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Characterization, sorption, and exhaustion of metal oxide nanoparticles as metal adsorbents

Dissertation
Author: Karen Elizabeth Engates
Abstract:
Safe drinking water is paramount to human survival. Current treatments do not adequately remove all metals from solution, are expensive, and use many resources. Metal oxide nanoparticles are ideal sorbents for metals due to their smaller size and increased surface area in comparison to bulk media. With increasing demand for fresh drinking water and recent environmental catastrophes to show how fragile water supplies are, new approaches to water conservation incorporating new technologies like metal oxide nanoparticles should be considered as an alternative method for metal contaminant adsorbents from typical treatment methods. This research evaluated the potential of manufactured iron, anatase, and aluminum nanoparticles (Al2 O3 , TiO2 , Fe2 O3 ) to remove metal contaminants (Pb, Cd, Cu, Ni, Zn) in lab-controlled and natural waters in comparison to their bulk counterparts by focusing on pH, contaminant and adsorbent concentrations, particle size, and exhaustive capabilities. Microscopy techniques (SEM, BET, EDX) were used to characterize the adsorbents. Adsorption experiments were performed using 0.01, 0.1, or 0.5 g/L nanoparticles in pH 8 solution. When results were normalized by mass, nanoparticles adsorbed more than bulk particles but when surface area normalized the opposite was observed. Adsorption was pH-dependent and increased with time and solid concentration. Aluminum oxide was found to be the least acceptable adsorbent for the metals tested, while titanium dioxide anatase (TiO 2 ) and hematite (α-Fe2 O3 ) showed great ability to remove individual and multiple metals from pH 8 and natural waters. Intraparticle diffusion was likely part of the complex kinetic process for all metals using Fe2 O3 but not TiO 2 nanoparticles within the first hour of adsorption. Adsorption kinetics for all metals tested were described by a modified first order rate equation used to consider the diminishing equilibrium metal concentrations with increasing metal oxides, showing faster adsorption rates for nanoparticles compared to bulk particles. Isotherms were best fit with most correlations of r=0.99 or better using the Langmuir-Freundlich equation which describes a heterogeneous surface with monolayer adsorption. Calculated rate constants and distribution coefficients (Kd ) showed TiO2 nanoparticles were very good sorbents and more rapid in removing metals than other nanoparticles studied here and reported in the literature. Desorption studies concluded Pb, Cd, and Zn appear to be irreversibly sorbed to TiO2 surfaces at pH 8. TiO2 and Fe2 O3 nanoparticles were capable of multiple metal loadings, with exhaustion for both adsorbents at pH 6. Exhaustion studies at pH 8 showed hematite exhausted after four consecutive cycles while anatase showed no exhaustion after 8 cycles. Their bulk counterparts exhausted in earlier cycles indicating the lack of ability to adsorb much of the multiple metals in solution. The increased surface area of TiO 2 and Fe 2 O3 nanoparticles, coupled with strong adsorption at the pH of most natural waters and resistance to desorption of some metals, may offer a potential remediation method for removal of metals from water in the future.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ........................................................................................................... iv ABSTRACT .................................................................................................................................... v LIST OF TABLES .......................................................................................................................... x LIST OF FIGURES ...................................................................................................................... xii CHAPTER ONE: INTRODUCTION ............................................................................................. 1 1.1 Organization of Thesis .......................................................................................................... 4 1.2 Study Objectives ................................................................................................................... 4 CHAPTER 2: LITERATURE REVIEW ........................................................................................ 6 2.1 Metals in the Environment .................................................................................................... 6 2.1.1 Lead................................................................................................................................ 7 2.1.3 Cadmium ........................................................................................................................ 9 2.1.4 Copper .......................................................................................................................... 11 2.1.5 Nickel ........................................................................................................................... 13 2.1.6 Zinc .............................................................................................................................. 15 2.2 Metal Oxides in the Environment ....................................................................................... 17 2.2.1 Iron Oxides................................................................................................................... 17 2.2.2 Titanium Oxides........................................................................................................... 18 2.2.3 Aluminum Oxides ........................................................................................................ 20 2.2.4 Zero Valent Iron ........................................................................................................... 21 2.3 Sorption Processes of Metal Oxides ................................................................................... 23 2.3.1 Isotherms ...................................................................................................................... 27 2.3.2 Kinetics ........................................................................................................................ 32 2.3.3 Surface Complexation Models ..................................................................................... 34 2.3.4 Spectroscopic studies ................................................................................................... 37 2.4 Nanomaterials ..................................................................................................................... 38 2.4.1 Metal Oxide Nanoparticles .......................................................................................... 42 2.4.2 Carbon Nanotubes ........................................................................................................ 44 2.4.3 Zeolites ......................................................................................................................... 44 2.4.4 Dendrimers .................................................................................................................. 45 2.5 Current Drinking Water Technologies for Metal Removal ................................................ 46 2.6 Reuse and Regeneration of Nanoparticle Metal Oxides ..................................................... 50 2.7 Summary ............................................................................................................................. 53 CHAPTER 3: MATERIALS AND METHODS .......................................................................... 55

