Phosphate Species Research Articles

Unexpected nanoscale CeO2 structural transformations induced by ecologically relevant phosphate species

In the present study, the dissolution and microstructural transformation of CeO2 nanoparticles (NPs) in a phosphate-containing milieu were investigated. The dissolution behaviour of 2 nm and 5 nm CeO2 NPs in phosphate buffer solutions was found to differ markedly from that observed in 0.01 M NaClO4. Through synchrotron X-ray diffraction analysis and X-ray absorption spectroscopy, the interaction between CeO2 NPs and phosphate species was examined, revealing the transformation of the oxide into sodium-cerium double phosphate, with cerium predominantly existing in the Ce(IV) state. According to scanning and transmission electron microscopy observations, thus formed Na-Ce(IV) phosphate consists of spindle-like aggregates of nanocrystalline rods, presumably formed during phosphate anions sorption on the initial CeO2 surface. Pair distribution function analysis revealed that Na-Ce(IV) phosphate has a three-dimensional framework crystal structure, similar to NaTh2(PO4)3, as reported earlier, with large channels along the c-axis containing disordered sodium atoms. This study represents the first detailed analysis of phosphate-induced speciation and microstructural transformation of CeO2 NPs, resulting in the formation of Ce(IV) phosphate. Similar processes may occur in natural ecosystems upon the introduction of CeO2 NPs.

Beam‐Generated Sample Evolution to Emphasize Spatial Inhomogeneities: The Example of HPO42−‐Containing Amorphous Calcium Phosphate

ABSTRACTAmorphous calcium phosphates (ACPs) represent a family of bioactive compounds particularly relevant to bone regeneration. However, due to their intrinsic metastability, their processing into 3D‐shaped materials cannot be undergone by conventional sintering methods and requires cold sintering approaches. Also, their microstructure and local compositional changes still have to be explored in detail. To this aim, spectroscopy techniques are particularly appealing to probe local chemical environments at the microscale. Concerning ACPs, one question regards the distribution of (hydrogenated) phosphate species, as they may lead to various evolutionary trends. In this contribution, we purposely exploited the laser–beam interaction through Raman mapping to trigger the in situ HPO42−‐to‐P2O74− transformation. Analysis by a multivariate approach allowed us to spot P2O74− clusters resulting from HPO42−‐rich initial domains. Moreover, a blue shift of the ν1PO43− band was noticed in the close vicinity of these clusters, thus evidencing a local evolution of the chemical composition of the ACP. These results, corroborated by differential thermal analysis, demonstrate the relevance of using the laser–sample interaction through local heating to probe the spatial evolution induced, in our case, by an ultrafast compaction process. Comparison of outcomes obtained using two different laser/power strategies finally evidenced the need to adapt the Raman analytical conditions to the behavior of the metastable material to analyze.

Conversion of Isopropanol to Diisopropyl Ether over Cobalt Phosphate Modified Natural Zeolite Catalyst

This study aims to produce diisopropyl ether (DIPE) via isopropanol dehydration using cobalt-phosphate-supported natural zeolite catalysts. The catalytic activities of the zeolite/CoO and zeolite/Co(H2PO4)2 were compared. The as-prepared catalysts were assessed using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared (FTIR) spectroscopy, and N2 adsorption-desorption. Surface acidity was determined using the gravimetric method with pyridine as the probe. The results of this study showed that natural zeolite was favorably impregnated by CoO and Co(H2PO4)2 species. The impregnation process affected the textural and acidic features of the catalysts. The zeolite/Co(H2PO4)2 catalyst with a loading of 8 mEq.g-1 exhibited the highest surface acidity of 1.827 mmol.g-1. This catalyst also promoted the highest catalytic activity towards isopropanol dehydration, with an isopropanol conversion of 66.19%, DIPE selectivity, and yield of 46.72% and 34.99%, respectively. The cobalt phosphate species promoted higher catalytic activity for isopropanol dehydration than the CoO species. This study demonstrated the potential of cobalt phosphate-supported natural zeolite catalysts for DIPE production with adequate performance. Copyright © 2024 by Authors, Published by BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Catalytic hydrodeoxygenation of lignin-derived phenolic compounds with Ni-based aluminum phosphate catalyst

