Steven A. Banwart

Characterization of redox conditions in groundwater contaminant plumes.

Evaluation of redox conditions in groundwater pollution plumes is often a prerequisite for understanding the behaviour of the pollutants in the plume and for selecting remediation approaches. Measuring of redox conditions in pollution plumes is, however, a fairly recent issue and yet relative few cases have been reported. No standardised or generally accepted approach exists. Slow electrode kinetics and the common lack of internal equilibrium of redox processes in pollution plumes make, with a few exceptions, direct electrochemical measurement and rigorous interpretation of redox potentials dubious, if not erroneous. Several other approaches have been used in addressing redox conditions in pollution plumes: redox-sensitive compounds in groundwater samples, hydrogen concentrations in groundwater, concentrations of volatile fatty acids in groundwater, sediment characteristics and microbial tools, such as MPN counts, PLFA biomarkers and redox bioassays. This paper reviews the principles behind the different approaches, summarizes methods used and evaluates the approaches based on the experience from the reported applications.

Biological weathering and the long-term carbon cycle: integrating mycorrhizal evolution and function into the current paradigm

The dramatic decline in atmospheric CO2 evidenced by proxy data during the Devonian (416.0-359.2 Ma) and the gradual decline from the Cretaceous (145.5-65.5 Ma) onwards have been linked to the spread of deeply rooted trees and the rise of angiosperms, respectively. But this paradigm overlooks the coevolution of roots with the major groups of symbiotic fungal partners that have dominated terrestrial ecosystems throughout Earth history. The colonization of land by plants was coincident with the rise of arbuscular mycorrhizal fungi (AMF),while the Cenozoic (c. 65.5-0 Ma) witnessed the rise of ectomycorrhizal fungi (EMF) that associate with both gymnosperm and angiosperm tree roots. Here, we critically review evidence for the influence of AMF and EMF on mineral weathering processes. We show that the key weathering processes underpinning the current paradigm and ascribed to plants are actually driven by the combined activities of roots and mycorrhizal fungi. Fuelled by substantial amounts of recent photosynthate transported from shoots to roots, these fungi form extensive mycelial networks which extend into soil actively foraging for nutrients by altering minerals through the acidification of the immediate root environment. EMF aggressively weather minerals through the additional mechanism of releasing low molecular weight organic chelators. Rates of biotic weathering might therefore be more usefully conceptualized as being fundamentally controlled by the biomass, surface area of contact, and capacity of roots and their mycorrhizal fungal partners to interact physically and chemically with minerals. All of these activities are ultimately controlled by rates of carbon-energy supply from photosynthetic organisms. The weathering functions in leading carbon cycle models require experiments and field studies of evolutionary grades of plants with appropriate mycorrhizal associations. Representation of the coevolution of roots and fungi in geochemical carbon cycle models is required to further our understanding of the role of the biota in Earths CO2 and climate history.

Biotite dissolution at 25°C: The pH dependence of dissolution rate and stoichiometry

Abstract The rate and stoichiometry of biotite dissolution were studied in the pH range 2–10 using thin-film continuous flow reactors. The release of interlayer K is relatively fast and becomes diffusion-controlled within a few days. The release rates of framework ions (Mg, Al, Fe, Si) are much slower and reach an apparent steady-state within ten days. The stoichiometry and rate of dissolution vary greatly with pH. Consistent with surface reaction control of release rates, an empirical rate law, R + kH[H+]m + k0 + kOH[H+]n (moles m−2 h−1) describes proton- and hydroxyl-catalysed dissolution for each ion. Si Fe Mg Al log k H −4.45 −5.10 −4.93 −5.31 m 0.91 0.51 0.57 0.40 log k 0 −7.31 log k OH −11.29 −15.36 −10.57 −12.58 n −0.48 −0.81 −0.29 −0.54 Rapid K+ release provides a tracer for the extent of the hydrated reacting layer on the biotite surface and within interlayers. An altered reaction layer composition, calculated from mass balances for released ions, results from preferential leaching of some ions and is consistent with that of vermiculite. X-Ray powder diffractometry confirmed the formation of both vermiculite and kaolinite during the weathering reaction. The pH dependence of release rates, normalised to the corresponding ion concentrations in the reacting layer, correlate with those for the respective binary oxides (SiO2, Al2O3, Fe2O3, MgO). Release rates for Al, Mg, and Fe at neutral pH are much slower when the mineral has been previously reacted at low pH where these ions are released rapidly. Model simulations suggest that, for ions that initially dissolve rapidly, release rates will decrease as the ion is depleted in the reacting layer. Rates will eventually approach those of the most slowly dissolving ion. At 25°C and pH 7, this process would lead to stoichiometric dissolution within 50 y.

