Development of Novel Biochar-Molecularly Imprinted Polymer Composites for Targeted Adsorption of Per- and Polyfluoroalkyl Substances in Water Treatment Applications

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a class of emerging contaminants which have received considerable research and regulatory attention in the last decade because of their toxicity, persistence, and prevalence in environmental and human reservoirs. Adsorption by activated carbon is the industry standard for PFAS removal from water; however, biochar produced from agricultural food waste has recently been explored as a sustainable, cost-effective alternative for PFAS treatment. For example, a novel spent coffee grounds biochar (“SCGKOH”) produced and tested herein possessed perfluorooctanesulfonate (PFOS) removal capabilities (43.4 mg/g maximum adsorption capacity) comparable to a commercially available Filtrasorb® F300 activated carbon (55.7 mg/g) and a wood-based fly ash char (79.5 mg/g). PFOS adsorption increased in the presence of divalent cations but decreased when simulated effluent organic matter was added. This phenomenon has been observed in a number of studies which have shown that PFAS removal by biochar and activated carbon decreases significantly in matrices with high concentrations of organic matter or competing organic and inorganic species. Thus, there is a need for a more selective PFAS removal method for water treatment. Molecularly imprinted polymers (MIPs) possess high selectivity for a template compound used to create adsorption binding sites with tailored size, shape, and affinity; however, MIP morphology hinders deployment in water treatment. In this study, the aforementioned SCGKOH was modified with a MIP coating to facilitate targeted PFAS adsorption in (waste)water. Waste derived biochar presents a low cost, widely available, easily tunable, and high surface area substrate ideal for MIP functionalization. Amine groups (either native or functionalized on the biochar) served as attachment points for MIP during thermally activated radical initiated polymerization. A quaternary nitrogen-containing monomer was chosen to confer positive surface charge over a wide pH range—an important characteristic for effective PFAS adsorption. Initial synthesis of a PFOS-templated biochar-MIP (BC@MIP) composite demonstrated adsorption of perfluoroalkyl acids (PFAAs) comparable to unmodified biochar in ultrapure water (0.043 and 0.039 mg PFAA/g*g/m2, respectively), and increased adsorptive removal (by 0.012 mg PFAA/g*g/m2) in synthetic wastewater due to reduced MIP swelling and non-specific binding. However, the high selectivity of the BC@MIP (Kselectivity of 4.52 for perfluorobutanesulfonate [PFBS] and 3.76 for perfluorooctanoic acid [PFOA]) resulted in lower removal of non-template PFAS which is not ideal for water treatment. In particular, good removal of short chain PFAS compounds—which are replacing longer chain PFAS in manufacturing applications and have shown similar toxic end points to their longer chain counterparts—is a requirement for high performing PFAS adsorbents. To overcome this challenge, a multi-template BC@mMIP was synthesized via simultaneous templating with 6:2-fluorotelomer sulfonate (6:2-FTS), perfluorobutanesulfonate (PFBS), and perfluoropentanoate (PFPeA). The BC@mMIP adsorbent was capable of treating PFOA, PFBS, and perfluorohexanesulfonate (PFHxS) to below their proposed US EPA MCL in ultrapure water containing nine PFAS at (waste)water-relevant concentrations. Column testing in real wastewater effluent revealed competition between total dissolved solids (TDS) species and PFAS; thus, this material is recommended for use in water treatment with low TDS or used in series with a pre-treatment like ion exchange for optimal performance These BC@MIP materials present a novel, cost-effective option for targeted removal of PFAS from (waste)waters.