Two Birds With One Stone: Artificially Enhanced Olivine Weathering for Sediment Management and CO2 Sequestration in the Port of Rotterdam

Every year about 10-14 million m³ of sediments are dredged in the Port of Rotterdam (PoR) as part of harbor maintenance.  Approximately 10% of these sediments are stored in a confined disposal facility (CDF) Slufter due to high levels of toxic metals (cadmium, zinc, mercury, lead, etc.) and persistent organic pollutants, whereas over 90% of the sediments that are not contaminated are currently being relocated to the sea. Whilst the majority of these sediments have the potential to be used in nature-based environmental management projects, there are concerns that the oxidation of these sediments will release greenhouse gases and contaminants to the environment.

The idea of spreading ground olivine in terrestrial and coastal environments to capture CO₂ is becoming increasingly popular due to the urgency to combat climate change. This technique (termed artificially enhanced olivine weathering, EOW) capitalizes on the natural process of olivine weathering that encourages gaseous CO₂ to transform into dissolved bicarbonate ions (HCO₃^-).  In addition, the dissolution of olivine increases soil water pH and allows precipitation of secondary minerals (e.g. Fe oxyhydroxides) that can immobilize toxic metals through adsorption and co-precipitation mechanisms. As a result, EOW could be a promising geo-engineering solution for sediment management at PoR and reduce the negative environmental impacts associated with dredging. However, the specific controls on the drawdown of CO₂ and toxic metal dynamics via silicate weathering are not well constrained.

Through laboratory experiments and field trials, we aim to investigate whether the addition of various commercial olivine-rich mineral mixtures (Greensand, Sibelco sand, etc.) can transform the dredged material from the PoR into a sustainable resource. Several bulk sediment and intact core samples, representing the majority of sediment supplied to the Slufter, were collected from the fluvial stretch of the PoR area. Laboratory batch experiments using artificial seawater were conducted for 90 days (at 1 bar and 12^oC) with (1) only fine-grained (10 – 30 μm) Greensand containing ~62 weight-% forsteritic olivine, 2) only fluvial harbor sediment, and 3) mixtures of Greensand and fluvial harbor sediment. Our results show that olivine dissolution caused significant increases in alkalinity, dissolved inorganic carbon (DIC), and seawater-pH. Nickel concentrations in the aqueous phase remained below the environmental standards in most of the experiments and only slightly exceeded the standard value in experiments with the highest solid/liquid ratio. Furthermore, the mobilization of toxic metals like Zn and Mn from the harbor sediment to the solution was limited in the olivine-sediment mixed experiments, most likely due to adsorption with olivine or with precipitated byproducts of olivine dissolution. Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM/EDS) analysis of the reacted olivine samples shows the presence of Ca carbonates precipitation but no clear evidence of Mg carbonates or secondary Mg silicate phases (in contrast to results from thermodynamic calculations using PHREEQC). ­Overall, these preliminary laboratory findings indicate that EOW applications in PoR are likely to be viable from an environmental geochemical point-of-view, but further testing in long-term experiments and field trials planned in the project will provide a more accurate assessment.