Mechanistic insight into the interactions of EDDS with copper in the rhizosphere of polluted soils

Water-soluble carboxymethyl chitosan and rhamnolipids promote the remediation of Cd-contaminated soil by mediating the growth of Hylotelephium spectabile and regulating the rhizospheric ecological environment

The application of biodegradable chelating agents in phytoremediation is a promising approach. This study aimed to investigate the effects and roles of underlying mechanisms of water-soluble carboxymethyl chitosan (WSCC) and rhamnolipids (RLs) on the remediation of Cd-contaminated soil by Hylotelephium spectabile. WSCC and RLs mediated the growth of H. spectabile by increasing chlorophyll content and the activity of antioxidant enzymes as well as promoted the conversion of water-extractable Cd to HAc-extractable Cd in leaves. WSCC and RLs promoted the secretion of malic acid, acetic acid, and succinic acid by the roots; decreased soil pH; increased the number of functional groups, such as hydroxyl, amino, and carboxyl groups, in the soil; and changed the diversity and structure of bacterial communities in the soil, thereby improving the bioavailability of Cd in the soil and creating a good ecological environment of the rhizosphere. The combined application of WSCC and RLs had a better auxiliary effect than single application of either, especially under CR2 treatment (1.5 g·kg-1 WSCC + 0.2 g·kg-1 RLs), where the accumulation of Cd in plants significantly increased by 159.86 % compared with the control. These findings indicated that WSCC and RLs enhanced the remediation efficiency of H. spectabile by regulating both plant growth and the ecological environment of the rhizosphere.

EDDS application destabilizes soil organic matter in phytoremediation: Insights from quantity and molecular composition of dissolved organic matter

The stability of soil organic matter (SOM) is crucial for metal transport and carbon cycling. S,S-ethylenediaminedisuccinic acid (EDDS) is widely used to enhance phytoremediation efficiency for heavy metals in contaminated soils, yet its specific impacts on SOM have been underexplored. This study investigates the effects of EDDS on SOM stability using a rhizobox experiment with ryegrass. Changes in soil dissolved organic matter (DOM) quantity and molecular composition were analyzed via Fourier transform ion cyclotron resonance mass spectrometry. Results showed that the use of EDDS increased the uptake of Cu, Cd and Pb by ryegrass, but simultaneously induced the destabilization and transformation of SOM. After 7 days of EDDS application, dissolved organic carbon (DOC) and nitrogen (DON) concentrations in rhizosphere soils increased significantly by 3.44 and 10.2 times, respectively. In addition, EDDS reduced lipids (56.3%) and proteins/amino sugars-like compounds (52.1%), while increasing tannins (9.11%) and condensed aromatics-like compounds (24.4%) in the rhizosphere DOM. These effects likely stem from EDDS's dual action: extracting Fe/Al from SOM-mineral aggregates, releasing SOM into the DOM pool, and promoting microbial degradation of bioavailable carbon through chain scission and dehydration. Our study firstly revealed that the application of EDDS in phytoremediation increased the mineralization of SOM and release of CO2 from soil to the atmosphere, which is important to assess the carbon budget of phytoremediation and develop climate-smart strategy in future.

Effect of EDDS on the rhizosphere ecology and microbial regulation of the Cd-Cr contaminated soil remediation using king grass combined with Piriformospora indica

