Release of metals from synthetic Cr-goethites under acidic and reductive conditions: Effect of aging and composition.

ANA E. TUFO; ELSA E. SILEO; PEDRO J. MORANDO

APPLIED CLAY SCIENCE

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2012 p. 88 - 95

0169-1317

The dissolution mechanism of pure and Cr-substituted, synthetic goethites with different aging times, as well as the changes in morphology, crystallinity and structural changes, were investigated. The Cr-goethites were obtained by synthesizing goethite in solutions that contained different concentrations of Cr(III) ions. The obtained suspensions were aged in the parent solutions for different time spans. Two samples of pure goethites, prepared using different KOH concentrations, were also studied for comparative purposes. Chemical analyses showed that Cr-for-Fe substitution was greatest in samples aged for longer time. Rietveld simulation of XRD powder data indicated that the unit cell parameters of pure goethite increased for samples prepared at a higher KOH concentration, and decreased with Cr-incorporation. Simulation also showed that the mean coherence path dimension (MCP), or crystallite size, in the direction perpendicular to crystal plane (110), did not change much in the samples, whereas the crystallite size in the direction parallel to crystal plane (110) increased with Cr-content, KOH concentration and aging time. The obtained MCP values indicate an increasing elongated form in the domains. Chromium substituted goethites presented a good capacity for immobilizating Cr when the dissolution rates were studied in 3.98 M HCl, 0.10 M oxalic acid (H2Oxal), and reductive media (H2Oxal/Fe(II)). The shape of the f vs. t profile, where f stands for the fraction of Fe dissolved (dissolved Fe mass/total Fe mass) and t refers to the dissolution time, follows a contracting bidimensional model, and when dissolved in similar acid concentrations,the reactivity follows the trend HClbH2Oxal≪H2Oxal/Fe(II), indicating that dissolution is noticeably increased in complexing-reducing media. The dissolution rate constant k values were highly depend on crystallite size, and the dissolution rate decreased in the samples aged longest and in goethites containing higher concentrations of Cr. The release of Fe and Cr is congruent and shows a homogeneous distribution of Fe and Cr in the samples. The calculated activation energies values support a surface reaction control for the dissolution process.