Stefan Peiffer

Effect of pH on the anaerobic microbial cycling of sulfur in mining-impacted freshwater lake sediments

Lakes caused by coal strip-mining processes are characterized by low pH, low nutrient status, and high concentrations of Fe(II) and sulfate due to the oxidation of pyrite. Most microbiological studies of sulfidic mine tailings concentrate on processes in the oxic zone, and on the ability of acidophilic bacteria to promote the production of toxic leachates. Few studies have studied microbial processes in anoxic sediment zones, where the reduction of Fe(III) often is the dominant electron-accepting process in acidic sediments, and the reduction of sulfate occurs in sediments of elevated pH. In this study, microcosm experiments were performed with sediments of a coal mining-impacted lake to study the microbial turnover of sulfur under anoxic conditions. Microcosms with sediment of the zone of maximum sulfate-reducing activity indicated that sulfidic minerals subsequently formed due to the reduction of sulfate were not stable under anoxic conditions. Supplemental Na2S stimulated the formation of Fe(II) and sulfate, whereas supplemental S0 had no effect under pH 5 conditions. Low numbers of S0-utilizing Fe(III)-reducing bacteria were enumerated in this sediment zone. In contrast, sediment microcosms of the upper acidic sediment zone demonstrated that supplemental elemental S0 stimulated the formation of Fe(II) and sulfate, whereas supplemental Na2S had no effect. Most probable number estimates demonstrated that S0-utilizing Fe(III)-reducing bacteria cultivated at pH 3 approximated to 1% of the total direct counts of bacteria in the upper acidic sediment zone. Sulfate reducers cultured at pH 5.2 were more abundant than sulfate reducers cultured at pH 6.4 in both sediment zones. A sulfate-reducing bacterium, Lau III, was isolated at the in situ pH of 5.2 from the highest growth positive dilution series from the zone of maximum sulfate-reducing activity. Analysis of the 16S rRNA gene sequence (1498 base pairs) of Lau III demonstrated that it was a member of the Clostridium–Bacillus subphylum of gram-positive bacteria related to Desulfosporosinus orientis Y11571 (96.4% 16S rRNA gene sequence similarity). Lau III was a spore-forming, lactate-utilizing sulfate reducer capable of growing at a pH range of 4.9–6.1 and a pH optimum of 5.5. These collective results indicate that, (i) acid-tolerant sulfate reducers are involved in the reduction of sulfate; (ii) sulfide or sulfidic minerals are reoxidized under anoxic conditions; and (iii) the anaerobic turnover of sulfur is affected by the pH gradient in this impacted heterogeneous lake sediment.

Redox measurements in aqueous solutions — A theoretical approach to data interpretation, based on electrode kinetics

Abstract Redox electrodes are commonly used to measure redox potentials in complex natural systems. In this paper, an approach based on electrode kinetics to interpreting measured redox potentials is presented. Redox couples dissolved in aqueous solutions are classified as sensor effective redox couples (SERCs), which exchange electrons with the electrode surface at a significant rate, and sensor ineffective redox couples (SIRCs), which do not control the potential of the electrode. Going through the corresponding mathematical formalism, we arrive at a Butler-Volmer-type equation which states that a quantitative interpretation of redox potentials is not possible without understanding the nature of SERCs and their electrode kinetic parameters. A sensitivity analysis of effects of these unknown parameters on the redox potential is presented. Implications for interpretations of potential readings in natural samples are discussed. Finally, outlooks for necessary future experimental studies are given.

Support for an anaerobic sulfur cycle in two Canadian peatland soils

concentrations of dissolved sulfate and H2S adjusted to 5–20mmol L � 1 and 0–9mmol L � 1 , respectively, whereas concentrations of CO2 ,C H4, and DOC reached millimolar levels. CO2 production was not explained by methanogenesis and net reduction of inorganic electron acceptors. In the shallow peat, H2S was produced and 34 S in sulfate enriched by 3.6 to 6%, indicating occurrence of BSR. Sulfate reducers also accounted for much of the metabolic activity. Addition of molybdate suppressed CO2 production by 20 to 50%. Deeper into the peat, the sulfate pool was apparently replenished from the peat matrix as sulfate became enriched in 32 S, likely stemming from TRIS or organic sulfur in the peat. Sulfur was thus anaerobically cycled between oxidized and reduced pools. An electron acceptor capable of driving this cycle could not be conclusively identified. Regardless of this uncertainty, the results suggest that anaerobic S cycling can maintain BSR and potentially contribute to low methane production in soils of ombrotrophic bogs.

