Mineralogical and Hydrochemical Behaviour of a DAS-Passive Treatment System In SW Spain

Many of the passive system treatments treating acid mine drainage (AMD) have been focused on treating AMD with low to very low metal concentration, for instance, as those existing in coal mining districts. However all these passive treatment systems (anaerobic wetlands, sulphate reducing bioreactors, anoxic limestone drains (ALD), etc) show severe problems of clogging or reactivity loss when exposed to AMD with high metal concentrations. To overcome all these problems we have developed a novel Dispersed Alkaline Substrate (DAS) consisting of a mixture of wood chips (coarse inert matrix increasing hydraulic conductivity) and an alkaline reactive substrate (fine-grained limestone to raise pH and generate alkalinity).

Mina Esperanza DAS passive system treatment comprises a DAS-reactive pool (15m*8m*4m length-width-depth) and a decantation pond (10m*3m*2m), both connected by stepped concrete open channels.

At present the system has been working for a period of 12 months having treated a mean inflow rate of 0.5 l/s. Water analyses are available for the firsts 6 months of work and show encouraging results. The system eliminates a net acidity of 1500 mg/L as CaCO₃ from the inflow water and removes 100% Al, Cu, Pb, Cd, V, 90-100% As, Cr, 75-90% Zn, 45% Fe and 20% SO₄^2-.

Iron removal takes place along all the system although the greater precipitation happens at the surface of the DAS-reactive pool. This iron removal is controlled by the precipitation of schwertmannite, Fe₈O₈(OH)₆(SO₄). The precipitation of this mineral leads to the complete removal of As and Ti by processes of adsorption or coprecipitation.

The precipitation of hydrobasaluminite (Al₄ (SO₄)(OH)₁₀ •(12-36)H₂O), at pH around 5, controls the removal of Al inside the reactive material and promotes the sorption or coprecipitation of Cu, Cd, V, Be and Cr.