Formatted Title
Solar Thermal Activation of Persulfate for ISCO Source Zone Treatment at Former Industrial Site
Background/Objectives
The site to be presented in this talk is a former industrial site in the São Paulo Metropolitan area. In 2000 the land was rezoned and has become part of an educational center. After that, soil and groundwater investigations revealed substantial contamination with chlorinated volatile organic compounds (CVOCs). In situ chemical oxidation (ISCO) was selected for the remediation of a source zone (former electroplating area) located at the inner courtyard of an academic building. Maximum concentrations of the main contaminants TCE and 1,1,1-TCA before the start of remediation were 1,500 mg/kg and 13,000 mg/kg in soil and 4,000 and 94,000 µg/L in groundwater, respectively. Considering the presence of 1,1,1-TCA, activated persulfate was chosen as the oxidant. It was decided to use a thermal activation mechanism as both the high natural groundwater temperatures in this region (more than 20°C) and high insolation energy present favorable conditions for the development and application of green energy solutions.
Approach/Activities
An ISCO recirculation design was selected for the remediation of the 380 square meter (m²) treatment zone in the inner courtyard. To account for the heterogeneous geological conditions, a dense well network consisting of 70 wells was installed in two different soil layers up to 10 below grade surface (bgs). Several recirculation cells consisting of up to four injection and one center extraction well were defined to create overlapping radii of influence (ROI) and enable an appropriate distribution of the oxidant in the whole treatment area. The 10% oxidant solution was prepared within a mixing station outside of the remediation zone to exclude any health and safety risks in the sensitive usage area of the educational center. A bivalent heating system was used to heat the 20°C oxidant solution to the target temperature of 50°C. To achieve that, 18 solar thermal panels were installed on the roof of the academic building. Model calculations were carried out to define the required number of panels. To compensate for days with insufficient solar thermal supply, a supplementary gas heater was implemented. Heat was transferred to the oxidant solution via a heat exchanger to raise the temperature of the solution to 50°C and initiate activation of persulfate prior to distribution in the injection wells. Groundwater was extracted and transferred to a treatment plant after an incubation period to keep the water balance and avoid an accumulation of sulfate in the aquifer. The processed water was re-used for the preparation of oxidant solution. After the first treatment cycle, recirculation cells have been modified to induce altered flow regimes and target areas that still showed relevant concentrations.
Results/Lessons Learned
After two subsequent treatment cycles in 2020 and 2022, remedial goals in groundwater have been met in 97% of the wells with an average concentration reduction of 97% for TCE and 89% for 1,1,1-TCA. This demonstrates that persulfate activation and rapid destruction of the recalcitrant 1,1,1-TCA can also be achieved by a sustainable solar thermal activation mechanism. It underlines that innovative green energy applications not only contribute to the sustainability of a project, but can also provide an effective tool for achieving remedial goals. The presentation will address the technical details of the design as well as insights from the process and performance monitoring. Result from the long-term success control monitoring as a basis for decision-making for the next steps will be shared. In this regard, the use of the solar thermal system to stimulate microbiology for natural degradation is currently being considered as a polishing step.