Iron Oxide Phase Transformation of a Commercial Granular Iron Oxyhydroxide Based Arsenic Adsorbent: A Mineralogical Investigation

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1 Iron Oxide Phase Transformation of a Commercial Granular Iron Oxyhydroxide Based Arsenic Adsorbent: A Mineralogical Investigation Arun Kumar, Graduate Research Assistant, Department of Civil, Architectural and Environmental Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, U.S.A.; ak385@drexel.edu Patrick L. Gurian, Assistant Professor, Department of Civil, Architectural and Environmental Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, U.S.A.; pgurian@drexel.edu Zhorro S. Nickolov, Director of Spectroscopy, College of Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, U.S.A.; znikolov@drexel.edu ABSTRACT The objective of this research work was to understand the combined effects of ph, ion types and time on phase transformation of a commercially available granular iron hydroxide media, Bayoxide E33, exposed to a synthetic groundwater at ph 6 and ph 8 for 90 days in batch experiments, using X-ray diffraction (XRD) technique. No new iron/arsenic bearing mineral was observed for spent E33 media indicating that arsenic was not incorporated within the crystalline structure of the media. However, XRD peaks of spent media at 2θ = and 36.9 were observed to widen compared to fresh media. Greater widening of peaks was observed for samples exposed to ph 6 solutions than that exposed to ph 8 solutions. Observed changes in peak-widths of spent media suggest the possibility of structural changes in media over time, which may also influence its arsenic adsorptive capacity over time. These findings are relevant to small utilities that treat groundwater with high arsenic and ferrous iron concentrations. Concerns appear to be greater for reduced, alkaline groundwater as the ph 8 sample with ferrous iron present showed the greatest structural change. Further experiments are underway to collect more information about changes in degree of crystallinity and arsenic adsorption capacity of iron oxide media over time, which could be useful in understanding the combined effects of different environmental factors on its stability and time-dependent arsenic removal effectiveness. Keywords: Arsenic, crystallinity, granular iron hydroxide, media aging, phase transformation, X-ray diffraction INTRODUCTION Iron oxide can be found in different phases such as amorphous iron oxide, ferrihydrite, akaganeite, goethite, hematite, etc. depending on time, solution ph, and ions present in solution [1], and these different iron oxide phases possess different arsenic adsorptive capacities. Amorphous iron oxide possesses higher arsenic adsorption capacity than other iron oxide phases [1-3]. Arsenic forms surface complexes with amorphous iron oxide and remains adsorbed on iron-oxide surface until the surface area becomes too small to 1

2 adsorb all arsenate ions [4]. Additional adsorption also happens through structural incorporation of ions during phase transformation [4-7]. Iron oxide-based media is continuously exposed to different ions for long periods during treatment of groundwater for arsenic. Granular iron hydroxide (GIH) media, for example, has been reported to be used for more than 1.5 years of operation without any regeneration [8]. Continuous exposure of different ions may influence the stability of iron oxide-based media and may interfere with its arsenic adsorptive capacity [1-4, 9]. Some studies have addressed the effect of aging on arsenic adsorptive capacity of iron oxidebased media. For example, a study conducted by the Sandia National Laboratories observed the effect of high temperature on crystallinity of iron oxide-based media and its influence on arsenic adsorption capacity during treatment of Socorro groundwater for arsenic [10]. In that study, no significant change in X-ray diffraction (XRD) patterns of iron oxide media were reported when it was aged in deionized water at 20 ºC or 37 ºC for 9 to 11 weeks. To understand the effect of aging on arsenic adsorptive capacity of arsenic treatment residuals, Ela et al. [11] monitored the leaching of iron and arsenic from a column packed with paper, arsenic treatment residuals, compost, and anaerobic digester sludge and observed leaching of 70% iron and 75% arsenic after 27 months and suggested that the solubilization of iron and reduction in surface area might have helped in increasing the arsenic leaching rate. Very few studies, so far, have focused on studying the combined effects of time, temperature, ph, and ion types on phase transformation of these iron oxide based adsorbents, which is imperative to understand the fate of adsorbed arsenic with time. These factors have been reported to influence the rate and extent of phase transformation of iron oxide solids [1, 4, 15]. The objective of this research work was to understand the combined effects of ph, ion types and time on phase transformation of an iron oxide-based media. Batch studies were conducted using a granular iron hydroxide media and combined effects of time, ph, and ions were studied by observing changes in mineralogy of media using XRD technique. MATERIALS AND METHODS 1. Batch Adsorption Studies Batch studies were conducted using a commercially available granular iron hydroxide media, Bayoxide E33 1, for this study (GIH E33, hereafter). A sample of 10g/L media was incubated for 90 days in the presence of synthetic groundwater (water quality characteristics are given in Table 1). Some batch studies were conducted in the presence of ferrous iron as it has been reported to catalyze dissolution and re-crystallization of iron oxide phases [4]. Two levels of ferrous iron, 0 and 0.3 mg/l, were used to prepare synthetic groundwater (Table 1). All batch studies were conducted at two different nearneutral solution ph values, ph 6 and ph 8 as this ph range is optimal for removal of both types of arsenic, arsenite and arsenate, from groundwater using iron oxide-based media 1 Severn Trent Services, FL, U.S.A. 2

