Manganese reductive dissolution coupled to Sb mobilization in contaminated shooting range soil

Abstract

A "redox-stat" R_MnR bioreactor was employed to simulate moderately reducing conditions (+ 420 mV) in Sb-contaminated shooting range soils for approximately 3 months, thermodynamically favoring Mn(IV) reduction. The impact of moderately reducing conditions on elemental mobilization (Mn, Sb, Fe) and speciation [Sb(III) versus Sb(V); Fe²⁺/Fe³⁺] was compared to a control bioreactor R_CTRL without a fixed redox potential. In both bioreactors, reducing conditions were accompanied by an increase in effluent Sb(V) and Mn(II) concentrations, suggesting that Sb(V) was released through microbial reduction of Mn oxyhydroxide minerals. This was underlined by multiple linear regression analysis showing a significant (p < 0.05) relationship between Mn and Sb effluent concentrations. Mn concentration was the sole variable exhibiting a statistically significant effect on Sb in R_MnR, while under the more reducing conditions in R_CTRL, pH and redox potential were also significant. Analysis of the bacterial community composition revealed an increase in the genera Azoarcus, Flavisolibacter, Luteimonas, and Mesorhizobium concerning the initial soil, some of which are possible key players in the process of Sb mobilization. The overall amount of Sb released in the R_MnR (10.40%) was virtually the same as in the R_CTRL (10.37%), which underlines a subordinate role of anoxic processes, such as Fe-reductive dissolution, in Sb mobilization. This research underscores the central role of relatively low concentrations of Mn oxyhydroxides in influencing the fate of trace elements. Our study also demonstrates that bioreactors operated as redox-stats represent versatile tools that allow quantifying the contribution of specific mechanisms determining the fate of trace elements in contaminated soils. KEY POINTS: • "Redox-stat" reactors elucidate Sb mobilization mechanisms • Mn oxyhydroxides microbial reductive dissolution has a major role in Sb mobilization in soils under moderately reducing conditions • Despite aging the soil exhibited significant Sb mobilization potential, emphasizing persistent environmental effects.

Keywords: Antimony environmental fate; Antimony mobility; Metalloid risk assessment; Redox-stat bioreactor; Trace metal fate.

Fig. 1

Mobilization of Sb (a), Mn (b), Fe (c), Pb (d), redox potential (e), and DOC (f) in R_MnR (open symbols) and R_CTRL (closed symbols). Elemental mobilization is indicated as concentration in the effluent [μg L⁻.¹] (triangles; on primary y-axis in a–d) and as cumulative element mobilized [% of initial] (circles; secondary axis in a–d). The dashed horizontal lines in a and d represent the most stringent drinking water limit values for Sb and Pb, respectively (details see text)

Fig. 2

Metal fractionation of Sb (a), Mn (b), and Fe (c) in the initial soil and the soils after treatment in R_MnR and R_CTRL (for details refer to text)

Fig. 3

Actual versus Sb effluent concentrations [µg/L] predicted by multiple linear regression in R_MnR (a) and R_CTRL (b) (for details refer to text and supplementary materials 1.3)

Fig. 4

Relative abundance of genera with more than 1% abundance of all sequences in the initial soil and the soils after treatment in R_MnR and R_CTRL. Arrows up mark genera that increased by more than 3% relative abundance during the incubation, arrows down indicate genera that decreased by 3%, and the equal sign represents genera that underwent no significant change of the incubated soil after 3 months of incubation, in comparison with the initial soil

Fig. 5

Possible electron donor mechanisms for Mn(IV) reductive dissolution based on a “conventional” understanding where DOM serves as electron donor (A) and a hypothetical alternative (indicated by metagenomic sequencing soil genera) where nitrification is coupled with Mn reduction (B). Figure created with BioRender.com