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

Abstract

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.

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1. Introduction

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 classified 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 often 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 determination 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 countries, 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 classified as groups 1 (carcinogenic to humans), 2A (probably carcinogenic to humans) and 2B (possibly carcinogenic to humans) some of these amines that could release by reductive cleavage.

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 oxidation processes (AOPs),¹¹ like ozonation,¹² biological treatments,^13–16 including anaerobic,¹⁷ coagulation–flocculation,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 original ones, especially important from nitrogenated or chloronitrogenated compounds. In AOPs under substoichiometric amounts of oxidant (H₂O₂ or O₃) those substances remain in the final 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.

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

In this context, catalytic hydrotreatment appears as a potential solution which can overcome the aforementioned drawbacks of other technologies.²³ Some previous works have reported on the use of catalytic hydrotreatment with different catalysts for the abatement of colour. This treatment has been used in the reduction of Congo red and methyl orange with TiO₂,²⁴ Ni²⁵ and Au and Ag nanoparticles^26,27 as catalysts, leading to amines as reaction products. Besides, other works have reported on the use of Pd (>10% wt) on polypropilene membranes for the removal of Congo red,²⁸ Pd on hydroxyapatite and magnetite for the elimination of methyl orange²⁹ and Pd nanoparticles with sodium borohydride.³⁰ The hydrotreatment of methyl orange^31–33 and other azo dyes³⁴ using zerovalent Fe or Zn has been also described.

Precious metals (Pd, Pt, Rh) supported on alumina and pillared clays have been used as catalysts for hydrodechlorination (HDC)^35–39 and hydrodenitrogenation (HDN).⁴⁰ This technology could be potentially used for the treatment of textile dyeing wastewaters since it can not only convert the chromophore groups of some organic dyes but also some species usually found in this kind of wastewaters, such as chlorinated anilines^40,41 and chloronitrobenzenes,⁴⁰ which are used in dyes manufacture. The Pd catalysts have demonstrated fairly high stability and scarce or no leaching of the active phase^39,40 which would allow treating chlorinated and chloronitrogenated pollutants on a long-term basis. They arise as the more suitable for catalytic dechlorination, showing a wider range of applicability in water and wastewater treatment than other noble metals as Rh or Pt.³⁹ Thus, the use of that catalytic system offers a competitive advantage over other technologies such as ZVI when metal leaching should be avoided. The main advantage of catalytic hydrotreatment is the low reagent (H₂) needs to achieve complete decoloration compared to oxidation processes.

The aim of this work is to analyze the application of catalytic hydrogenation to the decoloration of two different groups of organic dyes (azo and triarylmethane) in aqueous solution using Pd on pillared clay and alumina as catalysts. The activity as well as the adsorption capacity of the supports are studied and the stability of the catalysts is tested in long-term experiments.

2. Materials and methods

2.1. Dyes

The organic dyes used were (supplier, CAS number): azobenzene (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).

2.2. Preparation of the catalysts

The starting material used to prepare the pillared clay was a purified-grade bentonite supplied by Fisher Scientific Company (Loughborough, Oregon, USA) (<100 μm). The spheric alumina (Sigma-Aldrich, CAS: 1344-28-1, 3 mm diameter) was used only for the treatment of Congo red in long-term experiment. The Al-pillared clay (Al-PILC) was obtained following the method described elsewhere.⁴² Summarizing, an aluminium pillaring solution was prepared (OH/Al molar ratio = 2) and added to a bentonite suspension (1 wt%) providing 10 mmol of Al per gram of clay. The resulting material was washed by centrifugation with deionized water, dried overnight at 110 °C and calcined at 350 °C for 2 h. The metallic active phase (Pd) was introduced into the Al-PILC structure or spheric alumina by wet impregnation from PdCl₂ dissolved in HCl (1 M). The wet solid was dried for 2 h at 25 °C, 14 h at 110 °C and finally calcined in air atmosphere for 2 h at 500 °C, reached at 2 °C min⁻¹ heating rate. The nominal Pd load was 1 wt% for Al-PILC and 5% for alumina spheres.

2.3. Catalyst characterization

The porous texture of the pillared clay and the catalysts was assessed from the N₂ adsorption–desorption isotherms obtained at −196 °C in a Micromeritics Tristar 3000 apparatus. The samples were previously outgassed at 160 °C and 5 × 10⁻³ Torr for 16 h. The specific surface area was calculated from the BET method (S_BET). A value of 35 m² g⁻¹ was obtained for the raw bentonite and 212 m² g⁻¹ for the Al-PILC whereas the Pd/PILC catalyst yielded 162 m² g⁻¹.

