Supporting Information. for

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1 Supporting Information for Zero-Valent Iron Nanoparticles Reduce Arsenites and Arsenates to As(0) Firmly Embedded in Core-Shell Superstructure: Challenging Strategy of Arsenic Treatment under Anoxic Conditions Jiří Tuček,*,1 Robert Prucek, 1 Jan Kolařík, 1 Giorgio Zoppellaro, 1 Martin Petr, 1 Jan Filip, 1 Virender K. Sharma, 1,2 and Radek Zbořil*,1 1 Regional Centre of Advanced Technologies and Materials, Departments of Physical Chemistry and Experimental Physics, Faculty of Science, Palacký University in Olomouc, Šlechtitelů 27, Olomouc, Czech Republic. 2 Department of Environmental and Occupational Health, School of Public Health, Texas A&M University, 1266 TAMU, College Station, Texas 77843, United States. * Corresponding authors: Phone: , Fax: , address: jiri.tucek@upol.cz (Jiří Tuček); Phone: , Fax: , address: radek.zboril@upol.cz (Radek Zbořil). Number of pages: 7 Number of figures: 5 Number of tables: 1 S1

2 Supporting Figures Figure S1. Physicochemical characterization of commercial oxidic-shell-free nanoscale zerovalent iron (OSF-nZVI) sold under the name Nanofer 25P (NANO IRON, Ltd., Rajhrad, Czech Republic): (a, b) Representative TEM images of OSF-nZVI; (c) Magnified TEM image of one S2

3 (OSF-nZVI) particle with an inset showing in details its surface without any signs of surface oxidation; (d) Particle size distribution of OSF-nZVI derived from a statistical analysis of a series of TEM images, obeying a log-normal distribution curve; (e) Representative energydispersive X-ray spectroscopy mappings of an assembly of OSF-nZVI particles showing dominantly peaks from Fe. Peaks belonging to C, Cu, Al, and Si originate from the copper grid and holey carbon on which the samples were dried. No traces from O were detected demonstrating absence of any oxidic shell on the surface of OSF-nZVI particles. Adopted with the permission of the manufacturer. S3

4 Figure S2. Physicochemical characterization of the commercial oxidic-shell-free nanoscale zerovalent iron (OSF-nZVI) sold under the name Nanofer 25P (NANO IRON, Ltd., Rajhrad, Czech Republic): (a) X-ray powder diffraction pattern of OSF-nZVI with the diffraction peaks assigned to α-fe (PDF card No , > 99% in the solid state); (b) Room-temperature 57 Fe Mössbauer spectrum of OSF-nZVI with the values of the Mössbauer hyperfine parameters typical of α-fe (where δ is the isomer shift, ΔE Q is the quadrupole splitting, B hf is the hyperfine magnetic field, and Γ is the linewidth of the Mössbauer resonant line); (c) Survey and highresolution Fe 2p XPS patterns of OSF-nZVI with most intense peaks indicated; (d) Nitrogen adsorption and desorption isotherms of OSF-nZVI. Adopted with the permission of the manufacturer. S4

5 Figure S3. Kinetics of arsenite and arsenate removal by OSF-nZVI (for Fe/As = 10:1 and at ph 7) under anoxic conditions, monitored for 166 h. Figure S4. (a, b) Representative HRTEM images and (c) energy-dispersive X-ray chemical mappings (Fe and Fe/O) showing the core/shell architecture of the Fe(0)/Fe(III) nanoparticles. The shell is ~4 nm thick, as shown schematically in the inset of panel (a). S5

6 Figure S5. Physicochemical characterization of the Fe(0)/Fe(III) core/shell nanoparticles: (a) X- ray powder diffraction pattern of the Fe(0)/Fe(III) core/shell nanoparticles with the diffraction peaks assigned to α-fe (PDF card No ) and γ-fe 2 O 3 (PDF card No ). The inset shows the portion of α-fe and γ-fe 2 O 3 identified from the refining of the X-ray powder diffraction pattern; (b) Room-temperature 57 Fe Mössbauer spectrum of the Fe(0)/Fe(III) core/shell nanoparticles. For the Fe(0) spectral component, δ = 0.00 mm/s, ΔE Q = 0.00 mm/s, B hf = 32.9 T, and Γ = 0.31 mm/s. For the Fe(III) spectral component, δ = 0.35 mm/s, ΔE Q = 0.79 mm/s, and Γ = 0.48 mm/s. The Fe(0)/Fe(III) spectral ratio is 94/6. S6

7 Supporting Table Table S1. Values of the Mössbauer hyperfine parameters, derived from fitting of the Mössbauer spectra of As(III)-containing solution treated with oxidic-shell-free nanoscale zero-valent iron (OSF-nZVI) under oxic (air) and anoxic (N 2 ) conditions (i.e., labeled as OSF-nZVI+As(III)-air and OSF-nZVI+As(III)-N 2, respectively), As(V)-containing solution treated with OSF-nZVI under oxic (air) and anoxic (N 2 ) conditions (i.e., labeled as OSF-nZVI+As(V)-air and OSFnZVI+As(V)-N 2, respectively), and As(III)- and As(V)-containing solution treated with Fe(0)/Fe(III) core/shell nanoparticles under anoxic (N 2 ) conditions (labeled as Fe(0)/Fe(III)+As(III)-N 2 and Fe(0)/Fe(III)+As(V)-N 2, respectively), where T is the temperature of the measurement, δ is the isomer shift, ΔE Q is the quadrupole splitting, B hf is the hyperfine magnetic field, and RA is the spectral area of individual spectral components indentified during spectra processing. T-sites and O-sites denote tetrahedral and octahedral cation sites in the Fe 3 O 4 crystal structure, respectively. Sample T (K) Component δ ± 0.01 (mm/s) ΔE Q ± 0.01 (mm/s) B hf ± 0.3 (T) RA ± 1 (%) Assignment OSF-nZVI+ 5 Sextet Fe(0) As(III)-air Sextet Fe 3 O 4 T-sites (Fe 3+ ) Sextet Fe 3 O 4 O-sites (Fe 3+ ) Sextet Fe 3 O 4 O-sites (Fe ) Sextet Fe 3 O 4 O-sites (Fe ) Sextet Fe 3 O 4 O-sites (Fe 2+ ) OSF-nZVI+ 5 Sextet Fe(0) As(III)-N Sextet Fe(0) Doublet Fe(III) oxide OSF-nZVI+ 5 Sextet Fe(0) As(V)-air Sextet Fe 3 O 4 T-sites (Fe 3+ ) Sextet Fe 3 O 4 O-sites (Fe 3+ ) Sextet Fe 3 O 4 O-sites (Fe ) Sextet Fe 3 O 4 O-sites (Fe ) Sextet Fe 3 O 4 O-sites (Fe 2+ ) OSF-nZVI+ 5 Sextet Fe(0) As(V)-N Sextet Fe(0) Doublet Fe(III) oxide Fe(0)/Fe(III)+ 300 Sextet Fe(0) As(III)-N 2 Doublet Fe(III) oxide Fe(0)/Fe(III)+ 300 Sextet Fe(0) As(V)-N 2 Doublet Fe(III) oxide S7