Thermodynamics and organic matter: constraints on neutralization processes in sediments of highly acidic waters

Abstract The controls on the internal neutralization of low productivity, highly acidified waters by sulfide accumulation in sediments are yet poorly understood. It is demonstrated that the neutralization process is constrained by organic matter quality and thermodynamic effects which control the relative rates of SO4 and Fe reduction, and the fate of the reduced Fe and S in the sediments. The investigated sediments were rich in dissolved Fe(II) (0.005–12 mmol l−1) and SO4 (1.3–22 mmol l−1). The pH ranged from 3.0 to 6.8. Contents of reduced inorganic S (0.1–9.5%), molar C/N ratios of the organic matter (12–80) and metabolic turnover rates (1–110 μeq cm−3 a−1) varied strongly. Substantial amounts of Fe sulfides were only found at a simultaneous partial thermodynamic and solubility equilibrium of the involved biogeochemical processes. Sulfide oxidation was apparently inhibited, and SO4 and Fe reduction coexisted. In this type of sediment increases in C availability cause enhanced neutralization rates. In the absence of a partial equilibrium, the sediments were in a sulfide oxidizing and Fe reducing state, and did not accumulate Fe sulfides. The latter type of sediment will increase neutralization rates in response to decreasing deposition of reactive Fe oxides but not necessarily in response to increases in lake productivity by e.g. fertilization measures.

Organic Matter Diagenesis in Acidic Mine Lakes

The significance of organic matter origin for carbon oxidation via sulfate and iron reduction in the sediments of three acid mine lakes is analyzed. Carbon reactivity was estimated by fitting first-order expressions to measured rates. Carbon oxidation rates via sulfate and ferric iron reduction ranged from 3.4 to 4.7 mmol m 2 d -1 and resembled those reported for freshwater lakes. The estimated reaction constants increased from about 10 -3 a -1 at the interface to the former mine grounds to 0.05 to 0.2 a -1 at the current sediment-water interface. Aquatic organic matter accounted for an estimated 45…75% of total carbon oxidation rates while it amounted only to about 5…14% of the total organic matter that had been deposited. The results of this study suggest that in highly acidic mine lakes the reactivity of the deposited organic matter can rapidly increase after flooding, enhancing carbon oxidation and internal neutralization rates in the sediments.

The Interaction of Natural Organic Matter with Iron in a Wetland (Tennessee Park, Colorado) Receiving Acid Mine Drainage

Pore water from a wetland receiving acid mine drainage was studied for its iron and natural organic matter (NOM) geochemistry on three different sampling dates during summer 1994. Samples were obtained using a new sampling technique that is based on screened pipes of varying length (several centimeters), into which dialysis vessels can be placed and that can be screwed together to allow for vertical pore-water sampling. The iron concentration increased with time (through the summer) and had distinct peaks in the subsurface. Iron was mainly in the ferrous form; however, close to the surface, significant amounts of ferric iron (up to 40% of 2 mmol L-1 total iron concentration) were observed. In all samples studied, iron was strongly associated with NOM. Results from laboratory experiments indicate that the NOM stabilizes the ferric iron as small iron oxide colloids (able to pass a 0.45μm dialysis membrane). We hypothesize that, in the pore water of the wetland, the high NOM concentrations (>100 mg C L-1) allow formation of such colloids at the redoxcline close to the surface and at the contact zone to the adjacent oxic aquifer. Therefore, particle transport along flow paths and resultant export of ferric iron from the wetland into ground water might be possible.

Parameter uncertainty in chemical equilibrium calculations using fuzzy set theory

A method based on fuzzy set theory is presented to incorporate imprecise thermodynamic parameters into chemical equilibrium calculations of aqueous systems. Imprecision may arise from uncertainties in experimental parameter determination as well as from inconsistency of available data in the literature. Fuzzy numbers with different shapes of membership functions are used to express imprecision in a non-probabilistic sense. A solution algorithm for a system of nonlinear algebraic equations calculating the chemical equilibrium composition is combined with level set operations to solve the fuzzy chemical equlibrium problem. The method results in multiple minimizing/maximizing procedures from which the membership functions of equilibrium species concentrations are determined. An application of the proposed method to an aqueous cadmium-sulfide system illustrates the acquisition of membership functions for the thermodynamic constants out of given information. Stochastic information on measurement data are appropriately transformed into fuzzy numbers to allow for the combined calculation of different kinds of uncertainty. The results of four calculation scenarios show their strong impact on the resulting membership functions of chemical equilibrium composition and are discussed in the context of data evaluation and decision making in geochemistry.

