Biological and electrochemical treatments of coal mine-impacted water (MIW): toxicological evaluation on the duckweed Landoltia punctata

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• Two treatments for coal mine-impacted water (MIW) were evaluated toxicologically; • Potential secondary use of treated water was assessed; • Duckweeds are adequate for toxicological evaluation; • MIW treated via electrocoagulation has potential for non-potable reuse; • MIW treated via biostimulation of sulfate-reducing bacteria was highly toxic. Background: Two different coal mine-impacted water (MIW) treatments (biological via biostimulation of sulfate-reducing bacteria (SRB), and electrocoagulation (elC)) were proposed, reaching efficiencies of up to 99.79% in relation to SO 4 2− , Fe, Mn, and Al ions, as well as acidity removals. The MIW, a high pollutant potential effluent, consists of river water impacted by coal acid mine drainage (AMD), which is acid, rich in sulfate and dissolved metallic ions, and consequently toxic. The aim of this study was to make a toxicological evaluation of MIW after biological and electrochemical treatments on Landoltia punctata, in order to verify the safeness and usability of the two treated waters. Material and method: Duckweeds were exposed to different dilutions (0, 25, 50, 75, and 100% of samples) of the two treated waters, and the growth (r) and inhibition of growth ( I r ) rates were calculated, based on 50% effect concentration (EC 50 ). Results: The water from the biological treatment (microcosm assay) presented the highest toxicity (EC 50 = 33.42%), even higher when compared to the raw MIW (EC 50 = 42.78%), probably due to the hydrogen sulfide, that even after a purge removal, remained in solution. The results showed that this water, despite being within the standards in physicochemical terms, demonstrated risks in terms of toxicity. The water from electrocoagulation (elC) treatment, in the opposite way, showed much less toxicity, even lower than the control, and therefore not reaching EC 50 , also suggesting a possible nutrient function of the treated water. Consequently, the treated water by elC could, for example, has potential for non-potable use. Conclusions: The study made it possible to prove the efficiency of elC treatment (72%, 99.79%, 85%, and 87.36% for SO 4 2− , Fe, Mn, and Al, respectively), the importance of post-treatment toxicological assessments, and the potential of the duckweeds as an option for a test organism in these types of evaluations. Graphical Abstract. .