Environ. Sci. Technol. 2004, 38, 4860-4864
Reduction of Hexavalent Chromium
with the Brown Seaweed Ecklonia
Biomass
DONGHEE PARK,†
YEOUNG-SANG YUN,‡ AND
J O N G M O O N P A R K * ,†
Department of Chemical Engineering,
Pohang University of Science and Technology, San 31,
Hyoja-dong, Pohang 790-784, Korea, and
Division of Environmental and Chemical Engineering and
Research Institute of Industrial Technology,
Chonbuk National University, Chonbuk 561-756, Korea
A new type of biomass, protonated brown seaweed
Ecklonia sp., was used for the removal of Cr(VI). When
synthetic wastewater containing Cr(VI) was placed in contact
with the biomass, the Cr(VI) was completely reduced to
Cr(III). The converted Cr(III) appeared in the solution phase
or was partly bound to the biomass. The Cr(VI) removal
efficiency was always 100% in the pH range of this study
(pH 1 ∼ 5). Furthermore, the Cr(VI) reduction was
independent of the Cr(III) concentration, the reaction
product, suggesting that the reaction was an irreversible
process under our conditions. Proton ions were consumed
in the ratio of 1.15 ( 0.02 mol of protons/mol of Cr(VI),
and the rate of Cr(VI) reduction increased with decreasing
the pH. An optimum pH existed for the removal efficiency
of total chromium (Cr(VI) plus Cr(III)), but this increased
with contact time, eventually reaching approximately pH 4
when the reaction was complete. The electrons required
for the Cr(VI) reduction also caused the oxidation of the organic
compounds in the biomass. One gram of the biomass
could reduce 4.49 ( 0.12 mmol of Cr(VI). From a practical
viewpoint, the abundant and inexpensive Ecklonia biomass
could be used for the conversion of toxic Cr(VI) into less toxic
or nontoxic Cr(III).
Introduction
Chromium and its compounds are widely used in industry,
with the most usual and important sources coming from the
electroplating, tanning, water cooling, pulp producing, and
ore and petroleum refining processes (1). The effluents from
these industries contain both Cr(VI) and Cr(III) in concentrations ranging from tens to hundreds of mg/L. Cr(VI) is
known to be toxic to both plants and animals, as a strong
oxidizing agent and potential carcinogen (2). In contrast,
Cr(III) is generally only toxic to plants in very high concentrations and is less toxic, or nontoxic, to animal (3). Because
of these differences, the discharge of Cr(VI) to surface water
is regulated to below 0.05 mg/L by the U.S. EPA, while total
Cr, including Cr(III), Cr(VI), and its other forms, is regulated
to below 2 mg/L. Therefore, the reduction of Cr(VI) is
* To whom correspondence should be addressed. Phone: +8254-279-2275. Fax: +82-54-279-2699. E-mail: [email protected].
† Pohang University of Science and Technology.
‡ Chonbuk National University.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 18, 2004
imperative to meet the discharge levels, and recycling and
reuse are promoted. Consequently, the removal of Cr(VI)
from industrial wastewater has attracted much research
interest.
Existing chemical treatment processes for the lowering of
Cr(VI) concentrations generally involve the aqueous reduction of Cr(VI) to Cr(III) using various chemical reagents, with
the subsequent adjustment of the solution pH to near-neutral
conditions, for the precipitation of the Cr(III) ions produced.
However, these methods have been considered undesirable
due to the use of expensive and toxic chemicals, poor removal
efficiency for meeting regulatory standards, and the production of large amounts of chemical sludge (4, 5). An alternative
for dealing with the problem of Cr(VI) wastewater may be to
remove Cr(VI) via sorption-based processes, wherein synthetic resin (6), activated carbon (7), fly ash-wollastonite (8),
carbon slurry (9), inorganic sorbent materials (10), or the
so-called biosorbents derived from dead biomass. Of these,
biosorbents are considered the cheapest, most abundant,
and environmentally friendly option (4, 5). Because of these
advantages, there has been extensive research exploring
appropriate biosorbents able to effectively remove Cr(VI),
such as sawdust (11-14), moss peat (15), agricultural
byproduct (16-18), food industrial waste (19), plants (2022), fungi (23-26), bacteria (27-29), microalgae (30-33),
and seaweed (4, 5, 34).
