Abstract
It is well known that speciation transformations of As(III) vs. As(V) in acid mine drainage (AMD) are mainly driven by microbially mediated redox reactions of Fe and S. However, these processes are rarely investigated. In this study, columns containing mine water were inoculated with two typical acidophilic Fe/S-oxidizing/reducing bacteria [the chemoautotrophic Acidithiobacillus (At.) ferrooxidans and the heterotrophic Acidiphilium (Aph.) acidophilum], and three typical energy substrates (Fe2+, S0, and glucose) and two concentrations of As(III) (2.0 and 4.5 mM) were added. The correlation between Fe/S/As speciation transformation and bacterial depth distribution at three different depths, i.e., 15, 55, and 105 cm from the top of the columns, was comparatively investigated. The results show that the cell growth at the top and in the middle of the columns was much more significantly inhibited by the additions of As(III) than at the bottom, where the cell growth was promoted even on days 24–44. At. ferrooxidans dominated over Aph. acidophilum in most samples collected from the three depths, but the elevated proportions of Aph. acidophilum were observed in the top and bottom column samples when 4.5 mM As(III) was added. Fe2+ bio-oxidation and Fe3+ reduction coupled to As(III) oxidation occurred for all three column depths. At the column top surfaces, jarosites were formed, and the addition of As(III) could lead to the formation of the amorphous FeAsO4⋅2H2O. Furthermore, the higher As(III) concentration could inhibit Fe2+ bio-oxidation and the formation of FeAsO4⋅2H2O and jarosites. S oxidation coupled to Fe3+ reduction occurred at the bottom of the columns, with the formations of FeAsO4⋅2H2O precipitate and S intermediates. The formed FeAsO4⋅2H2O and jarosites at the top and bottom of the columns could adsorb to and coprecipitate with As(III) and As(V), resulting in the transfer of As from solution to solid phases, thus further affecting As speciation transformation. The distribution difference of Fe/S energy substrates could apparently affect Fe/S/As speciation transformation and bacterial depth distribution between the top and bottom of the water columns. These findings are valuable for elucidating As fate and toxicity mediated by microbially driven Fe/S redox in AMD environments.
Highlights
Arsenic (As) pollution in acidic environments has been a global concern due to its harmfulness to the ecosystem and human health
Through monitoring Fe/S/As speciation transformations and the dynamic change in the microbial community along the depths of the water columns, we comparatively investigated their correlations at three different depths of the water columns
The total cell density was determined by direct counting with a blood corpuscle counter (XB-K-25). [SO42−] was measured by the barium sulfate turbidimetric colorimetric method according to Zhu et al (2011). [total Fe (TFe)] and [Fe(III)] were determined by 5-sulfosalicylic acid spectrophotometry according to Karamanev et al (2002)
Summary
Arsenic (As) pollution in acidic environments has been a global concern due to its harmfulness to the ecosystem and human health. On the one hand, during the oxidative dissolution of Ascontaining sulfide minerals driven by microbial Fe/S oxidation in acidic environments such as acid mine drainage (AMD) sites, a series of iron/sulfur-containing intermediates and secondary products are formed, e.g., orpiment (As2S3), jarosite (M[Fe3(OH)6(SO4)2], M = K+, Na+, NH4+, or H3O+), scorodite (FeAsO4·2H2O), and schwertmannite [Fe8O8(OH)8−2x(SO4)x (where 1 ≤ x ≤ 1.75)], leading to changes in As speciation and occurrence forms due to As adsorption to or coprecipitation with the Fe/S secondary products (Zhang et al, 2019; Tabelin et al, 2020; Battistel et al, 2021). Several acidophilic bacteria and archaea in acidic environments as well as neutrophilic Fe(III)- and sulfate-reducing bacteria (IRB and SRB, e.g., Shewanella and Desulfovibrio, respectively) in pH-neutral and mildly alkaline environments can lead to the reductive dissolution of Fe(III) minerals with the formation of Fe/S secondary products, such as mackinawite (FeS) and siderite (FeCO3), with a release of As species and the reduction of As(V) to As(III) (Drahota et al, 2013; Wang et al, 2016; Hedrich and Schippers, 2021)
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