Repeated phytoextraction of metal contaminated calcareous soil by hyperaccumulator Sedum plumbizincicola

Phytoextraction of highly cadmium-polluted agricultural soil by Sedum plumbizincicola: An eight-hectare field study

Phytoextraction with Sedum plumbizincicola is an in-situ, environmentally friendly and highly efficient remediation technique for slightly Cd-polluted soils but it remains a challenge to remediate highly Cd-polluted soils under field conditions. Here, an 8-ha field experiment was conducted to evaluate the feasibility of repeated phytoextraction by S. plumbizincicola of a highly Cd-polluted acid agricultural soil (pH 5.61, [Cd] 2.58 mg kg-1) in Yunnan province, southwest China. Mean shoot dry biomass production, Cd concentration and Cd uptake were 1.95 t ha-1, 170 mg kg-1 and 339 g ha-1 at the first harvest, and 0.91 t ha-1, 172 mg kg-1 and 142 g ha-1 at the second harvest. After two seasons of phytoextraction, soil total and CaCl2-extractable Cd concentrations decreased from 2.58 ± 0.69 to 1.53 ± 0.43 mg kg-1 and 0.22 ± 0.12 to 0.14 ± 0.07 mg kg-1, respectively. Stepwise multiple linear regression analysis shows that the shoot Cd concentration and uptake of S. plumbizincicola were positively related to soil CaCl2-extractable Cd concentrations, especially in the first crop. A negative relationship indicates that soil organic matter content played an important role in soil Cd availability and shoot Cd concentration in the first crop. In addition, the rhizosphere effect on soil CaCl2-extractable Cd concentration was negatively correlated with soil pH in the first crop. The accuracy of the calculation of soil Cd phytoextraction efficiency at field scale depends on all of the following factors being considered: shoot Cd uptake, cropping pattern, standardized sampling points, and the leaching and surface runoff of Cd. Phytoextraction with S. plumbizincicola is a feasible technique for efficient Cd removal from highly polluted soils and wide variation in soil properties can influence phytoextraction efficiency at the field scale.

Does phytoextraction with Sedum plumbizincicola increase cadmium leaching from polluted agricultural soil?

Sedum plumbizincicola is a cadmium (Cd) and zinc hyperaccumulator that can activate Cd by rhizosphere acidification. However, there is little understanding of the Cd leaching risk from polluted soil during phytoextraction process. Here, pot and column experiments were conducted to monitor soil Cd leaching characteristics under different rainfall simulation conditions during S. plumbizincicola phytoextraction. Soil Cd leaching increased significantly with increasing simulated rainfall intensity. Compared with normal rainfall (NR), weak rainfall (WR) resulted in a 34.3% decrease in Cd uptake by S. plumbizincicola and also led to a 68.7% decline in Cd leaching. In contrast, Cd leaching under heavy rainfall (HR) was 2.12 times that of NR in the presence of S. plumbizincicola. After two successive growing periods, phytoextraction resulted in a 53.5-66.4% decline in the amount of soil Cd leached compared with controls in which S. plumbizincicola was absent. Even compared with maize cropping as a control, S. plumbizincicola did not instigate a significant increase in Cd leaching. The contribution of Cd leaching loss to the decline in soil total Cd concentration was negligible after phytoextraction in the pot experiment. Overall, the results contribute to our understanding of soil Cd leaching risk by phytoextraction with S. plumbizincicola.

An Assessment of the Feasibility of Phytoextraction for the Stripping of Bioavailable Metals from Contaminated Soils

Phytoextraction has been proposed in many papers as a low-cost method for remediating contaminated soil. However, if national regulation is based on total metal(loid) concentrations in soil, phytoextraction is generally infeasible because of the long time required for remediation. Assessing phytoextraction requires determination of the dynamic rate of metal removal from soil. Phytoextraction may be feasible if the main goal is to reduce the soluble fraction of the metal(loid) with the goal of reducing bioavailability. However, it has been reported that there is a large mass balance mismatch between the reduction of the soluble metal fraction in contaminated soil and metal uptake by plants. Several studies report that the decrease of soluble fraction of metals in soil is higher than can be accounted for by plant uptake. In other words, studies generally overestimate the feasibility of bioavailable contaminant stripping. Therefore, a more rigorous approach is advisable to ensure that papers on bioavailable contaminant stripping include relevant information on mass balances. Furthermore, to implement the concept of bioavailable contaminant stripping, regulations must distinguish between the bioavailable fraction and the total metal concentration in soil. Environ Toxicol Chem 2023;42:558-565. © 2022 SETAC.

