Influences and Mechanisms of As(V) Concentration and Environmental Factors on Hydrosulfate Green Rust Transformation
Received date: 2017-02-09
Online published: 2017-04-12
Supported by
Project supported by the China Postdoctoral Science Foundation (No. 2016M590700) and National Natural Science Foundation of China (Nos. 41601228, 41471194).
Green rusts can coexist with As(V) in some anoxic environments, such as soils, sediments, and groundwater, the interaction between them will affect the transformation of green rusts and the environmental behaviors of As(V), but the influences of As(V) on the processes and mechanisms of green rust transformation have not been fully understood. In this study, the effects of As(V) concentration, pH, temperature, and air rate on hydrosulfate green rust (GR2(SO42-), GR) transformation have been systematically studied by solution chemistry methods combined with spectroscopic analysis, including synchrotron based X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and As K-edge X-ray absorption near edge structure (XANES) spectroscopy. GR shows extremely strong removal capability of As(V) via adsorption and co-precipitation during its transformation, meanwhile the presence of As(V) increases the stability of GR and significantly affects the crystallinity and phases of the transformation products and the transformation mechanisms. With increasing the As(V) concentration (0~20 mmol/L As) at pH 7.3 and 25℃ under open and stirring conditions, the mechanisms change from dissolution-oxidation-precipitation (DOP) to solid state oxidation (SSO), and the transformation products of GR change from mixed phases of goethite and lepidocrocite to pure lepidocrocite to mixed phases of poorly crystalline lepidocrocite, ferrihydrite, and ferric GR, and their crystallinity gradually decreases. The transformation processes of GR exhibit strong accumulation capability towards As(V), leading to the formation of amorphous FeAsO4 surface precipitation at high As(V) concentrations (Fe/As molar ratio<24). When the molar ratio of Fe/As=24, lepidocrocite is the main product at the conditions of pH 6.5~9, temperature of 5~45℃, and air rate of 0~0.05 m3/h, its crystallinity decreases with increasing pH and air rate or decreasing temperature. High pH and air rate, and low temperature favors the formation of ferric GR (same structure with GR but only contains Fe(Ⅲ)) and ferrihydrite, while high temperature favors the formation of goethite. These new insights provide important implications for understanding the formation and transformation mechanisms of various iron oxides and the environmental behaviors of As(V).
Key words： green rust; As(V); pH; temperature; air rate; transformation
Wang Xiaoming , Peng Jing , Xu Huanhuan , Tan Wenfeng , Liu Fan , Huang Qiaoyun , Feng Xionghan . Influences and Mechanisms of As(V) Concentration and Environmental Factors on Hydrosulfate Green Rust Transformation[J]. Acta Chimica Sinica, 2017 , 75(6) : 608 -616 . DOI: 10.6023/A17020046
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