Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Doodzonde om op te stoken. Lignine de grondstof van de toekomst
    Zundert, M. ; Gosselink, R.J.A. - \ 2014
    Chemie Magazine 2014 (2014)3. - ISSN 1572-2996 - p. 24 - 27.
    biopolymeren - lignine - toepassingen - bioraffinage - innovaties - chemie op basis van biologische grondstoffen - biobased economy - aromatische koolwaterstoffen - biopolymers - lignin - applications - biorefinery - innovations - biobased chemistry - biobased economy - aromatic hydrocarbons
    Onhandelbaar, wee barstig en recalcitrant. Zo staat het houtpolymeer lignine bekend. Maar de stof vervangt steeds vaker fenol. En onderzoekers zijn hoopvol over de ontwikkeling van bio-BTX en koolstofvezels uit lignine.
    Physiology and biochemistry of aromatic hydrocarbon-degrading bacteria that use chlorate and/or nitrate as electron acceptor
    Oosterkamp, M.J. - \ 2013
    Wageningen University. Promotor(en): Fons Stams, co-promotor(en): Caroline Plugge; Peter Schaap. - Wageningen : Wageningen University - ISBN 9789461737779 - 191
    bacteriën - aromatische koolwaterstoffen - fysiologie - biochemie - elektronen - genomen - nitraten - chloraten - microbiologie - bacteria - aromatic hydrocarbons - physiology - biochemistry - electrons - genomes - nitrates - chlorates - microbiology
    Degradation of benzene and other aromatic hydrocarbons by anaerobic bacteria
    Weelink, S.A.B. - \ 2008
    Wageningen University. Promotor(en): Fons Stams. - S.l. : S.n. - ISBN 9789085852391 - 128
    benzeen - aromatische koolwaterstoffen - microbiële afbraak - benzene - aromatic hydrocarbons - microbial degradation
    Accidental spills, industrial discharges and gasoline leakage from underground storage tanks have resulted in serious pollution of the environment with monoaromatic hydrocarbons, such as benzene, toluene, ethylbenzene and xylene (so-called BTEX). High concentrations of BTEX have been detected in soils, sediments and groundwater. The mobility and toxicity of the BTEX compounds are of major concern. In situ bioremediation of BTEX by using naturally occurring microorganisms or introduced microorganisms is a very attractive option. BTEX compounds are known to be transformed (or degraded) by microorganisms under aerobic and anaerobic conditions. As BTEX compounds are often present in the anaerobic zones of the environment, anaerobic bioremediation is an attractive remediation technique. The bottleneck in the application of anaerobic techniques is the lack of knowledge about the anaerobic biodegradation of benzene. In particular, little is known about the bacteria involved in anaerobic benzene degradation and the anaerobic benzene degradation pathway has still not been elucidated. The aim of the research presented in this thesis was to gain more insight in the degradation of benzene and other aromatic hydrocarbons by anaerobic bacteria. In particular, the physiology and phylogeny of the bacteria responsible for the degradation were studied and the results have been presented in this thesis.
    Anaerobic benzene and toluene degradation was studied with different electron acceptors in batch experiments inoculated with material from an aquifer polluted by BTEX-containing landfill leachate (Banisveld landfill near Boxtel, The Netherlands). Benzene was not degraded during one year of incubation. Toluene degradation, on the other hand, was observed with nitrate, MnO2 and Fe(III)NTA as electron acceptors. After further enrichment and several isolation attempts, a novel betaproteobacterial bacterium, strain G5G6, was obtained in pure culture. Strain G5G6 is able to grow with toluene as the sole electron donor and carbon source, and amorphous and soluble Fe(III)-species, nitrate and MnO2 as electron acceptors. Strain G5G6 has several other interesting physiological and phylogenetic characteristics, which will be subject of future research. Strain G5G6 represents a novel species in a novel genus for which we propose the name Georgfuchsia toluolica.
    In general, aerobic degradation of BTEX is a faster process than anaerobic BTEX degradation. However, for a number of reasons application of oxygen-dependent processes in the subsurface are technically and financially often not appealing. Therefore, an alternative bioremediation strategy would be to introduce oxygen in an alternative way, e.g. by in situ production. Chlorate reduction is a way to produce molecular oxygen in situ under anaerobic conditions. The formation of oxygen during chlorate reduction may result in rapid oxidation of compounds which are slowly degraded under anaerobic conditions; an example of such a compound is benzene. Therefore, benzene degradation coupled to the reduction of chlorate (ClO3-) was studied in this thesis. With mixed material from a wastewater treatment plant and soil samples obtained from a location contaminated with benzene, a benzene-degrading chlorate-reducing stable enrichment culture was obtained. This stable enrichment consisted of about five different bacterial species. Cross feeding involving interspecies oxygen transfer is a likely mechanism in this enrichment. One of these species, strain BC, was obtained in pure culture. Phylogenetic analysis showed that strain BC is a Alicycliphilus denitrificans strain. Strain BC is able to degrade benzene in conjunction with chlorate reduction. Oxygenase genes putatively encoding the enzymes performing the initial steps in aerobic degradation of benzene, were detected in strain BC. This demonstrates the existence of aerobic benzene bacterial biodegradation pathways under essentially anaerobic conditions. Thus, aerobic pathways can be employed under conditions where no external oxygen is supplied.
    The new insights into toluene degradation under anaerobic conditions and benzene degradation coupled to chlorate reduction, as described in this thesis, can be applied for the improvement or development of in situ bioremediation strategies for BTEX contamination.
    Modelling maximum adsorption capacities of soot and soot-like materials for PAHs and PCBs
    Noort, P.C.M. van; Jonker, M.T.O. ; Koelmans, A.A. - \ 2004
    Environmental Science and Technology 38 (2004)12. - ISSN 0013-936X - p. 3305 - 3309.
    aromatische koolwaterstoffen - polycyclische koolwaterstoffen - adsorptie - sorbaten - hydrofobiciteit - organische verbindingen - monitoring - waterkwaliteit - waterbodems - aromatic hydrocarbons - polycyclic hydrocarbons - adsorption - sorbates - hydrophobicity - organic compounds - monitoring - water quality - water bottoms - polycyclic aromatic-hydrocarbons - hydrophobic organic-chemicals - partition-coefficients - aqueous solubilities - black carbon - sorption - water - sediment - extraction - biphenyls
    Recent studies have shown that not partitioning but adsorption is the main mechanism for sorption of hydrophobic organic compounds to soot and soot-like materials. For compounds that adsorb by van der Waals forces only, variation in soot-water distribution coefficients will result from differences in these forces for adsorption, as well as the maximum number of accessible sites. This maximum number of accessible sites may a priori be expected to vary due to differences in both sorbent characteristics and sorbate dimensions. In this modeling study, variation in maximum adsorption capacities is explained from sorbent and sorbate properties. Maximum adsorption capacities were calculated using (a) literature values for soot-water distribution coefficients for polycyclic aromatic hydrocarbons and polycholorobiphenyls on 10 different soot and soot-like materials and (b) Langmuir affinities for adsorption at a carbonaceous surface estimated using a recently reported method
