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Separation Science and Technology
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RECOVERY OF COPPER (II) AND CHROMIUM (III,VI) FROM
ELECTROPLATING-INDUSTRY WASTEWATER BY ION EXCHANGE
Sibel Yalçina; Reşat Apaka; Jülide Hizala; Hüseyin Afşarb
a Department of Chemistry, Faculty of Engineering, Istanbul University, Avcilar, Istanbul, Turkey b
Department of Chemistry, Faculty of Science and Letters, Yildiz Technical University, Şişli, Istanbul,
Turkey
Online publication date: 31 August 2001
To cite this Article Yalçin, Sibel , Apak, Reşat , Hizal, Jülide and Afşar, Hüseyin(2001) 'RECOVERY OF COPPER (II) AND
CHROMIUM (III,VI) FROM ELECTROPLATING-INDUSTRY WASTEWATER BY ION EXCHANGE', Separation Science
and Technology, 36: 10, 2181 — 2196
To link to this Article: DOI: 10.1081/SS-100105912
URL: http://dx.doi.org/10.1081/SS-100105912
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RECOVERY OF COPPER (II) AND
CHROMIUM (III,VI) FROM
ELECTROPLATING-INDUSTRY
WASTEWATER BY ION EXCHANGE
Sibel Yalçin,
1
Res¸at Apak,
1,*
Jülide Hizal,
1
and
Hüseyin Afs¸ar
2
1
Department of Chemistry, Faculty of Engineering,
Istanbul University, Avcilar, 34850, Istanbul, Turkey
2
Department of Chemistry, Faculty of Science and Letters,
Yildiz Technical University, S¸is¸li, Istanbul, Turkey
ABSTRACT
Two laboratory-scale separation processes have been developed
for the recovery of copper (II) from acidic and cyanide-containing
alkaline wastewater of electroplating industries. Acidic bath
wastes were treated with Dowex 50X8, a strongly acidic cation-ex-
change resin, and the retained copper was eluted with H
2
SO
4
. The
cyanide-containing alkaline bath waste was first oxidized with ex-
cessive hypochlorite, then neutralized, and recovered by the use of
Amberlite IRC-718 chelating resin. Copper was eluted with
H
2
SO
4
.
The two different valencies of chromium have been recov-
ered from electroplating-industry wastewater by different separa-
tion processes: The predominant valency, Cr(VI), was retained on
a strongly basic Dowex 1X8 resin and eluted with a NaCl and
SEPARATION SCIENCE AND TECHNOLOGY, 36(10), 2181–2196 (2001)
Copyright © 2001 by Marcel Dekker, Inc. www.dekker.com
2181
*Corresponding author. E-mail: rapak@istanbul.edu.tr
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NaOH solution. Alternatively, Cr(III), either existing originally in
electroplating-industry waste-rinse mixtures or converted from
Cr(VI) by reduction with Na
2
SO
3
, could be recovered by a weakly
acidic Amberlite IRC-50 resin and eluted with a solution contain-
ing H
2
O
2
and NaOH. Where plating industry wastes contain high
levels of organic contamination, Cr(VI) would be naturally
reduced to Cr(III) upon acidification, and it may be more econom-
ical to recover all chromium as Cr(III).
Key Words: Copper (II); Chromium (VI); Chromium (III);
Recovery; Removal; Electroplating-Industry Wastewater; Ion Ex-
change; Wastewater Treatment
INTRODUCTION
Copper and chromium are important constituents of modern alloys, and
these elements are quite abundant in untreated wastewaters of iron and steel,
leather tanning, metal plating, textile, battery, electrowinning, and metal-finishing
industries. Both elements pose a contamination risk to the natural environment
(1,2) because at trace level amounts they are essential micronutrients to biota
while at more elevated concentrations they may be toxic or carcinogenic (such
as Cr(VI))(3). Chromium speciation basically involves the Cr(III) and Cr(VI)
oxidation states; the latter is of much more of environmental concern because
of its toxicity (4). Thermodynamic and kinetic considerations predict that
[Cr(H
2
O)
5
(OH)]
2
and [Cr(H
2
O)
4
(OH)
2
]
species of the trivalent state and
HCrO
4
and CrO
42
species of the hexavalent state should be the predominant
forms of chromium in natural waters at common pH levels (5,6). The maximum
tolerable limits of Cr(VI) (0.05 mg/L), of Cr(III) (0.17 mg/L), and of Cu(II) (1.0
mg/L) in drinking water have been specified by the World Health Organization
and Environmental Protection Agency standards (7–9).
