Participants

DTU (LMO and Pernille E. Jensen), AAU(JM and MLC), AU (AMS), SDU (KR)

Description

Many Danish lake sediments have heavy metal concentrations, which are too high for direct spreading at agricultural land (in comparison to limiting values in “Slambekendtgørelsen”). Especially cadmium is problematic, as the limiting values are low due to both the high toxicity and uptake in plants. Soils and sediments polluted with heavy metals remain an unsolved challenge in relation to the implementation of remediation methods for the removal of the pollutants. 

For heavy metal polluted sediments, remediation requires desorption of the heavy metals. Electrokinetic methods have proven efficient for remediation of soil and harbor sediments but have not yet reached the stage of implementation. At DTU, electrodialytic (ED) methods for remediation or recovery of elemental resources have been developed over the past decades. ED is based on the application of an electric DC field and the use of ion-exchange membranes. Optimizing the placement of electrodes and ion exchange in the setup allow for targeting mobilization of different chemical elements from the particulate material. ED methods have wide applications and have been tested successfully for remediation of various polluted matrices. For example, most recently, a two-compartment electrodialytic cell was developed for the recovery of P and simultaneous removal of heavy metals from sewage sludge ash (Ottosen et al. 2016). Developing an ED method for remediation of heavy metal polluted lake sediment takes offset in the knowledge gained from other matrices. Development of an ED method to lake sediments is a focus in this WP. The main task will be to remove heavy metals to obtain a sediment, which is considered non-hazardous. 

Objectives

The objective of WP4 is to optimize the electrodialytic remediation method for the removal of heavy metals from polluted lake sediments and simultaneous separation of P.

Activities

Task 4.1: Binding mechanisms and mobility of heavy metals in the sediments

Sediment from different heavy metal polluted lakes (3-5) is collected and characterized. The pH-dependent desorption of P and heavy metals, sequential extraction and size fractionation are used to describe the heavy metal mobility in the different sediments. This will be in collaboration with WP7, where DGT´s will be tested as tools for estimating Cd mobility in sediments and soils. Multivariate analysis is used to extract the generic knowledge. This task aims to evaluate how different the sediments are and how differently they need to be treated by EDR. A reference sediment (most representative of the investigated) is chosen in which the major part of the following tasks takes offset. The metal binding may also be influenced by choice of polymers tested in WP2. This will be investigated.

Task 4.2: Choice of electrodialytic cell or combination of cells

Different EDR (Electrodialytic Remediation) cells have been developed at DTU for remediation of different particulate materials. The number of compartments (and ion exchange membranes) and the placement of the electrodes (in separate compartments or in a suspension of the material) are optimized for the specific materials and target pollutants. Combining different EDR setups can also be beneficial. In this task, a screening of the remediation efficiency for different setups for treatment of the reference sediment is conducted. The setup is tested for remediation of other investigated sediments, possibly from other lakes. The best setup/combination of setups is chosen for further work in the following WPs.

Task 4.3: Optimization of treatment parameters

Treatment parameters are optimized for the reference sediment for the setup/combination of setups chosen from WP2. The parameters are current, duration, stirring rate, liquid to solid ratio, and flocculation polymer. Chemometric modeling is used to optimize the parameters mutually. EDR experiments are with the other investigated sediments (task 1). The results are evaluated by using the chemometric model to see if processing parameters need to be optimized for each sediment type or if the sediments are similar enough for using the same parameters. During EDR, fouling of the ion exchange membranes due to electrophoretic movement of organic molecules may become an issue, which needs to be solved. We have previously seen it during electrodialysis of sewage sludge, where periodical reversal of the current (short time) could be the solution, which is also tested in this WP. Finally, a suggestion is given on designing a full-scale plant, enabling simultaneous electroosmotic dewatering and electrodialytic removal of heavy metals. The treated sediment goes to WP7 for soil testing.

Task 4.4: Evaluation of solution with recovered P

To remove heavy metals, desorption from the sediment is necessary. This is obtained by lowering pH and depending on the EDR setup; this is obtained from the anode process or by water splitting at the ion exchange membranes. It is expected that P is partly released from the sediment during EDR. We aim at separating P and heavy metals into two different electrolyte solutions, so the solution with P can be treated for P recovery in WP3. This WP evaluates the P concentrations in the solution to WP3 and whether sufficiently low heavy metal concentrations are present in the solution.

Task 4.5: Precipitation of heavy metals for safe disposal

When heavy metals are transferred from the sediment to the aqueous phase and into the catholyte, a feasible treatment process for its immobilization will be researched and demonstrated so that the effluent is environmentally safe for discharge. For this purpose ion commercial exchange resins and more novel biological waste materials such as maize cope and husk will be investigated.

Task 4.6: Design of EDR full scale plant

On the basis of the work in the other tasks, a suggestion to design a full-scale plant for EDR is developed. In addition to the upscaling of the process itself the possibilities for basing the energy input on green sources is outlined, e.g., solar panels linked to the plant.

Milestones Structure

LMO will be the leader of WP4. A PhD student will conduct the major part of the experimental work in this WP. The PhD student will be hosted at DTU Civil Engineering and supervised by LMO (DTU) with MLC and JM (AAU) as co-supervisor. The PhD student will focus on sediment dewatering and modelling of the process. Main workplace will be DTU but with research stays at AAU. The international research stay is planned at Universidade Aberta, Portugal with associate prof. Celia Diaz-Ferreira, who is working with electrodialytic recovery of P from different organic residues.

Risks associated with work package 

The major risk towards the commercialization of electrodialytic remediation of lake sediments is that the process will be too expensive in terms of energy consumption. The process optimization will aim at the highest heavy metal removal at the lowest cost, and further, the option for basing the energy supply on fossil-free sources is explored.

The Results