Catalytic wet air oxidation with Ni- and Fe-doped mixed
oxides derived from hydrotalcites
G. Ovejero, A. Rodríguez, A. Vallet, P. Gómez and J. García
ABSTRACT
Catalytic wet air oxidation of Basic Yellow 11 (BY11), a basic dye, was studied in a batch reactor.
Layered double hydroxides with the hydrotalcite-like structure containing nickel or iron cations have
been prepared by coprecipitation and subsequently calcined leading to Ni- and Fe-doped mixed
oxides, respectively. Compared with the results in the wet air oxidation of BY11, these catalysts
showed high activity for total organic carbon (TOC), toxicity and dye removal at 120
W
C and 50 bars
after 120 min. It has been demonstrated that the activity depended strongly on the presence of
catalyst. The results show that catalysts containing nickel provide a higher extent of oxidation of the
dye whereas the reaction carried out with the iron catalyst is faster. The Ni and Fe dispersion
determined from the TPR results was higher for the catalysts with a lower Ni or Fe content and
decreased for higher Ni or Fe contents. On the basis of activity and selectivity, the Ni containing
catalyst with the medium (3%) Ni content was found to be the best catalyst. Finally, a relationship
between metal content of the catalyst and reaction rate has been established.
G. Ovejero
A. Rodríguez
A. Vallet
P. Gómez
J. García (corresponding author)
Grupo de Catálisis y Procesos de Separación
(CyPS), Departamento de Ingeniería Química,
Facultad de Ciencias Químicas, Universidad
Complutense de Madrid, Avda. Complutense
s/n, 28040 Madrid, Spain
E-mail: juangcia@quim.ucm.es
Key words | catalytic wet air oxidation, dyes, heterogeneous catalysts, wastewater
INTRODUCTION
Wastewater from textile dyeing and graphic arts industry
may cause severe environmental damage because of its
strong colour, high level of total organic carbon (TOC)
and low rates of biodegradability. It is reported that 10–
20% of dyes are lost in wastewater as a result of inefficiency
during the dyeing process (Srinivasan & Viraraghavan ).
The large amount of wastewater generated from these indus-
tries represents a major environmental danger due to the
refractory nature of the dyes and the potential carcinogeni-
city of their anaerobic metabolites. Therefore, these
effluents must be treated before their discharge, having to
achieve the standards required by the legislation (Rajkumar
et al. ).
Oxidation processes constitute a promising technology
for the treatment of wastewater containing organic refrac-
tory compounds, as a pre-treatment step to the
conventional biological treatment. Some of the chemical
oxidation processes are advanced oxidation processes
(AOPs), wet air oxidation (WAO), supercritical water
oxidation (SCWO) and incineration. Wet air oxidation
(WAO) has already been applied successfully to treat efflu-
ents from printing and dyeing industry. A wide range of
products have been treated successfully due to the capacity
of this process to eliminate products with low solubility such
as polymers or fatty acids (Oliviero et al. ). The main
disadvantages of WAO are the high requirements of
temperature (200–320
W
C) and pressure (20–200 bar) which
results in high operation costs. The use of a catalyst strongly
improves the degradation of organic pollutants by using
milder conditions of temperature and pressure (Melero
et al. ).
Synthetic hydrotalcite-type (HT) compounds are layered
double hydroxides (LDH) comprising positively charged
layers and charge balancing anions located in the interlayer
region. HT are characterized by the general formula
½M
2þ
1x
M
3þ
x
ðOHÞ
2
xþ
½A
n
x=n
:mH
2
O where M is the metal and
A the anion. This kind of material has aroused considerable
interest because of the variety of its chemical composition
and its structural properties. HT has been widely employed
as catalyst, catalyst support, ion exchanger, stabilizer and
adsorbent.
In particular, high surface area mixed oxides, which are
obtained by thermal decomposition of HT at relatively low
temperature, are employed as catalysts and catalysts support
2381 © IWA Publishing 2011 Water Science & Technology | 63.10 | 2011
doi: 10.2166/wst.2011.513