Investigating sorption behaviour of Perfluoroalkyl Substances to biochar
Creators: name: Askeland, Matthew
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401199
subject: Environmental engineering not elsewhere classified

description: The toxicity, mobility and recalcitrant nature of the chemical class known as Perfluoroalkyl Substances (PFAS) is increasingly being demonstrated to pose a threat to human and environmental health globally. The identification of the risk posed by PFAS by numerous governments and scientific organisations has been a driver for further investigation into potential PFAS management options. Biochar has been considered as a possible sustainable approach to PFAS immobilisation due to being derived from waste biomass materials and having a demonstrated ability to sorb a variety of organic and inorganic contaminants.<br><br>This study was undertaken to determine if manipulation of biochar characteristics by varying pyrolysis conditions, such as feedstock and pyrolysis temperature, significantly impacted upon biochar PFAS sorption capacity. In addition, the influence of soil matrix was studied, accumulatively demonstrating that with further research biochar could be reverse engineered as an effective and sustainable PFAS sorbent.<br><br>It was found that by varying feedstock type, and pyrolysis temperature, a wide variety of physiochemical parameters could be manipulated. To better understand what type of physiochemical characteristic were more beneficial for PFAS sorption, a kinetic and sorption study was undertaken at environmentally relevant concentrations for a suite of biochars (pine and pea-straw feedstocks pyrolyzed at 300, 500 and 700°C). A specially developed direct aqueous injection liquid chromatography mass spectrometry method, associated sample preparation technique, and serial sorption method were developed to undertake appropriately sensitive, accurate and robust sample analysis for serial sorption biochar experiments.<br><br>Through the study of kinetic and sorption behaviour it was found that all tested biochars did not adequately sorb short chain PFAS, PFBA and PFBS. Further, low temperature biochar (350°C) sorbed far less PFAS from solution (&lt;50 %) than its higher temperature counterparts. Sorption equilibriums were found to be reached within 0 - 96 hours to for most PFAS, with the bulk of sorption occurring in the first hour. High temperature pine biochar was demonstrated to be the most effective sorbent for PFHxA, PFHxS, PFOA and PFOS. This suggests that key characteristics for the sorption of longer chain PFAS compounds are high surface area, high aromaticity and hydrophobicity. Isotherms demonstrated complex sorption behaviour, including the formation of monolayers and micelles for some PFAS compounds. This behaviour was greatly influenced by PFAS molecular structure, primarily functional group type, and subsequently chain length. Desorption was less reversible for higher temperature biochars, with all PFAS desorbing less than 20% of sorbed fraction. Sorption behaviour was demonstrated to be impacted by intra PFAS conger interaction, with positive or negative impacts on sorption and desorption dependant on the specific biochar - PFAS congener combination.<br><br>Further investigation explored the efficacy of pine biochar produced at 750°C as a PFAS sorbent in soils. Soils used in experiments where characterised by their contrasting levels of organic matter and clay. It was found that soil type had a notable impact on biochar efficiency, with biochar being more efficient as a sorbent in soils characterised by higher clay, than in that with higher organic matter.<br><br>This work strongly suggests biochar can be employed as sustainable sorbents for PFAS. To achieve this goal, further investigation surrounding reverse engineering of biochar over a greater range of temperatures, residence times and feedstocks is required. Additionally, biochar application should be tailored to matrix type, environment type and target PFAS, for optimum performance at environmentally relevant PFAS concentrations.