Abstract:
The rate of oxidation of substituted phenols in wastewater was
studied by reaction of phenols with polyoxometalate catalysts to
activate H2O2. The substituted phenols used in this study are, phenol,
para-Cresol, meta-Cresol, para-Chlorophenol, and para-nitrophenol.
The polyoxometalates used are, H3PW12O40 (HWPA), ph-H3PW12O40
(ph-HWPA), H3PMo12O40 (HMoPA), and Na5Mo10V2PO40(HMoVPA).
The
catalyzed
rate
and
catalyzed
rate
constants
were calculated for these catalytic reactions. meta-Cresol has the
highest value of rate constant, while para-nitrophenol has the lowest
value. The rate of these reactions increases with increasing the
concentration of H2O2,
catalyst, and substituted phenol. Also,
increase in temperature increases the rate of these reactions while
increase in ionic strength increases them only slightly.
Hammet equation was used to study the effect of the presence
of electron donating and withdrawing groups on phenol on the rate
of these reaction. Electron donating groups (eg. -CH3 group) on
phenol increase the rate of these reactions, but electron withdrawing
groups (NO2, and CL groups) on phenol decrease the rate. Also, the
reaction constant (ρ) was calculated for these reactions by using
Hammet equation and it was observed to be negative value, which
indicates that, the reaction follows free-radical mechanism.
Eyring equation was used to calculate the enthalpy of
activation for these reactions. HWPA has the lowest value of
enthalpy of activation (∆H# =38.7 kJ/mole), but HMoVPA has the
highest value ((∆H# =110.8 kJ/mole). It means that, HWPA is the
most reactive catalyst, while HMoVPA is the lowest reactive among
the catalysts studied.
q
The effect of oxygen (air) was studied. It was found that it has
no effect on the rate of these reactions. The rate of these reactions
decreases with increase in acidity, and also with increase in volume
percentage of Acetonitrile. Immidazole decreases the rate of these
reactions.
Water was found to be the best solvent for these reactions.
HWPA was the most effective catalyst among the group of catalysts
studied, for complete degradation of substituted phenols in
wastewater. GC-Mass spectroscopy and IR - spectroscopy were used
to identify the products of these reactions, which were CO2, H2O, and
some organic acids. Methacrylic acid test was used to prove that, the
reaction follows free-radical mechanism.
