Iron oxide–coated sand has been demonstrated to be an effective adsorbent material for the
removal of arsenic and other metals and metalloids from drinking water and wastewater. In
this study, fibrous materials were evaluated for their ability to offer a high specific surface area
alternative to sand as the substrate for the iron oxide coating. Four types of fibrous materials,
polypropylene, polyester, fiberglass, and cellulose, were evaluated for their ability to retain iron
coatings and to remove arsenate. Sand was also evaluated to provide a basis for comparison
with previous research. In isotherm experiments, all four fibrous materials showed higher
arsenate adsorption densities than iron oxide–coated sand. Arsenate adsorption densities were
highest for iron oxide–coated fiberglass and cellulose, suggesting that these fibrous materials
may offer advantages over iron oxide–coated sand.
Iron oxide–coated
fibrous sorbents for arsenic removal
BY ARUN KUMAR,
PATRICK L. GURIAN,
ROBIN H. BUCCIARELLI-TIEGER,
AND JADE MITCHELL-BLACKWOOD
Additional tables that further
illustrate the material discussed
here can be found in an
Appendix at the end of this
article.
rsenic can be found in natural waters such as rainwater (0.02–16 µg/L),
river water (< 0.02–21,800 µg/L), lake water (< 0.2–1,000 µg/L), estuarine
water (0.7–16 µg/L), seawater (0.7–3.7 µg/L), groundwater (< 0.5–
50,000 µg/L), and industrial waters from mine drainage (< 1–850,000
µg/L), oilfields, and related brine (230–243,000 µg/L; Marquez et al,
2005; Smedley & Kinniburgh, 2002). Arsenic(V) (arsenate) predominates in oxidizing environments, and arsenic(III) (arsenite) is found under reducing conditions.
Arsenic has been associated with cancer (skin, lung, and urinary bladder) and noncancerous (keratosis, i.e., skin lesions) health effects (Brown & Ross, 2002).
In 2001 the US Environmental Protection Agency (USEPA) lowered the
arsenic maximum contaminant level (MCL) from 50 µg/L to 10 µg/L (USEPA,
2001). A number of treatment processes for removing arsenic from drinking
water are available to meet the lower arsenic standard, including conventional
coagulation/precipitation, iron/manganese removal, membrane filtration, ionexchange adsorption, reverse osmosis, and electrodialysis (Gurian et al, 2004;
Kartinen & Martin, 1995). Arsenate can more easily be removed using these
treatment processes than arsenite (Kartinen & Martin, 1995) and is less toxic
than arsenite (Manning & Goldberg, 1997). For water from a reducing environment (typically waters with high soluble iron and manganese), it may be necessary to oxidize arsenite to arsenate prior to treatment. The oxidation can be
accomplished by numerous methods such as chemical oxidation using manganese oxide and potassium permanganate (Jekel & Seith, 2000), biological oxi-
A
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151
dation (Lievremont et al, 2003), solar oxidation (Lara et
al, 2006), and ultraviolet irradiation (Lee & Choi, 2002).
When soluble iron is present, this pretreatment may
remove arsenic by adsorption to the iron hydroxide precipitates that will be formed.
Despite the availability of different treatment processes
for arsenic removal, concerns remain as to the ability of
US public water suppliers to comply with the new MCL,
particularly for small systems that lack the expertise and
economies of scale to implement new treatment processes
in an affordable manner (Gurian et al, 2001a). Adsorption on iron oxide surfaces has received significant attention, and research is under way because of its effective
removal of arsenic and ease of operation and handling
(Thirunavukkarasu et al, 2003). One strategy for developing low-cost adsorbents has been to put an iron oxide
coating on an inexpensive material such as sand (Vaishya
& Gupta, 2003; Benjamin et al, 1996). In addition to
sand, different raw materials such as zeolites (Payne &
Abdel-Fattah, 2005), calcium alginate beads (Banerjee et
al, 2007; Zouboulis & Katsoyiannis, 2002; Min & Hering, 1998), activated carbon (Payne & Abdel-Fattah,
2005), resin (Matsunaga et al, 1996), ion exchangers
(Cumbal & Sengupta, 2005), and cellulose (Ghimire et al,
2003) have been used as substrates for the iron oxide
coating. The extent of iron loading depends on material
characteristics such as surface area; presence of surface
functional groups capable of binding iron such as silanol,
hydroxyl, carboxyl, and ammonium; and coating condi-
TABLE 1
tions used. Table A1 (see Appendix), summarizes the
experimental conditions and iron loadings obtained by
previous researchers on iron oxide–coated materials.
Many fibrous materials such as activated carbon
fibers (Suzuki, 1991), polypropylene (Blackwood, 2007;
Konrath & Hsuan, 2002), polyester (Koerner, 1998),
and fiberglass (Bismarck et al, 2004) possess high surface
area because of their small diameter (Ghimire et al,
2003). Activated carbon fibers have been shown to offer
faster kinetics and greater removal of metals and organic
matter compared with their granular counterparts
(Dominguez et al, 2002; Suzuki, 1991). Anion-exchange
fibers with interspersed iron oxide particles have been
successfully used to adsorb arsenic (Vatutsina et al, 2007;
Greenleaf et al, 2006). Despite their small diameter, the
high porosity of many fibrous materials allows for permeability values that are comparable to or greater than
permeability values for conventional granular materials (Blackwood, 2007).
The objective of this study was to investigate the
potential of four commercially available fibrous materials—polypropylene, polyester, fiberglass, and cellulose—
to be used as substrates for the development of iron
oxide–coated adsorbents (Table 1). All of these fibrous
materials are widely available (fiberglass is a common
insulation; polypropylene and polyester fibers are used in
geosynthetic mats; cellulose fibers are found in many
agricultural waste products such as straw). In addition,
sand was considered to provide a basis of comparison
Characteristics of different fibrous materials
Material
Sand
Fiberglass
Insulation*
Cellulose
Sponge†
Silanol
Silanol
Carboxyl
Ester
500
10
NA
48**
40
0.04††
0.17
1.5
0.09
0.14‡‡
0.25 M Fe,
pH 7, 110oC
0.25 M Fe,
pH 1.3, 110oC
0.25 M Fe,
pH 1.3, 25oC
2.5 M Fe,
pH 8.5, 25oC
0.25 M Fe,
pH 7, 25oC
5.6§§
9.3
1.3
2.4
8.4‡‡
51
210
19
16
4
1.3 × 103
1.2 × 103
0.01 × 103
0.18 × 103
0.03 × 103
Characteristic
Functional groups
Diameter—µm
BET surface area (raw materials)—m2/g
Preferred coating conditions
BET surface area, coated—m2/g
Iron loading—mg Fe/g***
Iron loading/BET surface area—mg Fe/m2
Freundlich arsenate density (Kf)—mg/g mg1/n/L1/n
Freundlich arsenate density/iron
loading—mg As/mg Fe mg1/n/L1/n
Polypropylene
Mat§
None
0.12
2.2
7.9
0.47
0.13
2.4 × 10–3
10 × 10–3
416 × 10–3
29 × 10–3
33 × 10–3
BET—Brunauer, Emmett, and Teller, Fe—iron, NA—not available
*Wrap-on 6525; Ace Hardware, Philadelphia, Pa.
†HWI 600 059 O/M 1-24; HWI, Fort Wayne, Ind.
‡NER-3500-04-02100; Jamestown Distributors, Bristol, R.I.
§90NW, US Fabrics, Cincinnati, Ohio
**Garai & Pompoli, 2005
††Benjamin et al, 1996
‡‡Blackwood, 2007
§§Average of BET surface area values from Benjamin et al (1996), Vaishya and Gupta (2003)
***Under preferred coating conditions
152
Polyester
Fiber‡
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between these novel fibrous materials and a granular material that has been widely studied by previous researchers (Thirunavukkarasu et al, 2003;
Vaishya & Gupta 2003; Benjamin et al, 1996;
Joshi & Chaudhuri, 1996).
MATERIALS AND METHODS
Materials. All chemicals used were of reagent
grade; deionized water1 (18.2 µmho) was used to
prepare solutions. Prior to use, all glassware and
polyethylene bottles were washed in 10% nitric
acid2 (HNO3) for 12 h and rinsed with deionized
water. An iron chloride solution was prepared
using ferric chloride2 (> 98% purity). Hydrochloric acid (HCl) and sodium hydroxide2 (NaOH)
(> 98% purity) were used for pH adjustment.
