ABSTRACT
BACKGROUND: The presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others. Understanding the dissolution reactions of jarosite-type compounds is notably important for an environmental assessment (for water and soil), since some of these elements could either return to the environment or work as temporary deposits of these species, thus would reduce their immediate environmental impact. RESULTS: This work reports the effects of temperature, pH, particle diameter and Cr(VI) content on the initial dissolution rates of K-Cr(VI)-jarosites (KFe3[(SO4)2 - X(CrO4)X](OH)6). Temperature (T) was the variable with the strongest effect, followed by pH in acid/alkaline medium (H3O(+)/OH(-)). It was found that the substitution of CrO4 (2-)in Y-site and the substitution of H3O(+) in M-site do not modify the dissolution rates. The model that describes the dissolution process is the unreacted core kinetic model, with the chemical reaction on the unreacted core surface. The dissolution in acid medium was congruent, while in alkaline media was incongruent. In both reaction media, there is a release of K(+), SO4 (2-) and CrO4 (2-) from the KFe3[(SO4)2 - X(CrO4)X](OH)6 structure, although the latter is rapidly absorbed by the solid residues of Fe(OH)3 in alkaline medium dissolutions. The dissolution of KFe3[(SO4)2 - X(CrO4)X](OH)6 exhibited good stability in a wide range of pH and T conditions corresponding to the calculated parameters of reaction order n, activation energy E A and dissolution rate constants for each kinetic stages of induction and progressive conversion. CONCLUSIONS: The kinetic analysis related to the reaction orders and calculated activation energies confirmed that extreme pH and T conditions are necessary to obtain considerably high dissolution rates. Extreme pH conditions (acidic or alkaline) cause the preferential release of K(+), SO4 (2-) and CrO4 (2-) from the KFe3[(SO4)2 - X(CrO4)X](OH)6 structure, although CrO4 (2-) is quickly adsorbed by Fe(OH)3 solid residues. The precipitation of phases such as KFe3[(SO4)2 - X(CrO4)X](OH)6, and the absorption of Cr(VI) after dissolution can play an important role as retention mechanisms of Cr(VI) in nature.
ABSTRACT
The widespread use of jarosite-type compounds to eliminate impurities in the hydrometallurgical industry is due to their capability to incorporate several elements into their structures. Some of these elements are of environmental importance (Pb(2+), Cr(6+), As(5+), Cd(2+), Hg(2+)). For the present paper, AsO4 (3-) was incorporated into the lattice of synthetic jarosite in order to carry out a reactivity study. Alkaline decomposition is characterized by removal of sulfate and potassium ions from the lattice and formation of a gel consisting of iron hydroxides with absorbed arsenate. Decomposition curves show an induction period followed by a conversion period. The induction period is independent of particle size and exponentially decreases with temperature. The conversion period is characterized by formation of a hydroxide halo that surrounds an unreacted jarosite core. During the conversion period in NaOH media for [OH(-)] > 8 × 10(-3) mol L(-1), the process showed a reaction order of 1.86, and an apparent activation energy of 60.3 kJ mol(-1) was obtained. On the other hand, during the conversion period in Ca(OH)2 media for [OH(-)] > 1.90 × 10(-2) mol L(-1), the reaction order was 1.15, and an apparent activation energy of 74.4 kJ mol(-1) was obtained. The results are consistent with the spherical particle model with decreasing core and chemical control.
ABSTRACT
El objetivo de este trabajo fue determinar si a los 45 y 60 días, el crecimiento del hongo comestible Pleurotus ostreatus en paja de cebada, aumenta el valor nutritivo y digestibilidad de ésta para la alimentación de rumiantes, dada la capacidad lignificante del mismo. Para el caso, se realizaron las siguientes determinaciones en paja de cebada sin tratar (testigo), y en paja de cebada incubada con el hongo en cuestión por 45 ó 60 días: pH, humedad, proteína cruda, cenizas, hemicelulosa, celulosa, lignina, energía bruta y digestibilidad in vitro de la materia seca. Los resultados mostraron que el porcentaje de proteína se mantuvo constante (P < = 0,05) en todos los tratamientos (x 2.67%), aumentando el contenido de cenizas en la paja incubada con el hongo por 60 días. El porcentaje de hemicelulosa y celulosa disminuyó significativamente (P <= 0,05) en la paja incubada por 45 ó 60 días (16.74, 32.24; 17.43, 32.41%, respectivamente), en relación a la paja testigo (24.54; 40.15%), mientras que el de lignina aumentó aunque no en forma significativa en la paja incubada por 45 ó 60 días, en relación con la paja testigo (8.36; 9.10; 9.06%, respectivamente. Los valores de energía fueron menores en la paja incubada por 45 ó 60 días (2.70; 2.74 Kcal/g) que en la testigo (2.80 Kcal/g), no habiendo diferencia en el porcentaje de digestibilidad in vitro de la materia seca entre la paja testigo y la incubada por 45 ó 60 días (56.04; 52.65; 53.06%, respectivamente). Se concluye que la cepa de Pleurotus ostreatus utilizada en este trabajo no fue capaz de delignificar la paja, por carecer de fenoloxidasas, enzimas necesarias para degradar la lignina. Por consiguiente, se estima necesario realizar otros estudios al respecto, a fin de aislar cepas con gran capacidad delignificante, y lograr con ello, un mayor aprovechamiento de las pajas en el país
