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Combined ion exchange treatment for removal of dissolvedorganic matter and hardness

Jennifer N. Apell 1, Treavor H. Boyer*

Department of Environmental Engineering Sciences, University of Florida, P.O. Box 116450, Gainesville, FL 32611-6450, USA

a r t i c l e i n f o

Article history:

Received 9 November 2009

Received in revised form

5 January 2010

Accepted 6 January 2010

Available online 14 January 2010

Keywords:

Anion exchange

Calcium

Cation exchange

Dissolved organic matter

Magnetic ion exchange

Membrane fouling

Softening

* Corresponding author. Tel.: þ1 352 846 335E-mail address: thboyer@ufl.edu (T.H. Bo

1 Present address: CDM, 50 Hampshire Str0043-1354/$ – see front matter ª 2010 Elsevidoi:10.1016/j.watres.2010.01.004

a b s t r a c t

Dissolved organic matter (DOM) and hardness cations are two common constituents of

natural waters that substantially impact water treatment processes. Anion exchange

treatment, and in particular magnetic ion exchange (MIEX), has been shown to effectively

remove DOM from natural waters. An important advantage of the MIEX process is that it is

used as a slurry in a completely mixed flow reactor at the beginning of the treatment train.

Hardness ions can be removed with cation exchange resins, although typically using

a fixed bed reactor at the end of a treatment train. In this research, the feasibility of

combining anion and cation exchange treatment in a single completely mixed reactor for

treatment of raw water was investigated. The sequence of anion and cation exchange

treatment, the number of regeneration cycles, and the chemistry of the regeneration

solution were systematically explored. Simultaneous removal of DOM (70% as dissolved

organic carbon) and hardness (>55% as total hardness) was achieved by combined ion

exchange treatment. Combined ion exchange is expected to be useful as a pre-treatment

for membrane systems because both DOM and divalent cations are major foulants of

membranes.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction membranes (Yoon et al., 1998; Kimura et al., 2004; Saravia

Dissolved organic matter (DOM) and hardness cations (i.e.,

calcium and magnesium) are common constituents of natural

water that have a substantial impact on physical-chemical

unit processes and finished water quality. DOM is undesirable

because it imparts taste, odor, and color to water; increases

chemical requirements for oxidation, coagulation, and disin-

fection; and is a precursor to disinfection byproducts (DBPs).

Hardness cations are primarily an economic concern for

domestic water users. In addition, many industrial processes

require hardness-free water to prevent scaling. Of increasing

importance is the fact that both DOM and calcium have been

shown to cause reversible and irreversible fouling of

1; fax: þ1 352 392 3076.yer).eet, Cambridge, MA 02139er Ltd. All rights reserved

et al., 2006; Gray et al., 2007; Jin et al., 2009).

Coagulation is a common unit process used to remove DOM

(Dempsey et al., 1984), while lime softening is commonly used

for removal of hardness (Mercer et al., 2005). Coagulation and

lime softening, however, have limitations. For example, coag-

ulation is limited to removal of ultraviolet-absorbing DOM

(Archer and Singer, 2006), while lime softening is limited by the

solubility of calcite and removal of carbonate hardness

(Stumm and Morgan, 1996). Therefore, alternative treatment

processes for removal of DOM and hardness are sought that

could provide benefits over traditional treatment. A combined

anion and cation exchange process is envisioned that would

remove both DOM and hardness, and thereby replace

, USA..

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 02420

coagulation and lime softening with a single unit process. The

basis for combined ion exchange treatment is discussed below.

Anion exchange, and in particular magnetic ion exchange

(MIEX), is an alternative to coagulation for DOM removal

(Singer and Bilyk, 2002; Fearing et al., 2004; Boyer and Singer,

2005; Jarvis et al., 2008). MIEX resin is designed to be used as

a slurry in a completely mixed flow reactor or fluidized bed

reactor (Boyer and Singer, 2006; Singer et al., 2009). As a result,

MIEX resin is used as a pre-treatment process to treat unfiltered

water at the beginning of a treatment train. MIEX resin has

been previously shown to be very effective for removal of DOM

(Humbert et al., 2005; Kitis et al., 2007; Mergen et al., 2008;

Zhang et al., 2008). The substantial reduction in DOM by MIEX

pre-treatment results in decreased chemical requirements and

reduced formation of DBPs (Singer and Bilyk, 2002; Johnson and

Singer, 2004). In addition, research has shown that anion

exchange and MIEX pre-treatment have the potential to reduce

membrane fouling by DOM when resin carryover is controlled

(Fabris et al., 2007; Zhang et al., 2008).

Cation exchange is an alternative to lime softening for

hardness removal, and has been extensively used for point-

of-use water softening. In municipal water treatment plants,

cation exchange resin is traditionally used in a fixed bed

reactor at the end of a treatment train. Orica Watercare, the

manufacturer of MIEX resin, recently developed a weak-acid,

magnetic cation exchange resin specifically designed for

removal of hardness. This resin is designed to be used in

a suspended manner as a pre-treatment process for hardness

removal, similar to traditional MIEX resin for DOM removal.

Although cation exchange treatment is less common than

softening in municipal water treatment plants, recent research

has shown that cation exchange is beneficial as a pre-treat-

ment for membrane systems (Cornelissen et al., 2009; Heijman

et al., 2009). Cation exchange is used to remove calcium and

other divalent cations to prevent precipitation of sparingly

soluble minerals, such as calcium sulfate and calcium

carbonate, and to minimize enhanced fouling by DOM on

membrane surfaces (Li and Elimelech, 2004). For example,

Cornelissen et al. (2009) showed a decrease in irreversible

fouling on an ultrafiltration membrane when raw water was

treated with cation exchange resin in a fluidized bed. Heijman

et al. (2009) were able to achieve a 97% recovery in a nano-

filtration system with the use of a cation exchange fluidized

bed that achieved nearly complete removal of divalent cations.

Thus, combined anion and cation exchange is expected to

substantially decrease membrane fouling by simultaneously

removing DOM and divalent cations.

Although previous researchers have investigated anion

exchange for removal of DOM and cation exchange for

removal of hardness, none of the previous work combined

both anion and cation exchange into a single unit process for

simultaneous removal of DOM and hardness. It is also not

known how the interactions between DOM and hardness

cations would affect the anion and cation exchange reactions.

The potential benefits of combined ion exchange for removal

of DOM and hardness are elimination of sludge from coagu-

lation and lime softening, ability to use a single completely

mixed flow reactor or fluidized bed reactor at the head of the

treatment train, and removal of both organic and inorganic

membrane foulants.

The overall goal of this work is to evaluate the removal of

DOM and hardness by combined anion and cation exchange

treatment. The specific objectives of this work are: (1) to

evaluate the effectiveness of a magnetically-enhanced cation

exchange resin; (2) to compare removal efficiencies for anion,

cation, and combined ion exchange treatment; (3) to evaluate

the effect that simultaneous versus sequential combined ion

exchange treatment has on removal efficiencies; (4) to deter-

mine the influence of regeneration parameters on removal

efficiencies; and (5) to discuss additional applications of

combined ion exchange treatment.

