Existing methods for dissolving poorly crytalline Fe oxides (primarily ferrihydrite) in soils and mineral mixtures are simple and rapid but often lack selectivity. In this work, we tested a more selective, alternative ascorbate‐based extractant. Ascorbate combined with citrate was found to be highly effective in dissolving poorly crystalline Fe oxides while dissolving little goethite and hematite. Citrate‐ascorbate and oxalate extracted similar amounts of Fe from most of the soils and Fe‐rich materials studied. However, citrate‐ascorbate was more selective than oxalate, since it dissolved only negligible amounts of allophane or imogolite and magnetite. The recommended citrate‐ascorbate extraction procedure (0.2 M sodium citrate‐0.05 M sodium ascorbate, pH 6, 16 h) is simple to implement and relatively inexpensive, and uses nontoxic chemicals. Thus, it can be used on a routine basis for estimating poorly crystalline Fe oxides in soils. By subtracting citrate‐extractable Fe and Al from citrate‐ascorbate‐extractable Fe and Al values, respectively, one can estimate Fe and Al contained in poorly crystalline Fe oxides. The citrate‐ascorbate extraction, combined with others (oxalate, Tiron, hydroxylamine) can help detect and quantify other soil components such as allophane and magnetite.

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Jarosites são compostos que podem sofrer substituições com vários elementos de importância ambiental (tais como As 5+) 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: r 0 /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: r 0 /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 As 5+) 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: r 0 /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: r 0 /v [1 − (1 − x) 1/3 ] = 9.22 × 10 11 exp(−48.610/RT) [OH − ] 1.51 t.

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 [K 0,75 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 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 (Pb2+, Cr6+, As5+, Cd2+, Hg2+). For the present paper, AsO43- 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.