3 [13]. All solutions were purged with nitrogen gas to created reducing conditions. Detailed experimental design is given in Table 2. After incubation, the GIH E33 media was contacted with a synthetic groundwater (Table 1) spiked with 60 mg/l As 5+ (0.8mM) for 15 days at ph 6 and ph 8. A fresh GIH E33 media was also studied under the same conditions. 2. Analytical Methods To study the mineralogy of raw and spent iron oxide media, X-ray powder diffraction (XRD) was carried using the Siemens D500 X-ray Powder Diffractometer (1500 W Cu fine focus tube, graphite receiving monochromator) 2. Prior to analysis, the sample was washed, filtered, and then dried at room temperature for 48 h following the method used in previous studies [5,6,12]. Material was ground into a fine powder using mortar and pestle, dispersed using acetone on a microscopic slide, and dried at room temperature before analysis. Diffraction data were obtained for 2θ angle between 15 to 65 (radiation source: CuK α1 at Ǻ, voltage: 40 kv, current: 30 ma, method: step-scan, step size: 2 o 2θ, dwell time: 3 seconds). X-ray diffractograms were analyzed using the Jade+ analysis software (version 7.1) 3. Mineralogical phases were determined using the computer-based library of the Joint Committee on Powder Diffraction Standards (JCPDS) and published literature. Table 1. Water Quality Parameters of Synthetic Groundwater Parameters Value Alkalinity-mg/L CaCO Calcium-mg/L 56 Silica-mg/L 33 Sulfate-mg/L 276 Chloride-mg/L 232 Nitrate-mg/L 1.4 CaCO 3 -calcium carbonate. Source: EPWUPSB [14] 2 Centralized Research Facilities, Drexel University, Pa, U.S.A. 3 Materials Data, Inc., Ca., U.S.A. 3

4 Table 2. Experimental Design * (Indicator for Incubation = -1 (0 day) and 1 (90 days), Ferrous iron = -1(0) and 1 (0.3mg/L), Initial ph = -1 (ph6) and 1(pH8)) Incubation Fe 2+ conc. Initial ph Label (days) (mg/l) -1 (0) -1 () -1 (6) 1A 1 (90) -1() -1 (6) 13A -1 (0) 1(0.3) -1 (6) 2A 1 (90) 1(0.3) -1 (6) 14A -1 (0) -1 () 1(8) 3A 1 (90) -1() 1(8) 15A -1 (0) 1(0.3) 1(8) 4A 1 (90) 1(0.3) 1(8) 16A * Fe 2+ / Fe 3+ = 47µmol/mol Intensity (a.u.) Figure 1: XRD Spectrum of GIH E33 Media RESULTS 1. Mineralogy of Iron Oxide Media Figure 1 shows the X-ray diffractogram of GIH E33 media. Mineralogies of different spent GIH E33 samples were also investigated, and their X-ray diffractograms are shown in Figure 2. X-ray diffractogram of GIH E33 is also shown in Figure 2 for comparison purposes. XRD peaks of GIH E33 at 2θ = and 36.9 were observed to be dominant than other peaks (Figure 1). The same peaks at 2θ = and 36.9 were also observed for spent GIH E33 samples. Thus, XRD peaks at 2θ = and 36.9 were considered for studying possible changes in mineralogy of GIH E33 media exposed to synthetic groundwater (Table 2). For all samples studied, only goethite iron oxide phase was found. No new peaks corresponding to any iron/arsenic or other ions bearing mineral in synthetic water were observed to be introduced. 4

