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PII: S0043-1354(97)00349-7
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
Department 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 treatment 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 biopolymer 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 CaCl2; Ca beads were then washed and equilibrated with 0.1 M FeCl3 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 content (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,
Of the many toxic metals that may be present as
contaminants in natural waters and wastewaters, a
few (speci®cally arsenic, chromium(VI), and selenium) occur as oxyanions. Removal of arsenic is,
in particular, an issue of increasing concern. Reevaluation 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 generated 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
*Author to whom all correspondence should be addressed
[Tel: +1-626-3953644, Fax: +1-626-3952940, E-mail:
[email protected]].
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-contaminated 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 Fe2+ and hydrogen peroxide, arsenic was eciently removed from contaminated 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 subsequent sludge disposal costs. Thus, this study was
As(V) sorption by Fe(III) alginate gel
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 Pb2+ and Cu2+ (Deans and Dixon,
(Hassan et al., 1993), Cu2+ (Jang et
1992), UO2+
al., 1990), Cu2+, Zn2+, Cd2+, and Ni2+, (Jellinek
and Sangal, 1972), Nd3+ and Yb3+ (Konishi et al.,
1992), and 226 Ra (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 contaminants. 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 binding cationic metals.
In this study, the sorption properties of metaldoped anionic biopolymer were systematically
examined using alginic acid, a linear, binary heteropolymer of mannuronic (M) and guluronic (G) residues (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 alginate, often form hydrogels in the presence of cations (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-linking 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 studied, principally as H2AsOÿ
4 , and with As(III),
which occurs as the neutral species H3AsO4.
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 (Na2HAsO4 7H2O,
Sigma) dissolved in Milli Q water. Secondary standard solutions 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, As2O3 (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 Na2SO4 H2O and
NaH2PO4 H2O, respectively, dissolved in Milli Q water.
Calcium, copper, and iron solutions (0.100 M) used in gel
CaCl2 2H2O,
CuCl2 2H2O, and FeCl3 6H2O, respectively, by dissolution 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 solution. 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 showing conformation of M and G units. (b) Ca bead is produced by forming junctions with Ca (adapted
from Jang et al., 1991). (c) Fe3+ displaces loosely bound Ca to produce Ca±Fe beads.
Joon H. Min and Janet G. Hering
(Burrell, Model 75) overnight until a yellowish brown, viscous solution was obtained. A peristaltic pump was used
to dispense the polymer solution. At the end of the dispensing 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 (ColeParmer) 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 system (015 ml/min) with in-line acid neutralizing unit. For
preparation of Ca±Fe beads, washed Ca beads were transferred to 0.100 M FeCl3 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 centrifuge 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 NaNO3 unless otherwise speci®ed.
The pH of the solution was adjusted with either HNO3
(0.1 N) or NaOH (0.1 N) solution, and the pH was
measured with an Orion 720A pH meter. For each replicate 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 bottles 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 volumetric ¯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 membrane ®lter (Sartorius).
Fe analysis by Inductively Coupled Plasma Mass
Spectrometry (ICP-MS, Perkin-Elmer/Sciex Elan 5000A)
was done by measuring the 57 Fe 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
Both the initial As(V) and the dissolved As(V) concentration in the solution for each batch experiment were
measured with ICP-MS for low initial As(V) concentrations (i.e., 1.00 to 400 mg/l) at 75 As 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 calibration standards. All samples were measured with ICPMS in 1% HNO3 solution.
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 demonstrates that As(V) accumulation in the pore volume
of Fe and Ca±Fe beads by passive di€usion is negligible 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,
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 initially 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 network. 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 2 0.5 mg (0.452 0.01 mmol).
All the subsequent experiments were performed
with the Ca±Fe beads.
As(V) sorption kinetics and e€ects of initial As(V)
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
As(V) sorption by Fe(III) alginate gel
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 NaNO3, pH 4, 50 ml
reaction volume) with 5 beads and 4 mg/l initial As(V) concentration.
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 approximately 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) concentration 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. 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).
Joon H. Min and Janet G. Hering
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.
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 modeled 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 ˆ
K‰AsŠdiss fAlgFegT
1 ‡ K‰AsŠdiss
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 K is the conditional anity constant in l/
mg. Values of K and {AlgFe}T were obtained by ®t-
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.
As(V) sorption by Fe(III) alginate gel
ting values of [As]diss and {As}sorb from Fig. 6(a) to
equation 1 using a non-linear least square regression in the SigmaPlot (Jandel) software.
Good agreement between the model and data
was observed over the range in initial As(V) concentrations used in this study as shown in Fig. 6(a).
At initial As(V) concentrations below 200 mg/l,
however, the model consistently underpredicted the
observed As(V) sorption at 120 h equilibration
time [Fig. 6(b)]. The isotherm constants {AlgFe}T
and K were found to be 352 mg As/g Fe and
1.68 l/mg, respectively, and the saturation As(V):Fe
molar ratio predicted from the constant was 0.26
to 1 in agreement with the observed maximum
value (Fig. 5).
