Phytoextraction of cadmium-contaminated soil by Celosia argentea Linn.: A long-term field study

Enhancement of cadmium uptake in Sedum alfredii through interactions between salicylic acid/jasmonic acid and rhizosphere microbial communities

The focus on phytoremediation in soil cadmium (Cd) remediation is driven by its cost-effectiveness and eco-friendliness. Selecting suitable hyperaccumulators and optimizing their growth conditions are key to enhance the efficiency of heavy metal absorption and accumulation. Our research has concentrated on the role of salicylic acid (SA) and jasmonic acid (JA) in facilitating Cd phytoextraction by "Sedum alfredii (S. alfredii)" through improved soil-microbe interactions. Results showed that SA or JA significantly boosted the growth, stress resistance, and Cd extraction efficiency in S. alfredii. Moreover, these phytohormones enhanced the chemical and biochemical attributes of the rhizosphere soil, such as pH and enzyme activity, affecting soil-root interactions. High-throughput sequencing analysis has shown that Patescibacteria and Umbelopsis enhanced S. alfredii's growth and Cd extraction by modifying the bioavailability and the chemical conditions of Cd in soil. Structural Equation Model analysis further verified that phytohormones significantly enhanced the interaction between S. alfredii, soil, and microbes, leading to a marked increase in Cd accumulation in the plant. These discoveries emphasized the pivotal role of phytohormones in modulating the hyperaccumulators' response to environmental stress and offered significant scientific support for further enhancing the potential of hyperaccumulators in ecological restoration technologies using phytohormones.

Rotation of Celosia argentea and Sedum plumbizincicola promotes Cd phytoextraction efficiency

Most hyperaccumulators cannot maintain vigorous growth throughout the year, which may result in a low phytoextraction efficiency for a few months. In the present study, rotation of two hyperaccumulators is proposed to address this issue. An 18-month field experiment was conducted to evaluate the phytoextraction efficiency of Cd by the monoculture and rotation of Celosia argentea and Sedum plumbizincicola. The results showed that rotation increased amount of extracted Cd increased by 2.3 and 1.6 times compared with monoculture of C. argentea and S. plumbizincicola. In rotation system, the biomass of S. plumbizincicola and Cd accumulation in C. argentea increased by 54.4% and 40.7%, respectively. Rotation reduced fallow time and increased harvesting frequency, thereby enhancing Cd phytoextraction. Planting C. argentea significantly decreased soil pathogenic microbes and increased the abundances of plant growth-promoting rhizobacteria (PGPR) and 1-aminocyclopropane-1-carboxylate (ACC) deaminase genes, which may be beneficial for the growth of S. plumbizincicola. Planting S. plumbizincicola increased the abundance of sulfur oxidization (SOX) system genes and decreased soil pH (p < 0.05), thereby increasing the Cd uptake by C. argentea. These findings indicated that rotation of C. argentea and S. plumbizincicola is a promising method for promoting Cd phytoextraction.

Transcriptomic and physiological analyses of Trichoderma citrinoviride HT-1 assisted phytoremediation of Cd contaminated water by Phragmites australis

Plant growth promoting microbe assisted phytoremediation is considered a more effective approach to rehabilitation than the single use of plants, but underlying mechanism is still unclear. In this study, we combined transcriptomic and physiological methods to explore the mechanism of plant growth promoting microbe Trichoderma citrinoviride HT-1 assisted phytoremediation of Cd contaminated water by Phragmites australis. The results show that the strain HT-1 significantly promoted P. australis growth, increased the photosynthetic rate, enhanced antioxidant enzyme activities. The chlorophyll content and the activity of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) were increased by 83.78%, 23.17%, 47.60%, 97.14% and 12.23% on average, and decreased the content of malondialdehyde (MDA) by 31.10%. At the same time, strain HT-1 improved the absorption and transport of Cd in P. australis, and the removal rate of Cd was increased by 7.56% on average. Transcriptome analysis showed that strain HT-1 induced significant up-regulated the expression of genes related to oxidative phosphorylation and ribosome pathways, and these upregulated genes promoted P. australis remediation efficiency and resistance to Cd stress. Our results provide a mechanistic understanding of plant growth promoting microbe assisted phytoremediation under Cd stress.

Effect of hesperidin on growth, photosynthesis, antioxidant systems and uptake of cadmium, copper, chromium and zinc by Celosia argentea plants

Rapid industrialization and extensive agricultural practices are the major causes of soil heavy metal contamination, which needs urgent attention to safeguard the soils from contamination. However, the phytotoxic effects of excessive metals in plants are the primary obstacle to efficient phytoextraction. The present study evaluated the effects of hesperidin (HSP) on metals (Cu, Cd, Cr, Zn) phytoextraction by hyperaccumulator (Celosia argentea L.) plants. For this purpose, HSP, a flavonoid compound with strong antioxidant potential to assist metal phytoextraction was used under metal stress in plants. Celosia argentea plants suffered significant (P ≤ 0.001) oxidative damage due to the colossal accumulation of metals (Cu, Cd, Cr, Zn). However, HSP supplementation notably (P ≤ 0.001) abated ROS generation (O2•‒, •OH, H2O2), lipoxygenase activity, methylglyoxal production, and relative membrane permeability that clearly indicated HSP-mediated decline in oxidative injury in plants. Exogenous HSP improved (P ≤ 0.001) the production of non-protein thiol, phytochelatins, osmolytes, and antioxidant compounds. Further, HSP enhanced (P ≤ 0.001) H2S and NO endogenous production, which might have improved the GSH: GSSG ratio. Consequently, HSP-treated C. argentea plants had higher biomass alongside elevated metal accumulation mirrored as profound modifications in translocation factor (TF), bioaccumulation coefficient (BAC), and bioconcentration factor (BCF). In this context, HSP significantly enhanced TF of Cr (P ≤ 0.001), Cd (P ≤ 0.001), and Zn (P ≤ 0.01), while BAC of Cr (P ≤ 0.001), Cd (P ≤ 0.001), and Zn (P ≤ 0.001). Further, BCF was significant (P ≤ 0.05) only in plants grown under Cr-spiked soil. Overall, HSP has the potential for phytoremediation of metals by C. argentea, which might be a suitable strategy for metal-polluted soils.

