Binding mechanism of arsenate on rutile (110) and (001) planes studied using grazing-incidence EXAFS measurement and DFT calculation

Self-Competitive Adsorption Behavior of Arsenic on the TiO2 Surface

TiO2 is a commonly used material to remove arsenic from drinking water by adsorption as well as photocatalytic oxidation (PCO). In the present paper, arsenic adsorption and PCO at different pH environments are studied on the (1 1 0) facet of rutile TiO2 (r-TiO2). A self-competitive adsorption (SCA) behavior of arsenic is observed; i.e., arsenic species compete to adsorb on the surface. Related DFT calculations are carried out to simulate adsorption. SCA behavior is the key to connecting calculation results with experimental results. Furthermore, PCO of arsenite is performed at different pH values. Of note, PCO is related to adsorption; namely, the adsorption process determines the whole PCO reaction speed. Therefore, SCA is also helpful for the PCO reaction. The SCA behavior is useful not only for arsenic on r-TiO2 but also for arsenic on anatase TiO2 (a-TiO2). It may be helpful to further study arsenic adsorption and PCO on other materials such as Fe2O3 and MnO2. The SCA behavior extends our understanding of arsenic and provides new insights into arsenic removal and its cycle in nature.

Nanometer effect promoting arsenic removal on α-MnO2 nano-surface in aqueous solution: DFT+U research

The nanometer effect in the process of arsenic ions removal on α-MnO₂ nano-surface is studied by the first-principle method through microfacet models. Several parameters, such as adhesion energy, electrostatic potential, and Mulliken population were calculated to illuminate the internal mechanism. The results show that the adsorption energies of As(OH)₃ molecules on MnO₂[(100×110)] nanostructure are smaller than that on the bulk surface with the same concentration, which means the nanometer effect is beneficial to enhance the adsorption ability of MnO₂ nano-surface. In an aqueous solution, there exist two possible removal ways of As ions. One is the direct reaction of As(OH)₃→As(OH)₆^-, which occurs both in bulk surface and nano-surface. However, to nanomaterials, there exists another removal way of As(OH)₃→As(OH)₄→As(OH)₆^- through an intermediate As(OH)₄ molecule produced by nanometer effect. Furthermore the smaller electrostatic potential of As ions on [(100×110)] nano-surface is beneficial to enhance the removal capability of As ions. Then the reason why MnO₂ nanomaterials have better catalytic activity than the bulk materials is originated from its much less adhesion energy, much more removal ways, and much smaller electrostatic potential. So this research provides a detailed understanding of the removal capability of toxic ions influenced by a nanometer effect.

U(VI) adsorption on hematite nanocrystals: Insights into the reactivity of {001} and {012} facets

Predicting the environmental behavior of U(VI) relies on identification of its local coordination structure on mineral surfaces, which is also an indication of the intrinsic reactivity of the facet. We investigated the adsorption of U(VI) on two facets ({001} and {012}) of hematite (α-Fe₂O₃) by coupling experimental, spectroscopic and theoretical studies. Batch experiments results indicate higher removal capacity of the hematite {012} facet for U(VI) with respect to the {001} facet, due to the existence of extra singly and triply coordinated oxygen atoms with higher reactivity on the {012} facet while only doubly coordinated oxygen atoms exist on the {001} facet. The formation of surface complexes containing U(VI) is responsible for the appearance of a new sextuplet by Mössbauer spectra. The local structures of an inner-sphere edge-sharing bidentate complex on the hematite {001} and a corner-sharing complex on the {012} facet was deciphered by extended X-ray absorption fine structure spectroscopy. The chemical plausibility of the proposed structures was further verified by density functional theory calculation. This finding reveals the important influence of surficial hydroxyl groups reactivity on ions adsorption, which is helpful to better understand the interfacial interactions and to improve the prediction accuracy of U(VI) fate in aquatic environments.

The adsorption characteristics and degradation mechanism of tinidazole on an anatase TiO2 surface: a DFT study

The adsorption characteristics and degradation mechanism of tinidazole on TiO₂(101) and (001) surfaces under vacuum and aqueous solution conditions were studied by density functional theory (DFT). The results show that tinidazole can adsorb on the surfaces of TiO₂(101) and (001) under different conditions. The hydrogen bond generated during the adsorption process can enhance the stability of the adsorption configuration, which makes the bond length of C-N of tinidazole longer and finally facilitates the ring-opening degradation reaction. As for the mechanism of the ring-opening degradation reaction, it was found that ring-opening can be carried out along reaction route II on both crystal surfaces, and the reaction activation energy is lower on (101) surface. Under the conditions of aqueous solution, the decrease of the activation energy of the ring-opening degradation reaction indicates that the solvent conditions can promote the degradation reaction.