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3.1 Introduction ......................................................................................................................... 55 3.2 Materials ............................................................................................................................. 55 3.3 Methods............................................................................................................................... 57 3.3.1 Characterization of Nanoparticles and Bulk Particles ................................................. 57 3.3.2 Vessel Apparatus and Analytical Methods .................................................................. 59 3.3.3 Effect of pH on Individual Metal Adsorption .............................................................. 60 3.3.4 Adsorption Isotherms ................................................................................................... 62 3.3.5 Effect of Particle Size and Solid Concentration on Individual Metal Removal .......... 62 3.3.6 Effect of Simultaneous Multiple Metal Adsorption..................................................... 62 3.3.7 Effect of Ionic Strength and Natural Waters on Adsorption ........................................ 63 3.3.8 Exhaustion.................................................................................................................... 63 3.3.9 Desorption Experiments............................................................................................... 63 3.4 Summary ............................................................................................................................. 64 CHAPTER 4: EFFECT OF ALUMINUM OXIDE NANOPARTICLE USE ON METAL REMOVAL ................................................................................................................................... 65 4.1 Results and Discussion ....................................................................................................... 65 4.1.1 Characterization of Aluminum Oxide Nanoparticles .................................................. 65 4.1.2 Effect of solid concentration and metal concentration on adsorption.......................... 67 4.1.3 Simultaneous Multiple Metal Adsorption .................................................................... 68 4.1.4 Desorption .................................................................................................................... 69 4.2 Summary ............................................................................................................................. 70 CHAPTER 5: EFFECT OF TITANIUM DIOXIDE NANOPARTICLE USE ON METAL REMOVAL ................................................................................................................................... 71 5.1 Results and Discussion ....................................................................................................... 71 5.1.1Characterization of TiO 2 nanomaterials ........................................................................ 71 5.1.2 Isotherms ...................................................................................................................... 74 5.1.3 Intraparticle Diffusion Model ...................................................................................... 81 5.1.4 Effect of Particle Size and Sorbent Concentration on Individual Metal Adsorption ... 83 5.1.5 Effects of Multiple Metal Adsorption and Natural Water Samples ............................. 87 5.1.6 Desorption experiments ............................................................................................... 94 5.1.7 Exhaustion.................................................................................................................... 95 5.1.8 EDX Element Mapping................................................................................................ 98 5.2 Summary ........................................................................................................................... 100 CHAPTER 6: EFFECT OF IRON OXIDE NANOPARTICLE USE ON METAL REMOVAL ..................................................................................................................................................... 102

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6.1 Results and Discussion ..................................................................................................... 102 6.1.1 Characterization of Iron Oxide (Hematite) Nanomaterials ........................................ 102 6.1.2 Isotherms .................................................................................................................... 105 6.1.3 Intraparticle Diffusion Model .................................................................................... 109 6.1.4 Adsorption Experiments Regarding Individual Metal Removal ............................... 111 6.1.5 Effects of Multiple Metal Adsorption and Natural Water Samples ........................... 115 6.1.6 Desorption Experiments............................................................................................. 124 6.1.7 Exhaustion Experiments ............................................................................................ 125 6.1.8 EDX Element Mapping.............................................................................................. 128 6.2 Summary ........................................................................................................................... 132 CHAPTER 7: COMPARISON OF METAL REMOVAL ADSORBENTS .............................. 133 7.1 Langmuir Adsorption Capacity and Kinetic Rates for TiO 2 and Fe 2 O 3 Nanoparticles 133 7.2 Statistical Analyses of Bulk and Nanoparticles ............................................................ 134 7.3 Multiple Metal Adsorption ........................................................................................... 135 7.4 Nanoparticle and Bulk Particle Desorption .................................................................. 136 7.5 Nanoparticle and Bulk Exhaustion ............................................................................... 137 7.6 Comparison to Metal Adsorbents in Literature ............................................................ 138 CHAPTER 8: CONCLUSIONS AND FUTURE WORK .......................................................... 140 8.1 Conclusions ................................................................................................................... 140 8.2 Future Work .................................................................................................................. 142 BIBLIOGRAPHY ....................................................................................................................... 144 VITA

