Cadmium Isotope Fractionation during Adsorption onto Edge Sites and Vacancies in Phyllomanganate.

Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd2+ adsorption on edge sites (Δ114/110Cdedge-solution = -1.54 ± 0.11‰) is smaller than that on vacancies (Δ114/110Cdvacancy-solution = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

Kinetics of polystyrene nanoplastic deposition on SiO2 and Al2O3 surfaces: Ionic strength effects.

Nanoplastic pollution is an emerging environmental threat to the critical zone. The transport of nanoplastic particles in subsurface environments can be determined mainly by soil minerals because they provide surfaces that interact with nanoplastic particles. However, the interactions between mineral surfaces and nanoplastics are poorly understood. In this study, the deposition kinetics of polystyrene-nanoplastic particles onto representative oxide surfaces SiO2 and Al2O3 at circumneutral pH were investigated using a quartz crystal microbalance, with variations in the ionic strength (0.1-100 mM) of the well-dispersed nanoplastic particles suspension. While polystyrene-nanoplastic particles deposited minimally on the SiO2 surface at an ionic strength of < 100 mM (∼10 ng/cm2), substantial deposition occurred at 100 mM (3.7 ± 0.4 µg/cm2). On the Al2O3 surface, a significant amount of polystyrene-nanoplastic particle was deposited from the lowest ionic strength (4.5 ± 0.8 µg/cm2). The deposition mass at 100 mM NaCl was two times higher (7.2 ± 0.2 µg/cm2) than on the SiO2 surface, while the deposition rates were similar between the two surfaces (10-15 Hz/min). Our results indicate that alumina most likely exerts a stronger influence than quartz on the transport of nanoplastic particles in soils and groundwater aquifers. The deposition kinetics strongly depends on the mineral surface and solution ionic strength, and these quantitative results can serve as validation data in developing transport modeling of nanoplastic in subsurface environments.

Toward a mechanistic understanding of cesium adsorption to todorokite: A molecular dynamics simulation study.

A mechanistic understanding of cesium (Cs) adsorption to soil mineral phases is essential for effective mitigation of Cs mobility in the subsurface environment. Todorokite, a common tunnel-structured manganese oxide in soil, exhibits sorption capacity for Cs comparable to the capacities of clay minerals. However, the adsorption sites and molecular species of Cs+ adsorbed to todorokite remain uncertain in comparison with those of clay minerals. In this study, we explored adsorption of Cs+ to hydrated todorokite surfaces via atomistic molecular dynamics (MD) simulations. We performed the first MD simulations based on atomic pair potentials for Mn-oxide edge surfaces interfaced with an aqueous solution. MD simulations predicted that Cs+ forms only inner-sphere (IS) complexes within todorokite tunnels; however, Cs+ forms both IS and outer-sphere (OS) complexes at the external (010) and (100)/(001) external surfaces. On the (010) surface, the positions between IS and OS complexes of Cs+ were interchangeable during MD simulations. Detailed molecular structures of IS and OS Cs+ surface complexes are compared to those of Cs+ in an aqueous solution. The current MD simulation results can be used as an atomistic structural proxy for spectroscopic analysis of adsorbed metal speciation and surface complexation modeling of metal adsorption to Mn oxides.

Molecular speciation of phosphorus in phosphogypsum waste by solid-state nuclear magnetic resonance spectroscopy.

Phosphogypsum (PG), a waste by-product of the phosphate fertilizer industry as well as a point-source P contaminant, has caused serious environmental problems particularly in estuarine and coastal regions. However, in-depth understanding of P speciation in PG, which is critical for its restoration and management, remains largely unknown. Using solid-state 31P NMR spectroscopy, density functional theory calculations of the NMR parameters and NanoSIMS, we for the first time reported that P in PG ubiquitously exists as phosphate incorporated into gypsum and minor fluorapatite. The occasional presences of mineral phosphate phases mainly associated with Ca and Al were also detected. The molecular environment of the incorporated phosphate is HPO42- substituting for SO42- in the gypsum lattice with the H atom away from the H2O molecules and almost parallel to the a-c plane. A high spatial heterogeneity was observed for the distribution of this phosphate species in PG at the submicron scale. Upon heating, at least 64% of the incorporated phosphate could be converted to the easy-to-recover fluorapatite or amorphous calcium phosphate by thermal treatments at above 750 °C for 2-4 h. This information of P speciation transformation may pave a solid basement for the sustainable recovery of P from PG.

