Application of functionalized layered double hydroxides for heavy metal removal: A review

Long-term soil remediation using layered double hydroxides: Field evidence for simultaneous immobilization of both cations and oxyanions

Layered double hydroxides (LDHs) have great potential for immobilizing potentially toxic elements in soil. Nevertheless, their practical effectiveness under field conditions remains largely unknown. In this study, we conducted a 2.5-year field trial using pristine Mg-Al LDHs, Ca-Al LDHs, and iron (Fe)-modified LDHs to simultaneously immobilize both oxyanions (including As and Sb) and cations (including Cd and Pb) in historically contaminated soil affected by mining activities since the 1950s. The immobilization performance of LDHs was examined using various batch tests, including water and DTPA extraction, and by measuring metal(loid) concentrations in Coriandrum sativum (coriander). We found that both pristine and Fe-modified LDHs showed promising initial immobilization performance 7 days after application, achieving significant reductions in DTPA-extractable concentrations of As, Sb, Cd, and Pb by 45.6%-68.3%, 55.4%-94.2%, 11.2%-50.9%, and 62.9%-64.9%, respectively, compared to the control soil without amendment. Notably, pristine LDHs showed diminished immobilization performance in the long term, while Fe-modified LDHs exhibited long-term stability over 2.5 years. A conditional probability-based model was used to depict long-term metal(loid) leaching characteristics in LDH-amended soils. Temporal changes in metal(loid) concentrations in the aboveground edible parts (namely, stems and leaves) of coriander corroborated well with DTPA extraction results. Coriander grown in Fe-modified LDH-amended soils had much lower metal(loid) concentrations compared to those grown in pristine LDH-amended soils. As a result, reductions of 35.1%-42.2% for As, 54.4%-66.2% for Sb, 8.5%-22.8% for Cd, and 56.0%-62.7% for Pb concentrations in coriander were still observed 2.5 years after soil amendment with Fe-modified LDHs. To the best of our knowledge, this is the first field-based evidence using LDHs to simultaneously stabilize both cations and oxyanions in soil. The findings support the potential of LDHs for long-term immobilization of metal(loid)s in soil.

Synthesis of magnetic multi-walled carbon nanotubes with layered double hydroxide (M-MWCNTs@MnAl-LDH) nanocomposite as an adsorbent for lead extraction

MnAl layered double hydroxide hybrid with magnetic-multiwalled carbon nanotubes was synthesized by a hydrothermal method and used for the extraction of Pb(II) (lead) from spices and water samples in the dispersive solid phase microextraction (dSPμE) technique using FAAS. The as-prepared adsorbent MMWCNTs@MnAl-LDH was characterized by XRD, FTIR, EDX, and SEM techniques. Various analytical parameters were optimized, including pH 8, adsorbent dosage of 5 mg, HNO3 eluent concentration of 1 mol L-1, eluent volume of 3 mL, eluent time of 60 s, and sample volume of 20 mL, for quantitative lead recoveries, with an LOD of 0.314 μg L-1, an LOQ of 1.048 μg L-1, and PF of 11.53. Under the optimized conditions, the linearity ranges from 0.5 to 500 μg L-1 (R2 = 0.9997). For the validation test of the established dSPμE procedure, Certified reference materials (CRMs) were used, yielding satisfactory recovery results ranging from 97.8 to 102.7 %. The method was applied to determine lead in turmeric, tap water, and industrial water samples.

Memory Effect of Double Oxides Compared to Simple Ion Exchange for Controlled Fluoride Ion Capture and Release

A layered double hydroxide (LDH) containing Mg and Al was synthesized from a nitrate solution using a coprecipitation method. The resulting material exhibited a homogeneous structure, which, upon calcination at 450 °C, was converted into a layered double oxide (LDO). When rehydrated in a fluoride-containing aqueous solution, the original hydroxide structure was successfully regenerated, demonstrating the LDH's memory effect. During this transformation, fluoride anions from the solution were incorporated into the interlayer galleries to maintain electroneutrality, as confirmed by energy-dispersive X-ray spectroscopy (EDS) analysis. Separately, the process was tested in the presence of ethanol, which significantly enhanced the incorporation of fluoride ions into the interlayer spaces. The material's potential for controlled fluoride release was evaluated by monitoring its release into demineralized water. For comparison, a simple ion-exchange process was carried out using the as-synthesized MgAl LDH. The memory effect mechanism displayed a notably higher fluoride incorporation capacity compared to the ion-exchange process. Among all the specimens, the sample reconstructed in the presence of ethanol exhibited the highest fluoride ion content. Fluoride release studies revealed a two-phase pattern: an initial rapid release within the first three hours, followed by a substantially slower release over time.

