Environmental remediation and monitoring of cadmium

Fluorescent Switch-on Detection of Cadmium(II) Using Salicylaldehyde-Decorated Gold Nanoclusters

In this study, salicylaldehyde (SA) conjugated gold nanoclusters were synthesized, characterized, and applied for the fluorescent turn-on sensing of Cd2+. The trypsin-stabilized fluorescent gold nanocluster (Tryp-AuNCs, λem = 680 nm) was modified with SA to form the spherical-shaped SA_Tryp-AuNCs. After modification, the red-emitting Tryp-AuNCs turned to green-emitting SA_Tryp-AuNCs because of the formation of imine linkage between the -CHO group of SA with the -NH2 group of functionalized trypsin. The modified SA_Tryp-AuNCs selectively interacted with Cd2+ and exhibited a fluorescence enhancement at 660 nm. The Cd2+ detection with SA_Tryp-AuNCs is simple and rapid with an estimated nanomolar detection limit of 98.1 nM. The practical utility of SA_Tryp-AuNCs was validated by quantifying Cd2+ in real environmental water samples.

Tripolyphosphate-functionalized cellulose: A green solution for cadmium contamination

This study introduces a highly efficient tripolyphosphate -tethered cellulose sorbent for cadmium (Cd2⁺) removal from aqueous solutions. Characterization through FTIR and SEM revealed the material's structural properties. The sorbent achieved 99% Cd2⁺ removal even at a minimal dosage of 0.05 g. Optimal sorption occurred within the pH range of 4-6, influenced by the sorbent's weak acidic functional groups. Rapid kinetics, reaching equilibrium within a minute, and a high sorption capacity (up to 18.03 mg/g at 50 °C) were observed. Langmuir isotherm modeling confirmed monolayer sorption, and thermodynamic studies indicated a spontaneous, endothermic process with increased randomness at the solid-liquid interface. Selectivity studies demonstrated strong Cd2⁺ removal performance in the presence of competing ions, with minimal interference from monovalent ions but notable effects from divalent ions. The sorbent exhibited consistent reusability over multiple cycles. XPS analysis conclusively established an ion exchange mechanism between Cd2⁺ and negatively charged P3O105- groups as the primary removal pathway. This research highlights the potential of TPP-tethered cellulose as a promising sorbent for effective Cd2⁺ remediation.

Design and application of Cd2+ polypeptide fluorescent probes based on Aggregation Induced Emission (AIE)

Cadmium is a toxic heavy metal, which is both an environmental pollutant, and a threat to human health. A fluorescent probe was developed to detect Cd2+ selectively, sensitively, and quickly. This study reports the successful development of a polypeptide fluorescent probe TPE-HC (TPE-His-Pro-Gly-Cys) which selectively detects Cd2+ by Aggregation-Induced Emission effect. After fluorescence excitation, Cd2+ can be effectively detected based on the change of fluorescence intensity. The detection limit of Cd2+ in buffer solution was determined to be 151 nM (R2 = 0.9933). This probe exhibits high sensitivity, high cell permeabilit y, and low biological toxicity, and can perform live cell imaging under biological conditions. This study indicates that TPE-HC can detect Cd2+ in biological environments.

Differential immobilization of cadmium and changes in soil surface charge in acidic Ultisol by chitosan and citric acid: effect of their functional groups

Soil organic matter plays an important role in cadmium adsorption and immobilization. Since different organic matter components affect cadmium adsorption processes differently, selecting the right organic substrate and knowing how to apply it could improve cadmium remediation. This study compares the effects of two contrasting organic molecules; chitosan and citric acid, on cadmium adsorption and speciation in acidic Ultisol. The adsorption of chitosan to Ultisol significantly increased the soil positive charge while adsorption of citric acid increased the soil negative charge. At pH 5.0, the maximum amount of cadmium adsorbed in excess chitosan was 341% greater than that in excess citric acid. About 73-89% and 60-62% of adsorbed cadmium were bound to Fe/Mn oxides and organic matter/sulfide at pH 4.0 while this fraction was 77-100% and 57-58% for citric acid and chitosan at pH 5.0, respectively. This decrease in the complexing ability of chitosan was related to the destabilizing effect of high pH on chitosan's structure. Also, the sequence through which chitosan, citric acid, and cadmium were added into the adsorption system influenced the adsorption profile and this was different along a pH gradient. Specifically, adding chitosan and cadmium together increased adsorption compared to when chitosan was pre-adsorbed within pH 3.0-6.5. However, for citric acid, the addition sequence had no significant effect on cadmium adsorption between pH 3.0-4.0 compared to pH 6.5 and 7.5, with excess citric acid generally inhibiting adsorption. Given that the action of citric acid is short-lived in soil, chitosan could be a good soil amendment material for immobilizing cadmium.

