Surface speciation of myo-inositol hexakisphosphate adsorbed on TiO2 nanoparticles and its impact on their colloidal stability in aqueous suspension: A comparative study with orthophosphate

Transformation of zinc oxide nanoparticles in the presence of aluminum oxide with pre-sorbed phosphorus ligands

Naturally occurring oxides could react with zinc oxide (ZnO) nanoparticles (NPs) and then change its transformation and toxicity to ecological receptors. The reaction may be affected by a variety of environmental factors, yet the relevant processes and mechanisms are limitedly investigated. Natural prevalent ligands, as an important factor, can sorb on natural oxide minerals and change its surface property, finally affecting ZnO NP transformation. This study investigated the interactions of ZnO NPs with phosphorus ligands (i.e., phytate and orthophosphate) pre-sorbed γ-alumina (γ-Al₂O₃) via batch experiments and multi-technique analyses. A limited amount of aqueous Zn²⁺ is observed when the concentration of ZnO NPs is relatively low (<64.8 mg L^-1) in the presence of phytate pre-sorbed γ-Al₂O₃. Solid Zn(II) species includes binary/ternary surface Zn(II) complexes on γ-Al₂O₃ with minor amounts of zinc phytate precipitates. As the concentration of ZnO NPs increases, surface Zn(II) complexes gradually transform into zinc phytate and Zn-Al layered double hydroxide (Zn-Al LDH) precipitates. The quantitative analysis indicates that, as the concentration of ZnO NPs increases from 32.4 to 388.8 mg L^-1, the proportion of Zn(II) species as binary/ternary surface complexes decreases from 81.9 to 30.2%; and the proportion as zinc phytate and Zn-Al LDH increases from 17.9 to 27.6% and 0 to 43.8%, respectively. The pre-sorption of orthophosphate can also inhibit ZnO NP transformation into Zn-Al LDH precipitates on γ-Al₂O₃. This study suggests that natural ligands pre-existed on natural oxide minerals could greatly influence the solubility, stability, transformation, and fate of easily dissoluble metal oxides (e.g., ZnO) in the environments.

Modified biochar improves the storage capacity and adsorption affinity of organic phosphorus in soil

The loss of soil organic phosphorus can easily cause water eutrophication. In order to effectively reduce the loss of soil organic phosphorus, this manuscript investigated the adsorption of soil organic phosphorus by lanthanum modified biochar (BC), traditional adsorbent gypsum (GY) and zeolite (ZE) by taking phytic acid as the representative. The adsorption isotherm model and kinetic models were used to fit the phosphorus absorption characteristics of the adsorbents. The effects of initial pH and temperature on the adsorption capacity were discussed, and the adsorption mechanism of each adsorbent was explained by means of FTIR and XRD. The results showed that the adsorption capacity of phytate phosphorus followed the trend of BCTS > GYTS > ZETS > TS (soil), and the maximum phosphorus adsorption capacity obtained from Langmuir isotherm for treatment with BCTS was 2.836 mg g^-1, and the treatment had the strongest affinity for phytate phosphorus and also the ability to store phosphorus. The adsorption process fits well with Langmuir isotherm equation and pseudo-second-order kinetic equation, and the adsorption behavior of phytate phosphorus was mainly controlled by the chemisorption of monolayer. When the concentration of phytate phosphorus was 100 mg L^-1, percentage of modified biochar added to the soil was 3% and the pH was 6, the adsorption capacity reached the maximum, and the maximum adsorption capacity was 2.000 mg g^-1. The results of FTIR and XRD characterization showed that complexation was the main adsorption mechanism. In this study, the combination of modified biochar and soil phytate phosphorus can provide a good theoretical basis for reducing the loss of soil organic phosphorus.