2. Materials and methods

2.1. Materials

All experiments were conducted using groundwater from

Cedar Key, FL collected from Well 4 of the Cedar Key Water &

Sewer District. Groundwater was collected in November 2008

and January, February, and April 2009. Finished water was

collected from the Cedar Key Water Treatment Plant in August

2009. The finished water was produced by the following

treatment train: permanganate oxidation at the well head for

iron; MIEX to remove DOM; lime softening to remove hard-

ness; sand filtration; and chlorine disinfection.

Magnetically enhanced anion and cation exchange resins,

manufactured by Orica Watercare, were evaluated in this work.

In previous literature, the magnetic anion exchange resin is

referred to as MIEX resin. In this work, the magnetic anion

exchange resin is referred to as MIEX-Cl (i.e., chloride is the

mobilecounteranion)andthemagneticcation exchange resin is

referredtoasMIEX-Na(i.e., sodiumisthe mobile counter cation).

Both resins have a polyacrylic backbone, macroporous struc-

ture, and contain magnetic iron oxide. In addition, the MIEX-Cl

and MIEX-Na resins are designed to be used in a suspended

manner in a completely mixed flow reactor, as discussed

previously. The MIEX-Cl resin is a strong-base anion exchange

resin with quaternary amine functional groups, and has a volu-

metric anion exchange capacity of 0.52 milliequivalents (meq)

per mL resin (Boyer and Singer, 2008). Additional discussion of

anion exchange resin properties is provided elsewhere (Boyer

and Singer, 2008). The MIEX-Na resin is a weak-acid cation

exchange resin with carboxylic acid functional groups. Weak-

acid cation exchange resins are typically used in the hydrogen-

form at acidic pH values (Clifford, 1999). At neutral to basic pH

values, weak-acid resins function much like strong-acid resins,

and are typically used in the sodium form (Clifford, 1999). The

MIEX-Na resin was assumed to have a cation exchange capacity

of 0.52 meq/mL because it was functionalized from the same

starting material as the MIEX-Cl resin. All ion exchange resins

were dosed volumetrically by measuring the volume of wet

settledresinusing agraduatedcylinder.The resinwassettled for

one hour to ensure consistent volume measurements.

ACS grade chemicals were used for all experimental

procedures and analytical methods. Standard chemicals used

for total organic carbon analysis were provided by the manu-

facturer. Deionized (DI) water was used to prepare all chemical

reagents and standards. Glassware was cleaned by rinsing with

DI water and, if necessary, a 6% nitric acid solution.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 0 2421

2.2. Jar test procedure

A Phipps & Bird PB-700 jar tester with 2 L square jars was used

to conduct batch tests with ion exchange resin. Two liters of

Cedar Key raw water was added to each jar. The ion exchange

resin was measured, as described in 2.1., and added to the jars.

The resin was mixed for 20 min at 100 rpm and allowed to

settle for 30 min. A sample was taken from each jar from

a spigot in the jar. All ion exchange experiments were

conducted using duplicate doses of ion exchange resin, and all

results are shown as average values with error bars corre-

sponding to one standard deviation for duplicate resin doses,

except where noted otherwise.

Individual anion and cation exchange jar tests were

conducted as described in the previous paragraph. In addition,

three types of combined ion exchange experiments were

performed: (1) simultaneous anion and cation exchange, (2)

sequential anion exchange followed by cation exchange

(Sequence 1), and (3) sequential cation exchange followed by

anion exchange (Sequence 2). For all combined ion exchange

experiments, anion and cation exchange resins were

measured separately in graduated cylinders and then added

to a single jar at the appropriate time during the experiment.

Initial jar tests were conducted with fresh ion exchange resin,

which is defined in 2.3 below. After the initial jar test, the resin

from the duplicate jars was combined for regeneration, which

is also described in 2.3. The combined resin was split into

duplicate doses with the assumption that the anion and

cation exchange resins were evenly distributed. Subsequent

jar tests were conducted with regenerated resin, and the tests

are referred to as the number of times the resin was regen-

erated (e.g., one regeneration cycle¼ regen. 1�). Sequences 1

and 2 followed the general procedure described above, with

the following additional steps. Three jars were used for the

first stage of treatment with either anion or cation exchange

resin. After the first treatment stage, at least 4 L of treated

water was decanted from the three jars, and 2 L each of

treated water was transferred to two clean jars. The comple-

mentary ion exchange resin was added to the new jars for the

second stage of treatment. A sample from each jar was taken

after the second treatment stage.

Raw and treated water samples were measured for pH,

total hardness, alkalinity, ultraviolet (UV) absorbance, dis-

solved organic carbon (DOC), fluorescence intensity, chloride,

and sulfate.

2.3. Regeneration of ion exchange resin

Virgin anion and cation exchange resins were regenerated

before their initial use to become fresh resin. Both MIEX-Cl

and MIEX-Na resins were regenerated in a solution that

contained 10 times more sodium or chloride than was theo-

retically available on the resin, based on an ion exchange

capacity of 0.52 meq/mL. For example, 2 mL/L of MIEX-Cl resin

has a capacity of 1.04 meq/L, and a 10 times sodium chloride

solution has a concentration of 10.4 meq/L Cl� (or 10.4 mM

Cl�). Although MIEX-Cl is shipped in the chloride form,

preliminary jar tests showed an increase in DOC removal with

regeneration, suggesting that the virgin resin was not fully

saturated with chloride, possibly due to contamination of the

resin. MIEX-Na resin is shipped as a mixture of sodium and

hydrogen mobile ions, so it was regenerated to convert all

mobile ions to sodium.

The resins were regenerated after each jar test as follows.

Excess water was decanted from the jars and the resin was

rinsed once with DI water. All regeneration solutions had

a sodium chloride concentration of w2 M. The baseline

regeneration procedure used a brine solution that contained

25 times more sodium chloride (on a meq/L basis) than was

theoretically available on the resin. This was achieved by

adjusting the ratio of the volume of regeneration solution to

the volume of MIEX resin. The regeneration solution and resin

were mixed on a stir plate for 30 min and allowed to settle for

10 min before decanting the brine. The container was filled

with DI water, mixed for 10 min, settled for 10 min, decanted,

and repeated for a second time. The cation and anion

exchange resins were combined for the simultaneous ion

exchange tests, so the amount of sodium chloride used for

regeneration was dependent on the amount of cation

exchange resin present. Consequently, the brine solution was

8 times stronger for the anion exchange resin than it was for

the cation exchange resin because of the different dosages of

MIEX-Cl and MIEX-Na resins. For Sequences 1 and 2, the cation

and anion exchange resins were regenerated separately, and

therefore, the ratio of sodium chloride to resin, on a meq/L

basis, was constant at 25 for both resins.