Sodium arsenate3 (> 98% purity) was used to prepare arsenic solutions. River sand and four fibrous
materials—polypropylene (mat4 and fiber5 forms),
polyester,6 cellulose (wood fibers7 and cellulose
sponge8 forms) and fiberglass (cloth,9 insulation,10
and mat11 forms)—were used. Characteristics of
the different materials are summarized in Table 1.
To remove surface impurities and oxide coatings,
sand was soaked in an acid solution (1.0 M HCl)
for 24 h, rinsed with deionized water three times,
and dried at 110oC for 20 h before use (Xu &
Axe, 2005; Benjamin et al, 1996). The fibrous
materials were submerged in deionized water
overnight at room temperature (25oC), rinsed with
deionized water, and dried again at room temperature for 20 h.
Coating procedure. The coating procedure consisted of first adding the material to be coated to
a concentrated ferric chloride solution and then
precipitating the iron by neutralization with NaOH
(Xu & Axe, 2005). Alternatively, neutralization
can be accomplished by evaporating HCl during
heating at higher temperature (Benjamin et al,
1996). During this procedure, water and hydrochloric acid evaporate, and iron oxide precipitates
as the solution becomes neutralized and more concentrated (Schwertmann & Cornell, 2000; Benjamin et al, 1996). Coating experiments were performed using different combinations of coating
temperature (tempcoating), coating pH (pHcoating),
and initial iron concentration (Fecoating). The materials were cut into 1-cm × 1-cm pieces before coating. In a typical coating experiment, 2 g of material was completely submerged in an iron solution.
After 24 h the media was washed with deionized
water to remove loose iron oxide particles until
the water was clear (Chen et al, 2007; Zouboulis
& Katsoyiannis, 2002; Joshi & Chaudhuri, 1996)
and again dried for a specified period (24 h). Iron
loading (expressed as milligrams of iron per gram
Surface morphology (magnification: 1000 ×) of fiberglass insulation fibers:
(A) uncoated fibers; (B) coated fibers at 1.3 pHcoating, 0.25 M Fecoating, and
110oC; (C) coated fibers at 1.3 pHcoating, 2.5 M Fecoating, and 110oC.
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153
of media) was used as a metric to compare different
materials and coating conditions. After every coating
experiment, the individual media was weighed and stored
in capped polyethylene bottles for subsequent iron analysis. BET (Brunauer, Emmett, and Teller) surface area
analyses 12 were conducted for all fibers. The most
promising fibrous material was selected for additional
characterization of surface morphology by scanning electron microscopy13 and of iron oxide mineralogy by X-ray
diffraction (XRD) analysis.14
Arsenic removal. Arsenic adsorption capacities were
measured in the presence of co-solutes to assess the performance of these fibers under realistic conditions. This
study used a single set of background ion concentrations representative of typical groundwater. The study
did not vary the levels of co-solutes because previous
studies have documented the effects of many common
water constituents such as silica, sulfate, phosphate, calcium, and nitrate on arsenate adsorption on iron oxide
surfaces (Montoya & Gurian, 2004; Holm, 2002; Gao
& Mucci, 2001; Swedlund & Webster, 1999; Manning
& Goldberg, 1996; Wilkie & Hering, 1996; Edwards,
1994; Dzombak & Morel, 1990). A number of adsorption models are available that account for speciation of
arsenic and iron oxide as well as competitive effects
with other ions (Edwards, 1994; Dzombak & Morel,
1990; Westall et al, 1976).
Water quality characteristics (Table 2) from an arsenicbearing well in the southwestern United States (El Paso
[Texas] Water Utilities well 303) were simulated because
the Southwest has the highest percentage of systems that
exceed the new MCL (Gurian et al, 2001a). Well 303 is
located in El Paso Water Utilities’ Canutillo well field
and draws from the intermediate stratum of the Santa
Fe formation at a depth of 167 m, a portion of the aquifer
TABLE 2
Water quality parameters of well 303
Parameters
23
pH
7.6
Arsenic—µg/L
33
Alkalinity—mg/L CaCO3
134
Calcium—mg/L
56
Silica—mg/L
33
Iron—mg/L
< 0.3
Sulfate—mg/L
276
Chloride—mg/L
232
Nitrate—mg/L
Total organic carbon
Source: EPWUPSB (2005)
CaCO3—calcium carbonate
154
Value
Temperature—oC
1.4
Not reported
with moderately reducing conditions (Marquez et al,
2005). Experiments comparing the synthetic water used
in this study with actual water from well 303 found generally similar results at arsenic concentrations ranging
from 20 to 50 µg/L (Kumar et al, 2007a). Arsenic removal
studies were conduced using arsenate only.
Given the amount of previous research addressing
effects of arsenic speciation on sorption to iron hydroxides, including both experimental (Vaishya & Gupta,
2003; Hering et al, 1997) and modeling studies (Montoya
& Gurian, 2004; Gao & Mucci, 2001; Swedlund &
Webster, 1999; Smith, 1998; Wilkie & Hering 1996;
Manning & Goldberg, 1996; Edwards, 1994; Goldberg,
1985), further research in this area was not considered
a priority. There is already considerable evidence that
an iron oxide–adsorption process that is effective for
removing arsenate at the moderately alkaline pH values
typical of southwestern groundwater will also be effective at removing arsenite (Daus et al, 2004; Katsoyiannis & Zouboulis, 2002; Hering et al, 1997). If necessary, water utilities can oxidize arsenite to arsenate prior
to treating for arsenic.
Equilibrium studies were conducted at pH 7.6 using a
24-h equilibrium time ([As(V)]0 = 1,000 µg/L As(V); temperature = 25oC; ionic strength = 0.0175 M; system =
closed to atmosphere; adsorbent concentration = 2.5–10
g/L). These conditions yield equilibrium arsenic concentrations representative of many drinking water applications (generally < 500 µg/L even in highly contaminated
areas such as Bangladesh; Mandal & Suzuki, 2002).
Reports in the literature indicate that there is an initial
rapid sorption step followed by a slower adsorption
process (Payne & Abdel-Fattah, 2005; Kuriakose et al,
2004). During an even longer time frame the iron oxide
phase may transition to forms that have less affinity for
arsenic, resulting in desorption into solution (Kumar et
al, 2007b; Pedersen et al, 2006; Ford, 2002; Schwertmann & Cornell, 2000; Fuller et al, 1993). It is generally
not considered feasible to allow the reaction to reach
complete equilibrium because the slower processes may
require weeks or longer. A duration of 24 h was selected
for this study because this allows ample time for the
rapid, initial sorption step to reach completion and is
comparable to the time scales used by other researchers
(Guo & Chen, 2005; Zeng, 2004; Vaishya & Gupta,
2003). A small kinetic study that is reported by Kumar
et al (2007a) verified that 24 h is an appropriate equilibration time for these materials.
Nonlinear regression was used to estimate adsorption
isotherm parameters by minimizing a chi-square error
function using the a spreadsheet software15 add-in. The
coefficient of determination (R2) was used to compare
different isotherm models.
Column studies were conducted to evaluate the arsenate breakthrough behavior of the most promising
adsorbent by operating a column (30-cm height, 2.52-
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2008 © American Water Works Association
25
ot
cl
la
s
rg
Fi
be
in
s
s
la
s
Fi
be
rg
s
ul
a
s
la
s
rg
h
25
tio
at
m
ib
rf
Fi
be
es
n
25
25
er
25
te
er
st
ly
e
Po
ly
ro
ly
p
Po
Po
py
l
yl
en
en
e
e
fib
m
at
25
er
25
m
at
d
Po
ly
pr
op
W
oo
Sa
nd
11
0
25
Iron Loading—
mg Fe/g media
certified laboratory.16 The minimum detection limit for
cm diameter) in an up-flow mode using a constant-flow
peristaltic pump and transparent vinyl plastic tubing at
arsenic is 2 µg/L (5–10% analytical errors).
25 mL/min flow rate, unless otherwise mentioned. In a
typical column study, the coated media, cut in small
RESULTS AND DISCUSSION
pieces, was packed manually into the column followIron oxide–coated fibrous materials. The first phase of the
ing the approach used by Vatutsina et al (2007). The
experimental work involved screening a range of fibrous
media was initially washed with deionized water at a
materials for their ability to retain an iron oxide coating.
