Decoloration of azo and triarylmethane dyes in the aqueous phase by catalytic hydrotreatment with Pd supported on pillared clays

Article abstract of DOI:10.1039/c6ra24542k

Catalytic hydrotreatment of synthetic dyes as azo compounds and triarylmethanes has been carried out under ambient-like conditions (25-50 °C, 1 atm) with H₂ using Pd supported on Al-pillared clays and Al₂O₃ as catalysts. The treatment leads to complete decoloration of aqueous solutions of the azo dyes (azobenzene, methyl orange, Congo red and oil red) due to the catalytic hydrogenation of the N=N bonds. Under acid pH the color removal was enhanced and a solid residue was formed in the case of Congo red, being completely removed. With aqueous solutions of the triarylmethane dyes crystal violet, fuchsine and malachite green complete decoloration was also achieved upon hydrogenation of the chromophore groups or modification of the chemical structure.

Full text of DOI:10.1039/c6ra24542k

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Decoloration of azo and triarylmethane dyes in the
aqueous phase by catalytic hydrotreatment with Pd
supported on pillared clays
Cite this: RSC Adv., 2016, 6, 113820
*
*
A. H. Pizarro, C. B. Molina and J. J. Rodriguez
Catalytic hydrotreatment of synthetic dyes as azo compounds and triarylmethanes has been carried out
under ambient-like conditions (25–50 ꢀC, 1 atm) with H2 using Pd supported on Al-pillared clays and
Al2O3 as catalysts. The treatment leads to complete decoloration of aqueous solutions of the azo dyes
(azobenzene, methyl orange, Congo red and oil red) due to the catalytic hydrogenation of the N]N
bonds. Under acid pH the color removal was enhanced and a solid residue was formed in the case of
Congo red, being completely removed. With aqueous solutions of the triarylmethane dyes crystal violet,
fuchsine and malachite green complete decoloration was also achieved upon hydrogenation of the
chromophore groups or modification of the chemical structure.
Received 2nd October 2016
Accepted 28th November 2016
DOI: 10.1039/c6ra24542k
www.rsc.org/advances
carcinogenic to humans) some of these amines that could
release by reductive cleavage.
1. Introduction
The triarylmethane dyes group includes synthetic organic
compounds with a central carbon atom bonded to three
aromatic rings, the colour and properties depending on the
number and nature of auxochromic groups. They are the most
important among the arylmethane dyes, characterized by their
intense colours, being mainly used as staining agents.7
The presence of these dyes in aqueous wastes produces
intense coloration even at very low concentrations. Destructive
and non-destructive technologies have been used for the
removal of these compounds from industrial effluents.8 These
include catalytic wet air oxidation (CWAO),9,10 advanced oxida-
tion processes (AOPs),11 like ozonation,12 biological treat-
ments,13–16 including anaerobic,17 coagulation–occulation,18
adsorption19 or combinations of these processes.20,21 The main
drawback of the treatments based on catalytic oxidation is the
formation of highly toxic species, more harmful than the orig-
inal ones, especially important from nitrogenated or chloroni-
trogenated compounds. In AOPs under substoichiometric
amounts of oxidant (H2O2 or O3) those substances remain in the
nal effluent at unacceptable levels. Some other issues of wet
oxidation processes, like expensive equipments and severe
temperature and pressure conditions, particularly in the case of
WAO, are also important limitations to the use of these
technologies.
The synthesis of organic dyes represents a very important
activity of the chemical industry and two-thirds of their world-
wide production is consumed by the textile industry. Azo
compounds are, by far, the most widely used synthetic organic
colorants. More than 10 000 azo dyes are classied in the colour
index. They are mostly synthesized from primary aromatic
amines by diazotisation and coupling with e.g. phenols or
secondary aromatic amines. The commercial products oen
contain high levels of other components, being aromatic
amines especially relevant from a toxicological point of view.
