Molecular mechanisms and physiological responses of rice leaves co-exposed to submicron-plastics and cadmium: Implication for food quality and security.

The effects of co-exposure to aged submicron particles (aSMPs) and Cd as model contaminants on rice leaves via the foliar route were investigated. Thirty-day-old rice seedlings grown in soil were exposed to Cd (nitrate) through foliar spraying at concentrations of 1, 10, 50, 100, and 500 µM, with or without aSMP at a rate of 30 µg d-1. It was observed that Cd translocated from leaves to roots via stems even without co-exposure to SMP. Co-exposure can reduce cadmium levels in leaves. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis confirmed a significant reduction (29.3 - 77.9%) in Cadmium accumulation in the leaves of rice plants during co-exposure. Exposure to Cd resulted in physiological, transcriptomic, and metabolomic changes in rice leaves, disrupting 28 metabolism pathways, and impacting crop yield and quality. Exposure to both Cd and aSMPs can interfere with the Cd distribution in plants. Rice leaves exposed solely to Cd exhibit higher toxicity and Cd accumulation, compared to those co-exposed to Cd and aSMPs. The accumulation of Cd in plant leaves is enhanced with aSMPs, which may lead to more pronounced gene expression regulation and changes in metabolic pathways, compared to Cd exposure. Our study found that the independent Cd exposure group had higher Cd accumulation and toxicity in rice leaves compared to the combined exposure of Cd and aSMPs. We hypothesize that aged negatively charged SMPs can capture Cd and reduce its exposure in the free state while jointly inhibiting Cd-induced oxidative and chloroplast damage, thereby reducing the potential risk of Cd exposure in rice plants.

Cr(VI) removal from contaminated waters using ultra-thin layered meixnerite.

Cr(VI) is of great concern to public health and environmental safety due to its high toxicity. Here, we report a low-cost yet highly efficient method to prepare a novel LDH, ultra-thin layered meixnerite, which performed superiorly in treatment of aqueous Cr(VI) with little secondary pollution being induced. The produced ultra-thin layered meixnerite was composed of nanoparticles with a thickness of around 7 nm, less than 9 times the thickness of a single LDH layer. The XRD patterns of the ultra-thin layered meixnerite, in which the characteristic diffraction peaks of a typical LDH were weakened or even disappeared, confirmed the successful delamination. This special morphology of the ultra-thin layered meixnerite was not only helpful to its full dispersion in the Cr(VI)-bearing solutions but also facilitated the formation of more active sorption sites on its external surface. As a result, the maximum sorption capacity of UTLM for Cr(VI) removal was 480.9 mg g-1, far higher than that of OM (196.9 mg g-1). In addition to electrostatic attraction and anion exchange, the ultra-thin layered meixnerite could also become restacked during removal of aqueous Cr(VI) to generate inner-sphere complexation, finally inducing an enhanced Cr(VI) uptake. Furthermore, XPS analysis characterized the promotion of the break of Al-OH bond with the increase in temperature, and the Cr-O peak increased correspondingly from 29.69% at 25 °C to 48.77% at 85 °C, resulting that the ultra-thin layered meixnerite could remove Cr(VI) more effectively at higher reaction temperatures. Therefore, ultra-thin layered meixnerite is very suitable for future application in treatment of industrial wastewaters with elevated temperatures.

Hydrocalumite as well as the Formation of Scheelite Induced by Its Dissolution, Removing Aqueous Tungsten with Varying Concentrations.

As a toxic element, tungsten (W) in elevated concentrations, originating from human activities or geological sources, poses a severe threat to the environment. However, there has been a lack of robust remediation techniques focusing on aqueous tungsten contamination with varying initial concentrations, because only recently have the toxicity and the environmental threat of tungsten been fully realized. In this study, the removal of tungsten from an aqueous solution by hydrocalumite was investigated for the first time. Systematic removal experiments were carried out at designated contact time, temperature, and initial tungsten concentration. The results showed that hydrocalumite is capable of effectively removing tungsten under various conditions, especially at high initial tungsten concentrations, with the maximum uptake capacity being up to 1120.5 mg (tungsten)/g (hydrocalumite). The mechanisms of tungsten removal were studied based on the measured chemical compositions of the solution samples and their PHREEQC simulations as well as the solid sample characterization by XRD, SEM-EDX, and XPS. At low initial tungsten concentrations (below 1 mmol/L), anion exchange between the tungsten in solution and the Cl in the hydrocalumite interlayers played a critical role in tungsten removal. At high initial tungsten concentrations (higher than 5 mmol/L), the removal of W from the solution was solely caused by the precipitation of scheelite (CaWO4), facilitated by the substantial release of Ca2+ from hydrocalumite dissolution. At moderate tungsten concentrations (1-5 mmol/L), however, both mechanisms were responsible for the uptake of tungsten, with scheelite precipitation being more important. Hydrocalumite is promising for wide use in the treatment of high-tungsten natural waters or wastewaters.