TheMIEX-Na resinwasalso regeneratedusing aseriesofacid

and base solutions as follows. The resin was stirred in DI water

while hydrochloric acid was added until pH 3 was reached. This

step converts the resin to the hydrogen form. Sodium hydroxide

was then added until pH 11 to convert the resin to the sodium

form. The same rinsing procedure was followed.

2.4. Analytical methods

Samples requiring filtration were filtered through 0.45 mm

nylon membrane filters (Millipore). All filters were pre-rinsed

with 500 mL of DI water followed by 15 mL of sample. Filtered

water was used for all analyses except pH, alkalinity, and total

hardness. An Accumet AP71 pH meter with a pH/ATC probe

was used to measure pH. The pH meter was calibrated before

each use with pH 4, 7, and 10 buffer solutions. Alkalinity and

total hardness were determined following Standard Method

2320 and 2340, respectively (American Public Health Associa-

tion, 1998).

UV absorbance at 254 nm (UV254) was measured on a Hita-

chi U-2900 spectrophotometer using a 1 cm quartz cell. Fluo-

rescence excitation-emission matrix (EEM) spectra were

collected on a Hitachi F-2500 fluorescence spectrophotometer

using a 1 cm quartz cell. Samples were scanned at 5 nm

increments over an excitation (EX) wavelength¼ 200–500 nm

and at 5 nm increments over an emission (EM) wave-

length¼ 200–600 nm. The raw EEMs were processed in

MATLAB (The MathWorks) following published procedures

(Cory and McKnight, 2005). A DI water EEM, which was

analyzed daily, was subtracted from the sample EEM; the area

under the Raman water peak (EX¼ 350 nm) was calculated for

DI water; intensity values of the sample EEM were normalized

by Raman water area; and EEMs were plotted in MATLAB using

the contour function.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 02422

DOC was measured on a Shimadzu TOC-VCPH total organic

carbon analyzer equipped with an ASI-V autosampler. All DOC

samples were measured in duplicate with average values

reported. The relative difference between DOC duplicates,

defined as the absolute difference between duplicate samples

divided by the mean of duplicate samples, was <10%. Stan-

dard checks for DOC were within 10% of the known value.

Specific UV254 absorbance (SUVA254), defined as (UV254/

DOC)� 100, was also calculated.

Chloride and sulfate were measured by suppressed

conductivity on a Dionex ICS-3000 ion chromatograph

equipped with IonPac AG22 guard column and AS22 analytical

column. The operating conditions were as follows: 25 mL

sample loop, 4.5 mM Na2CO3/1.4 mM NaHCO3 eluent compo-

sition, 1.2 mL/min eluent flow rate, suppressor/column

compartment maintained at 30 �C, and conductivity cell

maintained at 35 �C. All inorganic anions were measured in

duplicate with average values reported. The relative differ-

ence between duplicates was <5% and standard checks were

within 10% of the known value.

The aqueous concentration of metal cations was deter-

mined by acidifying samples to pH<2 with concentrated nitric

acid (Trace Metal Grade, Fisher Scientific) and measured on an

ICP-AES (Thermo Jarrell Ash) as described in Method 6010C

(U.S. EPA, 2007).

3. Results and discussion

The average composition of Cedar Key groundwater is shown

in Table 1. The minimum and maximum parameter values

show that the water quality was relatively constant over the

study timeframe, as would be expected for a groundwater.

The relatively high concentrations of DOC and hardness in

Cedar Key water illustrate a water source that requires

substantial treatment to prevent the problems associated

with elevated concentrations of DOM and hardness, such as

DBP formation and membrane fouling. Properties of Cedar Key

water relevant to anion exchange treatment are SUVA254 and

sulfate. Previous research has shown that removal of DOM by

anion exchange is more effective in waters with SUVA254> 3

Table 1 – Characteristics of Cedar Key water used in ionexchange experiments.

Parameter Averagea Minimuma Maximuma

pH 7.6 7.1 8.1

UV254 (cm�1) 0.171 0.168 0.186

DOC (mg C/L) 5.6 5.0 6.1

Cl- (mg/L) 11.8 10.5 14.3

SO42� (mg/L) 20.9 16.9 31.5

Ca2þ (mg/L) 103b – –

Hardness

(mg/L CaCO3)

275 265 288

Alkalinity

(mg/L CaCO3)

244b – –

a November 2008 and January, February, April 2009.

b January 2009; other cations (mg/L): Kþ¼ 0.38, Naþ ¼ 5.5,

Mg2þ ¼ 4.2, Sr2þ ¼ 0.87.

L/mg C$m and sulfate< 50 mg/L (Boyer and Singer, 2005; Boyer

and Singer, 2006). Thus, MIEX-Cl treatment of Cedar Key water

is expected to be effective given an average SUVA254 of

3.1 L/mg C$m and sulfate< 32 mg/L. The speciation of the

hardness cations is relevant to DOM interactions and cation

exchange treatment. In Cedar Key water, >90% of the hard-

ness was as calcium. This is important because calcium and

DOM form strong inner-sphere complexes, while magnesium

and DOM do not interact (Kalinichev and Kirkpatrick, 2007).

Furthermore, cation exchange resins typically have a greater

affinity for calcium over magnesium (Helfferich, 1995). For

these reasons, it will be assumed that calcium dominates all

interactions with DOM and cation exchange reactions.

3.1. Magnetically-enhanced cation exchange treatment

Preliminary jar tests were conducted using the magnetic

cation exchange resin (i.e., MIEX-Na) to evaluate the rela-

tionship between hardness removal and resin dose. The

treatment goal was to achieve 50% hardness removal so that

changes in removal could be measured when varying the

regeneration procedure. This was also the approximate

hardness removal goal at the Cedar Key Water Treatment

Plant. The change in water chemistry following magnetic

cation exchange treatment is shown in Table 2, where positive

values indicate removal and negative values indicate release.

The results are from jar tests using fresh MIEX-Na resin that

was regenerated with sodium chloride. A linear regression

line was fit to the resin dose and hardness removal data

(R2¼ 0.997), and showed that 3.6% hardness removal is

achieved per mL/L of MIEX-Na resin. Furthermore, MIEX-Na

resin removed 0.40 meq of hardness as calcium per meq of

resin at 16 mL/L, which means that the resin was 40% satu-

rated with calcium. For comparison, complete removal of

hardness from Cedar Key water at 16 mL/L MIEX-Na resin is

equal to 68% of the cation exchange sites occupied with

calcium. Thus, the resin has sufficient cation exchange

capacity to remove all hardness cations at 16 mL/L MIEX-Na

resin. The previous calculations used a MIEX-Na resin

capacity of 0.52 meq/mL and assumed that 20 min was suffi-

cient time for ion exchange. The resin capacity is a reasonable

assumption based on previous work using MIEX-Cl resin

(Boyer and Singer, 2008) and information from the manufac-

turer. The mixing time is also reasonable for exchange of

inorganic cations (Kunin and Barry, 1949). Weak-acid cation

exchange resin in the sodium form has been previously

reported to have a high affinity for calcium (Kunin and Barry,

1949), so the excess cation exchange capacity remaining after

treatment suggests that MIEX-Na resin was incompletely

converted to the sodium form. Moreover, weak-acid resin in

the hydrogen-form has a very low affinity for sodium and

calcium (Kunin and Barry, 1949). Therefore, incomplete

conversion of magnetic cation exchange resin to the sodium-

form is a likely explanation for the hardness removal results.

