Bioleaching of Indian Ocean nodules with in situ iron precipitation by anaerobic Mn reducing consortia

Highlights

• Reduction of PMN under facultative anaerobic conditions by Mn reducing consortia.

• Dissolution of metals through dissimilatory reduction of MnO₂ and FeOOH by the consortia.

• In-situ reoxidation of reduced iron by MnO₂ and precipitation as FePO₄.

• Reduction by direct anaerobic respiration envisaged especially when using acetate as electron donor.

Abstract

In the present study selective iron-free extraction of Cu, Ni, Co and Mn from Indian Ocean manganese nodules by manganese reducing consortium is described. The consortium isolated from manganese ore mines of Joda, Odisha (India) and enriched with Indian Ocean manganese nodules was used for bioleaching under facultative anaerobic conditions in mineral salt media using organic carbon source. Bioleaching was carried out at 30 °C without agitation. Recoveries of Mn, Ni and Co increased with time, while maximum Cu recovery occurred during the initial period followed by a decrease. A notable finding was that Fe dissolution remained extremely low, i.e. less than 42 mgL^− 1 (3.0%), possibly due to in situ precipitation of iron. X-ray diffraction patterns of bioleach residues indicated formation of FePO₄ and MnCO₃ phases. Iron dissolution remained consistently low even with increasing pulp density and decreasing particle size. The Mn reducing consortia may thus help in selective leaching of Mn while preventing Fe from contaminating the leach liquor, which is desirable in downstream processing of metal production from Mn-nodule and ferromanganese ores.

Introduction

Continuous demand of industrially important metals has led to global exhaustion of metal resources at rapid rates. Metals like Co and Ni are widely used in steel making and super alloys but their deposits are comparatively less. For instance, India has its only Ni deposit in the form of nickel lateritic ore at Sukinda, Orissa containing 0.8% Ni and 0.049% Co (Pradhan et al., 2012). The increased need for alternative source of metals has driven the research in alternative extraction methods (Senanayake, 2011). Multi-metal containing deposits are one such resource with high strategic significance as more than one metal can often be obtained by processing such resources. In particular manganese nodules abundantly available on ocean floor are potentially important natural sources of metals such as Cu, Co, Ni and Mn. Polymetallic manganese nodules, characterized by high porosity and surface area are formed of concentric layers of iron and manganese oxide around a core. The mineralogical composition of Indian Ocean nodules is birnessite, δ-MnO₂, amorphous hydrated iron oxide, clays, quartz and zeolites (Kanungo and Das, 1988). Although manganese is the major metal constituent of the nodule occurring up to 33%, significant level of Cu, Co, Ni, Zn and Fe are found in the form of oxides and hydroxides distributed within the δ-MnO₂ and FeOOH phases of nodule. Due to the high moisture content (30–40% by weight) of nodules many hydrometallurgical extraction routes have been proposed and studied for the recovery of constituent metals (Kanungo and Das, 1988, Anand et al., 1988, Acharya et al., 1999).

With the increased emphasis on environmental friendly processes, ‘biohydrometallurgy’ can be considered in nodule processing. To recover the metals found in nodule matrix one needs to break down the highly oxidized and interlaminated MnO₂ and FeOOH phase, which can be completely dissociated only under reducing conditions (Acharya et al., 1999). Bioleaching of nodules must also address the reduction of oxide phases to liberate the locked elements. Bacteria mediated manganese reduction can occur through direct or indirect methods. In the direct method, organisms use the oxides as terminal electron acceptor and transfer electron to mineral surface by oxidizing organic matter, while in indirect method the primary or secondary metabolites produced by the organisms carry out the reduction of oxide (Lovley, 1991). In general, the organisms capable of reducing oxide minerals are either mixotrophic, which derive nutrition from minerals as well as organic carbon, or heterotrophic, depending completely on organic carbon for growth and reduction of mineral does not support nutritional requirements (DeVrind et al., 1986, Ehrlich, 1987). Certain organisms which can support growth by complete oxidation of organic compounds to CO₂ with Mn(IV) or Fe(III) as sole electron acceptors, known as dissimilatory reducers, have been identified as responsible for reduction of Mn(IV) and Fe(III) in sediment environments and are known to accumulate Mn(II) or Fe(II) under anaerobic conditions in their organically complex culture media (Lovley, 1991).

Several geo-microbiological studies establish the redox reactions of nodule bacteria in reduction of Mn(IV) present in the nodule. The process of manganese bioreduction has been reported to occur in aerobic as well as anaerobic environments (Myers and Nealson, 1988). Geo-microbiology studies of nodule bacterium Bacillus 29 were among the earliest studies that showed the organism to be capable of both oxidation and reduction of manganese under appropriate conditions (Ehrlich, 1963). However, biohydrometallurgical processing of nodules has mostly been addressed in the context of aerobic leaching and the approach has been restricted mostly to indirect leaching methods using pure cultures of heterotrophic (Bacillus sp.) or chemolithotrophic organisms (Acidithiobacillus ferroxidans, Thiobacillus ferroxidans, Thiobacillus thiooxidans) (Konishi and Asai, 1995, Kumari and Natarajan, 2001, Mukherjee et al., 2003). Other studies on nodule leaching for metal recovery involving heterotrophic bacteria and fungi have also been reported (Kumari and Natarajan, 2001, Mehta et al., 2010). While significant numbers of investigation exist on aerobic leaching of nodules, anaerobic leaching has been studied rather less extensively. Some bacteria have been shown to reduce Mn(IV) under both aerobic and anaerobic conditions, while some are strictly anoxic manganese reducers (Trimble and Ehrlich, 1968, Burdige and Nealson, 1985, Myers and Nealson, 1988). Oxygen depleted cells of Bacillus sp. was demonstrated to reduce MnO₂ with the possible involvement of electron transport system and oxidation of type b and c cytochromes (DeVrind et al., 1986). In yet another study, anaerobic leaching of nodules catalyzed by Thiobacillus ferrooxidans in the presence of FeS₂ was reported (Li et al., 2005), wherein pyrite acted as nutrient for bacteria and reductant for MnO₂.

However selectivity in bioleaching has not been achieved especially for a multi-metal target material such as manganese nodule. Due to the structure of nodule it is imperative to reduce the iron and manganese oxide phases to recover the other elements distributed within these phases. Co-dissolution and generation of large quantities of Fe(II) make the downstream separation of metals more challenging, necessitating precipitation steps that lead to the loss of extracted metals by co-precipitation or adsorption. Earlier bioleaching studies with Polymetallic nodule have not addressed the selectivity in leaching or elimination of iron, as the manganese reducing bacteria were usually developed by acclimation of Fe-reducing organisms in nodule (Lee et al., 2001). A very selective bioleaching strategy for nodule, capable of recovering target metals with minimum impurities is thus necessary. In the present study, selective bioleaching of polymetallic nodules carried out under facultative anaerobic conditions by a consortium developed from manganese ore mines of Joda (India) is reported for the first time. Effect of the parameters controlling the bioreduction and dissolution of metals are discussed here.