Enhanced Removal of Polyfluoroalkyl Substances by Simple Modified Biochar: Adsorption Performance and Theoretical Calculation

Abstract

To cope with the widespread polyfluoroalkyl substance (PFAS) pollution in the global aquatic environment, this study prepared an acid-modified biochar adsorbent derived from sludge with satisfactory performance. The theoretical maximum adsorption capacities of the adsorbent for perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) were determined to be 72.17 and 45.88 mg·g–1, respectively. The theoretical calculation of the adsorption of biochar surface functional groups with PFOS and PFOA was first performed in this study, and the mechanism of the adsorption process was inferred with a combination of experimental results and redundancy analysis. The adsorption mechanism is composed mainly of the following three aspects: (I) electrostatic interaction between the positively charged biochar and the deprotonated PFAS; (II) hydrogen bonding between anionic species of PFAS and functional groups; and (III) hydrophobic interaction between long-chain hydrophobic PFAS and biochar. Electrostatic interaction plays a major role in the three main adsorption mechanisms under acidic media. Meanwhile, different functional groups have different adsorption energies for PFAS, and CO has the highest adsorption energy. The findings in this work could provide great insights into the adsorption of PFAS on modified biochar as an environmentally friendly adsorbent to promote resource reuse, improve water quality, and ensure human health.

Effects of Environmental Factors on the Sorption of Per- and Polyfluoroalkyl Substances by Biochars

“…For example, Zhang et al observed a blue-shift of nonaromatic ketone (C=O) peaks and new bands that were assigned to −CF 2 – before and after PFAS sorption by biochars using Fourier transform infrared spectrometry analysis. Similarly, density functional theory calculations confirmed that the C=O groups had the highest adsorption energy for PFAS . Nevertheless, high R 2 values obtained from the pseudo-first-order kinetic model suggest that physical interactions, such as pore filling, hydrogen bonding, and hydrophobic interactions, also likely contribute to PFAS removal by biochars.…”

“…Similarly, density functional theory calculations confirmed that the C=O groups had the highest adsorption energy for PFAS . Nevertheless, high R 2 values obtained from the pseudo-first-order kinetic model suggest that physical interactions, such as pore filling, hydrogen bonding, and hydrophobic interactions, also likely contribute to PFAS removal by biochars. The measured contact angles of the commercial biochar and Douglas Fir 900 biochar were 135.3 and 145.5°, respectively, indicating that both PFAS and biochar had a great hydrophobicity and hydrophobic interaction, which was one of the factors facilitating PFAS sorption by biochars …”

“…These observations suggest that chemical interactions are likely involved in PFAS sorption by the two biochars, which is consistent with literature-reported findings. 19,32,35,36 Both commercial biochar and Douglas Fir 900 biochar are rich in functional groups (e.g., C=O, C−O−C, C=C, and −OH); 11 it is expected that electrostatic interactions likely contribute to PFAS removal by the two biochars. For example, Zhang et al 19 observed a blue-shift of nonaromatic ketone (C=O) peaks and new bands that were assigned to −CF 2 − before and after PFAS sorption by biochars using Fourier transform infrared spectrometry analysis.…”

“…Similarly, density functional theory calculations confirmed that the C=O groups had the highest adsorption energy for PFAS. 35 Nevertheless, high R 2 values obtained from the pseudo-firstorder kinetic model suggest that physical interactions, such as pore filling, hydrogen bonding, 37 and hydrophobic interactions, 35 also likely contribute to PFAS removal by biochars.…”

“…The measured contact angles of the commercial biochar and Douglas Fir 900 biochar were 135.3 and 145.5°, respectively, 11 indicating that both PFAS and biochar had a great hydrophobicity and hydrophobic interaction, which was one of the factors facilitating PFAS sorption by biochars. 35…”