Spatial dynamics of pH in the rhizosphere of Leersia hexandra Swartz at different chromium exposure

The roots of hyperaccumulators can significantly alter soil pH and thus change the chromium (Cr) availability in the rhizosphere. The pH dynamics in the rhizosphere of Cr hyperaccumulator Leersia hexandra Swartz remains unknown. In this study, the spatial dynamics of pH in the rhizosphere of L. hexandra at different Cr exposure were examined using planar optode (PO). The effects of different Cr concentrations on the biomass, physiological parameters, and soil enzyme activity were investigated. The results showed that pH in the rhizosphere of L. hexandra was highly heterogeneous and followed the root shape. There were obvious soil acidification in all groups and the average pH values in the control, Cr50, and Cr100 groups decreased by 0.26, 0.27, and 0.35 pH unit, respectively. At a certain concentration (50 mg kg-1), Cr significantly increased the plant height and biomass of L. hexandra compared to the control (p < 0.05). The concentrations of chlorophyll a, chlorophyll b, and total chlorophyll in the leaves increased with increasing Cr concentrations. The acid phosphatase, urease, and catalase activities in the rhizosphere were higher than those in the bulk soil. These results provide new insights into elucidating the hyperaccumulating mechanism of Cr and improving the phytoremediation efficiency.

Phytoremediation strategies for heavy metal-contaminated soil by selecting native plants near mining areas in Inner Mongolia

Phytoremediation technology, as an eco-friendly and cost-effective approach, is widely used to restore soil contaminated by heavy metal(loid)s. However, the adaptability and absorption capacity of plants to multiple elements are the crucial factors affecting the application of phytoremediation in mining areas. In this study, dominant native plant species and their paired soils were collected near a lead-zinc mine in Inner Mongolia, to assess the ecological risk of heavy metal(loid)s and phytoremediation potential. The results showed that Cd and As were the dominant soil pollutants, with levels of 90.91% and 100%, respectively, exceeding the risk intervention values for soil contamination of agricultural land. The rates of Pb, Cu, and Zn exceeding the risk screening values were 69.70%, 60.61%, and 96.97%, respectively. Extremely high ecological risk of heavy metal(loid)s was observed in this area. The ability of native plants accumulating heavy metals varied among species. The bioconcentration factor (BCF) varied from 0.14 to 2.59 for Cd, 0.02 to 0.45 for As, 0.06 to 0.76 for Pb, 0.05 to 2.69 for Cr, 0.15 to 1.00 for Cu, and 0.22 to 4.10 for Zn. Chinese Cinquefoil Herb (Potentilla chinensis Ser.) showed the potential to accumulate multiple toxic elements based on the biomass, shoot content, translocation factor (TF), BCF, and metal extraction rate (MER), while, other species showed the potential to accumulate single toxic element: goosefoot (Chenopodium album L.), Lespedeza daurica (Laxm.) Schindl. and peashrubs (Caragana korshinskii Kom.), Herba Artemisiae Scopariae (Artemisia capillaris Thunb.), alfalfa (Medicago sativa L.), and Moldavian Dragonhead (Dracocephalum moldavica L.) for Cd, As, Cr, Cu, and Zn, respectively. Furthermore, wild leek (Allium ramosum L.), cogongrass (Imperata cylindrica (L.) Beauv.), fringed sagebrush (Artemisia frigida Willd.), and field bindweed (Convolvulus arvensis L.) were selected for phytostabilization of specific elements, considering the heavy metal contents in the roots and low TF values. This study provides a reference for selecting appropriate species for the remediation of heavy metal-contaminated soils in certain mining areas.

Phytomanagement of trace element polluted fields with aromatic plants: supporting circular bio-economies

Trace elements pollution of soils became a global concern because of their persistence in the environment which can lead to accumulation in food chains up to toxic levels. At the same time, there is a shortage of arable land for growing food, fodder and industrial crops, which highlights the need for remediation/use of polluted land. Restoration of degraded lands has been included as a vital component of UN Sustainable Development Goals (SDGs). We summarize various sources of entry of important trace elements in the environment, available biological reclamation and management strategies and their limitations. Recent advances in phytomanagement approaches using aromatic crops to obtain economically valuable products such as essential oils and revalorize such polluted areas are reviewed. The worldwide application of this strategy in the last 10 years is illustrated through a choropleth map. Finally, the emerging concept of phytomanagement as a restorative and regenerative circular bio-economy is also discussed.

High-resolution imaging of O2 dynamics and metal solubilization in the rhizosphere of the hyperaccumulator Leersia hexandra Swartz

The mobilization of trace metals in the rhizosphere can be affected by the redox potential, which is closely related to the O₂ dynamics. This study examined the distributions of O₂ and trace metals in the rhizosphere of the subaquatic hyperaccumulator Leersia hexandra Swartz under chromium (Cr) stress using planar optodes and the diffusive gradients in thin films technique coupled with laser ablation inductively coupled plasma mass spectrometry. The O₂ concentrations and oxidized areas in the rhizosphere significantly increased with increases in the light intensity, air humidity, and atmospheric CO₂ concentrations (p < 0.05). The O₂ concentration first increased with increasing ambient temperatures, then decreased when the temperature increased from 25 to 32 ℃. The O₂ concentration in the rhizosphere was significantly decreased under Cr stress (p < 0.05), with a prolonged response time to the altered ambient temperature. Cr stress led to decreased mobilities of As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sb, V, W, and Zn in the rhizosphere, which were negatively correlated with the concentrations of O₂. These results provide new insights into the role of changes in the O₂ concentration induced by the roots of hyperaccumulator plants in controlling the mobility of trace metals in soils.

