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

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 (R_cont) 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 (R_nat, 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 R_cont) and microbial arsenate [As(V)] reduction (189 μg g^-1 released vs. 46 μg g^-1 As(III) in R_cont). A redox-stat bioreactor thus represents a versatile tool to study processes underlying mobilisation and sequestration of other trace elements as well.