Most phenomena described in introductory university courses of chemistry have a solid thermodynamic conceptual foundation. It is unavoidable, at this initial level, to treat thermodynamic concepts partially (especially the second principle), and to use examples and applications corresponding to very definite and simple cases. In these conditions, great care must be exerted to help students avoid the misconceptions that they frequently adopt after being exposed to this partial vision of thermodynamics. The following are the most frequent ones: 1) to incorrectly extend to all isothermal processes in any system, the conclusion that the internal energy change is null for all isothermal expansion-compression processes of an ideal gas, 2) to unrestrictedly associate enthalpy changes with transferred thermal energy, 3) to unrestrictedly use free energy changes as spontaneity criteria. We discuss here how these false interpretations commonly develop, and propose possible alternatives to decrease their frequency.

La termodinámica es una herramienta fundamental en cualquier curso universitario inicial de química ya que constituye una base conceptual sólida sobre la cual se pueden justificar los aspectos fenomenológicos de diversos temas. Naturalmente, en el nivel inicial, la termodinámica, y especialmente el segundo principio, son tratados sólo parcialmente, con la consecuencia de que los ejemplos y aplicaciones suelen corresponder a casos muy particulares y sencillos que no sólo transmiten una visión incompleta sino que, con frecuencia, suelen conducir a desconceptos o a errores de interpretación por parte de los alumnos. Entre estos últimos, podemos enumerar los siguientes: 1) la extensión errónea de una variación de energía interna nula a procesos distintos de la expansión-compresión isotérmica de un gas ideal, 2) la asociación sin restricciones de la variación de entalpía al calor puesto en juego en un proceso y 3) el uso de la variación de energía libre como criterio de espontaneidad sin restricciones. En este trabajo, presentaremos éstos y otros ejemplos en los cuales suelen detectarse falsas interpretaciones además de una discusión sobre posibles alternativas para disminuir la frecuencia y la incidencia de estos desconceptos.

The microbial reduction of chromium(VI) to chromium(III) has been one of the most widely studied forms of metal bioremediation. Recently, we have found that Thiobacillus ferrooxidans and Thiobacillus thiooxidans, growing on elemental sulphur, can indirectly promote chromium(VI) reduction by producing reducing agents such as sulphite and thiosulphate, which abiotically reduce chromium(VI). Those species of Thiobacillus are acidophilic bacteria which grow optimally at pH values lower than 4. However, most of those reducing agents are stabilised at higher pH values. Thus, the present paper reports on the ability to reduce chromium(VI) using another specie of Thiobacilli, Thiobacillus thioparus, which is able to grow at pH close to 7.0. T. thioparus cultures were carried out in a fermentation vessel containing medium and sulphur as the sole energy source and maintained at 30ºC and 400 rpm. The pH was adjusted to 6.0, 7.0 or 8.0 and maintained with the automatic addition of KOH. Our results show high chromium (VI) reduction values (close to 100% at the end of bacterial growth) at the three pH values. The results of these experiments are very promising for development of a microbiological process to be used in the detoxification of chromium(VI)-polluted effluents.

Copper recoveries using two methodologies of acid leaching of a low-grade ore are reported in this work. The ore, abounding in oxidized species, was from Barda González reservoir in Neuquén province, Argentina. In the first methodology, cells of Acidithiobacillus thiooxidans were inoculated into a column containing the ore and sulfur as energy source. In the second one, a bioreactor with immobilized cells of Acidithiobacillus thiooxidans placed on small pieces of elemental sulfur was used to generate acid medium. This medium was then transferred to another column containing the ore. In the first methodology low cell counts and also low copper recovery were obtained. However, in the other, about 56 % of copper was recovered after four loads of fresh medium.

Cyanidation is a relatively simple and cheap technology to treat gold-containing ores. However, this process is not completely effective in refractory gold-bearing sulfide ores. In the case of very low-grade ores, biooxidation is an attractive alternative pre-treatment to render the gold amenable to extraction. This process utilizes the ability of acidophilic bacteria to oxidize the sulfide matrix, physically freeing the gold. The aim of this work is to evaluate the recovery of other available metals (Fe, Mn, Cu and Zn) during the biooxidation pre-treatment of an ore from Andacollo with low gold recovery (less than 50 %). This pre-treatment was carried out in shaken flasks using pure cultures of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans and a mixed culture with both bacteria.

In the absence of iron, dissolution of zinc sulfide was enhanced by the action of Acidithiobacillus caldus at 40ºC. The bioleaching mechanism was similar to that observed for mesophilic species of Acidithiobacillus at 30ºC, although the final metal recovery was lower. When iron was added to the cultures, the solubilization of zinc and copper from the sulfides was higher than that in sterile controls. The activity of the cells was through two indirect mechanisms (acid and oxidant mechanisms, for zinc and copper sulfides respectively). A. caldus did not enhance the dissolution of nickel sulfide neither in the absence nor in the presence of iron.