Lesley A. Warren

Biogeochemical controls on metal behaviour in freshwater environments

Abstract The biogeochemical controls on metal behaviour in aqueous environments involve complex linkages of biological, principally bacterially driven, and geochemical processes, which occur at both microscopic and macroscopic scales. The framework of aqueous surface chemistry and aquatic geochemistry continues to provide the foundations of the emerging paradigm: (1) metal behaviour (e.g., transport, toxicity, bioaccumulation) is governed by solid-solution reactions; (2) pH, ionic strength, redox potential, the types and concentrations of solution elements, and solid surfaces all interact to determine metal behaviour in any given system; (3) metal sorption reactions show both metal ion and solid surface specificity; (4) sorption reactions are dynamic and reversible; and (5) processes are at sufficient pseudo-equilibrium or dynamic steady state that thermodynamics can be applied to describe such reactions. Reactions controlling metal behaviour are increasingly modelled, with some success, using a variety of geochemical modelling approaches all based on this framework. However, not yet considered in the majority of these thermodynamic treatments of metal dynamics is that these reactions are highly influenced by biological factors, which will affect their location, magnitude and rate. The extent of this influence will be largely driven by microbial ecology, and thus, a fundamental identification and mechanistic understanding of how these factors will drive the geochemistry of a particular system is required. The lack of substantive biogeochemical understanding stems from the fact that the field of environmental microbiology, with its crossover to environmental geochemistry, has only recently begun to receive attention. The developing evidence strongly underscores the impact of bacterial reactions for a number of highly relevant processes related to metal dynamics such as solid solution partitioning, mineral precipitation and dissolution reactions, and intense changes in system geochemical conditions. The development of new molecular level microscopic and spectroscopic techniques provides powerful tools to promote an integrated approach to understanding the mechanisms underlying metal dynamics, which encompasses both the geochemical and biological components of this dynamic and complex cycle. Particularly, when used in conjunction with new molecular biological tools, these multiple lines of evidence will provide a mechanistic model of the controls on metal behaviour that will reflect the inherently complex reality of natural systems.

Microbially Mediated Calcium Carbonate Precipitation: Implications for Interpreting Calcite Precipitation and for Solid-Phase Capture of Inorganic Contaminants

Microbial degradation of urea was investigated as a potential geochemical catalyst for Ca carbonate precipitation and associated solid phase capture of common groundwater contaminants (Sr, UO2, Cu) in laboratory batch experiments. Bacterial degradation of urea increased pH and promoted Ca carbonate precipitation in both bacterial control and contaminant treatments. Associated solid phase capture of Sr was highly effective, capturing 95% of the 1 mM Sr added within 24 h. The results for Sr are consistent with solid solution formation rather than discrete Sr carbonate phase precipitation. In contrast, UO2 capture was not as effective, reaching only 30% of the initial 1 mM UO2 added, and also reversible, dropping to 7% by 24 h. These results likely reflect differing sites of incorporation of these two elements-Ca lattice sites for Sr versus crystal defect sites for UO2. Cu sequestration was poor, resulting from toxicity of the metal to the bacteria, which arrested urea degradation and concomitant Ca carbonate precipitation. Scanning electron microscopy (SEM) indicated a variety of morphologies reminiscent of those observed in the marine stromatolite literature. In bacterial control treatments, X-ray diffraction (XRD) analyses indicated only calcite; while in the presence of either Sr or UO2, both calcite and vaterite, a metastable polymorph of Ca carbonate, were identified. Tapping mode atomic force microscopy (AFM) indicated differences in surface microtopography among abiotic, bacterial control, and bacterial contaminant systems. These results indicate that Ca carbonate precipitation induced by passive biomineralization processes is highly effective and may provide a useful bioremediation strategy for Ca carbonate-rich aquifers where Sr contamination issues exist.Microbial degradation of urea was investigated as a potential geochemical catalyst for Ca carbonate precipitation and associated solid phase capture of common groundwater contaminants (Sr, UO2, Cu) in laboratory batch experiments. Bacterial degradation of urea increased pH and promoted Ca carbonate precipitation in both bacterial control and contaminant treatments. Associated solid phase capture of Sr was highly effective, capturing 95% of the 1 mM Sr added within 24 h. The results for Sr are consistent with solid solution formation rather than discrete Sr carbonate phase precipitation. In contrast, UO2 capture was not as effective, reaching only 30% of the initial 1 mM UO2 added, and also reversible, dropping to 7% by 24 h. These results likely reflect differing sites of incorporation of these two elements-Ca lattice sites for Sr versus crystal defect sites for UO2. Cu sequestration was poor, resulting from toxicity of the metal to the bacteria, which arrested urea degradation and concomitant Ca carbonat...