    Recent studies have shown that not partitioning but adsorption is the main mechanism for sorption of hydrophobic organic compounds to soot and soot-like materials. For compounds that adsorb by van der Waals forces only, variation in soot-water distribution coefficients will result from differences in these forces for adsorption, as well as the maximum number of accessible sites. This maximum number of accessible sites may a priori be expected to vary due to differences in both sorbent characteristics and sorbate dimensions. In this modeling study, variation in maximum adsorption capacities is explained from sorbent and sorbate properties. Maximum adsorption capacities were calculated using (a) literature values for soot-water distribution coefficients for polycyclic aromatic hydrocarbons and polychlorobiphenyls on 10 different soot and soot-like materials and (b) Langmuir affinities for adsorption at a carbonaceous surface estimated using a recently reported method. The variation in maximum adsorption capacities could be explained by the variation in sorbent specific surface area, sorbent organic carbon content, and the sorbent-sorbate contact area. Furthermore, increasing sorbate thickness was related to a decrease in maximum adsorption capacities, which points to adsorption in micropores. Maximum adsorption capacities decreased by 1-2 orders of magnitude as the contact area increased by 50%. This points to adsorption sites being hardly larger than sorbates.
    Voeding en gezondheid - toxische stoffen in de voeding
    Rietjens, I.M.C.M. ; Alink, G.M. - \ 2003
    Nederlands Tijdschrift voor Geneeskunde 147 (2003)48. - ISSN 0028-2162 - p. 2365 - 2370.
    voeding - toxinen - voedselbesmetting - verontreiniging - zware metalen - dioxinen - polychloorbifenylen - aromatische koolwaterstoffen - polycyclische koolwaterstoffen - aminen - acrylamiden - nutrition - toxins - food contamination - pollution - heavy metals - dioxins - polychlorinated biphenyls - aromatic hydrocarbons - polycyclic hydrocarbons - amines - acrylamides
    With respect to food, the most important factors causing adverse health effects are: an unbalanced diet, overconsumption of alcohol or fat, the presence of microbial contamination and the presence of natural toxines. Two additional factors, the presence of environmental contaminants and products formed on heating food, may also be of importance. It is generally assumed that, when combined, food-related factors contribute to around 35% of overall cancer incidence
    Validatie van biomarkers voor de bepaling van effecten van doorvergiftiging in terrestrische fauna : effecten van TCDD en PAK in de huisspitsmuis (Crocidura russula)
    Bosveld, A.T.C. ; Bie, P.A.F. de - \ 2001
    Wageningen : Alterra (Alterra-rapport 154) - 75
    crocidura russula - benzopyreen - aromatische koolwaterstoffen - toxische stoffen - dioxinen - toxicologie - verontreiniging - biochemische merkers - biologische indicatoren - ecotoxicologie - crocidura russula - benzopyrene - aromatic hydrocarbons - toxic substances - dioxins - toxicology - pollution - biochemical markers - biological indicators - ecotoxicology
    Om eventuele effecten van PAK's bij kleine zoogdieren uit de wilde fauna te kunnen herkennen is in een viertal blootstellingstudies onderzocht wat de effecten zijn van benzo[a]pyreen (BaP) of een mengsel van negen verschillende polycyclische aromatische koolwaterstoffen (PAK). Daarnaast is het effect onderzocht van tetrachloordibenzo-p-dioxine (TCDD), als modelstof voor de veroorzakers van Ah receptor gemedieerde effecten. Het onderzoek is verricht aan de huisspitsmuis (Crocidura russula), een algemeen in het wild voorkomend klein zoogdier. In de verschillende studies zijn het verloop van het lichaamsgewicht en verschillen in orgaangewichten beschouwd naast de effecten op cytochroom P450 enzymen (EROD, MROD, PROD, BROD en specifieke testosteronhydroxylases) en de histopathologie van de geslachtsorganen.
    Ecologische risico's van bodemverontreinigingen in toemaakdek in de gemeente De Ronde Venen
    Bosveld, A.T.C. ; Klok, T.C. ; Bodt, J.M. ; Rutgers, M. - \ 2000
    Wageningen : Alterra (Alterra-rapport 151) - 92
    sorex araneus - limosa limosa - lumbricus - vuilnis - bodemverontreiniging - zware metalen - lood - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - bodemfauna - milieueffect - aardwormen - vogels - zoogdieren - nederland - ecotoxicologie - bioaccumulatie - utrecht - veenweiden - sorex araneus - limosa limosa - lumbricus - refuse - soil pollution - heavy metals - lead - polycyclic hydrocarbons - aromatic hydrocarbons - soil fauna - environmental impact - earthworms - birds - mammals - netherlands - ecotoxicology - bioaccumulation - utrecht - peat grasslands
    De gemeente De Ronde Venen ligt in het gebied de Venen in het Groene Hart. De veengronden in dit gebied zijn vanaf de achttiende eeuw regelmatig opgehoogd met stadsafval om hun draagkracht en vruchtbaarheid te verbeteren. De opgebrachte laag wordt toemaakdek genoemd. In dit toemaakdek komen hoge concentraties zware metalen en PAK voor. Om de ecologische risico's van deze verontreinigingen in kaart te brengen is onderzoek gedaan naar de concentraties van de afzonderlijke verontreinigingen en de effecten die deze verontreinigingen hebben op bodemorganismen en daarvan voor hun voedsel afhankelijke vogels en zoogdieren.
    Ruimtelijke patronen van zware metalen en PAK's in de gemeente Utrecht; uitbreiding Leidsche Rijn en weglating verdachte locaties
    Bierkens, M.F.P. - \ 1999
    Wageningen : DLO-Staring Centrum - 45
    nederland - aromatische koolwaterstoffen - polycyclische koolwaterstoffen - koper - lood - zink - bemonsteren - cartografie - geografische informatiesystemen - utrecht - bodemkwaliteit - geostatistiek - netherlands - aromatic hydrocarbons - polycyclic hydrocarbons - copper - lead - zinc - sampling - mapping - geographical information systems - utrecht - soil quality - geostatistics
    Kaarten van de Gemeente Utrecht met gehalten aan koper, lood, zink, PAK's, humus en lutum, alsmede de bijbehorende betrouwbaarheidskaarten, zijn uitgebreid met het gebied Leidsche Rijn, ten westen van Utrecht. Naast kaarten op basis van alle monsterlocaties (feitelijke bodemkwaliteit), zijn ook kaarten gemaakt gebaseerd op monsters van onverdachte locaties (gebiedseigen bodemkwaliteit). De gebruikte geostatistische methode houdt rekening met voorinformatie zoals bodemtype of landgebruik en gaat op een statistisch correcte wijze om met problematisch waarnemingsmateriaal, zoals waarden onder de detectielimiet, gemengde monsters en monsters die preferent op vervuilde plaatsen zijn genomen. Aan de hand van de gehaltekaarten, percentielenkaarten en betrouwbaarheidskaarten kan bodembeleid worden uitgevoerd dat rekening houdt met de onzekerheid van de geschatte gehalten.
    Remediation of soils, sediments and sludges by extraction with organic solvents
    Noordkamp, E.R. - \ 1999
    Agricultural University. Promotor(en): W.H. Rulkens; J.T.C. Grotenhuis. - S.l. : S.n. - ISBN 9789058080714 - 143
    slib - volksgezondheidsbevordering - aromatische koolwaterstoffen - aceton - oplosmiddelen - extractie - waterbodems - sludges - sanitation - aromatic hydrocarbons - acetone - solvents - extraction - water bottoms