Azo dyes represent the 60–70% of the world production of dyes
(680 000 t per year).1 Some works estimate that around 2 and
50% of the dyeing materials are wasted due to inefficiencies
in the staining process.2 The discharge of dyes is forbidden
by the EU legislation as well as the use of azobenzenes which are
regulated by the REACH directive. The genotoxicity of some
azo dyes, used also as food additives, has been analyzed by
several authors,3,4 existing many uncertainties in the determi-
nation of the NOAEL values of some azo derivatives.5 Among the
organic dyes which are recognized to be carcinogens, a great
number are azo dyes. Furthermore, in many European coun-
tries, e.g. in Germany, it is generally accepted that all azo dyes
which may be split into carcinogenic aromatic amines by means
of enzymatic reactions inside the human body6 are potential
carcinogens. The International Agency for Research on Cancer
(IARC) list classied as groups 1 (carcinogenic to humans), 2A
(probably carcinogenic to humans) and 2B (possibly
An alternative to oxidation and non-destructive technologies
is the use of reductive processes. Some reducing agents, like
sodium sulphite and dithionite, have been used, which can
promote the reduction of the azo bonds. Sodium borohydride
has been employed in combination with aerobic biological
degradation for the treatment of textile dyeing wastewater.22
´
Chemical Engineering Area, Faculty of Sciences, Universidad Autonoma de Madrid,
Cantoblanco, 28049, Madrid, Spain. E-mail: alejandro.herreropizarro@gmail.com;
carmenbelen.molina@uam.es; Fax: +34-914973516; Tel: +34-914972878
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In this context, catalytic hydrotreatment appears as a poten- used only for the treatment of Congo red in long-term experi-
tial solution which can overcome the aforementioned draw- ment. The Al-pillared clay (Al-PILC) was obtained following the
backs of other technologies.23 Some previous works have method described elsewhere.42 Summarizing, an aluminium
reported on the use of catalytic hydrotreatment with different pillaring solution was prepared (OH/Al molar ratio ¼ 2) and
catalysts for the abatement of colour. This treatment has been added to a bentonite suspension (1 wt%) providing 10 mmol of
used in the reduction of Congo red and methyl orange with Al per gram of clay. The resulting material was washed by
TiO2,24 Ni25 and Au and Ag nanoparticles26,27 as catalysts, leading centrifugation with deionized water, dried overnight at 110 C
ꢀ
ꢀ
to amines as reaction products. Besides, other works have re- and calcined at 350 C for 2 h. The metallic active phase (Pd)
ported on the use of Pd (>10% wt) on polypropilene membranes was introduced into the Al-PILC structure or spheric alumina by
for the removal of Congo red,28 Pd on hydroxyapatite and wet impregnation from PdCl2 dissolved in HCl (1 M). The wet
magnetite for the elimination of methyl orange29 and Pd solid was dried for 2 h at 25 ꢀC, 14 h at 110 ꢀC and nally
nanoparticles with sodium borohydride.30 The hydrotreatment calcined in air atmosphere for 2 h at 500 C, reached at 2 C
ꢀ
ꢀ
of methyl orange31–33 and other azo dyes34 using zerovalent Fe or minꢁ1 heating rate. The nominal Pd load was 1 wt% for Al-PILC
Zn has been also described.
and 5% for alumina spheres.
Precious metals (Pd, Pt, Rh) supported on alumina and pil-
lared clays have been used as catalysts for hydrodechlorination
(HDC)35–39 and hydrodenitrogenation (HDN).40 This technology
2.3. Catalyst characterization
could be potentially used for the treatment of textile dyeing The porous texture of the pillared clay and the catalysts was
wastewaters since it can not only convert the chromophore assessed from the N2 adsorption–desorption isotherms ob-
groups of some organic dyes but also some species usually tained at ꢁ196 ꢀC in a Micromeritics Tristar 3000 apparatus.
found in this kind of wastewaters, such as chlorinated The samples were previously outgassed at 160 ꢀC and 5 ꢂ 10ꢁ3
anilines40,41 and chloronitrobenzenes,40 which are used in dyes Torr for 16 h. The specic surface area was calculated from the
manufacture. The Pd catalysts have demonstrated fairly high BET method (SBET). A value of 35 m2 gꢁ1 was obtained for the
stability and scarce or no leaching of the active phase39,40 which raw bentonite and 212 m2 gꢁ1 for the Al-PILC whereas the Pd/
would allow treating chlorinated and chloronitrogenated PILC catalyst yielded 162 m2 gꢁ1
pollutants on a long-term basis. They arise as the more suitable
The metal content in the catalysts was measured by X-ray
.
