Comparison of Various Types of Modified Biochar for Optimal PFOS/PFOA Adsorption

Abstract

The increasing abundance of per- and polyfluoroalkyl substances (PFAS) has attracted growing interest due to concern about the potential hazards to public health and ecosystems. Due to PFAS toxicity, mobility, environmental persistence, and strong bioaccumulation, they are being further investigated. Long-chain perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are most often detected in aquatic environments even though their use has been limited by recent regulations. However, most adsorption studies are focused on their removal using deionized water instead of wastewater. In this research, the removal of PFOS in wastewater was investigated using four different types of municipal sludge-produced biochars, where the metal-enhanced ferric chloride (FeCl3)-modified biochar showed the most promising adsorption of PFOS and PFOA. FeCl3-modified biochar was characterized via x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), thermogravimetric analyzer (TGA), and Brunauer–Emmett–Teller (BET) method. The results indicated that physical adsorption might play a dominant role in the removal process. The FeCl3-modified biochar exhibited a high removal efficiency of PFOS and PFOA, 99.89% and 94.34%, respectively, with an initial concentration of 25 mg L-1, for PFOS and PFOA and an adsorbent dosage of 0.5 g L-1 and 0.75 g L-1, respectively. The possible adsorption mechanism was proposed, and the enhanced removal ability was due to the improved specific surface area, and pore volume. Functional groups in the biochar, such as Fe, were also responsible for the enhanced removal ability, which also might be the reason for the better performance of the FeCl3-modified biochar than other modified biochars. Moreover, the adsorption process was best described using the Langmuir and pseudo-second-order models. The desorption and regeneration of FeCl3-modified biochar for the removal of PFOS and PFOA were also determined. No desorption of PFOS was observed at 24 hrs or 7 days and a 5.3% desorption of PFOA was seen after 7 days. The regeneration of FeCl3-modified biochar for PFOS and PFOA adsorption was evaluated over five cycles. After the first regeneration cycle, the FeCl3-modified biochar removed 84.68% of PFOS with the removal decreasing with each cycle to 33.47%. PFOA removal increased with the first regeneration cycle to 100% and was maintained for four cycles before reducing to 99.08% after the fifth regeneration cycle. Overall, the FeCl3-modified biochar would be a promising and effective adsorbent for PFOS and PFOA removal, due to its removal efficiency and cost-effective and environmentally friendly advantages.