1-s2.0-S0043135497003497-main

ARSENATE SORPTION BY FE(III)-DOPED ALGINATE
GELS
JOON H. MIN1and JANET G. HERING2*
1Department of Civil and Environmental Engineering, University of California, Los Angeles, 5732
Boelter Hall, Los Angeles, CA 90095-1593, U.S.A. and 2California Institute of Technology,
Environmental Engineering Science (138-78), Pasadena, CA 91125, U.S.A.
(First received February 1997; accepted in revised form August 1997)
AbstractÐAlthough cationic metal contaminants can be eectively removed from wastewaters by treat-
ment with biopolymers, application of biopolymers for the removal of anionic contaminants (such as
As, Cr(VI), and Se) has been limited. The objective of this study was to examine the fundamental
aspects of a possible remediation strategy for removal of anionic metal species employing the biopoly-
mer alginic acid pretreated with Ca and Fe(III). Spherical gel beads (2 mm in diameter) were formed by
dispensing the biopolymer solution dropwise into 0.1 M CaCl
2
; Ca beads were then washed and equili-
brated with 0.1 M FeCl
3
to achieve partial substitution of Fe(III) for Ca. The resulting Ca±Fe beads
were found to be eective at removing As(V) from solution on a time scale of approximately 100 h.
As(V) sorption was pH dependent; optimal removal and stability of the Ca±Fe beads was achieved at
pH 4. At a given initial As(V) concentration, As(V) removal eciency increased with increasing Fe con-
tent (number of beads); at an initial As(V) concentration of 400 mg/l, up to 94% removal was achieved
at pH 4 after 120 h. For a given Fe content, uptake of As(V) increased with increasing initial As(V)
concentration until saturation was reached. Sorption data was modeled using a single type of As(V)
binding site. Data on As(V) sorption as a function of dissolved As(V) concentration and Fe content
were used to obtain and validate the site density and conditional anity constant for As(V) sorption.
#1998 Elsevier Science Ltd. All rights reserved
Key wordsÐoxyanion, arsenic, arsenate, biopolymer, alginate, gel bead, Fe(III)-doped, removal,
wastewaters
INTRODUCTION
Of the many toxic metals that may be present as
contaminants in natural waters and wastewaters, a
few (speci®cally arsenic, chromium(VI), and sel-
enium) occur as oxyanions. Removal of arsenic is,
in particular, an issue of increasing concern. Re-
evaluation of the current Maximum Contaminant
Level (MCL) for arsenic is mandated under the
1996 re-authorization of the Safe Drinking Water
Act. A decrease in the current standard (50 mg/l)
would impact the required levels of treatment not
only for potable water, but also for hazardous
wastewaters and euents under the Resource
Conservation and Recovery Act. Arsenic is a major
contaminant of concern at many Superfund sites
(Brewster and Passmore, 1994; Mariner et al.,
1996). Arsenic containing wastestreams are gener-
ated in the microelectronics industry due to the use
of arsenic in the form of gallium arsenide (GaAs)
for the manufacturing of semiconductor devices
(Gilles and Loehr, 1994; Vagliasindi and Poulsom,
1994). The average concentration of arsenic in
wastewater at facilities which produce GaAs has
been estimated to be 2.4 mg/l (USEPA, 1982).
For arsenic-contaminated wastestreams, various
physical/chemical treatment technologies have been
applied for contaminant removal. Arsenic-contami-
nated water collected during clean up at a former
pesticide facility was treated in a full-scale process
involving co-precipitation/adsorption with ferric
chloride, ®ltration and carbon adsorption (Harper
and Kingham, 1992). In pilot-scale studies using
electrochemically generated Fe
2+
and hydrogen per-
oxide, arsenic was eciently removed from con-
taminated waters at both a Superfund site and a
wood-preserving facility (Brewster and Passmore,
1994). Other processes, such as adsorption onto
activated carbon (Huang and Fu, 1984; Huang and
Vane, 1989) or ¯y-ash (Diamadopoulos et al., 1993)
have been tested at bench scale.