for catalytic dechlorination, showing a wider range of applica- uorescence with a TXRF EXTRA-II (Rich & Seifert, Germany)
bility in water and wastewater treatment than other noble spectrometer aer digestion of the solid samples by acid
metals as Rh or Pt.39 Thus, the use of that catalytic system offers treatment (nitric, hydrochloric and sulphuric acids mixture) at
ꢀ
a competitive advantage over other technologies such as ZVI 100 C. The blue solid formed upon hydrotreatment of Congo
when metal leaching should be avoided. The main advantage of red was separated using a PTFE membrane lter with 0.45 mm
catalytic hydrotreatment is the low reagent (H2) needs to achieve pore size (Sigma-Aldrich). Aer drying, the solid was charac-
complete decoloration compared to oxidation processes.
terized by elemental analysis (LECO elemental analyzer) and
The aim of this work is to analyze the application of catalytic total reection X-ray uorescence (Chemical Analysis Seifert
hydrogenation to the decoloration of two different groups EXTRA-II spectrometer).
of organic dyes (azo and triarylmethane) in aqueous solu-
tion using Pd on pillared clay and alumina as catalysts. The
activity as well as the adsorption capacity of the supports
2.4. Hydrotreatment experiments
are studied and the stability of the catalysts is tested in long- The catalytic experiments were performed in a stirred (360 rpm)
term experiments.
batch glass jacketed reactor (1.1 L total capacity) at 25 or 50 ꢃ
0.5 ꢀC and 1 atm using a catalyst concentration of 1 g Lꢁ1. The
Pd-PILC catalyst was used in powder with a particle size lower
than 100 mm. Previous experiments were performed with
different particle sizes and stirring velocities to exclude mass-
transfer limitations under the working conditions. The cata-
2. Materials and methods
2.1. Dyes
The organic dyes used were (supplier, CAS number): azo-
benzene (Sigma-Aldrich, 103-33-3), methyl orange (Panreac,
547-58-0), Congo red 35 wt% (Sigma-Aldrich, 573-58-0), oil red
(Merck, 1320-06-5), fuchsine (Merck, 632-99-5), crystal violet
(Merck, 548-62-9) and malachite green solution (Sigma-Aldrich,
2437-29-8).
lyst was reduced in situ under hydrogen ow (35 N mL minꢁ1
)
for 2 h at 90 ꢀC. The hydrogen was purged with a nitrogen ow
of 35 N mL minꢁ1 during 30 min in order to remove hydrogen
from the catalyst surface. 1 L of 100 mg Lꢁ1 aqueous solution of
the target dye was added to the reactor and stirring was main-
tained for 15 min to allow complete wetting of the catalyst.
Then, H2 was continuously passed in excess, at 50 N mL minꢁ1
Samples were withdrawn at 5, 10, 15, 30 min and each hour
.
2.2. Preparation of the catalysts
The starting material used to prepare the pillared clay was until 4–9 h reaction time. The catalyst was separated by ltra-
puried-grade bentonite supplied by Fisher Scientic tion using 0.45 mm PTFE lters and the resulting HDC effluents
a
Company (Loughborough, Oregon, USA) (<100 mm). The spheric were adjusted to neutral pH when necessary. HCl was used in
alumina (Sigma-Aldrich, CAS: 1344-28-1, 3 mm diameter) was order to reach pH 3 in the reaction medium in some tests.
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The stability of the catalysts was tested in long-term (100 h)
experiments in a xed-bed reactor (Pyrex glass, 30 cm length,
9 mm internal diameter). The catalyst (0.1 g of Pd-PILC for
methyl orange or 0.5 g of Pd–Al2O3 for Congo red at pH 3) was
mixed with glass beads of 2 mm external diameter in order to
avoid channelling. The aqueous solutions used were fed at
0.42 mL minꢁ1 ow rate, so that the space-time was adjusted to
0.13 kg h molꢁ1 for methyl orange solution (100 mg Lꢁ1) aꢁn1d
Pd-PILC or 4.02 kg h molꢁ1 for Congo red solution (10 mg L
)
.
with Pd–Al2O3. H2 was passed in excess, at 1 N mL minꢁ1
Liquid samples were periodically taken from the reactor exit
upon the time on stream.