5 To determine change in inter-planar spacing of crystalline structure, peak locations of fresh GIH E33 and spent GIH E33 media were plotted for two different cases: (1) No ferrous iron and (2) 0.3 mg/l ferrous iron (Figure 3). A linear model was fitted for sample and GIH E33 peak locations for every case (Figure 3). For both cases, a 45 straight line was observed to fit sample and GIH E33 peak locations very well (coefficient of determination (R 2 ) = ) indicating that probably no net change in interplanar spacing of spent GIH E33 media occurred during exposure of GIH E33 to different ions during the 90-day incubation time. (a) Intensity GIH E33 0d_pH6 (1A) 0d_pH8(3A) 90d_pH6(13A) 90d_pH8(15A) (b) Intensity GIH E33 0d_pH6 (2A) 0d_pH8(4A) 90d_pH6(14A) 90d_pH8(16A) Figure 2: Mineralogy of Spent GIH E33 Media: (a) No Ferrous Iron, and (b) 0.3 mg/l Ferrous Iron (Legend indicates Incubation Time, Solution ph and Sample Label) 5

6 2. Effect of Time, ph, and Ferrous Iron on Mineralogy of Iron Oxide Media The effects of time, ph, and ferrous iron on mineralogy of iron oxide media was studied using full-width at half-maximum (FWHM) peak values, which depends on the shape of the peak and intensities of neighboring peaks. FWHM values at 2θ = and 36.9 for GIH E33 and samples were calculated using the Jade + software. A comparison of peakwidths of samples over time is shown in Figures 4 and 5. Peak-widths generally broadened over time (except for the 2θ = 36.9 peak of the ph 6 sample for which no trend is evident). 70 (a) 2θ (Sample) line θ (GIH E33) 0d_pH6(1A) GIH E33 0d_pH8(3A) 90d_pH8(15A) 90d_pH6(13A) Linear (GIH E33) 70 (b) 2θ (Sample) line θ (GIH E33) 0d_pH6(2A) GIH E33 0d_pH8(4A) 90d_pH8(16A) 90d_pH6(14A) Linear (GIH E33) Figure 3: Peak Locations of GIH E33 and Spent GIH E33 Media: (a) No Ferrous Iron, and (b) 0.3 mg/l Ferrous Iron (Legend indicates Incubation Time, Solution ph and Sample Label) 6

7 (a) Full-width at half-maximum GIH E33 0d_pH6(1A) 90d_pH6(13A) (b) Full-width at half-maximum GIH E33 0d_pH8(3A) 90d_pH8(15A) Figure 4: Effect of Time on Full-width at Half-maximum Heights, an Indicator of Phase Transformation of GIH E33 Media Exposed to Synthetic Groundwater without Ferrous Iron for Two Initial ph Levels: (a) ph 6, and (b) ph 8. 7

8 (a) Full-width at half-maximum GIH E33 0d_pH6(2A) 90d_pH6(14A) (b) Full-width at half-maximum GIH E33 0d_pH8(4A) 90d_pH8(16A) Figure 5: Effect of Time on Full-width at Half-maximum Heights, an Indicator of Phase Transformation of GIH E33 Media Exposed to Synthetic Groundwater With 0.3 mg/l Ferrous Iron for Two Initial ph Levels: (a) ph 6, and (b) ph 8. DISCUSSION Mineralogical investigation of GIH E33 media indicates that it is primarily composed of goethite iron oxide phase as also observed in previous studies [1,11,15,16]. Arsenate primarily adsorbs on goethite by forming inner-sphere surface complexes [4], and thus does not contribute to the observed change in crystallinity of the media. No iron/arsenic bearing mineral was observed for spent GIH E33 media indicating that arsenic was not incorporated within the crystalline structure of the media. Other ions present in solution such as sulfate, chloride, nitrate, and silicate (Table 1) were also not observed to be incorporated in the crystalline structure of goethite. Similar observations were also reported by Pedersen et al [4] for goethite during reductive dissolution at ph 6.5 and 8