Table 2 shows that MIEX-Na resin also removed UV-

absorbing substances and DOC. This is surprising because

DOM is rich in carboxylic acid functional groups, which gives

DOM a net negative charge over the pH range of natural waters

(Ritchie and Perdue, 2003) and allows DOM to take part in

anion exchange reactions (Boyer et al., 2008). The increase in

Table 2 – Preliminary jar test results for fresh MIEX-Na resin.a

MIEX-Na (mL/L) Hardness UV254 DOC Chloride Sulfate

2b 7.7 1.6 3.3 �0.4 0.1

4b 12.3 3.2 4.4 �2.6 �2.6

16c 57.4� 0 16.0� 0 6.7� 3.5 �8.7� 5.0 3.8� 0.1

Amberlite 200c,d 76.5� 0 �1.1� 0.8 �2.3� 1.2 �1.0� 0.1 �1.2� 0.3

a All results are percent removal or release.

b Single resin dose.

c Duplicate resin dose; average value � one standard deviation reported.

d Amberlite 200 manufactured by Rohm and Haas; jar test experiment with resin dose of 8 mL/L.

Fig. 1 – Impact of brine and acid/base regeneration

procedures on hardness removal by magnetic cation

exchange using 16 mL/L MIEX-Na resin.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 0 2423

chloride suggests the possibility of anion exchange between

aqueous DOM and resin-phase chloride. Because MIEX-Na

resin is synthesized from the same starting material as

MIEX-Cl resin it is possible that there are residual anion

exchange functional groups on the cation exchange resin.

However, minimal removal of sulfate does not support the

anion exchange hypothesis, and suggests that chloride

release is an artifact of regenerating the resin in sodium

chloride solution. Alternative explanations for DOM removal

by cation exchange resin include adsorption of DOM to the

resin matrix and cation exchange uptake of DOM-calcium

complexes. Boyer and Singer (2008) previously showed no

removal of DOC by a weak-acid, magnetic cation exchange

resin, so adsorption is unlikely.

Complexes of DOM and calcium are formed by the binding

of calcium to carboxylic acid groups of DOM (Leenheer et al.,

1998; Kalinichev and Kirkpatrick, 2007). As a result,

DOM-calcium complexes will be represented as DOM-Caþ.

The fraction of DOM that is complexed with calcium (i.e.,

[DOM-Caþ]/[DOM]) can be estimated using the work of Lin

et al. (2005) as follows: [DOM-Caþ]/[DOM]¼ Ks[Ca2þ], where Ks

is the DOM stability constant. Using a calcium concentration

of 2.57� 10�3 M (see Table 1) and Ks¼ 50 M�1 (see Lin et al.,

2005), 13% of the DOM is complexed with calcium. Hence,

there is a fraction of DOM that is complexed with calcium and

is likely removable by cation exchange. Cation exchange

uptake of DOM-Caþ is further supported by results for

Amberlite 200 cation exchange resin shown in Table 2.

Amberlite 200 shows substantial removal of hardness and no

removal of UV254, DOC, chloride, or sulfate. It is hypothesized

that the polystyrene matrix of Amberlite 200 allows transport

of calcium and other inorganic ions, but hinders the transport

of DOM-Caþ because of size exclusion. Similar results were

reported for a polystyrene anion exchange resin, whereby

bicarbonate was removed but size exclusion prevented the

uptake of DOM (Boyer and Singer, 2008). Thus, cation

exchange uptake of DOM-Caþ is a reasonable explanation for

DOM removal by MIEX-Na resin. All subsequent cation

exchange jar tests were conducted using 16 mL/L MIEX-Na

resin, because this resin dose achieved >50% hardness

removal.

The impact of the regeneration procedure on the efficiency

of hardness removal by MIEX-Na resin was also investigated.

The MIEX-Na resin was regenerated using a brine solution and

an acid/base solution, as described in 2.3. Fig. 1 shows the

effect of the regeneration procedure on hardness removal. For

fresh resin test conditions, regeneration of MIEX-Na resin

with brine solution results in a measurable advantage in

hardness removal as compared with acid/base regeneration.

During the acid/base procedure, the equivalents of sodium

added to solution were approximately equal to 1 times the

resin capacity, while the brine regeneration was conducted

with 25 times more sodium than resin. The subsequent

regeneration test results show that the regeneration proce-

dure had a dramatic impact on hardness removal. For

example, hardness removal by resin regenerated with brine

decreased from 66% for the fresh resin to 52% for the regen-

erated resins (i.e., regen. 1� and 2�), while hardness removal

by resin regenerated with acid/base solution decreased from

52% for the fresh resin to <10% for the regenerated resins

(regen. 1� and 2�). The difference in hardness removal due to

the brine and acid/base regeneration procedures is a result of

the affinity of the carboxylic acid functional groups on the

resin for hydrogen, sodium, and calcium (Kunin and Barry,

1949). Thus, the acid/base regeneration procedure was found

to be ineffective at regenerating the resin. All subsequent

regenerations were conducted using the brine regeneration

procedure.

3.2. Combined cation and anion exchange treatment

MIEX-Na and MIEX-Cl resins were used separately and

combined to treat Cedar Key water, and removal of hardness,

DOC, and UV254 was measured as shown in Fig. 2. MIEX-Na

Fig. 2 – Comparison of DOM and hardness removal by

cation, anion, and combined ion exchange treatment using

2 mL/L MIEX-Cl and 16 mL/L MIEX-Na resins after three

regeneration cycles. Cation and anion exchange data are

from single jar tests.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 02424

resin was tested at 16 mL/L based on 3.1. Preliminary jar tests

were conducted using MIEX-Cl resin and Cedar Key water

(results not shown), and a dose of 2 mL/L was chosen based on

DOC and UV254 removal. Combined anion and cation

Fig. 3 – Fluorescence EEMs for (a) Cedar Key water (5.4 mg C/L,

273 mg/L as CaCO3), and (c) 16 mL/L MIEX-Na resin (4.7 mg C/L,

exchange treatment was conducted at the same doses. All

results are for ion exchange resin that has gone through three

regeneration cycles, which will be discussed in more detail in

3.3. Regenerated MIEX-Na resin removed 54% hardness and

w20% DOC and UV254, which is in agreement with fresh

MIEX-Na resin (see Table 2). Regenerated MIEX-Cl resin

removed 3% hardness and>75% DOC and UV254. These results

are in agreement with the preliminary MIEX-Cl experiments

(results not shown). When MIEX-Na and MIEX-Cl resins were

combined, hardness removal appeared to be cumulative,

while DOM removal was not cumulative. These results are

explained by the fact that DOM-Caþ complexes retain depro-

tonated carboxylic acid groups in the presence of calcium

(Bose and Reckhow, 1997), so DOM-Caþ can theoretically be

removed by anion and cation exchange, depending on the

ratio of calcium to carboxylic acid groups.