2.5. Analytical methods
The absorbance of the solutions of azobenzene, methyl orange,
Congo red, oil red, fuchsine, crystal violet and malachite green
were measured by means of UV 1603 Shimadzu UV/Vis spec-
trophotometer at different wavelengths which will be indicated
in each case in the corresponding section of results and
discussion. All the samples of the catalytic tests carried out with
100 mg Lꢁ1 initial concentration were diluted 1/10 v/v before
their analysis. Total Organic Carbon (TOC) was determined with
a Shimadzu TOC-VCSH/CSN E200V analyzer. Cyclohexanone
was analyzed by means of a GC/FID using a CP-Wax 52 CB
Varian capillary column (30 m length and 0.25 mm i.d.). The
ecotoxicity of the samples from catalytic experiments and initial
solutions was evaluated by means of the Microtox® Acute
Toxicity Test (SCI 500 Analyzer). The results were expressed in
Fig. 1 Hydrotreatment of methyl orange with Pd-PILC catalyst at pH0
(a) 6 and (b) 3 (50 ꢀC, 1 atm, [methyl orange]0 ¼ 100 mg Lꢁ1, [Pd-PILC]
¼ 1 g Lꢁ1, QH2 ¼ 50 N mL minꢁ1). Inset: initial methyl orange solution
and samples obtained at different reaction times.
two aromatic amines.32 Previous works reported that the
hydrogenation of methyl orange produces sulfanilic acid and
N,N-dimethylbenzene-1,4-diamine.28,32 Sulfanilic acid presents
a low toxicity value.43 Cyclohexanone was found in the nal
effluent, probably formed from N,N-dimethylbenzene-1,4-
diamine or from some impurity. A Total Organic Carbon
(TOC) reduction of 20% due to adsorption was measured.
Similar behaviour was observed at pH 3 (absorbance measured
at 468 nm in this case). Complete decoloration was achieved
aer 30 min with a TOC reduction of 10% in the resulting
effluent, attributed to adsorption. The stability of the Pd-PILC
catalyst was evaluated in a long-term (100 h) xed bed reactor
experiment with methyl orange (100 mg Lꢁ1 inlet concentra-
tion) at 25 ꢀC and pH 3. As depicted in Fig. 2, the catalyst
showed a fairly stable performance during the 100 h of the
terms of EC50 and IC50
.
3. Results and discussion
3.1. Hydrotreatment of monoazoic dyes
The hydrotreatment of azobenzene (5 mg Lꢁ1 initial concen-
tration) was carried out at ambient-like conditions (25 ꢀC, 1
atm) in a 1 : 99 v/v methanol : water solution with the Pd-PILC
catalyst at 1 g Lꢁ1. The main reaction products identied and
quantied by GC/FID were aniline and cyclohexanone. The
scission of N]N bond allows the formation of aniline from
azobenzene which further reacts with hydrogen on the Pd active
sites, producing cyclohexylimine. The imine group reacts with
+
water giving rise to cyclohexanone and NH4 as reported in
a previous work on the hydrotreatment of chloronitrobenzene.40
The hydrotreatment of the aromatic amines gives rise to deni-
trogenation in some cases. Complete decoloration of the
resulting effluent was achieved aer total conversion of azo-
benzene. The hydrotreatment in this case reduce the ecotoxicity
since the molecules formed present higher EC50 values (>70 mg
Lꢁ1 for aniline, 18.7 mg Lꢁ1 for cyclohexanone and less than
2 mg Lꢁ1 for azobenzene).43
Hydrotreatment of methyl orange was tested at two different
initial pH values (3 and 6) and 50 ꢀC in batch experiments. The
results are depicted in Fig. 1. The absorbance of methyl orange
at pH 6 was measured at 464 nm as in previous works.31
A complete decoloration was achieved aer 1 h reaction time at
Fig. 2 Hydrotreatment of methyl orange with Pd-PILC catalyst in
a long-term experiment (25 ꢀC, 1 atm, pH0 3, [methyl orange]0
¼
100 mg Lꢁ1, [Pd-PILC] ¼ 0.1 g, Q ¼ 0.416 mL minꢁ1, QH2 ¼ 1 N mL
pH 6. The scission of the azo bond gives rise to the formation of minꢁ1).