Efficient and Fast Removal of Aqueous Tungstate by an Iron-Based LDH Delaminated in L-Asparagine.

High concentrations of tungstate in aqueous systems pose a severe threat to the environment and human health. This study explored the potential of iron-based LDHs to remove tungstate from water. To improve its tungstate uptake capacity, environment-friendly L-asparagine was used to delaminate iron-based LDH synthesized via a coprecipitation method. The successful delamination was proved by AFM, revealing that the thickness of the obtained nanoparticles was approximately 1-2 times that of a single LDH layer. XRD, TEM, and XPS analyses confirmed that the delaminated LDHs were amorphous and ultrathin and had surface defects within their nanosheets that acted as active sites, leading to a very fast tungstate sorption rate and superior tungstate uptake capacity. Notably, the original layered structure of the L-asparagine-treated LDH was recovered upon its reaction with tungstate-bearing solutions, and therefore, the high availability of aqueous tungstate to the interlayer regions during the structural restoration of the delaminated iron-based LDH contributed to its excellent capability of tungstate removal as well. In addition, the tungstate uptake by the delaminated iron-based LDH was not affected substantially by the presence of coexisting anions, implying that the strong inner-sphere complexation between the tungstate and LDH layers with defects (i.e., Fe-O bonds) was the primary mechanism responsible for the tungstate removal. The delamination process described in this paper was validated to be an effective way to enhance the immobilization of tungstate by iron-based LDHs without inducing secondary pollutions, and delaminated iron-based LDHs are promising to be used extensively in the practice of treating tungstate-rich waters.

Simultaneous removal of harmful anions from geothermal waters using OH- intercalated Mg-Fe-LDH: batch and field column studies.

Unlike collectively treatable industrial wastewaters where only one or a few pollutants have concentrations much higher than the relevant standards, geothermal waters, in which multiple harmful constituents coexist, are usually discharged dispersedly, provoking a big challenge for their effective treatment. Here, a Mg/Fe layered double hydroxide with OH- intercalated (Mg-Fe-OH-LDH) was synthesized in a mechanochemical way and then applied in the treatment of various types of high-temperature geothermal waters in western Yunnan (China) containing a variety of harmful anions (As, Sb, W, and F) and inducing local environmental pollution. Due to the endothermic nature of removal of aqueous As, Sb, W, and F by Mg-Fe-OH-LDH, the original high temperatures of the geothermal waters could promote their sorption effectively. Batch sorption experiments demonstrated that over 94% and 80% of the As and W removal amounts could be reached within the first 10 and 20 min, respectively. On-site column experiments confirmed that the sorbent could remove the targeted harmful constituents from the investigated geothermal waters efficiently. In fact, the performance of the sorbent in the column studies was even better than that in the batch experiments, which can be ascribed to the continuous impetus for sorption caused by the concentration gradient in the flowing sorption system. Specifically, Mg-Fe-OH-LDH displayed the best sorption performance for As(V) among various harmful constituents, and the sorption of As along with W and F was little affected by the coexisting common anions in the geothermal waters, including Cl-, SO42-, and HCO3-/CO32-. In contrast, the removal of Sb(V) from geothermal waters may be impeded to a certain extent by SO42- and CO32-, which possessed stronger electronegativity or smaller ionic radii. This study is the first attempt to apply Mg-Fe-LDH in treatment of geothermal waters with multiple harmful constituents and sheds a light on providing a practical approach for field treatment of geothermal water-derived pollution.

