Nitrogen rich core–shell magnetic mesoporous silica as an effective adsorbent for removal of silver nanoparticles from water
Graphical abstract
Introduction
With the rapid development of nanotechnology, environmental issues from nanomaterials have caused tremendous attention of the scientific community [1]. In particular, the development and application of silver nanoparticles (AgNPs) have been conducted over the past 120 years [1]. AgNPs have favorable antibacterial property [2] and have been widely used for cosmetics, paints and textile fabrics in human daily life [3], [4]. The annual production of AgNPs is about 500 tons [5]. The wide use of AgNPs inevitably results in their release into wastewater from manufacturing and recovery of commodities, and finally entering into wastewater treatment plants [6]. Due to its antimicrobial activity, the potential adverse effects of AgNPs on human health and the environment have been concerned [7], [8]. For example, the previous reports have demonstrated AgNPs are toxic to fish [9], algae [10], animal [11], human cells [12], and nitrifying bacteria [13], [14]. Therefore, the removal of AgNPs from water is urgently needed. However, investigations on the removal of AgNPs were limited in the literature until now. For example, Valiyaveettil's group studied adsorption removal of AgNPs by the amine functionalized polyvinyl alcohol nanofibers (PVA NFs) [15], PVA/gluten hybrid nanofibers [16], chitosan coated cellulose nanofibers [17], metal oxides derived from eggshell membrane as biotemplate [18], PEI modified carbon spheres [19], and amine-modified block copolymers [20]. Khan et al. illustrated the potential of the resistant bacterial species Aeromonas punctata for the removal of AgNPs [21]. More recently, the group of Černík reported the use of methane plasma treated electrospun nanofibres for the removal of various NPs in water [22], [23]. These studies demonstrated the adsorption of engineered NPs onto adsorbents is an economical and simple way for their removal. However, these materials showed low adsorption capacities ranging from 5 to about 170 mg/g level. Hence, it will be interesting to develop new materials with low-cost, improved adsorption, and easy separation from aqueous solution, which is highly desired to reduce a risk to the environment due to the toxic effects of AgNPs.
The core–shell magnetic mesoporous silica NPs have attracted significant attention in the field of chemical sensors, drug delivery and water treatment [24], [25], [26] due to their easy separation, ordinary synthetic methods and low cost. However, owing to their poor adsorption properties, it is highly desirable to functionalize the surface of magnetic mesoporous silica NPs. And functionalization with aminonaphthalimide, porphyrin and AgNO3 has been reported to prepare the desirable magnetic SiO2 NPs for removal of toxic heavy metal ions [26], [27], [28], [29]. In particular, poly(ethylenimine) (PEI) has been widely selected for the surface functionalization of metal oxides to obtain nitrogen rich composites, including alumina, silica and zirconia [30], [31], [32] because it is a cationic polymeric with branched chains containing a large number of amino groups [33], which possess good metal chelation properties. Thus, it is anticipated that notable adsorption ability of AgNPs onto the nitrogen rich magnetic mesoporous silica may be obtained. However, to the best of our knowledge, the capability of the nitrogen rich magnetic mesoporous silica materials through PEI functionalization toward removal of AgNPs has not been investigated. Here, we reported the synthesis of the PEI functionalized magnetic mesoporous silica and further investigated the feasibility of the prepared material for the adsorption removal of AgNPs from water. The adsorption characteristics between AgNPs and adsorbent, such as the kinetics, isotherms and factors affecting adsorption capacity were further investigated. Also, the catalytic performance and reusability of AgNPs onto Fe3O4@SiO2-PEI were evaluated using the reduction of 4-nitrophenol (4-NP) by NaBH4 as a model system. The result demonstrates that the obtained Fe3O4@SiO2-PEI-AgNPs composite exhibits highly catalytic performance toward 4-nitrophenol reduction. Our result provides a sustainable way to merge the recovery of noble metallic NPs as pollutants and reuse of the recovered noble metallic NPs as recyclable catalyst for environmental remediation.
Section snippets
Reagents and chemicals
All the chemicals were of analytical grade. A branched PEI with average molecular weight of ∼25,000 was purchased from Sigma–Aldrich (Shanghai, Sigma–Aldrich, China). Tetraethyl orthosilicate (TEOS) was from Kelong Chemical Reagents Company (Chengdu, China). All of other chemicals were purchased from Chongqing Chemical Reagents Company (Chongqing, China).
Preparation of magnetic Fe3O4
Magnetic Fe3O4 was prepared through the chemical co-precipitation method. Typically, 6.1 g of FeCl3·6H2O and 4.2 g of FeSO4·7H2O were dissolved
Characterization
Fig. 1 shows the TEM images of the prepared materials, in which the Fe3O4 NPs display sphere like morphology with an average diameter of 13.0 ± 3.0 nm (Fig. 1A). The Fe3O4@SiO2 NPs exhibit core–shell structure with an average diameter of 13.9 ± 3.3 nm and have a smooth silica shell coating with about 1 nm thickness (Fig. 1B). After PEI functionalization, the Fe3O4@SiO2 NPs reserved the spherical shape and the core–shell structure with an average diameter of 14.4 ± 3.0 nm (Fig. 1C). Fig. 1D shows the
Conclusion
In summary, we have successfully demonstrated that the nitrogen rich core–shell magnetic mesoporous silica is a promising adsorbent for the removal of AgNPs with a maximum adsorptive capacity of 909.1 mg/g, which is 5–181 times higher than that of the previously reported adsorbents for AgNPs. Moreover, after adsorption of AgNPs by Fe3O4@SiO2-PEI, the resultant Fe3O4@SiO2-PEI-AgNPs composite exhibits highly catalytic performance toward 4-nitrophenol reduction with convenient separation ability
Acknowledgment
Financial support from the National Natural Science Foundation of China (No. 21277111 and No. 51678485) is gratefully acknowledged.
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