Physico-chemical assessment of [Mg-Al-PO4]-LDHs obtained by structural reconstruction in high concentration of phosphate

In-depth characterization of phosphate intercalated Mg Al Layered double hydroxides and study of the PO4 release properties

Slow-release fertilizers (SRFs) form the core of innovative strategies in sustainable agriculture. Layered Double Hydroxides (LDH), known for their high capacity to sequester plant nutrients, especially phosphate, are emerging as promising candidates for SRF synthesis. The phosphate release properties of MgAl LDH (with a targeted Mg/Al ratio of 2.0) intercalated with HPO42- anions were assessed in various aqueous environments. A comprehensive analysis, including in-depth chemical and structural characterizations (ICP-OES, XRD, PDF, 27Al NMR, 31P NMR, FTIR, SEM) of the as-prepared phase unveiled a more intricate composition than anticipated for a pure or ideal Mg2Al-HPO4 LDH, encompassing an excess of intercalated phosphate in conjunction with K+. Beyond the intercalated phosphate, solid state 31P NMR speciation identified multiple HxPO4(-3+x) environments, indicating a portion of the phosphate reacting with intralayer Mg2+ to form K-struvite. Additionally, some phosphates were adsorbed onto the surface of amorphous aluminum hydroxide, a side phase formed during MgAl coprecipitation. The phosphate release demonstrated rapid kinetics, occurring within 6 days. Moreover, the released phosphate increased significantly when reducing the Solid/Liquid (S/L) ratio (58%) and further increasing in the presence of carbonate ions (90%). The released phosphate varied from 12% to 90% under different release conditions, transitioning from water to a 3.33 mM NaHCO3 aqueous solution at a low S/L ratio (from 20 mg LDH per mL to 0.02 mg LDH per mL). The simultaneous release of K+, Mg2+, Al3+ indicated the complete dissolution of the K-struvite and partial dissolution of phosphate intercalated MgAl LDH. These results enhanced our understanding of the mechanism governing phosphate release from MgAl LDH, paving the way for potential phosphate recovery by LDH or for the development of LDH-based SRFs.

Phosphate recovery from sludge-incinerated ash by adsorption with hydrotalcite synthesized by metals in the ash

Selective adsorption of phosphorus (P) from the acidic leachate of sludge-incinerated ash (SIA) becomes more attractive due to avoiding removing heavy metals. Especially, layered double hydroxides (LDHs) as an anion adsorbent could be applied into this area owing to their good capacity on P-adsorption and low cost on preparation. Interestingly, SIA contains more aluminum (Al) and iron (Fe) needed to be removed prior to P-recovery, and removed Al and Fe could be utilized to synthesize LDHs, like Mg/Al-LDH and Mg/Fe-LDH. With this study, Mg/Al-LDH-r and Mg/Fe-LDH-r were economically synthesized with Al and Fe removed from SIA, which were similar in their chemical structures to commercial LDHs. The synthesized LDHs had a high P-adsorption capacity, up to 95.0%. The maximal phosphate capacity of the recovered LDHs (Mg/Al-LDH-r and Mg/Fe-LDH-r) was 239.0 and 199.8 mg P/g LDHs, respectively. "NaOH + desalinated brine" as a new desorption solution could achieve a desorption ratio at about 80%, which could reduce the liquid-solid ratio by at least 60%, greatly decreasing the desorption cost. Pot trials demonstrated that the desorbed and precipitated CaP could promote the growth of maize as well as a commercial P-fertilizer. Furthermore, the adsorbed phosphate by LDHs could be directly used as a slow-released P-fertilizer and also improve the pH value of acidic soil, completely deleting the desorption process.

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.

