The decomposition of the solid solution of potassium jarosite with chromium(VI) in Ca(OH)2 media was studied in the present work. According to experimental results, the incorporation of CrO42- into the crystal structure of jarosite resulted in a solid solution with the following approximate formula: [K0.86(H3O)0.14]Fe2.67[(SO4)1.23(CrO4)0.77][(OH)5.01(H2O)0.99]. The experimental data describe a reaction based on the shrinking core model with chemical control for spherical particles. A reaction order of n = 0.67 and an activation energy (Ea) of 63.75 kJ mol-1 were obtained in the induction period (tind). The progressive conversion period is characterized by the diffusion of K+, SO42- and CrO42- ions into the solution. In this stage, n = 1.99 with respect to OH-, and Ea = 51.56 kJ mol-1. The CrO42- diffusion is slower compared to that of sulfate, a slight amount of chromate is adsorbed in the layer of the solid residue consisting on Fe(OH)3. Finally, the equations that satisfactorily describe the reaction process were established from the data obtained.

The efficiency of phenol degradation by the iron complexes ([Fe(TBMA)Cl3∙3H2O] and [Fe(terpy)Cl2]) is compared with that of the Fenton reaction. The results show that although the Fenton reaction efficiently oxidizes phenol at low pH's, the Fenton modified reagents (iron complexes/H2O2) effectively oxidize phenol at neutral pH. Besides, the factorial designing study is performed by considering three independent variables: (i) [Fe] (A), (ii) pH (B), and (iii) [H2O2] (C). For the Fenton reaction, the normal probability plot reveals that two factors, such as concentration of Fe2+ and interaction{[H2O2]∙pH} have considerable influence on the phenol oxidation; in the normal probability plot of the complexes, factors C (concentration of H2O2) and AC {[Fe]∙[H2O2]} have an effect on the oxidation of the phenol by [Fe(terpy)Cl2]/H2O2, while for [Fe(TBMA)Cl3∙3H2O], factors B and AC significantly influence the degradation. Of both iron complexes, [Fe(TBMA)Cl3∙3H2O]/H2O2 is an excellent oxidant, showing a good response at pH 7.0.

La eficiencia en la degradación de fenol por complejos de hierro ([Fe(TBMA)Cl3∙3H2O] y [Fe(terpy)Cl2]) es comparada con la reacción Fenton. Los resultados muestran que aunque la reacción Fenton oxida al fenol a bajo pH, la reacción Fenton modificada (complejos de hierro/H2O2) oxida efectivamente al fenol a pH neutro. También, un diseño experimental se desarrolla considerando tres variables independientes: (i) [Fe] (A), (ii) pH (B) y (iii) [H2O2] (C). Para la reacción Fenton la gráfica de probabilidad normal revela que dos factores, la concentración de Fe2+ y la interacción {[H2O2]∙pH} tienen una influencia considerable en la oxidación de fenol; en las graficas de probabilidad normal para los complejos, los factores C (concentración de H2O2) y AC {[Fe]∙[H2O2]} tienen una influencia considerable en la oxidación de fenol por [Fe(terpy)Cl2]/H2O2, mientras que para [Fe(TBMA)Cl3∙3H2O]/H2O2, los factores B y AC significativamente influencian la degradación. De ambos complejos, [Fe(TBMA)Cl3∙3H2O]/H2O2 es un excelente oxidante, mostrando una buena respuesta a pH 7.0.

This paper studies and determines the dispersion properties (Jg, Eg and Db), kinetics parameters and hydrodynamics of the process and its effect on the recovery of silver contained in spent diluted fixers by techniques of ion flotation in columns. The experimental results show silver recoveries of 97 % using sodium isopropyl xanthate (SIX) 0.06 g-L-1 and 0.04 g-L-1 of frother, at a Jg of 1.0 cm-s-1 and Jl of 0.72 cm-s-1. Xanthate-promoter combinations do not improve the separation; however, recoveries of 91.5% are achieved. The control of pH at 6.0 during the ion flotation of silver helps to improve the separation, compared to pH values of 7.0 and 8.0. The kinetic constant of apparent flotation k (min-1) maintains a linear behavior with the recovery and the dispersion properties, until reaching a slope change in the plot of Eg Vs Jg, where the system changes bubble flow regime, from homogeneous to turbulent.

