Application of Innovative Remediation Processes to Mining Effluents contaminated by Heavy Metals

The scope of the paper was to demonstrate the technical feasibility of the remediation processes by electrowinning and selective sequential precipitation, for toxic metals removal from acid mine drainage. By electrochemical experiments, high metals removal has been achieved: in particular, by Zn and Mn electrodeposition, it was possible to achieve about 93-99% Zn and Mn removal (as MnO2), with a relatively low energetic consumption. The principle of the heavy metals selective sequential precipitation is the combined application of sodium hydroxide solution and hydrogen sulfide produced by sulfate-reducing bacteria. For the hydrogen sulfide production the sulphate-reducing bacteria of genus Desulfovibrio was used. The selective sequential precipitation process reaches the selective precipitation of chosen metals with 99% efficiency – Fe, As, Al and Mn in the form of metal hydroxides, Cu and Zn as metal sulfides.


Introduction
The elimination of metals from the mining effluents is a severe environmental problem and has been a long-standing major concern to scientists, engineers, industry, and governments, both in this country and throughout the world (Beolchini et al., 2007;Beolchini et al., 2009a;Beolchini et al., 2009b;Vegliò et al., 2003).
Various methods are used for the metals removal from waters, but any of them have been applied under commercial-scale conditions.Applications of methods depend on geochemical, technical, natural, financial and other factors.In addition each mining effluent i.e. acid mine drainage (AMD) is unique in regard to different chemical status and require different methods of treatment (Ubaldini et al., 2010).The AMD under study coming from a lead and zinc mine located in Tùnel Kingsmill outlet of the Rio Yaulì (district of Yauli -Perù).The river is contaminated by water of the tunnel when they are discharged into their flow, as these are oxidized prior to making contact with the minerals and metals.Peruvian mining companies are finalizing the feasibility studies using conventional remediation technologies involving the addition of lime (Luptakova et al., 2010).However, these methods present negative drawbacks -the production of secondary wastes (e.g.lime precipitation generates high volumes of solid wastes).The main aim of the present experimental work was to test innovative remediation processes in order to remove heavy metals (Al, As, Cu, Fe, Mn, Zn) from Peruvian AMD, in a relatively cheap and environmentally friendly way, by means of electrowinning and selective sequential precipitation (SSP) (Luptakova et al., 2011;Ubaldini et al. 2009).

Materials and Methods
The experimental activity has been carried out at laboratory scale by synthetic solutions, starting from AMD coming from the lead and zinc mine located in Tùnel Kingsmill outlet of the Rio Yaulì -Perù.The elemental composition of the peruvian AMD is reported in Table 1.The natural pH was of 3.5.
A little amount of nitric acid (HNO3) has been added to the synthetic solution, with the aim to oxidise Fe2+ eventually present to Fe3+.In a subsequent step, sodium hydroxide (NaOH) was added to reach pH 4.0 (Ubaldini et al., 2010b).Electrowinning tests have been performed in a cylindrical glass laboratory cell of 200 cm3 volume.The cell was connected to a potentiostat-galvanostat (Luptakova et al., 2012).Liquid samples have been whit drawn and submitted to chemical analysis by ICP-MS.Purity of the solid deposit, was determined by X-Ray Diffraction technique (XRD).Metallic content of the deposit was analysed by ICP-MS (Vegliò et al., 2003).The combined application of sodium hydroxide solution and hydrogen sulfide produced by sulfate-reducing bacteria i.e. selective sequential precipitation (SSP) was realized in too principal steps: 1addition of 0,2M NaOH solution by the automatic titrator TitraLab 850; 2 -addition of bacterially produced hydrogen sulfide by the equipment consist of too interconnected bioreactors.A little amount of H2O2 has been added to the synthetic solution with the aim to provide the presence of Fe, As and Mn in the form of Fe3+, As5+ and Mn2+.The determination of suitable pH values for selective precipitation of metals from studied model solution was performed by the acid-base titrationalkalimetry using automatic titrator TitraLab 850 in connection with PC program TitraMaster 85.
For the hydrogen sulphide production in the framework of 2.step the cultures of SRB (genus Desulfovibrio) were used.Bacteria were isolated in Kosice-north, Slovakia.The genus Desulfovibrio was enriched from the mixed cultures SRB using the nutrient Postgate's medium C (Luptakova et al., 2008).The SRB cultivation for the bacterial production of hydrogen sulfide was carried out using the same nutrient medium.After precipitation of individual metals accrued precipitates were removed from AMD by filtration.The concentration of metals during SSP was determined by AAS.

Results and Discussion
Innovative technology such as electrowinning has been developed during the experimental work with recovery of purified useful metals as Zn and Mn (Table 2).All metals deposited on the cathode, while manganese deposited on the anode as MnO 2 .
In the first stage of the process, cathodic deposition of Zn was achieved.In this phase, Cu and traces of Cd, Ni, Mn (as metallic Mn) metals codeposited with the Zn.A decreasing of all metals concentrations (Table 2) under the recommended limit suggested from Peruvian law directives (Quality III).
MnO 2 anodic deposition was achieved on the residual solution, adding a little amount of concentrated H 2 SO 4 for pH adjusting at the main experimental conditions.
The selectivity of the metals precipitation from AMD depends on the buffering systems.Its strength provides the titration curves, which determines the range of pH values for precipitation of individual metals.For all that the first part of experiments was centered on the study of titration curve by acid-base titration of the AMD synthetic solution with 0.2M NaOH.The results of alkalimetry were prepared in the form of titration curves.The shapes of the AMD synthetic solution integral titration curve (data not shown), its the first derivation (data not shown) and chemical analysis of the AMD synthetic solution during titration allocated five points of inflexion for precipitation metals: Fe + As (pH 4.5); Al (pH 5.8); Cu (pH 6.5); Zn (pH 8.5) and Mn (10.2).After determination of the suitable pH values for metals selective precipitation were realized experiments concerning of the SSP.The metals selective precipitation as hydroxides or sulfides at the various values of pH AMD is the fundamental of the examined process.The working conditions and obtained results of the selective sequential precipitation of heavy metals form AMD synthetic solution (Table 2).Bacterially produced hydrogen sulfide reacts with the available metal ions in AMD to form insoluble metal sulfides at the appropriate values of pH.When pH of the studied solution is adjusted by sodium hydroxide come to consequent precipitation of metals in the form of hydroxides.
Table 3 describes some characteristics, advantages, disadvantages and improvements of studied processes.

Conclusion
Biohydrometallurgical applications constituted by bioprecipitation/electrowinning have demonstrated the technical feasibility of the process aimed at the removal of toxic metals from Peruvian AMD samples; in fact, at the end of the process, the metals' concentration decreased under the recommended legislation limit.
In particular, electrowinning permitted 99 % of Zn recovery, whit an energetic consumption of 118 kWh/kg, while 93 % of manganese was recovered as MnO2 at grade of purity, with an energetic consumption of 619.05 This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2 0 , which .permits unrestricted use, distributi and reproduction in any medium, provided the original work is properly cited.on, E3S Web of Conferences DOI: 10.1051/ C Owned by the authors, published by EDP Sciences,

Table 2 .
The metal removal efficiency of the integrated process applied to synthetic AMD sample.