25-mL/min flow rate (hereafter referred to as singleFeinitial of 2.5 M and pHcoating of 8.53 were used, followstep washing) and subsequently exposed to the synthetic
ing the work of Joshi and Chaudhuri (1996). The sand was
El Paso groundwater spiked with 1,000 µg/L As(V) (soludried at 110oC tempcoating, also in accordance with the
tion pH 7.6). Liquid samples were collected at specimethod of Joshi and Chaudhuri (1996), whereas cellufied times and analyzed for pH, aqueous iron, and
lose, polyester, and polypropylene were dried at 25oC
arsenic concentrations.
because these materials were observed to be unstable at
Analytical methods. To determine iron in solid samhigher temperatures in preliminary experiments. Figure
ples, samples were acid-digested according to method
1 compares iron loadings for sand with eight fibrous mate3050 B (USEPA, 1996) and analyzed using method 200.7
rials during screening experiments (conditions: pHcoating =
(inductively coupled plasma–atomic emission spec8.53, Fecoating = 2.5 M). With the exception of the fibertroscopy; USEPA, 1994a) at a USEPA-certified laboraglass mat, the loadings on the fibrous materials exceeded
tory16 (minimum detection limit = 0.25 mg/L, analytical
the value for sand. Iron loadings were comparable for a
number of fibrous materials, including loose polypropyerrors = 5–10%). For acid digestion, 1 g of media was
lene fiber (41 mg Fe/g), polypropylene mat (35 mg Fe/g),
added to 20 mL of 2 M HNO3, and the solution was
wood (39 mg Fe/g), fiberglass cloth (34 mg Fe/g), and
heated for 10 min without boiling. The sample was
fiberglass insulation (33 mg Fe/g; Figure 1). A higher iron
allowed to cool for 5 min, and another 10 mL of HNO3
loading was achieved on cellulose sponge (51 mg Fe/g)
was added and heated for 10 min. This step was repeated
compared with wood shavings (20 mg Fe/g). Consequently,
until no brown fumes were observed from the samples.
cellulose sponge was selected as a representative of celluSubsequently, 10 mL of deionized water and 6 mL of
lose for further coating experiments. These results reflect
30% hydrogen peroxide17 (H2O2) was added and heated
retention under fairly mild conditions (washing). Retention
for 5 min. Further, 2 mL of H2O2 was added to the soluof iron under more vigorous conditions (physical abration and heated for an additional 5 min.
sion) is substantially lower (for results see Mitchell-BlackThe last stage involved adding 10 mL of 1 M HCl and
wood et al, 2007).
heating for 15 min. At the end of the final digestion step,
Because fiberglass is available in a number of physical
the iron attached to the media was completely dissolved
forms and is tolerant of high-temperature coating condi(solution turned yellow in color). The acid-digested solution was filtered using filter paper18
and diluted to 200 mL total volume using deionized water. The filFIGURE 1 Iron loading on different media during screening experiments
trate was preserved in a polypropylene bottle at 4 o C until iron
50
analysis. To determine suspended
iron concentration in water sam40
ples, the samples were also filtered
30
using filter paper.18 The filter paper
20
was digested to determine sus10
pended iron concentration follow0
ing the method described. During
adsorption studies, suspensions
were filtered immediately through
a 0.45-µm nominal pore size membrane filter19 at the end of every
Media
experiment and were preserved
using nitric acid at 4oC until arFe—iron
senic analysis. Arsenic concentraError bars show 10% analytical error. Coating conditions: pHcoating = 8.53, Fecoating = 2.5 M;
tion was determined using inNumbers that follow media indicate °C at which media was coated, e.g., wood 25 = wood
ductively coupled plasma–mass
coated at 25°C.
spectrometry according to method
200.8 (USEPA, 1994b) at a USEPA-
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2008 © American Water Works Association
155
sand using a sequence of two coating
steps—one at 110 o C and one at
550oC. Coating of fiberglass insulation fibers at 110 o C resulted in
approximately eight times higher iron
Material
pHcoating
Tempcoating—oC
Iron Loading*—mg/g
loading (206 mg Fe/g) as compared
Sand
1.3
25
16 ± 11
with the iron loading of 26 mg Fe/g
Fiberglass
1.3
25
26 ± 12
at 25oC for acidic pH 1.3. The higher
Sand
7.0
25
26 ± 17
temperature coating conditions also
Fiberglass
7.0
25
21 ± 16
produced much higher iron loadings
Sand
1.3
110
21 ± 9
for the experiments conducted at neuFiberglass
1.3
110
206 ± 32
tral pH. An iron loading of 66 mg
Sand
7.0
110
30 ± 14
Fe/g for fiberglass was obtained at
Fiberglass
7.0
110
66 ± 50
110oC as compared with 21 mg Fe/g
for
sand at 25oC.
*Average ± 1 standard deviation (four replicates)
An analysis of variance (ANOVA)
identified four significant factors
affecting iron retention: material form
(i.e.,
fiberglass
>
sand),
temperature (110oC > 25oC), intertions, a wider variety of screening experiments was conducted for fiberglass. Fiberglass cloth and fiberglass insuaction between material and coating pH (fiberglass shows
lation had similar iron retentions when coated at 25oC
added iron retention under low pH conditions), and interaction between material and temperature (fiberglass shows
(Figure 1). In follow-up experiments in which the mateadded iron retention under high-temperature conditions;
rials were heated during drying (tempcoating = 110oC,
see Table A2 for ANOVA results). The combination of
pHcoating = 8.5 , and Fecoating = 2.5 M), a higher iron loadlow pH and high temperature consistently leads to
ing was obtained for the insulation than for the cloth (76
markedly higher iron loadings on fiberglass than on sand.
versus 32 mg Fe/g, respectively). Use of a combination of
Fiberglass is initially smoother than sand, and the develelevated drying temperature (110oC) and acidic pHcoating
opment of small-scale pitting under acidic conditions could
dramatically improved the iron loading of the insulation
aid the fiberglass more in retaining a coating compared
(206 mg/g) but did not improve the performance of the
with the already fairly irregular sand particles. The avercloth (22 mg Fe/g).
age of 206 mg Fe/g for fiberglass, which was obtained
These results indicated that temperature, pH, and
under these conditions, exceeds the values for sand
material form all influence iron retention and that these
obtained both in this study (Table 3) and others (Xu &
factors may interact in complex ways. To understand
Axe, 2005; Thirunavukkarasu et al, 2003). On the basis
the effects of these factors individually and in combinaof the higher iron loadings obtained on fiberglass compared
tion, a full-factorial experiment design was used. Two
with sand, fiberglass appears to be a promising substrate
factors, pH and temperature, were varied between two
for developing iron oxide–coated adsorbents.
levels for a total of four experimental conditions for each
To study the effect of initial iron concentration, fiberglass
of two materials (sand and fiberglass). Four replicates
was coated using different initial iron concentrations
were performed for each combination of experimental
ranging from 0.05 to 2.5 M Fecoating (pHcoating = 1.3,
conditions. Fiberglass was selected as the most promising of the fibrous materials, and sand was selected to
tempcoating = 110oC; Figure 2). Saturation of the iron loadprovide a basis of comparison with previous research
ing was approached after 0.25 M Fecoating (Figure 2). An
(Benjamin et al, 1996; Schiedegger et al, 1993). Because
Fecoating (2.5 M) of even 10 times higher resulted in only
both sand and fiberglass are composed primarily of siltwo times higher iron loading (438 versus 206 mg Fe/g).
ica, the comparison provides insight into the effect of
The attainment of a plateau in iron loading with increasthe form of silica on iron retention.
ing Fecoating is also reported in previous studies (Banerjee
Table 3 compares iron loadings on sand and fiberglass
et al, 2007; Zouboulis & Katsoyiannis, 2002).
insulation fibers obtained during these two-level full-factorial
The highest iron loading of 438 mg Fe/g fiberglass
coating experiments. Average iron loadings of 16–21 mg
obtained here exceeds the iron loading of 154 mg Fe/g
Fe/g (pHcoating = 1.3) and 26–30 mg Fe/g (pHcoating = 7)
obtained by previous researchers on modified activated
carbon (Chen et al, 2007) and 93 mg/g on polyHIPE, an
were obtained on iron oxide–coated sand, with the low
organic polymer (Katsoyiannis & Zouboulis, 2002). The
end of these ranges obtained at the lower coating temiron retention of fiberglass is close to the value of 468 mg
perature. These iron loadings on sand are comparable
Fe/g on cellulose beads reported by Guo and Chen (2005).
with previous reports in the literature, for example
Iron loadings obtained in a number of other studies are
the 21-mg Fe/g sand obtained by Xu and Axe (2005).
summarized in Table A1.