Lignin-derived products possess high carbon content and offer significant potential to be used as liquid fuels. However, they also contain high oxygen element and unstable functional groups. Hydrodeoxygenation (HDO) is an effective method for upgrading lignin-derived phenolic compounds to produce high-quality liquid biofuels. In this study, a series of Ni-based phosphate catalysts incorporating various metals (Al, Ce, La) were prepared and employed for the HDO of lignin-derived phenolic compounds. The results showed that Ni/AlPO4 had the most efficient HDO performance, with guaiacol conversion of 99.9 % and cyclohexane selectivity of 91.1 % at the reaction temperature of 250 °C. Various catalyst characterization measurements were conducted to assess the impact of metal phosphate species on the crystallography, morphology, physicochemical characteristics and surface properties. Results indicated that Ni/AlPO4 catalyst exhibited stronger acidity, smaller Ni particle sizes and larger specific surface area than other Ni-based phosphate catalysts. Based on the characterization results and experimental data, the corresponding HDO reaction pathways and mechanisms were also proposed. Ni/AlPO4 catalyst was also applied to the HDO of various lignin-derived phenol compounds and achieved a good HDO effect. Moreover, the catalytic upgrading of lignin-oil was also carried out, and a high proportion of hydrocarbon products that increased from 16.0 % to 88.6 % was obtained. This Ni/AlPO4 catalyst provides potential reference for the HDO application of lignin-derived phenolic compounds for the production of hydrocarbon liquid fuels.

Phosphorus-Water Interaction Drives Active Center Evolution into the Water-Adaptive Structure in the High-Humidity NH3-SCR Reaction.

The impact of water on catalyst activity remains inconclusive due to its dependence on the specific reaction environment. To maximize the exploitation of water's promoting effect, we employed ammonia selective catalytic reduction (NH3-SCR) as a probe reaction and proposed a phosphorus modification strategy for Cu-ZSM-5 catalysts. The objective of this approach was to construct water-adaptive microstructures through directional arrangement. To investigate the effect of phosphorus on the transformation of framework copper sites in humid environments, we conducted comprehensive characterizations and density functional theory calculation. Results reveal that water molecules cleave the oxygen bridges between phosphorus oxide and copper, leading to the formation of active isolated [Cu(OH)]+ groups and phosphate. The phosphate species weaken the interaction between exchanged Cu2+ groups and the zeolite framework, leading to the generation of highly migratory hydrated Cu2+ species. This work will potentially guide the rational design of water-adaptive catalysts for gas pollution abatement in a humid environment.

Effect of Crystalline Phase and Facet Nature on the Adsorption of Phosphate Species onto TiO2 Nanoparticles.

The current use of TiO2 nanoparticles raises questions about their impact on our health. Cells interact with these nanoparticles via the phospholipid membrane and, in particular, the phosphate head. This highlights the significance of understanding the interaction between phosphates and nanoparticles possessing distinct crystalline structures, specifically anatase and rutile. It is crucial to determine whether this adsorption varies based on the exposed facet(s). Consequently, various nanoparticles of anatase and rutile TiO2, characterized by well-defined morphologies, were synthesized. In the case of the anatase samples, bipyramids, needles, and cubes were obtained. For the rutile samples, all exhibited a needle-like shape, featuring {110} facets along the long direction of the needles and facets {111} on the upper and lower parts. Phosphate adsorption experiments carried out at pH 2 revealed that the maximum adsorption was relatively consistent across all samples, averaging around 1.5 phosphate·nm-2 in all cases. Experiments using infrared spectroscopy on dried TiO2 powders showed that phosphates were chemisorbed on the surfaces and that the mode of adsorption depended on the crystalline phase and the nature of the facet: the anatase phase favors bidentate adsorption more than the rutile crystalline phase.

