Biogeochemical Redox Processes Controlling the Element Cycling: Insights from Karst-Type Bauxite, Greece

: The occurrence of very-high-grade bauxite ores of karst-type deposits resulting from Fe-leaching is of particular importance, because they are widespread in the Mediterranean metallogenetic province and result in the natural beneﬁciation of the ore quality. The present study focuses on mineral transformations and variations of major and trace elements, including platinum-group elements (PGE) and mineral chemistry along a bauxite proﬁle from the Parnassos-Ghiona deposit, Greece underlying a fault. The most salient feature of the multicolor ores (grey–whitish, yellowish, deep red, deep grey to brown-red color, from top to bottom) is their association with fossilized and present-day microorganisms, which, by their reducing and/or oxidizing activity, catalyze redox reactions and provide nucleation sites for the precipitation of secondary minerals. Texture relationships between mineral and variations in the mineral chemistry, suggesting the sequence in their formation, indicate a multistage evolution. The recorded compositional variations show that the Al enrichment is accompanied by increase in the TOC, As, Pd and U, and chondrite-normalized REE patterns exhibit a similar trend and positive Ce anomalies. A lower (Pt + Pd) content and higher Pd/Pt ratio, ranging from 1.0 to 5.5 in the bauxite proﬁle compared to those in Fe-Ni laterite deposits with the Pd/Pt ratio ranging from 0.1 to 0.68, reﬂect the higher solubility and mobility of Pd compared to that of Pt, and differences in their origin and genesis. A positive correlation between Pd and As and the elevated As content (up to 960 mg/kg) in multicolor ores compared to brown-red samples (average 10 mg/kg As) conﬁrms their mobilization and redeposition.


Introduction
The role of microorganisms in transforming organic and inorganic substrates is of particular significance in geology, mineralogy, and biogeochemical cycles of metals/metalloids [1]. Some of the activities of microorganisms in biogeochemical processes resulting in metal distributions and speciation may take place in aquatic and terrestrial ecosystems and are mainly related to physicochemical parameters. Many minerals are of biogenic origin and provide evidence for the role of microorganisms in geological processes [1][2][3][4]. Specially, uptake, assimilation, degradation and metabolism of organic and inorganic compounds, respiration (CO 2 production), photosynthesis, biosynthesis of polymers, and humus formation are a few representative examples showing the role of microorganisms in elemental cycles [1]. Natural weathering of rocks and ores/mining and/or smelting wastes, industrial and urban waste leachates, and agricultural activities are common sources of contamination for the soil-groundwater-plant/crop system [5,6]. On the other hand, bioremediation has been used to remove harmful elements from contaminated soil and groundwater, and applications of microorganisms as a green alternative process to the metal recovery have been widely recommended, due to high capital and energy costs of the very expensive involved techniques to reduce contamination of degraded ecosystems and pyrometallurgical techniques [7][8][9].

Materials and Methods
Morphological and geochemical characteristic have been described in previous studies [10,11,18,19]. Fourteen bauxite samples along a profile (~2 m) underlying a fault from the B 3 formation of the Parnassos-Ghiona deposit ( Figure 1) were chosen for this study to be representative of all multicolor ore types. Due to the heterogeneous character of the bauxite deposit, samples of approximately 2 kg weight were collected. They were prepared and analyzed for major and trace elements, including rare-earth elements (REE) by inductively coupled plasma mass spectrometry (ICP-MS) analysis, after multi-acid digestion (HNO 3 -HClO 4 -HF-HCl) at the ACME Laboratories Ltd., Vancouver, BC, Canada (Table 1). Detection limits, quality control samples and the precision of the analyses are in agreement with international standards (~10%). The platinum-group elements (PGE) analyses were obtained using lead collection fire-assay preconcentration technique from large (25 g) samples at Genalysis Laboratory Services, Perth, Australia. This method allows for complete dissolution of samples. The detection limits were 0.5 µg/kg for Pd, 0.1 µg/kg for Pt, and 1.0 µg/kg for Au. Sulfur was determined at the Technical University of Athens using a LECO instrument, and total organic carbon at the Athens University, following the titration method of [20].