Surface speciation of phosphate on boehmite (gamma-AlOOH) determined from NMR spectroscopy.

Interaction of phosphate with the surfaces of clays and metal oxyhydroxides is important for nutrient cycling in natural and agricultural systems. We examined the specific adsorption of phosphate to boehmite (gamma-AlOOH) by solid-state (31)P NMR spectroscopy, which yields evidence for the presence of two bridging bidentate surface complexes differing in protonation. For samples prepared along the sorption isotherm at pH 5, distinct phosphate environments are observed as two major peaks in (31)P NMR spectra (chemical shifts of 0 and -6 ppm) that show little change in relative intensity with adsorbate loading. Both peaks correspond to rigid phosphate in close proximity to H, as indicated by (31)P{(1)H} cross-polarization magic-angle-spinning (CP/MAS) data, and yield nearly identical (31)P{(27)Al} dephasing curves in rotational echo adiabatic passage double resonance (REAPDOR) experiments. The REAPDOR results indicate that both phosphate environments have similar coordination to Al and are best fit by dephasing curves simulated for bridging bidentate configurations. The two resolved phosphate species exhibit distinct (31)P chemical shift anisotropy (CSA) and intensity variations with pH, the peak near 0 ppm being dominant at pH > 7. (31)P CSA's from quantum chemical calculations of hydrated bidentate cluster models with varying protonation state show that the CSA for monoprotonated phosphate is unique and closely matches that for the peak at -6 ppm. The CSA for the peak at 0 ppm is consistent with both di- and nonprotonated phosphate, but assignment to the latter is suggested based on the dominance of this peak in samples prepared at high pH and with trends in (31)P NMR chemical shifts.

Defect-induced photoconductivity in layered manganese oxides: a density functional theory study.

Enhanced photoconductivity of layered Mn(IV)O2 containing protonated Mn(IV) vacancy defects has been recently demonstrated, suggesting new technological possibilities for photoelectric conversion based on visible light harvesting. Using spin-polarized density functional theory, we provide the first direct evidence that such defects can indeed facilitate photoconductivity by (i) reducing the band-gap energy and (ii) separating electron and hole states. Our results thus support the proposition that nanosheet MnO2 offers an attractive new material for a variety of photoconductivity applications.

Solid-state NMR and computational chemistry study of mononucleotides adsorbed to alumina.

Solid-state NMR spectroscopy and ab initio computational chemistry are used to determine the structure of the complex formed upon adsorption of the mononucleotide 2'-deoxyadenosine 5'-monophosphate (dAMP) to the surface of a mesoporous alumina. In this multi-technique approach, rotational-echo double-resonance NMR results reveal that the phosphate group of dAMP interacts predominantly with octahedrally coordinated aluminum species at the surface, and therefore, adsorption is modeled with both mono- and bidentate sorption of the nucleotide phosphate group with octahedral aluminum. 31P chemical shielding tensors are calculated from the structure of the lowest energy conformations, and these results are compared to tensor values extracted from analysis of spinning-sideband patterns in the experimental 31P cross-polarization magic-angle-spinning NMR spectrum. The chemical shift anisotropy and asymmetry parameter indicate that the binding is via a monodentate, inner-sphere complex.

Molecular orbital theory study on surface complex structures of glyphosate on goethite: calculation of vibrational frequencies.

