Study of the spatial distribution of mercury in roots of vetiver grass (Chrysopogon zizanioides) by micro-pixe spectrometry

Roles and significance of chelating agents for potentially toxic elements (PTEs) phytoremediation in soil: A review

Phytoremediation is a biological remediation technique known for low-cost technology and environmentally friendly approach, which employs plants to extract, stabilise, and transform various compounds, such as potentially toxic elements (PTEs), in the soil or water. Recent developments in utilising chelating agents soil remediation have led to a renewed interest in chelate-induced phytoremediation. This review article summarises the roles of various chelating agents and the mechanisms of chelate-induced phytoremediation. This paper also discusses the recent findings on the impacts of chelating agents on PTEs uptake and plant growth and development in phytoremediation. It was found that the chelating agents have increased the rate of metal absorption and translocation up to 45% from roots to the aboveground plant parts during PTEs phytoremediation. Besides, it was also explored that the plants may experience some phytotoxicity after adding chelating agents to the soil. However, due to the leaching potential of synthetic chelating agents, the use of organic chelants have been explored to be used in PTEs phytoremediation. Finally, this paper also presents comprehensive insights on the significance of using chelating agents through SWOT analysis to discuss the advantages and limitations of chelate-induced phytoremediation.

Effect of monoethanolamine salt-containing dicarboxylic acid and plant growth regulators on the absorption and accumulation of mercury

In the modern world, mercury has become an extremely dangerous pollutant due to intensive human activity. Currently, sources of mercury are wastes from chemical industries, as well as mines, oil combustion products, and household waste. Phytoextraction of heavy metals from soil is considered one of the most promising and cost-effective technologies. The efficiency of this process can be increased by introducing various amendments. The use of additives in phytoextraction can enhance the absorption of heavy metals and increase their concentration in various parts of the plant. This article presents the results of a study of various chelating agents for effective phytoextraction of mercury with white clover (Trifolium repens L.) and watercress (Lepidium sativum). In the present study, the monoethanolamine salt of dithiodiacetic acid (MEDBA) was used. The optimal concentration of MEDBA on watercress and creeping clover has been determined for highly efficient phytoextraction of mercury. Research has been carried out with a complex of exogenous growth regulators (GA / IAA / Fe-EDDHA). The results showed that the use of phytohormones and plant growth regulators led to a synergistic effect in combination with thiosulfate, but a pronounced inhibitory effect was observed with the use of MEDBA.

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.

Growing food in polluted soils: A review of risks and opportunities associated with combined phytoremediation and food production (CPFP)

Innumerable private households and small-scale producers currently operate on polluted soils. Phytoremediation is one of the most cost-effective remediation options but as a stand-alone technology, it is often not lucrative enough to make it appealing for farmers, especially in economically vulnerable regions. Economic incentives are crucial for remediation projects to materialise and synergies can be obtained by integrating phytoremediation with other profitable activities including food production. This review aims to synthesise state-of-the-art scientific data to provide a general understanding of opportunities and risks for sustainable remediation of agricultural soil by the use of combined phytoremediation and food production (CPFP). The results show that strategies based on CPFP may be appropriate options for most pollutants in virtually all climatic or socioeconomic contexts but a number of challenges need to be surpassed. The challenges include remediation-technological issues such as undeveloped post-harvest technology and inadequate soil governance. The need for remediation solutions for polluted fields is increasingly urgent since many farmers currently operate on polluted land and the scarcity of soil resources as the human population continuously increases will inevitably force more farmers to cultivate in contaminated areas. We conclude that, although large scale CPFP has not yet reached technological maturity, appropriate combinations of soil types, plant species/cultivars, and agronomic practices together with thorough monitoring of the pollutants' pathways can potentially allow for safe food production on polluted soil that restricts the transfer of a number of pollutants to the food chain while the soil pool of pollutants is gradually reduced.