Enhanced Cr(vi) removal by Co and PPy co-modified Ca-Al-layered double hydroxides due to adsorption and reduction mechanisms

Co and polypyrrole co-modified hierarchical CaAl-LDH microspheres (CCALP) were synthesized via hydrothermal and in situ polymerization methods. The synergistic effect of PPy and Co endowed CCALP with higher surface area and more reduction sites than CaAl-LDHs modified by Co or PPy alone, maintaining good recyclability for Cr(vi) removal efficiency over four cycles without any treatment. Compared to Co, PPy doping was the dominant reason for Cr(vi) reduction on CCALP. Under optimized conditions, the theoretical maximum adsorption capacity reached 845.25 mg g-1, and the removal efficiency of Cr(vi) achieved 98.83%. The Langmuir model fitted well with the Cr(vi) adsorption on CCALP, supporting the monolayer adsorption hypothesis. The adsorption process followed the Avrami fractional kinetics (AFO) model, suggesting complex and multiple kinetic stages. Thermodynamic experiments confirmed that the adsorption was a spontaneous exothermic process. The density functional theory (DFT) and electrostatic potential (ESP) calculations confirmed that the oxygen-containing parts of Cr2O7 2- and HCrO4 - were the affinity sites, and the co-doping of Co and PPy significantly improved the Cr(vi) adsorption energy on CCALP. Therefore, the Cr(vi) removal mechanism on CCALP was proposed with electrostatic interaction, ion exchange, complexation and reduction.

Interactions between antibiotic removal, water matrix characteristics and layered double hydroxide sorbent material

Sorption by layered double hydroxides (LDH) is gaining substantial interest for remediating emerging contaminants, including pharmaceuticals from wastewaters. Findings from a sorbent material performing successfully in lab-based studies using non-environmental (laboratory-sourced) water cannot be assumed to translate to equal performance under environmental downstream applications. However, studies evaluating sorbent material performance for removal of pollutants and understanding material interactions with environmental waters are limited. This study evaluates the removal of the antibiotic amoxicillin (AMX) using a Mg2Al-NO3-LDH sorbent material from laboratory-grade water and wastewater effluent (WWE). AMX is successfully removed (94.53 ± 4.30 % within 24 h) in laboratory-grade water (under batch sorption conditions: 100 μg/L AMX, 0.2 g/L LDH, 20 °C). The comparison of LDH removal performance in laboratory grade and WWE shows a decreased maximum removal of AMX in WWE (13.39 ± 5.53 %). A lower final AMX concentration is observed in the WWE without the presence of LDH, compared to the 'removal' experiments in WWE with the presence of LDH, indicating a contribution of non-sorption removal pathways of AMX. This is proposed to be due to the difference in metal concentrations in the WWE with and without LDH present. The presence of LDH is found to decrease concentrations of metal pollutants in WWE, such as Zn concentration decreasing by 85 % over 24 h, changing water characteristics. Overall, this paper reports that an LDH performs differently in laboratory-sourced water and a wastewater effluent. This provides evidence that sorbent material performance needs to be evaluated in complex water matrices to ensure that it is representative of how a sorbent material will perform in an environmental application, which is the end goal of developing such technologies. Finally, good practice recommendations are provided for future lab-scale sorption experiments evaluating the performance of any new sorbent materials for water treatment applications.

Enhancing Analytical Potential for Ultratrace Analysis of Inorganic Oxyanions Using Extraction Procedures with Layered Double Hydroxides

This article provides an overview of the use of layered double hydroxides (LDHs) as effective sorbents in various extraction methods, including column-based solid-phase extraction (SPE), dispersive solid-phase extraction (DSPE), and magnetic solid-phase extraction (MSPE), for the separation and preconcentration of inorganic oxyanions of chromium, arsenic, and selenium. The primary focus is on enhancing the analytical performance of spectrometric detection techniques, particularly in terms of sensitivity and selectivity when analyzing low concentrations of target analytes in complex matrices. LDHs, which can be readily prepared and structurally modified with various substances, offer promising potential for the development of novel analytical methods. When used in analytical extraction procedures and following careful optimization of experimental conditions, the developed methods have yielded satisfactory results, as documented by studies reviewed in this paper. This review is intended to assist analytical chemists in scientific laboratories involved in developing new extraction procedures.

