Implementation of electrochemical advanced oxidation processes for the treatment of soil washing solutions from polycyclic aromatic hydrocarbon contaminated soils
Remediation of soil contaminated by polycyclic aromatic hydrocarbons (PAHs) is an important societal, environmental, economical and technological challenge, due to the high number of sites contaminated by these persistent and toxic compounds. Environmental engineering companies using conventional bioremediation processes often fails to reach sufficient PAH removal rates from historically contaminated soils. Therefore, there is a real need for the development of innovative solutions.
In the present work, the characterization of 6 historically tar oil-contaminated soils showed that 42 to 86% of PAH are located in the sand fraction, either adsorbed on various coal/coke/wood particles or integrated in resinified and weathered tar oil particles. Thus, either selective separation of the most contaminated fraction or surfactant-enhanced soil washing (SW) appears to be promising alternatives to bioremediation, according to the level of sequestration of PAHs. Further investigations were performed on the surfactant-enhanced SW process, which is based on the transfer of PAHs from the soil-sorbed fraction to the washing solution. This process generates SW solutions containing a large amount of surfactant and pollutant. They have to be treated in order to avoid environmental contamination and ensure the cost-effectiveness of the whole process.
Anodic oxidation (AO) was identified as a suitable and promising process for the treatment of SW solutions containing PAHs and Tween® 80 as extracting agent. The detailed understanding of mechanisms involved in the removal of organic compounds from SW solutions during AO allowed the implementation of two different treatment strategies:
- First, it was emphasized that the use of AO at low current intensity during long treatment times (23 h) allows the selective degradation of target pollutants (PAHs) and the reuse of the SW solution for additional SW steps. Thus, the large amount of Tween® 80 saved strongly improves the cost-effectiveness and ecological footprint of SW processes, particularly when the high sequestration of pollutants requires several SW steps using high surfactant concentrations.
- In contrast, high removal rates of organic compounds and production of more biodegradable by-products was observed during short treatment times (3 h) by AO at high current intensity. Thus, synergistic effects were observed for the combination of AO with a biological post-treatment. High removal rates with optimized operating costs can be achieved. Moreover, AO can also be used as a post-treatment (polishing step) for the removal of hardly-biodegradable compounds. This treatment strategy aims at avoiding environmental contamination by SW solutions.
Both treatment strategies have to be considered for a suitable management of SW solutions. In view of promising results obtained, scientific challenges related to the scale-up of this process were discussed.