Cadmium Isotopic Fractionation in the Soil–Plant System during Repeated Phytoextraction with a Cadmium Hyperaccumulating Plant Species

Abstract

Analysis of stable metal isotopes can provide important information on biogeochemical processes in the soil–plant system. Here, we conducted a repeated phytoextraction experiment using the cadmium (Cd) hyperaccumulator Sedum plumbizincicola X. H. Guo et S. B. Zhou ex L. H. Wu (Crassulaceae) in four different Cd-contaminated agricultural soils over five consecutive crops. Isotope composition of Cd was determined in the four soils before and after the fifth crop, in the plant shoots harvested in all soils in the first crop, and in the NH4OAc extracts of two contrasting soils with large differences in soil pH (5.73 and 7.32) and clay content (20.4 and 31.3%) before and after repeated phytoextraction. Before phytoextraction NH4OAc-extractable Cd showed a slight but significant negative isotope fractionation or no fractionation compared with total Cd (Δ114/110Cdextract‑soil = −0.15 ± 0.05 (mean ± standard error) and 0.01 ± 0.01‰), and the extent of fractionation varied with soil pH and clay content. S. plumbizincicola preferentially took up heavy Cd from soils (Δ114/110Cdshoot‑soil = 0.02–0.14‰), and heavy isotopes were significantly depleted in two soils after repeated phytoextraction (Δ114/110Cdsoil:P5‑soil:P0 = −0.15 ± 0.02 and −0.12 ± 0.01‰). This provides evidence for the existence of specific Cd transporters in S. plumbizincicola, leading to positive isotope fractionation during uptake. After phytoextraction by five sequential crops, the NH4OAc-extractable Cd pool was significantly enriched in heavy isotopes (Δ114/110Cdextract:P5‑extract:P0 = 0.07 ± 0.02 and 0.18 ± 0.05‰) despite the preferential uptake of heavy isotopes, indicating the occurrence of root-induced Cd mobilization in soils, which is supposed to favor heavy Cd in the organo-complexes with root exudates. Our results demonstrate that Cd is taken up by S. plumbizincicola via specific transporters, partly after active mobilization from the more strongly bound soil pool such as iron/manganese (hydr)­oxide-bound Cd during repeated phytoextraction. This renders S. plumbizincicola a suitable plant for large-scale field phytoremediation.

Theoretical isotope fractionation of cadmium during complexation with organic ligands

“…Isotope geochemistry has been increasingly applied in the past years for tracing anthropogenic Cd pollution (Cloquet et al, 2006;Salmanzadeh et al, 2017;Yang et al, 2019), tracing the source of ore-forming fluids (Zhu et al, 2016), to advance the understanding of the past and present marine biogeochemical cycling of Cd Zhang et al, 2018;, as well as to trace processes that control the Cd homeostasis in unicell organisms and distinct types of plants such as grasses Imseng et al, 2018;Zhang et al, 2021), cacao (Barraza et al, 2019, and Cd accumulating plants Zhou et al, 2020). Cd has eight stable isotopes which atomic masses range from 106-116, their ratios can vary significantly in the environment .…”

“…The Cd isotope fractionation induced by chelating thiols may have implications on Cd isotope fractionation in other plants than grasses. Cd hyperaccumulator plants can cope with high Cd concentrations and store, unlike cereals, the majority of the Cd taken up from soils in their shoots(Zhou et al 2020). To cope with the high Cd shoot concentrations, Cd is mostly bound to O donor atoms of e.g., organic acids and to a lesser extent to S donors(Tian et al 2011;Isaure et al 2015).…”

Cadmium Isotope Fractionation during Complexation with Humic Acid

“…Pilot studies involving Cd isotopes date back to the 1970s, when large variations were measured in meteoritic and extraterrestrial materials. 1,2 Cadmium has six stable isotopes ( 106 Cd, 108 Cd, 110 Cd, 111 Cd, 112 Cd, 114 Cd) and its isotope systematics can be used in different fields: (i) marine biogeochemistry and oceanography, given the nutrient-like behavior of Cd and its close correlation with algal nutrients, e.g., nitrate and phosphate, [3][4][5][6] (ii) Cd plant uptake in the soil-plant system influenced by the applications of mineral P fertilizers containing Cd and the risk of Cd transfer into the food chain, [7][8][9][10][11] and (iii) natural and anthropogenic source tracking based on the large isotope fractionation induced by evaporation and condensation processes during industrial Cd emissions. [12][13][14][15] Despite these numerous environmental studies, there is a surprising lack of a fundamental understanding of Cd isotope fractionation at specific conditions.…”

Preparation of CdS@C Photocatalyst Using Phytoaccumulation Cd Recycled From Contaminated Wastewater

“…Cadmium is one of the most important heavy metal contaminants in soils and water bodies and is extremely toxic to plants, animals, and humans ( Zhao et al, 2015 ; Gong et al, 2019 ; Xing et al, 2020 ; Zhou et al, 2020 ). Therefore, the remediation of Cd-contaminated aqueous and soil environments has become a matter of great concern in recent years.…”

“…Phytoextraction is considered as a feasible and cost-efficient in situ remediation strategy for slightly or moderately contaminated environments with heavy metals, including Cu, Mn, As, Cd, and Zn ( Hou et al, 2017 ; Li et al, 2018 ; Ent et al, 2020 ). In fact, phytoextraction has been applied for achieving a substantial removal of arsenic (As) and cadmium (Cd) from environments at large-scale field trials in China ( Li et al, 2018 ; Zhou et al, 2020 ). In past decades, the majority of present works have focused on enhancing the removal capacity of heavy metals from environments using hyperaccumulator plants.…”