x

LIST OF TABLES

Table 1: Drinking water regulation, contaminant sources, and health impacts of Pb, Cd, Cu, Ni, and Zn ............................................................................................................................................. 2 Table 2. Classification of nanoparticles adapted from Nowack and Bucheli [181] ..................... 40 Table 3. Removal effectiveness for eight processes by inorganic contaminant [239] .................. 47 Table 4. Most effective treatment methods for removal of inorganic contaminants [239] .......... 47 Table 5. Advantages and disadvantages of inorganic-contaminant removal processes [241] ...... 48 Table 6. Composition and other water quality parameters for San Antonio, Texas, tap water; Comal Springs, Texas, water; Austin, Texas, tap water; and Carrizo Aquifer, TX, groundwater, USA, before spiking a . All units in mg/L unless otherwise stated. ............................................... 57 Table 7. Adsorption of individual metals after 24 hours with 0.1 g/L Al 2 O 3 nanoparticles and C o

= 100 µg/L M 2+ ............................................................................................................................. 67 Table 8. Individual metal adsorption after 24 hours in 50 mL containers using 0.1 or 0.5 g/L Al 2 O 3 nanoparticles at pH 8.0 ....................................................................................................... 68 Table 9. Percent metal adsorbed in simultaneous multiple metal adsorption batch after 24 hours in pH 8 using 0.1 or 0.5 g/L Al 2 O 3 nanoparticles with C o = 100 or 500 µg/L M 2+ ...................... 69 Table 10. Two cycle desorption data for individual metals using 0.1 g/L aluminum oxide nanoparticles after 24 hour adsorption .......................................................................................... 70 Table 11. Freundlich, Langmuir, and Langmuir-Freundlich (LF) isotherm parameters for isotherms at pH 5 and 8 for five metals using titanium dioxide nanoparticles ............................. 79 Table 12. The calculated distribution coefficient (K d ) and percent metal adsorbed at 120 minutes. Rate constants (k) were determined by least-squares fitting of experimental data using equation 11 and ‗r‘ is the data correlation coefficient. Initial metal concentration was 100 µg/L at pH 8.0 and room temperature. .................................................................................................................. 86 Table 13. Metal adsorption, pH, conductivity, and adsorption rates for pH 8 and natural water samples in multiple metal adsorption using 0.1 g/L TiO 2 nanoparticles and initial concentration of 500 µg/L for each metal. .......................................................................................................... 89 Table 14. Two-cycle 24 hour desorption results for 100, 500, and 1000 µg/L M 2+ using 0.1 g/L TiO 2 nanoparticles at pH 8.0 and room temperature. ................................................................... 95 Table 15. Freundlich, Langmuir, and Langmuir-Freundlich (LF) isotherm parameters for hematite nanoparticles in pH 4.0, 6.0, and 8.0 ............................................................................ 109 Table 16. The calculated distribution coefficient (K d ) and percent metal adsorbed at 120 minutes. Rate constants (k) were determined by least-squares fitting using equation 11 and 'r' is the correlation coefficient. Initial metal concentration was 100 µg/L at pH 8.0 and room temperature. ................................................................................................................................ 114 Table 17. Multiple metal adsorption rates, pH, conductivity for natural and synthetic waters .. 119 Table 18. Metal adsorption for pH 8 and natural waters in multiple metal adsorption using 0.1 g/L Fe 2 O 3 nanoparticles and initial concentration of 500 µg/L for each metal .......................... 123

xi

Table 19. Two cycle 24-hour desorption results for 100, 500, and 1000 µg/L M 2+ using 0.1 g/L Fe 2 O 3 nanoparticles at pH 8.0 and room temperature................................................................. 125 Table 20. Percent multi-element adsorption to 0.1 and 0.5 g/L nanoparticles at pH 8.0 using 100 or 500 µg/L for each contaminant M 2+ during a 24 hour period in 50 ml containers at room temperature. ................................................................................................................................ 136 Table 21. Two cycle desorption process data for anatase and hematite bulk particles in pH 8 for 24 hours at room temperature ..................................................................................................... 137 Table 22. Comparison of distribution coefficients between TiO 2 and Fe 2 O 3 nanoparticles used in this work and other sorbents when multiple metals are present ................................................. 139