An experimental approach for determining thermodynamic surface complexation descriptors of weak-acid oxyanions onto metal (hydr)oxides: Case study of arsenic and titanium dioxide

The extensive literature review suggests that there are two main reasons for contradictory thermodynamic parameter values obtained via sorption experiments: (1) many of the studies are conducted under unrealistic conditions where the sorbate/sorbent ratios are so high that physisorption is artificially induced; or (2) many of the studies incorrectly calculate the equilibrium constants. The goal of this study is to demonstrate a methodology that describes how to properly determine and verify theoretically predicted thermodynamic descriptors. The study employs arsenate and titanium dioxide as a model sorbate-sorbent pair, which is equilibrated under realistic conditions for a period of 3 days at two different pH conditions (~6.5 and ~8.5) and three different temperatures (7 °C, 25 °C and 35 °C) in 10 mM NaHCO₃. At pH ≈ 8.55, ΔG^o values were -83.38 ± 1.62 kJ/mol, -88.13 ± 0.66 kJ/mol, and -90.78 ± 0.61 kJ/mol for sorption performed at 7 °C, 25 °C and 35 °C, respectively. Decreasing the pH to about 6.65 resulted in slightly less negative values of ΔG^o to -73.38 ± 1.58 kJ/mol, -77.14 ± 1.52 kJ/mol, and -78.75 ± 1.53 kJ/mol for sorption conducted at the same respective temperature conditions. These values overlap with the ΔG^o ranges reported for sorption of arsenate on metal oxides. Change in enthalpy values of ΔH^o = -19.04 kJ/mol at pH ≈ 6.65 and ΔH^o =-9.35 kJ/mol at pH ≈ 8.55 were observed. Based on reports, which suggest that at lower pH more bidentate ligands are being formed, these values are expected. The change in entropy values ranged from ΔS^o = 0.19 kJ/mol K at pH ≈ 6.55 to ΔS^o = 0.26 kJ/mol K at pH ≈ 8.55, which suggests lower level of disorder among the created complexes at lower pH and it is in line with the rationale that bidentate complexes are better organized on the surface of the sorbent and less susceptible with desorption. These findings clearly demonstrate that experimentally obtained ΔG⁰ and other thermodynamic values and trends could be obtained to reflect and confirm model predictions when the existing sorption theory is properly translated into experimental practice. The sorbate-sorbent bond in chemisorption has covalent character, characterized with short bond length and higher bond energy, which makes it less reversible when compared to physisorption, and therefore highly significant from a sorbent remediation-performance practical point of view and long-term waste sorbents disposal. While thermodynamic parameter modeling represents a good first step in determining the suitability of an initial design, experimental techniques potentially have the ability to provide far more superior description of the thermodynamic sorbent/sorbate interactions under realistic conditions.

First-principles and Molecular Dynamics simulation studies of functionalization of Au32 golden fullerene with amino acids

With the growing potential applications of nanoparticles in biomedicine especially the increasing concerns of nanotoxicity of gold nanoparticles, the interaction between protein and nanoparticles is proving to be of fundamental interest for bio-functionalization of materials. The interaction of glycine (Gly) amino acid with Au₃₂ fullerene was first investigated with B3LYP-D3/TZVP model. Several forms of glycine were selected to better understand the trends in binding nature of glycine interacting with the nanocage. We have evaluated various stable configurations of the Gly/Au₃₂ complexes and the calculated adsorption energies and AIM analysis indicate that non-Gly, z-Gly and also tripeptide glycine can form stable bindings with Au₃₂ at aqueous solution via their amino nitrogen (N) and/or carbonyl/carboxyl oxygen (O) active sites. Furthermore, cysteine, tyrosine, histidine and phenylalanine amino acids bound also strongly to the Au₃₂ nanocage. Electronic structures and quantum molecular descriptors calculations also demonstrate the significant changes in the electronic properties of the nanocage due to the attachment of selected amino acids. DFT based MD simulation for the most stable complex demonstrate that Gly/Au₃₂ complex is quite stable at ambient condition. Our first-principles findings offer fundamental insights into the functionalization of Au₃₂ nanocage and envisage its applicability as novel carrier of the drugs.

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.

A theoretical investigation of the removal of methylated arsenic pollutants with silicon doped graphene

Density functional theory calculations show the ability of silicon embedded graphene for the removal of methylated arsenic(iii, v) pollutants.