Non-phytoremediation and phytoremediation technologies of integrated remediation for water and soil heavy metal pollution: A comprehensive review

Since the 1980s, there has been increasing concern over heavy metal pollution remediation. However, most research focused on the individual remediation technologies for heavy metal pollutants in either soil or water. Considering the potential migration of these pollutants, it is necessary to explore effective integrated remediation technologies for soil and water heavy metals. This review thoroughly examines non-phytoremediation technologies likes physical, chemical, and microbial remediation, as well as green remediation approaches involving terrestrial and aquatic phytoremediation. Non-phytoremediation technologies suffer from disadvantages like high costs, secondary pollution risks, and susceptibility to environmental factors. Conversely, phytoremediation technologies have gained significant attention due to their sustainable and environmentally friendly nature. Enhancements through chelating agents, biochar, microorganisms, and genetic engineering have demonstrated improved phytoremediation remediation efficiency. However, it is essential to address the environmental and ecological risks that may arise from the prolonged utilization of these materials and technologies. Lastly, this paper presents an overview of integrated remediation approaches for addressing heavy metal contamination in groundwater-soil-surface water systems and discusses the reasons for the research gaps and future directions. This paper offers valuable insights for comprehensive solutions to heavy metal pollution in water and soil, promoting integrated remediation and sustainable development.

Nanostructured layered double hydroxide (NLDH) - Zn/Al-based materials: strategy to improve performance for zirconium sorption from acidic sulfate solution

Zirconium is a highly stable radionuclide commonly used in various nuclear operations. However, removing zirconium from wastewater streams is crucial to protect the environment and human health. To achieve this, a zinc and aluminum nanostructured layered double hydroxide (Zn/Al-NLDH) was prepared and investigated for effective removal of zirconium from aqueous solutions. This study examined the prepared Zn/Al-NLDH's structural and textural properties and the impact of various factors on its adsorption performance. The Langmuir isotherm and Pseudo-second order kinetic models were found to be the best fit for the adsorption process of Zr(vi). This suggests that the adsorption process is uniform, involves the formation of a monolayer, and is chemisorption in nature. The maximum uptake capacity was 117.6 mg g-1, and the process was endothermic, spontaneous, and feasible. About 96% of Zr(vi) was successfully desorbed from the loaded sorbent using 1.0 M hydrochloric acid, and the Zn/Al-NLDH sorbent remained stable for six consecutive sorption/desorption cycles. These findings emphasize the high potential of Zn/Al-NLDH to act as a remarkable sorbent for efficiently tackling water contaminants.

Highly selective and efficient Pb2+ capture using PO4-loaded 3D-NiFe layer double hydroxides derived from MIL-88A

Lead (Pb) contamination in water requires improved decontamination technologies. The addition of phosphate to precipitate Pb2+ is a widely used method for remediating Pb in soil and water, though it has certain limitations. This study focuses on novel 3D mesoporous layered double hydroxide (LDH) sorbents functionalized with phosphate anions for Pb2+ removal from contaminated waters. Our innovative strategy involves converting a sacrificial template metal-organic frameworks (MOFs) structure (MIL-88A(Fe)) into NixFe LDH, followed by an anion exchange reaction with phosphate anions. This process preserves the 3D microrod architecture of MIL-88A and prevents deleterious LDH particle aggregation. The synthesis results in stable microrod crystals, 1-2 μm long, composed of 3D assemblies of NixFe-PO4 LDH nanoplatelets with a specific surface area exceeding 110 m2/g. The novel LDH materials display fast adsorption kinetics (pseudo-second order model) and remarkably high Pb2+ removal performances (Langmuir isotherm model) with a capacity of 538 mg/g, surpassing other reported adsorbents. LDH-PO4 exhibits high selectivity for Pb2+ over competing ions like Ni2+ and Cd2+ (selectivity order is: Pb2+ > Ni2+ > Cd2+). Removal of Pb2+ from NixFeLDH/88A-PO4 involves various mechanisms, including surface complexation and surface precipitation of lead phosphate or lead hydroxide phases as revealed by structural characterization techniques.

