Activated carbon in sediment remediation : benefits, risks and perspectives
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Met lit. opg. - Met samenvatting in het Engels en Nederlands
|Koelmans, Prof. dr. A.A. ; Grotenhuis, Dr. ir. J.T.C.|
|Samenvatting door auteur||
Aquatic sediments form an ultimate sink for legacy contaminants such as hydrophobic organic compounds (HOCs) and may become a source of secondary pollution, thus posing a risk to aquatic organisms and humans. Traditional remediation approaches of contaminated sediment, like dredging and in situ capping, are disruptive to benthic environments, not always efficient, and very expensive. Therefore, new developments in remediation approaches are needed that either supplement or provide less laborious, less expensive, and less disruptive alternatives to existing methods. Over the past years the potential of adding sorptive materials, like activated carbons (AC), to polluted sediment, as a means to reduce aqueous HOC concentrations, has been explored. The current research was meant to increase our understanding of the effects of AC application on HOC exposure and toxicity reduction for benthic species and communities, and to bridge the gap between laboratory and field settings for AC remediation. In Chapter 2, we review the current state of the art for the use of AC as an extensive method for sediment remediation, covering both technical and ecological angles. In the review we discuss how factors such as AC type, particle size and dosage, sediment characteristics, properties of adsorbates, and presence of other carbonaceous materials like BC, OM, and oil affect the AC amendment efficiency to adsorb HOCs and to reduce HOC bioaccumulation in benthic macroinvertebrates. We also review to what extent AC may reduce toxicity of contaminants and whether it has negative effects on benthic species and communities. In the end we discuss if the effects of AC addition can be predicted using fate and transport models. Since sediments and AC types differ in their characteristics, it is highly relevant to identify the affinity parameters for in situ sorption of HOCs to AC in order to be able to design and evaluate applications of AC in sediment remediation. A conceptual framework to assess affinity constants for HOC sorption to BC and AC is provided in Chapter 3. It appears that sorption to BC increases with LogKOW, whereas sorption to AC shows a relatively narrow range of affinity properties with a median Freundlich LogKF,AC value of 7.2 for polychlorinated biphenyls (PCBs) and 8.6 for polycyclic aromatic hydrocarbons (PAHs). Sorption to AC is stronger than to BC for chemicals below LogKOW=6.3–6.6. The developed model and optimized in situ KBC and KAC can be used, when designing an AC dosage, to predict the efficiency of AC amendments. In Chapter 4, effects of three different sediment treatments with AC, viz. powdered AC addition, granular AC addition, and addition and subsequent removal of granular AC (sediment stripping), on PCB pore water concentrations, sediment-to-water fluxes, and mass transfer coefficients, are assessed. Sediment treatment with AC is shown to decrease pore water concentrations of PCBs. Moreover, the efficiency of the AC-treatments decrease in the order PAC > sediment stripping > GAC. AC addition is shown to decrease mass fluxes but increase apparent mass transfer coefficients due to dissolved organic carbon (DOC) facilitated transport across the benthic boundary layer (BBL). The presence of bioturbators may also stimulate DOC facilitated transport of HOC, increase fluxes and thus decrease efficiency of sediment treatment with AC. A dual BBL resistance model, combining AC effects on PCB concentration gradients, DOC facilitated transport and biodiffusion, is evaluated against the experimental data. The analysis of the results illustrates the variation in resistance between different treatments and PCB congeners with different hydrophobicity. In Chapter 5, we provide data on effects of AC addition on locomotion and ventilation, sediment avoidance, mortality, and growth of two benthic species, Gammarus pulex and Asellus aquaticus, in clean versus polycyclic aromatic hydrocarbon (PAH) contaminated sediment. The series of whole-sediment behavioural toxicity tests with clean sediment demonstrates the absence of behavioural responses like locomotion, ventilation or avoidance. Our tests with A. aquaticus identify no negative effects for up to 28 d, whereas the application level of 2-4% AC would affect populations of G. pulex to some extent. It is not likely, however, that this would lead to extinction of this species. Thus, a series of laboratory single species bioassays has shown that sediment treatment with AC reduces the risks associated with HOCs and increases the habitat quality for benthic organisms (Chapter 4 and 5). At the same time, it has been shown that AC itself may have negative impacts on benthic organisms (Chapter 4 and 5). A key question is under what conditions the improvement of habitat quality due to HOC sequestration would compensate for the