Layered Double Hydroxides: Potential Release-on-Demand Fertilizers for Plant Zinc Nutrition

Ageing influences the toxicity of two innovative nanofertilizers to the soil invertebrates Enchytraeus crypticus and Folsomia candida

The increasing global food demand is threatening the sustainability of agrifood production systems. The intensification of agricultural practices, with inadequate use of pesticides and fertilizers, poses major challenges to the good functioning of agroecosystems and drastically degrades the soil quality. Nanotechnology is expected to optimize the current farming practices and mitigate some associated impacts. Layered double hydroxides (LDHs) are a class of nanomaterials with high potential for use in agricultural productions, mostly due to their sustained release of nutrients. Considering its novelty and lack of studies on the terrestrial ecosystem, it is essential to assess potential long-term harmful consequences to non-target organisms. Our study aimed to evaluate the effect of Zn-Al-NO3 LDH and Mg-Al-NO3 LDH ageing on the survival and reproduction of two soil invertebrate species Enchytraeus crypticus and Folsomia candida. We postulated that the toxicity of nanomaterials to soil invertebrates would change with time, such that the ageing of soil amendments would mediate their impacts on both species. Our results showed that the toxicity of LDHs was species-dependent, with Zn-Al-NO3 LDH being more toxic to E. crypticus, while Mg-Al-NO3 LDH affected more F. candida, especially in the last ageing period, where reproduction was the most sensitive biological parameter. The toxicity of both nanomaterials increased with ageing time, as shown by the decrease of the EC50 values over time. The influence of LDH dissolution and availability of Zn and Mg in the soil pore water was the main factor related to the toxicity, although we cannot rule out the influence of other structural constituents of LDHs (e.g., nitrates and aluminium). This study supports the importance of incorporating ageing in the ecotoxicity testing of nanomaterials, considering their slow release, as effects on soil organisms can change and lead to more severe impacts on the ecosystem functioning.

Soil habitat function after innovative nanoagriproducts application: Effect of ageing on the avoidance behaviour of the soil invertebrates Enchytraeus crypticus and Folsomia candida

Research on nanotechnology with applications in agriculture has been gathering attention because it may achieve a good balance between agricultural production and environmental integrity. Among the vast nanomaterials, layered double hydroxides (LDHs) are a promising solution for supplying crops with macro- and/or micronutrients. Still, little is known about their safety implications for non-target organisms, such as soil invertebrates. The habitat function of soils might be impacted by potential stressors, which can be assessed through avoidance behaviour tests. This study aimed to assess the effect of two innovative agriproducts, Zn-Al-NO3 LDH and Mg-Al-NO3 LDH, on the avoidance behaviour of the enchytraeid Enchytraeus crypticus and the collembolan Folsomia candida, over time. Simultaneously, Zn and Mg potential release from LDHs to soil was evaluated. Overall, the behaviour of soil invertebrates differed between species, with enchytraeids being more sensitive to LDHs-treated soils than collembolans, possibly explained by their different physiological traits. The behaviour of soil organisms also depended on the LDH structural composition and was time-variable. Soil treated with Zn-Al-NO3 LDH was perceived as less favourable compared to Mg-Al-NO3 LDH, which was preferred to clean soil at most tested concentrations. LDHs toxicity was partly, but not exclusively, related to Zn and Mg release. Cations release over time was demonstrated in the chemical assessment. Still, LDHs toxicity to soil invertebrates decreased as increasing AC50 values were derived over time. Slower dissolution over time might explain the decrease in toxicity. Our study demonstrates that both soil invertebrates could sense LDHs in soil and eventually adapt their behaviour by avoiding or preferring, according to the type and level of LDH present.