Geological Outline
The Parnassos-Ghiona bauxite deposits of Greece are part of the Mediterranean karst bauxite belt. These deposits are hosted within carbonate rocks and have different ages. Three bauxite formations, B 1 , B 2 , and B 3 , from the oldest to the youngest, can be distinguished (Figure 1), and are intercalated with shallow water limestone within a Late Jurassic to Cretaceous sequence of the Parnassos-Ghiona zone [21] (Figure 1). The organic matter, as thin coal layers, overlying the bauxite deposits has been described [13]. Intense tectonism has affected those bauxite deposits, producing overthrusting, faulting, and disruption of the continuity of the bauxite bodies [21], facilitating the meteoric fluid circulation and mineral deposition. The studied profile belongs to the B 3 formation (or horizon), hosting both high-Fe and high-Al bauxite ores, under exploitation. The significant proportion (approximately 30% volume) of transformed bauxite ore, similar to that of the studied profile (Figure 1), is the most characteristic feature. Specifically, at the Pera-Lakkos bauxite deposit, grey to whitish-grey bauxite gradually grades downwards through yellow, deep red, and deep grey to brown-red-colored bauxite ore ( Figure 1).

Mineralogical Characteristics
The main mineralogical characteristics of the Parnassos-Ghiona bauxite deposits have been described by previous authors [10][11][12][13][14]21,22]. Typical bauxite ore is dominated by pelitomorphic (fine-grained), ooidic-pisolitic and clastic texture, and by dark red to redbrown color. The AlOOH polymorphs boehmite and diaspore, forming pisoliths and oolites, are the main components of the Parnassos-Ghiona bauxite ores, while gibbsite (Al 2 O 3 ·3H 2 O), goethite and/or hematite are present in lesser amounts. Kaolinite, illite and pyrite are common in thrust-fault-affected parts of the deposits, whereas chamosite, quartz and zircon are present in lesser amounts. The present study is focused on bauxite samples affected by the presence of microorganisms ( Figure 2). Bauxite ores from the transition zone of the from the Parnassos-Ghiona bauxite deposit show whitish-grey, yellow, deep red color (Figure 2a), and red to deep grey for the sulfide-rich ore (Figure 2b). Thin sulfide veins may crosscut earlier-transformed grey bauxite ore, including goethite and/or occur as remnants in neoformed goethite (Figure 2d

Bauxite Ores
The most salient feature in the geochemical composition of the Parnassos-Ghiona bauxite deposit is a wide variation occurring along and near faults of the deposit. Arsenic ranges from 7.2 to 16 mg/kg in the typical brown-red-colored ore and from 34 to 960 mg/kg in yellow-, deep red-, and deep grey-colored ore (average 390 mg/kg), while in whitish ore it ranges from 17 to 76 mg/kg ( Table 1). The multicolor bauxite ore is characterized by elevated total organic carbon (TOC) content (up to 5.4 wt.%) compared to the brown-red ore (0.08-0.26 wt.%). In addition, 0.66 wt.% O.M. was determined in a whitish material grown after a few weeks on the surface of grey bauxite samples, exposed to air oxidation, in the room conditions (20-25 • C) and moderate air humidity (atmospheric water). The main components of that epigenetic material are hydrated Al-and Fe-sulfates (Figure 2j-l) which are similar to those identified on polished and unpolished sections [19]. The highest S and FeO contents occur in the samples with deep grey color, reaching values up to 10.8 wt.% S and 26.4 wt.% FeO (Table 1), which are consistent with the presence of abundant pyrite ( Figure 2). The lowest contents of REE were recorded in a sulfide-rich bauxite sample (Table 1).
Based on the presented analytical data (Table 1), the calculated correlation matrix ( Table 2) shows positive values (≥0.5) between TOC and Al, As, Sb, S, and Co, and negative values with Yb, Lu, and Ta. Also, Pd increases with increasing As content ( Figure 3a) and decreases with the increasing Ce and Sc contents (Figure 3b,c). Scandium shows a negative correlation with As ( Figure 3d).  The schematic compositional variations present the Al enrichment, accompanied by REE, As, Pd, U, and TOC in the transformed multicolor ores compared to brown-red bauxite ores, and a very good correlation (r = 0.89) between Sm and Eu (    Palladium shows positive correlation with As and Sb and negative with Ce ( Figure 5). The correlations between S and FeO, Co, and U are positive, as well as the interelement correlation between heavy rare-earth elements (HREE). It is obvious that a very good positive correlation between S and Co (r = 0.93) reflects the association of Co with sulfides. Scandium content varies from 33 to 77 mg/kg (average 53 mg/kg), with the highest content in a sample of brown-red color (Table 1) and it shows a negative correlation with As and Pd (Figure 3c,d).
The chondrite-normalized REE patterns for bauxite samples from the studied profile of the Parnassos-Ghiona bauxite exhibit a similar trend to those analyzed and presented for the total of REE [11], showing positive Ce anomalies for all samples, although the anomaly is greater compared to sulfide-rich samples. The LREE enrichment compared to HREE is obvious ( Figure 6).

