Beyond solubilization: Interface PAH biodegradation in nonaqueous-phase liquids treated with a plant biosurfactant

Biodegradation of polycyclic aromatic hydrocarbons (PAHs) in non-aqueous phase liquids (NAPLs) is constrained by limited bioavailability. Plant-derived biosurfactants offer sustainable alternatives to enhance remediation, yet their efficacy in NAPL systems remains unexplored. We investigated how Quillaja saponin affects PAH partitioning and biodegradation in biphasic NAPL systems using heptamethylnonane (HMN) and creosote/HMN mixtures with Mycobacterium gilvum and the microbial population from a creosote-polluted soil. Saponin enhanced aqueous-phase PAH concentrations in all systems. However, enhanced partitioning did not consistently translate to faster biodegradation. In HMN, saponin inhibited phenanthrene (PHE) mineralization by reducing bacterial adhesion to the NAPL interface, whereas pyrene degradation was enhanced due to slower diffusivity counteracting adhesion effects. In creosote-containing NAPLs, saponin substantially enhanced PHE partitioning but did not increase degradation rates, which remained controlled by continuous intra-NAPL diffusion of multiple high-molecular-weight PAHs sustaining interfacial bacterial activity. Community analysis revealed that saponin promoted growth of Gram-negative bacteria (primarily Pseudomonas) in the aqueous phase or enhanced Gram-positive bacterial colonization at the NAPL/water interface, at high and low saponin-to-PAH ratio, respectively. These divergent outcomes reveal a context-dependent dual mechanism: saponin simultaneously solubilizes PAHs and provides biostimulation, but efficacy depends critically on NAPL composition, PAH diffusion kinetics, and carbon availability. Our findings demonstrate that a dual mechanism combining solubilization and biostimulation offers a cost-effective, low-toxicity approach for sustainable remediation of persistent organic contaminants across diverse subsurface scenarios, from NAPL source zones to distal contamination plumes. SYNOPSIS: The dual solubilization and biostimulation mechanism of saponin determines NAPL-PAH remediation efficacy based on composition and diffusion kinetics.