Per- and Polyfluoroalkyl Substances Removal Using Carbonaceous Adsorbents in a Brackish Water Circular Economy

Abstract

Circular economy concepts are becoming increasingly attractive in a world with limited resources. This study proposed one that would exist at the intersection of two pressing issues, per- and polyfluoroalkyl substances (PFAS) contamination and dwindling freshwater resources. The concept was developed around treating PFAS-contaminated, brackish irrigation water (BIW) using unconventional carbonaceous adsorbents, including an evaluation of realistic implementation hurdles. PFAS would be removed from BIW, halting their environmental cycling, using powdered bone char (PBC) or powdered halophyte biochar (PHB), while their production would be integrated with livestock grazing and halophyte cultivation, respectively. Halophytes would be irrigated with treated BIW, potentially paired with reverse osmosis for freshwater production, while cattle would use halophytes for fodder/forage. Using a PFAS-contaminated brackish groundwater (BGW) as a representative BIW, higher doses of PBC and PHB relative to powdered activated carbon (PAC) (200-500 mg/L vs. 10-50 mg/L) were required to achieve >50% PFAS removal. Though results highlighted inferior performance, PBC and PHB did achieve comparable PFAS loadings to PAC on a surface area basis, indicating adsorption deficiencies were more related to the lack of specific surface area than surface properties. Increasing PBC- and PHB-specific surface areas by at least 3.5-4.0 times would be required to match PAC performance. Cattle and land requirements were projected to be ∼6,700 animals and 35 ha, respectively, assuming either a 500 mg/L (PBC) or 350 mg/L (PHB) dose to achieve substantial perfluorooctane sulfonic acid (PFOS) removal from 1,000 L/min of BIW. In addition, a simple approach using PFOS dose-response data and the homogeneous surface diffusion model was developed to predict kinetic curves without testing.