Insights into the Selective Transformation of Ceria Sulfation and Iron/Manganese Mineralization for Enhancing the Selective Recovery of Rare Earth Elements.

To cope with the urgent and unprecedented demands for rare earth elements (REEs) in sophisticated industries, increased attention has been paid to REE recovery from recycled streams. However, the similar geochemical behaviors of REEs and transition metals often result in poor separation performance due to nonselectivity. Here, a unique approach based on the selective transformation between ceria sulfation and iron/manganese mineralization was proposed, leading to the enhancement of the selective separation of REEs. The mechanism of the selective transformation of minerals could be ascribed to the distinct geochemical and metallurgical properties of ions, resulting in different combinations of cations and anions. According to hard-soft acid-base (HSAB) theory, the strong Lewis acid of Ce(III) was inclined to combine with the hard base of sulfates (SO42-), while the borderline acid of Fe(II)/Mn(II) prefers to interact with oxygen ions (O2-). Both in situ characterization and density functional theory (DFT) calculation further revealed that such selective transformation might trigger by the generation of an oxygen vacancy on the surface of CeO2, leading to the formation of Ce2(SO4)3 and Fe/Mn spinel. Although the electron density difference of the configurations (CeO2-x-SO4, Fe2O3-x-SO4, and MnO2-x-SO4) shared a similar direction of the electron transfer from the metals to the sulfate-based oxygen, the higher electron depletion of Ce (QCe = -1.91 e) than Fe (QFe = -1.66 e) and Mn (QMn = -1.64 e) indicated the higher stability in the Ce-O-S complex, resulting in the larger adsorption energy of CeO2-x-SO4 (-6.88 eV) compared with Fe2O3-x-SO4 (-3.10 eV) and MnO2-x-SO4 (-2.49 eV). This research provided new insights into the selective transformation of REEs and transition metals in pyrometallurgy and thus offered a new approach for the selective recovery of REEs from secondary resources.

New Insights into the Accumulation, Transport, and Distribution Mechanisms of Hexafluoropropylene Oxide Homologues, Important Alternatives to Perfluorooctanoic Acid, in Lettuce (Lactuca sativa L.).

Hexafluoropropylene oxide (HFPO) homologues, which are important alternatives to perfluorooctanoic acid, have been frequently identified in crops. Although exposure to HFPO homologues via crops may pose non-negligible threats to humans, their impact on crops is still unknown. In this study, the accumulation, transport, and distribution mechanisms of three HFPO homologues in lettuce were investigated at the plant, tissue, and cell levels. More specifically, HFPO trimer acid and HFPO tetramer acid were primarily fixed in roots and hardly transported to shoots (TF, 0.06-0.63). Conversely, HFPO dimer acid (HFPO-DA) tended to accumulate in lettuce shoots 2-264 times more than the other two homologues, thus resulting in higher estimated daily intake values. Furthermore, the dissolved organic matter derived from root exudate enhanced HFPO-DA uptake by increasing its desorption fractions in the rhizosphere. The transmembrane uptake of HFPO homologues was controlled by means of a transporter-mediated active process involving anion channels, with the uptake of HFPO-DA being additionally facilitated by aquaporins. The higher accumulation of HFPO-DA in shoots was attributed to the larger proportions of HFPO-DA in the soluble fraction (55-74%) and its higher abundance in both vascular tissues and xylem sap. Our findings expand the understanding of the fate of HFPO homologues in soil-crop systems and reveal the underlying mechanisms of the potential exposure risk to HFPO-DA.

Plasma-Membrane-Localized Transporter NREET1 is Responsible for Rare Earth Element Uptake in Hyperaccumulator Dicranopteris linearis.

