Anion Recovery Using Cation-Exchanged ZSM-5

by

Mitchell Adam Keller

A thesis submitted to Johns Hopkins University in conformity with the requirements for

the degree of Master of Science in Chemical and Biomolecular Engineering

Baltimore, Maryland

May, 2018

ii

______________________________________________________________________

Abstract: The world is in need of new methods for handling aqueous, inorganic

pollutants such as phosphate, nitrate, sulfate, and chloride because of the decreasing

water quality in global water supplies. Phosphate and nitrate are of special interest

because they are used in fertilizer and require tremendous amounts of resources to

produce them for crop growth. Here, I offer the use of cation-exchanged ZSM-5 zeolites

as a novel method for the recyclable extraction and regeneration of such anions from

wastewater. Previous work has proven the effectiveness of different zeolites for both

adsorption and catalytic applications due to their unique geometric and electromagnetic

properties. I use Fe, Co, Ni, Cu, Sn, La, and Ce substituted ZSM-5 sorbents to

selectively capture the anions of phosphate, nitrate, sulfate and chloride from model

wastewater solutions. I also use a brine solution based regeneration process to remove

the anions from the zeolites’ framework and regenerate the material. I then use

extensive characterization techniques to identify the trends that may give each sorbent

its unique capacity and affinity for each anion. My results show that the sorbents are

able to capture as much as 7.36 ± 0.26 molanion/molcation with different affinities for each

anionic species with noticeable trends by oxidation state and hydration radius of the

exchanged cations. I also propose an ion-exchange mechanism using the exchanged

cation as the active site for the recovery process.

______________________________________________________________________

iii

Acknowledgements

This work, in its entirety, is dedicated to my loving and supportive family, without

whom none of my accomplishments would have been attainable.

This work is supported by an incredible group of colleagues and advisors who

inspired and guided my research on a daily basis. Special thanks to Dr. Chao Wang for

his guidance and intellect and Dr. Michael J. Manto for his encouragement, honest

feedback, and support.

I would also like to thank my colleagues in the Wang lab, especially Pengfei Xie

and Tiancheng Pu for their help in this project’s completion. I would like to give credit to

the Johns Hopkins University teaching staff for teaching me all of the fundamental

science of the field and giving me the tools required to pursue the rest on my own.

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Table of Contents:

Contents List of Figures........................................................................................................................................ v

List of Tables ........................................................................................................................................ vi

Introduction ............................................................................................................................................ 1

Phosphorus and Nitrogen in the Environment ............................................................................... 1

Phosphate, Nitrate, Sulfate, and Chloride as Pollutants ............................................................... 3

Ion Recovery Using Zeolites ............................................................................................................ 6

Cation-Substituted ZSM-5 Preparation and Characterization .......................................................... 7

Materials and Synthesis of Sorbents............................................................................................... 7

X-ray Diffraction Spectroscopy (XRD) ............................................................................................ 8

X-ray Photoelectron Spectroscopy (XPS)....................................................................................... 9

Diffuse Reflectance UV-Vis Spectroscopy (DRS-UV-Vis) ........................................................... 11

Physical Properties of the Sorbents .............................................................................................. 13

Experimental Results and Discussion............................................................................................... 16

Anionic Capture............................................................................................................................... 16

Anionic Release .............................................................................................................................. 21

Mechanism and Active Site for Ion Exchange Process ............................................................... 24

Conclusion ........................................................................................................................................... 28

Results Summary............................................................................................................................ 28

Applications ..................................................................................................................................... 29

Future Steps ........................................................................................................................................ 30

References .......................................................................................................................................... 32

Supplementary Information ................................................................................................................ 36

Materials .......................................................................................................................................... 36

Equipment Specifications ............................................................................................................... 36

v

List of Figures

Figure 1. (Cordell D. R., 2011) If phosphorus use continues as it has, the world’s supply of phosphorus will become quickly diminished. It is only through improvements in technology and increased food-chain and agricultural efficiency that the problem can be mitigated. (2)

Figure 2. (Ator, 2015) Shows the levels of nitrogen and phosphorus pollution off of the eastern

shore. The levels indicated are significantly higher than would negatively affect aquatic life. (5)

Figure 3. Each of the zeolite sorbents after synthesis and drying in the order of (left to right) Unexchanged-ZSM-5, Sn-ZSM-5, Co-ZSM-5, Cu-ZSM-5, Fe-ZSM-5, Ni-ZSM-5, Ce-ZSM-5, La-ZSM-5. (8)

Figure 4. XRD patterns of each ZSM-5 sample compared to JCPDS PDF cards for MFI. (9)

Figure 5. XPS spectra of each of the M-ZSM-5 sorbents with the electron orbital signals and corresponding cations labelled. (10)

Figure 6. Diffuse reflectance UV-Vis spectra of M-ZSM-5 sorbents with labeled electron

domains. (12)

Figure 7. N2 adsorption and desorption profiles for the unexchanged-ZSM-5 and the seven M-ZSM-5 samples. (14)

Figure 8. Shows the anionic uptake ratio of chloride, nitrate, sulfate, and phosphate to each M-

ZSM-5 sorbent. The ratio is defined as the amount of anion adsorbed divided by the amount of

exchanged cation present in the zeolite. (17)

Figure 9. Shows the capture of each individual anion as its percentage of total anionic uptake.

