Adsorption of PFCAs using polyethyleneimine modified Biochar: Role of chain length and effects of water matrices

Perfluorocarboxylic acids (PFCAs) are emerging organic pollutants posing a threat to human health and the environment. This study investigates the efficacy of polyethyleneimine-modified biochar (BC-PEI) as an adsorbent for removing PFCAs from a mixed solute system, focusing on competitive adsorption among PFCAs with varying chain lengths. It includes perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA), hexafluoropropylene-oxide-dimer-acid (GenX), and perfluorobutanoic acid (PFBA). BC-PEI (1:1) (w_PEI/w_BC = 1) exhibited the highest adsorption capacities for PFOA, PFHxA, GenX, and PFBA at 1.302, 0.850, 0.711, and 0.397 mmol/g, respectively. It follows the Sips isotherm (Langmuir-Freundlich isotherm) model, which becomes Langmuir at high concentration and Freundlich at low concentration. Surface functional groups, as well as electrostatic and hydrophobic interactions, influenced the adsorption mechanism. Long-chain PFCAs demonstrated higher adsorption capacities due to stronger hydrophobic interactions, while short-chain PFCAs were primarily adsorbed via electrostatic interactions. Kinetics data were best described by the pseudo-second-order (PSO) model, with surface adsorption and minor micropore contributions governing the process. The presence of humic acid reduced the adsorption capacity by competing for adsorption sites. The background ions in the aqueous matrix further diminished capacity due to double-layer compression. Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed PFCAs adsorption onto BC-PEI. These findings underscore the potential of BC-PEI as a promising adsorbent for PFCA remediation in wastewater systems, highlighting its engineering applications.