Supporting Information

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1 Supporting Information Enhancement of Arsenic Adsorption during Mineral Transformation from Siderite to Goethite: Mechanism and Application Huaming Guo 1, 2, *, Yan Ren 2, Qiong Liu 2, Kai Zhao 1, 2, Yuan Li 1, 2 1 State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing , P.R. China 2 School of Water Resources and Environment, China University of Geosciences, Beijing , P.R. China * Corresponding author: Tel.: Fax: address: hmguo@cugb.edu.cn (H. Guo) 35 pages, 7 figures, and 2 tables S1

2 Materials. Artificial siderite was synthesized with ferrous sulfate (FeSO 4 7H 2 O) and ammonium bicarbonate (NH 4 HCO 3 ). Ferrous carbonate was precipitated by mixing 1 M Fe 2+ with 2 M HCO - 3 at room temperature. The precipitate was filtered with 0.45 µm membrane. After rinsed with deionized water for several times, the artificial siderite was dried for 24 h and ground to powder (200 mesh). The product was then kept in a desiccator. The whole process was performed in a grove box with 92.5% N 2 and 7.5% H 2 (Coy lab, USA). The identity was confirmed by X-ray diffraction. Synthetic goethite was prepared from Fe(NO 3 ) 3 9H 2 O solution under highly alkaline conditions according to the procedure described by Schwertmann & Cornell. 20 Briefly, 100 ml of 1 M Fe(NO 3 ) 3 solution was rapidly mixed with 180 ml of 5.0 M KOH solution. The suspension was immediately diluted to 2 L with deionized water and kept in a closed polyethylene flask at 70 ºС for 60 h. This suspension ageing was necessary to prevent Ostwald ripening of goethite nano-particles during the experiments. The precipitation was centrifuged and washed several times to remove OH - and NO - 3 until ph of the supernatant was around 7.0. The product was dried and kept in a desiccator. The identity of goethite was confirmed by X-ray diffraction. All reagents used were of analytical grade. Batch Tests. Stock solutions (10 g/l As) were prepared from sodium arsenite (AsNaO 2 ; >99.0%, Fluka Chemical) for As(III) and sodium arsenate (Na 2 HAsO 4 7H 2 O; >98.5%, Fluka Chemical) for As(V). Batch experiments to study As removal from solution were carried out by reacting 50 ml of As(V)/As(III) solutions in 100 ml polyethylene bottles with 0.10 g of the adsorbent (e.g. synthetic S2

3 siderite or goethite) under anoxic conditions or oxic conditions. The ranges of As(III)/As(V) concentration were between 2.0 and 600 mg/l in both oxic experiments and anoxic experiments, which were checked using ICP-MS. The bottles were immersed in a shaking water bath at 150 rpm at room temperature (25ºС) for 6 h. Solution ph was kept around 7.0 during the reactions by using 0.01 M NaOH and 0.05 M HCl for all batches. Six hours were enough for As adsorption equilibrium on both siderite and goethite. The aqueous sample in each bottle was decanted and centrifuged at 4500 rpm for 5 min, and then filtered through a 0.22 µm cellulose acetate filter. The supernatant was analyzed for dissolved total As and As species. Concerning As adsorption on siderite under anoxic conditions, experiments were carried out in a glove box (Coy lab, USA) to maintain an anaerobic environment with 92.5% N 2 and 7.5% H 2. Others were performed under oxic conditions. All experiments were carried out in duplicate and reported as a mean value. Arsenic Adsorption on Modified Natural Siderite. Natural siderite and modified natural siderite were used to evaluate enhancement of As adsorption on bi-mineral adsorbent during partial mineral transformation from siderite to goethite. The natural siderite was composed of siderite (69.2%), clay minerals (14.1%), quartz (9.6%), dolomite (6.6%), and calcite (0.5%). During modification, natural siderite was calcinated at 300 for about 4 h. After modification, siderite content decreased from 69.2% to 40.4%, while goethite increased from <1.0% to 23.9%. Arsenic adsorption on both natural siderite and modified natural siderite was characterized under experimental conditions with the grain size of mm, initial As(III) S3

