Ecotoxicity with Allium cepa to Determine the Efficiency of ARD Treatment by Neutralization/Precipitation from a Brazilian Coal Mine
dos Santos, Karine Batista (1); de Almeida, Vítor Otacílio (1); Weiler, Jéssica (1); Menezes, Jean Carlo Salomé dos Santos (2); Schneider, Ivo André Homrich (1)
1: Universidade Federal do Rio Grande do Sul, Brazil; 2: Universidade de Passo Fundo, Brazil
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. The ARD studied came from coal mine located Paraná State, Brazil, highly associated with pyrite. This wastewater is concentrated in iron and sulfate 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, 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.
It is possible to conclude that, in terms of chemical parameters and the toxicity with Allium cepa, active treatment of ARD from brazilian coal mines should operate with Ca(OH)2 at pH 8.7 +/- 0.1.