Ion exchange processes have been frequently used for the treatment of
wastewater containing copper and chromium and for the recovery of these ele-
ments (10). Cr(III) has been removed from leather tanning effluents by a 4-step
adsorption process involving oxidation-reduction reactions between Cr(III) and
Cr(VI) and utilizing anion- and cation-exchanger resin columns (11). In another
study, tanning effluents were treated with a carboxylic-acid cation exchanger for
the removal of Cr(III). Elution followed by regeneration was not carried out by
mineral acids (HCl, H
2
SO
4
, etc.) but by H
2
O
2
in an alkaline medium (12–15) for
preventing surface passivation of the resin beads and prolonging the useful life of
the resin.
Hexavalent chromium held by a strongly basic anion resin was eluted by the
use of strong reducing agents. For performing the reductive elution of Cr(VI), var-
2182 YALÇIN ET AL.
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ious reductant solutions, such as 9% ascorbic acid in 1 M HCl (16), hydroxy-
lamine in 1 M H
2
SO
4
(17), ammonium ferrous sulfate in 1 M HCl (18), 10%
NH
2
OH HCl (19), 5% Na
2
SO
3
in 0.1 M HCl (20), and 0.01M Na
2
SO
3
in 0.1 M
H
2
S0
4
(21), were used.
In cases where reducing agents were not preferred, Cr(VI) elution from the
strongly basic anion exchanger was realized by salt solutions containing relatively
high anion concentrations, such as 2 M KNO
3
(22), 1 M NaCl (18), 1 M KSCN
(23), and 0.4 M NaClO
4
(24). Other than ordinary salts, the use of a 8–10%
Na
2
CO
3
solution or the more effective mixture of Na
2
CO
3
and NaHCO
3
(25,26)
has the advantage of combining basicity with a high anion concentration.
The uptake of chromate-containing wastewater by a strongly basic resin in
OH-form, regeneration of the resin with NaOH, and production of chromic acid
coupled to removal of excessive NaOH by the use of an acidic resin can be repre-
sented by a series of chemical reactions (10):
2ROH H
2
CrO
4
→R
2
CrO
4
2H
2
O
R
2
CrO
4
2NaOH →2ROH Na
2
CrO
4
2RH Na
2
CrO
4
→2 RNa H
2
CrO
4
RH NaOH →RNa H
2
O
When chrome-plated parts are removed from the bath, the adhering film of
chromic acid–plating electrolyte passes to the rinse water. As a result, bath waste
combined with rinse water can be evaluated for the recovery of chromic acid.
As for the recovery of Cu(II) from plating effluents, strongly acidic resins
normally prefer Cu
2
to Na
and other alkali metal cations, but it is not feasible
to recover trace Cu(II) from solutions containing major amounts of alkali metal
ions. In such cases, chelating resins (e.g., having iminodiacetate functional
groups) are very effective for copper recovery, especially when other relatively
stable complex-forming ligands, such as EDTA (ethylenediaminetetraacetate),
citrate, and organic compounds, are present in the solution (10). However, recov-
ery of copper from acidic solution obtained from leaching operations in copper
production is not usually feasible with conventional chelating resins. In that case,
N-(hydroxylalkyl)picolyamine–type resins may be used to concentrate Cu(II)
from acidic solutions, but these resins require much higher acidity to be regener-
ated (27–29).
The main form of copper in cyanide-containing plating-bath wastes is the
Cu(CN)
32
complex species. Although anion exchangers may show a high selec-
tivity for the cyanide complex of copper, special regeneration procedures
employing both an acid and caustic may be required, bringing the risk of generat-
ing extremely toxic HCN gas (10).
In the Istanbul Metropolitan Ikitelli Organized Industrial Area, the electro-
plating industries produce high levels of metal-contaminated wastewater that may
contain heavy metal ions (200–900 ppm), such as Cu(II), Ni(II), Cr(III,VI), as
RECOVERY OF CU(II) AND CR(III,VI) BY ION EXCHANGE 2183
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2184 YALÇIN ET AL.
well as acids, bases, salts, and cyanide. Thus the aim of this work was to design
simple ion-exchange processes (of laboratory scale) to solve the wastewater treat-
ment problems of individual plating plants and to develop combined processes for
the treatment of mixtures (as these bath-rinse effluents are frequently mixed in the
area). Connected to these aims, both simple acidic and cyanide-containing alka-
line bath wastes containing copper were treated. Also, both valencies of
chromium, individually or in mixtures, were treated.