Rare earth elements (REEs) are critical for numerous modern technologies, and demand is increasing globally; however, production steps are resource-intensive and environmentally damaging. Some plant species are able to hyperaccumulate REEs, and understanding the biology behind this phenomenon could play a pivotal role in developing more environmentally friendly REE recovery technologies. Here, we identified a REE transporter NRAMP REE Transporter 1 (NREET1) from the REE hyperaccumulator fern Dicranopteris linearis. Although NREET1 belongs to the natural resistance-associated macrophage protein (NRAMP) family, it shares a low similarity with other NRAMP members. When expressed in yeast, NREET1 exhibited REE transport capacity, but it could not transport divalent metals, such as zinc, nickel, manganese, or iron. NREET1 is mainly expressed in D. linearis roots and predominantly localized in the plasma membrane. Expression studies in Arabidopsis thaliana revealed that NREET1 functions as a transporter mediating REE uptake and transfer from root cell walls into the cytoplasm. Moreover, NREET1 has a higher affinity for transporting light REEs compared to heavy REEs, which is consistent to the preferential enrichment of light REEs in field-grown D. linearis. We therefore conclude that NREET1 may play an important role in the uptake and consequently hyperaccumulation of REEs in D. linearis. These findings lay the foundation for the use of synthetic biology techniques to design and produce sustainable, plant-based REE recovery systems.

Rare earth elements, aluminium and silicon distribution in the fern Dicranopteris linearis revealed by µPIXE Maia analysis.

BACKGROUND: The fern Dicranopteris linearis is a hyperaccumulator of rare earth elements (REEs), aluminium (Al) and silicon (Si). However, the physiological mechanisms of tissue-level tolerance of high concentrations of REE and Al, and possible interactions with Si, are currently incompletely known. METHODS: A particle-induced X-ray emission (µPIXE) microprobe with the Maia detector, scanning electron microscopy with energy-dispersive spectroscopy and chemical speciation modelling were used to decipher the localization and biochemistry of REEs, Al and Si in D. linearis during uptake, translocation and sequestration processes. RESULTS: In the roots >80 % of REEs and Al were in apoplastic fractions, among which the REEs were most significantly co-localized with Si and phosphorus (P) in the epidermis. In the xylem sap, REEs were nearly 100 % present as REEH3SiO42+, without significant differences between the REEs, while 24-45 % of Al was present as Al-citrate and only 1.7-16 % Al was present as AlH3SiO42+. In the pinnules, REEs were mainly concentrated in necrotic lesions and in the epidermis, and REEs and Al were possibly co-deposited within phytoliths (SiO2). Different REEs had similar spatial localizations in the epidermis and exodermis of roots, the necrosis, veins and epidermis of pinnae of D. linearis. CONCLUSIONS: We posit that Si plays a critical role in REE and Al tolerance within the root apoplast, transport within the vascular bundle and sequestration within the blade of D. linearis.

Bacterial community diversity in the rhizosphere of nickel hyperaccumulator plant species from Borneo Island (Malaysia).

The Island of Borneo is a major biodiversity hotspot, and in the Malaysian state of Sabah, ultramafic soils are extensive and home to more than 31 endemic nickel hyperaccumulator plants. The aim of this study was to characterize the structure and the diversity of the rhizosphere bacterial communities of several of these nickel hyperaccumulator plants and factors that affect these bacterial communities in Sabah. The most abundant phyla were Proteobacteria, Acidobacteria and Actinobacteria. At family level, Burkholderiaceae and Xanthobacteraceae (Proteobacteria phylum) were the most abundant families in the hyperaccumulator rhizospheres. Redundancy analysis based on soil chemical analyses and relative abundances of the major bacterial phyla showed that abiotic factors of the studied sites drove the bacterial diversity. For all R. aff. bengalensis rhizosphere soil samples, irrespective of studied site, the bacterial diversity was similar. Moreover, the Saprospiraceae family showed a high representativeness in the R. aff. bengalensis rhizosphere soils and was linked with the nickel availability in soils. The ability of R. aff. bengalensis to concentrate nickel in its rhizosphere appears to be the major factor driving the rhizobacterial community diversity unlike for other hyperaccumulator species.

Spatially Resolved Localization of Lanthanum and Cerium in the Rare Earth Element Hyperaccumulator Fern Dicranopteris linearis from China.

