synthesis of anion exchange/activated carbon fiber hybrid materials
TRANSCRIPT
SYNTHESIS OF ANION EXCHANGE/ACTIVATED CARBON FIBER HYBRID MATERIALS FOR AQUEOUS ADSORPTION OF TOXIC HEXAVALENT
CHROMIUM
BY
WEIHUA ZHENG
THESIS
Submitted in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering
in the Graduate College of the University of Illinois at Urbana-Champaign, 2011
Urbana, Illinois
Adviser: Professor James Economy
ii
ABSTRACT
Hexavalent chromium, also known popularly as the “Erin Brockwich
chemical”, has recently been identified as a carcinogen by ingestion. Unfortunately, it
is present in drinking water supplies in United States. An effective, Point-of-Use
(POU) based technology to remove Cr(VI) in tap water is urgently needed to protect
human beings against this carcinogen. Due to the inadequate state-of-the-art
technology for rapid, selective removal of Cr(VI), a unique hybrid activated
carbon/anion exchange system (HACAX) has been developed for the first time in our
group. HACAX was synthesized by methylation of PAN based Chemically Activated
Carbon Fibers. The HACAX was characterized by BET, SEM, XPS,TGA and
elemental analysis. The ion exchange property and adsorption performance towards
Cr(VI) were also studied comprehensively. HACAX displays not only very high
mesopore content with a surface area up to 1085 m2/g based on coating, but also good
strong base ion exchange capacity up to 1.5 meq/gC and exceptionally fast kinetics
due to multi-scale facilitated mass transfer. Compared with conventional activated
carbon and ion exchange resin systems, HACAX has excellent adsorption capacity
towards Cr(VI) even at natural pH and very low concentrations of Cr(VI); besides,
HACAX has very fast kinetics and high selectivity which can remove Cr(VI) from 4
ppm down to ppb levels or below; moreover, this hybrid material is also able to
convert Cr(VI) to non-toxic Cr(III) to prevent the leaking of Cr(VI). These results
showed that HACAX is a unique and effective adsorbent for POU Cr(VI) removal
and potentially other anion treatments.
iii
To My Love and Be Loved Family….
iv
ACKNOWLEDGEMENTS
At this moment, my gratefulness is beyond words and I wish I could be a
writer or a poet to express my feelings as how I feel. First, I would like to express my
appreciation, thankfulness and gratitude to my advisor, Professor James Economy, for
his support, guidance, inspiration, also for his trust and freedom to me. His vision,
optimistic attitude, enthusiasm and continuous pursuing for his belief set me a good
example of what kind of person I want to be and try to be.
I also would like to thank the MatSE department office personnel, Judy
Brewer, Terri Brantley, Debbie Kluge, Bryan Kieft and special thanks to Michelle
Malloch and Jay Menacher for their kindness help during my research. Your presence
makes my days bright and sunny.
Deep thanks to MRL stuff: Julio Soares, Vania Petrova and special thanks to
Rick Haasch for their help on UV-Vis, SEM and XPS characterizations.
Special thanks to former group member Dr. Zhongren Yue. Although I ‘ve
never seen him before, Dr. Yue is always, always there in Tennessee for help by email
and by phone like he never left, providing help, suggestions, guidance on every detail
of the lab and my research.
Also I thank a lot to our group member Yaxuan Yao, Jacob Meyer – the time
we together make my life wonderful which I cherish. Celia Guo’s coming also adds
our group more fun and joy. Special thanks to James Langer who is also literally my
half-mentor for lab and research - It is hard to imagine the life without you.
Thanks to the past group members, Jinwen Wang, Chaoyi Ba, Jing Zhang,
Xuan Li, and especially Gordon Nanmeni who teach me a lot about research and
about life, and all these are in my memory. I also thank visiting scholars and
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professors in our group, Prof. Yi Zhang, Prof. Zhen Zheng, and Ms. Shanshan Zhao.
They always have helpful suggestions for my research.
I really appreciate my undergraduate students, Samuel Rappeport, Rujia
Wang, Erick Yu, Tianqi Ren, especially Jingtian Hu; without their dedicated work in
the lab, I cannot finish my work.
Last but not least, I want to thank my parents and parents-in-law for their
endless love, rise and support to me and my wife since we were born. Also, my wife
and my one and a half years old son are always the sources of my joy, happiness and
power. I wish I could have been spending more time with them.
See you soon in my PhD dissertation for a more comprehensive study of this
hybrid material and chromium removal and God bless us all.
vi
TABLE OF CONTENTS
CHAPTER 1 INTRODUCTION ................................................................................... 1
1.1 Drinking Water Contamination - Cr(VI) ............................................................. 1
1.2 Treatment Technology ......................................................................................... 2
1.3 Activated Carbon Fiber System Development and Derivation ........................... 2
1.4 Objective and Organization ................................................................................. 3
1.5 References ............................................................................................................ 4
CHAPTER 2 LITERATURE REVIEW ........................................................................ 7
2.1 Chromium ............................................................................................................ 7
2.2 Cr(VI) Removal Technologies ........................................................................... 14
2.3 Novel Activated Carbon Fiber System .............................................................. 18
2.4 Vision for the Hybrid Materials ......................................................................... 19
2.5 References .......................................................................................................... 22
CHAPTER 3 EXPERIMENTAL ................................................................................. 27
3.1 Chemicals and Reagents .................................................................................... 27
3.2 Preparation of HACAX ..................................................................................... 28
3.3 HACAX Characterization .................................................................................. 29
3.4 Cr(VI) Adsorption Test ...................................................................................... 31
3.5 References .......................................................................................................... 32
CHAPTER 4 RESULTS AND DISCUSSION ............................................................ 34
4.1 Ion Exchange Properties .................................................................................... 34
4.2 Cr(VI) Removal by Hybrid Materials ................................................................ 39
4.3 Summary and Future Work ................................................................................ 47
4.4 References .......................................................................................................... 48
1
CHAPTER 1
INTRODUCTION
1.1 Drinking Water Contamination - Cr(VI)
Hexavalent chromium, also named chromium-6 or Cr(VI), is a toxic anion in aqueous
solutions. Cr(VI) is well known as a notorious chemical to the public from the movie “Erin
Brockovich”, which highlighted the lawsuits on the drinking water contaminated by Cr(VI) in
Hinkley, California. The case was settled in 1996 for $333 million, which was the largest
settlement ever paid in US history[1, 2].
After 15 years, it turns out that Hinkley was not alone: according to a recent
nationwide drinking water supply survey conducted in December 2010, 31 out of 35 cities in
the United States tested were contaminated by Cr(VI)[3]; more importantly, Cr(VI) in
drinking water not only causes health issues like anemia, damage of gastrointestinal tract and
kidney/ liver malfunctions, but also was recently identified as carcinogen which causes
stomach cancer [4-7]. The widespread carcinogenic Cr(VI) in drinking water supply has
already aroused wide public concerns and panic about whether their tap water was
contaminated by this carcinogen[8, 9].
To date, USEPA hasn’t established a legal limit for Cr(VI) in drinking water yet; but
an MCL (maximum contaminant level) for total chromium has been set as 100 ppb to protect
against allergic dermatitis[10]. The state of California has established a very strict public
health goal (PHG) of 0.02 ppb for Cr(VI) as of July, 2011[11, 12].
Recognizing the carcinogenic character and widespread presence of Cr(VI), currently
EPA is in the process of establishing a new standard for Cr(VI)[13, 14]. However, before the
new regulations come out demanding better treated drinking water by Public Water Systems
2
(PWS)[15] , it is critical to develop and implement an effective, Point of Use (POU) based
technology to remove Cr(VI) in tap water and thus protect against this carcinogen, especially
for those vulnerable populations and areas.
1.2 Treatment Technology
In the past few decades, several technologies have been developed for wastewater
Cr(VI) removal, including chemical precipitation[16], ion exchange[17], membrane
separation[18], sedimentation[19], reduction[20] and adsorption[21], etc. Of all these
methods, activated carbon emerges out as the most popular and widely used adsorbent in
wastewater treatment applications due to its high surface area, porous internal structure,
controllable surface chemistry, cost-effectiveness and ease of use[22].