Thirunavukkarasu et al (2003) obtained 45 mg Fe/g
TABLE 3
156
Effect of material form, coating pH, and coating temperature
on iron oxide loading (coating conditions: 0.25 M Feinitial,
24-h drying time)
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500
400
300
200
100
0
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
Initial Iron—mol/L
Fe—iron
Error bars show the range of values. Coating conditions: tempcoating = 110°C, pHcoating = 1.3
FIGURE 3 X-ray diffraction patterns of coated fiberglass insulation fibers
1.2
Intensity—arbitray units
favorable material. Previous research on sand has suggested
that covalent bonding is possible between a silicate surface and
iron oxide coating (Xu & Axe,
2005). Polyester performed the
best of the remaining materials,
possibly because of the electrical dipole associated with the
ester bond, which could interact
with the electrical dipole of the
iron–oxygen–hydrogen bonds in
the coating.
Fiberglass insulation, which
was selected as the most promising of the materials because of
its high iron retention, was further characterized by studying its
surface morphology and iron
oxide mineralogy. Electron
microscopy indicated that the
uncoated fiber diameter was
roughly 10 µm (see photo, part
A, page 153). When an initial
iron concentration of 0.25 M was
used, the iron oxide deposits
were observed to have an irregular, patchy distribution (see
photo, part B, page 153) similar
to that reported by the previous
studies on iron oxide–coated
materials (Blackwood, 2007;
Konrath & Hsuan, 2002; Benjamin et al, 1996). The patchy
distributions of iron oxide on the
fibers suggested the underuti-
Iron Loading—mg Fe/g media
lization of available fiber surface area (see photo, part
Characterization. Raw and iron oxide–coated fibrous
B, page 153). The use of a more concentrated iron solumaterials were characterized using BET surface area valtion (2.5 M) increased the coating thickness but did not
ues (Table 1). An increase in BET surface area values after
produce a uniform coating (see photo, part C, page 153).
coating was observed for most materials because of the
These results suggest that the coating process is far from
irregular, rough surface formed by the iron oxide precipoptimized. If pretreatment methods can improve the abilitate (see photo on page 153). The only exception was
ity of the fiber surface to retain iron oxide, then the perthe cellulose. The sponge form of the cellulose may have
formance of these adsorbents could be improved, percontained smaller pores that could be clogged by the iron
haps dramatically.
oxide deposits. The iron retention of the fiberglass and
The mineralogy of the iron oxide coating on fiberglass
sand is similar when normalized by BET surface area,
fibers was determined by analyzing the XRD spectra. Figpossibly because of the chemically similar nature of these
ure 3 shows the XRD spectra of fiberglass fibers (condisubstrates (both are made of silica). The iron loadings of
tions: pHcoating = 1.3, Fecoating = 2.5 M, tempcoating = 110oC).
these two silica-based materials were substantially higher
than the values for the other materials. This indicates that
Background noise from the XRD spectra was removed by
specific surface area is not the only characteristic of imporsubtracting background peaks from the sample peaks using
tance when selecting a substrate for iron oxide coating; the
specialized software,20 and XRD peaks were normalized
chemistry of the surface bonding
of the iron oxide with the subFIGURE 2 Effect of initial iron concentration on iron loading of coated
strate is important as well.
fiberglass insulation fibers
Clearly, silica appears to be a
1.0
A, G
A
0.8
A, G
0.6
A, G
A, G
0.4
0.2
0.0
15
20
25
30
35
40
45
50
55
60
65
70
75
2θ—copper anode
A—akaganeite, Fe—iron, G—goethite, 2—diffraction angle
Peak intensities shown in arbitrary units are normalized with respect to maximum peak.
Coating conditions: pHcoating = 1.3, tempcoating = 110°C, Fecoating = 2.5 M
KUMAR ET AL | 100:4 • JOURNAL AWWA | PEER-REVIEWED | APRIL 2008
2008 © American Water Works Association
157
and 0.98 for sand coated at 110oC and 25oC, respectively; Table 4). Both Langmuir and Freundlich models
provided good descriptions of arsenate adsorption on
coated fiberglass insulation (conditions: 0.25 M Fe, pH
1.3, 110oC; hereafter FGI110), cellulose sponge (conditions: 0.25 M Fe, pH 1.3, 25oC; hereafter Cell25),
and polypropylene mat (conditions: 0.25 M Fe, pH 7,
25oC; hereafter PPm25) fibers (R2 > 0.96; Tables 1 and
4). The arsenate adsorption on polyester fiber (conditions: 2.5 M Fe, pH 8.53, 25 o C;
hereafter PETf25) could not be
modeled well using either of the
models (R2 < 0.50), an indication
that performance of these materihese results indicated that temperature, pH,
als was not consistent.
A comparison of variance of reand material form all influence iron retention and
siduals of these models for each adthat these factors may interact in complex ways.
sorbent, performed using F-tests,
found no significant difference in
variances for a 0.05-level test, indicating that one model cannot be
established as superior to the other (see Table A4). The
0.25 M (pHcoating = 1.3, tempcoating = 110oC) indicated
Freundlich model was chosen for the adsorbents considered
peaks similar to those of coated fiberglass fibers using
in this study, because it allows for a range of different asso2.5 M Fecoating for similar coating conditions. No signifciation constants between the solute and the adsorbent
icant change in full width at half-maximum values was
rather than the single association constant assumed by the
observed between these coated fibers at 2 = 27o, 35.32o,
Langmuir model. Iron oxide coatings on these materials are
39.64o, and 55.96o, indicating no significant change in
likely a mixture of different iron oxide species (e.g., akadegree of crystallinity (see Table A3).
ganeite, goethite), which would be expected to have someSchwertmann and Cornell (2000) reported that akawhat different association constants.
ganeite can be formed by the hydrolysis of ferric chloride
Solid-phase arsenate adsorption concentrations, calsolution at 40oC for eight days and can subsequently
culated for an aqueous-phase arsenate concentration of
transform to hematite or goethite at higher temperatures.
20 µg/L As using the Freundlich model, were used to
Similarly, Lo et al (1997) observed that the mineralogy
compare the performance of these adsorbents at an arseof iron oxide coatings on sand varied from amorphous
nate concentration that is realistic for US groundwater
iron oxide phase (tempcoating = 60oC) to more crystalline
(most water supplies that exceed 10 µg/L are in the range
phases such as goethite (tempcoating = 150oC) and hematite
of 10–30 µg/L; Gurian et al, 2001b). The highest arsen(tempcoating = 300oC) with increasing temperature. Thus
ate adsorption concentration was achieved by coated
the coating conditions used in this study (110oC) appear
cellulose (0.23 mg As/g media for Cell25) followed by
to allow the formation of some crystalline phases. Cryscoated fiberglass (0.13 mg As/g media for FGI110) and
talline iron oxide phases have a lesser affinity for arsenate
polypropylene (0.11 mg As/g media for PPm25), all of
than amorphous iron oxide (Schwertmann & Cornell,
which are markedly higher than coated sand
2000; Fuller et al, 1993). The use of higher-temperature
(0.002–0.004 mg As/g media). Arsenate adsorption concoating processes can increase the amount of iron retained
centrations on the fibrous materials developed here are
by the fiberglass, but at the expense of producing a coatlower than values of 3 mg/g reported for hybrid
ing that is less reactive toward arsenic.
organic–inorganic fibers (Vatutsina et al, 2007) and 1–6
Arsenic removal. Equilibrium arsenate adsorption
mg/g for metal hydroxide media (CH2M HILL, 2004;
capacity. Materials used in the equilibrium studies are
Driehaus et al, 1998). The arsenate adsorption concenshown in Table 1. The materials and coating conditions
trations of Cell25 and FGI110 fibers were markedly
were selected based on the results of the initial screening
higher than the values of 0.02 mg As/g for bead cellulose
experiments. Figure 4 shows the adsorption isotherms
iron oxyhydroxide (Guo & Chen, 2005) and 0.01 mg
for various iron oxide–based materials in the presence of
As/g for ferruginous ore (Chakravarty et al, 2002). Table
realistic levels of co-solutes. Arsenate adsorption was
A5 provides a more detailed comparison of these results
modeled using the nonlinear forms of the Langmuir and
with results from previous studies.