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LIST OF FIGURES

Figure 1. Eh-pH diagram of Pb [29] ............................................................................................... 9 Figure 2. Eh-pH diagram of Cd [29] ............................................................................................. 11 Figure 3. Eh-pH diagram of Cu [29] ............................................................................................. 12 Figure 4. Eh-pH diagram of Ni [29] ............................................................................................. 15 Figure 5. Eh-pH diagram of Zn [29] ............................................................................................. 16 Figure 6. Atomic crystal structure of hematite (α-Fe 2 O 3 ) showing iron atoms (red) surrounded by oxygen atoms (grey) [72] .............................................................................................................. 18 Figure 7. Atomic crystal structure for anatase TiO 2 showing titanium atoms (grey) and oxygen atoms (blue) [88] ........................................................................................................................... 20 Figure 8. Atomic crystal structure of aluminum oxide (α-Al 2 O 3 ) showing aluminum atoms (red) surrounded by oxygen atoms (grey) [88] ...................................................................................... 21 Figure 9. Comparison of Langmuir, Freundlich, and Langmuir-Freundlich isotherm plots from Umpleby et al [145] ...................................................................................................................... 30 Figure 10. Adsorption and desorption isotherms for Ni 2+ on four synthetic Fe(III)-oxides (solid line) and desorption isotherms for the same samples (dashed line). [71] ..................................... 31 Figure 11. Kinetic studies of adsorption on Cr +6 , Cu +2 , and Ni +2 [155] ....................................... 33 Figure 12. Langmuir isotherms for adsorption of metals from Figure 11 [155]........................... 33 Figure 13. Electric double layer defined by electrostatic planes in surface complexation modeling. Adapted from [163, 164] ............................................................................................ 36 Figure 14. Oxide surface coordination with ligands, adapted from Auffan et al [7] .................... 37 Figure 15. Selected nanomaterials currently being evaluated as functional materials for water purification [4] .............................................................................................................................. 41 Figure 16. Controlled experimental batch apparatus .................................................................... 60 Figure 17. SEM image of 0.1 g/L 58.77 nm Al 2 O 3 nanoparticles in nano-water suspension on 300 mesh copper grids ......................................................................................................................... 66 Figure 18. Energy dispersive X-ray analysis plot for Al 2 O 3 nanoparticles performed on a 300 mesh copper grid. Unlabeled peaks correspond with insignificant background elements in the SEM. ............................................................................................................................................. 66 Figure 19. Potentiometric curve for Al 2 O 3 nanoparticles ............................................................. 67 Figure 20. SEM images of 0.1 g/L (a) 8.3 nm TiO 2 nanoparticles and (b) 329.8 nm bulk TiO 2

particles in nano-water suspension on 300 mesh copper grids. .................................................... 72 Figure 21. Energy dispersive X-ray analysis plot for TiO 2 nanoparticles performed on a 300 mesh copper grid. Unlabeled peaks correspond with the Cu mesh background. ......................... 72 Figure 22. Potentiometric curve for TiO 2 nanoparticles ............................................................... 73 Figure 23. XRD analysis plot of 8.3 nm TiO 2 nanoparticles ........................................................ 74 Figure 24. Nature of surfaces at titanium oxide water interfaces, adapted from Schindler, 1981 [282] .............................................................................................................................................. 75 Figure 25. Deprotonated surface hydroxyls create a negatively charged surface and readily adsorbed cations, adapted from Schindler, 1981 [282] ................................................................. 75

xiii

Figure 26. Isotherms at pH 5 and pH 8 for Pb (a,b), Cd (c,d), Cu (e,f), Ni (g,h), Zn (i,j) ............ 77 Figure 27. Intraparticle Diffusion Model plot fits for Pb, Cd, Cu, Zn, and Ni adsorption using TiO 2 anatase nanoparticles. ........................................................................................................... 82 Figure 28. Individual 100 µg/L M 2+ adsorption in 0.01M THAM and 0.01M NaNO 3 with 0.01 g/L for nanoparticle (solid) and 0.01 g/L bulk (open) TiO 2 normalized by mass, µmol/g (a) and normalized by surface area, µmol/m 2 (b) versus time at pH 8. Error bars represent 5% error in the measurement of each metal. .......................................................................................................... 84 Figure 29. Simultaneous adsorption of five metals at 500 ug/L M 2+ with 0.1 g/L TiO 2