The influence of temperature and NaCl on cadmium, copper and zinc partitioning among suspended particulate and dissolved phases in an urban river

Abstract Distribution coefficients (Kd) were used to estimate the most important geochemical phases within the suspended particulate matter (SPM) pool for sorption of Cd, Cu and Zn. Given that pH effects were expected to be minimal, as the don is a well buffered system, the possible influence of secondary environmental variables [temperature, dissolved organic carbon (DOC), dissolved Ca2+ and NaC] on trace metal partitioning between SPM and the dissolved phase was evaluated using a series of multiple linear regressions for total (KdT) as well as the phase specific KdL leachable phase; KdR reducible phase; and KdO oxidizable phase) distribution coefficient estimates. The three metals varied in their sorptions patterns. Cd and Zn showed the same relative affinities for three SPM pools (leachable = reducible > oxidizable), while Cu affinities ranked oxidizable = leachable > reducible. Secondary environmental factors were identified as more important influences on trace metal partitioning than pH. Temperature and NaCl (from road salt runoff) were found to be key environmental variables influencing trace metal partitioning. A decrease in water temperature caused decreases in the accumulation of Cd, Cu and Zn in the particulate pool. Increasing NaCl concentrations decreased the concentrations of Cd and Zn associated with the particulate leachable phase and the Cd, Zn and Cu content in the oxidizable SPM phase. These results suggest that in running waters during winter months, or even during summer months in the hypolimnia of sufficiently deep lakes, a relatively higher proportion of these metals remains in the dissolved and potentially more bioavailable pool.

Influence of ionic strength on strontium sorption to bacteria, Fe(III) oxide, and composite bacteria-Fe(III) oxide surfaces

Abstract A series of experiments were conducted to investigate the influence of ionic strength on Sr 2+ sorption by the bacteria Shewanella alga , hydrous ferric oxide (HFO) and bacteria-HFO composite solids. Sorption of Sr 2+ to S. alga exhibited a strong dependence on ionic strength with the maximum concentration of solid phase Sr (BSr max ) decreasing from 79 μmol g −1 under dilute aqueous conditions to 4 μmol g −1 at high ionic strength (0.1M NaNO 3 ). Corresponding apparent surface complex formation (K S Sr ) values for S. alga increased from 10 −0.51 to 10 −0.26 with increasing ionic strength, implying that only high affinity sites remain to bind Sr 2+ under conditions of increased ionic strength. In contrast, Sr 2+ sorption to HFO surfaces was independent of ionic strength with BSr max and K S Sr remaining relatively constant (approximately 1 μmol g −1 and 10 −2.1 , respectively) under increasing ionic strength conditions. The ionic strength dependent sorptive behaviour exhibited by S. alga is consistent with electrostatic outer-sphere complexation reactions occurring in the diffuse layer, whereas inner-sphere complexation reactions account for the Sr 2+ sorption behaviour of HFO. The bacteria-HFO composite solid exhibited moderate ionic strength dependence with maximum binding capacities decreasing from 34 μmol g −1 (dilute conditions) to 24 μmol g −1 (0.1 M NaNO 3 ). These results suggest that Fe 3+ sorption and precipitation at the bacterial surface alters the electrochemical surface properties of the composite solid, buffering the effects of increased ionic strength on subsequent Sr 2+ sorption.