The metal content in the catalysts was measured by X-ray fluorescence with a TXRF EXTRA-II (Rich & Seifert, Germany) spectrometer after digestion of the solid samples by acid treatment (nitric, hydrochloric and sulphuric acids mixture) at 100 °C. The blue solid formed upon hydrotreatment of Congo red was separated using a PTFE membrane filter with 0.45 μm pore size (Sigma-Aldrich). After drying, the solid was characterized by elemental analysis (LECO elemental analyzer) and total reflection X-ray fluorescence (Chemical Analysis Seifert EXTRA-II spectrometer).

2.4. Hydrotreatment experiments

The catalytic experiments were performed in a stirred (360 rpm) batch glass jacketed reactor (1.1 L total capacity) at 25 or 50 ± 0.5 °C and 1 atm using a catalyst concentration of 1 g L⁻¹. The Pd-PILC catalyst was used in powder with a particle size lower than 100 μm. Previous experiments were performed with different particle sizes and stirring velocities to exclude mass-transfer limitations under the working conditions. The catalyst was reduced in situ under hydrogen flow (35 N mL min⁻¹) for 2 h at 90 °C. The hydrogen was purged with a nitrogen flow of 35 N mL min⁻¹ during 30 min in order to remove hydrogen from the catalyst surface. 1 L of 100 mg L⁻¹ aqueous solution of the target dye was added to the reactor and stirring was maintained for 15 min to allow complete wetting of the catalyst. Then, H₂ was continuously passed in excess, at 50 N mL min⁻¹. Samples were withdrawn at 5, 10, 15, 30 min and each hour until 4–9 h reaction time. The catalyst was separated by filtration using 0.45 μm PTFE filters and the resulting HDC effluents were adjusted to neutral pH when necessary. HCl was used in order to reach pH 3 in the reaction medium in some tests.

The stability of the catalysts was tested in long-term (100 h) experiments in a fixed-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 min⁻¹ flow rate, so that the space-time was adjusted to 0.13 kg h mol⁻¹ for methyl orange solution (100 mg L⁻¹) and Pd-PILC or 4.02 kg h mol⁻¹ for Congo red solution (10 mg L⁻¹) with Pd–Al₂O₃. H₂ was passed in excess, at 1 N mL min⁻¹. 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 spectrophotometer 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 L⁻¹ 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 terms of EC₅₀ and IC₅₀.

3. Results and discussion

3.1. Hydrotreatment of monoazoic dyes

The hydrotreatment of azobenzene (5 mg L⁻¹ initial concentration) 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 L⁻¹. The main reaction products identified and quantified 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 denitrogenation in some cases. Complete decoloration of the resulting effluent was achieved after total conversion of azobenzene. The hydrotreatment in this case reduce the ecotoxicity since the molecules formed present higher EC₅₀ values (>70 mg L⁻¹ for aniline, 18.7 mg L⁻¹ for cyclohexanone and less than 2 mg L⁻¹ 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 after 1 h reaction time at pH 6. The scission of the azo bond gives rise to the formation of 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 final 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 after 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) fixed bed reactor experiment with methyl orange (100 mg L⁻¹ inlet concentration) at 25 °C and pH 3. As depicted in Fig. 2, the catalyst showed a fairly stable performance during the 100 h of the experiment, maintaining almost constant decoloration (more than 98%).

Fig. 1 Hydrotreatment of methyl orange with Pd-PILC catalyst at pH₀ (a) 6 and (b) 3 (50 °C, 1 atm, [methyl orange]₀ = 100 mg L⁻¹, [Pd-PILC] = 1 g L⁻¹, QH₂ = 50 N mL min⁻¹). Inset: initial methyl orange solution and samples obtained at different reaction times.

Fig. 2 Hydrotreatment of methyl orange with Pd-PILC catalyst in a long-term experiment (25 °C, 1 atm, pH₀ 3, [methyl orange]₀ = 100 mg L⁻¹, [Pd-PILC] = 0.1 g, Q = 0.416 mL min⁻¹, QH₂ = 1 N mL min⁻¹).

The TXRF analyses showed the absence of Pd in the exit stream. This behaviour with regard to Pd leaching was also observed in the batch experiments described before, thus confirming the stability of the catalyst in that respect.