Comparative study on plant growth-promoting bacterial inoculation by irrigation and spraying for promoting Bidens pilosa L. phytoremediation of cadmium-contaminated soil

A field study was conducted to compare FM-1 inoculation by irrigation and spraying for promoting Bidens pilosa L. phytoremediation of cadmium (Cd)-contaminated soil. Cascading relationships between bacterial inoculation by irrigation and spraying and soil properties, plant growth-promoting traits, plant biomass and Cd concentrations in Bidens pilosa L. were explored based on the partial least squares path model (PLS-PM). The results indicated that inoculation with FM-1 not only improved the rhizosphere soil environment of B. pilosa L. but also increased the Cd extracted from the soil. Moreover, Fe and P in leaves play vital roles in promoting plant growth when FM-1 is inoculated by irrigation, while Fe in leaves and stems plays a vital role in promoting plant growth when FM-1 is inoculated by spraying. In addition, FM-1 inoculation decreased the soil pH by affecting soil dehydrogenase and oxalic acid in cases with irrigation and Fe in roots in cases with spraying. Thus, the soil bioavailable Cd content increased and promoted Cd uptake by Bidens pilosa L. To address Cd-induced oxidative stress, Fe in leaves helped to convert GSH into PCs, which played a vital role in ROS scavenging when FM-1 was inoculated by irrigation. The soil urease content effectively increased the POD and APX activities in the leaves of Bidens pilosa L., which helped alleviate Cd-induced oxidative stress when FM-1 was inoculated by spraying. This study compares and illustrates the potential mechanism by which FM-1 inoculation can improve the phytoremediation of Cd-contaminated soil by Bidens pilosa L., suggesting that FM-1 inoculation by irrigation and spraying is useful in the phytoremediation of Cd-contaminated sites.

Integrated transcriptome and metabolome analysis reveals the mechanism of tolerance to manganese and cadmium toxicity in the Mn/Cd hyperaccumulator Celosia argentea Linn

Understanding the molecular mechanism of tolerance to heavy metals in hyperaccumulators is important for improving the efficiency of phytoremediation and is interesting for evolutionary studies on plant adaption to abiotic stress. Celosia argentea Linn. was recently discovered to hyperaccumulate both manganese (Mn) and cadmium (Cd). However, the molecular mechanisms underlying Mn and Cd detoxification in C. argentea are poorly understood. Laboratory studies were conducted using C. argentea seedlings exposed to 360 μM Mn and 8.9 μM Cd hydroponic solutions. Plant leaves were analyzed using transcriptional and metabolomic techniques. A total of 3960 differentially expressed genes (DEGs) in plants were identified under Cd stress, among which 17 were associated with metal transport, and 10 belonged to the ATP transporter families. Exposures to Mn or Cd led to the differential expression of three metal transport genes (HMA3, ABCC15, and ATPase 4). In addition, 33 and 77 differentially expressed metabolites (DEMs) were identified under Mn and Cd stresses, respectively. Metabolic pathway analysis showed that the ABC transporter pathway was the most affected in Mn/Cd exposed seedlings. Conjoint transcriptome and metabolome analysis showed that the glutathione (GSH) metabolic pathway was over-represented in the KEGG pathway of both DEGs and DEMs. Our results confirm that the ABC transporter and GSH metabolic pathways play important roles in Mn and Cd detoxification. These findings provide new insight into the molecular mechanisms of tolerance to Mn and Cd toxicity in plants.

Film mulching alters soil properties and increases Cd uptake in Sedum alfredii Hance-oil crop rotation systems

Film mulching (FM) is an agronomic measure worldwide, yet its effect on cadmium (Cd) accumulation in plants is unknown. This study investigated the potential for phytoremediation with FM treatment of Cyperus esculentus L. (chufa) and Sedum alfredii Hance (S. alfredii)-oil crop rotation system. The FM increased the biomass and Cd content of the chufa, resulting in an increase of 65.0-193.5% in the Cd accumulation. S. alfredii also was planted using non-film mulching and film mulching (FMSA), followed by rotation oil plants using non-film mulching. Soil pH and dissolved organic carbon content were significantly reduced, and the Cd grain size fraction of macro-aggregates was significantly increased by FMSA, which increased the uptake of available Cd by S. alfredii. This phenomenon further promoted the accumulation of Cd in S. alfredii and reduced the Cd content of aboveground tissues and seeds in subsequent oil crops. Vegetable oils were safely produced in all treatments due to their low Cd content. Compared with non-film mulching, FM increased the Cd accumulation of rotation systems by 66.8-96.4%, and the Cd remediation efficiency reached 11.8-12.9%. Collectively, the FM treatment effectively improved the remediation efficiency of Cd in the rotation system and ensured the safe production of vegetable oil.

Mn Pretreatment Improves the Physiological Resistance and Root Exudation of Celosia argentea Linn. to Cadmium Stress

Phytoextraction using Celosia argentea Linn. by Mn pretreatment can potentially decontaminate Cd-contaminated soils. However, the mechanism that accelerates the Cd bioaccumulation is still unknown. In order to study the effect and mechanism of Mn pretreatment on Cd bioaccumulation in C. argentea, the hydroponic experiments were set to determine the chlorophyll content, antioxidant enzyme activity, malondialdehyde content, and root exudation of C. argentea. The results indicated that after seven days of Mn pretreatment, both the biomass and Cd concentrations in plants increased compared to the control group. One of the mechanisms for this was the improvement in the physiological resistance of C. argentea following pretreatment with Mn. Compared with Cd stress alone, Mn pretreatment increased photosynthesis and reduced membrane lipid peroxidation. Meanwhile, the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) were significantly reduced in leaves of C. argentea after Mn pretreatment through the reduction in the production of reactive oxygen species. In addition, Mn promoted the exudation of organic acids in the roots of C. argentea. The contents of citric and malic acids increased by 55.3% and 26.4%, respectively, which may be another important reason for Mn pretreatment increasing Cd bioaccumulation in C. argentea. Therefore, the present work shows that the pretreatment of seedlings with Mn can provide a meaningful strategy to improve the remediation efficiency of Cd-contaminated soils by C. argentea.

