Colloidal transport of heavy metals in low‐advective‐velocity environmental systems: Reactive transport model on biogeochemical and hydrodynamic impacts

Abstract In this study, the impact of colloid facilitated transport of heavy metals on the overall biogeochemical processes is demonstrated in example Lake Coeurd'Alene sediments. Release and transport of heavy metals (Pb and Zn) on initially sorbed colloidal Fe (hydr)oxide minerals are compared with immobile surfaces under various advective flow velocities. The reactive transport model integrates a coupled biotic reaction network with multiple terminal electron acceptors, including multicomponent diffusion and electrostatic double layer (EDL) treatment effects, illustrating the impact of colloidal transport under competing biogeochemical reaction dynamics for the first time to the authors’ knowledge. The model results illustrate the sensitivity of the results under low‐flow‐velocity conditions. Although enhanced Fe reduction prevails with immobile Fe (hydr)oxide mineral surfaces, the desorbed metal ions with aqueous sulfide complexes are rather “washed out” from the system along with advective transport of solutes, whereas the reductive dissolution of colloidal Fe (hydr)oxides from freshly coming colloidal surfaces results in the accumulation of metal and sulfide ions in the system. The results show that when the potential transport of sorbed contaminants with colloidal particles are ignored, the contaminant concentrations might be underestimated under low‐flow‐velocity conditions, especially around 10−8 or 10−9 m s−1, where the underestimation for the worst case scenario at the lowest bound of low‐flow‐velocity conditions may reach around 90% with depth. On the other hand, this impact may be less significant under cases of higher flow velocity, even around higher limits of low‐velocity environments around 10−7 m s−1, as well as in pure diffusive transport cases

Impact of different environmental conditions on the aggregation of biogenic U(IV) nanoparticles synthesized by Desulfovibrio alaskensis G20.

This study investigates the impact of specific environmental conditions on the formation of colloidal U(IV) nanoparticles by the sulfate reducing bacteria (SRB, Desulfovibrio alaskensis G20). The reduction of soluble U(VI) to less soluble U(IV) was quantitatively investigated under growth and non-growth conditions in bicarbonate or 1,4-piperazinediethanesulfonic acid (PIPES) buffered environments. The results showed that under non-growth conditions, the majority of the reduced U nanoparticles aggregated and precipitated out of solution. High resolution transmission electron microscopy revealed that only a very small fraction of cells had reduced U precipitates in the periplasmic spaces in the presence of PIPES buffer, whereas in the presence of bicarbonate buffer, reduced U was also observed in the cytoplasm with greater aggregation of biogenic U(IV) particles at higher initial U(VI) concentrations. The same experiments were repeated under growth conditions using two different electron donors (lactate and pyruvate) and three electron acceptors (sulfate, fumarate, and thiosulfate). In contrast to the results of the non-growth experiments, even after 0.2 μm filtration, the majority of biogenic U(IV) remained in the aqueous phase resulting in potentially mobile biogenic U(IV) nanoparticles. Size fractionation results showed that U(IV) aggregates were between 18 and 200 nm in diameter, and thus could be very mobile. The findings of this study are helpful to assess the size and potential mobility of reduced U nanoparticles under different environmental conditions, and would provide insights on their potential impact affecting U(VI) bioremediation efforts at subsurface contaminated sites

Impact of different environmental conditions on the aggregation of biogenic U(IV) nanoparticles synthesized by Desulfovibrio alaskensis G20.

This study investigates the impact of specific environmental conditions on the formation of colloidal U(IV) nanoparticles by the sulfate reducing bacteria (SRB, Desulfovibrio alaskensis G20). The reduction of soluble U(VI) to less soluble U(IV) was quantitatively investigated under growth and non-growth conditions in bicarbonate or 1,4-piperazinediethanesulfonic acid (PIPES) buffered environments. The results showed that under non-growth conditions, the majority of the reduced U nanoparticles aggregated and precipitated out of solution. High resolution transmission electron microscopy revealed that only a very small fraction of cells had reduced U precipitates in the periplasmic spaces in the presence of PIPES buffer, whereas in the presence of bicarbonate buffer, reduced U was also observed in the cytoplasm with greater aggregation of biogenic U(IV) particles at higher initial U(VI) concentrations. The same experiments were repeated under growth conditions using two different electron donors (lactate and pyruvate) and three electron acceptors (sulfate, fumarate, and thiosulfate). In contrast to the results of the non-growth experiments, even after 0.2 μm filtration, the majority of biogenic U(IV) remained in the aqueous phase resulting in potentially mobile biogenic U(IV) nanoparticles. Size fractionation results showed that U(IV) aggregates were between 18 and 200 nm in diameter, and thus could be very mobile. The findings of this study are helpful to assess the size and potential mobility of reduced U nanoparticles under different environmental conditions, and would provide insights on their potential impact affecting U(VI) bioremediation efforts at subsurface contaminated sites

