Antimony speciation in the environment: Recent advances in understanding the biogeochemical processes and ecological effects

Abstract

Antimony (Sb) is a toxic metalloid, and its pollution has become a global environmental problem as a result of its extensive use and corresponding Sb-mining activities. The toxicity and mobility of Sb strongly depend on its chemical speciation. In this review, we summarize the current knowledge on the biogeochemical processes (including emission, distribution, speciation, redox, metabolism and toxicity) that trigger the mobilization and transformation of Sb from pollution sources to the surrounding environment. Natural phenomena such as weathering, biological activity and volcanic activity, together with anthropogenic inputs, are responsible for the emission of Sb into the environment. Sb emitted in the environment can adsorb and undergo redox reactions on organic or inorganic environmental media, thus changing its existing form and exerting toxic effects on the ecosystem. This review is based on a careful and systematic collection of the latest papers during 2010–2017 and our research results, and it illustrates the fate and ecological effects of Sb in the environment.

Introduction

As a toxic element of growing environmental concern, antimony (Sb) is ubiquitous throughout the environment as a result of natural phenomena such as weathering, biological activity, and volcanic activity together with anthropogenic inputs, and it has been considered a priority pollutant by the United States Environmental Protection Agency and the European Union. Worldwide, Sb is the ninth-most mined metal, and it primarily occurs in nature as Sb₂S₃ and Sb₂O₃ (Filella et al., 2002a, He et al., 1992, Okkenhaug et al., 2011). Some records in peat bogs (Shotyk et al., 2005) and Arctic polar ice cores (Krachler et al., 2005) show a dramatic increase of Sb in the environment as a result of its rapid growth in industrial use. Mining and smelting operations, waste incineration, coal and petroleum combustion, and spent ammunition are all associated with elevated concentrations of Sb in the environment, e.g., abandoned and active Sb mines, mineral processing facilities and smelters, waste incinerators, power plants, polyethylene terephthalate (PET) esterification industries, battery factories, shooting ranges and highways (Okkenhaug et al., 2011, Reimann et al., 2010).

Most Sb is emitted as Sb₂O₃ into air, water and soil from pollution sources (He et al., 2012, Tian et al., 2012). Then, it dissolves and is released into environmental media (Hu et al., 2014), and its transformation affects its mobility and bioavailability. Sb can exist in four oxidation states (-III, 0, III, and V) with the III and V states being most frequently encountered in the environment (Filella et al., 2002a). Sb(V) is the predominant species, and it exists in the form of Sb(OH)₆⁻ in oxic environments; Sb(III) primarily occurs as Sb(OH)₃, which is predominant under anoxic conditions. Sb(III) is more toxic than Sb(V) (Filella et al., 2002a, Filella et al., 2002b). In addition, Sb can also exist as organic compounds (methylated species) (Wilson et al., 2010). The mobility and toxicity of Sb strongly depend on its oxidation speciation (Kong et al., 2015). Similar to the metalloid As, Sb has no known biological function but is toxic. Therefore, it is significant to understand Sb speciation and transformation in the ecological environment to assess its fate and risk. In recent years, a series of studies on the occurrence of Sb, the solution (Hu et al., 2014, Hu et al., 2015a, Hu et al., 2015b, Hu et al., 2016, Hu et al., 2017, Hu and He, 2017) and transformation features associated with its chemistry (Kong and He, 2016, Kong et al., 2015, Kong et al., 2016), its interactions with microbiota (Abin and Hollibaugh, 2014, Kulp et al., 2014, Lehr et al., 2007, Li et al., 2015a), and its toxicity and biological effects on organisms (Corrales et al., 2014, Fu et al., 2010, Li et al., 2014, Sun et al., 2016a, Sun et al., 2017) have drawn the greater attention of the global scientific community on Sb contamination over different environmental, biological and geological fields.

In this paper, we attempt to review the biogeochemistry and ecological effect of Sb in environmental media and illustrate the key processes of the migration and transformation of Sb by combining the results of our research. The main topics of this review include the following: (1) pollution sources and emission characteristics; (2) distribution and speciation in different environmental media; (3) transformation processes associated with chemical and biological processes; and (4) toxicity to and bio-effects on different types of organisms. The main pollution source, environmental process and ecological effect of antimony in the environment are shown in Fig. 1.