Cloning and Functional Characterization of SpZIP2

Zinc (Zn)-regulated and iron (Fe)-regulated transporter-like proteins (ZIP) are key players involved in the accumulation of cadmium (Cd) and Zn in plants. Sedum plumbizincicola X.H. Guo et S.B. Zhou ex L.H. Wu (S. plumbizincicola) is a Crassulaceae Cd/Zn hyperaccumulator found in China, but the role of ZIPs in S. plumbizincicola remains largely unexplored. Here, we identified 12 members of ZIP family genes by transcriptome analysis in S. plumbizincicola and cloned the SpZIP2 gene with functional analysis. The expression of SpZIP2 in roots was higher than that in the shoots, and Cd stress significantly decreased its expression in the roots but increased its expression in leaves. Protein sequence characteristics and structural analysis showed that the content of alanine and leucine residues in the SpZIP2 sequence was higher than other residues, and several serine, threonine and tyrosine sites can be phosphorylated. Transmembrane domain analysis showed that SpZIP2 has the classic eight transmembrane regions. The evolutionary analysis found that SpZIP2 is closely related to OsZIP2, followed by AtZIP11, OsZIP1 and AtZIP2. Sequence alignment showed that most of the conserved sequences among these members were located in the transmembrane regions. A further metal sensitivity assay using yeast mutant Δyap1 showed that the expression of SpZIP2 increased the sensitivity of the transformants to Cd but failed to change the resistance to Zn. The subsequent ion content determination showed that the expression of SpZIP2 increased the accumulation of Cd in yeast. Subcellular localization showed that SpZIP2 was localized to membrane systems, including the plasma membrane and endoplasmic reticulum. The above results indicate that ZIP member SpZIP2 participates in the uptake and accumulation of Cd into cells and might contribute to Cd hyperaccumulation in S. plumbizincicola.

A meta-analysis about the accumulation of heavy metals uptake by Sedum alfredii and Sedum plumbizincicola in contaminated soil

Sedum alfredii and Sedum plumbizincicola typically have high heavy metal (such as Zn and Cd) accumulation capacities with fast growth rates and relatively high Pb tolerance in contaminated soils. We compared the accumulation characteristics of heavy metals in Sedum species through meta-analysis. Furthermore, we analyzed the effects of soil organic matter (SOM) and soil pH on Cd, Pb and Zn accumulation by S. alfredii and S. plumbizincicola and the correlation between various metals. Results showed that the accumulations of Cd and Zn in shoots were higher than that of roots, but Pb accumulated in roots more than shoots. Moreover, there is a significant positive correlation between the accumulation of Zn and Cd in shoots. We found that the heavy metal accumulation rate in shoots was higher with lower soil pH. Sedum species had the highest Cd adsorption capacity in 20-30 g/kg SOM and the highest Zn adsorption capacity in SOM less than 20 g/kg. The accumulation rate of Cd in shoots of S. plumbizincicola was increased with exposure time, while the accumulation rate of Zn was slightly decreased.

Comparative effects of Tagetes patula L. extraction, mercapto-palygorskite immobilisation, and the combination thereof on Cd accumulation by wheat in Cd-contaminated soil

Phytoextraction and in situ immobilisation are two of the most commonly used remediation techniques for Cd-contaminated farmland. In theory, phytoextraction followed by immobilisation can reduce the total Cd and available Cd contents of the soil, making it suitable for the remediation of heavily Cd-contaminated alkaline soil. However, the real remediation efficiency is uncertain, and it is also unknown whether phytoextraction affects subsequent wheat Cd accumulation. In this study, two seasonal pot experiments were conducted to determine the effects of S,S-ethylenediamine disuccinic acid (EDDS)-assisted Tagetes patula L. (T. patula) extraction, mercapto-palygorskite (MPAL) immobilisation, and the combination thereof on subsequent Cd accumulation in wheat. EDDS application significantly increased the Cd content in the subsequent-soil solution, but the EDDS-activated Cd could not be absorbed by wheat roots. T. patula extraction decreased the subsequent soil pH value by 0.1-0.2 pH units, increased the available Cd content by 0.19 mg/kg, but had no effect on subsequent wheat Cd accumulation. The Cd absorption capacity of wheat roots and the Cd translocation capacity of wheat stems to grains of high-Cd wheat were higher than that of low-Cd wheat cultivar. The application of MPAL had no effect on soil pH value, but significantly decreased soil available Cd and exchangeable Cd contents by 17.78-36.76% and 21.13-52.63%; it also increased the Fe/Mn oxide-bound Cd fraction by 14.02-64.00%. MPAL application decreased the wheat grain Cd concentrations from 0.51 to 0.13 mg/kg (high-Cd wheat) and 0.35 to 0.05 mg/kg (low-Cd wheat), but had no negative effect on Fe, Mn, Cu, and Zn elements. Compared with the single MPAL application treatments, the combination treatments had no inhibition effect on Cd accumulation in wheat. MPAL is an efficient amendment, and considering the remediation efficiency, stability, and time of these methods, the combination of MPAL application with a low-Cd accumulation wheat cultivar represents a suitable proposal to ensure the safe production of wheat in Cd-contaminated alkaline soil.