Three-season rotation of chicory-tobacco-peanut with high biomass and bioconcentration factors effectively remediates cadmium-contaminated farmland

Traditional phytoremediation is one approach to remediate heavy metal pollution. In developing countries, the key factor in promoting practical application of phytoremediation in polluted soils is selecting suitable plants that are tolerant to heavy metals and also produce products with economic value. Therefore, a field experiment was conducted with a three-season chicory-tobacco-peanut rotation to determine effects on remediation of cadmium (Cd)-contaminated farmland in China. All crops had strong Cd accumulation capacity, with bioconcentration factors of 6.61 to 11.97 in chicory, 3.85 to 21.61 in tobacco, and 1.36 to 7.0 in peanut. Yield of total dry biomass reached 32.4 t ha-1, and the Cd phytoextraction efficiency was 10.3% per year. Aboveground tissues of the three crops accounted for 83.9 to 91.2% of total biomass in the rotation experiment. Cd content in peanut grain and oil met the National Food Safety Standard of China (0.5 mg kg-1, GB 2762-2017) and the Food Contaminant Limit of the European Union (0.1 mg kg-1, 18,812,006). Therefore, in addition to phytoremediation of Cd-contaminated soils, the chicory-tobacco-peanut rotation system can also produce economic benefits for local farmers.

Role of plant growth-promoting rhizobacteria in boosting the phytoremediation of stressed soils: Opportunities, challenges, and prospects

Soil is considered as a vital natural resource equivalent to air and water which supports growth of the plants and provides habitats to microorganisms. Changes in soil properties, productivity, and, inevitably contamination/stress are the result of urbanisation, industrialization, and long-term use of synthetic fertiliser. Therefore, in the recent scenario, reclamation of contaminated/stressed soils has become a potential challenge. Several customized, such as, physical, chemical, and biological technologies have been deployed so far to restore contaminated land. Among them, microbial-assisted phytoremediation is considered as an economical and greener approach. In recent decades, soil microbes have successfully been used to improve plants' ability to tolerate biotic and abiotic stress and strengthen their phytoremediation capacity. Therefore, in this context, the current review work critically explored the microbial assisted phytoremediation mechanisms to restore different types of stressed soil. The role of plant growth-promoting rhizobacteria (PGPR) and their potential mechanisms that foster plants' growth and also enhance phytoremediation capacity are focussed. Finally, this review has emphasized on the application of advanced tools and techniques to effectively characterize potent soil microbial communities and their significance in boosting the phytoremediation process of stressed soils along with prospects for future research.

Pathways of cadmium fluxes in the root of the hyperaccumulator Celosia argentea Linn

In order to study the mechanism of cadmium (Cd) uptake by the roots of Celosia argentea Linn. (Amaranthaceae), the effects of various inhibitors, ion channel blockers, and hydroponic conditions on Cd²⁺ fluxes in the roots were characterized using non-invasive micro-test technology (NMT). The net Cd²⁺ flux (72.5 pmol∙cm^-2∙s^-1) in roots that had been pretreated with Mn was significantly higher than that in non-pretreated roots (58.1 pmol∙cm^-2∙s^-1), indicating that Mn pretreatment enhanced Cd uptake by the roots. This finding may be explained by the fact that the addition of Mn significantly increased the expression of the transporter gene and thus promoted Cd uptake and transport. In addition, Mn pretreatment resulted in an increase in root growth, which may in turn promote root vigor. The uncoupler 2,4-dinitrophenol (DNP) caused a significant reduction in net Cd²⁺ fluxes in the roots, by 70.5% and 41.4% when exposed to Mn and Cd stress, respectively. In contrast, a P-type ATPase inhibitor (Na₃VO₄) had only a small effect on net Cd²⁺ fluxes to the plant roots, indicating that ATP has a relatively minor role in Cd uptake by roots. La³⁺ (a Ca channel inhibitor) had a more significant inhibitory effect on net Cd²⁺ fluxes than did TEA (a K channel inhibitor). Therefore, Cd uptake by plant roots may occur mainly through Ca channels rather than K channels. In summary, uptake of Cd by the roots of C. argentea appears to occur via several types of ion channels, and Mn can promote Cd uptake.

Mechanism underlying how a chitosan-based phosphorus adsorbent alleviates cadmium-induced oxidative stress in Bidens pilosa L. and its impact on soil microbial communities: A field study