Water Leachates of Bauxite Ore and Red Mud
Since the mining and smelting processes are potential sources of contamination and can be a threat for the environment, water leachates from representative samples of bauxite ore from the studied profile and bauxite-processing residue or red mud from the Aspra Spitia plant, Greece (May 2008), are given as well (Table 3). These leachates show relatively elevated concentrations of Zn, Co, Cr, Mn, Ni, Cu, U, Ca, Mg, Na, K, and S. In particular, very high Cr(VI) concentrations exhibit the water leachate from the red mud. The concentrations for all REE, PGE, and As in both water leachates are lower than the detection limit of the method (Table 3).

Discussion
Microorganisms are considered as a key controlling factor for mineral transformation, the metal/metalloid mobilization, enrichment, and biomineralization in the Earth system [1,4,[24][25][26][27][28]. A consortium of microorganisms of varying morphological forms may produce enzymes and catalyze redox reactions and provide nucleation sites for the precipitation of secondary minerals [24,29,30]. The mineralogical and biogeochemical features reflecting the interaction with microorganisms at highly faulted zones throughout Parnassos-Ghiona bauxite deposits have been described in detail [10][11][12][13][14]18,22]. The spatial association of microorganisms and main minerals in the bauxite deposits, the described bacteriomorphic goethite and pyrite, and the negative δ 34 S values for sulfide-bearing samples point to the existence of the appropriate conditions for iron bioleaching, Al enrichment, and biomineralization [10,11]. In addition, Cretaceous white bauxite of high quality is the type of the economic importance in the Montenegro (Bijele Poljane) bauxite deposits [16], Cretaceous bauxites in Serbia [31], Cretaceous reddish, yellowish, and greyish bauxites at Ayranci, Karaman, and Turkey [32], Late Permian grey bauxites at the Kanirash deposit, NW Iran [17], and elsewhere, suggesting an important role of microorganisms to biogeochemical processes.

