The kinetic study of the oxidation process was based on Ozawa's non-isothermal method and was applied to the thermogravimetric (TG) curves. The results obtained using the thermal analysis system TGA-50 (Shimadzu, Japan) were under the following conditions: 4mg of sample were used in an open alumina crucible in an air flow of 150 mL min1; the samples were heated from 30 C to 600 C at heating rates of 10, 15, 20, 25 and 30 C min1. The instrument was first calibrated with standard weight and calcium oxalate monohydrate. 1H NMR.