Ecotoxicity with Allium cepa to determine the efficiency of conventional ARD treatment by neutralization/ precipitation from a brazilian coal mine

摘要

ARD (acid rock drainage) represents a major problem in the mining industry worldwide due to the risk of water and soil pollution. The active treatment of ARD involves the addition of alkaline reagents to increase the pH and precipitate the dissolved metals as hydroxides. The ARD treatment plants in Brazil mostly use sodium hydroxide or lime for neutralization. However, even after the treatment the water contain substantial amounts of dissolved ions. The aim of this work was to assess the chemical and physical characteristics of the treated effluent and evaluate the toxicity using Allium cepa (onion) as the organism test. Onion as a bioindicator has been widely used in tests to examine effects caused by toxic metals present in wastewaters. The ARD studied came from coal mine located in Brazil, highly associated with pyrite. This wastewater is concentrated in iron, sulfate ions with the presence of aluminum, manganese, zinc and small amounts of lead and arsenic. The following treatment condition were tested accordingly the alkaline chemical reagent and neutralization pH: NaOH - pH 7.0 +/0.1;;NaOH - pH 8.7 +/-0.1;;Ca(OH)_2 - pH 7.0 +/0.1;;Ca(OH)_2 - pH 8.7 +/- 0.1. All treatment conditions resulted in a considerable reduction of the concentration of the metals, although Mn an As still remained above or very near the maximum limits for discharge accordingly to the Brazilian legislation. Best results in terms of overall metal removal, including manganese and sulfate removal and final conductivity were achieved with Ca(OH)_2 at pH 8.7. Phytotoxicity tests were performed with Allium cepa in order to verify the efficiency of the adopted processes. The results showed that the raw ARD drainage was toxic, since it was able to cause total inhibition of root growth in the phytotoxicity assay. On the other hand, the treatments generated clarified water that does not present toxicity. There were no inhibition nor a delay in the growth of Allium cepa roots when compared to the control sample. Treatment Ca(OH)_2 at pH 8.7 presented a little bit better performance in terms of roots growth compare to the other conditions applied.