MATERIALS AND METHODS
The chemicals CuCl
2
2H
2
O, CrCl
3
6H
2
O, K
2
Cr
2
O
7
, H
2
O
2
, Na
2
SO
3
,
HCl (concentrated), H
2
SO
4
(concentrated), NaOH, NaCl, CH
3
COONa, NaOCl
were of E. Merck, analytical reagent grade. The ion exchangers used were as
follows: Dowex 50X8 (H-form) strongly acidic cation-exchange resin, Dowex
1X8 (Cl-form) strongly basic anion-exchange resin, Amberlite IRC-50 weakly
acidic cation-exchange resin, and Amberlite IRC-718 weakly acidic, chelating,
cation-exchange resin. The strongly acidic and basic resins were converted to
the Na- and OH-forms when required. The electroplating-industry wastewater
samples were supplied from plants located in the Istanbul Ikitelli Organized In-
dustrial Area.
The ion exchangers filled glass thermostatic columns of 9.5 mm and 25
cm
3
. All pH levels were adjusted by the use of a Metrohm E 512 pH meter
equipped with a glass electrode. Metal (copper and chromium) analyses were per-
formed with a Varian 220AA-Spectrometer using an air-acetylene flame. When
the analysis of a Cr(III) and Cr(VI) mixture was required, Cr(VI) was adsorbed on
a melamine-formaldehyde resin and eluted with 0.1 M sodium acetate. Total
chromium was analyzed after H
2
O
2
oxidation, as described elsewhere (30).
For the recovery of Cu(II) from synthetic, acidic, copper-plating wastewa-
ter with strongly acidic, Dowex 50X8 resin, 1 L of 400 ppm Cu(II) solution
adjusted to pH 1 was passed through a 4 g resin column at 2.6 mL/min. The
capacity of the resin was saturated after the flow of 950 mL Cu(II) solution. (See
Fig. 1.) The retained copper was eluted with 75 mL of 1 M H
2
SO
4
, and the resin
breakthrough capacity was found as mg Cu/g resin. An acidic copper-plating
wastewater sample containing 400 ppm Cu(II) was run simultaneously (Fig. 1).
For the recovery of Cu(II) from synthetic, alkaline, copper-plating waste-
water (containing cyanide) with weakly acidic, chelating, Amberlite IRC-718
resin, the resin capacity was found by passing 700 mL of 450 ppm Cu(II) solution
(at pH 2.5) through 4 g resin at 1.3 mL/min.
For oxidation–ion exchange treatment of synthetic, alkaline, plating
wastewater, 2.7 g CuCl
2
2H
2
O and 3.1 g NaCN were dissolved in water to con-
vert copper to the Cu(CN)
32
complex and leave some free excessive cyanide in
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RECOVERY OF CU(II) AND CR(III,VI) BY ION EXCHANGE 2185
solution. This solution was treated with 85 mL of 2 M hypochlorite solution (con-
taining 37.25 g NaOCl per 250 mL solution) and left overnight. The final alkaline
suspension was treated with 1 M H
2
SO
4
to dissolve the copper (II) precipitate; the
pH was adjusted to 2.5; and 1 L water was added to yield a 1,000 ppm copper
solution. A 450-ml aliquot of this solution was diluted to 1 L with water. Seven
hundred milliliters of this final solution (containing 450 ppm Cu) was passed
through 4 g of resin at 1.3 mL/min to confirm the previously found capacity of the
resin. Simultaneously, a real alkaline wastewater sample containing 450 ppm cop-
per (II) was treated with excessive NaOCl, treated with 1 M H
2
SO
4
to yield a
final pH of 2.5, and passed through the resin column under identical conditions.
(See Fig. 2 for breakthrough curves.) The retained copper from each separation
was eluted with 50 mL of 1 M H
2
SO
4
.
For finding the capacity of the Dowex 1X8 strongly basic resin for chromate
(VI) and for testing the treatability of plating wastewater containing chromate, a
5,000 ppm Cr(VI) stock solution (containing 7.07 g K
2
Cr
2
O
7
per 500 mL) was pH
adjusted and diluted tenfold to yield a 500-ppm Cr(VI) working solution at pH 2;
2,000 mL of this final solution was passed through a 5-g resin column at 2.5
mL/min to yield the breakthrough curves (see Fig. 3). Because an elution trial
Figure 1. Breakthrough curves of synthetic and real acidic copper-plating wastewater on
a Dowex 50X8 strongly acidic cation exchanger. Solution pH : 1.0; Cu(II) concentration:
400 ppm; amount of resin: 4 g; throughput rate: 2.6 mL/min. C/C
o
represents concentration
of Cu in the effluent to that in the influent solution.
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with 1 M NaOH was not efficient, complete elution was realized by using 125 mL
of a mixture solution containing 15% NaCl and 1 M NaOH.
For testing the treatability of Cr(III) effluents with a Dowex 50X8 strongly
acidic resin, 300 ppm of Cr(III) solution (prepared from CrCl
3
6H
2
O) at pH 3.4
was passed through 4 g of resin at 2.6 mL/min. The resin capacity was determined
after eluting the retained chromium with 1.5 M H
2
SO
4
.