The fern Dicranopteris linearis (Gleicheniaceae) from China is a hyperaccumulator of rare earth element (REE), but little is known about the ecophysiology of REE in this species. This study aimed to clarify tissue-level and organ-level distribution of REEs via synchrotron-based X-ray fluorescence microscopy (XFM). The results show that REEs (La + Ce) are mainly colocalized with Mn in the pinnae and pinnules, with the highest concentrations in necrotic lesions and lower concentrations in veins. In the cross sections of the pinnules, midveins, rachis, and stolons, La + Ce and Mn are enriched in the epidermis, vascular bundles, and pericycle (midvein). In these tissues, Mn is localized mainly in the cortex and mesophyll. We hypothesize that the movement of REEs in the transpiration flow in the veins is initially restricted in the veins by the pericycle between vascular bundle and cortex, while excess REEs are transported by evaporation and cocompartmentalized with Mn in the necrotic lesions and epidermis in an immobile form, possibly a Si-coprecipitate. The results presented here provide insights on how D. linearis regulates high concentrations of REEs in vivo, and this knowledge is useful for developing phytotechnological applications (such as REE agromining) using this fern in REE-contaminated sites in China.

Mobility of metal(loid)s in Pb/Zn tailings under different revegetation strategies.

Metal tailings are potential sources of strong environmental pollution. In situ remediation involves the installation of a plant cover to stabilize materials and pollutants. Whether metal(loid)s are effectively immobilized in remediated tailing ponds submitted to heavy rainfall remains uncertain. In this study, tailing materials were collected from bare tailings (control), grass-planted (G) and grass-shrub planted (GS) areas on a former Pb/Zn mine site. Batch column experiments were performed with three rainfall intensities of 0.36, 0.48, and 0.50 mL min-1 for 18 d in the lab. The pH, Eh, Cd, Pb, Zn and As concentration in leachate were recorded. Selected leached tailing materials were finally characterized. Results showed that leachates from control were strongly acidic (pH 3.11-4.65), and that Cd, Pb, Zn and As were quickly released at high rate (e.g., 945 mg L-1 Zn). During the experiment up to 4% Cd present in the material was released and almost 1% Zn. With material collected from the G area, leachates were even more acidic (2.16-2.84) with a rainfall intensity of 0.50 mL min-1 and exhibited a high redox potential (588-639 mV). However, concentrations of metals in leachates were much lower than that in the control, except for Zn (e.g., 433 mg L-1), and they tended to decrease with time. Cumulative leaching rate was still relatively high (e.g., 0.68% Cd; 0.75% Zn) during the first eight days (stage I). However, with the GS treatment, leachate pH gradually raised from acid to alkaline values (3.9-8.2) during stage I, then remained high until the end of the experiment (stage II). Also, amounts of elements released during the 18 d were low in general. The releasing ratios of Cd (R2 > 0.95), Pb (R2 > 0.95), As (R2 > 0.87), and Zn (R2 > 0.90) fitted well with a two-constant model. In conclusion, under subtropical climate with heavy rainfall, phytostabilization is effective but immobilization of metals is higher with a combination of grass and shrub than with only grass to reduce leaching of As and Zn.

Accumulation and fractionation of rare earth elements (REEs) in the naturally grown Phytolacca americana L. in southern China.

The widespread use of rare earth elements (REEs) has resulted in problems for soil and human health. Phytolacca americana L. is a herbaceous plant widely distributed in Dingnan county of Jiangxi province, China, which is a REE mining region (ion absorption rare earth mine) and the soil has high levels of REEs. An investigation of REE content of P. americana growing naturally in Dingnan county was conducted. REE concentrations in the roots, stems, and leaves of P. americana and in their rhizospheric soils were determined. Results showed that plant REEs concentrations varied among the sampling sites and can reach 1040 mg/kg in the leaves. Plant REEs concentrations decreased in the order of leaf > root > stem and all tissues were characterized by a light REE enrichment and a heavy REE depletion. However, P. americana exhibited preferential accumulation of light REEs during the absorption process (from soil to root) and preferential accumulation of heavy REEs during the translocation process (from stem to leaf). The ability of P. americana to accumulate high REEs in the shoot makes it a potential candidate for understanding the absorption mechanisms of REEs and for the phytoremediation of REEs contaminated soil.

Metal Immobilization on Wood-Derived Biochars: Distribution and Reactivity of Carbonate Phases.

