The hydrolysis of organophosphorus pesticides is facilitated by
suspension of manganese dioxide. The concentration of organophosphorus
pesticides in solution apparently decayed showing the second order kinetics. It
was found that the rate constant value was highest at strong acidic conditions
and it continued to decrease as the pH of the solution was increased gradually.
In basic conditions, the rate constant value decreased to minimum. Manganese
dioxide under acidic conditions converted into Mn
$^{2+}$
ions,
then these ions in water formed hexaaquomanganese (II) ion
–[Mn(H
$_2$
O)
$_6$
]
$^{2+}$
.
This ion then adsorbed itself on the S or O atom of the organophosphorus
compound, thus weakens the bond between P–S. This reaction facilitated
the attack of H
$_2$
O or OH
$^-$
ion and thus
enhanced the efficiency of hydrolysis. It was studied that methidathion
hydrolyzed more efficiently than omethoate. The rate constants of catalytic
hydrolysis were increased with increasing the amount of
MnO
$_2$
. It was found that the pesticides had undergone
adsorption on catalyst in the first few minutes and there was rapid drop of
total phosphorus concentration. The decrease of total phosphorus adsorption and
rate of hydrolysis with increasing pH was also observed. After the addition of
alkaline earth metal cations (Ca
$^{2+}$
and
Mg
$^{2+}$
) along with magnesium, the rate of hydrolysis at
near neutral conditions was successfully increased.
