Evaluation of the effectiveness of in situ stabilization in the field aged arsenic-contaminated soil: Chemical extractability and biological response
Graphical abstract
Introduction
In situ stabilization of soils and sediments contaminated with metals and/or metalloids has been proposed due to concerns about disturbance to their ecosystems [[1], [2], [3]]. The mobility of metals and metalloids in soil can be alleviated by amending an appropriate sorbent that makes their chemical forms more stable via sorption and/or precipitation, thereby decreasing bioavailability and toxicity [3]. It can be effectively applied in areas where it might be difficult to carry out ex situ treatment that requires excavation and site closure [4].
Arsenic (As) contamination in the vicinity of an abandoned smelter in South Korea has been garnering attention due to its adverse effects on humans and surrounding ecosystem. Nonferrous metal smelting took place from 1936 to 1989 [5], and the As contamination of the surrounding soils has occurred through various sources, including arsenic trioxide (As2O3) emitted from the smelting stack, dust associated with ores (e.g., lead and copper ores containing arsenopyrite (FeAsS) and arsenic sulfide (As2S3)), and field-disposed sludge produced from the smelting activity [4,5]. As concentrations in surface soils within 1.5 km from the smelting stack was found to frequently exceed the Korean soil regulatory levels for As (25 mg/kg for a rice paddy field, farmland, residential area, school, or park; 50 mg/kg for a forest, commercial area, or recreational area) [5]. Considering the As background concentration in soil (i.e., 6.83 mg/kg) in South Korea, it indicates that the smelter site is severely contaminated and the appropriate action should be taken. Hence, the Korean government purchased the area to apply soil remedial actions. Some forest lands in the purchased area have limitations in carrying out ex situ treatments such as soil washing because preserving vegetation is important given the present land use as a tourist site [4]. In this respect, appropriate measures, such as in situ stabilization, must be carried out at this site to manage human health risk and ecotoxicity.
A variety of sorbents are reported to be capable of stabilizing As in soil [6]. Among them, iron (Fe) oxides such as ferrihydrite, goethite, hematite, and magnetite have been recognized for several decades as efficient stabilizing agents for As-contaminated soils [7,8]. By forming the outer- and/or inner-sphere complexes between the positively charged surface of Fe oxides and pentavalent arsenate (HAsO42− and H2AsO4-) in soil porewater [9,10], the amendment of Fe oxides renders As in soil stable.
For in situ stabilization to be a valid risk mitigation measure, the stabilized As in soil should be chemically and biologically stable [1]. Since in situ stabilization does not physically remove As from soil, but retains it strongly bound, issues regarding the human health risk and ecotoxicity of the stabilized As in soil are critical. This study evaluated the effectiveness of in situ stabilization of the historically long-term As-contaminated soil in the vicinity of an abandoned smelter in South Korea by means of (i) chemical extractability and the corresponding human health risk, and (ii) ecotoxicity to barley Hordeum vulgare as an indicator of biological responses. In situ stabilization was performed through the amendment of a Fe-based sorbent. To elucidate why chemical extractability and biological responses have changed, the X-ray absorption spectroscopy with the aid of linear combination fitting (LCF), and the analysis of soil physicochemical properties were performed.
Section snippets
In situ stabilization
In situ stabilization at the forest near the old abandoned smelter in South Korea (longitude: 126°39′59.40″, latitude: 36°00′27.46″) was performed as follows: (i) pH adjustment, (ii) sorbent amendment, (iii) water spraying, and (iv) reaction for one week (April 3–10, 2017). Because this site is covered with vegetation, and because the preservation of vegetation is important according to the land use as a sightseeing site, the application of ex situ treatments is limited [4]. All application
Chemical extractability and human health risk
Aqua regia digestible As concentrations in the original and stabilized soils were 142 ± 4 and 152 ± 9 mg/kg, respectively (Table 1). Since stabilization process is a kind of risk reduction, not concentration reduction, it is noteworthy that SBRC-extractable As concentrations slightly decreased after in situ stabilization. Since the adsorbed As in the Fe-based sorbent is resistant to SBRC extraction, the SBRC-extractable As concentration decreased from 16.0 ± 1.0 to 13.0 ± 0.2 mg/kg (Table 1).
Conclusions
In situ stabilization as a risk mitigation measure was performed in historically long-term As-contaminated soil in the vicinity of an abandoned smelter via the amendment with the Fe-based sorbent. The effectiveness of in situ stabilization was assessed by means of chemical extractability and biological responses. Chemical extractability of As in soil determined using SBRC extraction slightly decreased, thereby lowering the human health risk through oral ingestion. Nonspecifically bound As in
Acknowledgements
This work was financially supported by Mid-Career Researcher Program through National Research Foundation (NRF) grant funded by the Ministry of Science and ICT (MSIT) (No. 2016R1A2B2015399) and Korea Environment Industry and Technology Institute (KEITI) through SEM (Subsurface Environment Management) project funded by Korea Ministry of Environment (RE201805067). The authors thank the Institute of Engineering Research at Seoul National University for technical assistance.
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Current address: Memory Environment & Safety Group, Samsung Electronics, 1 Samsungjeonja-ro, Hwaseong-si, Gyeonggi-do 18448, Republic of Korea.