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Magnetic Ion Exchange: Is There Potential for International Development?
P. A. Neale, A. I. Schäfer
School of Engineering and Electronics, University of Edinburgh, Edinburgh, EH9 3JL,
United Kingdom, email: p.neale@ed.ac.uk
Submitted to Desalination
Date Submitted: January 2008
Date Re-submitted: May 2008
Abstract: Magnetic ion exchange (MIEX®) is an ion exchange resin developed as an additive to
existing water treatment plants where additional organic matter is to be removed. The smaller size,
magnetic properties and simple regeneration using NaCl distinguishes MIEX® from conventional
ion exchange resins and hence its use in international development applications is investigated in
this review article. MIEX® has been demonstrated to remove varying levels of dissolved organic
matter, inorganic anions such as nitrate and sulphate and micropollutants including non-ionic
pesticides. The removal efficiency can also be influenced by temperature, pH and presence of other
anions. As MIEX® is unable to disinfect water, the most likely application within international
development is as pre-treatment before disinfection or membrane filtration.
Keywords: Magnetic ion exchange, international development, natural organic matter removal,
micropollutant removal
1. Introduction
Magnetic ion exchange (MIEX®) is a strong base anion exchange resin with magnetic properties
that can be used to adsorb weak organic acidic ions from water [1]. The magnetic properties
differentiate it from other ion exchange resins, as it allows for faster resin agglomeration and
recovery [2]. The resin was developed with the purpose of removing dissolved organic carbon
(DOC) from drinking water supplies [3]. While DOC by itself is relatively harmless, problems can
occur when DOC is combined with chlorine, a common drinking water disinfectant, or bromide as
they can form disinfection by-products (DBP), which are potential carcinogens [2]. In addition, the
removal of DOC from water reduces the need for residual disinfection of bacterial regrowth during
distribution, as well as customer complaints relating to taste, odour and colour of the water [4]
The objective of this paper is to review potential applications of this ion exchange resin in
international development. One of the main problems facing international development is the
provision of safe drinking water, with over 1.1 billion people lacking access to this basic need [5].
Waterborne diseases, associated with pathogenic bacteria, viruses and protozoa, are the most
common cause of illness and death related to water and sanitation within developing countries [6].
Therefore, improved microbiological safety through the removal of pathogens is essential for
international development. While MIEX® can not be used to disinfect or physically remove bacteria
or viruses, it can remove DPB precursors, reduce chlorine demand, and therefore reduce DPB risk
[7]. In addition, it can be coupled with disinfection processes such as ozonation or membrane
filtration such as ultrafiltration.
This review will describe the principles of MIEX® and focus on its applicability to international
development by discussing its ability to remove DOC, as well as a range of inorganic and organic
contaminants from water and examine the possibility of MIEX® integration with existing water
treatment options. The issue of brine waste disposal options will conclude the paper.
2. Process Principles
2.1. Resin Characteristics
MIEX® is an anionic exchange resin which consists of a magnetic core with a polymer shell (Fig. 1)
[8]. The polymer, polyacrylate, is macroporous and contains quaternary amide functional groups,
which assist with DOC removal through ion exchange [9]. MIEX® differs from traditional ion
exchange resins due to its small size and magnetic properties. These changes were implemented to
maximise organic removal and resin reuse. The resin beads have a mean diameter of 150 – 180 µm
which is approximately 2 to 5 times smaller than other ion exchange resins [10]. As a result it has
an increased surface area to volume ratio compared to other resins, meaning there are more
exchange sites which increases exchange kinetics, therefore more DOC can be removed [2, 7]. The
magnetic core enables fast agglomeration and settling of resin particles, and this leads to high (up to
99.9%) resin recovery rates [11].
Figure 1: Light microscope image of MIEX® resin
2.2. Adsorption and Desorption
Adsorption and desorption are the chemical reactions that allow the process to function (Fig. 2).
The DOC is adsorbed to the resin through ion exchange with the chloride ions at the active sites on
the surface. Quaternary amide moieties act as DOC chloride exchange sites [12]. The presence of
negatively charged carboxylic groups in DOC enables removal as they are attracted to the active
sites [12]. The resin is not limited to the adsorption of DOC. MIEX® also has the ability to adsorb
other anions, including nitrate, sulphate, bromide, chromium, arsenic and certain pesticides [13, 14].
The removal efficiency of these compounds will depend on the anion competition for exchange
sites.
