Elevated concentrations of mercury (Hg) in aquatic systems can be a threat to ecosystems and human health. Mercury-bearing sediment particles from eroding riverbanks can be an ongoing source of bioavailable Hg to aquatic ecosystems. Hyporheic zones in particular can be important sources of both inorganic and organic-complexed Hg, which can be rapidly transported to adjacent surface waters. The objective of this study was to investigate the release and treatment of dissolved and particle-bound Hg in water derived from the riverbank sediments of the South River, Virginia. The solid-phase forms of Hg in riverbank sediment samples were characterized by sequential extraction and synchrotron based techniques. The analyses suggest that 79-93% of Hg in the sediment samples is in the form of insoluble sulfides (βHgS metacinnabar); however significant masses of more-soluble Hg phases (0.4-33 μg/g) are also present. Simulated erosion events resulted in elevated concentrations of Hg in the river water up to 80 μg/L. There was no correlation between the mass of water-soluble Hg in the sediment and the concentrations of Hg released in the river water following sediment suspension. Column transport experiments were conducted to assess Hg release from the sediment under water:sediment ratios typical of those that might occur at the bottom or in the banks of the river. Concentrations of Hg in the 0.45 μm-filtered fraction of the effluent varied from 0.15 μg/L for samples collected from the base of the riverbank to 8 μg/L for samples collected from the top of the riverbank. Filter size-fractionation of water column effluent suggested approximately 50% of the leached Hg was present in the dissolved phase, with the remainder in particulate form. Riverbank sediments were amended with various types of reactive material including complexing agents, reductants and charcoals. Batch experiments indicate that the mass of Hg released from the sediment could be lessened by 64-99% with the addition of reactive media, and that aerating and re-wetting the sediment amendments resulted in equal or greater removal of Hg from the water. The greatest removal of Hg was observed when more amendment was added to the sediment, however the greatest Hg uptake capacity (Hg captured per mass of material) was observed for the lowest doses of reactive media. The Hg uptake capacities ranged from 35-500 ng/g and were greatest for treatment of water with elevated concentrations of Hg. The Hg uptake capacities were a function of the Hg concentrations in the untreated water, and were generally lower relative to values reported in the literature. Column studies were used to simulate the flow of river water containing elevated concentrations of Hg through a reactive zone containing a charred hardwood material. The concentration of filtered Hg was < 120 ng/L for treated effluent from columns, resulting in > 98% removal of Hg from the water. Assuming that the majority of removal occurred within the initial 2-3 cm along the length of the column, the calculated uptake of Hg2+ ranged from 1.2-7.7 μg/g. The uptake capacity for charred hardwood material was much greater for the column experiments relative to the batch experiments, suggesting that the uptake capacity is limited by Hg loading. The chemical composition of the treated column effluent was similar to the South River water, and suggests the material did not add or remove significant constituents during the course of these experiments.

Elevated concentrations of mercury (Hg) in aquatic systems can be a threat to ecosystems and human health. Mercury-bearing sediment particles from eroding riverbanks can be an ongoing source of bioavailable Hg to aquatic ecosystems. Hyporheic zones in particular can be important sources of both inorganic and organic-complexed Hg, which can be rapidly transported to adjacent surface waters. The objective of this study was to investigate the release and treatment of dissolved and particle-bound Hg in water derived from the riverbank sediments of the South River, Virginia. The solid-phase forms of Hg in riverbank sediment samples were characterized by sequential extraction and synchrotron based techniques. The analyses suggest that 79-93% of Hg in the sediment samples is in the form of insoluble sulfides (βHgS metacinnabar); however significant masses of more-soluble Hg phases (0.4-33 μg/g) are also present. Simulated erosion events resulted in elevated concentrations of Hg in the river water up to 80 μg/L. There was no correlation between the mass of water-soluble Hg in the sediment and the concentrations of Hg released in the river water following sediment suspension. Column transport experiments were conducted to assess Hg release from the sediment under water:sediment ratios typical of those that might occur at the bottom or in the banks of the river. Concentrations of Hg in the 0.45 μm-filtered fraction of the effluent varied from 0.15 μg/L for samples collected from the base of the riverbank to 8 μg/L for samples collected from the top of the riverbank. Filter size-fractionation of water column effluent suggested approximately 50% of the leached Hg was present in the dissolved phase, with the remainder in particulate form. Riverbank sediments were amended with various types of reactive material including complexing agents, reductants and charcoals. Batch experiments indicate that the mass of Hg released from the sediment could be lessened by 64-99% with the addition of reactive media, and that aerating and re-wetting the sediment amendments resulted in equal or greater removal of Hg from the water. The greatest removal of Hg was observed when more amendment was added to the sediment, however the greatest Hg uptake capacity (Hg captured per mass of material) was observed for the lowest doses of reactive media. The Hg uptake capacities ranged from 35-500 ng/g and were greatest for treatment of water with elevated concentrations of Hg. The Hg uptake capacities were a function of the Hg concentrations in the untreated water, and were generally lower relative to values reported in the literature. Column studies were used to simulate the flow of river water containing elevated concentrations of Hg through a reactive zone containing a charred hardwood material. The concentration of filtered Hg was < 120 ng/L for treated effluent from columns, resulting in > 98% removal of Hg from the water. Assuming that the majority of removal occurred within the initial 2-3 cm along the length of the column, the calculated uptake of Hg2+ ranged from 1.2-7.7 μg/g. The uptake capacity for charred hardwood material was much greater for the column experiments relative to the batch experiments, suggesting that the uptake capacity is limited by Hg loading. The chemical composition of the treated column effluent was similar to the South River water, and suggests the material did not add or remove significant constituents during the course of these experiments.