Freundlich models (Zeng, 2004).
Cellulose showed the best performance of any materThe Freundlich model provided a better fit than the
ial considered in this study. However, in order to treat
Langmuir model to the data for coated sand (R2 = 0.94
with respect to the maximum peak (Figure 3). The XRD
peaks at 2 = 27o, 35.32o, 39.64o, 46.5o, and 55.96o of
fiberglass fibers coated at Fecoating = 2.5 M matched the
XRD peaks of akaganeite (Schwertmann & Cornell, 2000).
In addition, some of the XRD peaks at 2 = 39.64o, 46.5o,
and 55.96o also matched the XRD peaks of goethite (Schwertmann & Cornell, 2000). Peaks for two-line and six-line
ferrihydrite were not observed. Analysis of the XRD spectra of fiberglass fibers coated using a lower Fecoating of
T
158
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2008 © American Water Works Association
Sorption Density—μg As/g media
Sorption Density—mg As/g media
large quantities of water for
FIGURE 4 Arsenate adsorption isotherms of various sorbents.* Remaining
arsenic, adsorbents should be staaqueous arsenic range: 0–600 µg/L (A) and expanded view for the
ble when used in continuous-flow
remaining aqueous arsenic range: 0–80 µg/L (B)
column operations for months.
Cellulose is biodegradable and
may degrade during column operFGI110
Sand110
Sand25
ation. On the other hand, fiberCell25
PPm25
PETf25
Freundlich FGI110
Freundlich Sand110
Freundlich Sand25
glass is not biodegradable, which
Freundlich Cell25
Langmuir PPm25
Freundlich PETf25
may be an advantage for drinking
A
water applications. For this rea2.00
son, coated fiberglass was selected
for evaluation in a column study.
1.50
Column study. In initial sin1.00
gle-step washing of FGI110 fibers
in column studies, iron levels in
0.50
the effluent were observed to
exceed the iron MCL of 0.3 mg/L
0.00
(USEPA, 2006). Although this is a
0
0.1
0.2
0.3
0.4
0.5
0.6
secondary MCL (i.e., a nonhealthRemaining Aqueous Arsenic—mg/L
based guideline value that is not
enforced), any treatment process
FGI110
Sand110
Cell25
PPm25
PETf25
Freundlich FGI110
intended for drinking water
Freundlich Sand110
Freundlich Cell25
Langmuir PPm25
should be able to meet this stanFreundlich PETf25
dard. Because the fibers had
B
already been washed, this release
10,000
of iron during the column experiments was not anticipated. Nev1,000
ertheless, it does not preclude the
use of the fibers in water treat100
ment if the loss of iron is partial
and is limited to the initial period
10
of column operation, such that it
could be undertaken as the con1
0
10
20
30
40
50
60
70
80
cluding step of media preparation.
Remaining Aqueous Arsenic—μg/L
With this in mind, the effect of
an initial high-flow washing step
As—arsenic; FGI110—fiberglass insulation coated using coating conditions 0.25 M Fe, pH
1.3, 110°C; Freundlich—Freundlich adsorption model for indicated media; Langmuir
on the retention of iron on FGI110
PPm25—Langmuir adsorption model for PPm25; PETf25—polyester fiber with coating
fibers was examined by passing
conditions of 2.5 M Fe, pH 8.53, 25°C; PPm25—polypropylene mat with coating conditions of
0.25 M Fe, pH 7, 25°C; Sand25—sand coated with conditions of 0.25 M Fe, pH 7, 25°C;
100 bed volumes (BVs) of deionSand110—sand coated with conditions of 0.25 M Fe, pH 7, 110°C; Cell25—cellulose sponge
ized water through the column at
coated with conditions of 0.25 M Fe, pH 1.3, 25°C
a flow rate of 50 mL/min, fol*Synthetic El Paso groundwater spiked with 1,000 µg/L As(V), pH 7.6, sorbent concentration
lowed by an additional 100 BVs of
2.5–10 g/L, equilibrium time 24 h
deionized water at a flow rate of
The larger error bars on data show 10% analytical error, and smaller error bars on model
25 mL/min (two-step washing). A
show one standard deviation of residual from modeled results. Figure 4 part B is a semilog
significant release of iron was
plot with the y-axis on a logarithmic scale.
observed for the first 25 BVs of
the washing experiment (Figure
5). The aqueous iron concentrations decreased continuhereafter FGI110_7d), was also assessed. This dramatiously and reached a level below 0.3 mg Fe/L after passing
cally reduced the loss of iron (Figure 5). After 15 BVs, the
an additional 25 BVs of deionized water in the two-step
aqueous iron concentration of the treated water conwashing experiments (initial iron loading = 231 mg/g media,
formed to the USEPA secondary standard for iron (initial
total iron lost during washing = 40%, remaining average
iron loading = 270 mg/g media, total iron lost = 1.5%,
iron loading = 138 mg/g media).
remaining average iron loading = 266 mg/g media). The
An alternate strategy of heating the coated fiberglass
greater iron loss observed during two-step washing of
at 110oC for a full week, rather than the 24-h drying
FGI110 could be attributed to the application of higher
shear force on the media during washing compared with
time used previously (conditions: 0.25 M Fe, pH 1.3;
KUMAR ET AL | 100:4 • JOURNAL AWWA | PEER-REVIEWED | APRIL 2008
2008 © American Water Works Association
159
trations from the two columns matched fairly well, both
in the effluent (points 100 and 200 BVs in Figure 6) and
at the column midpoint (data shown in Kumar et al,
2007a). Had the FGI110 column been run longer, the
authors expect that breakthrough would have been similar to that observed for the FGI110_7d column. In previous research on iron oxide–coated sand, Joshi and
Chaudhuri (1996) and Vaishya and Gupta (2003) reported
breakthrough at 150 BVs and 125 BVs, respectively, when
treating an influent of 1,000 µg/L arsenic. Results of column studies presented in this article
exceeded those run lengths, indicating that iron oxide–coated fiberFIGURE 5 Iron retention study of coated fiberglass fibers in column
glass may offer advantages over
experiments (deionized water)
iron oxide–coated sand.
Longer runs (i.e., larger volumes
FGI110 two-step
of
water
treated before the adsorFGI110_7d single-step
bent
is
exhausted) would be
USEPA limit
1.0E+05
expected to be obtained at lower
arsenic concentrations found at US
1.0E+03
public water supplies. A rough estimate based on an influent arsenate
1.0E+01
concentration of 20 µg/L and the
assumption that the equilibrium
1.0E-01
solid-phase concentration of 0.13
mg As/g found by the isotherm stud1.0E-03
ies can be achieved indicates that
0
20
40
60
80
100
120
140
160
180
200
220
approximately 2,000 BVs could be
Bed Volumes Passed—number
treated. In contrast, US pilot studies
Fe—iron, FGI110 two step—two-step washing of fiberglass insulation using coating
indicate that > 100,000 BVs can be
conditions 0.25 M Fe, pH 1.3, 110°C; FGI110_7d single-step—single-step washing of
fiberglass insulation coated using same conditions as FGI110 with seven-day drying
treated with metal hydroxide media
period; USEPA—US Environmental Protection Agency
(CH2M HILL, 2004; these run
lengths were achieved with pH
Semilog plot with the y-axis on a logarithmic scale. Error bars show 10% analytical error.