THE OXIDATION OF PYRITE AT PH 7 IN THE PRESENCE OF REDUCING AND NONREDUCING FE(III)-CHELATORS

Abstract The oxidation rate of pyrite at pH 7, 25°C and at constant partial pressure of oxygen (0.21 and 0.177 atm) was measured in the presence of the Fe(III)-chelators NTA, oxalate, leucine, EDTA, citrate, IDA and the Fe(III)-reductant ascorbic acid. With the exception of leucine and EDTA, non-reducing Fe(III)-chelators increased the oxidation rate relative to the reference state of formation of the Fe(OH)2+ complex at pH 7. The rate increase was proportional to the logarithm of the conditional stability constant of the ligands for the complexation of Fe3+. No effect on the oxidation rate was observed in the presence of EDTA, which shifted the redox potential of the redox couple Fe2+/Fe3+ to a value below that in the absence of any ligand at pH 7. Ascorbic acid decreased the pyrite oxidation rate by a factor of 5 at ascorbic acid concentrations between 10−4 and 10−2 mol L−1. Comparison of the rate constants for the oxidation of ascorbic acid by surface bound Fe(III) in the absence and presence of pyrite shows that the pyrite surface accelerates this reaction by a factor of 10. The oxidation of both pyrite and ascorbic acid is of fractional order with respect to ascorbic acid (HAsc): rpy=0.55 c(HAsc)−0.35 rHAsc=3.6 c(HAsc)0.59. Both the results from experiments with Fe(III)-chelating ligands and the Fe(III)-reductant, suggest a very efficient interference in the electron cycling between Fe(II) and Fe(III) at the pyrite surface. The interference seems to be mainly related to the reductive side of the iron cycling. It is therefore concluded that the electron transfer between ferric iron and pyritic sulfur limits the pyrite oxidation rate at pH 7.

Phosphorus mobility in interstitial waters of sediments in Lake Kinneret, Israel

Monthly samples of sediment cores from maximum depth (∼ 42 m) in Lake Kinneret were taken from May 1988 until January 1989. The chemical composition of the interstitial and overlying water was investigated with respect to phosphate, Fe2+, Fe3+, Ca2+, alkalinity and electric conductivity. pH, pH2S and pe (electron-activity) were measured by microelectrodes inserted directly into the sediment core immediately after sampling.Ion activity products of vivianite, siderite, ironsulfides, Ca-P complexes and Ca-P solid phases were calculated; in addition, Ca/P ratios for the overlying and pore water were obtained by using the potential diagram technique. Despite the fact that anoxic conditions prevail for most of the year, no control of phosphate solubility by a Fe-P relationship could be found. Determination of IAPs, together with calculated molar Ca/P-ratios, suggests that hydroxyapatite as well as surface complexes like dicalciumphosphate are the solubility-controlling species in pore water. For the overlying water a Ca3(HCO3)3PO4 surface complex is assumed to fix the phosphorus, accompanied by a subsequent transformation of the bound P into apatite.

Slope deposits and water paths in a spring catchment, Frankenwald, Bavaria, Germany

The spring catchment under study is underlain by shale, on which several layers of slope sediments (cover-beds) are deposited. The upper of these layers contain eolian fines mixed into shale-derived debris, which latter material entirely comprises the basal cover-bed. Due to its dislocation by solifluction, the shale debris has a tegular structure. This leads to hydraulic anisotropy, particularly where no fine earth occurs. Thus, water that seeps into such material is forced to flow laterally while vertical movement is impeded. The basal cover-bed therefore hosts a deep aquifer. Only during major discharge events, excess hydrostatic pressure makes water flow into upper parts of the soils, being mixed with surficially interflowing water. Most of the time, however, there are two levels of water flow at different depths with different dwelling times, which finding is supported by probably geogenic sulphur isotopes in the deeply flowing water that are different from precipitation water.KurzfassungDas bearbeitete Quelleinzugsgebiet liegt in Tonschiefer, welcher mit Deckschichten bedeckt ist. In die oberen Deckschichten wurde Löß eingearbeitet, während die untere ausschließlich aus umgelagertem Anstehenden besteht. Dieses Material wurde solifluidal verlagert und dabei mit einer dachziegelartigen Struktur abgelagert. Diese führt zu hydraulischer Anisotropie besonders in feinerdearmen Schichten. Wasser, das darin einsickert, fließt bevorzugt lateral, während vertikale Bewegungen stark behindert werden. Deshalb ist in der Basislage ein tieferes Aquifer entstanden. Während ausgeprägter Abflußereignisse wird der Überdruck darin jedoch so groß, daß es zu einem Aufpressen des Wasser in hangende Schichten und dort zur Vermischung mit höherem Zwischenabfluß kommt. Zumeist aber gibt es zwei unterschiedliche Interflow-Stockwerke in verschiedenen Tiefen. Dies wird bestätigt durch eine wahrscheinlich geogene Sulfatbeimischung im tieferen Stockwerk, die sich vom Niederschlagswasser in den Isotopen unterscheidet.