A sol-gel templating route for the synthesis of hierarchical porous calcium phosphate glasses containing zinc

Hierarchical porous phosphate-based glasses (PPG) have great potential in biomedicine. Micropores (pore size <2 nm) increase the surface area, mesopores (pore size 2–50 nm) facilitate the absorption and diffusion of therapeutic ions and molecules making them ideal controlled delivery systems, while macropores (pore size >50 nm) facilitate the movement and diffusion of cells and fluids. In addition, the bioresorbability of PPG allows for their complete solubility in body fluid, alongside simultaneous formation of new tissue. Making PPG via the traditional melt-quenching (MQ) synthesis method used for phosphate-based glasses (PG), is not straightforward. Hence, we present here a route for preparing such glasses using a combination of sol-gel (SG) and templating methods. Hierarchical PPG in the P2O5–CaO–Na2O system with the addition of 1, 3 and 5 mol % of Zn2+ were prepared with pore dimensions ranging from the micro-to the macro scales using Pluronic 123 (P123) as a surfactant. The presence of micropores (0.30–0.46 nm), mesopores (1.75–9.35nm) and macropores (163–207 nm) was assessed via synchrotron-based Small-Angle X-ray Scattering (SAXS), with the presence of the latter two confirmed by Scanning Electron Microscopy (SEM). Structural characterisation performed using 31P solid state magic angle spinning nuclear magnetic resonance (MAS NMR) and Fourier Transform Infrared (FTIR) spectroscopies shows the presence of Q2, Q1 and Q0 phosphate species with a predominance of Q1 species in all compositions. Dissolution studies in deionised (DI) water confirm that controlled release of phosphates, Ca2+, Na+ and Zn2+ is achieved over a period of 7 days. In particular, the release of Zn2+ is proportional to its loading, making its delivery particularly easy to control.

Dual isotopic (33P and 18O) tracing and solution 31P NMR spectroscopy to reveal organic phosphorus synthesis in organic soil horizons

Soil microorganisms can do both, mineralize and synthesize organic and condensed phosphate (P) species. Whereas P mineralization has been extensively studied, few studies have assessed the biological synthesis of organic P species, which can potentially accumulate in soil. The goal of this study was to investigate biotic and abiotic P transformations, particularly the synthesis of organic P species, upon water-soluble P addition in the organic (O) horizons of two beech forest sites with contrasting P availability.The two O horizons (low-P and high-P) were subjected to four different nutrient addition treatments (Control without addition, CN, P, and CNP additions) in an incubation experiment of up to 104 days. We combined isotopic tracing (33P-labelled P addition and 18O-enriched soil water) into sequentially extracted P pools with the characterization of organic P species (solution 31P nuclear magnetic resonance (NMR) spectroscopy) and soil respiration measurements.The P availability of the two O horizons shaped the microbial response to the nutrient additions. In the low-P O horizon, P addition stimulated microbial activity together with the increase of organic (phosphodiesters and phosphonates) and condensed (polyphosphates) P species, most likely from microbial origin. In the high-P O horizon, microbes were unaffected by the added P and abiotic processes controlled its fate. CN addition had no effect on P fate in the high-P O horizon but reduced the transformation of added P into organic P and increased soil-derived P in the resin P pool in the low-P O horizon. The 18O isotopic values in phosphate of the resin P pool suggest that the released P was biologically cycled.Our study confirms with a unique multi-analytical approach the microbial synthesis of phosphodiesters, phosphonates, and polyphosphates upon inorganic P addition under low P availability.

