In situ reactive capping for PAH-contaminated marine sediments: experimental investigations and numerical simulations

The distribution of contaminants in water bodies is a very complex topic due to several origins of pollution, with major concerns for human health and the environment. Several coastal-marine areas coexist with large industrial and port settlements that have led, over time, to compromise the whole marine ecosystem. In this context, the investigation of remediation strategies has given rise to great scientific and public concern throughout the world. The present thesis aims at evaluating the effectiveness of in situ reactive capping (ISC) technology and at investigating the best reactive material to reduce propagation of pollutants from marine sediments contaminated by polycyclic aromatic hydrocarbons (PAHs). Experimental activities were based on lab-scale batch column tests and aimed at assessing the performance of two reactive materials: • AquaGate®+5% of Powdered Activated Carbon (AG PAC), made of composite particles coated with bentonite and PAC; • Organophilic Clay Reactive Core Mat (OC RCM), a thin permeable composite mat with a layer of organophilic clay encapsulated between two geotextiles. The experimental activity has been carried out on contaminated marine sediments sampled from the Mar Piccolo of Taranto, Southern Italy, considered as a case study. The Mar Piccolo is a strongly anthropized environment, as a final receptor of a series of pollutants coming from all urban, industrial, and agricultural activities, municipal wastewater treatment plants, and other anthropic activities, especially due to its natural hydrogeological network conformation. A control column with no cap was also tested to investigate how pollutants behave and move through the sediment layer. After a first characterization, PAH concentrations in water were measured after 5, 10, and 20 days, while sediments were sampled after 20 days, at the end of the experiment. Benzo[a]pyrene, pyrene, and anthracene were chosen as target PAHs to be monitored as they were the most abundant pollutants in the sample. In the second step, a predictive model of the distribution of concentrations, in sediments and overlying water, was developed in order to evaluate the long-term effectiveness of the ISC intervention. A model was set up in COMSOL Multiphysics® where the physical-chemical properties measured during the experimental tests were considered as input values of the model. The basic equation describing the contaminant transport in one-dimensional porous media, over the time, was considered. The PAH fate and transport, through the multi-layered model to the upper water column, was predicted by considering the phenomena of diffusion, advection, and desorption in the sediments for all scenarios, and by adding the adsorption process in the two amendments tested, AG PAC and OC RCM. The design objective for the cap was to keep breakthrough concentrations of pollutants below the toxicity targets. The model was validated with the experimental values from the first phase. The results demonstrated a good effectiveness of both AG PAC and OC RCM for PAH contamination treatment over the time. The best performance, in terms of higher contamination reduction in the water column, was given by AquaGate®+5% PAC for the cases of benzo[a]pyrene and pyrene. Part of the experimental activities were funded in the framework of the Cooperation Agreement between the Special Commissioner of the Italian Government for urgent measures of remediation and environmental requalification of the Taranto area (south Italy) and the Polytechnic University of Bari.