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Lignin-based activated carbon as an effective adsorbent for the removal of polystyrene nanoplastics: Insights from adsorption kinetics and equilibrium studies
Summary
Scientists created activated carbon filters from lignin, a natural plant material, that effectively removed polystyrene nanoplastics from water. The filters worked through a combination of physical trapping in tiny pores and chemical interactions between the carbon surface and plastic particles. This research demonstrates a sustainable approach to filtering the smallest and most harmful plastic particles from water, potentially reducing human exposure through drinking water.
• The adsorption of nanoplastics in aqueous solution was effective for all adsorbents. • The porous structure and the nature of the surface controlled the removal efficiency. • The adsorption process was endothermic and dominated by physical adsorption. • L-FeCl 3 showed higher adsorption capacity due to its high macroporosity. In this study, activated carbons were prepared from lignin using different activation agents (FeCl 3 , H 3 PO 4 and KOH) and tested as adsorbents of polystyrene nanoplastics (PS). The prepared activated carbons were completely characterized with special attention to their porous structure, morphology and surface chemistry. Activation with KOH resulted in the highest BET surface area (1389 m 2 ∙g −1 ) with a remarkable contribution of mesopores, while FeCl 3 produced mainly a microporous solid with a slightly lower surface area (1063 m 2 ∙g −1 ). H 3 PO 4 resulted in the lowest surface area (467 m 2 ∙g −1 ) mainly in the form of micropores. FTIR analysis revealed π-π interactions between the benzene rings of PS and the FeCl 3 activated carbon surface, while H-bonding seems to play a major role with H 3 PO 4 and KOH carbons. The pseudo-second-order kinetic model provided the best fit to the experimental data for the FeCl 3 and KOH samples, suggesting a significant contribution of chemical adsorption in the adsorption mechanism. However, the results for H 3 PO 4 carbon fit better to the pseudo-first order model, suggesting that physisorption is the rate-controlling step. The adsorption process was quite rapid and increased with temperature, indicating an endothermic process consistent with chemical interactions between adsorbate and adsorbent (ΔH = 7.1 kJ·mol -1 of FeCl 3 ). The Toth isotherm model provided the best fit to the equilibrium data at all temperatures and resulted in an adsorption process of endothermic nature, with physical adsorption being the dominant process. Breakthrough curves were obtained for different adsorption temperatures and fit accurately to a logistic type of equation representative of the Bohart-Adams, Thomas, and Yoon-Nelson models. Thermodynamic analysis indicated that the adsorption was spontaneous and feasible. In conclusion, lignin-based activated carbons, especially in the case of FeCl 3 activation, showed promising adsorption capacities (49.53 mg∙g -1 at 75 °C) and kinetics for the removal of polystyrene nanoplastics from water.