Utilization of coal fly ash waste for effective recapture of phosphorus from waters

Reutilization of the waste by-products from industrial and agricultural activities is crucially important towards attainment of environmental sustainability and the 'circular economy'. In this study, we have developed and evaluated a sustainably-sourced adsorbent from coal fly ash, which was modified by a small amount of lanthanum (La-FA), for the recapture of phosphorous (P) from both synthetic and real natural waters. The prepared La-FA adsorbent possessed typical characteristic diffraction peaks similar to zeolite type Na-P1, and the BET surface area of La-FA was measured to be 10.9 times higher than that of the original FA. Investigation of P adsorption capability indicated that the maximum adsorption (10.8 mg P g^-1) was 6.14 times higher than that (1.8 mg P g^-1) of the original fly ash material. The ζ potentials measurement and P K-edge X-ray Absorption Near Edge Structure (XANES) spectra demonstrated that P was bonded on La-FA surfaces via an adsorption mechanism. After applying the proposed adsorbent to real lake water with La/P molar ratios in the range from 0.5:1 to 3:1, the La-FA adsorbent showed the highest phosphate removal ability with a La/P molar ratio 1:1, and the P adsorption was similar to that performance with the synthetic solution. Moreover, the La-FA absorbent produced a negligible effect on the concentrations of total dissolved nitrogen (TDN), NH₄⁺-N and NO₃^--N in water. This study thus provides a potential material for effective P recapture and details of its operation.

Competitive arsenate and phosphate adsorption on α-FeOOH, LaOOH, and nano-TiO2: Two-dimensional correlation spectroscopy study

Competitive adsorption of arsenate (AsO₄^3-) and phosphate (PO₄^3-) on α-FeOOH, LaOOH, and nano-TiO₂ was studied using batch adsorption experiments and in-situ flow cell ATR-FTIR coupled with two-dimensional correlation spectroscopy (2D-COS) for the first time. With a higher temporal resolution, our results found a highly dynamic adsorption sequence for AsO₄^3- and PO₄^3-. When AsO₄^3- and PO₄^3- were simultaneously exposed to the adsorbents at the same concentrations, AsO₄^3- was preferentially adsorbed by α-FeOOH and TiO₂, but PO₄^3- adsorption was dominant on LaOOH. The results implied that the PO₄^3- adsorbed on LaOOH had to be remobilized to allow for AsO₄^3- adsorption, but that PO₄^3- adsorption on α-FeOOH and TiO₂ was hindered by faster AsO₄^3- adsorption. Crystal orbital Hamilton population (COHP) analysis revealed that AsO₄^3- complexes bonded more strongly on α-FeOOH and TiO₂, whereas PO₄^3- complexes were more stable on LaOOH. Different adsorption sequences and the stability of the complexes were attributed to the diverse geometric configurations of AsO₄^3- and PO₄^3- on metal oxides surfaces with specific bonding chemistry. The presence of Ca²⁺ did not affect AsO₄^3- and PO₄^3- adsorption sequence on α-FeOOH or LaOOH, but it reversed the adsorption sequence on TiO₂ due to the formation of ternary surface complexes on TiO₂ surfaces.

Binding of Cd(II) by Amorphous Aluminum Hydroxide-Organophosphorus Coprecipitates: From Macroscopic to Microscopic Investigation

The mobility of Cd(II) in soils, sediments, and aquatic systems is strongly dependent on adsorption behaviors occurring at the mineral-water interface, and this process may be influenced by the presence of organic phosphorus (OP). In this study, we investigate Cd(II) adsorption onto amorphous aluminum hydroxide (AAH), both in the presence and absence of OP, represented by the widely abundant myo-inositol hexakisphosphate (IHP). Isothermal adsorption experiment coupled with attenuated total reflection Fourier transform infrared (ATR-FTIR) and 1H solid-state NMR spectra were employed. Physiochemical characterization shows that IHP can increase the surface negative charge and the number of surface sites. Isothermal results show that high IHP loading enhances Cd(II) adsorption while no obvious increase is observed at low IHP loading. The overall effect of IHP on Cd(II) sorption depends on the extent of two positive processes, i.e., (1) IHP can form ternary complexes with adsorbed Cd(II) on AAH and (2) IHP can increase the negative surface charge of AAH, and a negative process, i.e., AAH competes with Cd(II) for AAH surface sites. ATR-FTIR results confirm the possible formation of three structurally distinct ternary complexes, i.e., the AAH-IHP-Cd, AAH-Cd-IHP, and AAH-Cd-IHP-Cd. The analysis of 1H solid-state NMR demonstrates that IHP only increases the number of surface OH groups rather than changes their chemical environment and speciation. Cd does not bind to the AAH surface but mainly binds with the OH groups of IHP. All findings of this work suggest that the presence of high dose of OP promotes the retention of Cd(II) in soils, thereby decreasing their bioavailability to biota.

