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Ammonium ions (NH4+) are a crucial part of the nitrogen cycle and are widely present in natural water bodies and industrial wastewater. However, the toxic effects of excessive ammonium ions on ecosystems and biological health have received increasing attention. For example, high concentrations of ammonium in water can damage the nervous system of fish, and the problem of ammonium toxicity in plants also restricts the sustainable development of agriculture. Traditional adsorbents such as zeolites and activated carbon can remove some ammonium ions, but they suffer from low selectivity and insufficient adsorption capacity.
In recent years, sodium cobalt hexacyanoferrate (NaCoHCF) has become a research focus due to its high ammonium adsorption capacity and selectivity. Compared with directly using powdered NaCoHCF, combining it with a substrate to make a filter can prevent powder leakage, enhance reusability, and have more functions. Electrospinning nanofibers, which are produced by an electrospinning machine, with their extremely fine fiber diameters and large specific surface areas, are conducive to ammonium adsorption and are thus selected as the carrier for NaCoHCF. Previous studies have found that micro - sized NaCoHCF particles entangle with multiple nanofibers. If nano - sized NaCoHCF can be embedded in a single nanofiber, more efficient adsorption is expected to be achieved.
This article systematically presents the research progress on significantly improving the ammonium adsorption performance of sodium cobalt(II) hexacyanoferrate (NaCoHCF) through nanonization technology. Nanoscale NaCoHCF was synthesized using microfluidics and combined with electrospinning technology (enabled by an electrospinning device) to prepare composite nanofibers, resulting in a three - fold increase in adsorption capacity. The article details the influence mechanism of nanoparticle size on adsorption efficiency and explores the application prospects of this technology in environmental governance and biomedicine.
(1) Preparation of Micro - sized and Nano - sized NaCoHCF
In this study, two different synthesis methods were used to prepare micro - sized NaCoHCF (micro - NaCoHCF) and nano - sized NaCoHCF (nano - NaCoHCF). When preparing micro - NaCoHCF, aqueous solutions of Na4[Fe(CN)6]⋅10H2O and Co(NO3)2⋅6H2O were stirred and mixed in a beaker, and then the resulting particle dispersion was freeze - dried. For the preparation of nano - NaCoHCF, the two aqueous solutions were mixed in a Y - shaped microchannel. Particles formed during the mixing process, and then the obtained dispersion was freeze - dried (Fig. 1). In this synthesis method, the turbulence generated when the two solutions collide can promote nucleation and inhibit particle growth, thus enabling the synthesis of nanoparticles.
The results of dynamic light scattering (DLS) measurements (Fig. 2a, b) show that the average particle size of micro - NaCoHCF is 4473.2 nm, and that of nano - NaCoHCF is 45.9 nm. The particle size of nano - NaCoHCF is approximately 1/100 that of micro - NaCoHCF, and nano - NaCoHCF has a lower polydispersity index (PDI), indicating that its particles are monodispersed. This is mainly because the turbulence during the mixing of the two solutions in the microchannel forms multiple locally supersaturated regions, promoting nucleation, consuming a large amount of solute, and inhibiting particle growth.
Scanning electron microscope (SEM) images (Fig. 2c - f) confirm that the particle size of nano - NaCoHCF is smaller than that of micro - NaCoHCF. However, the particle size of nano - NaCoHCF observed by SEM is approximately 200 nm, which is larger than the DLS measurement result. This may be due to particle aggregation during the drying process of the nano - NaCoHCF dispersion for SEM sample preparation, and the limited resolution of SEM makes it difficult to accurately observe its morphology. Transmission electron microscope (TEM) images show the presence of particles with a size of approximately 100 nm and cubic structures.
(2) Incorporation of NaCoHCF into Nanofibers by Electrospinning Technology
The synthesized micro - sized NaCoHCF and nano - NaCoHCF were incorporated into poly(ethylene - co - vinyl alcohol) (EVOH) nanofibers by electrospinning technology (facilitated by an electrospinning device). SEM images show that micro - sized NaCoHCF particles are only attached to the fiber surface, while nano - sized particles are successfully embedded inside the fibers (Fig. 3).
SEM images (Fig. 3a, b) show that in EVOH/micro - NaCoHCF nanofibers, inclusions with a diameter of approximately 20 - 50 μm can be observed, which are presumed to be micro - NaCoHCF. These particles are present on the nanofiber sheet, covered by a thin polymer layer, and are not embedded inside the nanofibers. In contrast, no obvious inclusions are found in EVOH/nano - NaCoHCF nanofibers. TEM imaging (Fig. 3c) and energy - dispersive X - ray spectroscopy (EDX) elemental analysis further confirm that nano - NaCoHCF has been successfully embedded in the nanofibers, as inclusions are observed inside the nanofibers and Fe and Co atoms are detected, while EVOH only contains carbon and oxygen atoms.
(3) Significant Improvement in Ammonium Adsorption Performance
The results of adsorption tests (Fig. 4) show that pure EVOH nanofibers have no ammonium adsorption capacity. The ammonium adsorption amounts of both EVOH/micro - NaCoHCF and EVOH/nano - NaCoHCF nanofibers increase with time. After 2 hours of testing, the ammonium adsorption amounts are 38.1 mg/g and 126.8 mg/g, respectively. The adsorption performance of nano - NaCoHCF is more than three times that of micro - NaCoHCF. This is due to the higher specific surface area of nano - NaCoHCF, which allows for more efficient utilization of adsorption sites. In the case of micro - NaCoHCF, only surface adsorption sites may be utilized, while the internal adsorption sites are less efficiently used.
Environmental Governance
This material can be used as an efficient ammonium - adsorbing filter for wastewater treatment, especially suitable for high - salinity or complex water quality conditions.
Biomedical Potential
The flexible structure of nanofibers is adaptable to blood purification devices and is expected to be used in the treatment of metabolic diseases such as hyperammonemia.
Feasibility of Large - scale Preparation
The compatibility of microfluidic synthesis and electrospinning technology (using an electrospinning machine) provides a technical route for industrial production.
In this study, nano - NaCoHCF particles with a size of approximately 50 nm were successfully synthesized and incorporated into nanofibers. This significantly enhanced the ammonium adsorption performance of NaCoHCF in nanofibers. The adsorption capacity is more than three times that of micro - sized particles, exceeding that of common ammonium adsorbents such as zeolites (about 8 - 50 mg/g) and biochar (about 5 - 43 mg/g), demonstrating good practical adsorption performance. Moreover, since NaCoHCF is embedded in nanofibers, the risk of leakage can be minimized, making it a promising ammonium - adsorbing filter material.
In addition, the research results are not only applicable to NaCoHCF but also have reference value for many adsorbent materials, providing a direction for the further development of electrospun nanofiber/inorganic hybrid materials. In the future, it is expected to develop more efficient and stable adsorbent materials based on this research to address ammonium pollution and other issues in the environment and promote the development of related fields.
Article Source: DOI: 10.1039/d5cc00625b