adjustment). It would be difficult
for coated fiberglass to be economically competitive with the metal
FIGURE 6 Column effluent aqueous arsenic concentration
hydroxide media on a single-use
basis. An alternative approach
1,000
FGI110
would be to regenerate the coated
FGI110_7d
Breakthrough
media. At least some of the micro100
porous metal hydroxide media are
vulnerable to decomposition under
10
pressurized flow conditions (CH2M
HILL, 2004), and they are usually
1
not regenerated. Recent reports have
investigated hybrid organic–inorganic fibers for arsenic removal, a
0
0
100
200
300
400
500
600
strategy similar to the one proposed
Bed Volumes Passed—number
here. Run lengths of 8,200 BVs are
reported by Vatutsina et al (2007)
Fe—iron, FGI110—fiberglass insulation coated using coating conditions 0.25 M Fe, pH 1.3,
110°C, FGI110_7d—fiberglass insulation coated using same coating conditions as FGI110
using an influent arsenic concentrawith seven-day drying period, breakthrough—conditions corresponding to 20 µg/L As
tion of 50 µg/L.
remaining aqueous phase arsenic
Material properties and perforSemilog plot with the y-axis on a logarithmic scale. Error bars show 10% analytical error.
mance
comparisons. Table 1 sumEmpty bed contact time of 6 min. Synthetic El Paso groundwater spiked with 1,000 µg/L
marizes
the properties and arsenic
arsenic(V), pH 7.6, taken from exit port
removal performance of the variRemaining Arsenic—µgL
Remaining Iron Concentration—mg/L
the single-step washing of FGI110_7d media. In addition, the extended heat treatment used in the preparation of FGI110_7d media may have resulted in stronger
chemical bonding between the iron and silica (Zeng,
2003; Benjamin et al, 1996).
Figure 6 shows the breakthrough curve of arsenic in the
column effluent. Breakthrough occurs for the FGI110_7d
at between 290 and 450 BVs, with saturation reached at
around 550 BVs. For the FGI110, breakthrough was not
observed after 205 BVs. In general the arsenic concen-
160
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2008 © American Water Works Association
ous materials considered in this study. In order to gain
insight into what materials are the most promising, Table
1 shows results for the coating conditions that resulted in
the best performance for each material. The BET surface
area of uncoated fiberglass is approximately four times
higher than that of sand. This is primarily because of the
smaller diameter of the fiberglass. Given that specific surface area scales with diameter, the fact that the fiberglass
is 150 the diameter of the sand yet has only four times
higher BET surface area indicates that small-scale surface roughness on the sand contributes significantly to
only partially explained by the higher iron retention of the
fiberglass. When normalized for iron, the fiberglass still
has four times the arsenic adsorption density (2.4 × 10–3
versus 10 × 10–3 mg As/g Fe). This study does not provide
an explanation for this advantage, although it can be
speculated that the coating on the sand may be thicker and
that therefore some of the coating is not accessible to the
bulk solution.
The remaining three materials all show an advantage
over sand in BET surface area; however, iron retention per
unit surface area is actually quite poor (all three materials have values well
below those of sand
and fiberglass). Cellulose did not show good
iron retention, partichere is already considerable evidence that an iron
ularly given its high
oxide–adsorption process that is effective for removing arsenate
BET surface area, yet
actually had the highat the moderately alkaline pH values typical of southwestern
est arsenic adsorption
groundwater will also be effective at removing arsenite.
density. Cellulose has
a variety of functional
groups that may be
able to bind arsenic, even without an iron oxide coating.
sand’s BET surface area and ability to retain iron. The iron
The arsenic adsorption densities for polypropylene and
loading per unit BET surface area for the two silica-based
polyester are well below fiberglass, although they are
materials is virtually identical. The coated surface area for
somewhat higher than sand. These two materials are
sand represents a nearly 100-fold increase over uncoated
coated at room temperature, which results in a relatively
surface area, whereas the fiberglass has only a 50-fold
small mass of iron retained but tends to produce an amorincrease. The patchy nature of the coating achieved here
phous precipitate (ferrihydrite) that has a higher affinity
(see photo on page 153) may not provide as large an
for arsenic. Although polypropylene and polyester have
increase in surface area as would be achieved by a more
high arsenate adsorption densities (Freundlich Kf values)
uniform coating. However, fiberglass achieves a much
higher arsenic adsorption density than sand (Freundlich
per unit iron retained, overall performance is mediocre at
Kf value of 2.2 mg/g versus 0.12 mg/g for sand), which is
best because of relatively low iron retention.
T
TABLE 4
Estimated isotherm parameters of different models for arsenate adsorption
Model
Langmuir
Parameters
Sand110
FGI110
Cell25
PPm25
PETf25
b—L/mg
0.06
4.9
4.3
1.5
15
61
Qm—mg/g
1.6
0.04
1.5
7.5
0.36
0.12
r—C0 = 1,000 µg As/L
0.94
0.17
0.19
0.40
0.06
0.02
R2
0.96
0.90
0.97
0.99
0.99
0.28
2
3
119
223
82
65
Kf—mg/g mg1/n/L1/n
0.12
0.04
2.2
7.9
0.47
0.13
nf
0.90
1.6
1.4
1.1
2.7
8.2
R2
0.98
0.94
0.98
0.99
0.97
0.33
2
4
130
218
108
81
Qeq|Ceq = 20 µg/L—µg/g
Freundlich
Sand25
Qeq|Ceq = 20 µg/L—µg/g
25oC),
b—Langmuir constant, C0—initial arsenic concentration, Cell25—cellulose sponge coated using coating conditions (0.25 M Fe, pH 1.3,
Ceq—equilibrium
aqueous arsenic concentration, FGI110—fiberglass insulation coated using coating conditions (0.25 M Fe, pH 1.3, 110oC), Kf—adsorption density, nf—adsorption
intensity, PETf25—polyester fiber coated using coating conditions (2.5 M Fe, pH 8.53, 25oC), PPm25—polypropylene mat coated using coating conditions (0.25 M
Fe, pH 7, 25oC), Qeq—equilibrium arsenic adsorption capacity, Qm—maximum adsorption density, R2—coefficient of determination, r—separation factor, Sand110—
sand coated using coating conditions (0.25M Fe, pH 7, 110oC), Sand25—sand coated using coating conditions (0.25 M Fe, pH 7, 25oC)
Synthetic El Paso groundwater spiked with 1,000 µg/L As(V), pH 7.6, 24-h equilibrium time, sorbent concentration: 2.5–10 g/L, Ceq range: 0–500 µg/L As(V)
The R2 for the better-fitting model is shown in boldface.
KUMAR ET AL | 100:4 • JOURNAL AWWA | PEER-REVIEWED | APRIL 2008
2008 © American Water Works Association
161
These comparisons suggest that the high specific surface area offered by small-diameter fibers can lead to
improved performance. This is best illustrated by the
comparison between the two silica-based materials—
sand and fiberglass. These comparisons also suggest some
avenues for future research. Fiberglass has only a fourfold BET surface area advantage over sand, despite a
10-fold smaller diameter. The performance of fiberglass
might be improved by pretreatment designed to roughen
the fibers, because such small-scale roughness might
increase the BET surface area of fiberglass. In addition,
further efforts are needed to improve the retention of
iron. Although heat treatment can do this effectively, it
also leads to the formation of a less-reactive iron phase.
Chemical modification of the fiber surfaces may allow for
retention of iron without the use of a high-temperature
coating process.
CONCLUSIONS
This study compared four fibrous materials and sand
for their ability to retain iron oxide coatings and adsorb
arsenate from water. Iron oxide–coated cellulose showed
the highest arsenate adsorption concentration of the materials evaluated here, despite retaining less iron than the
fiberglass. This may be because of a large variety of
arsenic-adsorbing functional groups (e.g., carboxylic
acids) that are naturally present in organic materials. Cellulose is biodegradable but may be stable in applications
where biological activity is limited by low nutrient or
dissolved oxygen levels.
Fiberglass achieved the second highest arsenate adsorption concentration and has the advantage of not being
subject to biological degradation. Fiberglass was found to
retain more iron and adsorb more arsenate than sand.
Much of this advantage is attributable to the higher specific surface area of fiberglass. However, the advantage
over sand is actually greater than would be predicted
based on BET surface areas. Polyester and polypropylene retained somewhat more iron than sand but appear
inferior to fiberglass based on this study.