Fluorescence EEMs were analyzed to help understand the

differences in hardness and DOM removal by anion and cation

exchange. Fig. 3 shows fluorescence EEMs for Cedar Key raw

water, anion exchange treated water, and cation exchange

treated water, and the corresponding DOC and hardness

concentrations. The anion and cation exchange data are after

two regeneration cycles. The EEM for Cedar Key water had two

peaks (see Fig. 3a): Peak 1 at EM¼ 445 nm/EX¼ 265 nm and

Peak 2 at EM¼ 305 nm/EX¼ 270 nm. Peak 1 was present in all

raw water samples collected, and is attributed to terrestrially

derived DOM (Coble, 1996). Peak 2 was not present in all raw

277 mg/L as CaCO3), (b) 2 mL/L MIEX-Cl resin (1.3 mg C/L,

120 mg/L as CaCO3).

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 0 2425

water samples. Nevertheless, Peak 2 is likely attributed to

microbially derived DOM (Coble, 1996). Anion exchange

treatment substantially decreased the fluorescence intensity

of Peaks 1 and 2 (see Fig. 3b), with a corresponding decrease in

DOC of 76%. Cation exchange treatment also decreased the

fluorescence intensity of Peaks 1 and 2 (see Fig. 3c), but to

a lesser extent than anion exchange, which is in agreement

with the DOC data. Thus, cation exchange resin appears to

remove a wide range of DOM fluorophores. It is not known to

what extent DOM-Caþ complexes are contributing to the

fluorescence EEM spectra. Previous researchers have shown

that DOM-metal complexes affect fluorescence intensity

(Ohno et al., 2008; Yamashita and Jaffe, 2008). Additional work

is underway studying the interactions between alkaline earth

metals, DOM, and ion exchange reactions.

It is important to emphasize that combined anion and

cation exchange treatment is an effective strategy whereby

a single unit process can remove 71% DOC and 58% hardness,

as shown in Fig. 2. Furthermore, Table 3 shows a comparison

of water quality data from laboratory-scale, combined ion

exchange treatment and full-scale treatment at the Cedar Key

Water Treatment Plant. The combined ion exchange process

produces water that has a water quality near the finished

water quality of Cedar Key.

3.3. Simultaneous versus sequential combined ionexchange treatment

Sequential cation and anion exchange treatment was tested

and compared with simultaneous ion exchange treatment,

which was the focus of the previous section. The basis for

sequential ion exchange was to evaluate the effect that

interactions between DOM and calcium have on the ion

exchange process. Sequence 1 treated raw water with MIEX-Cl

resin first and then with MIEX-Na resin. Sequence 2 was the

reverse with MIEX-Na resin in the first stage of treatment and

MIEX-Cl resin in the second treatment stage. Fig. 4a–c shows

the removal of hardness, DOC, and UV254 as a function of the

ion exchange treatment scenario and the number of regen-

eration cycles. For fresh resin, removal of hardness showed

the following trend: Simultaneous> Sequence 1 w Sequence 2

(see Fig. 4a). In contrast, removal of DOC and UV254 showed the

trend of Sequence 1 w Sequence 2> Simultaneous (see Fig. 4b

and c). Although not shown, sulfate removal followed

Table 3 – Comparison of finished water quality forcombined ion exchange and municipal drinking water.

Parameter Combined ionexchangea

Municipal drinkingwaterb

pH 7.7 8.1

DOC (mg C/L) 1.7 1.1

Hardness

(mg/L as CaCO3)

112 173

Chloride (mg/L) 48.8 59.7

Sulfate (mg/L) 3.1 1.1

a Cationþ anion in Fig. 2.

b Cedar Key Water & Sewer District; August 2009.

a similar trend as DOC and UV254, with Sequence 1 (84%

removal)¼ Sequence 2 (84% removal)> Simultaneous (77%

removal). Hence, there was a greater difference in removal

efficiencies between simultaneous and sequential ion

exchange than between the two sequences. These results

suggest that hardness cations, DOM, and sulfate interact

differently in single ion exchange reactions than in simulta-

neous, combined ion exchange reactions.

Evaluating the performance of ion exchange resin over

multiple regeneration cycles is an important contribution of

this work, because previous studies have focused on testing

fresh resin (Mergen et al., 2008 and references therein). This is

the first study to comprehensively investigate the regenera-

tion of MIEX resin on a batch treatment basis. The examina-

tion of regenerated resin is useful as it would be impractical

for water treatment plants to use only fresh resin. The

importance of the regeneration process is illustrated in

comparing the removal of hardness, DOC, and UV254 as

a function of the number of regeneration cycles. For example,

over the course of three regeneration cycles total hardness

removal decreased by 8–15% for simultaneous and sequential

ion exchange treatment scenarios (see Fig. 4a). This is similar

to the results seen in Fig. 1. In contrast to the hardness find-

ings, removal of DOC and UV254 increased for the three ion

exchange treatment scenarios by 8–16% over the course of

three regeneration cycles (see Fig. 4b and c). Furthermore,

UV254 removal increased by a greater extent than DOC

removal as indicated by SUVA254. For fresh resin, SUVA254

values for Simultaneous, Sequence 1, and Sequence 2 treated

samples were 2.3, 2.1, and 2.1 L/mg C$m, respectively.

Following three regeneration cycles, SUVA254 values for

Simultaneous, Sequence 1, and Sequence 2 treated samples

were 1.7, 1.8, and 1.6 L/mgC$m, respectively. Additionally, the

increased ratio of chloride to MIEX-Cl resin in the simulta-

neous resin regeneration did not result in improved perfor-

mance over the sequential treatments (see Fig. 4b and c).

Increased removal of DOM upon regeneration was unex-

pected, because the fresh resin was regenerated before its first

use to ensure that it had full anion exchange capacity. Sulfate

removal was analyzed in an attempt to understand the

increased removal of DOC and UV254 over the course of several

regeneration cycles. However, sulfate removal changed by

<5% for fresh resin and regenerated resin (i.e., regen. 1�, 2�,

and 3�). The decrease in calcium removal when using

regenerated resins suggests incomplete removal of calcium

from the resin during the regeneration procedure. If calcium

built up on the resin, DOM complexation with resin-phase

calcium could result in increased DOM removal. This is a topic

of follow up work.