    Remediation of contaminated soils, sediments and sludges by extraction with organic solvents is still in the initial stages of development. So far hardly any scientific research has been carried out into this approach. Therefore, the main objective of the present investigation was to study the effect of several process parameters on the efficiency of the extraction step of a solvent extraction process for soils, sediments and sludges (generally indicated as solids) contaminated with polycyclic aromatic hydrocarbons (PAHs). The process parameters of interest were: type of solvent, extraction time, extraction procedure, water content of the extraction solvent, type of solids, size of the solid particles, and water content of the solids.

    In the development of a remediation method, a critical factor is the analysis of the PAH concentration in the solids. Therefore, several extraction methods were investigated to remove PAHs from an aged sandy soil, a harbour sediment and an extraction sludge. Extraction with N-methyl-2-pyrrolidinone in a microwave oven at 130°C for one hour was the most efficient method for the extraction of PAHs. The second best method was extraction with a mixture of 80 vol.% acetone and 20 vol.% water in a microwave oven at 100°C for one hour.

    Acetone is suitable for use in a solvent extraction process, because it is less toxic than most other organic solvents, easily biodegradable, and socially acceptable in the Netherlands. In addition, it showed high removal efficiencies in the extraction of spiked pyrene and benzo[a]pyrene from sandy, silty and clayey soils, even when water was present in the soil or extraction agent. In the extraction of air-dried soil with mixtures of acetone and water, maximum efficiencies were reached with 10 to 20 vol.% water in the mixture.

    To determine the extraction efficiency of a mixture of 80 vol.% acetone and 20 vol.% water, a procedure comprising seven extraction steps was carried out at 20°C using a rotary tumbler for mixing. In this way, an aged extraction sludge and an aged harbour sediment were remediated. After seven extractions, the Dutch target level established for clean soil was reached in the sludge and almost reached in the sediment. The PAH concentrations decreased from about 1,000 mg kg -1in the sludge and about 650 mg kg -1in the sediment to about 2 mg kg -1in both the sludge and the sediment. Of the PAHs studied, those of low molecular weight (phenanthrene, anthracene and fluoranthene) were found to be most difficult to remove from the solids.

    Another important aspect of the extraction process is the rate at which PAHs are removed. Experiments revealed that about 90% of the PAHs concerned were desorbed from the aged sludge within 10 minutes of extraction with a mixture of 80 vol.% acetone and 20 vol.% water. Within 40 minutes, at least 95% of the PAHs concerned were removed from the sludge and the extraction was then assumed to be complete. The experimental desorption curves were fitted by means of a radial diffusion model and a first-order reaction model. The diffusion model fitted the curves best for a situation in which it is assumed that the PAH contamination is accumulated in the core of the sludge particles.

    Because of the high extraction efficiencies and high desorption rates attained, realisation of the solvent extraction process with acetone is probably worthwhile. This process is especially suitable for the remediation of sediments and sludges containing high amounts of water and clay and contaminated with barely (bio)available or biodegradable organic contaminants.

    Use of soft data in a GIS to improve estimation of the volume of contaminated soil.
    Hendriks, L.A.M. ; Leummens, H. ; Stein, A. ; Bruijn, P.J. de - \ 1998
    Water Air and Soil Pollution 101 (1998). - ISSN 0049-6979 - p. 217 - 234.
    bodemchemie - anorganische verbindingen - mineralen - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - geografische informatiesystemen - geostatistiek - soil chemistry - inorganic compounds - minerals - polycyclic hydrocarbons - aromatic hydrocarbons - geographical information systems - geostatistics
    Ruimtelijke interpolatie van zware metalen en PAK's ten behoeve van de bodemkwaliteitskaart van de Gemeente Utrecht
    Bierkens, M.F.P. - \ 1997
    Wageningen : DLO-Staring Centrum - 73
    bodem - zware metalen - fysicochemische eigenschappen - bodemeigenschappen - bodemchemie - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - stedelijke gebieden - nederland - geostatistiek - utrecht - gemeenten - soil - heavy metals - physicochemical properties - soil properties - soil chemistry - polycyclic hydrocarbons - aromatic hydrocarbons - urban areas - netherlands - geostatistics - municipalities - utrecht
    Voor de gemeente Utrecht is een geostatistische methode ontwikkeld om gehaltekaarten en achtergrondgehaltekaarten te maken voor koper, lood, zink, polycyclische aromatische koolwaterstoffen (PAK's), humus en lutum, alsmede kaarten van de bijbehorende schattingsvarianties (betrouwbaarheidskaarten). De methode houdt rekening met voorinformatie zoals bodemtype of landgebruik en gaat op een statistisch correcte wijze om met problematisch waarnemingsmateriaal, zoals waarden onder de detectielimiet, gemengde monsters, en monsters die preferent op vervuilde plaatsen zijn genomen. Aan de hand van de gehaltekaarten en betrouwbaarheidskaarten kan bodembeleid worden uitgevoerd dat rekening houdt met de onzekerheid van de geschatte gehalten.
    Toepassingsmogelijkheden van schimmeltechnologie bij verontreinigde baggerspecie
    Toorn, A. van den; Gerrits, J.P.G. ; Dijk-Hooyer, O.M. van; Harmsen, J. ; Wieggers, H.J.J. - \ 1997
    Wageningen : DLO-Staring Centrum - 77
    havens - bagger - stort - sediment - modder - volksgezondheidsbevordering - bodem - decontaminatie - schoonmaken - microbiële afbraak - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - mineraaloliën - industrie - schimmels - mycologie - capaciteit - toepassingen - Nederland - petrochemische industrie - harbours - dredgings - spoil - sediment - mud - sanitation - soil - decontamination - cleaning - microbial degradation - polycyclic hydrocarbons - aromatic hydrocarbons - mineral oils - industry - fungi - mycology - capacity - applications - Netherlands - petrochemical industry
    Landelijk meetnet bodemkwaliteit; resultaten 1994
    (RIVM) Groot, M.S.M. ; (RIVM) Bronswijk, J.J.B. ; (RIVM) Willems, W.J. ; Haan, T. de; (AB-DLO) del Castilho, P. - \ 1997
    Bilthoven : RIVM - 151
    bodem - zware metalen - bodemverontreiniging - gezondheid - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - gewasbescherming - pesticiden - pesticidenresiduen - persistentie - grondanalyse - nederland - soil - heavy metals - soil pollution - health - polycyclic hydrocarbons - aromatic hydrocarbons - plant protection - pesticides - pesticide residues - persistence - soil analysis - netherlands
    Het Landelijk Meetnet Bodemkwaliteit (LMB) heeft als primaire doelstelling de trendmatige veranderingen na te gaan in de kwaliteit van de bodem ten gevolge van diffuse belasting van de bodem. Het object van onderzoek is de toplaag van de bodem (0-10 cm); daarnaast wordt ook een diepere bodemlaag en het bovenste grondwater onderzocht. Het LMB wordt in samenwerking met LEI-DLO en AB-DLO uitgevoerd. Jaarlijks worden een 2-tal combinaties van bodemgebruik en grondsoort bemonsterd, bestaande uit ca. 20 locaties per combinatie. In 1993 is gestart met de bemonstering van landbouwgrond op zandgrond. Voor het bodemgebruik landbouw is het bedrijf de schaal van de locatie. De categorieen die in 1994 zijn onderzocht, zijn melkveehouderijbedrijven met een groot aandeel intensieve veehouderij op zandgrond en bos op zandgrond. Naast algemene kwaliteitsparameters zijn parameters onderzocht die gerelateerd zijn aan de milieuthema's vermesting en verspreiding. Vermestingsparameters zijn fosfaat (bodem en grondwater) , nitraat, kalium en ammonium (grondwater). Voor verspreiding zijn zware metalen onderzocht (bodem en grondwater). Voorts zijn bodemgehalten aan PAK en een aantal organochloorbestrijdingsmiddelen (OCB) bepaald.
    Gifstoffen in het rivierengebied; een belemmering voor natuurontwikkeling?
    Hendriks, J. ; Jonge, J. de; Besten, P. den; Faber, J. - \ 1997
    Landschap : tijdschrift voor landschapsecologie en milieukunde 14 (1997)4. - ISSN 0169-6300 - p. 219 - 233.
    toxicologie - chemicaliën - rivieren - waterlopen - kanalen - water - oppervlaktewater - waterverontreiniging - waterkwaliteit - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - toxicology - chemicals - rivers - streams - canals - surface water - water pollution - water quality - polycyclic hydrocarbons - aromatic hydrocarbons
    Contaminant variability in a sedimentation area of the river Rhine = Variabiliteit van verontreinigingen in een sedimentatiegebied van de Rijn
    Winkels, H.J. - \ 1997
    Agricultural University. Promotor(en): S.B. Kroonenberg; L. Lijklema. - Lelystad : Directoraat Generaal Rijkswaterstaat, Directie IJsselmeergebied - ISBN 9789036912105 - 161
    rivieren - waterlopen - kanalen - geologische sedimentatie - oppervlaktewater - waterverontreiniging - waterkwaliteit - stroomgebieden - drainage - milieueffect - schade - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - binnenwateren - rijn - waterbodems - rivers - streams - canals - geological sedimentation - surface water - water pollution - water quality - watersheds - drainage - environmental impact - damage - polycyclic hydrocarbons - aromatic hydrocarbons - inland waters - river rhine - water bottoms

