Patricia Acero

Kinetics of chalcopyrite dissolution at pH 3

The management of mine wastes and the quantitative forecast of Acid Rock Drainage ( ARD ) generation have aroused widespread interest. In order to be coupled with gas and water flux models, chalcopyrite dissolution kinetics at pH 3, at temperatures from 25 to 70 °C and at variable dissolved oxygen concentrations was studied in flow-through experiments. In the range of conditions under study, the rate of chalcopyrite dissolution is independent of the concentration of dissolved oxygen. This enables chalcopyrite to dissolve throughout the whole waste piles. The rise in temperature from 25 to 70 °C produces an increase of one order of magnitude in the chalcopyrite dissolution rate. As a result of this study, the following expression for chalcopyrite dissolution rate was obtained: [mathit{R_{chalcopyrite}} = 10^{{-}5.52{pm}0.07}mathit{e}^{frac{{-}32{pm}5}{mathit{RT}}}] where R chalcopyrite is the chalcopyrite dissolution rate (molm −2 s −1 ), R is the gas constant (kJmol −1 uK −1 ) and T is the temperature (K). In line with earlier studies carried out under different conditions, iron was released to solution preferentially over copper in all the experiments carried out. XPS examination of the samples showed that reacted surfaces were formed by phases enriched in sulfur and copper (relative to iron) compared with the initial, pristine chalcopyrite surface. However, these surface layers allow the progress of dissolution with out passivating the chalcopyrite surface. According to the apparent activation energy obtained (32±5 kJ mol − 1 ) and to the lack of rate variation with stirring, the chalcopyrite dissolution rate seems to be controlled by surface reactions and is independent of the non-stoichiometric surface layer.

Field rates for natural attenuation of arsenic in Tinto Santa Rosa acid mine drainage (SW Spain)

Reactive transport modelling of the main processes related to the arsenic natural attenuation observed in the acid mine drainage (AMD) impacted stream of Tinto Santa Rosa (SW Spain) was performed. Despite the simplicity of the kinetic expressions used to deal with arsenic attenuation processes, the model reproduced successfully the major chemical trends observed along the acid discharge. Results indicated that the rate of ferrous iron oxidation was similar to the one obtained in earlier field studies in which microbial catalysis is reported to occur. With regard to the scaled arsenic oxidation rate, it is one order of magnitude faster than the values obtained under laboratory conditions suggesting the existence of a catalytic agent in the natural system. Schwertmannite precipitation rate, which was represented by a simple kinetic expression relying on Fe(III) and pH, was in the range calculated for other AMD impacted sites. Finally, the obtained distribution coefficients used for representing arsenic sorption onto Fe(III) precipitates were lower than those deduced from reported laboratory data. This discrepancy is attributed to a decrease in the schwertmannite arsenate sorption capacity as sulphate increases in the solution.