Philippe Van Cappellen

Biogeochemical Redox Processes and their Impact on Contaminant Dynamics

Life and element cycling on Earth is directly related to electron transfer (or redox) reactions. An understanding of biogeochemical redox processes is crucial for predicting and protecting environmental health and can provide new opportunities for engineered remediation strategies. Energy can be released and stored by means of redox reactions via the oxidation of labile organic carbon or inorganic compounds (electron donors) by microorganisms coupled to the reduction of electron acceptors including humic substances, iron-bearing minerals, transition metals, metalloids, and actinides. Environmental redox processes play key roles in the formation and dissolution of mineral phases. Redox cycling of naturally occurring trace elements and their host minerals often controls the release or sequestration of inorganic contaminants. Redox processes control the chemical speciation, bioavailability, toxicity, and mobility of many major and trace elements including Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg, Tc, and U. Redox-active humic substances and mineral surfaces can catalyze the redox transformation and degradation of organic contaminants. In this review article, we highlight recent advances in our understanding of biogeochemical redox processes and their impact on contaminant fate and transport, including future research needs.

A surface complexation model of the carbonate mineral-aqueous solution interface

A surface complexation model for the chemical structure and reactivity of the carbonatewater interface is presented. The model postulates the formation of the hydration species >CO3H0 and >MeOH0 at the surface of a divalent metal carbonate MeCO3 (Me = Ca, Mn, Fe, etc.). The existence of these primary hydration species is supported by spectroscopic data. The following reactions are proposed to govern surface speciation in the MeCO3(s)-H2O-CO2 system: CO3H0a2>CO3− + H+(1) CO3H0 + Me2+a2>CO3Me+ + H+(2) MeOH2+a2 >MeOH0 + H+(3) MeOH0a2 >MeO− + H+(4) MeOH0 + CO2a2 >MeHCO3o (5) MeOH0 + CO2a2 >MeCO3−+ H+(6) It is shown in this paper that the surface complexation model provides a systematic explanation of surface charge development and dissolution kinetics of carbonate minerals. The intrinsic stability constants of the surface complexation reactions agree well with the equilibrium constants of the corresponding complexation reactions in homogeneous solution.

Benthic phosphorus regeneration, net primary production, and ocean anoxia: A model of the coupled marine biogeochemical cycles of carbon and phosphorus

We examine the relationships between ocean ventilation, primary production, water column anoxia, and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon (C) and phosphorus (P). The elemental cycles are coupled via the Redfield C/P ratio of marine phytoplankton and the C/P ratio of organic matter preserved in marine sediments. The model assumes that on geologic timescales, net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone. The model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water-sediment interface. Evidence from marine and lacustrine settings, modern and ancient, demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia-dysoxia expands. Steady state simulations show that a reduction in the rate of thermohaline circulation, or a decrease of the oxygen content of downwelling water masses, intensifies water column anoxia-dysoxia and at the same time increases surface water productivity. The first effect reflects the declining supply of oxygen to the deeper parts of the ocean. The second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides. Sedimentary burial of organic carbon and authigenic calcium phosphate mineral (francolite), on the other hand, is promoted by reduced ocean ventilation. According to the model, global-scale anoxia-dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system. Consequently, higher rates of primary production and organic carbon burial can be achieved, even when the continental supply of reactive phosphorus to the oceans remains unchanged.

Influence of water column anoxia on the burial and preservation of carbon and phosphorus in marine shales