Multifaceted potential applicability of hydrotalcite-type anionic clays from green chemistry to environmental sustainability

Hydrotalcite-like anionic clays (HTs) also known as Layered double hydroxides (LDHs) have been developed as multifunctional materials in numerous applications related to catalysis, adsorption, and ion-exchange processes. These materials constitute an important class of ionic lamellar solid clays of Brucite-like structure which comprise of consecutive layers of divalent and trivalent metal cations with charge balancing anions and water molecules in interlayer space. These materials have received increasing attention in research due to their interesting properties namely layered structure, ease of preparation, flexible tunability, ability to intercalate different types of anions, electronic properties, high thermal stability, high biocompatibility, and easy biodegradation. Moreover, HTs/LDHs have unique tailorable and tuneable characteristics such as both acidic and basic sites, anion exchange capability, surface area, basal spacing, memory effect, and also exhibit high exchange capacities, which makes them versatile materials for a wide range of applications and extended their horizons to diverse areas of science and technology. This study enlightens the various rational researches related to the synthetic methods and features focusing on synthesis and/or fabrication with other hybrids and their applications. The diverse applications (namely catalyst, adsorbent to toxic chemicals, agrochemicals management, non-toxic flame retardants, and recycling of plastics) of these multifunctional materials related to a clean and sustainable environment were also summarized.

The tetracyclines removal by MgAl layered double oxide in the presence of phosphate or nitrate: Behaviors and mechanism exploration

Pollution of tetracyclines (TCs) in swine wastewater has been a critical concern worldwide. Notably, multiple anions (e.g. PO₄^3-, NO₃^-) coexist in the actual environments, which could significantly influence the TCs removal. In the current study, MgAl layered double oxide (MgAl-LDO) was adopted for investigating the TC removal performance with/without PO₄^3- or NO₃^-. In all systems, the adsorption performance exhibited two different approaches between low and high TC concentrations. In the single system, pseudo-second-order and the Freundlich model fitted well to the equilibrium adsorption data when TC concentration was below 125 mg·L^-1, while the pseudo-first-order and the linear model could describe the removal process at high TC concentration (>125 mg·L^-1). The maximum adsorption capacity was 83.56 mg·g^-1. In the co-existing system, the adsorption capacity was slightly enhanced when TC concentration below 150 mg·L^-1 however was inhibited at high concentration (>150 mg·L^-1). Combined with the characterization analyses, the interaction mechanism at low concentration was primarily surface adsorption on reconstructed LDH from LDO in the TC-alone system. It is worth mention that both PO₄^3- and NO₃^- facilitated the formation of LDH via rehydration of LDO which enhanced surface adsorption in the co-existing system. At high TC concentration, the formation of tetracycline-metal complexes played a dominant role in TC removal in the single system, whereas diminished complexation in the binary system led to the decreased TC removal. This study provides a theoretical and practical guidance for MgAl-LDO on the efficient remediation of actual tetracyclines wastewater.

Adsorption kinetic and thermodynamic studies of the 2, 4 - dichlorophenoxyacetate (2,4-D) by the [Co-Al-Cl] layered double hydroxide

[Co-Al-Cl] layered double hydroxide (LDH) obtained by co-precipitation at constant pH 8 presented a single phase in a hexagonal unit cell parameters similar to the hydrotalcite (JCPDS 14-191) belonging to the rhombohedral crystal system and space group R (-3)m . The adsorption kinetics of 2,4-D onto [Co-Al-Cl] LDH was better described by the Pseudo Second-Order (best adjust R2 = 0.9998 for 60 mg L-1 2,4-D adsorption). Intra-particle diffusion model was not the sole rate-controlling factor, indicating the adsorption of 2,4-D by the [Co-Al-Cl] LDH is a complex process for the experimental conditions performed, involving both boundary layer and intra-particle diffusion. The adsorption isotherm adjusted better to the Freundlich model (R2 = 0.9845) and the ΔH° value of - 51.18 kJ mol-1 indicated the predominance of the physical adsorption. The FT-IR spectrum of LDH after adsorption presented 2,4-D bands together with those of LDH and XRD showed an increase in the interlamellar distance (d 003) due to the intercalation of 2,4-D in the interlayer structure of the [Co-Al-Cl] LDH, corroborating inter and intra-particle adsorption data. Thus, [Co-Al-Cl] LDH, commonly used as electrodes in supercapacitors, can be effectively used as an adsorbent for the removal of 2,4-D from contaminated waters.

