Traitement de la salinité sulfatée et de la toxicité associée des effluents miniers au moyen de l'électrocoagulation

Bio-electrochemical processes (e.g. electrocoagulation and microbial fuel cells (MFCs)) represent promising alternatives to conventional processes, for wastewater treatment in general and mine drainage (MD) in particular. They expanded for several years for the removal of metals (Fe, Cu, Zn, Pb, Cr, Al, Ni) and sulfates. However, their applicability for the treatment of highly contaminated MD depends strongly on the quality of the effluent to be treated, the metal and sulfate removal objectives as well as the operating conditions used. Depending on the quality of MD, the mechanisms for removing sulfates, one of the major contaminants common to MD, may differ. Indeed, for acid mine drainage (AMD), removal can be carried out either by precipitation with the cations present in the solution (e.g. metal ions) using electrostatic forces, or by coagulation, following the formation of the coagulant in the solution following anodic dissolution. For neutral mine drainage (NMD) or alkaline mine drainage, sulfates can be adsorbed on neoformed amorphous metal hydroxides, depending on the charge of their surfaces. Metals are removed by precipitation in the form of metal hydroxides because of oxidation-reduction reactions occurring on both sides of the anode and cathode. A co-precipitation mechanism can also be complementary to precipitation to ensure better metal removal. As part of this project, an in-depth evaluation of the performance of electrocoagulation for the treatment of MD highly contaminated with iron and/or sulfates and the toxicity associated with salinity was carried out. In this context, the main objectives of the thesis are the following: (1) the comparison of the effectiveness of MFC and electrocoagulation for the treatment of synthetic mine effluents highly contaminated with iron; (2) the evaluation of the effectiveness of electrocoagulation, for the treatment of sulfated salinity present in synthetic and real mine effluents of variable qualities; and (3) the comparative evaluation of the toxicity of sulfated salinity contained in highly contaminated MD before and after electrocoagulation treatment on two aquatic species (Daphnia magna and Daphnia pulex). To achieve these objectives, the methodological approach used was to perform laboratory tests (in batch mode), without flow, by varying several operating parameters and then to perform toxicity tests in the laboratory of the Centre d'Expertise en Analyse Environnementale du Québec (CEAEQ) to evaluate the effectiveness of electrocoagulation in treating toxicity. In the first part of the project, batch tests were carried out to evaluate the comparative performance of MFC and electrocoagulation for the treatment of iron-rich AMD (500 mg/L). The purpose of these tests was to evaluate the efficiency of the two processes in neutralizing acidity and removing iron as well as to estimate their operating costs to select the most efficient process for future tests. In this context, MFC tests were compared with electrocoagulation tests performed in a batch reactor with a working volume of 0.25 L containing a synthetic AMD (pH = 2.5 and [Fe] = 500 mg/L). An evaluation of the influence of the electrode material (iron and aluminium the reaction time (15, 30, 45, 60 and 90 min) as well as the current intensity (100, 200, 300, 400 and 500 mA) on the removal efficiency of iron by electrocoagulation was performed. The results showed that both technologies neutralized acidity and removed >99% of the iron, producing a treated effluent that met Canadian discharge requirements (pH 6.0-9.5; < 3 mg/L Fe). Electrocoagulation under optimal conditions (Fe electrodes, 400 mA and 60 min) proved to be more efficient than MFC in terms of reaction time (minutes vs. days) and operating costs (0.58 vs. $2731/m3). Subsequently, electrocoagulation tests were continued for the treatment of AMD highly contaminated with iron and sulfates (pH = 2; [Fe] = 500 mg/L and [SO42-] = 1 280 mg/L). These tests were performed under the same operating conditions as the previous tests, including electrode material (iron vs. aluminum), current densities (3.8-20 mA/cm2) and reaction time (15-90 min) following small-scale batch tests (0.25 L of working volume). The objective was to evaluate the influence of the presence of sulfate ions in MD on the efficiency of electrocoagulation. Thus, a scaling-up (3.3 L of working volume) was performed to evaluate the effect of other operating parameters such as the number of electrodes (2-10) and the inter-electrode distance (0.5-4 cm) on the treatment efficiency. All these tests were carried out with the addition of salt to counter the resistance of the