Mineralization of perfluorooctanesulfonate (PFOS) and perfluorodecanoate (PFDA) from aqueous solution by porous hexagonal boron nitride: adsorption followed by simultaneous thermal decomposition and regeneration†
Poly- and perfluoroalkyl substances (PFASs) are of global concern due to their toxicity, high persistency, bioaccumulation, and worldwide occurrence. Boron nitride (BN), consisting of light elements and bearing excellent thermal stability, has shown great potential in wastewater purification as a readily-recyclable sorbent. In this study, porous hexagonal BN nanosheets (h-BNs) were synthesized and for the first time their sorption capacities toward perfluorooctanesulfonate (PFOS) and perfluorodecanoate (PFDA) (two representative PFASs) were evaluated under various solution compositions. The h-BNs used after sorption were regenerated by calcining at 600 °C in air for 20 min. The h-BNs synthesized were found to have fast sorption kinetics for both PFOS and PFDA, and the sorption processes fitted well with the Freundlich model and pseudo-second-order kinetics. Under the conditions of 50 mg L−1 PFDA or PFOS, 0.2 g L−1 h-BNs, and pH 6.0, sorption capacities of ∼0.72 mg m−2 and ∼0.45 mg m−2 were achieved for PFDA and PFOS, respectively. The effects of H+ and Ca2+ showed that electrostatic interactions were responsible for the sorption. The reutilization experiments revealed that the h-BNs had a persistent sorption capacity after three cycles. To reduce the production of fluorine-containing gases, calcium hydroxide was used as a calcination additive and the fluorine-fixing product calcium fluoride was successfully detected. The results suggest that h-BN sorption may be a promising approach for the removal of PFASs from an aqueous solution.