Reductive Dissolution of Tl(I)–Jarosite by <i>Shewanella putrefaciens</i>: Providing New Insights into Tl Biogeochemistry

Thallium (Tl) is emerging as a metal of concern in countries such as China due to its release during the natural weathering of Tl-bearing ore deposits and mining activities. Despite the high toxicity of Tl, few studies have examined the reductive dissolution of Tl mineral phases by microbial populations. In this study we examined the dissolution of synthetic Tl­(I)–jarosite, (H₃O)_0.29Tl_0.71Fe_2.74(SO₄)₂(OH)_5.22(H₂O)_0.78, by <i>Shewanella putrefaciens</i> CN32 using batch experiments under anaerobic circumneutral conditions. Fe­(II) concentrations were measured over time and showed Fe­(II) production (4.6 mM) in inoculated samples by 893 h not seen in mineral and dead cell controls. Release of aqueous Tl was enhanced in inoculated samples whereby maximum concentrations in inoculated and cell-free samples reached 3.2 and 2.1 mM, respectively, by termination of the experiment. Complementary batch Tl/<i>S. putrefaciens</i> sorption experiments were conducted under experimentally relevant pH (5 and 6.3) at a Tl concentration of 35 μM and did not show significant Tl accumulation by either live or dead cells. Therefore, in contrast to many metals such as Pb and Cd, <i>S. putrefaciens</i> does not represent a sink for Tl in the environment and Tl is readily released from Tl–jarosite during both abiotic and biotic dissolution

Linking Spectral Induced Polarization (SIP) and Subsurface Microbial Processes: Results from Sand Column Incubation Experiments

Geophysical techniques, such as spectral induced polarization (SIP), offer potentially powerful approaches for in situ monitoring of subsurface biogeochemistry. The successful implementation of these techniques as monitoring tools for reactive transport phenomena, however, requires the deconvolution of multiple contributions to measured signals. Here, we present SIP spectra and complementary biogeochemical data obtained in saturated columns packed with alternating layers of ferrihydrite-coated and pure quartz sand, and inoculated with <i>Shewanella oneidensis</i> supplemented with lactate and nitrate. A biomass-explicit diffusion-reaction model is fitted to the experimental biogeochemical data. Overall, the results highlight that (1) the temporal response of the measured imaginary conductivity peaks parallels the microbial growth and decay dynamics in the columns, and (2) SIP is sensitive to changes in microbial abundance and cell surface charging properties, even at relatively low cell densities (<10⁸ cells mL^–1). Relaxation times (τ) derived using the Cole–Cole model vary with the dominant electron accepting process, nitrate or ferric iron reduction. The observed range of τ values, 0.012–0.107 s, yields effective polarization diameters in the range 1–3 μm, that is, 2 orders of magnitude smaller than the smallest quartz grains in the columns, suggesting that polarization of the bacterial cells controls the observed chargeability and relaxation dynamics in the experiments

Simultaneous Release of Fe and As during the Reductive Dissolution of Pb–As Jarosite by <i>Shewanella putrefaciens</i> CN32

Jarosites are produced during metallurgical processing, on oxidized sulfide deposits, and in acid mine drainage environments. Despite the environmental relevance of jarosites, few studies have examined their biogeochemical stability. This study demonstrates the simultaneous reduction of structural Fe­(III) and aqueous As­(V) during the dissolution of synthetic Pb–As jarosite (PbFe₃(SO₄,AsO₄)₂(OH)₆) by <i>Shewanella putrefaciens</i> using batch experiments under anaerobic circumneutral conditions. Fe­(III) reduction occurred immediately in inoculated samples while As­(V) reduction was observed after 72 h. XANES spectra showed As­(III) (14.7%) in the solid phase at 168 h coincident with decreased aqueous As­(V). At 336 h, XANES spectra and aqueous speciation analysis demonstrated 20.2% and 3.0% of total As was present as As­(III) in the solid and aqueous phase, respectively. In contrast, 12.4% of total Fe was present as aqueous Fe­(II) and was below the detection limits of XANES in the solid phase. TEM-EDS analysis at 336 h showed secondary precipitates enriched in Fe and O with minor amounts of As and Pb. Based on experimental data and thermodynamic modeling, we suggest that structural Fe­(III) reduction was thermodynamically driven while aqueous As­(V) reduction was triggered by detoxification induced to offset the high As­(V) (328 μM) concentrations released during dissolution