In the present study, field experiments were conducted in Side village, Yangshuo, Guilin, Guangxi Province, China, using four C-BPA application levels (control (0 mg m^-2), T1 (100 mg m^-2), T2 (200 mg m^-2) and T3 (400 mg m^-2)) to clarify the mechanism by which a chitosan-based phosphorus adsorbent (C-BPA) applied as a passivator helps Bidens pilosa L. (B. pilosa L.) alleviate cadmium (Cd)-induced oxidative stress in Cd-contaminated soil. In the aqueous phase, C-BPA successfully adsorbed Cd²⁺ on the surface primarily via ion exchange, and C-BPA has potential Cd²⁺ adsorption capacity, enabling its use as a passivator in real Cd-contaminated environments. In Cd-contaminated soils, under C-BPA application at the T3 level, the pH value increased by 11.2%, and the acid-soluble form of Cd decreased by 26.5%. Additionally, the application of C-BPA improved the rhizosphere soil environment and impacted the soil microbial community diversity and structure. Among soil microbes, the soil fungal community was more sensitive than bacteria to C-BPA application. Dehydrogenase, acetic acid, soil pH and Eurotiomycetes or Dothideomycetes significantly impacted Cd accumulation in the leaves of B. pilosa L.; Cd accumulation in leaves was decreased by 68.1% under C-BPA application at the T3 level. Additionally, the variation of increased catalase (CAT) and peroxidase (POD) jointly promoted plant growth; the plant weight was increased by 112.7% under the C-BPA application at the T3 level. Notably, the production of CAT and POD by B. pilosa L. was more effective than the synthesis of glutathione (GSH) in helping B. pilosa L. eliminate excess reactive oxygen species (ROS). Therefore, our findings demonstrated that the application of C-BPA to Cd-contaminated soil can greatly improve the rhizosphere soil environment, help B. pilosa L. eliminate ROS and promote plant growth.

Mixed Contaminants: Occurrence, Interactions, Toxicity, Detection, and Remediation

The ever-increasing rate of pollution has attracted considerable interest in research. Several anthropogenic activities have diminished soil, air, and water quality and have led to complex chemical pollutants. This review aims to provide a clear idea about the latest and most prevalent pollutants such as heavy metals, PAHs, pesticides, hydrocarbons, and pharmaceuticals—their occurrence in various complex mixtures and how several environmental factors influence their interaction. The mechanism adopted by these contaminants to form the complex mixtures leading to the rise of a new class of contaminants, and thus resulting in severe threats to human health and the environment, has also been exhibited. Additionally, this review provides an in-depth idea of various in vivo, in vitro, and trending biomarkers used for risk assessment and identifies the occurrence of mixed contaminants even at very minute concentrations. Much importance has been given to remediation technologies to understand our current position in handling these contaminants and how the technologies can be improved. This paper aims to create awareness among readers about the most ubiquitous contaminants and how simple ways can be adopted to tackle the same.

Phytoremediation potential of Bermuda grass (Cynodon dactylon (L.) pers.) in soils co-contaminated with polycyclic aromatic hydrocarbons and cadmium

Soils co-contaminated with polycyclic aromatic hydrocarbons (PAHs) and cadmium (Cd) have serious environmental impacts and are highly toxic to humans and ecosystems. Phytoremediation is an effective biotechnology for the remediation and restoration of PAH- and Cd-polluted soils. Pot experiments were conducted to investigate the individual and combined effects of PAHs (1238.62 mg kg-1) and Cd (23.1 mg kg-1) on the phytoremediation potential of Bermuda grass grown in contaminated soils. Bermuda grass exhibited a significant decrease in plant growth rate, leaf pigment content, root activity, plant height and biomass and a remarkable increase in malondialdehyde content and electrolyte leakage when grown in PAH- and Cd-contaminated soils compared with grass grown in uncontaminated soils. The activity of soil enzymes, including urease, alkaline phosphatase, sucrose, and fluorescein diacetate hydrolysis, were reduced in soil with PAH and Cd stress. Furthermore, the toxicity of combined PAHs and Cd on Bermuda grass growth and soil enzyme activity was much higher than that of PAH or Cd stress alone, suggesting a synergistic effect of PAHs and Cd on cytotoxicity. To scavenge redundant reactive oxygen species and avoid oxidative damage, Bermuda grass increased plant catalase, superoxide dismutase, and peroxidase activity and soluble sugar and proline content. The bioconcentration factor of Cd in Bermuda grass grown under Cd alone and combined PAH and Cd exposure was greater than 1 for both, suggesting that Bermuda grass has a high Cd accumulation ability. Under PAH alone and combined PAH and Cd exposure conditions, a higher PAH removal rate (41.5-56.8%) was observed in soils planted with Bermuda grass than in unplanted soils (24.8-29.8%), indicating that Bermuda grass has a great ability to degrade PAHs. Bermuda grass showed great phytoremediation potential for the degradation of PAHs and phytoextraction of Cd in co-contaminated soils.

Biochar combined with phosphate fertilizer application reduces soil cadmium availability and cadmium uptake of maize in Cd-contaminated soils

Cadmium (Cd) has become the primary pollution factor in farmland, which seriously threatens crop growth and food safety. A pot experiment was conducted to investigate the effect of combined application with biochar and P fertilizer on soil Cd availability and translocation, in which biochar was 0 (C₀) and 20 g kg^-1 (C₂₀), P fertilizer was 0 (P₀), 20 (P₂₀), and 40 mg P kg^-1 (P₄₀). Results showed that, compared with C₀ level, the content of DTPA-Cd in soil was significantly decreased with biochar addition after 60 days of cultivation, under C₂₀ level, soil DTPA-Cd in C₂₀P₄₀ treatment were significantly increased. Under both C levels, the percentage of exchangeable Cd fraction at P₄₀ rate was significantly lower than that at P₂₀ rate, because the excess P in soil could precipitate Cd. The percentage of residual-Cd fraction was significantly increased with the combined addition of biochar and P fertilizer, particularly in C₂₀P₄₀ treatment, which was 75.95%, while it was only 61.65% in C₀P₀ treatment. The Cd translocation factor (TF) and bioconcentration factor (BCF) were also significantly reduced in C₂₀P₂₀ and C₂₀P₄₀ treatments compared with C₀P₀ treatment. Therefore, the combined high P and biochar application was a good choice in inhibiting soil Cd availability and plant Cd uptake, which benefited to the safe utility of the Cd contaminated soil.