The negative impacts of soil heavy metals composite pollution on agricultural production and human health are becoming increasingly prevalent. The applications of green chelating agents and microorganisms have emerged as promising alternate methods for enhancing phytoremediation. The regulatory effects of root secretion composition, microbial carbon source utilization, key gene expression, and soil microbial community structure were comprehensively analyzed through a combination of HPLC, Biolog EcoPlates, qPCR, and high-throughput screening techniques. The application of EDDS resulted in a favorable rhizosphere ecological environment for the king grass Piriformospora indica, characterized by a decrease in soil pH by 0.41 units, stimulation of succinic acid and fumaric acid secretion, and an increase in carbon source metabolic activity of amino acids and carbohydrates. Consequently, this improvement enhanced the bioavailability of Cd/Cr and increased the biomass of king grass by 25.7%. The expression of dissimilatory iron-reducing bacteria was significantly upregulated by 99.2%, while there was no significant difference in Clostridium abundance. Furthermore, the richness of the soil rhizosphere fungal community (Ascomycota: 45.8%, Rozellomycota: 16.7%) significantly increased to regulate the proportion of tolerant microbial dominant groups, promoting the improvement of Cd/Cr removal efficiency (Cd: 23.4%, Cr: 18.7%). These findings provide a theoretical basis for the sustainable development of chelating agent-assisted plants-microorganisms combined remediation of heavy metals in soil.

Non-negligibly negative role of e-waste-derived pyrogenic carbon in the soil washing of copper and polybrominated diphenyl ethers

The open incineration of electrical and electronic waste (e-waste) results in the accumulation of pyrogenic carbon in the soil. However, the effect of the e-waste-derived pyrogenic carbon (E-PyC) on the performance of soil washing at e-waste incineration sites remains unclear. In this study, the effectiveness of a citrate-surfactant mixed solution in removing copper (Cu) and decabromodiphenyl ether (BDE209) at two e-waste incineration sites was evaluated. The removal efficiencies of Cu (24.6-51.3%) and BDE209 (13.0-27.9%) were low in both soils and were not significantly improved by ultrasonic. Soil organic matter analysis, hydrogen peroxide and thermal pretreatment experiments, and microscale soil particle characterization demonstrated that the poor removal of soil Cu and BDE209 was due to steric effects of E-PyC on the release of the solid fraction of pollutants and the competitive sorption of the labile fraction of pollutants by E-PyC. Weathering of soil Cu weakened the influence of E-PyC but strengthened the negative impact of natural organic matter (NOM) on soil Cu removal by promoting complexation between NOM and Cu2+ ions. This study demonstrates that the negative effect of E-PyC on Cu and BDE209 removal by soil washing is non-negligible, which has implications for decontaminating e-waste incineration sites by soil washing.

5-Aminolevulinic acid mitigates the chromium-induced changes in Helianthus annuus L. as revealed by plant defense system enhancement

Chromium (Cr) in the soil is one of the major pollutants for agricultural production. This study examined the efficiency of sunflower plants to remediate Cr-contaminated soils using a plant growth regulator, 5-aminolevolinic acid (ALA). At six leaf stage, sunflower plants were exposed to soil-applied Cr (0.15 g kg^-1), manganese (Mn, 0.3 g kg^-1) and trisodium (S,S)-ethylenediamine-N,N'-disuccinic acid (EDDS, 2.5 mmol kg^-1), ALA (10 mg L^-1) was sprayed. After ALA treatment, the plants were harvested for further biochemical analyses. Results showed that EDDS and Mn improved the Cr accumulation but restrained plant growth. Conversely, ALA improved the growth of Cr-stressed plants by promoting chlorophyll concentration in the top fully expanded leaves. The bioaccumulation quantity and removal efficiency of sunflowers treated by Cr + EDDS + ALA was improved by 47.92% and 47.94%, respectively, as compared to the Cr treatment. This was further supported by qRT-PCR analysis, where the expression of heavy metal transport genes such as ZIP6 and NRAMP6 and subsequently Cr accumulation in sunflower tissues increased by EDDS, Mn, and ALA application. However, compared with other treatments, ALA ameliorated cellular injury from Cr-stress by uptake or movement of Cr prevention, modulation of antioxidant enzymes, and elimination of reactive oxygen species. Our study suggested that ALA as an ideal option for the phytoremediation of Cr-contaminated soils.

