The redox reaction of manganese (Mn) is of great environmental, geological, and public health significance, as Mn oxides control the distribution and electron flow of numerous nutrients and contaminants in natural and engineered environments. Current understanding on the oxidation pathways of Mn(II) to Mn(III/IV) mainly focuses on biotic processes due to their much higher oxidation rates than those of abiotic processes. This study demonstrates rapid photocatalytic oxidation of Mn 2+ (aq) under circumneutral conditions catalyzed by naturally abundant semiconducting TiO 2 minerals. Notably, the photocatalytic oxidation rates are comparable to or even higher than those of reported biotic/abiotic processes. In addition, the rapid photocatalytic oxidation leads to the formation of large tunnel structured Mn oxides (todorokite and romanechite) on the surface of TiO 2 . These findings suggest that photocatalytic oxidation of Mn 2+ (aq) by natural semiconducting minerals is likely an important yet previously overlooked pathway for understanding the occurrence of natural Mn oxide coatings on rock surfaces. In addition, considering the increasing input of photoreactive engineered nanoparticles into environmental systems, this study shows the potential impacts of nanoparticles on influencing natural redox cycles.
A thermodynamic analysis is made of heat of adsorption based on the Myers-Prausnitz theory of ideal solution of the adsorbed phase. Ray and Box's adsorption equilibrium data are analyzed in light of the theory for the calculation of heat of adsorption of methane, ethane, ethylene, propane, and n-butane and their mixtures on activated charcoal at temperatures of 1 00°to 400°F and pressures to about 200 psi. ^Xdsorption provides a useful means for separating the components in natural gas and other industrial gas streams. Several industrial processes for recovering ethane and heavier hydrocarbons from natural gas are based on adsorption on activated carbon. Thermodynamic properties of the adsorption systems are needed in engineering studies of such processes.
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