Sensitive electrochemical sensor for Cd2+ with engineered short high-affinity aptamer undergoing large conformation change

Cadmium ion (Cd2+) is a highly toxic heavy metal ion that threatens the environment and human health. To achieve rapid and sensitive detection of Cd2+, here we developed a reagent-less aptamer electrochemical sensor by immobilizing an engineered high-affinity DNA aptamer with a redox tag of methylene blue (MB) on the gold electrode. After testing a series of engineered aptamer sequences, we employed an optimal and new 15-mer aptamer with a short 3-bp stem for sensor fabrication, which underwent large conformation change upon Cd2+ binding. This aptamer retained high affinity with a Kd about 360 nM, verified by isothermal titration calorimetry (ITC) analysis. In the presence of Cd2+, this aptamer folded into a stem-loop structure, drawing the MB into a close proximity to the electrode surface and generating enhanced current in square wave voltammetry (SWV). Under the optimized conditions, this aptamer sensor enabled us to sensitively detect Cd2+ in a wide concentration range from 0.5 nM to 4 μM, and the detection limit was 90 pM. The developed electrochemical aptasensor has the advantages in easy preparation, rapid response, high stability, high selectivity and easy regeneration and reuse, showing the potential for Cd2+ detection in broad applications.

SbYS1 and SbWRKY72 regulate Cd tolerance and accumulation in sweet sorghum

MAIN CONCLUSION

SbYS1 and its upstream transcription factor SbWRKY72 were involved in Cd tolerance and accumulation and are valuable for developing sweet sorghum germplasm with high-Cd tolerance or accumulation ability through genetic manipulation. Cadmium (Cd) is highly toxic and can severely affect human health. Sweet sorghum, as an energy crop, shows great potential in extracting cadmium from Cd-contaminated soils. However, its molecular mechanisms of Cd-tolerance and -accumulation remain largely unknown. Here, we isolated a YSL family gene SbYS1 from the sweet sorghum genotype with high Cd accumulation ability and the expression of SbYS1 in roots was induced by cadmium. GUS staining experiment exhibited that SbYS1 was expressed in the epidermis and parenchyma tissues of roots. Further subcellular localization analysis suggested that SbYS1 was localized in the endoplasmic reticulum and plasma membrane. Yeast transformed with SbYS1 exhibited a sensitive phenotype compared to the control when exposed to Cd-NA (chelates of cadmium and nicotianamine), indicating that SbYS1 may absorb cadmium in the form of Cd-NA. Arabidopsis overexpressing SbYS1 had a longer root length and accumulated less Cd in roots and shoots. SbWRKY72 bound to the promoter of SbYS1 and negatively regulated the expression of SbYS1. Transgenic Arabidopsis of SbWRKY72 showed higher sensitivity to cadmium and increased cadmium accumulation in roots. Our results provide references for improving the phytoremediation efficiency of sweet sorghum by genetic manipulation in the future.

Cd2+-Selective Fluorescence Enhancement of Bisquinoline Derivatives with 2-Aminoethanol Skeleton

The development of fluorescent Cd2+ sensors requires strict selectivity over Zn2+ because of the high availability of Zn2+ in the natural environment. In this paper, bisquinoline-based fluorescent sensors with a 2-aminoethanol backbone were investigated. The weak coordination ability of quinoline compared to well-studied pyridine is suitable for Cd2+ selectivity rather than Zn2+. In the presence of 3 equiv. of metal ions, TriMeO-N,O-BQMAE (N,O-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methylaminoethanol (3)), as well as its N,N-isomer TriMeO-N,N-BQMAE (N,N-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methoxyethylamine (6)), exhibits Cd2+-selective fluorescence enhancement over Zn2+ in DMF-HEPES buffer (1:1, 50 mM HEPES, 0.1 M KCl, pH = 7.5) (IZn/ICd = 26-34%), which has similar selectivity in comparison to the corresponding ethylenediamine derivative TriMeOBQDMEN (N,N'-bis(5,6,7-trimethoxy-2-quinolylmethyl)-N,N'-dimethylethylenediamine) under the same experimental condition (IZn/ICd = 24%). The fluorescence mechanisms of N,O- and N,N-isomers of BQMAE are quite different, judging from the fluorescence lifetimes of their metal complexes. The Cd2+ complex with TriMeO-N,O-BQMAE (3) exhibits a long fluorescence lifetime similar to that of TriMeOBQDMEN via intramolecular excimer emission, whereas the Cd2+ complex with TriMeO-N,N-BQMAE (6) exhibits a short lifetime from monomer emission.

Mechanisms of ssDNA aptamer binding to Cd2+ in aqueous solution: A molecular dynamics study

ssDNA aptamers have been increasingly used to detect heavy metal ions as recognition elements due to their high affinity and specificity. However, the specific recognition and binding mechanisms between aptamers and most heavy metals were still unclear, which limits the development of aptamer-based detection methods. In this work, the interaction mechanisms of CD-2-1 aptamers with Cd2+ in aqueous solutions were investigated using molecular dynamic simulations. The most stable complex was found where Cd2+ binding at aptamer's stem-loop junction and preferred at the phosphate backbone or bases. Noteworthily, two binding modes of Cd2+ combining aptamer in aqueous solution were discovered: direct and indirect. In the former mode, Cd2+ directly coordinated O atoms of bases. For the latter, Cd2+ connected to bases with coordinated water molecules as bridges. Electrostatic interaction was found to be the main driving force, and differences of residues role between two binding modes were elucidated. Buffer molecules in aqueous solutions can stabilize aptamer-Cd2+ complex by hydrogen bonds. This study revealed the specific interaction mechanisms of aptamer with Cd2+ at an atomic level, which provided theoretical references for aptamer-based Cd2+ detection methods establishment as well as an efficient technical route of screening potential aptamers for heavy metal ions.