Biomimetic Charge‐Neutral Anion Receptors for Reversible Binding and Release of Highly Hydrated Phosphate in Water

AbstractControl of phosphate capture and release is vital in environmental, biological, and pharmaceutical contexts. However, the binding of trivalent phosphate (PO43−) in water is exceptionally difficult due to its high hydration energy. Based on the anion coordination chemistry of phosphate, in this study, four charge‐neutral tripodal hexaurea receptors (L1–L4), which were equipped with morpholine and polyethylene glycol terminal groups to enhance their solubility in water, were synthesized to enable the pH‐triggered phosphate binding and release in aqueous solutions. Encouragingly, the receptors were found to bind PO43− anion in a 1 : 1 ratio via hydrogen bonds in 100 % water solutions, with L1 exhibiting the highest binding constant (1.2×103 M−1). These represent the first neutral anion ligands to bind phosphate in 100 % water and demonstrate the potential for phosphate capture and release in water through pH‐triggered mechanisms, mimicking native phosphate binding proteins. Furthermore, L1 can also bind multiple bioavailable phosphate species, which may serve as model systems for probing and modulating phosphate homeostasis in biological and biomedical researches.

Biomimetic Charge-Neutral Anion Receptors for Reversible Binding and Release of Highly Hydrated Phosphate in Water.

Control of phosphate capture and release is vital in environmental, biological, and pharmaceutical contexts. However, the binding of trivalent phosphate (PO4 3-) in water is exceptionally difficult due to its high hydration energy. Based on the anion coordination chemistry of phosphate, in this study, four charge-neutral tripodal hexaurea receptors (L1-L4), which were equipped with morpholine and polyethylene glycol terminal groups to enhance their solubility in water, were synthesized to enable the pH-triggered phosphate binding and release in aqueous solutions. Encouragingly, the receptors were found to bind PO4 3- anion in a 1 : 1 ratio via hydrogen bonds in 100 % water solutions, with L1 exhibiting the highest binding constant (1.2×103 M-1). These represent the first neutral anion ligands to bind phosphate in 100 % water and demonstrate the potential for phosphate capture and release in water through pH-triggered mechanisms, mimicking native phosphate binding proteins. Furthermore, L1 can also bind multiple bioavailable phosphate species, which may serve as model systems for probing and modulating phosphate homeostasis in biological and biomedical researches.

On the Structure and Role of Avian Eggshells: A 31P, 1H, and 13C Solid-State NMR Study.

The eggshell is a composite and highly ordered structure formed by biomineralization. Besides other functions, it has a vital and intricate role in the protection of an embryo from various potentially harsh environmental conditions. Solid-state nuclear magnetic resonance (SSNMR) has been used for detailed structural investigations of the chicken, tinamou, and flamingo eggshell materials. 31P NMR spectra reveal that hydroxyapatite and β-tricalcium phosphate in the ratio 3:2 represent major constituents of phosphate species in the eggshells. All three eggshells exhibit similar spectra, except for the line widths, which implies different structural order of phosphate species in the chicken, tinamou, and flamingo eggshells. 1H NMR spectra for these materials are comparable, differentiating overlapped peaks in three spectral regions at around 7, 4-5, and 1-2 ppm. These spectral regions have been attributed to protons from NH or CaHCO3, water, and possibly isolated monomeric water molecules or hydroxyl groups in calcium-deficient hydroxyapatite. 1H-13C CP MAS NMR revealed the presence of organic matter in the form of lipids and proteins. Two overlapped resonances in the carbonyl region at around 173 and 169 ppm are assigned to the carbonyls of the peptide bonds and the bicarbonate unit in calcite, respectively. Fourier-transform infrared spectroscopy (FTIR) spectra confirmed the presence of structural units detected in the NMR spectra.

Effects of low-molecular-weight organic acids on the transformation and phosphate retention of iron (hydr)oxides