For testing the treatability of effluents containing Cr(VI) by the use of a re-
duction-cation exchange procedure, 5 mL of 1 M H
2
SO
4
and 2.5 mL of 2 M
Na
2
SO
3
were added to 30 mL of 5,000 ppm Cr(VI) stock solution (originally at
pH 2). The mixture was diluted to 500 mL with water to obtain a Cr(III) solution
at approximately pH 3. A 625-mL volume of this final solution (containing 300
ppm of converted Cr(III)) was passed through a 5.4-g, Amberlite IRC-50, weakly
acidic resin bed at 1.3 mL/min to obtain the breakthrough curve (See Fig. 4) en-
abling the calculation of the resin capacity. The same reduction was performed on
the Cr(VI)-plating wastewater (originally at pH 2), and 600 mL volume of the fi-
nal 300-ppm Cr(III) solution at approximately pH 3 was enough to saturate the
same amount of resin (Fig. 4). The retained Cr(III) was eluted with 300 mL of an
oxidizing mixture solution consisting of 0.15 M H
2
O
2
and 0.01 M NaOH.
2186 YALÇIN ET AL.
Figure 2. Breakthrough curves of synthetic and real alkaline copper-plating wastewater
(after NaOCl oxidation of cyanide and followed by neutralization) on an Amberlite IRC-
718 weakly acidic cation exchanger. Solution pH: 2.5; Cu(II) concentration: 450 ppm;
amount of resin: 4 g; throughput rate: 1.3 mL/min.
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Figure 3. Breakthrough curves of synthetic and real acidic chrome-plating wastewater on
a strongly basic Dowex 1X8 anion exchanger. Solution pH: 2.0; Cr(VI) concentration: 500
ppm; amount of resin: 5 g; throughput rate: 2.5 mL/min.
Figure 4. Breakthrough curves of synthetic and real chrome-plating wastewater (after
Na
2
SO
3
reduction of chromate in acidic medium and followed by neutralization) on an Am-
berlite IRC-50 weakly acidic cation exchanger. Solution pH: 3.0; Cr(III) concentration: 300
ppm; amount of resin: 5.4 g; throughput rate: 1.3 mL/min.
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2188 YALÇIN ET AL.
All adsorption and desorption data were obtained from 3–5 experiments,
and the efficiencies are given within 95% confidence limits as mean [standard
deviation x t
.95
/ (number of measurements)
1/2
].
RESULTS AND DISCUSSION
Experiments on the treatability of copper-electroplating industry wastewa-
ter obtained from the Istanbul Ikitelli Organized Industrial Area showed that
acidic wastewater could be treated with a Dowex 50X8 strongly acidic, cation ex-
changer, and after the oxidation of bound- and free-cyanide with NaOCl, alkaline
(cyanide-containing) wastewater could be treated with an Amberlite IRC-718
chelating resin. The postulated reaction scheme for oxidation and ion exchange is
as follows:
Cu
2
4CN
→Cu(CN)
32
1/2 (CN)
2
CN
OCl
→CNO
Cl
2CNO
3OCl
H
2
O →2CO
2
N
2
3Cl
2OH
2R-N-(CH
2
COOH)
2
Cu
2
↔[R-N-(CH
2
COO)
2
]
2
Cu 2H
(on chelating resin)
2R-SO
3
H Cu
2
↔(R-SO
3
)
2
Cu 2H
(on strongly acidic resin)
The breakthrough curves presented in Fig.1 enable the calculation of the
strongly acidic resin capacity for Cu(II) at pH 1 as 1.25 mmol/g. The 400 ppm
acidic, plating wastewater that contained Cu (pH 1) was treated with the resin, and
the overall efficiency of uptake and elution was 93.3 (0.43)% (294 mg Cu re-
covery on 4 g resin). When the retained Cu(II) was eluted with 75 mL of 1 M
H
2
SO
4
, the first 50 mL-portion of the eluant was responsible for 99.7 (0.23)%
of the total recovery, while the remaining eluant volume (25 mL) desorbed the rest
of copper (0.3%). Economical reasons may dictate the acid elution method from
acid resins may be preferentially chosen for the recovery of the overwhelming
proportion of Cu(II); as long as its capacity for the metal is not seriously
affected, the resin may be reused for repetitive adsorption-desorption cycles. Gen-
erally, strongly acidic cation exchangers with a high degree of cross-linking have
excellent chemical stability and a life expectancy of up to 5 years (31).