    Aquatic sediments in sedimentation zones of major rivers are in general sinks for pollutants. The sedimentation zone Ketelmeer/IJsselmeer is an important sink for contaminants of the river Rhine (i.e. river IJssel). Recent and historical pollution interact here. Redistribution of suspended solids and erosion of deposited sediment in the shallow Dutch lakes (due to wave action) are likely to change contamination levels of sediments in these lakes, which is the subject of this thesis. The aim of this research was to study and explain the variability of contaminants in the sedimentation area Ketelmeer/IJsselmeer in order to predict the fate of the contaminants in the future. For this purpose a number of methodologies and models were developed and/or adapted.

    Chapter 2 describes the collection and analysis of sediment cores, top-layer sediments and geologically different layers in Lake Ketelmeer. Sediment cores were sectioned into thin slices and the year of deposition of each layer was determined using radio-chemical analyses. The contaminant concentrations were plotted versus the year of deposition of each sediment layer to (re-)construct the history of contamination. Similar vertical changes in contaminant concentrations were found as in a number of sediment cores sampled in sandpits in Lake Ketelmeer. Further, differences in concentration between the top-layer sediments and the degree of contamination in the entire recent IJsselmeer deposits (IJm-deposits) of Lake Ketelmeer were found. The older Zuiderzee deposits (Zu-deposits) underlying the IJm-deposits have low background values for heavy metals, PAHs and PCBs. This indicates that downward transport of these contaminants with infiltrating water is negligible in this lake. The concentrations of metals and PAHs in the sediment cores reflect, without any serious alterations, the historical input of the past five decades. The pollution history is characterized by, in the early 1940s, low concentrations of metals and already elevated levels of PAHs; a possible reduction of these contaminants during the Second World War and attaining, their highest levels between 1955 and 1970. Rather low levels occur in recently deposited sediments, some of which are the lowest ever observed over the last five decades (Pb, As, and all studied PAHs). Almost all chlorinated compounds showed a certain decline in concentration in anaerobic sediments as compared to samples of the top-layer collected in 1972 and stored in the laboratory, which still reflect the original pollution input. For several PCBs this decline proved to be significant; it may have been caused by microbial dechlorination reactions in the anaerobic sediment. Consequently, the concentration profiles of the chlorinated compounds do not reflect the original pollution history directly. Despite the attenuation of concentration, peaks in PCB concentration profiles were still observed. The following trends in concentrations of PCBs can be currently observed in Lake Ketelmeer sediment:
    - Almost all PCBs studied had rather low concentrations in the early 1940s.
    - The highest levels of PCBs occurred between 1960 and 1975.
    - Recently deposited sediments also have elevated levels of PCBs as compared to the levels in layers from the early 1940s.
    Overall, recently deposited material is far less-polluted than sediment deposited in the 1960s and 1970s. These findings prove that in this lake older, highly polluted sediments are buried under a younger, less- polluted layer. However, at some locations, such as in dredged parts or erosive zones, the highly polluted layers may remain uncovered, so aquatic organisms may still be exposed to highly polluted sediments from the 1960s and 1970s through the benthic food chain.

    In Chapter 3 attention is focused on the distribution and geochronology of the sediments of Lakes Ketelmeer and IJsselmeer The concentrations of metals, PCBs, PAHs and various sediment characteristics were determined in 77 samples of the surface sediments and one 3 in core of both lakes. Absolute concentrations of these pollutants were normalized for sediment composition (e.g. clay fraction and organic matter contents). In Lake IJsselmeer the youngest geological layer (IJm-deposit) is mainly found in deep sedimentation areas (25%). This deposit is severely polluted in Lake Ketelmeer (Chapter 2). Concentrations of all polluting compounds in the IJm-deposit of Lake Ketelmeer proved to be 1.6 - 9 times higher than in Lake IJsselmeer Concentrations in the same deposit in Lake IJsselmeer were 2 - 4 times higher than those in the older sandy sediments of the lake. Concentrations of heavy metals, As and PCBs initially increase with depth, but then decrease to lower or even background levels. This corresponds with the inputs of the river IJssel). during the past five decades. As the distance from the river mouth (i.e. Lake Ketelmeer) to Lake IJsselmeer increases, there is a decrease in the degree of pollution in this (IJm-deposit) The hypothesis is developed that primary production (with related calcite formation) and mixing with eroded sediment from elswhere in Lake IJsselmeer are together responsible for this dilution.

    Chapter 4 describes the core sampling and analysis for two similar sedimentation zones of two major river deltas. Uniformly soft anoxic sediments in the Volga and Danube deltas were collected, using satellite images, which reflect the concentration of suspended solids. Cesium-isotope dating and measurement of the concentration profiles of heavy metals and PAHs, which reflect (without serious alterations) the historic pollution input into these rivers, were used in the comparison. The contents of the 7 PCBs investigated and of cadmium were below the detection limits for all sediment samples in the Volga and Danube deltas. Low, more or less constant concentrations of arsenic, copper, zinc and all studied PAHs were observed in sediments of the last five decades in the Volga river. Nickel concentrations in Volga delta sediments were rather high, and recently deposited sediments seemed to show slightly increasing levels for zinc, chromium and arsenic. The pollution history of the Danube is characterized by low concentrations of metals but elevated PAH levels in the early 1940s; increasing levels of metals and PAHs between 1950 and 1987; and decreasing levels in more recently deposited sediments. When comparing the concentrations of heavy metals, PAHs and PCBs in the aquatic sediments of the rivers Rhine, Danube and Volga deltas for the past five decades it is evident that the Volga delta was, and still is, the cleanest of the three. A combination of natural (background) inputs, industrial inputs and man made technical changes in the river systems (like the building of storage lakes) can explain most differences in the historical contaminant profiles of the three deltas. Nowadays the concentrations of heavy metals (except copper and nickel), PAHs and PCBs in sediments of the river Rhine are still higher than in the other two rivers, but the sediment loading rate for heavy metals (except cadmium and zinc) of the Danube is higher than for the other two rivers.