Layered Double Hydroxides for Regulating Phosphate in Water to Achieve Long-Term Nutritional Management

Hunger and undernourishment are increasing global challenges as the world's population continuously grows. Consequently, boosting productivity must be implemented to reach the global population's food demand and avoid deforestation. The current promising agricultural practice without herbicides and pesticides is fertilizer management, particularly that of phosphorus fertilizers. Layered double hydroxides (LDHs) have recently emerged as favorable materials in phosphate removal, with practical application possibilities in nanofertilizers. This review discusses the fundamental aspects of phosphate removal/recycling mechanisms and highlights the current endeavors on the development of phosphate-selective sorbents using LDH-based materials. Specific emphasis is provided on the progress in designing LDHs as the slow release of phosphate fertilizers reveals their relevance in making agro-practices more ecologically sound. Relevant pioneering efforts have been briefly reviewed, along with a discussion of perspectives on the potential of LDHs as green nanomaterials to improve food productivity with low eco-impacts.

Application of Layered Double Hydroxides as a Slow-Release Phosphate Source: A Comparison of Hydroponic and Soil Systems

The utilization of slow-release fertilizer materials capable of responding to their environment and releasing nutrient ions efficiently over a prolonged period is an emerging research area in agricultural materials sciences. In this study, two-dimensional layered materials were prepared to release phosphor ions (P) slowly into the soil as well as in the hydroponic system. Various P-intercalated layered double hydroxides (LDHs) (Mg/Al, Zn/Al, and Mg-Zn/Al-LDHs) with a molar ratio of 2:1 were synthesized using an ion-exchange method from corresponding LDHs containing NO₃ ^- ions within the layers. Sodium alginate (SA) was used to encapsulate P-intercalated Mg/Al-LDH to produce bionanocomposite beads (LB) to check the effect of the biopolymer matrix on the release characteristics. The prepared materials were characterized by XRD and FTIR to confirm the incorporation of P in LDHs. TGA, SEM, and elemental analysis were also performed to study the thermal decomposition pattern, surface morphology, and chemical composition of synthesized materials. The P-release experiments were conducted in a soil solution. The performance of the prepared materials was investigated in soil as well as in a hydroponic system for tomato plants under a controlled atmosphere of humidity, temperature, and light. The fertilization ability of the prepared materials was compared with that of a soluble P source (KH₂PO₄), commercial hydroponic fertilizer (Nutrifeed), and a commercial soil slow-release fertilizer (Wonder plant starter). The prepared materials demonstrated a slow release of P in the soil solution. P-intercalated LDHs were not very effective under hydroponic conditions; however, the LDHs were more effective in the soil system in terms of dry matter production and P content in dry matter. Furthermore, LDHs were able to increase the soil pH value over time.

A Comparison of Nitrate Release from Zn/Al-, Mg/Al-, and Mg-Zn/Al Layered Double Hydroxides and Composite Beads: Utilization as Slow-Release Fertilizers

Nitrate-loaded Zn/Al, Mg/Al, and Mg-Zn/Al layered double hydroxides (LDHs) were synthesized using the coprecipitation method. The slow-release properties of LDHs were measured in powder form at various pH conditions. Sodium alginate was used to encapsulate Mg/Al LDH to produce composite beads (LB) to further slow down the release of nitrate ions. The prepared LDH samples and LB were characterized by X-ray diffraction, attenuated total reflectance Fourier transform infrared spectroscopy, thermogravimetric analysis, and inductively coupled plasma optical emission spectroscopy. The surface morphologies of LDHs and LB were obtained from scanning electron microscopy analysis. The slow-release properties of the materials were evaluated using a kinetic study of nitrate release in tap water, soil solution, as well as plant growth experiments using coriander (Coriandrum sativum). The nitrate release ability of LDHs and LB was compared with a soluble nitrate source. The plant growth experiments showed that all three LDHs were able to supply an adequate amount of nitrate to the plant similar to the soluble fertilizer while maintaining the availability of nitrate over extended periods. The ability of LDHs to increase soil pH was also demonstrated.