After hypochlorite oxidation of the synthetic alkaline Cu(II) solution con-
taining cyanide, a reaction shown to leave no residual cyanide (32), the chelating
resin (Amberlite IRC-718) capacity for Cu(II) at pH 2.5 was 0.78 mmol/g. This
amount is slightly less than that obtained for pure Cu(II) solution at the same pH
(0.81 mmol). The iminodiacetate-based Amberlite resin is particularly useful for
Cu(II) recovery from complex solutions (33,34) because, per the Irwing-Williams
order, Cu(II) is the strongest complex-forming divalent metal ion among the first-
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RECOVERY OF CU(II) AND CR(III,VI) BY ION EXCHANGE 2189
row transition elements. For example, the selectivity factor of Cu(II) on IRC-718
resin relative to Ca
2
is approximately 2,300 (35). The selection of the working
pH was made on the basis of assuring the dissolution of the hydrolytic copper (II)
precipitate upon hypochlorite oxidation. Above approximately pH 3, the hy-
drolytic Cu(II) precipitate could partly suspend in solution forming a colloidal so-
lution, which would negatively affect overall copper recovery. The breakthrough
curves of pure, synthetic, and real samples are shown in Fig. 2 and represent over-
all copper yield efficiency of 99.0 (0.39)%. Elution of copper from the chelat-
ing resin was made with 50 mL of 1 M H
2
SO
4
, yielding a desorption efficiency of
99.9 (0.25)%.
Chelating ion exchangers, such as the iminodiacetic acid–functionalized
IRC-718 resin, have been tested for stability upon prolonged exposure to EDTA.
After 3,000 cycles at room temperature, the resin compressed and lost 36% of its
original Cu(II)-retention capacity (36).
Chromium (VI) removal from aqueous media by reduction to Cr(III) fol-
lowed by Cr(OH)
3
precipitation, though seemingly practical and cost-effective,
may not be efficient for dilute solutions nor environmentally acceptable because
Cr in the disposed sludge may be oxidized and mobilized into groundwater (37).
Thus, synthetic electroplating-industry wastewater, which consisted of simulated
effluents of an organized industrial area, was used, and removal of individual
metal contaminants by selective ion-exchange processes was both efficient and
environmentally friendly.
Cr(III,VI) recovery from aqueous solution by ion exchange may be more
difficult than expected due to the complexity of Cr speciation (e.g., polymeric
species, sulfato- and other anionic complexes of hydrolytic Cr ions) created as a
function of pH, concentration, and storage time (38).
On quaternary ammonium–functionalized basic resins, a small fraction of
Cr(VI) is reduced to nonadsorbed Cr(III) in H
2
SO
4
solutions of 0.01–4 N (39).
This reaction would negatively affect the separation of Cr as well as the stability
of the resin (40). However, the use the anion exchange resin in the hydroxide form
(at higher pH) may increase the alkalinity of the resin bed, which in turn may en-
tail the precipitation of metal hydroxides (such as Cr(OH)
3
).
Because sulfate and chloride salts would normally accompany chromate in
plating effluents, the selectivity of chromate over these anions is important in ion-
exchange recovery. The preferred resin should have a polystyrene, rather than
polyacrylate, matrix, and high degrees of cross-linking, hydrophobicity, func-
tional group basicity, and longer alkyl groups bound to quaternary amines all in-
crease the selectivity of chromate over sulfate (37).
Generally, as a result of an equilibrium phenomenon characteristic of some
unusual interactions between the anion exchanger and chromate species at acidic
pH, an early Cr(VI) breakthrough is observed when a strongly basic anion ex-
changer is used during a fixed-bed column run. Thus, the total available chromate-
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2190 YALÇIN ET AL.
removal capacity cannot be fully utilized in conventional single-unit, fixed-bed
runs (41). Chromate-sulfate and chromate-chloride isotherms at slightly acidic pH
are unfavorable (concave upward) for two strongly basic anion exchangers and
may also lead to early breakthroughs (37).
Cr(III) recovery by cation exchangers are problematic because of its strong
adsorption on acidic resins, such as Dowex 50, and the need for strong acid elu-
tion (42) or oxidative desorption (43). Chelating resins have also been used with
limited success due to the slow kinetics of Cr(III) retention (44,45) and difficulty
in pH control.