While the effectiveness of commercially available Granular Activated Carbon
(GACs) for wastewater treatment of Cr(VI) at levels of 100-1000 ppm worth noted, GACs
suffer from a number of drawbacks including slow adsorption kinetics, poor selectivity and
low capacity for Cr(VI). These features limit its application for drinking water treatment,
where Cr(VI) ions are present at very low levels of ppm or even ppb[3, 23]. Besides, GACs
needs to be operated under low pH for adequate capacity, which is impractical for POU
application [21, 24, 25]. There remains no indication of an adequate material for fast and
selective removal of Cr(VI) from drinking water in the literature. So it is very important to
develop a novel technology to be able to selectively and rapidly to remove this carcinogen at
trace level.
1.3 Activated Carbon Fiber System Development and Derivation
To address the slow kinetics of GACs, the Economy group [26-33] has developed a
new family of chemically activated carbon fibers by using low-cost glass fibers coated with
the activated carbon fibers (CAFs). CAFs offers a number of advantages over GACs,
3
including lower activation temperatures < 450 °C; cost-effectiveness ( $1-2/lb) due to use of
glass fiber as core substrate; excellent wear characteristics; and more importantly, the ~7 µm
diameter of CAFs greatly improved contact efficiency and thus provides much faster
intraparticle adsorption kinetics compare with the mm size of GACs. CAFs are very effective
in removing trace levels of organic contaminants in drinking water below 1 ppb, such as
BTEX (benene, toluene, ethylbenzene and p-xylene)[33], TCE (trichloroethylene), atrazine,
half mustard, humic acid,etc.[28].
In 2002, in order to expand this novel system for inorganic ions removal, the
Economy group [34, 35] developed a hybrid activated carbon/cation exchange (HACCX)
system through sulfonation of phenolic based activated carbon coated on glass fibers.
HACCX combines a cation exchange functional group in a micropore structure of activated
carbon fibers, which shows excellent adsorption properties towards cationic heavy metals
such as Cs+, Sr2+, etc.[35].
1.4 Objective and Organization
Motivated by the need to address the problem of Cr(VI) in drinking water as well as
the inadequate state-of-the-art technology, the current study was undertaken to design a novel
materials system with rapid kinetics and good selectivity to bring trace Cr(VI) down to below
ppb level. In light of success of cation exchangeable activated carbon system for cation
heavy metal removal[35], a concept of novel anion exchangeable activated carbon system is
proposed and explored in this work for Cr(VI) anion removal.
Analogous to the HACCX system [34, 35], this hybrid activated carbon/anion
exchange (HACAX) material would have high surface area with more active sites consisting
of a permanently positive charged surface to attract and adsorb anions such as Cr(VI), and
greatly improved kinetics due to hydrophilic surface and the small fiber diameter of 7 µm.
4
All of these characteristics would contribute to the feasibility for POU applications of this
hybrid material bring trace level of Cr(VI) down to below the ppb level. To the author’s
knowledge, there is no work done on this novel material yet, since it is very difficult to
introduce a permanently positive charge onto the surface of activated carbon, which makes it
exhibits anion exchange property at natural pH.
In this work, positive charge functional group was introduced to the surface by
choosing a special nitrogen-rich PAN (polyacrylonitrile) as precursor to make chemically
activated carbon fiber (CAFs). CAF has pyridine-like ladder structure, which could be
further methylated to introduce a methyl group onto pyridine-N, and thus display a permanent
positive charge for anion exchange adsorption.
In Chapter 2, basic background information of Cr(VI) and state-of-the-art
technologies for Cr(VI) removal are reviewed, and feasibility of design of the hybrid material
is justified.
In Chapter 3, experimental procedures and methods are described for the synthesis,
characterization and Cr(VI) adsorption test for the HACAX material.
In Chapter 4, in the first part, the ion exchange properties and characterization of the
hybrid materials is discussed. In the second part, Cr(VI) adsorption kinetics and equilibrium
performance at trace level is discussed, including the characterization by XPS, elemental
analysis and adsorption kinetic tests, an novel Cr(VI) electrostatic attraction – Cr(VI)
accumulation – Cr(VI) surface reduction to Cr(III) – Cr(III) repulsion adsorption mechanism
for this hybrid material is proposed. In the end, a summary and a future work plan will be
described.
1.5 References
[1] Donovan, T. Hexavalent Chromium, Erin Brockovich Chemical, Found in Tap Water of 31 U.S. Cities. 2010 02/19/11; Available from:
5
http://www.huffingtonpost.com/2010/12/20/hexavalent-chromium-erin-_n_799310.html.
[2] Cernansky, R. Hexavalent Chromium, Erin Brockovich Chemical, Found in 31 of 35 U.S. Cities' Tap Water. 2010; Available from: http://www.treehugger.com/green-food/hexavalent-chromium-erin-brockovich-chemical-found-in-31-of-35-us-cities-tap-water.html.
[3] EWG, EWG-commissioned testing for hexavalent chromium in tap water from 35 cities.
[4] Sedman, R.M., et al., Review of the evidence regarding the carcinogenicity of hexavalent chromium in drinking water. Journal of Environmental Science and Health - Part C Environmental Carcinogenesis and Ecotoxicology Reviews, 2006. 24(1): p. 155-182.
[5] NTP, Actions on Draft NTP Technical Reports Reviewd by the NTP Board of Scientific Counselors Technical Reports Review Subcommittee, 2007.
[6] NTP, NTP Technical Report on the Toxicology and Carcinogenesis Studies of Sodium Dichromate Dihydrate in F344/N Rats and B6C3F1 Mice (Drinking Water Studies), 2008, NIH, U.S. Department of Health and Human Services.
[7] Zhang, J. and X. Li, Chromium pollution of soil and water in Jinzhou. Chinese Journal of Preventive Medicine, 1987. 21(5): p. 262-264.
[8] CDPH. Frequently Asked Questions: Hexavalent Chromium in Drinking Water. 2011; Available from: http://www.cdph.ca.gov/certlic/drinkingwater/Documents/Chromium6/FAQs-chromium6-07-27-2011.pdf.
[9] NTP. Hexavalent Chromium. 2007; Available from: http://ntp.niehs.nih.gov/files/ntphexavchrmfactr5.pdf.
[10] EPA. Basic Information about Chromium in Drinking Water. Available from: http://water.epa.gov/drink/contaminants/basicinformation/chromium.cfm.
[11] OEHHA. Peer Review Comments regarding 2001 Draft Public Health Goal for Hexavalent Chromium in Drinking Water. 2010; Available from: www.oehha.ca.gov/water/phg/chrom092010.html.
[12] OEHHA. FINAL TECHNICAL SUPPORT DOCUMENT ON PUBLIC HEALTH GOAL FOR HEXAVALENT CHROMIUM IN DRINKING WATER. 2011; Available from: http://www.oehha.org/water/phg/072911Cr6PHG.html.
[13] EPA. Toxicological Review of Hexavalent Chromium 2010 Sep 30; Available from: http://cfpub.epa.gov/ncea/iris_drafts/recordisplay.cfm?deid=221433.
[14] EPA. EPA Issues Guidance for Enhanced Monitoring of Hexavalent Chromium in Drinking Water. [News] 2011 January, 11; Available from: http://yosemite.epa.gov/opa/admpress.nsf/6427a6b7538955c585257359003f0230/93a75b03149d30b08525781500600f62!OpenDocument.
[15] EPA. Drinking Water Treatment. Safe Drinking Water Act 1999; Available from: http://esa21.kennesaw.edu/modules/water/drink-water-trt/drink-water-trt-epa.pdf.
[16] Paterson, J.W., Wastewater Treatment Technology1975: Ann Arbour Science. [17] Rengaraj, S., K.H.Yeon, and S.H.Moon, Removal of chromium from water and
wastewater by ion exchange resins. J. Hazard. Mater. , 2001. 87(1-3). [18] Ozaki, H., K. Sharma, and W. Saktaywin, Performance of an ultra-low-pressure
reverse osmosis membrane for separating heavy metal: effects of interference parameters. Desalination, 2002. 144: p. 287-294.
