Although it is known that manganese oxides are solubilized by reduction in anoxic waters, the chemical processes are poorly understood. A study of the reduction and dissolution of manganese oxide suspensions by twenty-seven organic substrates that have chemical structures similar to those of natural organics was undertaken to determine the rates and mechanisms of the solubilization reactions. Dissolution of suspensions by hydroquinone in the pH range 6.5 < pH < 8.5 is described by the following experimental rate law:
d[Mn^(2+)]/dt = k_l{H^+}^(0. 46) [HQ]^(1.0) (Mn_T - [Mn^(2+)])
where [Mn^(2+)] is the amount of dissolved manganese, [HQ] is the hydroquinone concentration, and Mn_T is the initial amount of manganese oxide. The apparent activation energy of the reaction was found to be +37 kJ/mole. The Mn(III,IV) oxide suspension was prepared by oxidizing a Mn(OH)_2(s.) suspension with oxygen, and has a composition characterized by MnO_(1.66). Suspension particles were between 0.2 and 1.0 microns in diameter. Calcium and phosphate were found to inhibit the dissolution reaction, by adsorbing on the oxide surface.
Dihydroxybenzenes and methoxyphenols dissolved the suspensions at appreciable rates. Of the aliphatic substrates examined, only ascorbate, oxalate, and pyruvate dissolved the oxide. Dissolution by marine fulvic acid was found to be photocatalyzed.
A model was developed to explain the observed rate dependence and the relative reactivity of different organic substrates. The model assumes that complexes between substrate and surface sites form prior to electron transfer and dissolution. The pH dependence is not explained by this model; involvement of H^+ in the dissolution of reduced surface sites may be responsible for the observed fractional order with respect to H^+
