Carbon-dot hydrogels as superior carbonaceous adsorbents for removing perfluorooctane sulfonate from water

Efficiently removing perfluorooctane sulfonate (PFOS) from wastewaters is a hot topic in environmental field. Herein, crosslinking the nano-sized and zero-dimensional carbon dots (CDs) with polyethylene glycol diglycidyl ether (PEG) and polypropylene glycol diglycidyl ether (PPG) resulted in CD hydrogels that were verified by TEM, SEM, FTIR and XPS. The obtained CD hydrogels could efficiently adsorb PFOS from water through their CD moieties. The kinetic curves were fitted well by the pseudo-second order model, indicating the involvement of chemisorption in the rate-limiting step. The adsorption isotherm measurements showed that the hydrogel with the highest CD content had a PFOS maximum uptake capacity of ca. 2.82 g/g (5.64 mmol/g) in the neutral water, which was much higher than the ever-reported carbonaceous materials, and also ranked the top three among the superior positively-charged adsorbents. The superior PFOS adsorption of CD hydrogels could be attributed to good dispersion of the nanoscale CDs in the PEG/PPG hydrogel matrix, leading to the substantial exposure of the binding sites of CDs towards PFOS molecules. Moreover, CD hydrogels did not show pronounced loss of adsorption capacity after experiencing five adsorption-desorption cycles. As for the real wastewaters, CD hydrogels could efficiently treat not only the high concentration of PFOS in the neutral firefighting wastewaters, but also the extremely low concentration of PFOS in the neutral environmental wastewater, where the residual PFOS concentration could be reduced less than 5 ng/L, satisfying the standard of the U.S. Environmental Protection Agency.