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experiment, maintaining almost constant decoloration (more 15 min of reaction. The main difference with respect to the
than 98%). experiment at pH 6 was the formation of a blue precipitate as
The TXRF analyses showed the absence of Pd in the exit can be appreciated in Fig. 3a (inset). A 48% of TOC was removed
stream. This behaviour with regard to Pd leaching was also aer 4 h, attributed to adsorption onto the catalyst and
observed in the batch experiments described before, thus con- precipitation of the blue solid produced. The formation of this
rming the stability of the catalyst in that respect.
solid is promoted by the interaction with the catalyst (Pd-PILC
and Pd–Al2O3) as it is also produced even without H2 addi-
tion. The elemental analysis of this residue gave the following
composition: C ¼ 28.9%, H ¼ 3.5%, N ¼ 6.2, S ¼ 4.2%. Thus,
the treatment of this dye under acidic conditions allowed
decoloration by both catalytic reaction and precipitation. The
precipitate can be easily separated by ltration. The occurrence
of this precipitate is favored by the presence of HCl and the
formation of the protonated chinone form. The performance of
the Pd–Al2O3 catalyst in the hydrotreatment of Congo red was
tested in a long term experiment in xed bed. The results are
shown in Fig. 3b. The removal of color was maintained close to
100% upon the 100 h of the experiment. During the test,
formation of a blue precipitate was observed in all samples aer
several hours.
The hydrotreatment of oil red was carried out batchwise in
the same experimental conditions than with Congo red, but
using now a methanol : water (95 : 5, v/v) mixture due to the low
water solubility of oil red. The results are shown in Fig. 4. The
pH was not adjusted and the absorbance was measured at
515 nm. Complete color removal was achieved in less than 4 h.
Congo red and methyl orange solutions and their respective
effluents aer the treatment showed moderate ecotoxicity
values (Table 1).
3.2. Hydrotreatment of disazoic dyes
The hydrotreatment of Congo red (100 mg Lꢁ1, 50 ꢀC) with the
Pd-PILC catalyst at pH 6 in batch reactor allowed complete
colour removal aer 120 min as shown in Fig. 3a.
The maximum absorption peak of Congo red at this pH value
was measured at 498 nm, corresponding to the sulfonazo form
in the terminal naphthalene moieties, with intense red color.44
Congo red that leads to the formation of 3,4-diamino-
naphthalene sulfonate and benzidine. Benzidine is one of the
harmful intermediates that is produced in the reduction of
Congo red and it is classied by the IARC and the UE as one of
the carcinogenic substances to humans. A 27% TOC reduction
was observed attributed to adsorption. The removal of Congo
red by adsorption with natural clays has been proved to be very
efficient as previously reported.45 Previous works reported the
hydrogenation of Congo red with Pd supported on porous pol-
ypropilene membranes identifying 3,4-diamino-naphthalene
sulfonate and benzidine as the main reaction products of the
azo bond splitting.28 Congo red presents blue color at acid pH
due to the change of the molecular structure of the sulfonic
group which is in the chinone form.25 Hydrotreatment at pH
3 showed that the reaction proceeds at higher rate, giving rise to
complete decoloration (maximum absorption at 575 nm) aer
3.3. Hydrotreatment of triarylmethane dyes
The results obtained with crystal violet are shown in Fig. 5a.
Hydrogenation with the Pd-PILC catalyst at pH 3 allowed
complete removal of color (measured at 590 nm) aer 8 h under
ambient-like conditions (25 ꢀC, 1 atm). Almost 50% TOC
reduction was achieved. Therefore, adsorption might be
contributing substantially to color removal as a decrease in the
absorbance of 30% was obtained.
Thus, catalytic hydrogenation represents the main contribu-
tion to decoloration. The results obtained with fuchsine under
Fig. 3 Hydrotreatment of Congo red with (a) Pd-PILC catalyst (50 ꢀC,
1 atm, pH0 6, [Pd-PILC] ¼ 1 g Lꢁ1, QH2 ¼ 50 N mL minꢁ1), Congo red
influent and effluent after 10 min of reaction ([Congo red]0 ¼ 10 mg Fig. 4 Hydrotreatment of oil red with Pd-PILC catalyst (25 ꢀC, 1 atm,
L
ꢁ1) (inset) and (b) hydrotreatment of Congo red with Pd–Al2O3 natural pH, [cat] ¼ 1 g Lꢁ1, [oil red]0 ¼ 10 mg Lꢁ1, QH2 ¼ 50 N mL minꢁ1
,
catalyst in a long-term experiment (25 ꢀC, 1 atm, pH0 3, [Congo red]0 ¼ diluted in a mixture MeOH/H2O 95 : 5 v/v). Inset: initial oil red solution
10 mg Lꢁ1, [Pd–Al2O3] ¼ 0.5 g, QH2 ¼ 1 N mL minꢁ1).
and samples obtained at different reaction times.