E€ect of total Fe content on As(V) sorption
Fig. 6. Model ®t (ÐÐÐ) and data for As(V) sorption by
Ca±Fe beads; background reaction conditions as in Fig. 2
with 5 Ca±Fe beads of 25 mg Fe per bead and 120 h equilibration time. (a) 0±10 mg/l dissolved As(V) concentration.
(b) Expanded view for 0±0.8 mg/l dissolved As(V) concentration.
The total Fe content in the batch system was varied by using di€erent number of beads, with an
average amount of Fe in a single Ca±Fe bead of
25.0 2 0.5 mg (0.45 2 0.01 mmol). With an initial
As(V) concentration of 400 mg/l, the percent As(V)
removed in 120 h at pH 4 reached plateau at 94%
with 20 beads (Fig. 7).
The amount of As(V) removed by varying the
total iron concentrations in the reactor volume,
[Fe]T (g/l), can be easily predicted under conditions
where the Ca±Fe beads are undersaturated with
respect to As(V) sorption. Then, the concentration
of free Fe sites in the gel phase, {AlgFe}, is
fAlgFegT 1fAlgFeg
Fig. 7. E€ect of total Fe (number of Ca±Fe beads of 25 mg Fe per bead) on % As(V) removed (W) and
mole of As(V) sorbed per mole of Fe (w); background reaction conditions as in Fig. 2 with 400 mg/l initial As(V) concentration and 120 h equilibration time. Lines show model ®ts for (ÐÐÐ) undersaturated case (equation 7) and ( ) not undersaturated case.
Joon H. Min and Janet G. Hering
The concentration of As(V) sorbed in the reactor
volume, [As]sorb in mg/l,
imental conditions, however, is not entirely negligible compared to the model saturation value
(0.26:1). Therefore the result of model prediction
accounting for sorbed As in the mass balance for
alginate binding sites, that is
‰AsŠsorb ˆ fAsgsorb ‰FeŠT ˆ K‰AsŠdiss fAlgFegT ‰FeŠT
AlgFeT 1AlgFe ‡ Assorb
fAsgsorb ˆ K‰AsŠdiss fAlgFeg1K‰AsŠdiss fAlgFegT
is used in the mass balance for the total As concentrations
‰AsŠT ˆ ‰AsŠdiss ‡ ‰AsŠsorb
‰AsŠsorb ‡ ‰AsŠdiss
KfAlgFegT ‰FeŠT
KfAlgFegT ‰FeŠT ‡ 1
K = 1.68 l/mg
{AlgFe}T=352 mg/g, the % As removed can be
predicted by
% Asremoved ˆ
592‰FeŠT ‡ 1
is also shown in Fig. 7. The second prediction (not
undersaturated case) shows a better ®t than the ®rst
prediction (undersaturated case) at lower total Fe
region where the As(V) sorbed is not negligible.
E€ects of pH on As(V) removal
to obtain an expression for percent As removed.
% Asremoved ˆ
Under these conditions, the Ca±Fe beads are
undersaturated with respect to As(V) sorption with
a As(V):Fe molar ratio below 0.1:1 compared with
0.26:1 for a saturated case. The model (equation 7)
predicted arsenic removal to within 10% of the
measured values for the entire range of total Fe studied. The model prediction for % As removed is
shown with the data in Fig. 7.
The As(V):Fe molar ratio of approximately 0.1:1
achieved at low total Fe contents under these exper-
The pH was a predominant factor a€ecting the
removal of As(V) under the conditions employed
(Fig. 8). The Ca±Fe beads used were pre-equilibrated in various pH solutions before As(V) batch
experiments to isolate the swelling or shrinking
e€ects from the sorption process. With an initial
As(V) concentration of 400 mg/l, a minimum in the
residual dissolved As(V) concentration after a 120 h
exposure to 5 Ca±Fe beads was observed at pH 3.
A sharp increase in the residual, dissolved As(V)
concentration was observed with a decrease in pH
from 3 to 2 and a more gradual increase with
increasing pH from 3 to 6.4.
The poor As(V) removal by the gel at pH 2 may
be due to partly to formation of the neutral species
H3AsO4 (with pKa112.2), but also appears to be associated with instability of the Ca±Fe gel under
these conditions. At pH 2, more than 60% of the
total Fe (125 mg) in the Ca±Fe gel matrix leached to
solution during pre-equilibrium and sorption experiments. Competition between proton and metal
binding by alginic acid at pH values below 3 (the
Fig. 8. E€ect of pH on As(V) sorption (W) and Fe leaching (w); concentration of leached Fe were
determined for un®ltered samples and could indicate colloidal iron; reaction conditions: 0.5 mM acetic
acid, 5 mM NaNO3, 50 ml reaction volume with 5 Ca±Fe beads of 25 mg Fe per bead, 400 mg/l initial
As(V) concentration, and 120 h equilibration time.
