Tim Mansfeldt

The oxygen isotope composition of nitrate generated by nitrification in acid forest floors

Abstract The oxygen isotope composition of nitrate is used increasingly for identifying the origin of nitrate in terrestrial and aquatic ecosystems. This novel isotope tracer technique is based on the fact that nitrate in atmospheric deposition, in fertilizers, and nitrate generated by nitrification in soils appear to have distinct oxygen isotope ratios. While the typical ranges of δ18O values of nitrate in atmospheric deposition and fertilizers are comparatively well known, few experimental data exist for the oxygen isotope composition of nitrate generated by nitrification in soils. The objective of this study was to determine δ18O values of nitrate formed by microbial nitrification in acid forest floors. Evidence from laboratory incubation experiments and field studies suggests that during microbial nitrification in acid forest floor horizons, up to two of the three oxygen atoms in newly formed nitrate are derived from water, particularly if ammonium is abundant and nitrification rates are high. It was, however, also observed that in ammonium-limited systems with low nitrification rates, significantly less than two thirds of the oxygen in newly formed nitrate can be derived from water oxygen, presumably as a result of heterotrophic nitrification. It can be concluded from the presented data that the δ18O values of nitrate formed by microbial nitrification in acid forest floors typically range between +2 and +14‰, assuming that soil water δ18O values vary between −15 and −5‰. Hence, oxygen isotope ratios of nitrate formed by nitrification in forest floors are usually distinct from those of other nitrate sources such as atmospheric deposition and synthetic fertilizers and, therefore, constitute a valuable qualitative tracer for distinguishing among these sources of nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of δ18O values, particularly for atmospheric nitrate deposition and for nitrate from microbial nitrification.

Contaminants at Former Manufactured Gas Plants: Sources, Properties, and Processes

In the course of industrialization in the 19th century, manufactured gas plant sites were built in almost every larger town in Europe and the United States. Organic and inorganic contaminations pose a continuing threat to groundwater resources at these locations even though the operations were terminated decades ago. The different coal tars and tar oils present in the subsurface are composed of a huge variety of organic compounds often possessing a high toxic potential, such as monocyclic, polycyclic, and heterocyclic aromatic hydrocarbons. In addition to these inorganic contaminants, cyanides and heavy metals can be found due to dumping of by-products and wastes. A multitude of factors influence the fate of these compounds and an increasing number of studies deal with this subject. However, key processes are still not well understood. One example is the mobilization of colloid associated contaminants by extreme single events, which can be responsible for the bulk contaminant load in the seepage water. Another example is the role of microbial populations and communities in cooperative degradation pathways of organic pollutants. We aim for a comprehensive overview on the present knowledge about the processes, which determine the fate of organic and inorganic contaminant at such sites. This way, a sound basis is provided for the identification of future research directions, evaluation of risk, and development of remediation techniques.

Speciation of Zn in blast furnace sludge from former sedimentation ponds using synchrotron X-ray diffraction, fluorescence, and absorption spectroscopy.

Blast furnace sludge (BFS), an industrial waste generated in pig iron production, typically contains high contents of iron and various trace metals of environmental concern, including Zn, Pb, and Cd. The chemical speciation of these metals in BFS is largely unknown. Here, we used a combination of synchrotron X-ray diffraction, micro-X-ray fluorescence, and X-ray absorption spectroscopy at the Zn K-edge for solid-phase Zn speciation in 12 BFS samples collected on a former BFS sedimentation pond site. Additionally, one fresh BFS was analyzed for comparison. We identified five major types of Zn species in the BFS, which occurred in variable amounts: (1) Zn in the octahedral sheets of phyllosilicates, (2) Zn sulfide minerals (ZnS, sphalerite, or wurtzite), (3) Zn in a KZn-ferrocyanide phase (K(2)Zn(3)[Fe(CN)(6)](2)·9H(2)O), (4) hydrozincite (Zn(5)(OH)(6)(CO(3))(2)), and (5) tetrahedrally coordinated adsorbed Zn. The minerals franklinite (ZnFe(2)O(4)) and smithsonite (ZnCO(3)) were not detected, and zincite (ZnO) was detected only in traces. The contents of ZnS were positively correlated with the total S contents of the BFS. Similarly, the abundance of the KZn-ferrocyanide phase was closely correlated with the total CN contents, with the stoichiometry suggesting this as the main cyanide phase. This study provides the first quantitative Zn speciation in BFS deposits, which is of great relevance for environmental risk assessment, the development of new methods for recovering Zn and Fe from BFS, and potential applications of BFS as sorbent materials in wastewater treatment.