Because Cr-plating effluents are basically composed of hexavalent
chromium, the design of ion exchange treatment was focused on chromate (VI) re-
moval. Typical Cr-plating wastewater (at approximately pH 2) contained 200–900
ppm Cr in the hexavalent form. However, some minor amounts of Cr(III), either
originally present in such effluents or converted from Cr(VI) by reduction with or-
ganic matter, should also be treated. In this regard, strongly basic resins offer
greater Cr(VI) removal capacity at acidic pH as measured by removed Cr atoms
per exchange site (37). The capacity of the strongly basic Dowex 1X8 resin for
500 ppm Cr(VI) synthetic solution at pH 2 was found as 2.78 mmol/g. The Cr(VI)
uptake efficiency of the anionic resin at pH 2 was 98.0 (0.35)%, and the retained
Cr could be quantitatively recovered (with an efficiency of 99.9 (0.27)%) by us-
ing 125 ml of a 15% NaCl and 1 M NaOH mixture solution. (See Fig. 3 for break-
through curves.) A lower concentration of NaCl in the eluting mixture solution
(such as 10% NaCl in 1M NaOH) required a higher volume for quantitative Cr re-
covery; for example, 220 mL of this mixture eluted Cr with 99.8 (0.39 )% effi-
ciency. When NaNO
3
was replaced with NaCl, even better results were obtained;
150 mL of 5% NaNO
3
in 1 M NaOH yielded 99.9 (0.24)% Cr recovery, and 100
mL of 10% NaNO
3
in 1 M NaOH yielded 99.9 (0.20)% Cr recovery. The use
of NaCl (or NaNO
3
) along with NaOH weakens the adsorption tendency of the
trapped Cr(VI) followed by ultimate elution (46). A single eluting solution con-
taining both NaOH and NaCl may deprotonate the ion exchange–held HCrO
4
and exchange CrO
42
with Cl
in the resin phase (37).
A low level of chromate leakage compared to that associated with weakly
basic resins is an advantage of using a strongly basic resin for Cr(VI) removal;
however the necessity of using greater amounts of NaOH in regeneration is a
drawback (29). Quaternary ammonium groups of strongly basic anion exchangers
may be partly converted into tertiary amines on prolonged standing in strongly al-
kaline solutions. The life expectancy of a particular anionic resin may involve
such chemical characteristics as the aqueous influent (e.g., other trace metals,
anions, oxidizing agents, and dissolved O
2
), presence of fouling substances, and
regeneration level (31). Then the treatability of Cr(III) contaminants was investi-
gated by passing 750 mL of 300 ppm Cr(III) solution (prepared from CrCl
3
6H
2
O) through 4 g of Dowex 50X8 strongly acidic resin bed at pH 3.4 and using
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RECOVERY OF CU(II) AND CR(III,VI) BY ION EXCHANGE 2191
a throughput rate of 2.6 mL/min. A breakthrough capacity of 0.83 mmol/g was
found for the strongly acidic resin.
The uptake of Cr(III) by the strongly acidic cation-exchanger resin strongly
depended on the aqueous phase speciation of Cr(III) with respect to pH. The
Cr(III) uptake efficiency at pH 2.7 was 98.3%. The species distribution calcula-
tions for Cr(III) within the studied concentration (300 ppm total Cr(III) (47)) re-
vealed that 92% of Cr(III) is solely in the [Cr(H
2
O)
6
]
3
form while 8% is in the
hydroxo-complex [Cr(H
2
O)
5
OH]
2
form. However, Cr(III) recovery efficiency
was 99.9 (0.18)% at pH 3.4 and the speciation diagram indicated that 70% was
[Cr(H
2
O)
6
]
3
and 30% was [Cr(H
2
O)
5
OH]
2
. The [Cr(H
2
O)
6
]
3
-
[Cr(H
2
O)
5
OH]
2
conjugate acid–base buffer system may prevent the extreme pH
drop that would normally occur as a result of the H
release associated with Cr
3
uptake (3 H
liberated per Cr
3
retained), thereby maintaining Cr removal effi-
ciency. In regard to overall Cr(III) removal efficiency, best results were obtained
in buffer media containing both [Cr(H
2
O)
6
]
3
and [Cr(H
2
O)
5
OH]
2
species at
appreciable levels. When the basicity was increased to pH 4.8, the Cr(III) uptake
efficiency was maintained at 99.9 (0.14)%, but the relative amount of
[Cr(H
2
O)
4
(OH)
2
]
should have increased at the same time. The anticipated
increase in [Cr(H
2
O)
4
(OH)
2
]
was expected to give rise to some surface-precipi-
tated hydroxo-Cr(III) species, e.g., Cr(OH)
3
, which would be adsorbed irre-
versibly on the resin and would not be recovered by acid elution. Though the
Cr(III) uptake efficiency was about the same for both 3.4 and 4.8 pH values, Cr
desorption proved to be lower for the latter. At pH 3.4, the retained Cr(III) could
be eluted with 140 ml of 1.5 M H
2
SO
4
at a throughput rate of 1 mL/min. The first
100-mL portion of the sulphuric acid solution released 99.0 (0.46)% of bound
chromium while the rest (40 mL) eluted an additional 0.6%, making the overall
recovery 99.6%. For 100% elution, a total acid volume of 170 mL was necessary.