Layered Double Hydroxides for Sustainable Agriculture and Environment: An Overview

Agricultural practices in modern society have a detrimental impact on the health of the ecosystem, environment, and consumers. The significantly high usage rate of chemicals causes serious harm, and the sector demands the development of innovative materials that can foster improved food production and lessen ecological impacts. The majority of layered double hydroxides (LDH) are synthetic. At the same time, some of them occur in the form of natural minerals (hydrotalcite), which have recently emerged as favorable materials and provided advanced and ingenious frontiers in various fields of agriculture through practical application possibilities that can replace conventional agricultural systems. LDH can exchange anions intercalated between the layers in the interlayer structure, and there is evidence that atmospheric carbon dioxide and moisture can completely break down LDH over time. Due to certain unique properties such as tunable structure, specific intercalation chemistry, pH-dependent stability, as well as retention of the guest molecules within interlayers and their subsequent controlled release, LDHs are increasingly investigated as materials to enhance yield, quality of crops, and soil in recent times. This review aims to present the current research progress in the design and development of LDH-based materials as nanoscale agrochemicals to illustrate its relevance in making agro-practices more sustainable and efficient. Specific emphasis is given to the functionality of these materials as effective materials for the slow release of fertilizers and plant growth factors as well as adsorption of toxic agrochemical residues and contaminants. Relevant research efforts have been briefly reviewed, and the potential of LDH as new generation green materials to provide solutions to agricultural problems for improving food productivity and security has been summarized.

Boosting nitrogen fertilization by a slow releasing nitrate-intercalated biocompatible layered double hydroxide-hydrogel composite loaded with Azospirillum brasilense

Indiscriminate use of chemical fertilizers leads to soil environmental disbalance and therefore, preparation and application of environment-friendly slow-release multifunctional fertilizers are of paramount importance for sustainable crop production in the present scenario. In this study, we propose a slow-release multifunctional composite nitrogen (N) fertilizer, which possesses the ability to supply plant accessible N in the form of ammonium (NH₄⁺) and nitrate (NO₃⁻) to improve nitrate assimilation coupled with zinc (Zn, a major micronutrient for plants in the soil) after its degradation. For this purpose, NO₃⁻-intercalated zinc–aluminum (Zn–Al) layered double hydroxide (LDH) was synthesized using a co-precipitation protocol. The prepared LDH was added as 25.45% of total polymer weight to a sodium carboxymethyl cellulose/hydroxyethyl cellulose citric acid (NaCMC/HEC-CA) biodegradable hydrogel. A. brasilense, commonly used nitrogen-fixing bacteria in soils, was added to the LDH–hydrogel composite along with LDH alone to augment the availability of NH₄⁺ and NO₃⁻. Adjusting the pH under acidic (pH 5.25) and neutral (pH 7) conditions, the release pattern of NO₃⁻ from LDH and the composite was monitored for 30 days at normal temperature. The pH was selected based on the soil analysis data of North East India. The LDH-composite released 90% (w/w) and 85.45% (w/w) of intercalated NO₃⁻ at pH 5.25 and 7.00 respectively in 30 days. However, 100% (w/w) and 87% (w/w) of intercalated NO₃⁻ at pH 5.25 and 7.00 respectively were released in 30 days when only LDH was applied, which indicated the lower performance of LDH alone in comparison to the LDH-composite for the nitrate holding pattern. The pH of the bacteria-loaded system was observed to be acidic (pH = 5–6) during the study of nitrate assimilation and Zn²⁺ release. A. brasilense improved nitrate assimilation and increased the NH₄⁺ ion concentration in the studied system. A significant increase in Zn²⁺ release was observed from day 5 in the presence of A. brasilense in the LDH-composite compared with that in the absence of A. brasilense. In conclusion, the prepared LDH–hydrogel–A. brasilense composite fertilizer system increases the availability of plant accessible N form (both NO₃⁻ and NH₄⁺) and can potentially improve soil fertility with the addition of Zn and bacteria to the soil in the extended course.

Indiscriminate use of chemical fertilizers leads to soil environmental disbalance, therefore, use of environment-friendly slow-release multifunctional fertilizers are of paramount importance for sustainable crop production in the present scenario.