(18)

Figure 10. Shows the molar sum of total anionic uptake for each zeolite (sum of all anions

together) normalized by the loading of metal in the zeolite framework. (19)

Figure 11. Shows the anionic uptake ratio of each of the zeolite samples plotted against the oxidation state of the cation exchanged into the zeolite framework. (20)

Figure 12. Normalized capture and release of (a) phosphate, (b) nitrate and (c) sulfate for each

M-ZSM-5 sorbent. (22)

Figure 13. The synthesis of the M-ZSM-5 sorbents is meant to stabilize a positive charge in the

framework of the ZSM-5. The zeolite itself is composed of repetitive silica-alumina subunits in a

ratio of 11.5:1. (25)

Figure 14. Beginning with the metal exchanged zeolite framework, anions pass over the

cationic active site and some are captured by the cation. After capture, the solution is exposed

to the high salt content regeneration solution and the chloride ions outcompete the anion for the

active site. The final step of release is a relatively dynamic equilibrium in which the anion is

released from the framework in exchange for a chloride ion but where the anion has potential to

reattach to the active site due to the high affinity for the active site from anions like phosphate

that exhibit high hydrogen bonding potential. (26)

vi

List of Tables

Table 1. Si/Al ratios, M/Al ratios, M content, and BET surface area for each of the eight zeolite samples. (14)

Table 2. A quantitative table showing the quantity of each anion adsorbed alongside the

percent of adsorbed anion that is released from the sorbent after exposure to regeneration

solution. (22)

Table 3. Shows pH measurements of the solution before and after the capture and release

phases of the experiment to verify the ion-exchange mechanism. (27)

1

Introduction

Phosphorus and Nitrogen in the Environment

Phosphorus and nitrogen are two of the most essential elements in biology and

in nature. Phosphorus is a key component in DNA and RNA, which is the chemical

basis for the genetics on which all reproducing organisms rely to procreate, as well as in

adenosine triphosphate, which is the primary energy transportation system in humans

and many other living creatures. Phosphorus is a key component of the phospholipid

bilayer in cellular membranes, which serves as the protective housing for all living cells.

Nitrogen, on the other hand, is one of the key components of proteins, which are the

structural and operational building blocks of living things. Proteins are used by both

plants and animals and serve not only as the mechanical building blocks of tissues and

muscles but also as enzymes that maintain biological homeostasis. Due to the

importance of these two elements, any obstacle that challenges their availability to

humans should be treated with concern. Through the past several centuries, the

primary source of dietary phosphorus has been the mining of phosphate rock that is

used, in a solid state, to fertilize the plants that humans consume. These mined

phosphate reserves, however, are finite and depreciating at rates that threaten global

food-stocks within the next fifty years. (Cordell D. D., 2009; Cordell D. W., 2011)

Nitrogen (in its active forms of either nitrate (NO3-) or ammonium (NH4

+)) faces

similar challenges in that man-made forms of fixed nitrogen production require large

amounts of energy in processes such as the Haber-Bosch process, which supplies

2

roughly 70% of the world’s reactive nitrogen but produces nearly 2% of the world’s

carbon pollution. (Bodirsky, 2014; Smil, 1999; Matassa, 2015) With the deteriorating

state of the earth’s environment, high-energy processes like the Haber-Bosch process

might not be environmentally or economically feasible in the near future. These

challenges must be met with alternate technology, in the form of either alternate

sources of phosphorus and nitrogen or ways to regenerate the nutrients that have

already been extracted from the earth.

Figure 1. (Cordell D. R., 2011) If phosphorus use continues as it has, the world’s supply of phosphorus will become quickly diminished. It is only through improvements in technology and increased food-chain and agricultural efficiency that the problem can be mitigated.

The need for enhancement of both the phosphorus and nitrogen cycles may be

more pressing than initially predicted. The decrease in use of and reliance on

phosphorus as a means for fertilizer remains viable, but is unlikely that large scale

3

changes in diet preference would occur without mandate. (Cordell D. S.-N., 2009) As

mentioned, the global supply of phosphate rock is expected to run out within the century

and large portions of the world use soil that is drastically undernourished. For one

example, areas of Sub-Saharan Africa have close to a 30% undernourishment rate, with

75% of their farming soil being nutrient deficient. (Cordell D. D., 2009) These problems

are not going away, and are only expected to compound as global population

exponentially increases while relying on the same nonrenewable sources of fertilizer.

Phosphate, Nitrate, Sulfate, and Chloride as Pollutants

Apart from phosphorus and nitrogen’s important roles in human and plant

biology, a parallel concern is the fact that these nutrients are harmful contaminants in

natural bodies of water that lead to eutrophication, the death of a body of water.

(Mainston, 2002; Tilman, 2001; Carpenter, 1998; Conley, 2009) The vast majority of

nutrients applied to crop fields (>80%) run into natural bodies of water in the form of

agricultural runoff, where they feed algae that impairs natural flow and removes

essential nutrients, making the water useless to feed crops or people. (Lewis, 2011)

Phosphate and nitrate have been chosen in this study for the high degree of impact that

would come from finding a sustainable process for the removal and reutilization of them.

All four anions in this study (phosphate, nitrate, sulfate, and chloride) contribute to the

fact that approximately 1.1 billion people in the world have no access to clean drinking

water, a number that grows annually. (Gadgil, 1998) Chloride concentrations even in

water in the northeastern United States have reached concentrations as high as 25% of

ocean concentrations and are expected to increase unless mitigation is introduced.

4

(Kaushal, 2005) Chloride is also one of the most abundant ions in sea water which,

pending efficient advances in desalination technology, might be a key source of drinking

water for humans as world population increases. Sulfate poses a unique problem as a

pollutant in that it leads to increased production of mineral sulfides in inland aquatic

ecosystems which have long term adverse effects on water systems such as

acidification and mobilization of heavy metals. (Baldwin, 2012) Although sulfate and

nitrate are not typically thought of as pollutants, they can serve as such if they exceed

the appropriate levels for drinking. The levels that we will use in our experiments far

exceed safe drinking levels. All four of these pollutants have been chosen in this study

for their unique affects that they have on water systems.

These pollutants have special relation to waterways local to Johns Hopkins

University such as the Chesapeake Bay. Inputs of nitrogen and phosphorus into the

Chesapeake Bay have increased 7- and 18-fold respectively since the first government

attempt to control the eutrophication problem in the 1950’s. (Boesch, 2000) The same

projection source states that this problem has the potential to reach a point of

irreversibility. The increase in sediment in the Bay has increased to the point that large

flocks of algae and sediment are visible from above. A water quality increase of at

least 40% in the coming years is going to be required to mitigate the issue. (Boesch,

2000)

5

Figure 2. (Ator, 2015) Shows the levels of nitrogen and phosphorus pollution off of the eastern

shore. The levels indicated are significantly higher than would negatively affect aquatic life.