Innovative recovery of matrix layered double hydroxide from simulated acid mine wastewater for the removal of copper and cadmium from wastewater

This study innovatively added biochar to optimize regulation in the neutralization process of simulated acid mine drainage (AMD) and recovered a new type of matrix layered double hydroxides (MLDH), which can be used to remove copper (Cu(II)) and cadmium (Cd(II)) from wastewater. A series of batch experiments show that MLDH with strong selective removal ability of Cu(II) and Cd(II) can be successfully obtained by adding biochar (BC) at pH = 5 end in the neutralization process. Kinetic and isotherm modeling studies indicated that the removal of Cu(II) and Cd(II) by the MLDH was a chemical multilayer adsorption process. The removal mechanism of Cu(II) and Cd(II) was further analyzed through related characterization analysis with contribution rate calculation: the removal rates of Cu(II) and Cd(II) by ion exchange were 42.7% and 26%, while that by precipitation were 34.5% and 49.9%, respectively. This study can provide a theoretical reference and experimental basis for the recovery and utilization of valuable by-products in AMD and the treatment of heavy metal wastewater.

Elimination of copper obstacle factor in anaerobic digestion effluent for value-added utilization: Performance and resistance mechanisms of indigenous bacterial consortium

The presence of excessive residual Cu(II), a high-risk heavy metal with potential toxicity and biomagnification property, substantially impede the value-added utilization of anaerobic digestion effluent (ADE). This study adapted indigenous bacterial consortium (IBCs) to eliminate Cu(II) from ADE, and their performances and resistance mechanisms against Cu(II) were analyzed. Results demonstrated that when the Cu(II) exposure concentration exceeded 7.5 mg/L, the biomass of IBCs decreased significantly, cells produced a substantial amount of ROS and EPS, at which time the intracellular Cu(II) content gradually decreased, while Cu(II) accumulation within the EPS substantially increased. The combined features of a high PN/PS ratio, a reversed Zeta potential gradient, and abundant functional groups within EPS collectively render EPS a primary diffusion barrier against Cu(II) toxicity. Mutual physiological and metagenomics analyses reveal that EPS synthesis and secretion, efflux, DNA repair along with coordination between each other were the primary resistance mechanisms of IBCs against Cu(II) toxicity. Furthermore, IBCs exhibited enhanced resistance by enriching bacteria carrying relevant resistance genes. Continuous pretreatment of actual ADE with IBCs at a 10-day hydraulic retention time (HRT) efficiently eliminated Cu(II) concentration from 5.01 mg/L to ∼0.68 mg/L by day 2. This elimination remained stable for the following 8 days of operation, further validated their good Cu(II) elimination stability. Notably, supplementing IBCs with 200 mg/L polymerized ferrous sulfate significantly enhanced their settling performance. By elucidating the intricate interplay of Cu(II) toxicity and IBC resistance mechanisms, this study provides a theoretical foundation for eliminating heavy metal barriers in ADE treatment.

Sorption of cadmium by layered double hydroxides: Performance, structure-related mechanisms, and sequestration stability assessment

Layered double hydroxides (LDHs) have been recognized to have great potential for the treatment of heavy metals in wastewater and soil through various mechanisms. Isomorphic substitution is an important mechanism for the sorption of heavy metal cations with LDH reconstruction and highly stable product formation. However, sorption performance, structure-related relationships, and, more importantly, stability are still poorly understood. In this study, a series of LDHs with different structures were synthesized to evaluate their cadmium (Cd) sorption performance and stability concerning the isomorphic substitution mechanism. Divalent cation types in the LDH lattice determined the Cd sorption capacity as well as the isomorphic substitution possibility, following the order of hydroxide solubility of divalent cations (MII): Ca2+>Mg2+>(Cd2+) > Ni2+>Zn2+. In addition, CaAl-LDH exhibited a super-high Cd sorption capacity of 625.0 mg g-1. Cd sorption by LDHs with different interlayer anion types and divalent/trivalent cation molar ratios varied due to crystallite size-related MII release through cation-exchange/isomorphic substitution. Coexisting cations (e.g., Zn2+, Ni2+, Mg2+) influence the sorption performance of MII-LDH mainly through isomorphic substitution mechanism, largely depending on the solubility of MII(OH)2 with a trend of stable product formation. Furthermore, Mg2.9Cd0.1AlCl-LDH was fabricated, and limited Cd dissolution without destruction of the LDH structure was observed under various conditions. For example, only 7.69%, 2.16% and 0.96% of Cd was released from as-prepared Mg2.9Cd0.1AlCl-LDH in NaCl solution (0.02 mol L-1, pH 5), soil extract, and soil matrix, respectively. The very low leaching of Cd from Cd-containing LDHs indicated the high stability of LDH-sorbed Cd via isomorphic substitution and feasible practical application in Cd sequestration in wastewater treatment and soil remediation.

