Mechanistic understanding of perfluorooctane sulfonate (PFOS) sorption by biochars

Biochar has recently emerged as a cost-effective solution to combat per-and polyfluoroalkyl substances (PFAS) pollution in water, but mechanistic understanding of which physicochemical properties of biochars dictate PFAS sorptive removal from water remains elusive. Herein, 15 biochars were pyrolyzed from five feedstocks (corn, Douglas fir, eucalyptus, poplar, and switchgrass) at three pyrolysis temperatures (500, 700, and 900 degrees C) to investigate their removal efficiencies and mechanisms of perfluorooctane sulfonate (PFOS) from water. A commercial biochar was also included for comparison. Biochar physiochemical properties, including elemental composition, pH, specific surface area (SSA), pore structure, hydrophobicity, surface charge, surface functional groups, and crystalline structure were systematically characterized. Batch sorption data showed that the Douglas fir 900 biochar (Douglas fir and 900 are the feedstock type and pyrolysis temperature, respectively; this naming rule applies to other biochars), poplar 900 biochar, and commercial biochar can remove over 95% of PFOS from water. Structural equation model (SEM) was used to elucidate which biochar properties affect PFOS sorption. Interestingly, biochar pore diameter was identified as the most critical factor controlling PFOS removal, but pore diameter/pore volume ratio, SSA, pyrolysis temperature, hydrophobicity, and elemental composition all played variable roles. Hypothetically, biochars with small pore diameters and large pore volumes had a narrow yet deep pore structure that traps PFOS molecules inside once already sorbed, resulting in an enhanced PFOS sorption. Biochars with small pore diameter, low nitrogen content, and high pyrolysis temperature were also favorable for enhanced PFOS sorption. Our findings advance the knowledge of using biochars with optimized properties to remove PFOS and possibly other similar PFAS compounds from water.

Automated summary

Biochar is a cost-effective solution for removing per- and polyfluoroalkyl substances (PFAS) from water, but the specific physicochemical properties of biochars that determine their PFAS removal remain unknown. In this study, 15 biochars were produced from five feedstocks at three pyrolysis temperatures to investigate their removal efficiency of perfluorooctane sulfonate (PFOS), a type of PFAS, from water. It was found that biochar pore diameter was the most critical factor for PFOS removal, but other factors such as pore diameter/pore volume ratio, specific surface area, pyrolysis temperature, hydrophobicity, and elemental composition also played roles. Biochars with small pore diameter, low nitrogen content, and high pyrolysis temperature showed enhanced PFOS sorption. These findings contribute to the understanding of using biochars with optimized properties to remove PFAS compounds from water.