Nano-zero-valent iron (nZVI) refers to Fe particles with a particle size of 1-100 nm and a specific surface area of 10-70 m2/g. It has high catalytic performance and reaction activity. Researchers have found that zero-valent iron has good environmental friendliness and strong reducibility, and can be used as a medium for in-situ treatment of seepage reaction barrier in groundwater. Since then, more and more researchers began to study this material. At present, it has excellent performance in the degradation of organic matter (degradation of organic pesticides) and inorganic pollutants (denitrification, denitrification, perchlorate reduction and heavy metal ions).
The mechanism of removing heavy metals is that when nZVI contacts oxygen or water in the air, it will form oxides, hydroxides or coprecipitate with them, thus adsorbing heavy metals. It has been proved that Cr6+ can be reduced by nZVI to form precipitation. At present, nZVI is mainly prepared by reduction of FeCl3 and NaBH4 or reduction of goethite and hematite at 200-600 C. However, the size, morphology and iron phase of nanoparticles will affect their adsorption properties for heavy metals. For example, under the protection of N2, Kanel reduced FeCl3 with NaBH4 to prepare nZVI with particle size of 10-100 nm and used it to adsorb As 3+ in groundwater. It was found that the presence of HCO 3-, H 4 SiO 40 and H 2 PO 4 interfered with the adsorption of As 3+.
Laser scattering shows that nZVI and As3+ form inner surface complexes, and the surface reactivity is very high, so it can be used to treat groundwater in situ and in situ.
Although this liquid-phase reduction method is simple and yields high, a large number of raw materials and reagents are needed in the reaction. The increase of production cost hinders the wide application of this material in industry. In order to reduce the cost of production, Fang prepared nZVI by sodium borohydride reduction, polyvinylpyrrolidone as dispersant and adding 2.46 mL diluted acidic waste liquid from iron and steel plant, which reduced the production cost to a certain extent and effectively utilized the steel-making wastewater. The removal rate of Cr6+ by the nanoparticles is very high, reaching 99.9%. The main product of the reaction is Cr0.67Fe0.33OH or (Cr0.67Fe0.33) (OH) 3. The reaction principle is shown in Figure 1. Compared with nZVI or nZVI-activated carbon, the absorption efficiency of this substance for Cr6+ is increased by 36%. It can not only remove Cr6+, but also completely remove Cr3+. Although nZVI, as a potential environmental remediation agent, provides a new development direction for environmental remediation, its application in surface shallow water and other fields is limited by its weakness in reduction, difficulty in preservation and large particle size. Therefore, the stability of nZVI and how to use it in actual industry are limited. The application needs further study.