Dissolution of fe(iii)(hydr)oxides in natural waters; laboratory assessment on the kinetics controlled by surface coordination☆

Abstract Dissolution of Fe (III) (hydr)oxides, of importance in oceanographic and limnological cycles of iron, occurs (1) in anoxic environments, (2) at the oxic-anoxic boundary of the water-sediment column, and (3) as a light-induced process in oxic surface waters. The various pathways of dissolution of Fe (III) (hydr) oxides, especially by reductants in thermal and in photochemical processes, are discussed and assessed on the basis of laboratory experiments. The relevance of these mechanisms for the transformation of iron in natural waters is discussed. It is shown that surface processes and not transport processes control the dissolution kinetics and that all the dissolution reactions are critically dependent on the type of complexes formed on the surface of the Fe (III) (hydr) oxides. Thus, a reductant such as ascorbate exchanges electrons with a surface Fe (III) ion subsequent to its inner-sphere coordination to the oxide surface. The Fe (II) thereby formed becomes more easily detached from the surface, because of the larger lability in the crystalline lattice surface of the Fe(II)-O bond than of the Fe(III)-O bond. A catalytic dissolution that appears to be important in natural waters is accomplished by Fe (II) in the presence of a bifunctional complex former (dicarboxylic acid, hydroxycarboxylic acid, diphenol). The latter is able to form on the surface of Fe (III)(hydr)oxides a ternary complex with Fe(II): >FeIII−X−FeII (X≠bridging ligand). Electron transfer from Fe (II) to the surface Fe (III) ions occurs through the bridging ligand. In light-induced reductive dissolution of Fe (III) (hydr) oxides as well as in thermal reductive dissolution, the inner-sphere surface coordination of the electron donor to the oxide surface is essential for the efficient occurrence of the photochemical (or thermal) redox reaction.

Mine Water Hydrology

Hydrology is a relatively young science, which has developed gradually out of the hydraulic expedients which underpin much of civil engineering practice. Indeed, the emergence of truly scientific hydrology is such a recent development that major textbooks and learned journals have until recently devoted many pages to discussing its scientific credentials (Bras, 1990; Wilby, 1997). Until the 1980s most hydrological analysis was concerned with purely physical processes and practices of engineering interest, such as rainfall-runoff modelling and flow-net analysis of seepage pathways. By the start of the 21st Century, the scope of hydrology has expanded to such an extent that it now embraces relevant areas of chemistry and ecology. Sub-disciplines such as hydrogeochemistry (e.g. Appelo and Postma, 1994) and hydroecology (or ecohydrology; Baird and Wilby, 1999) are now firmly established, and account for a large proportion of the innovation in hydrological science (see, for instance, Wilby, 1997, and Wheater and Kirby, 1998). The “scientification” of hydrology has now proceeded to such an extent that Wilby (1997) could claim that hydrology provides the most logical basis for ‘holistic environmental science’, since water is a prominent medium in all of the earth and life sciences which deal with the natural environment. In the light of these trends, a contemporary definition of hydrology might be given as follows: “Hydrology is the science which deals with the nature, movement and environmental functions of terrestrial natural waters”

Processes controlling the distribution and natural attenuation of dissolved phenolic compounds in a deep sandstone aquifer

