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

Abstract

BackgroundTwo 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 SO42−, 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 methodsDuckweeds 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 (Ir) rates were calculated, based on 50% effect concentration (EC50). ResultsThe water from the biological treatment (microcosm assay) presented the highest toxicity (EC50 = 33.42%), even higher when compared to the raw MIW (EC50 = 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 EC50, 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. ConclusionsThe study made it possible to prove the efficiency of elC treatment (72%, 99.79%, 85%, and 87.36% for SO42−, 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. Statement of noveltyThis paper with duckweed enabled the identification of the most suitable treatment to allow the treated water to be (re)used, since both treatments presented high efficiencies in terms of sulfate, acidity, and metal removals. This approach therefore gives a complementary view of the treatment then beyond complying with current environmental legislation to the treated effluent.