Many of these processes generated a signi®cant
quantity of sludges or other solid wastes; in some
cases, these materials had to be disposed of as
hazardous waste (Harper and Kingham, 1992). In
some applications, selective removal of oxyanions
may provide options for more economical treatment
of wastestreams, for example, by reducing sub-
sequent sludge disposal costs. Thus, this study was
Wat. Res. Vol. 32, No. 5, pp. 1544±1552, 1998
#1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0043-1354/98 $19.00 + 0.00
PII: S0043-1354(97)00349-7
*Author to whom all correspondence should be addressed
[Tel: [phone], Fax: [phone], E-mail:
[email protected]].
1544

designed to investigate the fundamental aspects of a
possible remediation strategy for oxyanion removal
using a negatively-charged biopolymer pretreated
with metal cations.
At the bench scale, biosorbents have been tested
primarily for removal of heavy metal cations, which
are removed preferentially to alkali and alkaline
earth metals and anionic species (Brierley, 1990).
Alginic acid has been shown to be eective at
removing many cationic metals from solution
including Pb
2+
and Cu
2+
(Deans and Dixon,
1992), UO
2
2+
(Hassan et al., 1993), Cu
2+
(Jang et
al., 1990), Cu
2+
,Zn
2+
,Cd
2+
, and Ni
2+
, (Jellinek
and Sangal, 1972), Nd
3+
and Yb
3+
(Konishi et al.,
1992), and 226Ra (Torma et al., 1991).
Pretreatment or doping of an anionic biosorbent,
such as alginic acid, with cations allows (indirect)
interaction between the biosorbent and anionic con-
taminants. Metal recovery (Co, Cu, As, Fe, Mg, Al,
Ca) from an acidic (pH 2±3) cobalt ore leachate
with alginic acid has been investigated by Jang et
al. (1991). The removal of arsenic observed with
this anionic biopolymer may be attributable to in
situ metal doping in the presence of strongly bind-
ing cationic metals.
In this study, the sorption properties of metal-
doped anionic biopolymer were systematically
examined using alginic acid, a linear, binary hetero-
polymer of mannuronic (M) and guluronic (G) resi-
dues (Fig. 1(a)). The proportions of MM-, GG- and
MG-blocks vary with the source of the alginic acid.
GG blocks have a strong anity for divalent metal
ions and are responsible for gelation (Yotsuyanagi
et al., 1990).
Charged polysaccharides, such as sodium algi-
nate, often form hydrogels in the presence of cat-
ions (Mikkelsen and Elgsaeter, 1995); the gel
characteristics depend on the speci®c cation. In suf-
®ciently concentrated solutions of sodium alginate,
divalent cations form junction zones by cross-link-
ing functional groups of polymer chains to form a
gel matrix, illustrated schematically in Fig. 1(b). In
this study, Ca gel beads were further treated with
Fe(III) to optimize both sorptive capacity for
arsenate and the physical properties of the gel
beads [Fig. 1(c)]. Arsenic removal was studied both
with As(V), which occurs, within the pH range stu-
died, principally as H
2
AsO
4
ÿ
, and with As(III),
which occurs as the neutral species H
3
AsO
4
.
MATERIALS AND METHODS
Reagents and stock solutions
Low viscosity grade sodium alginate (250 cps at 258C
and 2% w/v) was obtained from Sigma and used without
further puri®cation. Other chemicals were reagent grade
and used as received. All solutions were prepared with
water puri®ed by reverse osmosis and deionized using a
Millipore System (Milli Q following Milli RO), referred to
here as Milli Q water.
