PNNL

Abstract

Delivery of carbon sources (nutrients) to the vadose zone and establishing a slow release carbon source in this unsaturated zone is essential for promoting long-term, enhanced contaminant bioremediation at sites with deep vadose zones such as Hanford in southeastern Washington State, USA. Conventional solution-based injection and infiltration approaches face challenges in achieving delivery goals. Aqueous colloidal silica suspension has characteristics that can potentially be used for nutrient delivery and slow release source setup. The present research was conducted to (1) demonstrate that delayed gelation of colloidal silica suspensions occurs when nutrients are present; (2) prove suspension gelation takes place in sediment pore spaces and the gel slowly releases nutrients; and (3) show that silica suspensions are ready for injection. Sodium lactate, vegetable oil, ethanol, and molasses were used as nutrients. The rheological properties of the silica suspensions during the gelation are presented, as well as the kinetics of nutrient release from silica-nutrient gel for molasses. The injection behavior of the suspensions was investigated by monitoring viscosity changes during preparation and the injection pressures when the suspensions were delivered into sediment columns. Results demonstrated that nutrient-laden colloidal silica suspensions have low initial viscosity, but then undergo viscosity increases with time until reaching gelation, allowing for a slow release of nutrients to the environment.