Development of a Novel Biochar-Molecularly Imprinted Polymer Composite for Targeted Adsorption of Perfluoroalkyl 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 current industry standard for PFAS removal during water treatment. Biochar materials produced from agricultural food waste have recently been explored as more sustainable, cost-effective alternatives to granular activated carbon for PFAS removal in water. For example, a novel spent coffee grounds biochar (“SCGKOH”) produced and tested in this study possessed PFOS removal capabilities comparable to a commercially available activated carbon at environmentally relevant concentrations. SCGKOH demonstrated a maximum adsorption capacity of 43.4 mg/g compared to Filtrasorb® F300 activated carbon (55.7 mg/g) and a wood-based fly ash char (79.5 mg/g). PFOS adsorption by all materials 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. Additionally, short chain PFAS compounds—which are replacing longer chain PFAS in some manufacturing applications, and have shown many of the same toxic end points as their longer chain counterparts—are poorly removed by activated carbon and biochar. Thus, there is a need for a more selective PFAS removal method for water treatment. Molecularly imprinted polymers (MIPs) are a class of materials designed with high selectivity for a template compound used during synthesis to create an adsorption binding site with tailored size, shape, and affinity. MIPs have limited applications due to their small size which hinders deployment. In this study, the aforementioned SCGKOH biochar was modified with MIPs to facilitate targeted PFAS adsorption in water. Waste derived biochar presents a low cost, widely available, easily tunable, and high surface area substrate ideal for functionalization with a MIP coating. The biochar was first modified with amine groups via electrophilic aromatic substitution followed by reduction, or pyrolysis in the presence of melamine or ammonium chloride. Amine groups served as attachment points for MIP coating achieved via 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. Materials were characterized using various physicochemical characterization techniques to confirm the success of each modification. PFAS adsorption capabilities were evaluated in the presence of simulated wastewater effluent organic matter, common ions, and co-occurring organics. Results indicated that the MIP coated biochar have higher selectivity for PFAS than the unmodified biochar with adsorption of PFOS > PFOA > PFBS. Regeneration of spent adsorbent was successfully achieved with a 70% methanol, 1% sodium chloride solution. These materials present a novel, cost-effective option for targeted removal of PFAS from wastewaters.