As(V)浓度和环境因子对硫酸盐绿锈转化的影响及其机制

Influences and Mechanisms of As(V) Concentration and Environmental Factors on Hydrosulfate Green Rust Transformation

绿锈与As(V)可共存于土壤、沉积物和地下水等缺氧环境,但As(V)如何影响绿锈转化过程和机制了解甚少.本工作通过溶液化学和光谱学方法,系统研究了As(V)浓度、pH、温度和空气流速对硫酸盐绿锈(GR)转化的影响.GR转化过程中通过吸附和共沉淀作用对As(V)有极强的去除能力,同时As(V)增强了GR的稳定性,显著影响转化产物的结晶度、矿物类型和生成机制.随As(V)浓度增加,GR氧化转化由溶解-氧化-沉淀机制向固态氧化机制过渡,产物由针铁矿和纤铁矿混合相向纯纤铁矿向纤铁矿、水铁矿和高铁绿锈混合相转变;高As(V)浓度时形成无定形FAsO_4表面沉淀.Fe/As=24时,pH 6.5~9、温度(5~45℃)和空气流速(0~0.05 m3/h)条件下纤铁矿均为主要产物,随pH和空气流速增加或温度减小纤铁矿结晶度逐渐减弱;高pH或高空气流速或低温有利于高铁绿锈和水铁矿形成,高温有利于针铁矿形成.上述结果对深入理解环境中各种铁氧化物的形成转化机制和As(V)的环境行为有重要意义.

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）.