A series of paired t-tests were conducted to determine

whether the ion exchange treatment scenarios were statisti-

cally different. The following pairs were tested: Simultaneous/

Sequence 1, Simultaneous/Sequence 2, and Sequence

1/Sequence 2. The null hypothesis for all tests was that the ion

exchange treatment pairs had equal means, i.e., there was no

statistical difference between treatment performance. The

t-tests were conducted in MATLAB using data for both fresh

and regenerated resin and regenerated resin alone for hard-

ness, DOC, UV254, and sulfate. Table 4 summarizes the results

of the paired t-tests, and shows that a significant difference

Fig. 4 – Comparison of simultaneous and sequential ion exchange treatment on removal of (a) hardness, (b) DOC, and (c)

UV254. All jar tests used 16 mL/L MIEX-Na resin and 2 mL/L MIEX-Cl resin.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 02426

between ion exchange scenarios was influenced by the resin

state and chemical removal parameter. For fresh and regen-

erated resin, there was a statistical difference between all

three pairs of ion exchange scenarios for UV254 removal,

whereas there was no statistical difference between the ion

exchange scenarios for sulfate removal. Both hardness and

DOC removal showed mixed results. For regenerated resin

only, Simultaneous/Sequence 1 showed the most consistent

statistical difference. In summary, the ion exchange treat-

ment scenario and the state of the resin (i.e., fresh versus

regenerated) had an effect on removal performance.

Table 4 – Paired t-tests for ion exchange treatmentscenarios.a

Parameter Simultaneous/Sequence 1

Simultaneous/Sequence 2

Sequence 1/Sequence 2

Hardness h¼ 0b, 0c h¼ 0b, 0c h¼ 1b, 0c

DOC h¼ 1b, 1c h¼ 1b, 0c h¼ 0b, 0c

UV254 h¼ 1b, 1c h¼ 1b, 1c h¼ 1b, 0c

Sulfate h¼ 0b, 1c h¼ 0b, 0c h¼ 0b, 0c

a h¼ 0: null hypothesis cannot be rejected at the 5% significance

level; h¼ 1: null hypothesis can be rejected at the 5% significance

level.

b Fresh and regenerated resin.

c Regenerated resin only.

3.4. Influence of regeneration parameters on removalefficiency

It was shown in 3.1 that regeneration with brine was more

effective than regeneration with an acid/base solution. As

a result, the impact of the meq Naþ/meq MIEX-Na resin ratio,

regeneration time, and regeneration solution chemistry were

investigated to learn more about the brine regeneration

process. Hardness removal as a function of sodium chloride

concentration in the regeneration solution is shown in Fig. 5,

where 25 meq Naþ/meq MIEX-Na resin is the baseline regen-

eration concentration. The data correspond to treatment with

16 mL/L MIEX-Na resin after one regeneration cycle. There is

a clear trend of increasing hardness removal with increasing

concentration of sodium chloride in the regeneration solution.

For example, hardness removal increased from 36% at

a regeneration level of 10 meq Naþ/meq MIEX-Na resin to 69%

at a regeneration level of 50 meq Naþ/meq MIEX-Na resin.

However, the lowest regeneration level is the most efficient

when hardness removal is normalized by the regeneration

level. For example, 3.6% hardness removal per 10 meq Naþ/

meq MIEX-Na resin versus 1.4% hardness removal per 50 meq

Naþ/meq MIEX-Na resin. Thus, absolute hardness removal

and regeneration efficiency must be considered when

choosing a regeneration level.

In Fig. 6, the reaction time and regeneration time were

varied to measure the effects on hardness removal. The

Fig. 5 – Effect of the ratio of NaCl to MIEX-Na resin on

regeneration efficiency and hardness removal. Data

correspond to treatment with 16 mL/L MIEX-Na resin after

one regeneration cycle.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 0 2427

reaction time is defined as the length of time fresh resin is

mixed in raw water, while the regeneration time is the length

of time exhausted resin is mixed in concentrated sodium

chloride solution. The results show that the exchange of

hardness ions with sodium ions can take place within 5 min in

both the raw water and the regeneration solution. Although

these results show that the cation exchange process is

relatively quick, reaction times >5 min are needed to transfer

DOM to/from the anion exchange resin (Boyer and Singer,

2005; Humbert et al., 2005). As a result, the reaction time for

combined ion exchange will be determined by the time

required for DOM removal.

All regeneration experiments, up to this point, were

conducted using regeneration solution prepared with DI water

that contained negligible amounts of hardness and alkalinity.

At a full-scale water treatment plant, however, chemical

Fig. 6 – Effect of varying reaction time and regeneration

time on regeneration efficiency and hardness removal by

magnetic cation exchange. Reaction time data based on

16 mL/L MIEX-Na resin and regeneration time data based

on 16 mL/L MIEX-Na D 2 mL/L MIEX-Cl resins.

reagents are prepared with finished drinking water that may

contain measurable inorganic chemicals. Thus, a set of

regeneration experiments were conducted to compare hard-

ness removal using regeneration solutions prepared from DI

water and tap water. The tap water was from Gainesville, FL

and had a hardness of 146 mg/L as CaCO3 and an alkalinity of

42 mg/L as CaCO3. The combined ion exchange resins were

regenerated using a tap water regeneration solution following

the baseline procedure described in 2.3. Table 5 shows that

hardness removal by 16 mL/L of fresh MIEX-Na resin was

approximately equal for regeneration solution prepared from

DI water and tap water. This means that hardness cations

present in the tap water had no effect on the regeneration

process. In addition, removal of UV254-absorbing substances

was consistent regardless of the use of DI or tap water to

prepare the regeneration solution (results not shown).

The impact of reusing the regeneration solution was also

investigated. Hardness removal decreased by 12–17% after

each regeneration cycle with ‘‘used’’ regeneration solution for

both DI water and tap water, as shown in Table 5. Before the

last regeneration, 2.563 g/L of sodium carbonate was added to

the regeneration solution prepared using tap water. This

amount corresponded to the theoretical meq/L of hardness

cations added to the ‘‘used’’ regeneration solution during the

previous regeneration cycles, based on calculations.

A precipitate was immediately formed by the addition of

sodium carbonate to the used regeneration solution. The

precipitate was not characterized, but it was likely a calcium

carbonate mineral. The regeneration solution was then

filtered through a 1.6 mm GF/A filter (Whatman) to remove the

precipitate. The resin was regenerated using the sodium

carbonate treated solution and tested in a jar test. As a result

of the sodium carbonate addition, hardness removal

increased by 13% from the previous jar test. This suggests that

the regeneration solution can be more effectively reused if

calcium is precipitated out of solution, especially if a sodium

salt of carbonate is used. Furthermore, calcium sulfate may

precipitate during regeneration of combined ion exchange

resin, which would benefit both anion and cation exchange

regeneration. In summary, MIEX-Na resin regeneration

performance is a function of the sodium concentration;

regeneration is accomplished in 5 min; regeneration solution

prepared from hardness-containing tap water does not affect

regeneration; and regeneration solution can be effectively

reused by precipitating minerals, such as calcium carbonate

and calcium sulfate.

Table 5 – Comparison of regeneration solutions preparedfrom DI water and tap water.