Anaerobic microbial growth near thermodynamic equilibrium as a function of ATP/ADP cycle: the effect of maintenance energy requirements

Summarization: For predicting microbial metabolism in low energy yielding environments, various rate laws have been proposed to account for the effects of thermodynamic state (as a measure of product-inhibition) as well as maintenance requirements on energetics of mediated reactions. Explicit or implicit modeling of simplified ATP reactions allows distinction between energy and ATP producing (catabolic, treated as kinetic and reversible) and energy and ATP consuming (anabolic) processes including maintenance requirements. Here, we provide a comparison of several approaches for modeling microbial metabolism in anaerobic environments considering thermodynamic factors, and maintenance energy requirements. We develop a mathematical model for microbial metabolism in anaerobic systems, which couples catabolic and anabolic processes considering the limiting effects of intermediate concentrations on reaction rate through the reduction of chemical potential and reversibility, and that explicitly partitions energy (ATP) allocation between cell growth and maintenance. We include an approach where maintenance energy requirements are assumed to take precedence over ATP-consuming cell synthesis reactions. Also, substrate utilization terminates when the catabolic reactions reach thermodynamic equilibrium with respect to ATP formation, including maintenance energy. The comparison of the proposed model to other modeling approaches shows the benefits of incorporating product inhibition and maintenance requirements in situations which maintenance energy requirements are comparable in size to growth energy requirements. An example application is also presented, where the proposed model is applied to an experimental study of arsenate reduction by Bacillus arsenicoselenatis conducted by Blum et al. (Arch Microbiol. 171 (1998) 19-30), in which the rate of metabolism is controlled by thermodynamics.Presented on: Biochemical Engineering Journa

Microbially mediated kinetic sulfur isotope fractionation: reactive transport modeling benchmark

Microbially mediated sulfate reduction is a ubiquitous process in many subsurface systems. Isotopic fractionation is characteristic of this anaerobic process, since sulfate-reducing bacteria (SRB) favor the reduction of the lighter sulfate isotopologue (S32O42−) over the heavier isotopologue (S34O42−). Detection of isotopic shifts has been utilized as a proxy for the onset of sulfate reduction in subsurface systems such as oil reservoirs and aquifers undergoing heavy metal and radionuclide bioremediation. Reactive transport modeling (RTM) of kinetic sulfur isotope fractionation has been applied to field and laboratory studies. We developed a benchmark problem set for the simulation of kinetic sulfur isotope fractionation during microbially mediated sulfate reduction. The benchmark problem set is comprised of three problem levels and is based on a large-scale laboratory column experimental study of organic carbon amended sulfate reduction in soils from a uranium-contaminated aquifer. Pertinent processes impacting sulfur isotopic composition such as microbial sulfate reduction and iron-sulfide reactions are included in the problem set. This benchmark also explores the different mathematical formulations in the representation of kinetic sulfur isotope fractionation as employed in the different RTMs. Participating RTM codes are the following: CrunchTope, TOUGHREACT, PHREEQC, and PHT3D. Across all problem levels, simulation results from all RTMs demonstrate reasonable agreement.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Sanitary Engineerin

Anaerobic microbial growth near thermodynamic equilibrium as a function of ATP/ADP cycle: The effect of maintenance energy requirements