Zur Restaurierung eutrophierter Seen kommen unterschiedliche chemische Fällmittel zum Einsatz. Eine neuartige Fällmittelkombination von Aluminat und Calciumhydroxid wurde in einem whole-lake-Experiment zur Restaurierung des Tiefwarensees (Mecklenburg-Vorpommern) eingesetzt. Dabei wurden im Laufe der 5 Jahre währenden Fällmittelzugabe ca. 2 t P zusätzlich im Sediment festgelegt. Dadurch wechselte der vorher hocheutrophe See in den mesotrophen Zustand. Mit Sedimentuntersuchungen und einer modellhaften Prognose wird nachgewiesen, dass die P-Retention erhöht und damit eine nachhaltige Wirkung der Restaurierung erzielt wurde. Weiterhin wurde ein neuentwickelter Nitrat-Depotstoff in Labortests und einem Enclosureversuch im Dagowsee (Brandenburg) angewandt. Der Depotstoff stellte das Nitrat zeitverzögert über mehrere Wochen nach der Applikation an der Sedimentoberfläche zur Verfügung. Die erhöhten Nitratkonzentrationen bewirkten zusammen mit dem gleichzeitig eingebrachten P-Bindungspartner Fe eine Vergrößerung der P-Bindungskapazität der behandelten Sedimente. Dadurch wurde die P-Rücklösung aus den Sedimenten mindestens ein Jahr lang beinahe vollständig unterdrückt. Beide Fällmittelkombinationen förderten zunächst hauptsächlich die Bildung anorganischer partikulärer P-Verbindungen im Sediment. Die bakteriellen Mineralisationsprozesse wurden nur indirekt bzw. zeitlich versetzt beeinflusst. Die sich andeutenden Veränderungen in Art und Intensität der mikrobiellen Stoffumsetzungen werden als Folge der Entwicklung der Sedimentation sowie des Sedimentmilieus diskutiert. Um das Gefahrenpotenzial beim Einsatz aluminiumhaltiger Fällmittel in der Seenrestaurierung einschätzen zu können, wurde unter Berücksichtigung der relevanten Mechanismen der Al-Toxizität und der Al-Chemie ein geeignetes Analyse-Verfahren für die Anwendung in karbonatreichen Seen angepasst und damit erstmals die gelösten monomeren Al-Hydroxide, die als potenziell toxisch wirkende Al-Spezies beschrieben werden, vor und während einer Maßnahme analytisch überwacht. Das Risiko einer Al-Giftwirkung auf Organismen im behandelten Tiefwarensee wird als sehr gering eingestuft. / In order to restore eutrophicated lakes different chemical phosphorus precipitation agents are utilized. A novel combined hypolimnetic aluminium and calcium precipitation treatment was first applied to restore Lake Tiefwaren (Mecklenburg Vorpommern, Germany) in a whole-lake-experiment lasting five years. About 2 t of phosphorus, also from deep (> 10 cm) sediment layers, were permanently bound during the restoration and the lake changed from a highly eutrophic to a mesotrophic state. Different investigations of the sediments and a model based prognosis demonstrate that the sediment’s P retention capacity increased indicating that a sustainable restoration effect was obtained. In addition a newly developed nitrate storing compound was tested in the laboratory as well as in a mesocosm experiment in Lake Dagow (Brandenburg, Germany). This compound provides nitrate at the sediment surface for some weeks. The nitrate together with the simultaneously added iron increased the P-retention of the treated sediments. Therefore the P release from sediments was suppressed for at least one year. The addition of both chemical compounds initially enhanced the P precipitation, whereas microbial processes of the P diagenesis were affected indirectly or temporally delayed. The observed changes in microbial activities are discussed as an effect of the changing sedimentation rate and chemical characteristics (pH, redox). In order to assess the risk of toxicity on aquatic organisms by the aluminium compounds used for lake restoration an analytical method was adapted for use in carbonate-rich natural waters. This enabled us for the first time to monitor dissolved aluminium hydroxide, a potentially toxic aluminium species, before and during a restoration process. The risk of a toxic effect on organisms in Lake Tiefwaren was found to be neglectible.