The importance of surface area in metal sorption by oxides and organic matter in a heterogeneous natural sediment

Abstract This study provides empirical validation of current trace metal sorption theory in a small urban river. We demonstrate that trace metal complexation reactions occur predominantly at the suspended particulate surface involving surface layers of Fe oxides and organic matter. Associated surface areas of these geochemical fractions were calculated where possible, using the total surface area (TSA) of the suspended particulate matter pool (SPM) in conjunction with estimates of suspended iculate Fe and Mn oxides (SPOX) and organic matter (SPOM) concentrations. Iron and Mn oxides concentrations were estimated using an extraction scheme. For two samples where no SPOM or Mn oxides were present, estimates of Fe oxides associated surface area were determined which compared favourably to literature estimates, providing further evidence for acceptance selectivity of extraction schemes. The utility of literature estimates of surface areas for single component sediments in heterogeneous sediments was also assessed. In mixed sediment samples, exposed surface areas of discrete phases are probably reduced due to mixed layering effects of the coatings, and the use of constants to estimate the surface areas of individual fractions does not work, since the relationship between the concentration of a given sedimentary fraction and its exposed surface area is no longer predictable.

Microbial Architecture of Environmental Sulfur Processes: A Novel Syntrophic Sulfur-Metabolizing Consortia

Microbial oxidation of sulfur-rich mining waste materials drives acid mine drainage (AMD) and affects the global sulfur biogeochemical cycle. The generation of AMD is a complex, dynamic process that proceeds via multiple reaction pathways. The role of natural consortia of microbes in AMD generation, however, has received very little attention despite their widespread occurrence in mining environments. Through a combination of geochemical experimentation and modeling, scanning transmission X-ray microscopy, and fluorescent in situ hybridization, we show a novel interdependent metabolic arrangement of two ubiquitous and abundant AMD bacteria: chemoautotrophic sulfur-oxidizing Acidithiobacillus sp. and heterotrophic Acidiphilium sp. Highly reminiscent of anaerobic methane oxidation (AOM) consortia, these bacteria are spatially segregated within a planktonic macrostructure of extracellular polymeric substance in which they syntrophically couple sulfur oxidation and reduction reactions in a mutually beneficial arrangement that regenerates their respective sulfur substrates. As discussed here, the geochemical impacts of microbial metabolism are linked to the consortial organization and development of the pod structure, which affects cell-cell interactions and interactions with the surrounding geochemical microenvironment. If these pods are widespread in mine waters, echoing the now widespread discovery of AOM consortia, then AMD-driven CO(2) atmospheric fluxes from H(2)SO(4) carbonate weathering could be reduced by as much as 26 TgC/yr. This novel sulfur consortial discovery indicates that organized metabolically linked microbial partnerships are likely widespread and more significant in global elemental cycling than previously considered.

Physical and Ecological Controls on Freshwater Floc Trace Metal Dynamics

Significantly higher concentrations of Ag, As, Cu, Co, Ni, and Pb are found in suspended floc compared to surficial bed sediments for a freshwater beach in Lake Ontario. Contrasting observed element-specific bed sediment metal partitioning patterns, floc sequestration for all elements is dominated by one substrate: amorphous oxyhydroxides. More specifically, floc metal scavenging is controlled by floc biogeochemical architecture. Floc organics, largely living microbial cells and associated exopolymeric substances (EPS), act as scaffolds for the collection and/or templating of amorphous Fe oxyhydroxides. While interactions between floc organics and amorphous Fe oxyhydroxides affected floc sorption behavior, specific element affinities and competition for these limited substrates was important for overall floc partitioning. Further, assessment of metal dynamics during stormy conditions indicated energy-regime driven shifts in floc and bed sediment partitioning that were specifically linked to the exchange of floc and bed sedimentary materials. These novel results demonstrate that the microbial nature of floc formation exerts an important control on floc metal dynamics distinguishable from surficial bed sediments and that hydrologic energy-regime is an important factor to consider in overall floc metal behavior, especially in beach environments.