Molecular-level investigations of effective biogenic phosphorus adsorption by a lanthanum/aluminum-hydroxide composite

Biogenic phosphorus (P), such as organic P and inorganic pyrophosphates, could substantially contribute towards eutrophication in aquatic systems by internal loading of P from sediment through P species transformation. Previous eutrophication management studies mainly focus on the removal of orthophosphate (Ortho-P), however, an effective approach for biogenic P control from water sources, prior to incorporation in sediment, is still lacking. In this study, a lanthanum/aluminum-hydroxide (LAH) composite was demonstrated to provide both superior removal of Ortho-P and biogenic P, employing myo-inositol hexakisphosphate (IHP) and pyrophosphate (Pyro-P) as model compounds. The maximum IHP and Pyro-P adsorption capacities by LAH attained 36.4 and 21.8 mg P g^-1, respectively. In order to understand the mechanisms of adsorption, zeta potential, ³¹P solid-state nuclear magnetic resonance (NMR) spectroscopy and P K-edge X-ray absorption near edge structure (XANES) techniques were used to characterize the LAH after adsorption. The results supported the hypothesis that the interaction between LAH and P species was through surface adsorption, by the formation of inner-sphere complexes. Linear combination fitting results of XANES data indicated that IHP and Pyro-P preferentially bonded with La-hydroxide in LAH. This study elucidates the adsorption properties and binding mechanisms of IHP and Pyro-P on lanthanum-bearing compounds at the molecular level, indicating that LAH is a promising material for the control of eutrophication.

Quantitative investigation of ZnO nanoparticle dissolution in the presence of δ-MnO2

The widespread use of zinc oxide nanoparticles (ZnO NPs), the second most produced nanomaterial, inevitably leads to their release into the environment. In this study, dissolution and transformation of ZnO NPs in the presence of δ-MnO₂, an abundant and ubiquitous manganese (Mn) oxide mineral, was investigated via a suite of techniques covering bulk to molecular scales. Dissolution kinetics indicated that the presence of δ-MnO₂ significantly affected ZnO NP dissolution rate/trend and equilibrium Zn²⁺ concentration, which were found to be mainly dependent on the concentration and mass ratio of ZnO NPs and δ-MnO₂. Approximately 300 mg ZnO NPs per g δ-MnO₂ was expected for ZnO NP uptake at pH 7.0 via ZnO NP dissolution and surface Zn²⁺ adsorption. X-ray diffraction (XRD), ζ potential, high-resolution transmission electron microscopy (HR-TEM), and Zn K-edge X-ray absorption spectroscopy (XAS) results revealed that when the mole content of ZnO NPs was less than the total adsorption sites of δ-MnO₂ surface, ZnO NPs were completely dissolved and adsorbed on δ-MnO₂ surface in the form of inner-sphere complexes. A fraction of ZnO NPs persisted when the mole ratio of ZnO to δ-MnO₂ further increased. These results suggest that the transformation and fate of ZnO NPs is affected by environment-relevant minerals such as Mn oxides due to their huge capacity of fixing dissolved metal cations at the surface or interlayer structure.

Effects of myo-inositol hexakisphosphate, ferrihydrite coating, ionic strength and pH on the transport of TiO2 nanoparticles in quartz sand

Evaluating the fate and transport of nanoparticles (NPs) in the subsurface environment is critical for predicting the potential risks to both of the human health and environmental safety. It is believed that numerous environmental factors conspire to control the transport dynamics of nanoparticles, yet the effects of organic phosphates on nanoparticles transport remain largely unknown. In this work, we quantified the transport process of TiO₂ nanoparticle (nTiO₂) and their retention patterns in water-saturated sand columns under various myo-inositol hexakisphosphate (IHP) or phosphate (Pi) concentrations (0-180 μM P), ferrihydrite coating fractions (λ, 0-30%), ionic strengths (1-50 mM KCl), and pH values (4-8). The transport of nTiO₂ was enhanced at increased P concentration due to the enhanced colloidal stability. As compared with Pi at the equivalent P level, IHP showed stronger effect on the electrokinetic properties of nTiO₂ particles due to its relatively more negative charge and higher adsorption affinity, thereby facilitating the nTiO₂ transport (and thus reduced retention) in porous media. At the IHP concentration of 5 μM, the retention of nTiO₂ increased with increasing λ and ionic strength, while decreased with pH. In addition, the retention profiles of nTiO₂ showed a typical hyperexponential pattern for most scenarios mainly due to the unfavorable attachment, and can be well described by a hybrid mathematical model that coupled convection dispersion equations with a two-site kinetic model and DLVO theory. These quantitative estimations revealed the importance of IHP on affecting the transport of nTiO₂ typically in phosphorus-enriched environments. It provides new insights into advanced understanding of the co-transport of nanoparticles and phosphorus in natural systems, essential for both nanoparticle exposure and water eutrophication.