Release and mobility of polycyclic aromatic hydrocarbons and iron‐cyanide complexes in contaminated soil

Export of organic and inorganic pollutants from the unsaturated zone of manufactured gas plant (MGP) sites may endanger groundwater quality. The release and mobility of polycyclic aromatic hydrocarbons (PAH) and iron-cyanide complexes (FeCN) in soils from a former MGP site were studied in column and batch experiments. The unsaturated column experiment involved two parts. (1) The development of steady-state effluent concentrations was studied under constant irrigation. (2) Rate-limited contaminant release was studied by the systems response to flow interruption. Transport data were supplemented by a sequential batch-desorption experiment. During constant irrigation the initial concentrations of total PAH and FeCN, but also the concentration of dissolved organic carbon (DOC) were higher than the steady state level, indicating that a specific fraction of the DOC and the contaminants was readily mobilized. Cyanide reached a constant effluent concentration after seven pore volumes (pvs), whereas equilibration of effluent PAH took 20 pvs. After flow interruption, rate-limited release of DOC, PAH, and FeCN was observed by increased effluent concentrations. Comparison of FeCN levels with theoretical equilibrium concentrations indicated that in addition to the dissolution of iron(II)ferrocyanide (Berlin Blue), Fe4[Fe(CN)6]3, other cyanide-species may control the release and such the mobility of FeCN. This was confirmed by decreasing FeCN concentrations during sequential batch extractions. Due to rate-limited release, seepage water concentrations of DOC, PAH, and FeCN in alkaline MGP soils are influenced by the flow regime. Experiments performed at continuous irrigation may therefore severely underestimate the release of PAH and FeCN under field conditions. The effect of DOC on the release and mobility of PAH and FeCN under transient conditions has to be further investigated. Freisetzung und Mobilitat von PAK und Eisen-Cyanid-Komplexen in kontaminierten Boden Der Austrag von Schadstoffen aus der ungesattigten Bodenzone von Gaswerken stellt eine grose Gefahr fur die Qualitat des Grundwassers dar. Die Freisetzung und die Mobilitat von polyzyklischen aromatischen Kohlenwasserstoffen (PAK) und Eisen-Cyanid-Komplexen (FeCN) im belasteten Boden eines ehemaligen Gaswerkes wurde in Schuttel- und Saulenexperimenten untersucht. Die Saulenexperimente, die unter wasserungesattigten Transportbedingungen durchgefuhrt wurden, bestanden aus zwei Teilen: Unter stationaren Fliesbedingungen wurde die Einstellung einer konstanten Zusammensetzung des Effluenten untersucht. Eine mogliche Ratenlimitierung der Freisetzung der Schadstoffe wurde anhand der Konzentrationsanderung im Effluenten als Reaktion auf eine Flussunterbrechung untersucht. Unter stationaren Fliesbedingungen war der Export von PAK und FeCN, aber auch von gelostem organischem Kohlenstoff (DOC), durch eine zweiphasige Durchbruchskurve gekennzeichnet: Die Effluentkonzentration von DOC, PAK und FeCN nach Aufnahme der Beregnung war zunachst sehr hoch und deutlich hoher als die Konzentration nach Einstellung des Fliesgleichgewichts. Dies weist auf einen leicht verfugbaren Schadstoffanteil hin, der aufgrund der Aufsattigung gelost und mobilisiert wird. Fur die Eisen-Cyanid-Komplexe wurde ein Fliesgleichgewicht nach 7 ausgetauschten Porenvolumen erreicht, fur die PAK nach 20 Porenvolumen. Fur FeCN und PAH, aber auch fur DOC, konnte eine erhohte Effluentkonzentration nach Flussunterbrechung beobachtet werden. Der Transport der Schadstoffe scheint damit eng an den Transport von DOC geknupft zu sein. Ein Vergleich der FeCN-Effluentkonzentration mit berechneten Gleichgewichtskonzentrationen lasst vermuten, dass die Auflosung von Berliner Blau, Fe4[Fe(CN)6]3, nicht der einzige Prozess ist, der die Freisetzung von FeCN bestimmt. Aufgrund der ratenlimitierten Freisetzung wird die Konzentration von DOC, FeCN und PAK im Sickerwasser vom Fliesregime des Bodenwassers bestimmt. Unter stationaren Fliesbedingungen konnen die Sickerwasserfrachten von Schadstoffen daher falsch eingeschatzt werden. Inwieweit die Mobilitat von DOC den Transport von PAK und FeCN unter instationaren Fliesbedingungen bestimmt, muss in weiteren Experimenten geklart werden.