For predicting microbial metabolism in low energy yielding environments, various rate laws have been proposed to account for the effects of thermodynamic state (as a measure of product-inhibition) as well as maintenance requirements on energetics of mediated reactions. Explicit or implicit modeling of simplified ATP reactions allows distinction between energy and ATP producing (catabolic, treated as kinetic and reversible) and energy and ATP consuming (anabolic) processes including maintenance requirements. Here, we provide a comparison of several approaches for modeling microbial metabolism in anaerobic environments considering thermodynamic factors, and maintenance energy requirements. We develop a mathematical model for microbial metabolism in anaerobic systems, which couples catabolic and anabolic processes considering the limiting effects of intermediate concentrations on reaction rate through the reduction of chemical potential and reversibility, and that explicitly partitions energy (ATP) allocation between cell growth and maintenance. We include an approach where maintenance energy requirements are assumed to take precedence over ATP-consuming cell synthesis reactions. Also, substrate utilization terminates when the catabolic reactions reach thermodynamic equilibrium with respect to ATP formation, including maintenance energy. The comparison of the proposed model to other modeling approaches shows the benefits of incorporating product inhibition and maintenance requirements in situations which maintenance energy requirements are comparable in size to growth energy requirements. An example application is also presented, where the proposed model is applied to an experimental study of arsenate reduction by Bacillus arsenicoselenatis conducted by Blum et al. (Arch Microbiol. 171 (1998) 19-30), in which the rate of metabolism is controlled by thermodynamics. (C) 2013 Elsevier B.V. All rights reserved

A reactive transport benchmark on heavy metal cycling in lake sediments

Sediments are active recipients of anthropogenic inputs, including heavy metals, but may be difficult to interpret without the use of numerical models that capture sediment-metal interactions and provide an accurate representation of the intricately coupled sedimentological, geochemical, and biological processes. The focus of this study is to present a benchmark problem on heavy metal cycling in lake sediments and to compare reactive transport models (RTMs) in their treatment of the local-scale physical and biogeochemical processes. This benchmark problem has been developed based on a previously published reactive-diffusive model of metal transport in the sediments of Lake Coeur d'Alene, Idaho. Key processes included in this model are microbial reductive dissolution of iron hydroxides (i.e., ferrihydrite), the release of sorbed metals into pore water, reaction of these metals with biogenic sulfide to form sulfide minerals, and sedimentation driving the burial of ferrihydrite and other minerals. This benchmark thus considers a multicomponent biotic reaction network with multiple terminal electron acceptors (TEAs), Fickian diffusive transport, kinetic and equilibrium mineral precipitation and dissolution, aqueous and surface complexation, as well as (optionally) sedimentation. To test the accuracy of the reactive transport problem solution, four RTMs-TOUGHREACT (TR), CrunchFlow (CF), PHREEQC, and PHT3D-have been used. Without sedimentation, all four models are able to predict similar trends of TEAs and dissolved metal concentrations, as well as mineral abundances. TR and CF are further used to compare sedimentation and compaction test cases. Results with different sedimentation rates are captured by both models, but since the codes do not use the same formulation for compaction, the results differ for this test case. Although, both TR and CF adequately capture the trends of aqueous concentrations and mineral abundances, the difference in results highlights the need to consider further the conceptual and numerical models that link transport, biogeochemical reactions, and sedimentation

Synthesis of graphene oxide/magnesium oxide nanocomposites with high-rate adsorption of methylene blue

A series of graphene oxide/magnesium oxide nanocomposites (GO/MgO NCs) were and applied for the removal of Methylene Blue (MB) from aqueous solutions. The prepared NCs were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The results showed that MgO particles was successfully decorated on GO. The impacts of different experimental variables on the removal of MB including GO/MgO NCs dosage, pH, contact time, and initial MB concentration were investigated. The experimental analysis of adsorption isotherms indicated that adsorption data was best fit with the Langmuir isotherm model. Among the three different synthesized weight ratios of GO/MgO (5:1, 1:1, and 1:5), 5:1 ratio showed the maximum adsorption capacity as 833 mg/g, which is higher than any previously reported GO-based composites. The synthesized GO/MgO NC is also observed to have higher adsorption capacity for MB removal, in comparison with pure GO and MgO. The kinetic adsorption data was best described by pseudo-second-order kinetic model. The pH of point of zero charge (pH(pzc)) of GO/MgO NCs was determined to be 9.7, 10.5, and 10.5 for ratios 5:1, 1:1, and 1:5, respectively. The results revealed that electrostatic attraction can be the dominant mechanism of adsorption between GO/MgO NCs and MB for pH above pH(pzc); whereas for pH below pH(pzc), other adsorption mechanisms such as hydrogen bonding and pi-pi interaction may attribute to adsorption. The high adsorption capacity of GO/MgO composites, thus makes it a promising adsorbent for water and wastewater treatment. (C) 2016 Elsevier B.V. All rights reserved