[19] Song, Z., C.J. Williams, and R.G.J. Edyvean, Sedimentation of tannery wastewater. Water Research, 2000. 34(7): p. 2171-2176.
6
[20] Chen, J.M.N. and O.J.N. Hao, Microbial chromium-6 reduction. Critical Reviews in Environmental Science and Technology, 1998. 28(3): p. 219-251.
[21] Mohan, D. and C.U. Pittman Jr, Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water. Journal of Hazardous Materials, 2006. 137(2): p. 762-811.
[22] Babel, S. and T.A. Kurniawan, Low-cost adsorbents for heavy metals uptake from contaminated water: A review. Journal of Hazardous Materials, 2003. 97(1-3): p. 219-243.
[23] Huang, G., J.X. Shi, and T.A.G. Langrish, Removal of Cr(VI) from aqueous solution using activated carbon modified with nitric acid. Chemical Engineering Journal, 2009. 152(2-3): p. 434-439.
[24] Yin, C.Y., M.K. Aroua, and W.M.A.W. Daud, Review of modifications of activated carbon for enhancing contaminant uptakes from aqueous solutions. Separation and Purification Technology, 2007. 52(3): p. 403-415.
[25] Owlad, M., et al., Removal of hexavalent chromium-contaminated water and wastewater: A review. Water, Air, and Soil Pollution, 2009. 200(1-4): p. 59-77.
[26] Yue, Z., et al., Chemically activated carbon on a fiberglass substrate for removal of trace atrazine from water. Journal of Materials Chemistry, 2006. 16(33): p. 3375-3380.
[27] Yue, Z., J. Economy, and G. Bordson, Preparation and characterization of NaOH-activated carbons from phenolic resin. Journal of Materials Chemistry, 2006. 16(15): p. 1456-1461.
[28] Yue, Z. and J. Economy, Nanoparticle and nanoporous carbon adsorbents for removal of trace organic contaminants from water. Journal of Nanoparticle Research, 2005. 7(4-5): p. 477-487.
[29] Yue, Z., et al., Elucidating the porous and chemical structures of ZnCl2-activated polyacrylonitrile on a fiberglass substrate. Journal of Materials Chemistry, 2005. 15(30): p. 3142-3148.
[30] Yue, Z., C.L. Mangun, and J. Economy, Characterization of surface chemistry and pore structure of H3PO4-activated poly(vinyl alcohol) coated fiberglass. Carbon, 2004. 42(10): p. 1973-1982.
[31] Yue, Z., J. Economy, and C.L. Mangun, Preparation of fibrous porous materials by chemical activation 2. H3PO4 activation of polymer coated fibers. Carbon, 2003. 41(9): p. 1809-1817.
[32] Yue, Z., C.L. Mangun, and J. Economy, Preparation of fibrous porous materials by chemical activation: 1. ZnCl2 activation of polymer-coated fibers. Carbon, 2002. 40(8): p. 1181-1191.
[33] Yue, Z., et al., Removal of chemical contaminants from water to below USEPA MCL using fiber glass supported activated carbon filters. Environmental Science and Technology, 2001. 35(13): p. 2844-2848.
[34] Benak, K.R., et al., Sulfonation of pyropolymeric fibers derived from phenol-formaldehyde resins. Carbon, 2002. 40(13): p. 2323-2332.
[35] Economy, J. and K. Benak, Cationic Ion Exchange Carbon Fiber. Patent, 2002.
7
CHAPTER 2
LITERATURE REVIEW
2.1 Chromium
2.1.1 Sources and Uses
Chromium (Cr) is a steely-gray, lustrous, hard, transition metal, and it is the
21st most abundant element in Earth’s crust[1]. Cr is widely used in industry such as
electroplating, leather tanning and wood preserving, etc.[2]. Due to its high corrosion
resistance and hardness, Cr is a crucial element for producing stainless steel and
superalloys. United States is one of the biggest consumers for Cr with 538,000 tons
chromium consumption annually. 80% of Cr is used for stainless steel industry, and
currently there is no viable substitute for it.
Due to extensive use in industry, Cr has become one of the major
contaminants in drinking water. In some places, chromium is released into natural
water on level of 5 ppm via leakage, unsuitable storage or improper disposal
practices[2, 3]. In 1988, 142,000 tons of chromium was discharged into drinking
water around the world[4], which presents serious environmental and human health
problems[5, 6].
2.1.2 General Chemistry
In natural water, Cr exists almost exclusively as Cr(III) oxidation state or
Cr(VI) oxidation state. Cr(III) is a hard acid, and it is insoluble in water and thus
immobile under ambient conditions; Cr(III) could exist as Cr3+, CrOH2+, Cr(OH)3,
Cr(OH)4-, the form of which depends on the pH of aqueous solution.
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9
Besides, Cr(III) can also regulate blood cholesterol level by reducing low density
lipoproteins in blood and aid fat metabolism [2].
In contrast to Cr(III) , Cr(VI) is dangerous to human beings due to its high
toxicity and carcinogenetic properties[9]. It has been well-known for half a century
that inhalation or skin contact of Cr(VI) causes acute toxicity, irritation and ulceration
of the nasal septum, asthma, dermatitis and even cancers [2, 7, 11].
However, Cr(VI) by ingestion was not commonly accepted as a carcinogen
before 2009. The reason is that a forged paper published in the Journal of
Occupational and Environmental Medicine in 1997 [12] claimed that Cr(VI) ingestion
would not cause cancer. This work greatly influenced EPA to make MCL assessments
for Cr(VI) as well as held back research study activities on carcinogenicity of Cr(VI).
It was not until 2005 when this error was uncovered by the Environmental
Working Group (EWG) and the Wall Street Journal[13]. It turned out that the paper
was forged and financially supported by Pacific Gas & Electric Co. (PG&E), an
energy company who provides 2/3 of California’s natural gas and electricity, as well
as disposing large amounts of chromium into water. Interestingly, PG&E is also the
company who contaminated the drinking water in Hinkey with Cr(VI) and then was
defeated by Erin Brockovich and paid $333 million in fines. Apparently, PG&E hired
people to forge this paper in order to avoid the strict standard for Cr(VI) in drinking
water by EPA[13].
After that, this journal editor retracted the paper, and researchers and
regulators started a new round of re-evaluation research activities to investigate the
toxicity as well as human and animal cancer studies by ingestion of Cr(VI) from
drinking water, in order to estimate cancer risk and establish possible new
regulations[14].
10
In 2006, Sedman et al. [15] studied toxicokinetic and genotoxicity of Cr(VI) to
animal cells. The result shows that part of hexavalent chromium ingested can be
absorbed and enters cells of several differnt tissues and causes DNA damage.
After that, California EPA conducted a rigorous re-analysis of the original
data[16] from the forged publications, which showed that there is a statistically
significant increase in stomach cancer by Cr(VI) exposure compared with general
populations without exposure[13].
Moreover, National Toxicology Program (NTP, part of NIH (National
Institutes of Health)) [17, 18] carried out comprehensive animal studies in 2008 and
2009. The results revealed statistically significant, dose-related increases in tumors in
the duodenum and the small intestine of mice, and tumors in the oral cavity of rats.
This report provided additional evidence of the link between Cr(VI) to cancer.
The increases in stomach tumors in both human and animal studies, along with
the toxicology and mechanistic data suggest that Cr(VI) ingested orally poses a
carcinogenic risk and more strict standards for Cr(VI) in drinking water should be
considered.
2.1.4 Spread of Cr(VI) in Drinking Water
2.1.4.1 Cr(VI) in U.S.
In United States, monitoring of Cr(VI) in drinking water supply was not
mandatory in most states except in California, due to the lack of regulations by
USEPA as there was no connection between cancer and Cr(VI) by ingestion before. In
2010, EWG conducted a nationwide survey of drinking water supplies in 35 cities in
the U.S, which showed that 31 cities’ of 35 (89%) drinking water supply was
contaminated by carcinogenic Cr(VI)[13]; Moreover, 25 cities in this survey have
even higher levels than California’s proposed public health goal (PHG); The highest
11
one, Norman city from Oklahoma has Cr(VI) in their drinking water measured at 200
times California’s proposed safe limit, as shown in Figure 2.2.