Figure 2: MIEX® adsorption and desorption chemistry with NR+3 representing the quaternary
amide exchange sites (Adapted from [15])
Brine&
Raw&Water&
DESORPTION&
DOC&ADSORPTION&
-OOC
NR+
3%
NR+
3%
Cl(%
Cl(%
Cl(%
H+%
H+%
DOC&
DOC&
Na+%
Cl(%
NR+
3%
NR+
3%
Cl(%
Cl(%
Na+%
Na+%
DOC&
DOC&
Neale, P.A. ; Schäfer, A.I. (2009) Magnetic ion exchange: Is there potential for
international development?, Desalination, 248, 160–168. doi:10.1016/j.desal.
2008.05.052
The resin is regenerated through desorption, which is a reversal of the adsorption process.
Desorption is achieved with the assistance of a 10% w/w NaCl brine. Through a reverse of the ion
exchange process, the DOC sorbed to the quaternary amide active sites are substituted for chloride
ions, and the exchanged DOC goes to the brine [10]. This process occurs due to the high chloride
concentration, as the chloride ions have a strong affinity for the exchange sites and force the DOC
from the resin [15]. Depending on water quality 2% w/w NaOH can be added to assist regeneration,
as the increase in pH increases the solubility of DOC [15]. Conventional ion exchange resins are
fouled by organic matter, and thus can not be reused effectively. Therefore, the ability to regenerate
is advantageous to application in international development where ease of operation is crucial.
3. Contaminant Removal
3.1. Natural Organic Matter
Natural organic matter (NOM) is a complex mixture of different types of organic molecules
including fulvic and humic acids, hydrophilic acids and various low molecular weight compounds
[16]. The origins of NOM in water supplies include soil, swamps and plant decomposition, as well
as wastewater effluent if present. NOM has colour, taste and odour implications for drinking water,
and can also pose a threat to human health when combined with chlorine disinfection as NOM is a
DBP precursor [2].
MIEX® was developed for the purpose of removing the dissolved fraction of NOM from water.
Previous studies have demonstrated that it effectively removes the ultraviolet absorbing and acidic
fractions of DOC over a wider molecular weight range compared to alternative treatment options,
such as coagulation [1, 10, 11]. In addition, the resin can also effectively remove both hydrophobic
and hydrophilic fractions of DOC [7].
The removal of DOC from raw water using MIEX® is generally around 80% [2, 11]. However, the
removal efficiency of DOC can be affected by the quality of the raw water as well as the solution
chemistry. Due to the presence of carboxylic groups [12], DOC is in an anionic form from pH 4 - 5.
At low pH (<4) it cannot be used to effectively remove DOC, however as most applications are in
the pH range of 6.5 – 7.5 it can remove DOC rich in carboxylic groups. However, the removal
efficiency would also be greatly reduced if the raw water came from a source rich in tannins.
Tannins primarily contain phenolic hydroxyl groups, which are not in an anionic form until
approximately pH 8 - 9 [1]. Temperature also has the potential to affect removal. Humbert et al.
[14] found no significant difference in DOC removal between 6 and 26°C, however removal
increased slightly as the temperature increased to 36°C. This is because an increase in temperature
leads to an augmentation of reaction kinetics.
There are several factors that can impair DOC removal by the resin. Firstly, there is the issue of
seasonal variations in the quantity of DOC in raw water. After periods of high rainfall the level of
DOC in water supplies can increase [11]. In these situations it may be necessary to alter the dosage
of resin, or increase the contact time, to ensure that it can successfully remove DOC. This may
make this process vulnerable in international development applications where trained personnel and
monitoring may not be easily available. In addition, a high concentration of inorganic anions in the
raw water can compete for ion exchange sites, reducing DOC removal [2]. This indicates the studies
with specific waters are important before process adaptation.
3.2. Inorganic Matter
MIEX® has the potential remove inorganic anions from water, and the presence of anions such as
fluoride or nitrate are common problems in many developing countries. Inorganic anions such as
bromide can increase the formation of DBP [3], while trace metal anions such as chromium and
arsenic can have serious implications for human health as they are carcinogenic [13]. Several
studies have shown that inorganic anion removal is dependant on MIEX® resin concentration [9,
14]. At high resin doses (8 mL/L) as much as 65% of bromide can be removed, while this decreases
significantly at lower doses (0.5 mL/L) to 8% removal [14]. The removal efficiency of bromide is
further dependant on pH. A study by Singer and Bilyk [7] showed that bromide removal decreased
significantly as alkalinity increased. At high pH there is a higher concentration of bicarbonate ions
and these compete with bromide for ion exchange sites, reducing the bromide removal.