Anaerobic digestion of mercury phytoextraction crops with intermediary stage bio-waste polymer treatment

In laboratory experiments, Lepidium sativum L. and Mentha spicata L. were grown in compost spiked with mercury. After cultivation for 20 and 68 days, respectively, translocation factors of 0.05 ≤ TF ≤ 0.2 (Lepidium sativum) and accumulation factors of 2.2 ≤ AF ≤ 12 (Mentha spicata) were recorded. Plants were then harvested and used as feedstock for bench-scale anaerobic digesters. The reactors operated in continuously-stirred batch mode for a period of ten days. Inhibition of anaerobic biogas production was apparent with one sample set evidencing mercury-induced bacteriostatic toxicity. Otherwise, ex-situ characterization of digestate showed that the reactors were within stable operating range. A canola oil-sulphide polymer derived from bio-waste was also used as an intermediary treatment stage to test its capacity for extracting mercury from half the samples prior to anaerobic digestion, and also from the post-experimentation reactor digestate. The polymer removed mercury from digestate with a 40-50% efficacy across all samples, suggesting its potential as a sludge clean-up option. Anaerobic digestion combined with staged polymer extraction offers a potential route for the disposal of phytoremediation crops and ultimately the recovery of mercury, coincident with the production of a bioenergy vector.

Localization of mercury and gold in cassava (Manihot esculenta Crantz)

The potential of cassava (Manihot esculenta Crantz.) for simultaneous Hg and Au phytoextraction was explored by investigating Hg and Au localization in cassava roots through Micro-Proton Induced X-Ray Emission, High-Resolution Transmission Electron Microscopy (HR-TEM) and X-Ray Diffractometry (XRD). The effect of Hg and Au in the cyanogenic glucoside linamarin distribution was also investigated using Matrix Assisted Laser Desorption Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (MALDI-FT-ICR-MS) imaging. Hg was located mainly in the root vascular bundle of plants grown in 50 or 100 μmol L^-1 Hg solutions. Au was localized in the epidermis and cortex or in the epidermis and endodermis for 50 and 100 μmol L^-1 Au solutions, respectively. For 50 μmol L^-1 solutions of both Hg and Au, the two metals were co-localized in the epidermis. When the Hg concentrations were increased to 100 μmol L^-1, Au was still localized to a considerable extent in the epidermis while Hg was located in all root parts. HR-TEM and XRD revealed that Au nanoparticles were formed in cassava roots. MALDI-FT-ICR-MS imaging showed linamarin distribution in the roots of control and plants and metal-exposed plants thus suggesting that linamarin might be involved in Hg and Au uptake and distribution.

Cadmium and Mercury phytostabilization from soil using Miscanthus × giganteus

The determination of the effects of cadmium and mercury on the growth, biomass productivity and phytoremediation potential of Miscanthus × giganteus (MxG) grown on contaminated soil was the main aim of this paper. The use of bioenergy plants as an innovative strategy in phytotechnology gives additional benefits, including mitigation and adaptation to climate change, and soil remediation without affecting soil fertility. An experiment was set up as a randomized complete block design with the treatments varied in concentrations of Cd (0, 10 and 100 mg kg⁻¹ soil) and Hg (0, 2 and 20 mg kg⁻¹ soil) added to the soil. Three vegetative years were studied. Yield values ranged from 6.3–15.5 t_DM ha⁻¹, cadmium concentration in plants varied from 45–6758 µg kg⁻¹ and Hg varied from 8.7–108.9 µg kg⁻¹. Values between treatments and years were significantly different. MxG can accumulate and remove very modest amount (up to 293.8 µg Cd and 4.7 µg Hg) per pot per year in aboveground biomass. Based on this data it can be concluded that MxG, as a valuable energy crop, is a potential candidate for the phytostabilization and biomass production on soils contaminated with Cd and Hg moderately.