Review of analytical studies on TiO2 nanoparticles and particle aggregation, coagulation, flocculation, sedimentation, stabilization

Titanium dioxide (TiO₂) nanoparticles (NPs) have been widely used in industrial and consumer products. Comprehensive and accurate detection, characterization, and quantification of TiO₂ NPs are important for understanding the specific property, behavior, fate, and potential risk of TiO₂ NPs in natural and engineered environments. This review provides a summary of recent analytical studies of TiO₂ NPs and their aggregation, coagulation, flocculation, sedimentation, stabilization under a wide range of conditions and processes. Much attention is paid on sample preparation prior to an analytical procedure, analysis of particle size, morphology, structure, state, chemical composition, surface properties, etc., via measurements of light scattering and zeta potential, microscopy, spectroscopy, and related techniques. Recently, some advanced techniques have also been explored to characterize TiO₂ NPs and their behaviors in the environment. Many issues must be considered including distinction between engineered TiO₂ NPs and their naturally occurring counterparts, lack of reference materials, interlaboratory comparison, when analyzing low concentrations of TiO₂ NPs and their behaviors in complex matrices. No "ideal" technique has emerged as each technique has its own merits, biases, and limitations. Multi-method approach is highlighted to provide in-depth information. Improvements of analytical method for determination of TiO₂ NPs have been recommended to be together with exposure modelers and ecotoxicologists for maximum individual and mutual benefit. Future work should focus on developing analytical technology with the advantages of being reliable, sensitive, selective, reproducible, and capable of in situ detection in complicated sample system.

Enhanced oxidation of arsenite to arsenate using tunable K+ concentration in the OMS-2 tunnel

Cryptomelane-type octahedral molecular sieve manganese oxide (OMS-2) possesses high redox potential and has attracted much interest in its application for oxidation arsenite (As(III)) species of arsenic to arsenate (As(V)) to decrease arsenic toxicity and promote total arsenic removal. However, coexisting ions such as As(V) and phosphate are ubiquitous and readily bond to manganese oxide surface, consequently passivating surface active sites of manganese oxide and reducing As(III) oxidation. In this study, we present a novel strategy to significantly promote As(III) oxidation activity of OMS-2 by tuning K⁺ concentration in the tunnel. Batch experimental results reveal that increasing K⁺ concentration in the tunnel of OMS-2 not only considerably improved As(III) oxidation kinetics rate from 0.027 to 0.102 min^-1, but also reduced adverse effect of competitive ion on As(III) oxidation. The origin of K⁺ concentration effect on As(III) oxidation was investigated through As(V) and phosphate adsorption kinetics, detection of Mn²⁺ release in solution, surface charge characteristics, and density functional theory (DFT) calculations. Experimental results and theoretical calculations confirm that by increasing K⁺ concentration in the OMS-2 tunnel not only does it improve arsenic adsorption on K⁺ doped OMS-2, but also accelerates two electrons transfers from As(III) to each bonded Mn atom on OMS-2 surface, thus considerably improving As(III) oxidation kinetics rate, which is responsible for counteracting the adverse adsorption effects by coexisting ions.

Role of pH and ionic strength in the aggregation of TiO2 nanoparticles in the presence of extracellular polymeric substances from Bacillus subtilis

