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Comparative dissolution of natural goethite samples in HCl and HNO3
Abstract
The dissolution of goethite samples in mineral acids from to extreme parts of Nigeria (Osogbo and Kaduna) have been investigated. The rate of dissolution was found to be slightly faster in HCl than HNO3. The higher
rate of dissolution in HCl could be attributed to the complexing ability of Cl- in addition to the H+ effect while lowering effect by NO3
- was attributed to adsorption of NO3 on to the goethite surface. Mononuclear complexes, especially bidentate of oxyanion are found to accelerate dissolution as opposed to binuclear complexes. A binuclear
bidentate surface complex formation between NO3 - and Fe3+ was therefore proposed to account for the inhibition observed. The possible formation of Fe—Cl reduces both the surface positive charge and the repulsion between the oxide surface and protons in solution. This produced accelerated proton dissolution. The effect of temperature on the
initial dissolution rate fit into Arrhenius equation, with Kaduna goethite exhibiting higher rate than Osogbo type in both acids. The activation energies for Kaduna goethite sample are 66.11 and 82.43 kJmol-1 in 0.1M HCl and 0.1M HNO3 respectively while for Osogbo goethite sample are 54.60 and 76.32 kJmol-1 in 0.1M HCl and 0.1M HNO3 respectively. The frequency factors of 8.4 x 10-4 & 11.17 x10-4 and 5.0 x 10-5 and 3.09 x 10-3 for Kaduna and Osogbo goethite samples were obtained. @JASEM
rate of dissolution in HCl could be attributed to the complexing ability of Cl- in addition to the H+ effect while lowering effect by NO3
- was attributed to adsorption of NO3 on to the goethite surface. Mononuclear complexes, especially bidentate of oxyanion are found to accelerate dissolution as opposed to binuclear complexes. A binuclear
bidentate surface complex formation between NO3 - and Fe3+ was therefore proposed to account for the inhibition observed. The possible formation of Fe—Cl reduces both the surface positive charge and the repulsion between the oxide surface and protons in solution. This produced accelerated proton dissolution. The effect of temperature on the
initial dissolution rate fit into Arrhenius equation, with Kaduna goethite exhibiting higher rate than Osogbo type in both acids. The activation energies for Kaduna goethite sample are 66.11 and 82.43 kJmol-1 in 0.1M HCl and 0.1M HNO3 respectively while for Osogbo goethite sample are 54.60 and 76.32 kJmol-1 in 0.1M HCl and 0.1M HNO3 respectively. The frequency factors of 8.4 x 10-4 & 11.17 x10-4 and 5.0 x 10-5 and 3.09 x 10-3 for Kaduna and Osogbo goethite samples were obtained. @JASEM