The retention and mobilization of phosphate in soils are closely associated with the adsorption of iron (hydr)oxides and root exudation of low-molecular-weight organic acids (LMWOAs). This study investigated the role of LMWOAs in phosphate mobilization under incubation and field conditions. LMWOAs-mediated iron (hydr)oxide transformation and phosphate adsorption experiments revealed that the presence of LMWOAs decreased the phosphate adsorption capacity of iron (hydr)oxides by up to ~74 % due to the competition effect, while LMWOAs-induced iron mineral transformation resulted in an approximately six-fold increase in phosphate retention by decreasing the crystallinity and increasing the surface reactivity. Root simulation in rhizobox experiments demonstrated that LMWOAs can alter the contents of different extractable phosphate species and iron components, leading to 10 % ~ 30 % decreases in available phosphate in the near root region of two tested soils. Field experiments showed that crop covering between mango tree rows promoted the exudation of LMWOAs from mango roots. In addition, crop covering increased the contents of total phosphate and available phosphate by 9.08 % ~ 61.20 % and 34.33 % ~ 147.33 % in the rhizosphere soils of mango trees, respectively. These findings bridge the microscale and field scale to understand the delicate LMWOAs-mediated balance between the retention and mobilization of phosphate on iron (hydr)oxide surface, thereby providing important implications for mitigating the low utilization efficiency of phosphate in iron-rich soils.

In-Depth Analysis of the Species and Transformations during Sol Gel-Assisted V2PC Synthesis.

The sol-gel reaction mechanism of 211 MAX phases has proven to be very complex when identifying the intermediate species, chemical processes, and conversions that occur from a mixture of metal salts and gelling agent into a crystalline ternary carbide. With mostly qualitative results in the literature (Cr2GaC, Cr2GeC, and V2GeC), additional analytical techniques, including thermal analysis, powder diffraction, total scattering, and various spectroscopic methods, are necessary to unravel the identity of the chemical compounds and transformations during the reaction. Here, we demonstrate the combination of these techniques to understand the details of the sol-gel synthesis of MAX phase V2PC. The metal phosphate complexes, as well as amorphous/nanocrystalline vanadium phosphate species (V in different oxidation states), are identified at all stages of the reaction and a full schematic of the reaction process is suggested. The early amorphous vanadium species undergo multiple changes of oxidation states while organic species decompose releasing a variety of small molecule gases. Amorphous oxides, analogous to [NH4][VO2][HPO4], V2PO4O, and VO2P2O7 are identified in the dried gel obtained during the early stages of the heating process (300 and 600 °C), respectively. They are carbothermally reduced starting at 900 °C and subsequently react to crystalline V2PC with the excess carbon in the reaction mixture. Through CHN analysis, we obtain an estimate of left-over amorphous carbon in the product which will guide future efforts of minimizing the amount of carbon in sol gel-produced MAX phases which is important for subsequent property studies.

Probing Chemical Equilibrium in Frozen Sodium Phosphate Buffer Solution by 31P Solid-State NMR.

Phosphate buffers are crucial for cryopreservative stability in pharmaceuticals, food processing, biomedical sciences, and biology. However, their freeze concentrates lack quantitative characterization, especially regarding the physicochemical properties of phosphate salt species in equilibrium at subzero temperatures. This study employs 31P solid-state NMR (ssNMR) to analyze frozen sodium phosphate (NaP) solutions, providing insights into phase composition, ionic strength, and pH. For the first time, we have directly quantified phosphate species in frozen NaP buffer, including crystallized disodium phosphate dodecahydrate (Na2HPO4·12H2O) content and the concentrations of H2PO4- and HPO42- in the freeze concentrate. This enabled the calculation of the pH as well as the ionic strength in the freeze concentrate. Trehalose effectively mitigated pH shifts in buffer solutions by preventing the selective crystallization of salt, a spectroscopic phenomenon not previously observed experimentally.