6

Ion Recovery Using Zeolites

The use of zeolites as adsorbents in water and wastewater is not a novel process

in itself and has been performed using many different zeolite frameworks for different

applications such as phosphate and nitrate adsorption and heavy metal removal.

(Wang, 2010; Baileyab, 1999; Kesraoui-Ouki, 1994; Morse, 1998; Manto, 2017)

Zeolites tested include ZSM-5, clinoptilolite, mordenite, chabazite, and Ferrierite (with

many others) which have structure types of MFI, HEU, MOR, CHA, and FER

respectively. (Onyango, 2007; Wang, 2010) Zeolites are unique, however, in that they

provide countless numbers of substitution and geometric variability that make any

process involving them further optimizable. The concept of recyclability is also a

pressing issue in the field of zeolite based adsorption because cost is a factor for any

industrial process and many substrates for adsorption, such as heavy metals, have high

binding affinity and do not allow for easy regeneration and reuse of sorbent materials.

The factors of regeneration, adsorption capacity, and selectivity for adsorption

substrates will all be considered in the study presented here.

7

Cation-Substituted ZSM-5 Preparation and Characterization

In an attempt to study not only the optimization of the anion recovery process but

also what causes the different capacities that different cations possess for recovery,

extensive characterization is performed on each of the zeolite samples used in this

study.

Materials and Synthesis of Sorbents

In order to perform this study using a variety of different types of metals, the

cations of iron, cobalt, nickel, copper, tin, lanthanum and ceria were chosen for cation

exchange. An unexchanged precursor is also included as a background. Synthesis of

the sorbents begins with commercially available NH4-ZSM-5 (Si/Al = 11.5, Alfa Aesar).

The zeolite is calcined at 450 ºC for 4 h to convert the zeolite to H-ZSM-5, releasing

NH3 gas. 10 g of the resulting H-ZSM-5 powder was suspended in 100 mL of 1 M

NaNO3 solution and stirred for 4 h at 80 oC. The zeolite was removed, washed, and the

sodium nitrate exchange is repeated two more times. The resulting zeolite is dried at 95

oC overnight and labeled as the unexchanged-ZSM-5 to be used as the background.

The metal-cation-substituted ZSM-5 derivatives are made by suspending 3.0 g of

unexchanged-ZSM-5 in 220 mL of 0.01 M metal acetate aqueous solution, covered, and

stirred at constant stirring for 24 hours. The zeolite is removed from solution and

washed and the metal acetate exchange is repeated two more times for a total of three

metal-cation exchanges. Each sample of sorbent is washed with 1.5 L of DI water in a

Buchner Funnel and dried overnight at 95 oC in static air. Henceforth, the cation

8

exchanged sorbents will be denoted as M-ZSM-5, where M = (Fe, Co, Ni, Cu, Sn, La,

Ce ).

Figure 3. Each of the zeolite sorbents after synthesis and drying in the order of (left to right) Unexchanged-ZSM-5, Sn-ZSM-5, Co-ZSM-5, Cu-ZSM-5, Fe-ZSM-5, Ni-ZSM-5, Ce-ZSM-5, La-ZSM-5

X-ray Diffraction Spectroscopy (XRD)

The purpose of the metal-cation exchange is to stabilize a cation onto the

framework of the ZSM-5 but not to make any modifications to the MFI (mordenite

framework inverted) framework of the zeolite itself or to form any metal oxides

anywhere in the zeolite (within the pores or on the surface of the crystals).

Accomplishment of this is confirmed using X-ray Diffraction Spectroscopy (XRD) shown

in Figure 4.

9

The XRD scans of each sorbent show no sign of metal oxide formation, which

would be manifested as additional peaks present in the XRD scans indexed to unique

metal oxides. Comparison of the unexchanged scan against the exchanged scans

show that no modifications were made to the MFI framework during exchange and the

structure remains intact.

X-ray Photoelectron Spectroscopy (XPS)

A possible source of performance variation between cation species for this

process would be differences in the oxidation state of the cations on the framework of

the zeolites. The oxidation state of the cations in the framework are analyzed using

XPS. The results of the XPS analysis are shown in Figure 5.

Figure 4. XRD patterns of each

ZSM-5 sample compared to

JCPDS PDF cards for MFI

10

Figure 5. XPS spectra of each of the M-ZSM-5 sorbents with the electron orbital signals and corresponding cations labelled.

For Na-ZSM-5 (unexchanged-ZSM-5), the 1s signal at a binding energy of 1072

eV is indicative of Na+. (Liese, 1997) For Cu-ZSM-5, 2p1/2 signal at a binding energy of

955 eV, 2p3/2 at 935 eV, and satellite signal at 945 eV indicate Cu2+ present in the ZSM-

5 network. (Xu W. Z., 2015; Manto, 2017) For Co-ZSM-5, the 2p3/2 signal at 785 eV as

well as the satellite peak at 787 eV correspond to Co2+. (da Cruz, 1998) For Ni-ZSM-5,

the 2p3/2 signal at 851 eV and 2p3/2 signal at 868 eV indicate cations are present in the

Ni2+ state. (Kónya, 2004) For Fe-ZSM-5, the strong 2p3/2 signal at 711 eV and 2p1/2

signal at 722 eV indicate the majority presence of Fe3+ in the framework. (Long, 2000)

For Ce-ZSM-5, the 3d signals (both 3d3/2 and 3d5/2) at 886 eV and 900 eV as well as the

absence of a signal at 905 eV indicate the presence of Ce3+ and absence of Ce4+.

(Carja, 2007) For La-ZSM-5, 3d5/2 signals at 839 eV and 835 eV as well as 3d3/2 signals

11

at 853 eV and 857 eV indicate La3+ presence. (Zhang Q. Y., 2015) For Sn-ZSM-5, the

3d3/2 signal at binding energies of 487 eV and 495 eV are indicative of Sn4+. (Zhang Y.

Z., 2011; Morales, 2005; Xia, 2014) The state of Sn is further confirmed as Sn4+ by

DRS-UV-Vis.

