Introduction Iron sulfides, in particular, troilite, pyrite and the less crystalline mackinawite have demonstrated effective uptake of As(III) in batch solutions. This type of behavior is promising for applications as a sequestration agent in arsenic remediation designs such as above-ground adsorption units as well as in-situ subsurface permeable barriers. Direct application, however, in such flow- through systems is dependent not only upon ability to synthesize the mackinawite on a large scale industrial level but also the ability to produce the mackinawite at various range of particle sizes. Synthesis and Characterization of Mackinawite Coated Sand Tanya J. Gallegos 1, Kim F. Hayes 1, Linda M. Abriola 2 1 University of Michigan, 2 Tufts University Conclusion FeS is effectively coated onto iron oxide coated sand. Methods of Synthesis Further Study FTIR examination to determine surface species Objective The purpose of this study was to develop a method for emplacing a mackinawite coating onto the surface of clean quartz sand for arsenic uptake. Hypotheses 1.Physical stabilization of FeS coating onto SiO2 is expected to also occur upon aging via temperature and time. 2.of iron oxide coatings to a FeS coating is expected due to reduction with Na2S. 3. FeS will electrostaticlly coat the layer of iron oxide coating SiO2 SEM Total Iron Extraction Comparison to Pure Mackinawite Acid/Base Resistivity XRD Analysis Peaks: d-spacings of 5.05, 2.31 and Arsenic Redox Sensitivity Results Three methods produced coatings: MFCS- ferrihydrite coated sands /FeS/27C/2 days MCSH -clean quartz sand/FeS/70C/24 hour MCST-clean quartz sand/FeS/27C/20day MFCSMCSHMCST Characterization Results of these coated sands are provided below Implications The ability to produce mackinawite as a coating for sands provides for a promising application in both in-situ and ex- situ processes. The abundance of sands and availability of a wide range of particle sizes that may be implemented in remediation process such as permeable reactive barriers as well as above-ground adsorption treatment units.. Arsenic Reactivity

Introduction Mackinawite coated sand is under consideration as a reactive porous media for use in flow-through remediation schemes. Effective application of this reactive porous media in permeable reactive barriers will rely on reactive transport models which accurately describe sorption chemistry which relies on an accurate depiction of FeS acid- base chemistry. Contaminants such as cadmium, arsenic and mercury have been show to be sequestered by FeS. This sequestration is through to be due to surface complexation. In fact, spectroscopic studies by others have confirmed the presence of hydroxyl (Me≡OH) and sulfhydryl (S≡H) surface functional groups on the surface of generic metal sulfides. These amphoteric reactive units are thought to undergo independent protonation and deprotonation reactions to produce reactive sites for sorption. Acid-Base Properties of Mackinawite Coated Sand Tanya J. Gallegos 1, Kim F. Hayes 1, Linda M. Abriola 2 1 University of Michigan, 2 Tufts University Protonation: HOFe≡SH 0 + H + = H 2 OFe≡SH +1 K + =6 Deprotonation: HOFe≡SH 0 = OFe≡SH -1 + H + K - = -6.7 Ion Exchange: H 2 OFe≡SH +1 + H + = H≡SH+ Fe 2+ + H 2 O K ix =8.9 Conclusion Results Further Study Measurement of total dissolved S 2- as a function of pH EXAFS examination of the FeS coated sand at pH 4, 7, 8, 10 to confirm the presence of the proposed surface species.. Objective The purpose of this study is to complete the first step in developing a reactive model that describes mackinawite chemistry by assessing the acid base properties of the mackinawite surface. Experimental Development of probable surface species and conceptual model Measurement of BET Surface Area Potentiometric titration of the mackinawite surface at various ionic strengths Solubility measurements FITEQL fitting of acidic potentiometric data to a conceptual model resulting in protonation and deprotonation constants. BET Surface Area is m2/g Other Model ParametersValue Specific Surface Area (m 2 /g)0.149 Total Number of Surface Sites (M) =S tot = (OFe≡SH - )+ (H≡SH 0 )+ (H 2 OFe≡SH + )+ (HOFe≡SH 0 )=3.1-4 M Capacitance (F/m 2 )20 PZC6.67 F (error) 119 Titration Data Fit Modeled Predominance Diagram Conceptual Model Modeled Reactions Dissolved Iron Measurements Titration Data Comparison to Pure FeS Implications These species may form complexes in solution for any number of contaminants providing for effective sequestration. Amount of acid taken up by solid~ 3.3e-4 M

