DNA Facilitates the Sorption of Polycyclic Aromatic Hydrocarbons on Montmorillonites

The sorption of polycyclic aromatic hydrocarbons (PAHs) to montmorillonites is largely influenced by their interactions with dissolved organic matter (DOM). However, the role of DOM rather than humic and fulvic acids (e.g., extracellular DNA) in the PAH sorption to soil clays is little known. Here, we demonstrate that extracellular double-stranded salmon testes DNA substantially increased the sorption of phenanthrene and pyrene to Na-, Ca-, and Fe-modified montmorillonites. All PAH sorption isotherms fitted the linear and Freundlich models reasonably well ( R² = 0.918-0.999). Distribution coefficients were increased from 0.0458-0.103 and 0.0493-0.141 L/g at 0 mg/L DNA to 0.413-0.589 and 0.385-0.560 L/g at 10 mg/L DNA for phenanthrene and pyrene, respectively. Spectroscopic and computational chemistry analyses confirmed that PAHs were first inserted into DNA by binding with the nucleobases via van der Waals and π-π electron donor-acceptor interactions. Compared to PAHs, the DNA-PAH complex can be more easily sorbed to cation-modified montmorillonites by complexation between DNA phosphate and exchangeable cations in addition to intercalation into clay interlayers. This work highlights the importance of understanding the control on contaminant sorption by many organic compounds that are ubiquitous in soils but not represented by humic and fulvic acids.