Metals can be immobilized on biochars by precipitation with carbonate. The distribution of metal-carbonate phases at the surface of biochars and the conditions of their formation, however, are unknown. Electron microscopy and X-photon spectroscopy were used to characterize carbonate phases in various morphological groups of particles of a wood-derived biochar, both before and after a metal-sorption experiment. Our results showed that the distribution of metals at the surface of biochar particles depended on the corresponding wood tissues and the presence of carbonate phases. Metals were particularly concentrated (i) within calcium carbonate crystals in bark-derived particles, which originated from calcium oxalate crystals formed prior to pyrolysis, and (ii) as new phases formed by the reprecipitation of carbonate on specific tissues of biochar. The formation of biochar carbonate phases and their redistribution by dissolution-precipitation mechanisms may primarily control the localization of metals on biochar particles and the durability of metals immobilization.

Zinc Isotope Fractionation in the Hyperaccumulator Noccaea caerulescens and the Nonaccumulating Plant Thlaspi arvense at Low and High Zn Supply.

On the basis of our previous field survey, we postulate that the pattern and degree of zinc (Zn) isotope fractionation in the Zn hyperaccumulator Noccaea caerulescens (J. & C. Presl) F. K. Mey may reflect a relationship between Zn bioavailability and plant uptake strategies. Here, we investigated Zn isotope discrimination during Zn uptake and translocation in N. caerulescens and in a nonaccumulator Thlaspi arvense L. with a contrasting Zn accumulation ability in response to low (Zn-L) and high (Zn-H) Zn supplies. The average isotope fractionations of the N. caerulescens plant as a whole, relative to solution (Δ(66)Znplant-solution), were -0.06 and -0.12‰ at Zn-L-C and Zn-H-C, respectively, indicative of the predominance of a high-affinity (e.g., ZIP transporter proteins) transport across the root cell membrane. For T. arvense, plants were more enriched in light isotopes under Zn-H-A (Δ(66)Znplant-solution = -0.26‰) than under Zn-L-A and N. caerulescens plants, implying that a low-affinity (e.g., ion channel) transport might begin to function in the nonaccumulating plants when external Zn supply increases. Within the root tissues of both species, the apoplast fractions retained up to 30% of Zn mass under Zn-H. Moreover, the highest δ(66)Zn (0.75‰-0.86‰) was found in tightly bound apoplastic Zn, pointing to the strong sequestration in roots (e.g., binding to high-affinity ligands/precipitation with phosphate) when plants suffer from high Zn stress. During translocation, the magnitude of isotope fractionation was significantly greater at Zn-H (Δ(66)Znroot-shoot = 0.79‰) than at Zn-L, indicating that fractionation mechanisms associated with root-shoot translocation might be identical to the two plant species. Hence, we clearly demonstrated that Zn isotope fractionation could provide insight into the internal sequestration mechanisms of roots when plants respond to low and high Zn supplies.

Agromining: farming for metals in the future?

Phytomining technology employs hyperaccumulator plants to take up metal in harvestable plant biomass. Harvesting, drying and incineration of the biomass generates a high-grade bio-ore. We propose that "agromining" (a variant of phytomining) could provide local communities with an alternative type of agriculture on degraded lands; farming not for food crops, but for metals such as nickel (Ni). However, two decades after its inception and numerous successful experiments, commercial phytomining has not yet become a reality. To build the case for the minerals industry, a large-scale demonstration is needed to identify operational risks and provide "real-life" evidence for profitability.

Nickel and zinc isotope fractionation in hyperaccumulating and nonaccumulating plants.

Until now, there has been little data on the isotope fractionation of nickel (Ni) in higher plants and how this can be affected by plant Ni and zinc (Zn) homeostasis. A hydroponic cultivation was conducted to investigate the isotope fractionation of Ni and Zn during plant uptake and translocation processes. The nonaccumulator Thlaspi arvense, the Ni hyperaccumulator Alyssum murale and the Ni and Zn hyperaccumulator Noccaea caerulescens were grown in low (2 µM) and high (50 µM) Ni and Zn solutions. Results showed that plants were inclined to absorb light Ni isotopes, presumably due to the functioning of low-affinity transport systems across root cell membrane. The Ni isotope fractionation between plant and solution was greater in the hyperaccumulators grown in low Zn treatments (Δ(60)Ni(plant-solution) = -0.90 to -0.63‰) than that in the nonaccumulator T. arvense (Δ(60)Ni(plant-solution) = -0.21‰), thus indicating a greater permeability of the low-affinity transport system in hyperaccumulators. Light isotope enrichment of Zn was observed in most of the plants (Δ(66)Zn(plant-solution) = -0.23 to -0.10‰), but to a lesser extent than for Ni. The rapid uptake of Zn on the root surfaces caused concentration gradients, which induced ion diffusion in the rhizosphere and could result in light Zn isotope enrichment in the hyperaccumulator N. caerulescens. In high Zn treatment, Zn could compete with Ni during the uptake process, which reduced Ni concentration in plants and decreased the extent of Ni isotope fractionation (Δ(60)Ni(plant-solution) = -0.11 to -0.07‰), indicating that plants might take up Ni through a low-affinity transport system of Zn. We propose that isotope composition analysis for transition elements could become an empirical tool to study plant physiological processes.