Este estudio investiga y determina las propiedades de la dispersión (Jg, Eg y Db), los parámetros cinéticos y la hidrodinámica del sistema y su efecto en la recuperación de plata contenida en líquidos fijadores diluidos agotados, mediante la flotación iónica en columnas. Los resultados experimentales muestran recuperaciones de plata del 97 %, usando xantato isopropílico de sodio (XIS) en concentración de 60 mg L-1 y 40 mg L-1espumante, Jg de 1.0 cm·s-1, y Jl de 0.72 cm s -1. Combinaciones de xantato y promotor no mejoran la separación, sin embargo, se logran recuperaciones del 91.5%. El control del pH en 6.0durante la flotacióniónica de plata contribuyea mejorar la separación respecto a los pH de 7.0 y 8.0. La constante cinética de flotación aparente mantiene un comportamiento lineal con la recuperación y las propiedades de la dispersión, hasta el cambio de pendiente en el grafico de Eg Vs Jg, donde el sistema cambia de régimen de flujo de homogéneo a turbulento.

Uma amostra de arsenojarosita de potássio foi sintetizada e completamente caracterizada. A amostra obtida é uma solução sólida de arsenojarosita de potássio, cuja fórmula aproximada é [K0,75(H3O)0,25 ]Fe1,84[(SO4)1,82(AsO 4)0,18][(OH)2,34(H 2O)3,66]. O processo de decomposição em meio alcalino foi estudado no período de indução e de conversão progressiva, e a ordem de reação e a energia de ativação foram determinadas para cada caso. Nas condições experimentais utilizadas, os resultados são consistentes com modelo de partícula esférica com núcleo decrescente e controle químico. Em ambos os processos, quatro modelos parciais e dois modelos globais foram desenvolvidos para descrever seu comportamento básico. Os modelos foram validados, e foi provado que descrevem favoravelmente o processo de decomposição em meio alcalino.

A sample of potassium arsenojarosite was synthesized and thoroughly characterized. The obtained sample is a solid solution of potassium arsenojarosite, whose approximate formula is [K0.75(H3O)0.25 ]Fe1.84[(SO4)1.82(AsO 4)0.18][(OH)2.34(H 2O)3.66]. The decomposition process in alkaline medium was studied in the induction and progressive conversion periods, and the reaction order and activation energy were determined for each case. Under the used experimental conditions, results are consistent with the spherical particle model with decreasing core and chemical control. In both processes, four partial models and two global models were developed in order to describe their basic behavior. The models were validated, and it was proved that they favorably describe the decomposition process in alkaline medium.

The nature of the reaction and the main parameters affecting Ag precipitation rate in the system Ag+-CN--S2O4(2-) are studied. From the results obtained, a process of chemical precipitation for Ag recovery is proposed, using an environmentally-friendly reducing reagent (sodium dithionite, Na2S2O4), leaving a residual content of Ag into the permitted environmental levels (<5 mg L-1 Ag). The levels of CN- in liquids wastes are of the order of 0.816 mol L-1, and they can be treated by ozonization processes for inerting cyanide. The results obtained indicate that Ag precipitation is achieved in less than 1 hour, with recoveries close to 99%, at room temperature. Silver precipitation is characterized by an induction period followed by a precipitation period, leaving a metal amount in solution less than 1 mg L-1. The main kinetic parameters studied lead to obtain the following apparent reaction orders: with respect to the initial [S2O4(2-)] was n = 1.675, with respect to [CN-] was -1.24, with respect to [OH-] was ≈0 (between 10-4 and 10-2 mol L-1) and with respect to the initial [Ag+] was 0.524. An activation energy of 77 kJ mol-1 was obtained. Under these conditions, Ag precipitation in the system Ag+-CN--S2O4(2-) is controlled by chemical reaction, characterized by high activation energy, and is independent of the hydrodynamic variables of the system.

