Removal of nanoplastics from aquatic environments using graphene oxide/chitosan sponges

A three-dimensional porous graphene oxide/chitosan (GO/CS) sponge (average pore size = 21.67 μm; pore size distribution = 0.75-319.26 μm, total pore area = 1.84 m/g; and pH = 5.76) was evaluated for the polystyrene (PS) NPs (hydrodynamic diameter = 31.1 ± 0.4 nm) removal from aqueous solutions. The adsorption kinetics followed a pseudo-second-order model, and intra-particle diffusion occurred in three distinct stages: film diffusion, pore diffusion, and surface sorption. Adsorption isotherms aligned best with the Langmuir model, indicating a maximum adsorption capacity of 33.30 mg/g. Thermodynamic analysis showed that PS NPs adsorption was spontaneous (ΔG° < 0) and endothermic (ΔH° > 0) within the temperature range of 283-293 K. FT-IR and XPS analyses confirmed that adsorption occurred through multiple mechanisms, including electrostatic attraction, hydrogen bonding, and π-π stacking. The adsorption behavior was significantly affected by pH, salinity, and competing ions. The highest adsorption capacity (17.99 mg/g) was observed at pH 5.5, which is close to the pH (5.76) of the sponge, due to favorable electrostatic and hydrophobic interactions. As NaCl concentration increased, adsorption capacity remained stable up to 20 mM, but declined sharply above 100 mM due to PS NPs aggregation. Divalent cations, particularly Ca, significantly reduced adsorption by compressing the electrical double layer and enhancing aggregation, while monovalent cations and anions had minimal impact. The sponge was reusable after a simple squeeze-wash regeneration, achieving 83.12 % removal efficiency after 12 consecutive adsorption-desorption cycles. Compared to deionized water, real water samples exhibited similar or improved adsorption capacities, with the highest capacity observed in seawater (15.78 mg/g).