Pedogenesis and nickel biogeochemistry in a typical Albanian ultramafic toposequence.

This study aimed at relating the variability of Ni biogeochemistry along the ultramafic toposequence to pedogenesis and soil mineralogy. Hypereutric Cambisols dominate upslope; Cambic Vertisols and Fluvic Cambisols occur downslope. The soil mineralogy showed abundance of primary serpentine all over the sequence. It is predominant upslope but secondary smectites dominate in the Vertisols. Free Fe-oxides are abundant in all soils but slightly more abundant in the upslope soils. Whereas serpentines hold Ni in a similar and restricted range in every soil (approx. 0.3 %), Ni contents in smectites may vary a lot and Mg-rich and Al-poor smectites in the Vertisol could hold up to 4.9 % Ni. Ni was probably adsorbed onto amorphous Fe-oxides and was also exchangeable in secondary smectites. High availability of Ni in soils was confirmed by DTPA extractions. However, it varied significantly along the toposequence, being higher in upslope soils, where Ni-bearing amorphous Fe-oxides were abundant and total organic carbon higher and sensibly lower downslope on the Vertisols: NiDTPA varied from 285 mg kg(-1) in the surface of soil I (upslope) to 95.9 mg kg(-1) in the surface of Fluvic Cambisols. Concentration of Ni in Alyssum murale shoots varied from 0.7 % (Hypereutric Cambisols) to 1.4 % (Hypereutric Vertisol). Amazingly, Ni uptake by A. murale was not correlated to NiDTPA, suggesting the existence of specific edaphic conditions that affect the ecophysiology of A. murale upslope.

Heavy metal migration dynamics and solid-liquid distribution strategy in abandoned tailing soils.

The tailings soil originating from an abandoned sulfur-iron mine in Sichuan Province, China, exhibits elevated concentrations of heavy metals (HMs) and possesses limited soil conservation capacity. Variability soil particle size fractions (PSFs) contributes to an increased risk of HMs ion migration. Existing research on HMs behavior has focused on the bulk soil scale, resulting in a dearth of comprehensive information concerning different particle sizes and colloid scales. We collected soil samples from upstream source (XWA), migration path (XWB), and downstream farmland (XWC) of an abandoned tailing and categorized into sand, silt, clay, colloid and dissolved, respectively. The investigation primarily aimed to elucidate the solid-liquid distribution trade-off strategies of soil HMs along migration pathway. Results show that PSFs composition predominantly influences HMs solid-liquid distribution. In the mining area, large particles serve as the principal component for HMs enrichment. However, along the migration pathway, the proportion of highly mobile fine particles increases, shifting HMs from solid to liquid phase. Furthermore, inorganic elements such as Mg, Al, and Fe influence on HMs distribution within PSFs through various reactions, whereas organic matter and glomalin-related soil protein (GRSP) also exert regulatory roles. Increasing the proportion of large particles can reduce the risk of HMs migration.

Fe(III) transporter OsYSL15 may play a key role in the uptake of Cr(III) in rice (Oryza sativa L.).