Removal of sulphate and nitrate has also been studied. The presence of sulphate in the water can
cause problems for DOC removal, as it can compete with DOC for exchange sites [3]. Sulphate
removal has been shown to be concentration dependant, with 92% removal by MIEX® at 2 mL/L
resin concentration, decreasing to 42% removal at 0.5 mL/L concentrations [14]. The resin can
contribute significantly to nitrate removal, up to 94% [10, 14].
In addition, the application of MIEX® to removal trace metal anions such as arsenic (As(V)) and
chromium (Cr(VI)) has been studied, and showed 37% removal of arsenic and 58% removal of
chromium [13]. As arsenic contamination is a problem in some developing countries such as
Bangladesh [17] the ability of MIEX® to remove moderate amounts of arsenic is very relevant and
promising for international development.
3.3.Micropollutants
Micropollutants, which include pesticides, natural and synthetic hormones and pharmaceutically
active products, can be considered ubiquitous in surface and wastewater [18]. Low concentrations
of many micropollutants (ng/L to µg/L) can have a significant effect on vertebrate and ecosystem
health. Such compounds are of concern where water reuse is practiced intentionally or
unintentionally because of polluted waters serving as water supplies downstream. Such sewage-
water supply cross-connections are very common in international development due to open sewers,
pit latrines and usage streams for disposal and water supply. In developing countries the usage of
pesticides that may be banned in developed countries is commonplace and it is likely that this is the
most severe micropollutant threat. In addition, many micropollutants are not removed effectively
using conventional water treatment, where such treatment schemes exist, and therefore MIEX® may
be applied to assist their removal. Within the literature, MIEX® has been applied to remove
micropollutants including pesticides atrazine and isoproturon [14, 19] and natural hormones such
as estrone [20, 21]. The removal of pesticides from water is particularly applicable to international
development due to extensive pesticide use in many developing countries [22].
The removal of atrazine and isoproturon using MIEX® was studied by Humbert et al. [14, 19]. The
results demonstrated only 7% of atrazine and 5% of isoproturon could be removed using MIEX®
after a contact time of 30 minutes [14]. By increasing the contact time to 24 hours slightly higher
(approximately 12%) removal was observed [19]. The poor performance was attributed to the non-
ionic nature of the pesticides. However, MIEX® may have greater applicability in pesticide removal
when used as a pre-treatment to powdered activated carbon (PAC). Humbert et al. [19] found the
use of MIEX® prior to PAC lead to a 20% increase in pesticide removal compared to removal by
PAC alone.
The removal of steroidal hormones from drinking water is also of importance. Hormones, such as
estradiol and estrone, are naturally excreted by humans, and they are amongst some of the most
potent endocrine disrupting chemicals [23]. Mastrup and Schäfer [20] and Schäfer et al. [21] studied
estrone removal by MIEX® and found removal was influenced by pH, temperature and ionic
strength. The charge of some hormones is affected by pH. For example, estrone changes from a
neutral to a negative charge above its acid dissociation constant of 10.4. At pH 11, MIEX® could
remove approximately 70% of negatively charged ionic estrone from solution, compared to only
30% when estrone has a neutral charge and is non-ionic [21]. This suggests that while ion exchange
interactions are important for estrone removal by MIEX®, estrone must still interact with MIEX®
through either specific (e.g. hydrogen bonding) or non-specific (eg van der Waal forces)
interactions as 30% is still removed below pH 11.
4. Process Integration
MIEX® can be added to water treatment processes in similar ways to coagulants or PAC. Its
magnetic characteristics allow accelerated floc formation and settling. The process, from the input
raw water to the output DOC removed water, is shown below in Fig. 3 and it can be divided into
three key parts, DOC removal, resin recovery and resin regeneration. In addition, MIEX® can be
retrofitted to existing treatment plants.
Figure 3: The MIEX® process (adapted from [23]).
4.1. MIEX Process
The raw water enters a stirred contact, and resin is added. The contractor is stirred to allow the resin
to disperse to provide maximum surface area for DOC adsorption [11]. The amount of resin added
is typically 5 to 10 mL/L, however this can vary depending on the quality of the raw water [15].
The contact time is typically 30 minutes, however a study by Humbert et al. [14] suggested
maximum DOC removal can be reached within 15 minutes using 8 mL/L of MIEX® resin.
