Enhanced Phytoextraction for Co-contaminated Soil with Cd and Pb by Ryegrass (Lolium perenne L.)

Extraction of heavy metals from copper tailings by ryegrass (Lolium perenne L.) with the assistance of degradable chelating agents

Heavy metal contamination is an urgent ecological governance problem in mining areas. In order to seek for a green and environmentally friendly reagent with better plant restoration effect to solve the problem of low efficiency in plant restoration in heavy metal pollution soil. In this study, we evaluated the effects of three biodegradable chelating agents, namely citric acid (CA), fulvic acid (FA) and polyaspartic acid (PASP), on the physicochemical properties of copper tailings, growth of ryegrass (Lolium perenne L.) and heavy metal accumulation therein. The results showed that the chelating agent application improved the physicochemical properties of copper tailings, increased the biomass of ryegrass and enriched more Cu and Cd in copper tailings. In the control group, the main existing forms of Cu and Cd were oxidizable state, followed by residual, weak acid soluble and reducible states. After the CA, FA or PASP application, Cu and Cd were converted from the residual and oxidizable states to the reducible and weak acid soluble states, whose bioavailability in copper tailings were thus enhanced. Besides, the chelating agent incorporation improved the Cu and Cd extraction efficiencies of ryegrass from copper tailings, as manifested by increased root and stem contents of Cu and Cd by 30.29-103.42%, 11.43-74.29%, 2.98-110.98% and 11.11-111.11%, respectively, in comparison with the control group. In the presence of multiple heavy metals, CA, FA or PASP showed selectivity regarding the ryegrass extraction of heavy metals from copper tailings. PCA analysis revealed that the CA-4 and PASP-7 treatment had great remediation potentials against Cu and Cd in copper tailings, respectively, as manifested by increases in Cu and Cd contents in ryegrass by 90.98% and 74.29% compared to the CK group.

Combining biochar and grass-legume mixture to improve the phytoremediation of soils contaminated with potentially toxic elements (PTEs)

The combination of soil amendments with plants can be a viable option for restoring the functionality of PTEs-contaminated soils. Soil recovery could be further optimized through the mixed cropping of plant species (e.g. legumes and grasses) with different physiological characteristics. The aim of this study was to assess the phytoremediation ability of Vicia villosa Roth. And Lolium rigidum Gaud. Grown alone or in mixture in a soil contaminated with PTEs (C), i.e. Cd (23 mg kg-1), Pb (4473 mg kg-1) and Zn (3147 mg kg-1), and amended with 3% biochar (C + B). Biochar improved soil fertility and changed PTEs distribution, reducing soluble fractions and increasing the more stable ones. The addition of biochar increased the plant biomass of hairy vetch and annual ryegrass, both in monoculture and when in mixture. For example, shoot and root biomass of the C + B intercropped hairy vetch and annual ryegrass increased 9- and 7-fold, and ∼3-fold respectively, compared to the respective C plants. The biochar addition decreased PTE-uptake by both plants, while mixed cropping increased the uptake of PTEs by shoots of hairy vetch grown in C and C + B. The bioaccumulation, translocation factors, and mineralomass showed that hairy vetch and annual ryegrass behaved as phytostabilising plants. PTE mineralomasses proved that mixed cropping in C + B increased the overall capacity of PTE accumulation by plant tissues, particularly the root system. Therefore, the combination of biochar and legumes/grasses mixed cropping could be an effective solution for the recovery of PTEs-contaminated soils and the mitigation of their environmental hazard.

Polyaspartic acid assisted-phytoremediation of cadmium-contaminated farmland: Phytoextraction efficiency, soil quality, and rhizosphere microbial community

