Mechanistic insights into fast adsorption of perfluoroalkyl substances on carbonate-layered double hydroxides

Layered double hydroxide (LDH) with the metal composition of Cu(II)Mg(II)Fe(III) was prepared as an adsorbent for fast adsorption of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA). 84% of PFOS and 48% of PFOA in relation to the equilibrium state were adsorbed in the first minutes of contact with 0.1 g/L of suspended mu m-sized LDH particles. The adsorption mechanisms of PFOS and PFOA on the CuMgFe-LDH were interpreted. Hydrophobic interactions were primarily responsible for the adsorption of these compounds in accordance with the different adsorption affinities of long-chain (C8, K-d = 10(5) L/kg) and short-chain (C-4, K-d = 10(2) L/kg) perfluorinated carboxylic acids. PFOA adsorption on CuMgFe-LDH was strongly suppressed under alkaline conditions while PFOS uptake was only slightly affected in the pH range from 4.3 to 10.7, indicating a significant role of electrostatic interactions for PFOA adsorption. The adsorption of PFOS and PFOA was rather insensitive to competition by monovalent anions. The previously reported 'memory effect' of calcined CuMgFe-LDH for sorption of organic anions was not confirmed in the present study. Spent CuMgFe-LDH could be easily regenerated by extraction with 50 vol% methanol in water within 1 h and maintained a high PFOS removal in subsequent usage cycles.

Automated summary

CuMgFe-LDH was prepared as an adsorbent for fast adsorption of PFOS and PFOA, primarily driven by hydrophobic interactions. While PFOA adsorption was suppressed under alkaline conditions, PFOS uptake was only slightly affected, indicating the significant role of electrostatic interactions for PFOA adsorption. The adsorption of PFOS and PFOA was insensitive to competition by monovalent anions, and spent CuMgFe-LDH could be easily regenerated for subsequent usage cycles.