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Application of Lemna minor in site remediation strategies

1. Context

Enhanced levels of pollutants such as radionuclides and heavy metals are present in the environment due to the exploitation of nuclear installations, mining and milling processes, industrial activities (e.g. involving naturally occurring radionuclides), nuclear accidents and the improper management of radioactive waste. This poses risks for people and other biota. Therefore, it is important to understand the behaviour of the pollutants released in the environment, use this knowledge to assess the impact and propose remediation strategies if needed. Phytoremediation exploits the biological processes of living plants and their associated microorganisms to remove, degrade or stabilise pollutants in environmental compartments such as soil, water and air. Within this PhD project, we will focus on phytoremediation using the macrophyte Lemna minor for cleaning radioactively polluted water. Previous research has already highlighted the potential of using photosynthetic organisms for the clean-up of radioactively polluted water [1-2]. The best remediation approach is always scenario-specific and depends on the mixture of radionuclides/metals, the pollution levels and the environmental and ecological factors involved. Both phytoremediation and conventional waste water treatment techniques have their specific advantages and limitations [3]. For example, chemical precipitation is highly effective for the treatment of water containing high metal levels but ineffective for lower concentrations. Other methods can be expensive (e.g. ion-exchange) or chemical consuming (e.g. coagulation-flocculation). Phytoremediation is not in competition with other conventional waste water remediation techniques but is a complementary technique that can be more appropriate for specific scenarios. Phytoremediation is particularly useful where there is need for on-site remediation of ponds polluted with relatively low amounts of pollutants, if it needs to be integrated in an existing ecosystem, or when it is not advisable to use an aggressive chemical treatment which can cause more harm than good to the environment.

SCK•CEN has a high level of expertise in risk assessment, site remediation and management related to planned, existing and emergency exposure/pollution scenarios. The general strategic agenda presents “remediation of NORM and legacy sites and their direct environment” as a high priority topic. Knowledge not only on conventional waste water treatment techniques but also on alternative biology driven water remediation systems will enhance the expertise of SCK•CEN and its competitiveness when applying for site remediation projects. As such, the present project falls squarely within the task force “site remediation”. Within the R&D and valorisation plan of EHS PL3, the proposed topic addresses right-on the stated criterion “Research to study uptake and sorption of radionuclides by macrophytes aiming to define the feasibility of using phytoremediation to remediate radioactively polluted water”. In addition, the project is set to increase process-based knowledge on the behaviour and fate of radionuclides and their co-pollutants in the aquatic environment for different scenarios (e.g. NORM-related legacy sites, nuclear plant related accidents). Impact assessments for people and environment can be improved based on this knowledge. All these aspects fall within the main objectives of PL3 and the BIS research lines. Combining an experimental and mathematical modelling approach is one of the assets of the BIS research group and here too the project fits the stated goal by developing a model of the uptake of radionuclides by Lemna minor in the presence of chemical and ecological factors. Hence, there is a strong potential for practical application not only insofar as the remediation process is concerned, but also regarding the model’s possible use as a tool for remediation optimisation and planning in its own right. Gaining knowledge on the toxicity effects caused by (mixtures of) pollutants on Lemna minor and including this in the modelling also contributes to PL1 and the research line of BIS related to the effects of radionuclides and ionizing radiation on biota. The PhD proposed here is grounded in solid foundations, laid over the past four years with research performed for ENGIE and within the internship of three bachelor students. This has resulted in three reports [4-6], two recent journal publications [1-2], one publication in progress, three bachelor theses [7-9] and several presentations at international conferences. On this basis, we are now fully ready to boost this research with a PhD study on the role of Lemna minor in site remediation, confident that this project is viable and may have a real-world impact.

2. State of the art

A literature review on the potential of using photosynthetic organisms for remediation of radioactively polluted water, revealed a low number of good quality studies for a limited amount of (mixtures of) radionuclides [1]. Most studies are performed under controlled laboratory conditions rather than under realistic conditions or on full (industrial) scale [1]. The final result and success of a phytoremediation application is known to be influenced by the biological characteristics of the plant species together with environmental physico-chemical factors such as pH, cations (e.g. K+, Na+, Ca2+), anions (e.g. PO43-, CO32-), radionuclide concentration and speciation, growth conditions (e.g. light, nutrients, temperature), set-up (e.g. biomass/water ratio, contact time), etc. Based on available literature and recent pilot studies performed in our laboratory, Lemna minor demonstrates good radionuclide and heavy metal removal capacities [1-2]. Different plant species (e.g. Lemna minor, Pistia stratiotes, Eichhornia crassipes) have been compared for their potential to remove 137Cs and 60Co from artificial river water under changing experimental parameters (e.g. pH). Results from these experiments reveal the strong potential for Lemna minor to remediate radioactively polluted water. For example, more than 95% of the initially added 60Co could be removed by Lemna minor after 1 day contact time [2].

To design a practical application, it is necessary to predict the performance of a Lemna-based remediation system building on the laboratory results, and to compare it with conventional waste water treatment processes. Therefore, we want to assess the capacities of Lemna minor in phytoremediation of radionuclides polluted water using an experimental and mathematical approach. Under which conditions can Lemna minor be used? Can the performance of Lemna minor be improved by modifying certain parameters? Can the outcome of a Lemna-based application be predicted using population models? Can we compare it to other remediation approaches when defining a site remediation strategy? Going beyond the laboratory experiments and trying to understand and model the system will contribute to bridge the gap between laboratory studies and field applications.

Universiteit UHASSELT
Opleiding gestart op


Vanhoudt Nathalie


Vives i Batlle Jordi


Vangronsveld Jaco


Van Dyck Isabelle

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