Layered Double Hydroxides as Slow-Release Fertilizer Compounds for the Micronutrient Molybdenum

Molybdenum (Mo) is an essential plant micronutrient. Despite low plant Mo requirements, deficiencies are not uncommon and soluble Mo fertilizers are often applied. However, soluble Mo may result in poor Mo use efficiency due to strong sorption (acid weathered soils) or leaching (lighter-textured soils). Here, ZnAl layered double hydroxides (LDHs), loaded with molybdate (MoO₄), were examined for their potential as slow-release Mo compounds. Chloride-exchanged LDHs with varying Zn/Al ratios (2, 3, and 4) were exchanged with MoO₄. Zn₂Al LDH indicated MoO₄ intercalation, whereas Zn₃Al and Zn₄Al LDHs bound MoO₄ merely on edge sites. Short-term Mo-LDH incubation identified sulfate, carbonate, and phosphate as the most competitive anions for MoO₄ exchange. Long-term Mo-LDH incubation in simulated pH-neutral soil solutions demonstrated slow Mo release from Zn₂Al LDH (half-life of 35 h), with a total Mo desorption of up to 85%. For Zn₃Al and Zn₄Al LDHs, Mo desorption was limited to <20%. Finally, several macronutrient fertilizers were tested as possible carriers for Mo-LDH fertilizer compounds.

Mesoporous Silica Nanoparticles Modified inside and out for ON:OFF pH-Modulated Cargo Release

Highly efficient pH-modulated cargo release was achieved with a new hybrid nanocarrier composed of a mesoporous silica core with functionalized pores and a grafted pH-responsive crosslinked polymer shell of 2-(diisopropylamino)ethyl methacrylate (pKa ≈ 6.5). The retention/release performance of the system was optimized by a novel approach using selective functionalization of the silica pores to tune the carrier-cargo interaction and by tunning the amount of grafted polymer. The system features excellent retention of cationic cargo at low pH and a burst release at higher pH. This results from the expanded-collapsed conformation transition of the pH-responsive polymer shell and the simultaneous change in the interaction between the cargo and the polymer shell and the modified pore walls. At low pH, the electrostatic interaction of the cationic cargo with the protonated amine groups of the extended polymer shell retains the cargo, resulting in very low leakage (OFF state). At high pH, the electrostatic interaction with the cargo is lost (due to deprotonation of the polymer amine groups), and the polymer shell collapses, squeezing out the cargo in a burst release (ON state). Pore functionalization in combination with the stimuli-responsive polymer shell is a very promising strategy to design high-performance ON:OFF smart hybrid nanocarriers for stimuli-actuated cargo release, with great potential for application in the controlled release of drugs and other biologically active agents.

Castanheira E, P. C. Alves S, Baleizão C, Farinha JP. Grafting with RAFT-gRAFT Strategies to Prepare Hybrid Nanocarriers with Core-shell Architecture

Stimuli-responsive polymer materials are used in smart nanocarriers to provide the stimuli-actuated mechanical and chemical changes that modulate cargo delivery. To take full advantage of the potential of stimuli-responsive polymers for controlled delivery applications, these have been grafted to the surface of mesoporous silica particles (MSNs), which are mechanically robust, have very large surface areas and available pore volumes, uniform and tunable pore sizes and a large diversity of surface functionalization options. Here, we explore the impact of different RAFT-based grafting strategies on the amount of a pH-responsive polymer incorporated in the shell of MSNs. Using a "grafting to" (gRAFT-to) approach we studied the effect of polymer chain size on the amount of polymer in the shell. This was compared with the results obtained with a "grafting from" (gRAFT-from) approach, which yield slightly better polymer incorporation values. These two traditional grafting methods yield relatively limited amounts of polymer incorporation, due to steric hindrance between free chains in "grafting to" and to termination reactions between growing chains in "grafting from." To increase the amount of polymer in the nanocarrier shell, we developed two strategies to improve the "grafting from" process. In the first, we added a cross-linking agent (gRAFT-cross) to limit the mobility of the growing polymer and thus decrease termination reactions at the MSN surface. On the second, we tested a hybrid grafting process (gRAFT-hybrid) where we added MSNs functionalized with chain transfer agent to the reaction media containing monomer and growing free polymer chains. Our results show that both modifications yield a significative increase in the amount of grafted polymer.

Layered double hydroxides intercalated with borate: effect of fertilization on boron leaching and successive sunflower cultivations

Layered double hydroxides intercalated with borate: reduction of boron leaching in sandy soil and increase in plant boron uptake.

