Copper migration and isotope fractionation in a typical paddy soil profile of the Yangtze Delta

A novel application of thallium isotopes in tracing metal(loid)s migration and related sources in contaminated paddy soils

Thallium (Tl) is a highly toxic heavy metal, which is harmful to plants and animals even in trace amounts. Migration behaviors of Tl in paddy soils system remain largely unknown. Herein, Tl isotopic compositions have been employed for the first time to explore Tl transfer and pathway in paddy soil system. The results showed considerably large Tl isotopic variations (ε²⁰⁵Tl = -0.99 ± 0.45 ~ 24.57 ± 0.27), which may result from interconversion between Tl(I) and Tl(III) under alternative redox conditions in the paddy system. Overall higher ε²⁰⁵Tl values of paddy soils in the deeper layers were probably attributed to abundant presence of Fe/Mn (hydr)oxides and occasionally extreme redox conditions during alternative dry-wet process which oxidized Tl(I) to Tl(III). A ternary mixing model using Tl isotopic compositions further disclosed that industrial waste contributed predominantly to Tl contamination in the studied soil, with an average contribution rate of 73.23%. All these findings indicate that Tl isotopes can be used as an efficient tracer for fingerprinting Tl pathway in complicated scenarios even under varied redox conditions, providing significant prospect in diverse environmental applications.

Fractionation behaviors of Cu, Zn, and S-O isotopes in groundwater contaminated with petroleum and treated by oxidation

Fractionation behaviors of Cu and Zn isotopes have been increasingly studied at the field scale, but those in various redox conditions of groundwater contaminated with petroleum and treated by oxidation have not been assessed. In this study, δ⁶⁵Cu and δ⁶⁶Zn as well as δ³⁴S_SO4 and Δδ¹⁸O_SO4-H2O were assessed in wells undergoing contamination by total petroleum hydrocarbons (TPH) and oxidation using H₂O₂ in 2021 and 2022. High δ³⁴S_SO4 and relevant parameters (e.g., dissolved sulfide and HCO₃^-) indicated the occurrence of sulfate reduction. The plot of δ⁶⁵Cu versus δ³⁴S_SO4 effectively indicated precipitation of Cu sulfides and their reoxidation at oxidation wells. Although the plot of δ⁶⁶Zn versus δ³⁴S_SO4 could also indicate reoxidation of Zn sulfides, the Zn isotopic fingerprint of sulfide precipitation may have been masked by fractionation by sorption. The advantage of using δ⁶⁵Cu in the redox reactions resulted from the wider range of δ⁶⁵Cu owing to the redox behavior of Cu. The plot combining isotopic fractionations of Cu and S can assist in assessing sulfide precipitation and oxidative treatment in TPH-contaminated groundwater.

Antimony isotope fractionation and the key controls in the soil profiles of an antimony smelting area

The controlling factors of antimony migration and transformation in soil profiles are still unclear. Antimony isotopes might be a useful tool to trace it. In this paper, antimony isotopic compositions of plant and smelter-derived samples, and two soil profiles were measured for the first time. The δ¹²³Sb values of the surface and bottom layers of the two soil profiles varied in 0.23‰-1.19‰ and 0.58‰-0.66‰, respectively, while δ¹²³Sb of the smelter-derived samples varied in 0.29‰-0.38‰. The results show that the antimony isotopic compositions in the soil profiles are affected by post-depositional biogeochemical processes. The enrichment and loss of light isotopes at 0-10 cm and 10-40 cm layers of the contrasted soil profile may be controlled by plant uptake process. The loss and enrichment of heavy isotopes in the 0-10 cm and 10-25 cm layers of the antimony from smelting source in the polluted soil profile may be controlled by the adsorption process, while the enrichment of light isotopes in the 25-80 cm layer may be related to the reductive dissolution process. The conclusion emphasizes that the promotion of the Sb isotope fractionation mechanism will play a crucial role in understanding the migration and transformation behaviors of Sb in soil systems.

Emerging applications of high-precision Cu isotopic analysis by MC-ICP-MS

As a component of many minerals and an essential trace element in most aerobic organisms, the transition metal element Cu is important for studying reduction-oxidation (redox) interactions and metal cycling in the total environment (lithosphere, atmosphere, biosphere, hydrosphere, and anthroposphere). The "fractionation" or relative partitioning of the naturally occurring "heavy" (⁶⁵Cu) and "light" (⁶³Cu) isotope between two coexisting phases in a system occurs according to bonding environment and/or as a result of a slight difference in the rate at which these isotopes take part in physical processes and chemical reactions (in absence of equilibrium). Due to this behaviour, Cu isotopic analysis can be used to study a range of geochemical and biological processes that cannot be elucidated with Cu concentrations alone. The shift between Cu⁺ and Cu²⁺ is accompanied by a large degree of Cu isotope fractionation, enabling the Cu isotope to be applied as a vector in mineral exploration, tracer of origin, transport, and fate of metal contaminants in the environment, biomonitor, and diagnostic/prognostic marker of disease, among other applications. In this contribution, we (1) discuss the analytical protocols that are currently available to perform Cu isotopic analysis, (2) provide a compilation of published δ⁶⁵Cu values for matrix reference materials, (3) review Cu isotope fractionation mechanisms, (4) highlight emerging applications of Cu isotopic analysis, and (5) discuss future research avenues.