This need for additional eluent is not surprising because Cr(III) has a strong ad-
sorption affinity for the strongly acidic resin (42). The Cr(III) retained at pH 4.8
required more concentrated H
2
SO
4
for desorption than did the Cr(III) retained at
the lower pH; that is, at pH 4.8, 100 mL of 3M H
2
SO
4
, flowing at 1 mL/min, was
needed to release 99.0 (0.47)% Cr. For successful Cr(III) release, the Cr-satu-
rated resin should be contacted with acid for nonprolonged periods. If the resin is
left standing in acid, the hydrolyzed and surface-precipitated hydroxo-Cr(III)
species would be irreversibly adsorbed.
In addition to adsorption-desorption studies with synthetic Cr(III) solu-
tions, the treatability of Cr(VI), the predominant form of Cr in plating effluents,
by reduction with Na
2
SO
3
to Cr(III) and cation-exchange recovery of the latter
was tested. After the reduction of Cr(VI) in acidic medium according to the
reaction
Cr
2
O
7
2
3SO
3
2
8H
→2Cr
3
3SO
4
2
4H
2
O
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ORDER REPRINTS
2192 YALÇIN ET AL.
the recovery of the converted Cr(III) by the use of Dowex 50X8 resin was tried.
The recovery efficiency was 10% for 300 ppm (converted) Cr(III) solution, which
is much less than that for pure Cr(III) solutions. This low efficiency was attributed
to complexes formed by SO
42
and Cr(III). Some anions, such as sulfate, phos-
phate, acetate, and oxalate are known to form relatively stable complexes with Cr
and prevent its recovery (3). Thus, Cr(III) removal from real solutions by the use
of strongly acidic cation exchanger would not be a suitable process, and treatabil-
ity of Na
2
SO
3
-reduced solutions was further tested by using Amberlite IRC-50
weakly acidic cation-exchange resin. Because this resin in H
form could not
quantitatively recover Cr(III), the weakly acidic resin was converted to the Na
form by treating it with 0.1 M NaOH. The necessity for the partial conditioning of
weakly acidic cation exchangers in the salt form by means of NaOH treatment has
been reported in literature (48). The RCOOH weak acidic groups of the resins
could, at pH pK
a
, decationize wash solutions of galvanized products that had
been submitted to acid metal plating.
Preliminary experiments revealed that the IRC-50 resin was successful for
Cr(III) removal in a sulfated medium. For Na
2
SO
3
-reduced Cr solutions contain-
ing 300 ppm Cr(III), the breakthrough capacity of the resin was 0.47 mmol/g at
pH 3 and a throughput rate of 1.3 mL/min. Cr(III) converted from real wastewa-
ter by Na
2
SO
3
reduction was tested under identical conditions as the preliminary
experiment to yield the breakthrough curves, shown in Fig. 4, at a Cr-removal ef-
ficiency of 98.3 (0.38)%.
Some irreversible adsorption on the resin was observed, and the use of a
0.15 M H
2
O
2
and 0.01 M NaOH mixture as eluant was required (12,13). Oxida-
tive elution (49) effectively overcame the difficulties encountered in Cr(III) des-
orption (50). The retained Cr(III) could be quantitatively released upon oxidation
to Cr(VI) in alkaline medium because the CrO
42
anion would be rejected by the
cation exchanger. At 0.8 mL/min, 300 mL of a 0.15 M H
2
O
2
and 0.01 M NaOH
mixture solution was capable of eluting chromium in the hexavalent state at an ef-
ficiency of 95.1 (0.36)%. For completely recovering Cr remaining in the solid
resin phase, 100 more mL of the eluant was necessary. Practically all the Cr (98.1
(0.49)% ) retained by the resin could be eluted using 130 mL of the 0.15 M
H
2
O
2
and 0.1 M NaOH mixture solution. However, the higher concentration of
alkali, in the presence of the H
2
O
2
oxidant, could react with the resin particles over
an extended period causing some structural changes in the resin. As a result, use
of this mixture is not a recommended approach to achieve higher efficiency. An-
other method of using more dilute (e.g., 0.001 M NaOH) alkali combined with
0.15 M H
2
O
2
yielded unquantitative recovery results. Therefore, 400 mL of the
0.15 M H
2
O
2
and 0.01 M NaOH mixture solution was determined as the optimal
volume and eluant, respectively.