4 concentrations between 0.2 and 10 mg/l, adsorbent dosage of 10 g/l, and contact time of 48 h, at 25, under oxic conditions. Sample Analysis. Dissolved total As was analyzed by ICP-MS (7500C, Agilent). Arsenic species were determined using an HPLC-ICP-MS. Details were provided in Supporting Information. A high performance liquid chromatography (HPLC, 1100 Series, Agilent) consisting of a system controller, a solvent delivery module, a column oven and a six-port injection valve was used. A reversed-phase C18 column (Capcell, Pak, 250 mm 4.6 mm, 5 µm particle size) was used for separation of As species. An ICP mass spectrometer (7500C, Agilent) was used as a detector, which was operated in the He mode to remove the ArCl interference. For total As, the detection limit was 0.1 µg/l and the relative standard deviation (RDS) was less than ±2%. The detection limit for As(III) and As(V) was 0.2 µg/l, and the relative standard deviation (RDS) was less than ±2%. The mineral composition of the adsorbents was determined by X-ray diffraction analysis (XRD), using a URD-6 powder diffractometer (Co Kα radiation, graphite monochromator, 2θ range o, step 0.01 o, counting time 5 s per step). Morphological analysis of the pristine and used adsorbent was performed by scanning electron microscopy (SEM) using Zeiss SUPRA 55 microscope (at 15 kv) with energy-dispersive X-ray analyses. Specific surface area was determined for solid samples by Brunauer-Emmett -Teller (BET) N 2 adsorption. Data Analysis. The data obtained from the isotherm studies were used to analyze S4

5 adsorption isotherms in order to estimate the constants, adsorption density and adsorption maxima. Both the Langmuir and the Freundlich isotherms were adopted for fitting, which are shown in Eqs.1 and 2, respectively. q q b C m e e= 1 + b Ce (1) n e K C e q = (2) where C e (mg L -1 ) is the equilibrium As concentration; q e (mg g -1 ) is the amount of As adsorbed at equilibrium; q m (mg g -1 ) and b (L mg -1 ) are the Langmuir constants related to the saturated monolayer adsorption capacity and the binding energy of the adsorption system, respectively; K and n are empirical constants of Freundlich isotherms, indicating adsorption capacity and adsorption intensity, respectively. S5

6 S1. Distances of As-Fe (R) and number (N) of Fe atoms in As-Fe shells of As(V)/As(III)-treated minerals Interatomic shell b Samples As-Fe 1 As-Fe 2 R( Ǻ) (±0.03) N (±0.5) References As(V) siderite (oxic) This study As(V) siderite (anoxic) This study As(V) goethite (oxic) This study As(V) goethite As(V) lepidocrocite As(V) maghemite As(V) siderite (anoxic) As(V) Green rust (anoxic, 2.7 µmol/m 2 ) As(III) siderite (oxic) This study As(III) siderite (anoxic) This study As(III) goethite (oxic) This study As(III) goethite As(III) lepidocrocite As(III) ferrihydrite (oxic) As(III) ferrihydrite (anoxic) As(III) goethite (anoxic) As(III) goethite (anoxic) As(III) goethite (oxic) As(III) goethite (oxic) As(III) lepidocrocite (anoxic) As(V) siderite (oxic) This study As(V) siderite (anoxic) This study As(V) goethite (oxic) This study As(V) goethite As(V) lepidocrocite As(V) Green rust (anoxic, 2.7 µmol/m 2 ) As(III) siderite (oxic) This study As(III) siderite (anoxic) This study As(III) goethite (oxic) This study As(III) goethite As(III) lepidocrocite As(III) goethite (anoxic) As(III) lepidocrocite (anoxic) As(III) maghemite (anoxic) S6

7 S2. Relation between As adsorption and initial As concentration (a), Langmuir isotherm plots of As adsorption (b), and Freundlich isotherm plots of As adsorption (c) on modified natural siderite and natural siderite (the grain size of mm; initial As(III) concentrations between 0.2 and 10 mg/l; adsorbent dosage of 10 g/l; contact time of 48 h, at 25 ). Reference (1) Manning, B.A., Hunt, M.L., Amrhein, C., Yarmoff, J.A. Arsenic(III) and arsenic(v) reactions with zerovalent iron corrosion products. Environ. Sci. Technol. 2002, 36, (2) Morin, G., Ona-Nguema, G., Wang, Y., Menguy, N., Juillot, F., Proux, O., Guyot, F., Calas, G., Brown Jr, G.E. Extended X-ray absorption fine structure analysis of arsenite and arsenate adsorption on maghemite. Environ. Sci. Technol. 2008, 42, (3) Jönsson, J., Sherman, D.M. Sorption of As(III) and As(V) to siderite, green rust (fougerite) and magnetite: Implications for arsenic release in anoxic groundwaters. Chem. Geol. 2008, 255, (4) Wang, Y., Morin, G., Ona-Nguema, G., Juillot, F., Guyot, F., Calas, G., Brown Jr, G.E. Evidence for different surface speciation of arsenite and arsenate on green rust: An EXAFS and XANES study. Environ. Sci. Technol. 2010, 44, (5) Ona-Nguema, G., Morin, G., Juillot, F., Calas, G., Brown Jr., G.E. EXAFS analysis of arsenite adsorption onto two-line ferrihydrite, hematite, goethite, and lepidocrocite. Environ. Sci. Technol. 2005, 39, (6) Manning, B.A., Fendorf, S.E., Goldberg, S. Surface structures and stability of arsenic(iii) on goethite: Spectroscopic evidence for inner-sphere complexes. Environ. Sci. Technol. 1998, 32, S7