Desorption behavior of phosphate species from layered double hydroxides

In phosphate species recovery, layered double hydroxides (LDHs) are good adsorbent, but show difficulties in its desorption. In this study, the desorption behavior was systematically investigated by changing several factors, which were drying condition, change with time, crystallinity and solution for desorption. Interlayer space of PO4-exchanged LDHs could swell by adding water one day after synthesis when the LDHs were vacuum dried at 25 °C. However, the swelling ability was deteriorating with time, and was also lost by oven-drying at 110 °C. In the case of high-crystallinity LDH, clear results were observed: desorption ratio of phosphate species from the vacuum-dried LDH decreased with time even by 1 M NaOH which was the most effective, and desorption ratios of phosphate species from the oven-dried LDH dropped sharply. In contrast, SO4-exchanged LDH, which were used for comparison, did not lose their swelling ability, and desorption ratios of sulphate were almost unchanged over time or by oven-drying at 110 °C. PO4-exchanged LDH stored in aqueous suspensions did not lose their swelling ability with time, and their desorption ratios of phosphate species remained unchanged over time. Therefore, above results suggest that interaction between phosphate species and host layers of LDHs in dried state causes the changes. In conclusion, the result that phosphate species desorption from dried high-crystallinity LDH deteriorated with time has not been reported previously, which will give a great impact on applications such as recovery of phosphate and slow release of phosphate.

Binding of Phosphate Species to Ca2+ and Mg2+ in Aqueous Solution.

Phosphate derivatives and their interaction with metal cations are involved in many important biological phenomena, so an accurate characterization of the phosphate-metal interaction is necessary to properly understand the role of phosphate-metal contacts in mediating biological function. Herein, we improved the standard 12-6 Lennard-Jones (LJ) potential via the usage of the 12-6-4 LJ model, which incorporates ion-induced dipole interactions. Via parameter scanning, we fine-tuned the 12-6-4 LJ polarizability values to obtain accurate absolute binding free energies for the phosphate anions H2PO4-, HPO42-, PO43- coordinating with Ca2+ and Mg2+. First, we modified the phosphate 12-6-4 LJ parameters to reproduce the solvation free energies of the series of phosphate anions using the thermodynamic integration (TI) method. Then, using the potential mean force (PMF) method, the polarizability of the metal-phosphate interaction was obtained. We show that the free energy profiles of phosphate ions coordinated to Ca2+ and Mg2+ generally show similar trends at longer metal-phosphate distances, while the absolute binding energy values increased with deprotonation. The resulting parameters demonstrate the flexibility of the 12-6-4 LJ-type nonbonded model and its usefulness in accurately describing cation-anion interactions.

Efficient electrooxidation of 5-hydroxymethylfurfural via phosphate intercalated hydroxides: A dual-cycle mechanism

The electrooxidation of 5-hydroxymethylfurfural (HMF) has emerged as a promising way to generate high-value-added products. However, evoking high-valence state NiOOH species as active sites to realize high current density remains a challenge. Herein, we report an efficient electrocatalyst for HMF electrooxidation based on phosphate anion intercalated layered double hydroxides (NiCo-Pi-LDHs). Onset potential at 1.16 V vs. RHE and current density of 200 mA/cm2 at 1.41 V vs. RHE were realized. The intercalated phosphates act as proton transfer intermediates, facilitating the dehydrogenation of hydroxides to form oxyhydroxides while the protonated phosphate generated. The obtained oxyhydroxide oxidized HMF to 2,5-furandicarboxylic (FDCA) while cycling backward to hydroxide, accompanied by the cycle of protonated phosphate to phosphate. This novel strategy with dual-cycle of phosphate and nickel species can effectively evoke NiOOH species, thereby speeding up the reaction rate and having a great potential for biomass upgrading.

New insights into defluoridation via induced crystallization: Implications of phosphate species regulation for process efficiency and economics

Excess fluoride (F) at low concentrations in groundwater remains a challenge. F removal via fluidized-bed induced crystallization has been receiving increasing attention; however, the precipitant cannot be fully utilized. This results in generation of by-products, inadvertently increases the treatment cost, and impedes the assessment of product purity. In this study, phosphate species regulation was employed to improve the F removal efficiency without decreasing the phosphate utilization efficiency. It shows that F concentration could be reduced from 4.75 to 0.45 mg·L−1, with P concentration in the effluent being only 0.05 mg·L−1 at a P:F molar ratio of 3, involving appropriately mixed phosphate species. Hydroxyapatite (HAP) crystallization, subsequent F adsorption, and directly induced fluorapatite (FAP) crystallization occurred simultaneously in the fluidized bed, which was indirectly confirmed through intermittent operation of the process. The products of the induced crystallization process were FAP and a small amount of HAP, as determined qualitatively using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR) tests. Overall, by regulating the phosphate species, the F removal efficiency is maintained, the cost of the precipitant is lowered, and the residual P concentration in the effluent is reduced. The process results in products with FAP as the main component, which can be used as raw material for the phosphoric acid purification process.

