. Room temperature VSM measurements resulted in moments of 81, 62, 46, and 44 emu g−1 for the undoped, 4.5%, 9%, and 13.7%

Figure 3. VSM of (a) Fe3O4 nanoparticles, (b) 4.5% Mn-doped Fe3O4, (c) 9% Mn-doped Fe3O4, and (d) 13.7% Mn-doped Fe3O4. The inset shows the small coercivity of the ferromagnetic material.

Mn doped, respectively (Figure 3). Based on the highly oxidizing conditions of the doping step, the transition of magnetite to maghemite with its lower Ms (saturation magnetization)34 dominates the measured values due to the distribution of Fe3+ ions on the tetrahedral and octahedral sites. The process removes the Fe cations from the lattice and incomplete site replacement with the Mn2+ ions results in a disordered structure and significant loss of magnetic strength. In addition, a strong dependence of the saturation magnetization on particle size has also been reported in MnFe2O4 nanoparticles due to spin canting at the surface.35 Despite the reduction in magnetic strength, the materials are still strongly 3933

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Table 2. Metal Uptake with Iron Oxide Nanoparticles with Varied Dopant Concentrations heavy metal analyte sorbenta iron oxide 30 nm core 4.5% Mn dopedb 9% Mn dopedb 13.7% Mn dopedb activated carbon

Kd % capture Kd % capture Kd % capture Kd % capture Kd % capture

Co

Ni

Cu

Zn

As

Ag

Cd

Hg

Tl

14000 59 39000 80 66000 87 160000 94 2700 21

15000 60 16000 61 22000 69 26000 72 6000 37

140000 93 200000 94 270000 96 210000 95 180000 95

220000 91 1900000 98 1200000 97 2400000 100 1500 11

11000 52 23000 70 35000 78 33000 76 0 0

7500 43 76000 88 460000 98 1300000 99 8400 46

21000 68 290000 97 2700000 100 16000000 100 8300 45

28000 74 90000 90 150000 93 230000 96 42000 81

5300 34 10000 51 68000 87 600000 98 2600 20

a

Batch capture was carried out at L/S (liquid to solid) concentration of 104 (10 mL of solution to 0.001 g of sorbent) in river water, pH 7. bThe Mndoped materials reported here were made using the 30 nm iron oxide nanoparticle as a precursor. The Kd values were obtained by ICP-MS analysis of the metal spiked matrix before and after contact with the sorbent material.

The increase in affinity for each metal species with increasing Mn content is undoubtedly due to changes in surface chemistry. Sorbent reactivity is often complicated in complex matrices such as river water where many interferents (i.e., humic acid, carbonates, various dissolved organic carbon (DOC), etc.) are present. One notable trend observed by XRD suggests that lattice expansion may change the surface reactivity based solely on M−O distances. The metal species that had the highest affinity with increasingly high Mn content also have similar ionic radii, where Ag2+, Cd2+, and Tl4+ have ionic radii of 94, 95, and 97 Å,37 respectively, for the given oxidation state. The notion that the lattice parameters may have such a significant impact on the tuning of the sorbent for analyte affinity suggests one could dope a ferrite with varying concentrations of different metal dopants to tailor the selectivity and affinity of the magnetic nanoparticle toward the metal species of interest. Since unfunctionalized iron oxide and manganese ferrite (MnFe2O4) nanoparticles exhibit a near neutral surface charge at pH ∼ 7,38−40 a change in surface charge with the introduction of the manganese dopant is therefore not believed to be the driving force of the uptake of the analyte to the sorbent with increasing Mn content. Capacity Measurements of Mn-Doped Iron Oxide for Cd. The use of Cd has escalated dramatically in the past decade in part due to its use in Ni−Cd batteries. The improper disposal of these materials can result in elevated levels of the extremely toxic41 heavy metal in wastewater streams. A variety of sorbent materials are available for Cd collection, but improvements in sorbent capacity (as well as affinity and kinetics) will enable new and better industrial and remediation processes. On the basis of the results from the metal uptake experiments described in the previous section, the 13.7% Mndoped material was selected for capacity measurements for Cd in filtered Columbia River water (pH 7). Several commercial sorbents were also contacted under the same conditions for comparison. The commercial sorbents selected have surface chemistry that should impart a high affinity for Cd2+. The Mndoped iron oxide was found to have a capacity of 35 mg g−1, exceeding the next best performer, activated carbon, at ∼7 mg g−1 (Figure 4). The undoped iron oxide had a capacity of 4.1 mg g−1, which was followed closely by the commercial MnO2 resin (3.8 mg g−1) and Chelex-100 (1.9 mg g−1) resin. For comparison, the Kd values for each of the tested sorbents (for 100 ppb Cd spiked river water, pH 7, L/S 25 000) were

magnetic and enable magnetic manipulation and recovery of the sorbent. Effect of Mn Dopant Content on Heavy Metal Capture. The iron oxide core and Mn-doped samples were contacted with heavy metal spiked river water to determine the effect of Mn content in the ferrite on its performance as a heavy metal sorbent. The results from the batch contact experiments in Columbia River water are shown in Table 2. The uptake data are represented by both the calculated solid phase partition coefficient, Kd, and the % capture. While the % capture gives a measure of the total analyte removed, the Kd value is a direct measurement of the sorbent affinity for the analyte under the conditions it is tested. This value is a massweighted partition coefficient between the liquid supernatant phase and the solid sorbent phase and is derived from the equation