The materials considered here all have lower arsenate adsorption densities than commercially available
microporous metal hydroxide media. Further research
has the potential to improve the performance of these
materials. Electron micrographs indicate that the coating on the materials is irregular, and some leaching of
iron was observed during column experiments. Pretreatment methods may improve the affinity of the fiberglass for the iron oxide and thereby improve media performance. Further research to improve the uniformity
and durability of the coating is warranted.
trative assistance in funding and managing the project
through which this information was discovered (project
3161). The comments and views detailed here may not
necessarily reflect the views of the AwwaRF, its offices,
directors, affiliates, or agents, or the views of the US federal government. The authors thank Fernando Rico for
supplying well 303 water; Kevin Owens for assisting with
analytical methods; Charles Haas, Grace Hsuan, and
Frank Ko for assisting with study design; and three anonymous reviewers for detailed comments on the manuscript.
ABOUT THE AUTHORS
Arun Kumar (to whom correspondence should be addressed) is a graduate research assistant in the Department of Civil, Architectural, and
Environmental Engineering at Drexel
University, 3141 Chestnut St.,
Philadelphia, PA 19104; e-mail
ak385@drexel.edu. He received his
bachelor’s degree in civil engineering and his master’s
degree in environmental engineering and management
from the Indian Institute of Technology Kanpur, Uttar
Pradesh, India. Patrick L. Gurian is assistant professor in the Department of Civil, Architectural, and
Environmental Engineering, Drexel University. Robin
H. Bucciarelli-Tieger is an associate design engineer
with Fluor Enterprises, Mt. Laurel, N.J.; and Jade
Mitchell-Blackwood is a graduate research assistant in
the Department of Civil, Architectural, and Environmental Engineering, Drexel University.
FOOTNOTES
1MilliQ
purified water, Millipore Corp., Billerica, Mass.
Pittsburgh, Pa.
Milwaukee, Wis.
490NW, US Fabrics, Cincinnati, Ohio
5Propex Inc., Chattanooga, Tenn.
6NER-3500-04-021000, Jamestown Distributors, Bristol, R.I.
721-711-88; Paper Mart, Philadelphia, Pa.
8HWI 600 059 O/M 1-24, HWI, Fort Wayne, Ind.
9USC77080, Alco Industries Co., Massillon, Ohio
10Wrap-on 6525, Ace Hardware, Philadelphia, Pa.
11FIB-961, Jamestown Distributors, Bristol, R.I.
12Quantachrome Autosorb Automated Gas Sorption System,
Quantachrome Instruments, Boynton Beach, Fla.
13FESEM, FEI/ Phillips XL30, Drexel University, Philadelphia, Pa.
14Siemens D500 X-Ray Powder Diffractometer, Drexel University,
Philadelphia, Pa.
15Microsoft Excel Solver, Microsoft, Redmond, Wash.
16QC Laboratories, Southampton, Pa.
17AC41188-5000, Fisher-Scientific, Pittsburgh, Pa.
18Whatman, 1442-042, Fisher-Scientific, Pittsburgh, Pa.
1909-719-2E, Fisher-Scientific, Pittsburgh, Pa.
20Jade + (version 7), an XRD pattern processing, identification, and
quantification software, Materials Data Inc., Livermore, Calif.
2Fisher-Scientific,
3Sigma-Aldrich,
ACKNOWLEDGMENT
Drexel University gratefully acknowledges the Awwa
Research Foundation (AwwaRF) and Sandia National
Laboratories for their financial, technical, and adminis-
162
If you have a comment about this
article, please contact us at
journal@awwa.org.
APRIL 2008 | JOURNAL AWWA • 100:4 | PEER-REVIEWED | KUMAR ET AL
2008 © American Water Works Association
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2008 © American Water Works Association
Appendix A: Supporting Information
This appendix contains additional information on previous research on iron oxide–coating the analysis of variance (ANOVA) of the results of the full-factorial iron
oxide–coating experiments for fiberglass and sand, mineralogy of iron oxide–coated fiberglass fibers, statistical
tests for comparison of isotherm models of different adsorbents, and performance comparison of iron oxide–coated
fibers with other arsenate adsorbents.
PREVIOUS RESEARCH ON IRON OXIDE COATINGS
Table A1 summarizes previous iron oxide–coating
research. As discussed in the main article, the 438 mg Fe/g
obtained for fiberglass in this study compares fairly well with
previous research, exceeded only by Guo and Chen (2005)
who used a seven-step coating process for cellulose beads.
ANOVA RESULTS
Table A2 shows the results of an ANOVA analysis of
the full-factorial iron oxide–coating experiments conducted
for fiberglass and sand. As discussed in the Journal AWWA
article (April 2008), material form and temperature both
have significant effects, with fiberglass retaining more iron
than sand and a temperature of 110oC producing greater iron
retention than a temperature of 25oC. Two of the interactions are significant—material with pH and material with
temperature. Fiberglass responds more to low pH and high
temperature (i.e., these more stringent conditions produce a
greater increase in iron retention on fiberglass than on sand).
These results suggest that exploring more stringent conditions
may lead to better iron-oxide retention by fiberglass.
MINERALOGY OF IRON OXIDE–COATINGS
Table A3 shows the X-ray diffraction results for the
fiberglass coated at high iron concentration (2.5 M) and
lower iron concentration (0.25 M). The peaks match fairly
well between the two samples, indicating that the mineralogy of the two is similar.
STATISTICAL ANALYSIS OF ISOTHERM MODELS
Table A4 shows the results of a statistical comparison
of the Langmuir and Freundlich models for each adsorbent
material. There were no statistically significant differences
in model fit as assessed by an F-test of model residuals.
PREVIOUS RESEARCH ON ARSENIC ADSORPTION
Table A5 summarizes arsenate adsorption concentrations. Where possible, isotherm parameters were used
to calculate solid-phase adsorption concentrations for an
aqueous arsenic concentration of 20 µg/L, which was
considered realistic for US drinking water applications
(indicated by isotherm parameters in the table). In other
cases, where column run lengths have been reported, a
mass balance on influent and effluent concentrations
has been used to estimate solid-phase concentrations
(indicated by mass balance in the table). The iron-coated
fiberglass and cellulose reported for this study fall in
an intermediate range, substantially below a variety of
metal hydroxide media (e.g., Pena et al, 2005; CH2M
HILL, 2004) but above iron ore (Chakravarty et al,
2002) and iron oxide–coated sand (e.g., Thirunavukkarasu et al, 2003).
TABLE A1 Coating conditions and iron loadings of different iron oxide–coated arsenic adsorbents
Reference
Material
Fecoating
mol/L
pHcoating
Tempcoating—oC
(Duration—h)
Iron Loading
mg/g
Min and Hering (1998)
Calcium alginate beads
0.001
NR
(72)
0.025
Zouboulis and Katsoyiannis (2002)
Alginate beads (iron coating)
0.3
5
(24)
1*
Zouboulis and Katsoyiannis (2002)
Sodium alginate (iron doping)
0.05
5
NR
3
Munoz et al (2002)
Cellulose
0.1
2
75 (24)
14
Banerjee et al (2007)
Sodium alginate + CaCl2·2H2O
0.1
NR
4–7 (24)
38*
Thirunavukkarasu et al (2003)
Sand (2 step)
2
Acidic
110 (44) + 550 (3)
+ 20 (100)
45
Matsunaga et al (1996)
Chelating resin LDA
0.1
3
25 (5)
50
Cumbal and Senguptal (2005)
Anion exchanger†
0.25
2
50–60 (2)
60
Blackwood (2007)
Polypropylene
0.25
8.5
25 (24)
62
Vatutsina et al (2007)
Fibrous ion-exchanger‡
NR
NR
NR
64
Ghimire et al (2003)
Phosphorylated orange waste
0.001
3
30 (24)
68
Katsoyiannis and Zouboulis (2002)
PolyHIPE
0.3
5
25 (3)
93
Chen et al (2007)
Activated carbon§
2
4-5
105 (12)
154
This study
Fiberglass
2.5
Acidic
110 (24)
438
Guo and Chen (2005)
Cellulose (7 step)
0.62
3.5–4
25
468
2H2O—water molecule, CaCl2—calcium chloride, LDA—lysine-N, N–diacetic acid, NR—not reported
*grams of wet alginate beads
†Purolite A-400, Purolite, Bala Cynwyd, Pa.
‡FIBAN-As, IMT Ltd., Minsk, Belarus
§SAI Carbon, Superior Adsorbents Inc., Emlenton, Pa.
