Increased Solubility and Bioavailability of Hydroxy-Cr(III) Precipitates in the Presence of Hydroxamate Siderophores.

Siderophores are a diverse group of low molecular weight biogenic metallophores with a particular affinity for Fe(III) but they also have potential to complex a number of other polyvalent metal cations, including Cr(III). Here we show that two hydroxamate siderophores, desferrioxamine B and rhodotorulic acid, at environmentally relevant concentrations, facilitate the dissolution of hydroxy-Cr(III) precipitates from a common layer silicate. Desferrioxamine B and rhodotorulic acid induced maximum initial Cr dissolution rates of 11.3 ± 1.7 × 10- 4  and 9.03 ± 0.68 × 10- 4 µmol m- 2 h- 1, respectively, yielding maximum solution Cr concentrations of 0.26 ± 0.01 and 0.20 ± 0.02 µmol m- 2, respectively. These data demonstrate that hydroxamate siderophores may play an important role increasing the dispersal of Cr in natural environments, thus facilitating greater bioavailability of this potential toxin.

Dissolution of Al-Substituted Goethite in the Presence of Ferrichrome and Enterobactin at pH 6.5.

Naturally occurring goethites often show Al for Fe substitution approaching 33 mol% Al. This substitution has potential to influence the rate of goethite dissolution and therefore the supply of bioavailable Fe. Siderophores such as ferrichrome and enterobactin have considerable potential to dissolve Fe from Fe3+ rich minerals, including Al-substituted goethites. Here, we show that Al substitution in synthetic goethites (0.021 ≥ x ≥ 0.098) gives rise to a significant increase in both ferrichrome- and enterobactin-mediated dissolution rates. In the presence of ferrichrome, Al-goethite (x = 0.033) yields a dissolution rate of 19.0 × 10-3 µmol m-2 h-1, nearly twice that of pure goethite, whereas dissolution of the most highly substituted Al-goethite (x = 0.098) is 36.9 × 10-3 µmol m-2 h-1, more than threefold greater than the pure mineral. Similarly, in the presence of enterobactin, the dissolution rate of Al-goethite increases with increasing Al substitution. Ferrichrome is a less effective ligand than enterobactin in its dissolution of both pure goethite and the range of Al-goethites, an observation we ascribe to the lower affinity of the hydroxamate functional groups of ferrichrome for both Fe3+ and Al3+. Despite greater affinity of both ferrichrome and enterobactin for Fe3+ over Al3+, we observe a broadly congruent dissolution of all our Al-goethites.

Simultaneous release of Fe and As during the reductive dissolution of Pb-As jarosite by Shewanella putrefaciens 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 (PbFe(3)(SO(4),AsO(4))(2)(OH)(6)) by Shewanella putrefaciens 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.

Influence of hydrology on heavy metal speciation and mobility in a Pb-Zn mine tailing.

Among the inorganic toxicants of greatest concern in mine tailings, Pb, Zn, Cu, Cd and As figure prominently due to their abundance and potential toxicity. Here we report on their biolability and solid-phase speciation in two sediment cores subject to variable hydrological regimes at an abandoned pyritic mine tailing. The oxic conditions of well-drained sediments induced pyrite oxidation and the subsequent liberation of H+, SO4(2-) and considerable quantities of Fe(III), which precipitated as goethite. Solubility of Pb, Zn, Cu and Cd was closely coupled to pH and goethite presence. Metal lability was particularly low in zones of neutralization, formed by the accumulation of calcite, first carried then deposited by percolating waters in both saturated and unsaturated cores. We conclude that differential hydrology induces variable heavy metal speciation and biolability in Pb-Zn mine tailings, and suggest that site-specific risk assessments must account for past and present hydrological regimes.

Copper-glyphosate sorption to microcrystalline gibbsite in the presence of soluble Keggin Al13 polymers.

Among the most reactive yet largely neglected adsorbents of toxicant species occurring in acidic aquatic environments are the epsilon-Keggin Al13 polyoxocations [AlO4Al12(OH)24(H2O)12(7+)], known generally as Al13 polymers. Here, we report on the sorption of Cu(II), a common ingredient of pesticides, and glyphosate {N-[phosphonomethyl]glycine (PMG)}, a widely applied herbicide, to microcrystalline gibbsite [gamma-Al(OH)3] in the presence of soluble Al13 polymers over the pH range 4-7. In the presence of gibbsite and soluble Al13 polymers, dissolved Cu(II) decreased gradually with pH, achieving a minimum at pH 5.5. Between pH 5.5 and 6.0, however, soluble Cu increased markedly, with approximately 80% of the added metal remaining in solution at pH 5.86. At pH > 6.0, soluble Cu once again decreased, becoming undetectable at pH 7. The anomalous Cu solubilization was attributed to a concomitant deprotonation of soluble Al13 polymers, yielding surface OH groups possessing high affinity for Cu(II). Removal of Cu from solution at pH > 6.0 is facilitated by flocculation of the Al13 polymers to which Cu had sorbed. The sorption behavior of the zwitterionic PMG in the presence of gibbsite and Al13 polymers was consistent with this interpretation, there being a dramatic increase in sorbed PMG at pH > 6.0 as the Al13 polymers deprotonated and flocculated. Copper and PMG loss from solution with increasing pH when both adsorptives were added to the gibbsite-Al13 polymer system was broadly similar to what was observed in the PMG-free systems, although small differences were detected in response to varying the order of adsorptive addition. The inclusion of soluble Al polymers in our experiments exposes a fundamental limitation of models based on but a single inorganic adsorbent as a means to predict the behavior of trace metals and xenobiotic organic compounds in natural systems.

Influence of organic ligands on Cr desorption from hydroxy-Cr intercalated montmorillonite.

Retention of metals as hydroxy precipitates on mineral surfaces is an important process maintaining environmental quality. Hydroxy precipitates of Cr(III) are particularly important because appreciable mobilisation of intercalated, hydrolysed Cr from montmorillonite is currently thought to occur only via oxidative dissolution. The present study examines the potential of oxalate, tartrate, and citrate to desorb Cr from both hydroxy-Cr and hydroxy Al-Cr precipitates sorbed to montmorillonite. For all intercalated clays there was an initial very rapid dissolution, followed by a slow but sustained Cr release that more closely followed a parabolic rate law, the latter indicative of transport-controlled dissolution. The initial rapid dissolution persisted longer for the pure Cr clay ( approximately 48 h) than for the coprecipitated Al-Cr clay ( approximately 24 h). At the end of the 30 d reaction, however, most Cr remained sorbed to the montmorillonite in all systems. Citrate, for example, solubilised only approximately 10% of the total sorbed Cr from the pure Cr clays. Aluminum presence decreased Cr solubilisation rate due to Al accumulation at the polymer margins. Moreover, Al presence maintained the integrity of the intercalated polymers and prevented c-axis expansion beyond 1.4 nm, thus restricting the supply of organic ligand to the inter-layer region. Oxalate, tartrate, and citrate were shown to be effective chelators of sorbed Cr and these ligands may therefore play an important role in the mobilisation and cycling of this metal in soils and sediments.