Performance Evaluation of Electro-Fenton for Thiosalts Treatment in Mine Effluents

Thiosalts are partially oxidized sulfur intermediates formed during the oxidation of sulfide minerals and are responsible for delayed acidity, leading to aquatic toxicity in receiving water bodies. Freeze/thaw (F/T) cycles, induced by season changes in northern regions, are known to accelerate sulfur oxidation kinetics, which entails increased thiosalts concentrations in mine water. Concentrations as high as 15 g/L of thiosulfates (S2O3 have recently been reported in a study evaluating the effect of F/T cycles on the stability of mine waste. The limitations of chemical neutralization, used as the current preferred thiosalts management practice, entail the development of emerging technologies. In addition, water treatment strategies in northern countries are mainly limited by their inability to deal with extreme thiosalt contamination. A particular focus on emerging technologies that are also suitable in cold climate is necessary. Recently, the electro-Fenton (EF) process has been reported as effective for thiosalts treatment in synthetic mine water, even when the temperature of the water was as low as 4C. To further advance the general understanding of this approach, a performance evaluation was conducted on both synthetic and real mine effluents. The term “performance” was defined as contaminant removal efficiency, associated residual toxicity, and operating costs, the three components of the project. EF treatment tests were performed using pre-optimized parameters (carbon fiber brush cathode, single-side coated Nb/boron-dopped diamond plate anode, current intensity = 0.3 A, current density = 0.006 A/cm2 and initial [Fe2+] = 0.2 mM). In this study, H2O2 concentrations were not measured, but H2O2 production was proven during the preliminary work to optimize the operating parameters. Contaminant removal efficiency was evaluated on four different real mine effluents, selected based on their geographical origin (northern Quebec, Canada) and differing S2O3 concentrations (219 ± 21 mg/L, 32 mg/L, 0 mg/L and 58 mg/L for E1, E2, E3 and E4, respectively). Treatability tests were then performed on “simplified”, corresponding synthetic effluents, with and without addition of alkalinity, to investigate underlying mechanisms. The influence of chlorinated salinity (related to the presence of Cl-) on the EF process performance was also studied on effluent E1 at 2 and 4 g/L CaCl2, at pH 3 and pH 5. Resulting solutions were treated as per usual. For the assessment of residual toxicity, E1 and E4 were tested for acute toxicity to Daphnia magna, both before and after EF treatment. Finally, a preliminary techno-economic analysis was conducted to evaluate operating costs, in terms of energy consumption, reagents and electrode costs, for northern Quebec vs the rest of Canada and the USA.Thiosulfate removal efficiency results were encouraging. Complete removal (> 99%) was achieved under 60 min for all effluents tested, including corresponding synthetic effluents, with and without alkalinity, and independent of the Cl- concentration. Alkalinity was identified as a process inhibitor, resulting in an increased treatment time from 0.25 sec in E2 to 12 min in its corresponding synthetic effluent when alkalinity was added. The influence of chlorinated salinity on the process performance seemed more complex. It was observed that S2O32- removal was faster at pH 5 than pH 3, and faster at 4 g/L CaCl2 than at 2 g/L CaCl2, with unmodified E1 showing the slowest S2O 2- oxidation kinetics. These observations were in accordance with the literature, which seems to indicate that Cl- interfere with thiosalts treatment via their interaction with Fe2+ at the optimal pH range of the Fenton reaction. Furthermore, oxidative Cl- species formed at the anode could accelerate the treatment, whereas their HO•-scavenging properties could slow down thiosalts oxidation. Results of the residual toxicity assessment revealed that the initial matrix had no impact on the acute toxicity to D. magna for all effluents tested. Effluent E1 showed D. magna mortality initially but was below the acute toxicity threshold (LC ˂ 50%). Complete oxidation of thiosalts in E1 resulted in the elimination of D. magna mortality. EF treatment of effluent E4, which was initially acutely toxic, resulted in the elimination of toxicity to D. magna. Results of the preliminary techno-economic analysis showed that energy consumption was the main contributor (58‒64%) to the total operating costs, when diesel was the main power supply (northern Quebec), whereas electrode costs were highly superior (76‒77%), when hydroelectricity was used (central jurisdictions of Canada and the USA). The results of this study provided a better understanding of the general mechanisms governing the oxidation of thiosalts by EF in real mine water. In addition, it has been shown that, in some cases, the process does not generate any residual toxicity and can even reduce or eliminate the effluent's initial toxicity. Further studies in terms of toxicity associated to EF treatment will need to be done in order to deepen the acquired knowledge. The preliminary techno-economic analysis showed that the process could be economically viable in the mining industry (except in the North where the implementation of electrochemical water treatment technologies is unfavored due to high electricity costs), but studies will need to be done to identify alternative electrode materials that are less expensive. Overall, during the execution of this project, crucial information was acquired, constituting a starting point in the field of applied research for the development of EF as a new efficient and environmentally sustainable mine water treatment technology.