Effect of redox variation on the geochemical behavior of Sb in a vegetated Sb(V)-contaminated soil column

Highlights

• The Sb released was higher under the oxic zone compared to anoxic zone.

• The dissolved Sb(III) fraction in soil pore water increased with incubation time.

• Sb(III) fraction of anoxic soil increased while oxic soil solely composed Sb(V).

• No significant transport of Sb occurred from Sb contaminated soil to plant.

• Changes in redox conditions induced a shift in soil microbial communities.

Abstract

This study examined the geochemical behavior of antimony (Sb) in a vegetated contaminated soil column consisting of unsaturated rhizosphere and a waterlogging layer. The results showed a reducing condition (Oxidation-Reduction Potential (ORP) of −171 mV) was formed in about 5 days in the waterlogging zone. The amount of Sb released was higher under the oxidizing unsaturated-rhizosphere compared to that in the waterlogging zone possibly because of the weaker affinity of Sb(V) to Mn- and/or Fe-oxides in soil. The fraction of Sb(III) in the dissolved total Sb increased with time when soil redox states were subjected to a further reduction. Solid phase Sb K-edge X-ray absorption spectroscopy (XAS) of soils showed that Sb(III) fraction of the deeper layer soil increased while the unsaturated upper soil solely composed Sb(V). In this study, 250 mg/kg of Sb pollution did not significantly affect plant growth and no significant transport of Sb occurred from the soil to plant. However, changes in redox conditions within the soil column induced a shift in soil microbial communities. Consequently, the importance of redox states of soil on geochemical behavior of Sb and the effects of soil flooding or waterlogging deserve attention in the management of Sb-contaminated soil.

Introduction

The mobility and toxicity change of redox sensitive toxic elements have received attention in the management of contaminated soil and sediment. Antimony (Sb) is a redox sensitive metalloid that has recently received attention as a toxic element. Its chemical reactivity under fixed redox conditions is actively studied in relation to changing redox conditions (Arsic et al., 2018). Antimony displays a wide range of oxidation states (from -3 to +5) and mostly exists as Sb(V) and Sb(III) in oxidizing and reducing soil environments, respectively (Gebel, 1997; Mitsunobu et al., 2006). The toxicity of Sb species is different depending on theirs oxidation state. For example, Sb(III) is 10 times more toxic than Sb(V) (Tschan et al., 2009; Rakshit et al., 2011).

Major sources of Sb contamination is mining and industrial emission (He et al., 2012). Antimony is used in industry as flame retardant, catalyst in plastics, pigment in paints, in glassware production, and in batteries (Oorts et al., 2008). Antimony is toxic to plants causing retardation of growth, inhibition of photosynthesis, and prevention of the absorption of some essential elements and the synthesis of some metabolites (Feng et al., 2013a). Regarding human health, Sb is known to be hazardous by inhalation or ingestion and carcinogenic, and the Sb level is regulated in drinking water (He et al., 2012; Hansell, 2015). Antimony undergoes various reactions in soil. Silicate minerals, Fe-oxides, and organic matter can adsorb Sb in soil. Precipitation of Sb with Ca was suggested to control the dissolved Sb(V) in highly Sb-contaminated alkaline soil (Wilson et al., 2010).

The soil redox state is known to be a key factor in controlling the reactivity and mobility of many toxic elements in soil. The soil redox condition is closely related to the saturation degree of soil, amount of organic contents, and the type of dominant electron acceptors (Frohne et al., 2011; Pezeshki and DeLaune, 2012). For example, when oxygen depletes in a soil system, microorganisms use electrons from Mn(III, IV)- or Fe(III)- oxides that play essential roles in sequestering toxic elements like Sb (Mitsunobu et al., 2006; Leuz et al., 2006; Hockmann et al., 2014, 2015). The release of Sb to aqueous phase is then observed under metal oxide-reducing conditions (Leuz et al., 2006; Han et al., 2019a; Shaheen et al., 2014).

The oxidation states of some redox-sensitive elements are known to be an important factor in controlling bioavailability of those elements. For example, it is known that As(III) is more toxic and difficult to be removed from aqueous phase because of its higher mobility compared to As(V). Contrary to the latter, Sb(III) is less mobile and has a higher affinity to Mn/Fe-oxides in soil or sediment. Therefore, Sb concentration is found to be higher under oxic conditions where Sb(V) prevails (Arsic et al., 2018; Hockmann et al., 2014, 2015; Fan et al., 2016). Arsic et al. (2018) observed the release of Fe, As, and Sb under shifted oxic and anoxic transition zones. Their results demonstrated that the decoupling between Sb and Fe(II) release is opposite to the redox-related decoupling of closely related As and Fe(II) release (Han et al., 2019a). The enhanced immobilization of Sb(V) under reducing conditions was reported with a calcareous shooting range soil and the decrease of dissolved Sb was interpreted as the effect of increased Sb(III) that is more extensively bound to Fe-oxides (Hockmann et al., 2014). Many researchers have investigated geochemical reaction of Sb in soil or mineral systems compared to that of As due to the chemical similarity of these two elements (Wilson et al., 2010; Han et al., 2018; Ilgen and Trainor, 2012). Recent studies have reported apparent differences between As and Sb regarding redox-related reactions. Compared to the extensive studies on As, less is known about the geochemical reactions of Sb (Arsic et al., 2018; Mitsunobu et al., 2006; Okkenhaug et al., 2011). Especially, the lack of data about Sb geochemistry under various redox environments and Sb speciation in plants makes it difficult to develop future strategies and remedial actions for Sb-contaminated soil.

In this study, the geochemical reactions among Sb and other redox related elements were investigated using an Sb(V)-contaminated vegetated soil. Plants can change the redox conditions in rhizosphere by enhancing oxygen transport in soil pore space (Hartmann et al., 2009; Evans, 2004). Therefore, various redox conditions formed in plant rhizosphere and soil saturated zone change the dissolved concentration and speciation of Sb in the soil and its pore water. A simulated Sb-contaminated soil column test was conducted to examine the effect of water logging and plant vegetation on Sb behavior and microbial community shift with a corresponding change in geochemical conditions.