Cadmium is highly toxic and one of the most dangerous metal pollutants in soil, and poses a serious threat to human health through soil-crop-food chain transmission. Polyaspartic acid (PASP) is a biodegradable additive that is environment-friendly compared to traditional chelating agents. Current studies have explored its effect on auxiliary phytoextraction at a laboratory scale; however, the method is still rarely reported at the field scale. Therefore, this study used two ecotypes of Pennisetum sinese in a field experiment for 3 years in Jiaoxi Township, Liuyang City, Hunan Province, China, to understand the effect of PASP on the phytoremediation of Cd-contaminated soil and soil quality through long-term field studies. Moreover, because the soil microbial community responds well to the phytoremediation effect of heavy metal (including Cd)-contaminated soil, the changes in rhizosphere soil microbial community diversity and composition were analyzed. After 2 years of PASP-enhanced phytoremediation, the PASP application increased the total Cd reduction in soil by 237 % and 255 %, and the soil DTPA-extractable Cd content decreased to 0.092 and 0.087 mg kg^-1. When the application of PASP ceased in the third year, the two ecotypes of P. sinese obtained after harvest could achieve feed safety. Our study showed that the application of PASP could significantly increase the Cd extraction capacity and shoot biomass of P. sinese, and maintain soil health by optimizing the composition and structure of rhizosphere bacterial communities. The rhizosphere bacterial community structure was improved and dominated by Acidobacteriota, Proteobacteria, and Chloroflexi at the phylum level, and the increased abundance of Acetobacter, Enterobacter, Pseudomonas, and Stenotrophomonas at the genus level may promote heavy metal detoxification in soil, plant growth, and phytoremediation. Long-term field monitoring demonstrated that the low-cost and eco-friendly features of PASP made it a good candidate for enhancing phytoextraction efficiency and regulating soil microbial communities for remediation.

Effect of different amounts of fruit peel-based activator combined with phosphate-solubilizing bacteria on enhancing phytoextraction of Cd from farmland soil by ryegrass

To improve the uptake of heavy metals by plants and increase the effectiveness of phytoextraction, chelating agents are employed to change the speciation of heavy metals in soil and increase their bioavailability. However, the effect of a single activator is limited. In recent years, compound activators have been applied widely to improve phytoextraction efficiency. In this study, a fruit peel-based activator (OG) was prepared, containing a mixture of orange peel extracts and tetrasodium glutamate diacetate (GLDA) (1.6% v/v) in a ratio of 1:1 (v/v). The pot experiment was used to investigate the effects of different amounts of OG combined with phosphate-solubilizing bacteria (Acinetobacter pitti, AP) on the extraction of Cd from farmland soil by ryegrass (Lolium perenne L). The results indicated that the addition of OG and AP increased the pH and EC of the soil and improved the content of nutrient elements in the soil. The optimal combination of the application rates of OG and AP improved the growth of ryegrass and enhanced the phytoextraction of Cd. Redundancy analysis (RDA) showed that total soil nitrogen had the greatest influence on phytoextraction, with a contribution rate of 85.3%, followed by pH, with a contribution rate of 7.7%. Total nitrogen, pH, available phosphorus, alkaline nitrogen, and total organic matter were correlated positively with plant Cd, soil Cd decrease ratio, and the bioaccumulation factor but negatively with total Cd and available Cd. Based on the findings of this study, it is feasible to apply the fruit peel-based activator (amended with GLDA) and phosphate-solubilizing bacteria to enhance phytoextraction of Cd, which will provide a valuable reference for the treatment of heavy metal-contaminated soils and the reutilization of fruit peel waste. When applying the compound activator, it is recommended to consider the influence of the additional amount of compound activator on the extraction efficiency.

Transcriptomic Sequencing Analysis on Key Genes and Pathways Regulating Cadmium (Cd) in Ryegrass (Lolium perenne L.) under Different Cadmium Concentrations

