Article pubs.acs.org/est
Behavior of Antimony(V) during the Transformation of Ferrihydrite and Its Environmental Implications Satoshi Mitsunobu,*,† Chihiro Muramatsu,† Katsuaki Watanabe,‡ and Masahiro Sakata† †
Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 Japan
‡
S Supporting Information *
ABSTRACT: In this study, we investigated the behavior of Sb(V) during the transformation of poorly crystalline Fe(III) oxyhydroxides (two-line ferrihydrite) with various Sb/Fe molar ratios at pH 6.0. Both XRD and Fe EXAFS analyses confirmed that goethite and hematite are the primary transformation products of the ferrihydrite in the presence of Sb(V). The crystallization kinetics showed that the transformation rate with Sb(V) was approximately the same as that of the control (without Sb(V)), which indicates that the presence of Sb(V) does not influence the transformation rate to a noticeable extent. Throughout the transformation, Sb(V) dominantly partitioned in the solid phase and no desorption of Sb(V) was observed. Furthermore, Sb EXAFS analyses suggested that Sb(V) in the solid phase is structurally incorporated into crystalline goethite and/or hematite generated by the ferrihydrite transformation. Hence, Sb(V) transfers into the thermodynamically stable solids from the metastable ferrihydrite with aging, indicating a rigid immobilization of Sb(V). These findings are valuable for making predictions on the long-term fate of Sb associated with ferrihydrite in natural environments.
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environments such as soil and sediment.13,14 Scheinost et al.17 also reported the abundance of Sb(V) in shooting range soils under oxic conditions, but found no trace of Sb(III) in their investigation using X-ray absorption fine structure (XAFS). Evidence of the association of Sb with hydrous Fe(III) oxide (HFO) has been found by selective extraction analyses on sediments18 and sorption experiment studies.19 Sb(V) is strongly adsorbed with poorly ordered HFO (e.g., ferrihydrite) in oxic soils and sediments and can also form Sb and Fe bearing mineral such as tripuhyite with a wide range of Fe/Sb ratios in mine wastes and soils.13,14,18−21 XAFS spectroscopic investigation recently showed that Sb(V) associated with Fe(III) oxyhydroxides in natural soils primarily occurs as the coprecipitated species in natural soil, indicating structural incorporation of Sb(V) into the HFO.22 These findings suggest that both adsorption and incorporation processes to the HFO can control the mobility of Sb in the natural environments. Ferrihydrite, a short-ordered Fe(III) oxyhydroxide and a ubiquitous mineral observed in terrestrial systems such as soil and sediment, acts as a very efficient sink for trace metal(loid)s because of its positive charge below pH 7.8−8.123−25 and high specific surface area.26−28 Ferrihydrite is thermodynamically
INTRODUCTION Antimony (Sb) is a nonessential metalloid of increasing environmental concern because of its toxicity.1,2 Human ingestion of Sb results in hematic, gastrointestinal, and respiratory effects.3−5 The toxicity thresholds (EC50 values) of Sb (Sb2O3) to plant growth identified in laboratory test range from 0.5 to 8 mmol Sb kg−1.6−9 An and Kim10 reported the effect of dissolved Sb(III) on microbial growth inhibition. They showed that the 50% effects on the inhibition (EC50) widely varies with 16−555 mg Sb L−1. It is frequently used in a variety of industrial products such as flame retardants, catalysts in the synthesis of plastics, and alloys for ammunition.11,12 Although Sb and its compounds are considered as pollutants of priority interest (see ref 11 and references cited therein), the extent of industrial use is rapidly growing.2,12 The oxidation states of Sb most frequently observed in the environment are Sb(III) and Sb(V). In general, it is known that the natural abundance of Sb is low (in soil and sediment
Behavior of Antimony(V) during the Transformation of Ferrihydrite and Its Environmental Implications Satoshi Mitsunobu,*,† Chihiro Muramatsu,† Katsuaki Watanabe,‡ and Masahiro Sakata† †
Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 Japan
‡
S Supporting Information *
ABSTRACT: In this study, we investigated the behavior of Sb(V) during the transformation of poorly crystalline Fe(III) oxyhydroxides (two-line ferrihydrite) with various Sb/Fe molar ratios at pH 6.0. Both XRD and Fe EXAFS analyses confirmed that goethite and hematite are the primary transformation products of the ferrihydrite in the presence of Sb(V). The crystallization kinetics showed that the transformation rate with Sb(V) was approximately the same as that of the control (without Sb(V)), which indicates that the presence of Sb(V) does not influence the transformation rate to a noticeable extent. Throughout the transformation, Sb(V) dominantly partitioned in the solid phase and no desorption of Sb(V) was observed. Furthermore, Sb EXAFS analyses suggested that Sb(V) in the solid phase is structurally incorporated into crystalline goethite and/or hematite generated by the ferrihydrite transformation. Hence, Sb(V) transfers into the thermodynamically stable solids from the metastable ferrihydrite with aging, indicating a rigid immobilization of Sb(V). These findings are valuable for making predictions on the long-term fate of Sb associated with ferrihydrite in natural environments.
■
environments such as soil and sediment.13,14 Scheinost et al.17 also reported the abundance of Sb(V) in shooting range soils under oxic conditions, but found no trace of Sb(III) in their investigation using X-ray absorption fine structure (XAFS). Evidence of the association of Sb with hydrous Fe(III) oxide (HFO) has been found by selective extraction analyses on sediments18 and sorption experiment studies.19 Sb(V) is strongly adsorbed with poorly ordered HFO (e.g., ferrihydrite) in oxic soils and sediments and can also form Sb and Fe bearing mineral such as tripuhyite with a wide range of Fe/Sb ratios in mine wastes and soils.13,14,18−21 XAFS spectroscopic investigation recently showed that Sb(V) associated with Fe(III) oxyhydroxides in natural soils primarily occurs as the coprecipitated species in natural soil, indicating structural incorporation of Sb(V) into the HFO.22 These findings suggest that both adsorption and incorporation processes to the HFO can control the mobility of Sb in the natural environments. Ferrihydrite, a short-ordered Fe(III) oxyhydroxide and a ubiquitous mineral observed in terrestrial systems such as soil and sediment, acts as a very efficient sink for trace metal(loid)s because of its positive charge below pH 7.8−8.123−25 and high specific surface area.26−28 Ferrihydrite is thermodynamically
INTRODUCTION Antimony (Sb) is a nonessential metalloid of increasing environmental concern because of its toxicity.1,2 Human ingestion of Sb results in hematic, gastrointestinal, and respiratory effects.3−5 The toxicity thresholds (EC50 values) of Sb (Sb2O3) to plant growth identified in laboratory test range from 0.5 to 8 mmol Sb kg−1.6−9 An and Kim10 reported the effect of dissolved Sb(III) on microbial growth inhibition. They showed that the 50% effects on the inhibition (EC50) widely varies with 16−555 mg Sb L−1. It is frequently used in a variety of industrial products such as flame retardants, catalysts in the synthesis of plastics, and alloys for ammunition.11,12 Although Sb and its compounds are considered as pollutants of priority interest (see ref 11 and references cited therein), the extent of industrial use is rapidly growing.2,12 The oxidation states of Sb most frequently observed in the environment are Sb(III) and Sb(V). In general, it is known that the natural abundance of Sb is low (in soil and sediment