Most reports claim that the Cr(VI) was removed from
aqueous systems by anionic adsorption (12-14, 17-19, 2134). Some researchers have reported that the removal of Cr(VI)
was partly through reduction, as well as anionic adsorption,
and the partial reduction could take place only under strongly
acidic conditions (pH < 2.5) (4, 5, 11, 15, 16, 20).
This study presents a new type of biomaterial, protonated
biomass of brown seaweed Ecklonia, capable of completely
removing Cr(VI) even at pH 5.0, and the underlying Cr(VI)
removal mechanism has been elucidated. The factors affecting the Cr(VI) removal efficiency have been investigated.
Materials and Methods
Preparation of the Biomass. The brown seaweed, Ecklonia
sp., was collected along the seashore of Pohang, Korea. In
our previous study (35), this seaweed biomass was shown to
be a good biosorbent of Cr(III). After swelling and rinsing
with deionized-distilled water, the sun-dried biomass was
cut into approximately 0.5 cm sized pieces. The cut biomass
was treated with a 1 M H2SO4 solution for 24 h, which replaced
the natural mix of ionic species with protons and sulfates.
The acid-treated biomass, designated as protonated biomass
in this article, was washed with several times with deionizeddistilled water and then oven dried at 80 °C for 24 h. The
resulting dried biomass was later stored in a desiccator and
used for the following experiments.
Cr(VI) Removal Experiments. The Cr(VI) removal experiments were carried out in flasks. The following set of
factors was chosen as the standard conditions: 5 g/L biomass,
100 mg/L initial Cr(VI) concentration, 0 mg/L initial Cr(III)
concentration, 200 mL working volume, and pH 2 at room
temperature (20 ∼ 25 °C). Sufficient solution/biomass contact
time was allowed until the Cr(VI) concentration remained
unchanged, ranging from hours to weeks.
The stock solutions of Cr(VI) and Cr(III) used in all the
experiments were made by dissolving analytical grade K2CrO4 (Kanto) and CrCl3‚6H2O (Sigma), respectively, in deionized-distilled water and were freshly prepared every time.
Each trial was performed by bringing into contact the desired
amount of biomass with 200 mL of a Cr(VI) solution of known
10.1021/es035329+ CCC: $27.50
 2004 American Chemical Society
Published on Web 08/04/2004
concentration in a 500 mL Erlenmeyer flask. The flasks were
agitated on a shaker at 200 rpm. In the pH-stat experiments,
the solution pH was maintained at the desired value using
0.5 M H2SO4 or 1 M NaOH solutions. The changes in the
working volume due to NaOH or H2SO4 addition were
negligible. Meanwhile, in the pH-shift experiments, the
solution pH was not adjusted after bubbling of N2 gas to
remove O2 and CO2 from the system, but the final pH was
measured for comparison with the initial pH. With the
exception of the experiments conducted under biomasslimited conditions, the experiments were continued until
the Cr(VI) had been completely removed.
Samples for Cr(VI) and total chrome concentration
analyses were intermittently removed from the flasks and
appropriately diluted. The total volume of withdrawn samples
never exceeded 2% of the working volume.
XPS (X-ray Photoelectron Spectroscopy) Analysis. XPS
was employed to determine the valence state of the Cr bound
on the biomass. The Cr-laden biomass was obtained through
contact with 200 mg/L Cr(VI) at pH 2.0 for 2 days, while the
Cr(III)-laden biomass was obtained through contact with
200 mg/L Cr(III) at pH 4.0 for 2 days. Prior to mounting for
XPS, the biomass was washed with deionized-distilled water
several times and then freeze-dried. The resulting biomass
was transported to the spectrometer in a portable, gastight
chamber. CrCl3‚6H2O (Sigma) and K2CrO4 (Kanto) were used
as Cr(III) and Cr(VI) reference compounds, respectively.