Perennial ryegrass (Lolium perenne L.) is an important forage grass and has the potential to be used in phytoremediation, while little information is available regarding the transcriptome profiling of ryegrass leaves in response to high levels of Cd. To investigate and uncover the physiological responses and gene expression characteristics of perennial ryegrass under Cd stress, a pot experiment was performed to study the transcriptomic profiles of ryegrass with Cd-spiked soils. Transcriptome sequencing and comparative analysis were performed on the Illumina RNA-Seq platform at different concentrations of Cd-treated (0, 50 and 500 mg·kg−1 soil) ryegrass leaves and differentially expressed genes (DEGs) were verified by RT-qPCR. The results show that high concentrations of Cd significantly inhibited the growth of ryegrass, while the lower concentrations (5 and 25 mg·kg−1) showed minor effects. The activity levels of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and malondialdehyde (MDA) increased in Cd-treated ryegrass leaves. We identified 1103 differentially expressed genes (DEGs) and profiled the molecular regulatory pathways of ryegrass leaves with Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis in response to Cd stress. Cd stress significantly increased the membrane part, the metabolic process, the cellular process and catalytic activity. The numbers of unigenes related to signal transduction mechanisms, post-translational modification, replication, recombination and repair significantly increased. KEGG function annotation and enrichment analysis were performed based on DEGs with different treatments, indicating that the MAPK signaling pathway, the mRNA surveillance pathway and RNA transport were regulated significantly. Taken together, this study explores the effect of Cd stress on the growth physiology and gene level of ryegrass, thus highlighting significance of preventing and controlling heavy metal pollution in the future.

Polyaspartic acid enhances the Cd phytoextraction efficiency of Bidens pilosa by remolding the rhizospheric environment and reprogramming plant metabolism

The green soil chelator polyaspartic acid (PASP) can enhance heavy metal phytoextraction efficiency, but the potential mechanisms are not clearly understood from the whole soil-plant system. In this study, we explored the effects and potential mechanisms of PASP addition in soils on plant growth and cadmium (Cd) uptake in the Cd hyperaccumulator Bidens pilosa by analysing variations in chemical elements, rhizospheric microbial community, and plant metabolomics. The results showed that PASP significantly promoted the biomass yield and Cd concentration in B. pilosa, leading to an increase in the total accumulated Cd by 46.4% and 76.4% in shoots and 124.7% and 197.3% in roots under 3 and 6 mg kg^-1 PASP addition, respectively. The improved soil-available nutrients and enriched plant growth-promoting rhizobacteria (e.g., Sphingopyxis, Sphingomonas, Cupriavidus, Achromobacter, Nocardioides, and Rhizobium) were probably responsible for the enhanced plant growth after PASP addition. The increase in Cd uptake by plants could be due to the improved rhizosphere-available Cd, which was directly activated by PASP and affected by the induced rhizobacteria involved in immobilizing/mobilizing Cd (e.g., Sphingomonas, Cupriavidus, Achromobacter, and Rhizobium). Notably, PASP and/or these potassium (K)-solubilizing rhizobacteria (i.e., Sphingomonas, Cupriavidus, and Rhizobium) highly activated rhizosphere-available K to enhance plant growth and Cd uptake in B. pilosa. Plant physiological and metabolomic results indicated that multiple processes involving antioxidant enzymes, amino acids, organic acids, and lipids contributed to Cd detoxification in B. pilosa. This study provides novel insights into understanding how soil chelators drive heavy metal transfer in soil-plant systems.

Integrated physiological, transcriptomic and metabolomic analysis of the response of Trifolium pratense L. to Pb toxicity

Lead (Pb) interferes with plant gene expression, alters metabolite contents and affects plant growth. However, the molecular mechanism underlying the plant response to Pb is not completely understood. In the present study, Trifolium pratense L. was exposed to Pb concentrations of 0 (Pb0), 500 (Pb500), 1000 (Pb1000), 2000 (Pb2000) and 3000 (Pb3000) mg/kg in soils. Pb stress affected the ability of T. pratense to accumulate and transport Pb, increased the activity of peroxidase (POD) and the contents of malondialdehyde (MDA) and proline, decreased the amount of photosynthetic pigments and soluble proteins, and led to changes in growth and biomass. Transcriptomic and metabolomic analyses showed that Pb mainly affected eight pathways, and LHC, flavonoids, organic acids, amino acids and carbohydrates were upregulated or downregulated. Moreover, Pb500 induced the upregulation of serA, promoted the synthesis of citric acid, maintained photosynthetic pigment levels, and ultimately promoted an increase in stem length. Pb3000 induced the upregulation of ARF, GH3 and SAUR genes, but the saccharide contents and stem length decreased in response to Pb stress. We used a variety of methods to provide a molecular perspective on the mechanism underlying the response of T. pratense to Pb stress.

