Functionalized spent coffee grounds by citric acid and polydopamine for eliminating cationic dyes from wastewater: Investigation on recyclability, salt resistance and adsorption mechanism

Here, a novel approach for preparing PDA@SC adsorbents by chemically modifying spent coffee grounds with citric acid and polydopamine was proposed. Citric acid concentration (0.6–1.5 M) and dopamine polymerization time (6–18 h) were used as reaction factors in the study and were combined with each other to obtain optimal reaction conditions. The adsorbents obtained under optimized conditions were characterized by scanning electron microscopy, Fourier infrared spectroscopy, thermalgravimetric analysis and X-ray photoelectron spectroscopy. Maximum adsorption capacities of PDA@SC towards cationic dyes were found as 1361.5 and 291 mg g⁻¹ for Malachite green (MG) and Methylene blue (MB), respectively. The removal of each dye in the real water systems exceeds 75 %. Especially, a selectively removal of MG from mixed dye solutions with cationic MG and anionic Methyl orange reached beyond 97.5 %. The adsorption capacity of PDA@SC also displayed an increase/decrease with pH (4−10)/NaCl (CaCl₂) concentration (0.01–0.07 M), and the adsorbent maintained above 62 % removal after five cycles of adsorption-desorption, showing good resistance to solution pH, salinity and outstanding recyclability. The adsorption process is verified as a spontaneous and exothermic process which complies with pseudo-second-order and Langmuir models. The adsorption mechanism explored that electrostatic interaction, π-π conjugates and pore filling are predominant effects. The advantages of feasible route, eco-friendly and high efficiency make PDA@SC a desirable adsorbent for the applications in dye wastewater treatment.