Controlled release of herbicides through glyphosate intercalated layered double hydroxides and enhancement of anti-scouring ability via poly-l-aspartic acid and chitosan modification

Nanocarrier preparations could effectively improve the utilization rate of pesticides, and reduce pesticide loss. In this study, glyphosate (GLY)-loaded MgAl layered double hydroxide (GLY@LDH) was synthesized via an in-situ method. Subsequently, GLY@LDH composite samples were prepared using a layer-by-layer self-assembly approach and modified with poly-L-aspartic acid (PASP) and chitosan (CS). XRD, FT-IR, SEM, and Zeta potential characterization confirmed that GLY was successfully loaded in the interlayer of LDHs and PASP/CS were successfully encapsulated on the surface of the composite sample. The release effect in different ionic solutions and soils was studied and analyzed. The release behavior conforms to the Ritger-Peppas kinetic model, and the release mechanism was ion exchange, which was further explored by means of XRD, SEM, and molecular simulation. The results of the anti-scouring experiment and contact angle measurement indicated that the layered self-assembly material enhanced the washing resistance of the material. The practical application effect of the sample was verified through a pot experiment. This study provides new insights into the simple preparation of pesticide-controlled release formulations that reduce leaching losses.

Unveiling glutamic acid-functionalized LDHs: understanding the Cr(VI) removal mechanism from microscopic and macroscopic view points

Interlayer functionalization modulation is essential for modifying LDHs and improving their selectivity and adsorption capacity for target pollutants. In this work, Glu@NiFe-LDH was synthesized using a simple one-step hydrothermal method and tested for its ability to remove CrO42- from wastewater. The modification significantly increased the composite material's removal ability by 2-3 times, up to 98.36 mg g-1. The behavior of CrO42- adsorption on Glu@NiFe-LDH was further studied by adjusting the affecting factors (i.e., temperature, pH, contact time, initial concentration, and interfering substance), and the adsorption behavior was confirmed as a spontaneous and chemisorption process. And the result was that Glu@NiFe-LDH demonstrated high capacity, efficiency, stability, and selectivity for the adsorption of CrO42- in a single electrolyte and natural water containing competing anions. Furthermore, molecular dynamics simulations (NVT ensemble) were employed to further reveal the mechanism of glutamic acid modification on LDH at the microscopic scale. Additionally, the IRI analysis method revealed the mechanism of weak interaction between glutamic acid molecules and CrO42-. This study provides a detailed understanding of the intercalation mechanism involved in the amino acid modification of LDHs. It explains the adsorption mechanism of metal oxo-acid radicals by amino acid-modified LDHs from a theoretical perspective. The findings offer experiments and a theoretical basis for designing targeted adsorbents in the future.

Biological and green remediation of heavy metal contaminated water and soils: A state-of-the-art review

Contamination of the natural ecosystem by heavy metals, organic pollutants, and hazardous waste severely impacts on health and survival of humans, animals, plants, and microorganisms. Diverse chemical and physical treatments are employed in many countries, however, the acceptance of these treatments are usually poor because of taking longer time, high cost, and ineffectiveness in contaminated areas with a very high level of metal contents. Bioremediation is an eco-friendly and efficient method of reclaiming contaminated soils and waters with heavy metals through biological mechanisms using potential microorganisms and plant species. Considering the high efficacy, low cost, and abundant availability of biological materials, particularly bacteria, algae, yeasts, and fungi, either in natural or genetically engineered (GE) form, bioremediation is receiving high attention for heavy metal removal. This report comprehensively reviews and critically discusses the biological and green remediation tactics, contemporary technological advances, and their principal applications either in-situ or ex-situ for the remediation of heavy metal contamination in soil and water. A modified PRISMA review protocol is adapted to critically assess the existing research gaps in heavy metals remediation using green and biological drivers. This study pioneers a schematic illustration of the underlying mechanisms of heavy metal bioremediation. Precisely, it pinpoints the research bottleneck during its real-world application as a low-cost and sustainable technology.