Abstract Organic P and organic C concentrations were measured in several well-characterized Phanerozoic marine shale sequences with the primary focus being the Camp Run Member of the Devonian-Mississippian New Albany Shale. Sequences were selected with close spatial association of bioturbated and laminated sediments which reflect deposition from oxic and anoxic waters, respectively. Average organic C/P mole ratios calculated from the data are 150 for bioturbated shales and 3900 for the laminated shales of the New Albany. Differences in the extent and mechanisms of early diagenesis related to the oxygenation of waters at the sediment-water interface can account for the C/P ratios of buried organic matter. Low C/P ratios of bioturbated sediments are attributed to 1. (1) the increased ability of bacteria to store P in well-oxygenated environments which leads to the production of low C/P ratio bacterial biomass and 2. (2) the extensive oxidation of sedimentary organic matter resulting in the formation of residual organic phases with low C/P ratios. High organic C/P ratios of laminated sediments are explained by a combination of 1. (1) the limited ability of bacteria to store P under anoxic conditions, 2. (2) extensive P regeneration from sedimentary organic matter, 3. (3) enhanced C preservation relative to the bioturbated shales. It is also shown that relative to organic C, laminated shales are not as effective a P sink as are bioturbated shales. This provides a mechanism in anoxic environments for burying large quantities of organic matter without simultaneously sequestering the P needed to sustain further productivity.

Kinetic modeling of microbially-driven redox chemistry of subsurface environments: coupling transport, microbial metabolism and geochemistry

This paper deals with the treatment of subsurface environments as reactive biogeochemical transport systems. We begin with an overview of the effects of microbial activity on the chemical dynamics in these environments. Then, after a review of earlier modeling efforts, we introduce a one-dimensional, multi-component reactive transport model that accounts for the reaction couplings among the major redox and acid–base elements, O, C, H, N, S, Mn, Fe and Ca. The model incorporates kinetic descriptions for the microbial degradation pathways of organic matter, as well as for the secondary redox reactions and mineral precipitation–dissolution reactions. Local equilibrium only applies to fast homogeneous speciation reactions and sorption processes. The model is used to simulate the distributions of chemical species and reaction rates along flow paths in two subsurface environments. In the first case, waters containing moderate levels of natural soil-derived organics supply a regional groundwater system. In the second case, a pristine aquifer is contaminated by an organic-rich leachate from a landfill. In both environments, the microbial oxidation of organic matter causes the disappearance of dissolved and solid oxidants and the appearance of reduced species, albeit over very different spatial scales. In the second case, a pronounced reaction front develops at the downstream edge of the contaminant plume. The reactivity, or biodegradability, of the organic matter is shown to be a major factor governing the biogeochemical dynamics in the plume. The simulations predict different distributions of the biodegradation pathways, depending on whether the organics of the leachate have uniform or variable reactivity. The secondary reactions also have a significant impact on the concentration profiles of inorganic species and the spatial distributions of the biodegradation pathways. Within the downstream reaction front, large fractions of O2, Mn(IV), Fe(III) and SO2−4 are reduced by secondary reactions, rather than being utilized in the oxidative degradation of leachate organics. Overall, the model simulations emphasize the strong coupling between subsurface heterotrophic activity and an extensive network of secondary reactions.

A multicomponent reactive transport model of early diagenesis: Application to redox cycling in coastal marine sediments