Processes controlling the distribution and natural attenuation (NA) of phenol, cresols and xylenols released from a former coal-tar distillation plant in a deep Triassic sandstone aquifer are evaluated from vertical profiles along the plume centerline at 130 and 350 m from the site. Up to four groups of contaminants (phenols, mineral acids, NaOH, NaCl) form discrete and overlapping plumes in the aquifer. Their distribution reflects changing source history with releases of contaminants from different locations. Organic contaminant distribution in the aquifer is determined more by site source history than degradation. Contaminant degradation at total organic carbon (TOC) concentrations up to 6500 mg l(-1) (7500 mg l(-1) total phenolics) is occurring by aerobic respiration NO3-reduction, Mn(IV)-/Fe(III)-reduction, SO4-reduction, methanogenesis and fermentation, with the accumulation of inorganic carbon, organic metabolites (4-hydroxybenzaldehyde, 4-hydroxybenzoic acid), acetate, Mn(II), Fe(II), S(-II), CH4 and H2 in the plume. Aerobic and NO3-reducing processes are restricted to a 2-m-thick plume fringe but Mn(IV)-/Fe(II)-reduction, SO4-reduction, methanogenesis and fermentation occur concomitantly in the plume. Dissolved H2 concentrations in the plume vary from 0.7 to 110 nM and acetate concentrations reach 200 mg l(-1). The occurrence of a mixed redox system and concomitant terminal electron accepting processes (TEAPs) could be explained with a partial equilibrium model based on the potential in situ free energy (deltaGr) yield for oxidation of H2 by specific TEAPs. Respiratory processes rather than fermentation are rate limiting in determining the distribution of H2 and TEAPs and H2 dynamics in this system. Most (min. 90%) contaminant degradation has occurred by aerobic and NO3-reducing processes at the plume fringe. This potential is determined by the supply of aqueous O2 and NO3 from uncontaminated groundwater, as controlled by transverse mixing, which is limited in this aquifer by low dispersion. Consumption to date of mineral oxides and SO4 is, respectively, <0.15% and 0.4% of the available aquifer capacity, and degradation using these oxidants is <10%. Fermentation is a significant process in contaminant turnover, accounting for 21% of degradation products present in the plume, and indicating that microbial respiration rates are slow in comparison with fermentation. Under present conditions, the potential for degradation in the plume is very low due to inhibitory effects of the contaminant matrix. Degradation products correspond to <22% mass loss over the life of the plume, providing a first-order plume scale half-life >140 years. The phenolic compounds are biodegradable under the range of redox conditions in the aquifer and the aquifer is not oxidant limited, but the plume is likely to be long-lived and to expand. Degradation is likely to increase only after contaminant concentrations are reduced and aqueous oxidant inputs are increased by dispersion of the plume. The results imply that transport processes may exert a greater control on the natural attenuation of this plume than aquifer oxidant availability.

Kinetic modelling of geochemical processes at the Aitik mining waste rock site in northern Sweden

A steady-state geochemical model has been developed to study water-rock interactions controlling metal release from waste rock heaps at the Aitik Cu mine in northern Sweden. The Cu release in drainage waters from the site is of environmental concern. The waste rock heaps are treated as single completely mixed flow-through reactors. The geochemical model includes kinetices of sulphide and primary silicate mineral weathering, heterogeneous equilibrium with secondary mineral phases and speciation equilibrium. Field monitoring of drainage water composition provides a basis for evaluation of model performance. The relative rate of oxidative weathering of sulphides and dissolution of primary silicate minerals, using published kinetic data, are consistent with net proton and base cation fluxes at the site. The overall rate of Fe2+ oxidation within the heap is three orders of magnitude faster than that which could be explained by surface-catalysed reaction kinetics. This suggests significant activity of iron-oxidizing bacteria. The absolute weathering rates of sulphides and silicate minerals, normalized to a measured BET surface area, are approximately two orders of magnitude lower at field scale than published rates from laboratory experiments. Because of the relative absence of carbonate minerals, the weathering of biotite and plagioclase feldspar are important sources of alkalinity.

