Remediation of PAH‒contaminated marine sediments through sustainable techniques

The issue of polycyclic aromatic hydrocarbons (PAHs) is widespread in marine environments involving ecological systems and human health. At the same time, research on eco‒friendly alternatives for the treatment of PAH‒contaminated marine sediments and their reuse after remediation must be stepped up. For this purpose, this doctoral thesis firstly examined technical, economic and energy aspects to evaluate the most sustainable technology for removing phenanthrene (PHE) from marine sediments. Anaerobic bioremediation, sediment washing (SW) and thermal desorption were conducted under low liquid phase (i.e. 60% and solid‒to‒liquid ratio of 1:3) and low temperature (i.e. 200°C), whose resulted in a PHE removal of 68, 97 and 88%, respectively. On the other hand, the bioremediation exhibited the lowest cost (i.e. 228 €·m‒3) and a similar energy input (i.e. 16 kWh·m‒3) to SW due to a significant energy gain from the biogas produced during anaerobic digestion. Therefore, the use of digestate and organic fraction of municipal solid waste (OFMSW) was subsequently proposed as novel biostimulation amendments for improving the anaerobic bioremediation. A nutrient solution was also supplemented in some experiments to enhance the biostimulation yields. The simultaneous addition of OFMSW and nutrients increased the total PAH removal up to 55% and led to the highest biohydrogen and biomethane production of 80 and 140 mL·g VS‒1, respectively, indicating that bioenergy can be further recovered during PAH degradation by enhancing the sustainability of the entire process. Afterwards, the present thesis work focused on the treatment of spent SW solutions, since SW has proven to be the most effective remediation approach for PAH‒polluted sediments, but still raises waste handling concerns due to a considerable amount of effluents generated downstream of the SW process. The biological treatment of a PHE– and ethanol (EtOH)–containing spent SW solution was initially investigated as a highly–efficient, inexpensive and environmentally–friendly strategy. The experimental activity was conducted in a fed–batch bioreactor by evaluating several PHE concentrations (i.e. 20–140 mg·L–1) within six successive cycles under aerobic conditions. A PHE biodegradation up to 91% following a first–order kinetic model was achieved by an enriched PHE–degrading consortium mainly composed of Proteobacteria and Bacteroidota phyla, after a proper supplementation of nutrients (i.e. nitrogen and phosphorous). A techno–economic evaluation was carried out particularly considering EtOH recovery as a resource to be reused in SW treatment units, thus reducing the total cost of the whole remediation process by approximately 50%. Finally, this thesis assessed both the PHE desorption from sediments using Tween® 80 (TW80) as extracting agent and the treatment of the resulting spent SW solution in a novel biochar (BC) immobilized–cell bioreactor. The SW process reached a PHE removal of up to about 91% using a surfactant solution containing 10,800 mg·L–1 of TW80, and the generated amount of spent PHE–polluted SW solution can be controlled by keeping a solid to liquid ratio up to 1:4. A PHE degradation of up to 96% was subsequently achieved after 43 days of continuous reactor operation, aerobically treating the TW80 solution with a hydraulic retention time of 3.5 days. Brevundimonas, Chryseobacterium, Dysgonomonas, Nubsella, and both uncultured Weeksellaceae and Xanthobacteraceae genera were mainly involved in PHE biodegradation. A rough economic study showed a total cost of 342.60 €·ton−1 of sediment, including the SW operations, TW80 and BC supply and the biological treatment of the SW solution.