nanoparticles (a) in pH 8.0, (b) San Antonio, TX, tap water, (c) Comal Springs, TX, spring water, (d) Austin, Texas, tap water; (e) Carrizo Aquifer, TX, groundwater, (f) a synthetic brackish solution, and (g) 1M NaCl solution. Error bars indicate 5% error in the measurement of each metal. .................................................................................................................................... 88 Figure 30. Multiple metal exhaustion at pH 6 using 0.1 g/L TiO 2 (a) nanoparticles and (b) bulk particles at 500 ug/L M 2+ and pH 8.0 using 250 ug/L M 2+ for (c) nanoparticles and (d) bulk. Data reflect values for last samples collected in each 1 hr cycle, after which an additional 500 µg/L M 2+ was added. Error bars indicate 5% error in the measurement of each metal. ....................... 97 Figure 31. TiO 2 nanoparticle EDX element analysis plot (b). ...................................................... 99 Figure 32. Bruker EDX element mapping of adsorbed metals (Pb, Cd, Ni, Zn) onto TiO 2 anatase nanoparticles: (a) SEM image; (b) composite element map; and element distributions for (c) Ti; (d) O; (e) Ni; (f) Zn; (g) Pb, and (h) Cd. ..................................................................................... 100 Figure 33. SEM images of Fe 2 O 3 (a) nanoparticles and (b) bulk particles in nano-water suspension on 300 mesh copper grids. ........................................................................................ 103 Figure 34. Energy dispersive X-ray analysis for Fe 2 O 3 nanoparticles performed on 300 mesh copper grid. Unlabeled peaks correspond with elements considered insignificant in the analysis. ..................................................................................................................................................... 103 Figure 35. Potentiometric curve for hematite nanoparticles ....................................................... 104 Figure 36. XRD analysis plot for Fe 2 O 3 nanoparticles ............................................................... 105 Figure 37. Adsorption isotherms at pH 4,6,8 after 24 hours for Pb (a-c), Cd (d-f), Cu (g-i), Ni (j- l), and Zn (m-o) using 0.1 g/L Fe 2 O 3 nanoparticles with 100-1000 µg/L M 2+ . .......................... 108 Figure 38. Intraparticle diffusion model plot fits for Pb, Cd, Cu, Ni, and Zn adsorption using Fe 2 O 3 nanoparticles. .................................................................................................................... 111 Figure 39. Individual 100 µg/L M 2+ adsorption with 0.01 g/L for nanoparticle (solid) and bulk (open) Fe 2 O 3 normalized by mass, µmol/g (a) and normalized by surface area, µmol/m 2 (b) versus time at pH 8. Error bars represent 5% error in the metal measurements. ....................... 112 Figure 40. Adsorption of 500 ug/L M 2+ (Pb, Ni, Cd, Cu, Zn) simultaneously with 0.1 g/L Fe 2 O 3

nanoparticles at (a) pH 8.0, (b) spiked San Antonio, TX, tap water, (c) spiked Comal Springs, TX, spring water, (d) spiked Austin, Texas, tap water, (e) Carrizo Aquifer, TX, groundwater, (f) brackish solution, and (g) 1M NaCl solution. Error bars indicate 5% error in the measurement of each metal. .................................................................................................................................. 117

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Figure 41. Multiple metal exhaustion at pH 6 with 500 µg/L M 2+ using 0.1 g/L Fe 2 O 3 (a) nanoparticles and (b) bulk particles and in pH 8 with 250 µg/L M 2+ spikes using (c) nanoparticles and (d) bulk particles. Data reflect values for last samples collected in each 1 hr cycle, after which an additional spike of metals was added. Error bars indicate 5% error in the measurement of each metal. .............................................................................................................................. 127 Figure 42. EDX element analysis plot of Fe 2 O 3 nanoparticles with adsorbed Pb, Cd, Ni, and Zn. ..................................................................................................................................................... 130 Figure 43. Bruker EDX element mapping of adsorbed metals (Pb, Cd, Ni, Zn) onto hematite nanoparticles: (a) SEM image; (b) composite element map; and element distributions for (c) Fe; (d) O; (e) Ni; (f) Zn; (g) Pb; (h) Cd. ........................................................................................... 131

