Role of humic substances in the remediation of sites contaminated by PAHs

Polycyclic aromatic hydrocarbon (PAHs) are persistent organic contaminants that are widely present in nature mostly due to anthropogenic industrial activities. Their hydrophobic nature and tendency to adsorb strongly to organic matter, cause PAHs to accumulate in solid matrices, where they are subjected to processes of ageing that reduces their bioavailability, complicating the remediation processes. Conventional remediation technologies often face limitations related to high costs, secondary pollution, incomplete degradation, and reduced effectiveness in more complex co-contaminated environments. For these reasons, alternative remediation processes that are more efficient and greener are considered. The role of natural organic matter, and specifically humic substances (HS), in influencing the environmental behaviour and treatability of PAHs, has gained attention. In this thesis, a comprehensive literature review of the interaction between PAHs and HS, particularly humic acid, fulvic acid, and humin (with different significance), is presented, with a focus on their implications for remediation processes. The dual role of humic substances is discussed, as they may either enhance contaminant mobility and apparent solubility through surfactant-like and colloidal effects or promote long-term sequestration via strong sorption and organo-mineral associations. Laboratory studies, pilot-scale experiments, and field observations are critically observed in order to highlight that remediations carried by humic substances depends on the context; emphasizing the importance of identifying rate-limiting processes, such as desorption or biodegradation. Overall, the study aims to support a more realistic, risk-based, and integrated approach to the remediation and management of PAH-contaminated environments.