Are Grasses Really Useful for the Phytoremediation of Potentially Toxic Trace Elements? A Review

The pollution of soil, water, and air by potentially toxic trace elements poses risks to environmental and human health. For this reason, many chemical, physical, and biological processes of remediation have been developed to reduce the (available) trace element concentrations in the environment. Among those technologies, phytoremediation is an environmentally friendly in situ and cost-effective approach to remediate sites with low-to-moderate pollution with trace elements. However, not all species have the potential to be used for phytoremediation of trace element-polluted sites due to their morpho-physiological characteristics and low tolerance to toxicity induced by the trace elements. Grasses are prospective candidates due to their high biomass yields, fast growth, adaptations to infertile soils, and successive shoot regrowth after harvest. A large number of studies evaluating the processes related to the uptake, transport, accumulation, and toxicity of trace elements in grasses assessed for phytoremediation have been conducted. The aim of this review is (i) to synthesize the available information on the mechanisms involved in uptake, transport, accumulation, toxicity, and tolerance to trace elements in grasses; (ii) to identify suitable grasses for trace element phytoextraction, phytostabilization, and phytofiltration; (iii) to describe the main strategies used to improve trace element phytoremediation efficiency by grasses; and (iv) to point out the advantages, disadvantages, and perspectives for the use of grasses for phytoremediation of trace element-polluted soils.

Remediation of Cd-Contaminated Soil by Modified Nanoscale Zero-Valent Iron: Role of Plant Root Exudates and Inner Mechanisms

In this study, the role of exogenous root exudates and microorganisms was investigated in the application of modified nanoscale zero-valent iron (nZVI) for the remediation of cadmium (Cd)-contaminated soil. In this experiment, citric acid (CA) was used to simulate root exudates, which were then added to water and soil to simulate the pore water and rhizosphere environment. In detail, the experiment in water demonstrated that low concentration of CA facilitated Cd removal by nZVI, while the high concentration achieved the opposite. Among them, CA can promote the adsorption of Cd not only by direct complexation with heavy metal ions, but also by indirect effect to promote the production of iron hydroxyl oxides which has excellent heavy metal adsorption properties. Additionally, the H⁺ dissociated from CA posed a great influence on Cd removal. The situation in soil was similar to that in water, where low concentrations of CA contributed to the immobilization of Cd by nZVI, while high concentrations promoted the desorption of Cd and the generation of CA–Cd complexes which facilitated the uptake of Cd by plants. As the reaction progressed, the soil pH and cation exchange capacity (CEC) increased, while organic matter (OM) decreased. Meanwhile, the soil microbial community structure and diversity were investigated by high-throughput sequencing after incubation with CA and nZVI. It was found that a high concentration of CA was not conducive to the growth of microorganisms, while CMC had the effect of alleviating the biological toxicity of nZVI.

Process design and validation of a new mixed eluent for leaching Cd, Cr, Pb, Cu, Ni, and Zn from heavy metal-polluted soil

Chemical leaching, an emerging technology for treating heavy metal-polluted soils, requires a design for reasonable and new eluent and an evaluation of its efficiency on the simultaneous removal of different elements. In this study, the leaching effect and biodegradability of chelating agents were compared, and ethylenediamine disuccinic acid (EDDS) was selected to combine with ferric chloride (FeCl₃) for the design of a mixed eluent (EDDS + FeCl₃). Through batch experiments, the influences of the eluent concentration and solution pH on leaching were revealed, and leaching efficiencies of EDDS, FeCl₃, and EDDS + FeCl₃ on six heavy metals Cd, Cr, Pb, Cu, Ni, and Zn in the soil were separately analyzed. Results indicated that EDDS + FeCl₃ showed advantages over both EDDS and FeCl₃ alone, and it presented an excellent effect, especially for simultaneously leaching multiple heavy metals from the soil. The highest leaching efficiencies for Cd, Cr, Pb, Cu, Ni, and Zn reached up to 71.36%, 21.29%, 31.14%, 30.25%, 34.05%, and 4.96%, respectively. According to different soil types and target elements, the concentration, pH condition, and mass ratio of EDDS + FeCl₃ could be adjusted for soil remediation. Fourier transform infrared spectroscopy proved that the better leaching effect of EDDS + FeCl₃ was attributed to changes in the number and strength of functional groups in the solution, which enhanced the chelating ability of the mixed eluent and heavy metal ions. Therefore, chemical leaching by EDDS + FeCl₃ for the remediation of multiple heavy metal-contaminated soil is a potential feasible strategy.

