Resumo

REYES, Iván A. et al. Alkaline reactivity of arsenical natrojarosite. J. Braz. Chem. Soc. [online]. 2011, vol.22, n.12, pp. 2260-2267. ISSN 0103-5053.  http://dx.doi.org/10.1590/S0103-50532011001200004.

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 × 10⁹ 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 × 10¹¹ exp(-48.610/RT) [OH^-]^1.51 t.

Palavras-chave : reactivity; arsenical natrojarosite; kinetics; arsenate; decomposition; amorphous.