Screening Different Divalent and Trivalent Metals Containing Binary and Ternary Layered Double Hydroxides for Optimum Phosphate Uptake

The elements constituting a layered double hydroxides material provide many alternatives for its optimization. Ten different layered double hydroxides materials with various combinations of Ni, Cu, Zn, Al, Cr, and Fe elements were studied as sorbent materials for phosphate ion. The type of element used in the layered double hydroxides affected the uptake capacity of phosphate. The influence of a specific element alone was not the primary role of enhancing the sorption performance of phosphate ion on the LDHs material. However, using specific two or three elements together is the key to achieve the best result due to synergistic effects. BET surface area of the sorbent showed no correlation with phosphate uptake. From the examined materials, Four layered double hydroxides of Cu-Zn-Cr, Zn-Cr, Ni-Al, and Cu-Ni-Cr showed high phosphate sorption capability. Sorption equilibrium isotherm, reaction kinetics, and desorption of phosphate from the sorbent materials were also investigated.

Layered double hydroxides for phosphorus recovery from acidified and non-acidified dewatered sludge

Phosphate, which contains the essential element phosphorous (P), is a necessary fertilizer for agriculture, but the current phosphate deposits are running out and alternative sources are needed. Sludge obtained from wastewater treatment plants contains high concentrations of phosphorus and represents an alternative, sustainable source. In this study, sludge obtained from a wastewater treatment plant with biological and chemical phosphorus removal was acidified (pH = 3, 4, 5 and 6) to release orthophosphate followed by sequestration of the orthophosphate by a zinc aluminum layered double hydroxide (Zn₂Al-LDH). Sulfuric acid (H₂SO₄), nitric acid (HNO₃), and hydrochloric acid (HCl) was tested, which showed that only sulfate anions compete with phosphate and results in reduced phosphate recovery (25-35%). The orthophosphate concentration in the liquid phase increased from 20% (raw sludge) to 75% of the total phosphorus concentration at a pH of 3, which enhanced the phosphate uptake by the ZnAl-LDH from 1.7 ± 0.2% to 60.3 ± 0.6%. During acidification, the competing anion carbonate is degassed as CO₂, which further improved the phosphate uptake. PXRD showed the intercalation of carbonate in the LDH in the raw sludge at pH = 8, whereas orthophosphate was intercalated at lower pH values. ²⁷Al MAS NMR spectroscopy and powder X-ray diffraction (PXRD) proved preservation of the LDH at all pH values. Furthermore, about a fourth of the Al is present as an amorphous aluminum phosphate (AlPO₄) upon exposure to phosphate at low pH (pH = 3 and 5) based on ²⁷Al MAS NMR spectroscopy. At a pH of 6 about a third of the P is present as brushite (CaHPO₄·2H₂O).

Bromate removal from aqueous solution with novel flower-like Mg-Al-layered double hydroxides

A series of flower-like Mg-Al-layered double hydroxides (FHMA) with four Mg/Al ratios were successfully prepared via a simple method which put the mixed solution of magnesium and aluminum in the mixed solution of water and ethylene glycol (EG). After calcination under 600 °C, the calcination products (CFHMA) were used to adsorb bromate anionic from aqueous solution. The FHMA, CFHMA, and reconstructed CFHMA were characterized by FT-IR, XRD spectra, TG/DTA, N₂ adsorption-desorption isotherm, and SEM. It was found that FHMA had the flower-like morphology when Mg/Al ratio was 2 and 3. Moreover, FHMA lost the layered structures during calcination under 600 °C, but the disappeared structures were reconstructed after adsorption of bromate anionic, which was attributed to the "reformation effect" of FHMA with bromate as an interlayer anion. A series of adsorption studies were performed and the mechanism and reactivity of CFHMA were discussed including the effect of different kind of equilibration conditions, such as initial bromate anionic concentration, adsorbent dosage, contact time, initial solution pH, and co-existing anions. The results of adsorption displayed that the most suitable calcination temperature of FHMA is 600 °C, and the best ratio of Mg/Al is 2:1 to remove bromate anionic pollutant from water. The equilibrium adsorption data of CFHMA revealed a good compliance with the Langmuir model and the experimental data of CFHMA fitted well to the pseudo-second-order kinetic model. The calcination product of FHMA was a decent adsorbent of bromate anionic pollutant.

