Fraction and mobility of antimony and arsenic in three polluted soils: A comparison of single extraction and sequential extraction

Highlights

• Six extractants were applied to extract Sb and As from polluted soils simultaneously.

• Citric acids showed the highest extraction for Sb (24%) and As (41%).

• Fraction distribution of Sb and As indicated that As is more mobile than Sb.

• Elemental extractions order was citric acid > tartaric acid > EDTA > HCl > Na₂HPO₄ > CaCl₂.

• Citric acid extensively changed the fractionation distribution of Sb and As in soils.

Abstract

Co-contamination of arsenic (As) usually occurs with antimony (Sb) in Sb mine ores. However, the mobility and bio-availability of Sb and As in different types of mine impacted soils have received relatively little attention. This study aimed to investigate the fraction, mobility and removal of Sb and As in three types of polluted soils using environmentally friendly and cost-effective extractants. In the present study, lightly polluted (L), moderately polluted (M), and 3) highly polluted (H) soils were collected from the Xikuangshan (XKS) mine area in Hunan, China. Toxicity risk assessment, fraction and extraction of Sb and As were performed to evaluate Sb and As mobility and availability. According to the speciation fractions, the percent of residual Sb was larger than As in all studied soils, which suggested that As is far more mobile than Sb. Sb and As extractabilities from selected polluted soils were compared and ranked as: citric acid > tartaric acid > EDTA > HCl > Na₂HPO₄ > CaCl₂. Citric acid showed the highest extractabilities for both Sb and As (up to 24% for total Sb and 41% for total As respectively). Moreover, obvious alteration of Sb and As fractionations in three types of soils were observed after chemical extractions. The mobility of Sb and As increased after extraction by citric acid and tartaric acid, suggesting that these organic acids can make soil trace metals more bio-available and that, Sb/As polluted soils can be remediated via phytoextraction.

Introduction

Among potentially toxic elements (PTEs), Sb and As are considered as potential carcinogens (Shotyk et al., 2005; Filella et al., 2009). The European Community Council and U.S. Environmental Protection Agency listed Sb and As as priority pollutants in 1976 (EU, 1976) and 1979 (USEPA, 1979), respectively. These elements are toxic for humans and animals when exposed through the intake of water, air and food and/or skin/dermal contacts (Schnorr et al., 2010). Due to these hazards, these elements have attracted significant attention from the scientific community. Recently, intensive mineral/ore exploration and refining activities have released large amounts of Sb and As into the environment. Mining wastewater and dumping of mining and smelting waste residues are the major sources of these PTEs. Once distributed on soil surfaces, these mining and smelting residues were subjected to erosion and weathering processes, which release these contaminants to the surrounding area. Subsequently, they caused long-term and severe impacts on local environmental ecosystems (Shotyk et al., 2005). While Sb and As are unnecessary for plants growth and development, plants can uptake them along with nutrient or essential elements present in soil solution (Anawar et al., 2018). Recent studies have demonstrated that once Sb concentration reached to 5–10 mg kg⁻¹ in plant tissues, it can cause severe phytotoxicity towards to plants (Kabatapendias and Pendias, 2010). Similarly, another study was also claimed that 5 mg kg⁻¹ Sb in plants can produce toxicity (He, 2007). Regarding to As, recently research found that the predominant effect of As toxicity in plants is the inhibition of seed germination, root and shoot elongation and root surface (Anawar et al., 2018). Moreover, the critical As content in the primary soil types in China has been found as 10–13 mg kg⁻¹, the level that leads to the reduction and death of plant organisms (Capacity, 1991).

The presence of high-intensity Sb pollution generally co-occurs with As pollution in contaminated soils. It is generally assumed that the geochemical behavior and toxicity of Sb are similar to those of As (Li et al., 2014). Sb and As display the same range of oxidation states (trivalent (III) or pentavalent (V)). The pentavalent state is predominant in topsoil (Okkenhaug et al., 2012), while both elements present as trivalent under flooded conditions (Johnson et al., 2005; Mitsunobu et al., 2006). The toxicities of Sb and As in the environment highly depend on their oxidation state: 0 < V < III (Krachler et al., 2001; Wilson et al., 2010).

The sequential extraction is a direct and useful method to assess the fractions of toxic elements in soil by using specific chemical extractants that target different fractions bound to specific solid phases of soil components. Knowing the composition of different chemically bound forms of solid-state Sb and As in soils is key to evaluating the mobility and bio-availability of Sb and As. This characterization is especially important for the Fe/Mn oxides, which hold larger amounts of As and Sb in the soil matrix compared to other fractions (Lee et al., 2016). Besides, single extraction method is effective and widely used for determining the bio-availability of PTEs in soil (Ettler et al., 2007). Several extraction methods have been developed and used for Sb or As extraction (Lin, 2004), but simultaneously extraction of these two elements has been rarely studied. Previous work has shown that As contaminated soils can usually be extracted by alkali solution or even with mixed acids, such as aqua regia (Legiec et al., 1997), while synthetic chelants and organic acids can dissolve Fe/Mn oxides and extract oxide-bound metals present in soil (Nowack, 2002; Tang et al., 2011).

China has the largest Sb deposits and has been considered as the largest Sb production country of the world. Mining activities mostly occur in rural areas of China with field soils in their vicinity often highly contaminated with Sb and As (Waqas et al., 2014). Thus, local residents are frequently exposed to high concentrations of Sb and As through food chain contamination (Khan et al., 2008; Long et al., 2018). Acute and chronic PTE (Sb and As) exposure has caused a large number of people living in villages to fell ill with cancer and these villages are sometimes regarded as “cancer villages” (Khan et al., 2014). Recently, the Ministry of Environmental Protection (MEP) of China has shown a strong commitment for controlling PTE associated risks and developed a five-year plan (2011–2015) to meet objectives.

Focusing on soil contaminated with PTEs in China, this study aimed to investigate the fraction, mobility, and removal of toxic metals, particularly Sb and As, in three soil types using environmentally friendly and cost-effective extractants. Taking the XKS Sb mine in the region (Lengshuijiang, Hunan Province) as study area, the overall objectives of this study were to 1) assess the mobility of Sb and As in three types of contaminated soils; 2) study the extractabilities of PTEs (Sb and As) in the soil samples using six chemical agents; and 3) discuss the alteration in fractionation distributions of Sb and As after extracting with different extractants.