Regeneration solution Hardness removal

DI water Tap watera

Fresh regeneration solution 58% 62%

Reused regeneration solution (1�) 44% 45%

Reused regeneration solution (2�) – 33%

Na2CO3 added to reused solution – 46%

a Experiments with tap water were 1 L, single jar tests.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 02428

3.5. Applications of combined ion exchange treatment

Previous researchers have separately investigated anion and

cation exchange treatment and shown these processes to be

a possible pre-treatment for membrane systems to reduce

fouling (Fabris et al., 2007; Humbert et al., 2007; Cornelissen

et al., 2009; Heijman et al., 2009). However, the impact of

combined anion and cation exchange treatment on the

reduction of membrane fouling has not been previously

demonstrated. Fig. 7 shows the theoretical reduction in

membrane fouling potential as a result of the prevention of

calcium sulfate precipitation and removal of DOC, both of

which are major foulants of membrane systems (Shih et al.,

2005; Lin et al., 2006; Jarusutthirak et al., 2007; Ahn et al., 2008).

The membrane fouling potentials were defined as:

Inorganic fouling potential ¼�Ca2þ��SO2�

4

�

�Ca2þ�

0

�SO2�

4

�0

; (1)

Organic fouling potential ¼ ½DOC�½DOC�0

; (2)

where the subscript 0 indicates initial concentration. The

fouling potentials were calculated based on experimental ion

exchange treatment data, with cation¼ 16 mL/L MIEX-Na

resin, anion¼ 2 mL/L MIEX-Cl resin, and cation þanion¼ 16 mL/L MIEX-Na and 2 mL/L MIEX-Cl resins. Thus,

the fouling potential is presented as proportional to the

residual concentration of inorganic ions and DOC. A fouling

potential of 1 corresponds to untreated water. It is acknowl-

edged that the reduction in DOC and calcium would not have

a linear relationship with membrane fouling, because fouling

has been shown to be dependent on the characteristics of the

organic matter. However, combined ion exchange should

reduce membrane fouling because it removes two major

foulants. Although individual cation and anion exchange

treatment can reduce the fouling potential, the largest total

reduction in fouling is achieved with combined ion exchange

treatment. It is expected that combined ion exchange

treatment will be effective for reducing membrane fouling

potential for a wide range of calcium, sulfate, and DOC

concentrations.

Fig. 7 – Illustration of reduction in membrane fouling by ion

exchange treatment.

4. Conclusions

The overall goal of this work was to evaluate combined anion

and cation exchange treatment for removal of DOM and

hardness. The major conclusions of this work are summarized

as follows:

� Anion and cation exchange resins can be used in a single

completely mixed reactor to remove DOM and hardness

simultaneously. Combined ion exchange treatment

achieved >55% total hardness removal and 70% DOC

removal. Combined anion and cation exchange also allows

for the most efficient use of the brine regeneration solution

because both sodium and chloride are used as mobile

counter ions.

� Although simultaneous and sequential ion exchange treat-

ment showed different removal trends for fresh resin, the

differences between simultaneous and sequential treat-

ment were substantially dampened by the third regenera-

tion cycle. Paired t-tests showed that a statistical difference

between the ion exchange treatment scenarios was depen-

dent on the state of the resin and removal parameter.

� Increasing the ratio of meq Naþ/meq MIEX-Na resin from

10 to 50 resulted in increased hardness removal. However,

sodium was most effectively used at the lowest regenera-

tion ratio. Therefore, regeneration efficiency must include

hardness removal and sodium usage.

� Increasing the ratio of meq Cl-/meq MIEX-Cl resin from 25 to

200 did not result in additional removal of DOM.

� Exchange of hardness cations and sodium, during both

treatment and regeneration, was achieved in 5 min.

� Brine regeneration solution prepared using tap water, which

contained measurable hardness and alkalinity, provided the

same regeneration efficiency as regeneration solution

prepared using hardness-free, DI water.

� The regeneration solution can be used repeatedly, especially

if hardness cations are precipitated out of solution. Precip-

itation may also be used to precipitate anions such as

sulfate, which would improve the regeneration of anion

exchange resin.

Acknowledgements

The authors would like to thank Orica Watercare for providing

the MIEX-Cl and MIEX-Na resins. The authors also thank Neil

Doty at the Cedar Key Water & Sewer District for assistance

with collecting raw water samples.

r e f e r e n c e s

Ahn, W.Y., Kalinichev, A.G., Clark, M.M., 2008. Effects of backgroundcationsonthe fouling ofpolyethersulfonemembranes bynaturalorganic matter: experimental and molecular modeling study.Journal of Membrane Science 309 (1–2), 128–140.

American Public Health Association, American Water WorksAssociation, Water Environment Federation, 1998. Standard

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 0 2429

Methods for the Examination of Water and Wastewater, 20thed. American Public Health Association, Washington DC.

Archer, A.D., Singer, P.C., 2006. An evaluation of the relationshipbetween SUVA and NOM coagulation using the ICR database.Journal American Water Works Association 98 (7), 110–123.

Bose, P., Reckhow, D.A., 1997. Modeling pH and ionic strengtheffects on proton and calcium complexation of fulvic acid:a tool for drinking water-NOM studies. Environmental Science& Technology 31 (3), 765–770.

Boyer, T.H., Singer, P.C., 2005. Bench-scale testing of a magneticion exchange resin for removal of disinfection by-productprecursors. Water Research 39, 1265–1276.

Boyer, T.H., Singer, P.C., 2006. A pilot-scale evaluation ofmagnetic ion exchange treatment for removal of naturalorganic material and inorganic anions. Water Research 40 (15),2865–2876.

Boyer, T.H., Singer, P.C., 2008. Stoichiometry of removal of naturalorganic matter by ion exchange. Environmental Science &Technology 42, 608–613.

Boyer, T.H., Singer, P.C., Aiken, G.R., 2008. Removal of dissolvedorganic matter by anion exchange: effect of dissolved organicmatter properties. Environmental Science & Technology 42,7431–7437.

Clifford, D.A., 1999. Ion exchange and inorganic adsorption. In:Letterman, R.D. (Ed.), Water Quality and Treatment: AHandbook of Community Water Supplies. McGraw-Hill Inc.,New York, NY.

Coble, P.G., 1996. Characterization of marine and terrestrial DOMin seawater using excitation emission matrix spectroscopy.Marine Chemistry 51 (4), 325–346.

Cornelissen, E.R., Beerendonk, E.F., Nederlof, M.N., van der Hoek, J.P., Wessels, L.P., 2009. Fluidized ion exchange (FIX) to controlNOM fouling in ultrafiltration. Desalination 236, 334–341.

Cory, R.M., McKnight, D.M., 2005. Fluorescence spectroscopyreveals ubiquitous presence of oxidized and reduced quinonesin dissolved organic matter. Environmental Science &Technology 39, 8142–8149.