possible negative effects of AC. In Chapter 6, a conceptual model to quantify the trade-off between the advantageous and the disadvantageous effects of AC on populations of two benthic species of different sensitivity has been presented. The model describes population growth, incorporates concentration-effect relationships for PAHs in the pore water and for AC, and uses an equilibrium sorption model to estimate PAH pore water concentrations as a function of AC dosage. The model is calibrated using bioassay data (Chapter 5) and evaluated by calculating isoclines of zero population growth for two species. The current framework is evaluated for a specific case, but may be used as a tool in risk assessment or when designing sediment remediation scenarios and can be generally applicable to cases where sorbents are added to soils or sediments contaminated with HOCs. AC effects observed in laboratory single species tests may be less severe in field settings, where a better habitat quality may be maintained due to fresh input of nutrients and organic matter. Therefore, long term effects of AC application on benthic communities are investigated by evaluating the recolonization of benthic communities in Chapter 7. Sediment from an unpolluted site is amended with increasing levels of AC, placed in trays and randomly embedded in the original site, which acts as a donor system for recolonization of benthic species. After 3 and 15 months, the trays are retrieved and benthic organisms identified. From the univariate and multivariate statistical analyses, a considerable recovery in terms of species diversity and abundance is observed already after 3 months, and full recovery of the community after 15 months. This is explained by migration of individuals from the donor system, followed by further migration and reproduction of the species in the next year. AC treatments explain 3% of the variance in the community data. Negative trends with AC are detected for Lumbriculidae and Pisidiidae which most probably relate to species-specific life-history traits. So far, the remediation effectiveness and ecological side effects have been studied for the benthic compartment only. It is not clear whether such effects can be observed on the level of an entire aquatic food chain (including fish), in full scale i.e. not laboratory aquatic ecosystems. In Chapter 8, the effects of three different AC treatments, viz. powdered AC addition, granular AC addition, and addition and subsequent removal of granular AC (sediment stripping), on polycyclic aromatic hydrocarbon (PAHs) and polychlorinated biphenyl (PCBs) concentrations in pore water, benthic invertebrates, zooplankton and fish (Leuciscus idus melanotus) are investigated. Sediment treatments with AC result in a significant decrease in freely dissolved PCB and PAH concentrations. Sediment treated with PAC shows a reduction of accumulation of PCBs in fish by a factor of 100, which results in PCB levels in fish below toxic thresholds and had no significant negative effects on fish condition. Bioaccumulation in fish is mainly explained by uptake from water and food (i.e. zooplankton) with fish-zooplankton LogBMFs being up to 1.76 after 3 months of exposure. Sediment amendment with GAC does not yield reductions in bioaccumulation in fish, due to limited reductions of bioaccumulation in zooplankton and invertebrates, whereas components of the aquatic food web are considered to be major uptake pathways for fish. Finally, in Chapter 9, we summarize all benefits and risks associated with AC application, evaluate alternatives for the remediation of contaminated sediments, and give an outlook and recommendations for future studies.
|Trefwoorden (cab)||waterbodems / verontreinigde sedimenten / ecotoxicologie / remediatie / actieve kool / macroinvertebraten / benthos / waterorganismen / bioaccumulatie|
|Milieutechnologie / Milieutoxicologie, ecotoxicologie / Milieuverontreiniging|
|Toelichting||Klassieke verontreinigingen zoals hydrofobe organische verbindingen (HOCs) komen uiteindelijk vaak in waterbodems terecht. Deze waterbodems kunnen hierdoor zelf een bron van verontreiniging worden en zo een risico vormen voor aquatische organismen en voor de mens. Traditionele manieren om waterbodems te reinigen, zoals baggeren en in situ capping, zorgen voor een grote verstoring van het benthische milieu en zijn niet altijd effectief, terwijl zij wel hoge kosten met zich meebrengen. Daarom is het nodig om nieuwe methoden voor reiniging van waterbodems te ontwikkelen die makkelijker zijn, minder kosten en minder verstorend zijn dan de bestaande methoden. De afgelopen jaren is de mogelijkheid onderzocht om adsorberende materialen zoals actieve kool (AC) toe te voegen aan verontreinigde waterbodems om zo de HOC concentratie in het water te verminderen. Dit onderzoek heeft als doel om het effect van toevoegen van AC op HOC blootstelling en toxiciteitsafname voor bentische organismen en gemeenschappen beter te begrijpen, om zo het gat tussen laboratorium en veld te dichten.|
WUR, Leerstoelgroep Aquatische Ecologie en Waterkwaliteitsbeheer