Effect of dissolved cobalt(II) on the ozonation of oxalic acid

Cobalt(II) was examined as an ozonation catalyst. Laboratory-scale batch ozonation experiments were run at near-neutral pH and 24 degrees C. A hydroxyl radical probe compound, p-chlorobenzoic acid (pCBA), was also included in the solution matrix. Batch experiments showed that trace amounts of cobalt(II) accelerated the ozonation of oxalate. The rate of oxalate removal increased with decreasing pH from pH 6.7 to pH 5.3. The presence of cobalt(II) also increased the removal rate of pCBA indicating that the generation of hydroxyl radicals are byproducts of cobalt(II)-catalyzed ozonation of oxalate. It is proposed that the first step in the catalytic ozonation reaction pathway is the formation of a cobalt(II)--oxalate complex. Cobalt(II) oxalate is then oxidized by ozone to form cobalt(III) oxalate. The catalytic cycle is completed with decomposition of the cobalt(III) complex to form cobalt(II) and an oxalate radical. Assuming that cobalt, oxalate, and water were in equilibrium, the second-order reaction rate constants at pH 6 for ozonation of the cobalt(II) monooxalate and dioxalate species were 30 +/- 9 and 4000 +/- 500 M-1 s-1, respectively. These are both much greater than the reaction rate constant for the ozonation of free oxalate (kO3 < or = 0.04 M-1 s-1).