Perchlorate mobilization of metals in serpentine soils

Natural processes and anthropogenic activities may result in the formation and/or introduction of perchlorate (ClO₄⁻) at elevated levels into the environment. Perchlorate in soil environments on Earth and potentially in Mars may modify the dynamics of metal release and their mobilization. Serpentine soils, known for their elevated metal concentrations, provide an opportunity to assess the extent that perchlorate may enhance metal release and availability in natural soil and regolith systems. Here, we assess the release rates and extractability of Ni, Mn, Co and Cr in processed Sri Lankan serpentine soils using a range of perchlorate concentrations (0.10–2.50 w/v ClO₄⁻) via kinetic and incubation experiments. Kinetic experiments revealed an increase of Ni, Mn, Co and Cr dissolution rates (1.33 × 10⁻¹¹, 2.74 × 10⁻¹¹, 3.05 × 10⁻¹² and 5.35 × 10⁻¹³ mol m⁻² s⁻¹, respectively) with increasing perchlorate concentrations. Similarly, sequential and single extractions demonstrated that Ni, Mn, Co and Cr increased with increasing perchlorate concentrations compared to the control soil (i.e., considering all extractions: 1.3–6.2 (Ni), 1.2–126 (Mn), 1.4–34.6 (Co) and 1.2–6.4 (Cr) times greater than the control in all soils). Despite the oxidizing capability of perchlorate and the accelerated release of Cr, the dominant oxidation state of Cr in solution was Cr(III), potentially due to low pH (<2) and Cr(VI) instability. This implies that environmental remediation of perchlorate enriched sites must not only treat the direct hazard of perchlorate, but also the potential indirect hazard of related metal contamination.     

环境中高氯酸盐的来源、污染现状及其分析方法

高氯酸盐是一种持久性的无机污染物,它能够抑制碘的吸收并削弱甲状腺功能。随着高效灵敏的分离检测方法的发展,研究人员在多种环境介质中都检测到了高氯酸根,其环境污染问题引起了广泛的关注。最近的研究表明,环境中的高氯酸盐并非都来自于人为污染源及含有高氯酸盐的化肥,也有在大气中自然产生的,而且环境中存在大气来源的高氯酸根的背景值。在重点介绍了高氯酸盐的污染现状、分析方法以及自然条件下大气中产生高氯酸根的机理等方面的最新研究进展之后,提出今后应开展环境中高氯酸根背景值的调查研究。为此,应开发具有更低检出限的分析方法,并借助南极地区独特的地理优势开展南极雪冰中高氯酸盐的相关研究,从而获得不同历史时期环境中高氯酸根的背景值,并研究高氯酸根背景值的影响因素,为研究自然条件下大气中产生高氯酸根的机理提供基础数据。     

Perchlorate accumulation and migration in the deep vadose zone in a semiarid region

For 25 years, a plant in Israel manufacturing ammonium perchlorate disposed of untreated wastewater in four unlined ponds. This study explores the transport mechanisms of perchlorate infiltrated from 1965 to 1990 from one of these active storage ponds into a deep (40 m) layered vadose zone and the underlying Israeli coastal aquifer. Perchlorate migration from 1990, when wastewater disposal ceased, until today, with infiltration due only to natural rain (500 mm y⁻¹), was also studied. Several indirect methods were used, including: mass balance in the unsaturated zone profile, δ¹⁸O and δ²H profiles below the pond, and a comparison of the same sediment profiles in 2005 and 2007. The isotopic composition of the pore water could be divided into two separate groups: lighter (depleted) and heavier (enriched) samples. All samples in the lighter group were from the shallow vadose zone, above two clayey layers, and represent natural infiltration of rainwater. The enriched samples were from the deeper section of the unsaturated zone (20–40 m) and represent water used for perchlorate manufacturing 14 years prior to drilling. Consequently, the overall maximum infiltration rate was estimated to be 1.4 m y⁻¹. Below the clayey layer almost identical perchlorate concentrations were found along the sediment profile in 2005 and 2007 (two boreholes, 3 m apart). Very different perchlorate profiles were observed above the clayey layers. This suggests that perchlorate below the clay layers (20–40 m) is practically stagnant under the current natural conditions. The reduction in perchlorate concentration in groundwater below the ponds vs. its increased concentration further downgradient supports the contention that the current migration of perchlorate from the vadose zone to the groundwater is very small. We estimate that perchlorate concentration in the groundwater under the infiltration pond, which was 187 mg l⁻¹ in 2004, will reach 10 μg l⁻¹ within about 14 years. The existence of a clayey layer crossing the thick vadose zone was thus found to significantly change the infiltration rate when ponded conditions were replaced with natural precipitation.