Coupled Effects of Aging and Weak Magnetic Fields on Sequestration of Selenite by Zero-Valent Iron

The sequestration of Se­(IV) by zero-valent iron (ZVI) is strongly influenced by the coupled effects of aging ZVI and the presence of a weak magnetic field (WMF). ZVI aged at pH 6.0 with MES as buffer between 6 and 60 h gave nearly constant rates of Se­(IV) removal with WMF but with rate constants that are 10- to 100-fold greater than without. XANES analysis showed that applying WMF changes the mechanism of Se­(IV) removal by ZVI aged for 6–60 h from adsorption followed by reduction to direct reduction. The strong correlation between Se­(IV) removal and Fe²⁺ release suggests direct reduction of Se­(IV) to Se(0) by Fe⁰, in agreement with the XANES analysis. The numerical simulation of ZVI magnetization revealed that the WMF influence on Se­(IV) sequestration is associated mainly with the ferromagnetism of ZVI and the paramagnetism of Fe²⁺. In the presence of the WMF, the Lorentz force gives rise to convection in the solution, which narrows the diffusion layer, and the field gradient force, which tends to move paramagnetic ions (esp. Fe²⁺) along the higher field gradient at the ZVI particle surface, thereby inducing nonuniform depassivation and eventually localized corrosion of the ZVI surface

Enhanced Reactivity and Electron Selectivity of Sulfidated Zerovalent Iron toward Chromate under Aerobic Conditions

When zerovalent iron (ZVI) is used in reductive removal of contaminants from industrial wastewater, where dissolved oxygen (DO) competes with target contaminant for the electrons donated by ZVI, both the reactivity and the electron selectivity (ES) of ZVI toward target contaminant are critical. Thus, the reactivity and ES of two sulfidated ZVI (S-ZVI) samples, synthesized by ball-milling with elemental sulfur (S-ZVI^bm) and reacting with Na₂S (S-ZVI^Na2S), toward Cr­(VI) under aerobic conditions were investigated. Sulfidation appreciably increased the reactivity of ZVI and the ratio of the rate constants for Cr­(VI) removal by S-ZVI^bm or S-ZVI^Na2S to their counterparts without sulfur fell in the range of 1.4–29.9. ES of S-ZVI^bm and S-ZVI^Na2S toward Cr­(VI) were determined to be 14.6% and 13.3%, which were 10.7- and 7.5-fold greater than that without sulfidation, respectively. This was mainly ascribed to the greater improving effect of sulfidation on the reduction rate of Cr­(VI) than that of DO by ZVI. The improving effects of sulfidation on the performance of ZVI were mainly due to the following mechanisms: sulfidation increased the specific surface area of ZVI, the FeS_<i>x</i> layer facilitated the enrichment of Cr­(VI) anions on S-ZVI surface because of its anions selective property and favored the electron transfer from Fe⁰ core to Cr­(VI) at the surface because of its role as efficient electron conductor

Improving the Reactivity of Zerovalent Iron by Taking Advantage of Its Magnetic Memory: Implications for Arsenite Removal

Premagnetization was employed to enhance the reactivity of zerovalent iron (ZVI) toward As­(III) sequestration for the first time. Compared to the pristine ZVI (Pri-ZVI), the rate of As­(III) elimination by the premagnetized ZVI (Mag-ZVI) was greater over the pH_ini range of 4.0–9.0 and increased progressively with increasing intensity of the magnetic field for premagnetization. Mag-ZVI could keep its reactivity for a long time and showed better performance than Pri-ZVI for As­(III) removal from synthetic groundwater in column tests. The Fe <i>K</i>-edge XAFS analysis for As­(III)-treated ZVI samples unraveled that premagnetization promoted the transformation of ZVI to iron (hydr)­oxides and shifted the corrosion products from maghemite and magnetite to lepidocrocite, which favored the arsenic sequestration. The arsenic species analysis revealed that premagnetization facilitated the oxidation of As­(III) to As­(V). ZVI pretreated with grinding was very different from Mag-ZVI with regard to As­(III) removal, indicating that the improved reactivity of Mag-ZVI should not be associated with the physical squeezing effect of the ZVI grains during magnetization. The positive correlation between the remanence of Mag-ZVI and the rate constants of total arsenic removal indicated that the enhanced reactivity of Mag-ZVI was mainly ascribed to its magnetic memory, i.e., the remanence kept by Mag-ZVI