Recent advances in nanoremediation: Carving sustainable solution to clean-up polluted agriculture soils

Agriculture is one of the foremost significant human activities, which symbolizes the key source for food, fuel and fibers. This activity results in a lot of ecological harms particularly with the excessive usage of chemical fertilizers and pesticides. Different agricultural practices have remained industrialized to advance food production, due to the growth in the world population and to meet the food demand through the routine use of more effective fertilizers and pesticides. Soil is intensely embellished by environmental contamination and it can be stated as "universal incline." Soil pollution usually occurs from sewage wastes, accidental discharges or as byproducts of chemical residues of unrestrained production of numerous materials. Soil pollution with hazardous materials alters the physical, chemical, and biological properties, causing undesirable changes in soil fertility and ecosystem. Engineered nanomaterials offer various solutions for remediation of contaminated soils. Engineered nanomaterial-enable technologies are able to prevent the uncontrolled release of harmful materials into the environment along with capabilities to combat soil and groundwater borne pollutants. Currently, nanobiotechnology signifies a hopeful attitude to advance agronomic production and remediate polluted soils. Studies have outlined the way of nanomaterial applications to restore the eminence of the environment and assist the detection of polluted sites, along with potential remedies. This review focuses on the latest developments in agricultural nanobiotechnology and the tools developed to combat soil or land and or terrestrial pollution, as well as the benefits of using these tools to increase soil fertility and reduce potential toxicity.

Recent advances on nanocellulose biomaterials for environmental health photoremediation: An overview

As one of the potential bionanomaterials, nanocellulose has appeared as a favorable candidate for photoremediation of the environment because of its abundance in nature, inexpensive, eco-friendly, decomposable, high surface area, and outstanding mechanical properties. The current review carefully summarized the diverse type of nanocellulose, their preparation approaches, and several previous works on the use of nanocellulose for photoremediation. These include the role of nanocellulose for the increased surface active site of the hybrid photocatalysts by providing a large surface area for enhanced adsorption of photons and pollutant molecules, as a dispersing agent to increase distribution of metal/non-metal dopants photocatalysts, as well as for controlled size and morphology of the dopants photocatalysts. Furthermore, the recommendations for upcoming research provided in this review are anticipated to ignite an idea for the development of other nanocellulose-based photocatalysts. Other than delivering beneficial information on the present growth of the nanocellulose biomaterials photocatalysts, this review is expected will attract more interest to the utilization of nanocellulose photocatalyst and distribute additional knowledge in this exciting area of environmental photoremediation. This could be attained by considering that a review on nanocellulose biomaterials for environmental health photoremediation has not been described elsewhere, notwithstanding intensive research works have been dedicated to this topic.

Phytoextraction of metal(loid)s from contaminated soils by six plant species: A field study

Phytoextraction is an in situ remediation technique that uses (hyper)accumulator plant species to extract metal(loid)s from contaminated soils. Field studies can help in selecting appropriate plants for phytoextraction and in better understanding their phytoextraction performance. Hence, a field study was conducted using six (hyper)accumulator species (Solanum nigrum L., Bidens pilosa L., Xanthium strumarium L., Helianthus annuus L., Lonicera japonica T. and Pennisetum sinese R.) over two years in Jiaoxi town, Liuyang city, Hunan Province, China, to determine the effect of the (hyper)accumulator rhizospheres on field soils contaminated with multiple metal(loid)s and to analyze the variations in rhizosphere soil microbial community diversity and composition. After two years of field experiments, compared to the other four (hyper)accumulators, Bidens pilosa L. and Xanthium strumarium L. exhibited not only better metal(loid) phytoextraction abilities but also higher shoot biomasses. The contents of diethylenetriaminepentaacetic acid (DTPA)-extractable Pb, Cd and Zn decreased in the rhizosphere soils of all six (hyper)accumulators after repeated phytoextraction. Moreover, our findings illustrated that hyperaccumulator planting helps improve and rebuild the soil bacterial community composition and structure in contaminated soils by shifting the soil physiochemical properties. After repeated planting, the soil bacterial communities were reconstructed and dominated by Proteobacteria, Actinobacteriota, Chloroflexi and Acidobacteriota at the phylum level. The soil fungal communities were dominated by Ascomycota, Basidiomycota and Mortierellomycota at the phylum level. The reconstruction of soil microbial communities may help (hyper)accumulators adapt to metal(loid)-contaminated environments and improve their phytoextraction abilities.

Effects of environmental factors on soil bacterial community structure and diversity in different contaminated districts of Southwest China mine tailings

A mass of tailings left by mineral exploitation have caused serious environmental pollution. Although many studies have shown that soil microorganisms have the potential to remediate environmental pollution, the interaction mechanism between microorganisms and the surrounding environment of tailings is still unclear. In this study, 15 samples around pyrite mine tailing were collected to explore the ecological effects of environmental factors on bacterial community. The results showed that most of the samples were acidic and contaminated by multiple metals. Cadmium (Cd), copper (Cu), nickel (Ni) migrated and accumulated to into downstream farmlands while chromium (Cr) was the opposite. Proteobacteria, Chloroflex and Actinobacteria were the dominant phyla. Soil pH, total phosphorus (TP), total nitrogen (TN), available potassium (AK), available phosphorus (AP), the bacteria abundance and diversity all gradually increased with the increase of the distance from the tailing. Invertase, acid phosphatase, total organic carbon (TOC), pH, TP and Cr were the main influencing factors to cause the variation of bacterial community. This work could help us to further understand the changes in soil microbial communities around pollution sources.

