Long-term operation of a permeable reactive barrier with diffusive exchange

Abstract

In this study, we evaluate the long term operation of a bench-scale reactor which simulates a permeable reactive barrier with sulfidic diffusive exchange (SDES PRB) to treat acid mine drainage (AMD), considering that treatment costs are very sensitive to the useful life for passive reactors. Its functioning was evaluated for a much longer period of 591 days compared to previous SDES PRB studies, with two influents simulating moderately and highly acid groundwater contaminated by AMD. First, we fed water amended with 200 mg/L Zn²⁺ and 3300 mg/L SO₄^2- at pH 4.9; and after, water with 450 mg/L Fe²⁺, 100 mg/L Zn²⁺, 10 mg/L Ni²⁺, 5 mg/L Cu²⁺ and 3600 mg/L SO₄^2- at pH 2.5. Biologically produced sulfide and alkalinity were enough to remove both metals and acidity (~99%) from the moderately acidic water, while with the highly acidic water, they resulted in significant removal of the metals reaching up to 87% and 79% of total Fe and Zn, respectively. Furthermore, no inhibitory effect was apparent, as the sulfate reduction rates in the two experiments did not vary significantly (averages close to 0.2 mol/m³-d), despite the much higher acidity and metal load in the second case. Hence, the SDES PRB protected the microbial consortium from metal toxicity and acidity in the long-term, and thus is suitable for remediation of AMD contaminated groundwater with high concentrations of metals, extending the operational range of conventional biological PRBs. Furthermore, an economic evaluation shows that SDES costs can be competitive with the costs of conventional chemical precipitation if the enhanced reactivity that SDES technology offers is realized.

Keywords: Acid mine drainage; Diffusive exchange; Metal toxicity; Permeable reactive barrier; Sulfate reduction.