Mechanisms Of Removing Chromium Or Antimonite Using Zero-valent Iron

The discharge of heavy metal wastewater has posed a serious theat to our living environment and human health and heavy metal pollution has become a hot and difficult problem in the field of environment. Chromium and antimonite are extensively used in many fields of the national economy, and it is a common heavy metal contamination. Over the past 20 years, zero-valent iron (Fe0, ZVI) has been extensively applied for the remediation/treatment of groundwater and wastewater contaminated with various organic and inorganic pollutants. However, there are still some technical challenges associated with ZVI application. The major limitations of ZVI include low reactivity due to its intrinsic passive layer, narrow working pH, reactivity loss with time due to the precipitation of metal hydroxides and metal carbonates and limited efficacy for treatment of some refractory contaminants. Therefore, the removal of chromium or antimonite contaminated wastewater was studied in the laboratory based on ZVI technology and in combination with other technologies. These methods and techniques includes:preparing nanoscale zero-valent iron (nZVI) and nZVI supported on activated carbon (nZVI/AC) and zero-valent iron combined with weak magnetic field (WMF). The details are as follows:(1)Nanoscale zero-valent iron supported on acivated carbon (NZVI/AC) was synthesized, the effects of NZVI loading on AC, NZVI/AC dosage, pH, the initial concentration of Cr(Ⅵ), and temperature on the removal of Cr(Ⅵ) were investigated, and the removal mechanism of chromium were also explored. The experimental results indicate:NZVI/AC can remove hexavalent chromium (Cr(Ⅵ)) contamination in water pollution, and the mechanism is attributed to adsorption, oxidation-reduction, and coprecipitation. The SSA of the iron particles and the removal efficiency of Cr(Ⅵ) indicated that the optimum iron loading was 25%. Increase of nZVI/AC dosage and reaction temperature promoted the removal of Cr(Ⅵ), and increase of pH and initial Cr(Ⅵ) concentration abated the removal of Cr(Ⅵ). Kinetics studies showed that removal of Cr(Ⅵ) is a two-step reaction and each step could be expressed by pseudo first-order reaction kinetics, with initial Cr(Ⅵ) and temperature as variables. Total Cr was always almost equal to that of Cr(Ⅵ) under all tested conditions, which indicated that little Cr(Ⅲ) existed in solution. Iron ions, which could cause secondary pollution in the environment, are almost not released from this system.(2)Weak magnetic field (WMF), superimposed with a permanent magnet ring, was utilized to improve ZⅥ corrosion and thereby enhance Sb(Ⅲ) removal by ZⅥ. The effects of WMF on Sb(Ⅲ) removal by ZⅥ at various pH values, different initial Sb(Ⅲ) concentrations, and with the presence of coexisting anions were investigated. The experimental results indicate:XANES analyses showed that Sb(Ⅲ) was oxidated to Sb(Ⅴ). The WMF induced greater enhancement in Sb(Ⅲ) removal by ZⅥ at higher pH. The influence of WMF on Sb(Ⅲ) removal by ZⅥ was associated with a more rapid release of Fe ions and a more dramatic change in pH compared to the case without WMF. The addition of nitrate, chloride, ions hardly affected Sb(Ⅲ) ions removal, while the coexisting arsenite, phosphate, and silicate ions substantially depressed Sb(Ⅲ)) ions adsorption.