Amberlite IRC-50 carboxylic-acid resins possess good chemical stability in
acid, neutral, and alkaline solutions, and are relatively resistant to oxidizing agents
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RECOVERY OF CU(II) AND CR(III,VI) BY ION EXCHANGE 2193
(51). Their oxidative degradation rate is inversely proportional to the degree of
cross-linking, e.g., 1.4 10
2
meq g
1
h
1
for 8% divinylbenzene containing
types (31). It was suggested by Petruzelli et al. (12,13) that the carboxylate func-
tional abilities of the weakly acidic resins, once converted into polyvalent metal
form, could not be easily regenerated by using conventional chemicals, such as
acids and bases, because partial regeneration by mineral acids caused surface pas-
sivation of the resin beads. The passivity was created by sudden conversion of the
outer shells in H-form,
(RCOO)
3
Cr 3H
←
→3RCOOH Cr
3
Recovery of chromic species by the use of NaOH was also inefficient due to the
strong affinity of the RCOO
group for polyvalent metal (Cr(III)) species. Thus,
the following reactions can be proposed for the uptake and release of Cr(III):
3RCOONa Cr
3
→(RCOO)
3
Cr 3Na
2(RCOO)
3
Cr 3H
2
O
2
10NaOH →2Na
2
CrO
4
6RCOONa 8 H
2
O
Also, the same researchers (12,13) found the use of weakly acidic resin in
the Na form the most beneficial for Cr(III) uptake. As observed in this study, some
kind of a Brønsted buffer (RCOOH resin partly in Na form at optimal working pH,
i.e., pH 3) would be necessary to prevent the sudden decrease in pH that would re-
tard further metal uptake if only the H-form of the resin were used (3H
ions
would be normally liberated per every Cr
3
held by the adsorbent). Weakly acidic
resins are very sensitive to solution pH and are not able to take up metal ions in
acidic environments, i.e., at pHpKa of RCOOH (29).
CONCLUSIONS
A combined treatment scheme for the recovery of copper and chromium
from electroplating-industry wastewater has been proposed that is applicable to
small and medium metal-plating plants. Thus, when the amounts of rinse water are
relatively small, recycling of copper and chromic acid may be economical in such
plants, and consequently, the municipal sewage system would not be severely
contaminated with these toxic pollutants. The proposed treatment system is also
environmentally superior to the classical Cr(VI) reduction followed by Cr(OH)
3
precipitation with lime in which Cr in the disposed sludge may be oxidized and
mobilized into groundwater.
Two laboratory scale processes have been developed for copper: Cu(II)
from acidic plating wastewater is recovered by a strongly acidic cation exchanger
and eluted with H
2
SO
4
, while cyanide-containing alkaline plating effluents are
first oxidized with NaOCl and neutralized, and Cu(II) is retained on an iminodi-
acetate-type chelating resin and eluted by H
2
SO
4
. The selective uptake of Cu(II)
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ORDER REPRINTS
2194 YALÇIN ET AL.
by the iminodiacetate-based resin from a complex mixture (i.e., alkaline bath
waste constituents) has been fully exploited.
Two laboratory scale processes have been designed for the two valencies of
Cr. Cr(VI) from acidic plating waste-rinse water is taken up by a strongly basic
anion exchanger and eluted with a NaOH and NaCl mixture. Both original and re-
duced Cr(III) can be recovered by the use of a weakly acidic cation exchanger at
optimal pH and eluted by H
2
O
2
and NaOH, which also provides for resin-saving
oxidative regeneration. The combined process capable of treating both valencies
of Cr-contaminants is useful because plating bath-rinse effluents are frequently
mixed in the organized industrial area studied in the presented research and in
those in other countries (52).
Chromium (VI) was recovered at acidic pH on a strongly basic resin due to
increased chromate selectivity at this pH. However, the total chromate removal
capacity of such resins cannot be fully utilized in conventional single-unit fixed-
bed runs because other competing ions in the influent (SO
42
, Cl
, etc.) would
always lead to an early, gradual CrO
42
breakthrough at acidic pH. Therefore, the
original CrO
42
was reduced with Na
2
SO
3
to Cr(III) and total Cr ( initial and re-
duced Cr(III)) was recovered with a weakly acidic cation exchanger.
Because both Cr(III), on a weakly acidic cation exchanger and Cr(VI), on a
strongly basic anion exchanger resin, would show high adsorption affinities,
multi-complex eluting solutions were used to weaken affinities; for example, the
H
2
O
2
and NaOH solution affected Cr(III) release by oxidation to the cation ex-
changer–rejected chromate, and NaOH and NaCl mixture affected Cr(VI) release
by acid-base neutralization. Quantitative redox and neutralization reactions con-
stitute the driving force of these complex elutions by shifting weak equilibria to
completion.
ACKNOWLEDGMENT
The authors would like to thank the Research Fund of Istanbul University
for supporting this work under Project 1194/070998.
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Received March 2000
Revised July 2000
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