    In Chapter 5 the geostatistical sampling approach chosen for Lake Ketelmeer is explained. When monitoring contaminants and related sediment characteristics in an aquatic environment, their spatial variability needs to be taken into account. The sampling strategy covered short-distance variability (65 m) and long-distance variability (500 m) of the investigated variables. In Lake Ketelmeer we chose three sub-areas. The distances between sampling points takes into account the size of each sub-area. With this approach the number of sampling points needed to monitor trends of contaminants in sediments can be minimized, taking into account the necessary accuracy. The choice of sampling strategy for monitoring sub-areas, characterised by either water depth, sedimentation/erosion behaviour or sediment type, will result in different sampling spacings. For example, in Lake Ketelmeer the optimal sampling distance for monitoring Benzo(A)pyrene (BAP) in the central part of the lake was larger than near the harbour and shore, where gradiënts in water depth are steeper. Thus, when designing a dredging programme to remove seriously contaminated sediments, the identification of sub-areas is essential to ensure the adequate dredging of the sediments. If spatial variability is not taken into account for dredging contaminated layers, seriously contaminated spots may be overlooked or rather clean sediments may be dredged needless. Thorough (although expensive) spatial investigations of the contaminated layer before dredging starts, identifying critical sub-areas, is therefore recommended. Practical, cost-effective, geostatistical methods allow an efficient use of limited financial resources for monitoring aquatic sediments.

    Another important process affecting sediment concentration profiles is consolidation. Chapter 6 deals with this physical process of settling of suspended solids and the loss of water after deposition. Consolidation in principle can be described by mathematical models, but because of local circumstances in the Lake IJsselmeer area an empirical approach seemed more reliable. Five representative cores of the (IJm- deposit) were taken from deep zones of the lakes. Periodic water depth surveys at these locations over the last sixty years provided information on the net sedimentation rate and total thickness of the (IJm-deposit) at known time intervals. To calculate a time-equivalent of the depth scale, correction factors for sediment consolidation were needed. These factors were based on a simplification of the various stages of compression (i.e. 0%, 30% and 45%). A factor n , which represents changes of water content of the sediment as a dependent variable of clay content, was derived for each layer, making it possible to determine the initial, uncompressed thickness of each layer by an inverse calculation procedure. Hence, a fairly reliable time-scale for depth could be reconstructed. This time-scale was compared with radio-isotope-dated layers and the results showed close consistency.

    Annual variability of contaminants in the IJsselmeer area is described in Chapter 7. Measurements of the concentrations of six heavy metals in suspended solids, discharged by the river IJssel). and settling solids at two locations in Lake IJsselmeer showed a typical spatial gradient. The heavy metals concentrations decreased with increasing distance from the river IJssel). inlet. This spatial gradient corresponds with gradients observed in the bottom sediment (Chapter 3). Measurements in sediment cores from Lake Ketelmeer, i.e. the river mouth, and the central part of Lake IJsselmeer showed that the heavy metals concentration in sediments, deposited during the same periods, is 2 to 3 times higher in Lake Ketelmeer than in Lake IJsselmeer The concentration gradient in the settling solids is still significant when changes in the clay and organic matter content are accounted for by using normalized metal concentrations. A rough sediment mass balance for heavy metals, based on river input data and observed sedimentation fluxes, indicates that the total internal sedimentation fluxes of heavy metals in Lake IJsselmeer far exceed the external aereal loading by the river. Due to the complexity of the relationships between the measured variables, the heavy metal concentrations and variables related to primary production and erosion, single correlation analysis did not reveal clear relations and processes that could explain this dilution. Principal component analysis and stepwise multiple regression however showed that the variation in the heavy metals concentration in settling solids is related to wind velocity and clay content, both of which are related to resuspension/erosion of sediments; or alternatively, to pH, chlorophyll and CaCO 3 , which are in turn related to algal growth in the lake. Resuspension/erosion-related variables are the dominant factors explaining the variation in heavy metals concentration in the southern part of Lake IJsselmeer whereas algal-growth-related variables explain most of the variation in the metal concentrations in settling solids in the central part of the lake. In this central part, where algal concentrations are high, the negative relation between the concentration of most of the heavy metals in settling solids and the chlorophyll and organic matter concentration in the water compartment justifies the conclusion that dilution of contaminated suspended solids by primary production is active there. In the southern part of the lake, the heavy metals concentration is positively related to wind velocity and clay content. This indicates that resuspension of recent deposits contributes to the metal concentrations of the water compartment. The gradual increase in the δ 13C value with distance from the river IJssel). mouth (passing from the southern to the central part of Lake IJsselmeer shows that the freshwater sediments are mixed with eroded Zu deposits from within the lake, which were originally saline. It can be concluded, therefore, that the decrease in the heavy metals concentrations in suspended and bottom sediments in Lake IJsselmeer is the result of dilution of contaminated sediments due to erosion of old sediments and of primary production related to algal growth.

    In Chapter 8 the specific knowledge reported in all previous chapters on sediment contamination, dilution, transport and processes like consolidation and primary production occurring in this area, is integrated in a modeling framework. The sediment distribution processes and the related contaminant dilution of previous decades has been reconstructed. This exercise was the basis for prediction of the future sediment composition and contamination resulting from the expected changes in the contaminant load of the river IJssel). The adapted model STRESS-2d gives a reconstruction of resuspension, erosion, sedimentation and horizontal sediment transport processes, resulting in a good simulation of total-suspended-solids concentration and of the sediment fluxes. This model was nested into the more aggregated model DIASPORA, which simulates changes in morphology (net sedimentation fluxes) quite well. It also accounted for the effects of biogenic calcite production and the consolidation of sediment layers. The decrease of the lead concentration in sedimentation areas in Lake IJsselmeer reflects the reduced suspended input from the river IJssel). but this is dominated by the effects of internal redistribution of sediments in the area. The modelling results support the above mentioned conclusion of Chapter 7. Simulation of sediment fluxes and future contamination by lead are simulated for a constant sediment input and a 50% reduction of lead input from the river IJssel). Modelling results show a steady decrease of the lead concentration in sedimentation zones of Lake IJsselmeer and a purification effect in these zones due to the dredging of sediments in Lake Ketelmeer.

