Efficiency of electrocoagulation for simultaneous treatment of As and Mn in neutral mine water

Abstract

Despite the efficiency and ease of implementation of electrocoagulation (ECG), its industrial application for the treatment of contaminated mine water is limited due to high energy and electrode consumption. The main objective of the present study was to identify the most performant ECG conditions using Fe–electrodes for the simultaneous removal of As(III) and Mn(II) from synthetic and surrogate neutral mine water, while limiting energy and electrode consumption. Treatability tests were performed on i) single synthetic effluents containing only 3.5mg/L As (SAs) or 4.5mg/L Mn (SMn), ii)binary synthetic effluents containing both As and Mn (SAs+Mn), and iii) surrogate mine water (Elow and Ehigh). The effect of current density (CD), ionic strength, and retention time were first evaluated for the treatment of SAs, SMn, and SAs+Mn. Then, the most performant conditions were used to treat Elow and Ehigh. The toxicity to Daphnia magna of Elow and Ehigh was evaluated before and after ECG treatment. The results showed that the increase of CD from 0.25 to 10 mA/cm2 greatly improved the removal of Mn and to a lesser extent of As, as well as energy and electrode consumption. Despite a negligible effect on the removal efficiency of As and Mn, the ionic strength (from 0.25 to 2.5mS/cm) and inter-electrode distance (from 10 to 50 mm) strongly affected the energy consumption. Results showed that ECG efficiently removed As (>97 %) from SAs at low CD (0.5 mA/cm2), while Mn required a 3.3-fold higher CD to achieve satisfactory removal (>60 %) from SMn. For the treatment of SAs+Mn, As removal was more efficient and faster than Mn removal (99 vs. 60 % and 20 vs. 60 min, respectively) at a CD of 2.0 mA/cm2. Moreover, the simultaneous removal of As and Mn at 2.0 mA/cm2 produced a volume of sludge 3.3 times greater than at 0.5 mA/cm2. Lastly, high As (99 %) and variable Mn (40–57 %) removal efficiencies were obtained after Elow and Ehigh treatment, with potential positive effect of the presence of Ca2+ and Cl- on the removal of As, while SO42- may have negatively affected the removal of Mn. After ECG treatment, no addition of toxicity to D. magna was observed for Elow and Ehigh. The outcomes of this study may provide promising insights for the application of ECG for simultaneous removal of metal(loid)s from contaminated mine water.