Innovative thermal and physico-chemical treatments for the clean-up of marine sediments dredged from the Augusta Bay (Southern Italy)

The main objective of this study was to compare the energy cost-effectiveness of two innovative techniques, namely microwave (MW) heating and electrokinetic (EK) decontamination for the successful remediation of highly Hg-contaminated marine sediments from Augusta Bay (Sicily, Italy). Bench-scale experiments were conducted and obtained results were used to elaborate energy and economic considerations aimed at selecting the most effective and economic approach. Results revealed that without any appropriate conditioning agents, very poor contaminant mobilization can be achieved by EK due to the high percentage of carbonates, as sediment mineral, and high organic matter and sulphides. The best Hg-removal (∼71% in 400 h) was achieved using both methylglycinediacetic acid (MGDA) and polyoxyethylene (20) sorbitan monooleate (Tween 80) as processing agents. Related Hg residual concentrations exceed the desired regulatory limit of 1 mg kg−1 for Augusta Bay. Simulated MW heating showed the achievement of a very rapid sediment temperature increase (∼64 °C min−1, maximum ∼450 °C), mainly due to the mineral composition of the sediments and their high salinity. This jointly with the addition of both MGDA and Tween 80, led to a very low residual Hg concentration of 0.56 mg kg−1 (R = ∼99%) after 7-min treatment. Calculations from energy and economic analysis revealed that a specific energy consumption of 6.73 kWh ton−1, which corresponds to an energy cost of 0.81 € ton−1, is required for the EK, whereas slightly higher values (32.97 kWh ton−1 and 3.96 € ton−1) were assessed for MW heating. Therefore, the higher Hg-removal obtainable in such a short remediation time by MW heating along with its with other advantages offset a low specific energy cost increase, making this technique a preferable choice for the remediation of Hg-contaminated marine sediments such as those present in Augusta Bay.