PFAS removal from aqueous media using natural waste derived adsorbent: Synthesis optimization and performance evaluation in a closed loop sustainable process

Per- and poly-fluoroalkyl substances (PFASs) have attracted worldwide attention in recent years due to their persistent nature, potential toxicity and high bioaccumulation. This study deals with adsorptive removal of PFAS from aqueous media. The activated carbon of waste Karanja (Pongamia pinnata) was optimally synthesized. Surface functional group modification was performed using a cationic surfactant, Cetyltrimethylammonium bromide (CTAB). Selectivity analysis of the adsorbent for 500 µg/L PFOA against different background species present at their typical river concentration was performed. To make the process sustainable, the development of regenerant specifications and recovery of the regenerant and PFOA from the spent regenerant solution were also investigated. A biomass/acid ratio of 1:2.5, temperature 450 C, and 1% CTAB solution were found to be optimum for the adsorbent synthesis for PFOA removal. The adsorption of PFOA was fast, and equilibrium was achieved within 4 hours. The Langmuir model provided a good fit for the adsorption-isotherms data for both adsorbents. After functional group modification through CTAB, the PFOA uptake capacity of the adsorbent increased up to threefold. The maximum PFOA adsorption capacity exhibited by Karanja activated carbon (K_AC) & CTAB-modified activated carbon (CTAB_K-AC) was 155.5 mg/g & 455.8 mg/g, respectively. The adsorbent exhibited high PFOA selectivity against all background co-contaminants, i.e., NOM (humic acid), clay (bentonite), anions (Cl^-, NO₃^-, SO₄^2-, PO₄^3-, SiO₃^-, and CO₃^2-) and cations (Ca²⁺ and Mg²⁺) and removed around 90% PFOA. The regenerant (50% ethanol with 1% NH₄OH aqueous solution) showed 80-90% PFOA recovery. The distillation recovered 90% PFOA and 80-90% regenerant from the spent regenerant solution. The packed-bed column run of the adsorbent showed high PFOA removal capacity (>2000 bed volumes can be treated per gram of the adsorbent before break-through) from the river water matrix. The results showed that CTAB_K-AC can be an effective adsorbent for PFOA adsorptive remediation from aqueous media with high treatment selectivity towards the PFOA in a materially closed-loop sustainable process.