Study On The Enrichment And Separation Of AgNPs In Water By Iron Based Materials

With the development of nano-technology, more and more noble metal nano-particles are increasingly used in industrial and household products, thereby leading to the release of noble metal nano-particles into the environment as well as threating the ecological and biological health. The problem of environmental pollution caused by noble metal nanoparticles has gradually attracted widespread attention. At present, there are few studies reported on the enrichment and extraction of silver nanoparticles（Ag NPs） in the environment by IER, CPE and ion exchange resin, etc.However, these methods have shortcomings such as complex operation, small sample size and low enrichment efficiency. Therefore, it is necessary to develop a simple and efficient method for the enrichment and separation of Ag NPs from water. In addition,Ag NPs is not only rare and valuable but also an effective catalyst material. Therefore, it is significant to find a simple and effective method to enrich, separate and recycle. For this purpose, in this paper, we studied enrichment properties of Ag NPs by the magnetic polydopamine（Fe₃O₄@PDA）, MOF-235 and calcined GO/MIL-88（Fe）. We also explored the catalytic properties of dyes and 4-NP. The main contents are as follows:1. We used a simple method to synthesize Fe₃O₄@PDA core-shell micro-spheres.The as-prepared Fe₃O₄@PDA was characterized by XRD, FT-IR and TEM, showing that PDA was successfully grafted onto Fe₃O₄. The prepared magnetic material wasused as adsorbent for the enrichment and separation of GA-Ag NPs from aqueous solution. The effects of solution p H, different ligands, adsorption time, initial concentration of GA-Ag NPs, salt and humic acid（HA） concentration on the adsorption were studied. The experimental results showed that the adsorption capacity reached the maximum at p H=10, showing the best adsorption effect on GA-Ag NPs. The adsorption process could be described well by the Langmuir adsorption isotherm and the maximum adsorption capacity is 169.49 mg/g. In addition, we reclaimed the material after adsorption（Ag NPs-Fe₃O₄@PDA） which was used in catalytic removal of methylene blue（MB）. The results showed that the 20 m L 7.5 ppm MB was completely removed in30 min. Ag NPs-Fe₃O₄@PDA not only presented excellent stability but also can be reused for at least 8 times.2. We utilized one step hydrothermal method to synthesized metal organic framework material MOF-235, which was used to enrich and separate GA-Ag NPs from aqueous solution for the first time. The XRD, BET, TG,SEM and FT-IR results showed that we successfully synthesized MOF-235 with high purity and complete crystal structure. Ag NPs was successfully enriched onto the MOF-235 surface in the form of agglomeration. During the experiment, we studied the effects of p H, ionic strength, HA concentration and adsorption time on the adsorption process. The results showed that the maximum adsorption capacity at p H=6. External ion promoted the adsorption while HA inhibited adsorption, reaching to adsorption equilibrium after 12 h. The adsorption kinetics fitted well with pseudo-second-order model. The adsorption process could be described well by the Langmuir adsorption isotherm and the maximum adsorption capacity is 155.76 mg/g. Besides, the material after adsorption（MOF-235/Ag NPs） had good catalytic effect on 4-NP, Ks=2.7x10-3/sec.3. We incinerated GO/MIL-88（Fe） composite material to obtain the magnetic adsorbent material. It was used to enrich and separate Cit-Ag NPs from aqueous solution for the first time. The addition of GO not only improve the yield of MOF but also increase the adsorption capacity of the materials after calcined. The materials were characterized by XRD, TG, SEM and FT-IR. We studied the effects of GO ratio and calcination temperature as well as p H, ionic strength, HA concentration and adsorption time on the adsorption process. Results indicated that the best adsorption capacity happened when GO ratio was at 2.5%, alcination temperature at 700 ℃ and p H at 6.External ion promoted the adsorption while HA inhibited adsorption. The adsorption kinetics fitted well with pseudo-second-order model. Adsorption isotherm fitted wellwith the Langmuir adsorption isotherm and the maximum adsorption capacity was483.59 mg/g.