The arsenate stock solution (1.000 g As/l) was prepared
from sodium salt heptahydrate (Na
2
HAsO
4
7H
2
O,
Sigma) dissolved in Milli Q water. Secondary standard sol-
utions for calibration samples (1.000 mg As/l) and spiking
were freshly prepared for each experiment from the
1.000 g As/l stock solutions by dilution with Milli Q
water. The arsenite stock solution was prepared from solid
arsenic trioxide, As
2
O
3
(Aldrich, A.C.S. primary standard)
dissolved in 0.18 M (1.5% v/v) trace metal grade HCl
(Fisher). The sulfate (0.010 M) and phosphate (0.001 M)
stock solutions were prepared from Na
2
SO
4
H
2
O and
NaH
2
PO
4
H
2
O, respectively, dissolved in Milli Q water.
Calcium, copper, and iron solutions (0.100 M) used in gel
synthesis were prepared from CaCl
2
2H
2
O,
CuCl
2
2H
2
O, and FeCl
3
6H
2
O, respectively, by dissol-
ution of the salts in Milli Q water.
Gel bead synthesis and washing
The biopolymer powder (2.000 g) was dissolved in
100 ml of Milli Q water to obtain 2% w/v alginate sol-
ution. The solution was mixed on a wrist action shaker
Fig. 1. Schematics of Ca±Fe gel bead formation with sodium alginate. (a) Sodium alginate chain show-
ing conformation of M and G units. (b) Ca bead is produced by forming junctions with Ca (adapted
from Jang et al., 1991). (c) Fe
3+
displaces loosely bound Ca to produce Ca±Fe beads.
As(V) sorption by Fe(III) alginate gel 1545

(Burrell, Model 75) overnight until a yellowish brown, vis-
cous solution was obtained. A peristaltic pump was used
to dispense the polymer solution. At the end of the dispen-
sing tube, a pipette tip (Bioplas, Fisher; ID 0.30 mm) was
attached and positioned approximately 7 cm above the
surface of the cation solution used for gel formation. The
peristaltic pump was then programmed to dispense a drop
of polymer solution approximately every 3 s into 1 l of
0.1 M cation solution (Ca, Cu or Fe chloride solution) in
a beaker. A paddle stirrer unit (Phipps and Bird, Model
7790-400) was used to provide gentle stirring (60 rpm)
when forming the gel beads, since the beads were reported
to be susceptible to hydrodynamic forces (Ogbonna et al.,
1991). The acid resistant polypropylene paddles (Cole-
Parmer) were used as paddle assemblies for all mixing and
bead washing.
The beads were allowed to cure in cation solutions for
3 days and then removed from the cation solution and
rinsed twice (either with Milli Q water for Ca and Cu
beads or with 1 mM HCl for Fe and Ca±Fe beads). The
beads were then transferred to a continuous washing sys-
tem (015 ml/min) with in-line acid neutralizing unit. For
preparation of Ca±Fe beads, washed Ca beads were trans-
ferred to 0.100 M FeCl
3
solution for further doping of the
beads with Fe(III). The beads were continuously washed
for 3 days and stored in 1 mM HCl until used (up to
6 months) for As(V) sorption studies.
To con®rm that the bead synthesis was reproducible,
the size distribution of the beads was analyzed. Digitized
photographs of the beads were used in combination with
image analysis software (Optimetric, Optimas), which
automatically counts and measures the diameter of each
bead and calculates size distribution.
Conditions and apparatus for batch sorption experiments
For the batch experiments, 85 ml polycarbonate centri-
fuge tubes were used as reactors with 50 ml of solution as
reaction volume. All of the batch experiments were done
at pH 4 using acetate buer (0.5 mM) and background
electrolyte of 5 mM NaNO
3
unless otherwise speci®ed.
The pH of the solution was adjusted with either HNO
3
(0.1 N) or NaOH (0.1 N) solution, and the pH was
measured with an Orion 720A pH meter. For each repli-
cate in the batch experiments, 5 beads were equilibrated
(unless otherwise speci®ed) in acetate buer solution at
speci®ed pH for a day and transferred to the reaction bot-
tles after the surface moisture of the beads was wiped o.
