Redox-stat bioreactors for elucidating mobilisation mechanisms of trace elements: an example of As-contaminated mining soils.

The environmental fate of major (e.g. C, N, S, Fe and Mn) and trace (e.g. As, Cr, Sb, Se and U) elements is governed by microbially catalysed reduction-oxidation (redox) reactions. Mesocosms are routinely used to elucidate trace metal fate on the basis of correlations between biogeochemical proxies such as dissolved element concentrations, trace element speciation and dissolved organic matter. However, several redox processes may proceed simultaneously in natural soils and sediments (particularly, reductive Mn and Fe dissolution and metal/metalloid reduction), having a contrasting effect on element mobility. Here, a novel redox-stat (Rcont) bioreactor allowed precise control of the redox potential (159 ± 11 mV, ~ 2 months), suppressing redox reactions thermodynamically favoured at lower redox potential (i.e. reductive mobilisation of Fe and As). For a historically contaminated mining soil, As release could be attributed to desorption of arsenite [As(III)] and Mn reductive dissolution. By contrast, the control bioreactor (Rnat, with naturally developing redox potential) showed almost double As release (337 vs. 181 µg g-1) due to reductive dissolution of Fe (1363 µg g-1 Fe2+ released; no Fe2+ detected in Rcont) and microbial arsenate [As(V)] reduction (189 µg g-1 released vs. 46 µg g-1 As(III) in Rcont). A redox-stat bioreactor thus represents a versatile tool to study processes underlying mobilisation and sequestration of other trace elements as well.

Arsenic Mobilization from Historically Contaminated Mining Soils in a Continuously Operated Bioreactor: Implications for Risk Assessment.

Concentrations of soil arsenic (As) in the vicinity of the former Zloty Stok gold mine (Lower Silesia, southwest Poland) exceed 1000 µg g(-1) in the area, posing an inherent threat to neighboring bodies of water. This study investigated continuous As mobilization under reducing conditions for more than 3 months. In particular, the capacity of autochthonic microflora that live on natural organic matter as the sole carbon/electron source for mobilizing As was assessed. A biphasic mobilization of As was observed. In the first two months, As mobilization was mainly conferred by Mn dissolution despite the prevalence of Fe (0.1 wt % vs 5.4 for Mn and Fe, respectively) as indicated by multiple regression analysis. Thereafter, the sudden increase in aqueous As[III] (up to 2400 µg L(-1)) was attributed to an almost quintupling of the autochthonic dissimilatory As-reducing community (quantitative polymerase chain reaction). The aqueous speciation influenced by microbial activity led to a reduction of solid phase As species (X-ray absorption fine structure spectroscopy) and a change in the elemental composition of As hotspots (micro X-ray fluorescence mapping). The depletion of most natural dissolved organic matter and the fact that an extensive mobilization of As[III] occurred after two months raises concerns about the long-term stability of historically As-contaminated sites.

Biotransformation of copper from Kupferschiefer black shale (Fore-Sudetic Monocline, Poland) by yeast Rhodotorula mucilaginosa LM9.

This study describes the yeast Rhodotorula mucilaginosa strain LM9 isolated from copper-bearing, organic-rich Kupferschiefer black shale and its role in copper biotransformation. Strain LM9 exhibited great ability to simultaneously mobilize and immobilize copper from this sedimentary rock. In addition, it showed considerable resistance to copper and high uptake of this metal. Moreover, malic and oxalic acid as well as siderophore (rhodotorulic acid) produced by this strain enhanced its resistance by promoting the mobilization and complexation of copper from black shale. These processes, characterized here under laboratory conditions, are assumed to play a role in copper cycling in black shale as well as in the adaptation of strain LM9 to the conditions prevailing in its natural mine habitat. The findings of this study indicate that yeast strain LM9 might be used for the recovery of copper particularly from alkaline or slightly neutral ores in a non-chemical environmentally-friendly procedure.