Diffuse Reflectance UV-Vis Spectroscopy (DRS-UV-Vis)

Although XRD and XPS provide substantial evidence to the form and oxidation

state of each of the metal species, a secondary technique, DRS-UV-VIS, is employed

as confirmation. DRS-UV-Vis scans of each of the exchanged M-ZSM-5 samples are

shown in Figure 6.

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Figure 6. Diffuse reflectance UV-Vis spectra of M-ZSM-5 sorbents with labeled electron

domains.

In the Fe-ZSM-5 sample, the signal at 43,860 cm-1 indicates Fe3+ in a tetrahedral

coordination. (Liang T. C., 2016) The signal at 34,480 cm-1 in the same sample indicates

Fe3+ in the octahedral coordination. (Vélez, 2014) In the Co-ZSM-5 sample, signals at

16,000, 17,150, 18,600, and 21,200 cm-1 all indicate beta-type Al pairings in the channel

intersections while the signals at 20,100 and 22,000 cm-1 indicate gamma-type Al pairing

in the sinusoidal channels of the zeolite framework. (Liang T. C., 2016) It should also be

noted that there is no signal at 31,000 cm-1 which would indicate cobalt oxide formation.

(Liang T. C., 2016) In the Ni-ZSM-5 sample, the signals at 36,360 cm-1 and 25,000 cm-1

indicate the presence of Ni2+ in the trigonal coordination in sodelite cages and could also

13

represent Ni2+-O charge transfer. (Pullabhotla, 2009; Pawelec, 2004) The signal at

13,890 cm-1 in the same sample, indicates Ni2+ in the near-tetrahedral coordination.

(Pawelec, 2004) In the Cu-ZSM-5 sample, the signal at 39,000 cm-1 indicates an

interaction between Cu2+ species and oxygen in the ZSM-5 framework and the signal at

13,790 cm-1 indicates Cu2+ species in hexagonal coordination. (Urquieta-González, 2002)

In the Sn-ZSM-5 sample, the signal around 44,440 cm-1 indicates Sn tetrahedrally

coordinated. (Moliner, 2010) This indicates that the tin sits in an Sn4+ state. (Lefebvre,

1991) In the La-ZSM-5 sample, the signal around 39,200 cm-1 indicates La3+-O charge

transfer. In the Ce-ZSM-5 sample, the signal at 39,210 cm-1 indicates Ce3+-O transfer.

(Gu, 2013; Li, 2010) The signal at 33,330 cm-1 in the same sample indicates the minor

presence of Ce4+ but the signal strength is minimal compared to that from Ce3+. (Gu,

2013).

Physical Properties of the Sorbents

In order to make the data obtained from our eventual kinetic analysis more

comparable, we chose to analyze the physical properties of the sorbents. We started

this analysis by performing inductively coupled plasma mass spectroscopy (ICP) to

obtain the loading of exchanged metal in the framework of the ZSM-5. We then

measured the surface area of the zeolite samples using Brunauer-Emmett-Teller theory

(BET). The adsorption/desorption profiles from BET analysis are shown in Figure 7 and

a summary of the physical properties of all eight zeolite samples are shown in Table 1.

14

Figure 7. N2 adsorption and desorption profiles for the unexchanged-ZSM-5 and the seven M-ZSM-5 samples.

Table 1. Si/Al ratios, M/Al ratios, M content, and BET surface area for each of the eight zeolite samples

From this point forward, all kinetic analysis will be normalized according to the M

loading of each sample. This normalization is performed because such metal

15

exchanged sites are believed to be the active site for exchange via an ion exchange

mechanism. (Manto, 2017)

16

Experimental Results and Discussion

After all sorbents are synthesized and characterization confirms the integrity of

the synthesis reactions, experimental analysis of the sorbents’ performance can begin.

Anionic Capture

Each of the eight zeolites (unexchanged, Fe, Co, Ni, Cu, Sn, La, and Ce-ZSM-5)

were suspended in 10.0 mL of solution containing 32.3 mmol/L each of Na2HPO4,

NaNO3, Na2SO4, and NaCl (solution pH = 7.5, henceforth denoted as the capture

solution) with loading of 300.0 mg and stirred for 2 h. The resulting solution after

exposure to capture solution was analyzed by ion chromatography.

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Figure 8. Shows the anionic uptake ratio of chloride, nitrate, sulfate, and phosphate to each M-

ZSM-5 sorbent. The ratio is defined as the amount of anion adsorbed divided by the amount of

exchanged cation present in the zeolite.

Figure 8 shows the anionic uptake capacity of each sorbent for each anion. The

uptake numbers are calculated as the total molar ratio of anions removed relative to the

metal present in the zeolite. The cations in the zeolite framework have an overall

capture ability in the order of Sn4+ > La3+ > Co2+ > Ce3+ > Cu2+ > Fe3+ > Ni2+.

18

Figure 9. Shows the capture of each individual anion as its percentage of total anionic uptake

Due to the fact that this process could have a wide variety of applications to

different industrial processes, it is important to observe the selectivity of each zeolite for

a particular anion in the presence of all other anions. (Yilmaz, 2009) Figure 9 shows

this comparison. Phosphate is selected for at an observably higher percentage than the

rest of the anions on average. This affinity is followed by sulfate and nitrate which is

trailed by chloride. This is possibly due to the relative decrease in electronegativity and

capacity for hydrogen bonding in the order of HPO42- > NO3

-, SO42- > Cl- as it has been

shown that these anions adsorb to positive surfaces in an order of affinity that follows

the same trend. (Chitrakar, 2006; Zeltner, 1988)

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Figure 10. Shows the molar sum of total anionic uptake for each zeolite (sum of all anions

together) normalized by the loading of metal in the zeolite framework.

Another possible area of interest for this work would be total deionization of water

for purification purposes. Figure 10 compares the total anionic uptake of each of the

zeolite samples.

Notable performances from the sorbents are Sn-ZSM-5 with the highest total

anionic capture of 7.36 ± 0.258 molanion/molcation as well as the highest capture for each

individual anion. Selectivities for each anion are reported by Figure 9 . The metal-

exchanged ZSM-5 sorbent with the highest selectivity towards phosphate is Ni-ZSM-5

with 83.98%. Similarly, the highest selectivity for sulfate is by Co-ZSM-5 with 17.8%.