1

CHAPTER ONE: INTRODUCTION

Much of the world‘s population does not have adequate access to clean drinking water for basic survival needs. Demands are increasing as fresh water availability is decreasing due to extended drought, population growth, more stringent health-based regulations, and competitive user demands [1]. One of the biggest challenges is to create a simple, low-cost, environmentally- friendly and efficient method to remove contaminants, especially metals. Rock and soil may contain metals but natural levels present are usually not cause for concern; however, anthropogenic activities including industrial, mining, agricultural, and military sources have often distributed and circulated these metals in the environment [2]. According to the United States Environmental Protection Agency (US EPA) [3] report published in 2006, those anthropogenic activities previously suggested pose high human health risks for continued exposure to metal contamination. Although the US EPA monitors water quality for numerous contaminants, including metals, it appears that acceptable levels for metals in drinking water may need to be reduced as more studies on negative human health effects arise. Table 1 is a partial list retrieved from the US EPA which defines the drinking water regulation, sources, and human health impacts of various metals.

2

Table 1: Drinking water regulation, contaminant sources, and health impacts of Pb, Cd, Cu, Ni, and Zn Table 1. Drinking Water Regulation, Contaminant Sources, and Health Impacts of Pb, Cd, Hg, Cu, Ni, and Zn . Information retrieved on August 25, 2008, from USEPA (http://www.epa.gov/safewater/mcl.html)

Contaminant

MCLG (mg/L)

MCL or TT (mg/L)

Potential Health Effects from Ingestion of Water

Sources of Contaminant in Drinking Water

Lead

Zero

TT; Action Level = 0.015

Infants and children: Delays in physical or mental development; children could show slight deficits in attention span and learning disabilities

Adults: Kidney problems; high blood pressure

Corrosion of household plumbing syste ms; erosion of natural deposits

Cadmium

0.005

0.005

Kidney damage

Corrosion of galvanized pipes; erosion of natural deposits; discharge from metal refineries; runoff from waste batteries and paints

Copper

1.3

TT; Action Level = 1.3

Short term exposure: gastrointestinal distress Long term exposure: Liver or kidney damage

Corrosion of household plumbing systems; erosion of natural deposits

Nickel

0.1*

0.1*

Long term exposure: decreased body weight, heart and liver damage, dermatitis

Smelting, refining, s teelworks

Zinc

5**

5**

Excessive amounts may cause growth retardation, skin changes, poor appetite

Iron and steel products, household items, medicines, erosion of natural deposits

MCLG = Maximum Contaminant Level Goal

MCL = Maximum Contaminant Level

TT =

Treatment Technique

* MCL and MCLG remanded on 2/9/95 but are being reconsidered.

** Not currently regulated by EPA as a primary contaminant.

3

Various treatments exist for removing some metals from water, though popular current methods require large amounts of bulk media sorbents and increasingly more sophisticated treatment designs to produce potable water of acceptable quality. In addition, many of the adsorbents used to remove metals are one-time use sorbents. As the world‘s population continues to increase, more stringent demands will be placed on water treatment plants to supply safe drinking water to adequately protect this resource [4]. One possible solution is the use of nanoparticles in water treatment to adsorb metals. Particles ranging from 1 to 100 nanometers in size have significantly greater surface area than their bulk counterparts and may possess much greater sorption capacities in much smaller amounts, thereby being more cost effective in treatment processes with much less waste for disposal. As particle size decreases, more unsaturated surface atoms are exposed and functional groups may move closer, prompting greater reactivity and possibly a nanoscale effect with particles less than 20 nm in diameter [5-7]. Therefore, it has been suggested that nanoparticles can selectively adsorb metals and have high adsorption [8]. As society becomes increasingly more aware of the need for environmental sustainability methods, an additional benefit of nanoparticles may be their regeneration and reuse. When coupled with the fact that naturally created nanoparticles exist and that their sorption capabilities may possibly be manipulated for controlled release and proper disposal of contaminants, the potential for reducing treatment cost by using natural, regenerated nanomedia to eliminate metals should be cost effective for technological applications in water purification and remediation. Furthermore, although this study uses commercially available, laboratory-engineered nanomaterials for convenience and uniformity of variables, comparable natural nanoparticles

