RESUMO
Organic compounds are employed as additives to increase the dissolution speed of gold, in concentrations around 1 g/L when using cyanidation, thereby forming a residual aqueous effluent with high amounts of free cyanides and organic compounds, which generate metallic complexes difficult to degrade. To increase the photodegradation efficiency, promising niobium and titanium porous materials are proposed as photocatalysts, due to their role in simultaneous oxidation and reduction reactions. In the process of cyanide oxidation, NbO5 0.3H2O was doped with titanium oxalate (IV) of 0.5, 1, and 1.5%; and HTiNbO5 were synthesized, from the mixture of NbO5 with TiO2 Degussa-P25, by coprecipitation, impregnation, and solid state. The determination of its elemental composition, morphological and textural properties were carried out by using various XRD techniques, Raman spectroscopy, SEM/EDS and acidity by pyridine. The experiments of photocatalytic oxidation of cyanide used one semibatch reactor with ultraviolet irradiation 125 W in a pH range of 9.5-12. The catalyst with the highest percentage of degradation was HTiNbO5 93.7%, which is attributed to the microstructure of the double layer and Lewis acidity sites, followed by NbTi-1% 92.9% and the Nb2O5.3H2O 82.4%, being the majority product cyanate, proposing its mechanism of reaction. Characterization experiments indicated Nb-O-Ti bridges that have been associated with the control of redox properties of the niobium species and Ti-O-Nb = O, which could be generating a greater number of e-H +pairs, increasing the photocatalytic activity. It is considered that the method of synthesis has a strong influence in changing the morphology of the particles such as porosity, specific surface and factors such as the acidity of niobium-based catalysts, which are important to achieving efficiency in degradation. Niobium-Titanium photocatalysts proved to be an excellent new breakthrough in Advanced Oxidation Technologies (AOT), to eliminate cyanide in wastewater from mining activities.
RESUMO
Los monumentos pétreos declarados como patrimonio histórico son deteriorados a través del tiempo por efecto de agentes físicos, químicos y medioambientales. Estos últimos dependen en gran medida de la ubicación geográfica del inmueble, dichos agentes son: el régimen de vientos que desgasta la roca erosionándola; temporadas de lluvia que al contener sales disueltas causan corrosión por reacciones químicas de estas con la matriz del material calcáreo solubilizando el carbonato de calcio el cual es su principal componente; radiación solar durante todo el año causante de decoloramiento; humedad que permite el establecimiento de formas de vida. Las actividades antropogénicas que no son amigables con el ambiente son otro factor determinante, las emisiones de gases vehiculares por ejemplo, deterioran en gran medida los monumentos de piedra antiguos por ser agentes ácidos altamente corrosivos. Se crean así las condiciones para el establecimiento y proliferación de organismos vivos en la piedra tales como bacterias, algas, hongos, líquenes y plantas que por ser invasivos y colonizadores causan biodeterioro. Dado que los metabolismos de estas especies involucran el consumo de sustratos químicos orgánicos e inorgánicos presentes en la roca, son otro factor que va en detrimento de la estabilidad, durabilidad, y apariencia estética de la misma.
Stony monuments declared as cultural world heritage are deteriorated through the years because of physical, chemical and environmental agents which depend largely on their geographic location. Among these agents are: gust of, winds that wear away the stone eroding it; rainy seasons which bring dissolved salts and cause corrosion because of their chemical reaction with the calcareous material matrix which solubilizes calcium carbonate which is their main component; because of solar radiation all through the year stones lose their color; high humidity allows plants and living forms to grow. Anthropogenic activities which are not environmentally friendly are other important factor: vehicle gas emissions, for example, deteriorate greatly old stone monuments because they are highly corrosive acids. This way, optimal conditions are created for fungi, algae, bacteria, lichens and plants which are invasive and colonizing to live on the stone surface and cause biodeterioration. Since metabolisms of these species involve consumption of inorganic substrates that are present in the stones, they become another factor affecting monument's durability, stability, and aesthetic appearance.