The production of lactic acid by Lactobacillus helveticus from a cheese whey residue obtained from a dairy was investigated. Batch fermentation experiments showed that the optimal temperature to carry out the process was 42°C and the optimum pH was 5.9, giving at these conditions a maximum concentration of lactic acid of 53 g/l. The addition of yeast extract to the cheese whey residue as a nitrogen source for L. helveticus did not increase the lactic acid production significantly. The results obtained in this work confirmed that it is possible to increase the value of residues from the dairy industry for the production of value-added products such as lactic acid, which has important industrial applications. Also, the effect of the addition of a nitrogen source to the culture media of the fermentation experiments did not present a costbeneficial improvement on lactic acid production as other authors have shown. The last finding could reduce the cost of the culture media used for lactic acid production when scaling up the process.

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Some mine tailings pools in the Mine District of Guanajuato, Mexico, present a varied distribution and temporal and spatial concentration of elements that are potentially toxic, such as manganese, cadmium and zinc. These elements were detected in majority concentrations, and arsenic was present in the two major oxidation states As(III) and As(V). The highest arsenic concentration in the surrounding surface water reservoirs was detected when a rainy seasons occurred, which in turn is mainly a function of pH and the presence of bicarbonate ions. The conceptual model to describe the mobilization of arsenic from mining tailings towards the aqueous systems proposes a scenario where oxidation, the neutralization of acid drainage by carbonates, and arsenic desorption by bicarbonates takes place in different steps and at different times.

Resumen En este trabajo se presenta un método basado en el principio de estados correspondientes para predecir la densidad de un fluido con un mínimo de información. El método se basa en una formulación molecular del principio de estados correspondientes, utilizando metano como fluido de referencia. Las densidades de las distintas mezclas de gas natural se calcularon a temperaturas entre 95 y 150 K y se compararon con datos experimentales y estimaciones de densidad de ocho ecuaciones de estado y dos correlaciones. Este método de estados correspondientes es la correlación más exacta y confiable para predecir densidades de líquido saturado, presenta desviaciones menores al 0.1% para gas natural y menores al 0.2% para gas licuado de petróleo.

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