Heterogeneity of humic/fulvic acids derived from composts explains the differences in accelerating soil Cd-hyperaccumulation by Sedum alfredii

The hyperaccumulating mechanism concerning heavy metal activation or passivation and plant response triggered by fulvic acid (FA) and humic acid (HA) recruitments are investigated herein. We carefully examine the Cd activation effect by various FA and HA, tracing from pig, goat, and duck manure composts to straw compost and commercial materials (i.e., PC, GC, DC, SC, and CM), as well as their roles in plant growth promotion and Cd uptake. Our results indicate that due to the decrease of soil pH and their multiple functional groups, the contents of available Cd (AE-Cd) increased by 4.3-4.8% and 3.6-6.3% when all FA and HA sources were applied for 30 days. A 13.1-19.9% increase in AE-Cd was observed when CFA, DFA, and PFA were applied for five days, and a 9.5% increment was found when PHA was applied for 10 days. In the pot experiment, the Cd accumulation in plants increased by 2.78 and 2.17 folds with PFA and PHA applications, respectively, compared to the blank control group. This result can be attributed to the stimulative effects of the simultaneous Sedum alfredii growth and Cd phytoavailability. Notably, the Cd accumulation increased by 2.26 times with the SFA amendment due to the predominant stimulation effect to the phytoavailable Cd rather than plant growth. However, slight inhibitory effects were observed upon plant growth or Cd uptake, which led to the reduction of the Cd accumulation with DHA, SHA, and CHA employments. Consistently, the corresponding soil Cd removal efficiencies were 43.5% and 34.6% with PFA and PHA, respectively, which hold abundant O- and N-containing groups. Our research aims to gain insights into the ternary interaction in the presence of heavy metal, humic substances, and S. alfredii to simultaneously accelerate Cd activation and hyperaccumulation.

Plant Growth-Promoting Bacteria (PGPB) integrated phytotechnology: A sustainable approach for remediation of marginal lands

Land that has little to no utility for agriculture or industry is considered marginal land. This kind of terrain is frequently found on the edge of deserts or other arid regions. The amount of land that can be used for agriculture continues to be constrained by increasing desertification, which is being caused by climate change and the deterioration of agriculturally marginal areas. Plants and associated microorganisms are used to remediate and enhance the soil quality of marginal land. They represent a low-cost and usually long-term solution for restoring soil fertility. Among various phytoremediation processes (viz., phytodegradation, phytoextraction, phytostabilization, phytovolatilization, phytofiltration, phytostimulation, and phytodesalination), the employment of a specific mechanism is determined by the state of the soil, the presence and concentration of contaminants, and the plant species involved. This review focuses on the key economically important plants used for phytoremediation, as well as the challenges to plant growth and phytoremediation capability with emphasis on the advantages and limits of plant growth in marginal land soil. Plant growth-promoting bacteria (PGPB) boost plant development and promote soil bioremediation by secreting a variety of metabolites and hormones, through nitrogen fixation, and by increasing other nutrients' bioavailability through mineral solubilization. This review also emphasizes the role of PGPB under different abiotic stresses, including heavy-metal-contaminated land, high salinity environments, and organic contaminants. In our opinion, the improved soil fertility of marginal lands using PGPB with economically significant plants (e.g., Miscanthus) in dual precession technology will result in the reclamation of general agriculture as well as the restoration of native vegetation.

Plant growth regulators and EDTA improve phytoremediation potential and antioxidant response of Dysphania ambrosioides (L.) Mosyakin & Clemants in a Cd-spiked soil

Soil pollution due to potentially toxic elements is a worldwide challenge for health and food security. Chelate-assisted phytoextraction along with the application of plant growth regulators (PGRs) could increase the phytoremediation efficiency of metal-contaminated soils. The present study was conducted to investigate the effect of different PGRs [Gibberellic acid (GA3) and indole acetic acid (IAA)] and synthetic chelator (EDTA) on growth parameters and Cd phytoextraction potential of Dysphania ambrosioides (L.) Mosyakin & Clemants grown under Cd-spiked soil. GA3 (10-7 M) and IAA (10-5 M) were applied four times with an interval of 10 days through a foliar spray, while EDTA (40 mg kg-1 soil) was once added to the soil. The results showed that Cd stress significantly decreased fresh biomass, dry biomass, total water contents, and photosynthetic pigments as compared to control. Application of PGRs significantly enhanced plant growth and Cd phytoextraction. The combined application of GA3 and IAA with EDTA significantly increased Cd accumulation (6.72 mg pot-1 dry biomass) and bioconcentration factor (15.21) as compared to C1 (Cd only). The same treatment significantly increased chlorophyll, proline, phenolic contents, and antioxidant activities (CAT, SOD, and POD) while MDA contents were reduced. In roots, Cd accumulation showed a statistically significant and positive correlation with proline, phenolics, fresh biomass, and dry biomass. Similarly, Cd accumulation showed a positive correlation with antioxidant enzyme activities in leaves. D. ambrosioides showed hyperaccumulation potential for Cd, based on bioconcentration factor (BCF) > 1. In conclusion, exogenous application of GA3 and IAA reduces Cd stress while EDTA application enhances Cd phytoextraction and ultimately the phytoremediation potential of D. ambrosioides.

Abandoned Mine Lands Reclamation by Plant Remediation Technologies

Abandoned mine lands (AMLs), which are considered some of the most dangerous anthropogenic activities in the world, are a source of hazards relating to potentially toxic elements (PTEs). Traditional reclamation techniques, which are expensive, time-consuming and not well accepted by the general public, cannot be used on a large scale. However, plant-based techniques have gained acceptance as an environmentally friendly alternative over the last 20 years. Plants can be used in AMLs for PTE phytoextraction, phytostabilization, and phytovolatilization. We reviewed these phytoremediation techniques, paying particular attention to the selection of appropriate plants in each case. In order to assess the suitability of plants for phytoremediation purposes, the accumulation capacity and tolerance mechanisms of PTEs was described. We also compiled a collection of interesting actual examples of AML phytoremediation. On-site studies have shown positive results in terms of soil quality improvement, reduced PTE bioavailability, and increased biodiversity. However, phytoremediation strategies need to better characterize potential plant candidates in order to improve PTE extraction and to reduce the negative impact on AMLs.

