This research studied the bio-protection mechanism based on chemical gradients in diffusive exchange permeable reactive barriers, evaluating the thickness of the reactive layers in the treatment of concentrated acid mine drainage (AMD). Six bench-scale reactors were constructed with reactive layer thicknesses of 2.5, 5, and 7.5 cm in duplicate. The reactors were first fed a sulfated solution for 55 days, followed by concentrated AMD for 166 days. The change of feed to AMD mainly affected the reactors with thinner 2.5 cm layers in comparison to the reactors with 5 and 7.5 cm layers. Cu and Zn removal efficiency was practically 100% in all the reactors; however, in the thinner layer reactors, metal breakthrough occurred towards the end of the experiment concurrently with inhibitory metal concentrations in the reactive layers. On the contrary, the reactors with layer thicknesses of 5 and 7.5 cm evaluated did not present toxic concentrations of these metals at any of the monitoring points. The bio-protection criterion qD correctly predicted that the thin-layer reactor would be the most affected by the toxicity of AMD. The criterion also indicated that all the reactors should fail. Nevertheless, the fault in the thinner layer reactor registered in the effluent after >150 days; therefore, the possibility of failure in the 5 and 7.5 cm thickness reactors is not rejected, as it could have occurred if the experiment had continued.
Keywords: AMD; Bio-protection criterion; Diffusive exchange; Metal toxicity; Permeable reactive barrier.
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