Arsenic mobility and speciation in a gleysol with petrogleyic properties: a field and laboratory approach.

Naturally elevated levels of As have been observed in some iron-enriched lowland soils of the southern Münsterland, Germany. To determine whether As is mobilized by the reductive dissolution of As-hosting Fe oxides at reducing soil conditions, the release of arsenate and arsenite was investigated in the field for 24 mo and in laboratory experiments. The grassland Gleysol consists of oximorphic soil horizons (Ah 0-15, Bg 15-35, CrBg 35-70 cm) developed from fluvial loam overlaying sand with reductomorphic properties (2Cr, +70 cm). The soil has petrogleyic properties due to enrichment of oxidic Fe in the Bg and CrBg horizons (275 and 390 g kg, respectively). Most of the petrogleyic Fe belonged to nanosized goethite, whereas ferrihydrite was dominant in the topsoil. Arsenic levels of 149 mg kg were found in the topsoil. Levels peaked in the oximorphic soil horizons (626 and 999 mg kg, respectively) and were lowest in the sand (12 mg kg). Sequential fractionation revealed that 84 to 96% of the As in the oximorphic horizons was associated with Fe oxides. Water saturation in combination with soil temperatures above 5 to 10°C resulted in a fast decrease of the redox potential (up to -120 mV) and release of As (up to 35 μg L) with Fe into the soil solution only in the Ah horizon. Although the petrogleyic horizons were mostly water saturated with reducing conditions, no As release was observed. A lack of As mobilization was confirmed in experiments performed under fixed redox conditions; neither Fe nor As was released into solution at an appropriate redox potential. The As species distribution showed redox disequilibrium because arsenate was detected under reducing conditions and arsenite under oxidizing conditions. We conclude that microbial-mediated reductive dissolution of Fe oxides pushes the As mobilization in the topsoil; water saturation and soil temperature were key factors. The dominance of goethite, which is more resistant to microbial reduction, and/or the possible readsorption of released As onto remaining or newly formed Fe oxide surfaces is responsible for the absent As mobilization in the petrogleyic horizons.

Sorption and desorption of iron-cyanide complexes in deposited blast furnace sludge

Blast furnace sludge is a waste originating from pig iron production and contains small amounts of iron-cyanide complexes. Leaching of iron-cyanide complexes from deposited blast furnace sludge into the ground water seems to be possible in principle. We investigated the sorption of the iron-cyanide complexes ferrocyanide, [FeII(CN)6](4-), and ferricyanide, [FeIII(CN)6](3-), in 22 samples of deposited blast furnace sludge in batch experiments. Subsequently, desorption of iron-cyanide complexes was investigated using 1 M NaCl. Sorption in five samples was evaluated with Langmuir isotherms. The blast furnace sludge samples were neutral to slightly alkaline (pH 7.6-9) and consisted of X-ray amorphous compounds and crystalline Fe oxides primarily. X-ray amorphous compounds are assumed to comprise coke-bound C and amorphous Fe, Zn, and Al oxides. The experiments that were evaluated with Langmuir isotherms indicated that the extent of ferricyanide sorption was higher than that of ferrocyanide sorption. Saturation of blast furnace sludge with iron-cyanide complexes was achieved. Sorption of iron-cyanide complexes in 22 blast furnace sludge samples at one initial concentration showed that 12 samples sorbed more ferrocyanide than ferricyanide. The extent of sorption largely differed between 0.07 and 2.76 Micromol [Fe(CN)6] m(-2) and was governed by coke-bound C. Ferricyanide sorption was negatively influenced by crystalline Fe oxides additionally. Only small amounts of iron-cyanide complexes sorbed in blast furnace sludge were desorbed by 1 M NaCl (ferrocyanide, 3.2%; ferricyanide, 1.1%, given as median). This indicated strong interactions of iron-cyanide complexes in blast furnace sludge. The mobility of iron-cyanide complexes in deposited blast furnace sludge and consequently contamination of the seepage and ground water was designated as low, because (i) deposited blast furnace sludge is able to sorb iron-cyanide complexes strongly, (ii) the solubility of the iron-cyanide-containing phase, K2Zn3[FeII(CN)6] . 9H20, is known to be low, and (iii) a worst case scenario of the transport of iron-cyanide complexes within the blast furnace sludge deposit indicated strong retardation of the complexes within the next 100 years.

