187
A publication of
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 60, 2017
Guest Editors: Luca Di Palma, Elisabetta Petrucci, Marco Stoller
Copyright © 2017, AI DI C Servizi S.r.l.
I SBN 978-88-95608- 50-1; I SSN 2283-9216
The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet
DOI: 10.3303/CET1760032
Treatment of Dyeing and Finishing Waters Using Innovative
Photocatalysts
Daniela Caraccioloa, Vincenzo Vaiano*b, Mariantonietta Matarangolob, Olga
Saccob, Diana Sanninob, Tiziana Gambicortia, Gianluigi Calvanesea
a
Stazione Sperimentale Industria Pelli, Research Institute of Leather Industry, Via Nuova Poggioreale 39, 80134 Napoli
(NA), Italy
b
Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy
vvaiano@unisa.it
Waters coming from dyeing and finishing processes by leather industries produce serious problems because
they contain dyes and other pollutants. Toxic and recalcitrant for the environmental, these pollutants are
difficult to remove by conventional treatments. For this reason, the heterogeneous photocatalysis is a
promising and efficient method to remove them.
ZnO is an excellent and promising photocatalytic material to oxidize organic pollutants into CO2 and H2O. In
order to enhance the photocatalytic activity of ZnO, in this work, the doping of ZnO with praseodymium (Pr)
has been realized and studied in the photocatalytic treatment of dyeing and finishing waters with a total
-1
organic carbon (TOC) values in the range 540-1200 mg·L .
ZnO particles doped with Pr were prepared by a modified precipitation method and characterized by different
techniques (XRD, Raman, UV-Vis DRS). The photocatalytic activities of the synthesized samples were
investigated under UV light with a specifically photocatalytic apparatus designed for the experimental tests.
Photocatalytic results showed a discoloration in the range 56-98% after 4 h of irradiation while TOC removal
was about 40% after 3 h of UV irradiation for all the investigated dyeing and finishing waters. The obtained
results evidenced the efficiency of the synthetized photocatalyst in the treatment of real dyeing and finishing
water coming from leather industries.
1. Introduction
Leather industries are one of the biggest polluters in the world due to the complex nature of their discharged
waters. During the production of the skin, a multitude of chemicals and a large quantities of water are used to
convert raw skins into leather and so, large volumes of water are generated. These waters constitute one of
the main sources of environmental pollution due to the high value of the chemical oxygen demand (COD), , a
high concentration of heavy metals, strong odour and intense coloration. The consumption of process water,
and consequently the water that is discharged, changes significantly raw materials and products involved in
the tanning process (Saxena et al., 2017).
In this work, the attention was focused on dyeing and finishing processes in which the discharged water are
characterized by a high concentration of total organic carbon (TOC), presence of heavy metals and an intense
coloration of discharged water, which highlight the need for environment friendly technologies capable to
reduce the pollutant load of the effluents, avoiding both the use of harmful chemicals and the production of byproducts to be disposed of (Dixit et al., 2015). To prevent negative impact on the environment during leather
production, heterogeneous photocatalysis is an innovative and promising technology used for the removal of
pollutants. Even if TiO2 is the most used semiconductor in photocatalytic applications (Wang and Caruso,
2011), the use of zinc oxide (ZnO) is proving just as valid catalyst and in many cases more photoactive than
titania (Vaiano et al., 2015). Moreover, the doping is an effective strategy to enhance the photocatalytic
performance of semiconductors, as reported in the scientific literature (Vaiano et al., 2016).
Please cite this article as: Caracciolo D., Vaiano V., Matarangolo M., Sacco O., Sannino D., Gambicorti T., Calvanese G., 2017, Treatment of
dyeing and finishing waters using innovative photocatalysts, Chemical Engineering Transactions, 60, 187-192 DOI: 10.3303/CET1760032
188
In particular, the use of zinc oxide doped with rare earths showed interesting results for the degradation of the
model solutions containing different types of dyes and in the treatment of wastewater coming from industrial
processes for the production of hair dyes (Vaiano et al., 2017).
The present work is focused on the treatment of dyeing and finishing waters through a photocatalytic system
using ZnO doped with Pr under UV light irradiation.
2. Experimental
2.1 Synthesis of photocatalysts and their characterization
Undoped and doped ZnO with praseodymium were prepared by a modified precipitation method, (Vaiano et
al., 2017) in which ZnSO4 (Aldrich, 99%) was used as the precursor of ZnO while Pr(NO3)3·6H2O (Aldrich
99.9%) was used as Pr dopant source. The generated precipitate was centrifuged, washed and calcined at
450°C for 30 min. The synthetized photocatalyst (Pr-ZnO) was characterized by X-ray diffraction analysis
(XRD), Raman, and UV–Vis diffuse reflectance spectra (UV–Vis DRS).
2.2 Photocatalytic apparatus
The photocatalytic tests were conducted in a cylindrical pyrex photoreactor coupled with a peristaltic pump, an
-1
air distributor device (flow rate of 142 Ncc·min ), four UV lamps (Philips, nominal power at 8W each and main
emission peak at 365 nm) arranged at the same distance from the external surface of the photoreactor and
-1
the photocatalyst dosage was 3 g·L dispersed in 100 mL of dyeing and finishing waters.
The photocatalytic system was used to treat different waters coming from dyeing and finishing processes by
leather industries. Chemical characteristics of waters are shown in Table 1.
Pr-ZnO photocatalyst was dispersed in water to be treated and the suspension was left in dark conditions for
120 minutes. After that, the photocatalytic test was conducted up to 240 min. The discoloration of the solutions
collected at different test times was analysed by UV–vis spectrophotometer (Evolution 201) considering the
absorbance at a specific wavelength. At last, the total organic carbon (TOC) was measured by the high
temperature combustion method on a catalyst (Pt-Al2O3) in a tubular flow microreactor operated at 680°C, with
a stream of hydrocarbon free air to oxidize the organic carbon.
