Removal of Arsenic and Selenium with Nanoscale Zero-Valent Iron (nZVI)

Arsenic (As(III/V)) and selenium (Se(IV/VI)) are toxic inorganic contaminants in groundwater and industrial wastewater. The-pollution caused by As and Se has become an environmental concern throughout the world. A variety of treatment technologies have been applied for As and Se removal from aqueous solutions. Among them, nanoscale zero-valent iron (nZVI) has been found to have a remarkable capability to remove As and Se from waters. Although lots of studies on the process of As and Se removal with nZVI are published, a systematic comparative study is still limited. In this study, the removal capacities of As(III), As(V), Se(IV) and Se(VI) with nZVI in a single-specie system were compared. The performances of nZVI for As(III), As(V), Se(IV) and Se(VI) were investigated on different conditions (including dissolved oxygen, nZVI dosage, contact time, and initial solution pH). The morphology and structure of fresh and spent nZVI were also examined by spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) intergrated with energy-dispersive X-ray spectrometry (XEDS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The batch experiments were conducted at room temperature in the 50-mL glass vials sealed with screwcaps. According to the speciation diagram, H3AsO30 and H2AsO4- are the respective predominant dissolved As(III) and As(V) species respectively at pH 5.0; while HSeO3- and SeO42- are the predominant dissolved Se(IV) and Se(VI) species respectively at pH 5.0. The results showed that the removal capacities of As and Se investigated generally followed the order of Se(IV)> As(III)> Se(VI)> As(V). Dissolved oxygen (DO) was found no apparent effects on the removal of As(III) and Se(IV), while the removal performance of As(V) and Se(VI) was inhibited at high dissolved oxygen level (> 14 mg/L). The removal of As and Se were enhanced with increasing nZVI dosage. Initial solution pH had no significant effect on Se(IV) removal, whereas the removal of As(III), As(V), and Se(VI) appeared to be strongly dependent on the initial solution pH. The spent nZVI were different due to the different mechanisms of As(III/V) and Se(IV/VI) reactions with nZVI. The results will be useful for the application of nZVI to the treatment of As/Se-containing wastewater.