Due to the widely discharge of chromium (Cr) by mining and smelting industries, etc., contamination of paddy soils and rice has become serious problems. Therefore it is crucial to explore how rice takes up Cr. Cr(III) is the most common Cr form in the long-term water flooding paddy soils. Here, we demonstrate that OsYSL15, a key gene for Fe(III) uptake, is equally applicable for Cr(III) uptake in rice. Firstly, the antagonistic effect of Cr(III) and Fe(III) in the uptake process was found. Rice could accumulate more Cr(III) under Fe-deficient conditions. And the Fe(III) content in the protoplasts of rice root cells gradually decreased with the increase exposure of Cr(III). Knockdown of OsYSL15 in rice significantly reduced the Cr(III) uptake rate. Compared with wild type rice, the accumulation of Cr(III) in OsYSL15 mutant was decreased by 40.7%- 70.6% after gene editing. These results indicate that OsYSL15 is a key gene responsible for Cr(III) uptake in rice, which can guide the screening or genetic modification for low-Cr-accumulation rice varieties.

Nickel stocks and fluxes in a tropical agromining 'metal crop' farming system in Sabah (Malaysia).

Nickel hyperaccumulator plants play a major role in nickel recycling in ultramafic ecosystems, and under agromining the nickel dynamics in the farming system will be affected by removal of nickel-rich biomass. We investigated the biogeochemical cycling of nickel as well as key nutrients in an agromining operation that uses the metal crop Phyllanthus rufuschaneyi in the first tropical metal farm located in Borneo (Sabah, Malaysia). For two years, this study monitored nine 25-m2 plots and collected information on weather, biomass exportation, water, and litter fluxes to the soil. Without harvesting, nickel inputs and outputs had only minor contributions (<1 %) to the total nickel budget in this system. The nickel cycle was mainly driven by internal fluxes, particularly plant uptake, litterfall and throughfall. After two years of cropping, the nickel litter flux corresponded to 50 % of the total nickel stock in the aerial biomass (3.1 g m-2 year-1). Nickel was slowly released from the litter; after 15 months of degradation, 60 % of the initial biomass and the initial nickel quantities were still present in the organic layer. Calcium, phosphorus and potassium budgets in the system were negative without fertilisation. Unlike what is observed for nickel, sustained agromining would thus lead to a strong depletion of calcium stocks if mineral weathering cannot replenish it.

Biogenic manganese oxides promote metal(loid) remediation by shaping microbial communities in biological aqua crust.

Biological aqua crust (biogenic aqua crust-BAC) is a potentially sustainable solution for metal(loid) bioremediation in global water using solar energy. However, the key geochemical factors and underlying mechanisms shaping microbial communities in BAC remain poorly understood. The current study aimed at determining the in situ metal(loid) distribution and the key geochemical factors related to microbial community structure and metal(loid)-related genes in BAC of a representative Pb/Zn tailing pond. Here we showed that abundant metal(loid)s (e.g. Pb, As) were co-distributed with Mn/Fe-rich minerals (e.g. biogenic Mn oxide, FeOOH) in BAC. Biogenic Mn oxide (i.e. Mn) was the most dominant factor in shaping microbial community structure in BAC and source tailings. Along with the fact that keystone species (e.g. Burkholderiales, Haliscomenobacter) have the potential to promote Mn ion oxidization and particle agglomeration, as well as Mn is highly associated with metal(loid)-related genes, especially genes related to As redox (e.g. arsC, aoxA), and Cd transport (e.g. zipB), biogenic Mn oxides thus effectively enhance metal(loid) remediation by accelerating the formation of organo-mineral aggregates in biofilm-rich BAC system. Our study indicated that biogenic Mn oxides may play essential roles in facilitating in situ metal(loid) bioremediation in BAC of mine drainage.

Root Zn sequestration transporter heavy metal ATPase 3 from Odontarrhena chalcidica enhance Cd tolerance and accumulation in Arabidopsis thaliana.

The Ni hyperaccumulator Odontarrhena chalcidica (formerly Alyssum murale), exhibits a significant capacity to accumulate Zn in the roots. However, the molecular mechanisms underlying the variation in Ni and Zn accumulation are poorly understood. Here, we isolated a homolog of heavy metal ATPase 3 from O. chalcidica (OcHMA3) and characterized its functions using heterologous systems. Phylogenetic analysis revealed that OcHMA3 protein shares 87.6 % identity with AtHMA3, with similar metal binding sites to other HMA3 proteins. Heterologous expression of OcHMA3 in yeast increased sensitivity to Cd, Ni and Zn, suggesting it functions as a broad-specificity transporter. Further investigation showed OcHMA3 is constitutively expressed in the roots and localized to the tonoplast. Overexpression of OcHMA3 in A. thaliana shoots increased its roots Zn concentrations by 41.9 % - 74.1 %. However, overexpression of OcHMA3 in roots enhanced its tolerance to Cd and increased roots Cd concentrations by 50.9 % - 90.6 %. Our findings indicated OcHMA3 is responsible for Zn sequestration in root vacuoles, likely leading to Zn retention in roots and subsequent Ni hyperaccumulation in shoots. This study elucidates the molecular mechanism of Ni and Zn accumulation in O. chalcidica, and identifies OcHMA3 as a potential gene for developing Zn-rich plants and for phytoextraction in Cd-contaminated soils.