Figure 2.2 Chromium Distribution in 35 cities in United States[13]
In the survey by EWG, the contaminated cities’ drinking water served more than 26
million people, and it is estimated that at least 74 million people in 42 states are
affected by Cr(VI) contaminated drinking water, as indicated in Figure 2.3.
12
Figure 2.3 Chromium in Tap Water in U.S. [13]
Additionally, nearly 40% of California drinking water sources tested was
contaminated by hexavalent chromium above safety levels.
This survey has been cited and reported by major newspapers including The
New York Times, The Washington Post, Los Angeles Times, Chicago Tribune, CBS
News, etc., which brought public concerns and EPA’s quick response to promise to
speed up the new regulation process[19-24].
There are also several Cr(VI) contamination at very high levels in United
States. In 2009, the ground water in Midland, Texas had Cr(VI) levels up to 5.25
ppm[25][25]. Another occurrence of Cr(VI) in groundwater with 0.58 ppm was in
Hinkley, California, which was brought to the public attentions by the efforts of Erin
Brockovich[26].
13
2.1.4.2 Cr(VI) around the world
Water contamination with hexavalent chromium is also a worldwide problem.
In Sukinda, India, which is regarded as the world’s most polluted locale by the Times
Journal, 60% of drinking water contains double the Cr(VI) as compared with WHO
standards of 50 ppb[27]. With an exposed population of 2.6 million, it was estimated
that 85% of the deaths in this areas were due to chromium related diseases. In
Jinzhou, China, due to the chromium ore processing, the drinking water was polluted
by an industry wastestream which led to higher cancer risk for local residents. Dr.
Zhang’s survey and investigation was the first time that scientists found statistically
significant relations between human cancers and chromium in drinking water [16]. In
Aug., 2011, a chemical plant in Qujing, China dumped 5,000 tons of chromium waste
near Chachong Reservoir, whose downstream river feeds the Pearl River, one of
China’s longest waterways and threaten the water sources for tens of millions of
residents[28]. In Greece, ground water associated with industrial waste in central
Euboea and Asopos valley was contaminated by Cr(VI) at levels higher than 50 ppb.
Assopos aquifer also showed a wide range of Cr(VI) concentrations from 2 to 180 ppb
[29].
2.1.5 Regulations and New Regulation Progress
2.1.5.1 U.S.EPA
Starting from 1992, EPA has set a MCL (maximum contaminant level) for
total chromium at 100 ppb in order to protect against allergic dermatitis. This standard
includes both nutrient Cr(III) and toxic Cr(VI) [3].
In recent years, as more and more scientific evidence was published that
Cr(VI) may cause stomach cancer by ingestion [16-18], EPA began a rigorous and
comprehensive review of Cr(VI) health effect to establish a more strict standard. In
14
September 2010, EPA proposed to classify Cr(VI) as likely to be carcinogenic to
humans via ingestion[3], and a final determination about the carcinogenicity of Cr(VI)
will be made by 2011[30]. On December 22, 2010, in correspondence to 89% of the
cities in the USA is contaminated by Cr(VI), EPA administrator Lisa Jackson
announced that EPA is re-evaluating how wide-spread and prevalent this contaminant
is, and that “it is likely that EPA will tighten drinking water standards to address the
health risks posed by Cr(VI)”[30].
2.1.5.2 California EPA
As a frontier in environmental regulations, California currently regulates total
chromium under 50 ppb as MCL. In 2009, in response to the risk of gastrointestinal
tumors caused by Cr(VI) in drinking water studied by NTP[17, 18], California EPA
proposed setting a public health goal (PHG) of 0.06 ppb for Cr(VI) in drinking
water[31]. In December 2010, in order to better protect human health, California EPA
announced a new draft PHG of 0.02 ppb Cr(VI) in drinking water[32].
2.1.5.3 Other Countries
Most countries, including the supranational European Union (EU) have
currently established a limit of 50 ppb for total chromium in drinking water [33, 34].
India has set a India Standard (IS) of 50 ppb for Cr(VI) in drinking water[35].
Currently China has set a standard for the Cr(VI) at 50ppb in drinking water[36].
Italy has a more strict regulation in that Cr(VI) in drinking water needs to be lower
than 5ppb[34, 37].
2.2 Cr(VI) Removal Technologies
In literature, several methods have been developed for Cr(VI) removal from
wastewater and drinking water, including adsorption[7], ion exchange[38],
15
reduction[39], membrane separation[40], precipitation[41], etc. These methods are
reviewed in the following sections.
2.2.1 Membrane Separation
Membrane processes have been studied for chromium removal majorly from
wastewater. Aroua et.al[42] removed Cr(VI) from 10 ppm concentration in water
using either chitosan, PEI or pectin-enhanced ultrafiltration (UF) process with 30%
rejection rates. Muthukrishnan et.al[43] used two nanofiltration (NF) composites
polyamide membranes for Cr(VI) removal. At initial concentration of 1000 ppm
Cr(VI), the rejection rates are 94% and 99%, respectively. Reverse Osmosis (RO) has
also been applied for Cr(VI) removal both in wastewater and drinking water. At both
feed concentration of 10,000 ppm and 10 ppm, the rejection rate are up to 99%[44],
[45].
However, the current removal efficiency of UF and NF still needs
improvement in order to achieve ppb levels. Although RO membranes can achieve
higher effluent water purity, they must operate at higher pressure which can be
expensive, and there is no data available on removing Cr(VI) at the ppb level.
2.2.2 Ion Exchange
Ion Exchange Resins (IXR) have been widely used in industry for separation
of inorganic ions. The main advantages of ion exchange are high affinity towards
Cr(VI). Sapari et al.[46] studied Dowex 2-X4 anion exchange resin which can remove
Cr(VI) down to detection limit after 8 bed volumes of 8 ppm. Pehlivan et al.[9]
studied two macroporous strong base IXR (Lewatit MP64 and Lewatit MP 500),
which shows 20 mg/g adsorption capacity at 52 ppm initial Cr(VI) concentration and
pH 5.0.
16
Strong base ion exchange resins have charge sites which can attract and adsorb
Cr(VI) through electrostatic forces thus improving kinetics[47]. However, ion
exchange resins have some disadvantages: it is more expensive compare with other
adsorbents; the size of ion exchange resin is on the level of 1mm. Thus, the kinetics
for removing Cr(VI) is still very slow at the ppb level of contaminants, and flow rate
for flow-through test is remarkably slowed down[48]. This limits its applicability to
POU treatment (requires fast flow rate ~ 10 BV/min); also, there is still no indication
of capacity of ion exchange materials at drinking water contamination level.
Moreover, ion exchange resin does not have a pathway for converting Cr(VI) to less
toxic forms, such as Cr(III). This might lead to leaking of toxic Cr(VI) in POU
applications as more background ions flow through and exchange out the adsorbed
Cr(VI). All these factors limit use of ion exchange resins for POU treatment.
2.2.3 Activated Carbon
Activated Carbon is the main adsorbent in wastewater treatment due to its low
cost, effectiveness and ease of operation [49]. Activated carbon has a well developed
pore structure and a very high surface area from 500 to 1500 m2/g. Also, a wide
spectrum of surface functional groups could be prepared which interact, oxidize or
reduce the target compounds to enhance adsorption. All these properties make
activated carbon and its derivatives widely used in the treatment of Cr(VI)
contaminated waters[8].
Commercial activated carbon such as GACs has been studied for Cr(VI)
removal extensively. In the literature, most of the studies investigate the adsorption
performance at low pH and initial Cr(VI) concentration from 10 ppm up to 1000
ppm[7, 8, 50]. Han et al. [51] studied Filtrasorb 400 commercial activated carbon
(GACs) for Cr(VI), and the result showed an adsorption capacity of 0.18 mg/g.
17
Activated Carbon Fibers (ACF)[52] has also been used for Cr(VI) removal,
and has shown better adsorption capacity up to 22.29 mg/g and improved kinetics due
to the geometry of the fiber which is relatively small[53].