Ultrasonic and hydrothermal mediated synthesis routes for functionalized Mg-Al LDH: Comparison study on surface morphology, basic site strength, cyclic sorption efficiency and effectiveness

Amine functionalized layered double hydroxide (LDHs) adsorbents prepared using three different routes: co-precipitation, sono-chemical and ultrasonic-assisted high pressure hydrothermal. The prepared adsorbent samples were characterized using X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Scanning electron microscope-Energy dispersive X-ray spectroscopy (SEM-EDX), Temperature Programmed Desorption (TPD), Brunauer-Emmett-Teller (BET), and Thermogravimetric analysis (TGA), respectively. The performance of the prepared adsorbents was tested in a controlled thermal-swing adsorption process to measure its adsorption capacity, regeneration and cyclic efficiencies subsequently. The characterisation results were compared with those obtained using the conventional preparation routes but taking into account of the impact of sonochemical and hydrothermal pre-treatment on textural properties, adsorption capacity, regeneration and cyclic efficiencies. Textural results depicts a surge in surface area of the adsorbent synthesised by hydrothermal route (311m²/g) from 25 to 171m²/g for conventional and ultrasonic routes respectively. Additionally, it has been revealed from the present study that adsorbents prepared using ultrasonic-assisted hydrothermal route exhibit a better CO₂ uptake capacity than that prepared using sonochemical and conventional routes. Thus, the ultrasonic-assisted hydrothermal treatment can effectively promote the adsorption capacity of the adsorbent. This is probably due to the decrease of moderate (M-O) and weak (OH^- groups) basic sites with subsequent surge in the number of strong basic sites (O^2-) resulting from the hydrothermal process. Moreover, the cyclic adsorption efficiency of the ultrasonic mediated process was found to be 76% compared with 60% for conventional and 53% for hydrothermal routes, respectively. According to the kinetic model analysis, adsorption mechanism is mostly dominated by physisorption before amine modification and by chemisorption after the modification process.

Sonochemical surface functionalization of exfoliated LDH: Effect on textural properties, CO2 adsorption, cyclic regeneration capacities and subsequent gas uptake for simultaneous methanol synthesis

To improve CO₂ adsorption, amine modified Layered double hydroxide (LDH) were prepared via a two stage process, SDS/APTS intercalation was supported by ultrasonic irradiation and then followed by MEA extraction. The prepared samples were characterised using Scanning electron microscope-Energy dispersive X-ray spectroscopy (SEM-EDX), X-ray Photoelectron Spectroscopy (XPS), X-ray diffraction (XRD), Temperature Programmed Desorption (TPD), Brunauer-Emmett-Teller (BET), and Thermogravimetric analysis (TGA), respectively. The characterisation results were compared with those obtained using the conventional preparation method with consideration to the effect of sonochemical functionalization on textural properties, adsorption capacity, regeneration and lifetime of the LDH adsorbent. It is found that LDHs prepared by sonochemical modification had improved pore structure and CO₂ adsorption capacity, depending on sonic intensity. This is attributed to the enhanced deprotonation of activated amino functional groups via the sonochemical process. Subsequently, this improved the amine loading and effective amine efficiency by 60% of the conventional. In addition, the sonochemical process improved the thermal stability of the adsorbent and also, reduced the irreversible CO₂ uptake, CUirrev, from 0.18mmol/g to 0.03mmol/g. Subsequently, improving the lifetime and ease of regenerating the adsorbent respectively. This is authenticated by subjecting the prepared adsorbents to series of thermal swing adsorption (TSA) cycles until its adsorption capacity goes below 60% of the original CO₂ uptake. While the conventional adsorbent underwent a 10 TSA cycles before breaking down, the sonochemically functionalized LDH went further than 30 TSA cycles.

Synthesis and characterization of two-component acidic ion intercalated layered double hydroxide and its use as a nanoflame-retardant in ethylene vinyl acetate copolymer (EVA)

A series of two-component acidic ion intercalated layered double hydroxides (MgAlZn-X₁/X₂-LDHs) were synthesized by coprecipitation under microwave crystallization, and were further used as novel nanoflame-retardants for ethylene vinyl acetate (EVA).