Ecological improvement of antimony and cadmium contaminated soil by earthworm Eisenia fetida: Soil enzyme and microorganism diversity

Vermiremediation on improvement of antimony (Sb) and cadmium (Cd) contaminated soil was less reported. In this study, earthworm Eisenia fetida was exposed into soil spiked with Sb and Cd and their mixture for 30 days, and then we measured multiple soil enzyme activities and bacteria communities via enzymatic reaction and high-throughput sequencing of 16 S rRNA genes. The results showed that Sb and Cd at high treatment levels inhibited the activities of urease, neutral phosphatase and protease significantly, but earthworm could promote the activities of urease and neutral phosphatase by 17.75%-121.91% and 1.46%-118.97%, respectively. However, earthworms inhibited catalase and had no effect on protease. The Geometric Mean Index suggested that earthworms led to a higher soil biochemistry function. According to a taxonomic analysis, bacterial community structure predominantly consisted of phylum Proteobacteria, Actinobacteria, Firmicutes, etc. and class Gammaproteobacteria, Actinobacteria, Alphaproteobacteria, etc.; furthermore, Pielou index and Shannon index (Alpha diversity in the habitat) indicated that bacteria diversity and evenness increased in the presence of earthworms. The heating map revealed that earthworms made genus Sphingomonas, Flavobacterium, etc. and species Sphingomonas jaspsi, Conexibacter, etc. dominate. Overall, earthworm is a suitable remediation species to improve the ecological function of heavy metal polluted soil. However, the specific mechanism and causal relationship of how earthworm to control enzyme activity and bacteria community remained to be explored.

Remediation Techniques for Cadmium-Contaminated Dredged River Sediments after Land Disposal

This paper examines the remediation techniques of cadmium (Cd)-contaminated dredged river sediments after land disposal in a city in East China. Three remediation techniques, including stabilization, soil leaching, and phytoremediation, are compared by analyzing the performance of the techniques for Cd-contaminated soil remediation. The experimental results showed that the stabilization technique reduced the leaching rate of soil Cd from 33.3% to 14.3%, thus effectively reducing the biological toxicity of environmental Cd, but the total amount of Cd in soil did not decrease. Leaching soil with citric acid and oxalic acid achieved Cd removal rates of 90.1% and 92.4%, respectively. Compared with these two remediation techniques, phytoremediation was more efficient and easier to implement and had less secondary pollution, but it took more time, usually several years. In this study, these three remediation techniques were analyzed and discussed from technical, economic, and environmental safety perspectives by comprehensively considering the current status and future plans of the study site. Soil leaching was found to be the best technique for timely treatment of Cd contamination in dredged river sediments after land disposal.

Invasive Weed Asystasia gangetica as a Potential Biomonitor and a Phytoremediator of Potentially Toxic Metals: A Case Study in Peninsular Malaysia

The invasive weed Asystasia gangetica was investigated for its potential as a biomonitor and as a phytoremediator of potentially toxic metals (PTMs) (Cd, Cu, Ni, Pb, and Zn) in Peninsular Malaysia owing to its ecological resistance towards unfavourable environments. The biomonitoring potential of PTMs was determined based on the correlation analysis of the metals in the different parts of the plant (leaves, stems, and roots) and its habitat topsoils. In the roots, the concentrations (mg/kg dry weight) of Cd, Cu, Ni, Pb, and Zn ranged from 0.03 to 2.18, 9.22 to 139, 0.63 to 5.47, 2.43 to 10.5, and 50.7 to 300, respectively. In the leaves, the concentrations (mg/kg dry weight) of Cd, Cu, Ni, Pb, and Zn ranged from 0.03 to 1.16, 7.94 to 20.2, 0.03 to 6.13, 2.10 to 21.8, and 18.8 to 160, respectively. In the stems, the concentrations (mg/kg dry weight) of Cd, Cu, Ni, Pb, and Zn ranged from 0.03 to 1.25, 5.57 to 11.8, 0.23 to 3.69, 0.01 to 7.79, and 26.4 to 246, respectively. On the other hand, the phytoremediation potential of the five metals was estimated based on the bioconcentration factor (BCF) and the translocation factor (TF) values. Correlation analysis revealed that the roots and stems could be used as biomonitors of Cu, the stems as biomonitors of Ni, the roots and leaves as biomonitors of Pb, and all three parts of the plant as biomonitors of Zn. According to the BCF values, in the topsoil, the “easily, freely, leachable, or exchangeable” geochemical fractions of the five metals could be more easily transferred to the roots, leaves, and stems when compared with total concentrations. Based on the TF values of Cd, Ni, and Pb, the metal transfer to the stems (or leaves) from the roots was efficient (>1.0) at most sampling sites. The results of BCF and TF showed that A. gangetica was a good phytoextractor for Cd and Ni, and a good phytostabilizer for Cu, Pb, and Zn. Therefore, A. gangetica is a good candidate as a biomonitor and a phytoremediator of Ni, Pb, and Zn for sustainable contaminant remediation subject to suitable field management strategies.

Applications of Nanomaterials for Heavy Metal Removal from Water and Soil: A Review

Heavy metals are toxic and non-biodegradable environmental contaminants that seriously threaten human health. The remediation of heavy metal-contaminated water and soil is an urgent issue from both environmental and biological points of view. Recently, nanomaterials with excellent adsorption capacities, great chemical reactivity, active atomicity, and environmentally friendly performance have attracted widespread interest as potential adsorbents for heavy metal removal. This review first introduces the application of nanomaterials for removing heavy metal ions from the environment. Then, the environmental factors affecting the adsorption of nanomaterials, their toxicity, and environmental risks are discussed. Finally, the challenges and opportunities of applying nanomaterials in environmental remediation are discussed, which can provide perspectives for future in-depth studies and applications.