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exist. However, reports of the use of natural nanoparticles in literature are extremely sparse in comparison to engineering nanoparticles and their potential uses for environmental applications. Much work exists demonstrating the ability of bulk crystal media (e.g., iron oxides, activated carbon, and aluminum oxides larger than 100 nanometers in size) to remove isolated contaminants from water (e.g., [9-11]). Though these media are accepted as industry standards, they carry significant disadvantages: expensive reactivation, slow kinetics, lack of flexibility in design, and less than 100% removal [12]. As the field of nanotechnology expands, more research on using nanoparticles for contaminant removal is beginning to surface, but it is still in its infancy. Most of these studies focus solely on adsorption and little exists on the sorption kinetics or the exhaustion of these ideal sorbents. Studying these properties may lead to the establishment of certain trends based on sorption behaviors and provide the foundation for further studies that may lead to reuse and regeneration efficacy. 1.1 Organization of Thesis

The organization of this thesis is divided into eight chapters. Chapter 1 introduces the topic of metal oxide nanoparticles as metal sorbents and describes the objectives of this study. Chapter 2 represents the background literature review and Chapter 3 describes the materials and methods used in this research. Chapters 4, 5, and 6 discuss the results of experiments regarding aluminum, titanium, and iron oxide nanoparticles as adsorbents. Metal removal methods are compared in Chapter 7 and conclusions are presented in Chapter 8. 1.2 Study Objectives

The goal of this dissertation is to understand the interaction between metal contaminants and metal oxide nanoparticles when the nanoparticles are used as metal sorbents in drinking water technologies. The specific objectives of this research are to:

5

1. Evaluate the potential of manufactured metal oxide nanoparticles to remove certain metal contaminants (Pb (II), Cd (II), Cu (II), Ni (II), Zn (II)) in conditions representing natural waters and treatment processes compared to their bulk counterparts. (Hypothesis: The surface area-normalized sorption capacity of metal oxide nanoparticles is greater than bulk crystals due to enhanced physical properties involving surface area on the nanometer scale.) 2. Determine the kinetics of metal (Pb (II), Cd (II), Cu (II), Ni (II), Zn (II)) adsorption to metal oxide nanoparticles. (Hypothesis: Rates of adsorption for metals to metal oxide nanoparticles are proportional to both the concentration in solution and the surface area of the adsorbent.) 3. Establish metal oxide nanoparticle exhaustion limits to determine potential reuse of the particles. (Hypothesis: In multiple metal solutions, the capacity of certain metals to adsorb will exhaust before others due to their affinity for the metal oxide nanoparticle surface.)

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CHAPTER 2: LITERATURE REVIEW

Seven sections comprise the literature review: metals in the environment, metal oxides, sorption processes of metals to metal oxides, nanomaterials, current drinking water treatment technology to remove metals, reuse and regeneration of metal oxide nanoparticles, and a summary. The first two sections provide critical background information on the occurrence and effects of metals in the environment and the formation and importance of metal oxides. Section three focuses on sorption processes of metals to metal oxides with varying experimental conditions as well as the characterization of metal oxides to include surface complexation models, adsorption kinetics, and spectroscopic studies. The fourth section discusses nanomaterials and their potential for use in drinking water treatment strategies, especially focusing on metal oxide nanoparticles. Section five examines the current technologies used in drinking water treatment to remove metals. Discussion of the reuse and regeneration of metal oxide nanoparticles is presented in section six, and the final section summarizes all pertinent information for this study. 2.1 Metals in the Environment

Metals occur naturally in the environment in rock and soil and are usually not found at levels of concern, but human activities (e.g., industrial processes and mining) can dramatically alter their distribution. In addition, 20 to 50 million tons of electronic waste – or ‗e-waste‘ – generated annually from cell phones, computers, monitors, and other electronics contain potentially harmful levels of metals that may leach into the environment and enter drinking water sources [3]. High metal concentrations in the environment can disrupt the equilibrium in aquatic ecosystems and cause them to become unsuitable for industrial and domestic uses.