Spectra were collected on a VG Scientific model ESCALAB
220iXL. A consistent 2 mm spot size was analyzed on all
surfaces using a MgKR (hλ ) 1253.6 eV) X-ray source at 100
W and pass energy of 0.1 eV for 10 high-resolution scans.
The system was operated at a base pressure of 2 × 10-8 mbar.
The calibration of the binding energy of the spectra was
performed with the C 1s peak of the aliphatic carbons, which
is at 284.6 eV.
Chromium Analysis. A colorimetric method, as described
in the standard methods (36), was used to measure the
concentrations of the different Cr species. The pink colored
complex, formed from 1,5-diphenycarbazide and Cr(VI) in
acidic solution, was able to be spectrophotometrically
analyzed at 540 nm (Spectronic 21, Milton Roy Co.). To
estimate for total Cr, the Cr(III) was first converted to Cr(VI)
at high temperature (130 ∼ 140 °C) by the addition of excess
potassium permanganate prior to the 1,5-diphenycarbazide
reaction. The Cr(III) concentration was then calculated from
the difference between the total Cr and Cr(VI) concentrations.
The detection limit of this method was 0.03 mg/L.
Results and Discussion
Reduction of Cr(VI) into Cr(III). To examine the Cr(VI)
removal characteristic of the Ecklonia biomass, the Cr
concentrations and pH profiles were investigated, with no
pH adjustment (Figure 1). The Cr(VI) concentration was found
to sharply decrease and was removed to be below the lower
limit of detection for analytical method employed. Meantime,
the Cr(III), which was not initially present, appeared in
solution and increased in proportion to the Cr(VI) depletion.
These results indicated that the Cr(VI) was reduced into Cr(III)
when brought into contact with the biomass.
The solution pH, within the initial few minutes, decreased
abruptly from 2.00 to 1.94, then increased, and finally
equalized to 2.07 after 6 h of contact time. In a control
experiment, with no Cr(VI) present, the pH also decreased
abruptly, from 2.00 to 1.92, in the first few minutes, but
subsequently remained unaltered (data not shown). Thus,
the initial sharp decrease in the solution pH was considered
as a result of the efflux of protons from the protonated
biomass. While, the increase of the solution pH then likely
to be related to the removal of Cr(VI). In the region where
the solution pH increased, the amount of protons that
FIGURE 1. Dynamics of Cr(VI) removal by protonated Ecklonia
biomass during pH-shifting experiments. Conditions: 100 mg/L initial
Cr(VI) concentration, 5 g/L biomass concentration, initial pH 2.0.
Symbols: (O) Cr(VI); (2) Cr(III); and (3) pH.
FIGURE 2. Cr 2p spectra of the Cr-laden biomass and Cr(III)-laden
biomass; the former was obtained after Cr(VI) biosorption at pH 2.0,
and the latter was obtained after Cr(III) biosorption at pH 4.0.
disappeared was proportional to the amount of Cr(VI)
removed, reflecting the proton ion participation in the
removal of the Cr(VI). Thus, it could be expected for the
removal rate of Cr(VI) to increase as the solution pH
decreased.
After the complete Cr(VI) removal, the final Cr(III)
concentration in the solution remained at 48 mg/L, indicating
that 52 mg/L of total Cr was removed from the solution. It
could be assumed that the total Cr removed was bound to
the biomass. To characterize the valence state of the Cr bound
on the biomass, X-ray photoelectron spectroscopy (XPS) was
employed. Low-resolution XPS spectra of the Cr-unloaded
biomass indicated that other than C, N, and O, no significant
contributions were present from other elements associated
with biomass surfaces. High-resolution spectra collected from
the Cr 2p core region indicated that indeed there was no Cr
associated with the biomass surfaces (data not shown).
