Low-Temperature Conversion of Discarded Cigarette Filters into Magnetic Porous Carbon for Diethyl Phthalate Removal

Discarded cigarette filters represent a widespread cellulose-acetate waste stream with the potential for conversion into functional carbon materials. In this study, discarded cigarette filters were converted into magnetic porous carbon for diethyl phthalate (DEP) adsorption through hydrothermal carbonization, H3PO4 activation at 300 °C, and subsequent Fe(II)/Fe(III) coprecipitation. Activation increased the Brunauer–Emmett–Teller (BET) specific surface area of the hydrochar from 5.96 to 312.47 m2 g−1, while magnetic modification further increased it to 542.41 m2 g−1. The incorporation of Fe-containing species was confirmed by X-ray photoelectron spectroscopy, and magnetic functionality was demonstrated by superconducting quantum interference device measurements and separation using an external magnet. DEP removal by the resulting material (MAC-11) was influenced by the initial DEP concentration, solution pH, adsorbent dosage, and temperature. Response surface methodology identified a candidate operating condition of 30.4 °C, 14.6 mg adsorbent, and 236 min, under which an independent confirmation experiment achieved 87.7% DEP removal, compared with a model prediction of 85.0%. The pseudo-second-order model provided a better empirical description of the adsorption kinetics than the pseudo-first-order model, while both the Langmuir and Freundlich models described the concentration-dependent adsorption data. Although magnetic modification reduced DEP adsorption relative to the nonmagnetic activated carbon (AC-300), MAC-11 retained appreciable adsorption performance while acquiring magnetic separability. These findings demonstrate the feasibility of converting discarded cigarette filters into magnetically responsive porous carbon through comparatively low-temperature activation, highlighting the trade-off between adsorption performance and magnetic recoverability.