Preparation Of Entrapped Fe~0 Nanoparticles For Hexavalent Chromium (Cr(Ⅵ)) Removal From Wastewater

Due to the unreasonable discharge of wastewater, surface water was contaminated by hexavalent chromium (Cr (VI)). Cr (VI) is a potential carcinogen and often causes both short term and long term adverse effects to humans, animals, and plants. As a result, removal of Cr (VI) from surface water has caused more and more attention from researchers. In recent years, nanoscale zero-valent iron (NZVI) has been widely introduced into water treatment processes to remove numerous heavy metals. However, the agglomeration, oxidation by non-target compounds and higher mobility in the aqueous solution are the major challenges for NZVI use in environmental remediation. To overcome these problems, entrapped Fe0 nanoparticles have been successfully prepared. NZVI coated by citric acid and chitosan (CS) beads shows good dispersibility and stability. Carbon fiber (CF) and epichlorohydrin (ECH), were used to enhance the mechanical strength of CS-NZVI beads. The main objectives of the study are to:NZVI modified by citric acid are nearly spherical in shape and uniform in size with a mean diameter of 65.2 nm. NZVI particles are better protected by citric acid and oxidation is hindered. The reduction capacity for Cr (VI) increases with increasing temperature and NZVI dosage but decreases with the increase in initial concentration of Cr (VI) and humic acid (HA) and pH values. HA have a better ability to chelate iron ions or oxide surface and will hinder the formation of Fe(III)-Cr(III) precipitate. The chelation reaction will reduce reactive sites of NZVI available to Cr (VI), so HA adversely affects Cr (VI) removal from the wastewater.The optimum conditions of preparation of CS-NZVI beads are: an initial acetic acid solution concentration of 0.5 % (v/v), an initial NaOH concentration of 0.5 M, an initial CS concentration of 5.0 g/L, a height between the exit of the sample and the NaOH surface of 10 cm, 55℃and 4 h. CS-NZVI beads are black, nearly spherical in shape and uniform in size with a mean diameter of 3.1 mm. CS-NZVI beads are macroporous and the pore sizes in the CS-NZVI beads are heterogeneous. The pore size ranges from 9.5 to 108.8μm with an average aperture size of around 42.6μm. Entrapment of NZVI in CS beads prevents the particles from aggregation and oxidation. The Cr (VI) removal rates depended on the dosage of NZVI, the pH value, the initial concentration of Cr (VI) and the reaction temperature. With an initial Cr (VI) concentration of 20 mg/L, a NZVI dosage of 5.0 g/L, a solution pH of 3.9 and 20℃, the removal rate was more than 99.8% after 1 h.Cr (VI) removal by CS-NZVI beads is an endothermic process. The removal mechanism may include both physical adsorption of Cr (VI) on the surface or inside of CS-NZVI beads and subsequent reduction of Cr (VI) to Cr (III). The results indicate that there is no significant difference between the reaction rates of bare NZVI and entrapped NZVI. Cr (VI) reduction kinetics follows a pseudo-first-order rate expression. Freundlich isotherm agrees better than Langmuir isotherm with experimental data, which indicates that the adsorption of Cr (VI) by CS-NZVI beads is heterogeneous adsorption.Through either physical or chemical modifications by carbon fiber (CF) and epichlorohydrin (ECH), the mechanical strength of CS-NZVI beads were enhanced. The reduction capacity for Cr (VI) increases with increasing temperature and NZVI dosage but decreases with the increase in initial concentration of Cr (VI) and pH values. At the same condition, the results indicate that there is no significant difference between the reaction rates of CS-NZVI beads and ECH-CS-NZVI. However, the reaction rates of CF-CS-NZVI beads are obvious lower than these of CS-NZVI. ECH may be a better choice to enhance the mechanical strength of CS-NZVI beads.