The barrier function of plant roots: biological bases for selective uptake and avoidance of soil compounds

The root is the main organ through which water and mineral nutrients enter the plant organism. In addition, root fulfils several other functions. Here, we propose that the root also performs the barrier function, which is essential not only for plant survival but for plant acclimation and adaptation to a constantly changing and heterogeneous soil environment. This function is related to selective uptake and avoidance of some soil compounds at the whole plant level. We review the toolkit of morpho-anatomical, structural, and other components that support this view. The components of the root structure involved in selectivity, permeability or barrier at a cellular, tissue, and organ level and their properties are discussed. In consideration of the arguments supporting barrier function of plant roots, evolutionary aspects of this function are also reviewed. Additionally, natural variation in selective root permeability is discussed which suggests that the barrier function is constantly evolving and is subject of natural selection.

The role of arbuscular mycorrhiza in mercury and mineral nutrient uptake in maize

This work aimed to study the role of arbuscular mycorrhizal fungi (AMF) in Hg and major mineral nutrient uptake and tissue localization of these elements in the roots of maize plants. Maize plants were grown in pots filled with non- and Hg-contaminated substrate (50 μg Hg g^-1 as HgCl₂) and inoculated with two types of AMF inocula: a) Glomus sp. originating from Hg-polluted soil of a former Hg smelting site in Idrija, Slovenia, and b) commercial AM inoculum Symbivit. Controls were inoculated by corresponding bacterial extracts only. Tissue localization of Hg and major mineral nutrients was performed by laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) on cryofixed and freeze-dried root cross-sections. AMF colonization increased plant biomass in non-contaminated substrate, while this effect was not seen in Hg-contaminated substrate. Hg increased total plant biomass more than AMF inoculation, possibly through hormetic effects. AMF increased Hg uptake into the roots, as well as Hg transfer to the shoots. AMF affected plant mineral nutrient uptake, depending on the type of AMF inoculum and the presence of Hg. In the roots, Hg was mainly localized in rhizodermis and endodermis, followed by the cortex and the central cylinder. Higher Hg concentrations were detected in the central cylinder of AM plants than in that of the controls, pointing to a higher Hg mobility and potential bioavailability in AMF inoculated plants.

Thiosulphate-induced phytoextraction of mercury in Brassica juncea: Spectroscopic investigations to define a mechanism for Hg uptake

Thiosulphate is extensively used to enhance mercury (Hg) phytoextraction due to its efficient in prompting plant Hg uptake. However, the mechanism by which thiosulphate promotes Hg uptake is poorly understood. We determined the concentrations of Hg and potassium (K), and their spatial distribution, in the tissues of Brassica juncea grown in Hg-contaminated soils treated by thiosulphate and compared this to a non-treated soil (control). The spatial distribution of Hg and K was characterized using micro-X ray fluorescence spectroscopy. The subcellular localization and speciation of Hg in the root of plant treated by thiosulphate were elucidated using Transmission electron microscope coupled energy-dispersive X-ray (TEM-EDX) spectroscopy. Thiosulphate increased significantly the Hg concentration in the roots (mainly in the epidermis and xylem) and shoots (mainly in the vascular bundles), while Hg was accumulated in the root (mainly in the epidermis) of the control plant. Thiosulphate promoted the movement of Hg from the epidermis to the xylem of roots, with subsequent loading into the stem via vascular bundles. Thiosulphate decreased the K concentration in plant tissues, relative to the control plant, and we propose this is due to leakage of electrolyte from roots via increased plasma membrane permeability as a consequence of physiological damage caused by the added thiosulphate. Mercury was distributed mainly at the extracellular space in the roots and was shown by TEM-EDX to be predominately amorphous nano-clusters of HgS. We conclude that thiosulphate-promoted Hg accumulation in the plant may happen through increased plasma membrane permeability, a changed pathway of Hg movement within plants, and extracellular co-transportation of Hg-S complexes in the roots. Our results may underpin the ongoing development of phytomanagement as an environmental strategy for Hg contaminated soils around the world.