Assessing the Potential of Activated Bauxite Residue (ABR) to Remove PFAS in the Water Column
Creators: Pang, Jingya
Publisher: name: University of Alberta Library
Year: 2024

<p>Per- and polyfluoroalkyl substances (PFAS) belong to a group of synthetic organic compounds characterized by the substitution of at least one hydrogen atom with fluorine and the presence of other functional groups. Their high thermal and chemical stability makes PFAS resistant to removal and degradation. PFAS have been widely detected in the environment and food chain, with potential effects on humans. Wastewater treatment plant (WWTP) effluent is a significant source of PFAS emissions, and conventional wastewater treatment processes have proven ineffective against PFAS. This has sparked a growing interest in adsorption as a potential treatment option. Bauxite residue (BR), a by-product of alumina extraction, is highly alkaline material that poses environmental and health risks due to the potential release of toxic materials. However, BR has a potential to be reused for other purposes including wastewater treatment. This study investigates the efficacy of activated bauxite residue (ABR) as an adsorbent material for removing PFAS in the water column and evaluates its capacity as a viable solution for PFAS treatment technology. The potential of ABR for PFAS removal from wastewater was evaluated using a comprehensive approach involving three aspects: (1) characterization of ABR, focusing on porosity, elemental composition, bond types, and surface charge, (2) evaluation of adsorption kinetics and isotherms to quantify adsorption rate, capacity, and PFAS removal efficiency, and (3) biological toxicity testing via acute toxicity (Daphnia magna) and in-vitro bioassays to address cell toxicity pathways (cytotoxicity, estrogenicity, and mutagenicity). Brunauer-Emmett-Teller (BET) surface analysis revealed ABR is predominantly mesoporous, which can enhance the adsorption capacity and removal efficiency of PFAS. The pore size distribution indicated the heterogeneity of pore sizes on ABR. X-ray photoelectron spectroscopy (XPS) analysis showed an increase in fluorine (F) relative atom concentration from 1.37% to 57.69% after saturation with 100 mg/L PFAS (100 mg/L for individual of 10 PFAS), suggesting surface adsorption. The elemental concentration obtained from XPS and energy-dispersive X-ray spectroscopy (EDX) (detection depth of 1–10 nm vs up to 5000 nm) further confirmed the presence of F on the surface. Furthermore, carbon-fluorine bonds were detected on the “spent” ABR, supporting this conclusion. The most significant difference in surface charge between “virgin” and “spent” ABR occurred at pH 6–8, emphasizing the need to neutralize ABR solution to improve the removal efficiency. Various dosages of ABR (10, 15, 25, 50, and 100 g/L) and exposure periods (1 h vs 24 h) were tested. The highest sum removals (∑PFAS) of ~91% was achieved at 100 g/L, but even a 10 g/L dosage was nearly as effective (~87%). This was comparable to a commercial powdered activated carbon (PAC), suggesting the potential of ABR to exhibit better performance at higher concentrations. The removal efficiency was higher for long-chain PFAS (#C ≥6 for perfluoroalkyl sulfonic acids [PFSAs] and ≥8 for perfluoroalkyl carboxylic acids [PFCAs]), where the stronger electrostatic and hydrophobic interactions and higher molecular weight may contribute. Also, the short-term exposure was sufficient for the complete removal of long-chain PFAS, while a longer exposure period may be required for short-chain ones. Pseudo-second-order (PSO) adsorption kinetic models fit PFAS slightly better than other models (R2 ranged from 0.995 to 1), suggesting that chemisorption likely occurred on the ABR surface. However, individual PFAS followed different isotherm models (Langmuir, Freundlich, and Sips) in the mixture. These differences can be attributed to (1) the pore blocks of long-chain PFAS, (2) the interactions between PFAS molecules, and (3) the varying numbers of layers. Finally, no acute toxicity was observed in Daphnia magna exposed to standardized environmental water treated with 50 g/L ABR, indicating that the treatment process does not introduce additional toxicity into treated water at practical levels. Furthermore, cytotoxicity assessments using the Aliivibrio fisheri assay showed no significant difference across dosages, with the lowest 1/IC10 for 10 g/L ABR. Hence, the use of &lt;10 g/L ABR is effective as both removal efficiency is maximized, and the potential toxicity is minimized. Estrogenicity was not impacted by ABR through yeast-estrogen screen (YES) analysis. Similarly, there was no detectable DNA damage via the umuC assay. Overall, the use of ABR adsorption is a promising solution to remove PFAS from the water column, offering environmental and economic benefits for more sustainable water treatment practices.</p>