In order to improve the adsorption performance, various efforts have been
made to improve the affinity towards specific contaminants by chemical modification.
Acidic treatment is the most popular way to introduce functional groups to improve
the heavy metal adsorption. Huang et al. [49] treated activated carbon with nitric acid,
which showed improved adsorption capacity up to 16.1 mg/g at pH 4.0 and an initial
concentration of 25 ppm.
There are very few reports on low concentration of Cr(VI) removal. Pillay et.
al[54] studied the Multi-walled carbon nanotubes (MWCNT) for Cr(VI) removal at
ppb levels. The study showed that MWCNT can adsorb up to 98% of Cr(VI) at initial
concentration of 100 ppb. Another paper by Yue et al[55] studied Cr(VI) removal by
Activated Carbon Pellet (Pellet-600) with high mesopore content. The study showed
that 176 mg of Pellet-600 is able to remove 4ppm of 200 ml Cr(VI) down to 2 ppb.
The dominant mechanism for Cr(VI) adsorption by activated carbon includes
three steps: 1) surface attraction, 2)surface reduction from Cr(VI) to Cr(III) [56] and
3)adsorption of Cr(III) [7]. Since in water Cr(VI) is an anion in form of CrO42- or
HCrO4-, sorption of Cr(VI) only happens at low pH when the surface is positively
charged[57]. A spectrum of functional groups such as hydroxyl and ketone on the
surface of activated carbon is capable of reducing Cr(VI) to Cr(III) [7].
While the merits of the activated carbon system have been noted, activated
carbon also suffers from several problems: Cr(VI) adsorption has to be carried out at
low pH, which implies the usage of large amount of acid chemicals and is not
applicable for POU treatment; the surface of an untreated activated carbon is non-
18
polar and hydrophobic, and therefore the equilibrium is attained very slowly due to
slow pore diffusion of Cr(VI) ions[58]; The geometric size of GACs are very big on
the level of mm, which limits the mass transfer and thus kinetics. An AC system with
permanently positive charged surface and small mass transfer distance is necessary
for POU applications.
2.2.4 Others
There are also several other technologies which have also been studied.
Precipitation such as chemical precipitation [57] and electrochemical
precipitation [8] is used primarily for wastewater treatment. These techniques are only
effective for high concentration of chromium, and the disposal of residual Cr(VI)
from incomplete removal and sludge is still a challenge.
There are also numerous studies on low-cost adsorbents, such as chitosan have
very good capacity up to 100 mg/g, which is a very competitive process [7]. However,
they also suffer from problems including high pH dependence and slow kinetics due
to nonporous structure which limits POU applications [7].
Most of the literature for Cr(VI) removal is on wastewater system, where it is
present at ~100 or ~1000 ppm; more studies are needed to focus on drinking water,
where Cr(VI) presents at ppb to a few ppm level [54, 55]. Kinetics and selectivity are
the two limiting factors when applied the current technology for POU applications.
2.3 Novel Activated Carbon Fiber System
In order to improve the mass transfer and adsorption rates, a novel family of
chemically activated carbon fibers by using low-cost glass fibers as substrate (CAFs)
have been developed by the Economy group [59-66] . CAFs offers a number of
19
advantages over GACs: lower activation temperature < 450 °C; very low cost ( $1-
2/lb) due to use of glass fiber as a core substrate; excellent wear characteristics; and
most importantly, the ~7 µm diameter of CAFs greatly improves contact efficiency
and thus greater adsorption rates compared with mm size of GACs. CAFs are very
effective in removing trace level of organic contaminants in drinking water down to
below 1 ppb, such as BTEX (benene, toluene, ethylbenzene and p-xylene)[66], TCE
(trichloroethylene), atrazine, half mustard and humic acid[61],etc.
Besides, a hybrid cation exchange/ activated carbon fiber system was also
developed based on CAFs by Economy [67, 68]. This novel hybrid activated
carbon/cation exchanger (HACCX) material was prepared by sulfonation of phenolic
based activated carbon coated on glass fibers to introduce strong –SO4H- and weak -
COOH and -OH functional group. HACCX combines cation exchange functional
groups in a micropore structure of activated carbon fibers, which shows excellent
adsorption properties towards cation heavy metals such as Cs+, Sr2+, etc.[68].
In parallel to the opportunities for the of HACCX system, there remains the
need for an hybrid anion exchange/activated carbon fiber system, which potentially
can address anion contaminants problems in drinking water effectively, especially for
anion heavy metals, such as Cr(VI), As(III), etc.
2.4 Vision for the Hybrid Materials
Motivated by the gaps between the needs to remove trace Cr(VI) down to ppb
level for POU applications and lag behind state-of-the-art technology, the current
study is intended to design a novel materials system with rapid kinetics and great
selectivity ; In light of the success of heavy metal removal by cation exchangeable
activated carbon fiber system [68], a concept of novel anion exchangeable activated
20
carbon system is proposed and explored in this work for Cr(VI) like anion removal. If
successfully made, this hybrid activated carbon/anion exchange (HACAX) materials
potentially should display the properties indicated below:
High surface area to provide more functional sites and improve mass
transfer in small scale;
Positively charged sites in natural pH to attract Cr(VI) electrostatically
without changing pH of natural water;
Surface reducing functional group to convert Cr(VI) to low toxic Cr(III) at
the same time of adsorption process;
Fiber form with a diameter of 7 µm with glass fiber core to improve mass
transfer in larger scale as substrate without introducing massive pressure
drop.
With all these properties, HACAX combined the advantages of AC and IXR
plus enhanced kinetics in multi-scale, and hence could more effective for Cr(VI)
removal. It is very important to synthesis such a hybrid material and examines it.
To the author’s knowledge, there is no literature on introducing strong base
functional group onto activated carbon yet. The conventional approach to introduce
permanent positive charge surface, similar to the synthesis of an anion exchange resin,
is to add a chloromethyl group to activated carbon, and then introduce trialkylamine
to form positively charged amines; it should be noted that it is very difficult to
introduce a chloromethyl group to the rigid carbon structure.
One possible way to have the quarterized amines on activated carbon is
through direct N-methylation of a special nitrogen-containing activated carbon. If
nitrogen incorporates in the activated carbon in form of pyridine or pyridine-like
structures, quarterized amines could be formed by direct N-methylation. As a popular
prec
prec
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acce
stru
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stru
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oposed Mechan
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n in Figure 2
hus the meth
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et al.[62] st
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2.5. Both pa
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21
y, PAN (pol
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, 70]. The
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2.5
[1]
[2]
[3]
[4]
[5]
Figure 2.5
Figure 2.6 Pr
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EmsleyElemenLenntechttp://wEPA. Bhttp://wNriagu,contam333(616NTP, C
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27
CHAPTER 3
EXPERIMENTAL
There are two objectives of the experiment of design: 1) explore the synthesis
of anion exchange/activated carbon fibers(HACAX), and 2) study the Cr(VI) removal
performance of this novel HACAX.
3.1 Chemicals and Reagents
Polyacrylonitrile (PAN) ((C3H3N)n, Mw ~ 150,000, Aldrich Co. Cat. 181315)
and Zinc Chloride (ZnCl2, Reagent grade, ≥ 98% Sigma Cat. 208086) was used for
the synthesis of an activated carbon coating on a substrate fiber (Craneglass 230,
0.015 nominal, fiber diameter of 6.5 μm). This was a nonwoven glass fiber mat with
7% PVA (polyvinyl alcohol) binder made by Crane&Co. Iodomethane (CH3I, ≥ 99%,
Sigma-Aldrich, Cat. I8507) was used for the methylation reaction. Potassium
Dichromate (ACS reagent, ≥ 99.0% Sigma-Aldrich, Cat. 207802) was used as a stock
solution of hexavalent chromium, and 1,5-Diphenylcarbazide (DPC) (ACS Reagent,
Sigma-Aldrich, Cat. 259225) was used as the color reagent. Methanol (Fisher
Chemical, Cat. A454-1) was HPLC grade for dissolving color reagent. All reagents
are used without further purification.
