Impact of Variable Water Chemistry on PFOS-Goethite Interactions: Experimental Evidence and Surface Complexation Modeling

Abstract

Perfluorooctanesulfonate (PFOS) has become a major concern due to its widespread occurrence in the environment and severe toxic effects. In this study, we investigate PFOS sorption on goethite surfaces under different water chemistry conditions to understand the impact of variable groundwater chemistry. Our investigation is based on multiple lines of evidence, including (i) a series of sorption experiments with varying pH, ionic strength, and PFOS initial concentration, (ii) IR spectroscopy analysis, and (iii) surface complexation modeling. PFOS was found to bind to goethite through a strong hydrogen-bonded (HB) complex and a weaker outer-sphere complex involving Na + coadsorption (OS−Na + ). The pH and ionic strength of the solution had a nontrivial impact on the speciation and coexistence of these surface complexes. Acidic conditions and low ionic strength promoted hydrogen bonding between the sulfonate headgroup and protonated hydroxo surface sites. Higher electrolyte concentrations and pH values hindered the formation of strong hydrogen bonds upon the formation of a ternary PFOS-Na + -goethite outer-sphere complex. The findings of this study illuminate the key control of variable solution chemistry on PFOS adsorption to mineral surfaces and the importance to develop surface complexation models integrating mechanistic insights for the accurate prediction of PFOS mobility and environmental fate.

Multicomponent and Surface Charge Effects on PFOS Sorption and Transport in Goethite-Coated Porous Media under Variable Hydrochemical Conditions

“…Once this value was fixed, a similar calibration strategy was adopted to determine the log K values of the PFOS surface complexation reactions defined in the surface complexation model. In particular, PFOS breakthrough curves measured under static hydrochemical conditions (Figure a,c,e) were used to optimize the formation constants of the HB and OS-Na + complexes, which were found to be consistent with the values determined in a previous study based on a comprehensive set (triplicate experiments) of pH-dependent adsorption edges and isotherms . All complexation reactions and SCM parameters for the combined goethite-coated quartz model are reported in Table .…”

“…The quartz sand had a BET surface area ( A quartz ) of 0.07 m 2 /g of sand. The goethite used to coat the sand was synthesized following the method of Atkinson et al and had a BET surface area of 81 m 2 /g and a pH PZC of 8.9 . The quartz sand was coated with goethite using the method described by Scheidegger et al and detailed in Section 1 of the Supporting Information.…”

“…The goethite charging behavior was described assuming the surface as chemically heterogeneous with the multisite complexation (MUSIC) model by defining singly and triply coordinated Fe–OH(H) groups with site densities set equal to 3.45 and 2.7 sites/nm 2 , respectively . The electrical double layer at the goethite–water interface was defined by using the triple plane model (TPM) with inner and outer layer capacitances ( C 1 and C 2 ) set to 1.2 and 0.73 F/m 2 , respectively. , Ion-pair formations with background electrolyte ions were defined with the “extended charge distribution (CD) approach” …”

“…The resulting total surface area of the goethite-coated quartz sand ( A GCS ), calculated as the theoretical sum of the clean quartz sand surface area ( A quartz ) and the corresponding weight fraction of the goethite surface area, was in good agreement with the experimentally determined BET value. PFOS surface complexes, a strong hydrogen-bonded complex (HB) and a weaker outer-sphere complex involving Na + coadsorption (OS-Na + ), were implemented only for the goethite surface with the CD approach and by applying the Pauling’s bond valence …”

“…Perfluorooctanesulfonate (PFOS) is one of the most frequently detected, , toxic, − and environmentally recalcitrant , perfluorinated surfactants belonging to the PFAS class, and it is still widely measured at high concentrations in diverse environmental matrices. − In the aqueous phase, PFOS molecules behave as ionizable organic contaminants capable of electrostatically interacting with charge regulated surfaces. − The electrostatic mechanisms governing PFOS adsorption strongly depend on water chemistry conditions, and, in particular, on the pH and ionic strength of the solution. − A few studies of PFOS transport through saturated porous media pointed to the importance of hydrochemistry and its impact on PFOS solid adsorption especially on quartz and organic-carbon rich soils. , However, predictions of PFOS mobility relied on reactive transport models using empirically derived solid–water distribution coefficients (i.e., K d ) obtained under static hydrochemical conditions with respect to the pH and ionic strength. In natural subsurface environments, these water chemistry parameters often show dynamic spatio-temporal distributions as a consequence of natural processes, such as seawater intrusion, , mineral/water interactions and geochemical heterogeneities, , mixing processes at the fringes of contaminant plumes, and anthropogenic activities such as injection/extraction at remediation sites .…”