Biogeochemical Cycling of Metals/Metalloids in a Bauxite Profile
The investigation of bauxite samples along a profile (Figures 1-6, Tables 1-3) provides an approach in understanding the consequence of the biogeochemical processes during the bauxite formation, diagenesis/meta-diagenesis stages, the compositional variations along the profile, and relationships between redox-sensitive elements and TOC. Considering the transformation process of the bauxite, it seems likely that the interaction of Al-Fe-oxides with reducing microorganisms results in the Fe(III) reduction to soluble Fe(II), and, in turn, the Al, REE, and As enrichment (Figure 2a,b). Subsequently, part of the mobilized Fe(II) may be reprecipitated mainly as pyrite (Figure 2c,d), followed by the activity of oxidizing microorganisms and the replacement of pyrite by goethite and/or hematite (Figure 2d,e). Thus, under the activity of reducing microorganisms, Fe(II) may migrate, and fine-grained framboidal pyrite is present as veins crosscutting previous phases (Figure 2i). In addition, veinlets of goethite crosscutting all previous minerals (Figure 2e) reflect the multistage formation of minerals at a wide range of Eh-pH conditions during diagenetic and epigenetic stages. The higher As contents in the multicolor, compared to brown-red, bauxite samples (Table 1; Figure 5) may be the result of mobilization and redeposition, while the negative correlation with less mobile elements, such as As-Sc, r = −0.58, and As-Ta = −0.95, may indicate that As is more mobile than Sc and Ta. Both the oxidation of arsenite, As(III), to arsenate, As(V), and the reduction of arsenate to arsenite can be mediated by microorganisms [33][34][35]. The elevated As content (up to 960 mg/kg) in yellowwhitish, deep red to deep-grey color bauxite samples compared to brown-red samples (average 10 mg/kg As) (Table 1; Figure 5), the elevated total organic carbon (TOC) content (up to 5.4 wt.%) compared to the brown-red ore (0.08-0.26 wt.%), and the positive trend between As and TOC (r = 0.46) suggest that the As speciation is controlled by the presence of microorganisms. The creation of an acidic environment during meta-diagenesis stages, due to the decomposition of organic matter, may be responsible for the mobility and the elevated As contents, as well as for the potential presence of arsenate in association with goethite [11]. The investigation by synchrotron radiation (SR) spectroscopic techniques (SR µ-XRF and As K-edge µ-XAFS) of bauxite Fe-Ni-laterite ore from the Lokris area, C. Greece, having significant contents of As and TOC, has shown that As occurs in the form of arsenate anions As(V) or (AsO 4 3− ) in association with goethite as well [36]. Besides, a positive correlation (r > 0.50) between As and Pd, Sb (Table 2) may suggest the potential existence of (As, Pd, Sb)-bearing epigenetic minerals.
The lower PGE content in the Parnassos-Ghiona bauxite ores ranging from <0.1 to 1.9 µg/kg Pt and <0.5 to 5.5 µg/kg Pd (Table 1), compared to those in Fe-Ni ± Co laterites reaching up to 88 µg/kg Pt and 45 µg/kg Pd [37], is similar to that in other bauxite deposits and occurrences [38]. Such a difference may be related to their main parent rocks, as their contribution is reflected in the main components of ophiolites, such as Cr, Ni, and Co [21,38,39]. The relatively high values of the Pd/Pt ratio, ranging from 1.0 to 5.5 in the bauxite profile (Table 1), may be related to the higher solubility and mobility of Pd compared to that of Pt [40]. In addition, such higher Pd/Pt values in karst-type bauxites, compared to the Fe-Ni laterite deposits from the Balkan Peninsula (0.1 to 0.68, average = 0.40), may reflect a Pd-Pt decoupling during the transportation and redeposition at a longer distance from the former than the Fe-Ni laterites, lying on peridotites [37]. Scanning electron microscopy images showing biofilms containing abundant PGE nanoparticles and microcrystalline aggregates have shown that microorganisms can influence the mobility of PGE [41]. These authors described PGE minerals containing C, N, S, Se, and I, suggesting their biogenic origin, and concluded that biofilms are capable of forming or transforming PGE minerals, while secondary PGE minerals have been described at the border of Fe oxide(s) grains elsewhere [42]. Therefore, the Pd and Pt along the bauxite profile ( Figure 4) seems to be the result of their solubility and redeposition, facilitated by the interaction with the organic matter during the bauxite origin and redeposition by epigenetic processes.