28
3.2 Preparation of HACAX
3.2.1 Preparation of PAN-based Activated Carbon Fiber
PAN based Activated Carbon Fiber(PANCAF) was prepared using a patented
approach developed in Economy’s Group with minor modifications[1].
Polyacrylonitrile (PAN) was dissolved in DMF at 60 °C, then ZnCl2 was added to
make a viscous solution with weight ratio PAN: ZnCl2: DMF = 6.52:19:173.7. After
all solids dissolved in the solution, a glass fiber mat (Crane 230, Crane Co.) was dip-
coated into the above mixture. The coated fiber was then dip-washed with 5%wt ZnCl2
aqueous solution to make a uniform coating without losing ZnCl2. The mat was dried
in the hood overnight at room temperature, and then was put in a furnace at 200°C for
10 hours in air. After that, the mat was activated in flowing N2 by heating it to 450°C
at ~ 30°C/min and then isothermed for 30 mins. After cooling in flowing N2 for 3 hrs,
carbonized fiber mat was washed with 0.5 M HCl, then rinsed with DI water, then
washed with 0.5 M NaOH and DI water rinsed again. The sample was then dried in
the hood at room temperature overnight, then dried in 110 °C in the oven for 1 hr, and
transferred to a vacuum oven and dried further at 110 °C under vacuum overnight to
remove residual moisture and HCl. The final product, namely PAN-based Chemically
Activated Carbon Fiber (PANCAF) was stored for further use.
3.2.2 Anion Exchange Functionality of Activated Carbon Fibers
Anion Exchangeable Activated Carbon Fibers were prepared by PANCAF
methylation. PANCAF mat was cut into small pieces, and mixed into solutions with
NaI, CH3I and NMP with ratio PANCAF: NaI: CH3I: NMP = 0.6g: 0.6g: 1.6ml:
10ml. Methylation reaction is carried out in a sealed 50 ml flask at 20, 50, 70, 90 °C
for 2 hrs. After stop the heating and cool down the solution, the solid samples were
29
filtered and washed several times with ethanol and DI water to remove the residual
chemicals attached on the fiber. The sample was stored in DI water for further tests
and characterization.
3.3 HACAX Characterization
3.3.1 Ion Exchange Properties
Ion exchange capacity was measured by putting ~ 50 mg HACAX samples in
1M NaNO3 solution shaking for 1 day, and then using an iodide selective electrode to
measure iodide concentration in solution. Ion exchange kinetics of HACAX was
measured by putting small pieces of 70 mg of HACAX into 200 ml 1M NaNO3
solution with violent stirring. 10 ml aqueous sample was taken out for iodide ion
analysis at each preset time point until the iodide ion concentration was unchanged.
In comparison, original PANCAF before methylation and commercially available
strong base anion exchange resin Purolite A400 was used under the same conditions.
Iodide concentration was analyzed by iodide ISE electrode (double-junction, BNC,
Cole-Parmer, Cat. No. EW-27502-23) with detection limit from 6 ppb to 127,000
ppm. The ion exchange reaction is shown in Figure 3.1.
Figure 3.1 Ion Exchange of Iodide vs. NO3-
3.3.2 BET
Brunauer-Emmett-Tell (BET) Surface Area and micropore and mesopore
volumes of HACAX and PANCAF were measured by Quantachrome Autosorb-1
apparatus. All samples were degassed at 200°C until the outgassing pressure change
was below 5µHg/min before analysis. N2 is used to measure the surface area and pore
30
volume at 77 K. BET equation was used to calculate the surface area using P/P0
range from 0.05 to 0.3; The Dubinin-Radushkevitch (DR) equation was used to
determine micropore (< 2 nm) volumes. The total pore volume was estimated from
the amount of nitrogen adsorbed at P/P0 = 0.95. Mesopore volume was calculated by
the difference of total pore volume and micropore volume.
3.3.3 TGA
The coating content of HACAX was measured using TGA (Hi-Res TA
Instruments 2950 Thermogravimetric Analyzer) by burning off the carbon coating at
800°C in air.
3.3.4 XPS
X-ray photoelectron spectroscopy (XPS) (Physical Electronics PHI Model
5400) data was acquired for surface analysis of PANCAF, HACAX and Cr(VI)
loaded HACAX. XPS data acquisition was performed using an achromatic Mg K
(1253.6 eV) x-ray source operated at 300 W. Survey scans were accumulated from 0-
1100 eV. High-resolution scans were performed with the pass energy adjusted to
35.75 eV. The inside pressure of the vacuum chamber was maintained at
approximately 10-9 Torr during the experiments. The binding energy was calibrated
with Carbon 1s peak with 284.5 eV.
A non-linear least squares curve fitting program (XPSPEAK version 4.1)
program with a symmetric Gaussian-Lorentzian sum function and Shirley background
subtraction was used to deconvolute the XPS peaks.
3.3.5 SEM
SEM images were obtained using a Hitachi S-4700 with mixed field emission
with a 10 kV or 12 kV accelerating voltage. PANCAF and HACAX samples were
31
attached on an aluminum sample holder using a carbon tape. The sample were coated
using a gold palladium plasma spray.
3.4 Cr(VI) Adsorption Test
3.4.1 Cr(VI) Solution Preparation
Hexavalent chromium stock solution was prepared by dissolving 0.5658 g
Potassium dichromate (K2Cr2O7) in 200 ml DI water, giving a 1000 ppm Cr(VI). By
dilution of stock solution, test solutions and standard solutions were made by diluting
the 1000 ppm stock solution. In the whole procedure, pH was not adjusted.
3.4.2 Adsorption Isotherm
The adsorption isotherm was studied in Cr(VI) aqueous solution at room
temperature. Initial Cr(VI) solutions were prepared by diluting 1000 ppm stock
solution into different concentrations in DI water. 15 mg of HACAX sample was
weighed and put into these solutions for 6 days, and then 3 ml solution, or after
appropriate dilution if necessary, was analyzed for Cr(VI) concentration. The
following equation was used to calculate the removal capacity
3-1
where Q is removal capacity of Cr(VI) (mg/g), C0 and Ce means starting
Cr(VI) concentration and equilibrate Cr(VI) concentration(ppm), respectively. V is
the volume of the solution (ml) and m is the dry weight of HACAX sample (g).
3.4.3 Adsorption Kinetics
Adsorption kinetics was studied in aqueous solution at room temperature in
250 ml flask with magnetic stirring. Initial Cr(VI) concentration is 4 ppm, and ~150
mg HACAX sample was put into 200 ml solution mixed with a magnetic stir bar. The
32
solution was sampled at pre-set time points. 1 ml or 10 ml solution was sampled at
each time point for Cr(VI) concentration analysis depending on the concentration
estimation of solution. Several of other adsorbents, such as ACF, GAC-F400, Pellet-
600 in the literature is also used as comparison under the same condition.
3.4.4 Sample Concentration Analysis
The concentration of Cr(VI) was measured by UV-Vis Spectroscopy at λ=540
nm via photometric diphenylcarbohydrazide method [2, 3]. Color reagent was
prepared by dissolving 125 mg of 1,5-diphenylcarbohydrazide into 25 ml HPLC-
grade methanol, then 125 ml 5.5% H2SO4 (made by dissolving 14 ml 98% H2SO4 in
DI water) was added and diluted into 250 ml solution with DI water. Before UV-Vis
Spectroscopy measurement, a 3ml sample solution (or diluted sample ) and 1 ml color
reagent was mixed well and put into 3.5 ml polystyrene disposable cuvette. Standard
solutions were prepared at concentrations of 0, 5, 10, 20, 40, 80, 120, 400, 600, 800,
1000 ppb for calibration. After mixing with color reagent, the sample was measured
within 2 hrs to ensure a precise result.
In order to study the mechanism of adsorption of Cr(VI) and possible
transition from Cr(VI) to Cr(III), total chromium concentration is measured by ICP
(Inductively Coupled Plasma, OES Optima 2000 DV by Perkin Elmer) in the
Microanalysis Lab in the School of Chemical Science at UIUC.
3.5 References
[1] Yue, Z., C.L. Mangun, and J. Economy, Preparation of fibrous porous materials by chemical activation: 1. ZnCl2 activation of polymer-coated fibers. Carbon, 2002. 40(8): p. 1181-1191.