Tungstate removal from aqueous solution by nanocrystalline iowaite: An iron-bearing layered double hydroxide.

Tungstate enrichment in aquatic systems may cause negative environmental and health effects. This study addresses tungstate removal from aqueous solution by nanocrystalline iowaite, an iron-bearing layered double hydroxide, which has not been used for treatment of tungstate-rich waters so far. Tungstate sorption experiments were conducted with various contact times, temperatures, initial tungstate concentrations (0.001-2 mM), and solution pH values (2-13), the results indicating that iowaite sorbed aqueous tungstate effectively and quickly, and the sorption maximum can be up to 71.9 mg/g. Moreover, the tungsten sorption capacity keeps nearly constant at a wide pH range from 3 to 11. Duo to its pH buffering effect, the alkaline conditions were generated by the addition of iowaite, which are favorable for the removal of aqueous tungstate because the polymerization of tungstate can be prohibited at alkaline pH values. Zeta potential, XRD and XPS analyses were employed to clarify the sorption mechanisms, and it was concluded that tungstate was sorbed via its exchange with the chloride originally intercalated into iowaite interlayers as well as its stronger inner-sphere complexation with the Fe atoms located in iowaite layers. Nanocrystalline iowaite is suitable for treating both tungstate-bearing natural waters with moderately high tungstate concentrations and industrial wastewaters extremely rich in tungstate.

Enhanced Removal of Arsenic from Water by Synthetic Nanocrystalline Iowaite.

Nanocrystalline iowaite, a Mg/Fe-based layered double hydroxide (LDH) intercalated with chloride, was synthesized to evaluate its performance for arsenic removal from water and to investigate the contributing dearsenication mechanisms. It is characterized by fast arsenic sorption rates and has a much higher arsenic uptake capacity than other LDHs that are commonly used for water dearsenication. The surface adsorption of the solution arsenic onto the iowaite samples and the anion exchange of the arsenic in solution with chloride, which is originally in the iowaite interlayers, are the primary mechanisms for the uptake of arsenic by iowaite. In addition to the Coulombic attraction between arsenate/arsenite and positively charged layers of iowaite, the inner-sphere complexation of arsenic with Fe (instead of Mg) in the iowaite layers is responsible for the formation of more stable and stronger arsenic bonds, as indicated by both XPS and EXAFS analyses. Specifically, bidentate-binuclear and monodentate-mononuclear As-Fe complexes were detected in the arsenate removal experiments, whereas bidentate-mononuclear, bidentate-binuclear, and monodentate-mononuclear As-Fe complexes were present for the arsenite-treated iowaite samples. This study shows that nanocrystalline iowaite is a promising, low-cost material for arsenic removal from natural arsenic-rich waters or contaminated high-arsenic waters.

Boron sorption from aqueous solution by hydrotalcite and its preliminary application in geothermal water deboronation.

Hydrotalcite and its calcination product were used to treat pure water spiked with various concentrations of boron and geothermal water containing boron as a major undesirable element. The kinetics process of boron sorption by uncalcined hydrotalcite is controlled by the diffusion of boron from bulk solution to sorbent-solution boundary film and its exchange with interlayer chloride of hydrotalcite, whereas the removal rate of boron by calcined hydrotalcite rests with the restoration process of its layered structure. The results of isotherm sorption experiments reveal that calcined hydrotalcite generally has much stronger ability to lower solution boron concentration than uncalcined hydrotalcite. The combination of adsorption of boron on the residue of MgO-Al2O3 solid solution and intercalation of boron into the reconstructed hydrotalcite structure due to "structural memory effect" is the basic mechanism based on which the greater boron removal by calcined hydrotalcite was achieved. As 15 geothermal water samples were used to test the deboronation ability of calcined hydrotalcite at 65 °C, much lower boron removal efficiencies were observed. The competitive sorption of the other anions in geothermal water, such as HCO3-, SO4(2-), and F-, is the reason why calcined hydrotalcite could not remove boron from geothermal water as effectively as from pure boron solution. However, boron removal percents ranging from 89.3 to 99.0% could be obtained if 50 times of sorbent were added to the geothermal water samples. Calcined hydrotalcite is a good candidate for deboronation of geothermal water.