Then, As(V) spikes (to ®nal concentrations of 1.00 mg/l to
10.0 mg/l) were added. The bottles were placed in a rack,
which was mounted on a wrist action shaker for mixing.
After the speci®ed reaction times, the beads were removed
from the bottles to stop further reaction of arsenate with
the beads, and the supernatants and/or gel beads were
analyzed for As(V). All experiments were run in duplicate.
Analytical measurements
The total Fe in the beads was measured by disrupting
the gel phase by sonication in an EDTA solution (Murata
et al., 1993). The beads were transferred to a 50 ml volu-
metric ¯ask, 3 ml of 0.1 M EDTA was added and the ¯ask
and contents were sonicated (Branson B-220, Smithkline)
for 30 min. The ¯ask was then ®lled to volume with Milli
Q water and ®ltered using 0.1 mm cellulose nitrate mem-
brane ®lter (Sartorius).
Fe analysis by Inductively Coupled Plasma Mass
Spectrometry (ICP-MS, Perkin-Elmer/Sciex Elan 5000A)
was done by measuring the 57Fe isotope. The aqueous
samples were introduced at a rate of 1 ml/min from a
Gilson 212B autosampler and a peristaltic pump to a
cross-¯ow pneumatic nebulizer in a room temperature
spray chamber. High purity Ni skimmer and sampling
cones were used. Calibration standards were run in the
same matrix (by disintegrating Ca beads with EDTA) to
verify that matrix eects did not interfere with ICP-MS
analysis.
Both the initial As(V) and the dissolved As(V) concen-
tration in the solution for each batch experiment were
measured with ICP-MS for low initial As(V) concen-
trations (i.e., 1.00 to 400 mg/l) at 75As without ®ltration.
For initial As(V) concentrations higher than 400 mg/l,
As(V) in the gel phase was measured by the method used
for total Fe analysis in the beads with appropriate cali-
bration standards. All samples were measured with ICP-
MS in 1% HNO
3
solution.
RESULTS AND DISCUSSION
Eects of cation on As(V) sorption
Initially, four gel bead types, Ca, Cu, Fe, and
mixed Ca±Fe beads, were produced and tested for
As(V) sorption. Among the beads tested, Ca±Fe
beads showed highest As(V) removal at 24 h, and
Fe and Ca±Fe beads showed comparable As(V)
removal at 2 h (Fig. 2). Neither Cu beads nor Ca
beads showed any signi®cant As(V) sorption even
after 24 h. Even at an As(V) concentration of 5 mg/
l, the maximum uptake of As(V) by Ca beads was
0.013(20.001) mg per g of wet Ca bead compared
with 1.57(20.03) mg per g of wet Ca±Fe beads.
This absence of As(V) uptake by Ca beads demon-
strates that As(V) accumulation in the pore volume
of Fe and Ca±Fe beads by passive diusion is negli-
gible compared with speci®cally bound As(V). This
is consistent with the negligibly small water volume
in the beads as compared with the total solution
volume in the batch experiments. With Cu beads,
the poor As(V) removal may be attributable to the
formation of an impervious layer at the surface of
the Cu beads which inhibits diusion of solutes into
the beads as suggested by previous studies
(Martinsen et al., 1989; Velings and Mestdagh,
1995).
Although Fe beads were easier to produce than
Ca±Fe beads, the physical integrity of the Fe beads
was poor with extreme cracking and peeling at the
surface. The mixed Ca±Fe beads, which were in-
itially formed with Ca and then doped with Fe,
showed good mechanical stability. In the Ca±Fe
beads, Ca may act as primary gel forming cation
providing a stable structure for the polymer net-
work. The Fe then partly replaces the Ca in the gel
matrix to provide favorable sorption sites for
As(V). The average measured Fe content of a single
Ca±Fe bead was 25.0 20.5 mg (0.4520.01 mmol).
All the subsequent experiments were performed
with the Ca±Fe beads.