Elevated temperature induces contrasting transformation of exogenous copper to soil solution and solid phases in an arable soil

Heavy metal contamination of soils has been a global environmental issue over the past decades, threatening food security and human health. Understanding the migration and transformation of heavy metals in soils is critical for restoring an impaired environment and developing sustainable agriculture, particularly in the face of global warming. However, little effort has been devoted to investigating the impact of elevated temperatures on the migration and distribution of exogenous heavy metals in soils. This study experimented with a 180-day incubation at 15 °C, 30 °C, and 45 °C with an arable soil (Alfisol) of Huang-Huai-Hai River Basin, China, which was initially spiked with copper (Cu). A comparison of the results revealed that the percentage of soil water-soluble Cu doubled at 45 °C compared with 15 °C. The percentage of protein-like substances in dissolved organic matter (DOM) was the highest at 45 °C, suggesting that proteinaceous components play a more significant role in controlling the dissolution of Cu into DOM. Moreover, by sequential extraction and micro-X-ray fluorescence (μ-XRF), Cu was facilitatively transformed from exchangeable, and specifically adsorbed fractions, to iron (Fe)/manganese (Mn) oxides bound species by 7.75%23.63% with the elevation of temperature from 15 °C to 45 °C. The conversion of Cu speciation is attributed to the significant release of organic carbon from Fe/Mn oxides, especially the Mn oxide components, which are available for Cu binding. The findings of this work will provide an in-depth understanding of the fate of Cu in soils, which is fundamental for the risk assessment and remediation of Cu-polluted soils in the Huang-Huai-Hai River Basin under the context of global warming.

A critical review on biochar-assisted free radicals mediated redox reactions influencing transformation of potentially toxic metals: Occurrence, formation, and environmental applications

Potentially toxic metals have become a viable threat to the ecosystem due to their carcinogenic nature. Biochar has gained substantial interest due to its redox-mediated processes and redox-active metals. Biochar has the capacity to directly adsorb the pollutants from contaminated environments through several mechanisms such as coprecipitation, complexation, ion exchange, and electrostatic interaction. Biochar's electron-mediating potential may be influenced by the cyclic transition of surface moieties and conjugated carbon structures. Thus, pyrolysis configuration, biomass material, retention time, oxygen flow, and heating time also affect biochar's redox properties. Generally, reactive oxygen species (ROS) exist as free radicals (FRs) in radical and non-radical forms, i.e., hydroxyl radical, superoxide, nitric oxide, hydrogen peroxide, and singlet oxygen. Heavy metals are involved in the production of FRs during redox-mediated reactions, which may contribute to ROS formation. This review aims to critically evaluate the redox-mediated characteristics of biochar produced from various biomass feedstocks under different pyrolysis conditions. In addition, we assessed the impact of biochar-assisted FRs redox-mediated processes on heavy metal immobilization and mobility. We also revealed new insights into the function of FRs in biochar and its potential uses for environment-friendly remediation and reducing the dependency on fossil-based materials, utilizing local residual biomass as a raw material in terms of sustainability.

Biodegradation and effects of EDDS and NTA on Zn in soil solutions during phytoextraction by alfalfa in soils with three Zn levels

In chelator-enhanced Zn phytoremediation studies, it is crucial to understand how the degradable chelators and the competition from other ions influence the concentration of Zn in soil solutions. This study investigated the biodegradability of two chelators (EDDS: Ethylenediamine-N,N'-disuccinic acid, and NTA: Nitrilotriacetic acid) and their effects on the Zn concentration in the soil solution during the growth of alfalfa (Medicago Sativa L.). The chelators were added at four doses (0, 0.5, 2 and 5 mmol kg^-1) in soils with varying Zn levels (189, 265 and 1496 mg kg^-1). The results showed that the lag phase before EDDS and NTA biodegradation varied from 0 to 7 days in the three soils. EDDS and NTA were completely decomposed within the assessed 57 days regardless of the applied dosage, with a half-life of 1.3-3.0 days in highly Zn-contaminated soil and 4.2-10.8 days in the two other soils. In soil solutions, the change in solubilized Zn was in line with EDDS and NTA degradation kinetics. Cu, Al, Fe and Mn were the main metal ions that competed against Zn for chelation. Besides, Ni competed with Zn in the whole process. Ca did not compete effectively in the three soils, while Mg was a competitor only at the initial stage. Our results show the importance of considering both the biodegradation rate and the competition between the target cation and other elements present in the soil when using chelators to enhance phytoremediation. A 30-day explorative incubation experiment is recommended to evaluate the appropriate application time of chelators and the target Zn exposure time for plants during phytoremediation.