9 room temperature (As/Fe molar ratio: 01 and Fe 2+ : 7M). Ela et al [11] also did not observe any new iron/arsenic mineral phase while studying the effect of aging on amorphous ferric hydroxide loaded with a molar ratio of Fe/As of 5.7. However, they observed some peaks of pure pharmacosiderite (K 2 /Na 2 Fe 4 (AsO 4 ) 3 (OH) 5.7H 2 O) after aging for 1.5 months at 40 C, in which arsenic was incorporated within its crystalline structure. For most of the samples studied, peaks at 2θ = and 36.9 were observed to widen compared to fresh media. Different changes in peak-widths were observed for samples exposed to ph 6 and ph 8 solutions. Greater widening of peaks was observed for samples exposed to ph 6 (13 or 14) than samples exposed to ph 8 solution (15 or 16). Observed changes in peak-widths could be attributed to the combined effects of incubation time, ph and ion types, which have been reported to influence the rate and extent of phase transformation of iron oxide solids [1,4,12]. Wider peaks indicate less a structured and regular crystal. One would expect the crystal to become more ordered over time rather than less ordered. Wider peaks could be newly forming goethite, which is less structured than existing goethite. However, we cannot exclude the possibility that existing goethite is becoming less structures as adsorbing ions interact with crystals. CONCLUSIONS Observed changes in peak intensity and peak width at half-maximum values of different samples indicate that structural changes are occurring in the media over time, but no major phase change was observed. Based on previous research [9], these changes may decrease arsenic adsorption capacity of the media. Concerns appear to be greater for reduced, alkaline groundwater as the ph 8 sample with ferrous iron present showed the greatest change. The findings of this work support the need of observing changes in media over time due to consistent exposure to different ions present in groundwater, which may affect the long-term adsorption capacity of the media. Batch isotherm tests may overestimate the arsenic adsorption capacity of iron oxide based media. Long-term monitoring of media performance (i.e., arsenic adsorption capacity) with comparable water quality could provide realistic adsorption capacity data, useful for selecting among different adsorbents. Further experiments are underway to collect more information about changes in degree of crystallinity and arsenic adsorption capacity of iron oxide media over time, which could be useful in understanding the combined effects of different environmental factors on its stability and time-dependent arsenic removal effectiveness. ACKNOWLEDGMENTS Drexel University gratefully acknowledges the Awwa Research Foundation and Sandia National Laboratories for its financial, technical, and administrative assistance in funding and managing the project through which this information was discovered (Project 3161). The comments and views detailed herein may not necessarily reflect the views of the Awwa Research Foundation, its offices, directors, affiliates or agents, or the views of the 9

10 U.S. Federal Government. We gratefully acknowledge Richard S. Dennis of the Severn Trent Services for supplying the Bayoxide E33 media. REFERENCES [1] Schwertmann, U. & Cornell, R. M., Iron Oxides in the Laboratory: Preparation and Characterization. 2 nd ed. Weinhem: Wiley-VCH. [2] Fuller, C.C.; Dadis, J.A.; & Waychunas, G.A., Surface Chemistry of Ferrihydrite: Part 2. Kinetics of Arsenate Adsorption and Coprecipitation. Geochim. Cosmochim. Acta, 57:10: [3] Waychunas, G.A.; Rea, B. A.; Fuller, C. C.; & Davis, J. A., Surface Chemistry of Ferrihydrite: Part 1. EXAFS Studies of the Geometry of Coprecipitated and Adsorbed Arsenate, Geochim. Cosmochim. Acta, 57:2251. [4] Pedersen, H. D.; Postma, D.; & Jakobsen, R., Release of Arsenic Associated with the Reduction and Transformation of Iron Oxides. Geochim. Cosmochim. Acta, 70:4116. [5] Ford, R. G.; Bertsch, P. M.; & Farley, K. J., Changes in Transition and Heavy Metal Partitioning during Hydrous Iron Oxide Aging, Environ. Sci. & Technol., 31:2028. [6] Ford, R. G., Rates of Hydrous Ferric Oxide Crystallization and the Influence on Coprecipitated Arsenate. Environ. Sci. & Technol., 36:2459. [7] Jang, He-Hun; Dempsey, B.A.; Catchen, G. L.; & Burgos, W.D., Effects of Zn(II), Cu(II), Mn (II), NO - 2-3, or SO 4 at ph 6.5 and 8.5 on Transformations of Hydrous Ferric Oxide (HFO) as Evidenced by Mossbauer Spectroscopy. Colloid & Surf. A: Physicochem. Eng. Aspects., 221:55. [8] Driehaus, W., Arsenic Removal-Experience with the GEH Process in Germany. Water Sci. & Technol., 2:2:275. [9] Kumar, A.; Sarich, M.; Gurian, P. L.; & Montoya, T., Effect of Time and ph History on Arsenic Removal by Granular Iron Hydroxide Media. Proceedings of the AWWA Annual Conference and Exposition, Toronto, Ontario, Canada. [10] Siegel, M.; Aragon, A.; Zhao, H.; Everett, R. et al Pilot Test of Arsenic Adsorptive Media Treatment Technologies at Socorro Springs, New Mexico: Materials Characterization and Phase I Results (SAND ). Prepared for Sandia National Laboratories, Albuquerque, New Mexico & Livermore, California. [11] Ela, W., Saez, A.E.; Ghosh, A.; Mukiibi, M.; & Zelinski, B., Arsenic Residuals Stabilization: Motivation and Means. Proceedings of the AWWA Annual Fall Conference, Reno, Nevada, U.S.A. [12] Baltpurvins, K.A.; Burns, R. C.; & Lawrance, G. A., Effect of ph and Anion Type on the Aging of Freshly Precipitated Iron (III) Hydrous Sludges. Environ. Sci. & Technol., 30:

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