STEADYSED1 is a multicomponent reactive transport code for steady-state early diagenesis which fully incorporates the reaction couplings among the elements, C, O, N, S, Fe, and Mn. The model is tested against extensive datasets collected by Canfield et al. (1993a,b) at three coastal marine sites that exhibit high rates of combined iron and manganese (hydr)oxide reduction. It is shown that the model provides a consistent explanation of the entire body of multicomponent multisite observations. The measured concentration profiles of twenty-eight individual porewater and solid sediment species are satisfactorily reproduced. Furthermore, the model predicts the observed distributions of sulfate reduction rates, as well as diagnostic features of the porewater pH profiles. The coupled nature of the reactive species imposes stringent constraints when fitting model-calculated distributions to the data, because a single set of reaction and transport parameters must account for the profiles of all the species at each site. The parameters are further separated into site-specific (e.g., deposition fluxes, bottom water composition) and reaction-specific parameters (e.g., rate coefficients, apparent equilibrium constants, limiting substrate concentrations). By minimizing the variations of the reaction-specific parameters from one site to another, the fitting strategy emphasizes the retrieval from field data of reaction parameters that are mechanistically meaningful. The model reproduces the significant vertical overlap between organic carbon oxidation pathways observed in the sediments. The overlap is explained by the combination of high rates of organic carbon oxidation plus intense bioturbation and irrigation at the sites studied. The model-derived contributions of the various respiratory processes compare favorably with the incubation results of Canfield et al. (1993a,b). Aerobic respiration accounts for less than 32% of total organic carbon oxidation at the three sites. From 22 to 46% of the O2 uptake by the sediments is directly coupled to organic carbon oxidation, while the remainder is used for the oxidation of secondary reduced species, in particular dissolved, adsorbed and solid Fe(II) and Mn(II) species. According to the model, the oxidation of Fe2+ by Mn (hydr)oxides is responsible for the observed spatial separation of the porewater build-up of Mn2+ and Fe2+. At two of the sites, 75 and 97% of the total rate of Mn oxide reduction is due to chemical reaction with dissolved Fe2+. In contrast, at the same sites, iron (hydr)oxides are mostly utilized by bacteria for the oxidation of organic matter (dissimilatory iron reduction). As shown also by Canfield et al. (1993a,b), dissimilatory reduction is the principal dissolution pathway for manganese oxides at the third site. A sensitivity analysis suggests that, for given deposition fluxes of reactive Fe and Mn, the competition between dissimilatory and nondissimilatory metal reduction pathways depends primarily on the total carbon oxidation rate and the intensity of porewater irrigation. The simulations also highlight the importance of adsorption-desorption of Fe(II) and Mn(II) in the redox cycling of the metals, as well as their impact on porewater alkalinity and pH. Based on the calculated rate distributions, detailed budgets of Fe, Mn, and O2 in the sediments are presented. The model-calculated benthic exchange fluxes of solutes are dominated by irrigation. For nitrate, molecular diffusion and irrigation cause fluxes in opposite directions. As a result, there is a net transfer of nitrate from the water column to the sediments, although the interfacial porewater gradients predict diffusional fluxes out of the sediments. A significant fraction of the benthic Fe and Mn fluxes to the bottom waters may be due to desorption at the water-sediment interface.

Redox Stabilization of the Atmosphere and Oceans by Phosphorus-Limited Marine Productivity

Data from modern and ancient marine sediments demonstrate that burial of the limiting nutrient phosphorus is less efficient when bottom waters are low in oxygen. Mass-balance calculations using a coupled model of the biogeochemical cycles of carbon, phosphorus, oxygen, and iron indicate that the redox dependence of phosphorus burial in the oceans provides a powerful forcing mechanism for balancing production and consumption of atmospheric oxygen over geologic time. The oxygen-phosphorus coupling further guards against runaway ocean anoxia. Phosphorus-mediated redox stabilization of the atmosphere and oceans may have been crucial to the radiation of higher life forms during the Phanerozoic.

Biogenic silica dissolution in sediments of the Southern Ocean. II. Kinetics

Abstract The dissolution kinetics of biogenic silica in surface sediments collected during the ANTARES I cruise were measured in stirred flow-through reactors. The rate data exhibit a distinctly non-linear dependence on the degree of undersaturation. Near equilibrium, the rates of silica dissolution and precipitation define a single linear trend, i.e. the kinetics are symmetric about the equilibrium point. When the dissolved silica concentration drops below a critical level, however, the dissolution rate rises exponentially with increasing undersaturation. Hence, the data disagree with the linear rate law generally used to describe the dissolution kinetics of biogenic silica. It is hypothesized that the kinetic transition from the linear to the exponential regime represents the onset of localized dissolution centered on surface defects, e.g. small pores and crevices, or compositional defects. The effects of temperature and pH confirm that the critical process controlling the overall dissolution kinetics is the hydrolysis of bridging SiOSi bonds at the solid-solution interface. The rate measurements indicate that the reactivity of biogenic silica decreases substantially with depth in the sediment. The decrease in reactivity is explained by a progressive reduction of the defect density of the silica surfaces, through dissolution and reprecipitation of silica. It does not appear to result from the preferential dissolution of a more reactive fraction of biogenic debris deposited from the water column. Surface areas obtained by the N2-BET method or concentrations of extractable biogenic silica do not provide satisfactory proxies for the reactive surface area of silica in the sediments. However, a positive correlation was observed between the surface reactivity and the exchangeable Co2+ adsorption capacity of biogenic silica. Specific kinetic effects on silica dissolution of the aluminum content of the silica surfaces or organic matter coatings were not observed. Both the non-linear dissolution kinetics and the aging of the silica surfaces help restrict the dissolution of deposited biogenic silica to a narrow zone close to the water-sediment interface. The results of the flow-through experiments highlight the importance of in situ early diagenetic processes in controlling the behavior and fate of deposited biogenic silica: no evidence was found supporting a significant effect of differences in solubility or reactivity inherited from the biomineralization process in the water column.