Cu phytoextraction and biomass utilization as essential trace element feed supplements for livestock

Copper (Cu), as an essential element, is added to animal feed to stimulate growth and prevent disease. The forage crop alfalfa (Medicago sativa L.) produced during Cu phytoextraction may be considered a biofortified crop to substitute the Cu feed additives for livestock production, beneficially alleviating Cu contamination in soils and reducing its input into agriculture systems. To assess this, alfalfa was grown in three similar soils with different Cu levels, i.e., 11, 439 and 779 mg kg^-1 for uncontaminated soil (A), moderately Cu-contaminated soil (B) and highly Cu-contaminated soil (C), respectively. EDDS (Ethylenediamine-N,N'-disuccinic acid) was applied to the soils seven days before the first cutting at four rates (0, 0.5, 2 and 5 mmol kg^-1) to enhance bioavailable Cu uptake. Alfalfa grew well in soils A and B but not in the highly Cu-contaminated soil. After applying EDDS, a significant biomass reduction of the first cutting shoot was only observed with 5 mmol kg^-1 EDDS in the highly Cu-contaminated soil, with a 45% (P < 0.05) decrease when compared to the control. Alfalfa grown in the three soils gradually wilted after the first cutting with 5 mmol kg^-1 EDDS, and Cu concentrations in the first cutting shoot were augmented strongly, by 250% (P < 0.05), 3500% (P < 0.05) and 6700% (P < 0.05) compared to the controls, respectively. Cu concentrations in alfalfa shoots were found to be higher in this study than in some fodder plants and further augmented in soils with higher Cu levels and with EDDS application. These findings suggest that alfalfa grown on clean soils or soils with up to 450 mg Cu kg^-1 (with appropriate EDDS dosages) has the potential to be considered as a partial Cu supplementation for livestock. This research laid the foundation for the integration between Cu-phytoextraction and Cu-biofortification for livestock.

Roxarsone transformation and its impacts on soil enzyme activity in paddy soils: A new insight into water flooding effects

The aromatic arsenical roxarsone (ROX) has been used as feed additive for decades worldwide. The past or present application of animal manure containing ROX in paddy fields results in arsenic (As) accumulation in rice grain. However, the degradation and transformation mechanisms of ROX in paddy soil which determine As bioavailability and uptake by rice are still unclear. The current study investigated the variation of As speciation and soil enzyme activities in ROX-treated soils under flooded and non-flooded conditions for six months. Our results showed that 70.2% of ROX persisted in non-flooded paddy soils after 180 d while ROX degraded completely within 7 d in flooded soils. The rapid degradation of ROX under flooded conditions owed to the enhanced biotic transformation that was caused by the low Eh and the predominant presence of Clostridium spp. and Bacillus spp. ROX was not only transformed to As(III) and As(V) in non-flooded soils but also to 3-amino-4-hydroxyphenylarsonic acid and methyl arsenicals in flooded soils. The degradation products significantly inhibited soil enzyme activities for 7-30 d, but the inhibition effects disappeared after 90 d due to the sorption of transformed As products to amorphous Fe oxides. This study provides new insights into the flooding effect on ROX fate in paddy fields, which is important for the management of animal waste and risk control on polluted sites.