    Biomarkers for the assessment of exposure and sublethal effects of phahs in wildlife : with special emphasis on the aquatic food chain
    Murk, A.J. - \ 1997
    Agricultural University. Promotor(en): J.H. Koeman; A. Brouwer. - S.l. : Murk - ISBN 9789054856368 - 192
    milieu - verontreiniging - monitoring - biologische indicatoren - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - environment - pollution - monitoring - biological indicators - polycyclic hydrocarbons - aromatic hydrocarbons

    The research described in this thesis concerns the effects of polyhalogenated aromatic hydrocarbons (PHAHs) on wildlife species, and the development of biomarkers for exposure and effect based on the mechanism of toxic action of PHAHs. In addition, attention is paid to differences in vulnerability between species with regard to exposure, kinetics and (eco)physiology, and the possible impact of current PHAH levels in The Netherlands on fish-eating wildlife species. The research was performed within the framework of the ecotoxicological research program of the department of Toxicology.

    Natuurlijke afbraak van polycyclische aromatische koolwaterstoffen bij het op de kant zetten van baggerspecie op Goeree - Overflakkee
    Toorn, A. van den; Harmsen, J. ; Dijk - Hooijer, O.M. van - \ 1996
    Wageningen : DLO-Staring Centrum (Rapport / DLO-Staring Centrum 447) - 74
    aromatische koolwaterstoffen - bioremediëring - bagger - havens - stortterreinen - microbiële afbraak - modder - huisvuilverwijdering - polycyclische koolwaterstoffen - ontginning - vuilnisbelten - sediment - stort - afvalverwijdering - afvalstortplaatsen - Nederland - sludge farming - zuid-holland - zuidhollandse eilanden - aromatic hydrocarbons - bioremediation - dredgings - harbours - landfills - microbial degradation - mud - municipal refuse disposal - polycyclic hydrocarbons - reclamation - refuse tips - sediment - spoil - waste disposal - waste disposal sites - Netherlands - sludge farming - zuid-holland - zuidhollandse eilanden
    Een belangrijk deel van de waterbodemverontreiniging wordt veroorzaakt door polycyclische aromatische koolwaterstoffen (PAK). Vroeger werd de opgebaggerde waterbodem verspreid op landbouwpercelen. Voor sterk verontreinigde baggerspecie is deze procedure niet meer toegestaan. Momenteel mag licht verontreinigde baggerspecie op de kant worden verspreid, maar ook dit zal binnenkort worden verboden. Op Goeree-Overflakkee is onderzocht of op de kant zetten van baggerspecie heeft geleid tot een hoger PAK-gehalte naast de sloten. Hierbij is onderscheid gemaakt in korte- en langetermijneffecten. Op een proefstrook is nagegaan wat er met de PAK gebeurt nadat de baggerspecie op de kant is gezet (periode 1-2 jaar).
    Mogelijkheden van extensieve landfarming voor biologische reiniging van grond
    Harmsen, J. ; Akker, J.J.H. van den; Beurskens, A.G.C. ; Dijk-Hooyer, O.M. van; Ma, W. ; Wieggers, H.J.J. - \ 1996
    Wageningen : DLO-Staring Centrum - 114
    bioremediëring - ontginning - microbiële afbraak - bodemfysica - grondmechanica - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - sludge farming - waterbodems - bioremediation - reclamation - microbial degradation - soil physics - soil mechanics - polycyclic hydrocarbons - aromatic hydrocarbons - sludge farming - water bottoms
    Bioavailability and biodegradation of polycyclic aromatic hydrocarbons
    Volkering, F. - \ 1996
    Agricultural University. Promotor(en): W.H. Rulkens; A.M. Breure. - S.l. : S.n. - ISBN 9789054855750 - 175
    polycyclische koolwaterstoffen - aromatische koolwaterstoffen - microbiële afbraak - biologische beschikbaarheid - biodegradatie - bodemchemie - ecotoxicologie - bioaccumulatie - polycyclic hydrocarbons - aromatic hydrocarbons - microbial degradation - bioavailability - biodegradation - soil chemistry - ecotoxicology - bioaccumulation
    One of the main problems in biological soil remediation is the slow or incomplete degradation of hydrophobic organic pollutants. The principal reason for this problem is the fact that these compounds bind strongly to the soil matrix or occur as a separate non- aqueous phase in the soil. As most microbiological processes take place in the water phase, transport of the polluting compound to this phase is essential for biodegradation to occur. When this transport is the limiting factor in the biodegradation process, this is termed limiting bioavailbility.

    This thesis deals with the effect of bioavailability on the biodegradation of polycyclic aromatic hydrocarbons (PAHs). PAHs are hydrophobic organic pollutancs that are abundantly present in contaminated soils and give raise to ervironmental concern because of their toxicity and mutagenicity. Most PAHs are degradable by microorganisms and the important biochemical aspects of the PAH-degradation hav been revealed. PAHs are nevertheless considered persistent pollutants in soil, a fac that is attributed to their limited bioavailability.

    The first part of the research consisted of the isolation of bacteria capable of degrading the PAHs, naphthalene, phenanthrene, and anthracene. Subsequently a number of isolated bacterial strains were grown in batch and continuous cultures to determine the most important microbial growth parameters, such as the maximum growth rate, the Monod saturation constant, and the bacterial growth yield.

    The effect of bioavailability on the biodegradation of PAHs was studied in two model systems: (i) crystalline PAHs and (ii) PAHs bound to a matrix.

    For studying the bioavailability of crystalline PAHs the results of dissolution and biodegradation experiments were compared. In the degradation experiments it was found that two phases could be observed during batch growth: an exponential growth phase, followed by a linear growth phase, in which biomass formation was limited by the availability of the PAHs. By using a model in which Monod kinetics for bacterial growth were coupled to dissolution kinetics for substrate availability, it was shown that the observed degradation rates were matched by the rates of dissolution of the PAHs to the aqueous phase. Therefore it was concluded that in this system only aqueous phase PAHs were available for bacterial uptake and that the bioavailability of the PAHs was not directly stimulated by the presence of the microorganisms.

    With matrix-bound PAHs desorption and biodegradation experiments were conducted. The first matrices studied were the synthetic porous resins XAD4 and XAD-7. The desorption of naphthalene from these materials was studied in batch and continuous desorption experiments. The results from these experiments could be described using a two-compartment model in which the matrix is divided in a fraction with shallow pores and one with deep pores. In biodegradation experiments with naphthalene- loaded resins the same type of batch-growth kinetics was observed as described above for crystalline substrates: exponential growth, followed by a phase in which substrate availability limits the degradation rate. By comparing the results of the desorption experiments and the biodegradation experiments it was shown that the biodegradation proceeded faster than could be explained by desorption alone. Therefore it was concluded that the bacteria had a positive effect on the bioavailability of naphthalene that was adsorbed onto the resins. This effect was not caused by the presence of bacterial excretion products.

    In contrast to this it was found that the biodegradation of soil-bound naphthalene and phenanthrene could be explained by degradation of PAHs present in the aqueous bulk phase only. Thus, the bioavailability of sorbed PAHs depends on the type of matrix the PAHs are sorbed onto.

    The second part of this thesis deals with the most widely applied solution for the problem of limited bioavailability: the application of surface-active agents or surfactants. Surfactants are molecules that usually consist of a hydrophillic and a hydrophobic part. Due to this they have a tendency to concentrate at surfaces and interfaces and to form new interfaces. There are several different ways by which surfactants may increase the bioavailability of hydrophobic compounds in soil:
    - solubilization in the aqueous phase by the presence of micelles, aggregates of 20-200 surfactant molecules with a hydrophobic interior;
    - emulsification of liquid hydrocarbons in the waterphase;
    - facilitated transport, a term that covers several processes, such as mobilisation of pollutant present in soil pores or interaction pollutant with single surfactant molecules;
    Surfactants may also have a negative effect on pollutant bioavailability, for instance by the toxic effect or preferential degradation of the surfactant, or by interference with the natural interactions among microorganisms and pollutant.