3.2. Hydrotreatment of disazoic dyes

The hydrotreatment of Congo red (100 mg L⁻¹, 50 °C) with the Pd-PILC catalyst at pH 6 in batch reactor allowed complete colour removal after 120 min as shown in Fig. 3a.

Fig. 3 Hydrotreatment of Congo red with (a) Pd-PILC catalyst (50 °C, 1 atm, pH₀ 6, [Pd-PILC] = 1 g L⁻¹, QH₂ = 50 N mL min⁻¹), Congo red influent and effluent after 10 min of reaction ([Congo red]₀ = 10 mg L⁻¹) (inset) and (b) hydrotreatment of Congo red with Pd–Al₂O₃ catalyst in a long-term experiment (25 °C, 1 atm, pH₀ 3, [Congo red]₀ = 10 mg L⁻¹, [Pd–Al₂O₃] = 0.5 g, QH₂ = 1 N mL min⁻¹).

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 classified 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 polypropilene 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) after 15 min of reaction. The main difference with respect to the experiment at pH 6 was the formation of a blue precipitate as can be appreciated in Fig. 3a (inset). A 48% of TOC was removed after 4 h, attributed to adsorption onto the catalyst and precipitation of the blue solid produced. The formation of this solid is promoted by the interaction with the catalyst (Pd-PILC and Pd–Al₂O₃) as it is also produced even without H₂ addition. The elemental analysis of this residue gave the following composition: C = 28.9%, H = 3.5%, N = 6.2, S = 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 filtration. 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 fixed 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 after 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.

Fig. 4 Hydrotreatment of oil red with Pd-PILC catalyst (25 °C, 1 atm, natural pH, [cat] = 1 g L⁻¹, [oil red]₀ = 10 mg L⁻¹, QH₂ = 50 N mL min⁻¹, diluted in a mixture MeOH/H₂O 95 : 5 v/v). Inset: initial oil red solution and samples obtained at different reaction times.

Congo red and methyl orange solutions and their respective effluents after the treatment showed moderate ecotoxicity values (Table 1).

Table 1 Ecotoxicity values of the dye solutions (EC₅₀, 100 mg L⁻¹ initial concentration) and of the effluents obtained after the catalytic treatment (IC₅₀)

Compound	EC50 (mg L^−1)	IC50	Other authors EC50 (ref.)
Congo red	11.4	13.6	1623 (ref. 46)
Methyl orange	11.7	15.3	—
Crystal violet	0.3	48.0	4.7 (ref. 47)
Fuchsine	0.3	10.2	0.3 (ref. 48)
Malachite green	0.2	2.4	0.05 (ref. 49)

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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) after 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.

Fig. 5 Hydrotreatment and adsorption of triarylmethane dyes with Pd-PILC catalyst (25 °C, 1 atm, pH₀ 3, [cat] = 1 g L⁻¹), ([dye]₀ = 100 mg L⁻¹, QH₂ = 50 N mL min⁻¹). (a) Crystal violet; (b) fuchsine; (c) malachite green. Inset: initial dyes influents and samples obtained at different reaction times.

Thus, catalytic hydrogenation represents the main contribution to decoloration. The results obtained with fuchsine under 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.

Table 2 Initial rate constants calculated for hydrotreatment of dyes

Dye	Initial rates (min^−1)	r^2
Methyl orange (pH 6)	7.1 × 10^−2	0.988
Congo red (pH 6)	3.4 × 10^−2	0.987
Oil red	2.4 × 10^−2	0.997
Crystal violet (pH 3)	1.2 × 10^−2	0.968
Fuchsine (pH 3)	1.8 × 10^−2	0.976
Malachite green (pH 3)	0.9 × 10^−2	0.996

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The results obtained with malachite green are shown in Fig. 5c. Complete decoloration (λ = 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 decoloration 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 L⁻¹ 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 × 10⁻² to 7 × 10⁻² min⁻¹, 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 triarylmethane compounds) in aqueous solution with Pd on pillared clay allowed complete decoloration under ambient-like conditions (25 °C, 1 atm) in all cases. Color removal was accompanied by TOC reduction due to adsorption. This contribution was in some cases quite significant 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.

Acknowledgements

The authors want to thank the financial support from the Spanish Plan Nacional de I+D+i through the project CTQ2013-41963-R. They also express their gratitude to Alberto Puime Otín for his contribution.

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