The highest selectivity for nitrate is also by Co-ZSM-5 with 20.3%. The highest

selectivity for chloride is by Cu-ZSM-5 with 8.44%.

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Figure 11. Shows the anionic uptake ratio of each of the zeolite samples plotted against the oxidation state of the cation exchanged into the zeolite framework.

In order to further understand what gives different cations in the framework of

ZSM-5 different abilities to capture anions, the uptake ratios were compared to different

properties of the metal cations. Figure 11 shows an observed trend between the

cationic oxidation state and the anionic uptake ability. As confirmed by

characterizations shown in Figures 5 and 6, Ni, Cu, and Co exist in the zeolite

framework in the 2+ oxidation state, La, Ce, and Fe exist in the 3+ state, and Sn exists

in the 4+ state. An increasing trend is observed indicating that more strongly positive

oxidation states allow for more anionic capture (i.e. 4+ cations > 3+ cations > 2+

cations). This conclusion follows observations drawn from Figure 11. In order to

identify the trend that exists between cations of the same oxidation state, the anionic

uptake is subplotted against the hydration radius of the cation. (Yang, 2005) For both

the 2+ and 3+ cations, the anionic uptake increases as the hydration radius increases.

21

One possible explanation for this phenomenon is that as the charge radius of a cation

increases, the surface area of charge available to exchange with anions also increases,

making higher charge radius cations more capable of capturing anions. This trend

between uptake and oxidation state is a stronger correlation than the one between

uptake and hydration radius. (Person, 2010; Grzybkowski, 2006; Mahler, 2012; Rice,

2000; Yang, Density Functional Theory Calculations on Various M/ZSM-5 Zeolites:

Interaction with Probe Molecule H2O and Relative Hydrothermal Stability Predicted by

Binding Energies, 2005)

Anionic Release

In order to study the ability of the zeolites to release the anions they have

captured, and ultimately view the process of anionic recyclability as a whole, a

regeneration study was employed. Just as for the capture and selectivity trials, each of

the eight different zeolites (Fe-ZSM-5, Co-ZSM-5, Ni-ZSM-5, Cu-ZSM-5, Sn-ZSM-5, La-

ZSM-5, Ce-ZSM-5, and unexchanged ZSM-5) were suspended in 10 mL of capture

solution and recovered after 2 h of stirring. The dry powders were resuspended in 10

mL of regeneration solution (323.0 mmol NaCl). This regeneration solution manipulates

the equilibrium of the nutrient capture process to force what has been captured by the

zeolite back into solution at the exchange of an overabundance of chloride anions.

NaCl is used to study the regeneration capacity of the sorbents because, in an ideal

process, sea water would be used as the regeneration solution. The zeolites were

stirred in this solution for 2 h and a sample was taken for chromatographic

quantification. It should be noted that, due to the method for anion release chosen, the

22

release efficiency of chloride cannot be studied because the chloride captured and the

chloride in the regeneration solution could not be isolated from each other.

Table 2. A quantitative table showing the quantity of each anion adsorbed alongside the

percent of adsorbed anion that is released from the sorbent after exposure to regeneration

solution.

Before trends are identified, the release capacities of each sorbent are shown by

Table 2. Trends for the release and capture capacity of each sorbent are shown by

Figure 12.

Figure 12. Normalized capture and release of (a) phosphate, (b) nitrate and (c) sulfate for each

M-ZSM-5 sorbent.

23

Ideal performance of a sorbent for this regeneration process is a sorbent that

captures the greatest amount of the anion of interest possible and then releases all of

what is captured. Graphically, this would be a point it the upper, right-hand-side of the

capture/release graphs in Figure 12. Cu-ZSM-5 has the highest performance for

phosphate recovery (2.61 molP/molcation capture and 2.59 molP/molcation release). Sn-

ZSM-5 has the highest performance for nitrate recovery (1.33 molN/molcation capture and

0.33 molN/molcation release). Sn-ZSM-5 also has the highest performance for sulfate

recovery (1.24 molS/molcation capture and 0.31 molS/molcation release).

For phosphate and nitrate release, Fe-ZSM-5 has the highest release potential.

For sulfate, the unexchanged-ZSM-5 and the Fe-ZSM-5 have the highest release

potential, although the performance of Fe-ZSM-5 clearly exceeds unexchanged-ZSM-5

for sulfate capture because it captures 23x more anion to begin with. Figure 12 shows

interesting trends related to the capture affinity of different sorbents and the

corresponding release affinities. Sorbents that have especially high capture

performances such as (Sn, La, Co)-ZSM-5 ranging from 5.55 – 7.36 molanion/molcation

may hold onto their captured anions too strongly, such that they cannot be released in

regeneration solution. (Chitrakar, 2006; Zeltner, 1988) Sn-ZSM-5 (containing Sn

cations in the 4+ oxidation state) posts release percentages as low as 24.8%. This

means that the overload in in the chloride-anion equilibrium is not enough to overcome

the Sn-ZSM-5’s affinity for the anion and regenerate the material. This factor becomes

more important when considering these sorbents for industrial scale-up because

recyclability and reuse of materials becomes more of an issue for cost and

sustainability. Fe-ZSM-5 posts the highest percentages for regeneration (P: 94.3%, N

24

96.5%, and S:100%) making it a much more industrially viable sorbent, even though it

does not possess the highest capture affinity for any of the anions. It should be noted,

however, that high release potential alone does not indicate desirable or even practical

performance for the anion recovery process. A high level of release is merely indicative

of an anion’s relatively weaker affinity for the active site than chloride, so it is logical that

the unexchanged sample would release easily given its marginal ability to capture. The

capture and release need to be observed together in order to evaluate a sorbent’s

ability to perform the recovery process.