    The effect of several nonionic surfactants on the bioavailability of PAHs was studied in the same model systems as described above: crystalline PAHs and PAHs sorbed onto a matrix.

    Dissolution experiments with crystalline naphthalene and phenanthrene showed that the presence of surfactants caused an increase in the apparent solubility and in the maximum dissolution rate of these PAHs. Both phenomena have an effect on the bioavailability of PAHs. Although it was found that micellar PAHs were not readily available for uptake by the bacteria, the transport of PAHs from the micelles is sufficiently fast to allow almost complete exponential growth on solubilized PAHs. The effect on the maximum dissolution rate is probably more important because this is the most relevant factor under bioavailability-limiting conditions. Addition of surfactant to cultures growing on PAH in the dissolution-limited phase resulted in an increase in the linear growth rate. This shows that for crystalline PAHs surfactants can be used to increase the bioavailabilty

    For sorbed naphthalene similar results were found. In desorption experiments it was shown that in the presence of surfactant, the partitioning of naphthalene to the waterphase as well as the maximum desorption rate was increased. Addition of surfactants to cultures growing on sorbed naphthalene in the desorption-limited phase resulted in an increase in the degradation rate. This shows that surfactants can be used for enhancing the bioavailability of sorbed PAHs.

    The first general conclusion from this thesis is that the bioavailability of hydrophobic pollutants in soil is a complex matter and therefore difficult to quantify. In model systems under laboratory conditions, however, it was possible to simulate the essential processes. This experimental work revealed the most important mechanisms that play a role in bioavailability limtations. Because of the large impact of bioavailability on both the performance of biological soil remediation and on the risks posed by soil contamination, it is essential that standard methods be developed which provide criteria for bioavailability. These criteria may be used to predict the results of biological soil remediation processes and may form a basis for soil quality limits in which the bioavailability of the pollutant is considered.

    Secondly, the application of surfactants can be concluded to be a promising option for enhancing the bioavailability of hydrophobic pollutants. In two model sytems it was shown that addition of surfactants speeded up the biological degradation of PAHs markedly and some explanations for this phenomenon have been found. However, to allow the use of surfactants as a standard technique in biological soil remediation, more insight into the complex interactions involved in the introduction of surfactants into soil is necessary.

    Microbial transformation of chlorinated aromatics in sediments
    Beurskens, J.E.M. - \ 1995
    Agricultural University. Promotor(en): A.J.B. Zehnder; J. de Jong. - S.l. : Beursken - ISBN 9789054853954 - 173
    volksgezondheidsbevordering - decontaminatie - schoonmaken - microbiële afbraak - polycyclische koolwaterstoffen - aromatische koolwaterstoffen - waterbodems - sanitation - decontamination - cleaning - microbial degradation - polycyclic hydrocarbons - aromatic hydrocarbons - water bottoms

    Numerous contaminants like heavy metals, polycyclic aromatic hydrocarbons (PAHs), chlorinated benzenes (CBs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo- p -dioxins (PCDDs) and polychlorinated furans (PCDFs) are detected in the major rivers in the Netherlands. These contaminants have relatively low aqueous solubilities and bind substantially to the suspended solids in river water. Due to decreasing stream velocities in the downstream stretches of a river suspended solids will settle to the river bed. Thus, the deposition of suspended solids in the downstream stretches results in transport of contaminants from the water column to the sediments.

    In the past decade widespread contamination of sediments with heavy metals, PAHs, and chlorinated aromatics (CBs, PCBs, PCDDs and PCDFs) has been identified in the Netherlands. The extent and seriousness of sediment contamination is most pronounced in the sedimentation areas of the rivers Rhine, Meuse and Scheldt. The total amount of heavily contaminated sediments in these areas is estimated at more than 100 million m 3. The reduced fertility of cormorants in the Biesbosch, a sedimentation area of the Meuse, can be put forward as an example of the adverse ecological impact of contaminated sediments.

    Microorganisms (bacteria and fungi) play an important role in the decomposition of organic matter in sediments. The diversity of metabolic processes in microorganisms enables them to degrade a variety of organic contaminants as well. Two degradation processes can be distinguished: transformation and mineralization. Transformation is the process by which the chemical structure of the organic substrate (or contaminant) is altered and organic products are formed. Mineralization is the process whereby, besides biomass, only inorganic products (CO 2 , CH 4 , H 2 O, chemical elements) are formed during microbial metabolism.

    In the presence of oxygen, as in the water column of rivers, the microbial degradation of chlorinated aromatic compounds decreases with an increasing number of chlorine atoms in the molecule. The relative persistence of higher chlorinated aromatics in the water column, combined with the hydrophobic properties of the aromatics, results in deposition of these contaminants in sediments. The upper few centimeters of sediment contain oxygen; most of the sediment is anoxic. Higher chlorinated compounds can, particularly under these anoxic conditions, be transformed by anaerobic microorganisms into lower chlorinated compounds. In the last 5 to 10 years microbial dechlorination reactions, i.e. the replacement of chlorines in the contaminant molecule with hydrogen atoms, have been demonstrated in laboratory tests for a variety of chlorinated aromatics. Some of these tests, conducted with anaerobic sediment, suggest that dechlorination reactions may occur in contaminated sediments.

    Determining the occurrence of in situ microbial dechlorination was the general objective of the research described from which the following issues were deduced for application to the compound classes selected, i.e.: CBs, PCBs, PCDDs and PCDFs:
    1) What are the in situ dechlorination rates?
    2) What types of reactions occur and what are the reaction products?
    3) Does a general reaction pattern in dechlorination reactions exist?
    4) What are the consequences of in situ microbial dechlorination in sediments from an environmental point of view?

    A dual approach, including field studies and laboratory experiments, was followed to
    determine in situ reactions.

    Field studies

    The hypothesis that in situ dechlorination would lead to a shift from higher to lower chlorinated compounds in anaerobic sediments was verified in the field studies, which consisted of sediment cores collected from Lake Ketelmeer, a sedimentation area of the Rhine River. The cores were sectioned into thin layers and the year of deposition of each layer was determined using radiochemical analyses. In addition, the concentrations of the chlorinated aromatics were determined in each layer. The contaminant concentrations were plotted against the year of deposition of the individual sediment layers (chapters 2 and 3). The possibility of a shift from higher to lower chlorinated compounds was verified with the aid of historical samples, collected in 1972 from the Lake Ketelmeer sediment top layer, and subsequently dried and stored for almost 20 years. The historical samples were analyzed together with the sediment core layers, thus applying presently available, identical analytical techniques. The historical samples reflect the contaminant concentration at time of deposition in the past (around 1972), either without or with only limited influences of degradation processes. The concentrations of chlorinated aromatics in the historical samples were compared to the concentrations found in sediment core layers deposited around 1970. In this way, the chlorinated aromatic concentrations at the time of deposition are compared with the concentrations after 20 years of "environmental incubation". Six highly toxic PCBs, representing the so-called non-ortho and mono-ortho PCBs that cause dioxin-like toxic effects, were selected and compared with the concentrations in the historical samples. Four of the six PCB congeners showed a significant disappearance in the sediment core layers relative to the historical samples. Increased concentrations of lower chlorinated congeners were not observed, probably due to the limited number of congeners analyzed. Maximum half-lives (t 1/2 ) for the four PCB congeners showing significant disappearances were estimated at about 10 years. Of the 17 chlorinated dioxins and furans studied, only four congeners showed significant disappearances, indicating slow or complete absence of reactions (t 1/2≥ 12 years). Compared to historical input, significant disappearances of hexachlorobenzene (t 1/2≤ 7 years), pentachlorobenzene and 1,2,3,5- tetrachlorobenzene were found. On the other hand, the concentrations of 1,3,5-tri-, 1,2-di-, and 1,3- dichlorobenzene increased significantly as compared to the historical input, indicating their formation in the anaerobic sediment.