Increased use of commercial titanium dioxide nanoparticles (TiO₂ NPs) in consumer products most likely leads to their additional environmental release. Aggregation and disaggregation processes are expected to play an important role in the fate and transport of TiO₂ NPs in natural aquatic ecosystems. Therefore, in this work, we have studied the colloidal stability of TiO₂ NPs in the presence of extracellular polymeric substances (EPS) from Bacillus subtilis and the adsorption behavior of EPS on TiO₂ NPs in aqueous solutions at different pH values and ionic strengths (IS). The adsorption and aggregation processes were found to depend on the solution chemistry. The mass fraction of EPS on TiO₂ NPs decreased with increased pH and NaCl concentrations, which was verified by Fourier transform infrared spectroscopy. The presence of EPS can substantially influence the colloidal stability of TiO₂ NPs. In deionized water, the aggregation of NPs was induced by the addition of EPS only when the pH was below the TiO₂ NP point of zero charge (≈6). When the pH was equal to pH_{PZC, TiO2}, the TiO₂ NPs would rapidly form large aggregates, but the adsorption of EPS leads to partial fragmentation via electrostatic repulsion and steric hindrance. When the pH was greater than pH_{PZC, TiO2}, the aggregation rate was minimally affected by the increased EPS concentration. In NaCl solution, the aggregation rate of TiO₂ NPs obviously increased with increased NaCl concentration. The critical coagulation concentration (CCC) of TiO₂ NPs is 13.9 mM in the absence of EPS and increases to 155.6, 213.7 and 316.4 mM in the presence of 1, 5 and 10 mg/L EPS in NaCl solution, respectively, which indicates that the steric hindrance occurs after the addition of EPS. This study suggests that environmental conditions and EPS concentration greatly modify the colloidal stability of TiO₂ nanoparticles.

Enhanced Phosphorus Locking by Novel Lanthanum/Aluminum-Hydroxide Composite: Implications for Eutrophication Control

Lanthanum (La) bearing materials have been widely used to remove phosphorus (P) in water treatment. However, it remains a challenge to enhance phosphate (PO₄) adsorption capacity and La usage efficiency. In this study, La was coprecipitated with aluminum (Al) to obtain a La/Al-hydroxide composite (LAH) for P adsorption. The maximum PO₄ adsorption capacities of LAH (5.3% La) were 76.3 and 45.3 mg P g^-1 at pH 4.0 and 8.5, which were 8.5 and 5.3 times higher than those of commercially available La-modified bentonite (Phoslock, 5.6% La), respectively. P K-edge X-ray absorption near edge structure analysis showed that PO₄ was preferentially bonded with Al under weakly acid conditions (pH 4.0), while tended to associate with La under alkaline conditions (pH 8.5). La L_III-edge extended X-ray absorption fine structure analysis indicated that PO₄ was bonded on La sites by forming inner sphere bidentate-binuclear complexes and oxygen defects exhibited on LAH surfaces, which could be active adsorption sites for PO₄. The electrostatic interaction, ligand exchange, and oxygen defects on LAH surfaces jointly facilitated PO₄ adsorption but with varied contribution under different pH conditions. The combined contribution of two-component of La and Al may be an important direction for the next generation of commercial products for eutrophication mitigation.

Enhanced Dissolution and Transformation of ZnO Nanoparticles: The Role of Inositol Hexakisphosphate

The toxicity, reactivity, and behavior of zinc oxide (ZnO) nanoparticles (NPs) released in the environment are highly dependent on environmental conditions. Myo-inositol hexakisphosphate (IHP), a common organic phosphate, may interact with NPs and generate new transformation products. In this study, the role of IHP in mediating the dissolution and transformation of ZnO NPs was investigated in the laboratory kinetic experiments using powder X-ray diffraction, attenuated total reflectance Fourier transform infrared spectroscopy, (31)P nuclear magnetic resonance spectroscopy, high-resolution transmission electronic microscopy, and synchrotron-based extended X-ray absorption fine structure spectroscopy. The results indicate that IHP shows a dissolution-precipitation effect, which is different from citrate and EDTA that only enhances Zn dissolution. The enhanced dissolution and transformation of ZnO NPs by IHP (<0.5 h) is found to be strikingly faster than that induced by inorganic phosphate (Pi, > 3.0 h) at pH 7.0, and the reaction rate increases with decreasing pH and increasing IHP concentration. Multitechnique analyses reveal that interaction of ZnO NPs with IHP induces rapid transformation of ZnO NPs into zinc phytate complexes initially and poorly crystalline zinc phytate-like (Zn-IHP) phase finally. Additionally, ZnO NPs preferentially react with IHP and transform to Zn-IHP when Pi and IHP concurrently coexist in a system. Overall, results from this study contribute to an improved understanding of the role of organic phosphates (e.g., IHP) in the speciation and structural transformation of ZnO NPs, which can be leveraged for remediation of ZnO-polluted water and soils.