As(V) sorption by Fe(III) alginate gel
intrinsic pKa of alginic acid) has been observed
(Jang et al., 1989, 1995). Leaching of Fe was less
pronounced at pH 3 (approximately 15% leached).
Leaching of Fe is not desirable since the sites for
As(V) binding in the Ca±Fe beads would be
decreased by this process. The iron released from
the beads might also precipitate as an amorphous
hydroxide solid; this colloidal phase iron might provide binding sites for As(V) competing with the
alginate binding sites. The pH of 4 used in most of
the experiments reported here provides favorable
conditions both for Ca±Fe bead stability and As(V)
E€ects of competing ions and sorption of As(III)
Co-occurring inorganic anions such as sulfate
and phosphate may directly compete with As(V) for
binding sites in the gel matrix and a€ect the extent
of As(V) sorption. However, the presence of sulfate
(0.52 mM) and phosphate (0.16 mM) (singly or in
combination) did not a€ect As(V) sorption by Ca±
Fe beads at a low initial As(V) concentration of
4 mg/l (0.053 mM); under these conditions, the available binding sites are in excess. The amount of
As(V) sorbed in 24 h at pH 4 was 0.125 2 0.001 mg
per 125 mg Fe (5 beads), as compared with
0.128 2 0.004 mg with sulfate, 0.122 2 0.001 mg with
phosphate, and 0.123 2 0.006 mg with both. Under
conditions where the binding sites approach saturation (i.e., at high initial concentrations of both
arsenate and phosphate), the competitive e€ects
were observed (Min, in preparation).
In contrast to the signi®cant sorption of negatively-charged As(V) species by Ca±Fe beads,
uncharged As(III) species were poorly removed
under comparable conditions (Table 1). At initial
As concentration in the mg/l range, the sorption
densities observed with As(III) were <3% of those
observed with As(V). At a lower initial As concentration (4 mg/l), low sorption densities were
observed for both As(III) and As(V) with the sorption density for As(III) <25% that of As(V)
(Table 1). The weaker interaction of As(III) as compared with As(V) with Ca±Fe beads is consistent
with observed trends in As(III) and As(V) adsorption onto hydrous ferric oxide (Pierce and Moore,
1982; Wilkie and Hering, 1996). This comparison
supports the hypothesis that As(V) sorption by the
Ca±Fe beads is due to direct interaction between
As(V) and Fe(III).
Bench scale tests were conducted to investigate
the fundamental aspects of As(V) removal from
contaminated water by use of biopolymeric materials, which are selective for oxyanions, economical compared to commercial anion-exchange resins,
and environmentally benign. The experimental conditions studied here were chosen to elucidate the
fundamental aspects of As(V) interaction with the
metal-doped biosorbent and do not necessarily
re¯ect optimal conditions for practical applications.
Mixed Ca±Fe alginate beads were initially formed
with Ca and then further treated with Fe(III) to
optimize both sorptive capacity for As(V) and the
physical properties of the gel beads. The As(V)
sorption by Fe(III)-doped Ca alginate gel beads
was found to be e€ective in removing As(V) over a
wide concentration range (from mg to mg per liter).
Up to 94% removal of As(V) from solution was
achieved for an initial As(V) concentration of
400 mg/l by equilibration with 20 Ca±Fe beads at
pH 4 for 120 h. Under these conditions, the As(V)
sorption density in the Ca±Fe beads was approximately 10% of the saturation value. Slow sorption
kinetics were observed with a time scale for sorption of approximately 100 h. The possible role of
di€usion within the gel matrix in governing sorption kinetics is under investigation. The e€ects of
pH on As(V) sorption were studied over the pH
range from 2 to 6.4. Poor As(V) sorption was
observed at pH 2 with concomitant leaching of a
signi®cant amount of Fe (>60%) leached from the
gel matrix. Leaching of Fe decreased with increasing pH; an optimum combination of Ca±Fe beads
stability and As(V) sorption was achieved at pH 4.
In contrast with the ecient sorption of As(V) at
pH 4, removal of As(III) from solution by Ca±Fe
beads was negligible. The use of macro gel beads
allowed simple modi®cation of total Fe in the system by changing the number of the beads and easy
separation of spent beads from the treated solution.
In practice, some of the limitations associated with
slow sorption kinetics may be overcome by increasing the amount of biosorbent used. The sorption of
other environmentally-signi®cant oxyanions (such
as chromate, selenate and selenite) is also being
examined and will be compared with that of As(V).
AcknowledgementsÐThis work was supported by funding
from the National Science Foundation (BES-9258431 and
BES-9553208), the Fluor Foundation, and the Chevron
Research and Technology Company.
Table 1. E€ects of oxidation state on As sorption
mg As sorbed/g Fe
Initial As concentration
4 mg/l
1 mg/l
4 mg/l
0.031 2 0.002
3.4 2 0.2
7.5 2 0.1
0.132 20.001
121.8 21.3
261.4 22.4
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