Environ. Sci. Technol. 43, 9202–9207
Sorption of Arsenic(V) and Arsenic(III) to Schwertmannite E D W A R D D . B U R T O N , * ,† RICHARD T. BUSH,† SCOTT G. JOHNSTON,† KYM M. WATLING,† ROSALIE K. HOCKING,‡ LEIGH A. SULLIVAN,† AND GRETEL K. PARKER§ Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia, Monash Centre for Synchrotron Science and School of Chemistry, Monash University, Clayton, VIC 3800, Australia, and School of Biomolecular and Physical Sciences, Griffith University, Nathan, QLD 4111, Australia
Received August 11, 2009. Revised manuscript received October 8, 2009. Accepted November 4, 2009.
This study describes the sorption of As(V) and As(III) to schwertmannite as a function of pH and arsenic loading. In general, sorption of As(V) was greatest at low pH, whereas high pH favored the sorption of As(III). The actual pH of equivalent As(V) and As(III) sorption was strongly loading dependent, decreasing from pH ∼ 8.0 at loadings 4.6). Sorption of As(V) and As(III) caused significant release of SO42- from within the schwertmannite solid-phase, without major degradation of the schwertmannite structure (as evident by X-ray diffraction and Raman spectroscopy). This can be interpreted as arsenic sorption via incorporation into the schwertmannite structure, rather than merely surface complexation at the mineral-water interface. The results of this study have important implications for arsenic mobility in the presence of schwertmannite, such as in areas affected by acidmine drainage and acid-sulfate soils. In particular, arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic mobility in schwertmanniterich systems.
Introduction Arsenic poses a serious threat to water resources in areas affected by the oxidation of arsenic-bearing iron sulfide minerals (1). The oxidation of these minerals, which include arsenian pyrite and arsenopyrite, can occur through miningrelated disturbance or artificial drainage of sulfidic soils (1, 2). For this reason, arsenic can occur in acid-mine drainage (AMD) at hundreds of mg L-1 (1) and in acid-sulfate soil (ASS) groundwater at lower mg L-1 levels (2). As such, * Corresponding author e-mail:
[email protected]. † Southern Cross University. ‡ Monash University. § Griffith University. 9202
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 24, 2009
understanding the geochemical behavior of arsenic is important in designing AMD and ASS management strategies. Schwertmannite is a poorly crystalline Fe(III)-oxyhydroxysulfate mineral with a variable composition, typically represented as Fe8O8(OH)8-2x(SO4)x (where x usually spans 1 to 1.75) (3, 4). It forms in acidic (pH 3-4), sulfate-rich waters and is common in AMD and ASS landscapes (3-9). In such landscapes, schwertmannite may retard the mobility of arsenic by decreasing its solubility (10-13). This can occur by coprecipitation of arsenic-rich schwertmannite or via arsenic sorption to pre-existing schwertmannite (10). While several studies have addressed the coprecipitation process, the alternate sorption process has received much less attention (10, 12, 14-16). Sorption of arsenic to pre-existing schwertmannite is potentially important in situations where arsenic bearing ground- and surface waters migrate through schwertmannite-rich materials. In natural waters, arsenic exists predominantly as oxyanions of the As(V) and As(III) oxidation states (1). Although As(V) is stable under oxidizing-conditions and As(III) under reducing-conditions, both states often exist in either redox environment because of slow redox transformations (1). Despite this knowledge, previous investigations on arsenic sorption to schwertmannite have focused solely on sorption of As(V) (10, 12, 14-16). These previous studies have also only considered strongly acidic conditions (i.e., pH 3.0-3.5), and therefore the current understanding of As(V) sorption to schwertmannite is limited to a very narrow set of pH conditions. This is a fundamentally important limitation, as the pH in schwertmannite-rich systems can increase substantially because of natural and engineered neutralization processes (such as liming) (8, 9, 13, 17). For example, schwertmannite-rich soils and sediments can rapidly attain circumneutral pH conditions due to alkalinity generation by bacterial Fe(III)- and SO42--reduction (8, 13). To our knowledge, there are no published studies on As(III)-schwertmannite interactions which describe the pHor loading-dependent sorption behavior. This represents a considerable gap in the current understanding of arsenicschwertmannite interactions, since As(III) may predominate even under strongly oxidizing conditions (1). For example, in AMD from the Carnoule`s mine in France, As(III) persists at hundreds of mg L-1 and co-occurs with schwertmannite (18, 19). Therefore, resolving As(III) sorption to schwertmannite is an important step toward understanding arsenic behavior in AMD and ASS systems. This study examines sorption of As(V) and As(III) to schwertmannite as a function of pH and arsenic loading. Batch sorption experiments were used to investigate the effect of pH and level of arsenic loading on the partitioning of arsenic between the aqueous phase and the schwertmannite solid phase. The solid phase resulting from these experiments was examined using X-ray diffraction, Raman spectroscopy, and X-ray absorption near-edge structure (XANES) spectroscopy. The present study is significant because it is the first to quantify sorption of both As(V) and As(III) to schwertmannite under a wide range of environmentally relevant pH conditions and arsenic loadings.
Materials and Methods General Methods. All laboratory glass- and plasticware was cleaned by soaking in 5% (v/v) HNO3 for at least 24 h, followed by repeated rinsing with deionized water (Milli-Q). All reagents were analytical grade and all solutions were prepared with deionized water. 10.1021/es902461x
2009 American Chemical Society
Published on Web 11/18/2009
Schwertmannite Synthesis. Schwertmannite was synthesized via H2O2 oxidation of a FeSO4 solution following Regenspurg et al. (5). The schwertmannite suspension was rinsed three times in 0.1 M NaCl and adjusted to pH 3.0 using 1 M NaOH. The mineralogy was verified using X-ray diffractometry (XRD) and Raman spectroscopy as described below. The schwertmannite composition was determined by triplicate digestion of 3 mL of suspension with 5 mL of 10 M HCl, followed by analysis of Fe and SO42-. The aqueous (
