Stability of As- and Mn-sludge after neutral mine water treatment using Fe(VI) vs electrocoagulation

The electrocoagulation (ECG) and ferrate (Fe(VI))-based processes are increasingly acknowledged as efficient for the simultaneous removal of As and Mn from synthetic and real mine effluents. Prior to design of full-scale applications, more information on the physicochemical, mineralogical, and environmental characterization of the produced sludge is required. The main objective of this study was to characterize and evaluate the leaching potential of problematic elements in As- and Mn-rich sludge produced during ECG or Fe(VI) treatment of circumneutral surrogate mine water. To do so, PHREEQC modelling was carried out on the effluents, before and after ECG or Fe(VI) treatment, to calculate the saturation index of dissolved As, Fe, and Mn species. A physicochemical and mineralogical characterization of the sludge was also performed using powder X-ray diffraction (PXRD) and a scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS). Then, a non-sequential selective extraction procedure (N-SEP) combined with a USGS field leaching test (FLT) were conducted to evaluate the environmental behaviour of the As- and Mn-rich sludge. Geochemical modelling indicated that the Fe(VI) and ECG processes favor the precipitation of Fe-(oxy)hydroxides (lepidocrocite, schwertmannite, ferrihydrite). Chemical characterization showed that the Fe(VI)-sludge contained higher As and Mn concentrations and lower Fe concentrations than the ECG-sludge (3.8% As, 5.3% Mn, and 34% Fe for the Fe(VI)-sludge vs 1.2% As, 0.77% Mn, and 52% Fe for the ECG-sludge). These findings can be explained by the smaller amount of sludge produced during the Fe(VI) treatment and the higher removal efficiency of this method, especially for Mn. The PXRD patterns suggested the formation of poorly crystalline Fe-(oxy)hydroxides (lepidocrocite or βFeO(OH) in the ECG-sludge vs ferrihydrite in the Fe(VI)-sludge); however, no As- or Mn-bearing minerals were identified. Findings from N-SEP tests showed different speciation of As and Mn in the sludge, with a higher proportion of As bound to poorly crystalline Fe-(oxy)hydroxides in the Fe(VI) sludge than the ECG-sludge (97% and 71%, respectively), and higher proportion of Mn associated with the residuals in the Fe(VI)-sludge than the ECG-sludge (57% and 5.7%, respectively). Finally, FLT results indicated that very low concentrations of As (<0.05 mg/L) and Mn (<0.5 mg/L) were leached from the ECG- and Fe(VI)-sludge, with the Fe(VI) treatment resulting in slightly better As and Mn immobilization in the sludge relative to the ECG process. Nevertheless, both treatment processes were satisfactory in terms of efficient removal of As and Mn and their immobilization in the produced sludge.