Six possible complexes of glyphosate (O-PO(OH)-CH2NH2+CH2CO2H) with an Fe-hydroxide dimer were modeled with hybrid molecular orbital/density functional theory calculations to establish the nature of the bonds of glyphosate on goethite (alpha-FeOOH). Monodentate and bidentate coordination of the phosphonate moiety were considered, using three forms of the glyphosate molecule appropriate for different pH ranges: glyphosate with both phosphonate and amino moieties protonated, glyphosate with unprotonated phosphonates, and glyphosate with both unprotonated phosphonates and no hydrogen ion on the amino group. The calculated infrared vibrational modes were compared to experimental values, finding particularly good agreements with the monodentate complexes in all the pH ranges.

Model bacterial extracellular polysaccharide adsorption onto silica and alumina: quartz crystal microbalance with dissipation monitoring of dextran adsorption.

Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to investigate dextran adsorption to alumina and silica. Sensitive adsorption measurements combined with determination of nanometer-scale polymer conformations demonstrate the utility of this technique for studying biopolymer adsorption. The adsorbed amounts and polymeric structures of dextran were determined on A12O3 and SiO2 by real-time monitoring of resonance frequency and energy dissipation changes (deltaf and deltaD). After the sample was rinsed, the apparent mass of retained dextran was 83 ng/cm(2) on the alumina surface and 9 ng/cm2 on the silica surface based on the frequency and energy dissipation changes. The deltaD/deltaf ratios were significantly different on the two surfaces, indicating different conformations of the polymers. On alumina, the ratio changed as adsorption proceeded indicating changes of dextran conformation from the initial to latter adsorption steps. On silica, the ratio did not change during the experiments. Therefore, the dissipation and frequency data suggest significantly different mechanisms of dextran adsorption on alumina and silica surfaces. Molecular dynamics simulations of 12 monomeric units of dextran on a silica slab illustrated that H2O molecules lead to loosely bound dextran structure onto the SiO2 surface, consistent with the observed high-energy dissipation in the QCM-D experiments.

Adhesion of bacterial exopolymers to alpha-FeOOH: inner-sphere complexation of phosphodiester groups.

Extracellular polymeric substances (EPS) constitute a heterogeneous mixture of polyelectrolytes that mediate biomineralization and bacterial adhesion and stabilize biofilm matrixes in natural and artificial environments. Although nucleic acids are exuded extracellularly and are purported to be required for biofilm formation, direct evidence of the active mechanism is lacking. EPS were extracted from both Bacillus subtilis (a gram-positive bacterium) and Pseudomonas aeruginosa (a gram-negative bacterium) and their interaction with the goethite (alpha-FeOOH) surface was studied using attenuated total internal reflection infrared spectroscopy. Correspondence between spectral data and quantum chemical calculations demonstrate that phosphodiester groups of nucleic acids mediate the binding of EPS to mineral surfaces. Our data indicate that these groups emerge from the EPS mixture to form monodentate complexes with Fe centers on the goethite (alpha-FeOOH) surface, providing an energetically stable bond for further EPS or cell adhesion.

Molecular orbital theory study on surface complex structures of phosphates to iron hydroxides: calculation of vibrational frequencies and adsorption energies.

Quantum mechanical calculations were applied to resolve controversies about phosphate surface complexes on iron hydroxides. Six possible surface complexes were modeled: deprotonated, monoprotonated, and diprotonated versions of bridging bidentate and monodentate complexes. The calculated frequencies were compared to experimental IR frequency data (Persson et al. J. Colloid Interface Sci. 1996, 177, 263-275; Arai and Sparks J. Colloid Interface Sci. 2001, 241, 317-326.). This study suggests that the surface complexes change depending on pH. Four possible species are a diprotonated bidentate complex at pH 4-6, either a deprotonated bidentate or a monoprotonated monodentate complex at pH 7.5-7.9, and a deprotonated monodentate complex at pH 12.8. In addition, reaction energies were calculated for adsorption from aqueous solution to determine relative stability to form a monoprotonated monodentate complex and a deprotonated bidentate complex. According to these results, the monoprotonated monodentate complex should be favored. Vibrational frequencies of the monoprotonated monodentate and deprotonated bidentate complexes were analyzed with electronic effects on the Fe-OP and H-OP bonds.