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Metals can generally be defined as stable metallic elements with relatively high densities that are often toxic to organisms through biomagnification, thus inhibiting biological processes [13]. The human body needs some metals in trace amounts, but elevated levels can be dangerous. Metals like Pb, Hg, and Cd have no known human benefit and bioaccumulation can lead to serious illness and possible death. Other metals including As, Cd, Cr, Ni, and Pb are known to be genotoxic in vitro [14] and in vivo [15]. We live in a heavily industrialized world, so these substances may become more prevalent in our air, soil, and water resources in a wide range of products, so it is challenging to avoid exposure. Obviously, one area of critical importance is safe drinking water to which each of these metals may contaminate. 2.1.1 Lead

Lead is a naturally occurring element whose predominant oxidation state is +2 due to the rare occurrence of Pb 0 and extremely oxidizing conditions needed for Pb 4+ formation. Four stable Pb isotopes naturally exist, with abundance of each shown in percent: 208 Pb (51-53%), 206 Pb (23.5-27%), 207 Pb (20.5-23%), and 204 Pb (1.35-1.5%). Lead ore deposits are easily accessible and distributed worldwide, and Pb is a common metal in storage batteries, weights, solder, and pipes [16]. This metal is present in low concentrations in sedimentary rocks and soils, with average concentrations in shales, sandstones, and carbonate rocks of 20, 7, and 9 mg Pb/kg, respectively [17], and background soil concentrations of 17 to 26 mg Pb/kg in the United States [18]. Hydrothermal deposits and base metal ores contribute to natural lead enrichment, most often as galena (PbS), anglesite (PbSO 4 ) and cerussite (PbCO 3 ) ores [19]. Lead minerals usually have low solubility in most environmental conditions and are released only when low pH or high dissolved organic carbon concentrations are present.

Full document contains 177 pages
Abstract: Safe drinking water is paramount to human survival. Current treatments do not adequately remove all metals from solution, are expensive, and use many resources. Metal oxide nanoparticles are ideal sorbents for metals due to their smaller size and increased surface area in comparison to bulk media. With increasing demand for fresh drinking water and recent environmental catastrophes to show how fragile water supplies are, new approaches to water conservation incorporating new technologies like metal oxide nanoparticles should be considered as an alternative method for metal contaminant adsorbents from typical treatment methods. This research evaluated the potential of manufactured iron, anatase, and aluminum nanoparticles (Al2 O3 , TiO2 , Fe2 O3 ) to remove metal contaminants (Pb, Cd, Cu, Ni, Zn) in lab-controlled and natural waters in comparison to their bulk counterparts by focusing on pH, contaminant and adsorbent concentrations, particle size, and exhaustive capabilities. Microscopy techniques (SEM, BET, EDX) were used to characterize the adsorbents. Adsorption experiments were performed using 0.01, 0.1, or 0.5 g/L nanoparticles in pH 8 solution. When results were normalized by mass, nanoparticles adsorbed more than bulk particles but when surface area normalized the opposite was observed. Adsorption was pH-dependent and increased with time and solid concentration. Aluminum oxide was found to be the least acceptable adsorbent for the metals tested, while titanium dioxide anatase (TiO 2 ) and hematite (α-Fe2 O3 ) showed great ability to remove individual and multiple metals from pH 8 and natural waters. Intraparticle diffusion was likely part of the complex kinetic process for all metals using Fe2 O3 but not TiO 2 nanoparticles within the first hour of adsorption. Adsorption kinetics for all metals tested were described by a modified first order rate equation used to consider the diminishing equilibrium metal concentrations with increasing metal oxides, showing faster adsorption rates for nanoparticles compared to bulk particles. Isotherms were best fit with most correlations of r=0.99 or better using the Langmuir-Freundlich equation which describes a heterogeneous surface with monolayer adsorption. Calculated rate constants and distribution coefficients (Kd ) showed TiO2 nanoparticles were very good sorbents and more rapid in removing metals than other nanoparticles studied here and reported in the literature. Desorption studies concluded Pb, Cd, and Zn appear to be irreversibly sorbed to TiO2 surfaces at pH 8. TiO2 and Fe2 O3 nanoparticles were capable of multiple metal loadings, with exhaustion for both adsorbents at pH 6. Exhaustion studies at pH 8 showed hematite exhausted after four consecutive cycles while anatase showed no exhaustion after 8 cycles. Their bulk counterparts exhausted in earlier cycles indicating the lack of ability to adsorb much of the multiple metals in solution. The increased surface area of TiO 2 and Fe 2 O3 nanoparticles, coupled with strong adsorption at the pH of most natural waters and resistance to desorption of some metals, may offer a potential remediation method for removal of metals from water in the future.