Influence of continuous phase evolution of non-aqueous phase liquid on the retention and transport of coexistence perfluorooctanoic acid in saturated porous media

Non-aqueous phase liquid (NAPL) undergoing continuous evolution presents challenges in predicting the migration of co-contaminants within saturated porous media. This study explores the impact of the ongoing evolution of dimethyl phthalate (DMP-NAPL) on retention and transport of coexisting perfluorooctanoic acid (PFOA). The presence of PFOA facilitates the dispersion of free-phase DMP by mobilization and enhances the migration flux of solution phase through dissolution of both free and residual phase. The retardation of PFOA migration initially increases and then decrease as the evolution process continues. The retardation factor (R) demonstrates a negatively correlation with free-phase saturation (r = -0.987, p < 0.05), but a positively with residual-phase saturation (r = 0.948, p < 0.05). The primary factors controlling PFOA migration are its adsorption on the NAPL-water interface (f(Ki) = 0 similar to 58.51%) and on solid surface (f(Kd) = 41.18 similar to 100%). The contribution of partitioning to overall retention (f(Kn)) remains consistently below 2%, indicating limited partitioning into bulk DMP-NAPL. Dissolved DMP exhibits a slight retardation effect on low-concentration PFOA migration because of competitive adsorption. This study elucidates the migration characteristics of PFOA throughout the complete dynamic evolution of NAPL, providing valuable insights into contaminant transport in a multi-phase contaminated groundwater environment.