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Table 1 Ecotoxicity values of the dye solutions (EC50, 100 mg Lꢁ1 Table 2 Initial rate constants calculated for hydrotreatment of dyes
initial concentration) and of the effluents obtained after the catalytic
treatment (IC50
)
Initial rates
Dye
(minꢁ1
)
r2
EC50
Other authors
EC50 (ref.)
(mg Lꢁ1
)
IC50
Methyl orange (pH 6)
Congo red (pH 6)
Oil red
Crystal violet (pH 3)
Fuchsine (pH 3)
Malachite green (pH 3)
7.1 ꢂ 10ꢁ2
3.4 ꢂ 10ꢁ2
2.4 ꢂ 10ꢁ2
1.2 ꢂ 10ꢁ2
1.8 ꢂ 10ꢁ2
0.9 ꢂ 10ꢁ2
0.988
0.987
0.997
0.968
0.976
0.996
Compound
Congo red
11.4
11.7
0.3
0.3
0.2
13.6
15.3
48.0
10.2
2.4
1623 (ref. 46)
—
4.7 (ref. 47)
0.3 (ref. 48)
0.05 (ref. 49)
Methyl orange
Crystal violet
Fuchsine
Malachite green
The results obtained with malachite green are shown in
Fig. 5c. Complete decoloration (l ¼ 617 nm) was also achieved
under the ambient-like conditions of the experiment but now
took almost 9 h and TOC decreased only by 17%. This, together
with the results of adsorption allows concluding that decolor-
ation is essentially due to changes in the molecular structure
by catalytic hydrogenation of the chromophore group.
Triarylmethane dyes are highly ecotoxic showing EC50 values
lower than 1 mg Lꢁ1 obtained by the Microtox® test (15 min).
The effluents from catalytic treatment showed an important
reduction of the ecotoxicity as can be observed in Table 1.
The possible formation of leuco derivatives could explain the
reduction in ecotoxicity as these intermediates present higher
EC50 values.49
The initial rate constants (Table 2) were calculated varying
from 1 ꢂ 10ꢁ2 to 7 ꢂ 10ꢁ2 minꢁ1, values in the same range than
the obtained for the degradation of methyl orange and Congo
red in other works.26
4. Conclusions
The hydrotreatment of different organic dyes (azo and triaryl-
methane compounds) in aqueous solution with Pd on pillared
clay allowed complete decoloration under ambient-like condi-
tions (25 ꢀC, 1 atm) in all cases. Color removal was accompanied
by TOC reduction due to adsorption. This contribution was in
some cases quite signicant in terms of TOC reduction but even
in those cases played a fairly less important role than reaction
on color removal. The decoloration takes place with high
stability. The treatment showed not only complete decoloration
but also an important decrease in the ecotoxicity, especially
with triarylmethane dyes.
Fig. 5 Hydrotreatment and adsorption of triarylmethane dyes with
Pd-PILC catalyst (25 ꢀC, 1 atm, pH0 3, [cat] ¼ 1 g Lꢁ1), ([dye]0 ¼ 100 mg
Acknowledgements
L
ꢁ1, QH2 ¼ 50 N mL minꢁ1). (a) Crystal violet; (b) fuchsine; (c) malachite
The authors want to thank the nancial support from the
Spanish Plan Nacional de I+D+i through the project CTQ2013-
41963-R. They also express their gratitude to Alberto Puime
green. Inset: initial dyes influents and samples obtained at different
reaction times.
´
Otın for his contribution.
the same conditions are depicted in Fig. 5b. The removal of color
(measured at 550 nm) was now faster (Table 2). Complete
decoloration was achieved around 4 h and was accompanied by
52% TOC reduction. Again, decoloration can be attributed
mainly to reaction, so that the important reduction of TOC must
include in great part adsorption of colorless reaction byproducts.
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