As previously mentioned, optimal performance would be a sorbent that performs

in the upper-right most corner of the release vs. capture plot. This can be calculated by

using the Pythagorean theorem to find the distance away from the origin. Due to the

fact that adsorption can be a function of many conditions, I only propose to rank the

performance of each sorbent under the conditions of the experiment with loose

extrapolations to other systems. The process of phosphate recovery is optimally

performed by Cu2+ > Sn4+ > Ce3+ > La3+ > Fe3+ > Co2+ > Ni2+ followed by the control.

The processes of both nitrate and sulfate recovery are optimally performed by Sn4+ >

Co2+ > La3+ > Ce3+ > Fe3+ > Cu2+ > Ni2+ followed by the control.

Mechanism and Active Site for Ion Exchange Process

To better understand the regeneration process, it is important to determine and

examine the active site for the process. We believe the exchanged metal site is the

active site for exchange because, in other studies, processes such as adsorption and

25

catalysis have been shown to have performance that is directly proportional to the

amount of metal exchanged into the zeolite framework. (Manto, 2017; Chen, 1998;

Schwiddler, 2005; Beznis, 2010; Kauchky, 2000; Maia, 2010; Zhang Q. Y., 2015) The

materials themselves for our experiments are based on the stabilization of a positive

charge in the framework of the unexchanged zeolite as shown in Figure 13. The

experiment used to examine the active site and proposed ion-exchange mechanism

uses only Cu-ZSM-5 as a sorbent and a capture solution containing only 32.3 mmol/L of

Na2HPO4. The regeneration solution is still 323 mmol/L of NaCl as in the experimental

portion of this study.

Figure 13. The synthesis of the M-ZSM-5 sorbents is meant to stabilize a positive charge in the

framework of the ZSM-5. The zeolite itself is composed of repetitive silica-alumina subunits in a

ratio of 11.5:1.

After synthesis of the sorbents, the cations are initially stabilized by hydroxide

ions present in the aqueous solution. Upon exposure to the anion rich capture solution,

the anions outcompete the hydroxides on the metal sites for the positive charge and the

hydroxides are released into solution. The loaded zeolite is then removed from solution

by centrifugation and exposed to the high salt concentration regeneration solution.

When the overabundance of chloride ions outcompetes the anion of interest for the

26

active site, the anion is released into solution. (Manto, 2017) The entire capture and

release process is visualized by Figure 14.

Figure 14. Beginning with the metal exchanged zeolite framework, anions pass over the

cationic active site and some are captured by the cation. After capture, the solution is exposed

to the high salt content regeneration solution and the chloride ions outcompete the anion for the

active site. The final step of release is a relatively dynamic equilibrium in which the anion is

released from the framework in exchange for a chloride ion but where the anion has potential to

reattach to the active site due to the high affinity for the active site from anions like phosphate

that exhibit high hydrogen bonding potential.

In order to verify the capture and release ion-exchange mechanism, the pH of the

solution was measured before and after exposure to capture solution and before and

after exposure to regeneration solution. Releasing the hydroxide ions in the initial

phase of exposure to capture solution should cause an increase in pH and the

subsequent exposure to regeneration solution should also cause an increase in pH due

to the loss of chloride ions from solution (which are less basic than phosphate which is

27

released into solution upon regeneration). Table 3 confirms the predicted pH changes

for this hypothesis and confirms the ion-exchange mechanism for the anion

regeneration process. (Manto, 2017)

Solution Stage pH (± .01)

Na2HPO4 in H20 Before capture 8.6

After capture 9.5

NaCl in H20 Before release 6.9

After release 8.3 Table 3. Shows pH measurements of the solution before and after the capture and release

phases of the experiment to verify the ion-exchange mechanism.

28

Conclusion

Results Summary

We were able to successfully exchange seven different metal cations onto the

framework of ZSM-5. We verified the synthesis of the sorbents using XRD, measured

the oxidation state using DRS-UV-VIS and XPS, and characterized the physical

properties using BET and ICP. All characterization is shown in Figures 4-7 as well as in

Table 1. With those seven exchanged zeolites, and an eighth unexchanged-ZSM-5 as

a control, we captured the anions of phosphate, nitrate, sulfate, and chloride with

different affinity and selectivity according to Figures 8-12 as well as Table 2. Capture

capacities were attained with affinity in the order of Sn4+ > La3+ > Co2+ > Ce3+ > Cu2+ >

Fe3+ > Ni2+ with the highest capture capacity from Sn-ZSM-5 of 7.36 ± 0.258

molanion/molcation. A trend of capture capacity against oxidation state was observed in

the order of 4+ cations > 3+ cations > 2+ cations. We were then able to regenerate the

sorbents by over-exchanging chloride onto the active site using a brine regeneration

solution. Regeneration numbers and trends separated by anion are shown in Figure 12

but, by oxidation state, follow a general reverse trend of the capture affinity (2+ cations

> 3+ cations > 4+ cations). We also believe that the entire regeneration process is

governed by an ion exchange mechanism using the substituted cation site as the active

site for exchange as shown in Figures 13 and 14 as well as Table 3.

29

Applications

Seeing that this study has impactful engineering applications for the processes of

wastewater treatment and deanionization, conclusions are also drawn on the basis of

anionic selectivity for the complete regeneration process to recommend for industrial

scaleup. The process of phosphate recovery is optimally performed by Cu2+ > Sn4+ >

Ce3+ > La3+ > Fe3+ > Co2+ > Ni2+ followed by the control. The processes of both nitrate

and sulfate recovery are optimally performed by Sn4+ > Co2+ > La3+ > Ce3+ > Fe3+ > Cu2+

> Ni2+ followed by the control. The performance of each cation exchanged zeolite (with

the cation listed to represent M in M-ZSM-5) is compared in Figures 8-12.

30

Future Steps

Now that significant advancements have been made in using high capacity

zeolite based sorbents for anion recovery in model solutions, natural follow up studies

present themselves including anion recovery in real wastewater systems, cation

recovery, and desalination.