However, a longer contact time may be required depending on water quality, as it has been
suggested by Fearing, et al., [11] that a contact time of 60 minutes is required for optimal DOC
removal. In addition, as demonstrated in the case of atrazine and isoproturon, the contact time for
optimal removal will vary depending on the contaminant. After detention in the contactor, the resin
and water flow under gravity to the separator. Due to its magnetic properties, the resin rapidly
agglomerates and forms large particles which settle quickly, allowing fast separation from the
supernatant [10]. Only a small amount (<0.1%) is lost during the process, so the rate of resin
recovery is greater 99.9% [11], however any resin carryover can increase the turbidity of the treated
water [7]. Therefore, an additional water treatment process such as coagulation may be required
after to remove any turbidity [3]. Approximately 85 – 90% of the recovered resin is reused in the
process, while the remaining resin is regenerated using NaCl brine.
4.2. Pre-treatment
MIEX® cannot be used as a stand alone process to treat drinking water as it does not have the
capacity to remove turbidity, particulate matter or pathogens from water. However, it can be
coupled with existing water treatment options to produce safe drinking water. Recently research has
focused the usage of the resin as a pre-treatment step to coagulation (Fig. 4) ozonation and
membrane filtration [1, 2, 11]. Fearing et al. [11] compared DOC removal with MIEX® itself to the
resin combined with ferric sulphate coagulation, and found DOC removal increased by as much as
20% in the combined process. In contrast, Boyer and Singer [2] found there was no significant
difference in DOC removal between MIEX® itself and for a combined process with alum
coagulation. There are several advantages to applying the resin as a pre-treatment. Typically
MIEX® removes low to moderate molecular weight fractions of DOC, while coagulation is better
suited to remove high molecular weight fractions [24]. Allpike et al. [1] found that the use of
MIEX® pre-treatment before alum coagulation lead to a wider molecular weight range of DOC
removed compared to the resin or coagulation alone. In addition, pre-treatment with the resin can
reduce the coagulation dose required for treatment significantly (up to 85%) [7]. However, as
neither MIEX® nor coagulation can disinfect water, even the combined process is not suitable for
Contactor%
Resin%Separation%%
Resin%Regeneration%
Treated%
Water%
Raw%
Water%
Recovered%Resin%
international development alone, and would need to be coupled with chemical or physical
disinfection processes.
Figure 4: MIEX® pre-treatment option for alum coagulation (adapted from [15])
Ozonation is a water treatment method that disinfects water without the need for chlorine, however
there are some drawbacks which prevents greater ozonation application. Firstly, carcinogenic
bromate and bromate DBPs can form when using ozone to disinfect water containing bromide ions
[12]. Secondly, energy demand is high, and thirdly, any DOC present in the raw water can increase
the ozone demand, which reduces the effectiveness of ozonation [9]. Based on its ability to remove
approximately 80% DOC and up to 65% bromide, MIEX® is an attractive pre-treatment option for
ozonation. Johnson and Singer [9] found that MIEX® pre-treatment significantly increased the
dissolved ozone concentration following ozonation, compared to untreated water. Therefore, with
the removal of the majority of DOC the ozone demand could be reduced. MIEX® pre-treatment also
has implications for the ozone decay, as Wert et al. [12] demonstrated that MIEX® pre-treatment
decreased the ozone decay rate compared to untreated water which can lead to an increase in
bacteria and protozoan inactivation. In addition, bromate formation due to ozonation decreased
significantly when MIEX® was used for pre-treatment [9]. This is due to the removal of bromide
ions, as well as the reduced ozone demand. A hybrid MIEX® - ozonation process could be an option
to treat water in developing countries, however the operational and maintenance requirements of
ozonation may make it an inappropriate technology option at this stage [6].
Membrane filtration such as ultrafiltration (UF) may be a suitable water treatment option for
international development as it disinfects water by physically removing bacteria and viruses, and
has a lower chemical requirement compared to conventional treatment [25]. However, organic,
biological and particulate fouling which reduces the treated water output and increases costs
through high cleaning and maintenance requirements which again may be difficult to achieve in
international development. As MIEX® can remove significant amounts of DOC it may prevent
some of this membrane fouling. However, studies by Son et al. [26] and Humbert et al. [27] using
dead-end filtration configurations found that MIEX® pre-treatment for UF only lead to a minor
decrease in membrane fouling compared to no pre-treatment. Son et al. [26] showed that
coagulation was a better pre-treatment for UF, suggesting that more fouling was caused by high
molecular weight fractions which coagulation can remove, compared to low to moderate fractions
MIEX® typically removes. In addition, Schäfer et al. [21] has suggested that MIEX® resin is prone
to break up, and this may in fact contribute to membrane fouling. Zhang et al. [28] pre-treated
submerged UF membranes with MIEX® and PAC, and MIEX pre-treatment showed no increase in
transmembrane pressure, and allowed the process to run for longer, while PAC lead to an increase
in transmembrane pressure. Submerged membranes typically require lower pressure and less
cleaning than dead-end or cross flow membrane configurations [28], therefore submerged
membranes with MIEX® pre-treatment may be more manageable in international development
applications.
