Mechanistic insights into organic acid-enhanced polycyclic aromatic hydrocarbon release from soil: Disruption of metal cation bridges and charge-assisted hydrogen bonds

While low molecular weight organic acids (LMWOAs) are commonly used to enhance the dissolution and bioavailability of polycyclic aromatic hydrocarbons (PAHs) in soils, a comprehensive mechanistic framework is lacking. In this study, four representative organic acids (formic, acetic, citric, and oxalic acids) were systematically evaluated to their efficacy in promoting PAH desorption from a long-term contaminated soil, with particular emphasis on elucidating the governing mechanisms and environmental implications. The results demonstrated that these organic acids increased the release of PAHs and dissolved organic carbon (DOC) by 1.17-1.90 fold and 1.12-17.39 fold, respectively, compared to the NaCl control. The ratio of desorbed PAHs per unit of DOC followed the order: acetic acid > formic acid ≫ citric acid ≥ oxalic acid. Citric and oxalic acids solubilized significantly more Fe³⁺ and Al³⁺ (bridging cations between minerals and soil organic matter, SOM) than formic and acetic acids, thereby releasing larger amounts of dissolved organic matter (DOM) with relatively high molecular weights. However, additional solubilization of Fe³⁺ and Al³⁺ had negligible effects of on PAH desorption. The dominant mechanisms governing PAH desorption included Ca²⁺ dissolution and the disruption of negative charge-assisted hydrogen bonds, (-)CAHB, within SOM (including both intermolecular and intramolecular linkages). From the perspective of maintaining soil fertility and mitigating global climate change, formic/acetic acids outperform citric/oxalic acids in PAH desorption. The key advantage is that they release less biodegradable DOM, which reduces the risk of increased CO₂ emissions and helps preserve SOM.