New insights into the controversy of reactive mineral-controlled arsenopyrite dissolution and arsenic release.

Serious arsenic (As) contaminations could commonly result from the oxidative dissolution of As-containing sulfide minerals, such as arsenopyrite (FeAsS). Pyrite (Py) and calcite (Cal) are two typically co-existing reactive minerals and represent different geological scenarios. Previous studies have shown that a high proportion of Py can generate a stronger galvanic effect and acid dissolution, thereby significantly promoting the release of arsenic. However, this conclusion overlooks calcite's antagonistic effect on the release of As in the natural environment. That antagonistic effect could remodel the linear relationship of pyrite on the oxidative dissolution of arsenopyrite, thus altering the environmental risk of As. We examined As release from arsenopyrite along a gradient of Py to Cal molar ratios (Py:Cal). The results showed that the lowest As release from arsenopyrite was surprisingly found in co-existing Py and Cal systems than in the singular Cal system, let alone in the singular Py system. This phenomenon indicated an interesting possibility of Py assistance to Cal inhibition of As release, though Py has always been regarded as a booster, also evidenced in this research, for As release from arsenopyrite. In singular systems of Py and Cal, As continued to be released for 60 days. However, in co-existing Py and Cal systems, As was released non-linearly in three stages over time: initial release (0-1 Day), immobilization (1-15 Days), and subsequent re-release (>15 Days). This is a new short-term natural attenuation stage for As, but over time, this stage gradually collapses. During the re-release stage (> 15 Days), a higher molar ratio of Py:Cal (increasing from 1:9 to 9:1) results in a lower rate constant k (mg·L-1·h-1) of As release (range from 0.0011 to 0.0002), and a higher abundance of secondary minerals formed (up to 26 mg/g goethite and hematite at Py: Cal=9:1). This demonstrates that increasing the Py:Cal molar ratio results in the formation of more secondary minerals which compensate for the higher potential antagonistic mechanisms generated by pyrites, such as acid dissolution and galvanic effect. These results explain the mechanisms of the high-risk characteristics of As both in acidic mine drainage and karst aquifers and discover the lowest risk in pyrite and calcite co-existing regions. Moreover, we emphasize that reactive minerals are important variables that can't be ignored in predicting As pollution in the future.

Chrysotile dissolution in the rhizosphere of the nickel hyperaccumulator Leptoplax emarginata.

Ni phytoextraction processes need further understanding of the interactions between Ni availability in soils and its absorption by plant roots. The large metal uptake and root exudation by hyperaccumulator species could accelerate the weathering process of Ni-bearing phases in the rhizosphere. The aim of this work was to quantify the weathering of a Ni-bearing mineral phase in the rhizosphere of the Ni-hyperaccumulator Leptoplax emarginata. The studied mineral was chrysotile which was characterized by a low Ni solubility. Column experiments were performed to assess the effect of the Ni-hyperaccumulator L. emarginata and the contribution of rhizobacteria on the dissolution rate of chrysotile. Mineral weathering was monitored by measuring Ni and Mg transferred to leachates or plants throughout the experiment. Results showed that L. emarginata increased chrysotile dissolution by more than 2-fold . The hyperaccumulator L. emarginata accumulated 88% on average of total mobilized Ni. Inoculation with Ni-resistant bacteria in the rhizosphere of L. emarginata had no significant effect on chrysotile dissolution or plant accumulation of Ni in this context. Finally, after 15 weeks of culture, 1.65% of total Ni in the system was mobilized in the planted treatments compared with 0.03% in the unplanted treatments.