Rapid sensitive fluorescence detection of cadmium (II) with pyrene excimer switching aptasensor

Heavy metal cadmium (II) contamination often occurs, causing great health risk to human due to high toxicity of cadmium (II). Rapid, sensitive and simple detection of cadmium (II) are of great importance in environmental monitoring. Taking advantage of aptamer in specific recognition, easy modification, and capability of binding-induced structure change, here we reported a simple fluorescent sensor with rapid and sensitive response for Cd²⁺ using aptamer pyrene excimer switch. The aptamer was labeled with dual pyrene molecules at two ends of the sequence. The binding of Cd²⁺ to this aptamer probe brought the pyrene labels into close proximity and enhanced formation of a pyrene excimer, which generated increased fluorescence at 485 nm. By measuring the fluorescence of pyrene excimer, we achieved detection of Cd²⁺ with this aptasensor. Under the optimum experimental conditions, the detection limit of Cd²⁺ reached nanomolar levels. This method was selective and allowed for the detection of Cd²⁺ in tap water. This fluorescence aptasensor is promising for rapid detection of Cd²⁺ in broad applications.

Voltammetric sensing of Cd(II) at ZIF-8/GO modified electrode: Optimization and field measurements

In this work, a metal-organic framework/graphene oxide (MOF(ZIF-8)/GO) nanocomposite was utilized for the electroanalysis of trace level of Cd(II) after modification of a cheap graphite rod electrode (GRE). After closed circuit process on the modified electrode, the differential pulse anodic stripping voltammetry (DPASV) technique was used for measuring of Cd(II). In optimal conditions, the sensor showed a linear dependence of current with concentration range 0.1-30 ppb for Cd(II). Moreover, limit of detection 0.03 ppb were obtained. Besides good selectivity, the sensor also indicated good reproducibility (below 5%). Moreover, the sensor showed satisfactory sensing performance in river, dam and wastewater samples with recovery ranging from 97.2% to 102.4%. Additionally, possible interfering cations were examined, but no significant interference was found. For the detection of trace Cd(II) in real matrices, this sensor illustrated other good merits like high stability, rapidity and simplicity.

An Electrochemical Sensor for the Determination of Trace Concentrations of Cadmium, Based on Spherical Glassy Carbon and Nanotubes

The practical application of a novel, eco-friendly electrochemical sensor based on low-dimensional structures, spherical glassy carbon microparticles, and multiwall carbon nanotubes is described. This sensor, modified with a bismuth film, was used for the determination of Cd(II) by the anodic stripping voltammetric method. The instrumental and chemical factors influencing the sensitivity of the procedure were thoroughly investigated and their most favorable values were selected (acetate buffer solution pH = 3 ± 0.1; 0.15 mmol L-1 Bi(III); activation potential/time: -2 V/3 s; accumulation potential/time: -0.9 V/50 s). Under the selected conditions, the method exhibited linearity in the range of 2 × 10-9 to 2 × 10-7 mol L-1 Cd(II) with a detection limit of 6.2 × 10-10 mol L-1 Cd(II). The results obtained also showed that the application of the sensor for Cd(II) detection did not experience any significant interference in the presence of a number of foreign ions. The applicability of this procedure was evaluated using TM-25.5 Environmental Matrix Reference Material and SPS-WW1 Waste Water Certified Reference Material as well as river water samples through addition and recovery tests.

Design of a dual responsive receptor with oxochromane hydrazide moiety to monitor toxic Hg2+ and Cd2+ ions: Usage on real samples and live cells

In this work, we report a facile receptor OMB [N',N"'-(3-((4-oxochroman-3-yl)methylene)pentane-2,4- diylidene)bis(4-methoxybenzohydrazide)] for the simultaneous detection of toxic analytes (Hg²⁺ and Cd²⁺ ions) in environment and biological samples. The receptor OMB exhibits an excellent selectivity and sensitivity which was determined using absorption and emission spectra. The receptor OMB shows rapid detection with lowest LOD (0.62 nM for Hg²⁺ ions and 0.77 nM for Cd²⁺ ions) and LOQ (2.08 nM for Hg²⁺ ions and 2.57 nM for Cd²⁺ ions) values. In addition, the receptor OMB exhibits 1:1 binding stoichiometry towards Hg²⁺ and Cd²⁺ ions with binding constant values of 5.5 × 10⁶ M^-1 and 4.6 × 10⁶ M^-1. Moreover, the synthesized receptor OMB possess ability to detect these analytes (Hg²⁺ and Cd²⁺ ions) in realistic samples (food and water) which was recognized using photoluminescence spectroscopy technique. In addition, the receptor OMB is also utilized to detect both the analytes in live HeLa cells. Thus, the overall results indicate that the receptor OMB was more suitable to detect the toxic analytes (Hg²⁺ and Cd²⁺ ions) present in the environment.