Se ha estudiado la naturaleza de la reacción y los principales parámetros que afectan la velocidad de precipitación en el sistema Ag+-CN--S2O4(2-). De los resultados obtenidos, se propone un proceso de precipitación química para la recuperación de Ag usando un reactivo reductor amigable con el medio ambiente (ditionito de sodio, Na2S2O4), que deja un residuo de Ag dentro de los niveles ambientales permitidos (< 5 mg L-1 Ag). Los niveles de CN- en los líquidos residuales son del orden de 0.816 mol L-1 y pueden ser tratados por procesos de ozonización para la inertización del cianuro. Los resultados obtenidos indican que la precipitación de Ag se logra en menos de una hora, con recuperaciones cercanas a 99% a temperatura ambiente. La precipitación de la plata se caracteriza por un periodo de inducción, seguido de un periodo de precipitación, dejando una cantidad de metal en solución de menos de 1 mg L-1. Los principales parámetros cinéticos estudiados llevan a la obtención de los siguientes órdenes de reacción aparentes: Con respecto a la [S2O4(2-)] inicial, fue de n = 1.675; con respecto a [CN-], fue de -1.24; con respecto a [OH-], fue de ≈ 0 (entre 10-4 y 10-2 mol L-1), y con respecto a la [Ag+] inicial, fue de 0.524. Se obtuvo una energía de activación de 77 kJ mol-1. Bajo estas condiciones, la precipitación de Ag en el sistema Ag+-CN--S2O4(2-) está controlada por la reacción química, se caracteriza por una elevada energía de activación y es independiente de las variables hidrodinámicas del sistema.

Jarosites são compostos que podem sofrer substituições com vários elementos de importância ambiental (tais como As5+) durante a precipitação. Arsênico integrado na estrutura poderia influenciar a solubilidade da jarosita, potencialmente estabilizando a estrutura em uma ampla gama de condições que são toleradas pela jarosita pura. A reatividade álcali é caracterizada pela remoção de íons sulfato e sódio da rede e a formação de um gel composto de hidróxido de ferro com arseniato adsorvido. As curvas de decomposição mostram um período de indução seguida por um período de conversão. O período de indução é independente do tamanho das partículas e decresce exponencialmente com o aumento da temperatura. O período de conversão é caracterizado pela criação de um halo de hidróxidos em torno de um núcleo de natrojarosita arsênica não reagido. Dados cinéticos são consistentes com o controle químico do processo. A expressão obtida em meio NaOH para as concentrações de [OH-] que vão desde 3,84 × 10-3 para 1,08 × 10-1 mol L-1 é: r0/-v[1 - (1 - x)1/3] = 3,11 × 10(9) exp(-57,110/RT) [OH-]0.7 t. A expressão em meio Ca(OH)2 para as concentrações de [OH-] que vão desde 2,21 × 10-2 para 6,98 × 10-2 mol L-1 é: r0/-v[1 - (1 - x)1/3] = 9,22 × 10(11) exp(-48,610/RT) [OH-]1.51 t.

Jarosites are compounds that can undergo substitutions with several elements of environmental importance (such as As5+) during precipitation. Arsenic integrated in the structure could influence the solubility of the jarosite, potentially stabilizing the structure under a wide range of conditions that are tolerated by pure jarosite. Alkaline reactivity is characterized by the removal of sulfate and sodium ions from the lattice and by the formation of a gel consisting of iron hydroxides with adsorbed arsenate. The decomposition curves show an induction period, followed by a conversion period. The induction period is independent from the particle size and decreases exponentially as the temperature increases. The conversion period is characterized by the formation of a hydroxide halo around an unreacted arsenical natrojarosite core. The kinetic data are consistent with the chemical control of the process. The expression obtained in NaOH medium for [OH-] concentrations ranging from 3.84 × 10-3 to 1.08 × 10-1 mol L-1 is the following: r0/-v[1 - (1 - x)1/3] = 3.11 × 10(9) exp(-57.110/RT) [OH-]0.7 t. The expression in Ca(OH)2 medium for [OH-] concentrations ranging from 2.21 × 10-2 to 6.98 × 10-2 mol L-1 is the following: r0/-v[1 - (1 - x)1/3] = 9.22 × 10(11) exp(-48.610/RT) [OH-]1.51 t.