solution and to adjust the desired current density according to the voltage. The results showed that the use of two Fe electrodes, separated by 0.5 cm, meets Canadian discharge standards for pH (pH 6.0-9.5). Indeed, a final pH of 8.3 was obtained with iron electrodes, when only a pH of 4.3 was reached with aluminum electrodes. As for the removal, efficiencies have been similar (96% and 100% with iron electrodes, and 82% and 100% with aluminium electrodes for the removal of sulfates and iron, respectively). Nevertheless, it should be noted that the reaction time with iron electrodes was longer (420 vs 120 min). These results were used in the following experiments to treat different qualities of highly contaminated MD for a possible evaluation of system efficiency in treating sulfated salinity and associated toxicity. The third part of the project was devoted to evaluate the efficiency of electrocoagulation as a stand-alone process or as a polishing system for chemical treatment of sulfated salinity contained in synthetic and/or real MD: E1 (pH= 6.9; [Fe] = 0.04 mg/L and [SO42-] = 1 280 mg/L) and E2 (pH= 7.5; [Fe] = 0.07 mg/L and [SO42-]= 2 870 mg/L). The chemical treatment methods used are hydrated lime Ca(OH)2, and high density sludge (HDS). The experiments were performed in an electrocoagulation cell with a working volume of 1.5 L using the best operating conditions selected from the previous tests (2 iron electrodes, current density of 20 mA/cm2 and inter-electrode distance of 0.5 cm) and without adding salt. The results showed that the removal of sulfates by electrocoagulation alone for 15 minutes from synthetic AMD was better at lower concentrations (10% for 1 280 mg/L vs. 0% for 2 130 mg/L). The results were validated by the real effluents. Indeed, a low sulfate removal efficiency of 8% and 6% after 30 min of electrocoagulation treatment was obtained for E1 and E2, respectively. The results also showed that the removal efficiency of sulfates was improved by using electrocoagulation as a polishing system during the treatment of AMD. In addition, the HDS proved to be the best treatment alternative (44.8% of sulfate removal compared to 16.6% with Ca(OH)2 treatment). If HDS is produced industrially, the choice will be based on several reasons, including: (1) better management of industrial sludge itself through reuse, (2) a removal gain due mainly to its variable and element rich composition (e. g. neutralizing material) and (3) a gain in reducing the additional operating costs associated with the use of added chemicals. The last part of this thesis presents the evaluation of the efficiency of electrocoagulation for the treatment of toxicity associated with sulfated salinity of real effluents (E1 and E2) previously used. The tests were conducted using two aquatic organisms belonging to the daphnia family (Daphnia magna (D. magna) and Daphnia pulex (D. pulex)), which are the two most commonly used species in toxicity tests. The comparative evaluation of the sensitivity of these two species to NMD before and after treatment was carried out using acute (mortality) and sublethal (immobility) toxicity tests using five different dilution waters (one synthetic from the CEAEQ laboratory and 4 surface waters from site 1, 2, 3 and 4 from different lithologies). The results showed that only one surface water (site 4) was toxic for D. pulex (>35% mortality and immobility) while all dilution waters were nontoxic for D. magna. The E2 effluent, initially more contaminated than E1 ([SO42-] = 2 870 vs. 1 280 mg/L), showed a more toxic final effluent than the initial effluent, particularly for D. pulex (64% vs. 31% immobility and 41% vs. 19% mortality, for untreated and treated E2, respectively). The cause of this toxicity is still unknown. The absence of toxicity of the E1 effluent after treatment was demonstrated only with dilution water from Site 1 for D. pulex. Finally, all toxicological tests performed in this study showed that D. pulex is more sensitive than D. magna to the same NMD highly contaminated with sulfates (treated or untreated), as well as for dilution waters. This project contributes to improve the knowledge about the efficiency of electrocoagulation in treating synthetic and real MD that are highly contaminated with iron and/or sulfates. In addition, the evaluation of the toxicity associated with sulfated salinity on two daphnia species enabled to determine the efficiency of electrocoagulation for the treatment of MD. Finally, the knowledge acquired, and the methodological approach of this study could allow their application to other more complex qualities of MD and under other operating conditions. Keywords: mine drainage, electrocoagulation, chemical treatment, sulfated salinity, Daphnia magna, Daphnia pulex.