Kd =

C0 − C f V Cf M

where C0 and Cf are the initial and final concentrations of the target analyte in solution (as measured by ICP-MS), V is the volume of the solution in milliliters, and M is the mass of the sorbent in grams. A higher Kd indicates better capability for analyte collection and retention with values of 103 being reasonably good and values over 104 typically viewed as excellent.36 Every metal tested in the series showed higher affinity toward the sorbent when Mn content of the ferrite was increased. The increase in Mn content for metals such as Cd, Tl, and Ag resulted in a very dramatic increase in Kd from the undoped (no Mn) to the 13.7% doped material. Most notably, the Kd values for Cd are a respectrable 2.1 × 104 prior to doping but increases at least 1 order of magnitude for each ferrite material with higher Mn content, with a Kd increasing to 2.9 × 105 for 4.5% doped, 2.7 × 106 for 9% doped, and finally 1.6 × 107 for the 13.7% doped. Similar trends were also observed for Ag and Tl. In the case of Zn, doping had some effect on the affinity when compared to no dopant, but variations in the quantity of Mn seemed to have little effect on the ferrite affinity toward the metal. Although some of the other materials showed a less dramatic increase in analyte affinity with increasing Mn content, the changes in Kd are often on the order of 1−2 orders of magnitude higher than the undoped iron oxide nanoparticle core. 3934

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acid stripping using HCl of varying concentrations, as well as Ca(NO3)2, assuming a more gentle stripping environment for the nanoparticles (i.e., minimized Fe/Mn leaching)43 were explored. A 13.7% Mn-doped particle sample was first contacted with a Cd solution for 2 h, removed and rinsed, and then soaked for 1 h in each of the stripping solutions. The supernatant was collected to determine the removal of Cd from the surface as well as leaching of Mn and Fe from the particle using ICP-MS. Table 3 shows the relative amounts of each metal in the Table 3. Cd Desorption Conditions for HCl and Ca(NO3)2 at Various Concentrations conditions for Cd removal from sorbent stripping agent HCl

Figure 4. Cd Langmuir adsorption capacity curves of Cd in Columbia River water. Dotted line denotes Langmuir fit.

Ca(NO3)2

also determined. The Kd value for the Mn-doped iron oxide material was found to be 25 000 compared to 9400 for the undoped iron oxide. Activated carbon (Darco KB-B) had a Kd of 7600, while the Eichrom MnO2 resin and the Chelex-100 had Kd values of 4800 and 1600, respectively. Under these particular test conditions, the Mn doped iron oxide had a higher Kd value than the selected commercial sorbents. Clearly, the higher chemical affinity imparted by the dopant results in a sorbent material with superior heavy metal affinity when compared to many conventional commercial sorbents. The EPA designated safe drinking water limit for Cd is 5 parts per billion (ppb).30 After treatment with the Mn-doped sorbent, 100 ppb Cd spiked river water was reduced to 1.7 ppb (using 1 g of sorbent per 25 L of river water), which is well below the EPA limit. Absolute reduction in metal concentration will be determined based on analyte affinity and liquid-to-solid ratio, but the ability to rapidly clean heavy metal contaminated water to below EPA designated safety limits has been clearly demonstrated. Others have reported results for cadmium uptake under ideal conditions (DI water at controlled pH, low ionic strength, and no organic content) using an activated charcoal sorbent with capacities of ∼150 mg of Cd per gram of sorbent.42 This is much greater than the value we reported here for the uptake in river water (7 mg g−1 for activated charcoal) and highlights the challenge of operating in complex, real environmental matrices where fouling of the sorbent surface becomes an issue. In addition, this further demonstrates the extraordinary capacity of the Mn-doped iron oxide sorbent materials in matrices prone to fouling. Sorbent Recycling Experiments. The high affinity and capacity shown for Cd by the 13.7% Mn-doped material (see Table 2 and Figure 4) demonstrated excellent capability for capture but makes reversibility proportionally more challenging. If metal contact and subsequent metal stripping can be demonstrated for this system, it should be viable for the other heavy metals. First, it was necessary to determine what, if any, Cd stripping conditions were possible that did not destroy the sorbent material or involve elaborate (and costly) regeneration methods. After investigating various options,

conc. (M)

% Cd desorbed

Mn leached (mg g−1)

Fe leached (mg g−1)