However, high-resolution spectra of the Cr-laden biomass
and the Cr(III)-laden biomass indicated that there were
significant contributions of the Cr bound on the biomass
(Figure 2). Significant bands appeared at binding energies of
577.0 ∼ 578.0 and 586.0 ∼ 588.0 eV; the former corresponds
to Cr 2p3/2 orbitals, the latter to Cr 2p1/2 orbitals. The Cr 2p3/2
orbitals are assigned at 577.2 eV (CrCl3) and 576.2 ∼ 576.5
eV (Cr2O3) for Cr(III) compounds, while Cr(VI) forms are
characterized by higher binding energies such as 578.1 eV
(CrO3) or 579.2 eV (K2Cr2O7) (37). The spectra of the Cr-laden
biomass was same to that of the Cr(III)-laden biomass. There
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FIGURE 3. Effect of the initial Cr(III) concentration on the Cr(VI)
reduction. Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L
biomass concentration. Symbols: (O, b) initial Cr(III) ) 0 mg/L; (4,
2) initial Cr(III) ) 50 mg/L; (3, 1) initial Cr(III) ) 100 mg/L; open
symbols are Cr(VI); and closed symbols are Cr(III).
have been previous studies (16, 38) on the Cr species bound
to the biomass. Lytle et al. (38) reported that Cr(VI) taken
from the fine lateral roots of wetland plants was rapidly
reduced to Cr(III). Cardea-Torresdey et al. (16) reported that
Cr(VI) could be bound to an oat byproduct, but easily reduced
to Cr(III) by positively charged functional groups, and
subsequently adsorbed by available carboxyl groups. These
studies reached the same conclusions as this study, that is,
the bound Cr was in the Cr(III) state. Therefore, it can be
concluded that the Cr(VI) was removed from aqueous phase
and finally reduced to Cr(III).
Irreversibility of Cr(VI) Reduction. To investigate the
effect of Cr(III) on the Cr(VI) reduction, the initial Cr(III)
concentration was varied from 0 to 100 mg/L (Figure 3). The
presence of Cr(III) did not affect the Cr(VI) reduction. In
addition, no reoxidation of the reduced Cr(III) occurred in
these experiments. Therefore, it can be concluded that the
reduction of Cr(VI) by the Ecklonia biomass was an irreversible process under our experimental condition. From a
practical viewpoint, this result might encourage the application of the Ecklonia biomass as a Cr(VI) remover because
there was no interference of the final reaction product (Cr(III))
in the reduction of Cr(VI).
Effect of pH. As shown in the pH-shifting experiments
(Figure 1), protons were consumed during the Cr(VI) removal;
therefore, the Cr(VI) removal was studied at various solution
pHs (Figure 4). As seen in Figure 4, the Cr(VI) removal rate
increased with decreasing pH. The contact time required for
complete Cr(VI) removal varied from 12 to 480 h and was pH
dependent. In all the experiments conducted in this study,
the Cr(VI) was completely removed from aqueous phase,
even at pH 5. However, the removal efficiency of the total
Cr was 16.7% at pH 1, 73.2% at pH 3, and 57.2% at pH 5,
respectively. The existence of optimum pH for total Cr
removal will be discussed later. Although many previous
studies (10, 11, 13-17, 20-23) have shown the Cr(VI) removal
rate to increase with decreasing pH, no nonliving biomass
capable of completely removing Cr(VI) at pH 5 has been
reported.
Proton Consumption with Cr(VI) Reduction. To evaluate
the amount of protons required for Cr(VI) reduction, pHshift batch experiments were conducted under uniform initial
solution pH and biomass concentration conditions, while
the initial Cr(VI) concentration was varied from 0 to 200 mg/
L. To avoid the pH variation by carbon dioxide from air, the
experiments were carried out in a N2 environment. Figure
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 18, 2004
FIGURE 4. Dynamics of Cr(VI) removal by protonated Ecklonia
biomass at various pHs. Conditions: 100 mg/L initial Cr(VI)
concentration, 5 g/L biomass concentration. Symbols: (O) pH 1.0;
(2) pH 3.0; and (3) pH 5.0.