Suspended particulate oxides and organic matter interactions in trace metal sorption reactions in a small urban river

The relative scavenging abilities of suspended particulate oxides (SPOX), and organic matter (SPOM) for Cd, Zn and Cu were evaluated in a small, anthropogenically influenced river. In addition, the factor most important in influencing the sorption density (Ad: metal concentration associated with a given phase divided by the concentration of that geochemical phase in the suspended particulate pool) of each metal to SPOX and SPOM were identified through multiple linear regression analyses from the suite of: pH, temperature, dissolved metal concentration, and the concentration of the other particulate fraction. Results indicate that SPOX-SPOM interactions do occur in trace metal complexation reactions; and interactions are both phase and cation specific. Fe oxides are able to outcompete discrete organic binding sites for Cu and Zn as a relative decrease in the amount of these two cations sorbed to organic matter was observed with increasing particulate Fe oxides. SPOM concentration was identified as being most important in influencing Cu sorption densities associated with the SPOX fraction. Organic matter — oxide complexes are postulated to occur that enhance oxide sorption of Cu such that relatively more Cu is sorbed to particulate oxides with increasing particulate organic matter concentrations. Dissolved concentrations of Cd and Zn were found to be most important in influencing the sorption densities for these two metals associated with the oxides fraction. The sorption behaviour appears to follow Freundlich isotherm behaviour where the amount sorbed is a function of the dissolved concentration.

Identification of individual thiophene‐, indane‐, tetralin‐, cyclohexane‐, and adamantane‐type carboxylic acids in composite tailings pore water from Alberta oil sands

RATIONALE Naphthenic acids (NAs) accumulate in oil sands process-affected water (OSPW) as a result of the water-based extraction processes, and represent one of the toxic fractions in OSPW. They exist as a complex mixture and so the development of an analytical method to characterize and quantify individual acids has been an on-going challenge. The multidimensional separation technique of two-dimensional gas chromatography (GC × GC) has the potential to provide a fingerprint of the sources of NAs and can potentially resolve individual analytes for target analysis. However, the identity and toxicity of a large proportion of the acids present in tailing waters are still unknown. METHODS Comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC/TOFMS) was used to characterize NAs in a pore water sample from a Syncrude composite tailings (CT) deposit in Fort McMurray, Alberta, Canada. The extractable organic acid fraction was derivatized with diazomethane and the structures of selected resolved esters were elucidated through interpretation of their electron ionization (EI) mass spectra and, if available, confirmed by comparison with the spectra of reference standards. RESULTS The high resolving power of the GC × GC/TOFMS technique allowed for the structural elucidation of numerous as yet unidentified acids in the CT pore water sample such as carboxylic acids containing a thiophene, indane, tetralin or cyclohexane moiety. Seventeen members of the previously reported class of adamantane-type carboxylic acids in oil sands process water could also be identified in the sample. CONCLUSIONS This study underlines the complexity of naphthenic acid isomer distributions in composite tailings and provides a useful inventory of individual acids.

Diversity of integron- and culture-associated antibiotic resistance genes in freshwater floc.

ABSTRACT Clinically important antibiotic resistance genes were detected in culturable bacteria and class 1 integron gene cassettes recovered from suspended floc, a significant aquatic repository for microorganisms and trace elements, across freshwater systems variably impacted by anthropogenic activities. Antibiotic resistance gene cassettes in floc total community DNA differed appreciably in number and type from genes detected in bacteria cultured from floc. The number of floc antibiotic resistance gene cassette types detected across sites was positively correlated with total (the sum of Ag, As, Cu, and Pb) trace element concentrations in aqueous solution and in a component of floc readily accessible to bacteria. In particular, concentrations of Cu and Pb in the floc component were positively correlated with floc resistance gene cassette diversity. Collectively, these results identify suspended floc as an important reservoir, distinct from bulk water and bed sediment, for antibiotic resistance in aquatic environments ranging from heavily impacted urban sites to remote areas of nature reserves and indicate that trace elements, particularly Cu and Pb, are geochemical markers of resistance diversity in this environmental reservoir. The increase in contamination of global water supplies suggests that aquatic environments will become an even more important reservoir of clinically important antibiotic resistance in the future.