Synthesis of Porous N-doped TiO2 by Using Peroxo Sol-Gel Method for Photocatalytic Reduction of Cd(II)

Porous N-doped TiO2 photocatalyst was successfully synthesized by an environmentally friendly peroxo sol-gel method using polyethylene glycol (PEG) as a templating agent. Here, the effect of PEG addition to the aqueous peroxotitanium solutions on the structure, pore properties and photocatalytic activity of the obtained photocatalysts was systematically studied. The prepared photocatalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), and Brunauer-Emmett-Teller (BET). It was found that the doping of nitrogen narrows the band gap of TiO2 leading to enhance its visible-light response. The BET analysis shows that the prepared photocatalysts have a typical mesoporous structure with pore sizes of 3–6 nm. The photocatalytic activity of the prepared photocatalysts was evaluated by photocatalytic reduction of Cd(II) in an aqueous solution under visible light irradiation. The results show that porous N-doped TiO2 with the optimal PEG addition had the highest Cd(II) reduction of 85.1% after 2.5 h irradiation in neutral aqueous solution. This significant improvement in photocatalytic activity of the prepared photocatalysts was mainly attributed to the synergistic combination of N doping and porous structure, which could actively increase the catalytic active site of this photocatalysts. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Experimental and Theoretical Studies of the Pyrazoline Derivative 5-(4-methylphenyl)-3-(5-methylfuran-2-yl)-1-phenyl-4,5-dihydro-1H-Pyrazole and its Application for Selective Detection of Cd2+ ion as Fluorescent Sensor

A simple fluorescent chemosensor 5-(4-methylphenyl)-3-(5-methylfuran-2-yl)-1-phenyl-4, 5-dihydro-1H-pyrazole (PY) has been synthesized for the detection of Cd²⁺ ion.The fluorescent probe PY shows high selectivity for Cd²⁺in the presence of othermetal ions (Co²⁺, Cu²⁺, Hg²⁺, Mn²⁺, Zn²⁺, Fe³⁺, Pb²⁺, Ni²⁺, and Al³⁺). The fluorescence intensity of the PY has been strongly quenched with increasing concentration of Cd²⁺ (0-0.9 μM)via photoinduced electron transfer mechanism. The binding constant of Cd²⁺ to PY for the 1:1 complex isfound to be 5.3 × 10⁵ M^-1with a detection limit of 0.09 μM. The chemosensor was successfully applied for determination of Cd²⁺ in different water samples (tap, river, and bottled water) showing good recovery values in the range of 94.8-101.7% with RSD less than 3%. Density functional theory (DFT) calculations were also performed to investigate electronic and spectral characteristics which are quite agreeable with the experimental value. The results show that the synthesized fluorescent chemosensor shows good selectivity towards Cd²⁺ and can be readily applied for the detection of Cd²⁺ in real samples including water samples.

Determination of trace cadmium in saliva samples using spray assisted droplet formation-liquid phase microextraction prior to the measurement by slotted quartz tube-flame atomic absorption spectrophotometry

BACKGROUND

An effective, green and rapid analytical strategy namely the simultaneous spray assisted droplet formation-liquid phase microextraction (S-SADF-LPME) method was developed for the determination of trace quantity of cadmium in saliva samples by using the slotted quartz tube-flame atomic absorption spectrophotometry (SQT-FAAS). By the developed method, external dispersive solvent usage for droplet formation was reduced to obtain a more environmental-friendly method.

METHODS

Method consists of a simultaneous complexing and extraction step, which was based on spraying an extraction solvent containing a solid ligand into the aqueous sample solution, forming fine droplets without the use of dispersive solvent. The procedure was implemented using a customized, cost effective and portable spray apparatus to minimize the consumption of reagent, analysis time and operation steps. Thus, this methodology ensures better repeatability and accuracy while minimizing the relative errors caused by the experimental steps. Parameters including the buffer amount, extractant/ligand concentration, extraction solvent type, extraction/ligand solution volume, spraying number and vortex period were systemically optimized to lower the detection limit.

RESULTS

Under the optimal extraction conditions, 96.9-folds enhancement in the detection power of the traditional FAAS was achieved. The limit of detection and limit of quantification values of presented method were calculated to be 0.65 and 2.17 ng mL^-1, respectively. Accuracy and applicability of the optimized method was investigated by collecting saliva samples from smokers. Satisfactory percent recovery values wereachieved for cadmium with a low standard deviation in the acceptable range of 84.9-109.6 %.

CONCLUSION

The developed dispersive solvent-free S-SADF-LPME technique presents a fast, simple, cost-effective and eco-friendly microextraction method based on the use of an easily accessible and functional spray apparatus.