Dempsey, B.A., Ganho, R.M., Omelia, C.R., 1984. The Coagulationof humic substances by means of aluminum salts. JournalAmerican Water Works Association 76 (4), 141–150.

Fabris, R., Lee, E.K., Chow, C.W.K., Chen, V., Drikas, M., 2007. Pre-treatments to reduce fouling of low pressure micro-filtration(MF) membranes. Journal of Membrane Sciences 289, 231–240.

Fearing, D.A., Banks, J., Guyetand, S., Eroles, C.M., Jefferson, B.,Wilson, D., Hillis, P., Campbell, A.T., Parsons, S.A., 2004.Combination of ferric and MIEX (R) for the treatment ofa humic rich water. Water Research 38 (10), 2551–2558.

Gray, S.R., Ritchie, C.B., Tran, T., Bolto, B.A., 2007. Effect of NOMcharacteristics and membrane type on microfiltrationperformance. Water Research 41, 3833–3841.

Heijman, S.G.J., Guo, H., Li, S., van Dijk, J.C., Wessels, L.P., 2009.Zero liquid discharge: heading for 99% recovery innanofiltration and reverse osmosis. Desalination 236, 357–362.

Helfferich, F., 1995. Ion Exchange. Dover Publications, Inc., NewYork.

Humbert, H., Gallard, H., Suty, H., Croue, J.P., 2005. Performance ofselected anion exchange resins for the treatment of a highDOC content surface water. Water Research 39, 1699–1708.

Humbert, H., Gallard, H., Jacquemet, V., Croue, J.P., 2007.Combination of coagulation and ion exchange for thereduction of UF fouling properties of a high DOC contentsurface water. Water Research 41, 3803–3811.

Jarvis, P., Mergen, M., Banks, J., Mcintosh, B., Parsons, S.A.,Jefferson, B., 2008. Pilot scale comparison of enhancedcoagulation with magnetic resin plus coagulation systems.Environmental Science & Technology 42 (4), 1276–1282.

Jarusutthirak, C., Mattaraj, S., Jiraratananon, R., 2007. Influence ofinorganic scalants and natural organic matter on

nanofiltration membrane fouling. Journal of MembraneScience 287, 138–145.

Jin, X., Huang, X.F., Hoek, E.M.V., 2009. Role of specific ioninteractions in seawater RO membrane fouling by alginic acid.Environmental Science & Technology 43 (10), 3580–3587.

Johnson, C.J., Singer, P.C., 2004. Impact of a magnetic ionexchange resin on ozone demand and bromate formationduring drinking water treatment. Water Research 38,3738–3750.

Kimura, K., Hane, Y., Watanabe, Y., Amy, G., Ohkuma, N., 2004.Irreversible membrane fouling during ultrafiltration of surfacewater. Water Research 38, 3431–3441.

Kitis, M., Harman, B.I., Yigit, N.O., Beyhan, M., Nguyen, H.,Adams, B., 2007. The removal of natural organic matter fromselected Turkish source waters using magnetic ion exchangeresin (MIEX). Reactive & Functional Polymers 67, 1495–1504.

Kalinichev, A.G., Kirkpatrick, R.J., 2007. Molecular dynamicssimulation of cationic complexation with natural organicmatter. European Journal of Soil Science 58 (4), 909–917.

Kunin, R., Barry, R.E., 1949. Carboxylic, weak acid type, cationexchange resin. Industrial and Engineering Chemistry 41 (6),1269–1272.

Leenheer, J.A., Brown, G.K., MacCarthy, P., Cabaniss, S.E., 1998.Models of metal binding structures in fulvic acid from theSuwannee River, Georgia. Environmental Science &Technology 32 (16), 2410–2416.

Li, Q.L., Elimelech, M., 2004. Organic fouling and chemicalcleaning of nanofiltration membranes: measurements andmechanisms. Environmental Science & Technology 38 (17),4683–4693.

Lin, C.J., Shirazi, S., Rao, P., Agarwal, S., 2006. Effects ofoperational parameters on cake formation of CaSO4 innanofiltration. Water Research 40, 806–816.

Lin, Y.P., Singer, P.C., Aiken, G.R., 2005. Inhibition of calciteprecipitation by natural organic material: kinetics,mechanism, and thermodynamics. Environmental Science &Technology 39, 6420–6428.

Mercer, K.L., Lin, Y.P., Singer, P.C., 2005. Enhancing calciumcarbonate precipitation by heterogeneous nucleation duringchemical softening. Journal American Water WorksAssociation 97 (12), 116–125.

Mergen, M.R.D., Jefferson, B., Parsons, S.A., Jarvis, P., 2008.Magnetic ion-exchange resin treatment: impact of water typeand resin use. Water Research 42, 1977–1988.

Ohno, T., Amirbahman, A., Bro, R., 2008. Parallel factor analysis ofexcitation-emission matrix fluorescence spectra of watersoluble soil organic matter as basis for the determination ofconditional metal binding parameters. Environmental Science& Technology 42 (1), 186–192.

Ritchie, J.D., Perdue, E.M., 2003. Proton-binding study of standardand reference fulvic acids, humic acids, and natural organicmatter. Geochimica et Cosmochimica Acta 67 (1), 85–96.

Saravia, F., Zwiener, C., Frimmel, F.H., Boller, M., 2006.Interactions between membrane surface, dissolved organicsubstances and ions in submerged membrane filtration.Desalination 192, 280–297.

Shih, W.Y., Rrahardianto, A., Lee, R.W., Cohen, Y., 2005.Morphometric characterization of calcium sulfrate dehydrate(gypsum) scale on reverse osmosis membranes. Journal ofMembrane Science 252, 253–263.

Singer, P.C., Bilyk, K., 2002. Enhanced coagulation usinga magnetic ion exchange resin. Water Research 36, 4009–4022.

Singer, P.C., Boyer, T., Holmquist, A., Morran, J., Bourke, M., 2009.Integrated analysis of NOM removal by magnetic ionexchange. Journal American Water Works Association 101 (1),65–73.

Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry. John Wiley &Sons, Inc., New York.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 2 4 1 9 – 2 4 3 02430

U.S. EPA, 2007. Method 6010C (SW-846): Inductively CoupledPlasma-Atomic Emission Spectrometry, Revision 3. U.S. EPA,Washington DC.

Yamashita, Y., Jaffe, R., 2008. Characterizing the interactionsbetween trace metals and dissolved organic matter usingexcitation-emission matrix and parallel factor analysis.Environmental Science & Technology 42 (19), 7374–7379.

Yoon, S.H., Lee, C.H., Kim, K.J., Fane, A.G., 1998. Effect of calciumion on the fouling of nanofilter by humic acid in drinkingwater production. Water Research 32 (7), 2180–2186.

Zhang, R., Vigneswaran, S., Ngo, H., Nguyen, H., 2008. Fluidizedbed magnetic ion exchange (MIEX) as pre-treatment processfor a submerged membrane reactor in wastewater treatmentand reuse. Desalination 227, 85–93.