Remediation of a metal-contaminated soil by chemical washing and repeated phytoextraction: a field experiment

Agricultural soil contaminated with potentially toxic metals poses great health risk to humans and it requires long-term remediation. Here, we investigate the remediation of metal-polluted agricultural soil by combining chemical washing with repeated phytoextraction. The polluted field was initially washed with 40 mmol L^-1 FeCl₃ (F) or 20 mmol L^-1 FeCl₃ + 40 mmol L^-1 citric acid (F + C). After the application of organic fertilizer (O), lime (L), and sepiolite (S), Sedum plumbizincicola was cultivated for three successive crops from 2017 to 2019. Results showed that the soil washed with FeCl₃ had high removal efficiencies of Cd (35.2%), Pb (24.3%), and Zn (26.6%). Although the shoot biomass and metal concentrations of S. plumbizincicola decreased significantly in the first crop, there were no significant differences in the subsequent two crops. Throughout the remediation process, the higher total removal efficiencies of Cd, Pb, and Zn were conducted in F + OLS treatment which observed in 71.0, 34.0, and 47.7%, respectively. The results, therefore, conclusively indicated that combining chemical washing with repeated phytoextraction showed considerable potential for the remediation of agricultural soils polluted with multiple metals. However, further studies are required to focus on the amelioration of the degraded soil quality and safe agricultural production.

Acid buffering capacity of four contrasting metal-contaminated calcareous soil types: Changes in soil metals and relevance to phytoextraction

Four different metal-contaminated calcareous soil types, Carbonati-Perudic Cambosols (CPC), Fe-accumuli-Stagnic Anthrosols (FSA), Ochri-Aquic Cambosols (OAC) and Calci-Orthic Aridosols (COA), were investigated. The acid buffering capacity and metal-releasing behaviors of the soils were explored using an acid extraction method. Soil incubation and pot experiments were conducted to investigate changes in soil metal speciation and the enhancement of phytoextraction by soil acidification. There were several to tens of times differences in acid buffering capacities between soils. Soil calcium content may represent the major buffering system as indicated by significant linear correlations between the amount of Ca²⁺ released and H⁺ addition, and metal release into solution with H⁺ addition showed three stages, i.e. little release, slow release and rapid release stages. Soil carbonate-bound and Fe/Mn oxide-bound Cd and Zn decreased with the addition of H⁺ to all four soils, but organic matter-bound and residual metals remained unchanged. Based on the intensity of acidification, the efficiency Cd and Zn phytoextraction increased substantially with the addition of H⁺ in the case of the CPC but not the FSA which had a higher acid buffering capacity than the CPC. Hence, it may be concluded that the acid buffering capacity and changes in soil metal fractions with acidification of contaminated calcareous soil types should be determined before phytoextraction of these soils is attempted.

Biochar-assisted phytoextraction of Cd and Zn by Noccaea caerulescens on a contaminated soil: A four-year lysimeter study

Amendments of biochar, the residual solid of biomass pyrolysis, have been shown to enhance metal phytoextraction from contaminated soils with hyperaccumulating plants in specific situations. In order to investigate this phenomenon over successive harvests in field conditions, two identical undisturbed soil cylinders (1-m² section × 1.85-m height) were excavated from a contaminated agricultural plot and monitored with instrumented lysimeters. Wood-derived biochar was added at a rate of 5% (w/w) in the first 30 cm of one of the two lysimeters. The Cd/Zn-hyperaccumulator Noccaea caerulescens was then grown for the next four years on both lysimeters. Our results showed that the hyperaccumulating plant was able to remove about 2 g m^-2 of Cd and 12-16 g m^-2 of Zn within four years, representing about 40% and 4% of the initial Cd and Zn soil contamination, respectively. Biochar amendment improved plant germination and survival and increased root surface density. However, no significant effect of biochar on shoot metal content of N. caerulescens was observed. Mass balances suggested that up to 10% the metal contamination moved from the disturbed Ap horizon to the deeper horizons, particularly in the biochar-amended soil profile. Furthermore, shoot Cd and Zn concentration generally decreased over the successive harvests, together with soil metal availability. Depending on the way to account for this progressive decrease in efficiency, our estimations of the time necessary to remove the excess of metals in the topsoil in these conditions ranged from 11 to 111 years for Cd and from 97 years to an infinite time for Zn. In conclusion, the simultaneous use of N. caerulescens and biochar amendment can lead to a significant removal of specific metallic elements from the topsoil, but the risk of metal movement down the soil profile and the observed decrease in phytoextraction efficiency over time deserve further investigations.