Enhancement of cadmium accumulation in sweet sorghum as affected by nitrate

The Cadmium (Cd)-polluted soils are is an increasing concern worldwide. Phytoextraction of Cd pollutants by high biomass plants, such as sweet sorghum, is considered an environmentally-friendly, cost-effective and sustainable strategy for remediating this problem. Nitrogen (N) is a macronutrient essential for plant growth, development and stress resistance. Nevertheless, how nitrate, as an important form of N, affects Cd uptake, translocation and accumulation in sweet sorghum is still unclear. In the present study, a series of nitrate levels (N1, 0.5 mm; N2, 2 mm; N3, 4 mm; N4, 8 mm and N5, 16 mm) with or without added 5 μm CdCl₂ treatment in sweet sorghum was investigated hydroponically. The results indicate that Cd accumulation in the aboveground parts of sweet sorghum was enhanced by optimum nitrate supply, resulting from both increased dry weight and Cd concentration. Although root-to-shoot Cd translocation was not enhanced by increased nitrate, some Cd was transferred from cell walls to vacuoles in leaves. Intriguingly, expression levels of Cd uptake and transport genes, SbNramp1, SbNramp5 and SbHMA3, were not closely related to increased Cd as affected by nitrate supply. The expression of SbNRT1.1B in relation to nitrate transport showed an inverted 'U' shape with increasing nitrate levels under Cd stress, which was in agreement with trends in Cd concentration changes in aboveground tissues. Based on the aforementioned results, nitrate might regulate Cd uptake and accumulation through expression of SbNRT1.1B rather than SbNramp1, SbNramp5 or SbHMA3, the well-documented genes related to Cd uptake and transport in sweet sorghum.

Efficient separation of Zn(Ⅱ) from Cd(Ⅱ) in sulfate solution by mechanochemically activated serpentine

Clay minerals are widely used to treat sewage containing heavy metals such as zinc and cadmium. In this study, the chemical reactivity of natural serpentine was signally improved through mechanochemical activation, achieving the efficient separation of Zn(Ⅱ) and Cd(Ⅱ) ions in a mixed solution. The activated serpentine would release a large amount of Mg²⁺ and OH^- and thereby selectively precipitate Zn(Ⅱ) ions as an uncommon metamorphic zinc mineral, bechererite, in the presence of SO₄^2-. By adjusting the parameters including grinding intensity, reaction temperature, serpentine dosage and salt species, the optimum conditions were determined and a 92% separation rate of Zn(Ⅱ) and Cd(Ⅱ) ions was achieved. The mechanochemical activation of natural clay minerals expresses a great potential for purification of heavy metal contaminated sewage, as well as the simultaneous separation and recovery of multi-metal secondary resources.

Bacterially assembled biopolyester nanobeads for removing cadmium from water

Cadmium (Cd)-contaminated waterbodies are a worldwide concern for the environment, impacting human health. To address the need for efficient, sustainable and cost-effective remediation measures, we developed innovative Cd bioremediation agents by engineering Escherichia coli to assemble poly(3-hydroxybutyric acid) (PHB) beads densely coated with Cd-binding peptides. This was accomplished by translational fusion of Cd-binding peptides to the N- or C-terminus of a PHB synthase that catalyzes PHB synthesis and mediates assembly of Cd2 or Cd1 coated PHB beads, respectively. Cd1 beads showed greater Cd adsorption with 441 nmol Cd mg^-1 bead mass when compared to Cd2 beads (334 nmol Cd mg^-1 bead-mass) and plain beads (238 nmol Cd mg^-1 bead-mass). The Cd beads were not ecotoxic and did attenuate Cd-spiked solutions toxicity. Overall, the bioengineered beads provide a means to remediate Cd-contaminated sites, can be cost-effectively produced at large scale, and offer a biodegradable and safe alternative to synthetic ecotoxic treatments.

A colorimetric aptamer-based method for detection of cadmium using the enhanced peroxidase-like activity of Au-MoS2 nanocomposites

In this work, a colorimetric aptamer-based method for detection of cadmium using gold nanoparticles modified MoS₂ nanocomposites as enzyme mimic is established. In short, biotinylated Cd²⁺ aptamers are immobilized by biotin-avidin binding on the bottoms of the microplate, the complementary strands of Cd²⁺ aptamers are connected to the Au-MoS₂ nanocomposites which have the function of enhanced peroxidase-like activity. The csDNA-Au-MoS₂ signal probe and target Cd²⁺ compete for binding Cd²⁺ aptamer, the color change can be observed by addition of chromogenic substrate, thereby realizing visual detection of Cd²⁺. The absorbance of the solution at 450 nm has a clear linear relationship with the Cd²⁺ concentration. The linear range is 1-500 ng/mL, and the limit of detection is 0.7 ng/mL. The assay was used to test white wine samples, the results are consistent with those of atomic absorption spectrometry; which prove that this method can be used for detection of Cd²⁺ in real samples.

New insight into the removal of Cd(II) from aqueous solution by diatomite

Diatomite is an economical and environmentally friendly adsorbent, and its use has been applied widely for the treatment of water contaminated by heavy metals. Despite this, the mechanism for the removal of the heavy metal Cd(II) remains unclear. In this work, we explored the adsorption mechanism of Cd(II) by diatomite using batch experiment, and characterized the diatomite using scanning electron microscopy, energy-dispersive spectrometry, specific surface area, and pore size distribution analysis. Our results showed that, under the experimental conditions, the kinetic adsorption approached equilibrium within 5 min, and the Sips isotherm model was most suitable for data fitting. EDS characterization of the Cd-loaded diatomite indicated that Cd(II) was adsorbed onto the diatomite. Furthermore, desorption experiments showed that Ca²⁺ and Mg²⁺ in the diatomite caused an ion exchange interaction, and this was primarily responsible for Cd(II) adsorption. Moreover, we found that its contribution to the whole adsorption reaction could reach 80%, while the remainder of Cd(II) was probably trapped in the microporous structure of the diatomite. Additionally, our data indicated that the adsorption mechanism did not change significantly after regeneration. These results have provided special insight into the deep understanding of the mechanism of Cd(II) adsorption by diatomite, and could provide theoretical support and guidance for further development and application of diatomite in the treatment of Cd(II)-contaminated water. Graphical abstract.