It is worthwhile to underline that, the pH of the waters was never modified to demonstrate the possibility to
operate in a wide pH range (Table 1), through the developed photocatalytic system.
Table 1: Dyeing and finishing waters characteristics
Dyeing water
Finishing water 1
Finishing water 2
TOC [mg·L ] 452
1205
541
pH
2.3
6.4
6.2
Cd [mg·L-1]
<0.005
<0.005
<0.005
Cr [mg·L ]
30
0.25
2.5
Fe [mg·L-1]
3.5
1.8
14
-1
Mn [mg·L ]
0.1
0.1
<0.005
Ni [mg·L-1]
<0.005
<0.005
<0.005
-1
Pb [mg·L ]
<0.005
<0.005
<0.005
Cu [mg·L-1]
0.1
0.04
0.3
-1
-1
3. Experimental results
3.1 Characterization of the photocatalysts
Figure 1a shows the Raman spectra of the undoped ZnO and Pr-ZnO catalyst in the range of 150-750 cm-1 in
which the main bands related to the zinc oxide were observed.
The XRD patterns of the same photocatalysts (Figure 1b) evidenced five peaks at 2θ=32.06°, 34.74°,
36.53°,47.85° and 56.97°, respectively indexed to the (100), (002), (101), (102) and (110) planes of hexagonal
wurtzite crystal structure (Soares et al., 2017) and no signals related to praseodymium oxide (Khataee et al.,
2015) were detected, demonstrating the effectiveness of the doping process. Considering the peak at
2θ=36.53°, it was calculated the average crystallite sizes of the samples using the Debye-Sherrer’s equation
in which the crystallite size value of Pr-ZnO photocatalyst (21 nm) is lower than the undoped ZnO (25 nm).
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The UV-Vis DRS spectra, in terms of Kubelka-Munk (KM) of the undoped ZnO and Pr-ZnO photocatalyst is
showed in Figure 2. It is evident that the absorption threshold of undoped ZnO sample was at about 390 nm
(Figure 2a). It may also be noted a shift of the absorption threshold towards the visible region for the Pr-ZnO
photocatalyst (Figure 2a) with a wide absorption band in the range 410-660 nm. As a consequence a slight
reduction of the band gap value in comparison to the undoped ZnO has been achieved (Figure 2b).
Figure 1: Raman spectra of undoped ZnO and Pr-ZnO photocatalyst (a) and XRD patterns of undoped ZnO,
Pr-ZnO photocatalyst and praseodymium oxide (b)
Figure 2: UV-Vis DRS spectra of undoped ZnO and Pr-ZnO photocatalyst as a function of wavelength(a) and
band gap energy calculus (b).
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3.2 Photocatalytic tests
The photocatalytic experiments were carried out using Pr-ZnO photocatalyst. Figure 3 shows the results in
terms of discoloration for dyeing water and finishing waters as a function of run time.
In particular, the discoloration was obtained by monitoring over time the absorbance characteristic of the
colour of the treated water. A typical behaviour of the absorbance versus time is shown in Figure 4.
Figure 3: Photocatalytic discoloration of dyeing and finishing waters as a function of run time.
Figure 4: UV-Vis absorbance spectra during the irradiation time for the finishing water 1.
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In Figure 3, it is possible to observe that, in dark conditions, the adsorption equilibrium on Pr-ZnO surface is
reached in 60 min leading to a discoloration of about 40 and 60% for the dyeing water and finishing waters,
respectively. During the irradiation time, Pr-ZnO photocatalyst allowed to obtain 54 and 76% for the dyeing
and finishing water 1, respectively, after 240 min under UV light (Figure 3). As regards to finishing water 2, it
was obtained the almost complete discoloration in 240 min of UV irradiation (Figure 3).
Figure 5a reports the discoloration degree and TOC removal values after 180 min of test time for all the
treated waters. In particular, it was obtained a discoloration degree of about 56, 76 and 98% for dyeing water,
finishing water 1 and 2, respectively, while TOC removal was about 40% for all the treated waters. Possibly,
the lower value of TOC removal with respect to the discoloration degree is due to the presence of colourless
organic substances that are more difficult to degrade.
The discoloration of the treated waters is evident from the observation of Figures 5b, 5c and 5d. The obtained
results evidenced the efficiency the Pr-ZnO photocatalyst in the treatment of real dyeing and finishing water
coming from leather industries.
Figure 5: Discoloration and TOC removal of dyeing and finishing waters after 180 min of UV irradiation.
4. Conclusions
The photocatalytic treatment of waters coming from dyeing and finishing processes by leather industries has
been studied using Pr-ZnO photocatalyst. Characterization results showed that the Pr doping induced a shift
toward the visible region with a slightly decrease of the band gap value in comparison to undoped ZnO
photocatalyst. Photocatalytic tests of dyeing and finishing waters demonstrated that Pr-ZnO photocatalyst was
able to obtain a discoloration degree higher than 50% after 240 min under UV light with a TOC removal of
about 40% after 180 min under UV light irradiation. The obtained results evidenced the efficiency of the
synthetized photocatalyst in the treatment of real dyeing and finishing water coming from leather industries.
In general, the photocatalytic process is used in water treatment plants as a tertiary process. Given the
complexity of the industrial waters that has been treated here and with respect to the residual values of TOC
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(2701-723 mg·L ), it can be said that the photocatalytic process can be proposed as a pre-treatment step for
other treatment units (such as the biological process) in order to make the treated waters more easily to
degrade.
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