Cyanide in paper de-inking sludge used as a soil amendment

Paper de-inking sludge is processed during the recycling of paper, and is sometimes used as a soil amendment. In this study the effect of a compost application on the cyanide (CN) status in soils of a public park was investigated. The compost was a mixture of chipped limbs and paper de-inking sludge. Furthermore, the cyanide solubility was studied by conducting batch experiments with different pH levels. Total cyanide in the amended soils ranged from 540 to 740 mg CN kg—1, and water soluble cyanide from 170 to 370 μg CN l—1 as determined by means of an aqueous extract. Easily-liberatable cyanides, which include the toxic free cyanide (HCN and CN—) and weak metal-cyanide complexes, were not present in the soil. From this result and the fact that iron blue pigments are used during paper printing, it can be inferred that cyanides occurring here were exclusively stable iron-cyanide complexes [Fe(CN)6]. With increasing pH the solubility of cyanide increased. In contrast to soils of coking plants, in which cyanide occur as Berlin blue, Fe4[Fe(CN)6]3, the cyanide solubility in the paper de-inking sludge amended soils was substantially lower, especially in the neutral and alkaline range. Thus, cyanides in paper de-inking sludge could be present as sparingly soluble metal-cyanide compounds with the general formula A2B[FeII(CN)6] with A = K+, Na+ and B = Ca2+ or divalent transition metals and B2[FeII(CN)6] with B = divalent transition metals. Pollution exposure by the pathways soil human, and soil air human can be neglected. However, since leaching of iron-cyanide complexes into the ground water cannot be excluded, and since they are decomposed to HCN when exposed to day light, environmental hazards by the pathway soil ground water surface water are possible. This is the risk arising from paper de-inking sludge applications to soils. Cyanid im als Bodenverbesserungsmittel eingesetzten Papier-Deinkingschlamm Papier-Deinkingschlamm ist ein Ruckstand der Altpapieraufbereitung und wird manchmal als Bodenverbesserungsmittel eingesetzt. In dieser Arbeit wurde der Einfluss einer Kompostapplikation auf den Cyanid-Status in Boden eines offentlichen Parks untersucht. Der Kompost war eine Mischung aus Rindenmulch und Papier-Deinkingschlamm. Weiterhin wurde die Cyanid-Loslichkeit in Schuttelversuchen bei unterschiedlichen pH-Werten getestet. Die Gesamtcyanidgehalte in den behandelten Boden reichten von 540 bis 740 mg CN kg—1, und die der gelosten Gesamtcyanid-Konzen-trationen im wassrigen Extrakt von 170 bis 370 μg CN l—1. Leicht freisetzbare Cyanide, welche die toxischen freien Cyanide (HCN und CN—) und schwachen Metallcyankomplexe umfassen, konnten nicht nachgewiesen werden. Aus diesem Ergebnis und der Tatsache, dass wahrend des Papierdrucks Eisenblau-Pigmente verwendet werden, kann gefolgert werden, dass die Cyanide ausschlieslich als stabile Eisencyankomplexe, [Fe(CN)6], vorlagen. Mit steigendem pH nahm die Loslichkeit der Cyanide zu. Im Gegensatz zu Kokereiboden, die mit Berliner Blau, Fe4[Fe(CN)6]3, belastetet sind, war die Cyanid-Loslichkeit in diesen Boden wesentlich geringer, besonders im neutralen bis alkalischem Bereich. Cyanide im Papier-Deinkingschlamm konnten in Form schwer loslicher Metallcyanverbindungen mit der allgemeinen Formel A2B[FeII(CN)6] mit A = K+, Na+ und B = Ca2+ oder ein zweiwertiges Ubergangsmetall und B2[FeII(CN)6] mit B = ein zweiwertiges Ubergangsmetall vorliegen. Gefahrdungen uber den Pfad Boden Mensch und Boden Luft Mensch konnen ausgeschlossen werden. Da aber ein Eintrag von Eisencyankomplexen in das Grundwasser moglich ist und diese im Tageslicht Blausaure freisetzen, ist eine Umweltgefahrdung uber den Pfad Boden Grundwasser Oberflachenwasser denkbar. Hierin liegt ein Risiko in der Anwendung von Papier-Deinkingschlamm als Bodenverbesserungsmittel.