Bacterial and Fungal Communities Are Differentially Modified by Melatonin in Agricultural Soils Under Abiotic Stress

An extensive body of evidence from the last decade has indicated that melatonin enhances plant resistance to a range of biotic and abiotic stressors. This has led to an interest in the application of melatonin in agriculture to reduce negative physiological effects from environmental stresses that affect yield and crop quality. However, there are no reports regarding the effects of melatonin on soil microbial communities under abiotic stress, despite the importance of microbes for plant root health and function. Three agricultural soils associated with different land usage histories (pasture, canola or wheat) were placed under abiotic stress by cadmium (100 or 280 mg kg-1 soil) or salt (4 or 7 g kg-1 soil) and treated with melatonin (0.2 and 4 mg kg-1 soil). Automated Ribosomal Intergenic Spacer Analysis (ARISA) was used to generate Operational Taxonomic Units (OTU) for microbial community analysis in each soil. Significant differences in richness (α diversity) and community structures (β diversity) were observed between bacterial and fungal assemblages across all three soils, demonstrating the effect of melatonin on soil microbial communities under abiotic stress. The analysis also indicated that the microbial response to melatonin is governed by the type of soil and history. The effects of melatonin on soil microbes need to be regarded in potential future agricultural applications.

Effects of extracellular polymeric substances of Pseudomonas fluorescens, citrate, and oxalate on Pb sorption by an acidic Ultisol

The continuous production of low molecular weight (LMW) organic acids by plants and microorganisms coupled with the continuous presence of extracellular polymeric substances (EPS) in soils is a guarantee that the mobility of heavy metals in soils will be controlled. The effects of citrate, oxalate, and EPS on the adsorption of Pb by an acidic Ultisol were studied both as a function of pH and ionic strength. Electrokinetic potential measurements were also employed to observe to what extent each ligand affected the surface charge property of the Ultisol. All the ligands shifted the zeta potential of the Ultisol to the negative direction, implying that the surface charge of the soil became more negative. The effect on the zeta potential of the soil was observed in the order of oxalate ˃ citrate ˃ EPS. The quantity of Pb adsorbed at each pH (3.0-7.0) reflected the corresponding change in the zeta potential as induced by each ligand. The presence of the ligands shifted the isoelectric point of the Ultisol from 4.8 to 3.2 for the EPS system and below 3.0 for the citrate and oxalate systems. More Pb was adsorbed in the presence of oxalate than in the presence of citrate and EPS. The two most outstanding mechanisms that governed the adsorption of Pb by the Ultisol were (1) electrostatic attraction which was supported by the increase in negative zeta potential of the Ultisol and, (2) complexation which was supported by the lesser proportion of Pb adsorbed in the citrate system at higher pH and also by the spectroscopic data for EPS. The combination EPS + citrate + oxalate was more effective in enhancing the adsorption of Pb than the combination EPS + oxalate and EPS + citrate.

Low Molecular Weight Fluorescent Probes (LMFPs) to Detect the Group 12 Metal Triad

Fluorescence sensing, of d-block elements such as Cu2+, Fe3+, Fe2+, Cd2+, Hg2+, and Zn2+ has significantly increased since the beginning of the 21st century. These particular metal ions play essential roles in biological, industrial, and environmental applications, therefore, there has been a drive to measure, detect, and remediate these metal ions. We have chosen to highlight the low molecular weight fluorescent probes (LMFPs) that undergo an optical response upon coordination with the group 12 triad (Zn2+, Cd2+, and Hg2+), as these metals have similar chemical characteristics but behave differently in the environment.

Mechanism of Cu(II) and Cd(II) immobilization by extracellular polymeric substances (Escherichia coli) on variable charge soils

Extracellular polymeric substances (EPS) found in soils can reduce the mobility of heavy metals through the use of both electrostatic and non-electrostatic mechanisms. Their effects vary from one soil type to another. The influence of EPS from Escherichia coli on the adsorption behaviors of Cu(II) and Cd(II) by two bulk variable charge soils, Oxisol and Ultisol, was studied at constant and varied pH, and the results were compared to a constant charge Alfisol. The maximum adsorption capacities of the soils were significantly (P < 0.05) enhanced in the presence of EPS, with Cu(II) adsorption being greater. Interaction of EPS with soils made the soil surface charge more negative by neutralizing positive charges and shifting the zeta potentials in a negative direction: from -18.6 to -26.4 mV for Alfisol, +5.1 to -22.2 mV for Oxisol, and +0.3 to -28.0 mV for Ultisol at pH 5.0. The adsorption data fitted both the Freundlich and Langmuir isotherms well. Preadsorbed Cd(II) was more easily desorbed by KNO₃ than preadsorbed Cu(II) from both the control and EPS treated soils. The adsorption of both metals was governed by electrostatic and non-electrostatic mechanisms, although more Cu(II) was adsorbed through the non-electrostatic mechanism. The information obtained in this study will improve our understanding of the mechanisms involved in reducing heavy metals mobility in variable charge soils and hence, their bioavailability.