FIGURE 5. Ratio of proton consumption to reduced Cr(VI). Conditions: 5 g/L biomass concentration, 0 ∼ 200 mg/L of initial Cr(VI)
concentration. Symbols: (O) final pH and (9) amount of proton
consumed vs amount of Cr(VI) reduced.
5 shows that final solution pH increased as increasing the
amount of reduced Cr(VI). The proton consumption ratio
required for the reduction of Cr(VI) can be calculated by
subtracting the proton concentration of each trial from that
of the control. As shown in Figure 5, the ratio was almost
constant when reduced Cr(VI) was less than 2 mmol/L.
However, as reduced Cr(VI) increased to more than 2 mmol/
L, the ratio deviated from the constant. This likely resulted
from the complex participation of the protons in following
reactions: (a) protons can be released, or bound, from, or
to, various functional groups of the biomass depending on
the solution pH (35). (b) Protons are consumed during the
reduction of Cr(VI) as follows:
Cr2O72- + 14H+ + 6e- T 2Cr3+ + 7H2O
CrO42- + 8H+ + 3e- T Cr3+ + 4H2O
E° ) +1.33V (1)
E° ) +1.48V
(2)
HCrO4- + 7H+ + 3e- T Cr3+ + 4H2O
E° ) +1.35V (3)
H2CrO4 + 6H+ + 3e- T Cr3+ + 4H2O
E° ) +1.33V (4)
(c) Protons are released from the biomass during Cr(III)
binding via proton-Cr(III) ion exchange (35). In the case of
protonated Ecklonia biomass, the functional group related
to the interaction between proton and Cr(III) is a carboxyl
TABLE 1. Cr(VI) Removal under Biomass Limiting Conditiona
biomass concentration [g/L]
initial Cr(VI) concentration [mg/L]
pH [-]
final Cr(VI) concentration [mg/L]
Cr(VI) reduced/biomass [mmol/g]
0.0515
100
2.00
88.3
4.37
0.1685
100
2.00
59.6
4.61
a Experiments were conducted until the Cr(VI) concentration did not
nearly change (30 days).
group having a pKa of 4.6 ( 0.1 (35). Thus, the effects of
reactions a and c might have been relatively small below pH
3.0 as compared to reaction b. However, these effects cannot
be ignored at pH > 3, where Cr(III) can easily occupy the
functional groups by replacing protons. The decrease of the
proton/Cr(VI) ratio could be explained with the preferential
binding of the Cr(III) to the biomass at the high final pH
resulted from the reduction of high concentration of Cr(VI).
Since acidic conditions are practically favorable for the
fast removal of Cr(VI), the constant ratio (1.15 ( 0.02 mol of
proton consumption per mol of reduced Cr(VI)) at a low
final pH range (pH < 3) could be used for a rough estimation
of the proton consumption per Cr(VI) reduced in practical
operations.
Oxidation of the Biomass and Available Electrons. For
the reduction of Cr(VI) to Cr(III), not only protons, but also
electrons are required. The electrons required for the
reduction of Cr(VI) were possibly supplied from the biomass,
resulting in the oxidation of the organic compounds of the
biomass, resulting in the partial release of soluble organics.
When the biomass was long-term contacted with Cr(VI), there
was distinct increases in concentrations of dissolved organic
compounds and inorganic carbons (e.g., dissolved CO2,
HCO3-) in the solution phase, as compared with the Cr(VI)
free control (data not shown). The appearance of inorganic
carbons in the effluent implied that parts of organic carbons
of the biomass were completely oxidized into CO2. Furthermore, it was observed that the surface of the biomass was
rougher after contact with Cr(VI) (data not shown).