KUMAR ET AL | 100:4 • JOURNAL AWWA | PEER-REVIEWED | APRIL 2008
2008 © American Water Works Association
A1
TABLE A2 ANOVA to study the effect of material form, coating pH, and coating temperature on iron oxide loading
Source
Estimated Effect
Corrected model*
Sum of Squares
df
Mean Square
F
p-value
NA
21.720†
7
3.103
6.852
0.000
3.353
359.790
1
359.790
794.513
0.000
Material—1 = fiberglass, –1 = sand
0.798
5.089
1
5.089
11.238
0.003‡
pH—1 = pH 7, –1 = pH 1.3
–0.236
0.445
1
0.445
0.982
0.332
Temperature—1 = 110oC, –1 = 25oC
0.951
7.242
1
7.242
15.992
0.001‡
Material × pH
–1.96
4.566
1
4.566
10.083
0.004‡
Material × temperature
0.565
2.556
1
2.556
5.644
0.026‡
pH × temperature
–0.423
1.432
1
1.432
3.162
0.088
Material × pH × temperature
–0.221
0.391
1
0.391
0.862
0.362
Error
NA
10.868
24
0.453
Total
NA
392.378
32
Intercept
ANOVA—analysis of variance, df—degrees of freedom, F—F-statistic, NA—not applicable
*Model for log (iron loading)
†R2 = 0.67 (adjusted R2 = 0.57)
‡Significant at the 0.05 level
TABLE A3 A mineralogical comparison of coated fiberglass insulation fibers*
Fecoating—mol/L
2
Peak intensity†
FWHM‡
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
2
Peak intensity
FWHM
0.25
2.5
16.92
0.548
0.083
19.72
0.504
0.163
22.08
0.624
0.229
26.96
0.984
0.302
34.12
0.656
0.100
35.32
1.000
0.100
37.24
0.652
0.130
39.64
0.624
0.067
40.92
0.564
0.094
44.76
0.644
0.157
55.96
0.620
0.139
60.20
0.600
0.100
16.96
0.264
0.119
19.92
0.349
0.162
22.16
0.464
0.211
26.84
0.786
0.306
34.20
0.478
0.115
35.12
1.000
0.077
37.20
0.409
0.0713
39.20
0.551
0.081
40.60
0.233
0.086
44.84
0.301
0.104
56.04
0.548
0.177
60.88
0.219
0.112
—diffraction angle
*Coating conditions: pHcoating = 1.3, tempcoating = 110oC
†Peak intensity as a fraction of maximum peak
‡Full width at half-maximum value calculated using an X-ray diffraction pattern processing,
identification, and quantification softwater (Jade +, version 7, Materials Data Inc., Livermore, Calif.)
A2
APRIL 2008 | JOURNAL AWWA • 100:4 | PEER-REVIEWED | KUMAR ET AL
2008 © American Water Works Association
TABLE A4 Statistical analysis for comparing different isotherm models
Model
Parameters
Langmuir
Sand25
Average of residuals
FGI110
Cell25
PPm25
PETf25
0
0
0
0
0
0
1 × 10–4
2.4 × 10–5
8.5 × 10–3
7.9 × 10–3
1 × 10–4
6 × 10–4
Number of samples (N)
5
4
5
5
3
3
Number of parameters (Np)
2
2
2
2
2
2
Variance of residual [2 = SSE/(N – Np)]
1.9 × 10–5
1.2 × 10–5
2.8 × 10–3
2.6 × 10–3
1 × 10–4
6 × 10–4
Standard deviation of residual ()
4.4 × 10–3
3.5 × 10–3
5.3 × 10–2
5.1 × 10–2
0.96 × 10–4
2.5 × 10–2
SSE
Freundlich
Sand110
Average of residuals
0
0
0
0
0
0
Error sum of squares
3.3 × 10–5
1.5 × 10–5
7 × 10–3
1 × 10–2
4 × 10–4
6 × 10–4
Number of samples (N)
5
4
5
5
3
3
Number of parameters (Np)
2
2
2
2
2
2
Variance of residual [2 = SSE/(N – Np)]
1.1 × 10–5
0.8 × 10–5
2.3 × 10–3
3.4 × 10–3
4 × 10–4
6 × 10–4
Standard deviation of residual ()
3.3 × 10–3
2.8 × 10–3
4.8 × 10–2
5.9 × 10–2
2 × 10–2
2.3 × 10–2
F-ratio
1.73
1.50
1.22
1.31
4.00
1.00
F-critical*
9.28
19
9.28
9.28
161.45
161.45
Decision
Equivalent
models
Equivalent
models
Equivalent
models
Equivalent
models
Equivalent
models
Equivalent
models
Cell25—cellulose sponge coated with conditions of 0.25 M Fe, pH 1.3, 25°C; Fe–iron; FGI110—fiberglass insulation coated with conditions of 0.25 M Fe, pH 1.3, 110°C;
PETf25—polyester fiber with coating conditions of 2.5 M Fe, pH 8.53, 25°C; PPm25—polypropylene mat with coating conditions of 0.25 M Fe, pH 7, 25°C; Sand110—sand
coated with conditions of 0.25 M Fe, pH 7, 110°C; Sand25—sand coated with conditions of 0.25 M Fe, pH 7, 25°C; SSE—sum of squared error
*For a 0.05-level test
TABLE A5 Performance comparison of iron oxide–coated fibers with other arsenic adsorbents
Media
pH
Sorption
Concentration
mg As/g
Pena et al (2005)
Nanocrystalline TiO2
7.0
8.25
NL
IP
CH2M HILL (2004)
Granular ferric oxide*
6.8
6.22
135,000
MB
Zeng (2004)
Iron(III) oxide/silica
6.5
4.04
NL
IP
Vatutsina et al (2007)
Fibrous ion-exchanger†
7.2
2.8
8,200
MB
CH2M HILL (2004)
Granular ferric hydroxide
6.8
2.05
110,000
MB
Driehaus et al (1998)
GFH
7.8
1.4
37,000
Table 1
Reference
Deliyanni et al (2003)
Akaganeite-nanocrystal
Chen et al (2007)
Iron oxide-coated activated carbon
Lackovic et al (2000)
Gimenez et al (2007)
Su and Puls (2003)
BVs Treated
Basis
7.5
1.18
NL
IP
7.6–8.0
0.69
14,500
MB
Zero-valent iron
6.3
0.67
740
page 32
Natural goethite
6.5–7.5
0.45
NL
IP
Zero-valent iron
6.0–6.5
0.36
320
page 2,584
Murugesan et al (2005)
Iron chloride tea fungal biomass
7.2
0.3
NL
IP
This study
Cell25
7.6
0.23
NL
IP
This study
FGI110
7.6
0.13
NL
IP
Joshi and Chaudhuri (1996)
Iron oxide–coated sand
7.5–7.8
0.1‡
150
MB
Vaishya and Gupta (2003)
Iron oxide–coated sand
7.2–7.4
0.09‡
125
MB
Thirunavukkarasu et al (2003)
Iron oxide–coated sand
7.6
0.03
1,000
Figure 1
Benjamin et al (1996)
Iron oxide–coated sand
8.0
0.02
600
Figure 8
Guo and Chen (2005)
Cellulose
7.0
0.02
NL
IP
Chakravarty et al (2002)
Ferruginous ore
6.5
0.01
NL
IP
BVs—bed volumes, Cell25—cellulose sponge coated using coating conditions (0.25 M Fe, pH 1.3, 25oC), FGI110—fiberglass insulation coated using coating conditions
(0.25 M Fe, pH 1.3, 110oC), IP—isotherm parameters, MB—mass balance, NL—not listed, TiO2—titanium dioxide
Sorption density is calculated for 20 µg/L equilibrium arsenic(V) concentrations
*SORB 33, Severn Trent Services, Ft. Washington, Pa.
†FIBAN-As, IMT Ltd., Minsk, Belarus
‡Based on high-influent arsenic (1 mg/L); may not be realistic for US drinking water applications
KUMAR ET AL | 100:4 • JOURNAL AWWA | PEER-REVIEWED | APRIL 2008
2008 © American Water Works Association
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APRIL 2008 | JOURNAL AWWA • 100:4 | PEER-REVIEWED | KUMAR ET AL
2008 © American Water Works Association