Kinetics and Adsorption Studies of Mercury and Lead by Ceria Nanoparticles Entrapped in Tamarind Powder

In this study, novel adsorbent ceria nanoparticles (CeNPs) entrapped in tamarind powder (Tm@CeNPs) were efficiently utilized for the simultaneous adsorption of aqueous mercury [Hg(II)] and aqueous lead [Pb(II)]. Surface interactions between the adsorbent and heavy metal ions play an important role in the adsorption process, and the surface morphology can significantly improve the adsorption capacity of the adsorbent. The Langmuir adsorption capacity of Tm@CeNPs for Hg(II) and Pb(II) was found to be 200 and 142.85 mg/g, respectively. The surface area of utilized adsorbent was found to be very high, that is, 412 m2/g. The adsorption kinetics of Tm@CeNPs for both ions follow pseudo-second-order, and the adsorption process is also thermodynamically feasible. Column study favors multilayer adsorption of the heavy metal ion. The spectral analysis of the adsorbent revealed that hydroxyl, carboxylic, and ester groups, as well as CeNPs, are responsible for Hg(II) and Pb(II) adsorption. The cost-benefit analysis confirms the economic viability of the synthesized Tm@CeNPs composite for heavy metal removal. The adsorbent is best suited for Hg(II) adsorption as compared to Pb(II). This is a novel study on the utilization of tamarind leaf powder with CeNPs for heavy metal ion adsorption and its adsorption mechanism, which has not been reported to date.

Cadmium removal from simulated groundwater using alumina nanoparticles: behaviors and mechanisms

Cadmium (Cd), one of the most toxic contaminants in groundwater, can cause a severe threat to human health and ecological systems. In this study, alumina nanoparticles were synthesized and tested for high-efficiency Cd removal from simulated groundwater. Furthermore, the synthesized alumina nanoparticles were successfully modified using negatively charged glycerol, to alleviate the challenge of its low mobility in groundwater for the Cd removal. The maximum removal efficiency of both synthesized and glycerol-modified alumina nanoparticles were more than 99%. The sorption isotherm and kinetic data of both synthesized and glycerol-modified alumina nanoparticles were best fitted to the Freundlich model and the pseudo-second-order model, respectively, indicating that the sorption of Cd ions occurs on heterogeneous surfaces of both alumina nanoparticles via the chemisorption mechanism. X-ray photoelectron spectroscopy and energy dispersive X-ray analysis revealed the presence of Cd peak in both sorbents after contact with Cd. In addition, the FTIR analyses demonstrated that hydroxyl group participated in the sorption of Cd on both synthesized and glycerol-modified alumina nanoparticles, while other glycerol associated groups contributed to the removal of Cd ions by the glycerol-modified alumina nanoparticles. It was concluded that Cd removal by synthesized and glycerol-modified alumina nanoparticles were mainly due to ion exchange and electrostatic attraction, respectively. Desorption experiment suggested that both alumina nanoparticles are effective and practically significant sorbents to remediate Cd from contaminated groundwater. However, the stronger bond between Cd and glycerol-modified alumina, plus its potential of higher mobility due to the negative charge on the surface, warrant glycerol-modified alumina nanoparticles a better performance in remediating Cd contaminated groundwater than that of the synthesized alumina nanoparticles.

Biosorption of Cadmium by Non-Toxic Extracellular Polymeric Substances (EPS) Synthesized by Bacteria from Marine Intertidal Biofilms

Cadmium is a major heavy metal found in polluted aquatic environments, mainly derived from industrial production processes. We evaluated the biosorption of solubilized Cd²⁺ using the extracellular polymeric substances (EPS) produced by Bacillus sp. MC3B-22 and Microbacterium sp. MC3B-10 (Microbactan); these bacteria were originally isolated from intertidal biofilms off the coast of Campeche, Mexico. EPS were incubated with different concentrations of cadmium in ultrapure water. Residual Cd²⁺ concentrations were determined by Inductive Coupled Plasma-Optic Emission Spectrometry and the maximum sorption capacity (Qmax) was calculated according to the Langmuir model. EPS were characterized by X-ray photoelectron spectroscopy (XPS) before and after sorption. The Qmax of Cd²⁺ was 97 mg g⁻¹ for Microbactan and 141 mg g⁻¹ for MC3B-22 EPS, these adsorption levels being significantly higher than previously reported for other microbial EPS. In addition, XPS analysis revealed changes in structure of EPS after biosorption and showed that amino functional groups contributed to the binding of Cd²⁺, unlike other studies that show the carbohydrate fraction is responsible for this activity. This work expands the current view of bacterial species capable of synthesizing EPS with biosorbent potential for cadmium and provides evidence that different chemical moieties, other than carbohydrates, participate in this process.