0.00001 0.0001 0.01 0.015 0.03 0.15

3.1 27.8 93.2 28 34 48

1.9 6.9 18.5 3.2 3.8 4.4

1.9 0.18 0 5.1 10.9 56.4

supernatant for each stripping condition following desorption of Cd for both the HCl and Ca(NO3)2 as well as the amount of Fe and Mn leached in mg g−1 sorbent, also for each stripping solution. Based on this study, the 0.01 M HCl gives nearly complete stripping of the Cd from the particle surface with a minor amount of leaching of the Mn and no leaching of the Fe. The Ca(NO3)2 only removed ∼50% of the Cd with only minor leaching of the Mn, but the leaching of iron was very high. Based on these results, 0.01 M HCl was used for the recycling experiments. For the recycling experiments, a nanoparticle sample was initially contacted for 2 h (to ensure equilibrium) in 1028 ppb Cd spiked Columbia River water (pH 7) at a liquid-to-solid ratio of 104 (0.1 mg L−1). The kinetics demonstrated previously with nanoparticles support much faster processing rates,23 but these contact times were selected to avoid kinetic issues and focus on complete contaminant capture. The supernatant was then collected, and the Cd was stripped from the sorbent using a 1 h rinse in 0.01 M HCl. The supernatant from this was again collected and the sorbent rinsed briefly before it was again contacted with the Cd solution. The cycles of adsorption and desorption of the Cd are shown in Figure 5. The y-axis is split to allow for observation of the differences in the solution concentration of Cd for each adsorption/desorption cycle. The dotted blue and red lines in Figure 5 have been added to enhance clarity of the Cd adsorption and desorption steps of the cycles, respectively. Initial contact showed almost complete sorption of the analyte to the particle surface (>96%), followed by nearly complete stripping of the Cd by the HCl wash step. The cycle was repeated with Cd loading followed by nearly complete removal by the HCl wash step for three more subsequent cycles with capture efficiencies of 96, 98, and 97% Cd for the subsequent cycles (detailed data can be found in Table S3). While Table 3 clearly shows some leaching of the Mn with 0.01 M HCl, at least during the initial exposure, it is clear from the data that the minor Mn loss is insignificant with respect to the material’s performance as a Cd sorbent. 3935

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AUTHOR INFORMATION

Corresponding Author

*Tel (509) 375-6824; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NIH National Institute of Allergy and Infectious Diseases (R01-AI080502), the ONAMI Safer Nanomaterials Nanomanufacturing Initiative (SNNI), and the PNNL’s Laboratory Directed Research and Development (LDRD). TEM was carried out at the CAMCOR facility at the University of Oregon by Dr. Sujing Xie. BET surface area measurements were collected by Aleksandr Gerasimenko. Parts of the work was conducted in the Environmental Molecular Sciences Laboratory (EMSL), a DOE User Facility operated by Battelle for the DOE Office of Biological and Environmental Research. The Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-67RLO 1830.

Figure 5. Cd adsorption/desorption cycles in filtered Columbia River water (pH 7) at L/S ratio of 104. The y-axis is split to enable observation of the variations in adsorption/desorption values.

While regeneration and reuse of the Mn-doped iron oxide nanoparticle sorbents has been clearly demonstrated with Cd, many other metals should also be amenable to stripping and reuse as well. All the toxic heavy metals tested in Table 2 had lower affinities than Cd, suggesting stripping and sorbent reuse should be easier for the other metals with lower affinities. Stripping of the lower affinity toxic metals should be achievable with less acidic solutions or smaller volumes of acidic rinsing (assuming the absence of an irreversible binding process not observed in this work).

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CONCLUSION The results presented here demonstrate the ability to adjust an iron oxide nanoparticle sorbent reactivity through an inorganic doping process. When iron oxide is doped with Mn, the result is a magnetically manipulable sorbent material with much higher affinity and capacity when compared to the undoped iron oxide. The material has demonstrated utility for challenging aquatic matrices (i.e., unfiltered and untreated river water). Despite its high affinity for heavy metals, the sorbent can be stripped and reused. In addition, the uptake of radionuclides from complex matrices has also been shown to be drastically improved with the Mn-doped iron oxide nanoparticles compared to the undoped precursor iron oxide nanoparticles.44 Iron oxide nanoparticles are available commercially and can be made from simple inexpensive ingredients. In the experiments discussed in this paper, the doping process was carried out using a commercially available magnetite nanopowder to emphasize the cost-effectiveness of the procedure, but the doping process has been demonstrated on other iron oxide nanoparticle cores of varying morphology, size, and composition. The ability to simply modify these materials to provide cost-effective, high efficiency, and magnetically manipulable nanostructured sorbents may enable a range of remediation, recycling, and analytical solutions.

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ASSOCIATED CONTENT

S Supporting Information *

Details of materials characterization as well as more detailed results. This material is available free of charge via the Internet at http://pubs.acs.org. 3936

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dx.doi.org/10.1021/la2042235 | Langmuir 2012, 28, 3931−3937