Sorption of Arsenite Onto Mackinawite Coated Sand Tanya J. Gallegos 1, Kim F. Hayes 1, Linda M. Abriola 2 1 University of Michigan, 2 Tufts University Results Equilibrium in the FeS/H 3 AsO 3 system is reached within 2 hours. Arsenite sorption onto mackinawite coated sand increases with increasing pH reaching maximum removal at pH 7.5. Between pH 3-7.5, [Fe 2+ ] in solution increases as pH decreases approaching the total Fe 2+ in the coating at pH 3. [Fe 2+ ] in solution increases as ionic strength decreases. Arsenite removal is accompanied by a decrease of Fe 2+ in solution. At low As(III) concentrations, removal is Langmuirian in nature. Arsenite sorption abruptly converts to linear behavior at high concentrations, possibly attributed to formation of surface precipitates. At pH 10, As 3+ removal is linear whereas, it is Langmuirian at pH 6 and 8 suggesting a different removal mechanism. Increases in ionic strength enhance the removal of arsenite from solution, suggesting a possible inner-sphere surface complexation removal mechanisms. Further Study Development of probable surface species and data fitting of conceptual reactions with pH edge data are expected to provide appropriate surface complexation equilibrium constants These constants will result in a complete surface complexation model which can be further used to predict isotherms and subsequently can be used as input into a reactive transport model. Introduction Arsenic contamination of groundwater is a widespread problem affecting aquifers in the United States, as well as abroad. Recent strengthening of the US EPA MCL for arsenic has prompted the need for technology capable of removing both arsenite and arsenate from solution. Arsenite, the more toxic form of arsenic, is more difficult to remove from anoxic zones in the subsurface. Studies by others (Bostick and Fendorf, 2003; Farquhar et al., 2002; Wolthers et al., 2003, Zouboulis et al., 1993) have demonstrated the affinity of some types of iron sulfides (such as troilite, pyrite, amorphous iron sulfide and mackinawite) for arsenite. However, these studies have not provided a comprehensive investigation of the macroscopic behavior of arsenite in the presence of crystalline mackinawite in a form that can be readily applied to real-world treatment technologies. This study examines the behavior of arsenite in the presence of mackinawite coated sand (MCS). Objective It is the purpose of this study to characterize the macroscopic behavior of MCS in the presence of H 3 AsO 3 with respect to variations in time, ionic strength, pH and initial total arsenite concentrations. This information will be used to determine likely surface complexation reactions that occur between the arsenite species and surface functional groups thought to exist in the pH ranges as shown in the following figure. The acid-base reactions used to develop the arsenite surface complexes are consistent with those used to model complexation of anionic collectors in galena (PbS) and sphaelerite (ZnS) flotation systems (Sun et al., 1991). Kinetic Study Isotherm pH 10 isotherm pH 8 isotherm Probable Surface Species Iron in Solution pH Edge Implications This study clearly demonstrates that mackinawite coated sands have a capacity for reducing dissolved arsenite concentrations in solution over pH ranging from most notably in circumneutral pH regions. Probable AsO 3 3- /S/Fe surface complexes and reactions that may take place in a FeS(s)/AsO 3 3- system can be postulated to be consistent with observations of the macroscopic behavior with regard to pH and As(III) concentration of the system as well as with spectroscopic studies that suggest the formation of a FeSAs-like surface precipitate as reported by others (Bostick and Fendorf, 2003; Farquhar et al., 2002). References Bostick, B. C. and S. Fendorf (2003). "Arsenite sorption on troilite (FeS) and pyrite (FeS2)." Geochimica Et Cosmochimica Acta 67(5): Farquhar, M. L., J. M. Charnock, et al. (2002). "Mechanisms of arsenic uptake from aqueous solution by interaction with goethite, lepidocrocite, mackinawite, and pyrite: An X-ray absorption spectroscopy study." Environmental Science & Technology 36(8): Sun, Z. X., W. Forsling, et al. (1991). "Surface-Reactions in Aqueous Metal Sulfide Systems.3. Ion-Exchange and Acid-Base Properties of Hydrous Lead Sulfide." Colloids and Surfaces 59: Wolthers, M., L. Charlet, et al. (2003). "Arsenic sorption onto disordered mackinawite as a control on the mobility of arsenic in the ambient sulphidic environment." Journal De Physique Iv 107: Zouboulis, A. I., K. A. Kydros, et al. (1993). "Arsenic(Iii) and Arsenic(V) Removal from Solutions by Pyrite Fines." Separation Science and Technology 28(15-16): Experimental Batch Experiments consisted of 40 g/L mackinawite coated sand in 15 mL reactors. Sand exhibited a surface area of m 2 /g and the precipitate used to synthesize the coating was characterized by XRD as mackinawite. Batch experiments run were: Arsenite Sorption Edge at ionic strength M, 0.01 M and 0.1 M NaCl background electrolyte Arsenite Sorption Isotherms at pH 6, 8, 10 with initial As concentration of 1000 ppb As at 0.01 M NaCl Fe Dissolution at ionic strength M, 0.01 M and 0.1 M NaCl Kinetic Sorption experiment at 0.01 M NaCl