The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt

Abstract

This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD) waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters were selected to cover the spectrum of pH (2.2-3.5) and chemical composition (e.g., 319-2,103 mg/L Fe; 2.85-33.3 g/L SO ¼ 4 ) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH <3.5 (stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO ¼ 4 anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous free cations (Pb 2+ , Zn 2+ , Cu 2+ , Cd 2+ , Mn 2+ , Co 2+ , Ni 2+ ) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely scavenged during the oxidation stage at pH <3.5. In waters with high activities (>10 )1 ) of the SO ¼ 4 ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH <5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60-80% for Pb, to less than 20% for Zn, Co, Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB.

Impacto del drenaje ácido de mina en las características hidrogeoquímicas del estuario de los ríos Tinto y Odiel (SO España)

“…Likewise, a part of these dissolved metals is absorbed as solid particles that compose the SPM and cause a progressive increase of the concentration of these metals in this phase. This process has been observed in laboratory of neutralization of waterproof (Achterberg et al, 2003;España et al, 2006). Zone 3.…”

Use of Multivariate Statistics in Order to Understand the Flow of Acid Rock Drainage from an Abandoned Mining Site

“…Depending on the chemical composition of the leachates, precipitated phases would be ferrihydrite (Fe(OH) 3 ), goethite (FeOOH), jarosite ((K, Na, H)Fe 3 (SO 4 ) 2 (OH) 6 ), melanterite (FeSO 4 ⋅7H 2 O) or schwertmannite (Fe 8 O 8 (OH) 6 SO 4 ). Jarosite and schwertmannite are the most frequently reported phases in waters with low pH and high sulphate concentrations.…”

“…Jarosite and schwertmannite are the most frequently reported phases in waters with low pH and high sulphate concentrations. Jarosite precipitates in strongly acidic environments (pH <≈ 2.5) [2] whereas schwertmannite is formed under less acidic conditions (2.8 < pH < 3.2) and is generally the mineral favoured to precipitate at this pH and high redox conditions [3]. There is a latent acidity coupled with continued weathering and de-watering of these basic ferric sulphate minerals as all eventually will be converted to goethite [4].…”

“…When carbonate minerals are depleted, pH is regulated by iron-and aluminumoxyhydroxides (for example ferrihydrite (Fe(OH) 3 ) and gibbsite (Al(OH) 3 ), which involves a lowering of pH to around 4 [6]. Aluminium hydroxides generally buffer pH between 3.7 -4.4 while iron hydroxides buffer between pH 3.3 -3.7.…”

“…As Equation (1) proceeds and generates ferrous iron, the ferrous iron is further oxidized according to Equation (3). The following precipitation reactions of ferric hydroxides (Equation (4)) are highly acid producing.…”