Synthesis and Adsorption Characteristic Evaluation of Copper Hexacyanoferrate for Nanoplastic Adsorption Removal

Abstract

With the rapid increase in the production and consumption of plastics, the amount of plastic entering various ecosystems is expected to rise significantly. Once introduced into the environment, these plastics undergo diverse physicochemical processes that reduce their size to nanoplastics (NPs) under 1µm, posing unique challenges in removal and raising concerns regarding potential toxicity. In this study, we investigated the use of copper hexacyanoferrate (CuHCF), a metal–organic framework (MOF) with a high surface area and porous structure, as an effective adsorbent for NP removal. Notably, CuHCF exhibits metal-to-metal charge transfer (MMCT) upon visible light irradiation, which leads to rearrangement of surface charges and promotes electrostatic attraction with NPs. CuHCF was synthesized via a hydrothermal method involving the mixing of precursor materials, followed by solid–liquid separation and drying to obtain a powder. The synthesized product was characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). SEM images revealed nanoparticles with an average diameter of approximately 200nm, while XRD patterns, when compared with standard data in the JCPDS library, confirmed the successful formation of CuHCF. Polystyrene nanoplastics (below 100 nm in size) were selected as model adsorbates, and a calibration curve based on turbidity measurements was established to facilitate quantitative analysis. Adsorption kinetics and isotherm experiments were performed to assess the adsorption equilibrium time and maximum adsorption capacity (qmax). The results demonstrated that NP adsorption reached equilibrium within 20 minutes, and the qmax was 2586 mg/g, a value 75- to 1000-fold higher than those reported for other adsorbents (i.e., granular activated carbon, magnetic zeolite) used for NP removal. Furthermore, the adsorption capacity under visible light irradiation increased by about 20 times, suggesting that MMCT-driven surface charge rearrangement significantly enhances NP removal. These findings highlight the potential of CuHCF as a solar-driven adsorbent for water treatment applications and for the removal of nanoplastics in various environmental settings.

Keywords

• nanoplastics
• metal-organic framework
• polystyrene
• metal to metal charge transfer
• turbidity