Processes controlling solubility of biogenic silica and pore water build-up of silicic acid in marine sediments

Abstract Dissolution experiments in batch and flow-through reactors were combined with data on sediment composition and pore water silicic acid profiles to identify processes controlling the solubility of biogenic silica and the build-up of silicic acid in marine sediments. The variability of experimentally determined biogenic silica solubilities is due, in part, to variations in specific surface area and Al content of biosiliceous materials. Preferential dissolution of delicate skeletal structures and frustules with high surface areas leads to a progressive decrease of the specific surface area. This may cause a reduction of the solubility of deposited biosiliceous debris by 10–15%, relative to fresh planktonic assemblages. Dissolution of lithogenic (detrital) minerals in sediments releases dissolved aluminum to the pore waters. This aluminum becomes structurally incorporated into deposited biogenic silica, further decreasing its solubility. Compared to Al-free biogenic silica, the solubility of diatom frustules is lowered by as much as 25% when one out of every 70 Si atoms is substituted by an Al(III) ion. The build-up of silicic acid in pore waters of sediments with variable proportions of detrital matter and biogenic silica was simulated in batch experiments using kaolinite and basalt as model detrital constituents. The steady-state silicic acid concentrations measured in the experiments decreased with increasing detrital-to-opal ratios of the mixtures. This trend is similar to the observed inverse relationship between asymptotic pore water silicic acid concentrations and detrital-to-opal ratios in Southern Ocean sediments. Flow-through reactor experiments further showed that in detrital-rich sediments, precipitation of authigenic alumino-silicates may prevent the pore waters from reaching equilibrium with the dissolving biogenic silica. This agrees with data from Southern Ocean sediments where, at sites containing more than 30 wt.% detrital material, the pore waters remain undersaturated with respect to the experimentally determined in situ solubility of biogenic silica. The results of the study show that interactions between deposited biogenic silica and detrital material cause large variations in the asymptotic silicic acid concentration of marine sediments. The production of Al(III) by the dissolution of detrital minerals affects the build-up of silicic acid by reducing the apparent silica solubility and dissolution kinetics of biosiliceous materials, and by inducing precipitation of authigenic alumino-silicate minerals.

Biogenic silica dissolution in sediments of the Southern Ocean. I. Solubility

Abstract A stirred flow-through reactor technique was used to determine silica solubilities in sediments collected with a multicorer in the Indian sector of the Southern Ocean (ANTARES I cruise). The results show that the apparent silica solubility in the cores may decrease, increase or remain constant with depth. The silica solubility profiles are best explained by the early diagenetic interactions between biogenic silica and soluble aluminum derived from detrital material. By combining the solubility data with measured dissolved silica profiles, it is shown that the variable asymptotic pore water silica levels in the cores cannot be explained solely by differences in silica solubility. In sediments that experience a significant detrital input, the simultaneous reprecipitation of dissolved aluminum and dissolved silica prevents water silicic acid from reaching saturation with the dissolving biogenic silica. The principal oceanographic control on pore water silica build-up in the cores studied is the ratio of the deposition fluxes of biogenic silica and detrital material. Solubility differences inherited from the biomineralization process in the surface waters do not appear to have a significant effect on the observed pore water silica levels.