Characterization of the Cell Surface and Cell Wall Chemistry of Drinking Water Bacteria by Combining XPS, FTIR Spectroscopy, Modeling, and Potentiometric Titrations

Aquabacterium commune, a predominant member of European drinking water biofilms, was chosen as a model bacterium to study the role of functional groups on the cell surface that control the changes in the chemical cell surface properties in aqueous electrolyte solutions at different pH values. Cell surface properties of A. commune were examined by potentiometric titrations, modeling, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. By combining FTIR data at different pH values and potentiometric titration data with thermodynamic model optimization, the presence, concentration, and changes of organic functional groups on the cell surface (e.g., carboxyl, phosphoryl, and amine groups) were inferred. The pH of zero proton charge, pH(zpc) = 3.7, found from titrations of A. commune at different electrolyte concentrations and resulting from equilibrium speciation calculations suggests that the net surface charge is negative at drinking water pH in the absence of other charge determining ions. In situ FTIR was used to describe and monitor chemical interactions between bacteria and liquid solutions at different pH in real time. XPS analysis was performed to quantify the elemental surface composition, to assess the local chemical environment of carbon and oxygen at the cell wall, and to calculate the overall concentrations of polysaccharides, peptides, and hydrocarbon compounds of the cell surface. Thermodynamic parameters for proton adsorption are compared with parameters for other gram-negative bacteria. This work shows how the combination of potentiometric titrations, modeling, XPS, and FTIR spectroscopy allows a more comprehensive characterization of bacterial cell surfaces and cell wall reactivity as the initial step to understand the fundamental mechanisms involved in bacterial adhesion to solid surfaces and transport in aqueous systems.

The role of oxalate in accelerating the reductive dissolution of hematite (α-Fe2O3) by ascorbate

It is known that the dissolution of most oxide minerals is controlled by chemical reactions occurring at the mineral surface. Here the reductive dissolution of hematite (α-Fe2O3) by ascorbate follows an empirical rate law which is first order in the concentration of adsorbed ascorbate: rate = kobs {>FeIIIHA}. Such a rate law can be explained by a surface chemical mechanism where the formation and reactivity of a surface iron (III) —ascorbate complex determines the rate of dissolution. The adsorption of oxalate, a ligand capable of forming bidentate mononuclear surface complexes, in the presence of ascorbate leads to a significant increase in the rate of reductive dissolution even though the adsorption of oxalate displaces some of the ascorbate from the hematite surface. Detachment of iron (II) sites, rather than electron transfer, is most likely the rate-determining step in the overall reaction. Thus the role of oxalate in accelerating the reductive dissolution of hematite by ascorbate is believed to be similar to its role in promoting the non-reductive dissolution of metal oxides.

Experimental study of acidity-consuming processes in mining waste rock: some influences of mineralogy and particle size

Weathering reactions producing and consuming acid in fresh waste rock samples from the Aitik Cu mine in northern Sweden have been investigated. Batch-scale (0.15 kg) acid titrations with waste rock of different particle sizes were operated for 5 months. The pH was adjusted to a nearly constant level, similar to that in effluents from waste rock dumps at the site (pH near 3.5). The reactions were followed by analysing for all major dissolved elements (K, Na, Mg, Ca, Si, Al, SO4, Cu, Zn, Fe) in aliquots of solution from the reaction vessels. In addition, the solids were physically and chemically characterised in terms of mineralogy, chemical composition, particle size distribution, surface area and porosity. The results show that the alkalinity production is initially dominated by a rapid dissolution of small amounts of calcite and rapidly exchangeable base cations on silicate surfaces. Steady-state dissolution of primary silicate minerals also generates alkalinity. The total alkalinity is nearly balanced by input of acid from the steady-state oxidation of sulphides, such that the pH 3.1–3.4 can be maintained without external input of acid or base. There is a large difference in weathering rates between fine materials and larger waste rock particles (diameters (d) >0.25 mm) for both sulphides and silicates. As a result particles with d smaller than 0.25 mm contribute to approximately 80% of the sulphide and silicate dissolution. Calcite dissolution can initially maintain a neutral pH but with time becomes limited by intra-particle diffusion. Calcite within particles larger than 5–10 mm reacts too slowly to neutralise the acid produced from sulphides.