Manganese-Oxide-Coated Redox Bars as an Indicator of Reducing Conditions in Soils

Identification of reducing conditions in soils is of concern not only for pedogenesis but also for nutrient and pollutant dynamics. We manufactured manganese (Mn)-oxide-coated polyvinyl chloride bars and proved their suitability for the identification of reducing soil conditions. Birnessite was synthesized and coated onto white polyvinyl chloride bars. The dark brown coatings were homogenous and durable. As revealed by microcosm devices with adjusted redox potentials (E), under oxidizing conditions (E ∼450 mV at pH 7) there was no Mn-oxide removal. Reductive dissolution of Mn-oxides, which is expressed by the removal of the coatings, started under weakly reducing conditions (E ∼175 mV) and was more intensive under moderately reducing conditions (∼80 mV). According to thermodynamics, the removal of Mn-oxide coatings (225 mm d) exceeded the removal of iron (Fe)-oxide coatings (118 mm d) in soil column experiments. This was confirmed in a soil with a shallow and strongly fluctuating water table where both types of redox bars were inserted. Consequently, it was possible to identify reducing conditions in soils using Mn-oxide-coated bars. We recommend this methodology for short-term monitoring because tri- and tetravalent Mn is the preferred electron acceptor compared with trivalent Fe, and this additionally offers the possibility of distinguishing between weakly and moderately reducing conditions. If dissolved Fe is abundant in soils, the possibility of nonenzymatic reduction of Mn has to be taken into account.

Mercury in dumped blast furnace sludge

Blast furnace sludge (BFS) is a waste generated in the production of pig iron and was dumped in sedimentation ponds. Sixty-five samples from seven BFS locations in Europe were investigated regarding the toxic element mercury (Hg) for the first time. The charge material of the blast furnace operations revealed Hg contents from 0.015 to 0.097mgkg(-1). In comparison, the Hg content of BFS varied between 0.006 and 20.8mgkg(-1) with a median of 1.63mgkg(-1), which indicates enrichment with Hg. For one site with a larger sample set (n=31), Hg showed a stronger correlation with the total non-calcareous carbon (C) including coke and graphite (r=0.695; n=31; p<0.001). It can be assumed that these C-rich compounds are hosting phases for Hg. The solubility of Hg was rather low and did not exceed 0.43% of total Hg. The correlation between the total Hg concentration and total amount of NH4NO3-soluble Hg was relatively poor (r=0.496; n=27; p=0.008) indicating varying hazard potentials of the different BFS. Finally, BFS is a mercury-containing waste and dumped BFS should be regarded as potentially mercury-contaminated sites.

Molybdate adsorption from steel slag eluates by subsoils

Steel slags are industrial by-products which are generated in large amounts worldwide, e.g. 150-230×10(6) Mg in 2012, and which are partly used for construction. Molybdenum (Mo) can be added during steel processing in order to harden the steel. The objective of this study was to evaluate the adsorption behaviour of molybdate (MoO4(2-)) from slag eluates in subsoils. Molybdate batch adsorption experiments were carried out with eluates obtained from two different kinds of steel slags (i) LD slag (Linz-Donawitz operation, LDS) and (ii) electric arc furnace slag (EAF) to assess the risk that may arise from the contamination of groundwater by the leaching of molybdate. Six different subsoils were chosen in order to provide a wide range of chemical properties (pH 4.0-7.6; dithionite-extractable Fe 0.73-14.7 g kg(-1)). Molybdate adsorption experiments were carried out at the pH of the steel slag eluates (pH 11-12) as well as at pH values adjusted to the soil pH. The data were evaluated with the Freundlich equation. Molybdate adsorption exhibited a maximum near pH 4 for steel slag eluates adjusted to the soil pH, and decreased rapidly with increasing pH until adsorption was virtually zero at pH>11. Adsorption was greater for soils with high amounts of dithionite-extractable Fe oxides. The extent and behaviour of molybdate adsorption from both eluates was similar. After a reaction time of 24h, the pH of the EAF slag eluate was lower than that of the LD steel slag eluate, which was caused by different acid buffer capacities. Some soils were able to decrease the pH of the EAF slag eluates by about 4 pH units, enhancing the adsorption of molybdate. Transport simulations indicated that molybdate discharge is low in acidic soils.