This study used a model solution of sodium stabilized phosphate, nitrate, sulfate,

and chloride. These anions are present in natural wastewater sources but are present

in different concentrations and are in the presence of cations, other anions, dissolved

solids, and biological molecules depending on the source of the water. (Gadgil, 1998)

The presence of biological species remains a large obstacle to the use of molecular

sieves for large scale wastewater clean-up. (Han, 2007) Previous studies have begun

to examine the stress of these variables on zeolites’ abilities to capture and release ions

in real wastewater. Manto, et. al. show that Cu-ZSM-5 is resilient to the contaminants

found in wastewater samples taken downstream from an anaerobic digestion process

with high presence of biologics and solids. Such results are promising for the industrial

scale up potential of our zeolite based process. Due to the variations in contaminants in

wastewater, studies using the zeolite of interest on the actual wastewater to be treated

should be performed on a small scale to make sure that other anions do not outcompete

the ion of interest for the active site.

This study focused on exploiting a localized negative charge in the framework of

ZSM-5 to stabilize a local positive charge to capture passing anions. In theory, the

opposite process should be entirely viable. (Qui, 2009) For our synthesis process, the

31

cation exchange process can be skipped leaving the negative charge in the aluminum

sites of the ZSM-5 framework as H-ZSM-5. The possibility of only exchanging half of

the site in the framework of ZSM-5 making it capable of capturing both cations and

anions could also be explored. Due to the different geometric and electromagnetic

advantages that zeolites provide, other zeolites can be explored for this process as well.

(Tonetto, 2004; Chutoransky, 1973; Xu J. M., 2006; Martinez, 2005) Certain other

zeolites such as ETS-10 are titanosilicate based and have a stable local negative

charge in the framework of the zeolite that could be used to capture passing cations.

(Anderson, 1994)

The availability of fresh, desalinated drinking water is becoming a more pressing

issue. (Gadgil, 1998) The process I have presented shows minimal potential for

capturing chloride, especially in the presence of competing anions. The regeneration

process we have proposed also is not viable for a desalination process because it relies

on using high concentrations of NaCl to regenerate the sorbent. Our group is currently

working on a process that uses a combination of sorbents to capture sodium and

chloride simultaneously and regenerate those sorbents by thermally manipulating the

Gibb’s free energy of the solution creating a sustainable, recyclable way to produce

clean drinking water for society’s future.

32

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Supplementary Information

Materials

The following chemicals were purchased and used without further modification or

purification: Ammonium-ZSM-5 (Si/Al = 11.5, Alfa Aesar), cerium(III) acetate hydrate

(Ce(CO2CH3)3-H2O, 99.9%, Sigma-Aldrich), cobalt(II) acetate tetrahydrate

(Co(CO2CH3)2-(H2O)4, 98%, Strem Chemicals), copper(II) acetate monohydrate

(Cu(CO2CH3)2-H2O, 99.0%, Sigma-Aldrich), iron(II) acetate (Co(CO2CH3)2, 99.99%,

Sigma-Aldrich), lanthanum(III) acetate hydrate (La(CO2CH3)3-H2O, 99.9%, Aldrich),

nickel(II) acetate tetrahydrate (Ni(CO2CH3)2-(H2O)4, 99.998%, Sigma-Aldrich), tin(II)

acetate (Sn(CO2CH3)2, 98%, Sigma-Aldrich), sodium chloride (NaCl, 99%, Alfa Aesar),

sodium nitrate (NaNO3, 99.0%, Sigma-Aldrich), sodium phosphate dibasic anhydrous

(Na2HPO4, 99%, Sigma-Aldrich), sodium sulfate (Na2SO4, 99%, Sigma-Aldrich).

Deionized water was collected from an ELGA PURELAB flex apparatus.

Equipment Specifications

Scanning electron microscopy (SEM) images were taken on a JEOL 6700F field

emission electron scanning microscope operating at 10.0 kV.

X-ray diffraction (XRD) patterns were obtained from a PANalytical X’Pert3 X-ray

diffractometer equipped with a Cu Kα radiation source (λ = 1.5406 Å).

37

X-ray photoelectron spectroscopy (XPS) measurements were taken on a Thermo

Scientific ESCALAB 250Xi photoelectron spectrometer equipped with an Al Kα X-ray

source.

Nitrogen adsorption measurements were measured on a Micromeritics ASAP

2010 instrument with the samples degassed under vacuum at 300 °C for 4 h.

The M and Al contents were determined by inductively coupled plasma mass

spectrometry (ICP-MS) using a PerkinElmer Elan DRC II Quadrupole ICP-MS after

dissolution of the zeolites in HF.

Diffuse reflectance UV-Vis spectra were obtained on a PerkinElmer Lamba 950

UV-Vis-NIR spectrometer equipped with a 100 mm InGaAs integrating sphere.

Concentrations of each anion were quantified with a Thermo Scientific Dionex

ICS-2100 ion chromatography system equipped with an IonPac AS18 analytical column

(4 x 250 mm) and a Dionex AS40 automated sampler with a KOH eluent generator at a

sample flowrate of 0.9 mL/min.

38

39

Mitchell Keller

e-mail: mkelle29@jhu.edu alternate: keller.mitchell1995@gmail.com

Education Johns Hopkins University, Baltimore, Maryland

M.S.E. in Chemical and Biomolecular Engineering, 2017-2018 B.S. in Chemical and Biomolecular Engineering, 2013-2017

Awards and Honors Provost’s Undergraduate Research Award (PURA), 2016

Received a research award for my work in the Wang Nanoenergy lab for discovering ways to extract dissolved pollutants such as inorganic phosphorus and ammonium from wastewater using recyclable, transition metal bound zeolites

Sarah K. Doshna Undergraduate Research Award, 2017 Received this award from the Chemical and Biomolecular Engineering department for my overall undergraduate research accomplishments

AIChE Mideast Regional Paper Competition 3rd Place, 2017

Received this award from the Mideast Regional chapter of the AIChE for an abstract preparation and oral presentation of my research related to phosphorus recovery using copper exchanged ZSM-5 zeolites

ACS National Conference Certificate of Merit, 2017

Received this award from a national ACS committee for my Co-Authorship of the oral presentation of work related to the recovery of inorganic phosphorus via copper exchanged ZSM-5 given by my associate

Research Experience Undergraduate Research, 2015-2017

The focus of my undergraduate research was in the published work of using ceria nanocrystals of various morphologies to catalyze the dephosphorylation of model and real-world molecules such as para-nitrophenyl phosphate, adenosine triphosphate, and adenosine monophosphate. During this work, I gained experience in lab techniques including nanoparticle synthesis, pipetting, x-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), ultra-violet/visible light spectroscopy (UV-Vis), and other general lab techniques. I was also actively involved in larger scale hypothesizing, project direction, and manuscript preparation.

