Implication of cation-bridging interaction contribution to sorption of perfluoroalkyl carboxylic acids by soils

Abstract

Sorption of four perfluoroalkyl carboxylic acids (PFCAs) including perfluoropentanoic acid, perfluoroheptanoic acid, perfluorodecanoic acid, and perfluorododecanoic acid by three soils with cation exchange sites occupied by K+, Ca2+, or Fe3+ was measured using the batch equilibration method. We hypothesize that partitioning in soil organic matters (SOM) is the primarily operative mechanism for PFCA sorption by K+-soils, and sorption by Ca2+- or Fe3+-soils could be enhanced via cation-bridging interaction. The measured sorption isotherms for all four PFCAs by soils were linear within the aqueous concentration between 0 and 60 μg/L, and the distribution coefficients ranged between 14.8 and 173 L/kg. Long-chain PFCAs manifested greater sorption by the soils with higher SOM content. Compared to sorption by K+-soils, sorption of PFCAs by Ca2+- and Fe3+-soils increased by 19.9–90.2% and 38.5–219%, respectively. The relative contributions of cation-bridging interaction to the overall PFCA sorption were estimated to be 16.6–48.7% for Ca2+-soils and 27.8–67.7% for Fe3+-soils. These results demonstrate that multivalent exchangeable cations could play an important role, yet previously ignored, in controlling sorption and transport of PFCAs in soils.