[2] Yue, Z., et al., Removal of chromium Cr(VI) by low-cost chemically activated carbon materials from water. Journal of Hazardous Materials, 2009. 166(1): p. 74-78.
33
[3] Pehlivan, E. and S. Cetin, Sorption of Cr(VI) ions on two Lewatit-anion exchange resins and their quantitative determination using UV-visible spectrophotometer. Journal of Hazardous Materials, 2009. 163(1): p. 448-453.
34
CHAPTER 4
RESULTS AND DISCUSSION
In the first part of this chapter, properties of HACAX will be discussed. First,
ion exchange properties of HACAX were measured and discussed; then the physical
and chemical properties of HACAX are discussed. After that, the mechanism of
functionalizing ion exchange onto activated carbon is proposed.
The second part will discuss the Cr(VI) removal performance of HACAX
material. First, the result of adsorption kinetics and isotherm is presented and
discussed; then the characterization of HACAX is discussed and based on this, a
mechanism for adsorption of Cr(VI) is proposed.
4.1 Ion Exchange Properties
4.1.1 Ion Exchange Adsorption
Different reaction temperature gives different yields: As the equilibrium test
shown in Figure 4.1, the anion exchange capacity of HACAX material is range from
0.38 to 0.52 meq/g, with the highest reaction yield at 70 °C. The highest ion exchange
capacity is 1.5 mmol/g based on coating. This is the first time of such materials ever
been made; also, it is a reasonable capacity even compared with polystyrene based
anion exchange resins ~ 2.5 mmol/g.
35
20 40 60 80 1000.0
0.1
0.2
0.3
0.4
0.5
0.6
Ion Exchange Capacity IXC Coating
Temperature (C)
Ion
Exc
han
ge
Cap
aci
ty (
me
q/g
)
0.0
0.5
1.0
1.5
IXC
Co
ati
ng
(m
eq
/gC
)
Figure 4.1 Ion Exchange Functionality at different reaction temperatures
As for kinetics, Figure 4.2 shows the ion exchange adsorption of NO3- onto
HACAX compared with PANCAF. This graph shows that before methylation, there is
nearly no ion exchange reaction; after methylation, HACAX can exchange out iodide
ions and thus adsorb nitrate ions at very fast rates. The results show that PAN-based
activated carbon fibers has been well functionalized with strong base anion exchange
group, which represent the first report in the literature to the author’s knowledge.
36
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.0
0.5
1.0
1.5
2.0
Ion
Exc
han
ge
Ca
pac
ity
mm
ol/g
C
Time (hrs)
HACAX (after methylation)
PANCAF (before methylation)
Figure 4.2 Ion Exchange Kinetics of PANCAF vs. HACAX.
Kinetics comparison between HACAX and Purolite A400 was also made on a
log time scale. It shows HACAX has faster kinetics compared with polystyrene based
ion exchange resins. The reason for the fast kinetics is most likely due to the fiber
form of the material which could enhance particle and film diffusion of anions. After
10 mins, the relatively lower capacity of HACAX limits its exchange rate.
1E-3 0.01 0.1 10.0
0.2
0.4
0.6
0.8
1.0
Ion
Exc
han
ge
Cap
acit
y
Time (hrs)
HACAX
Purolite A400
Figure 4.3 Ion Exchange Kinetics of HACAX vs. Purolite A400 at log time scale
37
4.1.2 Morphology and Porous Structures
It is very important to understand how the carbon materials coated on glass
fiber. As shown in Figure 4.4, there are two patterns for active materials coating onto
glass fiber: uniform carbon coating on glass fiber and non-uniform bridging between
fibers.
Figure 4.4 SEM image of HACAX: two pattern of coatings
Surface area and pore volume including microporous and mesoporous volume
of PANCAF and HACAX are listed in Table 4.1. After modification, HACAX
surface areas increased slightly from 982 m2/g to 1045 m2/g based on coating
materials; mesopore content of HACAX increased greatly from 7% to 54%.
According to Yue et al’s[1] study, higher mesopores content is favorable for the
removal of Cr(VI) from water.
38
Table 4.1 Normalized surface area and pore volumes of PANCAF and HACAX
Sample
Surface Areaa Volume Volume Ratio
BET DR Micropore Mesopore Micropore Mesopore
m2/g ml/g ml/g % %
PANCAF 982 0.51 0.036 93% 7%
HACAX 1045 0.49 0.59 46% 54% aNote: all data are normalized based on the coating by TGA measurement.
The increased mesopore content could be further proven by use of SEM.
Compared with PANCAF (Figure 4.5 left), HACAX (Figure 4.5 right) shows more
porous structure in sub-microns scale and nano-scale, which suggested the increased
mesopore content of HACAX.
Figure 4.5 SEM of PANCAF (left) and HACAX (right)
4.1.3 Mechanism
According to the literature on heat treatment of PAN fiber and XPS studies
and adsorption property tested in this work, the following Figure 4.6 shows the
proposed mechanism of formation of anion exchange activated carbons.
4.2
4.2.
Figu
Gra
PVA
reas
rem
HA
Figure
Cr(VI) Rem
.1 Cr(VI) A
Cr(VI)
ure 4.7. Mo
anular Activ
A-300, and
sonable time
moval. Comp
CAX demo
4.6 Proposed M
moval by H
Adsorption
removal by
ost of activat
vated Carbon
PAN based
e. The Pelle
pare with ot
onstrates fas
Mechanism of
Hybrid Mat
Kinetics
y PANCAF
ted carbon m
n F400, hig
d CAF are n
et-600 and H
ther adsorbe
st kinetics an
39
formation of a
terials
and HACA
materials, in
gh performa
not able to re
HACAX sh
ents on the s
nd the abilit
anion exchange
AX vs. adsor
ncluding co
ance activate
emove Cr(V
ow superior
same weigh
ty to bring C
eable activated
rption time i
ommercially
ed carbon fi
VI) down to
r activity fo
ht level, Pell
Cr(VI) leve
d carbon
is shown in
y available
iber like AC
o EPA level
or Cr(VI)
let-600 and
el from 4 pp
n
CF,
at a
pm
40
down to 1 ppb.
-20 0 20 40 60 80 100 120 140 1601
10
100
1000
C
r C
on
cen
trat
ion
(p
pb
)
Time (hrs)
HXACF PANCAF Pellet-600 GAC-F400 PVA-300 ACF-10
EPA MCL=100 ppb
Figure 4.7 Cr(VI) removal by HACAX and various adsorbents vs. adsorption time Note: Data of Pellet 600,
GAC F400, ACF-10 were provided by Z. Yue.
Between the best two adsorbents Pellet-600 and HACAX , the HACAX
materials not only could remove Cr(VI) down to ppb levels (and potentially even
lower level, which is limited by the detection limit), but also with faster kinetics.
Time needed for removing Cr(VI) down to 2 ppb is 50 hrs and 145 hrs for HACAX
and Pellet-600, respectively. Furthermore, a better comparison on kinetics is shown in
Figure 4.8 starting with a Cr(VI) concentration of 4 ppm. It shows that within 30
mins, HACAX can remove ~90% of Cr(VI) compared with just 40% removal by
Pellet-600. There are two reasons for the improved kinetics by HACAX: The anion
exchange functionality provides a positively charged surface which facilitates the
adsorption by electrostatic force. Fiber form of HACAX with 7 µm-in-diameter
greatly increase the particle and film diffusion of Cr(VI) from bulk solution to
41
adsorbent surface, which also improves the adsorption rates. Moreover, HACAX also
showed superior performance on kinetics even compared with Purolite A400, as
shown below. HACAX is able to remove twice more of Cr(VI) than Purolite A400
within 10 mins.
0 5 10 15 20 25 300.0
0.2
0.4
0.6
0.8
1.0
HXACXPurolite A400
Cr
Co
nce
ntr
atio
n C
/C0
Time (mins)
Pellet-600
Figure 4.8 Kinetic Comparison between HACAX, Purolite A400 and Pellet-600
4.2.2 Cr(VI) Adsorption Equilibrium
The adsorption isotherms for the various materials are shown in Figure 4.9.