Synthesis, characterization and agronomic use of alginate microspheres containing layered double hydroxides intercalated with borate

Alginate microspheres containing layered double hydroxides intercalated with borate: reduction on boron leaching and increasing boron uptake by plants.

Alginate beads containing layered double hydroxide intercalated with borate: a potential slow-release boron fertilizer for application in sandy soils

Alginate beads containing layered double hydroxide intercalated with borate: a new fertilizer to reduce boron leaching and increase plant uptake.

Optimization of phosphate recovery from urine by layered double hydroxides

Urine contains sufficient phosphorus (P) to consider P recycling form urine as an interesting strategy. In this study, the potential of MgAl or ZnAl layered double hydroxides (LDHs) to be used in such recovery was assessed. LDHs are anion exchangers with a high P selectivity, and P-loaded LDHs have demonstrated fertiliser potential. A critical factor for efficient P recycling with LDH is the stability of these materials, which can be compromised by urinary citrate, complexing aluminium (Al³⁺) and by the low pH of fresh urine dissolving the alkaline LDHs. Different phase pure ZnAl and MgAl LDHs were synthesised by coprecipitation in scenarios of varying synthesis pH and Mg/Al or Zn/Al ratios. The obtained materials were incubated in P solutions at different pH, with or without citrate in full factorial combinations, and in fresh and stored human urine. The P sorption capacities increased for LDHs synthesised at lower pH, at increasing Al content and for sorption solutions with lower pH. These trends are explained by an increased anion exchange capacity (AEC) and by P speciation (charge) in the LDHs, an interpretation supported by XRD measurements. The P capacity reached 61mg P/g LDH, which equals 85% of the theoretical LDH exchange capacity. Only 1g LDH is required to remove 90% of P from 1L urine and evidence is found that sorption, not struvite precipitation, is the P removal mechanism involved. The ZnAl LDHs were equally effective in P uptake compared to the MgAl LDHs, but the ZnAl materials showed more irreversible P sorption in contrast with the high desorption yields (53mg P/g) of the MgAl LDHs. Therefore, the large potential of MgAl LDHs for P recovery from urine is supported by this study.

Nano-based smart pesticide formulations: Emerging opportunities for agriculture

The incorporation of nanotechnology as a means for nanopesticides is in the early stage of development. The main idea behind this incorporation is to lower the indiscriminate use of conventional pesticides to be in line with safe environmental applications. Nanoencapsulated pesticides can provide controlled release kinetics, while efficiently enhancing permeability, stability, and solubility. Nanoencapsulation can enhance the pest-control efficiency over extended durations by preventing the premature degradation of active ingredients (AIs) under harsh environmental conditions. This review is thus organized to critically assess the significant role of nanotechnology for encapsulation of AIs for pesticides. The smart delivery of pesticides is essential to reduce the dosage of AIs with enhanced efficacy and to overcome pesticide loss (e.g., due to leaching and evaporation). The future trends of pesticide nanoformulations including nanomaterials as AIs and nanoemulsions of biopesticides are also explored. This review should thus offer a valuable guide for establishing regulatory frameworks related to field applications of these nano-based pesticides in the near future.

Analytical Capabilities of the Community Bureau of Reference Protocol to Estimate the Mobility of Nutrients and Toxic Elements from Mineral Fertilizer

The sequential extraction procedure of the Community Bureau of Reference (BCR) was applied to investigate the mobility of potentially toxic elements (As, Cd, Cr, and Pb) and nutrients (P, Ca, Mg, Cu, Fe, Mn, and Zn) in a multinutrient mineral fertilizer based on phosphate rocks supplemented with 10% (w w^-1) micronutrient mixture (raw material used as a micronutrient source). For both samples, As and Cd were more mobile, whereas Cr remained in the solid residue. A higher mobility of Pb was observed in the micronutrient mixture; however, the high concentration of P (8.3% w w^-1) in the fertilizer could have decreased Pb mobility as a result of Pb₃(PO₄)₂ formation. The nutrients had great mobility, except Fe, which remained almost totally in the residual fraction in both samples. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses of solid residues showed that the ways in which elements were distributed in the sample particles could affect their mobility.