Lead transfer in the soil-root-plant system in a highly contaminated Andean area

Lead (Pb) is highly toxic heavy metal that is detrimental to the food system. There are large mining and metallurgical companies in the central highlands of Peru that have been active for almost a century and contribute to air, water, and soil pollution, affecting food quality and causing damage to the environment and human health. Our study, conducted in 2018, assessed the content and transfer of lead in the soil-root-plant system in the high Andean grasslands in a geographical area near the metallurgical complex of La Oroya. Lead levels were measured in 120 samples of top soil (0–20 cm), roots, and grass shoots by flame atomic absorption spectroscopy. No significant differences were found between the soil pH, organic matter content, and lead among the samples evaluated (P > 0.05). Mean Pb concentrations decreased in the order of soil > root > shoot (P < 0.01) (212.36 ±  38.40, 154.65 ±  52.85 and 19.71 ±  2.81 mg/kg, respectively). The soil-to-root Pb bioconcentration factor, root-to-shoot translocation factor, and soil-to-shoot bioaccumulation factor values were 0.74 ±  0.26, 0.14 ±  0.06 and 0.10 ±  0.03, respectively. Lead in the soil was 3.03 times higher than the maximum limit for agricultural soil, and was 1.97 times higher than the value limit for fodder. Our findings are important and show that soils and pasture in this geographical area have high Pb levels due to metallurgical emissions that have been occurring since 1922. Such pollution negatively impacts health and the socio-economic status of the exposed populations.

Potential use of king grass (Pennisetum purpureum Schumach. × Pennisetum glaucum (L.) R.Br.) for phytoextraction of cadmium from fields

Using king grass (Pennisetum purpureum Schumach. × Pennisetum glaucum (L.) R.Br.) for phytoextraction is a promising technology for producing large amounts of biomass fuel while remediating contaminated soil. To assess the practical phytoextraction capacity of king grass, we conducted a field experiment with three different soil types (loam, sandy loam, clay loam) and cadmium (Cd) concentrations (0, 0.25, 0.5, 1, 2, 4, 8, and 16 mg kg^-1, aged stably for 6 years). King grass were harvested at two different periods (elongation and maturity) to identify the optimal harvest time for extraction efficiency. The results showed that all treatments had bioconcentration factor (BCF) > 1 and translocation factor (TF) < 1; Cd is mainly stored in the roots. However, due to a high shoot biomass, the highest quantity of Cd extracted from shoots was 2.75 mg plant^-1, from the experimental group with 16 mg kg^-1 Cd added in sandy loam. A significant positive relationship (P < 0.05) was observed between the amount of Cd extracted from king grass stems, leaves, and roots from soil with the diethylene triamine pentacetate acid (DTPA) extractable Cd concentration. The Cd concentration in shoots at the maturity stage is lower than at the elongation stage, mainly due to the effect of biological dilution. Meanwhile, there is significantly more biomass (P < 0.05) at the maturity stage than at the elongation stage. At the latter, the extraction efficiency of the three soils was loam > sandy loam > clay loam, while at maturity it was sandy loam > clay loam > loam. This change in extraction efficiency can be attributed mainly to differences in soil DTPA-extractable Cd concentration and growth rate caused by differences in soil physical and chemical properties. According to calculations from multiple harvests using three types of soil, remediating contaminated soil with 0-16 mg kg^-1 Cd would take 13.9-224.5 and 19.5-250.6 years, extracting 7.21-265.23 and 4.96-330.52 g ha^-1 Cd while producing 33.62-66.50 and 73.8-110.5 t ha^-1 dry biomass at the elongation (90 days) and maturity (120 days) stages, respectively. In summary, king grass has major potential for remediating Cd-contaminated soil while producing large volumes of biofuel.