To evaluate the available electrons able to be supplied
from the biomass, Cr(VI) was brought into contact with a
small amount of the biomass (Table 1). In these biomasslimited experiments, some of the Cr(VI) could possibly remain
after the biomass was completely oxidized. As a result, 1 g
of the biomass could reduce 4.49 ( 0.12 mmol of Cr(VI) at
pH 2. In other words, since 3 mol of electrons are required
for the reduction of 1 mol of Cr(VI) to Cr(III), it can be
suggested that 1 g of the biomass possessed 13.47 ( 0.36
mmol of available electrons. From a practical viewpoint, only
223 g of Ecklonia biomass is required for the reduction of 1
mol of Cr(VI), while 834 g of FeSO4‚7H2O, which is a common
Cr(VI) reductant, is required for the same level of reduction.
Effect of Temperature. Temperature may play an important role in the reduction of Cr(VI) to Cr(III). Therefore,
batch experiments were performed at pH 2.0 to examine the
temperature dependency of Cr(VI) reduction by the biomass
in the range of 5 ∼ 45 °C (Figure 6). The increase of
temperature greatly increased the Cr(VI) removal rate. Some
previous studies (11, 15, 29) supported our results. In general,
the increase of temperature induces the rate of a redox
reaction (39).
Discussion on Existence of Optimum pH for the Removal
of Total Cr. The removal efficiency of total Cr in aqueous
phase was investigated at various pHs and with different
contact times (Figure 7). Removal efficiency was calculated
from the mass balance for total Cr in aqueous phase. As a
matter of course, the removal efficiency of total Cr increased
with increasing contact time. An optimum pH existed for the
total Cr removal efficiency at a fixed contact time. However,
FIGURE 6. Effect of the temperature on the Cr(VI) reduction.
Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L biomass
concentration, pH 2.0. Symbols: (O) 5 °C; (9) 15 °C; (4) 25 °C; (1)
35 °C; and (0) 45 °C.
FIGURE 7. Optimum pH for the removal efficiency of total Cr.
Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L biomass
concentration. Symbols: (O) 1 h; (9) 12 h; (4) 48 h; and (1) 480 h.
Cr(VI) in all the experiments was completely removed within 480
h.
the optimum pH increased with increasing contact time,
eventually reaching approximately 4 on completion of the
reaction. The reason the optimum pH increased can be
explained as follows: at a high pH, Cr(VI) is very slowly
reduced to Cr(III), and Cr(III) is easily bound to the biomass.
Thus, as the pH increases, the reduction rate of Cr(VI) is
rate-limited: as soon as Cr(III) is formed from Cr(VI), it is
bound to the biomass. However, since Cr(VI) reduction is
irreversible, although slow at a high pH, as the contact time
increases, Cr(VI) can be completely reduced to Cr(III), which
is then removed by biosorption. The question as to why the
optimum pH was 4 was likely due to the release, or
destruction, of Cr(III)-binding sites during the oxidation of
the biomass. The release of organic compounds from the
seaweed biomass is known to increase at elevated pHs (40).
The soluble form of Cr(III) binding sites, such as carboxyl
groups, can form complexes with Cr(III), which may still
exist in the aqueous phase.
The existence of an optimum pH for the efficient removal
of total Cr by the seaweed, Sargassum, biomass has previously
been observed (4, 5). However, these reports showed that
the optimum pH for the efficient removal of total Cr was in
the vicinity of 2 ∼ 2.5. To accurately evaluate the optimum
pH conditions, sufficient contact time should also be
considered. In our experiments, the complete reaction
required 480 h, while only 6 h was given in the previous
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reports (4, 5). It was interesting to note that the optimum pH
after 6 h of contact time was also between 1.5 and 2.5 in our
experiments.
Acknowledgments
This work was financially supported by the KOSEF through
the AEBRC at POSTECH and partially by Grant R08-2003000-10987-0 from the Basic Research Program of the KOSEF.
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Received for review November 30, 2003. Revised manuscript
received May 30, 2004. Accepted June 28, 2004.
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