    Laboratory studies

    Transformations in Lake Ketelmeersediment

    The field studies make the occurrence of selective disappearances of chlorinated aromatics in Lake Ketelmeer sediment clear, but these observations do not reveal the responsible mechanisms. Therefore laboratory experiments were conducted using Lake Ketelmeer sediment under conditions resembling the field conditions (chapter 3). These experiments demonstrated that the indigenous anaerobic microbial population in Lake Ketelmeer sediment is able to dechlorinate hexachlorobenzene (HCB) to 1,3,5-tri-, and 1,3-dichlorobenzene. Based on the field and laboratory observations it was concluded that in situ microbial dechlorination of HCB in Lake Ketelmeer had occurred, resulting in an 80% loss of HCB. The tri-, and dichlorobenzenes produced are less toxic and have a lower bioaccumulation potentials than the parent compound HCB. Moreover, tri-, and dichlorobenzenes are, opposite to HCB, suitable substrates for aerobic mineralization. The fact that the transformation products are less hydrophobic and therefore can be transported more easily with infiltrating water than HCB can be regarded as a disadvantage of dechlorination reactions in the environment.

    An enrichment culture from Lake Ketelmeer sediment that readily dechlorinates HCB under methanogenic conditions was obtained (chapter 4). Substantial dechlorinating activity was still present at low temperatures (3°C), whereas the optimum temperature for HCB dechlorination was around 30°C. The dechlorinating capabilities of the enrichment culture were verified with a variety of chlorinated aromatics. Individual incubations with 11 chlorinated benzenes demonstrated that from the 19 dechlorination reactions possible, only the seven reactions with the highest energy release took place.

    The enrichment culture was also incubated with a selected number of PCB congeners in accordance with those selected in the field study. The results show a selective removal of chlorines from para and meta positions if the departing chlorine atom was surrounded by chlorines on both sides (chapter 5). PCB 118 does not meet this structural prerequisite and was not dechlorinated by the enrichment. This experimental result is in agreement with the field observation where no significant disappearance of PCB 118 is observed. The structural prerequisite can also be deduced from the observed selectivity in chlorobenzene dechlorination. The correlation with thermodynamics, as observed for chlorinated benzenes, appeared to be valid for PCBs as well. Only the energetically most favourable dechlorination steps are catalyzed by the enrichment, except ortho-dechlorinations.

    The dechlorinating capacity of the enrichment culture for chlorinated dioxins was tested using 1,2,3,4-tetrachloro- p -dioxin (1,2,3,4-TeCDD) as a model compound (chapter 6). Microbial dechlorination of 1,2,3,4-TeCDD resulted in the formation of 1,3- and 2,3-dichlorodibenzo- p -dioxins as the main products. The results suggest that microbial dechlorination. of PCDDs in Lake Ketelmeer sediment might be possible. However, the field data indicate that in situ reactivity of PCDDs is extremely slow or absent. This may be due to extremely low concentrations of PCDDs in porewater and the slow desorption of PCDDs from sediments, resulting in a low microbial availability.

    The similar experimental protocols used for the laboratory incubations with CBs, PCBs and PCDDs allow the calculation of relative dechlorination. rates for these three compound classes. Dechlorination half-lives for tetra-, penta-, and hexachlorobenzene are between 1 and 2 days for 1,2,3,4-TeCDD 15.5 days, and for the tested reactive PCBs, between 10 and 120 days. Remarkable is the difference between half-lives determined in the laboratory and half-lives estimated in the field study (up to a decade). In the contaminated sediments densities of the actively dechlorinating microbial populations may be smaller and activities may be lower due to lower concentrations of co-substrates and/or nutrients and lower temperatures than in the laboratory incubations. In addition, in the laboratory interference of sediment was eliminated; experiments were conducted in liquid media. Binding of the hydrophobic chlorinated aromatics to sediments reduces the concentration in the sediment pore water substantially and the slow release of bound compounds to the pore water may limit in situ dechlorination rates.

    Transformation in coastal sediments

    The microbial dechlorination of HCB in sediment from the Ems estuary is described in chapter 7. Under sulfate-reducing conditions, prevailing in the estuarine sediment, an HCB dechlorinating- culture was obtained. The dechlorination of HCB occurred concomitantly with sulfate reduction but was not directly coupled to sulfate reduction. These results indicate that microbial dechlorination of chlorinated aromatics is not necessarily restricted to fresh water sediments but may also occur in estuarine sediments.

    Environmental significance

    The PCB data from the field and laboratory study support the conclusion that the occurrence of in situ dechlorination of PCBs in Lake Ketelmeer sediment is highly likely. The environmental significance of in situ dechlorination, particularly for those congeners that cause dioxin-like biochemical and toxic effects is the subject of chapter 8. The significant disappearance of four PCB congeners in Lake Ketelmeer sediment, as observed in the field study, has resulted in a 75% decrease in the dioxin-like toxicity during the last 20 years. The selectivity in the dechlorinating activity of the enrichment culture from Lake Ketelmeer was used to predict the reactivity of all non- ortho and mono-ortho PCBs. All congeners, except PCB 77 and PCB 118, appear to be reactive. Dechlorination could result almost exclusively in products that have no dioxin-like toxicity. Other advantages of PCB dechlorination, besides decreased toxicities, are a lowered bioaccumulation potential and dechlorination. products being more suitable substrates for aerobic mineralization than the parent compounds.

    The results of this study have been included in an overview of currently available in situ half-lives of chlorinated aromatics (chapter 9). Although the available data are limited, it is clear that in situ microbial dehalogenation is a relatively slow process that may proceed with half-lives of several years to decades in contaminated sediments. Despite the low rates, it is important to include these transformation processes in long-term sediment-quality prognoses. After all, these transformations with their significant toxicological implications are the essential first steps that may eventually lead to a complete elimination of PCBs from the environment. The correlation between energy yield and catalyzed dechlorination steps is further verified with data from the literature. The thermodynamically most profitable dechlorination steps appear to be catalyzed preferentially, except for ortho-dechlorinations. The correlation with thermodynamics appears to be an appropiate instrument for predicting microbial dechlorination pathways.

    New prospects for the biotechnological clean-up of sediments contaminated with halogenated aromatics have emerged from the results of this study. The clean-up process should include two steps; an anaerobic dehalogenation and a subsequent aerobic mineralization of dehalogenation products. The stimulation of dehalogenation rates and the application of the two-step process on pilot scale are two examples of future challenges.

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