A novel fluorescent peptidyl probe for highly sensitive and selective ratiometric detection of Cd(ii) in aqueous and bio-samplesviametal ion-mediated self-assembly

A fluorescent peptidyl probe based on a Cd(ii)-triggered self-assembling process was proposed for ratiometric detection for Cd(ii) in urine and live cells.

Integrated ion imprinted polymers-paper composites for selective and sensitive detection of Cd(II) ions

Paper-based sensor is a new alternative technology to develop a portable, low-cost, and rapid analysis system in environmental chemistry. In this study, ion imprinted polymers (IIPs) using cadmium ions as the template were directly grafted on the surface of low-cost print paper based on the reversible addition-fragmentation chain transfer polymerization. It can be applied as a recognition element to selectively capture the target ions in the complex samples. The maximum adsorption capacity of IIPs composites was 155.2mgg^-1 and the imprinted factor was more than 3.0. Then, IIPs-paper platform could be also applied as a detection element for highly selective and sensitive detection of Cd(II) ions without complex sample pretreatment and expensive instrument, due to the selective recognition, formation of dithizone-cadmium complexes and light transmission ability. Under the optimized condition, the linear range was changed from 1 to 100ngmL^-1 and the limit of detection was 0.4ngmL^-1. The results were in good agreement with the classic ICP-MS method. Furthermore, the proposed method can also be developed for detection of other heavy metals by designing of new IIPs.

Effective remediation of low-concentration cadmium in groundwater using nano-scale magnesia

Cadmium (Cd), one of the hazardous elements in groundwater, is a severe threat to human health and ecological systems even at low concentrations. This study explores the effectiveness of commercial and self-synthesized nano-scale magnesia (NMgO) for remediating low-concentration Cd in groundwater as well as their associated removal mechanisms. The sorption kinetic data for both NMgOs were well fitted to the pseudo-second-order model and the calculated q _e values matched the experimental q _e values for both commercial and self-synthesized NMgOs. The sorption equilibrium data for both NMgOs were well fitted to the Langmuir isotherm model, with the maximum Cd sorption capacity (q _e) of 19.25 and 16.54 mg/g at an initial concentration range of 5-200 μg/L and a temperature of 25 °C, for commercial and self-synthesized NMgOs, respectively. The combined sorption kinetics and equilibrium data suggest that the sorption onto both NMgOs follows a monolayer chemisorption. The scanning electron microscope-energy dispersive X-ray (SEM-EDX), Fourier transform infrared spectroscopy (FTIR), and X-ray diffractometer (XRD) analyses show that the chemisorption of Cd onto commercial NMgO is due to the formation of Cd(OH)₂, which precipitates on the sorbent surface. For the self-synthesized NMgO, it was demonstrated that the hydroxyl group plays a role in the chemisorption process and the amount of Cd sorbed on the sorbent was quantified. The results of batch experiments showed that both NMgOs removed Cd effectively, obtaining a removal efficiency of more than 99%, under different experimental conditions of pH, sorbent dosage, co-existing ions, and simulated groundwater. Results from both the sorption isotherm and desorption experiments indicated strong bonding between Cd and both NMgOs, suggesting that NMgOs are safe, effective, and practical sorbents to remediate Cd in groundwater.

Nanomembrane Canister Architectures for the Visualization and Filtration of Oxyanion Toxins with One-Step Processing

Nanomembrane canister-like architectures were fabricated by using hexagonal mesocylinder-shaped aluminosilica nanotubes (MNTs)-porous anodic alumina (PAA) hybrid nanochannels. The engineering pattern of the MNTs inside a 60 μm-long membrane channel enabled the creation of unique canister-like channel necks and cavities. The open-tubular canister architecture design provides controllable, reproducible, and one-step processing patterns of visual detection and rejection/permeation of oxyanion toxins such as selenite (SeO3(2-)) in aquatic environments (i.e., in ground and river water sources) in the Ibaraki Prefecture of Japan. The decoration of organic ligand moieties such as omega chrome black blue (OCG) into inorganic Al2O3@tubular SiO2/Al2O3 canister membrane channel cavities led to the fabrication of an optical nanomembrane sensor (ONS). The OCG ligand was not leached from the canister as observed in washing, sensing, and recovery assays of selenite anions in solution, which enabled its multiple reuse. The ONS makes a variety of alternate processing analyses of selective quantification, visual detection, rejection/permeation, and recovery of toxic selenite quick and simple without using complex instrumentation. Under optimal conditions, the ONS canister exhibited a high selectivity toward selenite anions relative to other ions and a low-level detection limit of 0.0093 μM. Real analytical data showed that approximately 96% of SeO3(2-) anions can be recovered from aquatic and wastewater samples. The ONS canister holds potential for field recovery applications of toxic selenite anions from water.

One-pot layer casting-guided synthesis of nanospherical aluminosilica@organosilica@alumina core–shells wrapping colorant dendrites for environmental application

Wrapping of dendritic colorant aggregates around core–double shell cavities afforded a container vehicle tracking architecture for recovering toxins in environments.

Modeling of spherical silver nanoparticles in silicone-based nanocomposites for marine antifouling

A non-toxic foul-release model of silicone/spherical AgNP hybrid nanocomposites with enhanced hydrophobicity, self-cleaning, and marine fouling release performance.