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Graduate Research, 2017-2018

As a graduate researcher, my interests have shifted towards the completion of the phosphorus cycle as a whole. With my team’s novel methods to catalyze the cleavage of phosphates and nitrates, we wanted to have the ability to selectively remove those phosphates and nitrates from reaction solution, which is heavily a matter of separation technology. We are currently employing cation exchanged zeolites to sustainably remove the molecules of interest from solution in a way that make the recovered nutrients reusable as solution-state fertilizers and preserves the structural integrity of our sorbent, making the process completely recyclable. We are also beginning to apply this technology to salt water as a way to desalinate the water to safe levels for drinking. During my graduate work I have added skills such as scanning electron micron microscopy with energy dispersive x-ray spectroscopy (SEM with EDX), inductively coupled plasma mass spectroscopy (ICP MS), ion chromatography (IC), and high-performance liquid chromatography (HPLC). I am also more actively involved in the grant proposal, publication, and patent aspects of the research process.

Research Interests

• Sustainable engineering

• High energy catalysts

• Wastewater treatment

• Desalination

• Ion Exchange Separation Publications Manto, M. J.; Xie, P.; Keller, M. A.; Liano, W. E.; Pu, T.; Wang, C. “Recovery of Inorganic Phosphorus Using Copper-Substituted ZSM-5.” ACS Sustainable Chem. Eng. 2017, 5 (7), 6192-6200. Manto, M. J.; Xie, P.; Keller, M. A.; Liano, W. E.; Pu, T.; Wang, C. “Recovery of Reactive Nitrogen from Aqueous Solutions Using ZSM-5.” Chemosphere. 2018, 198(May), 201-205 Manuscripts Under Review Manto, M. J.; Keller, M. A.; Guan, H.; Smith, K. J.; Xie, P.; Pu, T.; Wang, C. “Catalytic Deamination via Ceria Nanocrystals.” Nature: Catalysis. In Preparation.

41

Manuscripts In Preparation Manto, M. J.; Keller, M. A.; Liano, W. E.; Smith, K. J.; Guan, H.; Pu, T.; Xie, P.; Wang, C. “Selective Dephosphorylation of Nucleic Acids via Mesoporous Nanoceria.” Nat. Catal. In Preparation. Manto, M. J.; Pu, T.; Keller, M. A.; Guan, H.; Wang, C. “Effects of Framework Morphology on Recovery of Inorganic Phosphorus Using Cu-Exchanged Zeolites.” J. Am. Chem. Soc. In Preparation. Keller, M. A.; Manto, M. J.; Pu, T.; Wang, C. “Effects of Metal Cations Exchanged in ZSM-5 for Inorganic Phosphorus Recovery.” J. Am. Chem. Soc. In Preparation. Patents Wang, C.; Manto, M. J.; Xie, P. “Methods of Removing and Recovering Phosphorus from Aqueous Solutions.” 2017, US Patent Pending. Wang, C.; Manto, M. J.; Keller, M. A.; Xie, P. “Catalytic Deamination Using Ceria Nanocrystals.” 2017, Under Review. Presentations Manto, M. J.; Xie, P.; Keller, M. A.; Liano, W. E.; Pu, T.; Wang, C. “Recovery of Inorganic Phosphorus via Metal-Exchanged ZSM-5.” 2017 AIChE Annual Meeting, 1 November 2017, Minneapolis, MN. Manto, M. J.; Xie, P.; Keller, M. A.; Liano, W. E.; Pu, T.; Wang, C. “Phosphate Sequestration via Copper-Exchanged ZSM-5.” 254th ACS National Meeting, 23 August 2017, Washington, DC. Manto, M. J.; Xie, P.; Keller, M. A.; Liano, W. E.; Pu, T.; Wang, C. “Recovery of Inorganic Phosphorus Using Copper-Substituted ZSM-5.” 254th ACS National Meeting, 22 August 2017, Washington, DC. Manto, M. J.; Xie, P.; Keller, M. A.; Liano, W. E.; Pu, T.; Wang, C. “Recovery of Dissolved Ammonium and Inorganic Phosphorus via ZSM-5.” E2SHI Symposium: Addressing Food Energy and Water Challenges through Research for Development, 3 April 2017, Baltimore, MD.

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Manto, M. J.; Xie, P.; Keller, M. A.; Liano, W. E.; Pu, T.; Wang, C. “Phosphorus Recovery via Cation-Exchanged ZSM-5.” AIChE 2017 Mid-Atlantic Student Regional Conference, 25 March 2017, Glassboro, NJ Work Experience

RPA Engineering, Life Sciences Engineering Intern

• Wrote and assisted in the execution of installation and operational verification testing for a critical clean in place wash station upgrade and process chilling system required to support a sterile vial packaging line in a pharmaceutical manufacturing facility

Wang Nanoenergy Research Laboratory, Graduate Researcher

• Aid in all aspects of the research process including experimental design, data analysis and publication under Dr. Chao Wang

• Working in the field of nanocatalysis specifically related to renewable sources of energy

• Presented a poster at the 2017 ACS National Conference in Washington, D.C. Kaybrook Swim Club, Head Diving Coach and Lifeguard

Extracurricular Activities and Leadership

Collegiate Varsity Track and Field

• Indoor Conference Championship Silver Medalist in Shot Put

• School records in Hammer Throw, Javelin Throw and Shot Put Fellowship of Christian Athletes, President

• FCA is a group of college age Christian athletes that participate in weekly meetings, group retreats, and community/campus service projects.

AIChE/SBE/ACS Member Club Lacrosse Team