Mining/Smelting Processes and Environmental Implications
It is well known that smelting or pyrometallurgy is the traditional technique for Al extraction, but this process is energy-consuming and generates greenhouse gas emissions and metallurgical residue, or red mud, which is relatively toxic. The biogeochemical cycling of metals and metalloids during open-pit mining of bauxite and the large volume of smelting residues may cause change in the land use for large land zones, environmental hazards for terrestrial and aquatic ecosystems, and food quality and socioeconomic problems. Besides the emissions, due to the metallurgical process of bauxite, the metallurgical residue, or red mud, is relatively toxic due to its high alkalinity (pH 10-12.5) and the presence of heavy and radioactive metals (Cr, V, Cd, Ni, Zn, Pb, REE, U, and Th) [43,44].
Since soils provide plants with certain trace elements in small amounts, which are considered to be critical for the healthy growth of humans, the protection and increase of the soil organic carbon is a priority in the European Union (EU) [9]. The spatial and temporal exploration of the composition and dynamics of bacterial and fungal communities in bauxite residues [45] has revealed that both deposit age and ore origin affect the geochemistry of bauxite residue. These authors concluded that salinity, pH and TOC depend predominantly on the age of bauxite residue, while the content of REE is mainly influenced by the origin of the bauxite ore. In addition, the investigation of the fertility of natural forest soil affected by the bauxite mining activities in Indonesia revealed that the fertility decreases for total N, C and P contents, exchangeable Ca, Mg and Na contents, and the growth of tree species [46]. Fertilizers and microorganisms have been applied to improve the soil fertility of post-openpit mining land and develop bioindicators for the remediation of land contaminated by the mining of bauxite [47]. The soil biogeochemistry and arsenic mobility in soil-water has been investigated experimentally using As-bearing red mud from Ajka (Hungary) and it was found that As was present as arsenate, As(V), which is highly pH-dependent, while the addition of phosphate to red mud greatly enhanced As release to solution [48]. In addition, investigation of the area contaminated by red mud Ajka (Hungary) showed that trace element contents, such as Cu, Cr, Ni, B, and Fe in plant shoots increased significantly with increasing dose of red mud, but none of these exceeded toxic limits, and Na salinity should be the main concern for this red mud in soil [49].
In general, the accumulation of multi-oxidation-state elements or transfer factor, e.g., the ratio of the element content in plants relative to the total element content in the corresponding soil, depends on the amount of elements in the soil and their mobility, the physical/chemical parameters (pH, Eh), the concentrations of completing ions, organic matter content, and other parameters [50]. An integrated approach to the soil, plant crops, and groundwater system has been performed to several cultivated basins with potential contamination, to evaluate the bioaccumulation of toxic elements by plants, a potential threat for human health and ecosystems [51]. However, water leachates for red mud from the Aspra Spitia plant have shown unexpectedly high (2100 µg/LCr(VI) concentrations [23], suggesting the potential contamination of neighboring soil and groundwater. Microbemediated mineralization has important applications in the restriction and/or removal of contaminants, such as the potential transfer of Cr(VI) in cultivated basins from area contaminated by red mud. The toxic hexavalent chromium [(Cr(VI)] in the food chain has created an alarming situation for human life and ecosystems, and the ability of organic matter (leonardite) in reducing Cr uptake by crops for soil and irrigation water with Cr(VI) has been investigated [52]. These authors concluded that the highest Cr values in shoots and in roots were recorded in those plants cultivated in soil after the addition of Cr(VI)-water without organic matter, and the lowest Cr values were in shoots and in roots of plants cultivated in soil with addition of 30 wt.% organic matter. Thus, the organic matter may provide a cost-effective method for the restriction of Cr transfer from contaminated soils and irrigation water to plants/crops and contribute to the better management of soil and water use.

Conclusions
Mineral transformations and compositional variations along a bauxite profile underlying a fault at the Parnassos-Ghiona deposit, Greece, point to the following conclusions:

•
Multicolor ores: grey-whitish (top), yellowish, deep red, and deep grey to brown-red color (bottom) are all associated with fossilized and present-day microorganisms, with TOC content showing the highest values in the deep grey ores and the lowest in the brown-red ores.

•
The recorded compositional variations show that the Al enrichment is accompanied by an increase in the TOC, As, Pd, and U, while the chondrite-normalized REE patterns through the whole profile exhibit a similar trend and positive Ce anomalies.

•
The lower (Pt + Pd) content and Pd/Pt ratio higher in the bauxite profile, compared to those in Fe-Ni laterite deposits, may reflect the higher solubility and mobility of Pd compared to that of Pt, and differences in their origin and genesis. • A positive correlation between Pd and As and the elevated As content (up to 960 mg/kg) in multicolor ores compared to brown-red samples (average 10 mg/kg As) point to their mobilization and epigenetic enrichment. • Geochemical characteristics of bauxite ores and smelting residues and their water leachates suggest that cycling of metals and metalloids during open-pit mining of bauxite and the large volume of red mud may cause changes to the land use, ecosystems, and the food quality. • Microorganisms, by their reducing and/or oxidizing activity, catalyze redox reactions, resulting in the Fe(II) migration and precipitation of framboidal pyrite as veins crosscutting previous phases, and provide nucleation sites for the precipitation of secondary minerals.