Antimony (Sb) is an element of growing environmental concern due to the widespread use and uncontrolled release of Sb compounds into the environment (Filella et al. 2002b, He et al. 2019, Hu et al. 2014, Kong et al. 2015). In general, background concentrations of Sb in soil are lower than 1 mg/kg (Filella et al. 2002b). However, the massive smelting and utilization of Sb lead to severe contamination near the sites, posing a great risk to the local environment (Wang et al. 2018a, Wang et al. 2019, Wang et al. 2010a) and residents (Wu et al. 2011). More and more studies have shown that antimony pollution is a global issue (Amarasiriwardena &Wu 2011, Filella et al. 2009a) because of its toxicity to humans and its role in causing diseased of liver, skin, and respiratory and cardiovascular systems (Wu et al. 2011). Soil is an important medium for concentration and migration of Sb. In recent years, more attention has been paid to Sb contaminated soil in mining area and shooting ranges (Ahmad et al. 2014, Okkenhaug et al. 2013, Wang et al. 2018b, Wang et al. 2010b, Wu et al. 2011). However, there are little study on Sb contaminated soils caused by primary explosives due to the confidentiality and sensitivity. A significant input of Sb into the environment occurs through the production and utilization of primary explosives due to Sb used as combustible agent for classical primary explosives in history, which contain 33.4% Sb2S3 (Matyáš 2013). Weathering and corrosion of combustion residue lead to mobilization of metalloid Sb in anionic form (Hu et al. 2015). In addition, primary explosives sites, often characterized by critical concentrations of co-occurring copper (Cu), zinc (Zn) (Brede et al. 1996, Huynh et al. 2006, Jiang et al. 2020), can be of particular environmental concern since they represent hazardous multi-element contamination sources for sites zones. Leaching of Sb and co-occurring metals from primary explosives production and utilization areas pose a serious long-term threat to the environment and human health. Thus, immobilizing or reducing the mobility and bioavailability of Sb and co-occurring metals in primary explosives sites are crucial.
In the natural environment, the mobility, bioavailability and toxicity of Sb are primarily dependent on its chemical speciation (Filella et al. 2002b, a). Antimony is reported to exist in a variety of oxidation states (-III, 0, III, V), with oxidation states III and V being predominant in aqueous environment across a wide pH wide (4–10) (Ilgen et al. 2014). In the natural environment, Sb(III) primarily occurs as Sb(OH)3 under anaerobic conditions between pH 2 and 10 (Filella et al. 2002b, a), while Sb(V) is the predominant species existing as Sb(OH)6− in aerated environment (Filella et al. 2009b) and displays a high affinity to amorphous and crystalline Fe-(hydr)oxides with which it can form stable bidentate inner-sphere complexes (Guo et al. 2014b). These interactions are particularly favored by goethite in the pH range of 7.5-9.0, by hematite at pH of 8.5, by ferrihydrite in the pH range of 7.0-7.9, by akaganeite in the pH range of 9.5–10 (Garau et al. 2017). However, Sb with different valence states has different properties, in instance, Sb(III) can strongly adsorb on goethite over a wide pH range from 3 to 12, whereas the maximum adsorption of Sb(V) only occurs below pH 7(Leuz et al. 2006). In addition, these Fe-based metals not only can adsorb Sb strongly but also acting as oxidants in transforming Sb(III) to Sb(V) (Kong &He 2016). Laboratory-scale testing indicated that Fe2(SO4)3 is potentially applicable to Sb immobilization in soils (Okkenhaug et al. 2013). The sorption effects is based on the reaction of Sb(V) with surface hydroxyl group of Fe-based materials. However, the mobility of co-occurring Cu and Zn was enhanced after the addition of Fe-based sorbent (Okkenhaug et al. 2013). In the pH rang of 5–9, co-occurring metals behave quite differently, being commonly present in the soil solution as cations at acidic and circumneutral pH or as soluble SOM-metal(II) complexes at higher pH values (Garau et al. 2017). Moreover, at neutral and alkaline pH, substantial amounts of heavy metals are immobilized as Me-hydroxides, Me-carbonates or Me-hydroxycarbonates. Soluble heavy metals show limited affinity for hydroxyl groups due to their cationic nature but interact more strongly with negatively charged components (Garau et al. 2017). Thus, the different speciation, mobility and bioavailability between Sb and co-occurring metals make the identification of suitable amendments a very challenging task. Aluminum (hydr)oxides show important sorption properties for Pb, Cd and Zn (Wang et al. 2013), At the same time, Aluminum (hydr)oxides can be protonated, making the surface positively charged and thereby generating electrostatic interactive forces with negatively charged Sb(V) (Garau et al. 2014). To date, only a few amendments, mostly based on Fe- and Al-containing materials, have been tested with variable success as Sb-immobilizing agents. There limited number of studies highlight the need to deepen our knowledge on the mobility of Sb and co-occurring metals in soil and to select ideal sorbents to immobilize them.
Focusing on soil contaminated with Sb in primary explosives production site, the main goals of this work were to (i) investigate the mobility and speciation of Sb in primary explosive sites, (ii) evaluate the effects of combined application of Fe-Al mixed amendments for primary explosives sites using batch tests and column tests, (iii) investigate pH effect on Sb immobilization.