5. Brine Disposal and Treatment
Alum%Coagulation%Clarification%
Sand%Filtration%
MIEX®%Pre(Treatment%
In the final phase of the MIEX® process the DOC saturated resin is regenerated with 10% w/w
NaCl solution [10]. The by-product of regenerate is a waste brine containing NaCl as well as any
DOC, inorganics and micropollutants removed from the raw water and recovered from the resin.
Such brines are similar in characteristic to reverse osmosis concentrates from desalination or
wastewater reuse facilities. As such disposal or treatment can be expensive and have significant
environmental impacts it is important to consider brine disposal options suitable for developing
countries.
When geographically possible, the majority of such waste brines is disposed to the ocean [29]. This
disposal method is applied due to low cost, however disposal can be problematic, as the brine
salinity is considerably higher than seawater, disturbing the sea floor [29], while the presence of
micropollutants in the brine is likely to have negative implications for aquatic organisms. In
situations when ocean disposal is not suitable, such as for inland water treatment, other brine
disposal options include solar evaporation, landfill and bore injection [30]. For inland disposal,
solar evaporation ponds are the most suitable disposal method in warm or arid developing countries.
They are low cost, require little maintenance, and when the ponds are properly sealed there is no
contamination of surrounding soil or groundwater [31], unlike the other inland disposal options.
Another option is to treat the brine to separate the organic matter from brine. Using anion exchange
brine from a sugar refinery Wadley et al. [32] used nanofiltration to separate NaCl from organic
matter. The DOC concentration in brine can be as high as 20 g/L [15], therefore it could be used as
a soil supplement, while the NaCl could be reused to regenerate MIEX®. However, due to the
higher costs and complexities associated with brine treatment compared to disposal it is unlikely it
would be suitable for international development.
6. Integration in Developing Countries
As waterborne diseases are a major cause of mortality in developing countries [33] it is essential to
disinfect or physically remove pathogenic bacteria and viruses. As a result MIEX® can not be used
alone for international development. However, depending on levels of water turbidity and
particulate matter type, MIEX® can be coupled with either chlorine disinfection or submerged
ultrafiltration. Some advantages and disadvantages of MIEX® in the context of international
development are shown in Table 1. As well as the advantages of removing DBP precursors and
reducing chlorine demand, the ability of MIEX® to remove inorganic anions shows great potential
for international development in particular where physio-chemical water quality are of concern.
While MIEX® can not remove turbidity, and can even increase it, the presence of turbidity does not
affect removal of DOC. High turbidity waters are very common in developing countries,
particularly during rainy season, hence this is an important characteristic of MIEX® for international
development. Finally, waste brine produced by the process can create environmental problems if
not properly disposed of, however in arid or warm countries brine can be disposed of with low cost
and environmental impact using solar evaporation. Therefore, MIEX can be suitable for
international development, provided it is coupled with disinfection or membrane filtration and the
waste is dealt with responsibly.
Table 1: Advantages and disadvantages of MIEX® in international development
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ADVANTAGES
DISADVANTAGES
Effective removal of dissolved organic carbon
(up to 80%), and hence disinfection by-product
precursors
Cannot disinfect water, therefore cannot be used
alone for international development
applications where microbiology contaminants
are significant
Reduced chlorine demand and hence reduce
disinfection costs
Cannot remove turbidity or particulate matter,
therefore filtration or coagulation is required for
turbid raw water
Significant removal of inorganic anions, such as
nitrate and sulphate, and certain micropollutants,
such as ionic pesticides, pharmaceuticals and
hormones
Any resin carryover in the process can increase
turbidity of the treated water
Potential to couple MIEX® with other water
treatment applications, such as disinfection,
coagulation and membrane filtration
Seasonal fluctuations in DOC concentration can
require different MIEX® doses or retention
times, therefore the process requires monitoring
MIEX® waste brine can be disposed of through
solar evaporation at low cost and low
environmental impact
MIEX® waste brine requires treatment and
disposal
Presence of turbidity and particulate matter do
not affect removal of DOC by MIEX, which is
good characteristic for international development
where surface waters often have extreme
turbidity values
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