This graph shows that at most of concentration range, HACAX materials have better
adsorption capacity; this advantage is more obvious at low residual concentration,
which is orders of magnitude more adsorption capacity. This means that HACAX is a
good adsorbent at low level of Cr(VI) such as drinking water treatment.
42
1 10 100 1000 10000
0
5
10
15
20
25
30
35
Ad
sorb
ed C
r(V
I) (
mg
/g)
Residual Concentration (ppb)
PVA-300 Pellet-600 ACF-10 GAC-F400 HXACF
Figure 4.9 Isotherm of IXAC with Cr(VI)
There are basically two well established types of adsorption isotherms:
Langmuir adsorption isotherm and Freundlich adsorption isotherm. These two models
were applied here to this novel IXAC in order to find out the mechanism of
adsorption.
4.2.2.1 Langmuir isotherm
Langmuir adsorption isotherm describes a scenario where: adsorbent surface is
homogenous; all adsorption sites have equal adsorbate affinity with monolayer
coverage[2]. The Langmuir equation is
4-1
The linear form of the Langmuir equation is
4-2
43
Where Qe is the amount of adsorbate adsorbed (mg/g), Ce is equilibrium
concentration (mg/L or ppm), Q0 is monolayer adsorption capacity (mg/g), and b is
the constant related to free adsorption energy.
Langmuir plot is shown in Figure 4.10 :
-10000 0 10000 20000 30000 40000 50000 60000 70000 80000
0
500
1000
1500
2000
2500
Ce/
Qe
Ce (ppb)
Figure 4.10 Langmuir Fit of IXAC Isotherm
4.2.2.2 Freundlich isotherm
This isotherm developed by Freundlich in 1909 is an empirical expression
which describes the equilibrium on heterogeneous surfaces without necessarily
monolayer capacity.
The Freundlich equation is
/ 4-3
with the linear form described as:
4-4
44
Where Qe is the adsorbed amount per adsorbent weight (mg/g), Ce is the
equilibrium concentration (mg/L or ppm), K is a constant indicating adsorption
capacity (mg/g) and 1/n indicates the intensity of adsorption. The fit of Freundlich is
shown below.
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
log
Qe
log Ce
Figure 4.11 Freundlich isotherm
Summary of parameters and constants are in Table 4.2.
Table 4.2 Langmuir and Freundlich isotherm constants
Langmuir isotherm Freundlich isotherm
Q0 b R2 RL K n R2 33.11 0.00049 0.9887 0.998 0.7872 2.8019 0.8959
From R2, we can find that the Langmuir equation gives a better description of
the adsorption data. The values of Q0 and b were determined from the figure, namely,
33.11 mg/g and 0.00049 l/mg, respectively. RL <1 shows that this is a favorable
adsorption.
45
4.2.3 Cr(VI) Reduction and Characterization
In kinetic study, besides Cr(VI) concentration was measured by UV-Vis, total
Cr was also measured by ICP-MS. It turns out that after most of the Cr(VI) adsorbed
and accumulated onto the surface of activated carbon, there are Cr(III) coming out
and existing in the solution, as shown in Figure 4.12.
0 20 40 60 80 100 1201
10
100
1000
Cr
con
c. p
pb
Time (hrs)
Hexavalent Chromium Trivalent Chromium
Figure 4.12 Conversion from Cr(VI) to Cr (III) by Hybrid Materials
XPS study of Carbon 1s showed that there was 14% decrase in the signal for C-H and
C-C peak and a corresponding increase of C=O peak, as shown in Figure 4.13. This
might suggest that activated carbon surface has been oxidized, and Cr(VI) been
reduced by surface functional groups. The reduction of Cr(VI) to Cr (III) is helpful for
POU application since it not only converts toxic to beneficial Cr (III) to below 100
ppb and thus facilitate more adsorption of Cr(VI), but also prevents the leaking out of
Cr(VI) as more background ions flow through.
46
294 292 290 288 286 284 282
B.E.(eV)
COOH
C=O
HACAX-Cr
HACAX
C-H, C-C
decreased 14%
Figure 4.13 XPS of C 1s of HACAX and Cr(VI)-loaded HACAX
4.2.4 Mechanism of adsorption
According to the XPS study, the literature review[3] and adsorption measurements of
changes for Cr(VI) and Cr (III), the following electrostatic attraction – accumulation –
surface reduction – repulsion mechanism for the Cr(VI) removal by novel hybrid
materials is proposed:
1. multi-scale mass transfer of Cr(VI) is facilitated from bulk to surface by the 7
µm diameter of the fiber, presence of a macro-porous structure and high
mesopore content;
2. attracted through electrostatic force by surface positive charged functional
pyridium-like structure group and accumulated in the pores;
47
3. reductions of Cr(VI) by nearby surface functional group to positively charged
Cr(III);
4. Cr(III) is repelled by positive charge group nearby into bulk solution.
4.3 Summary and Future Work
At this time Cr(VI) has been identified as a carcinogen. Hence, an effective
POU system is urgently needed to bring trace Cr(VI) from ppm down to ppb levels.
In this study, we have successfully synthesized the first hybrid activated
carbon/anion exchange material. Compared with current state-of-the-art technology,
HACAX demonstrated a superior performance to rapidly remove Cr(VI) down to ppb
levels . Unlike earlier data on the activated carbon system described in the literature,
HACAX is able to work at natural pH with several orders of magnitude faster
kinetics; HACAX has faster adsorption rates and mechanisms to convert toxic Cr(VI)
to nutrients Cr(III). Besides, fabricating the hybrid material in the fiber form using
glass fiber as the substrate provides easy, cost-effective preparation, ease of handling
and higher throughput without introducing big pressure drop, which are very
important factors for considering a POU filter system.
Currently the author continues working on characterization such as FTIR, Zeta
Potential and thermal stability to study the general basic properties of HACAX. In
future the following works will also be explored:
Optimize the synthesis route, including temperature, time, methylating reagent
and catalysts to get the most process-feasible way for system scale-up;
Use Ion Chromography (IC) for Cr(VI) measurement to lower the detection
limit and study the performance around or below 1 ppb.
48
Explore other applications of these hybrid materials for phenol [4, 5],
perchlorate[6], and arsenic removal [7] in drinking water. Another important area is
for catalysis[8], since potentially HACAX have better thermal stability compare with
anion exchange resins.
4.4 References
[1] Yue, Z., et al., Removal of chromium Cr(VI) by low-cost chemically activated carbon materials from water. Journal of Hazardous Materials, 2009. 166(1): p. 74-78.
[2] Anandkumar, J. and B. Mandal, Removal of Cr(VI) from aqueous solution using Bael fruit (Aegle marmelos correa) shell as an adsorbent. Journal of Hazardous Materials, 2009. 168(2-3): p. 633-640.
[3] Park, D., Y.S. Yun, and J.M. Park, Comment on the removal mechanism of hexavalent chromium by biomaterials or biomaterial-based activated carbons. Industrial and Engineering Chemistry Research, 2006. 45(7): p. 2405-2407.
[4] Mohanty, K., D. Das, and M.N. Biswas, Adsorption of phenol from aqueous solutions using activated carbons prepared from Tectona grandis sawdust by ZnCl2 activation. Chemical Engineering Journal, 2005. 115(1-2): p. 121-131.
[5] Juang, R.S., F.C. Wu, and R.L. Tseng, Adsorption isotherms of phenolic compounds from aqueous solutions onto activated carbon fibers. Journal of Chemical and Engineering Data, 1996. 41(3): p. 487-492.
[6] Srinivasan, R. and G.A. Sorial, Treatment of perchlorate in drinking water: A critical review. Separation and Purification Technology, 2009. 69(1): p. 7-21.
[7] Vatutsina, O.M., et al., A new hybrid (polymer/inorganic) fibrous sorbent for arsenic removal from drinking water. Reactive and Functional Polymers, 2007. 67(3): p. 184-201.
[8] Gelbard, G., Organic synthesis by catalysis with ion-exchange resins. Industrial and Engineering Chemistry Research, 2005. 44(Compendex): p. 8468-8498.