As(V) sorption kinetics and eects of initial As(V)
concentration
Partitioning of As(V) between the solution and
gel phase at pH 4 is shown as a function of time in
Fig. 3. Excellent mass balance was obtained
throughout the experiment by summing the
Joon H. Min and Janet G. Hering1546

measured dissolved and sorbed As(V) concentration
with an initial As(V) concentration of 400 mg/l (or
total mass of 20 mg As(V)).
The time scale for As(V) sorption was observed
to be in the order of 100 h. With an initial As(V)
concentration of 400 mg/l, increasing As(V) sorption
was observed up to 93 h. At a 10-fold higher initial
As(V) concentration, slow increase in the mass of
As(V) sorbed was observed from 48 to 124 h
(Fig. 4). Most As(V) uptake, however, occurred
within 48 h; the mass of As(V) sorbed at 48 h is ap-
proximately 92% of that sorbed at 124 h.
The eect of initial As(V) concentration on the
As(V):Fe molar ratio attained was thus examined
with a 120 h equilibration time. The ratio increased
rapidly to 0.18:1 over a range in the initial As(V)
concentration from 0 to 2 mg/l; a slow increase in
the ratio was observed above an initial As(V) con-
centration of 4 mg/l until the plateau in As(V):Fe
molar ratio of 0.26:1 was reached (Fig. 5). The Ca±
Fe beads were nearly, if not fully, saturated with
As(V) at this As(V):Fe molar ratio.
In order to saturate the beads with arsenate and
determine the maximum sorption capacity, it was
Fig. 2. 2 h (Q) and 24 h (q) As(V) sorption (normalized by wet bead weight) by four gel bead types
produced with the same setup (2% sodium alginate solution, same size tip, gel formation in 0.1 M
chloride salts) and background reaction conditions (0.5 mM acetic acid, 5 mM NaNO
3
,pH4,50ml
reaction volume) with 5 beads and 4 mg/l initial As(V) concentration.
Fig. 3. As(V) sorption kinetics: % As(V) in solution phase (w), % As(V) in gel phase (q), and % total
As(V) in system (W); background reaction conditions as in Fig. 2 with 5 Ca±Fe beads of 25 mg Fe per
bead and 400 mg/l initial As(V) concentration (total mass As(V) 20 mg).
As(V) sorption by Fe(III) alginate gel 1547

necessary to employ conditions which may not be
practical in treatment systems. The slow sorption
observed with the current experimental conditions
can be mitigated by increasing the number of beads
or by decreasing the size of the beads, which can be
accomplished by modifying the dispensing setup
(Gilson and Thomas, 1995), to obtain a speci®c
removal eciency on a given time scale.
As(V) sorption model
Sorption of As(V) by Ca±Fe beads over a wide
range in initial As(V) concentrations could be mod-
eled by assuming equilibrium partitioning of As(V)
between the aqueous and gel phases, a uniform a-
nity of As(V) for the Ca±Fe gel, and a maximum
capacity of the Ca±Fe gel phase for As sorption as
expressed by equation 1
fAsgsorb KAsdissfAlgFegT
1KAsdiss
1
where [As]
diss
is the equilibrium (i.e., ®nal) dissolved
concentration of As(V) in mg/l, {As}
sorb
is the
sorbed concentration in mg As/g Fe, {AlgFe}
T
is
saturation (maximum) sorption capacity in mg As/g
Fe, and Kis the conditional anity constant in l/
mg. Values of Kand {AlgFe}
T
were obtained by ®t-
Fig. 4. As(V) sorption kinetics for initial As(V) concentrations of 400 mg/l (r) (total mass As(V) 20 mg)
and 4 mg/l (w) (total mass As(V) 200 mg); background reaction conditions as in Fig. 2 with 5 Ca±Fe
beads of 25 mg Fe per bead.
Fig. 5. Eect of initial As(V) concentration on As(V) sorption density; background reaction conditions
as in Fig. 2 with 5 Ca±Fe beads of 25 mg Fe per bead and 120 h equilibration time.
Joon H. Min and Janet G. Hering1548