Structural transformation investigations of δ to α manganese dioxides: Phosphate sorption and its mechanism

The elevation of environmental phosphate levels can negatively impact both the natural environment and human health. To address this issue, a series of MnO2 sorbents were investigated in this study. A facile one-pot synthesis method was employed to fabricate distinct morphologies and crystalline structures by controlling the hydrothermal duration. As the hydrothermal duration was prolonged, the samples demonstrated a pronounced enhancement of stability and crystalline phases, along with a corresponding increase in structural oxygen vacancies. A series of sorption performance tests were conducted, including pH effect, sorption isotherm, sorption kinetics, co-existing ions and sorption–desorption cycles experiments. It was revealed that manganese dissolution, the surface charge of the sorbents, and the phosphate species at different pH values collectively influenced the sorption process. The sorbent α-MnO2-120 h, with the highest oxygen vacancy ratio, exhibited the best sorption performance towards phosphate ions, with the maximum sorption capacity at pH 7. The highest removal rate and kd value were 82.63 % and 2.29 × 103 mL g−1, respectively, at the initial concentration of 30.7 mg PO4 g−1. The α-MnO2-120 h sorbent exhibited excellent selectivity in the presence of NO3−, SO42−, CO32−, and SiO32−. Furthermore, the regenerated material exhibited only a 6 % decrease in phosphate removal efficiency after five cycles, with no structural changes observed. A novel mechanism was proposed, highlighting the dominance of covalent chemical reactions in the sorption process. In particular, the participation of oxygen vacancies in sorbents contributed to enhancing the effective removal of phosphate ions. Overall, this study successfully demonstrated that synthesized α-MnO2 is a promising sorbent for phosphate removal.

Cadmium immobilization in soil using phosphate modified biochar derived from wheat straw

The phosphate-modified biochar (BC) immobilizes cadmium (Cd), yet little is known about how phosphate species affect Cd detoxification in contaminated soils. We developed phosphate-modified biochar through the pyrolysis of wheat straw impregnated with three types of phosphate: mono‑potassium phosphate (MKP), dipotassium hydrogen phosphate (DKP), and tripotassium phosphate (TKP). The Cd adsorption mechanism of modified biochar was investigated by biochar characterization, adsorption performance evaluation, and soil incubation tests. The results demonstrated that the efficiency of biochar in immobilizing Cd2+ followed the order: TKP-BC > DKP-BC > MKP-BC. The TKP-BC had the highest orthophosphate content, the fastest adsorption rate, and the largest adsorption capacity (Langmuir) of 257.28 mg/g, which is 6.31 times higher than that of the unmodified BC (CK). In contrast, pyrophosphate was predominant in MKP-BC and DKP-BC. The primary adsorption mechanism for Cd2+ was precipitation, followed by cation exchange, as evidenced by the formation of CdP minerals on the BC surface, and an increase of K+ in solution (compared to water-soluble K+) and a decrease of K+ in the biochar during adsorption. Desorption of Cd from the TKP-BC after adsorption was 9.77 %–12.39 % at a pH of 5–9, much lower than that of CK. The soil incubation test showed the diethylenetriaminepentaacetic acid extracted Cd of TKP-BC, MKP-BC, and DKP-BC was reduced by 67.93 %, 18.41 % and 31.30 % over CK, respectively. Using the planar optodes technique, we also found that TKP-BC had the longest effect enhancing in situ soil pH. This study provides a theoretical basis for developing heavy metal pollution control technology using green remediation materials and offers insights into the remediation mechanisms.